Analysis of a Residential Photovoltaic-Thermal (PVT) System in Two Similar Climate Conditions Madalina Barbu, George Darie, Monica Siroux

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Madalina Barbu, George Darie, Monica Siroux. Analysis of a Residential Photovoltaic-Thermal (PVT) System in Two Similar Climate Conditions. Energies, MDPI, 2019, 12 (19), pp.3595. 10.3390/en12193595. hal-03070747

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Article Analysis of a Residential Photovoltaic-Thermal (PVT) System in Two Similar Climate Conditions

Madalina Barbu 1,2,* , George Darie 1 and Monica Siroux 2

1 Department of Power Production and Usage, University Politehnica of Bucharest, 060042 Bucharest, Romania 2 INSA Strasbourg ICUBE, University of Strasbourg, 67000 Strasbourg, France * Correspondence: [email protected]; Tel.: +40-741-551-818

Received: 23 July 2019; Accepted: 18 September 2019; Published: 20 September 2019

Abstract: Photovoltaic-thermal (PVT) panels combine solar thermal and photovoltaic technologies and generate simultaneously both heat and electricity. This paper looks at the potential of integrating these systems into small domestic prosumer households for the climates of Bucharest, Romania, and Strasbourg, France. First, some brief background information on PVT systems and the concept of prosumers is introduced, highlighting their features as well as the solar energy market setting in Romania and France. Next, a PVT system is proposed for a given household consumer in Strasbourg and Bucharest with the variable weather conditions corresponding to the two locations. The PVT system and the coupled consumer are modelled in TRNSYS (v17, Thermal Energy System Specialists, Madison, USA). A performance analysis is carried out in order to establish the daily instantaneous energy output and the annual energy production. The results indicate a 10–12% better performance in Bucharest compared to Strasbourg due to slightly better weather conditions. The system efficiency was assessed through various methods (first law efficiency and primary energy saving). Depending on the method used, the location and time of year, the results vary from 15% for the first law efficiency to 90% for the primary energy saving efficiency. The most suitable efficiency assessment method for this study was found to be the primary energy saving method, as it takes into account the regional differences in energy production. This study concludes that the Romanian PVT market has a good potential for adopting the technology, especially since it is currently less mature than in France.

Keywords: renewable energy; solar energy; photovoltaic-thermal; micro-cogeneration; prosumer

1. Introduction Solar energy is the most wide-spread type of renewable energy, due to its multiple benefits: It is easily available in many locations, has low operational costs and can be adapted to small, decentralized systems. In theory, there is sufficient solar radiation reaching the Earth to meet 10,000 times the current energy needs of the world [1]. The established solar technologies currently available on the market can be broadly classified into photovoltaic (PV) panels and passive solar-thermal panels (ST). Hybrid photovoltaic-thermal systems (PVT) combine these two technologies and maximize their benefits into one single piece of equipment, producing both electricity and useful heat (Figure1). PVT systems are a type of micro-cogeneration technology that can be very efficiently integrated into individual households. This can achieve decentralized production of clean heat and energy, maximizing the South facing available space and obtaining an overall better payback of the investment, compared to separate side-by-side PV and ST systems [2].

Energies 2019, 12, 3595; doi:10.3390/en12193595 www.mdpi.com/journal/energies Energies 2019, 12, 3595 2 of 18 Energies 2019, 12, x FOR PEER REVIEW 2 of 18

43 FigureFigure 1. 1.Schematic Schematic diagram diagram of of the the functioning functioning principle principle of of solar-thermal solar-thermal (ST), (ST) photovoltaic, photovoltaic (PV) (PV and) 44 photovoltaic-thermal (PVT) systems.and photovoltaic-thermal (PVT) systems.

45 TheThe current current social social trend trend of of environmental environmental responsibility, responsibility, combined combined with with the the various various financial financial 46 incentivesincentives available available for for renewable renewable energy energy (Feed (Feed-in-in Tariffs, Tariffs, Green Green Certificates), Certificates), lead lead to to an an increasing increasing 47 numbernumber of of individual individual consumers consumers that that wish wish to to produce produce and and utilize utilize their their own own energy energy ( (‘prosumers’).‘prosumers’). 48 OnOn t thishis market market there there is is a a great great potential potential for for introducing introducing the the PVT PVT technology technology in in order order to to meet meet their their 49 energyenergy demand demand with with clean clean and and on on-site-site produced produced heat heat and and power. power. 50 ResidentialResidential PVT PVT systems systems and and their their applications applications in various in various climate climate conditions conditions have been have of beenresearch of 51 researchinterest ininterest the past in the years, past with years multiple, with multiple reviews andreviews state an ofdthe state art of papers the art on papers this topic on this [3, 4topic]. A study[3,4]. 52 Ahas study been has carried been outcarried to assess out to the assess performance the performance of building of building integrated integrated PVT (BIPVT) PVT compared(BIPVT) compared to simple 53 tobuilding simple integratedbuilding integrated PV (BIPV) PV in (BIPV) the context in the ofcontext New Delhi,of New India Delhi, [5 ],India and [ also5], and to analyzealso to analyze various 54 varioustypes of types PV cells: of PV Mono-crystalline cells: Mono-crystalline and amorphous and amorphous silicon. Itsilicon. was found It was that found although that although the first wasthe 55 firstmore was effi cient, more the efficient, latter is the more latter economical is more economical for the regional for the context. region Anotheral context study. Another [6] compares study [ the6] 56 comparesperformance the ofperformance PVT panels of in PVT Athens, panels Munich in Athens and Dundee,, Munichand, and asDundee expected,, and, Athens as expected, offers the Athens most 57 offerspromising the most conditions. promising Similar conditions. to Romania, Similar the to PVT Romania, market the in Greece PVT market is still underin Greece development is still under and 58 developmentoffers great opportunity and offers forgreat development. opportunity An for experimental development. study An [experimental7] also carried study out for [7 India,] also showedcarried 59 outthat for PVT India, system showed performed that PVT better system than performed the simple better PV and than solar the panels, simple whichPV and reinforces solar panels, the interestwhich 60 reinforcesfor further the developing interest for the further PVT technology. developing the PVT technology. 61 ThisThis paper paper usesuses the the software software TRNSYS TRNSYS to simulate to simulate and analyze and theanalyze energy the output energy and outputperformance and 62 performanceof PVT panels of inPVT a smallpanels sized in a prosumersmall sized household, prosumer household, comparing comparing two different two climate different conditions: climate 63 conditions:Bucharest, Bucharest, Romania, andRomania, Strasbourg, and Strasbourg, France. The France continental. The continental temperate temperate climates areclimates not the are ideal not 64 thelocations ideal locations for PVT systems,for PVT systems, as it can as be it noticed can be fromnoticed the from literature, the literature, and more and arid more and arid hot areasand hot are 65 areaspreferred. are preferred. However, However, there can there be a potential can be a growingpotential market growing even market in colder even locations. in colder Anlocations. interesting An 66 interestingcomparison comparison can be carried can outbe carried between out France, between where France, the marketwhere the is developed market is developed and there areand several there 67 aremanufacturers several manufacturer and distributerss and distributers of PVT products of PVT available, products andavailable, Romania, and Romania,where the where market the is 68 marketunder-developed. is under-developed. Another Another novelty ofnovelty this study of this is relatedstudy is to related the explicit to the TRNSYSexplicit TRNSYS model which model is 69 whichanalyzed is analyz by twoed performance by two performance indicators: indicators: First law eFffiirstciency law andefficiency primary and energy primary saving energy efficiency. saving 70 efficiency. 2. Residential Prosumers and the Energy Market 71 2. ResidentialA particularly Prosumers important and growingthe Energy market Market for renewable energy is the individual self-consumers 72 (householdsA particularly or businesses). important Self-consumptiongrowing market for represents renewable the energy production is the individual and utilization self-consumers of energy 73 (householdson-site, which or isbusines the mostses). e Selffficient-consumption way of using represents renewable the sources.production Decentralized and utilization systems of energy have on the- 74 site,advantage which of is generating the most efficient power where way of it using is needed, renewable and are sources. more e ffiDecentralizedcient in terms systems of transportation have the 75 advantagelosses, since of thegenerating consumer power is close where to the it energyis needed, source. and Theyare more are also efficient more in flexible, terms oofff ertransportation independent 76 losses,access since in remote the consumer places and is canclose benefit to the fromenergy localized source. smartThey are micro-grids. also more Anflexible, emerging offer termindependent for these 77 accessself-producers in remote is places ‘prosumers’, and can which benefit is from a combination localized smart of ‘producer’ micro-grids. and ‘consumer’.An emerging More term andfor these more 78 selfconsumers-producers are is inclined ‘prosumers’, to produce which their is a owncombination clean energy of ‘producer’ on-site by and using ‘consumer’. renewable More sources, and drivenmore 79 consumersby various are motivations: inclined to produce their own clean energy on-site by using renewable sources, driven 80 by variousIndependence motivations: from the regulated grid and safety against fluctuating energy prices; 81 • • IndependenceEnergy losses andfrom transport the regulated costs grid are reduced; and safety against fluctuating energy prices; 82 •• Energy losses and transport costs are reduced; Financial benefits from subsidies such as green certificates and ‘Feed in Tariffs’. 83 •• Financial benefits from subsidies such as green certificates and ‘Feed in Tariffs’. 84 The PVT systems were investigated and proved efficient for hot tropical climates, in both 85 experimental numerical studies [8,9]. Currently, in Europe there is also an active interest in the 86 research and development of PVT systems. The International Energy Agency (IEA) has carried out

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EnergiesThe 2019 PVT, 12, x FOR systems PEER REVIEW were investigated and proved efficient for hot tropical climates, in3 of both 18 experimental numerical studies [8,9]. Currently, in Europe there is also an active interest in the 87 theresearch research and project development Task 35 ofPV PVT-Thermal systems. Solar The Systems International [10], which Energy aimed Agency to catalyze (IEA) has the carrieddevelopment out the 88 andresearch adoption project of Task competitive 35 PV-Thermal high- Solarquality Systems and commercial [10], which aimed systems to catalyzeand to thecontribut developmente towards and 89 internationallyadoption of competitive accepted high-qualitystandards on and performance, commercial testing systems, and tomonitoring contribute of towards commercial internationally PVT. In 90 Mayaccepted 2018 standardsa new PVT on research performance, platform, testing, Task and 60- monitoringPVT Systems: of commercialApplication PVT.of PVT In MayCollectors 2018 a and new 91 NewPVT Solutions research platform, in HVAC Task Systems, 60-PVT part Systems: of the Solar Application Heating of and PVT Cooling Collectors Program, and New was Solutions launched, in 92 withHVAC 13 Systems,states testing part ofinnovativ the Solare Heatingtechnologies, and Cooling focusing Program, on HVAC was solutions launched, [1 with1]. Overall, 13 states Task testing 60 93 projectinnovative of the technologies, IEA has identified focusing a total on HVAC of 98 solutionsmodules in [11 Europe,]. Overall, of which Task 60 54 project are available of the IEAon the has 94 market,identified 32 aare total stopped of 98 modules from production in Europe, and of whichsix are 54 under are available R&D [12 on]. the market, 32 are stopped from 95 productionBoth the and Romanian six are underand French R & D markets [12]. are interesting contenders for the integration of the PVT, 96 for twoBoth different the Romanian reasons. andIn Romania, French markets the solar are energy interesting market contenders is growing, for with the 1.37 integration GW of PV of theenergy PVT, 97 th installedfor two di atff theerent end reasons. of 2017 In[13 Romania,] and 110 theMW solar of solar energy thermal market capacity is growing, [14]. withHowever, 1.37 GW the ofPVT PV market energy 98 hasinstalled not yet at theemerged, end of 2017with [only13] and a couple 110 MW of distributorsof solar thermal identified capacity on [ 14the]. market, However, which the PVT gives market the 99 th opportunityhas not yet of emerged, further exploiting with only this a couple niche. of In distributors France, the market identified for onPV theis growing market, even which more, gives with the 100 875opportunity MW of new of further installations exploiting in 2017, this niche.reaching In France,to a cumulative the market capacity for PV of is growing8.04 GW even[15]. more,The solar with 101 thermal875 MW market of new capacity installations is not in as 2017, impressive reaching as tothe a PV, cumulative but still capacitysignificantly of 8.04 higher GW than [15]. Romania: The solar 102 th 1.64thermal GW market in 2018 capacity [16]. PVT is not panels as impressive have been as introduced the PV, but stillon the significantly French marked higher since than Romania:2013, having 1.64 103 oneGW of inthe 2018 lead [16 market]. PVT producer panels have companies, been introduced DualSun on [ the17] French(Figure marked 2). The sinceconsumers 2013, having are already one of 104 th familiarthe lead with market the producerproduct and companies, are more DualSunwilling to [ 17adopt] (Figure it in 2their). The green consumers buildings are than already in Romania. familiar with the product and are more willing to adopt it in their green buildings than in Romania.

105 Figure 2. Example of a typical PVT panel from DualSun manufacturer [17]. 106 3. PVT SystemsFigure 2. Example of a typical PVT panel from DualSun manufacturer [17]. Hybrid PVT systems benefit from a number of performance advantages compared to their 107 3individual. PVT Systems solar thermal and photovoltaic counterparts: 108 Hybrid PVT systems benefit from a number of performance advantages compared to their The electrical efficiency of PV cells decreases with a temperature coefficient of 0.3–0.9%/◦C above 109 individual• solar thermal and photovoltaic counterparts: the standard test condition (STC) of 25 ◦C. Thus, the PV cells in hybrid PVT systems show an 110 • Theenergy electrical efficiency efficiency of 4–12% of PV higher cells decreases compared with to the a temperature same cell in ancoefficient individual of 0.3 PV–0.9%/ panel° [C18 above], due 111 theto thestandard fact that test the condition cell is cooled (STC) through of 25 °C. the Thus, extraction the PV of cells the thermal in hybrid energy. PVT systems show an 112 energy efficiency of 4–12% higher compared to the same cell in an individual PV panel [18], due A significant amount of the Sun’s spectral emissivity on a PV panel is lost in the form of heat 113 • to the fact that the cell is cooled through the extraction of the thermal energy. (either by non-absorption or thermalization) (Figure3). This heat can be recovered and used for 114 • A significant amount of the Sun’s spectral emissivity on a PV panel is lost in the form of heat domestic hot water (DHW) or heating applications, which leads to increasing the spectrum of 115 (either by non-absorption or thermalization) (Figure 3). This heat can be recovered and used for operation of the panels and improving the rate of solar energy conversion. 116 domestic hot water (DHW) or heating applications, which leads to increasing the spectrum of The global efficiency of the system is increased by combining the electrical and thermal components, 117 • operation of the panels and improving the rate of solar energy conversion. since the global efficiency can be described as the sum between the electrical and thermal efficiency 118 • The global efficiency of the system is increased by combining the electrical and thermal of the system, as shown in Equation (1). Furthermore, PVT reaches a higher reduction of the 119 components, since the global efficiency can be described as the sum between the electrical and non-renewable primary energy as well as a higher production of solar thermal energy compared 120 thermal efficiency of the system, as shown in Equation (1). Furthermore, PVT reaches a higher to the two separate systems (PV and ST) [19]. 121 reduction of the non-renewable primary energy as well as a higher production of solar thermal 122 energy compared to the two separate systems (PV and ST) [19]. ηGL = ηEL + ηTH (1)

휂퐺퐿 = 휂퐸퐿 + 휂푇퐻 (1) 123 • In urban locations, where South facing roof and façade space are limited, this solution is ideal 124 for fulfilling or partially contributing towards the heating and electricity needs of the consumer.

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125 In addition, the costs associated with transport, installation, and maintenance are significantly 126 reduced by implementing a single piece of equipment instead of two separate ones.

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In urban locations, where South facing roof and façade space are limited, this solution is ideal • Energies for2019 fulﬁlling, 12, x FOR orPEER partially REVIEW contributing towards the heating and electricity needs of the consumer.4 of 18 In addition, the costs associated with transport, installation, and maintenance are signiﬁcantly 125 Inreduced addition, by the implementing costs associated a single with piece transport, of equipment installation instead, and of maintenance two separate are ones. significantly 126 reduced by implementing a single piece of equipment instead of two separate ones.

127 Figure 3. Energy flow diagram of a PVT system: The blue area represents the energy that can be 128 extracted as electricity, the grey area represents the photons with a low level of energy that are unable 129 to excite the electrons to the conduction band and are lost through non-absorption, the green area 130 represents the excess energy that is lost as heat and the red line represents the band-gap of Si.

131 The PVT system can be configured in multiple ways: Flat or concentrated, with a liquid, air or 132 dual liquid-air working fluid. The geometry of the thermal collector can also take multiple forms 133 (harp, spiral, direct flow, fractal) and can be designed by various manufacturing methods (sheet and 134 tube, rollFigure bond, 3. Energy rectangular flow diagram channels) of a. PVTAste system: et al. [ The4] carries blue area out represents a comprehensive the energy review that can on be the 127 Figure 3. Energy flow diagram of a PVT system: The blue area represents the energy that can be 135 variousextracted geometrical, as electricity, thermo the grey-physical area represents, and manufacturing the photons with a solutions low level of for energy PVT that systems. are unable The 128 extracted as electricity, the grey area represents the photons with a low level of energy that are unable 136 configurationto excite investigated the electrons toin thethis conduction study is illustrated band and arein Figure lost through 4. non-absorption, the green area 129 to excite the electrons to the conduction band and are lost through non-absorption, the green area 137 represents the excess energy that is lost as heat and the red line represents the band-gap of Si. 130 representsThe model the developed excess energy for that this is study lost as is heat a water and the-based red line PVT represents system, the with band a direct-gap of flowSi. geometry 138 of the collector (Figure 4a), roll bond channels (Figure 4b), and mono-c-Si PV cells. The water-based The PVT system can be configured in multiple ways: Flat or concentrated, with a liquid, air or 131139 exchangerThe PVT was system chosen can due be to configured its better heatin multiple transfer ways capacity: Flat compared or concentrated, to air-based with systems,a liquid, andair or in dual liquid-air working fluid. The geometry of the thermal collector can also take multiple forms (harp, 132140 dualorder liquid to fulfill-air working the domes fluidtic. hotThe watergeometry (DHW) of the needs thermal of a collector prosumer can household. also take Themultiple direct forms flow spiral, direct flow, fractal) and can be designed by various manufacturing methods (sheet and tube, 133141 (harp,geometry spiral, of thedirect channels flow, fractal) provides and optim can beal experimentaldesigned by various performance manufacturing and geometrical methods simplicity (sheet and [4] . roll bond, rectangular channels). Aste et al. [4] carries out a comprehensive review on the various 134142 tube,The roll roll bond bond, channels rectangular are channels) thermally. Aste efficient, et al. easy [4] tocarries produce out , a relatively comprehensive inexpensive review an ond very the geometrical, thermo-physical, and manufacturing solutions for PVT systems. The configuration 135143 variousflexible in geometrical, terms of geometry thermo [-4physical]. The c-, Siand PV cells manufacturing are the most solutionswidely used for on PVTthe market, systems. with The the investigated in this study is illustrated in Figure4. 136144 configurationmono-crystalline investigated cells providing in this studybetter isefficiency illustrated than in theFigure poly 4-.crystalline cells. 137 The model developed for this study is a water-based PVT system, with a direct flow geometry 138 of the collector (Figure 4a), roll bond channels (Figure 4b), and mono-c-Si PV cells. The water-based 139 exchanger was chosen due to its better heat transfer capacity compared to air-based systems, and in 140 order to fulfill the domestic hot water (DHW) needs of a prosumer household. The direct flow 141 geometry of the channels provides optimal experimental performance and geometrical simplicity [4]. 142 The roll bond channels are thermally efficient, easy to produce, relatively inexpensive and very 143 flexible in terms of geometry [4]. The c-Si PV cells are the most widely used on the market, with the 144 mono-crystalline cells providing better efficiency than the poly-crystalline cells.

(a) (b)

145 FigureFigure 4. 4. PVTPVT configuration: configuration: (a) (a) Direct Direct flowflow collectorcollector geometry; geometry (; b(b)) cross cross section section of theof the PVT PVT collector collector [20]. 146 [20]. The model developed for this study is a water-based PVT system, with a direct flow geometry of the collector (Figure4a), roll bond channels (Figure4b), and mono-c-Si PV cells. The water-based exchanger was chosen due to its better heat transfer capacity compared to air-based systems, and in

order to fulﬁll the domestic hot water (DHW) needs of a prosumer household. The direct ﬂow geometry

(a) (b)

145 Figure 4. PVT configuration: (a) Direct flow collector geometry; (b) cross section of the PVT collector 146 [20].

Energies 2019, 12, 3595 5 of 18

of the channels provides optimal experimental performance and geometrical simplicity [4]. The roll bond channels are thermally efficient, easy to produce, relatively inexpensive and very flexible in terms of geometry [4]. The c-Si PV cells are the most widely used on the market, with the mono-crystalline cellsEnergies providing 2019, 12, x better FOR PEER effi ciencyREVIEW than the poly-crystalline cells. 5 of 18

147 4.4. MethodologyMethodology 4.1. PVT System 148 4.1. PVT System The PVT system modelled in this paper is illustrated in Figure5. The system comprises of a PVT 149 The PVT system modelled in this paper is illustrated in Figure 5. The system comprises of a PVT collector connected to a water storage tank through a heat exchanger, a circulation pump, connection 150 collector connected to a water storage tank through a heat exchanger, a circulation pump, connection to the household water main and an inverter/regulator with a battery bank. The input in the system 151 to the household water main and an inverter/regulator2 with a battery bank. The input in the system is the meteorological data: Solar irradiation (G (W/m )), ambient temperature (Tamb (◦C)) and wind 152 is the meteorological data: Solar irradiation (G (W/m2)), ambient temperature (Tamb (°C)) and wind speed (W (m/s)). The temperature at the outlet of the collector is Tout_coll, the temperature of the tank is 153 speed (W (m/s)). The temperature at the outlet of the collector is Tout_coll, the temperature of the tank Ttank, the temperature of the water main that feeds the tanks is Tmain, and the temperature at the inlet 154 is Ttank, the temperature of the water main that feeds the tanks is Tmain, and the temperature at the inlet of the collector is Tin_coll, all in ◦C. The PVT acts as a pre-heater when the temperature reached by the 155 of the collector is Tin_coll, all in °C. The PVT acts as a pre-heater when the temperature reached by the tank is not suﬃcient for providing domestic hot water. 156 tank is not sufficient for providing domestic hot water.

157 FigureFigure 5. 5.Schematic Schematic diagram diagram of of the the components components of of the the PVT PVT system. system. The PVT panel is assumed to be a flat plate collector, with mono-c-Si PV cells and a direct flow 158 The PVT panel is assumed to be a flat plate collector, with mono-c-Si PV cells and a direct flow roll bond thermal absorber, with the technical characteristics summarized in Table1. 159 roll bond thermal absorber, with the technical characteristics summarized in Table 1. Table 1. Properties of the PVT panel. 160 Table 1. Properties of the PVT panel. Parameter Value Unit Parameter Value Unit Collector Area 6 m2 ThermalCollector Efficiency Area 6 70m2 % ThermalElectrical EEfficiencyfficiency 70 15% % Fluid Water + Glycol - Thermal lossElectrical coefficient Efficiency for bottom and edge15 20% % SlopeFluid Water + Glycol 30 - ◦ Temperature coefficient of PV 0.5 %/ C Thermal loss coefficient for − ◦ Packing factor20 80% % bottom and edge Slope 30 ° 4.2. Meteorological Conditions Temperature coefficient of PV −0.5 %/°C The output of a PVTPacking system factor is dependent upon three80 meteorological%parameters: Ambient temperature, irradiation, and wind speed. The solar irradiation has the most significant impact on the 161 performance4.2. Meteorological of a PVT Conditions system, as it determines the electrical output, but also influences the temperature of the materials through radiative heat transfer. The temperatures in the PVT layers are also dependent 162 The output of a PVT system is dependent upon three meteorological parameters: Ambient on the ambient temperature, through convective and conductive heat transfer processes in the solid 163 temperature, irradiation, and wind speed. The solar irradiation has the most significant impact on 164 the performance of a PVT system, as it determines the electrical output, but also influences the 165 temperature of the materials through radiative heat transfer. The temperatures in the PVT layers are 166 also dependent on the ambient temperature, through convective and conductive heat transfer 167 processes in the solid and liquid layers of the system, but also on the speed of the wind which 168 determines the convective heat losses at the surface and at the back of the panel. The yearly profile 169 of each parameter is obtained from Meteonorm Database.

Energies 2019, 12, 3595 6 of 18

Energiesand liquid 2019, layers12, x FOR of PEER the system,REVIEW but also on the speed of the wind which determines the convective6 of 18 heat losses at the surface and at the back of the panel. The yearly profile of each parameter is obtained 170 The total radiation comprises of three components: Direct beam radiation, diffuse radiation, and from Meteonorm Database. 171 reflective radiation. Direct radiation is the direct incident beam radiation that hits the surface of the The total radiation comprises of three components: Direct beam radiation, diffuse radiation, and 172 panel, the diffuse radiation refers to the radiation that has been scattered through the molecules of reflective radiation. Direct radiation is the direct incident beam radiation that hits the surface of 173 the atmosphere, and reflective radiation (also referred to as albedo) is the radiation reflected by the the panel, the diffuse radiation refers to the radiation that has been scattered through the molecules 174 surroundings. of the atmosphere, and reflective radiation (also referred to as albedo) is the radiation reflected by 175 The meteorological conditions of both Strasbourg, France, and Bucharest, Romania are used in the surroundings. 176 parallel for the simulations. This allows a comparison of the performance of the PVT system in both The meteorological conditions of both Strasbourg, France, and Bucharest, Romania are used in 177 regions. The climate of Romania can be described as temperate continental, with cold, cloudy winters parallel for the simulations. This allows a comparison of the performance of the PVT system in both 178 and frequent snow and fog, and sunny summers with frequent showers and thunderstorms. In the regions. The climate of Romania can be described as temperate continental, with cold, cloudy winters 179 southern region, where Bucharest is located, the winters are milder and the summers hotter. France’s and frequent snow and fog, and sunny summers with frequent showers and thunderstorms. In the 180 climate is temperate, with four specific regions. Alsace, where Strasbourg is located, is characterized southern region, where Bucharest is located, the winters are milder and the summers hotter. France’s 181 by a continental climate which harbors cold winters and hot summers. Although, in broad terms, climate is temperate, with four specific regions. Alsace, where Strasbourg is located, is characterized 182 both cities have a similar climate (hot summers and cold winters), there are some small differences by a continental climate which harbors cold winters and hot summers. Although, in broad terms, both 183 in the average values of the meteorological parameters, which are summarized in Table 2. cities have a similar climate (hot summers and cold winters), there are some small differences in the average values of the meteorological parameters, which are summarized in Table2. 184 Table 2. Climate characterization of Strasbourg and Bucharest (source: WMO—World Meteorological 185 Organization). Table 2. Climate characterization of Strasbourg and Bucharest (source: WMO—World Meteorological Organization). Winter Spring Summer Autumn Annual

(DecWinter-Feb) (MarSpring-May) (JunSummer-Aug) Autumn(Sep-Nov)Annual Total Temperature Bucharest (Dec-Feb)4.0 (Mar-May)17.8 (Jun-Aug)29.1 (Sep-Nov)17.4 Total 17.1 (°TemperatureC) StrasbourgBucharest 5.24.0 17.815.2 29.124.2 17.414.5 17.1 14.8 (◦C) Strasbourg 5.2 15.2 24.2 14.5 14.8 Monthly Bucharest 36.9 48.1 64.9 47.9 594 Monthly Bucharest 36.9 48.1 64.9 47.9 594 rainfallrainfall (mm (mm)) StrasbourgStrasbourg 36.536.5 52.352.3 66.966.9 54.854.8 632 632 MonthlyMonthly rainyBucharestBucharest 9.39.3 10.510.5 9.99.9 7.5 7.5 113 113 rainy daysdays StrasbourgStrasbourg 8.48.4 9.89.8 10.210.2 8.9 8.9 112 112 MonthlyMonthly sun sun BucharestBucharest 72.672.6 189.7189.7 278.8278.8 163.0163.0 2121 2121 hourshours StrasbourgStrasbourg 54.554.5 160.3160.3 222.0222.0 108.7108.7 1637 1637

186 FigureFiguress 66 andand7 7 illustrate illustrate a a sample sample of of the the daily daily total total radiation radiation for for a a typical typical summer summer and and winter winter 187 week,week, respectively, inin StrasbourgStrasbourg andand Bucharest. The two cities have a similar average rage of values 2 188 during the summer days,days, ofof 700–900700–900 W W/m/m2.. TheThe dailydaily curve curve of of ambient ambient temperature temperature for for the the two two cities cities is 189 isshown shown in Figures in Figure8 ands 89 and. It can 9. beIt can observed be observed that summer that summer temperatures temperatures are slightly are higher slightly in Bucharest higher in 190 Bucharestcompared compared to Strasbourg, to Strasbourg, while the Romanian while the wintersRomanian are colder.winters Figures are colder. 10 and Figure 11 shows 10 and the variation11 show 191 theof wind variation speed of during wind thespeed summer during and the winter summer periods, and respectively.winter periods On,average, respectively. the wind On isaverage, stronger the in 192 windStrasbourg, is stronger with onlyin Strasbourg, a few higher with peaks only in Bucharest.a few higher Overall, peaks the in windBucharest. and temperature Overall, the data wind suggest and 193 temperaturethat the PVT data will performsuggest that better the in PVT Bucharest, will perform while thebetter solar in radiationBucharest, is while more orthe less solar equivalent. radiation is 194 more or less equivalent.

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Energies 2019, 12, 3595 8 of 18 Energies 2019, 12, x FOR PEER REVIEW 8 of 18

W_Buch W_Stras 12 Energies 2019, 12, x FOR PEER REVIEW 8 of 18 10

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0 0 0 0 0 0 0 0 4.3. DHW and Electricity Demand 201 2004.3. DHW and ElectricityFigure D 11.emand Wind speed during a winter period for Bucharest and Strasbourg. 202 InIn order order to carry out an accurate simulation, a a realistic realistic daily daily time time dependent dependent profile profile of of the the 201 4.3. DHW and Electricity Demand 203 householdhousehold DHW DHW demand demand is required. is required. The dailyThe daily profile profile of DHW of is DHW dependent is depe uponndent various upon parameters: various 204 202parameters:The numberIn orderT ofhe inhabitants, number to carry out of appliances, inhabitants, an accurate ambient simulation, appliances, conditions, a realistic ambient seasonal daily condition time variations, dependents, seasonal the profile average variations, of daily the thehot 205 203averagewater household volume, daily hot and DHW water the demand yearly volume demand is, and required. the [21 yearly ]. The daily demand profile [21 of]. DHW is dependent upon various 204 parameters: The number of inhabitants, appliances, ambient conditions, seasonal variations, the 206 TheThe profile profile used used for for this this simulation simulation wa wass created created using using the the DHW DHW-calc-calc software software (v1, (v1, University University of 205of Kassel,average Kassel, daily hot Germany) water volume developed, and the for yearly the IEA-SHC demand [21 Task]. 26, which can generate realistic DHW 207 206Kassel, Kassel, Germany) developed for the IEA-SHC Task 26, which can generate realistic DHW profiles forThe European profile used countries for this simulation for any required was created period using of the time DHW and-calc timestep software [ 22(v1,,23 University]. This software of 208 207profiles Kassel, for EuropeanKassel, Germany countries) developed for any for required the IEA period-SHC Task of time 26, which and timestepcan generate [22 ,realistic23]. This DHW software has been successfully used and validated in multiple research papers [21,24,25]. The user can define 209 208has beenprofiles successfully for European used countries and validated for any requiredin multiple period research of time papers and timestep [21,24 [,2225,].23 The]. This user software can define a number of parameters, such as the total mean daily water volume, probability distribution, flow 210 209a numberhas been of parameters,successfully used such and as validatedthe total inmean multiple daily research water papersvolume, [21 probability,24,25]. The user distribution, can define flow 211 210rates,rates, a holidays holidays,number ,of etc. etc. parameters, Figure Figure 112 2such showsshows as the an an totalexample example mean of ofdaily a a three three-day water-day volume, profile profile probability with with a a 1 1 h hdistribution, time time step step resolution resolutionflow 212 211fforor 200 200lrates,l/day/day holidays average , etc. consumption, Figure 12 shows which an example was usedof a three for -day this profile simulation. with a 1TheThe h time profile profile step resolution generated by by 213 212DHWDHW-calc for-calc 200 is l/day different diff erentaverage for consumption, weekday weekdayss and which for weekend wasweekends, used s for, and this the simulation. profile profile is The included profile in generated the DHW by Load Load 214 213functionfunction DHW of of- calcTR TRNSYS,N isS differentYS, shown for weekdayin in Figure s and1133. for weekends, and the profile is included in the DHW Load 214 function of TRNSYS, shown in Figure 13. 215 TheThe temperature temperature level for DHW was set to 45 °◦C,C, while while the the cold cold water water from from the the main main is is assumed assumed 215 The temperature level for DHW was set to 45 °C, while the cold water from the main is assumed 216 constantconstant across across the year at 10 °◦C. 216 constant across the year at 10 °C. 120120

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0 0 1 1 2 0 0 0 1 1 2 0 0 0 1 1 2 0 217 FigureFigure 12. 12.Three-day Three-day domestic domestic hothot waterwater ( (DHW)DHW) profile profile generated generated with with DHW DHW-calc.-calc. 217 Figure 12. Three-day domestic hot water (DHW) profile generated with DHW-calc. 218 An electricalAn electrical demand demand curve curve was was generated generated using a a forcing forcing function function in TRNSYS in TRNSYS and andconsidering considering 218 219weekday Anweekday electrical and and weekend demandweekend variations, variationcurve wass, asgenerated well well as as holidays, using holidays, a and forcing andthe system function the system includes in TRNSYS includes a battery and a bank. battery considering The bank. 220 219 weekdayThe electricityelectricity and weekenddemand demand also variation also includes includess, theas well thepower power as consumption holidays, consumption and of the ofpumpsystem the for pump includes circulating for circulatinga batterythe fluid. bank. theThe fluid. The 221 excess of produced electricity is exported to the grid when the battery is full, and when the battery 220 electricityThe excess demand of produced also electricityincludes the is exported power consumption to the grid when of the the pump battery for is circulating full, and when the thefluid. battery The 222 load is less than the demand, electricity is bought back from the grid. 221 excess of produced electricity is exported to the grid when the battery is full, and when the battery 223load is lessThe than same the DHW demand, and electricity electricity demand is bought profiles back were from used the for grid. both simulations, in Strasbourg 222 224load isTheand less Romania. same than DHW the The demand, andDHW electricity profile electricity generated demand is bought with profiles DHW back were- fromcalc has used the been grid. for developed both simulations, to be applicable in Strasbourg for 223 225and Romania.Theany same European TheDHW DHWcountry, and profile electricity and the generated electricity demand with demandprofiles DHW-calc waswere alsohas used been assumed for developed both to simulati be similar. to beons, applicable This in way,Strasbourg for a any 224 226andEuropean Romania.comparison country, The based andDHW solely the profile electricity on the generated input demand meteorological with was DHW also data assumed-calc can has be achieved. tobeen be similar.developed This to way, be applicable a comparison for 225 anybased European solely on country, the input and meteorological the electricity data demand can be achieved. was also assumed to be similar. This way, a 226 comparison based solely on the input meteorological data can be achieved.

Energies 2019, 12, 3595 9 of 18

4.4. TRNSYS Model TRNSYS (transient system simulation tool) is a flexible software that can simulate the behavior of dynamic transient systems. A TRNSYS model was developed to replicate the system in Figure 13, and it includes: Weather Data (Type 15), PVT collector (Type 50), Stratified Storage Tank with a capacity of 100l (Type 4), Single Speed Pump (Type 114), Inverter/Charge controller (Type 48), a Battery Bank (Type 47) and Controllers (Type 2). The Type 15 component is a tool that reads weather data either from the built-in database or from a user provided spreadsheet, and processes it to provide the input weather data for the simulation (temperature, radiation, wind speed). The storage tank (Type 4) models a tank that can be subjected to thermal stratification, by establishing the number of thermal nodes N. If N is equal to one, the tank is modeled as fully-mixed with no stratification, which was assumed for the purpose of this study. Type 114 is the component for a constant speed pump that maintains a constant fluid outlet mass flow rate. The pump control strategy modelled by a Type 2 component is based on the temperature of the liquid in the storage tank and the DHW demand. The collector pump circulates water from the tank when the energy can be collected, and the tank is emptied by the DHW load and refilled when it reaches a low volume. Type 48 models both the regulator of the system, which distributes DC power from the solar cell array to and from a battery, and the inverter converts the DC power to AC. The electrical controller decides whether to use the electrical load for charging the battery or for meeting the demand. Type 47 models the battery bank of the system, which is based on a lead-acid type of battery and can output data on the variation of the state of charge over time and the rate of charge or discharge. The simulation assumes that no heat losses occur on the pipes, the optical properties of materials are constant and no surroundings partial shading or dust is considered. The electrical flow is shown with purple arrows, the liquid flow in blue and the dotted arrows connect the control circuit and energy demand. The PVT component has eight operational modes, which vary the method of calculation for the thermal loss coefficient and transmittance-absorptance factor. This model used mode four, which calculates the parameters according to definitions of Klein (1976) [26]. The numerical model behind the PVT collector (Type 50) has been developed based on the work of Florschuetz [27], using the Hottel–Whillier–Bliss equations. Starting from the steady state energy balance of a system: d2T 1 = [UL(T Ta) S + qe] (2) dx2 kδ − − where T (◦C) is the temperature of the element dx, k is the thermal conductivity (W/mK), δ is the 2 thickness of the element (m), U is the thermal loss coefficient (W/(m K)), Ta is the ambient temperature L · (◦C), S is the absorbed solar energy (W) and qe is the electrical output (W), the paper defines the useful thermal output of the PVT collector as: h i Qu = AcFR S UL T Ta (3) − f ,i − 2 where Ac is the area of the collector (m ), FR is the heat removal factor (-) and Tf,i is the inlet temperature of the fluid (◦C). The electrical output of the system is defined as: ( ) AcSηa ηrβr S Qe = 1 FR T f ,i Ta + (1 FR) (4) α − ηa − UL − where ηa is the efficiency of the cell at ambient temperature (%), α is the effective absorbance of the cell (-), ηr is the reference cell efficiency (%) and βr is the temperature coefficient of the cell (%/K). The temperature at the outlet of the collector can be approximated as: T f ,o = T f ,i + QuAc/ V f lowρc f (5)

3 3 where Vflow is the volumetric flow in the collector (m /s), ρ is the fluid density (kg/m ) and cf is the heat capacity of the collector fluid (J/(kg K)). · Energies 2019, 12, 3595 10 of 18

This model is a simplified version of a complete numerical dynamic analysis that can be performed on aEnergies PVT 2019 based, 12, x on FOR the PEER heat REVIEW transfer in each layer of the panel [20,21], where a system of ordinary10 of 18 differential equations is solved simultaneously to find the temperatures at any time of the simulation. 270 Thesimulation detailed models. The detailed are used models for in-depth are used layer for analysis in-depth and layer geometrical, analysis and optical geometrical, or thermo-physical optical or 271 optimizationsthermo-physical and sizing. optimizations For the purposeand sizing of. aFor quantitative the purpose analysis of a quantitative of overall analysis daily, weekly of overall and daily, annual 272 productionweekly and of energy, annual theproduction TRNSYS of Type energy, 50 model the TRNSYS was validated Type 50 and model proved was accurate.validated and proved 273 accurate.The PVT component requires a set of input parameters that depend on the specific PVT that is 274 to be modelled.The PVT Thesecomponent can berequires found a in set the of manufacturer’sinput parameters specifications that depend on and the refer specific to: CollectorPVT that is fin 275 to be modelled. These can be found in the manufacturer’s specifications and refer to: Collector fin efficiency, plate absorbance, number of glass covers, optical efficiency, collector plate emittance, loss 276 efficiency, plate absorbance, number of glass covers, optical efficiency, collector plate emittance, loss coefficient, temperature coefficient, STC temperature, and packing factor. Thus, the module can be 277 coefficient, temperature coefficient, STC temperature, and packing factor. Thus, the module can be customized to replicate a product available on the market. 278 customized to replicate a product available on the market.

279 FigureFigure 13. 13.Components Components of of the the transient transientsystem system simulationsimulation tool ( (TRNSY)TRNSY) model model..

280 4.5.4.4. System System Performance Performance 281 TheThe energy energy performance performance of the of systemthe system can be can computed be computed in terms in terms of thermal, of thermal, electrical, electrical and overall, and 282 efficienciesoverall efficiencies (ηTH, ηEL, (ηoverallTH, ).EL, Theoverall electrical). The electrical efficiency efficiency is calculated is calculated as the as ratio the between ratio between the power the 283 generatedpower generated by the system by the (P ELsystem) and ( thePEL) amount and the ofamount solar radiationof solar radiation incident incident on the surface on the surface of the collector of the 284 (AxGcollectorirr): (AxGirr): PEL ηEL = 푃퐸퐿 (6) 휂퐸퐿 =AGirr (6) 퐴퐺푖푟푟 The standard equation for thermal efficiency is represented by the ratio between the amount of 285 The standard equation for thermal efficiency is represented by the ratio between the amount of thermal energy generated by the system (QTH) and the solar radiation incident on the surface of the 286 thermal energy generated by the system (QTH) and the solar radiation incident on the surface of the collector (AxGirr): . 287 collector (AxGirr): Q mc f T f out T f in TH − − − ηTH = 푄 = 푚̇ 푐 (푇 − 푇 ) (7) AGirr푇퐻 푓 AG푓−표푢푡irr 푓−푖푛 휂푇퐻 = = (7) 퐴퐺푖푟푟 퐴퐺푖푟푟

288 where 푚̇ is the mass flow rate of the fluid (kg/s) and cf is the specific heat capacity (J/(kg·K)), which 289 in this case for Glycol water 40%/60% is 3600 J/(kg·K).

Energies 2019, 12, x FOR PEER REVIEW 11 of 18 Energies 2019, 12, 3595 11 of 18 290 Another definition for the thermal efficiency was proposed by Bombarda et al. [28]. Here, a 291 distinction. is made between the electrical and thermal power of the system, and thus the electrical where m is the mass flow rate of the fluid (kg/s) and c is the specific heat capacity (J/(kg K)), which in 292 power production is subtracted from the total incidentf irradiation. This definition allows· for a more this case for Glycol water 40%/60% is 3600 J/(kg K). 293 accurate comparison between the thermal performance· of a PVT and stand-alone solar thermal 294 system.Another definition for the thermal efficiency was proposed by Bombarda et al. [28]. Here, a distinction is made between the electrical and thermal power of the system, and thus the electrical 푄 푚̇ 푐 (푇 − 푇 ) 휂 power production is subtracted∗ from the푇퐻 total incident푓 푓− irradiation.표푢푡 푓−푖푛 This definition푇퐻 allows for a more 휂푇퐻 = = = (8) accurate comparison between the thermal퐴퐺푖푟푟 − performance푃퐸퐿 퐴퐺 of푖푟푟 a− PVT푃퐸퐿 and stand-alone1 − 휂퐸퐿 solar thermal system.

295 From the point of view of the first law of thermodynamics,. the global efficiency of the system GL Q mc f T f out T f in η 296 (η ) can be calculated as the sum of theTH thermal and electrical− − efficiencies.TH This is also known as the ηTH∗ = = − = (8) 297 first law efficiency: AG PEL AG PEL 1 ηEL irr − irr − − From the point of view of the first law of thermodynamics, the global efficiency of the system 휂퐺퐿 = 휂퐸퐿 + 휂푇퐻 (9) (η ) can be calculated as the sum of the thermal and electrical efficiencies. This is also known as the 298 GL first lawAnother efficiency: approach is to consider the difference in grade between the two types of energy, i.e., 299 electrical energy is a higher form of energy compared to the thermal. Thus, due to the fact that a kWel ηGL = ηEL + ηTH (9) 300 cannot be directly compared to a kWth, some authors [19,29] propose the primary energy saving 301 efficiencyAnother (ηPES approach) as a more is accurate to consider way the of assessing difference the in performance grade between of the system: two types of energy, i.e., electrical energy is a higher form of energy compared to휂 the thermal. Thus, due to the fact that a 휂 = 휂 + 퐸퐿 kWel cannot be directly compared to a kWth푃퐸푆, some푇퐻 authors [19,29 ] propose the primary energy saving(10) 휂푇푝표푤푒푟 efficiency (ηPES) as a more accurate way of assessing the performance of the system: 302 where ηTpower is the average efficiency of producing electrical power, and it depends on the country of 303 reference. On average, it can be assumed to be 0.4 [1η9EL]. For Romania, the average efficiency of ηPES = ηTH + (10) 304 producing electrical power in 2016 was 0.532, while inηTpower France it was 0.601 [30]. These values were 305 used for computing the primary energy saving efficiency in Section 5. where ηTpower is the average efficiency of producing electrical power, and it depends on the country of reference. On average, it can be assumed to be 0.4 [19]. For Romania, the average efficiency of 306 5. Simulation Results producing electrical power in 2016 was 0.532, while in France it was 0.601 [30]. These values were used 307 for computingThe detailed the simulations primary energy were saving run for effi ciency the two in significant Section5. one-week periods of summer and 308 winter (7 Jan–13 Jan and 20 Jul–26 Jul) and for both the meteorological conditions of Bucharest and 309 Strasbourg,5. Simulation with Results a 15-minute time step. In addition, an annual simulation was performed to 310 determineThe detailed the weekly simulations electrical were and run thermal for the energy two significant production one-week for the periodsduration of of summer a year. and winter 311 (7 Jan–13The ele Janctrical and 20 power Jul–26 output Jul) and of forthe both system the for meteorological summer and conditions winter is illustrated of Bucharest in andFigures Strasbourg, 14 and 312 with15, respectively. a 15-minute timeThe variationstep. In addition, in the output an annual follow simulations closely was the performed variation to in determine solar radiation. the weekly The 313 ambientelectrical temperature and thermal also energy has productionan effect on forthe the power duration output: of aA year.s the operating temperature increases, 314 the efficiencyThe electrical of the power PV cells output drop ofs. the system for summer and winter is illustrated in Figures 14 and 15, 315 respectively.Figures 1 The6 and variation 17 show in the outputevolution follows of the closely temperature the variation at the inoutlet solar of radiation. the collector The ambient(Tout_coll) 316 andtemperature the temperature also has of an the eff storageect on thetank power (Ttank) output:for summer As theand operating winter in temperatureStrasbourg and increases, Bucharest. the 317 Duringefficiency winter, of the the PV tank cells reaches drops. a maximum of 15 °C, while during summer the maximum is around 318 30 °C.

20-Jul 21-Jul 22-Jul 23-Jul 24-Jul 25-Jul 26-Jul 27-Jul 1000 900 800

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319 Figure 14. InstantaneousInstantaneous electrical electrical power power generated generated by by the the system system during during the the summer summer..

Energies 2019, 12, 3595 12 of 18 Energies 2019, 12, x FOR PEER REVIEW 12 of 18

07-Jan 08-Jan 09-Jan 10-Jan 11-Jan 12-Jan 13-Jan 14-Jan 800 700 600 Energies 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 1212 of 1818

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30 ◦C. ) 34

320 Figure 15.C InstantaneousInstantaneous electricalelectrical powerpower generated by the system during the winter..

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T 0 322 T 0 Figure 17. Temperature of the collector tank during winter. -4-4 -8-8 323 Over a year, the-- 12weekly average temperature and solar radiation for Bucharest and Strasbourg -16 324 are illustrated in Figure-16 18. The solar radiation was averaged for the non-zero values, from sunrise to Tout_coll_Buch Tout_coll_Strasb T_tank_Stras T_tank_Buch 325 sunset. Given these conditions,Tout_coll _Btheuc thermalh Tout_c andoll_S electricaltrasb T_t energyank_Stra soutputT_tank_Bs areuc illustratedh in Figure 19. 326 Overall, the annual energy produced by the system in the two cities is summarized in Table 3. 322 FigureFigure 17. 17. TemperatureTemperature of the collector tank during winter winter... 327 Assuming an annual energy consumption of 2000 kWh and a thermal energy for DHW of 6000 323328 kWhOverTH, the a tableyear, alsothe weeklyshows the average percentage temperature of coverage and solarfor the radiation household. for BucharestIt can be observed and Stras thatbourg the 324 are illustrated in Figure 18.. TheThe solarsolar radiationradiation waswas averagedaveraged forfor thethe nonnon--zero values, from sunrise to 325 sunset. Given these conditions, the thermalthermal andand electricalelectrical energyenergy outputoutputs are illustratedillustrated inin FigureFigure 119.. 326 Overall, the annual energy produced by the system in the two cities is summarized in Table 3.. 327 Assuming an annual energy consumption of 2000 kWh and a thermal energy for DHW of 6000 328 kWhTHTH,, thethe tabletable alsoalso showsshows thethe percentage of coverage for the household. It can be observed that the

Energies 2019, 12, 3595 13 of 18 Energies 2019, 12, x FOR PEER REVIEW 13 of 18 Energies 2019, 12, x FOR PEER REVIEW 13 of 18

329329 Oversystemsystem a year,performs performs the weeklybetterbetter under under average the the temperature meteorological meteorological and conditions conditions solar radiation of of Bucharest, Bucharest, for Bucharest where where it it and is is capable capableStrasbourg of of are 330330illustrated coveringcovering in Figure moremore thanthan 18. 50% The50% solarofof thethe radiation annualannual consumption.consumption. was averaged for the non-zero values, from sunrise to sunset. 2 331331Given theseTheThe conditions, resultsresults indicateindicate the thermalthatthat aa 66 mm and2 systemsystem electrical wouldwould energy operateoperate outputs wellwell inin arebothboth illustrated climates,climates, butbut in with Figurewith aa 10 1019––.12%12% Overall, 332 better performance in Bucharest. The climatic conditions in Bucharest are more suitable (higher 332the annualbetter performanceenergy produced in Bucharest. by the system The climatic in the conditions two cities in is Bucharest summarized are more in Table suitable3. Assuming (higher an 333 radiation and temperature) during the summer but worse during the winter. The overall effect shows 333annual radiation energy and consumption temperature) of during 2000 the kWh summer and a but thermal worse during energy the for winter. DHW The of overall 6000 kWh effect shows, the table 334 a slight advantage of PVT systems in Bucharest compared to Strasbourg. TH 334also showsa slight the advantage percentage of PVT of coveragesystems in forBucharest the household. compared It to can Strasbourg. be observed that the system performs better under the meteorological conditions of Bucharest, where it is capable of covering more than 50% 335335 TableTable 3.3. TotalTotal annualannual energyenergy outputoutput forfor thethe twotwo locatiolocationnss.. of the annual consumption. Energy Total Annual Output Percentage of total consumption The results indicateEnergy that a 6 m2 systemTotal wouldAnnual operateOutput wellPercentage in both of climates, total consumption but with a 10–12% Thermal energy in Bucharest 3340.80 kWhTH 55.68% better performanceThermal energy in Bucharest. in Bucharest The climatic3340.80 conditions kWhTH in Bucharest55 are.68% more suitable (higher Thermal energy in Strasbourg 2491.96 kWhTH 41.53% radiation andThermal temperature) energy in Strasbourg during the summer2491.96 but kWh worseTH during the winter.41.53% The overall eﬀect shows Electrical energy in Bucharest 1173.24 kWhEL 58.66% a slight advantageElectrical energy of PVT in systems Bucharest in Bucharest1173.24 compared kWhEL to Strasbourg. 58.66% Electrical energy in Strasbourg 962.52 kWhEL 48.13% Electrical energy in Strasbourg 962.52 kWhEL 48.13%

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338338 FigureFigureFigure 19. 19.19.Weekly WeeklyWeekly energy energyenergy productionproduction forfor for aa a periodperiod period ofof ofoneone one yearyear year...

339339 AnotherAnother investigatedinvestigatedTable topictopic 3. Total isis the annualthe efficiencyefficiency energy ofof output thethe systemsystem for the inin two termsterms locations. ofof electrical,electrical, thermalthermal,, andand 340340 globalglobal performance.performance. AsAs discusseddiscussed inin SectionSection 4.4,4.4, thethe systemsystem performanceperformance cancan bebe describeddescribed inin multiplemultiple Energy Total Annual Output Percentage of Total Consumption 341341 ways:ways: FFirstirst law law efficiency efficiency and and primary primary energy energy saving saving efficiency. efficiency. TheThe thermal thermal efficien efficiencycy was was 342342 computedcomputedThermal energy according according in Bucharest to to Equation Equation ( (88),), by by 3340.80 subtracting subtracting kWh theTH the electrical electrical power power from from 55.68% the the total total incident incident Thermal energy in Strasbourg 2491.96 kWhTH 41.53%

Electrical energy in Bucharest 1173.24 kWhEL 58.66% Electrical energy in Strasbourg 962.52 kWhEL 48.13% Energies 2019, 12, 3595 14 of 18

Another investigated topic is the efficiency of the system in terms of electrical, thermal, and global performance. As discussed in Section 4.5, the system performance can be described in multiple ways: First law efficiency and primary energy saving efficiency. The thermal efficiency was computed

accordingEnergies to2019 Equation, 12, x FOR PEER (8), REVIEW by subtracting the electrical power from the total incident14 irradiation. of 18 The weekly efficiency of the system is calculated by taking into account the total weekly energy 343production irradiation. (shown The inweekly Figure efficiency 19) and of the the total system energy is calculated received by from taking the into sun account to the the surface total weekly of the panel 344during energy that production week. (shown in Figure 19) and the total energy received from the sun to the surface of 345 the panel during that week. Figure 20 illustrates the weekly electrical and modified thermal efficiency of the system. It can 346 Figure 20 illustrates the weekly electrical and modified thermal efficiency of the system. It can be observed that the electrical efficiency in both cases slightly drops during the summer, due to the 347 be observed that the electrical efficiency in both cases slightly drops during the summer, due to the 348increase increase in the in operating the operating temperature temperature of the of cells,the cells, while while the the thermal thermal effi efficiencyciency is atis itsat its maximum maximum during 349the summerduring the weeks, summer reaching weeks, 64% reaching in Bucharest 64% in Bucharest and 55% inand Strasbourg. 55% in Strasbourg. In addition, In addition, Table4 summarizes Table 4 350the monthlysummarizes energy the production monthly energy and effi productiociencies forn and both efficiencies locations, for for aboth better locations, comparison. for a The better monthly 351values comparison. of the energy The production monthly values are calculated of the energy by adding production the weekly are calculated production by adding illustrated the inweekly Figure 19, 352and theproduction efficiency illustrated values arein Figure obtained 19, and by averagingthe efficiency the values weekly are values obtained shown by averaging in Figure the 20 weekly. 353 values shown in Figure 20.

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355 FigureFigure 20. 20.Weekly Weekly electrical electrical andand thermal efficiency eﬃciency for for a one a one-year-year period period..

356 TableTable 4. 4.Monthly Monthly summary summary of energy production production and and efficiency eﬃciency..

MonthMonth Thermal Thermal ThermalThermal ElectricalElectrical ElectricalElectrical ThermalThermal ThermalThermal ElectricalElectrical Electrical EnergyEnergy EnergyEnergy EnergyEnergy EnergyEnergy EfficiencyEfficiency EfficiencyEfficiency EfficiencyEfficiency EfficiencyEfficiency BuchBuch StrasbStrasb BuchBuch StrasbStrasb BuchBuch StrasbStrasb BuchBuch Strasb (kWh(kWhTH) TH(kWh) (kWhTH) TH(kWh) (kWhEL) EL)(kWh(kWhEL) EL) (%)(%) (%)(%) (%)(%) (%)(%) JanJan 60.88 60.88 39.74 39.74 56.75 56.75 32.14 32.14 9.24 9.24 7.57 7.57 13 13.25.25 13.1413.14 FebFeb 117. 117.7575 110.44 110.44 75.76 75.76 67.26 67.26 20.87 20.87 20 20.18.18 13 13.25.25 13.2713.27 Mar 171.10 153.78 87.20 77.60 27.49 22.20 13.15 13.16 MarApr 171. 336.0510 153.78 250.27 87.20 122.32 77.60 105.41 27.49 39.92 22 32.99.20 13 13.04.15 13.0313.16 AprMay 336. 402.3305 250.27 341.51 122.32 123.80 105. 120.4941 39.92 47.26 32 38.56.99 13 12.83.04 12.8413.03 Jun 551.34 402.70 152.30 124.47 54.84 46.55 13.03 12.76 May 402.33 341.51 123.80 120.49 47.26 38.56 12.83 12.84 Jul 541.23 402.44 147.21 121.03 54.87 46.01 13.05 12.92 JunAug 551. 503.3134 402.70 447.15 152.30 135.44 124. 134.3847 54.84 57.03 46 47.79.55 13 13.25.03 13.2412.76 JulSept 541. 382.5623 402.44 214.26 147.21 129.13 121.03 82.09 54.87 55.74 46 46.69.01 13 13.20.05 13.0912.92 Oct 184.72 73.76 78.71 46.98 41.83 32.55 13.45 13.21 AugNov 503.31 55.33 447.15 48.44 135.44 35.58 134.38 29.09 57.03 15.18 47 12.33.79 13 13.28.25 13.2313.24 SeptDec 382.56 42.65 214.26 36.05 129.13 34.11 82.09 24.78 55.74 9.49 46. 6.9369 13 13.17.20 13.1313.09 Oct 184.72 73.76 78.71 46.98 41.83 32.55 13.45 13.21 NovAll the global55.33 performance48.44 indicators35.58 described29.09 in Section15.18 4.5 were12.33 computed 13 and.28 are described13.23 in FigureDec 21 . The42 first.65 law3 e6ffi.05ciency is34 simply.11 the24 sum.78 of the9 thermal.49 and6.93 electrical 13 effi.17ciencies illustrated13.13 in Figure 20. Overall, a better performance in Bucharest can be easily observed. The efficiency 357 All the global performance indicators described in Section 4.4 were computed and are described calculated by the PES method is higher than the first law efficiency, reaching 90% for operation in 358 in Figure 21. The first law efficiency is simply the sum of the thermal and electrical efficiencies 359Bucharest. illustrated Due in to Figure the fact 20. thatOverall, the average a better performanceefficiency of in producing Bucharest electrical can be easily power observed. in Bucharest The is 360 efficiency calculated by the PES method is higher than the first law efficiency, reaching 90% for

Energies 2019, 12, 3595 15 of 18

Energies 2019, 12, x FOR PEER REVIEW 15 of 18

361lower operation than in in Strasbourg Bucharest. (0.532Due to comparedthe fact that to the 0.601), average the efficiency difference of producing between theelectrical two cities power in in terms 362of PESBucharest increases is lower further than compared in Strasbourg to ( the0.532 first compared law effi tociency, 0.601), the which difference can be between observed the two in thecities graph. 363The overallin terms trend of PES follows increases closely further the compared trend of to thethe thermalfirst law efficiency efficiency, which in Figure can be 20 observed, with high in the values 364during graph. the The summer overall and trend low follows during closely the winter. the trend The of the inverse thermal variation efficiency of in the Figure electrical 20, with effi highciency is 365insignificant values during and doesthe summer not aff ectand thelow general during the trend. winter. The inverse variation of the electrical efficiency 366 is insignificant and does not affect the general trend.

100 90 80

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f 40 f E 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 Week First_Law_Stras First_Law_Buch PES_Stras PES_Buch 367

368 FigureFigure 21. 21. WeeklyWeekly overall overall efficiency eﬃciency..

3696. Discussion6. Discussion 370 The systemThe system presented presented in thisin this paper paper couples couples a typicala typical residential residential household household withwith a PVT system system that 371 that generates both heat and electricity, in two similar climate conditions. According to the generates both heat and electricity, in two similar climate conditions. According to the meteorological 372 meteorological data, the two cities, Bucharest and Strasbourg, have similar overall weather patterns data, the two cities, Bucharest and Strasbourg, have similar overall weather patterns with hot summers 373 with hot summers and cold winters. However, there are some small dissimilarities in terms of wind 374and coldspeed, winters. average ambient However, tempe thererature are, and some total small irradiation dissimilarities which would in terms indicate of that wind Bucharest speed, is average a 375ambient more temperature, suitable location and for total achieving irradiation the which best PVT would performance. indicate The that simulation Bucharests isw aere more run suitable for 376location representative for achieving seven the-day best summer PVT performance. and winter periods The simulations to assess the were electrical run for power, representative temperatur seven-daye in 377summer the storage and winter tank, and periods the temperature to assess the of electricalthe working power, fluid, and temperature also for a yearlong in the storage period tank,to assess and the 378temperature the global of the annual working performance fluid, and. As also expected, for a yearlong the electrical period performance to assess the flows global closely annual the performance. global 379As expected,irradiance the with electrical little influence performance from the flows ambient closely temp theerature global and irradiance wind speed. with The little tank influenceis coupled from 380the ambientwith a DHW temperature consumer, and thus wind the temperature speed. The does tank not isreach coupled high values, with a even DHW though consumer, the fluid thus at the 381 the outlet at the collector can reach up to 43 °C. temperature does not reach high values, even though the fluid at the outlet at the collector can reach 382 Overall, the yearly production of electrical and thermal energy, both supported by back-up up to 43 C. 383 solutions◦ (battery bank and auxiliary heater respectively), indicates that the energy requirements of 384 Overall,an average the family yearly household production could of be electrical half fulfilled and thermalover the energy,course of both one supported year, with a by slight back-up 385solutions advantage (battery in the bank Bucharest and auxiliary climate heater compared respectively), to Strasbourg. indicates This was that expected the energy due requirementsto the slightly of an 386average improved family weather household conditions. could be half fulfilled over the course of one year, with a slight advantage 387in the BucharestIn terms climateof system compared performance, to Strasbourg.there are various This ways was of expected assessing it due and to it theis debatable slightly which improved 388weather is the conditions. most accurate and suitable method. The first law efficiency is the simplest and widely used 389 Inmethod, terms where of system there performance, is no distinction there made are between various the ways electrical of assessing and thermal it and power. it is debatable The Primary which is 390the mostEnergy accurate Saving and efficiency suitable takes method. into account The first the regional law efficiency differences is the in simplest energy production, and widely and used can method,be 391 more useful to compare the efficiency of the system in two different locations, as is the case of this where there is no distinction made between the electrical and thermal power. The Primary Energy 392 study. Saving efficiency takes into account the regional differences in energy production, and can be more 393 Some of the limitations of this simulation and its results are: The simplicity of the PVT model 394useful compared to compare to a thefull efinitefficiency element of the analysis, system the in fact two that diff erentall the locations, components as isare the pre case-defined of this in study.the 395 SomeTRNSYS of software the limitations and thus of a thisfull customization simulation and is difficult, its results the are:consumer The simplicityprofiles were of obtained the PVT by model compared to a full finite element analysis, the fact that all the components are pre-defined in the TRNSYS software and thus a full customization is difficult, the consumer profiles were obtained by means of forcing functions and statistical methods and can vary significantly according to the individual end user and the location of the system. Energies 2019, 12, 3595 16 of 18

7. Conclusions and Perspectives This paper has performed a dynamic simulation of a PVT system for a small size domestic household using the software TRNSYS. The system includes both thermal and electrical storage, and is coupled to the simulated electrical and DHW demand of the household. The results show that, with a battery storage system, the household demand can be 58% covered in Bucharest and 48% in Strasbourg. The hot water heating system, provided that an auxiliary heater is utilized, can also be covered 51% and 41%, respectively. Overall, it appears that small scale PVT is a promising solution for maximal harvesting of the solar energy. Residential prosumers are the target market of this technology, which is quickly developing in the Western European countries, but is lagging behind in the East. More public awareness and demonstrative projects providing proof of concept are required to further push this technology on the mainstream market, next to conventional PV and solar thermal. An analysis of the costs, profitability, and return period of the system would be an interesting direction for further research, including consideration of the CO2 emission savings, from the cradle to the grave of the panel. This can be proved challenging due to the fluctuating market prices of energy, a non-homogeneity in the quoted prices of PVT on the market and the difficulty in accurately accounting for the true costs of CO2 emissions. Another issue to be further investigated is the coupling of the PVT panels with heat pumps for a more efficient utilization of the thermal energy and assessing the cost-benefits compared to those of a simple PVT system. This analysis can be carried out for a number of different consumer profiles, not only residential households, but also office buildings, schools and hospitals, and the system optimized to fit the demand curve. Finally, an energetic study of the PVT system would also be an interesting research direction.

Author Contributions: M.B., M.S. and G.D. all contributed in the conceptualization. The methodology and software implementation were done by M.B. G.D. obtained the data resources. M.B. prepared the original draft. Final editing was done by all authors. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations

PVT photovoltaic-thermal PV photovoltaic ST solar thermal STC standard testing conditions DHW domestic hot water EVA ethylene vinyl acetate c-Si crystalline silicon η efficiency (%) T temperature (◦C) k thermal conductivity (W/mK) δ thickness (m) U thermal loss coefficient (W/m2K) S solar energy absorbed (W) qe electrical energy (W) Qu thermal energy (W) A area (m2) Fr heat removal factor (-) α effective absorbance (-) βr temperature coefficient (%/K) Energies 2019, 12, 3595 17 of 18

References

1. EREC. Renewable Energy in Europe, 2nd ed.; EarthScan: London, UK, 2008; 116p. 2. Barbu, M.; Patrascu, R.; Darie, G.; Tutica, D. A technical-economical analysis of the implementation of hybrid solar energy systems in small energy prosumer applications. Qual. Access Success 2019, 20, 134–138. 3. Kumar, A.; Baredar, P.; Qureshi, U. Historical and recent development of photovoltaic thermal (PVT) technologies. Renew. Sustain. Energy Rev. 2015, 42, 1428–1436. [CrossRef] 4. Aste, N.; del Pero, C.; Leonforte, F. Water flat plate PV-thermal collectors: A review. Sol. Energy 2014, 102, 98–115. [CrossRef] 5. Agrawal, B.; Tiwari, G.N. Life cycle cost assessment of building integrated photovoltaic thermal (BIPVT) systems. Energy Build. 2010, 42, 1472–1481. [CrossRef] 6. Axaopoulos, P.J.; Fylladitakis, E.D. Performance and economic evaluation of a hybrid photovoltaic/thermal solar system for residential applications. Energy Build. 2013, 65, 488–496. [CrossRef] 7. Rawat, P.; Dhiran, T.S. Comparative Analysis of Solar Photovoltaic Thermal (PVT) Water and Solar Photovoltaic Thermal (PVT) Air Systems. Int. J. Civil Mech. Energy Sci. 2017, 3, 8–12. 8. Nualboonrueng, T.; Tuenpusa, P.; Ueda, Y.; Akisawa, A. Field Experiments of PV-Thermal Collectors for Residential Application in Bangkok. Energies 2012, 5, 1229–1244. [CrossRef] 9. Rejeb, O.; Dhaou, H.; Jemni, A. Parameters effect analysis of a photovoltaic thermal collector: Case study for climatic conditions of Monastir, Tunisia. Energy Convers. Manag. 2015, 89, 409–419. [CrossRef] 10. IEA SHC Task 35-PV/T-Systems. Available online: http://task35.iea-shc.org (accessed on 8 August 2019). 11. IEA SHC Hadorn. French PVT Market is Picking Up. Available online: http://www.iea-shc.org/article? NewsID=205 (accessed on 8 August 2019). 12. IEA SHC Task 60-PVT Systems: Application of PVT Collectors and New Solutions in HVAC Systems. Available online: http://task60.iea-shc.org (accessed on 20 June 2019). 13. PV Magazine. Romania’s PV Capacity Grew by 70 MW in 2017. Available online: https://www.pv-magazine. com/2017/03/08/romanias-pv-capacity-grew-by-70-mw-in-2016/ (accessed on 20 June 2019). 14. Weiss, W.; Spörk-Dür, M. Solar Heat Worldwide; IEA Solar Heating & Cooling Programme; AEE-Institute for Sustainable Technologies: Gleisdorf, Austria, 2017. 15. PV Magazine. France’s PV Hits 8 GW Milestone. Available online: https://www.pv-magazine.com/2018/02/ 28/frances-pv-hits-8-gw-milestone/ (accessed on 22 June 2019). 16. IEA SHC. France Country Report 2018. Available online: http://www.iea-shc.org/country-report-france (accessed on 22 June 2019). 17. Brottier, L. Optimisation Biénergie d’un Panneau Solaire Multifonctionnel: Du Capteur aux Installations in Situ. Ph.D. Thesis, Université Paris-Saclay, Paris, France, 2019. 18. BRE National Solar Centre. Hybrid Solar Photovoltaic Thermal Panels-Low Carbon Heating Technologies; BRE National Solar Centre: London, UK, 2016. 19. Zhou, C.; Liang, R.; Zhang, J. Optimization Design Method and Experimental Validation of a Solar PVT Cogeneration System Based on Building Energy Demand. Energies 2017, 10, 1281. [CrossRef] 20. Barbu, M.; Mey-Cloutier, S.; Siroux, M.; Darie, G. Dynamic modelling and sensibility analysis of a hybrid photovoltaic-thermal (PVT) system. In Proceedings of the 7th European Conference on Renewable Energy Sysstems, Madrid, Spain, 10–12 June 2019. 21. Guarracino, I.; Mellor, A.; Ekins-Daukes, N.J.; Markides, C.N. Dynamic coupled thermal-and-electrical modelling of sheet-and-tube hybrid photovoltaic/thermal (PVT) collectors. Appl. Therm. Eng. 2016, 101, 778–795. [CrossRef] 22. Jordan, U.; Vajen, K. DHWcalc: Program to generate domestic hot water profiles with statistical means for user defined conditions. In Proceedings of the ISES Solar World Congress, Orlando, FL, USA, 8–12 August 2005. 23. Jordan, U.; Vajen, K. Influence of the DHW load profile on the fractional energy savings: A case study of a solar combi-system with TRNSYS simulations. Sol. Energy 2001, 69, 197–208. [CrossRef] 24. Pflugradta, N.; Muntwylera, U. Synthesizing residential load profiles using behavior simulation. In Proceedings of the CISBAT 2017 International Conference—Future Buildings & Districts—Energy Efficiency from Nano to Urban Scale, Lausanne, Switzerland, 6–8 September 2017. Energies 2019, 12, 3595 18 of 18

25. Kelly, N.; Samuel, A.; Tuohy, P. The effect of hot water use patterns on heating load and demand shifting opportunities. In Proceedings of the BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, 7–9 December 2015. 26. Klein, S.A. Calculation of flat-plate collector loss coefficients. Sol. Energy 1975, 17, 79–80. [CrossRef] 27. Florschuetz, L.W. Extension of the Hottel-Whillier Model to the Analysis of Combined Photovoltaic/Thermal flat plate collectors. Sol. Energy 1979, 22, 361–366. [CrossRef] 28. Bombarda, P.; Di Marcoberardino, G.; Lucchini, A.; Leva, S.; Manzolini, G.; Molinaroli, L.; Pedranzini, F.; Simonetti, R. Thermal and electric performances of roll-bond flat plate applied to conventional PV modules for heat recovery. Appl. Therm. Eng. 2016, 105, 304–313. [CrossRef] 29. Huang, B.; Lin, T.; Hung, W.; Sun, F. Performance evaluation of solar photovoltaic/thermal systems. Sol. Energy 2001, 70, 443–448. [CrossRef] 30. European Environment Agency. Efficiency of Conventional Thermal Electricity and Heat Production in Europe 2018. Available online: https://www.eea.europa.eu/data-and-maps/indicators/efficiency-of- conventional-thermal-electricity-generation-4/assessment-2 (accessed on 8 August 2019).

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