Diurnal Thermal Behavior of Photovoltaic Panel with Phase Change Materials Under Different Weather Conditions

energies

Article Diurnal Thermal Behavior of Photovoltaic Panel with Phase Change Materials under Different Weather Conditions

Jae-Han Lim 1,* ID , Yoon-Sun Lee 1 and Yoon-Bok Seong 2

1 ELTEC College of Engineering, Ewha Womans University, Seoul 03760, Korea; [email protected] 2 Center for Climatic Environment Real-scale Testing, Korea Conformity Laboratories, Chungbuk 27873, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-3277-6874

Received: 16 October 2017; Accepted: 27 November 2017; Published: 1 December 2017

Abstract: The electric power generation efficiency of photovoltaic (PV) panels depends on the solar irradiation flux and the operating temperature of the solar cell. To increase the power generation efficiency of a PV system, this study evaluated the feasibility of phase change materials (PCMs) to reduce the temperature rise of solar cells operating under the climate in Seoul, Korea. For this purpose, two PCMs with different phase change characteristics were prepared and the phase change temperatures and thermal conductivities were compared. The diurnal thermal behavior of PV panels with PCMs under the Seoul climate was evaluated using a 2-D transient thermal analysis program. This paper discusses the heat flow characteristics though the PV cell with PCMs and the effects of the PCM types and macro-packed PCM (MPPCM) methods on the operating temperatures under different weather conditions. Selection of the PCM type was more important than the MMPCM methods when PCMs were used to enhance the performance of PV panels and the mean operating temperature of PV cell and total heat flux from the surface could be reduced by increasing the heat transfer rate through the honeycomb grid steel container for PCMs. Considering the mean operating temperature reduction of 4 ◦C by PCM in this study, an efficiency improvement of approximately 2% can be estimated under the weather conditions of Seoul.

Keywords: photovoltaic panel; phase change materials (PCMs); speciﬁc heat capacity; thermal conductivity; solar energy

1. Introduction The electric power generation efficiency of photovoltaic (PV) panels depends on the solar irradiation flux, and operating temperature of the solar cell. This means that the power output of PV panels is dependent mostly on direct sunlight. In addition, atmospheric conditions, such as dust and clouds can affect the power output. Moreover, the power generation output decreases with increasing operating temperature of the PV panels. For example, silicon-based PV shows an electric power drop at panel temperatures above 25 ◦C with a temperature coefficient ranging from −0.3%/K to −0.65%/K [1]. Therefore, the basic environmental variables and numerical parameters of a PV system should be examined carefully with a specific mounting geometry or building integration level to estimate the operating temperatures. For this reason, various mathematical equations have been developed to describe the operating temperature of different PV types and weather conditions [2]. In general, the solar irradiation cannot be controlled due to various weather conditions. Therefore, it is important to maintain the low operating temperature of solar cells for the better power generation efficiency. Several studies have examined passive and active heat removal technologies. To lower the operating temperature of a solar cell, free cooling on the back side of panel the due to the natural

Energies 2017, 10, 1983; doi:10.3390/en10121983 www.mdpi.com/journal/energies Energies 2017, 10, 1983 2 of 14 convection can generally be applied. In addition, passive cooling using phase change materials (PCMs) can also be used for heat absorption or release during the power generation process. Brinkworth examined the passive cooling performance in a free-standing PV panel with a duct, in which the heat removal depends on the buoyancy-driven air flow [3]. He suggested how the operating conditions and the proportions of the duct affect the air flow and heat transfer characteristics graphically. Yun et al. revealed a complex interrelationship between a ventilated PV façade and the overall energy performance of a building and suggested that a maximum 5 ◦C temperature reduction in the monthly average temperature can be anticipated due to the buoyancy air flow behind the PV façade [4]. Tonui et al. reported the use of a suspended thin flat metallic sheet at the middle, or fins at the back wall of an air duct as heat transfer augmentations in an air-cooled photovoltaic/thermal (PV/T) solar collector to improve its overall performance [5,6]. Huang et al. examined the passive cooling of BIPV with PCMs due to the experiments and simulations. They concluded that the operating temperature of the PV system could be reduced by more than 30 ◦C using PCMs with a single flat aluminum plate, and PCMs could be an effective means of reducing the operating temperature in PV devices [7–9]. Hasan et al. analyzed the energy efficiency and financial benefits of a PV system with PCMs [10]. They concluded that a PV system with PCMs were expected to be attractive systems in hot climates, such as Pakistan, but not in moderate climates, like Ireland. This means that the power generation efficiency and cost effectiveness can be strongly dependent on the climatic conditions for the application of PCMs to PV panels. Therefore, when PCMs are applied to a PV system, it is important to evaluate the annual thermal behavior of a PV panel under different weather conditions. The PCM based passive cooling technique for PVs could only be an option in future terms if a significant PCM material price drop were to occur. Furthermore, the future development of passive cooling techniques could be focused on the research of hybrid cooling options [11]. Recently comprehensive analysis of pork fat was addressed in Nižetićet al. for passive cooling applications in photovoltaics as a potential phase change material [12]. Since 2012, the Korean government decided to introduce a renewable portfolio standards (RPS) program to gradually increase the production of renewable energy by electricity companies [13]. In this respect, electricity companies in Korea have investigated a range of new and renewable energy resources, such as solar energy. In particular, to meet the government’s target through this RPS program, PV systems have been propagated as a desirable renewable energy source in urban areas of Korea. The main objective of this paper was to examine the feasibility of PCMs to reduce the temperature rise of solar cells under various weather conditions in Seoul, Korea to increase the power generation efficiency of a PV system. For this purpose, two different PCMs with different phase change characteristics were prepared and the phase change temperatures and thermal conductivities were compared. The diurnal thermal behavior of a PV panel with PCMs under the different weather conditions of Seoul was evaluated using the 2-D transient thermal analysis program. The heat flow characteristics of the PV cell with PCMs and effects of the PCM types and the macro-packed PCM (MPPCM) methods on the operating temperatures under the different weather conditions were examined.

2. Materials and Methods

2.1. Thermal Properties of PCMs The operating temperature of a PV system can be affected by different weather conditions [2,14] and previous research has shown that the maximum PV temperature in southern Libya is approximately 125 ◦C, resulting in a 69% power reduction [15]. In general, the operating temperatures of a PV module should be below 85 ◦C to prevent the temperature-induced power failure and PV cell delamination. In the case of a PV panel integrated with the PCMs, the operating temperature rise depends on the thermal characteristics of the PCMs because PCMs can absorb excess heat until the PCM Energies 2017, 10, 1983 3 of 14 has melted completely. On the other hand, as the operating temperature decreases, the solidification of the PCM can act as an insulation material due to the low thermal conductivity of the PCM. In previous research, Cellura et al. reported that a PCM with a melting temperature between 28 ◦C and 32 ◦C can improveEnergies the 2017 power, 10, 1983 generation efficiency by 20% in summer [16]. To reduce the operating temperature3 of 14 under different weather conditions, the thermal properties, such as phase change temperature and 28 °C and 32 °C can improve the power generation efficiency by 20% in summer [16]. To reduce the specificoperating heat capacity, temperature are under very importantdifferent weather when cond PCMsitions, are the applied thermal to properties, PV modules. such Inas thisphase study, two differentchange temperature PCMs were and prepared specific to heat compare capacity, the are thermal very important behaviors when due toPCMs the different are applied phase to PV change characteristics.modules. In Inthis this study, research, two different fatty acidPCMs ester were (coconutprepared to oil, compare referred the to thermal as PCM1) behaviors and due petroleum to jelly (Vaseline,the different referred phase change to as PCM2)characteristics. were obtainedIn this research, from thefatty Korea acid ester Similac (coconut Corp. oil, ofreferred Hanam, to as South KoreaPCM1) and DukSan and petroleum Corp. ofjelly Seoul, (Vaseline, South referred Korea, to respectively as PCM2) were and obtained prepared from as PCMsthe Korea for Similac a PV panel, whichCorp. possess of Hanam, different South phase Korea change and DukSan temperatures Corp. of Seoul, [17,18 South]. The Korea, phase respectively change temperatures and prepared and specificas PCMs heat capacitiesfor a PV panel, of the which PCMs possess were analyzeddifferent phase by differential change temperatures scanning calorimetry[17,18]. The phase (DSC: TA change temperatures and specific heat capacities of the PCMs were analyzed by differential scanning instrument Q 1000). The temperature accuracy and calorimetric precision of DSC equipment were calorimetry (DSC: TA instrument Q 1000). The temperature accuracy and calorimetric precision of ±0.1 ◦C and ±1%, respectively. DSC measurements were carried out at a 5 ◦C/min heating-cooling DSC equipment were ±0.1 °C and ±1%,◦ respectively. DSC measurements were carried out at a rate,5 and °C/min a temperature heating-cooling range rate, of and 0–75 a temperatureC. The latent range heat of 0–75 capacity °C. The of thelatent PCMs heat capacity was calculated of the by numericalPCMs integrationwas calculated of by the numerical area under integration the peaks of th ate area the under state ofthe the peaks phase at the transitions. state of the Thephase phase ◦ ◦ changetransitions. temperatures The phase of PCM1 change and temperatures PCM2 are around of PCM1 24 Cand and PCM2 44 C, are respectively, around 24 and°C and Figure 44 1°C, shows the specificrespectively, heat capacities.and Figure 1 The shows thermal the specific conductivities heat capacities. of the The PCMs thermal were conductivities measured throughof the PCMs the TCi thermalwere conductivity measured through analyzer. the TCi The thermal precision conductivity and accuracy analyzer. of thisThe precision equipment and were accuracy better of this than 1% and 5%equipment according were to ASTMbetter than 7984 1% respectively. and 5% according The thermal to ASTM conductivities 7984 respectively. of PCM1 The and thermal PCM2 are 0.321conductivities W/m·K and of 0.18 PCM1 W/m and·K, PCM2 respectively. are 0.321 W/m·K and 0.18 W/m·K, respectively.

FigureFigure 1. 1.Latent Latent heat heat capacitiescapacities of of prepared prepared PCMs. PCMs.

2.2. Thermal Model for PV Panel with PCMs 2.2. Thermal Model for PV Panel with PCMs The operating temperature of the PV panel can be affected by the physical variables of the PV Thecell materials, operating module temperature structure of and the its PV configuration, panel can beoutdoor affected weather by the conditions, physical and variables surrounding of the PV cell materials,environment. module In general, structure the PV and panel its configuration,is composed of various outdoor layers weather depending conditions, on the application and surrounding of environment.the PV system. In general, In this thestudy, PV a panel polycrystalline is composed PV module of various was layersunder consideration. depending on This the is application made of of the PVcrystalline system. silicon In this which study, is athe polycrystalline dominant semicond PV moduleucting material was under applied consideration. to PV technology This for is madethe of crystallineproducts silicon of solar which cells. is These the dominant solar cells semiconductingwere assembled into material PV panels applied to generate to PV technologyelectric power for the productsfrom ofsunlight. solar cells. PV module These has solar four cells different were assembledlayers, as shown intoPV in Figure panels 2: to a generateglass covering electric with power fromanti-reflective sunlight. PV coating module (ARC), has PV four cells, different ethylene layers, vinyl asacetate shown (EVA) in Figurelayer, and2: aa glassmetal coveringback sheet with anti-reflectivewith a polyvinyl coating fluoride (ARC), (PVF) PV cells, layer. ethylene Table 1 vinyllists the acetate physical (EVA) properties layer,and of the a metalPV layers. back In sheet this with study, PCM layer was attached to the backside of a PV panel not only by the macro-type container in a polyvinyl fluoride (PVF) layer. Table1 lists the physical properties of the PV layers. In this study, the nylon bag, but also the honeycomb grid steel container to improve the conduction and convection PCMheat layer transfer was attachedthrough the to external the backside surface. of The a PV thickness panel of not PCM only layer by is the 0.01 macro-type m. PCMs were container assumed in the nylonto bag, be assembled but alsothe on the honeycomb metal back grid sheet steel of the container PV panel, to so improve that some the part conduction of the thermal and energy convection from heat transfersolar through irradiation the externaland conduction surface. heat The transfer thickness could of be PCM absorbed layer isby 0.01 the m.PCM. PCMs It was were assumed assumed that to be assembledthere was on theno heat metal transfer back sheetin the ofcros thes-section PV panel, direction so that of the some PCM part layer. ofthe thermal energy from solar

Energies 2017, 10, 1983 4 of 14 irradiation and conduction heat transfer could be absorbed by the PCM. It was assumed that there was noEnergies heat 2017 transfer, 10, 1983 in the cross-section direction of the PCM layer. 4 of 14

(a) (b)(c)

Figure 2. Thermal model for PV panel with PCMs: (a) schematic diagram of thermal model; Figure 2. Thermal model for PV panel with PCMs: (a) schematic diagram of thermal model; (b) and (b) and example of a polycrystalline PV panel; and (c) an example of a PCM and macro-type container example of a polycrystalline PV panel; and (c) an example of a PCM and macro-type container in the in the nylon bag. nylon bag. Table 1. Physical properties of PV layers and PCMs. Table 1. Physical properties of PV layers and PCMs. Conductivity Density Specific Heat Layer Name Thickness (m) 3 Layer Name Thickness (m) Conductivity (W/m(W/m·K)·K) Density (kg/m3)) SpeciﬁcCapacity Heat Capacity(J/kg·K) (J/kg·K) Glass covering 0.003 1.8 3000 500 Glass covering 0.003 1.8 3000 500 PV cellsPV cells 0.00022 0.00022 148 148 2330 2330 677 677 EVAEVA 0.000500.00050 0.350.35 960960 2090 2090 MetalMetal back sheet back sheet 0.00001 0.00001 237 237 2700 2700 900 900 PCM1PCM1 0.01 0.01 0.321 0.321 872 872 Refer Refer to toFigure Figure 11 PCM2 0.01 0.18 850 Refer to Figure1 PCM2 0.01 0.18 850 Refer to Figure 1

To simulateTo simulate the the operating operating temperature temperature of the the PV PV panel panel with with PCMs PCMs under under different different weather weather conditions,conditions, heat heat transfer transfer through through solar solar irradiation,irradiation, convection convection and and radiation radiation should should be considered be considered altogether.altogether. In addition,In addition, the the heat heat capacity capacity of a PV PV cell cell and and PCMs PCMs should should be bemodelled modelled for transient for transient conditions.conditions. To evaluateTo evaluate the the thermal thermal model model for a PV PV panel panel with with PCMs, PCMs, thethe thermal thermal analysis analysis program, program, BISTRA,BISTRA, was was used used for for transient transient heat heat transfer transfer with a a two-dimensional two-dimensional geometry geometry [19]. [ 19In ].this In program, this program, the dynamic calculation parameters of time step interval and automatic triangulation and view factor the dynamic calculation parameters of time step interval and automatic triangulation and view factor calculation module were included. Multiple calculation cycles of linear equations for solving calculation module were included. Multiple calculation cycles of linear equations for solving non-linear non-linear properties were applied. All the temperature calculations at time intervals were carried propertiesout through were applied.the Cranck-Nicolson All the temperature finite difference calculations method. at In time a previous intervals study, were the carried heat transfer out through the Cranck-Nicolsonmodel of PCM was finite validated difference for the method. prediction In of a previousthe heat storage study, theand heatdissipation transfer characteristics model of PCM of was validatedPCM forconcrete the prediction for the application of the heat in storagebuildings. and Th dissipatione verification characteristics results showed of PCMthat the concrete overall for the applicationthermal inbehaviors buildings. of both The the verification experiment results and simulation showed that of PCM the overallconcrete thermal were similar behaviors [20]. In of this both the experimentprogram, and the simulation outdoor weather of PCM conditions concrete are were desc similarribed with [20]. the In thisexternal program, user-defined the outdoor functions weather conditionsbased on are the described typical weather with the data external given at user-defined a fixed time interval. functions PCMs based have on the the non-linear typical weatherthermal data givenproperties. at a fixed The time specific interval. heat PCMs capacity have of the the PCMs non-linear refer to thermal temperature-dependent properties. The specificfunctions, heat which capacity are described in the external text file inputs, between which linear interpolation is used. Direct and of the PCMs refer to temperature-dependent functions, which are described in the external text file diffuse solar irradiation data were used to consider the solar irradiation from the sun. For the fixed inputs, between which linear interpolation is used. Direct and diffuse solar irradiation data were used time interval, the direct solar irradiation and diffuse solar irradiation on a horizontal surface were to considerderived thefrom solar the corresponding irradiation from climate the data. sun. The For sola ther fixed position time was interval, calculated the from direct the solar time irradiationof the andcurrent diffuse time-step solar irradiation and the geographical on a horizontal location surface (latitude, were longitude, derived time from zone). the To corresponding consider the solar climate data.position The solar under position the different was calculatedclimate seasons, from a thelatitu timede of of 37.6 the N current and longitude time-step of 127 and E in the Seoul geographical were location (latitude, longitude, time zone). To consider the solar position under the different climate

Energies 2017, 10, 1983 5 of 14 seasons, a latitude of 37.6 N and longitude of 127 E in Seoul were assumed. The position of the sun was described by azimuth angle (horizontal clockwise angle from the south to the sun) and elevation angleEnergies (vertical 2017, angle10, 1983 from the horizon to the sun). 5 of 14 Natural convective heat transfer from the panel surface to the surrounding air was considered basedassumed. on ISO The 6946 position [21]. The of the convective sun was described heat transfer by azimuth coefﬁcients angle at (horizonta the frontl clockwise and rear ofangle the from PV panel werethe determined south to the by sun) the followingand elevation equations. angle (vertical For each angle case from of the horizon front and to rearthe sun). surface, the parameters Natural convective heat transfer from the panel surface to the surrounding air was considered of C2 and C3 were given different values. based on ISO 6946 [21]. The convective heat transfer coefficients at the front and rear of the PV panel

were determined by the following equations. For each case of theC3 front and rear surface, the parameters hc = 2 × C2 × (∆θsa) (1) of and were given different values.

∆θsa = maxℎ =2×(|θa − θs× min(∆|,|θ)s max − θa|) (1) (2) h C C (2) where c is convective heat transfer∆ coefficient=(| from− surface|, | to air,−|)2 and 3 are parameters based on heat flow direction, ∆θsa is the greatest difference between air temperature and surface temperature, where ℎ is convective heat transfer coefficient from surface to air, and are parameters based θa is an air temperature, θs min is the minimum surface temperature in contact with boundary condition, on heat flow direction, ∆ is the greatest difference between air temperature and surface and θ is the maximum surface temperature in contact with boundary condition. In the RADCON temperature,s max is an air temperature, is the minimum surface temperature in contact with moduleboundary in BISTRA, condition, each and grey surface is isthe connected maximum to surface a black temperature surface node in andcontact all blackwith boundary surface nodes are connectedcondition. toIn eachthe RADCON other in module a so-called in BISTRA, star diagram. each grey Radiative surface is heat connected transfer to betweena black surface the real node surface withand given all emissivityblack surface and nodes the correspondingare connected to blackeach other surface in a was so-called given star asfollows. diagram. Radiative heat transfer between the real surface with given emissivity and the corresponding black surface was given as follows. ε Qr = A· ·hrb·(θsb − θs) (3) 1− ε = ∙ ∙ℎ ∙( −) (3) 1− where Qr is the radiative heat transfer between real grey surface and black surface, A is the surface where is the radiative heat transfer between real grey surface and black surface, is the surface area, ε is the surface emissivity, hrb is radiation heat transfer coefficient of the black surface, θsb is area, is the surface emissivity, ℎ is radiation heat transfer coefficient of the black surface, is surface temperature of black surface, and θs is the surface temperature of the real surface. There can be surface temperature of black surface, and is the surface temperature of the real surface. There can a resultant radiation heat flow from the sky environment to the material surface. be a resultant radiation heat flow from the sky environment to the material surface. At the outer surface of the glass covering, an angle-dependent reflection factor, which is defined At the outer surface of the glass covering, an angle-dependent reflection factor, which is defined by aby function a function referred referred to to Figure Figure3, 3, was was considered. considered. ForFor normal normal incident incident solar solar irradiation, irradiation, the solar the solar propertiesproperties of theof the outer outer surface surface of of the the glass glass coveringcovering are as as follows: follows: ρ0 ρ=0 0.08= 0.08 and and τ0 =τ 0.83.0 = 0.83.Here, Here, ◦ the subscriptthe subscript 0 refers 0 refers to to the the incident incident angle angle ofof 00°,, irradiation normal normal to tothe the PV PVpanel panel surface. surface. It was It was assumedassumed that that the the PV PV panel panel faced faced the the south south andand the tilt tilt angle angle from from the the horizontal horizontal surface surface was fixed was fixed to 30to◦. 30°.

FigureFigure 3. Reﬂection3. Reflection factor factorof of glassglass material according according to tothe the incidence incidence angle. angle.

Energies 2017, 10, 1983 6 of 14 Energies 2017, 10, 1983 6 of 14

2.3.2.3. Validation Validation of of the the Thermal Thermal Model Model forfor PVPV Panel with PCMs PCMs TheThe thermal thermal model model for for PVPV panelspanels with PCMs wa wass validated validated through through the the experimental experimental results. results. For this purpose, field experiments were carried out at Seosan, South Korea where a PV system For this purpose, field experiments were carried out at Seosan, South Korea where a PV system coupled with PCM1 (coconut oil) was investigated. As shown in Figure 2b,c, polycrystalline PV panel coupled with PCM1 (coconut oil) was investigated. As shown in Figure2b,c, polycrystalline PV was installed to the south and a PCM layer was attached to the backside of the PV panel by the panel was installed to the south and a PCM layer was attached to the backside of the PV panel by the macro-type container in the nylon bag. The PV cell temperatures were measured by attaching a macro-type container in the nylon bag. The PV cell temperatures were measured by attaching a T-type T-type thermocouple between PCM container and the backside of the PV panel. Outdoor thermocoupletemperatures between and solar PCM irradiations container were and automati the backsidecally measured of the PV panel.every five Outdoor minutes temperatures near the test and solarfacility irradiations from sunrise were to automatically sunset. These measured data were every converted five minutes into input near data the in test one facility hour fromintervals sunrise for to sunset.verification These of data the werethermal converted model for into PV input panels data with in PCMs one hour using intervals the BISTRA for verification program. Based of the on thermal the modelmeasured for PV climate panels data with and PCMs specific using thermal the BISTRA property program. data, numerical Based on thesimulations measured were climate conducted data and specificfor the thermal case of PCM1. property Figure data, 4 numericalshows the comparison simulations of were simulation conducted results for and the caseexperimental of PCM1. results, Figure 4 showswhere the there comparison is good agreement of simulation between results the and simulati experimentalon results results, and experimental where there results. is good Based agreement on betweenthe validation the simulation results, it results can be and concluded experimental that the results. thermal Based model on for the PV validation panel with results, PCMs itof canthe be concludedBISTRA program that the is thermal acceptable model to evaluate for PV panel the th withermal PCMs performance of the BISTRA of PV-PCM program systems is acceptableand that it to evaluatecan provide the thermal reasonable performance results. of PV-PCM systems and that it can provide reasonable results.

FigureFigure 4. 4.Comparison Comparison of the simulation results results and and experimental experimental results. results.

2.4. Weather Conditions and Simulation Methods 2.4. Weather Conditions and Simulation Methods

2.4.1.2.4.1. Weather Weather Conditions Conditions The climate of Seoul is temperate with four distinct seasons. In summer, it is very hot and humid The climate of Seoul is temperate with four distinct seasons. In summer, it is very hot and humid with temperatures soaring as high as 35 °C due to the influence of the North Pacific high-pressure with temperatures soaring as high as 35 ◦C due to the influence of the North Pacific high-pressure zones. Most of the precipitation falls in summer between June and September. In winter, Seoul is zones. Most of the precipitation falls in summer between June and September. In winter, Seoul is influenced by the Siberian high-pressure zones and temperatures drop to as low as −20 °C. Figure 5 influenced by the Siberian high-pressure zones and temperatures drop to as low as −20 ◦C. Figure5 shows the four different weather conditions of Seoul, which are based on the typical weather data showsfromthe the four Korea different Meteorological weather conditions Administration of Seoul, (KMA). which These are based data onwere the referenced typical weather to the data input from the Korea Meteorological Administration (KMA). These data were referenced to the input functions functions of the outdoor temperatures (To), direct solar irradiation (Edir), and diffuse solar irradiation of( theEdif).outdoor In the case temperatures of an intermediate (To), directseason solar with irradiationcloudy skies ( andEdir), moderate and diffuse temperature, solar irradiation the outdoor (Edif ). Intemperatures the case of anrange intermediate from 10 °C season to 19.5 with °C, and cloudy the maximum skies and moderatedirect and temperature,diffuse solar irradiation the outdoor ◦ ◦ temperaturesreaches approximately range from 230 10 W/mC to2 and 19.5 65C, W/m and2. theIn the maximum case of directsummer and with diffuse clear solarskies irradiationand hot reachestemperatures, approximately the outdoor 230 temperatures W/m2 and 65 range W/m from2. In 21 the °C caseto 29.5 of °C, summer and the with maximum clear skies direct and and hot temperatures,diffuse solar irradiation the outdoor reaches temperatures approximately range 650 from W/m 21 2◦ andC to 260 29.5 W/m◦C,2. and In the the case maximum of summer direct with and diffusecloudy solar skies irradiation and hot temperatures reaches approximately, the outdoor 650 temperatures W/m2 and 260range W/m from2. In21 the°C to case 29.5 of °C, summer and the with ◦ ◦ cloudy skies and hot temperatures, the outdoor temperatures range from 21 C to 29.5 C, and the Energies 2017, 10, 1983 7 of 14 Energies 2017, 10, 1983 7 of 14 maximummaximum direct direct and and diffuse diffuse solar solar irradiation irradiation reachesreaches approximatelyapproximately 520 W/m 2 2andand 200 200 W/m W/m2. In2. the In the casecase of of winter winter with with cloudy cloudy skies skies andand coldcold temperatures,temperatures, the the outdoor outdoor temperatures temperatures range range from from −4− °C4 ◦C toto 4 ◦4C, °C, and and the the maximum maximum direct directand and diffusediffuse solarsolar irradiation reaches reaches approximately approximately 230 230 W/m W/m2 and2 and 100100 W/m W/m2.2.

(a) (b)

FigureFigure 5. 5.Four Four differentdifferent weatherweather conditions of of Seoul: Seoul: (a ()a )cloudy cloudy sky sky and and moderate moderate temperature temperature (intermediate season); (b) clear sky and hot temperature (summer season); (c) cloudy sky and hot (intermediate season); (b) clear sky and hot temperature (summer season); (c) cloudy sky and hot temperature (summer season); (d) cloudy sky and cold temperature (winter season). temperature (summer season); (d) cloudy sky and cold temperature (winter season). 2.4.2. Simulation Methods 2.4.2. Simulation Methods For the application of PCMs in buildings, leakage of the liquid-state PCM should be prevented. ForFor this the purpose, application many of studies PCMs have in buildings, been conducte leakaged to of evaluate the liquid-state the possibility PCM shouldof a container be prevented. that Forcan this prevent purpose, the manyleakage studies of liquid-state have been PCM conducted using the to evaluateshape-stabilized the possibility PCM (SSPCM), of a container or thatmicroencapsulated can prevent the PCM leakage (MPCM), of liquid-stateor macro-packed PCM PCM using (MPPCM) the shape-stabilized [22–25]. In this PCM study, (SSPCM), the orMPPCMs microencapsulated are applied to PCM the backside (MPCM), of a or PV macro-packed panel not only by PCM the macro-type (MPPCM) container [22–25]. in In the this nylon study, thebag MPPCMs [17], but also are appliedthe honeycomb to the grid backside steel containe of a PVr panelto improve not only the conduction by the macro-type and convection container heat in thetransfer nylon through bag [17 ],the but external also thesurface honeycomb [26]. Therefore, grid steel to evaluate container the thermal to improve behavior the conductionof a PV panel and convectionwith different heat transferPCMs under through the thedifferent external weather surface conditions [26]. Therefore, of Seoul, to two evaluate MPPCM the thermalmethods behavior were ofselected a PV panel as the with simulation different cases. PCMs As under listed the in differentTable 2, weatherthere are conditions five different of Seoul, simulation two MPPCMcases methodsaccording were to the selected different as the MMPCM simulation methods cases. and As PCM listed types. in Table Case12, there is the are general ﬁve differentPV panel simulationwithout casesPCMs, according and other to cases the different are PV panels MMPCM with PCMs. methods In this and study, PCM the types. calculation Case1 istime-step the general was 10 PV min. panel without PCMs, and other cases are PV panels with PCMs. In this study, the calculation time-step was 10 min.

Energies 2017, 10, 1983 8 of 14

Table 2. Details for the simulation cases.

Simulation Cases MMPCM Methods PCM Types Case1 - - Case2 PCM1 Honeycomb grid steel container Fatty acid ester (Coconut oil) Case2 PCM2 Honeycomb grid steel container Petroleum jelly (Vaseline) Case3 PCM1 Nylon bag container Fatty acid ester (Coconut oil) Case3 PCM2 Nylon bag container Petroleum jelly (Vaseline)

3. Results and Discussion The thermal behavior of a PV panel with PCMs under the different weather conditions of Seoul was evaluated using the 2-D transient thermal analysis program, BISTRA. The total heat ﬂux from a PV cell with PCMs to the surroundings and the effects of the PCM types and the MPPCM methods on the operating temperatures under different weather conditions are discussed.

3.1. Mean Opeating Temperatures of PV Cells In this section, to evaluate the thermal behavior of a PV panel with PCMs, the mean operating temperatures of a PV cell were analyzed under different weather conditions. Figure6 presents the results of the operating temperatures of a PV cell under different weather conditions. Due to the different phase change temperature ranges of which PCM1 and PCM2 were around 10 to 30 ◦C and 30 ◦C to 60 ◦C, respectively, the thermal behavior of the PV cells with PCMs were different. Especially since the phase change temperature of PCM1 is in the range of 10 ◦C to 30 ◦C, the thermal performance of Case3 PCM1 becomes worse than Case1 when the mean operating temperature of the PV cell becomes higher than 30 ◦C. In the morning, the mean operating temperatures increase gradually due to the solar heat gain and convective heat transfer at the panel surfaces. In the case of an intermediate season with cloudy skies and moderate temperatures, the mean operating temperatures of a PV cell with PCM1 and PCM2 decrease in the morning by 1.5 ◦C and 3.2 ◦C, respectively, compared to Case1. On the other hand, the mean operating temperatures of Case1 decrease considerably after 15:00 compared to those of PV cells with PCMs. This is related to the specific heat capacity and low thermal conductivity of PCMs. Owing to the small specific heat capacity of PCM1 compared to PCM2, the heat absorption rates of a PV cell with PCM1 can be smaller than the rates of a PV cell with PCM2. In addition, the heat release rates in Case2 and Case3 are reduced due to the low thermal conductivity of PCMs. Using a honeycomb grid steel container to increase the thermal conductance, a slight reduction of the mean operating temperatures is anticipated due to the low convective heat release to the surroundings. In the case of summer with clear skies and hot temperatures, the mean operating temperatures of the PV cell with PCM2 decreased in the morning by 4.0 ◦C in the Case2 PCM2 and 2.5 ◦C in the Case3 PCM2 compared to Case1. On the other hand, the mean operating temperatures of a PV cell with PCM1 decreased before 10:00 a.m. but increased after 10:00 a.m. In addition, in the case of summer with cloudy skies and hot temperatures, the results were similar to the case of summer with clear skies and hot temperatures. This means that PCM1 is unsuitable for use in thermal storage due to the small specific heat capacity in summer. In the case of winter with cloudy skies and cold temperatures, the mean operating temperatures of the PV cell with PCM2 was 2.0 ◦C lower until 16:00 compared to Case1. On the other hand, the mean operating temperatures of the PV cells with PCM1 were similar to Case1. As shown in Figure5, the operating temperature drop using a PCM2 with a phase change temperature of 44 ◦C was higher than that using a PCM1 with a phase change temperature of 24 ◦C under the weather conditions of Seoul due to the thermal properties, such as the phase change temperature, and specific heat capacity, and thermal conductivity. Therefore, the selection of PCM type was more important than the MMPCM methods when the PCMs are used for the performance enhancement of a PV panel. Energies 2017, 10, 1983 9 of 14 Energies 2017, 10, 1983 9 of 14

(a) (b)

Figure 6. Mean operating temperatures of PV cell under the different weather conditions: (a) cloudy Figure 6. Mean operating temperatures of PV cell under the different weather conditions: (a) cloudy sky and moderate temperature (intermediate season); (b) clear sky and hot temperature (summer sky and moderate temperature (intermediate season); (b) clear sky and hot temperature (summer season); (c) cloudy sky and hot temperature (summer season); and (d) cloudy sky and cold temperature season); (c) cloudy sky and hot temperature (summer season); and (d) cloudy sky and cold temperature (winter season). (winter season). 3.2. Total Heat Flux from the PV Panel to the Surroundings 3.2. Total Heat Flux from the PV Panel to the Surroundings In this section, the total heat flux from a PV panel to the surroundings was analyzed to estimate theIn heat this balance section, of the a totalPV panel heat fluxunder from different a PV panel weather to the conditions. surroundings Figure was 7 compares analyzed the to estimatemean theoperating heat balance temperatures of a PV of panel a PV under cell and different total heat weather flux from conditions. a PV panel Figure under7 comparesclear skies theand meanhot operatingtemperatures temperatures in summer. of In a PVCase1, cell the and total total heat heat flux flux changed from with a PV the panel solar under irradiation clear and skies outdoor and hot temperaturestemperature in due summer. to the Insmall Case1, heat the capacity total heat of th fluxe PV changed panel. In with the theafternoon, solar irradiation there was and a sudden outdoor temperaturechange in the due time to theof the small total heat heat capacityflux because of the of the PV sudden panel. decrease In the afternoon, in absorbed there solar was irradiation a sudden changedue to in the the reflection time of the factor total of heat the glass flux becausecovering. of When the sudden the sun decrease was not inshining, absorbed the solartotal heat irradiation flux duefrom to the the reflection PV panel factorshowed of little the glass change. covering. The maxi Whenmum the heat sun flux was was not approximately shining, the total 14.5 heat W/m flux when from thethe PV total panel solar showed irradiation little change.was at a The maximum maximum during heat the flux day. was In approximately Case2 PCM1, 14.5the total W/m heat when flux the totalchanged solar irradiation slightly until was 10:00, at a but maximum this increased during abruptly the day. after In Case2 10:00. PCM1, This is thebecause total heatthe specific flux changed heat slightlycapacity until of 10:00,PCM1 but was this too increasedsmall. In the abruptly afternoon, after the 10:00. difference This is in because the total the heat specific flux was heat similar capacity to of PCM1that wasof Case1. too small. In Case2 In the PCM2, afternoon, the total the heat difference flux from in thethe totalPV panel heat was flux smaller was similar than tothat that of ofCase1 Case1. In Case2due to PCM2,the thermal the total effect heat of PCM2. flux from The the maximum PV panel to wastal heat smaller flux was than approximately that of Case1 due 12.1 to W/m the when thermal there was a sudden decrease in the absorbed solar irradiation due to the reflection factor of the glass effect of PCM2. The maximum total heat flux was approximately 12.1 W/m when there was a sudden covering. When the sun was not shining in the afternoon, total heat flux from the PV panel decreased decrease in the absorbed solar irradiation due to the reflection factor of the glass covering. When the gradually to 3 W/m. In Case3 PCM2, the total heat flux change process was similar to Case2 PCM2. sun was not shining in the afternoon, total heat flux from the PV panel decreased gradually to 3 W/m. On the other hand, the maximum total heat flux of approximately 14.4 W/m was somewhat higher In Case3 PCM2, the total heat flux change process was similar to Case2 PCM2. On the other hand, the than Case2 PCM2 due to the difference in the heat transfer rate with the MPPCM methods. This maximum total heat flux of approximately 14.4 W/m was somewhat higher than Case2 PCM2 due to the difference in the heat transfer rate with the MPPCM methods. This means that the mean operating Energies 2017, 10, 1983 10 of 14 Energies 2017, 10, 1983 10 of 14

means that the mean operating temperature of the PV cell and total heat flux from the surface can be temperature of the PV cell and total heat ﬂux from the surface can be reduced by increasing the heat reduced by increasing the heat transfer rate through the honeycomb grid steel container for PCMs. transfer rate through the honeycomb grid steel container for PCMs.

(a) (b)

Figure 7. Comparison of mean operating temperatures of PV cell and total heat flux from panel to Figure 7. Comparison of mean operating temperatures of PV cell and total heat flux from panel to surrounding under the clear sky and hot temperature (summer season): (a) Case1; (b) Case2 PCM1; surrounding under the clear sky and hot temperature (summer season): (a) Case1; (b) Case2 PCM1; (c) Case2 PCM2; and (d) Case3 PCM2. (c) Case2 PCM2; and (d) Case3 PCM2. Figure 8 compares the mean operating temperatures of a PV cell and the total heat flux from the PVFigure panel 8under compares cloudy the skies mean and operating moderate temperatures temperatures ofin aintermediate PV cell and season. the total In heat Case1, flux the from total the PVheat panel flux under changed cloudy with skiesthe solar and irradiation moderate temperaturesand outdoor temperature in intermediate due to season. the small In heat Case1, capacity the total heatof the flux PV changed panel. In with addition, the solar when irradiation the sun was and not outdoor shining, temperature the total heat due flux to thefrom small PV panel heat capacityshowed of thelittle PV panel.change. In The addition, maximum when total the sun heat was flux not was shining, approximately the total heat 5.0 flux W/m from wh PVen panelthe direct showed solar little change.irradiation The maximumwas a maximum total heat during flux the was day. approximately Although the 5.0 specific W/m heat when capacity the direct of PCM1 solar irradiationwas too wassmall, a maximum the heat storage during theeffect day. of AlthoughPCM1 can thebe found specific by heat comparing capacity the of total PCM1 changes was too in small,heat flux the in heat storageCase1 effectwith ofthose PCM1 in Case2 can be PCM1. found byThe comparing approximatel they total two changes hour time in heatdelay flux effect in Case1 was confirmed. with those in Case2Under PCM1. the intermediate The approximately climatic conditions two hour of time low solar delay irradiation effect was intensity confirmed. and moderate Under the temperatures, intermediate climaticthe mean conditions operating of temperature low solar irradiationgenerally decreased intensity in and the morning moderate and temperatures, increased in the the afternoon. mean operating This temperatureis because the generally amount of decreased heat absorbed in the in morning the PCMs and in the increased morning in is thereleased afternoon. to the atmosphere This is because in the the afternoon. On the other hand, there was no large change in the mean operating temperature using PCM1 amount of heat absorbed in the PCMs in the morning is released to the atmosphere in the afternoon. because the amount of direct solar irradiation during the day was less than approximately 230 W/m2 and On the other hand, there was no large change in the mean operating temperature using PCM1 because the specific heat capacity of PCM1 was low. the amount of direct solar irradiation during the day was less than approximately 230 W/m2 and the specific heat capacity of PCM1 was low.

Energies 2017, 10, 1983 11 of 14

EnergiesEnergies2017 2017, 10, ,10 1983, 1983 11 of11 14 of 14

(a) (b) (a) (b)

(c()c ) (d()d) Figure 8. Comparison of mean operating temperatures of PV cell and total heat flux from panel to Figure 8. ComparisonComparison of of mean mean operating operating temperatures temperatures of of PV PV cell cell and and total total heat heat flux ﬂux from from panel panel to surrounding under the cloudy sky and moderate temperature (intermediate season): (a) Case1; tosurrounding surrounding under under the the cloudy cloudy sky sky and and moderate moderate temperature temperature (intermediate (intermediate season): season): ( (aa)) Case1; Case1; (b) Case2 PCM1; (c) Case2 PCM2; and (d) Case3 PCM2. (b) Case2 PCM1; ((c)) Case2Case2 PCM2;PCM2; andand ((d)) Case3Case3 PCM2.PCM2.

FigureFigure 9 9compares compares the the mean mean operatingoperating temperatures ofof the the PV PV cell cell and and the the total total heat heat flux flux from from Figure9 compares the mean operating temperatures of the PV cell and the total heat flux from PVPV panel panel under under cloudy cloudy skies skies and and coldcold temperaturestemperatures inin winter.winter. A A comparison comparison of of Case1 Case1 with with Case2 Case2 PVPCM1 panel showed under that cloudy the skiesdifference and was cold not temperatures significant due in winter.to the small A comparison specific heat of capacity Case1 withof PCM1 Case2 PCM1 showed that the difference was not significant due to the small specific heat capacity of PCM1 PCM1in winter. showed Using that PCM2 the difference for the PV was panel, not the significant total heat due flux to can the be small changed specific due heat to the capacity heat storage of PCM1 in winter. Using PCM2 for the PV panel, the total heat flux can be changed due to the heat storage ineffect winter. of PCM2, Using PCM2but the formean the operating PV panel, temper the totalature heat change flux was can not be changedlarge during due the to day. the heat storage effect of PCM2, but the mean operating temperature change was not large during the day. effect of PCM2, but the mean operating temperature change was not large during the day.

(a) (b) (a) (b)

Figure 9. Cont.

EnergiesEnergies2017 2017, 10, ,10 1983, 1983 12 of12 14 of 14

Figure 9. Comparison of mean operating temperatures of PV cell and total heat flux from panel to Figure 9. Comparison of mean operating temperatures of PV cell and total heat flux from panel to surrounding under the cloudy sky and cold temperature (winter season): (a) Case1; (b) Case2 PCM1; surrounding under the cloudy sky and cold temperature (winter season): (a) Case1; (b) Case2 PCM1; (c) Case2 PCM2; and (d) Case3 PCM2. (c) Case2 PCM2; and (d) Case3 PCM2. 3.3. Discussions about the Improvement of Power Generation Efficiency 3.3. Discussions about the Improvement of Power Generation Efficiency Several studies have expressed the PV cell temperature as a function of the weather variables, suchSeveral as the studies outdoor have temp expressederature and the the PV wind cell temperature speed, as well as aas function solar irradiation of the weather intensity variables, with suchsystem-dependent as the outdoor properties. temperature In andgeneral, the windthe ef speed,fect of asthe well PV ascell solar temperature irradiation on intensity the power with system-dependentgeneration efficiency properties. of a PV In cell general, can be the related effect ofto the the PV fill cell factor temperature (FF), open-circuit on the power voltage, generation and efficiencyshort-circuit of a PVcurrent. cell can As bethe related thermally-excited to the fill factor electron (FF),s begin open-circuit to dominate voltage, the andelectrical short-circuit properties current. of Asa thePV thermally-excitedcell, both the open-circuit electrons voltage begin and to the dominate FF decrease the electricalsubstantially properties with temperature, of a PV cell, whereas both the open-circuitthe short-circuit voltage current and increases the FF decrease slightly. substantiallyThe electrical power with temperature, output of a PV whereas module can the be short-circuit derived currentfrom increasesthe standard slightly. test conditions The electrical prepared power by output the manufacturer of a PV module of the can module. be derived Many from correlations the standard testhave conditions been reported prepared for byPV the power manufacturer generated ofas thea function module. of Many the cell/module correlations operating have been temperature reported for PVand power basic generated environmental as a function variables of [27]. the Many cell/module of them operating are linear temperatureequations, whereas and basic others environmental are more variablescomplex, [27 nonlinear]. Many multivariable of them are linearregression equations, equations whereas [28]. othersIn the arecase more of PCM complex, applications, nonlinear multivariablehowever, it is regression difficult to equations predict the [28 PV]. cell In thetemperature case of PCM based applications, on existing studies however, because it is difficultthe heat to predicttransfer the characteristics PV cell temperature are different based from onexisting those of studiesexisting because products. the The heat PV transfer cell temperature characteristics can be are predicted by evaluating the dynamic thermal behavior of the PCM, as in this study, and the different from those of existing products. The PV cell temperature can be predicted by evaluating the improvement of the power generation efficiency can be evaluated using these results. In a typical PV dynamic thermal behavior of the PCM, as in this study, and the improvement of the power generation panel, high operating temperatures induce an efficiency drop at an approximate rate of 0.5%/°C over efficiency can be evaluated using these results. In a typical PV panel, high operating temperatures a nominal PV cell operating temperature of 25 °C, as determined by the standard test conditions induce an efficiency drop at an approximate rate of 0.5%/◦C over a nominal PV cell operating (STC) [29]. Considering◦ the mean operating temperature reduction of 4 °C by PCM in this study, an temperatureefficiency improvement of 25 C, as of determined approximately by the 2% standardcan be estimated. test conditions On the other (STC) hand, [29]. the Considering effect of PV the ◦ meancell temperature operating temperature reduction by reduction PCM depends of 4 onC bythe PCMclimatic in thisconditions, study, ansuch efficiency as solar irradiation improvement and of approximatelyoutdoor temperature. 2% can be Therefore, estimated. it is On necessary the other to hand, design the and effect evaluate of PV the cell performance temperature considering reduction by PCMthe dependsannual climate on the characteristics. climatic conditions, such as solar irradiation and outdoor temperature. Therefore, it is necessary to design and evaluate the performance considering the annual climate characteristics. 4. Conclusions 4. Conclusions To lower the operating temperature of a solar cell, free cooling on the back side of the PV panel dueTo to lower the natural the operating convection temperature can be applied. of a solarNowa cell,days, free PV cooling systems on with the PCMs back sideare expected of the PV to panelbe dueattractive to the naturalsystems convection in hot climates. can be To applied. apply PCMs Nowadays, to PV systems, PV systems it is withimportant PCMs to areevaluate expected the to beannual attractive thermal systems behavior in hot of climates.a PV panel To under apply different PCMs weather to PV systems, conditions. it is Especially important in to Korea, evaluate there the annualare four thermal distinct behavior seasons, of so a the PV effect panel of under PCMs different on the power weather generation conditions. efficiency Especially should in Korea,be clearly there areevaluated four distinct depending seasons, on sothe the different effect ofclimate PCMs conditions. on the power This study generation aimed efficiency to examine should the feasibility be clearly evaluatedof phase dependingchange materials on the (PCMs) different to climatereduce the conditions. temperature This rise study of a aimedsolar cell to examineunder the the climate feasibility in ofSeoul, phase Korea. change For materials this purpose, (PCMs) the todiurnal reduce therma the temperaturel behavior of risea PV of panel a solar with cell PCMs under was the evaluated climate in Seoul,under Korea. the different For this weather purpose, conditions the diurnal of Seoul thermal through behavior the 2-D of atransient PV panel thermal with PCMs analysis was program. evaluated underThe heat the differentflow characteristics weather conditions though the of PV Seoul cell with through PCMs the and 2-D the transient effects of thermal the PCM analysis types and program. the The heat flow characteristics though the PV cell with PCMs and the effects of the PCM types and the

Energies 2017, 10, 1983 13 of 14 macro-packed PCM (MPPCM) methods on the operating temperatures under the different weather conditions were discussed. In this study, two different PCMs were prepared by considering a melting temperature range, and thermal properties (specific heat, thermal conductivity) were measured. The phase change temperatures of PCM1 and PCM2 were around 24 ◦C and 44 ◦C, respectively. Thermal conductivities of PCM1 and PCM2 were 0.321 W/m·K and 0.18 W/m·K, respectively. To evaluate the thermal model for a PV panel with PCMs, thermal analysis program, BISTRA, was used for transient heat transfer with a two-dimensional geometry. Thermal model for PV panel with PCM was validated through the experimental results. Based on the validation results, it can be concluded that the thermal model for PV panel with PCMs of BISTRA program is acceptable to evaluate the thermal performance of the PV-PCM system and it can provide reasonable results. The total heat flux from a PV cell with PCMs to the surroundings and the effect of the PCM types and MPPCM methods on the operating temperatures under different weather conditions are discussed. Considering the mean operating temperature reduction of 4 ◦C by PCMs in this study, an efficiency improvement of approximately 2% can be estimated under the weather conditions of Seoul. The selection of the PCM type was found to be more important than the MMPCM methods when the PCMs are used for the performance enhancement of a PV panel and the mean operating temperature of the PV cell and total heat flux from the surface can be reduced by increasing the heat transfer rate through the honeycomb grid steel container for PCMs. An improvement in the power generation efficiency of a PV panel can be anticipated by adding PCMs to the backside of PV panels.

Acknowledgments: This research was supported by a grant (15CTAP-C098006-01) from Technology Advancement Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. Author Contributions: Jae-Han Lim conceived and designed the simulations, and analyzed the data and wrote the paper; Yoon-Sun Lee performed the simulations; Yoon-Bok Seong contributed materials and analysis tools. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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