Development of a composite PVT panel with PCM embodiment, TEG modules, flat-plate solar collector, and thermally pulsing heat pipes

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Development of a composite PVT panel with PCM embodiment, TEG modules, flat-plate solar collector, and thermally pulsing heat pipes Birol Kılkış Polar Project & Technology, Hacettepe Teknokent 1st R&D Building, No: 22, 06800 Beytepe, Ankara, Turkey

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar PVT Exergy-levelized unit cost Phase change material Rational exergy management model Solar cogeneration Bottoming cycle TEG modules Thermally pulsing heat pipe Modular PVT

An integrated, multi-layered, composite photovoltaic thermal (PVT) module consisting of a multitude of standalone mini PVT cartridges was developed and a prototype was tested. Individual PVT cartridges consist of several sandwiched layers with photovoltaic cells, TEG units, packed-bed PCM layer for thermal storage, and thermally controlled heat pipes with dynamic control. These cartridges may be easily removed from the master PVT casing and re-installed for repair, inspection, and replacements. When cartridges are installed, the composite PVT module is completed by adding a flat-plate collector layer on the top. An internal array of pulse-control heat pipes maintains the total exergy output (power and heat) at a maximum by adjusting the heat flux. This PVT panel design eliminates the need for external thermal storage, pumping of the PVT coolant, and associated parasitic losses. PVT technology is made more energetically and exergetically rational as an economic asset for decarbonizing the environment. In such a configuration, it represents the solar equivalent of a conventional cogeneration system with a bottoming cycle. This paper summarizes the technological evolution of the new composite PVT system and provides examples of its use. Pilot-scale tests have shown that the Rational Exergy Management Model efficiency is about 25% more than a conventional PVT system and the total net electrical power output per unit solar insolation area is more than 30%. Total exergy output (power and heat) is twice as much as a conventional PVT unit in a typical summer month. The results obtained are discussed with further evolutionary recommendations. The importance of exergy-levelized unit panel cost is put forth to provide a basis for the fundamental procedure for a dedicated, well-accepted, and stand-alone PVT test method for the rating of system performance.

1. Introduction The European Union has set targets to increase the share of renewable energy sources in the general energy mix for decarbonization. While the successful utilization of solar thermal energy is expanding in different forms and technologies, European Union (EU) targets for 2020 and beyond are mainly based on utilizing solar and wind energy for electricity production with PV panels and then employing electricallydriven heat pumps for HVAC systems in buildings (Kilkis, 2017). When compared to a solar PV panel generating only electric power and a flatplate solar collector (FPC) generating only thermal power, a photovoltaic thermal (PVT) system, which generates both heat and power has advantages over electrically-driven heat pumps in the quest of decarbonization if optimally designed and operated. The generated heat may be transformed into cooling with adsorption-cycle chillers such that PVT technology may be a rational alternative to electrically-operated heat pumps. After all, even with the introduction of F-gas type refrigerants, heat pumps have both ozone depletion and global

warming potentials due to their compression-cycle components. The advent of net-zero energy and especially Low-Exergy (LowEx) buildings further reduce the need of heat pumps, while solar heat from PVT systems may be directly utilized without temperature peaking (in heating) or decreasing (in cooling). Therefore, it will be prudent to couple PVT panels with LowEx and other industrial applications with moderate temperature requirements. With denser urbanization with high-rise buildings, solar insolation area per building area decreases while demand for electrical and thermal power demand increase. Therefore, the available solar insolation area is becoming a restricted premium for satisfying both demands simultaneously. In this respect, PVT systems are becoming desirable, especially in hot climates and dense urban areas also considering the urban heat island effect where PV cells need to be cooled in order to preserve their rated efficiencies. During a typical summer day-time period, the First-Law efficiency of a PV cell decreases when its temperature increases while cooling loads peak. This contradiction can be solved by cooling the PV cells and

E-mail address: [email protected]. https://doi.org/10.1016/j.solener.2019.10.075 Received 19 September 2018; Received in revised form 31 August 2019; Accepted 26 October 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: Birol Kılkış, Solar Energy, https://doi.org/10.1016/j.solener.2019.10.075

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Nomenclature

Greek Symbols

Symbols

α β ε ηI ηII ηIPsc

A a ALT b AT APV c CO2a COP COPEX CP d e E ELC EM

EX EXT E1 E2 GWP ODI PC PEF PER Qs QH p, r, s Re Sc T tp TE TE’ Tdesign Tg Tf Ts In

V̇ X Y x,y,z,w W

Panel area, m2 Coefficient in Eq. (4) for heat pump COP, dimensionless Residence time in the atmosphere, years Coefficient in Eq. (4) for the heat pump performance COP, K−1 Total PV, PVT, FPC surface area in a PVT system observing the solar irradiation, m2 Net total insolation area of PV cell panel(s) served by the same PVT controls, m2 PVT circulation pump power demand to PV power output ratio, dimensionless Avoidable CO2 emissions due to exergy destructions, kg Coefficient of performance, dimensionless Exergy-based COP, dimensionless Specific heat of coolant at its mean temperature through the PVT panel, kJ/(kg·K) TES capacity ratio (Eq. (18)), dimensionless Heat exchanger temperature decrease in TES (Eq. (19)), dimensionless Quantity of total electric power generated in the PVT system, W Exergy-Levelized Unit Cost, €/m2 (Based on Rational Exergy Management Efficiency) Composite embodiment cost with respect to material, manufacture, and installation in terms of embodied CO2, energy, and destroyed exergy. Exergy, W Total exergy output, W Electric power generated by PV cells, W Electric power generated by TEG units, W Global warming potential, dimensionless Composite ozone-depletion index, dimensionless Unit power cost of a solar energy system, €/kWpeak Primary Energy Factor (Ideal:1) Primary Energy Ratio (Ideal: >1) Solar power impingent on the PVT panel surface, W Thermal power supplied by the PVT system, W Coefficients of Eq. (3-a), dimensionless Reynolds number, dimensionless Solar constant, 1366.1 W/m2 Temperature, K (or oC) Thermal pulse period, minute Average frame temperature of the PV layer, K Modified frame temperature of the PV layer after compensating for the circulating pump power demand, K Design supply temperature for building heating, K Ground temperature, K Carnot-Cycle equivalent virtual source temperature of solar insolation at a given In, K Surface temperature of the sun, 5778 K Total solar irradiation normal to the PVT panel surface, W/m2 The volumetric flow rate of the coolant in the PVT panel, m3/s Partial thermal load on PVT 1 (Fig. 21) Partial thermal load on PVT 2 (Fig. 21) Coefficients in Eqs. (15) and (16) Weight of a panel, kg

ηIPsco ηIPV ηIPVo ρ ΨR ΔCO2 ΔT Δt

Linearized temperature factor for FPC efficiency, K−1 Linearized temperature factor for PV efficiency, K−1 Unit exergy, W/W First-Law efficiency, dimensionless Second-Law efficiency, dimensionless First-Law Efficiency of heat generation in a PVT panel, dimensionless First-Law Efficiency of heat generation in a PVT panel according to standard test conditions, dimensionless First-Law Efficiency of PV cells at given operating conditions, dimensionless First-Law Efficiency of PV cell at standard test (reference) conditions, dimensionless Density of fluid at a mean fluid temperature, kg/m3 REMM Efficiency, dimensionless Avoidable, indirect CO2 emissions responsibility of a solar energy system, kgCO2/kW-h Temperature difference (rise) across the PVT hydronic inlet and outlet, K Thermal pulse interval of the heat pipe, minute

Subscripts a av B c D des E H m max min n o opt ret sc solar sup pcm PV ref TP x

Ambient (air) Average Temperature-peaking boiler or any other heater PVT cold (cooled) side (for Temperature) demand Destroyed, destruction Power (electric) Thermal (heat) Mean Maximum Minimum Normal to insolation surface Rated value at standard test conditions Optimum Return Flat-Plate Solar collector (FPC) Solar related variable Supply Phase-changing material Photo-voltaic Reference environment Temperature peaking Exergy

Acronyms ADS AG BIPV BIPVTC CPVT CW DHW EU F FPC GC GSHP HP HVAC 2

Adsorption Chiller Air gap Building-Integrated PV Building-Integrated Photo-Voltaic-Thermal and Cooling PVT Concentrator Cold Water Domestic Hot Water European Union Frame Flat-Plate Solar Collector Glass cover Ground-Source Heat Pump Heat pipe Heating, Ventilating, and Air-Conditioning

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IN IEA LowEx NS P REMM PCM PHVT PTHV

PV PVT PVTC TEC TEG TES TP VCHP

Insulation, Inverter (DC to AC power) International Energy Agency Low Exergy (Building) Nanosheet Pump Rational Exergy Management Model Phase-changing material Photo-Heat-Voltaic-Thermal Pulsing Heat Pipe

∑ ηII PVT

capturing the heat of the coolant for useful applications, like supplementary cooling by utilizing the heat in an adsorption-cycle chiller (Adarsh et al., 2015). Despite such an important advantage, PVT systems have certain drawbacks. One of them is the exergetic conflict between the electrical and thermal power exergy delivered and the electrical pumping power exergy that is required for circulating the coolant. If the aim is to preserve the PV efficiency at the design level, the temperature increase (ΔT) of the cooling fluid circulating under or in the PVT panel must be kept at a minimum. This requires a higher coolant flow rate accompanied with higher pumping power exergy demand, which may override the benefit of cooling the PV cells. Furthermore, high flow rates decrease the thermal power exergy since the coolant exit temperature from the PVT system will be insufficient for a useful thermal application. If the main objective is to generate a reasonable amount of thermal power at a reasonable level of exergy, then the average cooling temperature must be permitted to be high enough in the hydronic circuit. This reduces the pumping power demand but compromises the PV efficiency. Therefore, an optimum flow rate must be maintained.

(2)

The large difference between the First-Law and the Second-Law efficiencies in this example shows that from the perspective of net positive contribution of PVT systems to the environment, economy, and the rational use of renewable energy sources, the Second-Law becomes a primary rule, which deals with the quality of solar energy and separate qualities of heat and power outputs. Despite these facts, many studies in the literature do not undertake analyses based on the Second Law (Jiang et al., 2016; James et al., 2015; Ilhan and Ali, 2015) or concentrate on simple exergy efficiency about the PVT module without factoring in its peripherals like thermal storage, circulation pumps, and thermal peaking. For example, Ilhan and Ali (Ilhan and Ali, 2015) have carried out an exergy analysis of new design while only analyzing the exergy efficiency of the PVT unit alone without considering the thermo-electrical correspondence and connections of the PVT unit with its ancillaries and loads in the building. Srimanickam et al. (Srimanickam et al., 2015) conducted exergy analysis on a conventional air-cooled PVT panel. The solar insolation, current, voltage, inlet and outlet air temperature of the cooling duct, ambient air temperature, and solar panel surface temperature were their major parameters for calculating the energy and exergy efficiency. An improved electrical efficiency equation was used to estimate the electrical output. In their energy and exergy calculations, the exergy demand of ancillaries was not considered. Sobhnamayan et al. (Sobhnamayan et al., 2014) carried out research regarding the optimization of a PVT water collector, which is based on exergy analysis. A computer simulation program was developed in order to calculate the optimum inlet water velocity and coolant pipe diameter, along with a genetic algorithm to optimize the exergy efficiency. They indexed the exergy efficiency to the outdoor air temperature as usual in the literature. Such an exergy efficiency results in frequent changes with dynamic change in the outdoor air temperature. If a PVT system is considered as a system of system type of architecture, with many components attached, like circulation pumps (or fans), energy storage systems, temperature peaking systems when necessary, and if especially the thermal power output is connected to a ground-source heat pump in urban applications, then a more stable comparative reference temperature base will be the ground temperature Tg. In this research, the

The basics of a conventional, liquid-cooled PVT system, isolated from its surroundings is shown in Fig. 1. This figure does not yet show any connections to the demand points and ancillaries like an external thermal storage system, circulation pumps, controls, etc. According to a recent study, ancillaries may demand more exergy than obtained from the PV efficiency gain (Kilkis, 2016). A PVT panel becomes functional only with its external ancillaries, like thermal storage, coolant pumps or fans, and temperature peaking units as discussed in later sections. Therefore, any PVT system may only be analyzed with a holistic approach including all interrelated thermal, electrical, and hydro-mechanical connections, which expand the PVT concept to a complex domain of analysis which must be carried out by both the First and Second Law of thermodynamics. The First-Law of Thermodynamics only deals with the magnitudes (quantity) of the solar source power, Qs electrical power supplied, E and the thermal energy supplied, QH, each of them having quite a different exergy. Without referring to their qualities (exergy) with the SecondLaw, PVT systems seem to have very high total First-Law efficiency, ηI over 90% that is misleading.

= ηIPV + ηIH

= ηIIPV + ηIIH = ηIPV εE + ηIH εH = 0.18 × 0.96 + 0.75 × 0.06 = 0.218

1.1. Energy versus exergy efficiency of PVT systems

∑ ηIPVT

Photo-Voltaic Photo-Voltaic-Thermal Photo-Voltaic-Thermal-Cooling Thermo-electric Cooler Thermo-Electric Generator Thermal Energy Storage Temperature Peaking Variable Conductance Heat Pipe

(1)

For example, if the power generation efficiency of the PV cell is 0.18 and the thermal efficiency of the thermally cooling side of the PVT module is 0.75, then the total First-Law efficiency from Eq. (1) is 93%. This equation omits the large difference between the unit thermal power exergy, εH, in the range of 0.15 to 0.25 W/W, and a much higher unit electrical power exergy, εE of 0.96 W/W (sometimes rounded to 1 W/W). The unit exergy gained by the coolant at an input temperature, Tret of 40 °C (313 K) and rising to a supply temperature, Tsup of 60 °C (333 K) is only 0.06 W/W, which is derived from the ideal Carnot Cycle: (1–313 K/333 K). Eq. (2) gives the complete PVT performance according to the Second-Law.

Fig. 1. Isolated Layout of a Conventional Hydronic PVT Module (Kilkis, 2016). 3

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evaluation and rating metrics are generally directed to the First-Law of Thermodynamics, or to simple exergy analysis of PVT components towards pinpointing major exergy destruction points. If one needs to investigate an optimum solution such that both power and useful heat may be generated without much compromise, then the difference between unit exergy of power and heat must be taken into account. Then an optimum design and an optimum coolant flow rate may be dynamically achieved during operation by using a variable-speed pump controlled by a tailored, exergy-based algorithm (Kılkış et al., 2016). The extent that the power demand of the pump may be reduced is limited and a better solution is to eliminate the pump by replacing it by integrating heat pipes into the PVT system. Yet heat pipes facing variable electrical and thermal loads and solar heat, need to be dynamically controlled. The situation is further complicated with the thermal behavior of the embedded PCM layer if present (Kilkis, 2018). In order to more rationally utilize solar energy for power, heat, and cold in hybridized forms over the same solar insolation area, several sandwich designs were carried out as novel contributions, which are exemplified in Figs. 2–4. Fig. 2 shows the simplest form of solar cogeneration, which is a direct sandwich of thermoelectric generators (TEG) and PV layers, with the coolant layer at the bottom, which indirectly cools PV cells across the TEG units. Fig. 3 shows an improvement based on the relocation of the coolant layer between the PV cells and the TEG units such that a temperature gradient across them is formed by transferring the heat of PV cells to the back of TEG units with a heat-conducting film, thus actively cooling the PV cells. Preferably cadmium telluride (CdTe) type of PV cells are used with low β value, provided that environmental issues are taken care of. A composite wall type of PVT design, named solar brick was developed more recently, as shown in Fig. 4. The array of PV cells are mounted on the upper surfaces of optimally tilted bricks that are laid for constructing the wall. This angle that is molded during the manufacture of the bricks depends on the latitude and the relative dominance of the summer and winter seasons. A liquid coolant circulates at their backside. Heat pipes may also be used. On the inner side of each solar brick facing the indoor space, TEG units are laid, which actively establish the radiant indoor cooling panel surfaces. An electrical resistance mesh may be inlaid for winter heating, which uses part of the solar power-on-demand as a dual purpose wall. TEG units are activated on cooling demand by part of the solar power. In addition to the solar heat obtained from the façade, the indoor heat that is absorbed by TEG units is also collected by heat pipes and utilized in domestic use and cooling by adsorption units. Among the whole spectrum of innovative PVT designs, Ilhan and Ali (2015) have developed a pumpless, forced water-circulation PVT system in order to make the PVT system feasible. Their basic exergy analysis revealed that the overall exergy efficiency is about 17% and 21% for set temperatures of 45 °C and 55 °C, respectively. In some

annual average ground temperature was used (283 K). Atheaya et. al (Atheaya et al., 2016) attempted to investigate the exergetic performance of a PVT system that is compounded by a concentrating parabolic collector. The water circulation had two alternatives, namely constant temperature, and a constant flow rate. In the case of operation with constant temperature, there must be a pump that adjusts the flow rate accordingly. Again, they did not incorporate the pump features and its exergy demand into their calculations. Singh et al. (2015) developed a genetic optimization algorithm and demonstrated its effectiveness in the design of PVT panels with a singlechannel coolant layer. This feature resembled the one shown in Fig. 1. Chow (2010) recognized the PVT system as a collection of many ancillaries, like thermal storage unit while using only the First Law for analysis although a wide account of exergy related work in the literature was provided. The study analyzed different coolant channel positions like above the PV cells, below the PV cells, single pass or double pass. Double pass configuration seemed attractive but while channel absorbs more heat, the average temperature of the PV cells may be compromised, which can be distinguished by the model presented in this research based on exergy. Siecker et al. (2017) provided a very extensive account of cooling techniques compiled both from the field and the literature. They pointed out that the use of phase change materials (PCM), thermoelectric cooling, immersed PV cooling, and forced water circulation are important alternatives among many others. Parida et al. (2011) and Sathe and Dhoble (2017) in their review papers of PVT technologies emphasized the advantages of building-integrated PV (BIPV) applications when compared to roof-mounted applications and pointed out that relatively few efficient PVT systems are available in the commercial domain. They described the work carried out by many researchers on conventional and more recent PVT systems. Without focusing on PVT technology, Hansen and Mathiesen (2018) have referred to EU targets for solar thermal technologies in the context of targets to increase the share of renewable energy in the coming years. Solar thermal energy is currently expanding and could possibly contribute to achieving European targets. The authors evaluated the potentials of solar thermal energy from an energy perspective only for four European national energy systems, namely Germany, Austria, Italy, and Denmark. Their optimistic conclusion is that the national solar thermal potentials are in the range of 3–12% of the total heat supply. These values are open to improvement with PVT technology in especially Italy that has a Mediterranean climate. 1.2. Novel PVT systems Extensive research and development activities on PVT systems have been taking place at least for the past 35 years (Kumar et al., 2015). Although these research work ended up in many PVT types and applications, including heat pipe technology (Zhao, et al., 2008), their

Fig. 2. Simple PV Cell and TEG Combination (Kılkış and Kılkış, 2015). 4

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Fig. 3. Simple Sandwiched PV Cells and TEG Units Separated by the Coolant Layer (Çolak and Kılkış, 2014).

Fig. 4. Composite Solar Brick Wall (Bean and Kilkis, 2003).

a new PVT apparatus using PCM for cooling the PV cells but no detail was given about whether the heat is utilized in a useful work or not. In essence, these and other similar patents did not elaborate on how

further innovative solutions, a PVT concentrator (CPVT), wherein a parabolic concentrating system is provided to reduce the number of solar cells needed was patented by Escher et al. (2015). According to their US Patent (Escher et al., 2015), CPVT can achieve a high solar thermal performance compared to flat PVT collectors. Their system includes a photovoltaic-thermal hybrid solar receiver, a thermal collector, and a photovoltaic module at separate planes observing the concentrator. They claim that the main obstacles to CPVT are the need for providing effective cooling of the solar cells and a durable tracking system. The application of PCM materials in PVT systems is not entirely new. There are certain patents available in the literature with different claims. Xuan (2016) mention a PCM layer in their Patent as provided in Fig. 5. The TEG module (marked as 4) has a heat sink at the bottom (5). Between the PV layer with concentrating lens (1) and TEG module (5), there is a so-called thermostatic container (3), which contains the PCM. In another patent, Hi-Ki Lam (Lam, 2011) claims that the PV modules mounted on a heat sinking frame with TEG modules may be effectively cooled. Under the TEG modules, fins that are similar to the fins as shown in Fig. 5 remove the heat with convection to the ambient air, while additional power is generated. However, it was not made clear whether the air movement is assisted by an external fan. If this is the case, then the exergy demand of the fan power needs to be deducted from the electrical power exergy supplied. Mazor (2009) has described

Fig. 5. PVT Cooling with Fins (Lam, 2011). 5

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warming (GWP) potentials, which combine into an Ozone Depletion Index, ODI. Sometimes this effect may override the targeted CO2 emissions reductions (Kilkis, 2019). Eq. (3-a) represents the combined effect of ODP and GWP (Kilkis, 2019). Global coefficients p; r, and s depend upon the season and geographical distribution of emissions.

the PCM layer is operated and did not discuss its potentially practical disadvantages, such as leaking without pelletizing in hermetically sealed small capsules of optimum geometry and material. Furthermore, these studies did not incorporate FPC technology at the top or other assets, such as a reversing heat transfer mechanism that is operated after the sunset to continue to generate heat and thus some minor power generation by TEG units. In effect, these models reject the heat with the sole purpose of cooling the PV cells for a thermally stable state. The same argument holds true for another patent by Dinwoodie (1994). In this context, PV panels, preferably on the roof that are separated by adjustable spaces between them, are permitted to be variably cooled by the passing fluid, thus the PV panel temperatures are controlled and reduced. Anderson (2009) have modeled and tested a PVT system to be integrated into buildings as BIPVT with a fin section integrated beside the PV array for cooling purposes. The coolant enters the fin and exits on the other side after absorbing the heat that is transferred to the fin. Santbergen et al. (2010) have made a detailed discussion on the cooling advantages of PV cells in their paper. However, their analyses and results with respective comparisons of PV and PVT systems were based on the First Law, similar to Eq. (1). Massimo et al. (2014) have integrated a solar PVT system for a net-zero energy house retrofit in order to assist its HVAC system. The HVAC system was a low-temperature, PCM integrated system such that the thermal output of the PVT system was directly accommodated for sensible indoor comfort heating, while the peak load was satisfied by the PCM. More recently, Kılkış (2017) including the author have eventually comprised all solar-relevant power and heat components under one composite panel and eliminated the fluid circulation pump (or fan in gaseous cooling) with thermally self-adjusting-flow heat pipes, eliminating the need for external thermal storage by a PCM layer embodied in the same panel, and added a TEG layer and FPC technology (Kilkis, 2019). Their patent (Kilkis, 2019) claims that the system control is based on the rational use of solar exergy. Thus, an exergy-based control algorithm accompanies the composite panel, which is described in detail with prototype test results in the following sections to address a scientific and technological gap (see Fig. 6).

ODI =

pGWP r ALT s ⎞ ×⎛ (1 − ODP ) ⎝ 1 ⎠

(3-a)

PER =

COP PEF

(3-b)

More importantly, in a case where the heat pump delivers thermal power to the building at a temperature, Tsup of 333 K and the return temperature, Tret is 313 K, then the unit thermal power exergy, εH that is provided to the building by the heat pump according to the ideal Carnot Cycle is (1–313 K/333 K) = 0.06 W/W. On the contrary, even if the electric power supply is designed to be from on-site PV or a PVT system, the exergy break-even point will be 0.96 W/W unit electrical power exergy supplied to the heat pump divided by thermal power exergy of 0.06 W/W, which in terms of total exergy flow, EX requires a COP of 0.96/0.06 = 16. This is practically impossible with the current technology and suggests that there will be no net exergy gain. At first glance, this condition may seem to be relaxed by reducing Tsup from 333 K to 323 K for indoor space heating. According to Eq. (4), the COP may increase to 14.9 if a favorable heat pump design with a = 15 and b = 0.02 K−1 is innovated and used at a ground-source temperature, Tg of 330 K. Such a heat pump may involve reduced scale centrifugal compressor instead of a volumetric compressor with higher efficiency (Kilkis, 2019).

COP = a − b |Tsup − Tg|

(4)

Supplying heat at 323 K instead of 333 K further reduces εH supply from 0.06 W/W to 0.031 W/W. Then the break-even point COP moves to 31. Therefore, there is no real solution for this example in terms of the unit exergy balance. However, lowering Tsup in heating also lowers the heating load, QH of the building. Therefore, one may attempt to consider the total exergy flow, EX instead of εH also on the building demand side in order to correct the solution at an expense of further glazing and thermal insulation of LowEx buildings (Kilkis, 2019):

2. Importance of solar energy in a low-exergy built environment Especially for renewable and waste heat-originated district energy networks, medium to large-size solar and wind farms need to be collocated at a distance from the urban district. The same also holds true for thermal power plants and the industry with waste heat, which may be located at some distance from cities. While renewable and waste energy sources have low exergy, distance means more pumping power demand with higher electrical exergy input. Therefore, farther the plant and the district from each other, pumping exergy demand may start to exceed the thermal exergy supplied to the district (or collected in district cooling). The distance limit becomes more restrictive with the simultaneous advancement of LowEx buildings and districts on the demand side and the utilization of low-exergy, renewable, and waste energy resources on the supply side. The EU has adopted a decarbonization goal with the principle of electrification of all district services. This program includes electrically operated heat pumps, which satisfy heating or cooling demands. At first glance, electric power transmission to the districts and then using heat pumps at the demand points for heating and cooling seems a rational choice instead of transferring thermal exergy as in Fig. 7. Such a choice also eliminates the district pumping demand. However, the primary energy factor, PEF in EU countries currently average at about 2.5. This means that there is about 60% overall loss from fuel to the plug (heat pump) power transmission. Therefore, the primary energy ratio, PER, hardly exceeds one, which should ideally be much greater than one. For example, if the seasonal average COP of a ground-source heat pump is 3, then PER will be just 1.2. Furthermore, the heat pump compression cycle using refrigerants causes ozone depletion (ODP) and global

Tsup ⎞ T EX = εH QH = ⎛⎜1 − ret ⎞⎟/ ⎛⎜ ⎟ Tsup ⎠ ⎝ Tdesign ⎠ ⎝

(5)

In Eq. (5), Tdesign is the Tsup value at design conditions, which is 333 K. (Kilkis, 2019). However, calculations showed that this correction does not practically relax the condition. The solution is shown in Fig. 8. Until even better LowEx buildings coupled with more innovative heat pumps are developed, decarbonization with electrification and heat pumps remain exergetically unfeasible. This holds true especially for solar thermal energy, which has lower unit exergy. This leaves only wind energy, biogas, high-enthalpy geothermal, and industrial waste heat with higher exergy to be viable options for centralized district energy systems. Solar PV and concentrated solar power (CSP) systems may be centralized if they are going to serve the power grid. It is also noteworthy that the rational exergy efficiency of PV panels, ψR, which will be explained further below is around 0.6, compared to innovative PVT panels that increase up to 0.9. This kind of efficiency translates to

Fig. 6. Side View (Lam, 2011). 6

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Electricity

ODI

PEF=2.5

HP

Solar Energy

Tret

Tsup PV Plant

Building

without heat pumps. Optimum insulation and glazing may also help in improving LowEx buildings. Therefore, along with the decarbonization with electrification strategies of the EU, it is quite predictable that PVT systems will penetrate the solar market fast wherever the climate is appropriate like the Mediterranean region. The key aspect remaining for market penetration is to develop and innovate highly exergy-rational PVT systems with the highest PER and exergy rationality. The following section discusses the rational-exergy management of conventional and novel PVT systems, compared to single PV and single FPC systems. In order to lay a solid foundation for a holistic analysis, the quality of solar energy must be practically defined in terms of the Second-Law of Thermodynamics. This definition is based on the unit solar exergy, εsolar that is formulated in terms of the ideal Carnot Efficiency, which is given in Eq. (7) in terms of In, solar constant, Sc (1366.1 W/m2), radiant surface temperature of the sun, Ts (5778 K), which also gives 0.96 W/W for electric power unit exergy, and a reference environment temperature Tref (283 K).

COP Tg

Fig. 7. Solar Energy in Decarbonization With Central Electrification and Local Heat Pumps, PEF = 2.5, PER = 1.2.

Tref ⎞ ⎛ In ⎞ εsolar = ⎜⎛1 − ⎟ = Tf ⎠ ⎝ 0.96Sc ⎠ ⎝ ⎜

avoidable CO2 emissions, that are not accounted for by the First Law. In the quest of reducing the exergy demand in buildings for a better rationality of utilizing low-exergy resources, Low-Exergy (LowEx) buildings have been developed (LowEx, 2016). However transmisssion of low-exergy heat poses certain problems. In contrast, the transmission of low exergy sources like solar heat instead of solar electricity has serious restrictions because circulation pumps require power exergy in electricity form as mentioned above. Therefore, except for small neighborhood-scale district networks (Ciampi et al., 2018; Kilkis, 2019), solar energy must be decentralized (Kilkis, 2019). Even in very small mini solar heat networks, the pumping power demand may be an exergetic problem, such that if pump exergy demand is greater than the solar exergy that is circulated, additional exergy destructions and thus more CO2 emission impacts can take place (Kilkis, 2019). Therefore, with the technologies that are available today, electrification and decarbonization with renewables require de-centralized solar energy systems, such as the one shown in Fig. 9.

(

QH 1 −

Tret Tsup

EXpar

(7)

At the same time, Eq. (7) defines a Carnot Cycle equivalent (virtual) source temperature, Tf, which can be solved for mapping the quality of the normal solar flux, In to a thermal domain (Kılkış, 2011; Kılkış and Kılkış, 2018). If In is 800 W/m2, then rounded-off εsolar value will be 0.56 W/W and the corresponding Carnot Cycle equivalent (virtual) solar energy source temperature, Tf from Eq. (8) is 643 K:

Fig. 8. Heat Pump COP versus the Break-even Point for Exergy Supply and Demand.

E + EXE COPEX = XH = EXpar

⎟

Tf =

Tref (1 − εsolar )

(8)

2.1. 1. Solar Flat-Plate collector An FPC delivers only heat between supply and return temperatures, Tsup and Tret, respectively. Fig. 10 shows that exergy is destroyed between Tf -Tout (εdes1) and Tsup-Tref (εdes2). Major exergy destruction (εdes1) is upstream of the useful work because no electricity or any other useful work with higher-exergy is generated. In this case, the Rational Exergy Management Model (REMM) (Kılkış, 2011), gives the rationality metric of solar energy utilization, ψR, by Eq. (9).

)+E

XE

(6)

Solar PVT systems do not have fluid circulating pumps or fans, except small circulation systems within the building, which are standard for HVAC systems. The heat from PVT modules to the building is transferred with heat pipes and stored. Thus, there may be two levels of thermal storage, namely one in the PVT system and others in the building. Low-exergy heat is used in low-exergy sensible heating and cooling systems, such as the floor, ceiling, and wall radiant panels. In the cooling season, an adsorption type chiller converts the solar heat to cold. Both heat and cold are also thermally stored. Humidity control is achieved with a desiccant wheel. The value of PEF is practically equal to 1. Therefore, PER is equal to COP, which in this case is defined by Eq. (6) in exergetic form. If the parasitic losses are typically less than 10% and the amount of the thermal exergy that is supplied is about the same with the amount of electrical exergy that is supplied, then COP is about 20. Therefore, PER will be much greater than 1. Although the use of solar PV electricity in heat pumps for HVAC purposes in buildings seems to be a smart approach, especially for decarbonization efforts, there are better ways to use solar PV systems

Fig. 9. Pumpless, Thermally Self Storing, High ηIPV, De-centralized Decarbonization and Solar Electrification at Building Level with Heat Pipes. 7

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⎛1 − TE ⎞·(1 − c ) = ⎛1 − TE′ ⎞ ⎜ ⎟ ⎜ ⎟ Tf ⎠ Tf ⎠ ⎝ ⎝

Here, c is the ratio of the parasitic power exergy demand to the PV output. If c is 0.05, then from Eq. (11), the modified TE’ is 336 K. This means a decrease in the ψR value from 0.87 to 0.79, which is about a 10% decrease in exergy rationality, which points out that parasitic equipment like circulation pumps should be eliminated or minimized. Furthermore, due to the interrupted nature of solar energy, an external thermal storage system (TES) is necessary with additional exergy loss. In order to understand the mechanism better, a holistic model was developed, which is shown in Fig. 13. This figure shows how much the overall PVT system is complicated with five interconnected tiers, namely the PVT panel itself (Tier 1) the fluid circulation system (Tier 2), a thermal storage system with its own ancillaries (Tier 3), temperature peaking system (Tier 4) and load demand points in the building (Tier 5). The net total exergy output, EXT of such a composition is given in Eq. (12). EXE is the power exergy output, EXH is the thermal exergy output, EXP1 is the power exergy demand of the primary fluid circulation system, and EXTES is the exergy destruction due to heat loss of the thermal storage system to its environment. The insulation level may be another subject of optimization, which is excluded in this model. EXTP is the exergy required for temperature peaking by the unit TP in Tier 4, if necessary, on the demand side. All electrical systems in Tiers 1 to 5 operate on DC electricity without conversion to AC electricity. This eliminates inverter losses both from DC to AC and AC to DC on the demand side. However, on the demand side (Tier 5), depending on in-house demand, a partial inverter may be employed by the building system if there is AC current demand. All tiers depend upon the volumetric flow rate, V̇ . The exergy demand of the fluid circulation system i.e. pumps is crucial in such a manner that the exergy gain by preserving PV efficiency by active cooling may sometimes be less than the parasitic exergy loss of the pump. Therefore, the volumetric flow rate, V̇ which controls Tsup for the given solar irradiation, In, and Tret, must be optimized for maximum EXT while PV efficiency is maintained with little compromise.

Fig. 10. Flat-Plate Solar Water Collector.

ψR =

εHW εs

{Major exergy destroyed upstream}

ε ψR = HW = εs

(9)

εs = 0.557 W/W,

If for example, Tf = 638.8 K, Tsup = 333 K, and Tref = 283 K, then:

Tret = 288 K,

(1 − ) = 0.24. Tret Tsup

0.557

2.1.1. Solar PV PV cells generate only power at a higher exergy level. This is shown by the Exergy Flow Bar in Fig. 11. Starting from the frame temperature of the PV panel, TE, major exergy destruction takes place downstream of the useful application. Therefore, Eq. (10) applies (Kılkış, 2011).

(1 − ) {Major exergy destroyed downstream} Tref

ψR = 1 −

εdes =1− εsolar

(11)

TE

(1 − ) Tref Tf

(10) (12)

If for the same solar insolation level, the PV panel frame temperature, TE is 353 K, then ψR will be 0.64. Thus, with PV cells, the quality of solar energy is better harvested.

(

283

1 − 353 ε ψR = 1 − des = 1 − 0.56 εsolar

For Tier 1:

EXE = ηIPV × A × In × 0.96

) = 0.64

Tf

2.1.2. Conventional solar PVT Whether exergy is destroyed upstream or downstream of the useful work, a substantial portion of the available solar exergy is destroyed. However, when PV and FPC systems are coupled, exergy destruction decreases with two smaller exergy destruction points, namely between the PV modules and the cooling medium and between the water inlet point and the reference environment. The effect of active cooling of the PV cells is revealed in Fig. 12 by a lower TE (330 K). However, Tsup needs to be low for effective PV cooling (313 K). For Tret = 303 K:

ψR = 1 −

(

1εdes1 + εdes2 =1− εsolar

(13-a)

) + (1 (1 - )

313 K 330 K

283 K 638.8 K

283 K 303 K

TE

des

) = 0.79

In order to actively cool a PVT system, the coolant fluid must be forcefully circulated in the panel and its hydraulic ancillaries by a pump, if it is in the liquid state or by a fan if it is in a gaseous state. This parasitic loss may be substantial and is mapped to the exergy flow bar by TE′.

Tref Fig. 11. Simple Solar PV System. 8

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Fig. 14. Contradiction Between PV Power Output and Thermal Power Output (Kilkis, 2016).

E T EXTP = ⎛ XH ⎞ ⎛1 − TES ⎞ TDHW ⎠ ⎝ d ⎠⎝

Fig. 12. Conventional Solar PVT System Without Circulating Pump Losses.

⎜

TTES = eTsup

(13-b)

Here, ηPV is a function of Tsup for a given Tret, leading to the dependence of ηPV on V̇ (Eq. (17-a)).

Tret + Tsup

⎜

− 293K ⎞ ⎤ ⎠⎥ ⎦

(14)

For Tier 2:

EXP1 = xV̇ y

(15)

For Tier 3:

EXTES = zTsup w

(16)

where,

Tsup =

In A (1 − ηIPV ) ηsc + Tret , and V̇ ρw Cp

ηsc ~ηsc o ⎡1 − α ⎛ ⎢ ⎝ ⎣

Tsup + Tret

⎜

− 5K − Ta⎞ ⎤ ⎠⎥ ⎦

(17-a)

3. Need for the present research

(17-b)

In comparison, it is necessary to consider all tiers of the entire PVT system as shown in Fig. 13 together in order to optimally satisfy the expected functions. So far, these tiers are not exergetically interconnected in a holistic form in the literature for comprehensive

⎟

2

(19)

Here, the term d is the TES capacity ratio in terms of EXH. Eq. (18) assumes that the stored thermal energy in TES and the thermal demand are equal. The term e in Eq. (19) represents the heat-exchanger temperature loss in TES. Tret is fixed in the entire analysis as a known value. Because all relationships listed from Eq. (12) to Eq. (19) depend on the volume flow rate, Eq. (12) may be maximized in terms of the flow rate, V̇ . Typical results given in Fig. 14 shows that there is an optimum temperature rise ΔTopt for maximum EXT. However, Figs. 14 and 15 further show that even such an exergy-based optimization algorithm with a variable speed pump does not help much to improve the total exergy output. Fig. 15 shows that the maximum total exergy output EXT (74.5 W) is less than the output EXPV that a PV system alone could provide (97 W) (Kilkis, 2016).

⎟

2

(18)

where,

T EXH = ρV̇ Cp (Tret − Tsup) ⎜⎛1 − ret ⎞⎟ T sup ⎠ ⎝

ηIPV = ηIPVo ⎡1 − β ⎛ ⎢ ⎝ ⎣

⎟

For Tier 4:

Fig. 13. Holistic Model for a Conventional PVT System. 9

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pumping demand is minimized or eliminated. This shows that a pumpless PVT system is desirable. In aspects of the above, if for example In is 800 W/m2, net panel insolation area A is 1 m2, and thermal power is supplied at a supply and return temperature difference between 283 K and 333 K, the PV efficiency is 0.20 and the thermal power generation efficiency of the PVT is 0.60. For a 100 W electric motor for fluid circulation the comparison of EXT shows that pump power cannot be recovered by the PVT system in comparison to a PV-only system. PVT System:

EXT 283 K ⎞ ⎤ − 100 = 800W/m2 × 1m2 × ⎛0.96 W/ W× 0.20 + 0.60 ⎡1 − 333 K ⎦ ⎠ ⎣ ⎝

Fig. 15. A case study with Tsup = 293 K (Kilkis, 2016). The maximum value improves only by 4 W.

W× 0.96 W/ W= 125.7W PV-Only System: In this case, PV efficiency decreases from 0.20 to 0.17 because it is not cooled like in a PVT system:

optimization analysis. In addition, the demand side in buildings should be sufficiently integrated into the exergy analysis, which has not been the case. Moreover, available PVT modules are hard to disassemble, inspect, repair and assemble on-site. They need to be modularized, which also facilitates custom sizing of PVT panels and on-site installation with lighter cartridges transported, for example to the roof of an apartment for assembly. Other aspects that are not sufficiently addressed are listed below:

• • •

•

EXT = 800W/m2 × 1m2 × (0.96W/ W× 0.17) = 130.5W Therefore, except for very high-temperature climates, where PV efficiency decreases even further, a PVT system will have negative exergy gain. 4. Contributions of the research

The presence of water and electricity in the same panel may have power leakage-related risks. Water and power must be separated far away. The ideal solution is to have no water (or any kind of circulating fluid) in the panel itself. PVT panels with embedded PCM material may have the same kind of problem if they are not isolated hermetically or encapsulated in small sealed balls. There is not a universally accepted unified method of testing for rating PVT modules. Generally, PV and solar flat-plate collectors test standards are separately applied to a PVT unit and their results are summed up according to the First-Law as previously given in Eq. (1). For example, a PVT unit is first subjected to the ISO Standard 9806 for solar water collectors and then subjected to PV related tests like electrical performance and safety tests by the corresponding IEC (or EN) standards. A PVT system is not a collection of two separate systems, it is a composition of both thermal and electrical systems having a direct thermal influence on each other. Unless a dedicated PVT Test Standard is developed, PV performance will remain underestimated and under-rated. It is hard to exceed the PV output level with conventional PVT systems in terms of EXT. When total exergy output is considered, including circulation power exergy demand, current PVT systems cannot compete with PV systems. They can compete only if the

The research work provides multiple contributions based on aspects that address existing needs:

• Combine all tiers from 1 to 4 into one integrated module, • Eliminate in-PVT circulation pumping, • Collaborate FPC and PV in sandwiched form, • Use the PCM technology in the module itself, • Control the heat flow at heat pipes such that the best compromise for electrical power and thermal power outputs is maintained, • Develop an exergy-based control algorithm to regulate the heat flow in heat pipes, • Improve REMM efficiency, • Minimize weight (water in hydronic piping versus the heat pipe) • • •

and area (target maximum 10% more of the footprint of regular PVT) penalties, Optimize the weight of the PCM layer and compare its benefit compared to an external conventional energy storage tank weight, Provide a fundamental model for laying out an exergy-based test for a rating standard and its test procedures along with its instrumentation, Minimize temperature peaking requirements by thermal flow control and by optimally tandemizing two PVT modules,

Fig. 16. A Single PVT Cartridge When Cartridges are Installed into the Main PVT Frame, the Composite PVT Module Completes with a Flat-Plate Collector on the Top. 10

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• Modularize the complete PVT unit into stand-alone, interchange•

and additional DC power generation starts. After sunset, reverse heat flow from the bottom of the TEG units via the heat-conducting sheet radiates from the top surface to the atmosphere in terrestrial climates. Thus, for a short period of time, depending upon the climate and local weather, additional electrical power with reverse polarity is generated until thermal cooling brings the PVT panel to thermal equilibrium. A polarity control switch corrects the DC output. Power generation can be extended after sunset depending on the total PCM mass, temperature distribution, thermal mass, and the material of the module. In order to facilitate interchangeability, ease of maintenance, and construction of the composite PVT panel, it is so designed that it is composed of multiple, stand-alone, cartridge type PVT panels that can be stacked side-wise in order to form custom-length composite PVT modules. Cartridge-type PVT units are grouped in a main structural frame such that they may slide in and out for a replacement, repair, and inspection. A single cartridge in cross-sectional view is shown in Fig. 16. It is 1500 mm long, 240 mm wide, and 80 mm high. Therefore a 1 m wide and 1. 5 m high composite PVT panel unit has 6 cartridges side-by-side. A more detailed cutaway view of a single cartridge is shown in Fig. 17. While most of the heat is used to generate additional power at the TEG units, the overall exergy efficiency increases because the electric power has higher unit exergy when compared to heat. There is an optimum PCM layer thickness limited by the solar irradiation available in the region, year-round. This thickness is also governed by expected thermal load profiles. The PCM material is hermetically sealed in heat-resisting plastic pellets. The PCM performance is enhanced by filling the gaps by metal dust, which improves the heat transfer and thermal response and acts as an additional thermal storage medium. The packed-bed structure increases the total heat transfer surface area. Pellets are kept in touch with each other even when they slightly contract during their phase change and do not pile down in an inclined PVT position. This is achieved by maintaining some compression on the PCM layer as shown in Fig. 18 (Kilkis, 2019).

able, small PVT assemblies for ease of assembly on site, inspections, replacement, and repair Increase EXT output above a simple PV output level.

The following four targets emerged for this study in order to make the PVT technology more energy and exergy-rational and to accelerate decarbonization efforts (Kilkis, 2016): 1- Use short hydronic circuits or heat pipes and eliminate/minimize the circulation pump, 2- Relocate TES into the PVT module and integrate, 3- Use embedded TEG modules to supplement power generation by properly creating a sufficient temperature difference across TEG modules. Furthermore, enable any increase in the ηPV and reduction in the β value such that higher PV temperatures (Tc) in the PV panel becomes tolerable in the output (supply) temperature, thus increasing the thermal exergy while pumping power decreases. 4- Improve solar absorption by integrating the PVT module with FPC on top. 5. Design of the composite pvt module The above-mentioned measures were collectively implemented in an integrated, sandwiched module with a novel pumpless, composite PVT panel with its cross-section shown in Fig. 16. This module satisfies all of the targets of this study as mentioned above. The circulating pump is eliminated with special-design heat pipes with own pulsing capability and thermal energy storage is brought in to the panel itself by embedding a PCM layer, whose temperature is controlled by the heat transferred by the heat pipes. According to the latest modification, the 90 °C-bent heat pipe has now an inclination of 60° from the horizontal, which improves the heat flow in the heat pipe. The glass at the top covers the module and the air gap between the PV layer act as an ordinary FPC. After sunrise, the solar irradiation captured in this section by the greenhouse effect starts to heat the PV panels while PV panels pick up solar energy and generate power. At first glance, this may seem contrary since the aim is to cool the PV cells. Cooling is effectively achieved by transferring the solar heat to the backside of the TEG modules with heat-conducting nano-material sheets. While the encapsulated, packed-bed type of PCM layer is kept in thermally charging mode, a relatively cool temperature helps to cool the PV cells. A reasonable level of the temperature difference across the TEG units, whose top surfaces contact the PCM layer is gradually built up

6. Development of the thermal pulse control heat pipe method In a typical variable conductance heat pipe (VCHP), a non-condensable gas (NCG) is added to the heat pipe to allow the conductance to vary. A pressure controlled heat pipe (PCHP) is a VCHP variant, where the heat pipe operation is controlled by varying either the gas quantity or the volume of the gas reservoir for PCHPs. In this respect, the system requires:

Fig. 17. Composite, Stand-Alone PVT Module (not to scale), with Inclined Heat Pipes with Wick (Kilkis, 2019). 11

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Fig. 18. Packed Bed Hybrid Layer with Metal Dust and PCM-Filled Spherical Pellets.

Fig. 19. Heat Pipe Control (Kilkis, 2019). 12

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1. Precise Temperature Control, and 2. Switching thermal power between multiple sinks.

Tm2 =

EXTA =

These types all require external devices and connectors, which complicates the system and defeats the purpose (Kılkış, 2017). A new on-off type of control (thermal flow interrupter) was developed, which may also be adapted for partial heat transfer (Kilkis, 2019). This control unit is located between the two half sections of the heat pipe that interrupts the heat flow like a physical control gate (on-off) and it depends on either user input or maximum energy/exergy control algorithm. The objective is to maximize E1, E2, and thermal power, QH, in Eq. (20) by adjusting the pulse width, tp and the pulse interval of the heat flow by an interrupter disk at the mid-length of the heat pipe. This system (Kilkis, 2019) interrupts completely or restricts the heat flow with heat transferring and thermal insulating sections. When the heatconducting section touches the separated ends of the heat pipe, heat is transferred. Otherwise, heat transfer ends. This system is shown in Fig. 19. If the heat transferring and insulating parts partially touch each other, heat flow is pinched by a spring-loaded rotary disk, controlled by the pulse control model shown in Fig. 20. Fig. 21 shows the customized, exergy-based control algorithm.

T OF = ⎜⎛1 − ret ⎟⎞ T sup ⎠ ⎝

(24)

EXT 2.2A

(25)

An experimental prototype of PVT 1 with ¼ scale was constructed and tested. This set up is shown in Fig. 23. It was tested horizontally on the ground under typical summer conditions in Ankara. Fig. 24 shows the instrumentation points for temperature, flow, and electric power monitoring. Measurements were taken mostly on the symmetry line of PVT 1 for averaging the values on the horizontal plane. Temperatures were measured at 18 points by high-response and high-density thermocouples, most of them being located at the symmetry axis of the PVT module. Air temperature, wind velocity, and direction are recorded. Heat output QH is measured by the flow rate measurements and the return and supply temperature measurements. E1 and E2 are measured by watt-meters under proper power loads. Exergy-Based COP of the PVT 1, namely COPEX is calculated from the measurements by using Eq. (26) (see also Eq. (6)).

2

∑ QH + ∑ Ei

E COPEX = XT = Exsolar

{Maximize}

i=1

(20)

In A (1 − ηIPV 2 ) ηIsc 2 X + Tsup 1 V̇ ρCp

Tret Tsup

)

In A

1- FPC: This section absorbs heat and generates a greenhouse effect. On the surface of the glass, lenses may be installed in order to increase the solar flux. 2- PV: This section is the primary electric power generator. It also serves the FPC in terms of providing a solar absorber surface. This heat is transferred to TEG cells. 3- An energy storage layer based on PCM. 4- TEG layer: This layer is the secondary electric power generating layer. 5- Heat is delivered to the demand point at the required temperature and amount. The white strips in Fig. 25 are the intervals of exergy destruction. Cooling is possible by converting the heat generated (in total or part) by employing adsorption (ADS) units, which belongs to the building system. Therefore, the cooling option is not shown in Fig. 25. The temperatures that are given in Fig. 25 correspond to a typical summer day with In = 800 W/m2 at 2:00 pm afternoon (see Fig. 26) and for PVT 1 as shown in Fig. 21. PVT 2 is used for temperature peaking in an openloop DHW service. Because power is generated upstream of the heat supply, Eq. (10) is used in order to determine the ψR value. The 295 K return temperature corresponds to an open-loop case for domestic hot water (DHW). In a closed-loop system designed for heating or cooling purposes, 295 K will

(21)

(22)

Here X and Y are the partial thermal outputs of the heat pipes controlled by their individual pulses.

Tm1 =

Tret + Tsup 1 2

(26)

The composite PVT module in its final assembled form with the main casing and the FPC on top has five functional sandwiched sections, which are shown in the exergy flow bar in Fig. 25:

In composite PVT design, the external temperature peaking requirement that is necessary for reducing Legionella risk in open systems, for example in open-loop domestic water usage, is internalized to the new PVT system by arranging two of them in tandem with slightly different construction and functions, which is shown in Fig. 22. The first PVT module, numbered 1 in this cascade system is exactly the same as shown in Fig. 17 and it is the primary PVT. The primary task of the secondary PVT numbered 2 on the right has heat pipes with less thermal capacity and lower weight to peak the temperature to Tdemand in order to satisfy the condition Tsup ≥ Tdemand. Therefore, the mean temperature of the PV cells may be higher, which brings a compromise on power generation. After all, this will be a supplementary power generation. P stands for the external building HVAC pump and is regularly controlled separately with the building automation system. If temperature peaking is not required, then PV function with reduced efficiency remains. In order to avoid this condition, an external thermal storage system in the building may be installed. A predictive control algorithm with expected thermal and electrical loads must allocate and share the loads to PVT 1 and PVT 2 for maximum total exergy output by controlling the heat pipes. X and Y ratios in Eqs. (21) and (22) are collectively controlled for maximum EXTA, given in Eq. (25). Eqs. (23) and (24) define the mean temperatures.

In A (1 − ηIPV 1 ) ηIsc1 (Y ) + Tret V̇ ρCp

(

0.96(E1 + E 2) + QH 1 −

9. Results and discussion

7. Model for optimum temperature peaking

Tsup =

2

8. Prototype test setup

While Fig. 21 gives the exergy-based optimum control algorithm and Fig. 24 provides the basic instrumentation regarding testing and monitoring, both figures may be employed also for the developments of a stand-alone, dedicated PVT Method of Test for Rating.

Tsup 1 =

Tsup 1 + Tsup

(23)

Fig. 20. The Timing Model for the Pulsing Heat Flow Control. 13

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Fig. 21. Flow Chart of the Pulse-Control of the Heat Pipe.

Tests for a ¼-scale prototype with an insolation area of 0.25 m2 carried out for the PVT 1 version confirmed the model presented in this paper in such a manner that maximum measured EXT value was 60 W/ m2, which was estimated by the model as 66 W/m2 at In = 800 W/m2.

be higher. If for example 295 K is changed to 305 K that is destined for low-exergy radiant floor heating, then ψR reduces to 0.83. These results change during the day and the task that is left to PVT 2 for peaking also varies accordingly.

2

PVT 1 for power and heat

PVT 2 For Thermal Peaking and Power

Fig. 22. Two PVT Panels in Cascade with Different Functions (Kilkis, 2019). 14

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Fig. 23. Experimental Set-up for the Composite PVT 1.

ψR = 1 − =1−

∑ εdes εs

(1 -

350 K 353 K

) + (1 -

) + (1 (1 - )

318 K 320 K

283 K 638.8 K

315 K 318 K

) + (1 -

283 K 295 K

) = 0.88 Fig. 25. Exergy Flow Bar for the Composite PVT 1.

The difference is expected from minor parasitic losses, which were not accounted for. Fig. 26 shows the daily performance of the prototype during a typical, sunny day in Ankara on July 14, 2017. For the given data set COPEX from Eq. (26) is 60 W/(800 W/m2 × 0.25 m2) = 0.30. For a conventional FPC module, this value is only 0.07.

T εT = ηPV + ηTEG + ηH ·⎜⎛1 − ret ⎞⎟ Tsup ⎠ ⎝

(27)

According to Table 1, the total exergy output of the composite PVT 1 is almost four times than a conventional FPC and 14% more than a regular PVT. Thereby, it may be concluded that, without any innovative, hybrid PVT solutions, the current added exergy potential of conventional PVT systems will remain moderate compared to PV modules. The results presented in this paper indicate that external TES systems and TEG modules need to be embedded into the PVT panel for added power generation at the expense of slightly reducing the thermal exergy output. Due to these reasons, temperature peaking becomes even more important in these systems. The most feasible and exergy rational temperature peaking PVT 2 module must be selected and optimally controlled. In order to minimize the need for temperature peaking, PVT systems undoubtedly need to be coupled with LowEx Buildings with

Fig. 26. Combined Exergetic Heat and Power Performance on a Typical Summer Day in Ankara, A = ¼ m2, Inmax = 800 W/m2.

very moderate temperatures (Kılkış and Kılkış, 2018). Furthermore, REMM efficiency (ψR) is a strong indicator of the potential of reducing and replacing avoidable indirect CO2 emissions by solar panels (Kılkış and Kılkış, 2017). According to the following general relationship, emissions may be further reduced by a factor of 3 per square meter of solar insolation area when compared to PV systems and

Fig. 24. Instrumentation Points for the Test Prototype of PVT 1. 15

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Table 1 REMM Efficiency and Total Exergy Output of Different PVT Versions for unit A. REMM Efficiency

FPC

PV

PVT*

Composite PVT 1

ΨR Total exergy per unit solar input, εT***

0.24 0.09 W/W

0.64 0.20 W/W

0.79 0.26 W/W

0.88** 0.35 W/W

ELC =

(29)

This misconception also reduces the market potential of PVT systems in general, because PVT systems are compared side by side with either PV systems or FPC systems by the customer. For a fair basecomparison of ELC values of PC and FPC as given in Table 2 are added assuming that both of them are used to provide both heat and power independently occupying a total insolation area of 2.2 A. Only then does this approach enable a direct comparison of PVT systems with individual PC and FPC systems. In this respect, Table 2 clearly shows that neither PV nor FPC alone is exergy rational with a total ELC value of 15.2 €/kWEXpeak/m2. This analysis shows that the new combined PVT has the lowest ELC value with 7.1 €/kWEXpeak/m2 and regular PVT systems are the next best with an ELC value of about 9.9 €/kWEXpeak/ m2. These values also consider a 10% panel footprint area and 25% weight (material embodiment) penalties for PVT systems based on the variables in Eq. (29). This new approach for rating and comparing solar systems also has strong implications for preparing PVT standards. In this respect, ELC may be one of the combined rating metrics that should replace simple cost-per-power values of the products and systems.

* Excludes the power exergy demand of the PVT-dedicated circulation pump. Sometimes the net exergy output may be negative. ** Composite PVT 1 and 2 have no dedicated circulation pumps. They are operated with embedded heat pipes. *** See Eq. (27).

by a factor of 6 when compared with conventional FPC systems.

ΔCO2 ∝ (1 − ψR)

PC × EM ψR (A/ W )

(28)

9.1. Implications of the results Results have several implications such that novel PVT systems are potential solutions for decarbonization efforts globally.

9.1.2. Composite PVT with a ground-source heat pump In the Mediterranean region, PVT systems are more rational than PV systems due to prevailing high outdoor temperatures almost yearround. PV cooling makes sense to preserve their efficiency with relatively high coolant temperatures, which may be directly utilized in many applications. In some cases, temperature peaking even in such climatic regions may be necessary if the heat will be utilized in an open DHW system. In such hot summer climates, there is a conflict between hot water generated and cooling demand instead of heating. Therefore, besides DHW demand, heat must be converted to cold either by a heat pump or ADS Chiller. Their respective COP values of about 3.0 and 0.5, respectively make the choice an economic issue rather than a thermodynamic issue. However, even if the electric power requirement of the heat pump comes from the same PVT system on-site, the next question is whether it will be a more value-adding application if it is spent in cooling or using it in a more rational application on-site or through a grid in the built environment. The better performance of the composite PVT technology presented

9.1.1. Exergy-levelized cost According to IEA (Weiss and Spörk-dür, 2019), which included PVT systems into its database for the first time in 2018, global PVT installations in the same year just totaled 1,075,247 m2. This is a small installation rate when compared to the already installed solar thermal systems of glazed and glazed water collectors with 480 GWthermal global capacity, covering 686 million square m2. This latter capacity does not differentiate the exergy of the thermal capacities among applications and geographical regions because a breakdown of the supply temperature is not given or considered. Secondly, this is an incorrect way of comparing the installations in terms of panel surface area, which is taken to be a primary indicator of simple cost from the customer point of view. Although unit costs are also given in terms of peak W/m2, the quality of power in terms of heat or electricity generated is directly and indifferently compared or added without regarding whether it is a PV or FPC, or both, thus ignoring the large unit exergy difference of thermal and electric power. It is a common statement that PVT costs are about double of a PV system of the same square meter collector area (BRE and DELTA, 2016). A similar argument also holds for PV and FPC. This is the major reason why the PVT market is not increasing at the deserved rate, because overall reductions in the CO2 emissions responsibility, ozone depletion, and global warming potential parameters do not appear in a simple cost interpretation, even if the unit cost, PC is based on kWpeak or kWhpeak per year or season without respecting the split between heat and power outputs in these values according to their unit exergy. The stand-alone panel weight per square meter, W is also an important parameter, which has to be considered beyond static roof or foundation loads, extending to embodied energy, exergy, and CO2. In order to factor-in these parameters to be considered in the rating of solar systems, an exergy-levelized cost metric, namely ELC with weight correction was developed, which is given in Eq. (29). EM is the ratio of embodiment costs of energy, CO2 emissions, and exergy destruction during the manufacturing and installation stages. According to Table 2, when the solar PV system and the FPC system are compared separately, the PV system has a lower exergy-leveled cost, because of two reasons:

Table 2 Exergy-Levelized Panel Cost for Different Solar Systems with Weight Correction. Solar Energy System Variable

FPC

PC, €/kWpeak A, m2 W, kg EM ψR (See Table 1) ELC, €/kWEXpeak/m2 [1]

[1]

[2]

PV

PVT [3]

1 2.5 1 1 [6] 1.3 0.5 0.7 1.6 0.24 0.64 3.8 3.1 FPC + PV side by side[7] 15.2

[4]

2.9 1 1.5[6] 1.8 0.79 9.9

Combined PVT 3.3[5] 1.2 1.2 1.9 0.88 7.1

All costs are relatively indexed to FPC cost. Here peak power does not differentiate whether it is thermal, or power, or both. [2] Heat only. [3] Electric only. [4] Power and heat with unknown proportion. It includes the effect of improved electric power efficiency. [5] Adjusted according to the elimination of the external tank with respect to PVT but adding PCM layer. [6] With external thermal storage tank and ancillaries. [7] Occupying 2.2 A side by side (3.8 + 3.1) × 2.2 = 15.2.

a- PV modules have less weight, b- ψR value is much higher than FPC. However, this difference is not sufficiently high for substantiating EU goals of electrification in favor of PV systems rather than solar thermal. In spite of this fact, the PV market is increasing while the solar thermal market is decreasing. 16

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external TES.

in this study makes the heat pump choice a relatively feasible choice, mainly because of the fact that the heat pump investment may be also paid in heating seasons in addition to its cooling function. In contrary, ADS units can only be used for cooling. Fig. 27 shows a PV-driven ground-source heat pump that serves HVAC loads. PV cells may be sufficiently cooled by a DHW system in the summer, although the coolant temperature may be kept warm enough for direct utilization. In the winter, when the PV cooling load is relatively low, the thermal power output of PVT panels may be combined with the ground source, such that the COP of the ground-source heat pump is increased. Such an increase may be supplemented by low-temperature heating systems, like radiant panels in the building by decreasing the temperature difference between the supply and demand points across the heat pump. With more favorable performance factors of composite PVT makes such applications more feasible (see Table 1).

• It is a reality that a combined PVT like the one presented in this

paper will be heavier with all ancillaries grouped in one frame. This weight increase that is nearly 25% compared to a regular PVT with the same overall dimensions must be considered in static designs of the building, especially if PVT units are going to be installed on the roof or on the facades. Apart from raising this awareness to the reader, this paper draws attention that this weight increase may be compensated by its virtue of eliminating or minimizing the external TES system.

This paper has shown the importance of the need for temperature peaking, which is often ignored in theory and practice. Temperature peaking is an important element of PVT systems and has been widely discussed in this paper. The main novelties of the study are further given below.

9.1.3. Composite PVT on building facades Composite PVT may be used on vertical facades in a BIPV format. If the urbanization is not too dense and building heights are favorable such that they are high but they do not overcast others, then the total façade area with favorable solar insolation orientation is more abundant than roofs. Another advantage is that façade positions generate less glare, which is especially an important issue for air traffic near airports (Kılkış and Kılkış, 2017). The only disadvantage is the lower In value on vertical surfaces. Yet this penalty is compensated with higher performance factors.

• This paper has developed an exergy-levelized cost method. This

•

10. Discussion of the significance of the results

•

PVT systems are a promising technology given that the following points as addressed in this study are carefully examined and innovations are duly realized. This study has also increased the awareness of some shortcomings of current PVT technology and provided robust solutions.

11. Conclusions There are several challenges faced by conventional PVT systems in different climatic regions with different power and heat demands in different ratios, which generally conflict with the amount of cooling requirement of the PV cells that are involved in the system. There have been several configurations and types of PVT panels in the field, whether using heat pipes or not. These challenges are not recognized from an exergetic point of view and therefore the real performance, which in several cases may be negative, is not well understood. This study has brought a new rating method, based on exergy on a system level and developed a new autonomous PVT system, which seems to meet all the challenges involved and described in this paper. The main implication of this new PVT system is the further reduction of embodied CO2, energy, and exergy recovery rates although a 10% panel footprint area and 25% weight (material embodiment) penalties are included. The

• Generally, PVT units are independently designed, analyzed, rated,

•

•

•

method enables rating PVT systems more realistically and fairly in terms of their overall exergy rationality, ψR, which is a unique indicator of real economic and environmental value, a collection of the First and Second-Law of Thermodynamics, and the environmental footprint in terms of CO2 emission impacts This costing method will establish a fair basis of comparison among all solar energy systems. A new rating metric named ELC has been developed for solar thermal and power systems in any given system combination. This metric clarifies the ambiguity brought by simple power to cost approaches that are used in the industry and the market. In the latter respect, this paper laid out the fundamental base with measurement and rating parameters for a new method of evaluating and rating the performance of PVT panels in one comparison.

and sold. It has been shown how to identify the complete tiers of the PVT system in a holistic framework. This approach also revealed the reason why exergy rationality is important especially in PVT systems that generate both heat and power with substantially different exergy levels. This paper provided all exergy-based optimization tools for: o Optimum power-to-heat ratio, o Optimum temperature peaking layout design and optimum control, o PVT pump optimum flow control (if insisted to be used in a PVT pump), o Optimum heat pipe thermo-mechanical control, o Optimum heat pump connection (if used), o Optimum building heating supply temperature. This study clearly showed that even if the geometry and its internal hydronic layout are carefully optimized, the power demand of the PVT pump may override the benefit of PV cooling. Studies on the optimum piping layout as available in the literature are usually based on the total First-Law efficiency while neglecting the pumping loss in terms of energy and exergy (Fudholi, 2014). With case studies, this paper guides designers to the right path by showing that pump power demand is a problem, which may be eliminated only by replacing pumps with properly designed and selected heat pipes. This will substantially improve the PVT performance at large. The PVT system with all ancillaries is a system of systems. All components need to be designed and optimized comprehensively in the same integrated platform, particularly based on exergy rationality. In this respect, external thermal storage needs to be embedded to the PVT in the feasible range. This will reduce the need and size of

Fig. 27. PVT Coupled to a Ground-Source Heat Pump. 17

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overall power and heat exergy delivery are high enough compared to other PVT cases that these penalties are easily compensated by the field performance. Furthermore, the new PVT system has controls that are embedded to the panel with thermally adjustable heat pipes as cheap solutions such that even small buildings may afford the cost compared to more expensive external controls. So far, the literature, decision-makers, and energy agencies have been referring to energy transition against global warming based on the First-Law of Thermodynamics (Kilkis, xxxx; Kılkış and Kılkış, 2017). In view of IEA technology perspectives (Energy Technology, 2016), this research, in its seemingly simple scale of solar energy in urban life, has shown that the society needs an exergy transition beyond the energy transition efforts if a true and sustained decoupling between CO2 emissions and economic growth at large is to be reached (Kilkis, 2019). In the quest of maximizing the exergy rationality of PVT systems through many steps in the near past (Kılkış, 2010; Kılkış, 2010), this study shows that LowEx buildings (Kılkış, 2010) are a prerequisite. LowEx buildings both in space heating and cooling improve the COPEX value of PVT panels and ADS chiller by demanding very moderate supply temperatures. This feature also makes it more feasible for pumpless operations with heat pipes of different types (William et al., 2011; ACT; AAVID), including the type presented in this paper by reducing their temperature rise burden. In addition, if heat pumps even with advanced technologies (AAVID, 2018) are used to elevate the supply temperature, moderate demand temperatures make their COP values even higher. All these results indicate that a PVT system itself is only a part of an advanced solar energy system. Without a holistic view with ancillaries and awareness of the specific thermal and power loads, and an exergybased method of analysis, a PVT system will not be successful and inclusion of exergy rationality is one of the primary factors for future development and market penetration. Another market penetration driver is the development of the hydrogen economy and the need for hydrogen homes. The new composite PVT technology presented in this paper with higher performance characteristics of exergy rationality will help concepts like the hydrogen house and similar projects to be economically and technologically more feasible in the near future (Kilkis, 2019).

Strategy, UK. Chow, T., 2010. A Review on photovoltaic/thermal hybrid solar technology. Appl. Energy 87, 365–379. Ciampi, G., Ciervo, A., Rosato, A., Sibilio, S., and Di Nardo, A. 2018. Parametric Simulation Analysis of a Centralized Solar Heating System with Long-term Thermal Energy Storage Serving a District of Residential and School Buildings in Italy, Conference Paper, 3rd AIGE/IIETA International Conference - 12th AIGE Conference 2018 - Energy Conversion, Management, Recovery, Saving, Storage and Renewable Systems: Reggio Calabria-Messina (Italy), June 2018. Çolak, L., Kılkış, B., 2014, Irrigation by PHVT System, Global Conference on Energy, Soil, Water, Air & Environment, Antalya, November, 8-10. ESWAE 2013, Vol. 2. Also, in Global Journal on Advances in Pure and Applied Sciences (ISSN 2301-2706). Dinwoodie, T.L., 1994. Heat Regulated Photovoltaic Roofing Assembly, JP Patent 3777454B2. IEA. Energy Technology Perspectives 2016 – Towards Sustainable Urban Energy Systems. Paris: OECD/IEA; 2016. Escher, W., Michel B., Paredes, S., Ghannam, R. 2015. Photovoltaic Thermal Hybrid Solar Receivers, US Patent 9,219,183 B2, December 22, 2015. Fudholi, A., et al., 2014. Performance analysis of photovoltaic thermal (PVT) water collectors. Energy Convers. Manage. 78 (2014), 641–651. Hansen, K., Mathiesen, B.V., 2018. Comprehensive assessment of the role and potential for solar thermal in future energy systems. Solar Energy 169, 144–152. Ilhan, C., Ali, E.G., 2015. Exergetic Analysis of a New Design Photovoltaic and Thermal (PV/T) System. Am. Inst. Chem. Eng. Environ. Prog. 34 (4), 1249–1253. James, A., Zahir, D., Sinisa, S., Lascelle, M., 2015. Performance testing of thermal and photovoltaic thermal solar collectors, Energy Science & Engineering, Wiley O.L. Jiang, F., Toh, P.G., Goh, L.H., Leung, K.O., Kelvin, L., 2016. Design and thermal performance test of a solar photovoltaic/thermal (PV/T) collector, Journal of Clean Energy Technologies, Vol. 4, No. 6, November. Kılkış, B., Kılkış, Şiir., 2015. Yenilenebilir Enerji Kaynakları ile Birleşik Isı ve Güç Üretimi (Cogeneration with Renewable Energy Sources), TTMD Pub. No: 32, First Ed., ISBN: 978-975-6263-25-9, 371 pages, Doğa Pub. Co., İstanbul, 2015. Kılkış, B., Kılkış, Ş., Kılkış, Ş., 2016. Next-Generation PVT System with PCM Layer and Heat Distributing Sheet. In: Proceedings of the Solar TR2016 Conference and Exhibition, İstanbul, Turkey, 6–9 December 2016; Paper No: 0006, Conference Proceedings, pp. 20–28. Kılkış, Ş., Kılkış, B., 2017. Integrated circular economy and education model to address aspects of an energy-water-food nexus in a dairy facility and local contexts. J. Cleaner Prod. 167, 1084–1098. Kılkış, B., Kılkış, Ş., 2017. New exergy metrics for energy, environment, and economy nexus and optimum design model for nearly-zero exergy airport (NZEXAP) systems. Energy 140 (2), 1329–1349. https://doi.org/10.1016/j.energy.2017.04.129. Kılkış, B., Kılkış, Ş. Energies, 2018. Hydrogen economy model for nearly net-zero cities with exergy rationale and energy-water nexus. Energies, Special Issue, Published 10, May. Kılkış, B., 2010. A New Building Integrated Solar Facade System for Heating, Cooling, and Power (BIPVTC) in Green Buildings, TTMD J. (English Edition), No: 7, pp: 26–33, Ankara, 2010. Kılkış, B., 2010. Solar Tri-Generation Module for Heating, Cooling, and Power, Conference Proceedings on CD, Solar Future 2010 Conference, 11-12 February, İstanbul, 2010. Kılkış, Ş., 2011. A rational exergy management model to curb CO2 emissions in the exergy- aware built environments of the future, Ph.D. Thesis, Bulletin/Meddelande No. 204, ISBN 978-91-7501-129-5, KTH Royal Institute of Technology, Stockholm, Sweden. Kilkis, B., 2016. Optimum Operation of Solar PVT Systems: An Exergetic Approach, Solar TR2016 Conference, Paper No: 0025, Proceedings, pp: 72-80, Istanbul. Kilkis, B., 2017. TTMD, Sustainability and Decarbonization Efforts of the EU: Potential Benefits of Joining Energy Quality (Exergy) and Energy Quantity (Energy) in EU Directives, A State-of-the-Art Survey and Recommendations, Exclusive EU Position Report ©2017 Birol Kilkis, TTMD 2017-1, Ankara, Turkey. Kılkış, B., 2017. Energy and Exergy Analysis of Water and Air-Cooled PVT Systems with Heat Pipe Technology. Seminar 72 Low Energy Building Design Using Exergy Modeling, ASHRAE Winter Meeting, Las Vegas, USA, 27 January-1 February 2017. Kilkis, B., 2018. Controllable-Flow Heat Pipe, Patent Application, Turkish Patent Institute, Ankara, Turkey, 2019 (pending). Kilkis, B., 2019. Exergy-optimum coupling of heat recovery ventilation units with heat pumps in sustainable buildings, JSDEWES, in print. Kilkis, B., 2019. Analysis of Roof-Type Solar PVT System Layout for Maximum Exergy Output, 14th National HVAC Congress, TESKON, 17-20 April, Izmir, Proceedings, Vol.1, Paper No: 260, Turkish Chamber of Mechanical Engineers. Kilkis, B., 2019. Design of a Sustainable Hydrogen House for Future Hydrogen Cities, 4th International Hydrogen Technologies Congress (IHTEC-2019), June 20-23, 2019, Edirne, Online Proceedings: Turkey. http://ihtec2019.org/. Kilkis, B. Kilkis, Şiir, Kilkis Şan, 2019. PV and TEG Integrated PVT System with PCM Layer for Solar Power and Heat Generation, PCT/TR2018/050382 18 07 2018. Application no:2017/10622. Kilkis, B., 2019. An Exergy-Based Holistic Urban Development Model for The Distance Limit Between A Renewable Energy Plant and Its Low-Exergy District, 14th SDEWES 2014 Conference, October 1-6, 2019, Dubrovnik, Croatia. Kumar, A., Baredar, P., Qureshi, U., 2015. Historical and recent development of photovoltaic thermal (PVT) technologies. Renew. Sustain. Energy Rev. 42, 1428–1436. Hi-Ki Lam, 2011. US Patent 2011/01552 14 A1. LowEx. 2016. Low Exergy Systems for Heating and Cooling of Buildings Guidebook, IEA ECBS, Annex 37, ISBN 951–38– (softback edition). Massimo, F., Zhenjun, Ma., Paul, C., Mohammed, I.S., 2014. ASHRAE, Implementation of

Acknowledgment Kind co-operation with Mr. Murat Ali Altinsel, from ProMarin Technological Products Industry and Trade Inc. in developing 3-D modeling of the composite PVT that is used in the prototype for testing and support from my graduating students Mr. Varışlı and Mr. Aydoğan are greatly appreciated. References AAVID, Thermocore, Variable Conductance Heat Pipe (VCHP) Solutions, < https://www. thermacore.com/products/variable-conductance-heat-pipe.aspx > Last Visited on 18.09.2018. Data-Driven Predesign Tool for Small-Scale Centrifugal Compressor in Refrigeration, J. Eng. Gas Turbines Power 140(12), 121011 (Oct. 24, 2018), http://doi.org/10.1115/ 1.4040845. ACT, Advanced Cooling Technologies. John R. H., Kara L. W., and William G. A., Heat Pipe with Thermal Control Valve for Variable Heat Flow, Lancaster, PA, 17601, U. S.A. Adarsh, K.P., Pradeep, C.P., Origanti, S., Arun, K., Sudhir, K.T., 2015. Energy and exergy performance evaluation of a typical solar photovoltaic module. Therm. Sci. 19 (suppl. 2), s625–s636. Anderson, T.N., Duke, M., Morrison, G.L., Carson, J.K., 2009. Performance of a building integrated photovoltaic/thermal (BIPVT) solar collector. Solar Energy 83, 445–455. Atheaya, D., Tiwari, A., Tiwari, G.N., 2016. Exergy analysis of photovoltaic thermal (PVT) compound parabolic concentrator (CPC) for constant collection temperature mode. Solar Energy 135, 222–231. Bean, R., Kilkis, B., 2003. Fundamentals of Panel Heating and Cooling, ASHRAE Advanced Learning Institute (ALI) Continuing Education Course Notes, first delivered at ASHRAE Winter Meeting, Chicago, 2003, repeated in 2006 and 2010. BRE and DELTA. 2016. Evidence Gathering – Low Carbon Heating Technologies, Hybrid Solar Photovoltaic Thermal Panels, Department for Business, Energy and Industrial

18

Solar Energy xxx (xxxx) xxx–xxx

B. Kılkış

Sobhnamayan, F., Sarhaddi, F., Alavi, M.A., Farahat, S., Yazdanpanahi, J., 2014. Optimization of a solar photovoltaic thermal (PV/T) water collector based on exergy concept. Renew. Energy 68, 356–365. Srimanickam, B., Vijayalakshmi, M.M., Natarajan, E., 2015. Experimental study of exergy analysis on flat plate solar Photovoltaic Thermal (PV/T) hybrid system. Appl. Mech. Mater. 787, 82–87 Trans. Tech. Publ. Weiss, W., Spörk-dür, M., 2019. Solar Heat Worldwide-Global Market Developments and Trends 2018- Detailed Market Figures 2017, Report by IEA-Solar Heating and Cooling Program, 35 pages, May 2019, Germany. William, G.A., John, R.H., Calin, T., David, B.S., Kara, L.W., 2011. Pressure controlled heat pipes, Lancaster, PA, 17601, U.S.A. Xuan, Y., et al. 2016. Solar and Thermoelectric Coupling System Containing PCM, patent CN105471366 (A)-20160406. Zhao, Y., et al. Photovoltaic cell radiating and combined heat and power system. In: Patent CN 200820123998 U. 04.12.08; 2008.

a Solar PVT Assisted HVAC System with PCM Energy Storage on a Net-Zero Energy Retrofitted House, ASHRAE Conference Paper, December 2014, USA. Mazor, M.M., 2009. Solar Heat Management in Photovoltaic Systems Using Phase Change Materials, Patent WO2009/018016. Parida, B., Iniyan, S., Goic, R., 2011. A review of solar photovoltaic technologies. Renew. Sustain. Energy Rev. 15 (3), 1625–1636. Santbergen, R., Rindt, C.C.M., Zondag, H.A., van Zolingen, R.J.Ch., 2010. Detailed analysis of the energy yield of systems with covered sheet-and-tube PVT collectors. Solar Energy 84, 867–878. Sathe, T., Dhoble, A., 2017. A review on recent advancements in photovoltaic thermal techniques. Renew. Sustain. Energy Rev. 76, 645–672. Siecker, J., Kusakana, K., Numbi, B.P., 2017. A review of solar photovoltaic systems cooling technologies. Renew. Sustain. Energy Rev. 79, 192–203. Singh, S., Agrawal, S., Tiwari, A., Al-Helal, I.M., Avasthi, D.V., 2015. Modeling and parameter optimization of hybrid single-channel photovoltaic thermal module using genetic algorithms. Solar Energy 113, 78–87.

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