energies

Article Energy-Saving Analysis of Solar Heating System with PCM Storage Tank

Juan Zhao 1, Yasheng Ji 1, Yanping Yuan 1,*, Zhaoli Zhang 1 and Jun Lu 2 1 School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China; [email protected] (J.Z.); [email protected] (Y.J.); [email protected] (Z.Z.) 2 School of Urban Construction & Environment Engineering, Chongqing University, Chongqing 400045, China; [email protected] * Correspondence: [email protected]; Tel./Fax: +86-028-8763-4937

Received: 18 November 2017; Accepted: 15 January 2018; Published: 19 January 2018

Abstract: A solar heating system (SHS) with a phase change material (PCM) thermal storage tank is proposed with the view that traditional heat water storage tanks present several problems including large space requirements, significant heat loss and unstable system performance. An entire heating season (November–March) is selected as the research period on the basis of numerical models of the SHS-PCM. In addition, taking a public building in Lhasa as the object, the heating conditions, contribution rate of solar energy, and overall energy-saving capability provided by the heating system are analyzed under different PCM storage tanks and different terminal forms. The results show that an SHS with a PCM tank provides a 34% increase in energy saving capability compared to an ordinary water tank heating system. It is suggested that the design selection parameters of the PCM storage tank should specify a daily heat storage capacity that satisfies 70~80% of the entire heating season. A floor radiant system with supply/return water temperatures of 40/35 ◦C provides the optimal operation and the largest energy saving capability.

Keywords: flat-plate ; PCM storage tank; terminal forms; energy-saving analysis

1. Introduction Building heating with solar energy significantly reduces the use of non-renewable energy sources and relieves air pollution stress; these outcomes have obvious environmental and economic benefits. However, traditional solar heating systems present several issues, including intermittent operation, weather instability, and low energy density, etc. While traditional heat water storage tanks indicate problems, such as large space requirements and significant heat loss due to the sensible form of heat storage. Moreover, system performance is unstable owing to significant temperature fluctuation [1]. Extensive research has been performed on new types of solar thermal collectors [2–4] and phase change material (PCM) which together appear to be a promising solution as they possess high latent heat density and isothermal phase change characteristics [5,6]. Adding PCM into the heat storage unit cannot only reduce the volume but also allow heat to be stored and released at a nearly constant temperature, avoiding the shortcomings of traditional water tanks [7]. In order to address the issues of solar energy instability and heat storage insufficiency, an auxiliary heat source (AHS) is typically added into solar heating systems. Greater attention has been paid to the combination of solar energy and AHS, owing to its inconspicuous environmental benefit and operating cost; Moreover, it has become an important research topic. Extensive works have been carried out to explore the application of PCMs in solar thermal energy [8], such as solar power plants [9,10], solar air heaters [11], solar water heaters [12–14], solar desalination facilities, solar cookers, solar air conditioning [15,16], etc. Many forms of PCM

Energies 2018, 11, 237; doi:10.3390/en11010237 www.mdpi.com/journal/energies Energies 2018, 11, 237 2 of 18 storage units for use of building heating systems have been proposed by numerous scholars [17]. However, many issues still need to be overcome before they are being widely used. Belmonte et al. [18] presents a simple method for modeling the performance of the (TES) system that contains PCMs. The developed approach is similar in the concept to the bypass factor method, which is widely used in the air conditioning applications to estimate the outlet air conditions from a coil. The method can be implemented in whole-building simulation tools such as TRNSYS. An experimental investigation of a latent thermal energy storage (LTES) tank with a composite of paraffin was carried out by Zhang et al. [19]. PCMs and water temperature distribution were analyzed during the charging and discharging processes. A tube-in-tank LTES unit using paraffin as a PCM was built by Meng et al. [20]. The thermal performance of the LTES unit with the composite PCM was investigated experimentally, and a 3D numerical model was established to study the heat transfer mechanisms of the LTES unit. It was concluded that the LTES unit with the composite PCM showed good heat transfer performance. In addition, increasing the inlet flow velocity of the HTF and increasing the temperature difference between the HTF and PCM could enhance heat transfer and benefit thermal energy utilization. Lu et al. [21] proposed a water storage tank containing two types of PCMs with different melting points. Results show that the presence of PCMs with different melting points can significantly shorten heat storage time. An increase in PCM content in the tank leads to an evident increase in heat release. A performance comparison between SHS-PCM and SHS-without PCM has been put forward. Kumar et al. [22] analyzed the effect of adding PCM encapsulated in spherical capsules at the top of a hot water storage tank for stratification enhancements during the charging process. The temperature profile along the height of the storage tank was analyzed for TES systems with and without PCM capsules. The experimental results showed that the sensible-heat thermal energy storage system required less overall charging time than the system with PCM capsules. However, this difference in charging time was reduced with an increase in HTF inlet temperature. Talmatsky et al. [23] constructed a model to describe a heat storage tank with and without PCM, a collector, a pump, a controller, and an auxiliary heater. The results of all simulation scenarios indicated that the use of PCM in the storage tank does not yield a significant increase in the energy provided to the end user. The main reason for this undesirable effect was found to be the increased nighttime heat losses due to reheating the water by the PCM. Regarding SHS-PCM optimization, Najafian et al. [24] described how implementing PCM in a standard domestic hot water tank affects energy consumption and the discharge period, and suggested a tank optimization method that centers on their discharge time. Haillot et al. [25] placed PCM in the heat transfer fluid solar loop of an SDHW system, then studied the results under different weather conditions and system parameters. The parametric study highlighted a significant increase in system efficiency resulting from the PCM. Subsequently, a genetic algorithm allowed an optimized system configuration to be proposed. Padovan et al. [26] studied a simple SDHW that features a plane solar thermal collector, a boiler, and a PCM-enhanced storage tank. The PCM-enhanced tank was optimized using mono- and multi-objective genetic algorithms. At present, the dynamic performance of solar heating systems with PCM was mainly in experimental researches. The performance of a combined solar-heat pump system with energy storage in encapsulated PCM packing for residential heating was investigated both experimentally and theoretically at the Karadeniz Technical University by Kaygusuz et al. [27]. Zhang et al. [28] proposed a new solar energy-phase change storage-floor radiant heating system to provide a comfortable indoor environment in winter, which was installed in an office building in Urumqi, China. The results showed that it could provide a 30% increase in utilization of solar energy compared to a traditional solar heating system (SHS). A SHS-PCM should synthetically consider the energy efficiency over the entire heating cycle. Previous studies considered only a few days or hours and only particular operating conditions in their experimental investigations, and there is no available study that employs mathematical simulation or experiments to evaluate the performance of the entire system over an extended period. Energies 2018, 11, 237 3 of 18

Energies 2018, 11, 237 3 of 18 EnergiesBased 2018 on, 11 the, 237 numerical models of a solar heating system coupled with PCM thermal storage3 of 18 [29 ], the entireBased heating on the season numerical is chosen models as the of a research solar heating period, system and a coupled public building with PCM in Lhasathermal is storage taking as the[29], object. Basedthe Theentire on heating the heating numerical conditions, season models is chosen contribution of a assolar the heatingresearch rate of system solar period, energy, coupled and a and publicwith the PCM overallbuilding thermal energy in Lhasa storage saving is capability[29],taking the as provided entirethe object. heating by The the season heating heating is conditions, chosen system as contribution werethe research analyzed rateperiod, fromof solar and the energy,a following public and building the two overall aspects: in Lhasa energy daily is saving capability provided by the heating system were analyzed from the following two aspects: heattaking supply as the with object. AHS, The and heating the proportion conditions, of contribution heat supplied rate by of threesolar energy, sources and (AHS, the directoverall heating energy of savingdaily heat capability supply provided with AHS, by andthe heatingthe proportion system ofwere heat analyzed supplied from by threethe following sources (AHS,two aspects: direct flat-plate solar thermal collectors (FSTCs), and PCM storage tank). This analysis provides theoretical dailyheating heat of supplyflat‐plate with solar AHS,thermal and collectors the proportion (FSTCs), ofand heat PCM supplied storage by tank). three This sources analysis (AHS, provides direct support for the optimization and application of the system. heatingtheoretical of supportflat‐plate forsolar the thermal optimization collectors and (FSTCs), application and PCMof the storagesystem. tank). This analysis provides 2. Structurestheoretical support and Operation for the optimization Modes of SHS-PCM and application of the system. 2. Structures and Operation Modes of SHS‐PCM 2.1.2. Structures Structures and Operation Modes of SHS‐PCM 2.1. Structures 2.1.The Structures system under investigation is shown in Figure1 and represents a typical solar heating system The system under investigation is shown in Figure 1 and represents a typical solar heating with PCM storage tank (SHS-PCM). It is mainly composed of a solar collecting system (SCS), a phase systemThe with system PCM under storage investigation tank (SHS‐PCM). is shown It is mainlyin Figure composed 1 and represents of a solar collectinga typical systemsolar heating (SCS), change thermal storage system (PCTSS), and an indoor heating system (IHS). As the main thermal systema phase with change PCM thermal storage storage tank (SHS system‐PCM). (PCTSS), It is mainly and an composed indoor heating of a solar system collecting (IHS). system As the (SCS), main source, SCS is composed of FSTCs, a plate , pump 1, valves, pipelines, etc. As the athermal phase changesource, SCSthermal is composed storage system of FSTCs, (PCTSS), a plate and heat an exchanger, indoor heating pump system1, valves, (IHS). pipelines, As the etc. main As heatingthermalthe heating supply source, supply source, SCS issource, PCTSScomposed PCTSS is composed of FSTCs, is composed a of plate a PCM heatof a storage exchanger, PCM storage tank, pump an tank, AHS, 1, valves, an a plateAHS, pipelines, heat a plate exchanger, etc. heat As pumptheexchanger, heating 2, valves, pump supply pipelines, 2, valves,source, etc. pipelines,PCTSS The hourly is etc. composed The heating hourly of load aheating PCM of the loadstorage IHS of is thetank, calculated IHS an is calculatedAHS, by DeSTa plate by software DeSTheat (Tsinghuaexchanger,software University, (Tsinghua pump 2, Beijing, valves,University, China),pipelines, Beijing, to etc. determine China), The hourly to a suitabledetermine heating supply loada suitable of water the supplyIHS temperature is calculatedwater andtemperature by backwater DeST temperaturesoftwareand backwater (Tsinghua for the temperature form University, of indoor for theBeijing, heat form exchangers. China),of indoor to heat determine exchangers. a suitable supply water temperature and backwater temperature for the form of indoor heat exchangers.

FigureFigure 1. 1.Schematic Schematic diagram diagram of of the the solarsolar heatingheating system system-phase‐phase change change material material (SHS (SHS-PCM).‐PCM). Figure 1. Schematic diagram of the solar heating system‐phase change material (SHS‐PCM). Figure 2 shows a schematic of the PCM storage tank. The PCM is packed into flat metal boxes, whichFigureFigure are2 stacked shows 2 shows ain schematic alayers schematic in the of of phase the the PCM PCM change storage storage heat tank.storagetank. The tank. PCM PCM is is packed packed into into flat flat metal metal boxes, boxes, whichwhich are are stacked stacked in in layers layers in in the the phase phase change change heatheat storage tank.

(a) (b) (a) (b) Figure 2. (a) Front view of the phase change material (PCM) storage tank; (b) A–A cross section of the FigurePCM storage 2. (a) Front tank. view of the phase change material (PCM) storage tank; (b) A–A cross section of the Figure 2. (a) Front view of the phase change material (PCM) storage tank; (b) A–A cross section of the PCM storage tank. PCM storage tank.

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2.2. Operation Modes Considering the operational stability and the manageability of the control system, the following seven heating operation modes are proposed for the SHS-PCM:

Mode 1: Natural cooling Mode 2: Reserve the heat absorbed by the solar collection system in the PCM storage tank when no indoor heating is required. Mode 3: Directly supply the indoor heating demand using the solar collection system. Mode 4: Supply the heat absorbed by the solar collection system to the rooms, and store excess absorbed heat in the PCM storage tank. Mode 5: Supply the heat stored in the PCM storage tank to the rooms. Mode 6: Supply the room heating demands using the PCM storage tank and AHS simultaneously. Mode 7: Supply the room heating demands using only the AHS.

When there is shortage of solar radiation and without the terminal load, all the components will be suspended and naturally cooled (Mode 1). When the solar radiation is available, the FSTCs will collect solar radiation and use it to heat the contained water. If the water reaches the phase-transition temperature of the PCM, the PCM storage tank begins to store the heat absorbed by the water (Mode 2). If the water is heated to the indoor heat supply temperature, the heat can be directly supplied to the rooms (Mode 3). If excess heat is available beyond that demanded by the rooms, it can be stored in the PCM storage tank (Mode 4). If there is no useful solar radiation to collect, the solar collection system would be temporarily out of service and the room heating demands would first be supplied by the PCM storage tank (Mode 5). However, if the PCM storage tank contains insufficient thermal energy to meet the heating demands of the room, it would be assisted by the AHS (Mode 6). If the PCTSS and SCS are both incapable of contributing to the indoor heating requirements, the AHS would operate alone (Mode 7). With reference to the valve and temperature numbering in Figure1, the system operating modes and control parameters are listed in Table1.

Table 1. Operating modes and control parameters of the SHS-PCM. FSTCs: flat-plate solar thermal collectors; AHS: auxiliary heat source.

Flat-Plate Solar PCM Auxiliary Mode Detail Thermal Collectors Storage Heat Source Operation (FSTCs) Tank (AHS) Mode 1 Natural cooling Off Off Off All valves closed FSTCs for PCM Valves V7 and V1 opened; Mode 2 On On Off storage tank Pumps 1 and 2 on Valves V7, V2, V6, and V5 Mode 3 FSTCs for indoor heating On Off Off opened; Pumps 1 and 2 on FSTCs for PCM storage Valves V7, V1, V2. V6, and V5 Mode 4 On On Off tank and indoor heating opened; Pumps 1 and 2 on PCM storage tank for Valves V3, V6, V5, and V4 Mode 5 Off On Off indoor heating opened; Pump 2 on PCM storage tank and Valves V3, V6, V5, V4 opened; Mode 6 Off On On AHS for indoor heating Pump 2 on; AHS on Valves V6, V5, V4, and V2 Mode 7 AHS for indoor heating Off Off On opened; Pump 2 on; AHS on

3. Dynamic Simulation Model of System The dynamic simulation model of the system can be simplified into the four sub-models shown in Figure3, based on the physical model of the SHS-PCM. The dynamic simulation model of the system can be established by solving the problem based on the physical meaning of each sub-model, the equation of the heat transfer process, and the operation control strategy of the system [29]. Energies 2018, 11, 237 5 of 18 Energies 2018, 11, 237 5 of 18 Energies 2018, 11, 237 5 of 18

Figure 3. Simplified dynamic simulation model of SHS‐PCM. Figure 3. SimplifiedSimplified dynamic simulation model of SHS-PCM.SHS‐PCM. 3.1. Model of the Flat‐Plate Solar Thermal Collector 3.1. Model of the Flat-PlateFlat‐Plate Solar Thermal Collector The heat collection process includes heat transfer between the cover plate and the The heatheat collection collection process process includes includes heat transferheat transfer between between the glass the cover glass plate cover and the plate environment, and the environment, heat transfer between the glass cover plate and the heat absorption plate, heat transfer heatenvironment, transfer between heat transfer the glass between cover the plate glass and cover the plate heat absorptionand the heat plate, absorption heat transfer plate, heat between transfer the between the heat absorption plate and the bottom plate, and heat transfer between the heat heatbetween absorption the heat plate absorption and the bottom plate plate,and the and bottom heat transfer plate, betweenand heat the transfer heat absorption between platethe heat and absorption plate and the side panels. The corresponding heat transfer coefficient can be calculated theabsorption side panels. plate The and corresponding the side panels. heat The transfer corresponding coefficient heat can transfer be calculated coefficient by analyzing can be calculated each heat by analyzing each heat transfer process. Therefore, the heat efficiency, the efficiency factor, and the transferby analyzing process. each Therefore, heat transfer the process. heat efficiency, Therefore, the the efficiency heat efficiency, factor, and the the efficiency heat transfer factor, factor and the of heat transfer factor of the collector can be obtained using the energy conservation equation. A theheat collector transfer can factor be obtained of the collector using the can energy be obtained conservation using equation. the energy A schematic conservation illustration equation. of the A schematic illustration of the energy conservation of a FSTC is shown in Figure 4. energyschematic conservation illustration of of a the FSTC energy is shown conservation in Figure of4. a FSTC is shown in Figure 4.

Figure 4. Schematic of the energy conservation of a FSTC. Figure 4. Schematic of the energy conservation of a FSTC. According to the conservation of energy, the solar radiation energy per unit time projected to According to the conservation of energy, the solar radiation energy per unit time projected to the glassAccording cover is to equal the conservation to the sum of of the energy, light loss the Q solarC,l, heat radiation loss QC,r energy, internal per energy unit time increment projected of the to the glass cover is equal to the sum of the light loss QC,l, heat loss QC,r, internal energy increment of the thecollector glass Q coverC,n, and is equaluseful to energy the sum QC,us of obtained the light by loss theQ workingC,l, heat losssubstance,QC,r, internal as expressed energy in increment Equation collector QC,n, and useful energy QC,us obtained by the working substance, as expressed in Equation of(1). the collector Q , and useful energy Q obtained by the working substance, as expressed in (1). C,n C,us Equation (1). , , ,,, (1) , , ,,, (1)

IAC,d = QC,l + QC,r + QC,us + QC,n (1) 3.2. Model of the PCM Storage Tank 3.2. Model of the PCM Storage Tank 3.2. ModelDuring of thethe PCM transfer Storage of heat Tank between the fluid and the phase change heat storage unit at any During the transfer of heat between the fluid and the phase change heat storage unit at any givenDuring time, the the lower transfer part of of heat the PCM between is solidified the fluid /melted and the after phase the changerelease/absorption heat storage of unitlatent at heat. any given time, the lower part of the PCM is solidified /melted after the release/absorption of latent heat. givenThe lower time, solid/liquid the lower part phase of the region PCM and is solidified the upper /melted liquid/solid after thephase release/absorption region are formed of next latent to heat. the The lower solid/liquid phase region and the upper liquid/solid phase region are formed next to the Theboundary lower solid/liquidof the phase phasetransition region interface. and the The upper half liquid/solid‐layer PCM phaseplate and region fluid are channel formed are next used to the to boundary of the phase transition interface. The half‐layer PCM plate and fluid channel are used to boundaryinvestigate of the the heat phase transfer transition of the interface. PCM storage The half-layertank. As indicated PCM plate in Figure and fluid 5, the channel upper are and used lower to investigate the heat transfer of the PCM storage tank. As indicated in Figure 5, the upper and lower investigateboundaries theof heatthe PCM transfer units of the can PCM be storageconsidered tank. as As adiabatic indicated surfaces. in Figure This5, the is upper because and lowerof the boundaries of the PCM units can be considered as adiabatic surfaces. This is because of the boundariessymmetry of of the the flat PCM phase units change can be thermal considered storage as adiabatic unit model. surfaces. This is because of the symmetry symmetry of the flat phase change thermal storage unit model. of the flat phase change thermal storage unit model.

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Figure 5. Mathematical model of the phase change thermal storage unit. Figure 5. Mathematical model of the phase change thermal storage unit.

The following equations apply: The following equations apply: , PCM: , (2) ∂Am(x, t) h i PCM : Hm × ρm × = K f −m × p × Tf (x, t) − Tm (2) In Equation (2), Am is the cross‐sectional∂t area of the transformed PCM, Kf‐m is the heat transfer coefficient, p is the wetted perimeter, Tf is the fluid temperature, and Tm is the phase transition In Equation (2), Am is the cross-sectional area of the transformed PCM, Kf-m is the heat transfer temperature of the PCM. coefficient, p is the wetted perimeter, Tf is the fluid temperature, and Tm is the phase transition temperature of the PCM. ∂, Heat transfer fluid: , (3) , ∂ ∂Tf (x, t) h i Heat transfer fluid : m f × Cp, f × = −K f −m × p × Tf (x, t) − Tm (3) ∂x (4)

K f −m = h f × β (4) , R f (5) β(x, t) = , (5) R f + Rp + Rm(x, t) In Equation (5), Rf, Rp, and Rm are the convective thermal resistance of the fluid, conductive thermalIn Equation resistance (5), of Rthef, R packagep, and R platem are wall, the convective and conductive thermal thermal resistance resistance of the of fluid, the transformed conductive thermalPCM, respectively. resistance of the package plate wall, and conductive thermal resistance of the transformed PCM, respectively. 1⁄ , ⁄ , ⁄ = = = Initial condition: , R 0f A1/h,f ,Rm,y,m/ K m, 0R p, δp/ Kp Boundary condition: 0, , Initial condition: Am(x, t = 0) = Am,0(x), Tf (x, t = 0) = Tf ,0. Boundary condition: Tf (x = 0, t) = Tf ,in(t). 3.3. Model of Plate Heat Exchanger 3.3. ModelThe plate of Plate heat Heat exchanger Exchanger is a heat exchange component between the solar thermal collectors and theThe PCM plate storage heat exchanger tank. The is relationship a heat exchange between component the inlet between and outlet the parameters solar thermal of collectors the plate heat and theexchanger PCM storage is determined tank. The by the relationship principle betweenof heat balance the inlet with and no outlet heat loss. parameters of the plate heat exchanger is determined ∆ by the principle, of, heat balance, with, no heat, loss., , , (6)

Qh = Kh × Ah × ∆Tht = Cp,w1 × mh,1,× (Th1,,in − Th1,,o) =C,p,w2 × mh,2 × (Th2,o − Th1,in) (6) ∆ (7) 2 2

Energies 2018, 11, 237 7 of 18

Th1,in + Th1,o Th2,o + Th2,in Energies 2018, 11, 237 ∆Tht = − 7 of 18(7) Energies 2018, 11, 237 2 2 7 of 18 where Q is the heat quantity of the plate heat exchanger (W), K is the heat transfer coefficient whereh Qh is the heat quantity of the plate heat exchanger (W), Khh is the heat transfer coefficient ◦ (W/(mwhere2· QC)),h is Athe isheat the quantity area of theof the plate plate heat heat exchanger exchanger (m (W),2), andKh is∆ theT heatis the transfer logarithmic coefficient mean (W/(m2∙°C)), Ahh is the area of the plate heat exchanger (m2), and ΔThtht is the logarithmic mean (W/(m2∙°C)), Ah is the area of the plate heat exchanger (m2), and◦ ΔTht is the logarithmic mean temperaturetemperature difference difference between between both both sides sides of of the the heat heat exchangerexchanger ((°C),C), given given by by Equation Equation (7). (7). temperature difference between both sides of the heat exchanger (°C), given by Equation (7). TheThe mathematical mathematical heat heat transfer transfer model model of of the the FSTC,FSTC, PCM storage storage tank, tank, and and plate plate heat heat exchanger exchanger The mathematical heat transfer model of the FSTC, PCM storage tank, and plate heat exchanger hashas been been presented presented above. above. In In this this study,study, thethe loadload on the terminal terminal forms forms is is calculated calculated using using DeST, DeST, has been presented above. In this study, the load on the terminal forms is calculated using DeST, whilewhile the the dynamic dynamic coupled coupled heat heat transfer transfer modelmodel ofof the system is is developed developed with with respect respect to to the the while the dynamic coupled heat transfer model of the system is developed with respect to the operationoperation modes. modes. operation modes. 4. Design4. Design Parameter Parameter Settings Settings and and Methodologies Methodologies ofof thethe Object 4. Design Parameter Settings and Methodologies of the Object ThisThis study study uses uses a a multi-storymulti‐story public public building building in inLhasa Lhasa as the as theobject, object, as shown as shown in Figure in Figure 6. The6 . This study uses a multi‐story public building in Lhasa as the object, as shown in Figure 6. The Thebuilding building has has five five floors, floors, each each floor floor is 3.9 is 3.9m in m height, in height, and andthe heating the heating area areais 3778.87 is 3778.87 m2. m2. building has five floors, each floor is 3.9 m in height, and the heating area is 3778.87 m2.

FigureFigure 6. 6.DeST DeST model model ofof thethe public building. Figure 6. DeST model of the public building. The function of the PCM storage tank is to store the heat of solar radiation, and release heat to TheThe function function of of the the PCM PCM storagestorage tank tank is is to to store store the the heat heat of ofsolar solar radiation, radiation, and andrelease release heat heatto meet indoor heating needs when no solar radiation is available. Therefore, the amount of PCM is tomeet indoor indoor heating heating needs needs when when no no solar solar radiation radiation is available. is available. Therefore, Therefore, the amount the amount of PCM of PCM is determined by the difference between the heat collection efficiency and the indoor heating load. is determineddetermined by by the the difference difference between between the the heat heat collection collection efficiency efficiency and and the the indoor indoor heating heating load. load. Figure 7 gives the hourly graph of the useful energy of the collector and the building heat load for FigureFigure7 gives 7 gives the the hourly hourly graph graph of of the the useful useful energy energy ofof thethe collectorcollector and and the the building building heat heat load load for for December 1, while the building uses a fan coil as a terminal. DecemberDecember 1, 1, while while the the building building uses uses a a fan fan coil coil asas aa terminal.terminal.

Figure 7. Hourly graph of useful energy of collector and building heat load. Figure 7. Hourly graph of useful energy of collector and building heat load. Figure 7. Hourly graph of useful energy of collector and building heat load. A + B + C is the useful energy collected by FSTCs per day and B + D + E is the total indoor heat A + B + C is the useful energy collected by FSTCs per day and B + D + E is the total indoor heat load in the diagram. According to the diagram, the heat required for PCMs is the excess solar loadA +in B the + C diagram. is the useful According energy to collected the diagram, by FSTCs the perheat day required and B for + D PCMs + E is is the the total excess indoor solar heat radiation in the A + C region. A cumulative calculation of the daily heat storage in the heating season loadradiation in the diagram. in the A + According C region. A to cumulative the diagram, calculation the heat of required the daily for heat PCMs storage is the in excess the heating solar radiationseason is shown in Figure 8. in theis shown A + C in region. Figure A 8. cumulative calculation of the daily heat storage in the heating season is shown in Figure8.

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FigureFigure 8. 8.Daily Daily heat heat storagestorage in the heating season. season.

Figure 8 is the daily heat storage capacity of the public building with fan coil as terminals, with a maximumFigure8 is value the daily of 3.27 heat GJ. storage According capacity to Equation of the public (8), the building amount withof PCM fan is coil calculated. as terminals, with a maximum value of 3.27 GJ. According to Equation (8), the amount of PCM is calculated. , , (8) h i where Qm—heat quantityQm = Mofm PCM,× C kJ;p f × (Tm,2 − Tm) + Cps × (Tm − Tm,1) + Hm (8) Mm—Weight of PCM, kg; where QCmpf—Specific—heat quantity heat of of liquid PCM, PCM kJ; at constant pressure, kJ (kg °C)−1; −1 MmC—Weightps—Specific of heat PCM, of solid kg; PCM at constant pressure, kJ (kg °C) ; ◦ −1 CpfT—Specificm,1—Low temperature heat of liquid of PCMsolid PCM at constant endothermic, pressure, °C; kJ (kg C) ; ◦ −1 CpsT—Specificm,2—High temperature heat of solid of PCM liquid at PCM constant endothermic, pressure, °C; kJ (kg C) ; Tm—Phase‐transition temperature, °C; ◦ Tm,1—Low temperature of solid PCM endothermic, C; Hm—Latent heat, kJ/kg. ◦ Tm,2—High temperature of liquid PCM endothermic, C; The volume of the PCM storage ◦tank can be calculated according to Equation (9) after the Tm—Phase-transition temperature, C; amount of PCM is determined. Hm—Latent heat, kJ/kg. The volume of the PCM storage tank can be calculated/ according to Equation (9) after the amount(9) of PCM is determined. In which：Vph—Volume of water tank, m3; Vm—Volume of PCM, m3; V = V /ε (9) εv—Volume fraction ph m v Floor radiation, fan coil and radiator3 can be chosen as the heating terminals, and the operating In which: Vph—Volume of water tank, m ; temperatures are selected according3 to“Design Code for Heating Ventilation and Air Conditioning Vm—Volume of PCM, m ; of Civil Buildings (GB50736‐2012)” [30]: (1) 35~45 °C is the suitable range of supply heating medium εv—Volume fraction temperature for heating system with hot water floor radiations, which should not be above 60 °C. Floor radiation, fan coil and radiator can be chosen as the heating terminals, and the operating The difference between supply and return water temperature should not be greater than 10 °C, and temperatures are selected according to“Design Code for Heating Ventilation and Air Conditioning of not be less than 5 °C. (2) 40~65 °C is the suitable range of supply heating medium temperature for Civil Buildings (GB50736-2012)” [30]: (1) 35~45 ◦C is the suitable range of supply heating medium heating system with fan coils, and the difference between supply and return water temperature is temperature for heating system with hot water floor radiations, which should not be above 60 ◦C. within 4.2~15 °C. (3) 75 °C/50 °C is the suitable supply/return heating medium temperature◦ for Theheating difference system between with radiators. supply and The return supply temperature medium shouldtemperature not be should greater not than be 10aboveC, and85 °C not ◦ ◦ beand less thanthe difference 5 C. (2) 40~65betweenC supply is the suitable and return range heating of supply medium heating temperature medium should temperature not less for than heating 20 system°C. with fan coils, and the difference between supply and return water temperature is within ◦ ◦ ◦ 4.2~15 WhenC. (3) using 75 C/50 the floorC is radiation the suitable and supply/returnfan coil as the heating heating terminals, medium temperatureflat‐plate solar for thermal heating ◦ systemcollector with is radiators. the best choice The supply in Lhasa heating for its characteristics medium temperature of low cost should and not not easy be above to damage. 85 C When and the ◦ differenceusing the between radiator, supply the concentrating and return solar heating collector medium with temperature heat conducting should oil notwould less be than better. 20 C. When using the floor radiation and fan coil as the heating terminals, flat-plate solar thermal collector is the best choice in Lhasa for its characteristics of low cost and not easy to damage. When using the radiator, the concentrating solar collector with heat conducting oil would be better.

Energies 2018, 11, 237 9 of 18

Table 2. Parameter calculation table of the SHS-PCM for the public building.

Floor Radiation Fan Coil Radiator Architecture Type: Public Buildings Condition 1 Condition 2 Condition 3 Condition 4 Condition 5 Supply and return temperature of T /T ◦C 40/35 40/30 45/40 45/35 70/45 fluid 3 e,in e,o Design temperature difference of - ◦C 5 10 5 10 25 plate heat exchanger Supply and return temperature of T /T ◦C 45/40 50/40 50/45 55/45 120/70 fluid 1 C,in C,o Collector type FSTC FSTC FSTC FSTC Concentrating solar collector Heat transfer fluid of the collector side water-glycol water-glycol water-glycol water-glycol heat conducting oil 2 ◦ Heat transfer coefficient Kh W/(m C) 4124 4631 4124 4631 1996 2 Area of the plate heat exchanger Ah m 15.5 6.4 15 6.4 5.2 2 Area of single plate heat collector AC m 2.23 2.23 2.23 2.23 -

Number of heat collectors NC - 88 88 88 88 88

Total flow of fluid 1 mf1 kg/s 10.85 5.3 10.55 5.16 1.39

Total flow of fluid 2 mf2 kg/s 9.45 4.62 9.26 4.53 1.54

Specific heat of fluid 1 Cpf1 J/(kg·K) 3635 3635 3666 3666 2302

Specific heat of fluid 2 Cpf2 J/(kg·K) 4174 4174 4174 4174 4179 3 Solution density of fluid 1 ρf1 kg/m 1042 1042 1037 1037 850 3 Solution density of fluid 2 ρf2 kg/m 992.2 992.2 990 990 983 3 Volume of PCM storage tank Vhp m 3 × 2.5 × 1.9 3 × 2.5 × 1.9 3 × 2.5 × 2 3 × 2.5 × 2 3 × 2.5 × 1.6 PCM materials Lauric acid Lauric acid paraffin paraffin hydrated salts ◦ Phase-transition temperature Tm C 43 43 47 47 72 m × m × Volume of PCM plate V 3 × 2.5 × 0.08 3 × 2.5 × 0.08 3 × 2.5 × 0.08 3 × 2.5 × 0.08 3 × 2.5 × 0.08 pp m

Number of PCM plates Npp - 14 14 14 14 12 air-cooled heat air-cooled heat air-cooled heat air-cooled heat AHS - Gas fired boiler pump pump pump pump Note: Fluid 1 is the circulating solution in SCS; Fluid 2 is the circulating solution in PCTSS; Fluid 3 is the circulating solution in IHS. Energies 2018, 11, 237 10 of 18

The main design parameters are shown in Table2, according to different selection methods. The heating period ran from November 4th to March 19th of the following year (a total of 136 days), and the meteorological parameters and solar radiation intensity are selected from typical meteorological parameters in Lhasa, China. The specifications and quantities of solar thermal collectors, plate heat exchanger and AHS are calculated according to “Technical code for solar heating system (GB 50495-2009) [31]”. The methodologies are as follows: (1) Design the solar heating system with PCM storage tank based on select typical buildings firstly. Dynamic simulation models of SHS-PCM and the control strategy are established secondly. Then the design parameters are inputted in the simulation models to get the system operating parameters at each moment of the whole heating season; (2) Different design schemes are set up and simulated respectively, and the most energy saving design scheme is obtained by comparing the running parameters.

5. Results and Discussion The heat of the heating system is supplied by the AHS, the direct heating of the FSTCs, and the heat storage tank. The heat of the FSTCs and the storage tank comes from solar energy. The ratio of these two types of heat is defined as the energy saving rate. Therefore, the state of the heating system can be quantitatively analyzed by using the daily heating load of the building and the daily heat supplied by the AHS. The ratio of contribution of solar energy to the heating system and the general energy saving status can be determined according to the proportion of heat supplied by the three sources during the entire heating season. The following systems are compared and analyzed from the perspective of quantization, and by using the ratio that reflects (1) the daily heat supplied by AHS and (2) the proportion of heat supplied by the three sources.

(1) A coupled model of an ordinary water tank was established to analyze the operating conditions of a common water tank and a PCM storage tank combined with SHS, in order to explore how the ordinary water tank and the PCM storage tank contributed to the solar heating system. (2) Operating conditions with different sizes of heat water storage tanks are compared in order to optimize the selection of the heat water storage tank. (3) Operating conditions with different terminal forms and different supply and return water temperatures are compared in order to find the best match between the terminal form and the PCM storage tank.

5.1. Operating Conditions of Different PCM Storage Tanks

5.1.1. Operating Conditions Comparison between SHS-PCM and SHS-without PCM The reference parameters of the SHS-PCM model are shown in Table2, Case 5. The SHS-without PCM and SHS-PCM are consistent, except for the tank forms. The design parameter is performed on the basis of the “Technical code for solar heating system (GB 50495-2009) [31]” for ordinary water tanks selection calculation. The PCM storage tank is only considered latent heat storage with the same heat storage capacity. The selection parameters of both tanks are shown in Table3. Figure9 shows a box chart of daily energy delivered by the AHS of SHS-PCM and SHS-without PCM, along with the daily heat consumption of the building during the entire heating season. The data on the left of the box chart show the daily heat supplied by the AHS of SHS-without PCM. The average daily heat supply is 3964 MJ/day, and the distribution of the points is between 0~14,041 MJ/day. The data in the middle of the box chart show the daily heat supplied by the AHS of SHS-PCM. The average daily heat supply is 2202 MJ/day, and the distribution of the points is between 201~5956 MJ/day. The data on the right of the box chart show daily building energy consumption. The average daily energy consumption is 5225 MJ/day, and the distribution of the points is between 2088~8217 MJ/day. The figure shows that the daily heat supply of SHS-without PCM is occasionally Energies 2018, 11, 237 11 of 18 Energies 2018, 11, 237 11 of 18 even highertemperature than the building heat consumption. This is because of the serious heat dissipation of the ordinaryFigure water 9 shows tank. a The box AHS chart supplies of daily not energy only delivered indoor heating, by the butAHS also of SHS the heat‐PCM loss and of SHS the ordinary‐without waterPCM, tank.along with the daily heat consumption of the building during the entire heating season. The data on the left Tableof the 3. boxSelection chart parametersshow the daily for ordinary heat supplied water tank by and the PCM AHS storage of SHS tank.‐without PCM. The average daily heat supply is 3964 MJ/day, and the distribution of the points is between 0~14,041 MJ/day. The data in the middle of theNomenclature box chart show Units the Ordinary daily heat Water supplied Tank PCM by Storagethe AHS Tank of

SHS‐PCM. HeatThe storage average capacity daily heat supply Qism 2202 MJ/day,GJ and the 0.63 distribution of the 0.63 points is 3 between 201~5956Volume MJ/day. of tank The data on theVhp right of mthe box chart3 × 2.5 show× 4 daily building 3 × 2.5 × 0.8energy 3 × × consumption.Volume The of average PCM plate daily energy consumptionVpp mis 5225 MJ/day,- and the distribution 3 2.5 0.08 of the Number of PCM plates Npp - 6 points is betweenPCM materials2088~8217 MJ/day. The figure shows that the daily heat supply of paraffinSHS‐without ◦ PCM is Phase-transitionoccasionally even temperature higher than the buildingTm heat Cconsumption. -This is because of the 47 serious Terminal form fan coil fan coil heat dissipation of the ordinary water tank. The AHS supplies◦ not only indoor heating, but also the Supply and return heating temperature Te,in/Te,o C 45/40 45/40 heat loss of the ordinary water tank.

Figure 9. Box chart showing daily energy delivered by the AHS of SHS-PCM and SHS-without PCM, Figure 9. Box chart showing daily energy delivered by the AHS of SHS‐PCM and SHS‐without PCM, along with the daily heat consumption of the building. along with the daily heat consumption of the building.

FigureFigure 1010 showsshows thethe proportionsproportions ofof heatheat suppliedsupplied toto SHS-PCMSHS‐PCM andand SHS-withoutSHS‐without PCMPCM byby threethree heatheat sourcessources (AHS,(AHS, storagestorage tank,tank, andand FSTCsFSTCs directdirect heating)heating) duringduring thethe entireentire heatingheating seasonseason waterwater storagestorage tank.tank. As shown in the bar chart, for SHS-withoutSHS‐without PCM, the AHS provides 76% of the heat, whilewhile the the water water tank tank without without PCM PCM and and the the FSTCs FSTCs provide provide 14% 14% and and 10% 10% of the of heat, the respectively.heat, respectively. Thus, theThus, energy the energy saving ratesaving is 24%. rate For is 24%. SHS-PCM, For SHS the‐PCM, AHS provides the AHS 42% provides of the heat, 42% whileof the the heat, PCM while storage the tankPCM and storage the FSTCs tank and provide the FSTCs 5% and provide 53% of the5% heat, and respectively.53% of the heat, Thus, respectively. the energy saving Thus, ratethe energy is 58%. savingIt can rate be is seen58%. from Figures9 and 10 that SHS-PCM saves 34% more energy than SHS-without PCM.It In can addition, be seen after from the Figures PCM storage9 and 10 tank that is SHS adopted,‐PCM thesaves volume 34% more of the energy water tankthan isSHS reduced‐without to 1/5PCM. of anIn addition, ordinary waterafter the tank. PCM The storage primary tank reason is adopted, is that more the volume solar energy of the iswater supplied tank directlyis reduced from to the1/5 FSTCsof an ordinary to the indoor water terminals tank. The and primary the PCM reason storage is that tank more makes solar a relativelyenergy is lowsupplied contribution directly to from the heatingthe FSTCs of theto the terminal. indoor It terminals is necessary and to the optimize PCM storage the selection tank makes of the a PCM relatively storage low tank. contribution to the heating of the terminal. It is necessary to optimize the selection of the PCM storage tank.

Energies 2018, 11, 237 12 of 18 Energies 2018, 11, 237 12 of 18

Figure 10.10.Heat Heat supply supply proportions proportions of AHS, of AHS, water water storage storage tank, and tank, FSTCs and direct FSTCs heating direct for heating SHS-PCM for andSHS SHS-without‐PCM and SHS PCM.‐without PCM.

5.1.2. Operating Conditions Comparison with Different Sizes of PCM Storage Tanks 5.1.2. Operating Conditions Comparison with Different Sizes of PCM Storage Tanks In order to take advantage of the heating effect of the PCM storage tank, the operation condition In order to take advantage of the heating effect of the PCM storage tank, the operation condition of each heating system is analyzed during the full heating season, under the condition that only the of each heating system is analyzed during the full heating season, under the condition that only the PCM storage tank size and heat capacity are changed while the building characteristics and indoor PCM storage tank size and heat capacity are changed while the building characteristics and indoor and outdoor parameters are constant. and outdoor parameters are constant. The statistics on the daily storage capacity of the public building are given in Table 4. The statistics on the daily storage capacity of the public building are given in Table4.

Table 4. Different selection parameters of PCM storage tank. Table 4. Different selection parameters of PCM storage tank. Daily Heat Percentage Plate Actual Heat Days/Day Plate Volume/m3 Tank Volume/m3 Storage/GJDaily Heat of Days/%Percentage Number/Block3 Plate Tank ActualStorage/GJ Heat Days/Day Plate Volume/m 3 Storage/GJ0 5 4 ‐ ‐ ‐ ‐ of Days/% Number/Block Volume/m Storage/GJ 0.5 031 523 ‐ ‐ ‐ ‐ 4 - - - - 0.630.5 47 3135 233 × 2.5 × 0.08 -6 -3 × 2.5 - × 0.8 - 0.63 1 0.6372 4753 353 × 2.5 × 3 0.08× 2.5 × 0.0810 6 33× × 2.52.5 ×× 1.30.8 0.631.04 1.5 197 7271 533 × 2.5 × 3 0.08× 2.5 × 0.0814 10 3 ×3 ×2.5 2.5× × 1.32 1.041.46 2 1.5114 9784 713 × 2.5 × 3 0.08× 2.5 × 0.0819 14 33 ×× 2.52.5 ×× 2.52 1.461.98 2.5 2128 11494 843 × 2.5 × 3 0.08× 2.5 × 0.0824 19 33× × 2.52.5 ×× 3.22.5 1.982.5 × × × × 3 2.5133 12898 ‐ ‐ ‐ ‐ 94 3 2.5 0.08 24 3 2.5 3.2 2.5 3 133 98 - - - - 3.26 136 100 3 × 2.5 × 0.08 26 3 × 3 × 2.5 3.26 3.26 136 100 3 × 2.5 × 0.08 26 3 × 3 × 2.5 3.26

The daily heat storage is calculated in 0.5 GJ intervals in Table 4, and the percentage of days in the heatThe storage daily heat relative storage to isthe calculated total number in 0.5 of GJ days intervals in the in heating Table4, season and the is percentage obtained. ofIt can days be in seen the heatthat the storage proportion relative of to days the total of heat number storage of days below in 1 the GJ heating is 53%, seasonthe proportion is obtained. of days It can under be seen 2 GJ that is the84%, proportion and the proportion of days of of heat days storage under below 2.5 GJ 1 is GJ 94%, is 53%, while the the proportion daily heat of storage days under rate of 2 GJ0.63 is GJ 84%, is and35%. the proportion of days under 2.5 GJ is 94%, while the daily heat storage rate of 0.63 GJ is 35%. In accordance with the “Technical codecode forfor solarsolar heatingheating systemsystem (GB(GB 50495-2009)”50495‐2009)” for the Lhasa area, whichwhich specifiesspecifies a a 150 150 L L water water storage storage tank tank with with a square a square meter meter collector, collector, a large a large part ofpart the of solar the radiationsolar radiation is still is not still being not stored.being stored. Therefore, Therefore, PCM storage PCM storage tanks with tanks heat with storage heat storage capacities capacities of 0.63 GJ, of 1.040.63 GJ,GJ, 1.461.04 GJ, GJ, 1.98 1.46 GJ, GJ, 2.5 1.98 GJ, GJ, and 2.5 3.26 GJ, GJ and are 3.26 utilized GJ are in utilized the tests in described the tests in described this section in this while section other systemwhile other conditions system are conditions constant; are i.e., constant; the same buildingi.e., the same and fan building coil terminal and fan were coil used,terminal and were the supply used, ◦ ◦ and returnthe supply water and temperatures return water were temperatures 45 C and 40wereC, 45 respectively. °C and 40 The°C, respectively. operating conditions The operating of the buildingconditions were of the analyzed building for were the whole analyzed heating for seasonthe whole to determine heating season which to PCM determine storage tankwhich system PCM offersstorage the tank best system energy offers saving the capability. best energy saving capability. Figure 11 shows the daily energy delivered by the AHS of SHS‐PCM during the entire heating season, using PCM storage tanks with different heat storage capacities. It can be seen from the data and box chart that the average daily heat supplied by AHS has not obviously decreased or increased,

Energies 2018, 11, 237 13 of 18 Energies 2018, 11, 237 13 of 18 although the heat storage capacity of the PCM storage tank increases from 0.63 GJ to 3.26 GJ. For the six different heat storage capacities, the heat quantities supplied by AHS are 2202 MJ, 2083 MJ, 2020 MJ, 2023Figure MJ, 11 2050 shows MJ, theand daily 2072 energyMJ, respectively. delivered by the AHS of SHS-PCM during the entire heating season,Figure using 12 PCMshows storage the proportion tanks with of differentheat supplied heat storageto SHS‐PCM capacities. by three It can heat be sources seen from (AHS, the PCM data storageand box tank, chart and that theFSTCs average direct daily heating) heat suppliedover the byentire AHS heating has not season obviously water decreased storage or tank, increased, using PCMalthoughEnergies storage 2018 the, 11, heat 237tanks storage with capacitydifferent ofheat the storage PCM storage capacities. tank increasesThe heat from storage 0.63 capacity GJ to 3.26 of GJ. the For 13PCM of the 18 storagesix different tank heat increases storage from capacities, 0.63 GJ the to heat 3.26 quantities GJ; for the supplied six successive by AHS capacities, are 2202 MJ, the 2083 percentages MJ, 2020 MJ, of although the heat storage capacity of the PCM storage tank increases from 0.63 GJ to 3.26 GJ. For the heat2023 supplied MJ, 2050 MJ,by AHS and 2072are 42%, MJ, respectively.40%, 39%, 39%, 39%, and 40%, respectively. six different heat storage capacities, the heat quantities supplied by AHS are 2202 MJ, 2083 MJ, 2020 MJ, 2023 MJ, 2050 MJ, and 2072 MJ, respectively. Figure 12 shows the proportion of heat supplied to SHS‐PCM by three heat sources (AHS, PCM storage tank, and FSTCs direct heating) over the entire heating season water storage tank, using PCM storage tanks with different heat storage capacities. The heat storage capacity of the PCM storage tank increases from 0.63 GJ to 3.26 GJ; for the six successive capacities, the percentages of heat supplied by AHS are 42%, 40%, 39%, 39%, 39%, and 40%, respectively.

Figure 11. Box chart of daily energy delivered by AHS of SHS‐PCM using different capacities of PCM Figure 11. Box chart of daily energy delivered by AHS of SHS-PCM using different capacities of PCM storage tanks.

Figure 12 shows the proportion of heat supplied to SHS-PCM by three heat sources (AHS, PCM storage tank, and FSTCs direct heating) over the entire heating season water storage tank, using PCM storage tanks with different heat storage capacities. The heat storage capacity of the PCM storage tank increasesFigure from 11. Box 0.63 chart GJ to of 3.26 daily GJ; energy for the delivered six successive by AHS of capacities, SHS‐PCM the using percentages different capacities of heat suppliedof PCM by AHSstorage are 42%, tanks. 40%, 39%, 39%, 39%, and 40%, respectively.

Figure 12. Proportions of heat supplied by AHS, PCM storage tank, and FSTCs direct heating, with different capacities of PCM storage tanks.

The changes in the heat storage capacity of the PCM storage tank did not have a significant influence on the utilization of solar energy. The direct heat from the FSTCs played a greater role than the PCM storage tank in terms of heating proportions. Their respective proportions for the six successive PCM storage tank capacities were 53%, 52%, 51%, 49%, 48%, 46%, and 5%, 8%, 10%, 12%, 13%, Figure14%. With 12. Proportions the increase of heat in heat supplied storage by AHS, capacity, PCM storagethe proportion tank, and of FSTCs heat direct supplied heating, by withthe PCM storagedifferent tank gradually capacities of increases. PCM storage tanks. According to the above analysis, with the increase in heat storage, the heat supplied by AHS The changes in the heat storage capacity of the PCM storage tank did not have a significant does not decrease correspondingly; it decreases at first, rebalances, and then rises. The main reason influence on the utilization of solar energy. The direct heat from the FSTCs played a greater role than the PCM storage tank in terms of heating proportions. Their respective proportions for the six successive PCM storage tank capacities were 53%, 52%, 51%, 49%, 48%, 46%, and 5%, 8%, 10%, 12%, 13%, 14%. With the increase in heat storage capacity, the proportion of heat supplied by the PCM storage tank gradually increases. According to the above analysis, with the increase in heat storage, the heat supplied by AHS does not decrease correspondingly; it decreases at first, rebalances, and then rises. The main reason

Energies 2018, 11, 237 14 of 18

The changes in the heat storage capacity of the PCM storage tank did not have a significant influence on the utilization of solar energy. The direct heat from the FSTCs played a greater role than the PCM storage tank in terms of heating proportions. Their respective proportions for the six successive PCM storage tank capacities were 53%, 52%, 51%, 49%, 48%, 46%, and 5%, 8%, 10%, 12%, 13%, 14%. With the increase in heat storage capacity, the proportion of heat supplied by the PCM storage tank gradually increases. According to the above analysis, with the increase in heat storage, the heat supplied by AHS does not decrease correspondingly; it decreases at first, rebalances, and then rises. The main reason for this Energies 2018, 11, 237 14 of 18 situation is that the smaller the heat storage capacity of the PCM tank, the easier the heated water willfor reach this situation the required is that heating the smaller temperature the heat sostorage that the capacity indoor of heatingthe PCM can tank, be the carried easier out the as heated soon as possible.water will The reach larger the the required heat storage heating capacity temperature of the so PCM that the tank, indoor the longer heating period can be of carried time the out PCM as tanksoon will as operate possible. although The larger the the PCM heat tank storage is initially capacity starts of the slowly. PCM tank, As the the solarlonger radiation period of and time indoor the loadPCM at any tank time will in operate the change, although so sometimes the PCM tank the smalleris initially PCM starts tank slowly. can replace As the thesolar AHS radiation rapidly, and and sometimesindoor load the at larger any PCMtime in tank the canchange, delay so the sometimes operation the of smaller AHS. From PCM the tank simulation can replace results the AHS of the wholerapidly, heating and season,sometimes the the heat larger supplied PCM bytank the can auxiliary delay the source operation shows of AHS. little changeFrom the when simulation the heat storageresults capacity of the whole of the tankheating is 1.46~2.5 season, the GJ, andheat thesupplied energy by saving the auxiliary effect of source the system shows is little obvious change when thewhen heat storagethe heat capacity storage capacity is less than of the 1.46 tank GJ. is Therefore, 1.46~2.5 GJ, it isand considered the energy that saving an SHS-PCM effect of the with system a 1.46 is GJ heatobvious storage when capacity the isheat the storage best choice; capacity moreover, is less itthan is suggested 1.46 GJ. Therefore, that the design it is considered selection parameters that an forSHS the‐ PCMPCM storagewith a 1.46 tank GJ should heat storage specify capacity a daily heat is the storage best choice; capacity moreover, that satisfies it is suggested 70~80% of that the entirethe heatingdesign season. selection parameters for the PCM storage tank should specify a daily heat storage capacity that satisfies 70~80% of the entire heating season. 5.2. Operating Conditions of Different Terminal Forms 5.2. Operating Conditions of Different Terminal Forms The system design parameters of five different operating conditions under different terminal forms andThe differentsystem design supply parameters and return of water five different temperatures operating are listedconditions in Table under2. A different PCM storage terminal tank withforms a capacity and different of 1.46 supply GJ was and chosen return towater analyze temperatures the operation are listed conditions in Table 2. of A each PCM heating storage system tank duringwith thea capacity heating of season. 1.46 GJ was chosen to analyze the operation conditions of each heating system during the heating season. Figure 13 shows the daily energy delivered by the AHS of SHS-PCM with different terminal Figure 13 shows the daily energy delivered by the AHS of SHS‐PCM with different terminal forms and different supply/return water temperatures during the entire heating season. The data forms and different supply/return water temperatures during the entire heating season. The data and box chart show that the selected terminal forms and the supply/return water temperatures are as and box chart show that the selected terminal forms and the supply/return water temperatures are follows: floor radiation (40/35 ◦C), floor radiation (40/30 ◦C), fan coil (45/40 ◦C), fan coil (45/35 ◦C), as follows: floor radiation◦ (40/35 °C), floor radiation (40/30 °C), fan coil (45/40 °C), fan coil (45/35 °C), andand radiator radiator (70/45 (70/45 C).°C). The The corresponding corresponding average daily daily heat heat supplied supplied by by the the AHS AHS is is1898 1898 MJ/day, MJ/day, ◦ 28242824 MJ/day, MJ/day, 1988 1988 MJ/day, MJ/day, 2660 2660 MJ/day, MJ/day, andand 29392939 MJ/day. Among Among these, these, floor floor radiation radiation (40/35 (40/35 °C) C) ◦ hashas the the minimum minimum AHS AHS requirement requirement while while radiator radiator (70/45(70/45 °C)C) has has the the maximum maximum AHS AHS requirement. requirement.

FigureFigure 13. 13.Box Box chart chart of dailyof daily energy energy delivered delivered by by AHS AHS of SHS-PCMof SHS‐PCM with with different different terminal terminal forms forms and heatingand heating temperatures. temperatures.

Figure 14 shows the proportion of heat supplied to SHS‐PCM by three heat sources (AHS, PCM storage tank, and FSTCs direct heating) over the entire heating season, water storage tank with different terminal forms and heating temperatures. The selected terminal forms and the supply/return water temperatures are as follows: floor radiation (40/35 °C), floor radiation (40/30 °C), fan coil (45/40 °C), fan coil (45/35 °C), and radiator (70/45 °C). The corresponding proportions of heat supplied by AHS are 14%, 11%, 10%, 10%, and 4%. The proportions of heat supplied by FSTCs direct heating are 50%, 34%, 52%, 39%, and 39%. The above statistics show that the highest proportion of solar energy utilization is in the system with floor radiation (40/35 °C), in which the utilization rate of solar energy can reach 64%. This is mainly because the lower the supply heating

Energies 2018, 11, 237 15 of 18

Figure 14 shows the proportion of heat supplied to SHS-PCM by three heat sources (AHS, PCM storage tank, and FSTCs direct heating) over the entire heating season, water storage tank with different terminal forms and heating temperatures. The selected terminal forms and the supply/return ◦ ◦ waterEnergies temperatures2018, 11, 237 are as follows: floor radiation (40/35 C), floor radiation (40/30 C), fan15 of coil 18 (45/40 ◦C), fan coil (45/35 ◦C), and radiator (70/45 ◦C). The corresponding proportions of heat suppliedtemperature, by AHS the easier are 14%, the 11%, PCM 10%, tank 10%, and and the 4%.FSTCs The direct proportions heating of are heat satisfied supplied the by requirements, FSTCs direct heatingthereby arereducing 50%, 34%,the operation 52%, 39%, time and of 39%. AHS. The above statistics show that the highest proportion of ◦ solarAs energy shown utilization in Figures is in 13 the and system 14 for with different floor radiation terminal (40/35 forms C),and in different which the temperatures utilization rate of ofsupply/return solar energy water, can reach the optimal 64%. This operational is mainly conditions because theand lower energy the saving supply capability heating for temperature, SHS‐PCM theover easier the entire the PCM heating tank season and the are FSTCs provided direct by heating floor radiation are satisfied (40/35 the requirements,°C), followed by thereby fan coil reducing (45/40 the°C), operation fan coil (45/35 time °C), of AHS. floor radiation (40/30 °C), and radiator (70/45 °C).

Figure 14.14. HeatHeat supply supply proportions proportions of AHS,of AHS, PCM PCM storage storage tank, andtank, FSTCs and directFSTCs heating, direct heating, with different with terminaldifferent formsterminal and forms heating and temperatures. heating temperatures.

6. Conclusions As shown in Figures 13 and 14 for different terminal forms and different temperatures of supply/returnAn entire heating water, the season optimal (November–March) operational conditions is selected and energy as the savingresearch capability period on for the SHS-PCM basis of overnumerical the entire models heating of the seasonSHS‐PCM. are providedIn addition, by taking floor radiationa public building (40/35 ◦inC), Lhasa followed as the by object, fan coilthe (45/40heating◦ C),conditions, fan coil (45/35contribution◦C), floor rate radiation of solar energy, (40/30 ◦andC), andoverall radiator energy (70/45‐saving◦C). capability provided by the heating system are analyzed under different PCM storage tanks and different terminal forms. 6. ConclusionsThrough a comparative analysis of several system conditions, the following conclusions were obtained:An entire heating season (November–March) is selected as the research period on the basis of numerical(1) The operating models of conditions the SHS-PCM. of SHS In addition,‐without takingPCM and a public SHS building‐PCM were in Lhasa analyzed as the in object, order the to heatingexplore conditions, the contribution contribution rate rate of ofordinary solar energy, water and tanks overall and PCM energy-saving storage tanks capability in a solar provided heating by the heatingsystem. system The results are analyzed show that under an differentSHS‐PCM PCM system storage saves tanks 34% and more different energy terminal than an forms. ordinary Throughwater tank a heating comparative system. analysis In addition, of several after the system PCM conditions,storage tank theis adopted, following the conclusions water tank werevolume obtained: is reduced to 1/5 the size of the ordinary water tank. Because more solar energy is supplied directly from the FSTCs to the indoor terminals and owing to the PCM storage tank’s (1) The operating conditions of SHS-without PCM and SHS-PCM were analyzed in order to explore relatively low contribution to the heating of the terminal, it is necessary to optimize the the contribution rate of ordinary water tanks and PCM storage tanks in a solar heating system. selection of the PCM storage tank. The results show that an SHS-PCM system saves 34% more energy than an ordinary water tank (2) Operating conditions with different sizes of PCM storage tanks were compared in order to heating system. In addition, after the PCM storage tank is adopted, the water tank volume is optimize the selection of the PCM storage tank. The results show that with the increase in heat reduced to 1/5 the size of the ordinary water tank. Because more solar energy is supplied directly storage, the heat supply of AHS does not decrease correspondingly; it decreases first, from the FSTCs to the indoor terminals and owing to the PCM storage tank’s relatively low rebalances, and then rises. It is suggested that the design selection parameters of the PCM storage tank should specify a daily heat storage capacity that satisfies 70~80% of the entire heating season. (3) Operating conditions observed with different terminal forms and different temperatures of supply and return water were compared in order to find the terminal form that best matches the PCM storage tank. The results show that for different terminal forms and different designed temperatures of supply/return water, the best operational conditions and energy saving capability for SHS‐PCM over an entire heating season are provided by floor radiation (40/35

Energies 2018, 11, 237 16 of 18

contribution to the heating of the terminal, it is necessary to optimize the selection of the PCM storage tank. (2) Operating conditions with different sizes of PCM storage tanks were compared in order to optimize the selection of the PCM storage tank. The results show that with the increase in heat storage, the heat supply of AHS does not decrease correspondingly; it decreases first, rebalances, and then rises. It is suggested that the design selection parameters of the PCM storage tank should specify a daily heat storage capacity that satisfies 70~80% of the entire heating season. (3) Operating conditions observed with different terminal forms and different temperatures of supply and return water were compared in order to find the terminal form that best matches the PCM storage tank. The results show that for different terminal forms and different designed temperatures of supply/return water, the best operational conditions and energy saving capability for SHS-PCM over an entire heating season are provided by floor radiation (40/35 ◦C), followed by fan coil (45/40 ◦C), fan coil (45/35 ◦C), floor radiation (40/30 ◦C), and radiator (70/45 ◦C).

The analyses above show obviously energy-saving effect of the SHS-PCM. However, the mathematical models of the components on the heat sources side have been established such as PSTCs, PCM tank, AHSs and plate heat exchanger, but the terminal forms has just calculated the hourly heat load of the whole heating season but not established the mathematical model. In addition, ignoring the heat dissipation of the pipelines will also lead to the deviation. More meaningful advice is that PCMs with high heat transfer coefficient will lengthen the operation of the PCM tank and strengthen the energy-saving effect of the SHS-PCM.

Acknowledgments: We acknowledge funding from the Program of the Youth Science and Technology Innovation Team of Sichuan Province of Building Environment and Energy Efficiency (No: 2015TD0015) and Key Research and Development Program of Sichuan Province (No: 2017GZ0389). Author Contributions: Juan Zhao and Yanping Yuan conceived and designed the experiments; Juan Zhao performed the experiments; Yasheng Ji and Juan Zhao analyzed the data; Jun Lu contributed materials and analysis tools; Juan Zhao, Yasheng Ji, Yanping Yuan and Zhaoli Zhang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

A Area, m2 N Number Hm Enthalpy of PCM, kJ/kg p Wetted perimeter, m I Radiant intensity, W/m2 Q Quantity of heat, J K Heat transfer coefficient, W/(m2·◦C) T Temperature, ◦C L Length of PCM plate, m ∆t Temperature difference, ◦C l Length, m V Volume, m3 m Mass flow rate, kg/s x x-coordinate, m M Quality, kg y The height of the phase transition interface Cp Specific heat at constant pressure, J/(kg·K) h Coefficient of convective heat transfer, W/(m2·K) Greek symbols ρ Density, kg/m3 δ Thickness, m η Efficiency εV Volume fraction β Heat resistance of dimensionless number Subscripts C Solar thermal collector max Maximum e Load n Internal energy ev Environment o Outlet f Fluid p Wall of encapsulated version g Heat collection medium pp PCM plate h Plate heat exchanger r Thermal energy hp Heat storage tank S Auxiliary heat source Energies 2018, 11, 237 17 of 18

I Radiation s Solid in Inlet us Useful energy l Luminous energy W Water m PCM V Volume Abbreviations AHS Auxiliary Heat Source PCM Phase change material FSTC Flat-plate Solar Thermal Collector PCTSS Phase change thermal storage system HTF Heat transfer fluid SCS solar collection system IHS Indoor heating system SDHW Solar domestic hot water LHS Latent heat storage SHS-PCM Solar heating system with PCM storage tank LTES Latent thermal energy storage TES Thermal energy storage

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