energies

Article Detailed Comparison of the Operational Characteristics of Energy-Conserving HVAC Systems during the Cooling Season

Chul-Ho Kim 1 , Seung-Eon Lee 2, Kwang-Ho Lee 1 and Kang-Soo Kim 1,* 1 Department of Architecture, College of Engineering, Korea University, 145 Anam-Ro, Seongbuk-Gu, Seoul 02841, Korea; [email protected] (C.-H.K.); [email protected] (K.-H.L.) 2 Department of Living and Built Environment Research, Korea Institute of Civil Engineering and Building Technology, 283 Goyangdae-Ro, Ilsanseo-Gu, Goyang-Si, Gyeonggi-Do 10223, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-3290-3744



Received: 7 October 2019; Accepted: 27 October 2019; Published: 31 October 2019


Abstract: To provide useful information concerning energy-conserving heating, ventilation, and air-conditioning (HVAC) systems, this study used EnergyPlus to analyze in detail their operational characteristics and energy performance. This study also aimed to understand the features of the systems under consideration by investigating the dry-bulb temperature, relative humidity, and airflow rate at major nodes in each system’s schematic. Furthermore, we analyzed the indoor environment created by each HVAC system, as well as examining the cooling energy consumptions and CO2 emissions. The HVAC systems selected for this study are the variable air volume (VAV) commonly used in office buildings (base-case model), constant air volume (CAV), under-floor air distribution (UFAD), and active chilled beam (ACB) with dedicated outdoor air system (DOAS). For the same indoor set-point temperature, the CAV’s supply airflow was the highest, and VAV and UFAD were operated by varying the airflow rate according to the change of the space thermal load. ACB with DOAS was analyzed as being able to perform air conditioning only with the supply airflow constantly fixed at a minimum outdoor air volume. The primary cooling energy was increased by about 23.3% by applying CAV, compared to VAV. When using the UFAD and ACB with DOAS, cooling energy was reduced by 11.3% and 23.1% compared with VAV, respectively.

Keywords: energy-conserving HVAC systems; variable air volume (VAV); underfloor air distribution (UFAD); active chilled beam (ACB) with dedicated outdoor air system (DOAS); EnergyPlus; primary energy consumption; CO2 emissions

1. Introduction

1.1. Background and Purpose Due to global warming, average global temperatures have broken records each year, and abnormal weather phenomena have become more frequent. The Intergovernmental Panel on Climate Change (IPCC) predicts that the average global temperature will rise by 1.4–5.8 ◦C between 1990 and 2100 [1,2]. In addition, the IPCC has predicted that the frequency and duration of hot and cold periods (heat waves and cold waves) in each region will increase. Also, as temperatures continue to increase due to climate change, heat waves are expected to increase in intensity (e.g., hotter days and nights) and frequency (more frequent heat and cold waves) [3–6]. The 2015 United Nations Climate Change Conference adopted the Paris Agreement as a response to climate change to conserve energy and reduce greenhouse gases, implementing a plan for a new climate regime post-2020 [7–10]. In South Korea (henceforth Korea), building energy consumption accounts for 24.8% of the total energy consumption

Energies 2019, 12, 4160; doi:10.3390/en12214160 www.mdpi.com/journal/energies Energies 2019, 12, 4160 2 of 29 which, when considering current trends for developed countries, has the potential to increase to 40% [11,12]. The key to conserving energy associated with buildings lies in the planning of heating, ventilation, and air-conditioning (HVAC) systems. The energy uses in these systems accounts for a considerable portion approximately 40%–50% of a building’s total energy consumption [13]. An HVAC system is a component that unavoidably consumes energy with the purpose of providing a comfortable human living space. Energy-conserving HVAC systems continue to be studied and installed and can replace conventional HVAC systems to significantly reduce greenhouse gas (GHG) emissions [14,15]. To effectively reduce the energy used in an air-conditioning system, we should understand the characteristics of each system, and analyze each element that consumes energy. In the initial planning stage, an engineer’s experience usually determines the most suitable air conditioning method. However, due to the introduction of various systems, there is a need to develop design alternatives that are based on a more objective evaluation method. Therefore, to obtain useful information for designing and selecting energy-conserving HVAC systems, this study uses the EnergyPlus dynamic simulation program [16] to analyze the operational characteristics and energy performance, and to understand the features of the systems under consideration, by investigating the dry-bulb temperature, relative humidity, and airflow rate of major node points [17] in each system diagram. Furthermore, this study aims to examine the indoor space environment (i.e., zone dry-bulb temperature, relative humidity, and thermal comfort) that is conditioned by each system, as well as the cooling energy consumption, energy conservation contribution, and CO2 emissions of each system.

1.2. Literature Review The first works that were investigated for the literature review in this study were framework studies that outlined HVAC systems and evaluated their energy consumption. Pérez-Lombard et al. [13] analyzed the available information concerning building energy consumption, in particular, that related to HVAC systems. They found that office and retail buildings were the most energy-intensive typologies, and typically accounted for over 50% of the total energy consumption for non-domestic buildings. Therefore, they suggest that conservation of the energy consumed by HVAC systems is crucial. Pérez-Lombard et al. [17] also emphasized the complexity and variety of HVAC systems, and presented a consistent framework for energy efficiency analysis. To conduct an energy-efficiency analysis, HVAC systems were analyzed as energy conversion equipment, in which energy carriers are converted into heating and cooling, with a discussion of the HVAC energy consumption process principles. Trˇckaet al. [18] provided an overview of HVAC system modeling and simulation. They introduced the categorization of tools for HVAC system design and analysis with respect to the problems that require a solution. Furthermore, Trˇckaet al. [18] summarized the current approaches used for modeling, i.e., (A) HVAC components, (B) HVAC control, and (C) HVAC systems in general. After providing an overview of solution techniques for HVAC system simulation, they made suggestions for the selection of an appropriate HVAC modeling approach that was relative to the simulation objective. Terrill and Rasmussen [19] presented an in-depth analysis of HVAC systems, and the occupant comfort in two religious facilities. The analysis revealed that the most significant opportunities for energy use reduction occur during the proper maintenance and operation of HVAC equipment and schedules. Temperature setbacks were shown to be an important operational setting that reduced energy use. An accompanying analysis of thermal comfort revealed that temperature setbacks must be coupled with sufficient preconditioning of the space to ensure occupant comfort during intermittent building occupancy. Bellia et al. [20] modeled a modern museum building, and performed simulations to find an HVAC system that was suitable for the Italian climate. Using the dynamic simulation code, and hourly climatic data (TRY), the operating costs of different all-air systems (also with dehumidification by adsorption) were evaluated for exhibition areas and storage spaces, as well as system performance, with respect to controlling the thermal-hygrometric ambient parameters. Energies 2019, 12, 4160 3 of 29

Secondly, several previous studies have performed EnergyPlus simulations, which can properly model HVAC operating behavior. Crawley et al. [21,22] have introduced EnergyPlus, a new building energy simulation tool that combines two existing programs, the United States DOE-2 and BLAST. They have also explained that it is possible for EnergyPlus to perform an organic analysis of the thermal behavior that occurs between systems and buildings while properly modeling the organic connections between system components. Therefore, EnergyPlus can perform realistic modeling of HVAC systems by taking interactions between buildings and system into account. Third, previous studies have also compared the characteristics and energy performance of various HVAC systems. Gustafsson et al. [23] performed dynamic simulations to compare the energy performance of four innovative HVAC systems: (A) mechanical ventilation with heat recovery (MVHR) and a micro heat pump, (B) exhaust ventilation with an exhaust air-to-water heat pump and ventilation radiators, (C) exhaust ventilation with an air-to-water heat pump and ventilation radiators, and (D) exhaust ventilation with an air-to-water heat pump and panel radiators. All systems were tested using a model of a renovated single family house that varied the U-values, climate, and infiltration and ventilation rates. Korolija et al. [24] examined the relationship between the heating and cooling loads of buildings, and the subsequent energy consumption with different HVAC systems. Two common HVAC systems in use throughout UK office buildings, the variable air volume (VAV) system and the fan coil (FC) with a dedicated outdoor air system, were coupled with a typical office building with and without daylight control for both a cellular and open plan. For the two investigated systems, the difference between system demand and building demand varied from over 40% to almost +30% for − cooling and from 20% to +15% for heating. Storle et al. [25] compared the cooling and dehumidifying − capacities of two-liquid desiccant membrane air-conditioning (M-LDAC) systems installed in an office building in a hot-humid climate (Miami, Florida). The building HVAC system consisted of a radiant cooling system to cover the sensible load and either a 2- or 3-fluid M-LDAC system to meet the latent load. The systems were simulated during the warmest week of the year using the TRNSYS simulation software. Kim et al. [26] analyzed the energy saving potential of passive chilled beams in various climatic zones. A passive chilled beam model, developed based on full-scale experiments, was used as a system module in an entire building simulation tool to account for the convective and radiative effects from the passive chilled beams. The model was validated with measurements from a field study in an open-plan office equipped with multiple passive chilled beams. Furthermore, in an adjacent identical office space equipped with an air (VAV) system, a parallel field study was conducted to compare the resulting energy consumption between the two systems. Yu et al. [27] investigated both the VAV and variable refrigerant flow (VRF) systems in five typical office buildings in China, to compare their cooling energy use. Site surveys and field measurements were performed to collect building characteristics and operational data. Measurements of electricity used for cooling were collected based on sub-metering in the five buildings. Ho et al. [28] compared the thermal environment of two air distribution systems in an office setting. Airflow, as well as heat and mass (i.e., water vapor and contaminant gas) transfer at a steady-state condition, were modeled for underfloor air distribution (UFAD) and overhead air distribution (OHAD) systems. The results provided a detailed understanding of air transport and its consequence on thermal comfort and indoor air quality that are beneficial to office building air conditioner design. Based on these previous studies, we identified the necessity for a realistic simulation of energy-conserving HVAC systems in the Korean climate and performed a detailed analysis of dry-bulb temperature, relative humidity, and airflow rate at node points in HVAC system schematics. Although many studies have been conducted in the United States, Europe, and other parts of the world, studies that reflect the Korean climate and characteristics of Korean buildings are insufficient. Although findings of previous studies in other climate regions of the world can be indirectly applied to the situation in Korea, direct applicability of foreign studies is limited. In addition, although previous studies have been carried out for each system, studies comparing all four systems (constant air volume Energies 2019, 12, x FOR PEER REVIEW 4 of 29

Energies 2019, 12, 4160 4 of 29 four systems (constant air volume (CAV), VAV, UFAD, and active chilled beam with dedicated outdoor air system (DOAS)) are rare. Furthermore, it is difficult to find studies analyzing data of (CAV),primary VAV, consumption UFAD, and and active CO chilled2 emissions, beam indoor with dedicated environment outdoor (thermal air system comfort, (DOAS)) zone dry-bulb are rare. Furthermore,temperature, itzone is di ffihumidity),cult to find and studies HVAC analyzing systems data in detail of primary by using consumption a dynamic and analysis CO2 emissions, program. indoorDue to environment these reasons, (thermal we carried comfort, out this zone study. dry-bulb temperature, zone humidity), and HVAC systems in detail by using a dynamic analysis program. Due to these reasons, we carried out this study. 2. Methods and Theoretical Framework 2. Methods and Theoretical Framework 2.1. Methods and Overall Procedures of the Study 2.1. Methods and Overall Procedures of the Study This study aims to evaluate the cooling operation characteristics and energy performance for understandingThis study aimsthe tocharacteristics evaluate the coolingof the operationHVAC systems, characteristics and andfor energyproperly performance selecting foran understandingenergy-conserving the characteristics HVAC system. of the Figure HVACsystems, 1 summarizes and for the properly methods selecting and anprocedures energy-conserving used to HVACachieve system. the goals Figure of this1 summarizes study. the methods and procedures used to achieve the goals of this study.

FigureFigure 1.1. DiagramDiagram ofof thethe methodsmethods andand proceduresprocedures usedused inin thisthis study.study.

First,First, thethe dynamicdynamic simulationsimulation tooltool EnergyPlusEnergyPlus v9.1.0v9.1.0[ [16]16] was wasused usedto to analyzeanalyze thethe characteristicscharacteristics andand detailed detailed operating operating modes modes of each of air-conditioningeach air-conditioning method, andmethod, examine and their examine energy performance.their energy Second,performance. the input Second, class the lists input of the class models lists were of the reviewed models were to examine reviewed diff erencesto examine in the differences models, which in the reflectmodels, each which system’s reflect characteristics each system’s incharacteristics the EnergyPlus in the simulations. EnergyPlus Third, simula thetions. loop, Third, supply the side,loop, demandsupply side, side, demand and node side, modeling and node concepts modeling in EnergyPlus concepts were in EnergyPlus used to analyze were theused major to analyze nodes (i.e., the themajor dry-bulb nodes temperatures, (i.e., the dry-bulb relative humidity,temperatures, and airflow relative rates), humidity, and to understandand airflow the rates), characteristics and to ofunderstand each system the incharacteristics detail. Fourth, of each the indoorsystem environmentsin detail. Fourth, (i.e., the the indoor zone dry-bulbenvironments temperature, (i.e., the relativezone dry-bulb humidity, temperature, and thermal relative comfort) humidity, conditioned and thermal by each comfort) system wereconditioned examined; by each and finally,system wewere performed examined; calculations and finally, of we the performed cooling energy calculations consumption of the cooling (site andenergy primary consumption energy), (site energy and conservationprimary energy), contribution, energy conservation and CO2 emissions. contribution, and CO2 emissions.

2.2.2.2. EnergyPlus:EnergyPlus: AnAn IntroductionIntroduction andand ModelingModeling ConceptsConcepts (Loop(Loop andand Node)Node) EnergyPlusEnergyPlus isis aa simulationsimulation programprogram thatthat combinescombines thethe advantagesadvantages ofof thethe DOE-2DOE-2 andand BLASTBLAST models,models, andand isis usedused inin thethe USUS asas anan authorizedauthorized simulationsimulation programprogram toto designdesign newnew buildingsbuildings andand estimateestimate energyenergy performanceperformance [[16].16]. EnergyPlusEnergyPlus consistsconsists ofof threethree basicbasic modulesmodules (i.e.,(i.e., thethe HeatHeat andand MassMass BalanceBalance SimulationSimulation Module,Module, BuildingBuilding SystemSystem SimulationSimulation Module,Module, andand SimulationSimulation ManagerManager Module),Module), andand isis basedbased onon anan integratedintegrated simulationsimulation analysisanalysis technique.technique. EnergyPlusEnergyPlus isis advantageous,advantageous, becausebecause itit performsperforms anan organicorganic analysisanalysis ofof thethe thermalthermal behaviorbehavior thatthat occursoccurs betweenbetween aa systemsystem andand building,building, andand cancan properlyproperly modelmodel thethe organicorganic connectionsconnections betweenbetween systemsystem components.components. Therefore,Therefore, EnergyPlus is a suitable program for modeling the energy-conserving HVAC systems analyzed in

Energies 2019, 12, x FOR PEER REVIEW 5 of 29 EnergiesEnergies 20192019,, 1212,, 4160x FOR PEER REVIEW 55 of 2929 EnergyPlus is a suitable program for modeling the energy-conserving HVAC systems analyzed in EnergyPlus is a suitable program for modeling the energy-conserving HVAC systems analyzed in this study. Pérez-Lombard et al. [17] used the energy flow chain concept to understand energy flows, thisthis study.study. PPérez-Lombardérez-Lombard etet al.al. [[17]17] used the energyenergy flowflow chain concept to understand energy flows, flows, illustrated in Figure 2, which assumes that the HVAC system contains energy-converting equipment illustratedillustrated inin FigureFigure2 2,, which which assumes assumes that that thethe HVACHVAC system system contains contains energy-converting energy-converting equipment equipment that moves useful energy to the space being air-conditioned. thatthat movesmoves usefuluseful energyenergy toto thethe spacespace beingbeing air-conditioned.air-conditioned.

Figure 2. Heating, ventilation, and air-conditioning (HVAC) system thermal chain in the cooling and FigureFigure 2.2. Heating, ventilation, and air-conditioningair-conditioning (HVAC) systemsystem thermalthermal chainchain inin thethe coolingcooling andand heating mode [17]. heating mode [17]. heating mode [17]. In other words, the energy analysis method assumes that energy-converting equipment moves InIn otherother words,words, thethe energyenergy analysisanalysis methodmethod assumesassumes thatthat energy-convertingenergy-converting equipmentequipment movesmoves energy until the moment that cooling or heating is transferred to the indoor space of the building, energyenergy untiluntil thethe momentmoment that cooling or heating is transferred to the indoor space of the building, which is expressed as a chain. HVAC system can be modeled in EnergyPlus as the movement of a whichwhich isis expressedexpressed asas aa chain.chain. HVAC systemsystem cancan bebe modeledmodeled inin EnergyPlusEnergyPlus as thethe movementmovement of aa heating medium, similar to an energy flow chain. The main point to modeling in EnergyPlus is the heatingheating medium, similar to an energyenergy flowflow chain.chain. The main point to modelingmodeling in EnergyPlusEnergyPlus is thethe loop concept, which refers to the repeated circulation of the heating medium within the loop looploop concept,concept, which which refers refers to theto the repeated repeated circulation circulation of the of heating the heating medium medium within the within loop structure.the loop structure. Air loops are those in which the heating medium repeatedly circulates between a zone’s Airstructure. loops areAir those loops in are which those the in heating which mediumthe heatin repeatedlyg medium circulates repeatedly between circulates a zone’s between terminal a zone’s unit terminal unit and the air handling unit (AHU). In plant cooling loops (chilled water), the heating andterminal the air unit handling and the unit air (AHU). handling In plant unit cooling(AHU).loops In plant (chilled cooling water), loops the (chilled heating mediumwater), the repeatedly heating medium repeatedly circulates between the chiller and the AHU cooling coil, whereas in plant circulatesmedium repeatedly between the circulates chiller and between the AHU the cooling chiller coil,and whereasthe AHU in plantcooling heating coil, loopswhereas (hot in water), plant heating loops (hot water), it circulates between the boiler and the AHU heating coil. In condenser itheating circulates loops between (hot water), the boiler it circulates and the between AHU heating the boiler coil. Inand condenser the AHU loops, heating the coil. heating In condenser medium loops, the heating medium repeatedly circulates between the cooling towers and the heat source repeatedlyloops, the heating circulates medium between repeatedly the cooling circulates towers and between the heat the source cooling system. towers and the heat source system. system.Figure 3 shows that nodes are the connection points between elements (i.e., the AHU, fans, chillers, Figure 3 shows that nodes are the connection points between elements (i.e., the AHU, fans, boilers,Figure and cooling3 shows towers) that nodes in the are HVAC the networkconnection which points consists between of loops elements [16]. These (i.e., arethe theAHU, points fans, at chillers, boilers, and cooling towers) in the HVAC network which consists of loops [16]. These are the whichchillers, the boilers, supply and and cooling demand towers) sides in are the connected HVAC ne intwork the air, which plant, consists and condenser of loops loops.[16]. These EnergyPlus are the points at which the supply and demand sides are connected in the air, plant, and condenser loops. canpoints generate at which and the store supply status and data demand such as sides the temperature, are connected relative in the humidity, air, plant, and and airflow condenser rate forloops. the EnergyPlus can generate and store status data such as the temperature, relative humidity, and airflow nodeEnergyPlus locations can specified generate in and the store simulation. status data Such such data as can the help temperature, us understand relative the humidity, characteristics and airflow of the rate for the node locations specified in the simulation. Such data can help us understand the energy-conservingrate for the node HVAClocations systems specified used in in thisthe studysimulation. (Figure Such 9a–d data in Section can help4). us understand the characteristics of the energy-conserving HVAC systems used in this study (Figure 9a–d in Section 4). characteristics of the energy-conserving HVAC systems used in this study (Figure 9a–d in Section 4).

FigureFigure 3. 3. TheThe EnergyPlus EnergyPlus node node point point concept. concept. Figure 3. The EnergyPlus node point concept.

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2.3. Site Site Energy, Energy, Primary Energy Consumption, and CO2 Emissions Figure4 4 shows shows that that the the “building “building energy” energy” concept concept can can be dividedbe divided into into energy energy demand demand (Figure (Figure4A) 4A)and and site energysite energy consumption consumption (Figure (Figure4B). 4B).

2 Figure 4. Site, primary energy consumption, and building CO2 emissions.

The energy demanddemand (Figure(Figure4 A)4A) is is the the amount amount of of energy energy required required by by a building a building based based only only on itson itsarchitectural architectural conditions, conditions, such such as theas the building building envelope, envelope, and and does does not not include include its its HVAC HVAC systems. systems. In Inother other words, words, the the energy energy demand demand component component is the is the energy energy performance performance of the of the building building itself. itself. On theOn theother other hand, hand, site energy site energy consumption consum (Figureption (Figure4B) is calculated 4B) is calculated by adding by the adding energy the demand energy (Figure demand4A) (Figureto the energy 4A) lossto the caused energy by facility loss caused systems. by Therefore, facility tosystems. reduce siteTherefore, energy consumptionto reduce site (Figure energy4B), weconsumption should increase (Figure the 4B), effi ciencywe should to reduce increase energy the lossefficiency within to the reduce high-performance energy loss passivewithin andthe high-performanceHVAC systems. passive and HVAC systems. The primary energy consumption and CO2 emissions can be calculated by multiplying the site energy consumption (Figure 44B)B) byby thethe primaryprimary energyenergy andand COCO2 emissionsemissions factors, factors, respectively respectively [29]. [29]. The term “primary energy consumption” is define definedd as the primary energy from fossil fuels that a country must provide to meet a building’sbuilding’s energyenergy demand.demand. The primary energy is determined by multiplying thethe site site energy energy by by the the primary primary energy ener factor,gy factor, which which includes includes energy lossesenergy due losses to electricity due to productionelectricity production and fuel transportationand fuel transportation [30–32]. The[30–3 Building2]. The Building Energy EEnergyfficiency Efficiency Certification Certification System (BEECS)System (BEECS) [29] in Korea[29] in uses Korea diff useserent different primary prim energyary conversionenergy conversion factors thatfactors depend that depend on the energyon the energysupply sector,supply assector, listed as in listed Table 1in[ 29Table]. Therefore, 1 [29]. Ther in thisefore, study, in this we study, calculated we calculated the primary the energy primary by energymultiplying by multiplying the electric the power electric (2.75) power and fuel (2.75) (e.g., and coal, fuel gas, (e.g., and coal, oil) conversiongas, and oil) factors conversion (1.1) with factors the (1.1)final with energy the consumption. final energy consumption.

Table 1. Primary energy factors in Korea.

EnergyEnergy Supply Supply Sector Primary Primary Energy Energy Factors Factors in in Korea Korea FuelFuel (Coal, (Coal, Gas, Gas, and Oil)Oil) 1.1 1.1 ElectricityElectricity power 2.75 2.75 DistrictDistrict heating heating 0.728 0.728 DistrictDistrict cooling cooling 0.937 0.937

Table2 2 lists lists the the Korean Korean CO CO2 emission2 emission factor factor for for each each energy energy supply supply sector. sector. CO2 COemissions,2 emissions, the most the mostimportant important contributor contributor to global to warming, global warming, can be calculated can be calculated by multiplying by multiplying the site energy the consumption site energy consumptionby the CO2 emissions by the CO factor2 emissions of each energy factor supplyof each sector. energy Therefore, supply thissector. study Therefore, calculated this the study CO2 calculatedemissions andthe CO reduction2 emissions rates and by multiplyingreduction rates the siteby multiplying energy consumption the site energy by the consumption electric power by CO the2 electricand natural power gas CO (LNG)2 and CO natural2 emission gas factors (LNG) reported CO2 emission by the Koreafactors Energy reported Agency by (KEA)the Korea [33] andEnergy the AgencyIPCC guidelines (KEA) [33] [34 and]. the IPCC guidelines [34].

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Table 2. CO2 emission factors according to the energy supply sector. Table 2. CO2 emission factors according to the energy supply sector. CO2 Emission Factors CO2 Emission Factors Energy Supply Sector CO2 Emission Factors CO2 Emission Factors Energy Supply Sector (kg CO2/TJ) (kg CO2/kWh) (kg CO2/TJ) (kg CO2/kWh) Electric power 129,631 0.4663 Electric power 129,631 0.4663 LNGLNG (liquefied (liquefied natural natural gas)gas) 56,467 0.2031 0.2031 Gas/dieselGas/diesel oil 72,600 0.2612 0.2612 KeroseneKerosene 71,500 0.25720.2572 DistrictDistrict heating heating 34,277 0.1233 0.1233

3.3. SimulationSimulation ConditionCondition for for Heating, Heating, Ventilation, Ventilation, and and Air-Conditioning Air-Conditioning (HVAC) (HVAC) Analysis Analysis

3.1.3.1. EnergyPlusEnergyPlus Simulation Simulation Model Model and and Input Input Conditions Conditions ToTo improve improve the the reliability reliability of the of simulation,the simulation, a reference a reference building building representing representing the office buildingsthe office inbuildings Korea is in necessary. Korea is necessary. The United The States United Department States Department of Energy of (DOE) Energy and (DOE) the European and the European Union’s EnergyUnion’s Performance Energy Performance of Buildings of DirectiveBuildings (EPBD) Directive use (EPBD) the reference use the building reference concept building for simulations. concept for Insimulations. other words, In each other country’s words, each standards, country’s climate standa conditions,rds, climate standards conditions, for the standards thermal performance for the thermal of eachperformance building part,of each and building the effi ciencypart, and of buildingthe efficiency facility of elementsbuilding arefaci presentedlity elements in a are standard presented model, in a suchstandard that the model, user such can flexibly that the apply user can them. flexibly apply them. TheThe DOE DOE developed developed a prototype a prototype building building and the an DOE’sd the National DOE’s RenewableNational Renewable Energy Laboratory Energy reportsLaboratory [35] that reports these [35] models that servethese as models a baseline serve for as comparing a baseline and for improving comparing the and accuracy improving of energy the simulationaccuracy of software. energy simulation Therefore, softwa this studyre. Therefore, used the Americanthis study Societyused the of American Heating, Society Refrigerating of Heating, and Air-ConditioningRefrigerating and Engineers Air-Conditioning (ASHRAE) Engineers 90.1 prototype (ASHRAE) building 90.1 model prototype (medium building office) model [36] as (medium the base modeloffice) because[36] as the it contributes base model to because simulation it contribu accuracytes andto simulation convenience. accuracy This modeland convenience. reflects current This Koreanmodel buildingreflects current standards, Korean codes, building and the standards, Incheon (Seoul codes, metropolitan and the Incheon area) (Seoul climate metropolitan [37,38]. Figure area)5 showsclimate the [37,38]. base simulation Figure 5 shows model. the base simulation model.

FigureFigure 5. 5.The The EnergyPlus EnergyPlus simulation simulation model. model.

TableTable3 lists3 lists the EnergyPlusthe EnergyPlus simulation simulation base model’s base buildingmodel’s envelopebuilding performance envelope performance conditions, anconditions, outline of an the outline air-conditioning of the air-conditioning and plant system, and plant and detailedsystem, and input detailed conditions. input conditions.

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Table 3. Properties of the base simulation model.

Division Specifications of Base Model Usage Office Building Floor Area and Direction 1650 m2 (50 m 33 m 11.7 m) & South × × Simulation Program EnergyPlus v9.1.0 (dynamic simulation tool) Incheon 0.26 W/m2 K and 1.5 W/m2 K U-Value of Wall and Doors · · (The Korean energy-saving design standards) Incheon 0.22 W/m2 K U-Value of Floor · Base Model Envelope (The Korean energy-saving design standards) Incheon 0.15 W/m2 K U-Value of Roof · (The Korean energy-saving design standards) Double Low–E Pane Glazing (U-value = 1.5 W/m2 K, Glazing Type · Solar Heat Gain Coefficient (SHGC) = 0.458, (Low–E 6T + 12A + 6 CL) ◦ VLT = 0.698) (The Korean energy-saving design standards) Terminal Unit VAV Unit AHU Fan type Variable Speed Fan Cooling Temp. 20 ◦C, Relative Humidity Set point Temp and Relative Humidity 40–60% (The Korean energy-saving design standards) Base Model System Cooling Operation (June–August): Cooling Operation Schedule 07:00–18:00 (26.0 ◦C) Plant System Absorption Chiller (Cooling COP 1.0) Pump Type & Efficiency Variable Speed Pump, 0.6 (Design) Lighting & Equipment Occupancy 12 W/m2, 11 W/m2, 0.1–0.2 person/m2 density (The Korean energy–saving design standards, The MOTIE and KICT report) 3.0 ACH50 Infiltration (The Korean energy-saving design standards) Weekday: 08:00–18:00, Weekend: Off Schedule (The Korean energy-saving design standards) 117 W/person, 1.1 met (Office Work: Typing) People Metabolic (ASHRAE Handbook Fundamentals (2009)) 0.5 (Summer) Clothing Value (ASHRAE Standard 55–2004) 0.1 m/s Air Velocity (ASHRAE Standard 55–2004) Weather Data Incheon (4A, Dwa), Korea

For the building envelope conditions, which include the base model’s walls, roof, floor, and windows/doors, this study used the energy-saving design standards [39], which specify the Korean region’s legal standards. For the equipment, lighting load, and occupancy density, we used a report from a survey of existing buildings in Korea conducted by the Ministry of Trade, Industry, and Energy (MOTIE) and the Korea Institute of Civil Engineering and Building Technology (KICT) [40]. For the base air-conditioning system, we used a VAV system, which is commonly used in Korea, and adopted an absorption chiller (cooling coefficient of performance (COP) = 1.0) as the plant system [41]. The region selected for the simulation was the Incheon (Seoul metropolitan area) region in central Korea.

3.2. Validation of the Model As a final step in the development of the simulation model to be used in the study, a validation process was briefly carried out to ensure that the model could properly predict the thermal load and energy performance. Data by Kim et al. [17] were used for the validation process in this study since their study provides both conditions and results. Similar conditions of internal heat gain, diffuser conditions, and properties of the raised access floor were used in both Kim’s study and this study. Kim et al. [17] and our group have simulated that the primary energy consumption (CAV system model) of Incheon is 464.1 kWh/m2, which corresponds to level 5 in the BEECS [29]. The KEA and Korea Appraisal Board (KAB) [42] database indicates that the average actual primary energy consumptions of general office buildings in Incheon and Jeju is 457–489 kWh/m2a. Thus, in this study, the simulation results (CAV Energies 2019, 12, 4160 9 of 29 system model) confirmed that the base model’s primary energy consumption (464.1 kWh/m2a) met the Energies 2019, 12, x FOR PEER REVIEW 9 of 29 457–489 kWh/m2a range requirement of the KEA and KAB database for primary energy consumption ofdatabase general for offi primaryce buildings. energy Primary consumption energy of consumption general office for buildings. each system Primary (VAV, energy UFAD, consumption and active chilledfor each beam) system is also(VAV, the UFAD, same. and active chilled beam) is also the same. 3.3. Climate Analysis of Incheon, Korea 3.3. Climate Analysis of Incheon, Korea To reflect the effects of global warming on Korea over the past five years, we applied EnergyPlus To reflect the effects of global warming on Korea over the past five years, we applied with the outdoor temperature, humidity, wind velocity, atmospheric pressure, solar radiation, cloud EnergyPlus with the outdoor temperature, humidity, wind velocity, atmospheric pressure, solar cover, and precipitation data provided by the KMA [37]. We converted the KMA data into an radiation, cloud cover, and precipitation data provided by the KMA [37]. We converted the KMA EnergyPlus weather file (EPW) format data [38] for use in the EnergyPlus model. Figure6 and Table4 data into an EnergyPlus weather file (EPW) format data [38] for use in the EnergyPlus model. Figure show Incheon’s ASHRAE [43] and Köppen [44] climate zone and location information, as well as the 6 and Table 4 show Incheon’s ASHRAE [43] and Köppen [44] climate zone and location information, average heating degree days (HDD), cooling degree days (CDD), dry-bulb temperature, and relative as well as the average heating degree days (HDD), cooling degree days (CDD), dry-bulb humidity [37] over the last five years (2014–2018). temperature, and relative humidity [37] over the last five years (2014–2018).

Figure 6. Average monthly outdoor airair temperaturetemperature andand relativerelative humidityhumidity ofof Incheon,Incheon, Korea.Korea.

Incheon’s average TableHDD 4. (18Detailed °C) and climate CDD characteristics (10 °C) over of Incheon,the last Korea.five years are 2749 and 2327,

respectively. In the ASHRAE climate classifications,Outdoor Incheon Air is classified as a 4A (Mixed–Moist) Latitude N( ) Relative Humidity ASHRAE Köppen ◦ Temperature (Average HDD CDD climateRegions zone. Based on the Köppen/Longitude climate zones, Incheon is classified(Average Monthly) as Dwa (subarctic climate, Climate Climate Monthly) (18 ◦C) (10 ◦C) cold and dry winter, and hot summer).E(◦) Its maximum average monthlyMin./Avg./ Maxtemperature (%) is 27.7 °C, with Min./Avg./Max (◦C) an average maximum4A monthly humidity of 88.1%, which indicates that Incheon has hot and humid Incheon Dwa 37.45/126.70 2.4/12.8/27.7 46.1/69.1/88.1 2749 2327 summers. The(Mixed–Moist) average minimum monthly temperature− is −2.4°C, with an average minimum monthly humidity of 46.1%, which indicates that Incheon’s winters are cold and arid.

Incheon’s average HDD (18 ◦C) and CDD (10 ◦C) over the last five years are 2749 and 2327, respectively. In the ASHRAETable climate 4. Detailed classifications, climate characteri Incheonstics is of classified Incheon, asKorea. a 4A (Mixed–Moist) climate zone. Based on the Köppen climate zones,Outdoor Incheon Air is Temperature classified asRelative Dwa (subarctic Humidity climate, cold and ASHRAE Köppen Latitude N(°) HDD (18 CDD Regions (Average Monthly) (Average Monthly) dry winter,Climate and hotClimate summer). /Longitude Its maximum E(°) average monthly temperature is 27.7 ◦C, with°C) an average(10 °C) maximum monthly humidity of 88.1%, whichMin./Avg./Max indicates that(°C) IncheonMin./Avg./Max has hot and(%) humid summers. 4A TheIncheon average minimumDwa monthly 37.45/126.70 temperature is −2.4/12.8/27.72.4 C, with an average46.1/69.1/88.1 minimum monthly2749 humidity2327 (Mixed–Moist) − ◦ of 46.1%, which indicates that Incheon’s winters are cold and arid. Due to global warming, global temperatures are steadily increasing. Extreme weather events, such as heat waves, occur frequently throughout the world, and Korea is no exception [8,9,37,45]. Therefore, as a preliminary task, we should accurately analyze average yearly temperature changes during the Korean summer. The average temperatures in the summer for Incheon were analyzed using the Korea

Energies 2019, 12, 4160 10 of 29

Due to global warming, global temperatures are steadily increasing. Extreme weather events, such as heat waves, occur frequently throughout the world, and Korea is no exception [8,9,37,45]. Therefore, as a preliminary task, we should accurately analyze average yearly temperature changes duringEnergies 2019 the, Korean 12, x FOR summer. PEER REVIEW The average temperatures in the summer for Incheon were analyzed10 using of 29 the Korea Meteorological Administration (KMA) weather data [37] from the 1970 until 2018. Figure7 showsMeteorological Incheon’s Administration average temperatures (KMA) duringweather the data summer [37] seasonfrom the (June–August) 1970 until 2018. from Figure 1970 until 7 shows 2018. Incheon’s average temperatures during the summer season (June–August) from 1970 until 2018.

Figure 7.7. Changes in Incheon’s averageaverage summer temperatures (1970–2018).(1970–2018).

TheThe averageaverage summersummer temperature increased from 23.0 ◦°CC betweenbetween 1970 and 19991999 toto 23.423.4 ◦°CC betweenbetween 1980 andand 2009,2009, andand toto 23.723.7 ◦°CC betweenbetween 19881988 andand 2018.2018. TheThe averageaverage summersummer temperaturetemperature alsoalso steadilysteadily increased to 24.324.3 ◦°CC during the last 10 yearsyears (2009–2018)(2009–2018) and 24.7 ◦°CC in the lastlast 55 yearsyears (2014–2018).(2014–2018). These These characteristics characteristics give give rise rise to to ci circumstancesrcumstances during which it is necessarynecessary toto useuse energy-conservingenergy-conserving HVACHVAC systemssystems inin Korea.Korea.

3.4.3.4. Selection of HVACSystemHVAC System andand SimulationSimulation InputInput ConditionsConditions InIn this this study, study, we we selected selected the the energy-conservin energy-conservingg HVAC HVAC systems systems based based on the on results the resultsof Kim ofet Kimal. [15] et al.and [15 Li] and et al. Li et[46] al. that [46] thatinvestigated investigated trends trends in building in building technology technology through through an ananalysis analysis of ofcurrent current high-performance high-performance buildings buildings throughout throughout th thee world. world. The The VAV VAV system system was was selected as thethe basebase system, which is commonly used in oofficeffice buildings. The CAV, UFAD,UFAD, andand activeactive chilledchilled beambeam withwith DOAS,DOAS, whichwhich have have di differentfferent characteristics characteristics and and are are quite quite recent recent technologies, technologies, were were selected selected as the as conventionalthe conventional HVAC HVAC and and energy-conserving energy-conserving HVAC HVAC systems. systems. Figure Figure8 shows 8 shows a schematic a schematic layout layout and theand mechanisms the mechanisms of the of four the four HVAC HVAC systems. systems. The VAV system (Figure8A) changes the airflow based on increases and decreases in the load to control the indoor temperature [47,48]. In other words, the VAV compares the indoor temperature and set-point temperature while controlling the amount of air according to changes in the indoor load. In the VAV system, VAV terminal boxes are installed in each zone, where the amount of airflow is controlled according to a thermal load that matches the set-point temperature. The CAV system (Figure8B) is the most basic system of the four air conditioning systems. The CAV system was analyzed for purposes of comparison with the energy-conserving system and the conventional VAV system. The CAV always supplies a fixed amount of air to the indoors and adjusts the temperature via heat exchange in the coils within the air conditioner [49,50]. The UFAD system (Figure8C) is an air-conditioning method that cools by focusing on the occupied zone, which is the space between the floor surface and a height of approximately 2 m [51,52]. Typical air-conditioning systems, such as the CAV and VAV, are overhead air distribution (OHAD) systems, where the air produced by the AHU is supplied to a room via ceiling ducts and also has exhaust ports located in the ceiling. However, the UFAD system uses the space of an access floor rather than the ceiling as a space to diffuse conditioned air, which is supplied to the room via floor diffusers. The active chilled beam with DOAS (Figure8D) was proposed in response to problems with existing forced-air conditioning

Figure 8(a-d). Schematic layout of the HVAC systems: (A) variable air volume (VAV) system, (B) constant air volume (CAV) system, (C) underfloor air distribution (UFAD) system, and (D) active chilled beam with dedicated outdoor air system (DOAS).

Energies 2019, 12, x FOR PEER REVIEW 10 of 29

Meteorological Administration (KMA) weather data [37] from the 1970 until 2018. Figure 7 shows Incheon’s average temperatures during the summer season (June–August) from 1970 until 2018.

Figure 7. Changes in Incheon’s average summer temperatures (1970–2018).

The average summer temperature increased from 23.0 °C between 1970 and 1999 to 23.4 °C between 1980 and 2009, and to 23.7 °C between 1988 and 2018. The average summer temperature also steadily increased to 24.3 °C during the last 10 years (2009–2018) and 24.7 °C in the last 5 years (2014–2018). These characteristics give rise to circumstances during which it is necessary to use energy-conserving HVAC systems in Korea.

3.4. Selection of HVAC System and Simulation Input Conditions Energies 2019, 12, 4160 11 of 29 In this study, we selected the energy-conserving HVAC systems based on the results of Kim et al. [15] and Li et al. [46] that investigated trends in building technology through an analysis of systems, such as large amounts of conditioned airflow, conveyance power, comfort, and hygiene. In current high-performance buildings throughout the world. The VAV system was selected as the this system, the DOAS introduces the minimum airflow required for ventilation, where interior air is base system, which is commonly used in office buildings. The CAV, UFAD, and active chilled beam used to heat and cool, which is induced through an active chilled beam. When an active chilled beam with DOAS, which have different characteristics and are quite recent technologies, were selected as is combined with the DOAS, the DOAS handles the latent heat load [53,54]. Table5 lists the simulation the conventional HVAC and energy-conserving HVAC systems. Figure 8 shows a schematic layout input conditions of HVAC systems selected in this study. and the mechanisms of the four HVAC systems.

Figure 8(a-d). Schematic layout of the HVAC systems: (A) variable air volume (VAV) system, (B) Figure 8. Schematic layout of the HVAC systems: (A) variable air volume (VAV) system, (B) constant constant air volume (CAV) system, (C) underfloor air distribution (UFAD) system, and (D) active air volume (CAV) system, (C) underfloor air distribution (UFAD) system, and (D) active chilled beam chilled beam with dedicated outdoor air system (DOAS). with dedicated outdoor air system (DOAS).

Recently constructed office buildings in advanced nations use the UFAD, in which unoccupied zones are not air conditioned and only occupied zones are air conditioned using the access floor [55]. The cooling supply temperature was set with reference to several previous studies, which analyzed the conservational effects of the UFAD system in the Korean climate [56–58]. Pressurized diffusers (interior zone: swirl type; perimeter zone: linear bar grille type) were selected as the diffusers, after which we performed the simulations. The active chilled beam system with a DOAS cools and dehumidifies outdoor air. In Korea, there are no clear regulations regarding the operation of active chilled beam systems. Therefore, the heating and cooling water inlet temperature and temperature difference were set with reference to previous studies that verified the conservational effects of active chilled beam systems in a Korean climate [59–61], as well as Federation of European Heating, Ventilation and Air-conditioning Associations’ (REHVA) standards [62]. For the active chilled beam with DOAS, we installed the most basic form of a bidirectional diffusion chilled beam. The outdoor air cooled and dehumidified by the DOAS was used as the primary air for the chilled beam [62]. Table S1a–c in the Supplementary Material provide comparison of four HVAC system modeling in the EnergyPlus class list. Table S2a–d in the Supplementary Material provide diagram of four HVAC system layout in the EnergyPlus simulation. Energies 2019, 12, 4160 12 of 29

Table 5. Set of simulation variables.

Passive Systems (Envelope) Active Systems (HVAC) Item Wall, Floor, Glazing and Envelope and Roof Solar Shading Air Conditioning Systems Plant Systems Infiltration (U–Value) Systems VAV System Terminal Unit: VAV AHU fan type : Variable air volume Incheon Control logic Standard Double Low–E : Dual maximum control logic Wall 0.26 (No Blind) Fan efficiency: 75% VAV System W/m2 K (U–Value 1.5 3.0 ACH50 (Motor efficiency: 90%) Absorption · 2 (Base) Floor 0.22 W/m K, Damper heating Chiller · W/m2 K SHGC 0.458, action: Reverse (Cooling COP · Roof 0.15 VLT 0.698) Fan pressure 1.0) W/m2 K : 1100 (SA), 700 Pa(RA) · Maximum air flow (Heating) : 50% of max cooling air flow / Minimum air flow: 20% of max cooling air flow CAV System Incheon Terminal Unit: CAV Standard Double Low–E AHU fan type Wall 0.26 (No Blind) : Constant air volume 2 Absorption CAV W/m K (U–Value 1.5 Fan efficiency: 75% · 2 3.0 ACH50 Chiller System Floor 0.22 W/m K, (Motor efficiency: 90%) 2 · (Cooling COP W/m K SHGC 0.458, Fan Pressure · 1.0) Roof 0.15 VLT 0.698) : 1100 (SA), 700 Pa(RA) W/m2 K · Constant minimum airflow fraction: 1.0 UFAD System

Cooling SAT: 16–18◦C Diffuser: Swirl type (Core zone, n=242) Linear bar grille type (Perimeter UFAD zone, n=21–24) System Fan pressure : 1100 (SA), 700 Pa(RA) Incheon Transition height: 1.7m Standard Double Low–E Thermal comfort height: 1.2m Wall 0.26 (No Blind) Constant minimum Absorption W/m2 K (U–Value 1.5 airflow fraction: 0.3 Chiller · 2 Floor 0.22 W/m K, 3.0 ACH50 (Cooling COP · Active Chilled Beam W/m2 K SHGC 0.458, 1.0) · with DOAS Roof 0.15 VLT 0.698) W/m2 K Chilled beam type: Active · Entering water temperature Cooling: 15–17◦C Mean coil temperature to room Active Chilled design temperature Beam with difference: 2–4◦C, DOAS Coil surface area per coil length: 5.422m2/m Chilled beam tube inside and outside diameter: 0.0114, 0.0159 Leaving pipe inside diameter: 0.0145m

4. EnergyPlus Simulation Results

4.1. System Schematic Diagram of Node Temperature, Humidity, and Airflow Comparison

In Figure9a–d, the zone’s indoor temperature was set to 26 ◦C (cooling operation) and we compared in detail the dry-bulb temperature, relative humidity, and airflow rate at the major node points of the system schematics for each HVAC system (8.1 at 2:00 PM). The analysis date was selected as August 1 because it is representative of the general characteristics of summer in Incheon. Due to both the temperature and the humidity being high at 14:00 on August 1, it was selected as the analysis time (32.3 ◦C, 70.5%). Figure9a shows the cooling operation in the VAV. Energies 2019, 12, 4160 13 of 29 Energies 2019, 12, x FOR PEER REVIEW 13 of 29

(a)

(b)

Figure 9. Cont.

Energies 2019, 12, 4160 14 of 29 Energies 2019, 12, x FOR PEER REVIEW 14 of 29

(c)

(d)

FigureFigure 9. 9(a-d).(a) Analysis (a) Analysis of node of conditionsnode conditions in the in VAV the VAV system system network network (1 August). (1 August). (b) Analysis(b) Analysis of nodeof conditionsnode conditions in the CAV in the system CAV system network network (1 August). (1 August). (c) Analysis (c) Analysis of node of node conditions conditions in the in UFAD the UFAD system networksystem (1network August). (1 (August).d) Analysis (d) ofAnalysis node conditions of node conditions in the ACB in with the ACB DOAS with network DOAS (1 network August). (1 August). State 1 (Environment: Site Outdoor Air) is the point where outdoor air was introduced, and State 1 (Environment: Site Outdoor Air) is the point where outdoor air was introduced, and depicts the status of the outdoor air temperature, humidity, and airflow. Air at a temperature of 32.3 ◦C anddepicts relative the humiditystatus of the of 70.5%outdoor was air introducedtemperature, at hu 1.587kgmidity,/s and airflow. airflow. Here, Air toat calculatea temperature the minimum of 32.3 airflow°C and for relative the standard humidity floor’s of ventilation,70.5% was weintroduced used the at offi 1.587kg/sce building airflow. minimum Here, ventilation to calculate standard the (29minimum m3/person airflowh or more) for the from standard the Ministry floor’s of ventilation, Land, Infrastructure we used andthe Transportoffice building (MOLIT) minimum Building ventilation standard· (29 m3/person·h or more) from the Ministry of Land, Infrastructure and Act [63]. The minimum outdoor air inflow amount was calculated as 1.587 kg/s (1600 m2 29m3/ Transport (MOLIT) Building Act [63]. The minimum outdoor air inflow amount was calculated× as person h 0.1 person/m2 = 4640 CMH = 1.587 kg/s). State 2 (Air Loop AHU Mixed Air Outlet) was the 1.587· kg/s× (1600 m2 × 29m3/ person·h × 0.1 person/m2 = 4640 CMH = 1.587 kg/s). State 2 (Air Loop point where the return air from State 7 (Air Loop AHU Extract Fan Air Outlet) mixed with outdoor air at State 1. Energies 2019, 12, 4160 15 of 29

The air of State 2 had a temperature of 28.6 ◦C, and relative humidity of 59.4%. In the VAV, the airflow changed based on the indoor load and, therefore, the air was supplied at a rate of 3.081kg/s. State 3 (Air Loop Cooling Coil Outlet) was the point at which air passed the cooling coil in the AHU. Here, the air temperature was cooled to 13.7 ◦C by the cold water in the chiller, with an airflow of 3.081 kg/s, which was identical to the airflow at State 2. State 4 (Air Loop Supply Side Outlet) was the point where the air passed the supply fan, and was discharged. This was the stage before air was supplied to the zone, which had an AHU air discharge temperature of 13.9 ◦C which is identical to the AHU discharge air temperature set-point. The dehumidified air passed at a relative humidity of 56.1%, and the airflow was 3.081 kg/s. State 5 was air conditioned in the zone, with a conditioned air temperature set to the indoor temperature of 26 ◦C. At this time, the humidity was 47.7–53.8%. VAV adjusted the airflow and conditions the air to the set-point temperature of 26 ◦C. The total supplied airflow in the zones was maintained at 3.081 kg/s, which was identical to States 2, 3, and 4. State 6 (Air Loop AHU Extract Fan Air Inlet) was the air that was exhausted outside of the zone, just before the exhaust fan. Air was exhausted at a temperature of 26.0 ◦C, relative humidity of 52.5%, and airflow rate of 3.081 kg/s. State 7 (Air Loop AHU Extract Fan Air Outlet) was the state of the air that had passed through the exhaust fan. Due to heat generation associated with the fan, its temperature slightly increased to 26.2 ◦C, with a relative humidity of 53.5% and airflow of 3.081 kg/s, identical to States 2–6. Finally, State 8 (Air Loop AHU Relief Air Outlet) was the state of the air that was exhausted to the exterior. The exhausted airflow rate had a rate of 1.587 kg/s, which was identical to the airflow rate at which air was taken in from the exterior at State 1. The temperature was 26.2 ◦C and humidity was 53.5%, which was identical to State 7. State 9 (CHW Loop: Plant Supply Side Inlet-Outlet) showed the supply and return water temperatures of the chilled water supplied by the absorption chiller to the cooling coil, whose temperatures were 6 ◦C and 9.8 ◦C, respectively. Finally, State 10 (Condenser Loop: Plant Supply Side Inlet-Outlet) showed the supply and return water temperatures of the condenser water supplied by the cooling tower to the absorption chiller, whose temperatures were 29 ◦C and 32.3 ◦C, respectively. Figure9b shows the CAV during cooling operations. State 1 was the outside air conditions. At State 2, the return air temperature was 2.1 ◦C lower than the VAV system and, therefore, even though the outdoor air had an identical state with the air that was mixed in, the exterior had a temperature of 26.8 ◦C, which was 1.8 ◦C lower than in the VAV. The humidity was 61.7%, which was 2.3% higher than in the VAV. In the CAV, the airflow rate maintained a maximum airflow and, therefore, the air was supplied at a rate of 5.072 kg/s, that is, more than the VAV. State 3, which passes the cooling coil, had a temperature of 13.5 ◦C and an airflow of 5.072 kg/s. State 4, which passes the supply fan, had a temperature of 14 ◦C, humidity of 57.8%, and an identical airflow of 5.072 kg/s. Since the indoor discharge temperature was set to 14 ◦C, similar to the VAV in EnergyPlus, the temperature was maintained at approximately 14 ◦C and the humidity was dehumidified to 57.8%. State 5 (Zone) shows the condition of the air-conditioned zone. The temperature was conditioned to 23.4–24.5 ◦C, which was 1.5–2.6 ◦C below the set indoor temperature of 26 ◦C. At this time, the humidity was 51.2%–55.4%. Unlike the VAV adjusting the airflow and conditioning the air to the set-point temperature of 26 ◦C, the CAV conditions the air to a temperature that is lower than the indoor set-point indoor temperature. The air that had an identical condition to the indoor conditioned air was released as exhaust, and passed the return fan, which slightly increased the temperature to 24.1 ◦C. Since the conditioned indoor air was lower than in the VAV, State 7 was 2.1 ◦C lower. The airflow at State 8 was exhausted at an identical rate as the minimum outdoor air inflow of 1.587 kg/s, as well as the fact that both the temperature and humidity were identical to State 7. Figure9c shows the UFAD system during cooling operations. As State 1’s outdoor air passed State 2’s mixing box, the air changed to a temperature of 29.3 ◦C, humidity of 58.5%, and airflow of 2.752 kg/s. Since the temperature of State 7’s return air was high, State 2’s temperature was higher than in both the VAV and CAV. Energies 2019, 12, 4160 16 of 29

This is because air that had a higher temperature than the set indoor temperature was exhausted to the ceiling plenum due to thermal stratification, which is a typical characteristic of the UFAD. In the UFAD, it is possible to perform air conditioning at higher temperatures than typical air-conditioning systems, which supply air from the ceiling [56–58]. Therefore, the AHU discharge air temperature was set to 15 ◦C, which was higher than both the VAV and CAV. The floor diffuser’s discharge temperature was 17.2 ◦C, which was approximately 3 ◦C higher than both the VAV and CAV. This affected reductions in chiller energy, which is discussed in Chapter 5. Since airflow was only conditioned in the occupied zone, airflow was supplied at a rate of 2.752 kg/s, which is 0.329 kg/s lower than the VAV and 2.320 kg/s lower than the CAV. State 5, which was indoor air, was maintained at 26 ◦C via a lower airflow rate than that used in both the VAV and CAV, with the humidity regulated at a comfortable range of 40%–60%. Through the implementation of thermal stratification via the “room air model” in EnergyPlus, the temperature in the unoccupied zone increased, and the air at State 6 was exhausted at 28.1 ◦C. Based on this, the temperature of the ventilation/exhaust after passing through the occupied zone was higher in the UFAD than in both the VAV and CAV. If the set cooling temperature was 26 ◦C, a typical air-conditioning system, which supplies air from the ceiling, must maintain the entire indoor space at 26 ◦C but the UFAD only maintains the occupied zone at 26 ◦C, whereas the unoccupied zone can be maintained at above 26 ◦C. Thus, the UFAD system was able to conserve cooling energy. The air that was exhausted at State 7 passed the return fan, and was divided into a mixing box and relief air at 28.3 ◦C and 49.6% humidity. At State 8, the air was exhausted at an identical rate to the minimum exterior air inflow (1.587 kg/s). Figure9d shows the active chilled beam with DOAS during cooling operations. Normally, an active chilled beam is combined with a DOAS, which cools and dehumidifies outdoor air. The indoor sensible heat load was removed using the chilled beam and the latent heat load was handled by the DOAS. In other words, a conventional air-conditioning system moves a mixture of outdoor air (OA) and return air (RA) through an AHU to perform air conditioning. An active chilled beam with DOAS separates the OA from the air ventilated in the RA, and handles them independently. Since only OA is introduced and conditioned via the DOAS, State 1 introduced a minimum rate of OA at 1.587 kg/s. At State 2, the air passed through the DOAS heat exchanger, where heat exchange changed the temperature and humidity to 28.5 ◦C and 57.3%, respectively. The airflow was identical at 1.587 kg/s, which is the minimum OA inflow. The air that passed through State 3’s cooling coil was discharged at 14 ◦C and 50.6% humidity at State 4. In the zone (State 5), air conditioning occurred at a temperature of 26 ◦C and relative humidity of 49.6%–51.8%. Since indoor air was induced in the chilled beam system, the indoor zone can only be conditioned at a lower airflow than in the VAV, CAV, and UFAD, that is, with a minimum OA inflow of 1.587 kg/s. This air was released through the exhaust diffuser and passed the exhaust fan while the air at State 7 (26.1 ◦C, 51.5%, and 1.587 kg/s) exchanged heat in the heat exchanger. At State 8, unlike the VAV, CAV, and UFAD, the heat exchanged air that was at 29.7 ◦C and 68.9% was released as exhaust to the outdoor. In addition, the chilled beam increased the cooling effect by moving the primary air, which experienced heat exchange in the DOAS, through the chilled water coil installed in the beam [62]. Since water, which has a higher heat capacity than air, was used to perform heat exchange with indoor air through the water pipe within the chilled beam, it can reduce the conveyance energy produced by the heating medium. State 9–2 was the supply and return water temperatures of the secondary side cold water that supplied the chilled beam through the water pipe, whose temperatures were 15.5 ◦C and 17.5 ◦C, respectively. The supply and return water temperatures (State 10) of the condenser water supplied from the cooling tower to the absorption chiller were 29 ◦C and 30.8 ◦C, respectively.

4.2. Analysis of the Indoor Temperature and Humidity Figure 10 shows the outdoor temperature and humidity from 1 to 4 August in Incheon, which was used as a typical cooling period. It is evident that the climate of the cooling season in Incheon, Korea has both high temperatures and humidity. During these four days, the minimum and maximum Energies 2019, 12, x FOR PEER REVIEW 17 of 29

EnergiesFigure2019, 12 10, 4160 shows the outdoor temperature and humidity from 1 to 4 August in Incheon, 17which of 29 was used as a typical cooling period. It is evident that the climate of the cooling season in Incheon, Korea has both high temperatures and humidity. During these four days, the minimum and outdoormaximum temperatures outdoor temperatures were 24.7 ◦C andwere 33.6 24.7◦C, respectively,°C and 33.6 with °C, an respectively, average temperature with an of 28.3average◦C. Thetemperature minimum of and 28.3 maximum °C. The relativeminimum humidity and maxi weremum 59% relative and 86%, humidity respectively, were with59% an and average 86%, relativerespectively, humidity with ofan 79.9%. average relative humidity of 79.9%.

Figure 10. Outdoor temperature and relative humidityhumidity in Incheon (1–4 August).

Figure 11 shows the indoor temperature distributionsdistributions from August 1 to 4 when using the four HVAC systems.systems. The The AHU AHU was was set toset operate to operate from 7from AM to7 6AM PM, to during 6 PM, which during the CAVwhich conditioned the CAV theconditioned air to approximately the air to approximately 2–3 ◦C lower 2–3 than °C the lower cooling than set-point the cooling temperature set-point oftemperature 26 ◦C. It is consideredof 26 °C. It thatis considered the CAV conditionedthat the CAV the conditioned air to this lowerthe air temperature to this lower than temperature in the other than systems in the becauseother systems it had thebecause highest it had airflow the highest supply, airflow as mentioned supply, inas Chaptermentioned 4.1. in The Chapter VAV conditioned4.1. The VAV the conditioned air to 23.5–24 the ◦airC fromto 23.5–24 7 AM °C to 9from AM 7 and AM then to 9 to AM 26 ◦andC until then 6 to PM. 26 Unlike°C until the 6 CAV,PM. Unlike the VAV the changed CAV, the the VAV airflow changed based onthe theairflow interior based load on as the it conditionedinterior load the as air,it conditioned which yielded the aair, lower which airflow yielded than a lower the CAV. airflow The VAVthan maintainedthe CAV. The a setVAV indoor maintained temperature a set indoor of 26 ◦C. temperature The UFAD of conditioned 26 °C. The theUFAD air toconditioned 24.5–25.5 ◦ theC from air to 7 AM24.5–25.5 to 9 AM, °C from and then7 AM to to 26 9 AM,◦C until and 6 then PM. to Since 26 °C the until UFAD 6 PM. basically Since the adopts UFAD the basically VAV to controladopts the airflow,VAV to itcontrol adjusts the the airflow, airflow it basedadjusts on the the airflow indoor based load, on and the conditions indoor load, the airand at conditions a lower airflow the air than at a thelower CAV airflow system. than In the addition, CAV system. since the In UFAD addition, only since conditions the UFAD the occupied only conditions zone, it conditions the occupied the zone, air at ait lowerconditions airflow the than aireven at a thelower VAV. airflow During than initial even cooling the VAV. operations, During the initial temperature cooling in operations, the UFAD wasthe highertemperature than in in the the VAV. UFAD Among was thehigher three than systems, in the the VAV. active Among chilled the beam three conditioned systems, the the active air closest chilled to 26beam◦C, conditioned even during initialthe air operations, closest to 26 with °C, few even periods during during initial which operations, the air waswith conditionedfew periods to during lower thanwhich the the cooling air was set-point conditioned temperature, to lower indicating than the thatcooling the indoorset-point set-point temperature, temperature indicating was met that with the theindoor least set-point temperature temperature loss and was airflow. met with the least temperature loss and airflow.

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Energies 2019, 12, x FOR PEER REVIEW 18 of 29

Figure 11. Analysis of of mean mean zone zone air air temperature temperature when when ap applyingplying the the four four HVAC HVAC systems systems (1–4 (1–4 August). August).

Figure 1212 showsshows thethe indoor indoor relative relative humidity humidity distribution distribution from from 1 to 1 4to August 4 August when when using using the four the fourHVAC HVAC systems. systems. All four All systems four sy satisfiedstems satisfied the Korean the HVAC Korean thermal HVAC comfort thermal standard comfort of standard 40–70% [ 64of] 40–70%and ASHRAE’s [64] and thermalASHRAE’s comfort thermal standard comfort of standard 30%–60% of [30%–60%65]. During [65]. the During air-conditioning the air-conditioning analysis analysisperiod, theperiod, average the average relative relative humidity humidity of the conditioned of the conditioned interior interior air was air 50.9, was 54.7, 50.9, 49.9, 54.7, and 49.9, 53.4% and 53.4%for the for VAV, the CAV, VAV, UFAD, CAV, and UFAD, active and chilled active beam chil withled beam DOAS, with respectively. DOAS, respectively. Each HVAC Each system HVAC had systemdehumidifying had dehumidifying functions within functions its respectivewithin its respective AHU. The AHU. active The chilled active beam chilled with beam DOAS with systemDOAS systemmaintained maintained a comfortable a comfortable indoor environment indoor enviro evennment in Incheon’s even in Incheon’s high-humidity high-humidity climate because climate it becauseprocessed it processed latent heat latent and dehumidified heat and dehumidified it through theit through heat exchangers. the heat exchangers.

Figure 12. AnalysisAnalysis of of zone zone air air relative relative humidity humidity when when applying applying the the four four HVAC HVAC systems systems (1–4 (1–4 August). August).

4.3. Analysis Analysis of of Airflow Airflow Supply Figure 13a13a shows the indoor airflow airflow supply (sum of 5 zones) from 1 to 4 August during the operation of the four HVAC systems. Figure 13b13b sh showsows the airflow airflow divided into the five five zones when each system was used during the day on 1 August.

Energies 2019, 12, 4160 19 of 29 Energies 2019, 12, x FOR PEER REVIEW 19 of 29

(a)

(b)

Figure 13.13(a-b).(a) Analysis (a) Analysis of the of total the supplytotal supply airflow airflow rate (1–4 rate August). (1–4 August). (b) Analysis (b) Analysis of the supply of the airflowsupply rateairflow in the rate five in the zones five (1 zones August). (1 August).

In FigureFigure 13 13a,a, the the VAV VAV is ais method a method that maintainsthat maintains a constant a constant discharge discharge temperature temperature and controls and thecontrols airflow the according airflow according to increases to and increases decrease and in thedecrease load, whichin the results load, inwhich airflow results variations in airflow over time.variations The airflowover time. changed The airflow from a minimumchanged from of 1.311 a minimum kg/s to a maximumof 1.311 kg/s of 3.480to a maximum kg/s, exhibiting of 3.480 an increasingkg/s, exhibiting airflow an pattern increasing after airflow sunrise. pattern In Figure after 13 b,sunrise. by analyzing In Figure the 13b, 5 zones by analyzing over the course the 5 zones of the day,over wethe observedcourse of that the theday, interior we observed and south that perimeter the interior zones and experienced south perimeter a high zones airflow experienced rate, while a thehigh north airflow zone rate, experienced while the anorth low airflowzone experience rate. On thed a eastlow perimeterairflow rate. zone, On where the east the perimeter sun rises inzone, the morning,where the the sun airflow rises in increased the morning, in the the morning airflow based increased on changes in the morning in the solar based radiation. on changes In the in west the perimetersolar radiation. zone, whereIn the the west sun sets,perimeter the airflow zone, increased where the in the sun afternoon. sets, the On airflow the other increased hand, the in CAV the constantlyafternoon. suppliedOn the other air at hand, a maximum the CAV airflow constantly of 5.072 supplied kg/s from air 7 at AM a maximum to 6 PM, without airflow accounting of 5.072 kg/s for thefrom space 7 AM thermal to 6 PM, load. without The supply accounting airflow for during the space CAV thermal operation load. was The divided supply into airflow five zones,during which CAV indicatesoperation that was the divided airflow into supply five was zones, 1.279, which 1.198, indicates 1.070, 0.870, that and the 0.656 airflow kg/s insupply the interior, was 1.279, south, 1.198, east, 1.070, 0.870, and 0.656 kg/s in the interior, south, east, west, and north perimeter zones, respectively. In the interior and south perimeter zones, the airflow was high, while the north zone had the lowest

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Energies 2019, 12, x FOR PEER REVIEW 20 of 29 west, and north perimeter zones, respectively. In the interior and south perimeter zones, the airflow wasairflow. high, The while UFAD the north system zone also had controls the lowest the airflow. airflow The to UFAD maintain system temperature also controls and, the airflowtherefore, to maintainexhibited temperaturethe same pattern and, therefore, of changes exhibited in the airf thelow same as pattern the VAV. of changes However, in thesince airflow the UFAD as the VAV.only However,conditions since the occupied the UFAD zone only below conditions a height the of occupied 2 m from zone the below floor, ait height conditioned of 2 m fromthe air the with floor, less it conditionedairflow than the the air VAV. with The less airflow airflow thanchanged the VAV.from The a minimum airflow changed of 1.114 from kg/s ato minimum a maximum of 1.114 of 2.987 kg/s tokg/s, a maximum resulting ofin 2.987a trend kg of/s, air resulting conditioning in a trend that of had air the conditioning same airflow that pattern had the as same the VAV airflow in patterneach of asthe the 5 zones. VAV in The each active of the chilled 5 zones. beam The activewith DOAS chilled operates beam with at DOASa constant operates airflow, at a and constant uses airflow,indoor andinduced uses air, indoor such induced that by controlling air, such that the by flow controlling in the beam’s the flow cold in water the beam’s coil, it coldmeets water the thermal coil, it meets load. theTherefore, thermal the load. active Therefore, chilled the beam active was chilled able beamto provide was able conditioned to provide air conditioned and maintain air and the maintain indoor theset-point indoor temperature set-point temperature and humidity, and despite humidity, only despite conditioning only conditioning the air with the the air minimum with the outside minimum air outsideinflow (1.587 air inflow kg/s), (1.587 usingkg the/s), DOAS. using The the DOAS.minimum The outside minimum air inflow outside was air 0.460, inflow 0.382, was 0.460,0.312, 0.382,0.259, 0.312,and 0.174 0.259, kg/s and in 0.174the interior, kg/s in thesouth, interior, east, west, south, and east, north west, zones, and north respectively. zones, respectively.

4.4. Analysis of Indoor Thermal Comfort Figure 1414 shows shows the the distribution distribution of theof predictedthe predicted mean mean vote (PMV),vote (PMV), which iswhich the thermal is the comfortthermal indexcomfort for index the cooling for the period cooling between period between August 1 Augu and 4st when 1 and using 4 when the using four HVACthe four systems. HVAC systems.

Figure 14. Analysis of the thermal comfort during thethe operation of the four HVAC systems.systems.

The PMV comfortcomfort rangerange isis betweenbetween −0.5 and 0.5, based on ISO Standard 7730 [[65–68].65–68]. The PMV − during CAV airair conditioningconditioning fromfrom 77 AMAM toto 66 PMPM hadhad aadistribution distribution betweenbetween −1.271.27 andand −0.90 onon − − AugustAugust 1,1, −1.13 andand −0.79 on August 2,2, −1.21 and −0.84 on 3 August, andand −1.28 and −0.90 on August − − − − − − 4. Since the CAV conditions the air with the highest airflowairflow of all the systems used in this study, asas wellwell asas yieldingyielding aa temperaturetemperature thatthat waswas 2–32–3 ◦°CC belowbelow thethe set-pointset-point temperature of 26 ◦°CC due to thethe overcooling ofof thethe space,space, thethe PMVPMV waswas slightlyslightly lowerlower thanthan thethe otherother systems and was not within the comfort rangerange betweenbetween −0.5 and 0.5. During During VAV VAV air conditioning, the PMV was within the thermalthermal − comfort range betweenbetween −0.5 and 0.5. The The PMV values for the VAV hadhad aa distributiondistribution betweenbetween −0.41 − − and 0.03 on August 1, −0.180.18 and and 0.17 0.17 on on August August 2, 2, −0.390.39 and and 0.17 0.17 on on August August 3, 3,and and −0.410.41 and and 0.03 0.03 on − − − onAugust August 4. These 4. These values values were were higher higher and and lower lower than than the the PMV PMV distribution distribution of of the the CAV CAV and and UFAD, UFAD, respectively.respectively. DuringDuring UFADUFAD airair conditioning,conditioning, thethe PMVPMV hadhad aa distributiondistribution betweenbetween −0.36 and 0.28 on − AugustAugust 1,1, −0.06 and 0.42 on August 2, −0.28 and 0.42 on August 3, and −0.270.27 and 0.28 on August 4, − − − whichwhich waswas withinwithin the the comfort comfort range. range. Finally, Finally, during during air air conditioning conditioning with with the activethe active chilled chilled beam beam with DOAS,with DOAS, the PMV the hadPMV a distributionhad a distribution between between0.16 and −0.16 0.34 and on August0.34 on 1,August 0.01 and 1, 0.510.01 onand August 0.51 on 2, − August 2, −0.24 and 0.51 on August 3, and −0.17 and 0.39 on August 4. Among the four HVAC systems, these PMV values for the active chilled beam were a little high, but were within the comfort range.

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0.24 and 0.51 on August 3, and 0.17 and 0.39 on August 4. Among the four HVAC systems, these − − PMVEnergies values 2019, 12 for, x FOR the activePEER REVIEW chilled beam were a little high, but were within the comfort range. 21 of 29

5.5. Analysis of Cooling Energy Consumption andand COCO22 Emissions

5.1.5.1. Cumulative Zone Sensible CoolingCooling FigureFigure 1515 showsshows thethe cumulativecumulative amountamount ofof heatheat thatthat waswas removedremoved byby eacheach systemsystem toto coolcool thethe indoorindoor toto anan identicalidentical set-pointset-point temperaturetemperature (26(26◦ °C)C) duringduring June,June, July,July, and and August. August.

Figure 15. Analysis of the cumulative zone sensible cooling when operating the four HVAC systems. Figure 15. Analysis of the cumulative zone sensible cooling when operating the four HVAC systems.

WhenWhen usingusing the VAV, changeschanges inin thethe airflow,airflow, basedbased onon increasesincreases andand decreasesdecreases inin thethe load, controlledcontrolled thethe room room temperature, temperature, which which only only removed removed 29,646 29,646 kWh. kWh. When When using theusing CAV, the the CAV, amount the ofamount heat removed of heat removed from the fivefrom zones the five was zones 37,030 was kWh 37,030 of the kWh heat, of i.e.,the aheat, 24.9% i.e., increase. a 24.9% Whenincrease. using When the using the UFAD, the cooling load was reduced because only the occupied zone was maintained at 26 UFAD, the cooling load was reduced because only the occupied zone was maintained at 26 ◦C while maintaining°C while maintaining the unoccupied the unoccupied zone at a higherzone at temperature. a higher temperature. In other words, In other since words, only since the occupied only the zoneoccupied was zone air-conditioned, was air-conditioned, air conditioning air conditioning was performed was performed at a lower at airflow a lower rate airflow than rate in the than VAV, in whichthe VAV, reduced which the reduced amount the of heatamount removed of heat (27,082 removed kWh) (27,082 by 8.7% kWh) compared by 8.7% to compared the VAV. Whento the usingVAV. theWhen active using chilled the beamactive with chilled DOAS, beam air with conditioning DOAS, air was conditioning performed using was indoorperformed induced using air indoor at the DOASinduced minimum air at the outdoorDOAS minimum air inflow, outdoor which reducedair inflow, the which amount reduce of heatd the removed amount (24,502of heat kWh)removed by 17.3%,(24,502 compared kWh) by to17.3%, the VAV. compared Even though to the theyVAV. were Even set though to the samethey temperature,were set to the the same amount temperature, of indoor heatthe amount removed of by indoor each systemheat removed was diff byerent, each because system the was systems different, had because different the airflow systems and had temperature different controlairflow methodsand temperature based on control their own methods characteristics. based on their own characteristics.

5.2.5.2. Site and Primary CoolingCooling EnergyEnergy ConsumptionsConsumptions FigureFigure 1616 showsshows thethe sitesite andand primary cooling energyenergy for each system in June, July, and August. TheThe coolingcooling energyenergy consistsconsists ofof thethe absorptionabsorption chiller energy, coolingcooling towertower energy,energy, AHUAHU supplysupply andand exhaustexhaust fanfan energy,energy, andand pumppump energyenergy (absorption (absorption chiller chiller and and cooling cooling tower) tower) consumptions. consumptions.

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FigureFigure 16.16. Site-coolingSite-cooling energy, energy, primary-cooling primary-cooling energy, energy, and andenergy-saving energy-saving potential potential in each in HVAC each HVACsystem. system.

TheThe energyenergy requiredrequired forfor thethe absorptionabsorption chillerchiller accountsaccounts forfor mostmost ofof thethe sitesite energy,energy, followedfollowed sequentiallysequentially byby thethe fanfan energy,energy, coolingcooling towertower energy,energy, andand pumppump energy.energy. WhenWhen usingusing thethe CAV,CAV, site site coolingcooling energy energy consumptionconsumption waswas increasedincreased byby 27.6%,27.6%, compared to the VAV. WhenWhen usingusing thethe UFAD,UFAD, coolingcooling energy energy was was reduced reduced by 10.6%,by 10.6%, compared compared to the to VAV, the with VAV, a 15.4% with reductiona 15.4% inreduction the absorption in the chillerabsorption energy, chiller and energy, a 24.4% and reduction a 24.4% reduction in fan energy. in fan Sinceenergy. the Since UFAD the suppliesUFAD supplies air from air the from floor the (taskfloor and(task ambient and ambient HVAC), HVAC), only theonly occupied the occupied zone zone was was air-conditioned, air-conditioned, without without consideration consideration of theof the ceiling ceiling height. height. Therefore, Therefore, the the discharge discharge temperature temperature was was approximately approximately 3 3◦ C°C higher higher than than inin thethe VAV.VAV. This This resultedresulted inin a a reduction reduction of of chiller chiller use use and and conservation conservation of of fan fan power. power. However,However, inin buildingsbuildings that that use use the the UFAD, UFAD, systems systems should should be designed be designed by taking by taking into account into account the fact the that fact the that energy the conservationenergy conservation effect can effect vary can according vary according to the type to andthe type installation and installation conditions conditions of the lighting of the fixtures.lighting Whenfixtures. using When the active using chilled the active beam withchilled DOAS, beam cooling with energy DOAS, was cooling reduced energy by approximately was reduced 22.3% by comparedapproximately with the22.3% VAV, compared with 26.6% with and the 66.7% VAV, reductions with 26.6% in and the absorption66.7% reductions chiller andin the fan absorption energies, respectively.chiller and fan The energies, active chilledrespectively. beam withThe active DOAS ch decouplesilled beam sensible with DOAS cooling decouples and ventilation, sensible cooling while cooland airventilation, conditioning while occurred cool withair conditioning only the minimum occurred outdoor with inflowonly the required minimum for ventilation, outdoor whichinflow reducedrequired the for fan ventilation, energy. The which load reduced that must the fan be handledenergy. The by theload cooling that must coil be was handled also reduced by the cooling due to thecoil low was airflow also reduced rate. Additionally, due to the low the coolingairflow energyrate. Additionally, saving rate wasthe cooling high compared energy saving with the rate other was HVAChigh compared systems, via with the the effects other of radiationHVAC systems, and convection via the that effects induced of radiation air mixing and in theconvection zone. Water, that whichinduced has air a greatermixing heatin the capacity zone. Water, than air, which was alsohas a used greater via theheat water capacity pipes than within air, was the chilledalso used beam via tothe exchange water pipes heat withwithin the the indoor chilled air. Therefore,beam to exchan the conveyancege heat with power the could indoor be conserved,air. Therefore, due the to theconveyance heating medium. power could However, be conserved, with exposure due to to the the heating high-temperature medium. However, and high-humidity with exposure climate to the of Korea,high-temperature we should consider and high-humidity a control strategy climate that of respondsKorea, we to should the latent consider heat load a control and condensation, strategy that whichresponds is necessary to the latent in order heat to load carefully and condensation review performance, which inis necessary the design in and order operation to carefully stages. review performanceTo the right in the of design the site and cooling operation energy stages. in Figure 16, we compare the primary cooling energy. The primaryTo the right energy of the is calculated site cooling by energy multiplying in Figure by a16, conversion we compare factor the (Table primary1)[ cooling29] for eachenergy. nation The accordingprimary energy to its external is calculated energy by sources multiplying (delivered by a energy), conversion to apply factor both (Table power 1) generation[29] for each and nation fuel transportationaccording to its losses external to the energy energy sources consumption (delivered value. energy), As a to conversion apply both standard, power generation the electric and power fuel (2.75)transportation and fuel (1.1) losses (i.e., to coal,the energy gas, oil, consumption etc.) conversion value. factors, As a conversion proposed by standard, the BEECS the (Table electric1), power were used(2.75) to and calculate fuel (1.1) the primary(i.e., coal, energy gas, oil, consumption. etc.) conversion The factors, fans, pumps, proposed and coolingby the BEECS towers (Table have a 1), higher were used to calculate the primary energy consumption. The fans, pumps, and cooling towers have a higher electrical energy ratio (2.75) than the absorption chiller (1.1), which uses gas. The CAV

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Energies 2019, 12, x FOR PEER REVIEW 23 of 29 electrical energy ratio (2.75) than the absorption chiller (1.1), which uses gas. The CAV increased primaryincreased cooling primary energy cooling by23.3% energy compared by 23.3% to compar the CAV,ed whileto the theCAV, UFAD while reduced the UFAD energy reduced consumption energy byconsumption 11.3%, and by the 11.3%, active and chilled the beamactive with chilled DOAS beam by with 23.1%. DOAS by 23.1%.

5.3. CO2 Emissions 5.3. CO2 Emissions Figure 17 shows the CO2 emissions and the percent reduction in the CO2 emissions of each Figure 17 shows the CO2 emissions and the percent reduction in the CO2 emissions of each system in June, July, and August. The amount of CO2 emissions for each system can be calculated system in June, July, and August. The amount of CO2 emissions for each system can be calculated by by multiplying the site energy by the CO2 emission factor for each energy source. Therefore, the site multiplying the site energy by the CO2 emission factor for each energy source. Therefore, the site energy was multiplied by the electric power CO2 emission factor of 0.4663 kg CO2/kWh and the natural energy was multiplied by the electric power CO2 emission factor of 0.4663 kg CO2/kWh and the gas (LNG) CO2 emission factor of 0.2031 kg CO2/kWh, as listed in Table2 of Section 2.3. natural gas (LNG) CO2 emission factor of 0.2031 kg CO2/kWh, as listed in Table 2 of Section 2.3.

Figure 17. CO2 emissions and the reduction potential of each HVAC system. Figure 17. CO2 emissions and the reduction potential of each HVAC system.

WhenWhen analyzinganalyzing thethe COCO22 emissions,emissions, the CO2 emissions associated with the VAV systemsystem werewere 2 2 34,67934,679 kgCOkgCO2 while while for for the the CAV CAV system system the the emissions emissions were were 42,400 42,400 kgCO kgCO2, that, is,that an is, increase an increase of 22.3% of 2 compared22.3% compared with the VAV.with Whenthe VAV. using theWh UFADen using system, the theUFAD CO2 emissionssystem, werethe CO approximately emissions 31,185were 2 kgCOapproximately2, i.e., a reduction 31,185 kgCO of 10.1%, i.e., compared a reduction with of the10.1% VAV. compared Finally, whenwith the using VAV. the Finally, active chilled when beamusing 2 2 withthe active DOAS, chilled the CO beam2 emissions with DOAS, were 27,124 the CO kgCO emissions2, which were is a reduction 27,124 kgCO of 21.8%, which compared is a reduction to the VAV. of 21.8% compared to the VAV. 6. Conclusions and Discussions 6. Conclusions and Discussions 6.1. Summary and Conclusions 6.1. SummaryThis study and used Conclusions the EnergyPlus dynamic simulation program to analyze in detail the operational characteristicsThis study and used energy the performance, EnergyPlus anddynamic to understand simulation the featuresprogram of to the analyze HVAC systemsin detail under the consideration,operational characteristics by investigating and theenergy dry-bulb performance, temperature, and to relative understand humidity, the features thermal of comfort, the HVAC and airflowsystems rate under at majorconsideration, nodes. Furthermore,by investigating we the analyzed dry-bulb the temperature, cooling energy relative consumption, humidity, thermal energy reductioncomfort, contribution,and airflow andrate CO at2 emissions.major nodes. The conclusionsFurthermore, that we can beanalyzed drawn fromthe thiscooling study energy are as follows:consumption, energy reduction contribution, and CO2 emissions. The conclusions that can be drawn from this study are as follows: (1) We analyzed the changes in the average annual summer temperature in Incheon (Seoul 1) Wemetropolitan analyzed area), the changes Korea. The in resultsthe average indicated annu thatal thesummer temperature temperature is increasing in Incheon and abnormal (Seoul metropolitanclimate events, area), such asKorea. heat waves,The results are occurring indicated more that frequently. the temperature These characteristics is increasing give riseand abnormalto circumstances climate during events, which such it is necessaryas heat wa to useves, energy-conserving are occurring more HVAC frequently. systems in Korea.These characteristics give rise to circumstances during which it is necessary to use energy-conserving HVAC systems in Korea. 2) Each energy-conserving HVAC system has a different modeling configuration in EnergyPlus, according to its air-conditioning concept. In the VAV, the terminal unit was modeled as an “Air

Energies 2019, 12, 4160 24 of 29

(2) Each energy-conserving HVAC system has a different modeling configuration in EnergyPlus, according to its air-conditioning concept. In the VAV, the terminal unit was modeled as an “Air Terminal Single Duct: VAV Reheat”. In the CAV, the air loop terminal unit was modeled as an “Air Terminal Single Duct: Constant Volume: Reheat”. In the VAV, the fans were modeled as “Fan: Variable Volume”. In the CAV, they were modeled as “Fan: Constant Volume”. The UFAD was modeled as “Room air model Type” and “Room Air Setting: Underfloor Air Distribution” to implement floor diffusion and thermal stratification. For the active chilled beam with DOAS, “Air Terminal Single Duct: Cooled Beam” and “Heat Exchanger Air to Air: Sensible and Latent” were used to implement the beam’s induced air condition features, as well as the features of the DOAS, which entails the introduction of only outdoor air. (3) The dry-bulb temperature, relative humidity, and airflow rates were compared at the major nodes (10 node points) in the schematic of each HVAC system (8.1 at 2:00 PM). Even though the indoor temperature was set to be identical among each HVAC system, the node status analysis results show that the airflow supply rates were different according to each air-conditioning concept, as well as showing changes in the air temperature and humidity based on the airflow rates. (4) When the HVAC systems were used in the summer with high temperature and humidity, although the VAV, UFAD, and active chilled beam with DOAS matched the cooling set-point temperature of 26 ◦C, their initial indoor temperature distributions were slightly different. The CAV used a maximum airflow and conditioned the air to approximately 2–3 ◦C below the cooling set-point of 26 ◦C. All four systems have dehumidification functions in their AHUs. Since the active chilled beam with the DOAS handles latent heat and performs dehumidification via heat exchangers, this system conforms to the Korean and the ASHRAE thermal comfort standard. When we analyzed the indoor thermal comfort, the CAV had a slightly lower PMV than other systems at 1.27 to − 0.90 due to the overcooling by excessive air flow and, accordingly, a lower indoor temperature − than 26 C. Therefore, the CAV was unable to conform to the thermal comfort range from 0.5 to ◦ − 0.5. (5) The VAV maintained a constant indoor temperature, and controlled the airflow rate at 1.311–3.480 kg/s, according to the space thermal load. On the other hand, the CAV supplied air constantly at a maximum airflow of 5.072 kg/s during the air-conditioning period. The UFAD also controlled the airflow to maintain set-point temperature and, therefore, showed the same changes in airflow pattern as the VAV. However, since the UFAD only conditions air in the occupied zone, it used a lower airflow rate of 1.114–2.987 kg/s than the VAV. The active chilled beam with the DOAS operates using a constant airflow method, whose load response is achieved by controlling the flow in the beam’s chilled water coil. Therefore, the active chilled beam with DOAS was able to maintain the cooling set-point temperature and humidity, despite only conditioning air with a minimum outdoor air inflow of 1.587 kg/s. (6) Even though the same set-point temperatures were set, the amount of indoor heat removed by each system was different, because each system had different airflow and temperature control methods, based on their specific characteristics. The CAV constantly supplied air at a maximum airflow, which was capable of increasing the amount of removed heat by 24.9%, compared to the VAV. When using the UFAD, the amount of heat removed was reduced by 8.7%, compared with the VAV. When using the active chilled beam with DOAS, the beam’s indoor induced air at the DOAS minimum outdoor air inflow initiated air conditioning, which was capable of reducing the amount of heat removed by 17.3%, compared with the VAV. (7) Primary cooling energy, which takes into account electric power and fuel conversion factors, was increased by 23.3% when using CAV, compared to VAV. When using UFAD, the energy was reduced by 11.3%, whereas the active chilled beam with DOAS reduced the energy by 23.1%, compared to VAV. CO2 emission reduction rates were similar to the cooling energy saving rates; CO2 emissions when using CAV increased by 22.3%, compared to VAV. When using UFAD and active chilled beam, emissions were reduced by 10.1% and 21.8%, respectively, compared to VAV. Energies 2019, 12, 4160 25 of 29

6.2. Limitation of Research and Future Work Implementing the cooling mechanisms and understanding the energy conservation principles of each HVAC system in certain climate conditions is important for energy efficiency in future high-performance buildings. This study selected a reference model and performed evaluations of each energy-conserving HVAC system by focusing on its cooling mechanism, indoor environment, energy consumption, and CO2 emissions. However, future studies must also analyze energy reduction rates that occur when these systems are combined with other plant systems or renewable energy technology. Future studies will also need to analyze cooling seasons and heating mechanisms within each system. In addition, economic analyses that consider energy costs needed to be performed in the future. There are only simulation results through EnergyPlus in this study. Experimental study was not performed when four systems were applied in the actual building. Therefore, we need to discuss the results of the actual system operation through experimental study along with simulation data in a future study. Since there is no study that compares all four systems in various climates, we need to review the simulation data by applying it in different climates. However, data analyzed in this study showed similar results as those of Cho et al. [69] and Kim et al. [14] who studied various energy-saving systems in the Korean climate.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1073/12/21/4160/s1: Figure S1a. Comparison of CAV and VAV modeling in the EnergyPlus class list, Figure S1b. Comparison of CAV and UFAD modeling in the EnergyPlus class list, Figure S1c. Comparison of CAV and active chilled beam with DOAS modeling in the EnergyPlus class list, Figure S2a. Diagram of the CAV system layout in the EnergyPlus simulation (scalable vector graphics (SVG) of the CAV system), Figure S2b. Diagram of the VAV system layout in the EnergyPlus simulation (scalable vector graphics (SVG) of the VAV system), Figure S2c. Diagram of the UFAD system layout in the EnergyPlus simulation (scalable vector graphics (SVG) of the UFAD system), and Figure S2d. Diagram of the ACB with DOAS layout in the EnergyPlus simulation (scalable vector graphics (SVG) of the ACB with DOAS). Author Contributions: C.-H.K. performed the simulation and data analysis and wrote this paper based on the obtained results with the help of S.-E.L. and K.-H.L. K.-S.K. led and supervised this study. All of the authors have contributed for collecting ideas and concepts presented in the paper. Funding: This research was supported by a grant (19AUDP-B079104-06) from Architecture and Urban Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript:

ACB Active Chilled Beam ACH Air Change per Hour AHU Air Handling Unit ALT Altitude ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers BEECS Building Energy Efficiency Certification System CAV Constant Air Volume CDD Cooling Degree Day DOAS Dedicated Outdoor Air System DOE U.S. Department of Energy EPBD Energy Performance of Buildings Directive EPW EnergyPlus Weather File FC Fan Coil GHG Greenhouse Gas HVAC Heating, Ventilation, and Air-Conditioning HDD Heating Degree Day IPCC Intergovernmental Panel on Climate Change Energies 2019, 12, 4160 26 of 29

KEA Korea Energy Agency KICT Korea Institute of Civil Engineering and Building Technology KMA Korea Meteorological Administration LDAC Liquid Desiccant Membrane Air-conditioning LNG Natural Gas MOTIE Korea Ministry of Trade, Industry and Energy MVHR Mechanical Ventilation with Heat Recovery OHAD Overhead Air Distribution PMV Predicted Mean Vote REHVA Federation of European Heating, Ventilation and Air Conditioning Associations SHGC Solar Heat Gain Coefficient TRY Test Reference Year UFAD Underfloor Air Distribution VAV Variable Air Volume VLT Visible Light Transmittance VRF Variable Refrigerant Flow

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