Historical and recent development of photovoltaic thermal (PVT) technologies

Renewable and Sustainable Energy Reviews 42 (2015) 1428–1436

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Historical and recent development of photovoltaic thermal (PVT) technologies Anil Kumar n, Prashant Baredar, Uzma Qureshi Energy Centre, Department of Mechanical Engineering, Maulana Azad National Institute of Technology, Bhopal, India

art ic l e i nf o

a b s t r a c t

Article history: Received 19 June 2014 Received in revised form 20 October 2014 Accepted 4 November 2014

In the context of climate change in the world at the global level, various actions are taken for the development of renewable Energy and particularly solar energy which have potential for future energy applications. The current popular technology converts solar energy into electricity and heat separately. The photovoltaic thermal (PVT) system is designed to generate thermal and electrical energy simultaneously. A major research and development work on the photovoltaic thermal (PVT) hybrid technology has been done since last 30 years. Different types of solar thermal collector and new materials for PV cells have been developed for efficient solar energy utilization. The photovoltaic (PV) cells suffer efficiency drop as their operating temperature increases especially under high insolation levels. The overall electrical efficiency of the photovoltaic (PV) module can be increased by reducing the temperature of the PV module by withdrawing the thermal energy associated with the PV module. Both water and air either by forced or natural flow has been used for PV cooling through a thermal unit attached to the back of the module yielding photovoltaic thermal (PVT) collector. The main purpose of heat extraction unit is to extract heat from the photovoltaic system and keep its temperature at satisfactory level so that it can work efficiently. Till date many researchers have done a lot of work and number of studies have been carried out in designing, simulation, modeling, and testing of these systems. This paper reviews on the state and development of PVT technology around the world but the studies includes experimental and analytical are mainly focused on photovoltaic thermal technologies at the Indian subcontinent. & 2014 Elsevier Ltd. All rights reserved.

Keywords: PVT Heat extraction unit Design parameters Thermal modeling Solar insolation in India

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concept of photovoltaic–thermal (PVT) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of PVT systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Liquid PVT collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Air PVT collectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Building-integrated air PVT (BIPVT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Concentrator PVT system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Heat-pipe-based PVT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Advancements in recent years and future directions of PVT in India . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Case study of a PVT system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Details of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Analysis of the observed data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Historical review on PVT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Recent trend in PVT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n

Corresponding author. E-mail address: [email protected] (A. Kumar).

http://dx.doi.org/10.1016/j.rser.2014.11.044 1364-0321/& 2014 Elsevier Ltd. All rights reserved.

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1. Introduction The solar energy is a renewable, eco friendly and freely available energy resource on earth. Its use will ensure the conservation of conventional energy sources and at the same time its application protects the environment from its degradation. Among all the available renewable options, solar energy seems to be more promising, sustainable energy resources. Thus, the solar energy based systems can meet energy demands to some extent and keep the balance in the ecosystem. Solar radiations can be converted into either thermal energy or electrical energy or both [1]. Solar thermal energy collectors are special kind of heat exchangers that convert solar radiation into thermal energy through a transport medium and/or moving fluid. The major component of any solar system is the solar collector. This is a device which absorbs the incoming solar radiation, converts it into heat energy, and transfers it through a fluid (usually air, water, or oil) for useful purpose/applications. Generally, they are used as air dryer/heater for drying the agricultural products and/or heating/ cooling applications in combination with the auxiliary heaters for air conditioning of buildings [2]. Photovoltaic (PV) is the most useful way of utilizing solar energy by directly converting it into electricity. Energy conversion devices, which are used to convert sunlight to electricity by the use of the photoelectric effect are called solar cells. A photovoltaic system consists of solar cells and ancillary components. It converts the solar radiation directly into electricity. In 1954, researchers at the Bell Telephone Laboratories demonstrated the first practical conversion of solar radiation into electric energy by use of a p–n junction type solar cell with 6% efficiency [3]. With the advent of the space program, photovoltaic cells made from semiconductorgrade silicon quickly became the power source of choice for use in satellites. The common solar power conversion efficiencies are between 15 and 20% [4]. However, today a new area has emerged incorporating both the methods of energy conversion, which can be called photo thermo conversion [5]. The solar energy conversion into electricity and heat with a single device called hybrid photovoltaic thermal collector (PVT) as illustrated in Fig. 1. In this way, heat and power are produced simultaneously and it seems a logical idea to develop a device that can comply with both demands. A compressive review of all recent development and applications of PVT are presented in this work. In Section 1, introduction

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of the PVT is there; Section 2 describe about the concept of PVT system; Section 3 discuss the comprehensive review of various types of PVT systems; Section 4 investigates the advancement in recent years and future directions of PVT in India; Section 5 discuss the case study of a PVT System; Section 6 investigates the Historical Review on PVT; Section 7 discuss the recent trend on PVT; Section 8 includes the conclusion of this communication

2. Concept of photovoltaic–thermal (PVT) systems A PV–thermal (PVT) collector is a module in which the PV is not only producing electricity but also serves as a thermal absorber. In this way both heat and power are produced simultaneously [2]. The schematic of the PVT technologies is presented in Fig. 1. The dual functions of the PVT result in a higher overall solar conversion rate than that of solely PV or solar collector, and thus enable a more effective use of solar energy. Since the demand for solar heat and solar electricity are often supplementary, it seems to be a logical idea to develop a device that can comply with both demands. Photovoltaic (PV) cells utilize a fraction of the incident solar radiation to produce electricity and the remaining is turned mainly into waste heat in the cells and substrate raising the temperature of PV as a result, the efficiency of the module decreased. The photovoltaic thermal (PVT) technology recovers part of this heat and uses it for practical applications. The simultaneous cooling of the PV module maintains electrical efficiency at satisfactory level and thus the PVT collector offers a better way of utilizing solar energy with higher overall efficiency. There are alternative approaches in PVT integration. Among many others, there can be selections among air, water or evaporative collectors, monocrystalline/polycrystalline/amorphous silicon (c-Si/ pc-Si/a-Si) or thin-film solar cells, flat-plate or concentrator types, glazed or unglazed panels, natural or forced fluid flow, standalone or building-integrated features, etc. A major research and development work on the PVT technology has been conducted in the past few years with a gradual increase in the level of activities. The attractive features of the PVT system are [6]:


It is dual-purpose: the same system can be used to produce electricity and heat output.


It is efficient and flexible: the combined efficiency is always







higher than using two independent systems and is especially attractive in building integrated PV (BIPV) when roof-panel spacing is limited. It has a wide application: the heat output can be used both for heating and cooling (desiccant cooling) applications depending on the season and practically being suitable for domestic applications. It is cheap and practical: it can be easily retrofitted/integrated to building without any major modification and replacing the roofing material with the PVT system can reduce the payback period.

Different types of PV Thermal collector are being used presently such as, PVT/air, PVT/water and PVT concentrated collector [7]. The next section of this review article is focus on development of the PV Thermal technology and application.

3. Types of PVT systems 3.1. Liquid PVT collector

Fig. 1. Schematic of various solar technologies.

Similar to flat plate collector water heating system, liquid photovoltaic thermal (PVT) collectors are used to heat up the

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water and simultaneously electricity production for various domestic and industrial applications [8]. The domestic water heater generally uses flat plate collectors in parallel connection and run automatically with the thermo-siphon action whereas the industrial water heating system a number of flat plate collectors in series are used and hence, it uses a photovoltaic driven water pump to maintain a flow of water inside the collector. A schematic diagram of a PVT water collector is shown in Fig. 2 [9]. The Absorber materials generally used in liquid flat-plate PVT collectors are metallic sheet-and-tube absorber, while the use of

copolymer absorber was also examined extensively. This alternative offers several advantages:

i. The reduction in weight leads to less material utilization and easier installation. ii. The manufacturing process is simplified because of the fewer components involved. iii. The investment can be lowered as a result of the reduced material and installation costs.

Fig. 2. Schematic of PV/T air collector [2]. (a) Cross section view. (b) Exploded view. (c) Array view.

Fig. 3. Schematic of different types of air PV/T systems [10]. (a) Unglazed air PV/T collector. (b) Single glazed air PV/T collector. (c) Double glazed air PV/T collector. (d) Glazed air PV/T collector with double pass.

A. Kumar et al. / Renewable and Sustainable Energy Reviews 42 (2015) 1428–1436

Yet there are disadvantages like low thermal conductivity, large thermal expansion and limited service temperature. The copolymer in use has to be good in physical strength, UV-light protected and chemically stable. 3.2. Air PVT collectors Air and water both have been used as heat transfer fluids in practical PVT solar collectors, yielding PVT/air and PVT/water heating systems, respectively. PVT/water systems are more efficient than those of PVT/air systems due to the high thermophysical properties of water as compared to air, However, PVT/air systems are utilized in many practical applications due to low construction (minimal use of material) and operating cost among others. It is presented in Fig. 2 [2]. Fig. 3 illustrates various type of PVT air collectors [10]. 3.3. Building-integrated air PVT (BIPVT) From a holistic viewpoint, Bazilian et al. summarized the potential applications of PVT cogeneration in the built environment. The multi-functional external façade/roof was identified good for PVT installation that produces heat, light and electricity simultaneously. Other than the use of airflow behind the PV modules, a PVT system designed for light transmission requires no additional system cost except for ambient light sensors to

Fig. 4. Ventilated PV glazing operation for Building integrated PV/T systems [7]. (a) Cooling mode. (b) Heating mode.

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optimize the gain from day lighting as shown in Fig. 4 [7]. Wei et al. have surveyed the applicability of domestic solar water heater (DSHW) and roof type building integrated photovoltaic (BIPV) systems for China [11]. The survey was conducted in 96 samples in the Xi’an city, China. The photograph of the system is shown in Fig. 5. The optimal space required for DSWH 3–4 m2 however for BIPV require minimum 6 m2. Study shows that 84 % houses have roof about 3–4 m2 hence for this DSWH however only 18% of the houses are suitable for the installing roof type BIPV. 3.4. Concentrator PVT system Concentrating photovoltaic (CPV) systems can operate at higher temperatures than those of the flat plate collectors. Collecting the rejected heat from a CPV system leads to a CPV thermal (CPVT) system, providing both electricity and heat at medium temperatures. This approach is promising due to the significantly lower cost of the reflectors relative to the solar cells. A schematic diagram of CPV is shown in Fig. 6 [2]. The use of CPVT in combination with concentrating reflectors has a significant potential to increase the power production from a given solar cell area. Renno has optimised size as well as its electrical and thermal performance of the CPVT for the domestic applications [12]. The proposed system is presented in Fig. 7. Along with choice and size of the system, active cooling of the photovoltaic module is also taken into consideration. Concentration ratio is found in each condition and optimum value is evaluated. Due to decrease in the value, the shape and size decreases. Higher efficiency solar cells that handle higher current can be used though they are more expensive than the flat-plate module cells. Additional costs may also go to the complex sun tracking driving mechanism. Cell efficiency decreases when non-uniform temperature across the cell exists. Series connections of cells increase the output voltage and decrease the current at a given power output, thus reducing the ohmic losses. But the cell at the

Fig. 6. Schematic of concentrating PV/T collector [2].

Fig. 5. Photograph of DSWH and roof type BIPV [11].

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Fig. 7. Schematic diagram of concentrator photovoltaic system[12].

Fig. 8. Schematic of conventional heat pipe [14].

Fig. 9. Schematic of three types of flat plate heat pipes with micro channel array [15].

highest temperature will limit the efficiency of the whole string. This is known as the current matching problem. The coolant circuit should be designed to keep the cell temperature low and uniform, be simple and reliable, and keep parasitic power consumption to a minimum. Concentrators with the use of lenses or reflectors can be generally grouped into three categories: single cells, linear geometry, and densely packed modules. For highly concentrating systems, more concentrator material per unit cell/absorber area is needed. The use of lenses is then more appropriate than reflectors owing to their lower weight and material costs. However, concentrator systems that utilize lenses are unable to focus scattered light, and this limits their usage at places largely with clear weather. On the other hand, using “liquid” as the coolant is more effective than using “air” to obtain better electrical output. For these reasons, reflector-type CPVT systems are common for medium- to high-temperature hot water systems applicable for cooling, desalination, or other industrial processes. At lower operating temperatures, a flat-plate solar collector may give a higher efficiency than the concentrator-type collector when both are directly facing the sun. But the performance gap will diminish when the working temperature gradually increases. This is because at higher temperature differential, the large exposed surface of a flat-plate collector incurs more thermal loss. Presently, research is going to develop CPVT collector for more electricity as

well as heat generation. A novel concentrating photovoltaic thermal collector is being designed in the prototype form Buonomano et al. [13]. In this system, triple junction cells are attached with the collector. Both thermal and electrical efficiency is found will very high. 3.5. Heat-pipe-based PVT Heat pipes are considered efficient heat transfer mechanisms that combine the principles of both thermal conductivity and phase transition. A typical heat pipe, as indicated in Fig. 8 [14], consists of three sections namely, evaporated section (evaporator), adiabatic section and condensed section (condenser), and provides an ideal solution for heat removal and transmission. This prototype module comprises a photovoltaic layer and a flat plate heat pipe containing numerous micro-channel arrays acting as the evaporation section of the heat pipes. The other end of the heat pipe is the condensation section which releases heat to the passing fluid and the fluid within the section is condensed owing to the heat discharge [15] as shown if Fig. 9. The flat-plate geometry is more efficient due to the excellent thermal contact between the PV cells and heat extraction devices, which results in a smaller thermal resistance and higher overall solar conversion efficiency. In this way, the PV efficiency could increase by 15–30% compared to the sole PVs, if its surface temperature is controlled to

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Table 1 Remarks on previous study. S. no. Type of system

Author

1

Vats et al.

Building integrated air photovoltaic thermal system

2

3 4

Solar air heating photovoltaic thermal system

5

6 7

Concentrator photovoltaic thermal system

8

9

10 11

12

Heat-pipe-based photovoltaic thermal

Concluding remarks on previous research

A study has been conduct with different photovoltaic material and different packing factors for BIPV system with air duct. The packing factor 0.62 shows superior thermal and electrical performance as compared to packing factor 0.83 Yin et al. It is observed that airport structured received 90–95 % of global solar radiation. Hence PV technologies can be used to generate power. BAPV shows lower peak power capacity and lower annual energy density Ibrahim The energy and exergy of the BIPVT solar collector have been conducted. Authors have presented new et al. absorber design for the system. The energy saving efficiency has increased from 73% to 81% Tyagi et al. A study has been conducted on the solar air heater in the three different conditions namely without phase change material (PCM) - paraffin wax, with PCM and with hytherm oil. Study shows that efficiency in tthe paraffin wax condition is the highest as compared to other two conditions Touafek A new improved design of hybrid solar collector for hot air supply have been fabricated and studied. et al. Study shows better thermal and electrical performance as compared tradition solar air heater. It has thermal efficiency of 48% Amori et al. Different solar PV collectors of different configurations have been tested in the outdoor conditions. Mathematical model has been developed to predict the performance of these collectors Al-Alili et al. In this study concentrator photovoltaic system is applied to power the hybrid desiccant assisted air conditioner. Result shows that it provides better thermal comfort as compared to vapour compression system Sueto et al. In a concentrator photovoltaic system, the anti- soiling layer was coated on PMMA. This happens due to electrostatic charge. The system shows better performance than traditional parboiled PVT concentrator Li et al. A new static incorporated CPC-PV/T system have been designed and tested. Mathematic model to predict optical efficiency under outdoor conditions have been developed and was validated with experimental values. Results show good agreement with them Gang et al. A new heat pipe based photovoltaic thermal system has been designed and developed. It is compared with existing water system. Proposed system shows better performance than existing system Zhang et al. An innovative concept to incorporate heat pipe to PVT system. System was tested in the outdoor conditions for seven continuous days. The proposed system shows better performance than existing solar air system Moradgholi A novel concept of cooling of the solar panel by the array of heat pipe is presented. It enhances the et al. electrical efficiency of the system

References [47]

[48]

[49] [50]

[51]

[52] [53]

[54]

[55]

[56] [57]

[58]

around 40–50 1C. The overall solar conversion efficiency of the module was around 40%. Table 1 represents the comparative analysis of the various PVT systems.

4. Advancements in recent years and future directions of PVT in India The development of PVT systems around the world is reached at its peak in the last decade but in India utilization and commercialization of PVT has been started in the recent years, and it became so popular during the last decade. The study carried out on PVT in India as university research studies. BIPV and Solar photovoltaic thermal system for water as well as air heating/cooling are available commercially and simultaneously researchers of various Indian university are working for the introduction of new and effective designs for obtaining the better electrical as well as thermal efficiencies of the PVT systems. The team of researchers of the Indian Institute of Technology, Mumbai are working on the Power Generation—state-ofthe art PV and solar thermal technologies under the project named as Pan IIT Solar-research Initiative (PSI) [16]. The main aim of this project is to generate 1 MW 8 hours per day. Indian government also took some initiatives for the development of PVT at the rapid rate. National Action Plan on Climate Change(NAPCC) was launched by the prime minister of India on June 30, 2008, it is also known as “National Solar Mission”: The NAPCC aims to promote the development and use of solar energy for power generation and other uses with the ultimate objective of making solar competitive with fossil-based energy options [17]. The plan includes: Specific goals for increasing use of solar thermal technologies in urban areas, industry, and commercial establishments; A goal of increasing production of photovoltaic’s to 1000 MW/year; and a goal of deploying at least 1000 MW of solar

Fig. 10. Photograph of the low concentrating photovoltaic thermal system [18].

thermal power generation. The immediate aim of the Mission is to focus on setting up an enabling environment for solar technology penetration in the country both at a centralized and decentralized level.

5. Case study of a PVT system In this study, the performance for low concentrating photovoltaic thermal system is being conducted in the Energy and

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Building Design laboratory of Lund Technical University in Sweden (latitude 551440 N, longitude 131120 E)[18]. The result is being compared with simulated resulted and it is found to be close agreement with them. The photograph of the system is given in Fig. 10.

5.1. Details of the system In this system, tracking system is provided in the low concentrating parabolic photovoltaic thermal system. Reflector of the system concentrate the solar radiation to the water cooled PV modules which act as a thermal absorber. This system generates both electricity as well as thermal energy in the form of hot water. Here PV modules is kept in the two section form. Each section contains 32 solar cells. It is well laminated on both top and bottom side of the cell. It is kept in the V-shape on the thermal absorber of the system. Mono crystalline silicon solar cell which was special type so that it can bear concentrated light [19] and was spreaded in area of 0.33 m2. Water is the fluid which flows inside the thermal absorber. The reflector of the system is the steel sheet. This coated with silver colour. The tracking of the system happens by rotating through its axis of rotation. The area of parabolic trough is 4.6 m2.

5.2. Analysis of the observed data The reading of the various data is observed from 1/6/08 to 13/9/08. The variation of output electrical and thermal power is shown in Fig. 11. Results shows that both power become highest at the noon time. Fig. 12 shows the variation of electrical efficiency with respect to outlet water temperature. Results shows that lower outlet temperature provides higher electrical efficiency.

Fig. 11. Output on electric and thermal output [18].

Fig. 12. Variation of electrical with water outlet temperature [18].

6. Historical review on PVT Hendrie developed a theoretical model for the flat plate PVT solar collectors and by using the model [20]. He concluded that the air and liquid based units obtained thermal efficiencies of 40.4% and 32.9% respectively and electrical efficiency. Cox and Raghuraman explored numerous design features of air based flat-plate PVT collectors to determine their effectiveness on the basis of a computer simulation [21]. They found the air PVT types are usually less efficient than the liquid ones due to low PV cell packing factor, low solar absorptance, high infrared emittance and low absorber to air heat transfer coefficient. Huang et al. studied an integrated photovoltaic–thermal system (IPVTS) set-up [22]. The tested results showed that the solar PVT collector made of a corrugated polycarbonate panel can obtain a primary-energy saving efficiency of about 61.3%, while the temperatures difference between the tank water the PV module was around 4 1C. Bergene and Lovvik developed a PVT mathematical [23]. The model was based on analysis of energy transfers including conduction, convection and radiation initiated by Duffie and Beckman and the outcome of model operation suggested that the overall efficiency of PVT collectors are in the range 60–80% [24]. Grag and Agarwal developed a simulation model to examine the outcome of the design and operational parameters of a hybrid PVT air heating system [25]. It was found that PVT air heating system largely depended on the its design temperatures as the extra glass cover might lead to the increased transmission losses and beyond some critical point the single-glass cover can collect more heat than double glass does. Sopian et al. developed the steady-state models to analyze the performance of both single and double-pass PVT air collectors they concluded that the double-pass photovoltaic thermal solar collector produces better performance than the single-pass module at a normal operational mass flow rate range [26]. De Vries and Zondag et al. carried out testing of a PVT solar boiler with a water storage tank in the Dutch and found that the covered sheet-and-tube system was the most promising PVT concept for tap water heating [27–29]. This PVT system could achieve annual average solar efficiencies of between 34% and 39% for the covered designs, and 24% for the uncovered design. Kalogirou carried out the modelling and simulation of the performance of a hybrid PVT solar water [30]. The results showed that the hybrid system increases the mean annual efficiency of the PV solar water system from 2.8% to 7.7% and in addition covers 49% of the hot water needs in a house, thus increasing the mean annual efficiency to 31.7%. Jones and Underwood have studied the temperature profile of photovoltaic (PV) module in a non-steady state condition with respect to time [31]. They performed experiments for clear as well cloudy day condition and observed that the PV module temperature varies between 300 and 325 K (27–52 1C) for an ambient air temperature of 297.5 K (24.5 1C). Huang et al. have studied experimentally the unglazed integrated photovoltaic and thermal solar system (IPVTS) for water heating under natural mode of operation [32]. They observed that the primary energy saving efficiency of IPVTS exceeds 0.60 which is higher than that for a conventional solar water heater or pure PV system. Kalogirou has studied the monthly performance of unglazed hybrid PVT system under forced mode of operation for climatic condition of Southern Cyprus and observed an increase of the mean annual efficiency of PV solar system from 2.8 to 7.7% with thermal efficiency of 49%, respectively [33]. Sandnes and Rekstad have observed the behaviour of a combined photovoltaic thermal (PVT) collector which was constructed by pasting single-crystal silicon cells onto a black plastic solar heat absorber (unglazed PVT system) [34]. They recommended that the combined PVT concept must be used for low temperature thermal application for increasing the electrical efficiency of PV system,

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e.g., space heating of a building. Sandnes and Rekstad constructed a PVT unit by using a polymer solar heat collector combined with single-crystal silicon PV cell [35]. They found that pasting solar cells onto the absorbing surface would reduce the solar energy absorbed by the panel further there is an increased heat transfer resistance at the surface of absorber. Chow has carried out the analysis of PVT water collector with single glazing in a transient condition [36]. The tube underneath flat plate with metallic bond collector was used. He observed that photovoltaic conversion efficiency at the reduced temperature is increased by 2% at mass flow rate of 0.01 kg/s for 10,000 W/m2 K plate to bond heat transfer coefficient. An additional thermal efficiency of 60% was also observed. Tiwari and Sodha developed a thermal model for an integrated photovoltaic and thermal solar collector (IPVTS) system and compared it with the model for a conventional solar water heater by Huang et al. [37]. The simulations predicted a daily primaryenergy saving efficiency of around 58%, which was in good agreement with the experimental value (61.3%) obtained by Huang et al.

7. Recent trend in PVT Tripanagnostopoulos made some more improvements in the existing system by introducing an unglazed PVT/dual system with both water and air cooling modes and found that by attaching water tubes at the back surface of PV gives better thermal efficiency [38]. By adding a thin metal sheet (TMS), fins (FIN) and a combination of TMS with ribs (RIB) (TMS at the centre of the air duct and RIB at the opposite side of the PV surface) he compared the three modified systems with the reference PVT/ dual (water tube attached at back surface system and found a significant increase in thermal efficiency for the air heat extraction, which is respectively approximately 23, 33 and 36% higher for the said systems described above. Use of an additional booster diffuse reflector (REF) which further improves electrical performance 12% for the low (about 0 1C) operating temperatures, with electrical efficiency of 16.5 and 18.5% for the typical PV and the PV with booster diffuse reflector, respectively. This increase in electrical performance percentage is more significant for higher (about 55 1C) operating temperatures, as it happens usually in PVT collectors, thus the electrical performance can be higher by about 18% as electrical efficiency is 13 and 11%, respectively, for the PV and the PV with booster diffuse reflector systems. Tonui et al. constructed an air-based PVT solar collector which applied two low cost approaches to enhance heat transfer between the air flow and PV surface [39]. It is found that the induced mass flow rate and thermal efficiency decrease with increasing ambient temperature and increase with increasing tilt angle for a given insulation level. Solanki et al. designed and constructed a PVT solar air heater, and studied its performance over different operational parameters under steady indoor conditions [40]. They found that the thermal, electrical and overall efficiency of the solar heater obtained at indoor condition was 42%, 8.4% and 50%, respectively. Dubey et al. developed an analytical model with four different configurations [41]. It was found that the glass to glass PV module with duct gives higher electrical efficiency and the higher outlet air temperature among the all four cases. The annual average efficiency of glass to glass type PV module with and without duct was reported 10.41% and 9.75%, respectively. Ji et al. developed a novel solar PVT heat pump (PVTSAHP) system that combined a Rankine refrigeration cycle with a PVT solar collector [42]. The results indicated that the PV electrical efficiency and evaporator thermal efficiency are around 12% and 50% respectively during the testing period in Hefei, China.

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Shahsavar and Ameri designed and tested a direct-coupled PVT air collector with and without glass cover at Kerman, Iran [43]. They concluded that setting glass cover on photovoltaic panels leads to an increase in thermal efficiency and decrease in electrical efficiency of the system. Zhao et al. designed a novel PVT roof module to act as the roof element, electricity generator and the evaporator of a heat pump system [14]. Under a typical Nottingham (UK) operating condition, the modules would achieve 55% of thermal efficiency and 19% of electrical efficiency, while the module based heat pump system would have an overall efficiency of above 70%. It was also addressed that the integration of the PV cells and evaporation coil into a prefabricated roof would lead to large saving in both capital and running costs over separate arrangements of PV, heat pump and roof structure. Touafek et al., have design, developed and modelled the PVT collectors for the purpose of air heating and power generation [44]. It provides good thermal and electrical effect as compared to the classical hybrid collectors. Amrizal et al. have presented dynamic model for hybrid PVT solar collector [45]. This model is found to predict the thermal and electrical performance of the collector. It was validated with experimental data and found to be good agreement. Bakar et al. have upgraded the design of photovoltaic/thermal solar collector by integrating a roundabout-shape copper tube below the tedler of the PV panel [46]. It provides both hot air and water along with increase the electric production per unit as compared to the normal PV panel.

8. Conclusion The renewable energy become upcoming prominent energy sources due to following reasons namely fast rate of depletion of fossil fuels, volatile raise in price of gasoline and environmental pollution. Among all renewable energy sources, the solar photovoltaic technologies is found to be one of the most promising. Due to this, intense research work is going on this field and this leads to significant enhancement of its performance. A detailed reviews both historical and present trend of the solar photovoltaic thermal technologies have been presented. The various application of the solar photovoltaic system such as building integrated air PVT system, solar air heating PVT system, liquid PVT collector, concentrator PVT system, and heat-pipe-based PVT are also presented. Now various PVT applications on commercial level is there but still it is limited due product reliability and cost. Hence significant research is required in the field of PVT mainly in thermal absorber design and fabrication, material and coating selection, energy conversion and its effectiveness, cost minimization, performance testing, control and the reliability of the system. This communication is found to be helpful for the manufactures of the PVT collectors, researchers and students who is working in this field. References [1] Tiwari A, Barnwal P, Sandhu GS, Sodha MS. Energy metrics analysis of hybrid— photovoltaic (PV) modules. Appl Energy 2009;86:2615–25. [2] Tyagi VV, Kaushik SC, Tyagi SK. Advancement in solar photovoltaic thermal (PVT) hybrid collector technology. Renewable Sustainable Energy Rev 2012;16:1383–98. [3] Zondag H, Bakker M, Van Helden W. 2006. PVT roadmap/An European guide for the development and market production of PV–thermal technology. In: PV Catapult—contract no. 502775 (SES6). Energy Research Centre of the Netherlands ECN2006. [4] Chapin DM, Fuller CS, Pearson GL. A new silicon p–n junction photocell for converting solar radiation into electrical power. J Appl Phys 1954;25:676–7. [5] Grant CD, Schwartzberg AM, Smestad GP, Kowalik J, Tolbert LM, Zhang JZ. Characterization of nanocrystalline and thin film TiO2 solar cells with poly(3undecyl-2,20 -bithiophene) as a sensitizer and hole conductor. J Electroanal Chem 2002;1:522–40.

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