Evaluating the Use of Displacement Ventilation for Providing Space Heating in Unoccupied Periods Using Laboratory Experiments, Field Tests and Numerical Simulations

energies

Article Evaluating the Use of Displacement Ventilation for Providing Space Heating in Unoccupied Periods Using Laboratory Experiments, Field Tests and Numerical Simulations

Saqib Javed 1,* , Ivar Rognhaug Ørnes 2, Tor Helge Dokka 2, Maria Myrup 2 and Sverre Bjørn Holøs 3

1 Building Services, Lund University, 221 00 Lund, Sweden 2 Climate, Energy and Building Physics, Skanska Norway, 0107 Oslo, Norway; [email protected] (I.R.Ø.); [email protected] (T.H.D.); [email protected] (M.M.) 3 Architectural Engineering, SINTEF Community, 0373 Oslo, Norway; [email protected] * Correspondence: [email protected]; Tel.: +46-46-222-1745

Abstract: Displacement ventilation is a proven method of providing conditioned air to enclosed spaces with the aim to deliver good air quality and thermal comfort while reducing the amount of energy required to operate the system. Until now, the practical applications of displacement ventilation have been exclusive to providing ventilation and cooling to large open spaces with high ceilings. The provision of heating through displacement ventilation has traditionally been discouraged, out of concern that warm air supplied at the floor level would rise straight to the ceiling level without providing heat to the occupied space. Hence, a separate heating system is regularly integrated with the displacement ventilation in cold climates, increasing the cost and energy use of Citation: Javed, S.; Ørnes, I.R.; the system. This paper goes beyond the common industry practice and explores the possibility of Dokka, T.H.; Myrup, M.; Holøs, S.B. using displacement ventilation to provide heating without any additional heating system. It reports Evaluating the Use of Displacement on experimental investigations conducted in laboratory and field settings, and numerical simulation Ventilation for Providing Space of these studies, all aimed at investigating the application of displacement ventilation for providing a Heating in Unoccupied Periods Using comfortable indoor environment in winter by preheating the space prior to occupancy. The experi- Laboratory Experiments, Field Tests mental results confirm that the proposed concept of providing space heating in unoccupied periods and Numerical Simulations. Energies 2021, 14, 952. https://doi.org/ without a separate heating system is possible with displacement ventilation. 10.3390/en14040952 Keywords: indoor air quality; thermal comfort; CO2 concentrations; classroom; preheating; stratification Academic Editor: John Kaiser Calautit

Received: 16 January 2021 1. Introduction Accepted: 5 February 2021 In modern societies, people are spending increasingly more time in indoor environ- Published: 11 February 2021 ments. Due to the wide spectrum of pollutants and contaminants present in these confined environments, indoor air quality (IAQ) has become a matter of great importance [1]. It has Publisher’s Note: MDPI stays neutral been repeatedly and conclusively demonstrated that indoor air quality has a significant with regard to jurisdictional claims in influence on human health, comfort, and productivity [2,3]. The most common method published maps and institutional affil- for controlling and improving indoor air quality is the dilution of indoor pollutants and iations. contaminants by providing clean air through ventilation. It is well-established that a higher ventilation rate, i.e., bringing in more fresh outdoor air, is advantageous for reducing indoor air pollutants and thus achieving better indoor air quality [4]. However, improving indoor air quality by increasing ventilation rates is directly correlated with increased energy use Copyright: © 2021 by the authors. in buildings [5]. Today, the energy demands of newly constructed and renovated buildings Licensee MDPI, Basel, Switzerland. are tightly controlled through legislative building regulations and codes. Furthermore, This article is an open access article voluntary building certification programs like BREEAM (Building Research Establishment distributed under the terms and Environmental Assessment Method) [6] and LEED (Leadership in Energy and Environ- conditions of the Creative Commons mental Design) [7] also require buildings to reduce their energy consumption. In order to Attribution (CC BY) license (https:// fulfil the somewhat contradictory requirements of providing high indoor air quality while creativecommons.org/licenses/by/ simultaneously satisfying the requirements of low energy consumption, displacement ven- 4.0/).

Energies 2021, 14, 952. https://doi.org/10.3390/en14040952 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 34

Energies 2021, 14, 952 2 of 33 ship in Energy and Environmental Design) [7] also require buildings to reduce their energy consumption. In order to fulfil the somewhat contradictory requirements of providing high indoor air quality while simultaneously satisfying the requirements of low en- tilationergy consumption, has, in recent displacement years, emerged ventilation as an efficient has, in and recent a superior years, eme alternativerged as toan the efficient more commonlyand a superior used alternative mixing ventilation. to the more commonly used mixing ventilation. Figure1 1 presents presents a a conceptualconceptual illustration illustration of of the the mostmost salientsalient aspects aspects ofof displacementdisplacement ventilation. In displacement ventilation, relatively cold, and thus heavierheavier airair isis suppliedsupplied to spacespace atat lowlow levels.levels. The The cold cold air is warmed up by heat sources present in the space, including, for example,example, people,people, lightinglighting andand equipment,equipment, etc.etc. The air movesmoves upwardsupwards asas thermal plumes due toto buoyancybuoyancy effectseffects inducedinduced byby densitydensity differences.differences. TheThe ascendingascending thermal plumes reach the equilibriumequilibrium density level at thethe so-calledso-called stratificationstratification height, after which which they they spread spread horizontally. horizontally. The The warm warm and andlighter lighter air is airthus is accumulated thus accumulated below belowthe ceiling the ceilinglevel and level is extracted and is extracted from the from space the at spacehigh levels. at high The levels. thickness The thicknessof the upper of thelayer upper depends layer upon depends the plume upon and the the plume supply and airflows. the supply The airflows. pollutants The and pollutants contaminants and contaminantsthat are either thatwarmer areeither and/or warmer lighter and/orthan the lighter surrounding than the air surrounding are also led airupward are also by ledthe upwardascending by displacement the ascending flow. displacement Wherefore flow.the ventilated Wherefore space the ventilatedbecomes divided space becomesinto two dividedzones: A intolower two occupied zones: Azone lower with occupied clean air zone and withan upper clean unoccupied air and an upperzone with unoccupied contam- zoneinated with air. contaminatedIn order to provide air. In orderthe same to provide level of the ventilation, same level displacement of ventilation, ventilation displacement re- ventilationquires considerably requires considerably lower volume lower flows volume than flowsthe more than commonly the more commonly used mixing used ventila- mixing ventilation.tion. This is Thisbecause is because the air distribution the air distribution effectiveness effectiveness of displacement of displacement ventilation ventilation systems systemsis significantly is significantly higher than higher that than of mixed that of ventilation mixed ventilation systems systems [8]. This, [8 ]. in This, turn, in means turn, means lower energy consumption by fans as well as reduced energy consumption for lower energy consumption by fans as well as reduced energy consumption for thermal thermal conditioning of the ventilation air. conditioning of the ventilation air.

Figure 1. Illustration of displacement ventilation.

The supply air air may may quickly quickly rise rise from from the the occupied occupied zone zone to tothe the unoccupied unoccupied zone zone if its if itstemperature temperature is too is tooclose close or h origher higher than than the room the room temperature. temperature. In such In a such case, a the case, effective- the ef- fectivenessness of the displacement of the displacement ventilation ventilation may well may be well reduced be reduced significantly significantly due to duethe toshort the- short-circuitingcircuiting of the ofsupply the supply air. Since air. Sincethe supply the supply air in airdisplacement in displacement ventilation ventilation is provided is pro- videdat a temperatur at a temperaturee lower lowerthan the than space the spacetemperature, temperature, the applications the applications of displacement of displacement ven- ventilationtilation have have largely largely been beenconfined confined to providing to providing ventilation ventilation and cooling and cooling to the conditioned to the con- ditionedspaces. Sometimes, spaces. Sometimes, displacement displacement ventilation ventilation is also used is also together usedtogether with radiant with radiantcooling coolingsystems, systems, e.g., chilled e.g., chilled ceilings, ceilings, and floor and floor cooling. cooling. However, However, the the provision provision of ofheating heat- ingthrough through the thedisplacement displacement ventilation ventilation has hasgenerally generally been been discouraged discouraged because because of the of thepo- potentialtential short short-circuiting-circuiting of the of thewarm warm buoyant buoyant air to air the to unoccupied the unoccupied zone zone [9]. The [9]. use The of use an ofauxiliary an auxiliary heating heating system, system, e.g., floor e.g., floorheating, heating, ceiling ceiling panels, panels, and wall and wallradiators radiators and con- and convectors,vectors, has hasbeen been recommended recommended for forspace space heating heating when when using using displacement displacement ventilation ventilation[10]. [Using10]. Using a separate a separate heating heating system system not notonly only inflates inflates the thecapital capital and and operating operating costs costs of ofthe the overall overall system system [11] [11, ],but but also also leads leads to to increased increased environmental environmental impacts impacts due due to higher material and energy use [[12].12]. This paper is based on thethe hypothesishypothesis that displacementdisplacement ventilation can be used forfor providing spacespace heating,heating, primarilyprimarily to to avoid avoid the the added added cost cost and and environmental environmental impacts impacts of of a separatea separate heating heating system. system. A fewA few exploratory exploratory studies studies [13, 14[13,14]] have have shown shown that therethat there may existmay more potential for heating with displacement ventilation using slightly elevated supply temperatures than hitherto suggested in the literature. If the extraction and supply points are carefully located to avoid short-circuiting, e.g., at opposite ends of the room, supplying warm air through the displacement ventilation system destroys the vertical stratification in

Energies 2021, 14, 952 3 of 33

the space and results in mixing ventilation like air distribution [15]. Elsewise, special displacement diffusers with integrated heating sections, supplying slow-moving cold air for cooling from one part and fast-moving warm air for heating from the other part, may be used [16]. Another possible approach is to provide heating outside the occupied hours through the ventilation system. This way, space can be preheated to a suitable temperature level by supplying warm air during the non-occupancy periods, e.g., at night. In the occupied hours, space can then be provided with normal displacement ventilation, i.e., supplying air with a temperature a few degrees below the room temperature. The objective of this paper is to increase understanding of the application of displacement ventilation systems for providing heating through supply air, without any separate heating system. This is accomplished through a field test in a real classroom environment, laboratory tests on a scaled model of a classroom, and simulation studies of these tests. The paper first provides an extensive review of literature on the use of displacement ventilation. It then presents a field application of displacement ventilation providing night-time heating. The paper next describes the methodology of the field and the laboratory tests, and the simulation study, followed by the results from these investigations. A comparative discussion of the experimental and simulation results is then presented, together with design recommendations and lessons learned for practitioners and researchers interested in applying displacement ventilation with night-time heating. Finally, conclusions and final remarks are presented.

2. Literature Review Displacement ventilation has been extensively studied in the literature. Several reference manuals and design guides [9,17–19] offering a detailed description of design procedures and methods, design strategies and constraints, technical and performance requirements, and application examples and case studies of displacement ventilation, have been published. The typical installations of displacement ventilation include meeting rooms, lec- ture halls, auditoriums, theaters, conference rooms, shopping malls, and atriums, among others. One common application of displacement ventilation systems, also considered in this study, is in classrooms and school buildings, where indoor air quality and thermal comfort are of great significance due to their impact on both the learning environment and on students’ health and wellness. Schools have considerably higher occupancy densities than office buildings, which, in turn, results in higher internal gains and larger concentrations of indoor pollutants. Several studies, including [20–23], have noted that indoor air quality and thermal comfort problems are widespread in schools and other educational buildings. Ventilation rates are often inadequate in classrooms [24] and microbiological contaminants (e.g., allergens, fungi, and bacteria), formaldehyde, and total volatile organic compounds are commonly found in school and classroom environments [25]. Displacement ventilation is often used in classrooms and schools to provide a high level of air quality. Compared to mixing ventilation, displacement ventilation has been shown to result in lower concentrations of pollutants and contaminants in classrooms, at least in the breathing zone, and to improve the overall perception of air quality among students [26]. Moreover, displacement ventilation has also been shown to yield significant energy savings in schools [27]. Modeling for the design and simulation of displacement ventilation systems has been an open research topic. The most commonly used modeling approach for sizing displacement ventilation systems for non-industrial applications is the so-called temperature-based design approach. In this approach, the supply airflow and the supply air temperature are determined based on the heat balance of the occupied and upper zones in the room [9]. As stratification in the occupied zone is important for the thermal comfort of the occupants, the approach involves a calculation of the vertical air temperature gradient in the room using a temperature stratification model, such as those suggested presented by Mundt [28], Nielsen [29], or Mateus and da Graça [30], among others. These models differ in their assumptions about the temperature distribution in the room, and the number of temper- Energies 2021, 14, 952 4 of 33

ature nodes used to model the temperature profile. The Mundt model assumes a linear distribution of the indoor air temperature over the entire room height. The temperature profile of the room air is obtained using two temperature nodes, one at the floor level and the other at the ceiling level. The convective heat transfer from the floor surface to the supply air is taken equal to the radiative heat exchange between the ceiling and the floor surfaces. This model has been implemented in some building energy simulation software, such as EnergyPlus [31] and IDA Indoor Climate and Energy (IDA-ICE) [32]. The Nielsen model also considers a linear temperature distribution of indoor air but, unlike the Mundt model, the temperature gradient is only considered linear between the floor level and the stratification height, above which the air temperature is taken to be constant. The model calculates the vertical air temperature gradient based on the so-called Archimedes number of the flow and the type of heat source in the occupied zone. The Mateus and da Graça model considers a non-linear temperature distribution in the room. The model predicts the temperature profile of the indoor air using three temperature nodes, one at the floor level, one in the occupied zone, and one at the stratification height. Above the stratification height, the air temperature is considered constant by the model. The model considers four room surfaces, i.e., floor, ceiling, and two lateral wall portions. The radiative heat exchange between these surfaces and the convective heat exchange between each room surface and the corresponding air temperature node connected to it is accounted for by the model. The entrainment generated accumulated flows and the convective heat gains that get mixed into the occupied zone, and, are not directly carried to the stratification height are also considered by the model. A more complicated and consequently less used, modeling approach for sizing displacement ventilation systems is the so-called shift zone design approach. In this approach, the supply airflow at the stratification height, taken above the breathing zone, is set equal to the total upwards convective flows. The supply airflow is chosen to ensure that the contamination concentrations are below the threshold levels in the occupied zone and that the thermal comfort conditions in the occupied zone are met. Hence, in the shift zone approach, in addition to the modeling temperature gradient in the room, modeling contaminant concentration gradients are equally desirable. Calculation of the vertical contaminant gradient is generally carried out using zonal models, such as those proposed by Skåret [33], Koganei et al., [34], Sandberg et al., [35] Dokka [36], or Yamanaka [37], among others. Factors affecting the design and performance of displacement ventilation have been extensively studied using simulations, experiments, and field tests. Several laboratory and field studies have been undertaken to examine the underlying principles of thermal stratification and contaminant dilution. Okutan et al., [38] investigated the performance of displacement ventilation systems in open-plan office environments using a scale model, focusing in particular on vertical temperature distribution. Brohus and Nielsen [39] examined the effects of persons present in a displacement ventilated room on the contaminant distribution through full-scale measurements. In another study, the authors also probed the exposure of a seated and a standing person in proportion to the stratification height [40]. Akimoto et al., [41] studied the indoor thermal environment of the floor-supply displacement ventilation system in a controlled chamber altering the supply air volume, heat load, and position of heat sources. Yuan et al., [42,43] performed detailed measurements on the age of air and the vertical profiles of air temperature, air velocity, and contaminant concentration in a test chamber with displacement ventilation, simulating a small office, a large office with partitions, and a classroom. Xu et al., [44,45] examined the effect of heat loss through walls upon the distribution of temperature and contaminant concentration in an experimental room with displacement ventilation. Mundt [46] evaluated particle transportation and ventilation efficiency in a displacement-ventilated room with non-buoyant pollutant sources. Cheong et al., [47,48] assessed the effects of local and overall thermal sensations and comfort in a field environmental chamber served by a displacement ventilation system. Wachenfeldt et al., [49] evaluated the airflow rates and energy-saving potential of demand-controlled displacement ventilation systems in two Energies 2021, 14, 952 5 of 33

Norwegian schools. Trzeciakiewicz [50] investigated the two-zone airflow patterns and determined the stratification heights in a mock-up office room under conditions of various heat sources and airflow rates. Yu et al., [51] investigated the thermal influence of temperature gradient on overall and local thermal comfort at different room air temperatures in a large environment chamber served by displacement ventilation. These studies suggest that several factors are key to the design and performance of displacement ventilation systems, and thus must be considered in both modeling and experimental analysis of these systems. The effects of supply air conditions, heat and contaminant sources, and other practical issues concerning displacement ventilation have also been assessed using computer simulations. Lin et al., [52,53] and Kang et al., [54] examined the effects of supply air temperature and supply air location on the performance of displacement ventilation using CFD (computational fluid dynamics) analysis. Yuan et al., [43], Kobayashi and Chen [55], and Lin and Lin [56] studied the influence of supply airflow. Mathisen [57], Zhang et al., [58], and Cehlin and Moshfegh [59] reported on the effects of supply air diffuser on displacement ventilation. Several researchers including Park and Holland [60], Rees et al., [61], Deevy et al. [62], Zhong et al. [63], and Causone et al. [10] probed the effect of the heat and contaminant source location on the displacement ventilation performance using CFD simulations. Matsumoto et al., [64], Matsumoto and Ohba [65], and Mazumdar et al. [66] studied the impact of moving sources on the displacement ventilation. Li et al., [67], Faure and Le Roux [68], and Wu et al., [69] investigated the effects of heat losses, gains, and transfers from different room-envelope elements on the distribution of temperature and contaminant concentration in rooms with displacement ventilation. Lin et al., [70], and Hashimoto and Yoneda [71] studied the influence of ceiling height on the performance of the displacement ventilation. Lin et al., [72] and Mazumdar et al., [66] explored the impact of the door opening on thermal and contaminant distribution in the room through computer simulations. The results of the simulation studies indicate that supply air conditions, heat and contaminant source characteristics, envelope properties, and movements in the space all have a profound impact on displacement ventilation. Some experimental studies have examined air terminal and air-jet characteristics for displacement ventilation under a variety of supply air and indoor conditions. Xing et al. [73] tested three different types of displacement diffusers, including a flat-wall diffuser, a semi- circular diffuser, and a floor swirl diffuser, and measured the age of air distribution, air exchange index, and ventilation effectiveness for each diffuser type in a mock-up office room with varying thermal loads. In a related study, Xing and Awbi [74] assessed the relationship between stratification height and ventilation load under similar experimental settings. Kobayashi and Chen [55], and Lau and Chen [75] studied the performance of floor supply displacement ventilation system in a full-scale environmental chamber, simulating a two-person office room with swirl diffusers and a workshop with perforated panels and swirl diffusers, respectively. Fatemi et al., [76] analyzed the flow physics of a non-isothermal jet stream in a large room supplied by a relatively large corner-mounted quarter-round displacement diffuser. Fernández-Gutiérrez et al., [77] characterized a small-scale, low- velocity displacement diffuser in a laboratory test chamber through flow visualizations and velocity field measurements. Magnier [78] investigated velocity and temperature distribution in air jets from two different wall-mounted displacement ventilation diffusers, for different supply conditions. The experimental studies show that the effectiveness of the displacement ventilation system is directly affected by the type of the diffuser and characteristics of the supply air jet. A few studies have experimentally investigated the combination of displacement ventilation with radiant heating and cooling systems. Causone et al., [79] experimentally evaluated the possibilities and limitations of combining radiant floor heating and cooling with displacement ventilation. The profiles of air temperature, velocity and ventilation effectiveness were measured under typical office conditions. Wu et al., [69] investigated the air distribution and ventilation effectiveness of displacement ventilation systems with floor heating and ceiling heating systems in a laboratory investigation. Rees and Haves [80] Energies 2021, 14, 952 6 of 33

studied airflow and temperature distribution in a test chamber with displacement ventilation and a chilled ceiling over a range of operating parameters typical of office applications. Schiavon et al., [81] performed laboratory experiments to study the room air stratification and air change effectiveness in a typical office space with a combined radiant chilled ceiling and displacement ventilation system. All these studies indicate that radiant systems for supplemental heating and cooling are well-suited for displacement ventilation. In a particularly unique series of field studies in three Canadian schools, Ouazia et al., [14,82] evaluated the performance of displacement ventilation in heating mode with supplemen- tary perimeter heating systems. The contaminant removal effectiveness of the displacement ventilation in the heating mode was found to be higher than previously suggested in the literature. Moreover, thermal comfort indices, including vertical temperature gradient and draft ratio, were also found to be satisfactory. Several studies have focused on comparing the performance characteristics of displacement ventilation with mixing ventilation. Akimoto et al., [83] and Rimmer et al., [84] compared the two ventilation systems in terms of the mean age of the air in an environmental chamber simulating an office room, and an actual hospital building, respectively. Breum [85] studied the displacement and mixing ventilation systems in terms of exposure to a simulated body odor in an experimental chamber. Olmedo et al., [86] investigated the human exhalation flows for the two systems in a full-scale test chamber. Wu et al., [87] explored air distribution for the two systems with or without floor and ceiling heating in a multi-occupant room. Behne [88] evaluated the two systems with chilled ceilings. Cermak et al., [89] analyzed air quality and thermal comfort with the two systems in an office room. In a related study, Cremak and Melikov [90] probed the performance of personalized ventilation in conjunction with the two system types. Yin et al., [91] assessed the performance of the two ventilation systems in relation to the location of the exhaust in a full-scale experimental chamber. Smedje et al., [92] examined the two systems in the light of air quality, airborne cat allergen, and climate factors in the occupied zone in four classrooms of a school building. Hu et al., [93] carried out a comparison of energy consumption between displacement and mixing ventilation systems for different buildings and climates. Lin et al., [94,95] used CFD simulations to compare displacement and mixing ventilation in terms of thermal comfort and indoor air quality. These studies demonstrate that displacement ventilation systems normally have higher values of contaminant removal effectiveness and air change efficiency than mixing ventilation systems.

3. A Novel Application of Displacement Ventilation in Cold Climates In Norway and other Scandinavian countries, the heating, ventilation, and air conditioning (HVAC) system for school and office buildings is typically a hybrid air-water system. It is customary to use balanced mechanical ventilation with heat recovery to provide overall air quality, along with high energy efficiency in winter and thermal comfort in summer. In recent years, it has also become common to use mechanical cooling or free cooling from ground heat exchangers distributed centrally through the air system [96]. Heating is generally provided through a hydronic system, with radiators and underfloor heating being the most used terminal types. However, ventilation air is usually heated centrally to a supply setpoint, generally a few degrees below the room temperature (typically 17–20 ◦C). Until recently, heating buildings and individual rooms with an all-air system have been uncommon in Norway, except for storage facilities and certain industrial buildings with lower demands on thermal comfort. In recent years, however, the widespread introduction of low energy buildings and passive houses [97,98] have created new opportunities for innovative heating solutions. For such buildings with low heating demands, using an all-air system for heating provides a simple and energy-efficient alternative to the conventional hydronic heating systems [97,99]. Demand-controlled ventilation systems that vary airflow with changing heat load in the occupied zone have been used in some passive houses or similar energy standard office buildings [100], and have also been examined in laboratory Energies 2021, 14, x FOR PEER REVIEW 7 of 34

with lower demands on thermal comfort. In recent years, however, the widespread introduction of low energy buildings and passive houses [97,98] have created new opportunities for innovative heating solutions. For such buildings with low heating demands, using an all-air system for heating provides a simple and energy-efficient alternative to the conventional hydronic heating systems [97,99]. Demand-controlled ventilation systems that

Energies 2021, 14, 952 vary airflow with changing heat load in the occupied zone have been used in7 some of 33 passive houses or similar energy standard office buildings [100], and have also been examined in laboratory tests. These systems have been based primarily on the mixing ventilation principle, giving a rather uniform temperature and air quality in the occupied space. tests. These systems have been based primarily on the mixing ventilation principle, giving a The concept of providing heating and cooling through an air system has been taken rather uniform temperature and air quality in the occupied space. a stepThe further concept in of a providing newly built heating Montessori and cooling school through in Drøbak, an air system Norway. has The been lower taken asecondary stepschool further for in60 a newlystudents built has Montessori been built school with in a Drøbak, heated Norway. area of Theapproximately lower secondary 900 m2. The schoolschool for has 60 studentstwo level hass, been with built the with lower a heated floor areaunderparts of approximately of the building 900 m2. The where school the terrain hashas two a natural levels, fall. with The the lowerbuilding floor has underparts a compact of rectangular the building shape where oriented the terrain southeast has a -north- naturaleast and fall. is The intersected building by has an a compactinclined rectangular “solar slice”. shape A photograph oriented southeast-northeast of the front of the build- anding is is intersected shown in byFigure an inclined 2. The “solarschool slice”. has been A photograph built with of a thevision front to of become the building Norway’s is most shownenvironmentally in Figure2. The friendly school school. has been It built is the with first a visionschool to building become Norway’s to fulfil the most requirements envi- of ronmentally friendly school. It is the first school building to fulfil the requirements of the the Norwegian Powerhouse-concept [101]. The basis for the design is a well-insulated Norwegian Powerhouse-concept [101]. The basis for the design is a well-insulated building envelopebuilding with envelope minimized with heat minimized loss, a very heat efficient loss, a lighting very efficient system, lighting a high-performance system, a high-per- ventilationformance system, ventilation and asystem, ground-source and a ground heat pump-source system heat that pump provides system free coolingthat provides in free summer.cooling Thesein summer. measures These reduce measures the demand reduce for deliveredthe demand energy for radically,delivered and energy a building- radically, and integrateda building system-integrated with high-efficiency system with photovoltaic high-efficiency (PV) modules photovoltaic makes (PV) the school modules into amakes the plusschool energy into building a plus energy according building to the Powerhouse-definition.according to the Powerhouse The specifications-definition. for The the specifica- designtions for and the simulated designenergy and simulated performance energy are given performance in Tables1 are and given2 below. in Tables 1 and 2 below.

FigureFigure 2. 2.Drøbak Drøbak Montessori Montessori lower lower secondary secondary school. school. (Credit: (Credit: Robin Hyes, Robin Skanska). Hyes, Skanska).

Table 1. Design details of the Powerhouse Drøbak Montessori lower secondary school.

System Design Parameter Value 886 m2 Floor area (heated)Air volume (heated) 3340 m3 U-value external wall 0.14 W/m2K U-value roof 0.09 W/m2K U-value slab on ground 0.10 W/m2K U-value window (average value) 0.75 W/m2K Infiltration at 50 Pa 0.50 ach Normalized heat capacity (medium-heavy building) 81 Wh/m2K 4.0–8.0 m3/hm2 Average airflowHeat recovery efficiencySpecific fan power 86% 0.60 kW/m3/s 17 W/m2 Cooling capacity, cooling coilFree cooling from geothermal wells 15 kW 20 W/m2 Heating capacity, heating coilHeating from geothermal heat pump 15 kW Lighting average load 3 W/m2 145 m2 PV-system module areaEfficiencyPeak load 21% 30 kW Energies 2021, 14, 952 8 of 33

Table 2. Simulated net energy demand and delivered energy for the school. Energies 2021, 14, x FOR PEER REVIEW 9 of 34 Coefficient of Net Energy Demand Delivered Energy System Performance (kWh/m2/year) (kWh/m2/year) (COP) SpaceThe heating HVAC (by system air) of the 18.7Drøbak Montessori has 4.5 been designed to achieve 4.2 the high performanceDomestic hot needed water to be a plus 3.2 energy building, but 3.0 with as simple and robust 1.1 technology as possible.Fans Heating and cooling 3.7 to the school are - provided by a central 3.7 air system. The airPumps distribution is based on 1.0 displacement ventilation - that varies between 1.0 fully me- Lighting 6.6 - 6.6 chanicalPlug and loads hybrid ventilation 13.0 depending on the time - of the year. The supply 13.0 air is pro- videdVentilation to classrooms cooling and other 1.8spaces via rectangular 15.0 perforated displacement 0.1 diffusers installedTotal in the interior walls at 48.1 low levels. The extract- air is removed from29.6 the occupied zonesPV at Production high levels by overflow-- to the adjacent areas. During summer, the− exhaust37.0 air is directlyNet Delivered discharged Energy to the outside-- through an opening in the top of the atrium−7.4 in the center of the building. During winters, the air is mechanically exhausted through the air han- dlingThe unit HVAC after systemheat recovery. of the Drøbak Montessori has been designed to achieve the high performanceThe HVAC needed system to be of a plusthe Drøbak energy building,Montessori but is with unique as simple in the and aspect robust that technology it is based ason possible. the displacement Heating and ventilation cooling toprinciple the school and are provides provided heating by a central only outside air system. the occupied The air distributionperiods. During is based the onoccupied displacement hours between ventilation 7:30 that to 16:00 varies h, betweenthe system fully operates mechanical in the and“normal hybrid mode”, ventilation in which depending a VAV (variable on the time air volume) of the year. damper The regulates supply air flow is provided to each zone to classroomsto maintain and the otherdesired spaces indoor via CO rectangular2 (carbon dioxide) perforated concentration displacement and diffusers space temperature installed inset the-points. interior The walls supply at low air temperature levels. The extract during air the is normal removed mode from is the outdoor occupied temperature zones at - highcompensated levels by overflowas shown toin theFigure adjacent 3. In the areas. warmest During periods, summer, the the supply exhaust air airis provided is directly at discharged18.5 °C, whereas, to the outsidein the cold throughest periods, an opening the supply in the topair temperature of the atrium is in approximately the center of theiso- building.thermal with During the winters,zone air thetemperature air is mechanically (i.e., 21–22 exhausted °C). During through occupied the airhours, handling the system unit afteressentially heat recovery. operates as a conventional displacement ventilation system, by which the air is suppliedThe HVAC to th systeme conditioned of the Drøbakspace at Montessoria lower or at is most unique equal in temperature the aspect that to the it is average based onair the temperature displacement in the ventilation zone. principle and provides heating only outside the occupied periods.In hot During periods the in occupied summer hours the system between operates 7:30to in 16:00“night h, cooling the system mode” operates outside in occu- the “normalpied hours. mode”, The in night which-time a VAV cooling (variable starts air when volume) the inside damper air regulatestemperatu flowre is to above each zone23 °C toand maintain stops when the desired it reaches indoor 20.5 CO °C.2 During(carbon the dioxide) night concentrationcooling mode, and only space outside temperature air is used set-points.for cooling The the supplyspace. In air Norway temperature and several during other the normal European mode countries, is outdoor the temperature-outdoor air at compensatednight, even in as summer shown times, in Figure is frequently3. In the warmestcooler than periods, the indoor the supplyair and aircan is thus provided provide ◦ atfree 18.5 cooling.C, whereas, In the innight the coldestcooling periods,mode, the the system supply operates air temperature as a normal is approximately displacement ◦ isothermalventilation with system the zoneas the air temperature temperature of (i.e., the 21–22 supplyC). air During is lower occupied than the hours, average the systemair tem- essentiallyperature in operates the zone. as aHowever, conventional as there displacement are no heat ventilation sources system, to drive by the which displacement the air is suppliedprocess, tothe the temperature conditioned stratification space at a lower is less or pronounced at most equal and temperature there is almost to the an average isother- airmal temperature condition in in the the space. zone.

32 Normal mode (Occupied hours) 30 Heating mode (Non-occupied hours) 28 26 24 22 20 Supply Air Temperature (⁰C) Supply Air Temperature 18 −20 −10 0 10 20 30 Outside Air Temperature (⁰C)

FigureFigure 3. 3.Supply Supply air air temperature temperature as as a a function function of of outdoor outdoor air air temperature. temperature.

The “heating mode” outside occupied hours turns on when the outside air temperature is below 10 °C, and the room temperature in the three coldest rooms is below 20.5 °C. The heating stops when the temperature is above 22.5 °C in all rooms. In the heating mode, the ventilation system runs in recirculation with no fresh air intake to save energy for

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In hot periods in summer the system operates in “night cooling mode” outside occupied hours. The night-time cooling starts when the inside air temperature is above 23 ◦C and stops when it reaches 20.5 ◦C. During the night cooling mode, only outside air is used for cooling the space. In Norway and several other European countries, the outdoor air at night, even in summer times, is frequently cooler than the indoor air and can thus provide free cooling. In the night cooling mode, the system operates as a normal displacement ventilation system as the temperature of the supply air is lower than the average air temperature in the zone. However, as there are no heat sources to drive the displacement process, the temperature stratification is less pronounced and there is almost an isothermal condition in the space. The “heating mode” outside occupied hours turns on when the outside air temperature is below 10 ◦C, and the room temperature in the three coldest rooms is below 20.5 ◦C. The heating stops when the temperature is above 22.5 ◦C in all rooms. In the heating mode, the ventilation system runs in recirculation with no fresh air intake to save energy for heating. In both heating and night cooling modes, the airflow to a zone is set to its maximum design value. In the heating mode, the temperature of the supply air is outdoor temperature-compensated as shown in Figure3. In the coldest periods, the supply air is provided at 30 ◦C but the supply air temperature decreases down to 25 ◦C with increasing outside air temperature. Due to the elevated supply temperatures in heating mode, the vertical stratification in the zone ought not to be the typical distribution of displacement ventilation. The warm air supplied at low levels could rapidly move to the upper level of the space, resulting in a temperature stratification in the space. On the other hand, descending air flows along cold surfaces like windows and, to some degree, external walls would counteract the stratification in the space. Depending on the balance between the plume effect of the elevated supply temperature and the descending air flows from the cold surfaces, the hypothesis is that the temperature profile in the space will be somewhat intermediate between that of full mixing and displacement air distributions. Still, several issues related to the application and modeling of displacement ventilation for heating during non-occupied hours remain to be fully understood. Some of the most significant and still unresolved questions include: (1) Is heating during the non-occupied hours only enough and will it provide the desired thermal comfort level during the occupied hours? (2) Will the temperature gradient in classrooms be too high at the start of the school day and during the non-occupied hours? (3) Will the warm air supplied with low impulse just ascend to the ceiling, and not heat the lower and occupied space? (4) Is there a risk that with low occupancy in a zone, the internal loads may not be enough to ensure satisfactory thermal comfort conditions in the zone? And (5) is there a risk that with high occupancy in a zone after night mode preheating, the internal loads may be too high to ensure satisfactory thermal comfort conditions in the zone?

4. Method This paper explores the above-mentioned questions using a combination of experimental tests and numerical simulations. Full details of the experimental and simulation methods used in this study, for analyzing the aptness of displacement ventilation for providing heating during non-occupied periods without a separate heating system, is provided in the following sections.

4.1. Experimental Tests The experimental methods utilized in this study comprised of laboratory and field tests. The lab tests were performed on a scaled classroom model under carefully controlled laboratory conditions. The field test was carried out in an actual classroom under real uncontrolled field conditions. The methodology of the experimental tests is described below in detail. Energies 2021, 14, 952 10 of 33

4.1.1. Lab Tests The validity of the design concept has been verified through lab testing of a simplified unscaled model of the classroom. The laboratory testing was carried out at the Building and Infrastructure test facility of SINTEF in Oslo. The test room was built inside a lab hall with controlled indoor conditions of 18 ◦C. The mock-up classroom had a floor area of 4.0 m × 4.0 m and a height of 3.0 m. The room walls and ceiling were made of 100 mm polyurethane foam/aluminum sandwich elements. The floor consisted of two 40 mm precast concrete slabs bonded together with and overlaid with a bituminous mix, 10 mm EPS insulation, and 0.15 mm of building plastic. The joints between the construction Energies 2021, 14, x FOR PEER REVIEWelements were filled with foundry sand and then spackled. The U-value for walls and 11 of 34 ceiling was 0.2854 W/m2K and for the floor was 0.2816 W/m2K. Figures4 and5 show the floor plan and photos of the test room with installed equipment, measurement setup, and heat sources. A displacement ventilation system was installedthe classroom for providing was heatingprovided and through ventilation a tosemi the- mockupcircular classroom. displacement The supply diffuser air to mounted on thethe classroom south wall was connected provided through with aa semi-circular 125 mm circular displacement supply diffuser duct. mounted The part on theof the air duct south wall connected with a 125 mm circular supply duct. The part of the air duct located inlocated the room in was the insulated room was with insulated 4 mm cellular with plastic. 4 mm The cellular supply airflow plastic. was The controlled supply by airflow was meanscontrolled of a modulating by means damper, of a modulating which, in turn, damper, was regulated which, by in a turn, high-accuracy was regulated airflow by a high- sensor.accuracy Air wasairflow exhausted sensor. passively Air was via exhausted a 125 mm circular passively ventilation via a duct125 mm located circular at the ventilation oppositeduct located end of at the the room, opposite 0.2 m fromend theof the ceiling room, and 0.2 the northm from wall. the ceiling and the north wall.

FigureFigure 4. 4.Floor Floor plan plan of theof the test test room room with with measurement measurement points andpoints internal and heatinternal sources heat (people: sources (people: grey,grey, lighting: lighting: yellow, yellow, equipment: equipment: green). green).

1 Figure 5. Photos of the test room with measurement points and internal heat sources.

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the classroom was provided through a semi-circular displacement diffuser mounted on the south wall connected with a 125 mm circular supply duct. The part of the air duct located in the room was insulated with 4 mm cellular plastic. The supply airflow was controlled by means of a modulating damper, which, in turn, was regulated by a high- accuracy airflow sensor. Air was exhausted passively via a 125 mm circular ventilation duct located at the opposite end of the room, 0.2 m from the ceiling and the north wall.

Energies 2021, 14, 952 11 of 33 Figure 4. Floor plan of the test room with measurement points and internal heat sources (people: grey, lighting: yellow, equipment: green).

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1 FigureFigure 5. Photos 5. Photos of the of test the room test room with withmeasurement measurement points points and internal and internal heat sources. heat sources. In the test room, internal heat gains from people, equipment and lighting, and trans- missionIn thelosses test through room, internal the outer heat walls gains were from simulated people, equipment experimentally and lighting, for two and distinct trans- scenariosmission losses with throughdifferent theoccupancy outer walls patterns, were simulated and thus with experimentally different airflows, for two internal distinct loads,scenarios and with transmission different occupancy losses. The patterns, first scenario, and thus Scenario with different I, corresponded airflows, internal to a typical loads, situationand transmission in which losses.the classroom The first is scenario, fully occupied Scenario throughout I, corresponded the day, to from a typical 8:00 situa-until 16:00tion in h, which except the during classroom the lunch is fully break, occupied between throughout 11:00 and the12:00 day, h, fromwhen 8:00 the untilclassroom 16:00 is h, empty.except duringThe second the lunch scenario, break, Scenario between II, 11:00 corresponded and 12:00 to h, whena specific the classroomsituation when is empty. the classroomThe second is scenario, occupied Scenario only during II, corresponded the afternoon to between a specific 13:00 situation and 16:00 when h the with classroom 50% of theis occupied design occupancy only during level. the Both afternoon scenarios between were 13:00 simulated and 16:00 experimentally h with 50% for of the48 h. design Dur- ingoccupancy the experimental level. Both testing, scenarios thermal were manikins simulated with experimentally a nominal power for 48 of h. 40 During–120 W the and ex- a convectiveperimental heat testing, fraction thermal of manikinsapproximately with a0.5 nominal were used power for of simulating 40–120 W andheat a loads convective from peopleheat fraction and equipment. of approximately Heat loads 0.5 were from used lighting for simulating were provided heat loads through from incandescent people and bulbsequipment. with a Heatnominal loads power from of lighting 2 × 40 wereW. Figure provided 6 presents through the incandescent daily internal bulbs loads with from a people,nominal equipment power of 2and× 40 lighting, W. Figure and6 presentssupply air the temperatures daily internal and loads flows from simulated people, equip-for the twoment scenarios. and lighting, Transmission and supply losses air temperatures through the outer and flows walls simulated were simulated for the by two circulating scenarios. coldTransmission water in pipes losses integrated through the into outer the east walls and were west simulated side walls by of circulating the test room. cold The water sup- in plypipes temperature integrated intoand themass east flow and of west the circulating side walls ofwater the testwere room. chosen The to supply emulate temperature the trans- missionand mass losses flow through of the circulating the external water walls were during chosen the tooccupied emulate hours the transmission at an outside losses tem- − ◦ peraturethrough theof −15 external °C. walls during the occupied hours at an outside temperature of 15 C.

People (W/m²) Lighting (W/m²) People (W/m²) Lighting (W/m²) Equipment (W/m²) Airflow (m³/h/m²) Equipment (W/m²) Airflow (m³/h/m²) Supply Temperature (°C) Supply Temperature (°C) 40 40 32.2 35 Temperature 35 Temperature 30 30 Heat Gains 25 25 20 20 Heat Gains 15.9 15 15 Flow 10 10 5.0 5.0 5 Flow 5

0 3.8 0 2.5

Heat Gain / Temperature Flow / Gain Temperature / Heat

04:00 08:00 18:00 22:00 00:00 02:00 06:00 10:00 12:00 14:00 16:00 20:00 00:00

00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00 Time (hours) Time (hours) Figure 6. SpecificSpecific internal internal heat heat gains, gains, specific specific airflows, airflows, and supply air temperatures for Scenario I ( left) and Scenario II ( right).

The measurement setup used in the test room was shown above in Figures 4 and 5. The supply and exhaust air temperatures were measured in the supply air diffuser and the return air duct, respectively. The temperature profile of the room air was measured at two different positions in the SW and NE parts of the room. Two vertical poles, each with five measurement points at heights of 0.1, 1.1, 1.7, 2.3, and 2.9 m above the floor level, were used for measuring air temperature stratification in the room. Temperature distribution in the floor slab was measured at depths of 0, 4, and 8 cm below the floor level at four different positions in the room, with two positions being directly below the vertical poles. All the temperature measurements were made with calibrated thermocouples of type T, class 1, which had an accuracy of ±0.5 K in the measured temperature range. Each vertical pole also had an additional globe temperature sensor, with a measurement accuracy of ±0.1 K, installed at the sitting height to measure the radiant heat temperature in the room. The airflow measurements were taken in the supply air duct using a differential- pressure-based flow measurement station with a measurement tolerance of less than 4%. The transmission losses from the external walls were measured using calibrated heat meters.

4.1.2. Field Test An in-situ field test was carried out in a pentagon-shaped classroom on the lower level of the Drøbak Montessori school building. Figures 7 and 8 show the floor plan and

Energies 2021, 14, 952 12 of 33

The measurement setup used in the test room was shown above in Figures4 and5. The supply and exhaust air temperatures were measured in the supply air diffuser and the return air duct, respectively. The temperature profile of the room air was measured at two different positions in the SW and NE parts of the room. Two vertical poles, each with five measurement points at heights of 0.1, 1.1, 1.7, 2.3, and 2.9 m above the floor level, were used for measuring air temperature stratification in the room. Temperature distribution in the floor slab was measured at depths of 0, 4, and 8 cm below the floor level at four different positions in the room, with two positions being directly below the vertical poles. All the temperature measurements were made with calibrated thermocouples of type T, class 1, which had an accuracy of ±0.5 K in the measured temperature range. Each vertical pole also had an additional globe temperature sensor, with a measurement accuracy of ±0.1 K, installed at the sitting height to measure the radiant heat temperature in the room. The airflow measurements were taken in the supply air duct using a differential- pressure-based flow measurement station with a measurement tolerance of less than 4%. The transmission losses from the external walls were measured using calibrated heat meters.

4.1.2. Field Test Energies 2021, 14, x FOR PEER REVIEW An in-situ ﬁeld test was carried out in a pentagon-shaped classroom on the13 lowerof 34 level

of the Drøbak Montessori school building. Figures7 and8 show the floor plan and photos of the classroom with the measurement setup. The classroom had a floor area of 52.2 m2 andphotos a ceiling of the classroom height of with 3 m. the It had measurement a medium-weight setup. The construction, classroom had with a floor a well-insulated area of thermal52.2 m2 and envelope a ceiling and height a high of window-to-wall3 m. It had a medium ratio.-weight The classroom construction, had with two exteriora well- walls locatedinsulated on thermal the southwest envelope and and southeasta high window sides-to and-wall three ratio. interior The classroom walls located had two on ex- the north, east,terior andwalls northwest located on sides.the southwest The total and area southeast of the sides interior and andthree exterior interior walls located was 44.7 m2, andon the 42.9 north, m2 ,east, respectively. and northwest The sides. heat transferThe total coefficientarea of the (U-value)interior and and exterior heat walls capacity of interiorwas 44.7 wallsm2, and were 42.9 0.26 m2, W/mrespectively.2K and The 2.4 heat Wh/m transfer2K, respectively. coefficient (U The-value) exterior and heat walls had acapacity heat transfer of interior coefficient walls were and 0.26 an W/m inner2K heat and capacity2.4 Wh/m of2K, 0.14 respectively. W/m2K The and exterior 2.4 Wh/m 2K, respectively.walls had a heat Floor transfer and coefficient ceiling had and heat an inner transfer heat coefficients capacity of of0.14 0.10 W/m and2K and 0.25 2.4 W/m 2K, 2 respectively,Wh/m K, respectively. and heat Floor capacities and ceiling of 63, andhad heat3.0 Wh/m transfer2K, coefficien respectively.ts of 0.10 Windows and 0.25 (glazing 2 2 andW/m frames,K, respectively, combined) and covered heat capacities 43.8% of of the 63, totaland 3.0 exterior Wh/m wallK, respectively. area and had Windows a heat transfer (glazing and frames, combined) covered 43.8% of the total exterior wall area and had a coefficient of 0.75 W/m2K. heat transfer coefficient of 0.75 W/m2K.

Figure 7. Floor plan of the classroom with measurement points. Figure 7. Floor plan of the classroom with measurement points.

Energies 2021, 14, x FOR PEER REVIEW 13 of 34

photos of the classroom with the measurement setup. The classroom had a floor area of 52.2 m2 and a ceiling height of 3 m. It had a medium-weight construction, with a well- insulated thermal envelope and a high window-to-wall ratio. The classroom had two exterior walls located on the southwest and southeast sides and three interior walls located on the north, east, and northwest sides. The total area of the interior and exterior walls was 44.7 m2, and 42.9 m2, respectively. The heat transfer coefficient (U-value) and heat capacity of interior walls were 0.26 W/m2K and 2.4 Wh/m2K, respectively. The exterior walls had a heat transfer coefficient and an inner heat capacity of 0.14 W/m2K and 2.4 Wh/m2K, respectively. Floor and ceiling had heat transfer coefficients of 0.10 and 0.25 W/m2K, respectively, and heat capacities of 63, and 3.0 Wh/m2K, respectively. Windows (glazing and frames, combined) covered 43.8% of the total exterior wall area and had a heat transfer coefficient of 0.75 W/m2K.

Energies 2021, 14, 952 13 of 33

Figure 7. Floor plan of the classroom with measurement points.

Figure 8. Photos of the classroom with measurement points for temperature and CO2.

The classroom was supplied with air from a central air handling unit. The airflow in the classroom was driven by displacement ventilation. The supply air to the classroom was provided through two wall-embedded supply air diffusers installed 50 mm above the floor level at the far ends of the east-facing inner wall. The 600 mm × 900 mm rectangular air diffusers were connected to a 315 mm circular main supply duct via two feeder ducts. The airflow in the main duct was regulated by a modulating damper, which was controlled in response to air temperature and CO2 concentration in the classroom as described previously in Section3. The maximum allowable design airflow from each supply diffuser was limited to 264 m3/h to not exceed a sound power level of 25 dB(A). At maximum airflow, each diffuser had a near-zone distance of less than 1.5 m to the 0.20 m/s isovel, measured 0.1 m above the floor level with a 3 K temperature difference between the room and supply air temperatures. Air from the classroom was extracted via three extract air grills installed 2.8 m above the floor level in the top center of the north-facing inner wall. Passive overflow elements with sound attenuators were used to transfer the extract air to the corridor outside the classroom, from where it was collected and returned to the air handling unit. Figure9 presents the internal loads from people, equipment, and lighting, solar gains through the room fabric, and supply air temperatures and flows for the field test. The field test was performed for approximately 16 h on a cold winter day with ambient air temperatures ranging between −7 and +3 ◦C. From the start of the test at midnight, up until 7:30 a.m., the displacement ventilation system operated in the heating mode, supplying recirculation air to the classroom at a rate of approximately 10.5 m3/h/m2 floor area and at outdoor temperature-compensated supply temperatures between 27.1 and 28.1 ◦C. At 7:30 a.m., the displacement ventilation system switched from the heating mode to the normal mode. In this mode, the supply air temperature and flow to the room were originally designed to be regulated by outdoor temperature, and the indoor temperature and CO2 levels, respectively. However, for the field test, the supply air temperature and flow to the classroom were purposefully chosen to represent a nearly impossible worst-case scenario. The supply air to the classroom was provided at temperatures between 16 and 18 ◦C, which were 3 to 4 ◦C below the actual design values. The supply airflow to the classroom was set constant and approximately equal to the maximum design flow. The classroom was occupied between 08:00 a.m. and 03:30 p.m. The internal loads in the classroom varied throughout the day, as it normally happens in classroom contexts. Between 08:30 a.m. and 02:30 p.m., thirteen to nineteen persons were present in the classroom at any one time, Energies 2021, 14, 952 14 of 33

except the lunch break from 11:30 a.m. to 12:30 p.m. when the classroom was completely unoccupied. Internal loads from equipment and lighting were fairly constant throughout the day. Solar heat gains transmitted through the room fabric, including windows and walls deduced from the measured solar irradiance data from a nearby weather station using TEKNOsim 6 software [102] were as high as 450 W (i.e., 8.6 W/m2 ﬂoor area). Energies 2021, 14, x FOR PEER REVIEW 15 of 34 Heat losses through classroom envelope and ventilation and inﬁltration were simulated and are presented in Section 5.2.2.

People (W/m²) Lighting (W/m²) Equipment (W/m²) Solar (W/m²) Supply Temperature (°C) Airflow (m³/h/m²) 40 35 Temperature 30 25 20 15 Flow 10 5 Heat Gains

Heat Gain / Temperature Flow / Gain Temperature / Heat

04:00 12:00 20:00 00:00 02:00 06:00 08:00 10:00 14:00 16:00 18:00 22:00 00:00 Time (hours)

FigureFigure 9. 9.Specific Specific internal internal heat h gains,eat gains, specific specific airflows, airflows, and supply and supply air temperatures air temperatures for the field for test.the field test.

TheThe measurement measurement setup setup in the in classroom the classroom consisted consisted of sensors of for sensors measuring for measuring air temper- air tem- ature,perature, radiant radiant temperature, temperature, CO2 concentration, CO2 concentration, and airflow and and airflow airspeed. and The airspeed. temperatures The temper- ofatures supply of and supply extract and air extract to and air from to and the roomfrom werethe room measured were bymeasured sensors installedby sensors in installed supply air diffusers and return air grilles. Two instrumented vertical poles, placed in the in supply air diffusers and return air grilles. Two instrumented vertical poles, placed in south and east corners of the classroom, were used for measuring air temperature and CO2 the south and east corners of the classroom, were used for measuring air temperature and stratification in the room. Each pole had a set of temperature and CO2 sensors installed atCO four2 stratification different heights in the of 0.1,room. 1.1, Each 1.7, and pole 2.7 had m. Alla set air of temperature temperature sensors and CO had2 ansensors in- operatingstalled at range four ofdifferent−20 to 70heights◦C, an of accuracy 0.1, 1.1, of1.±7,0.21 and◦ 2.7C, andm. All a resolution air temperature of 0.024 ◦sensorsC. had Thean COoperatin2 sensorsg range had anof accuracy−20 to 70 of °C,±50 an ppm accuracy over theof ±0.21 measured °C, and concentration a resolution range. of 0.024 °C. EachThe verticalCO2 sensors pole also had had an anaccuracy additional of ±50 globe ppm temperature over the sensormeasured and anconcentration omnidirectional range. Each anemometervertical pole installed also had at an approximately additional globe the standing temperature height sensor for measuring and an omnidirectional the radiant an- heatemometer temperature installed and air at velocity,approximately respectively. the standing The measurement height for accuracies measuring of the the globe radiant heat sensor and anemometer were ±0.1 K, and ±0.04 m/s, respectively. The supply airflow to temperature and air velocity, respectively. The measurement accuracies of the globe sen- the room during the occupied period was measured using a thermal anemometer with asor measurement and anemometer tolerance were of less ±0.1 than K,± and4%. The±0.04 airflow m/s, respectively. outside the operating The supply hours airflow was to the obtainedroom during from the the centrally occupied measured period BMS was data. measured using a thermal anemometer with a measurement tolerance of less than ±4%. The airflow outside the operating hours was ob- 4.2.tained Simulation from Studiesthe centrally measured BMS data. Simulations of experimental studies of the previous sections were performed to assess the4.2. suitability Simulation of aStudies commonly used transient dynamic method for predicting the thermal and contaminant stratification in the displacement ventilation systems. The simulations Simulations of experimental studies of the previous sections were performed to as- were performed using IDA-ICE, which is a commercially available state-of-the-art building performancesess the suitability simulation of fora commonly performing used a multi-zonal transient and dynamic dynamic method study of for indoor predicting climate, the ther- energy,mal and and contaminant daylighting. stratification The software in is reportedthe displacement to be validated ventilation in accordance systems. with The simula- severaltions were European performed and International using IDA standards,-ICE, which including is a commercially Standard EN available 15265 [103 state] and-of-the-art ASHRAEbuilding Standard performance 140 [104 simulation], among others. for performing The Climate a model multi in-zonal IDA-ICE and version dynamic 4.8 SP1 study of in- wasdoor used climate, for performing energy, and the transientdaylighting. simulations The software of temperature is reported stratification to be validated and CO 2in accord- concentrations.ance with severa The Climatel European model and uses International multi-node calculations standards, to determineincluding temperatures Standard EN 15265 [103] and ASHRAE Standard 140 [104], among others. The Climate model in IDA-ICE version 4.8 SP1 was used for performing the transient simulations of temperature stratification and CO2 concentrations. The Climate model uses multi-node calculations to determine temperatures of different surfaces in the zone and different layers in the construction. The indoor environmental conditions in both horizontal and vertical directions are determined using a detailed physical model of the building and its components. A significant limitation of the Climate model is that it cannot be used for simulating irregular and asymmetrical zone geometries. The zone geometry must be simplified to a rectangular footprint before the simulations can be performed. In the climate model, air distribution to a zone is classified as mixing or displacement ventilation. For displacement ventilation, air temperature at the floor level is determined by using an energy balance between the convective heat transfer from the floor surface to

Energies 2021, 14, 952 15 of 33

of different surfaces in the zone and different layers in the construction. The indoor environmental conditions in both horizontal and vertical directions are determined using a detailed physical model of the building and its components. A significant limitation of the Climate model is that it cannot be used for simulating irregular and asymmetrical zone geometries. The zone geometry must be simplified to a rectangular footprint before the simulations can be performed. In the climate model, air distribution to a zone is classified as mixing or displacement ventilation. For displacement ventilation, air temperature at the floor level is determined by using an energy balance between the convective heat transfer from the floor surface to the air at the floor level and the ventilation heat flux from the supply air. The air temperature at the ceiling level is computed by considering the heat capacity of the zone air volume and accounting for all heat transfer to the zone air. Based on the calculated air temperatures at floor and ceiling levels, a linear temperature gradient is calculated for the zone using the Mundt model [28]. Temperatures at the zone surfaces are also interpolated between the floor- and ceiling-level air temperatures. Alternatively, a fixed linear temperature gradient can be specified directly by the user. In that case, the air temperature at the floor level is obtained using the air temperature at the ceiling level and the provided gradient. If the thermal gradient disappears or becomes negative, the air distribution is treated as mixing ventilation instead. The CO2 concentration in the Climate model is determined based on the balance between the CO2 generated in the zone and the CO2 concentration in the ventilation air supplied to the zone. The CO2 generated from occupants is modeled as a function of their activity level. A significant limitation of the model is that the CO2 concentration for each time step is calculated as a single average value over the zone volume. Therefore, the model does not account for the vertical stratification of the CO2 concentration in the zone. Moreover, it also does not consider the change in CO2 concentration with distance from the emission source.

4.2.1. Lab Test A simulation model of the mock-up classroom used for the lab tests was built in IDA- ICE. The simulation model was constructed using the actual geometry and construction parameters of the test room described in detail in Section 4.1.1. A constant temperature boundary condition of 18 ◦C was imposed on all envelope elements except the external walls to match the onsite test conditions. The transmission heat losses through the outer walls were modeled using a controller macro, which split the total losses equally over the two external walls. Inputs to the simulation model included internal loads, operating schedules, and supply air temperatures and ﬂows. These inputs were acquired from the site-controlled and measured test conditions shown in Figure6. The test data was however processed to 15-min time steps used for the simulation. The Climate model in IDA-ICE was used to simulate the vertical temperature gradients. Cyclic runs of each simulation scenario were performed before the result-generating simulation run to ensure stable conditions. As there were no emission sources in the zone, the CO2 concentrations in the mock-up classroom were not simulated.

4.2.2. Field Test The simulation model of the whole school building including the classroom used in the field test was built in IDA-ICE. For the simulation model, the geometry of the actual classroom used in the field test was simplified to a rectangular footprint. This was because the climate model in IDA-ICE could not simulate irregular and asymmetrical zone geometries. Nevertheless, the model was customized to match the volume, floor area, and climate envelope area of the actual classroom. Figure 10 shows the geometries of both the actual room and the simplified simulation model built in IDA-ICE. The actual classroom was shaded from terrain and trees surrounding it. The shading effect was estimated based on onsite observations and was incorporated in the simulation model through vertical Energies 2021, 14, 952 16 of 33

shading elements with 0% transparency for terrain-elements and 50% transparency for EnergiesEnergies 2021 2021, 14, 14, x ,FOR x FOR PEER PEER REVIEW REVIEW 17 of 34 17 of 34 tree-elements. Figure 11 shows the building model with modiﬁed classroom geometry and the surrounding vertical shading elements as implemented in IDA-ICE.

Figure 10. Actual and modeled geometries of the classroom. FigureFigure 10. 10.Actual Actual and and modeled modeled geometries geometries of the classroom. of the classroom.

FigureFigure 11. 11. ImplementedImplemented building building modelmodel in in IDA-ICE. IDA-ICE. As for the lab model, the actual construction details of the classroom, presented earlier inAs Section for the 4.1.2 lab, were model, used the to actual build theconstruction simulation details model. of The the inputs classroom, to the simulation presented earlier in Section 4.1.2, were used to build the simulation model. The inputs to the simulation model includedFigure 11. the Implemented specific schedule building used for model the test, in theIDA dynamic-ICE. internal loads, and the modelactual included supply airthe temperaturesspecific schedule and flows. used for The the climate test, the file dynamic used for the internal simulation loads, was and the actualbased supply on the air combination temperatures of weather and flows. data for The the actualclimate area file in used the simulated for the simulation period and was basedstandardized onAs the for combination valuesthe lab for model, a of typical weather the year actual data for Oslo. for construction the The actual outdoor area details temperature in the of simulated the and classroom, solar period data presentedand ear- standardizedwerelier in based Section onvalues actual 4.1.2, for measurements. awere typical used year Theto forbuild outdoor Oslo. the temperatureThe simulation outdoor data tempemodel. wasrature measured The andinputs on-site solar to datathe simulation werewhereasmodel based included theon solaractual data themeasurements. was specific taken fromschedule The a nearby outdoor used weather temperaturefor the station. test, the Thedata dynamic solar was datameasured wasinternal pro- on- lsiteoads, and the cessed using TEKNOsim algorithms [102] to determine direct and diffused components to whereasactual the supply solar data air temperatureswas taken from and a nearby flows. weather The climate station. fileThe usedsolar datafor the was simulation pro- was be used in the climate file of IDA-ICE. Temperatures in adjacent rooms and spaces were also cessedbased using on TEKNOsimthe combination algorithms of weather [102] to data determine for the direct actual and area diffused in the componentssimulated period and to bestandardized used in the climate values file for of aIDA typical-ICE. Temperaturesyear for Oslo. in The adjacent outdoor rooms tempe and spacesrature were and solar data alsowere based based on measured on actual values. measurements. The ground The reflection outdoor (albedo) temperature was set data to 0.6 was based measured on on-site observedwhereas values the onsolar the data test day.was Thetaken calculated from a nearby solar heat weather gains werestation. as highThe solaras 450 data W was pro- 2 (i.e.cessed, 8.6 W/m using floor TEKNOsim area). The model algorithms simulated [102] transmission to determine losses direct through and the diffused classroom components envelope based on the room envelope areas, heat transfer coefficients, and the tempera- to be used in the climate file of IDA-ICE. Temperatures in adjacent rooms and spaces were ture difference on each side of the envelope. One issue in simulating the transmission heat lossesalso was based to model on measured the ground values. heat losses The accurately. ground reflection This was because (albedo) the was field set test to was 0.6 based on performedobserved just values after theon completionthe test day. of The the building,calculated which solar did heat not g allowains were construction as high as 450 W (i.e., 8.6 W/m2 floor area). The model simulated transmission losses through the classroom envelope based on the room envelope areas, heat transfer coefficients, and the temperature difference on each side of the envelope. One issue in simulating the transmission heat losses was to model the ground heat losses accurately. This was because the field test was performed just after the completion of the building, which did not allow construction

Energies 2021, 14, 952 17 of 33

based on measured values. The ground reflection (albedo) was set to 0.6 based on observed values on the test day. The calculated solar heat gains were as high as 450 W (i.e., 8.6 W/m2 floor area). The model simulated transmission losses through the classroom envelope based on the room envelope areas, heat transfer coefficients, and the temperature difference on each side of the envelope. One issue in simulating the transmission heat losses was to model the ground heat losses accurately. This was because the field test was performed just after the completion of the building, which did not allow construction enough time to dry out and stabilize before the field test. To overcome this issue, the external floor was modeled considering a constant ground temperature of 0 ◦C. Furthermore, the thermal conductivity of the insulation in the external floor construction was adjusted to account for the moisture in the slab. The model also simulated heat losses due to ventilation and infiltration. The ventilation heat losses were modeled using the measured ventilation rates, whereas the infiltration heat losses were modeled based on the calculated air leakage to the outside and surrounding zones. The vertical temperature gradients were simulated using the Climate model in IDA- ICE. The simulations were performed using a dynamic startup and cyclic runs were made to ensure stable conditions before the result-generating simulation. The measurement points in the simulation run corresponded to actual sensor locations used in the field test. The CO2 concentrations in the zone over time were also simulated using the Climate model. An activity level value of 1 MET, corresponding to sitting quietly, and an atmospheric CO2 concentration of 400 ppm was used for the simulations.

5. Results In this section, the results of experimental tests and numerical simulations described above are critically presented and discussed, giving a detailed insight into the performance of the displacement ventilation for providing heating during non-occupied periods without the need for a separate heating system.

5.1. Experimental Results The results of two lab tests carried out in the scaled mock-up classroom under carefully controlled laboratory conditions and one ﬁeld test performed in the full-scale classroom under actual ﬁeld conditions are presented in the following sections.

5.1.1. Lab Test Figure 12, left and right, show measured values of supply, extract and operative temperatures, and airﬂows for Scenario 1 and Scenario 2 of the lab test. The only noticeable difference between the measured values of Figure 12 and the originally planned values of Figure6 is in the supply air temperature in the heating mode at night. For both scenarios, the actual supply air temperature in the heating mode was around 29 ◦C, which was 1 K lower than the originally planned value. This discrepancy was caused because the supply air temperature to the test room was regulated by a sensor located in the air duct, 2.8 m above the supply air diffuser, whereas the measurements of supply air temperature shown in Figure 12 were taken with a sensor placed in the supply air diffuser at 0.7 m height. Hence, in the heating mode, there was a duct heat loss of 1 K to the room due to the large difference in the temperature of the air in the duct and the temperature of the air in the room. In the normal mode, the supply air temperature in both scenarios was approximately 22 ◦C as originally planned. For both scenarios, the room operative temperature was between 22–24 ◦C when occupied. The extract air temperature from the room depended upon the operation mode of the system and the occupancy level. For both scenarios, the extract air temperature in the heating mode was 5–7 K lower than the supply air temperature. For Scenario 1, which corresponded to a situation in which the classroom was fully occupied between 8:00–11:00 and 12:00–16:00 h, the extract air temperature in the normal mode was on average 1.7 K and maximum 2.2 K higher than the supply air temperature. For Scenario 2, which corresponded to a situation in which the Energies 2021, 14, 952 18 of 33

classroom was occupied only during the afternoon between 13:00 and 16:00 h with 50% of Energies 2021, 14, x FOR PEER REVIEWthe design occupancy level, the extract air temperature in the normal operating-mode19 of was 34 Energies 2021, 14, x FOR PEER REVIEW 19 of 34 only occasionally and marginally higher (<0.5 K) than the supply air temperature.

32 18 32 18 32 SupplySupply TemperatureTemperature ExtractExtract TemperatureTemperature 18 32 SupplySupply TemperatureTemperature ExtractExtract TemperatureTemperature 18 Operative Temperature Airflow Operative Temperature Airflow

30 Operative Temperature Airflow 15 30 15

° C)

28 12 C) 28 12 °

28 12 ° 28 12

³/h/m²)

³/h/m²) ³/h/m²) 26 9 26 9 ³/h/m²)

24 6 24 6

Temperature ( Temperature

Airflow (m

Airflow (m Temperature ( Temperature

22 3 ( Temperature 22 3 Airflow (m 22 3 22 3 Airflow (m

20 0 20 0

04:00 08:00 00:00 04:00 08:00 20:00 00:00 00:00 12:00 16:00 20:00 12:00 16:00

04:00 08:00 00:00 04:00 08:00 20:00 00:00 00:00 12:00 16:00 20:00 12:00 16:00 Time (hours) Time (hours) Time (hours) Time (hours) Figure 12. MMeasuredeasured supply, extract,extract, andand operative operative temperatures, temperatures, and and airﬂows airflows for forfor Scenario ScenarioScenario I( IleftI ((leftleft) and)) and Scenario Scenario II (IIright (rightright) of)) ofthe the lab lab test. test.

FigureFigure 1313 showsshows the the vertical vertical temperature temperature profiles profiles at heights at heights of 0.1, of 1.1,0.1, 1.7,1.1, 2.3, 1.7,and 2.3, 2.9 and m 2.9above m above the floor the level floor for level the for two the scenarios. two scenarios.Each curve Each represents curve represents an average an average of two temper- of two temperaturetemperatureature sensors sensorssensors installed installedinstalled at the same atat thth heighte same on height the two on instrumentedthe two instrumented poles in thepoles opposite in the oppositecorners of corners the test of room. the test For room. Scenario For Scenario 1, the average 1, the average and maximum and maximum vertical vertical temperature tem- peraturegradient gradient between between the standing the standing head height head (1.7height m) (1.7 and m) ankle and heightankle height (0.1 m) (0.1 was m) 1.2 was K 1.2and K 1.4 and K, 1.4 respectively, K, respectively, during during the heating the heating mode mode between between 00:00–8:00, 00:00– and8:00,:00, 0.8 andand K and0.80.8 KK 1.2 andand K, 1.2respectively, K, respectively, during during the normal the normal mode mode between between 8:00–11:00 8:00–11:00 and and 12:00–16:00. 12:00–16:00. During During pe- periodsriods of of no no ventilation ventilation between between 16:00–00:00, 16:00–00:00, there there was was no no verticalvertical temperaturetemperature gradient andand consequently consequently no no stratification. stratiﬁcation. For For Scenario Scenario 2, the 2, theaverage average and maximum and maximum vertical vertical temperaturetemperature gradient gradient between between the standing the standing head head height height (1.7 m) (1.7 and m) ankle and ankle height height (0.1 m) (0.1 was m) 1.2was K 1.2 and K 1.4 and K, 1.4 respectively, K, respectively, during during the heating the heating mode mode between between 00:00 00:00–7:30,–7:30,:30, andand 0.40.4and KK 0.4 andand K 0.6and K, 0.6 resp K,ectively, respectively, during during the normal the normal mode mode between between 12:00– 12:00–15:00.15:00. During During periods periods of no of no occupancy between 07:30–12:00 and 15:30–18:00, there was no vertical temperature occupancy between 07:30–12:00 and 15:30–18:00, there was no vertical temperature gradi- gradient and consequently no stratiﬁcation. ent and consequently no stratification.

25 25 Height 0.1 m Height 1.1 m Height 1.7 m Height 0.1 m Height 1.1 m Height 1.7 m Height 2.3 m Height 2.9 m Height 2.3 m Height 2.9 m

24 24

C) C)

° °

C) C)

° ° 23 23

22 22

Temperature ( Temperature ( Temperature Temperature ( Temperature Temperature ( Temperature 21 21

20 20

00:00 04:00 16:00 04:00 08:00 16:00 20:00 08:00 20:00 04:00 08:00 16:00 08:00 12:00 20:00 00:00 12:00 00:00 00:00 04:00 12:00 16:00 00:00 12:00 20:00 00:00

00:00 04:00 16:00 04:00 08:00 16:00 20:00 08:00 20:00 04:00 08:00 16:00 08:00 12:00 20:00 00:00 12:00 00:00 00:00 04:00 12:00 16:00 00:00 12:00 20:00 00:00 Time (hours) Time (hours) Time (hours) Time (hours) Figure 13. Measured vertical temperature profiles for Scenario I (left) and Scenario II (right) of the lab test. Figure 13. MeasuredMeasured vertical temperature profiles proﬁles for Scenario I (left) and ScenarioScenario IIII ((rightright)) ofof thethe lablab test.test.

FigureFigure 1414 presentspresents the the vertical vertical temperature temperature distribution in the floor ﬂoor slab at the the depths depths ofof 0, 0, 4, 4, and and 8 8 cm cm below below the the floor ﬂoor level level for for the the two two test test scenarios. scenarios. Each Each curve curve represents represents an averagean average of fourfour of four thermocouplesthermocouples thermocouples installedinstalled installed atat thethe at thesamesame same depthsdepths depths inin differentdifferent in different positionspositions positions inin thethe in room.the room. In both In bothscenarios, scenarios, the slab the temperatures slab temperatures increased increased with time with in time the heating in the heating mode, andmode, its andthermal its thermal mass was mass charged. was charged. The slab The temperatures slab temperatures also increased also increased in the day in the oper- day atinoperatingg mode, mode, but at but different at different rates, as rates, occupancy as occupancy levels were levels dissimilar were dissimilar for the two for thescenar- two ios.ios. TheThe dischargingdischarging occurredoccurred inin nonnon--occupancy periods when the airflow to the test setup was reduced to zero. Throughout the experiment, temperatures at 8 cm depth remain no- ticeablyticeably higherhigher thanthan thethe temperaturestemperatures atat 00 andand 44 cmcm depths.depths.

Energies 2021, 14, x FOR PEER REVIEW 20 of 34

Energies 2021, 14, 952 19 of 33

25 25 Depth 0 cm scenarios.Depth 4 cm The dischargingDepth 8 cm occurred in non-occupancy periodsDepth when 0 cm the airﬂowDepth 4 cm to the Depth 8 cm Energies 242021, 14, x FOR PEER REVIEWtest setup was reduced to zero. Throughout the experiment,24 temperatures at 8 cm depth20 of 34

C) ° remain noticeably higher than the temperatures° at 0 and 4 cm depths. 23 23

22 25 25 22 Depth 0 cm Depth 4 cm Depth 8 cm Depth 0 cm Depth 4 cm Depth 8 cm

24 24 Temperature ( Temperature

21 ( Temperature 21

° ° 23 23 20 20

22 22

00:00 04:00 16:00 20:00 04:00 08:00 16:00 20:00 04:00 08:00 12:00 16:00 20:00 00:00 04:00 08:00 12:00 16:00 20:00 00:00 00:00 08:00 12:00 00:00 12:00 00:00 Temperature ( Temperature 21 ( Temperature 21 Time (hours) Time (hours) 20 20

Figure 14. Measured slab temperatures for Scenario I (left) and Scenario II (right) of the lab test.

00:00 04:00 16:00 20:00 04:00 08:00 16:00 20:00 04:00 08:00 12:00 16:00 20:00 00:00 04:00 08:00 12:00 16:00 20:00 00:00 00:00 08:00 12:00 00:00 12:00 00:00 Time (hours) Time (hours) 5.1.2. Field Test FigureFigure 14. 14. Measured slab temperatures for Scenario I (leftleft) and Scenario II right(right) of the lab test. Measured slabFigure temperatures 15 shows for Scenario measured I ( ) and values Scenario of IIsupply, ( ) of theextract lab test. and operative temperatures, and 5.1.2. Field Test 5.1.2.airflows Field Testfor the field test. Each temperature measurement in the figure is an average of twoFigureFigure different 1515 shows sensors. measuredmeasured The values values supply of of supply, supply, air temperature extract extract and and operative operative was temperatures,measured temperatures, in and each air- and of the two supply airflowsflowsair diffusers. for for the the field field Similarly, test. test. Each Each temperature the temperature extract measurement airmeasurement temperature in the in figurethe was figure is anmeasured averageis an average of twoin ofthe two extract air two different sensors. The supply air temperature was measured in each of the two supply differentgrilles. sensors. The radiant The supply air airtemperature temperature was was measured measured in each by of the twoglobe supply temperature air sensor in- airdiffusers. diffusers. Similarly, Similarly, the extractthe extract air temperature air temperature was measured was measured in the twoin the extract two airextract grilles. air grilles.Thestalled radiant The on radiant air each temperature air measurement temperature was measured was pole. bymeasured the The globe operative by temperature the globe temperature sensortemperature installed sensorwas on each calculatedin- based on stalledmeasurementthe measuredon each pole. measurement air The temperature, operative pole. temperature The meanoperative was radiant temperature calculated temperature, based was on calculated the and measured baairsed velocity air on according to thetemperature,ISO measured 7726 [105]. mean air temperature, radiant temperature, mean radiant and air temperature, velocity according and air to ISO velocity 7726 [105according]. to ISO 7726 [105]. 34 15 34 Supply Temperature Extract Temperature15 Supply Temperature Extract Temperature

OperativeOperative Temperature TemperatureAirflow Airflow

) )

30 12 2

° °

26 9 /h/m 3

26 9 /h/m 3 22 6

22 6 Temperature ( Temperature

18 3 Airflow (m Temperature ( Temperature 18 3 Airflow (m 14 0

14 0

20:00 00:00 04:00 10:00 14:00 18:00 18:00 22:00 02:00 06:00 08:00 12:00 16:00

Time (hours)

20:00 00:00 04:00 10:00 14:00 18:00 22:00 02:00 06:00 08:00 12:00 16:00 18:00 Figure 15. Measured supply, extract, and operative temperatures, and airflows for the field test. Time (hours) As seen from the figure, the supply air temperature in the heating mode was approx- FigureFigure 15. 15.Measured Measured supply, supply, extract, andextract, operative and temperatures, operative temperatures, and airflows for the and field airflows test. for the field test. imately between 27.3 and 28.3 °C. The supply air temperature in the normal mode varied between 16.1 and 18.3 °C. In both heating and normal modes, the actual supply air tem- AsAs seen seen from from the figure, the figure, the supply the air supply temperature air tempera in the heatingture mode in the was heating approxi- mode was approx- peraturesmately between were different 27.3 and from 28.3 ◦theC. Theoriginally supply plan air temperaturened outdoor in temperature the normal mode-compensated varied supplybetweenimately temperatures 16.1 between and 18.3 shown 27.3◦C. In andin both Figure 28.3 heating 3. °C. In and theThe normal normal supply mode, modes, air the temperature the supply actual temperature supply in airthe tem- wasnormal mode varied considerablyperaturesbetween were 16.1lower different and than 18.3 the from anticipated °C. the originallyIn both value heating planned of around outdoorand 20 normal°C, temperature-compensated primarily modes, due to the an anom-actual supply air tem- alysupplyperatures in the temperatures supply were temperature different shown in curve Figurefrom in 3the .the In originallycont the normalrol system. mode,plan Thisned the was supplyoutdoor because temperature temperature the school -compensated buildingwas considerably was not fully lower finished than the at anticipated the time of value field of testing, around and 20 ◦ C,the primarily control system due to anwas supply temperatures shown in Figure 3. In the normal mode, the supply temperature was beinganomaly tested in and the supplycommissioned. temperature Nevertheless, curve in the supplying control system.air at lower This temperatures was because thethan originallyschoolconsiderably building planned was lower in notthe fullynormalthan finished the operatio anticipated at then mode time provided ofvalue field testing,of an around even and more the 20 control stringent°C, primarily system test of due to an anom- thealy concept. in the As supply in the lab temperature test, the extract curve air temperature in the cont fromrol the system. room depended This was upon because the school thebuilding operation wasmode not of the fully system finished and the occupancyat the time level. of In field the heating testing, mode, and the the extract control system was airbeing temperature tested was and 6– 8commissioned. K lower than the supplyNevertheless, air temperature, supplying whereas, air in at the lower normal temperatures than operation mode, the extract air temperature was on average 3.0 K and at maximum 4.4 K higheroriginally than the planned supply air in temperature. the normal During operatio the lunchn mode break, provided the extract an air even tempera- more stringent test of turethe was concept. just under As 19 in ° Cthe due lab to lowertest, the internal extract heat airloads. temperature The operative from temperature the room in depended upon the operation mode of the system and the occupancy level. In the heating mode, the extract air temperature was 6–8 K lower than the supply air temperature, whereas, in the normal operation mode, the extract air temperature was on average 3.0 K and at maximum 4.4 K higher than the supply air temperature. During the lunch break, the extract air temperature was just under 19 °C due to lower internal heat loads. The operative temperature in

Energies 2021, 14, 952 20 of 33

was being tested and commissioned. Nevertheless, supplying air at lower temperatures Energies 2021, 14, x FOR PEER REVIEWthan originally planned in the normal operation mode provided an even more stringent test 21 of 34

of the concept. As in the lab test, the extract air temperature from the room depended upon the operation mode of the system and the occupancy level. In the heating mode, the extract air temperature was 6–8 K lower than the supply air temperature, whereas, in the normal operationthe room mode, was the rather extract airconstant. temperature In wasthe onheating average mode, 3.0 K and the at maximumoperative 4.4 temperature K was be- highertween than 19.0 the supplyand 19.5 air temperature. °C. In theDuring normal the mode, lunch break, the theoperative extract air temperature temperature ranged between was just under 19 ◦C due to lower internal heat loads. The operative temperature in the room19.3 was to 20.5 rather °C, constant. with Inan the average heating of mode, 20.2 the °C. operative During temperature the lunch was break, between the operative temper- 19.0ature and was 19.5 ◦ justC. In below the normal 19 mode,°C. the operative temperature ranged between 19.3 to 20.5 ◦C,Figure with an average16 shows of 20.2 the◦C. vertical During thetemperature lunch break, theprofiles operative at temperatureheights of was0.1, 1.1, 1.7, and 2.7 m ◦ justabove below the 19 floorC. level. Each curve is an average of two sensors mounted at the same height Figure 16 shows the vertical temperature profiles at heights of 0.1, 1.1, 1.7, and 2.7 m aboveon different the floor level.poles Each in the curve opposite is an average corners of two of sensors the classroom. mounted at As the seen same from the figure, a heightvertical on different temperature poles in thegradient opposite was corners present of the classroom.in the room As seen in fromboth the the figure, day operating and the aheating vertical temperature modes. The gradient air wastemperature present in the was room lowest in both at the the day floor operating leve andl and the rose steadily with heating modes. The air temperature was lowest at the floor level and rose steadily with height.height. The The average average and maximum and maximum vertical temperature vertical temperature gradient between gradient the standing between the standing headhead height height (1.7 m)(1.7 and m) ankle and height ankle (0.1 height m) in the(0.1 normal m) in mode the wasnormal 1.2 K mode and 1.6 was K, 1.2 K and 1.6 K, respectively,respectively, and inand the heatingin the heating mode was mode 0.5 K, andwas 0.7 0.5 K, respectively.K, and 0.7 K, respectively.

22 Height 0.1 m Height 1.1 m Height 1.7 m Height 2.7 m

C) ° 20

Temperature ( Temperature 18

18:00 22:00 04:00 08:00 10:00 14:00 20:00 00:00 02:00 06:00 12:00 16:00 18:00 Time (hours)

FigureFigure 16. Measured16. Measured vertical vertical temperature temperature proﬁles for the profiles ﬁeld experiment. for the field experiment.

Figure 17 presents the vertical CO2 profiles at heights of 0.1, 1.1, 1.7, and 2.7 m above Figure 17 presents the vertical CO2 profiles at heights of 0.1, 1.1, 1.7, and 2.7 m above the floor level. Again, each curve represents an average of two measurements taken at the samethe heightsfloor level. by sensors Again, mounted each oncurve two differentrepresents poles an in average two opposite of two corners measurements of the taken at the classroom.same heights The figure by shows sensors that inmounted the heating on mode two there different was no verticalpoles stratificationin two opposite of corners of the COclassroom.2 in the classroom. The figure This is shows because therethat werein the no heating CO2 sources mode in the there classroom. was no In the vertical stratification heating mode, the CO2 levels in the classroom decayed to nearly outdoor levels by dilution withof CO the2 recirculation in the classroom. air. During This the normalis because mode, there there were was a no vertical CO2 stratification sources in the classroom. In ofthe CO 2heatingin the classroom. mode, the The CO CO22 levelsconcentration in the wasclassroom lowest at decayed the floor levelto nearly in the outdoor levels by classroomdilution and with increased the recirculation non-linearly with air. height. During Above the the normal sitting headmode, height there (1.1 was m), a vertical stratifi- the CO concentration had a rather flat gradient. During the lunch break between 11:00 and cation2 of CO2 in the classroom. The CO2 concentration was lowest at the floor level in the 12:00 h, and at the end of the school day, the CO2 concentration in the classroom decayed veryclassroom rapidly. and increased non-linearly with height. Above the sitting head height (1.1 m), the CO2 concentration had a rather flat gradient. During the lunch break between 11:00 and 12:00 h, and at the end of the school day, the CO2 concentration in the classroom decayed very rapidly.

1400 Height 0.1 m Height 1.1 m 1200 Height 1.7 m Height 2.7 m

1000

800

600 Concentration (ppm) Concentration

2 400 CO

200

18:00 00:00 04:00 10:00 14:00 16:00 20:00 22:00 02:00 06:00 08:00 12:00 18:00 Time (hours)

Figure 17. Measured vertical CO2 concentrations for the field experiment.

Energies 2021, 14, x FOR PEER REVIEW 21 of 34

the room was rather constant. In the heating mode, the operative temperature was between 19.0 and 19.5 °C. In the normal mode, the operative temperature ranged between 19.3 to 20.5 °C, with an average of 20.2 °C. During the lunch break, the operative temperature was just below 19 °C. Figure 16 shows the vertical temperature profiles at heights of 0.1, 1.1, 1.7, and 2.7 m above the floor level. Each curve is an average of two sensors mounted at the same height on different poles in the opposite corners of the classroom. As seen from the figure, a vertical temperature gradient was present in the room in both the day operating and the heating modes. The air temperature was lowest at the floor level and rose steadily with height. The average and maximum vertical temperature gradient between the standing head height (1.7 m) and ankle height (0.1 m) in the normal mode was 1.2 K and 1.6 K, respectively, and in the heating mode was 0.5 K, and 0.7 K, respectively.

22 Height 0.1 m Height 1.1 m Height 1.7 m Height 2.7 m

C) ° 20

Temperature ( Temperature 18

18:00 22:00 04:00 08:00 10:00 14:00 20:00 00:00 02:00 06:00 12:00 16:00 18:00 Time (hours) Figure 16. Measured vertical temperature profiles for the field experiment.

Figure 17 presents the vertical CO2 profiles at heights of 0.1, 1.1, 1.7, and 2.7 m above the floor level. Again, each curve represents an average of two measurements taken at the same heights by sensors mounted on two different poles in two opposite corners of the classroom. The figure shows that in the heating mode there was no vertical stratification of CO2 in the classroom. This is because there were no CO2 sources in the classroom. In the heating mode, the CO2 levels in the classroom decayed to nearly outdoor levels by dilution with the recirculation air. During the normal mode, there was a vertical stratification of CO2 in the classroom. The CO2 concentration was lowest at the floor level in the classroom and increased non-linearly with height. Above the sitting head height (1.1 m), the CO2 concentration had a rather flat gradient. During the lunch break between 11:00 Energies 2021, 14, 952 21 of 33 and 12:00 h, and at the end of the school day, the CO2 concentration in the classroom decayed very rapidly.

1400 Height 0.1 m Height 1.1 m 1200 Height 1.7 m Height 2.7 m

1000

800

600 Concentration (ppm) Concentration

2 400 CO

200

18:00 00:00 04:00 10:00 14:00 16:00 20:00 22:00 02:00 06:00 08:00 12:00 18:00 Time (hours)

FigureFigure 17. Measured17. Measured vertical COvertical2 concentrations CO2 concentrations for the ﬁeld experiment. for the field experiment. 5.2. Simulation Results The results of numerical simulation conducted using IDA-ICE simulation software for the two lab tests and one ﬁeld test performed, respectively, in the scaled mock-up and the actual classroom are presented in the following sections.

5.2.1. Lab Test As mentioned earlier in Section 4.2.1, the simulation model of the test room was constructed using the actual geometry and construction details. The inputs to the simulation model including internal loads, operating schedules, and supply air temperatures and flows were based on actual measured data from the lab test. The simulated energy balances of the test room are shown in Figure 18 for the two scenarios of the lab test. As seen from the figure, the heat provided through the displacement ventilation system in the night heating mode was simulated to be partly stored in the room and partly lost through the room envelope. The simulation results implied that in normal mode the stored heat was released back to the room gradually over time, especially during periods of no occupancy and reduced ventilation flows. Moreover, for both Scenario I and Scenario II, the stored heat was simulated not to be released to the room when occupied. This was because the sum of internal heat gains from people, equipment and lighting, and heat gains from ventilation airflow did not exceed heat losses through the room envelope during the occupancy periods. Figure 19 shows the simulated vertical temperature profiles at heights of 0.1, 1.1, 1.7, 2.3, and 2.9 m above the floor level for the two scenarios of the lab test. For Scenario 1, the model did not simulate any stratification for the heating mode between 00:00– 8:00. For periods with normal mode between 8:00–11:00 and 12:00–16:00, the average and maximum vertical temperature gradient between the standing head height (1.7 m) and ankle height (0.1 m) simulated by the model was 0.4, and 0.8 K, respectively. For periods of no ventilation between 16:00–00:00, no stratification was simulated by the model, as expected. For Scenario 2, no stratification was predicted in heating or normal modes. The simulated vertical temperature gradients in both scenarios were lower than the experimentally measured gradients. Energies 2021, 14, x FOR PEER REVIEW 22 of 34

5.2. Simulation Results The results of numerical simulation conducted using IDA-ICE simulation software for the two lab tests and one field test performed, respectively, in the scaled mock-up and the actual classroom are presented in the following sections.

5.2.1. Lab Test As mentioned earlier in Section 4.2.1, the simulation model of the test room was constructed using the actual geometry and construction details. The inputs to the simulation model including internal loads, operating schedules, and supply air temperatures and flows were based on actual measured data from the lab test. The simulated energy balances of the test room are shown in Figure 18 for the two scenarios of the lab test. As seen from the figure, the heat provided through the displacement ventilation system in the night heating mode was simulated to be partly stored in the room and partly lost through the room envelope. The simulation results implied that in normal mode the stored heat was released back to the room gradually over time, especially during periods of no occupancy and reduced ventilation flows. Moreover, for both Scenario I and Scenario II, the stored heat was simulated not to be released to the room when occupied. This was because Energies 2021, 14, 952 the sum of internal heat gains from people, equipment and lighting, and heat gains22 from of 33 ventilation airflow did not exceed heat losses through the room envelope during the occupancy periods.

Energies 2021, 14, x FOR PEER REVIEW 23 of 34

periods with normal mode between 8:00–11:00 and 12:00–16:00, the average and maximum vertical temperature gradient between the standing head height (1.7 m) and ankle height (0.1 m) simulated by the model was 0.4, and 0.8 K, respectively. For periods of no ventilation between 16:00–00:00, no stratification was simulated by the model, as expected. For Scenario 2, no stratification was predicted in heating or normal modes. The Figure 18. Simulated energy balance for Scenario I (top) and Scenario II (bottom) of the lab test. simulatedFigure 18. Simulatedvertical temperature energy balance gradients for Scenario in Iboth (top) scenarios and Scenario were II (bottom lower) ofthan the labthe test.experimentally measured gradients. Figure 19 shows the simulated vertical temperature profiles at heights of 0.1, 1.1, 1.7, 2.3, and 2.9 m above the floor level for the two scenarios of the lab test. For Scenario 1, the 25 25 Height 0.1 m Heightmodel 1.1 m did Heightnot simulate1.7 m any stratification for Heightthe heating 0.1 m modeHeight 1.1 between m Height 00:0 1.7 0m–8:00. For

24 Height 2.3 m Height 2.9 m 24 Height 2.3 m Height 2.9 m

C) C) ° ° 23 23

22 22

21 21 Temperature ( Temperature Temperature ( Temperature 20 20

19 19

12:00 12:00 00:00 04:00 08:00 12:00 16:00 20:00 00:00 04:00 08:00 16:00 20:00 00:00 00:00 04:00 08:00 12:00 16:00 20:00 00:00 04:00 08:00 16:00 20:00 00:00 Time (hours) Time (hours) Figure 19. Simulated vertical temperature profiles for Scenario I (left) and Scenario II (right) of the lab test. Figure 19. Simulated vertical temperature proﬁles for Scenario I (left) and Scenario II (right) of the lab test.

5.2.2. Field Test 5.2.2. Field Test Figure 20 shows the simulated energy balance for the field test. As for the lab test, Figure 20 shows the simulated energy balance for the ﬁeld test. As for the lab test, thethe simulation simulation results results of of the the field ﬁeld test also demonstrateddemonstrated thatthat partpart ofof thethe heatheat provided provided throughthrough the the displacement displacement ventilation ventilation and stored inin thethe roomroom structurestructure in in the the night night heating heating modemode was was released released back back to to the room during the periodsperiods ofof nono occupancyoccupancy andand reduced reduced ventilationventilation in in the the normal normal operating operating mode. mode. Unlike Unlike the thelab labtest, test, heat heat gains gains from from solar solar radi- ationradiation through through the windows the windows in the in normal the normal mode mode provided provided a useful a useful contribution contribution to the to the en- ergyenergy balance balance of ofthe the classroom. classroom. Moreover, Moreover, ventilation ventilation losses losses i inn the normal modemode hadhad a a sizeablesizeable contribution contribution to to the the energy energy balance of the classroom forfor thethe reasonreason thatthat ventilation ventilation air to the classroom was supplied at fairly low temperatures in order to represent a worst- case scenario.

Figure 20. Detailed energy balance for the simulated classroom model.

Figure 21 shows the simulated vertical temperature profiles at heights of 0.1, 1.1, 1.7, and 2.7 m above the floor level. As seen from the figure, the model did not simulate any temperature stratification in the heating mode between 20:00–8:00. In the normal mode, the model simulated a vertical temperature gradient in the room. The air temperature was lowest at the floor level and rose linearly with height. The average and maximum vertical temperature gradient between the standing head height (1.7 m) and ankle height (0.1 m)

Energies 2021, 14, x FOR PEER REVIEW 23 of 34

25 Height 0.1 m Height 1.1 m Height 1.7 m 25 Height 0.1 m Height 1.1 m Height 1.7 m

24 Height 2.3 m Height 2.9 m 24 Height 2.3 m Height 2.9 m

C) C) ° ° 23 23

22 22

21 21 Temperature ( Temperature Temperature ( Temperature 20 20

19 19

5.2.2. Field Test Figure 20 shows the simulated energy balance for the field test. As for the lab test, the simulation results of the field test also demonstrated that part of the heat provided through the displacement ventilation and stored in the room structure in the night heating mode was released back to the room during the periods of no occupancy and reduced ventilation in the normal operating mode. Unlike the lab test, heat gains from solar radi-

Energies 2021, 14, 952 ation through the windows in the normal mode provided a useful contribution23 of 33 to the energy balance of the classroom. Moreover, ventilation losses in the normal mode had a sizeable contribution to the energy balance of the classroom for the reason that ventilation airair to to the the classroom classroom was suppliedwas supplied at fairly at low fairly temperatures low temperatures in order to represent in order a worst-to represent a worst- casecase scenario. scenario.

FigureFigure 20. 20.Detailed Detailed energy energy balance bala fornce the simulated for the simulated classroom model. classroom model.

FigureFigure 21 shows21 shows the simulated the simulated vertical temperaturevertical temperature profiles at heights profiles of 0.1, at heights 1.1, 1.7, of 0.1, 1.1, 1.7, and 2.7 m above the floor level. As seen from the figure, the model did not simulate any Energies 2021, 14, x FOR PEER REVIEWand 2.7 m above the floor level. As seen from the figure, the model did not simulate any24 of 34 temperature stratification in the heating mode between 20:00–8:00. In the normal mode, thetemperature model simulated stratification a vertical temperature in the heati gradientng mode in the between room. The air20:0 temperature0–8:00. In was the normal mode, lowestthe model at the floorsimulated level and a vertical rose linearly temperature with height. gradient The average in andthe maximumroom. The vertical air temperature was temperaturelowest at the gradient floor between level and the rose standing linearly head heightwith height. (1.7 m) andThe ankle average height and (0.1 maximum m) vertical simulatedsimulated by the by model the model was 1.5 was K, and 1.5 1.8 K K,, and respectively. 1.8 K, respectively. The simulated gradient The simulated values gradient values temperature gradient between the standing head height (1.7 m) and ankle height (0.1 m) werewere greater greater than thethan experimentally the experimentally measured ones. measured ones.

23 Height 0.1 m Height 1.1 m

22 Height 1.7 m Height 2.7 m C) ° 21

Temperature ( Temperature 18

22:00 14:00 18:00 20:00 00:00 02:00 04:00 06:00 08:00 10:00 12:00 16:00 18:00 Time (hours)

FigureFigure 21. Simulated21. Simulated vertical vertical temperature temperature proﬁles for the profiles ﬁeld experiment. for the field experiment.

Figure 22 presents the simulated CO2 concentration in the classroom during the field Figure 22 presents the simulated CO2 concentration in the classroom during the field experiment. As noted earlier in Section 4.2, the IDA-ICE model could only apply a simple COexperiment.2 balance in the As zone noted and could earlier not calculatein Section vertical 4.2, stratification the IDA-ICE or horizontal model gradientcould only apply a simple ofCO CO22 balanceconcentration in the in thezone zone. and The could figure not shows calculate that in thevertical heating str modeatification between or horizontal gradi- 20:00–8:00 when there were no CO sources present in the classroom, the simulated CO ent of CO2 concentration in2 the zone. The figure shows that in the heating2 mode between concentration in the classroom was equal to the outdoor concentration. In the normal mode 20:00–8:00 when there were no CO2 sources present in the classroom, the simulated CO2 concentration in the classroom was equal to the outdoor concentration. In the normal mode between 8:00–11:00 and 12:00–16:00, the model only predicted an average value of CO2 concentration at each time based upon the number of occupants in the classroom and their assigned activity levels. During the lunch break between 11:00 and 12:00 h and at the end of the school day, the model predicted a near exponential decay in CO2 concentration in the classroom.

1400

1200

1000

800

600 Concentration (ppm) Concentration

2 400 CO

200

06:00 18:00 20:00 22:00 00:00 02:00 04:00 08:00 10:00 12:00 14:00 16:00 18:00 Time (hours)

Figure 22. Simulated CO2 concentration for the field experiment.

6. Discussion and Conclusions The primary objective of this paper is to study and to enhance the understanding of using displacement ventilation for achieving a comfortable indoor environment in winter without the requirement of a separate heating system. As a first step towards this objective, a set of experimental tests was conducted under various realistic operating conditions. Firstly, a series of tests were performed in a scale model of a classroom under controlled laboratory conditions. It was followed by a field test in a real classroom environment with several uncontrollable disturbances influencing the operation of the system.

Energies 2021, 14, x FOR PEER REVIEW 24 of 34

simulated by the model was 1.5 K, and 1.8 K, respectively. The simulated gradient values were greater than the experimentally measured ones.

23 Height 0.1 m Height 1.1 m

22 Height 1.7 m Height 2.7 m C) ° 21

Temperature ( Temperature 18

22:00 14:00 18:00 20:00 00:00 02:00 04:00 06:00 08:00 10:00 12:00 16:00 18:00 Time (hours) Figure 21. Simulated vertical temperature profiles for the field experiment.

Figure 22 presents the simulated CO2 concentration in the classroom during the field experiment. As noted earlier in Section 4.2, the IDA-ICE model could only apply a simple CO2 balance in the zone and could not calculate vertical stratification or horizontal gradient of CO2 concentration in the zone. The figure shows that in the heating mode between

Energies 2021, 14, 952 20:00–8:00 when there were no CO2 sources present in the classroom,24 of 33 the simulated CO2 concentration in the classroom was equal to the outdoor concentration. In the normal mode between 8:00–11:00 and 12:00–16:00, the model only predicted an average value of

betweenCO2 concentration 8:00–11:00 and 12:00–16:00,at each time the based model onlyupon predicted the number an average of occupants value of CO in2 the classroom and concentrationtheir assigned at each activity time based levels. upon During the number the of lunch occupants break in the between classroom 11:00 and their and 12:00 h and at the assigned activity levels. During the lunch break between 11:00 and 12:00 h and at the end end of the school day, the model predicted a near exponential decay in CO2 concentration of the school day, the model predicted a near exponential decay in CO2 concentration in thein classroom. the classroom.

1400

1200

1000

800

600 Concentration (ppm) Concentration

2 400 CO

200

06:00 18:00 20:00 22:00 00:00 02:00 04:00 08:00 10:00 12:00 14:00 16:00 18:00 Time (hours)

FigureFigure 22. Simulated22. Simulated CO2 concentration CO2 concentration for the field for experiment. the field experiment. 6. Discussion and Conclusions 6. Discussion and Conclusions The primary objective of this paper is to study and to enhance the understanding of usingThe displacement primary ventilationobjective forof this achieving paper a comfortableis to study indoor and to environment enhance the in understanding of winterusing without displacement the requirement ventilation of a separate for achieving heating system. a comfortable As a first stepindoor towards environment in winter this objective, a set of experimental tests was conducted under various realistic operating conditions.without the Firstly, requirement a series of testsof a were separate performed heating in a scalesystem. model As of a afirst classroom step towards this objec- undertive, controlled a set of laboratoryexperimental conditions. tests It was followed conducted by afield under test invarious a real classroom realistic operating condi- environmenttions. Firstly, with a several series uncontrollable of tests were disturbances performed influencing in a scale the model operation of ofa theclassroom under con- system.trolled The laboratory experimental conditions. tests were specifically It was aimedfollowed to assess by thea field technical test and in practicala real classroom environ- suitability of the proposed concept in terms of thermal comfort, temperature stratification, ment with several uncontrollable disturbances influencing the operation of the system. and CO2 concentration. For this purpose, the measured data can be assessed against the design criteria laid out in standard EN 16798-1 [106]. The standard specifies several categories of thermal comfort and indoor air quality and stipulates requirements for operative temperature, vertical air temperature difference between head and ankle heights, and CO2 concentration above outdoor concentration, among others, for each category. Table3 summarizes these requirements for various categories for classrooms.

Table 3. Design criteria for thermal comfort, temperature stratiﬁcation, and CO2 concentration according to EN 16798-1 [106].

Vertical Air Minimum Operative CO Concentrations Temperature 2 Category Temperature for above Outdoors Difference [Seated] Heating (◦C) (ppm) (K) I 21 2 550 II 20 3 800 III 19 4 1350 1V 18 - 1350

The lab tests were conducted in a scale model of the classroom using two distinct scenarios. The test conditions for both scenarios were carefully controlled to simulate an Energies 2021, 14, 952 25 of 33

outside temperature of −15 ◦C. In the first scenario, the scale model of the classroom was subjected to normal operating conditions of regular occupancy and typical internal heat gains from lighting and equipment during a school day. In the second scenario, the occupancy and internal load profiles were chosen to represent an extreme case. The classroom was considered occupied only during the afternoon for three hours at half of its design occupancy and with much lower heat gains from equipment. The results of the lab tests showed that for the first scenario the operative temperatures during the occupancy periods remained 1–2 K higher than the supply temperature of 22 ◦C. The average and maximum temperature gradient between 0.1 and 1.1 m heights were approximately 0.7 and 1.0 K, respectively. The measured operative temperatures range under categories I of EN 16798-1, which correspond to a predicted percentage of dissatisfied (PPD) of 6%. Besides, the measured vertical temperature difference between head and ankle heights was significantly smaller than the design limit of 2 K for Category I of local thermal discomfort in EN 16798-1. For the second scenario, the operative temperature was below 21 ◦C at the beginning of the occupancy period but increased rapidly once the classroom was occupied. During the occupancy period, the average and maximum temperature gradient between 0.1 and 1.1 m heights were approximately 0.3, and 0.5 K, respectively. For this scenario, the measured operative temperatures ranged under categories I and II of EN 16798-1 corresponding to PPD of 6, and 10%, respectively. The measured temperature gradient between head and ankle heights was again significantly smaller than the 2 K design limit for Category I of local thermal discomfort in EN 16798-1. The occupancy and internal load profiles in the second scenario represented an extreme case. The case of no occupancy and no internal heat gains until noon, and lower than expected occupancy and internal heat gains in the afternoon, provided a stringent test of the concept under consideration. The field test was performed in an actual classroom under real dynamic conditions of occupancy, transmission and ventilation losses, and solar gains, among others. The field test lasted for approximately 20 h, during which the displacement ventilation system was operated in the heating mode during the first 12 h and in the normal mode for the remaining time. The supply air temperature and flow to the classroom used in the normal mode represented rather extreme conditions unlikely to be encountered in actual practice. During the test period, the outdoor temperature varied between −7 and +3 ◦C. The number and position of heat and pollutant sources, i.e., occupants, varied frequently throughout the testing period. Solar heat gains to the classroom varied with time of day, starting around mid-morning, peaking in late morning until mid-afternoon, and declining in the late afternoon. Transmission heat losses to the ambient and the ground also varied in time, depending upon the temperature differences between inside and outside. Sim- ilarly, ventilation losses from the classroom varied in time due to changes in supply air temperature and flow, as well as due to infiltration losses from recurring door openings. The results of the field test showed that even with the rather low supply temperatures of between 16 and 18 ◦C, not likely to occur during the actual operation, the operative temperatures during the occupancy periods were 2–4 K higher than the supply temperature. The average and maximum temperature and CO2 gradients between 0.1 and 1.1 m heights were approximately 0.8 and 1.1 K, and 240, and 350 ppm, respectively. The maximum average room and breathing zone CO2 concentrations above outdoor concentration were 670 and 730 ppm, respectively. The operative temperatures measured during the occupancy periods fall under categories II and III of EN 16798-1. The lowest operative temperatures, corresponding to category III, occurred early in the morning when supply air temperatures were at their minimum and solar heat gains were absent. As in the lab tests, the measured vertical temperature difference between head and ankle heights was smaller than the 2 K design limit for category I of local thermal discomfort in EN 16798-1. The measured CO2 concentrations in the classroom corresponded to categories I and II of recommended CO2 concentrations above the outdoor concentration in EN 16798-2 [107]. One type of commonly used indoor air quality indicator for displacement ventilation systems is ventilation effectiveness [69,73,79,81,82]. It is the ability of the ventilation system Energies 2021, 14, 952 26 of 33

to exchange the air in the zone and to remove air-borne contaminants from the zone [108]. Based on the field test measurements, the ventilation effectiveness of the classroom system could be evaluated in terms of contaminant removal effectiveness and air change efficiency. The contaminant removal effectiveness, providing a measure of how quickly an airborne contaminant is removed from the zone and determined as the ratio between the steady- state concentration of CO2 in the exhaust air and the steady-state mean concentration in the classroom, is computed to be 1.2. The air change efficiency, providing a measure of how quickly the air is replaced in the zone and calculated from concentration decay (step-down) of CO2 during the lunch break and after the end of school when the classroom was empty, using the method described in Mundt et al. [108], was found to be between 55% and 60%. The ventilation effectiveness indices of contaminant removal effectiveness and air change efficiency are somewhat lower than reported for displacement ventilation previously, which may be attributed to differences in boundary conditions between this and earlier studies. Cold climates, such as the one investigated in this study, cause descending air flows from cold surfaces like windows and external walls, which results in a mixing of contaminated air from the upper unoccupied zone air into the lower occupied zone, thus reducing the ventilation effectiveness. A few other studies have also reported similar results, implying lower-than-normal ventilation effectiveness indices for displacement ventilation systems in cold climates [45,109]. Nevertheless, the ventilation effectiveness indices calculated from the field measurements for the classroom are still superior to the maximum possible contaminant removal effectiveness of 1 and a maximum possible air change efficiency of 50% for mixing ventilation. The results of experimental tests clearly support the hypothesis that the proposed solu- tion of using displacement ventilation without a separate heating system is fully capable of maintaining a comfortable indoor climate in the classroom even in peak winter periods and under extreme operating conditions. In fact, the experimental results provide answers to the questions raised in Section3 on the application of displacement ventilation for heating during non-occupied hours. In answer to the first question, it has been found through the series of experiments that providing heating only during the non-occupied hours could be sufficient to achieve good indoor thermal comfort conditions during the occupied hours. Thermal comfort corresponding to Category I of EN 16798-1 could be achieved under typical winter operating conditions, whereas thermal comfort corresponding to Category II could be attained under extreme conditions, expected to occur only rarely in practice. In reply to the second question, it has been ascertained through the experimental tests that the vertical temperature gradient in classrooms would not be unreasonably high at the start of the school day and during the non-occupied hours. The experiments demonstrated that a temperature difference of less than 2 K between head and ankle heights, corresponding to Category I of local thermal discomfort in EN 16798-1, could be achieved during the occupancy, as well as non-occupancy periods. In relation to the third question, it has been exhibited both through the lab and field experiments that the warm air in heating mode does not ascend to the ceiling unimpeded, but rather is stratified along the height of the zone. The vertical temperature distribution in the heating mode could be noted to be a linear function of the zone height for all experimental tests. It has also been demonstrated that the warm air supplied with low impulse would indeed heat the floor and the space above. In response to the fourth question, it has been validated through Scenario II of the lab test that satisfactory thermal comfort conditions could be ensured even with exception- ally low occupancy and internal heat gains in the zone. The field test results also reaffirmed this conclusion. Finally, regarding the fifth question, while no specific experimental tests were conducted to directly analyze this issue, it could be deduced from the conducted experimental tests that satisfactory thermal comfort conditions could be achieved even if the occupancy and internal heat gains in the zone would be higher than the design values. The scenario I of the lab test showed that, under design conditions of occupancy and internal heat gains, thermal comfort in the zone could be comfortably maintained at a level corresponding to Category I of EN 16798-1. Although, the greater-than-design levels Energies 2021, 14, 952 27 of 33

of occupancy and internal loads would increase the operative temperatures and vertical temperature gradient in the zone, but an increase beyond the threshold levels of 25 ◦C for operative temperature and 3 K for temperature gradient between the head and ankle heights, corresponding to Category II of thermal comfort in EN 16798-1, would be unlikely. Moreover, it has been demonstrated through the field test that in case of greater-than-design levels of occupancy and internal loads, supply temperatures in heating mode could be lowered to maintain satisfactory thermal comfort conditions in the zone. As a second step towards realizing the research objective, numerical simulations of experimental tests were carried out using a state-of-the-art building indoor climate simulation program. The purpose of this step was to assess whether numerical simulations and state- of-the-art modeling methods are capable of accurately capturing experimentally observed temperature and CO2 distributions for the proposed concept. The results of numerical simulation of the lab tests showed that for the first scenario the simulated average and maximum temperature difference between head and ankle heights during the occupancy periods was 0.3, and 0.5, respectively. The simulated temperature gradient between head and ankle heights was zero during the non-occupancy periods, except for lunch breaks when internal loads from lighting and equipment were still present. For the occupancy periods, the simulated temperature gradients between head and ankle heights are only half of the experimental values. This is despite a very good match between the simulated and experimentally measured temperature gradients from floor to ceiling. This can be attributed to the stratification model used by the simulation program. The model has certain limitations, in terms of accurately modeling temperature stratification in the zone, as well as identifying the stratification height in the space, as previously suggested by other researchers, including Mateus and da Graça [30] and Lastovets et al. [110], among others. For the non-occupancy period, numerical simulations were unable to provide a good estimate of temperature gradient that can be compared to the experimental data. The numerical simulations were also unable to calculate the temperature stratification for both the occupancy, as well as the non-occupancy periods of the second scenario and were consequently not able to replicate the thermal gradients observed experimentally in the lab test. The results of the numerical simulation of the field test showed that the simulated average and maximum temperature difference between head and ankle heights during the occupancy periods was 0.9, and 1.1, respectively. During the non-occupancy period, the temperature gradient was simulated to be zero. The temperature gradients simulated between head and ankle heights for the occupancy periods were quite similar to the experimental values. However, there was a pronounced mismatch between the simulated and experimentally measured temperature gradients from floor to ceiling, with simulations overestimating the temperature gradients by a factor of over two. The simulated vertical temperature gradient was modeled to be linear over the classroom height, while the experimentally measured temperature gradient was observed to be non-linear. In point of fact, the measured temperature and contaminant distribution in the classroom indicated that space was divided into two distinct zones separated by a stratification height. Below the stratification height, the air temperature increased with height, whereas, above the stratification height, there existed a small negative temperature gradient. The stratification height was not evident in the simulated vertical temperature profile. As in the case of lab tests, the numerical simulations could not predict the temperature gradients observed in the field test during the non-occupancy period. Numerical simulations also could not compute the vertical CO2 concentration gradients due to the model limitations and could only predict the mean CO2 stratification. The simulated mean CO2 concentrations were slightly lower than the average CO2 levels measured experimentally. This could be because the numerical simulations used a constant rate of CO2 generation for each occupant throughout the day. In reality, the CO2 generation rate per occupant could vary widely based upon factors, such as occupant’s activity level, metabolic rate, and body surface area, among others [111]. Energies 2021, 14, 952 28 of 33

It is evident from the above discussion that the results of numerical simulations are inconsistent with the experimental results. The numerical simulations completely failed to predict the vertical temperature profiles when providing heating through displacement ventilation. The numerical simulations also could not correctly capture the experimental results in the normal mode when the supply air temperature was lower than the space temperature. This was even though the numerical simulations were carried out with great care and attention to detail to match the experimental conditions as closely as possible. The discrepancies between experimental and numerical results are relatively large and could not be explained by experimental uncertainties of temperature measurements that did not exceed ±0.2 and ±0.5 K for lab, and field tests, respectively. Therefore, other factors related to the experimental tests and/or the numerical analysis must be responsible for the differences in experimental and numerical results observed in this study. Some of the discrepancies between experimental tests and simulation results might be attributed to uncertainties and limitations associated with the analysis, particularly with regard to the simulation of dynamic conditions during the tests. For instance, the type and location of heat sources, such as occupants, equipment, and lights in numerical simulations were considered not to influence the temperature stratification in the zone. Heat gains from these sources were taken to be evenly distributed over the zone volume and zone surfaces. As a matter of fact, both the type and location of heat sources have a significant impact on the temperature stratification in the zone [10,60–63]. Moreover, while significant stochastic movements of occupants inside the classroom were observed during the field test, numerical simulations of the field test could not account for the occupant movement within the classroom. Previous research shows that movements in a zone with displacement ventilation can substantially affect the stratification in the zone [65,112,113]. Similarly, although, the volumetric exchange of air due to the combined effect of natural convection and forced airflow through the classroom door was implicitly included in the calculation of infiltration losses, the effect of classroom door openings on the temperature stratification could not be explicitly accounted for in the numerical simulations. Some earlier studies have shown that door openings may influence the airflow pattern in rooms with displacement ventilation [66,72]. Moreover, another factor contributing to the discrepancy between the field test and simulation might be the assumptions and simplifications made in developing the computational model of the classroom. First of all, the geometry of the classroom was simplified to a rectangular footprint in the computational model. Second, there was some level of uncertainty associated with estimating the heat loss to the ground. This was because the field test was performed just after the completion of the building when the concrete slab had not dried out completely. Next, the effects of external shading from terrain and vegeta- tion were approximated based on visual observations on the day of the field test. Finally, simulation inputs of direct and diffused components of solar radiation were approximated from global radiation measurements over the test period from a nearby weather station. Future work regarding this research should seek to repeat the numerical simulations of experimental tests using more-sophisticated nodal models, such as those proposed by Nielsen [29] and Mateus and da Graça [30]. At present, the implementation of these models in building performance simulation programs is largely lacking. Future work should also focus on advanced transient CFD simulations of experimental tests using actual zone geometries and design inputs matching experimental conditions. The future simulations should allow for a more precise analysis of displacement ventilation by capturing the non-linear vertical temperature distribution in the zone, by determining the stratification height, and by accounting for heat exchange between the space and the surrounding surfaces in a more comprehensive manner. These simulations are also expected to describe the thermal stratification in the heating mode more accurately, and thus, provide a better match with experimental results for heating the classroom through displacement ventilation. More broadly, future work should further develop the idea of providing heating through displacement ventilation only. It should also include finding general rules of Energies 2021, 14, 952 29 of 33

thumb regarding vertical temperature distribution in the zone when heating the zone with displacement ventilation. To conclude, this study was designed to explore whether displacement ventilation could be used for preheating a space outside occupancy periods to provide an acceptable indoor climate during the occupancy periods without utilizing any separate heating system. In exploring this concept, a series of experimental tests were performed in laboratory and field settings, under conditions ranging from normal to extreme. Extensive measurements were made to characterize the temperature and CO2 distributions during the tests. Numerical simulations of the experimental tests were performed to assess the suitability of a state-of-the-art building performance evaluation program that incorporates a commonly used temperature stratification model for predicting the design and performance of the proposed concept. The experimental results clearly demonstrated that the proposed concept of using displacement ventilation without a separate heating system is fully capable of maintaining a comfortable indoor climate even in peak winter periods and under extreme operating conditions. The results of numerical simulations suggested that existing design tools with simplistic stratification models are incapable of capturing the thermal behavior and actual performance of the proposed concept. Advanced numerical simulations utilizing detailed stratification models and taking into account actual geometry and layout of the zone, and the transient aspects of the system would be a more effective way of analysis.

Author Contributions: S.J.: Conceptualization (equal); data curation (lead); formal analysis (lead); funding acquisition (supporting); investigation (equal); methodology (equal); project administration (equal); resources (equal); software (supporting); validation (lead); visualization (lead); writing– original draft (lead); writing–review and editing (lead). I.R.Ø.: formal analysis (supporting); investigation (equal); methodology (equal); resources (equal); software (lead); validation (supporting); visualization (supporting); writing–original draft (supporting); writing–review and editing (supporting). T.H.D.: conceptualization (equal); funding acquisition (lead); investigation (equal); methodology (equal); project administration (equal); resources (equal); validation (supporting); writing–original draft (supporting); writing–review and editing (supporting). M.M.: data curation (supporting); formal analysis (supporting); investigation (equal); methodology (equal); resources (equal); visualization (supporting). S.B.H.: data curation (supporting); formal analysis (supporting); investigation (equal); methodology (equal); resources (equal). All authors have read and agreed to the published version of the manuscript. Funding: The research reported in this article was partly funded by the Research Council of Norway (Norges Forskningsråd) national program ENERGIX through research grants LowEx (269705) and SynHouse (310121). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors acknowledge the support offered by Drøbak Montessori school. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding source had no involve- ment in the study design; the collection, analysis, and interpretation of data; or in the decision to submit the article for publication.

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