Energy Performance Evaluation of a Desiccant Air Handling System to Maximize Solar Thermal Energy Use in a Hot and Humid Climate

Article Energy Performance Evaluation of a Desiccant Air Handling System to Maximize Solar Thermal Energy Use in a Hot and Humid Climate

Makiko Ukai 1,*, Masaya Okumiya 1 and Hideki Tanaka 2 1 Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8603, Japan; [email protected] 2 Campus Planning & Environment Management Oﬃce, Nagoya University, Nagoya 464-8603, Japan; [email protected] * Correspondence: [email protected]

Received: 3 February 2020; Accepted: 1 March 2020; Published: 3 March 2020

Abstract: A desiccant air handling unit is one of the major types of dehumidification handling systems and requires hot water or hot air to regenerate sorption materials. If solar thermal energy is used as the heat source for regeneration, in general, a backup electrical heater, backup boiler, or combined heat and power (CHP) is installed in order to maintain a stable hot water supply. In this study, effective control is proposed for a desiccant air handling system that uses solar thermal energy (flexible control), and its energy performance is compared to that of a traditional control (the fixed control) through a system simulation. The diurnal behavior shows that the system with a fixed control without a backup boiler cannot process the latent load properly (28 GJ of unprocessed latent load for July and August). On the other hand, the system with a flexible control without a backup boiler is able to process required latent heat load. Based on the fact that the fixed control needs a backup boiler to process the latent load, the system with a fixed control with a backup boiler is considered for the energy performance comparison. The simulation results show that the primary energy-based coefficient of performance (hereafter, COP) of the system with a flexible control without a backup boiler reaches 1.56. On the other hand, the primary energy-based COP of the system with a fixed control with a backup boiler reaches only 1.43. This proves that the flexible control contributes to the higher energy performance of the system and maximizes the use of solar thermal energy more than the fixed control.

Keywords: desiccant air handling unit; performance evaluation; ﬂexible control

1. Introduction Today, the concept of a net zero energy building (ZEB) is widely recognized, and the popularization and promotion of ZEBs is desired [1–3]. The indoor thermal load through the outer wall and windows has been decreasing due to the higher performance of insulation [4]. Furthermore, the indoor sensible heat load has become very small now thanks to light emitting diode (LED) lighting and the better energy efficiency of computers. On the other hand, the metabolic rates of humans do not change very much. Therefore, although the constitution of the thermal loads of buildings has changed, the latent load from humans still seems to be relatively large. In addition, global warming is a worldwide issue, thereby increasing the temperature and humidity in some areas of Japan (for example, Nagoya area) [5]. This results in a larger fresh air load. Thus, dehumidification in Japan is important for indoor environments. One of the major dehumidification systems involves desiccant air handling units, in which a solid adsorbent absorbs water content in the air. In order to regenerate the adsorbent, hot water or hot air is

Sustainability 2020, 12, 1921; doi:10.3390/su12051921 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 1921 2 of 18 necessary. Therefore, solar thermal energy and desiccant air handling units are a good combination. When solar energy is utilized, a backup electric heat heater, gas boiler, or combined heat and power (CHP) is installed due to the fluctuation and instability of solar energy. According to this paper [6], almost all of the reviewed systems have an auxiliary heater or boiler for solar water or an air collector. Angrisani [7] conducted a dynamic simulation on solar-assisted desiccant–based air handling units with backup boilers in two Italian cities. Angrisani’s results showed maximum primary energy saving of about 10% and 20% with flat plate and evacuated tube collectors, respectively. According to Fong [8], a solar desiccant cooling system with a backup gas heater achieves 35.2% savings of year-round primary energy consumption compared to conventional air-conditioning systems. Comino [9] conducted an experimental energy performance assessment of a solar desiccant cooling system in Southern European Climates. Comino’s desiccant air handling unit only had a solar thermal collector for the regenerating coil, so there were no backup heaters. However, the authors pointed out that it is not possible to achieve a humidity ratio point of 8 g/kg when the solar heat gain is low. The authors proposed to install thermal energy storage for solar thermal collectors to reduce the unprocessed latent load, but this possibility was not researched. Zied [10] conducted a numerical simulation without an auxiliary heater but did not consider a situation where solar thermal energy is not enough for regeneration. Enteria [11] conducted first and second law analyses of solar desiccant air conditioning over one day. The heater operated overnight to store thermal energy to support the early hours of its operation. The solar fraction was around 75% thermal energy. There is no research on desiccant air handling units without backup heaters, boilers, or CHP. For the desiccant material of the aforementioned papers, silica gel was adopted. In this study, a polymer sorbent was employed as a desiccant material. This material can achieve greater dehumidification than silica gel when dehumidifying high-relative-humidity air with low-temperature regeneration air [12]. This study proposes a desiccant (polymer sorbent) air-handling unit with solar thermal energy without using a backup heater or boiler and evaluates the energy performance of the desiccant air handling system through simulation. In order to avoid using a backup heat source, the proposed solution’s equipment uses variable control that considers the hot water temperature from solar thermal system and the air state entering the dehumidification material. This new control will contribute to the effective use of solar thermal energy. The purpose of this study is to

Analyze the air state in a desiccant air handling unit with the traditional control and the • proposed control; Evaluate the energy performance with the new control and compare it with the energy performance • with a normal control.

2. Targeted Building and System This section provides details on the targeted building, the desiccant air handling unit, and the heat source systems.

2.1. Targeted Building The targeted building is an office building located in Tokyo, Japan. It has five stories; the air-conditioning area of a typical floor is 983 m2, as shown in Figure1 (the total floor area is about 1106 m2). The opening ratio (the windowed area divided by the wall area) of the building is 41.5%. This building was designed with some energy saving methods, including eaves, high insulated walls and roofs, and LED lighting. The U-value of the outer wall is 0.6 W/m2K, and low-e double glazed windows are installed, with heat transmission resistance of 0.385 m2K/W. The room’s set temperature is 28 ◦C, and the relative humidity is 40% during the cooling period, according to the comfort zone described by Kato [13] and the “Cool Biz” [14] campaign, where appropriate room conditions and Sustainability 2020, 12, 1921 3 of 18 Sustainability 2020, 12, 1921 3 of 18

Sustainability 2020, 12, 1921 3 of 18 clothesperiodic were stationary proposed. calculations The load for calculation one year. Stan is conducteddard meteorological with Micro phenomena HASP/TES [data15], whichare used gives for periodic stationary calculations for one year. Standard meteorological phenomena data are used for the theperiodic thermal stationary load calculation. calculations In thisfor onepaper, year. July Stan anddard August, meteorological which are phenomena hot and humid data seasons,are used are for thermal load calculation. In this paper, July and August, which are hot and humid seasons, are used usedthe thermal for the loadsimulation. calculation. Figure In 2 this shows paper, the Julyload and ratio August, of the buildingwhich are for hot July and and humid August. seasons, Almost are for the simulation. Figure2 shows the load ratio of the building for July and August. Almost half of halfused of for the the thermal simulation. load is Figure latent heat2 shows load. the load ratio of the building for July and August. Almost the thermal load is latent heat load. half of the thermal load is latent heat load. Non air-conditioned space

Non air-conditioned space

Interior zone 6400 28800 N Interior zone 6400 West perimeter zone Eastperimeter zone 28800 N 6400 West perimeter zone Eastperimeter zone 6400

South perimeter zone 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 South38400 perimeter zone 6400 6400 6400 6400 6400 6400 38400 Figure 1. Typical floor plan. Figure 1. Typical ﬂoor plan. Figure 1. Typical floor plan.

33% 33% 50% 50% 2% 14% 2% 14% indoor sensible load indoor latent load fresh air sensible load fresh air latent load indoor sensible load indoor latent load Figure 2. Thermalfresh air load sensible of the load buildingfresh air for latent July load and August. Figure 2. Thermal load of the building for July and August. 2.2. Desiccant Air HandlingFigure Unit 2. Thermal load of the building for July and August. 2.2. Desiccant Air Handling Unit Figure3 shows the configuration of the desiccant air handling unit (DAHU), the sensible air handling2.2. DesiccantFigure unit, 3 Airshows the Handling solar the thermalconfiguration Unit system of and the thedesiccan heat sourcet air handling system. unit For DAHU,(DAHU), fresh the outdoorsensible air ishandling suppliedFigure unit, to 3 theshowsthe desiccantsolar the thermal configuration system system directly. ofand the the The desiccan heat system sourcet air in thissystemhandling study. For unit is DAHU, composed (DAHU), fresh of the aoutdoor pre-coolingsensible air airis coil,suppliedhandling a dehumidification to unit, the thedesiccant solar wheel, thermal system a sensibledirectly.system and heat The the exchanger,system heat insource this an after-cooling studysystem is. composedFor coil,DAHU, and of fresh a a regenerating pre-cooling outdoor aircoil, is asasupplied dehumidification shown to in the Figure desiccant 3wheel,. Fresh system a airsensible (1; directly. the heat number exchanger, The system in brackets an in after-cooling this is study correlated is coil,composed to and the a number regeneratingof a pre-cooling in Figure coil, coil,3 )as isshowna dehumidification provided in Figure to the 3. pre-cooling Freshwheel, air a sensible (1; coil the tonumber heat be exchanger, precooled in brackets andan after-coolingis dehumidified correlated coil,to (2),the and whichnumber a regenerating then in Figure passes coil, 3) the isas dehumidificationprovidedshown in toFigure the wheelpre-cooling3. Fresh (3), air the (1;coil sensible the to numberbe heat precooled exchanger in brac andkets (with dehumidifiedis correlated return air) to (4),(2), theand which number the after-coolingthen in Figurepasses coil3)the is (5).dehumidificationprovided The return to the air (RA)wheelpre-cooling (6) (3), passes the coil thesensible to sensible be heatprecooled heat exch exchangeranger and (withdehumidified (7) andreturn is then air) (2), heated (4), which and by the thethen after-cooling regenerating passes the coilcoildehumidification (8).(5). The return high wheel temperature air (RA) (3), the(6) air passessensible desorbs the heat moisturesensible exchanger heat from (withexchanger the polymericreturn (7) air) and adsorbents(4), is and then the heated and after-cooling is finallyby the exhaustedregeneratingcoil (5). The (9). coilreturn (8). airThe (RA) high (6)temperature passes the air sensible desorbs heat moisture exchanger from the(7) andpolymeric is then adsorbents heated by and the isregenerating finallyThe desiccantexhausted coil (8). air (9). The handling high temperature unit processes air the de entiresorbs moisture fresh outdoor from load the polymeric (sensible and adsorbents latent heat), and allis finally ofThe the desiccant indoorexhausted latent air (9). handling heat, and unit part processes of indoor the sensible entire heat.fresh outdoor The indoor load sensible (sensible heat and processed latent heat), by DAHUall of Thethe is indoor deriveddesiccant latent from air handlingheat, lighting and andunit part appliances. processes of indoor the se Each nsibleentire floor heat.fresh has Theoutdoor an air indoor handling load sensible (sen unitsible heat for and the processed latent remaining heat), by indoorDAHUall of the sensibleis derivedindoor heat latent from load, lightingheat, which and and is part called appliances. of indoor a sensible Eachsensible air floor handling heat. has Thean unit air indoor handling (SAHU). sensible unit heatfor the processed remaining by indoorDAHUFor sensible is chilled derived heat water, from load, air lighting sourcewhich and is heat called appliances. pumps a sensible are Each installed air floor handling tohas supply an unit aira handling(SAHU). pre-cooling unit andfor the after-cooling remaining coilindoor inFor DAHU sensible chilled and water,heat a cooling load, air sourcewhich coil for isheat SAHU.called pumps a sensible are installed air handling to supply unit a (SAHU). pre-cooling and after-cooling coil inFor DAHU chilled and water, a cooling air source coil for heat SAHU. pumps are installed to supply a pre-cooling and after-cooling coil in DAHU and a cooling coil for SAHU. Sustainability 2020, 12, 1921 4 of 18

For the regeneration of the sorbents, a solar thermal system is installed. Heat collected through the evacuatedSustainability tube2020, 12 type, 1921 solar thermal collector is ﬁrst stored in the solar thermal tank, and then hot water4 of 18 from the tank is supplied to the regenerating coil in DAHU when the temperature reaches its set point.

Solar thermal system solar thermal Solar collector Solar utilization pump Desiccant air Thermal Tank handling unit Solar collection pump (1)~(5): process side (6)~(9): regeneration side Dehumidification

Regenera- heat exchan EA(9) ting esbe Sensible RA(6) wheel (8) coil (7)

Pre- After- g cooling er cooling SA(5) OA(1) coil (2) (3) (4) coil

Cooling coil for SAHU ×5 HEADER Chilled water pump (AHU side) HEADER HEADER Air Source Heat Pump Chilled water pump Air Source (chiller side) Heat Pump

Figure 3. Diagram of the air handling unit and heat source system.

3. Design of the DAHU and Heat Source for Controls Figure 3. Diagram of the air handling unit and heat source system. This section provides the details of the traditional control, called a “fixed control”, and the newly3. Design proposed of the DAHU control, and called Heat a “flexible Source for control”, Controls as well as the design process for the DAHU and heat source. This section provides the details of the traditional control, called a “fixed control”, and the newly 3.1.proposed Fixed Controlcontrol, (Traditional called a “flexible Control) control”, as well as the design process for the DAHU and heat source. Figure4 shows the diagram of the traditional fixed control. The ordinary fixed control controls the3.1. airFixed temperature Control (Traditional by leaving Control) the pre-cooling coil at a fixed temperature (for example, 19 ◦C), and the relative humidity is 95%. Therefore, the dehumidification amount handled by the dehumidification Figure 4 shows the diagram of the traditional fixed control. The ordinary fixed control controls wheel and the regeneration coil demand do not fluctuate very much. The reason why fixed control the air temperature by leaving the pre-cooling coil at a fixed temperature (for example, 19 °C), and is traditionally used is that a fixed control provides stable pre-cooling, a stable regenerating coil, the relative humidity is 95%. Therefore, the dehumidification amount handled by the and stable after-cooling demand, which stabilizes the whole system’s balance. dehumidification wheel and the regeneration coil demand do not fluctuate very much. The reason why fixed control is traditionally used is that a fixed control provides stable pre-cooling, a stable regenerating coil, and stable after-cooling demand, which stabilizes the whole system’s balance. Sustainability 2020, 12, 1921 5 of 18 Sustainability 2020, 12, 1921 5 of 18 Absolute humidity [g/kg’] Absolute humidity

1 2 9 Fixed at 18.3°C

8 6 5 3

18.3°C Dry bulb temperature [℃]

Required regeneration temperature, which is 50°C, is determined because temperature leaving pre-cooling coil (2) is fixed at 18.3°C.

Figure 4. Diagram of the fixed control. Figure 4. Diagram of the fixed control. According to Stephan [12], a higher relative humidity in the air entering the dehumidification According to Stephan [12], a higher relative humidity in the air entering the dehumidification wheel at the process side achieves greater dehumidification with a polymer sorbent, even if the wheel at the process side achieves greater dehumidification with a polymer sorbent, even if the regeneration temperature is low. Thus, in a fixed control, the air state leaving the pre-cooling coil regeneration temperature is low. Thus, in a fixed control, the air state leaving the pre-cooling coil is is set at 18.3 ◦C, according to the preliminary simulation of the DAHU itself, without a heat source set at 18.3 °C, according to the preliminary simulation of the DAHU itself, without a heat source system. At this point, the value of thermal energy efficiency, which is given by the processed air system. At this point, the value of thermal energy efficiency, which is given by the processed air amount divided by the total coil demand (including the pre-cooling coil, regenerating coil, and after amount divided by the total coil demand (including the pre-cooling coil, regenerating coil, and after cooling coil), is the best. Depending on the load distribution and specification of the DAHU (shown in cooling coil), is the best. Depending on the load distribution and specification of the DAHU (shown Table1), the required total cooling coil demand for the pre-cooling and after-cooling coil and the hot in Table 1), the required total cooling coil demand for the pre-cooling and after-cooling coil and the water demand for the regenerating coil of the DAHU are 355 kW and 124 kW, respectively. For chilled hot water demand for the regenerating coil of the DAHU are 355 kW and 124 kW, respectively. For water, in addition to the cooling demand of the DAHU, the coil demands of the SAHU (24.8 kW/ chilled water, in addition to the cooling demand of the DAHU, the coil demands of the SAHU (24.8 floor and 33.5 kW/ floor on a typical floor and the top floor, respectively) are considered. In total, kW/ floor and 33.5 kW/ floor on a typical floor and the top floor, respectively) are considered. In total, the chilled water demand is 488.2 kW. For the solar collector, the average collection factor is set at 50%; the chilled water demand is 488.2 kW. For the solar collector, the average collection factor is set at therefore, the area of the collector is set at 242 m2. Table2 shows the specifications of each system. 50%; therefore, the area of the collector is set at 242 m2. Table 2 shows the specifications of each The specifications of the components with fixed controls are shown in Table2. system. The specifications of the components with fixed controls are shown in Table 2. Table 1. Design conditions for the desiccant air handling unit with fixed controls. Table 1. Design conditions for the desiccant air handling unit with fixed controls. Air state No. as Shown Temperature Humidity Air state No. as shown in Temperature Humidity Assumption and so on in Figure3 ◦C g/kg’ Assumption and so on Figure 3 °C g/kg’ 1 32.5 19.9 Outdoor condition given by load calculation Outdoor condition given by load 1 32.5 19.9 2 18.3 12.3 Relativecalculation humidity is 95% 32 33.3 18.3 6.5 12.3 Relative Relative humidity humidity efficiency is 95% is 90% 3 33.3 6.5 Relative humidity efficiency is 90% 4 29.1 6.5 Efficiency is 80% 4 29.1 6.5 Efficiency is 80% 5 15.4 6.5 SupplySupply air condition air condition at peak at peak time istime 15.4 is◦ C 5 15.4 6.5 6 28.0 9.4 Room air condition15.4°C (28 ◦C, 40%) 6 28.0 9.4 Room air condition (28°C, 40%) 7 31.8 9.4 Sensible heat exchange efficiency is 80% 7 31.8 9.4 Sensible heat exchange efficiency is 80% 8 50.0 9.4 AbsoluteAbsolute humidity humidity is the is the same same as returnas return air 8 50.0 9.4 1 In this assumption, the capacity of dehumidification wheel and heat transfer from regenerationair side to process 1 sideIn this due assumption, to rotation is notthe considered.capacity of dehumidification wheel and heat transfer from regeneration side to process side due to rotation is not considered.

Sustainability 2020, 12, 1921 6 of 18

Table 2. Speciﬁcations of the system.

Abbreviations Rated Specifications Number Cooling capacity: 265 kW; electricity Air-source HP ASHP 2 consumption: 66 kW Evacuated tube—type SOL 242 m2, tilt angle: 15, facing to the south solar collector Thermal storage tank for TANK 10 m3 1 the solar thermal system SOL Desiccant material: polymer sorbent Supplied air volume: 22,115 m3/h; exhaust air volume: 19,904 m3/h For the fixed control: Pre-cooling coil: 253.0 kW; after-cooling coil: Desiccant air handling DAHU 102 kW; regenerating coil: 123.0 kW 1 unit For the flexible control: Pre-cooling coil: 361 kW; after-cooling coil: 32 kW; regenerating coil: 123 kW (the regenerating coil and after cooling coil demand is based on the fixed control) Supply/ Return air: 6612 m3/h; cooling coil: Sensible air handling 4 SAHU 25 kW (on a typical floor) unit Supply/ Return air: 8941 m3/h; cooling coil: 1 33.5 kW (on the top floor) Flow rate: 760l/min Chilled water pump PUMP 2 chiller Head: 147 kPa Chilled water pump Flow rate: 474l/min PUMP 3 (AHU side) AHU Head: 441 kPa Flow rate: 240l/min Solar collection pump PUMPs 1 Head: 348 kPa Flow rate: 240l/min Solar utilization pump PUMPsu 1 Head: 258 kPa The capacity of the chiller is determined to cover the chilled water demand for both the fixed and flexible control.

3.2. Flexible Control Figure5 shows the flexible control proposed in this study. The flexible control controls the conditioned air leaving the pre-cooling coil with the absolute humidity of the supply air and the hot water temperature entering the regenerating coil. When the hot water temperature is not sufficiently hot, the temperature leaving the pre-cooling coil decreases, and the dehumidification amount at the pre-cooling coil increases. However, when the hot water temperature is very hot, the temperature leaving the pre-cooling coil increases, and the dehumidification amount at the pre-cooling coil decreases, which results in a larger dehumidification amount at the dehumidification wheel. The flow of the system is shown below: Step 1: When the temperature in the tank is above the set point, the hot water from the tank is supplied to the regenerating coil. Step 2: The regeneration temperature (T8) is determined based on the hot water from the tank. Step 3: Based on the regeneration temperature determined in Step 2, if the absolute humidity (AH3) is higher than the set point of supply for absolute humidity, then the set temperature leaving the pre-cooling coil (T2) decreases, and the absolute humidity leaving the pre-cooling coil (AH2) also decreases at the same time until the absolute humidity (AH3) fulfills the set point of absolute humidity. Sustainability 2020, 12, 1921 7 of 18

decreases at the same time until the absolute humidity (AH3) fulfills the set point of absolute Sustainabilityhumidity. 2020, 12, 1921 7 of 18

Absolute humidity [g/kg’]

(a) 9 2high 1

8high 6

5 3high

Dry bulb temperature [℃] Regeneration temperature becomes high Hot water Low High temperature at the regenerating coil Absolute humidity [g/kg’] (b) 1

2low 9 6 8low

5 3low Dry bulb temperature [℃] Regeneration temperature becomes low Hot water Low High temperature at the regenerating coil

Figure 5. Diagram of the flexible control. (a) Air state in the DAHU when the hot water temperature at the regenerating coil is high; (b) Air state in the DAHU when the hot water temperature at the Figure 5. Diagram of the flexible control. (a) Air state in the DAHU when the hot water temperature regenerating coil is low. at the regenerating coil is high; (b) Air state in the DAHU when the hot water temperature at the Inregenerating the flexible coil control, is low. for the design phase, the air state leaving the pre-cooling coil is set at 13 ◦C, which is sufficient to dehumidify the air without solar thermal energy. The required total pre-coolingIn the flexible cooling control, coil and for after-cooling the design phase, coil demand the air state for the leaving DAHU the ispre-cooling 393kW (361kW coil is +set32kW, at 13 respectively).°C, which is sufficient For the chiller,to dehumidify in addition the toair thewithout pre-cooling solar thermal and after-cooling energy. The coil required demands total of pre- the DAHU,cooling thecooling coil demandcoil and ofafter-cooling the SAHU (24.8coil kWdemand/floor andfor the 33.5 DAHU kW/ floor is on393kW a typical (361kW floor + and32kW, on therespectively). top floor, respectively)For the chiller, is considered.in addition to In th total,e pre-cooling the chilled and water after-cooling demand is coil 526.6kW. demands The of same the regenerationDAHU, the coil coil demand and solar of the thermal SAHU collector (24.8 kW/floor with fixed and controls33.5 kW/ were floor installed. on a typical Table floor2 shows and on the the specificationstop floor, respectively) of the system is consi withdered. both fixed In total, and flexible the chilled controls. water demand is 526.6kW. The same regeneration coil and solar thermal collector with fixed controls were installed. Table 2 shows the 4.specifications Outline of theof the Simulation system with both fixed and flexible controls. The simulation used in this study was done with the LCEM tool [16,17], which was developed 4. Outline of the Simulation under the supervision of the Ministry of Land, Infrastructure, Transport, and Tourism. This is a simulationThe simulation tool based used on Microsoft in this study Excel. was done with the LCEM tool [16,17], which was developed under the supervision of the Ministry of Land, Infrastructure, Transport, and Tourism. This is a simulation tool based on Microsoft Excel. Sustainability 2020, 12, 1921 8 of 18

4.1. Dehumidification Wheel In this tool, factor of dehumidification wheel is coefficient of relative humidity. Coefficient of relative humidity is defined as equation (1). The desiccant material in this study is a polymer sorbent [18,19]. The relative humidity entering the dehumidification wheel of the process also affects the performance of the DAHU. According to the measurement results for the polymer sorbent desiccant air handling unit installed in an office building in Japan (please see appendix Figure A1), when the relative humidity entering the dehumidification wheel is over 80%, the coefficient of relative humidity is constant at around 0.9. Thus, in this study, the value is set to 0.90.

RH RH η = 2 − 3 (1) RH RH RH 2 − 8 4.2. Coils The calculation method is based on the factor of a wetted surface, which is traditionally employed for designing coils in Japan. In this method, the process for calculating the number of coil columns is used inversely to provide the outlet conditions of the air and water. The outlet temperature of the water and its ﬂow rate will be calculated by Equations (2)–(19).

qc = ROW KF dtlm AF WSF (2) × × × × a1 KF = (3) 1 + 1 a u f a3+a a vwa6+a 2× 4 5× 7 WSF = a SHF2 + a SHF + a (4) 8 × 9 × 10 dt1 dt2 dtlm = − (5) dt1 ln dt2

dt1 = T Tcwo (6) cai − dt2 = Tcao T (7) − cwi dt1 Z = (8) dt2 D = ROW KF AF WSF (9) × × × dt2 (Z 1) qc = D × − (10) lnZ

qc = V ρ (H Hcao) (11) air × air cai − Q E = w (12) D dt2 × E lnZ = Z 1. (13) × − Equation (13) is a transcendental equation. Therefore, the approximate positive solution from Equations (14) to (16) are used:

1 If 0 < E < 0.1 Z = exp − (14) E

If 0.1 E < 1 (15) ≤ If 1 E < 100 ≤ 0.32473 + 0.320074 E + 1.27473 E2 + 0.0371635 E3 Z = − × × × (16) 1 + 0.304950 E + 0.00456563 E2 × × Sustainability 2020, 12, 1921 9 of 18

Tcwo = T Z dt2 ( f or cooling) (17) cai − × Tcwo = T + Z dt2 ( f or heating) (18) cai × 60 qc Wcoil = × . (19) Cw (T Tcwo) × cwi − Based on the ﬂow rate calculated by Equation (19), the ﬂow rate in the coils is provided again; then, the KF from Equation (3) continues to be calculated until reaching convergence.

4.3. Sensible Heat Exchanger The sensible heat exchanger is modelled with a face velocity and air volume ratio suﬃcient to provide eﬃciency, as shown in Equations (20) and (21); a to l are the default values.

2 SECpro = (br + c)Upro + (dr + e)Upro + ( f r + g) (20)

2 SECre = (h/r + i)Ure + (j/r + k)Ure + (l/r + m) (21)

4.4. Solar Collector The solar thermal collector object is modelled by Equations (22)–(24) [20]:

n∆T η = + o (22) col J

Tcol in + Tcol out ∆T = − − T (23) 2 − amb

cρWcol(Tcol out Tcol in) Q = η JA = − − − . (24) col col 3600 Coefficients n and o are 0.473 and 0.561, respectively, for the specified values of evacuated tube − type collectors. The collectors are divided into two modules. The area of each module is 121 m2, and the modules are arrayed in parallel. The collectors face the south, and their tilt angle is 15◦. For flat type collectors, n and o are 0.393 and 0.840, respectively. − The collection pump operates when the temperature difference between the inlet and outlet temperature of the collectors is larger than 1.0 ◦C, and the temperature in the tank is lower than 90 ◦C.

4.5. Thermal Storage Tank The characteristics of the thermal storage tank are assumed to be completely mixed. The thermal storage tank is modelled by Equation (25).

dTts cρVts = KtsSts(T Tts) + Q + Qsup. (25) dt amb − col

When the temperature in the tank is higher than 55 ◦C, hot water from the tank is supplied to the regenerating coil in the DAHU in both the ﬁxed and ﬂexible controls.

4.6. Boundary Conditions The same outdoor conditions, including temperature, humidity, and solar radiation, used for the load calculations are provided. The supply and return air volumes are stable at 22,155 m3/h and 19,904 m3/h, respectively. Based on the load calculations, the supply set point for the desiccant air handling unit is given as the boundary condition. Sustainability 2020, 12, 1921 10 of 18

Sustainability4.7. Simulation 2020, Cases12, 1921 10 of 18 In this study, we used four cases, as shown in Table3. For the fixed control, an evacuated tube type Table 3. Outline of simulation cases. of solar thermal collector is installed. For the flexible control, a flat plate collector is also considered, based on the reportCase name in [21 ]. In 2017, flat plate Control collectors were Solar ranked Collector second Type place in terms Backup of their Boile totalr installed capacityCase inFI-ET operation according Fixed to collector control type, but Evacuated the report tube says type that the share None of flat plate Case FL-ET Evacuated tube type None collectors increased by 10% in 2017. In bothFlexible fixed control and flexible control systems, chilled water at 7 ◦C is Case FL-FP Flat plate type None supplied to each coil. Furthermore, for the fixed control, a system with a backup boiler is considered. Case FI-ET with backup boiler Fixed control Evacuated tube type √ Table 3. Outline of simulation cases. 5. Simulation Results and Evaluation Case Name Control Solar Collector Type Backup Boiler The simulation is conducted for July and August. First, the diurnal behavior is verified to Case FI-ET Fixed control Evacuated tube type None determine the air state in the DAHU and the condition of solar thermal utilization. Then, energy Case FL-ET Evacuated tube type None performance is analyzed over two monthsFlexible control Case FL-FP Flat plate type None 5.1. DiurnalCase FI-ETBehavior with backup boiler Fixed control Evacuated tube type √

Figure 6, Figure 7 show the diurnal behavior in the DAHU on the representative day, 11th 5. Simulation Results and Evaluation August. In both figures, each number corresponds to the numbers in Figure 3. ForThe the simulation fixed control, is conducted as shown for in JulyFigure and 6, August.the temperature First, the leaving diurnal the behaviorpre-cooling is verifiedcoil (T2) tois stabledetermine at 18.8 the °C, air and state the in required the DAHU regeneration and the conditiontemperature of solar(T8) is thermal 50 °C. On utilization. the other Then,hand, energyfor the performanceflexible control is shown analyzed in Figure over two 7 (a), months the temperature leaving the pre-cooling coil (T2) varies between 14 and 20 °C, and the required regeneration temperature (T8) varies from 54 to 56 °C. During the calculation5.1. Diurnal period, Behavior the maximum regeneration temperature was 68.2 °C. TheFigures absolute6 and 7humidity show the supply diurnal (AH5) behavior (as shown in the DAHU in Figure on 6(b)) the representative for fixed control day, does 11th not August. fulfill theIn both set point figures, at 6.78 each g/kg’ number at 10:00, corresponds 12:00, 15:00, to the an numbersd 17:00. This in Figure is because3. the temperature in the tank is notFor high the enough, fixed control, and hot as water shown from in Figure the tank6, the is not temperature provided leavingto regenerate the pre-cooling the coil, as coil shown (T2) isin Figurestable at6(d). 18.8 ◦C, and the required regeneration temperature (T8) is 50 ◦C. On the other hand, for the flexibleOn controlthe other shown hand, in the Figure flexible7a, thecontrol temperature shown in leaving Figure the7(b) pre-cooling fulfills the coilset point (T2) varies of the betweenabsolute 14humidity and 20 supply◦C, and (AH5), the required even when regeneration the solar temperature energy is not (T8) provided varies from to the 54 regenerating to 56 ◦C. During coil (for the example,calculation at period,10:00, 11:00, the maximum 13:00, 15:00, regeneration and 17:00). temperature At these times, was 68.2the required◦C. chilled water demand becomesThe absolutelarge, and humidity absolute supply humidity (AH5) leaving (as shown the pre-cooling in Figure6b) coil for fixed(AH2) control is low does (below not fulfillaround the 9 g/kg’)set point in order at 6.78 to g /compensatekg’ at 10:00, 12:00,for the 15:00, lack of and hot 17:00. water. This is because the temperature in the tank is not high enough, and hot water from the tank is not provided to regenerate the coil, as shown in Figure6d.

40 25

35 ]

℃ 20

] 30 ℃ 25 15 20 15 10

temperature [ 10 5

5 g/kg'absolute humidity 0 0 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 time time

T1 T2 T3 T4 T5 AH1 AH2 AH3 AH4 AH5

(b) Absolute humidity of the (a) Temperature of the processed side processed side

Figure 6. Cont. Sustainability 2020, 12, 1921 11 of 18 Sustainability 2020, 12, 1921 11 of 18

60 60 500 50 59 450

] 58 400 ] ℃ 40 57 350 ℃ 56 300 Sustainability30 2020, 12, 1921 55 250 11 of 18 54 200 20 53 150 temperature [ temperature [ 52 100 [MJ/h] heat amount 10 51 50 60 50 0 0 60 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 500 50 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 59 time 450

] time 58 400

] solar collection coil demand to regenearting coil temp. in tank ℃ 40 57 350

T6 T7 T8 T9 ℃ 56 300 30 55 250 54 200 20 53 150 temperature [ temperature [ (d) Solar collection, coil demands of 52 100 [MJ/h] heat amount 10 (c) Temperature of the 51 regeneration coil, and temperature50 in 50 0 0 regeneration side 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 the tank time time solar collection coil demand to regenearting coil temp. in tank T6 T7 T8 T9 FigureFigure 6. 6. DiurnalDiurnal behavior behavior for for the the fixed fixed control control wi withth an an evacuated evacuated tube tube type type collector. collector. On the other hand, the flexible control shown in Figure(7db) Solar fulfills collection, the set point coil demands of the absolute of 45 (c) Temperature of the 25

humidity40 supply (AH5), even when the solar energy] is notregeneration provided to coil, the and regenerating temperature coil in (for 35 regeneration side ℃ 20 example,] at 10:00, 11:00, 13:00, 15:00, and 17:00). At these times,the tank the required chilled water demand ℃ 30 15 becomes25 large, and absolute humidity leaving the pre-cooling coil (AH2) is low (below around 9 g/kg’) 20 Figure 6. Diurnal behavior for the fixed control with an evacuated tube type collector. 10 in order15 to compensate for the lack of hot water. temperature [ 10 5

5 absoluteg/kg' humidity 45 25 0 0 40 ] 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 35 ℃ 20 ] time time

℃ 30 T1 T2 T3 T4 T5 15 AH1 AH2 AH3 AH4 AH5 25 20 10 15

temperature [ (b) Absolute humidity of the (a10) Temperature of the processed side 5 5 processedabsoluteg/kg' humidity side 0 0 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 60 time 60 time 600 50 T1 T2 T3 T4 T5 59 AH1 AH2 AH3 AH4 AH5 500

] 58

] ℃ 40 57

℃ 400 56 30 55(b) Absolute humidity of the 300 (a) Temperature of the processed side 54 20 53processed side 200 temperature [ temperature [ temperature 52 heat amount[MJ/h] 10 100 51 60 50 0 0 60 600 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 50 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 59 time 500

] time 58 ]

℃ 40 57 solar collection coil demand to regenearting coil temp. in tank T6 T7 T8 T9 ℃ 400 56 30 55 300 54 20 53 200 temperature [ ([ temperature d) Solar collection, coil demands of the 52 heat amount[MJ/h] 10 100 (c) Temperature of the regeneration side regeneration51 coil, and temperature in 50 0 0 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 the tank time time solar collection coil demand to regenearting coil temp. in tank T6 T7 T8 T9 Figure 7. Diurnal behavior of the flexible control with an evacuated tube type collector. 5.2. Performance Evaluation (d) Solar collection, coil demands of the (c) Temperature of the regeneration side regeneration coil, and temperature in For evaluation, the performance of the desiccant air handling unit itself is determined with the tank equation (8). For primary energy performance, the desiccant primary energy coefficient of performanceFigure (hereafter, 7. Diurnal COP) behavior (COPdes.primary of the flexibleflexible) and the control primary–energy with an evacuatedevacuated base tubetube system typetype COP collector.collector. (COP sys.primary) are defined with Equations (9) and (10), respectively. COPdes.primary provided us with the idea for a comparison5.2.5.2. Performance of normal Evaluation cooling, in which chilled water processes the sensible and latent heat load. The COP ofForFor the evaluation,evaluation, normal cooling the performanceperformance is predominated of thethe by desiccantdesiccantthe COP of airair the handlinghandling chillers unitbecauseunit itselfitself the isis total determineddetermined sensible withandwith latentequationequation heat (8). loads(8). For For are primary assumedprimary energy toenergy be performance, equal performance, to the the chilled desiccant the water desiccant primary demands. energyprimary Therefore, coe ffienergycient if the ofcoefficient COP performancedes.primary of is(hereafter,performance better than COP) the(hereafter, (COP COPdes.primary of COP) the chiller (COP) and des.primaryin the the primary–energy primary) and the energy primary–energy base base, system installing COP base (COP system a DAHUsys.primary COP is (COPeffective) aresys.primary defined for ) energyare defined conservation. with Equations (9) and (10), respectively. COPdes.primary provided us with the idea for a comparison of normal cooling, in which chilled water processes the sensible and latent heat load. The COP of the normal cooling is predominated by the COP of the chillers because the total sensible and latent heat loads are assumed to be equal to the chilled water demands. Therefore, if the COPdes.primary is better than the COP of the chiller in the primary energy base, installing a DAHU is effective for energy conservation. Sustainability 2020, 12, 1921 12 of 18

with Equations (9) and (10), respectively. COPdes.primary provided us with the idea for a comparison of normal cooling, in which chilled water processes the sensible and latent heat load. The COP of the normal cooling is predominated by the COP of the chillers because the total sensible and latent heat loads are assumed to be equal to the chilled water demands. Therefore, if the COPdes.primary is better than the COP of the chiller in the primary energy base, installing a DAHU is effective for Sustainability 2020, 12, 1921 12 of 18 energy conservation. COPdes = Qpro s/ Qpre + Qreg + Qa f ter (26) 𝐶𝑂𝑃 =𝑄 −⁄𝑄 +𝑄 +𝑄 (16) COPdes.primary = Qpro s/ Epre + Ere + Ea f ter (27) 𝐶𝑂𝑃. =𝑄−⁄𝐸 +𝐸 +𝐸 (17) COP = (Q + Q )/ E + E . (28) 𝐶𝑂𝑃.sys.primary = (𝑄c+𝑄H)⁄𝐸pump+𝐸HP. (18) FigureFigure 88 shows shows the the processed processed sensible sensible and and latent latent heat heat load load for for the the DAHU DAHU and and each each coil coil demand. demand. TheThe COP COPdesdes withwith a flexible a flexible control control and and fixed fixed control control has has the the almost almost same same value, value, around around 0.85. 0.85. The The total total unprocessedunprocessed latent latent heat heat load load with with a afixed fixed control control is is 28 28 GJ GJ for for two two months, months, which which accounts accounts for for 5% 5% of of thethe required required heat heat load load processed processed by by the the DAHU. DAHU. The The chilled chilled water water coil coil demand demand for for the the DAHU DAHU with with a a flexibleflexible control control is is larger larger than than that that with with a a fixed fixed control because itit isis necessarynecessary toto lowerlower the the supply supply air air to toabout about 14 14◦ C°C when when solar solar thermal thermal energy energy cannot cannot be be utilized. utilized.

500 1.0 450 0.9 0.84 0.85 0.85 400 114 113 0.8 350 138 0.7 300 94 94 0.6 246 246 250 218 89 0.5

200 0.4 COP [-] 150 0.3 253 254 100 207 0.2 [GJ/2months] 146 146 146 50 0.1 0 0.0 Case CaseFI-7- FI-ET Case FL-7-Case FL-ET Case FL-7-Case FL-FP ET ET FP processed heat amount, coil heat processeddemand amount, processed sensible heat processed latent heat coil demand at pre-cooling coil coil demand at regenerating coil coil demand at after-cooling coil COPdes

Figure 8. Processed heat, coil demands, and COP of the desiccant air handling unit over two months. Figure 8. Processed heat, coil demands, and COP of the desiccant air handling unit over two Figure9 shows the collected heat and solarmonths. energy supplied to the regenerating coil for two months. The collected heat is almost 106 GJ for both the fixed and flexible control with an evacuated tubeFigure type collector9 shows andthe collected 112 GJ for heat the and flexible solar control energy with supplied a flat typeto the collector. regenerating The systemcoil for with two a months.flexible The control collected can supply heat is more almost hot 106 water GJ for than both the the fixed fixed control and toflexible regenerate control the with coil. an In evacuated the system tubewith type a fixed collector control, and almost 112 GJ 84% for of the the flexible collected cont heatrol is with supplied a flat to type the regeneratingcollector. The coil. system In the with system a flexiblewith a control flexible can control supply and more an evacuated hot water tube than type the fi collector,xed control approximately to regenerate 88% the ofcoil. the In collected the system heat withis supplied a fixed control, to the regeneratingalmost 84% of coil. the collected For a flat heat plate is supplied collector, to the the ratio regenerating of solar thermalcoil. In the utilization system with(=83%) a flexible is slightly control lower and than an evacuated that of the tube system type with collector, an evacuated approximately tube type 88% collector. of the collected This is because heat isthe supplied temperature to the inregenerating the tank with coil. a flatFor type a flat collector plate collector, is lower thanthe ratio that of with solar an evacuatedthermal utilization tube type (=83%)collector. is slightly Even when lower hot than water that from of the the system tank is wi suppliedth an evacuated to the regenerating tube type collector. coil, the usable This is amount because of thehot temperature water becomes in the low. tank However, with a flat the type COP collecto of solarr is thermal lower than utilization that with ishighest an evacuated with the tube flat type type collector.collector Even among when the hot analyzed water casesfrom becausethe tank theis su lowerpplied temperature to the regenerating in the tank coil, results the usable in higher amount solar ofcollection hot water e ffibecomesciency andlow. aHowever, larger amount the COP of heat of solar collected thermal hourly. utilization is highest with the flat type collector among the analyzed cases because the lower temperature in the tank results in higher solar collection efficiency and a larger amount of heat collected hourly. Sustainability 2020, 12, 1921 13 of 18

120 14.0 83％ 100 84％ 88％ 12.0 10.0 11.8 9.2 10.0 80 SustainabilitySustainability 20202020, 12, 12, 1921, 1921 8.0 13 13of of18 18 60 112GJ 106GJ 107GJ 6.0 94GJ 93GJ COP[-] 40 89GJ 120 14.04.0 20 83％ heat amount [GJ/2months]100 84％ 88％ 12.02.0 10.0 11.8 0 9.2 10.00.0 80 Case FI-ET Case FL-ETCase FL-ET Case FL-FPCase FL-FP Case FI-ET 8.0 60 112GJ 106GJ 107GJ 6.0 collected heat amount 94GJsolar thermal energy to93GJ regenerating coilCOP[-] 40 89GJ 4.0 COP of solar themrmal utilization 20 heat amount [GJ/2months] 2.0 0 0.0 Figure 9. Collected heat and solar energy supplied to the regenerating coil over two months. Case FI-ETCase FI-ET Case FL-ETCase FL-ET Case FL-FPCase FL-FP As shown in Section 5.1, FI-ET does not process the latent load well because there is no backup heater in case thecollected temperature heat amount in the tank is too low tosolar supply thermal the energy regenerating to regenerating coil. coil For the system evaluation, a backup boiler with 80% primary energy efficiency is considered for FI-ET only. COP of solar themrmal utilization Figure 10 shows each coil demand for the DAHU and indoor sensible air handling unit, as well as the primaryFigure 9.energyCollected consumption. heat and solar energy supplied to the regenerating coil over two months. InFigure all cases, 9. Collected most of heat the and primary solar energy energy supplied consumption to the regenerating is dominated coil overby the two heat months. pump, which accountsAs shown for about in Section 80%. 5.1For, FI-ETthe FI-ET does notcase process with a the backup latent loadboiler, well additional because therehot water is no backupfor the heaterregeneratingAs shown in case in thecoil Section temperatureis necessary, 5.1, FI-ET in thesodoes the tank not regenerati is process too low theng to latent supplycoil demand load the well regenerating is because larger therethan coil. Foristhat no the backupof systemFI-ET. heaterevaluation,Furthermore, in case athe backup the temperature primary boiler energy with in the 80% consumption tank primary is too energylow for tothe esupply ﬃboilerciency isthe isadded. regenerating considered for coil. FI-ET For only.the system evaluation,FigureApproximately a backup10 shows boiler eacha 10% with coil reduction demand 80% primary forin the the ener primar DAHUgy efficiencyy and energy indoor isconsumption considered sensible air for handlingfor FI-ET FL-ET only. unit, and as FL-FP well asis theachievedFigure primary compared10 energyshows consumption.eachto the coil FL-ET demand case forwith the a backupDAHU andboiler. indoor sensible air handling unit, as well as the primary energy consumption. In all cases, most600 of the primary energy consumption is dominated by the heat pump, which accounts for about 80%. For the FI-ET case with a backup boiler, additional hot water for the regenerating coil is necessary, so the regenerating coil demand is larger than that of FI-ET. Furthermore, the primary400 energy consumption for the boiler is added. 339 311 308 306 Approximately a 10% reduction in the primary energy consumption for FL-ET and FL-FP is achieved compared to the FL-ET case with a backup boiler. 200 600 coildemand [GJ/2months]

primary energy primary[GJ/2months] energy 0 400 CaseCase FI- FI-ET Case FL- CaseCase FL- FL-FP 339CaseCase FI- FI-ET ET CaseET FL-ET FP ET (with 311 308 306 (with back up boiler) BB) primary energy of additional back up boiler primary energy of solar utilizatin pump primary200 energy of solar collectin pump primary energy for chilled water pump (AHU side) primary energy for chilled water pump (chiller side) Primary energy of HP coil demand for indoor SAHU coil demand at after-cooling coil coildemand [GJ/2months]

coil primary[GJ/2months] energy demand0 at regenerating coil coil demand at pre-cooling coil

CaseCase FI- FI-ET Case FL- CaseCase FL- FL-FP CaseCase FI- FI-ET ET CaseET FL-ET FP ET (with Figure 10. Coil demand for the DAHU and sensible AHU, as well as the(with primary back energy up boiler) consumption. Figure 10. Coil demand for the DAHU and sensible AHU, as well as the primaryBB) energy consumption. In allprimary cases, energy most of of additional the primary back up boiler energy consumptionprimary energy is dominatedof solar utilizatin by pump the heat pump, which accounts forprimary about energy 80%. of Forsolar thecollectin FI-ET pump case with a backupprimary boiler, energy additional for chilled water hot pump water (AHU for side) the regenerating primary energy for chilled water pump (chiller side) Primary energy of HP coil is necessary, so the regenerating coil demand is larger than that of FI-ET. Furthermore, the primary coil demand for indoor SAHU coil demand at after-cooling coil energy consumption for the boiler is added. coil demand at regenerating coil coil demand at pre-cooling coil Approximately a 10% reduction in the primary energy consumption for FL-ET and FL-FP is achieved compared to the FL-ET case with a backup boiler. Figure 10. Coil demand for the DAHU and sensible AHU, as well as the primary energy consumption. Figure 11 shows the COPdes.primary and COPsys.primary for each case. Both COPs for the ﬂexible control are higher than those for the ﬁxed control. Systems with an evacuated tube type collector or a

Sustainability 2020, 12, 1921 14 of 18 Sustainability 2020, 12, 1921 14 of 18

Figure 11 shows the COPdes.primary and COPsys.primary for each case. Both COPs for the flexible control are higher than those for the fixed control. Systems with an evacuated tube type collector or flat plate collector can achieve around 1.55 of COPdes.primary and 1.77 of COP sys.primary. When installing a flat plate collector can achieve around 1.55 of COPdes.primary and 1.77 of COP sys.primary. When installing a backup boiler, both COPs dropped to 1.43 of COPdes.primary and 1.67 of COPsys.primary. This drop is acaused backup by boiler, installing both the COPs backup dropped boiler, forto which1.43 of the COP primarydes.primary energy and 1.67 base of COP COP (=sys.primary0.8) is. lower This thandrop the is causedsolar system by installing COP (= theabout backup 9.2). boiler, for which the primary energy base COP (= 0.8) is lower than the solar system COP (= about 9.2). 1.90 1.76 1.77 1.80 1.73 1.67 1.70 1.60 1.50 1.55 1.56 1.51 1.40 COP[-] 1.43 1.30 1.20 1.10 1.00 Case FI-ET Case FL-ET Case FL-FP Ca s e FI-ET （with back up COPdes.primary COPsys.primary boiler) Figure 11. COP and COP for each case. Figure 11. COPdes.primarydes.primary and COPsys.primarysys.primary for each case. 6. Discussion 6. Discussion As shown in Figures6b and 11, for the fixed control without a backup boiler, there is the possibility As shown in Figure 6(b) and Figure 11, for the fixed control without a backup boiler, there is the that the required latent heat load is not processed properly, which will lead to uncomfortable indoor possibility that the required latent heat load is not processed properly, which will lead to environments. If there is no stable heat source for the regenerating coil (e.g., a CHP, gas boiler, or electric uncomfortable indoor environments. If there is no stable heat source for the regenerating coil (e.g., a heater), a flexible control is recommended to be installed for the DAHU system. CHP, gas boiler, or electric heater), a flexible control is recommended to be installed for the DAHU Table4 summarizes the pros and cons of the proposed controls and systems. In terms of primary system. energy performance in an indoor environment, the system with a flexible control is better than the Table 4 summarizes the pros and cons of the proposed controls and systems. In terms of primary system with a fixed control. In addition, for the flexible control, the mechanical room will be smaller energy performance in an indoor environment, the system with a flexible control is better than the than that of the fixed control with a backup heat source, resulting in a higher effective area for the office system with a fixed control. In addition, for the flexible control, the mechanical room will be smaller and a larger rentable floor area ratio. than that of the fixed control with a backup heat source, resulting in a higher effective area for the However, additional costs will be necessary for this flexible control since flexible control office and a larger rentable floor area ratio. is advanced. However, additional costs will be necessary for this flexible control since flexible control is In hot and humid climates, such as in South East Asia, heating demands and domestic hot water advanced. demands in office buildings are low, so backup heaters or boilers for DAHUs are excessive equipment. Instead, using affluent solar thermal energy during the summer for a DAHU can contribute to more Table 4. Summary of the pros and cons of the proposed system (based on this study and the authors’ sustainable buildings. There is also the possibility to install photovoltaic cells instead of solar thermal opinions). collectors, but the efficiency of photovoltaics is still lower than that of solar thermal collectors, and it is Case FI-ET with a not currently possible to compensate for the fluctuationCase of solar energy. This means thatCase systems Case with Details of Pros and Cons Backup photovoltaic cells need batteries, which will lead to a highFI-ET installation cost. FL-ET FL-FP Boiler/Heater As stated in the introduction, global warming is a serious problem. This DAHU system with Effective use of solar thermal energy in flexible control will contribute to ameliorating the effects√ of global warming√ from the√ perspective √ of summer maintaining indoor environments with smaller energy consumption, as well as allowing the maximum Proper processed latent heat load √ √ √ usePros of renewableHigher energy, primary such energy as solar base thermal COP energy, especially√ in hot and humid climates.√ √ Lower initial cost for a flat plate type collector √ than an evacuated tube type collector Smaller space for a mechanical room √ √ √ Cons Higher initial cost for a backup device √ Additional costs for advanced control √ √

In hot and humid climates, such as in South East Asia, heating demands and domestic hot water demands in office buildings are low, so backup heaters or boilers for DAHUs are excessive equipment. Sustainability 2020, 12, 1921 15 of 18

Table 4. Summary of the pros and cons of the proposed system (based on this study and the authors’ opinions).

Case FI-ET with Case Case Case Details of Pros and Cons a Backup FI-ET FL-ET FL-FP Boiler/Heater Eﬀective use of solar thermal energy √ √ √ √ in summer Proper processed latent heat load √ √ √ Higher primary energy base COP √ √ √ Pros Lower initial cost for a ﬂat plate type collector than an evacuated tube √ type collector Smaller space for a mechanical room √ √ √ Cons Higher initial cost for a backup device √ Additional costs for advanced control √ √

7. Conclusions This study proposed a new “flexible control” for a desiccant air handling unit, which only uses solar energy for its regenerating heat coil. System simulations, including an air conditioning system and a heat source, were conducted to compare flexible control and traditional “fixed control” systems, and the results are provided below.

The desiccant air handling unit with flexible control can correctly process the required latent heat • load, even when solar thermal energy cannot be supplied to the regenerating coil. The desiccant air handling unit with a traditional fixed control cannot process the required latent • heat load when solar thermal energy is not supplied to the regenerating coil. This unprocessed latent load reaches 28 GJ for July and August. This result proves that a traditional fixed control requires a backup heat source, such as a boiler, electric heater, or combined heat and power. Systems with flexible control have better primary energy performance than systems with fixed • control. Approximately 10% of primary energy consumption is achieved under flexible control compared to fixed control with a backup boiler. For the case with an installed evacuated tube type collector, the desiccant COP and primary energy-base system COP of the flexible control reached 1.76 and 1.55 respectively. The desiccant COP and primary energy-base system COP of the fixed control with a backup boiler decreased energy performance by about 5% (1.67 and 1.43, respectively). For flexible control, there is little difference in the primary energy performance between systems • with an evacuated tube type collector and those with a flat type collector. A desiccant air handling unit without a backup heat source will contribute to the wider installation of desiccant air handling units with solar thermal collectors, especially in hot and humid climates.

Author Contributions: Conceptualization, M.U.; Supervision, M.O. and H.T. All authors have read and agree to the published version of the manuscript. Funding: This work was supported by the JSPS Early-Career Scientists Grant Number 19K15148. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

A area of collector [m2] AV air volume [m3/h] C heat capacity [J/kgK] COP coefficient of performance Sustainability 2020, 12, 1921 16 of 18 dtlm logarithmic mean temperature difference [K] AF face area of coil [m2] E primary energy consumption H enthalpy [J/kg] J irradiation [J/m2] K heat loss factor [J/Km2] KF heat transfer coefficient [W/ROWm2K] Q heat amount [J] qc heat amount [W] r air volume ratio (=AVpro/AVre)[ ] − RH relative humidity [%] ROW number of coils [ ] − S surface area [m2] SEC sensible heat exchange coefficient [ ] − SHF sensible heat factor [ ] − T Temperature [◦C] U face velocity of DAHU [m/s] uf face velocity of coil [m/s] V volume [m3] vw flow rate of water in coil [m/s] W flow rata [m3/h] WSF coefficient of the wetted surface η relative humidity efficiency [ ] RH − η collection efficiency [ ] col − ρ density [kg/m3] Constant values from the manufacturers a1~a10 Constant value b, d, e, f, g, h, i, j, k, l, m, n, o

Subscript after after-cooling coil amb ambient air C chilled water cai air state entering coil cao air state leaving coil cwi water state entering coil cwo water state leaving coil col solar thermal collector col-in inlet of solar thermal collector col-out outlet of solar thermal collector des desiccant air handling unit des.primary primary energy base for desiccant air handling unit H hot water HP heat pump pre pre-cooling coil pro process side pro-s processed air Pump pumps for whole system re regeneration side reg regenerating coil RH relative humidity sup supplied to regenerate the coil from the thermal storage tank sys.primary primary energy base for the whole system Sustainability 2020, 12, 1921 17 of 18 ts solar thermal storage tank Sustainability 2020, 12, 1921 17 of 18 w chilled or hot water of coils 2,3,8 number in Figure3 Appendix A AppendixFigure A A1 shows the relationship between the relative humidity at Num. 2 and the coefficient of the relativeFigure A1 humidity shows the in relationship two different between office the buildi relativengs: humidity Building at Num. A is 2in and Tokyo, the coe andfficient building of the relativeB is in humidityShizuoka in (both two di infferent Japan). office Data buildings: for the Building two buildin A is ings Tokyo, were and recorded building every B is in 1 Shizuokamin. These (both data in Japan). were Dataconverted for the to two one-hour buildings average were recorded data, as every shown 1 min. in Figure These A1. data The were data converted for building to one-hour A was average collected data, in as shown in Figure A1. The data for building A was collected in July and August 2017, and the data for building B wasJuly taken and August in July and 2017, August and the 2014. data According for building to this B relation, was taken we setin theJuly coe andfficient August of relative 2014. According humidity at to a constantthis relation, value ofwe 0.9 set because the coefficient the relative of humidityrelative humidity entering the at dehumidification a constant value wheel of 0.9 on because the process the siderelative was almosthumidity 95%. entering the dehumidification wheel on the process side was almost 95%.

1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 coefficient of relatibe humidity [-] relatibe of coefficient 50 60 70 80 90 100 relative humidity at num. 2 [%] (air state leaving pre-cooling coil of process side) A 201707 A 201708 B 201407 B 201408 Figure A1. Actual measurement data for the relationship between relative humidity and the coefficient Figure A1. Actual measurement data for the relationship between relative humidity and the of relative humidity for two different office buildings in Japan. coefficient of relative humidity for two different office buildings in Japan. References References 1. Energy Efficiency and Conservation Division, Agency for Natural Resources and Energy Minsistry of 1. Energy Efficiency and Conservation Division, Agency for Natural Resources and Energy Minsistry of Economy, Trade and Industry. Definition of ZEB and Future Measures Proposed by the ZEB Roadmap Economy, Trade and Industry. Definition of ZEB and Future Measures Proposed by the ZEB Roadmap Examination Comiittee. 2015. Available online: https://www.enecho.meti.go.jp/category/saving_and_new/ Examination Comiittee, 2015. Available online: saving/zeb_report/pdf/report_160212_en.pdf (accessed on 25 February 2020). https://www.enecho.meti.go.jp/category/saving_and_new/saving/zeb_report/pdf/report_160212_en.pdf 2. European Commission. Nearly Zero-Energy Buildings. Available online: https://ec.europa.eu/energy/ (accessed on 25 February, 2020). en/topics/energy-efficiency/energy-performance-of-buildings/nearly-zero-energy-buildings (accessed on 2. European Commission. Nearly Zero-Energy Buildings. Available online: 25 February 2020). https://ec.europa.eu/energy/en/topics/energy-efficiency/energy-performance-of-buildings/nearly-zero- 3. Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. Available online: https: energy-buildings (accessed on 25 February , 2020). //www.energy.gov/eere/buildings/zero-energy-buildings (accessed on 25 February 2020). 3. Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. Available online: 4. Ahmed, K.; Yoon, G.; Ukai, M.; Kurnitski, J. How to compare energy performance requirements of Japanese https://www.energy.gov/eere/buildings/zero-energy-buildings (accessed 25 February , 2020). and European office buildings. In Proceedings of the E3S Web of conference 111, Bucharest, Romania, 4. Ahmed, K.; Yoon, G.; Ukai, M.; Kurnitski, J. How to compare energy performance requirements of Japanese 26–29 May 2019. and European office buildings. In Proceedings of the E3S Web of conference 111, Bucharest, Romania, 26– 5. Tanimoto, M.; Iizuka, S.; Xuan, Y. Numerical study on thermal and wind environments in the Nagoya 29 May 2019. metropolitan area using WRF (Part14) Future projections of dry island phenomenon and its impact on human 5. Tanimoto, M.; Iizuka, S.; Xuan, Y. Numerical study on thermal and wind environments in the Nagoya thermal comfort. In Summaries of Technical Papers of Annual Meeting; Architectural Institute of Japan: Kobe, Japan,metropolitan 2014; pp. area 931–932. using (InWRF Japanese) (Part14) Future projections of dry island phenomenon and its impact on 6. Ge,human T.S.; thermal Dai, Y.J.; comfort. Wang, R.Z. In Summaries Review on of solar Technical powered Papers rotary of Annual desiccant Meeting wheel; Architectural cooling system. InstituteRenew. of Sustain. Japan: EnergyKobe, Japan, Rev. 2014 2014;, 39 pp., 476–497. 931–932. [CrossRef (In Japanese)]

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