Overview of Photovoltaic Thermal (PVT) Water Collector

Received Jan 30, 2018 Revised Jun 6, 2018 Accepted Jun 20, 2018 The popular solar technology is the integration of solar thermal technology and photovoltaic (PV), called photovoltaic thermal (PVT) technology. This technology converts solar energy to electrical and thermal energy. The efficiency of solar energy conversion via PVT is higher than photovoltaic and solar systems. PV cell efficiency decreases if system operating temperature is higher. Therefore, solar systems attached to PV cells act to cool PV cells and increase the overall efficiency of the PVT system. PVT construction that saves space, is suitable for domestic consumption, and long-term saving costs makes PVT current research by researchers in the latest energy technology. This review presents descriptions and previous works conducted on performances analysis of PVT water collector. Results on the performances of PVT water collectors are summarized. The energy and exergy efficiency of PVT water collector ranges from 28.5% to 85% and 6.8% to 14%, respectively. Keyword:


INTRODUCTION
Growth in the field of renewable energy and energy efficiency has brought stability to the negative impact of carbon emissions as a result of increasing world energy consumption as well as the reduction of major fossil fuel resources. Global awareness in the exploration of renewable energy signaled that renewable energy would be an option in the future. Renewable energy ensures adaptation to climate change, continuous energy supply, new job opportunities, environmental health and energy delivery to remote areas. At present, advances in the field of renewable energy bring impact to areas such as transport, cooling, heating and power generation in rural areas. Nevertheless, renewable energy savings in each year are far from the rate of achievement set. The reason for this is that subsidized payments to fossil fuels are still ongoing, low energy efficiency technologies and expensive installation costs. Solar energy is one of the renewable energy sources and this clean energy has potential to meet a significant amount of the world's energy demand. The evolution of renewable energy sources such as solar energy is a source of energy that provides benefits to the environment and clean energy [1]- [6].
Photovoltaic (PV) technology converts solar energy into electricity for use in various applications such as water and space heating purposes, electrical use for home appliances and solar drying. The efficiency of PV cell conversion from solar energy to electricity decreases as operating temperatures increase. Combining solar thermal collectors and PV cells in a system known as thermal photovoltaic (PVT) can reduce operating temperature and improve system efficiency. PVT also produces thermal energy and thermal energy simultaneously. A PVT collector consisting of PV panel, insulation and frame. Accordingly, PVT collector consists of one or more cover (glass sheets) or a transparent material placed above an absorbing plate with air flowing around it. One way to enhance the collector's efficiency of PVT collector is use heat transfer area through absorber with finned absorber, corrugated surfaces, and porous media [7]- [8]. PVT collectpr can be classified into four types base on heat transfer medium; air-based PVT collector, water-based PVT collector, the combination of water/air-based PVT collector, and nanofluid-based PVT collector [9].
Various studies have shown that increased energy efficiency and energy-saving photovoltaic hybrid systems compared to self-developed solar and photovoltaic collector systems [10] [11]. PVT studies include experimental studies, analytical and simulation solutions. The lower operating temperature and the increase in electrical efficiency of the PV module were recorded by Bahaidarah et al. [12] when the PV module was fitted with water cooling.
Recently, enviroeconomic and exergoeconomic analyses for PVT water collectors were studied base on thermodynamics aspect [13]. Thermodynamics is the study of heat, work, energy, and the changes they produce in the states of systems. In wider sense, thermodynamics examines the relationships between the macroscopic properties of a system. A system is in the state of thermodynamic equilibrium when no change in any microscopic properties takes place. In this review, thermodynamic aspect is focused involving exergy analysis. Exergy analysis is conducted by utilizing the first and second law of thermodynamics. Exergy analysis has become an essential tool in the system design, analysis, and optimization of thermal systems [14]- [15]. Fudholi et al. [16] studied theoretical and experimental of PVT air collector with ▽-groove. They reported that PVT exergy efficiencies of were 12.66-12.91% and PVT energy efficiencies were 31.21-94.24%. The thermal and PV efficiency was 21.3-82.9% and 9.87-11.34% respectively. Sarhaddi et al. [17] [18] studied energy and exergy analysis of PVT air collector. They reported that PV, thermal and PVT efficiency were 10%, 17.18 and 45% respectively. PVT exergy efficiency was 10.75%. Recently, energy and exergy analysis for nanofluid based PVT systems were studied.Further theoretical approaches had been done such for the use of nanofluids as coolants as had been carried out by Mustafa et al. [19] which a theoretical model was performed to evaluate system's performance while studied numerically a new configuration of the photovoltaic thermal system which includes the collector tube and working fluid. Lari and Sahin [20] reported that PV energy efficiency was 13.2% for PVT nanofluid system. Aisyah et al. [21] studied PVT water collector with spiral thermal absorber design. They reported that the efficiency of PV, thermal and PVT were 4.18%, 76.68% and 80.86%. However, in this review we focused on energy and exergy analysis of PVT water collector. Figure 1 shows PVT water collectors, has similar structure as the conventional flat plate solar collectors. The absorber consist of tube is attached to the PV panel in order to use for heating purposes. The PVT water collector could achieve the enhance cooling effectiveness compared to the air based system due to the high thermal mass of the of water over the air. The PVT technology is a technology developed to remove accumulated heat from the PV module and use the heat waste in an appropriate way. It combines PV cells and solar collector into a single module. This approach allows the cooling of PV cell and simultaneously utilizing the extracted heat for domestic use. Base on energy analysis, the overall efficiency of PVT, is the sum of solar collector thermal efficiency, ℎ and PV's electrical efficiency, used to evaluate the performance of PVT written as follows:

DESIGN AND PERFORMANCE OF FLAT PLATE PVT WATER COLLECTORS
(1)

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Electrical energy ( ) is a high-grade from of energy gain, the primary energy-saving efficiency was proposes as another performance indicator of the energy-grade difference between electricity and thermal energy andwas given by [18], [22]: (2) Where is the electric-power generation efficiency of a conventional power plant; its value can be taken as 0.38.The evaluation indicator of the primary energy-saving efficiency also considers the quality and quantity of the energy that the PVT collector converts into solar energy [22].
The thermal efficiency of flat plate PVT collector is a ratio of the useful thermal energy, to the overall incidence irradiations, . ( The heat collected by the flat plate PVT collector can be measured by result of average mass flow rate,ṁ heat capacity of flowing medium, and a temperature difference of the medium at the collector inlets, and outlets, . The electrical efficiency of PV modules, which is the ratio of measured output power to the overall incident solar radiation. (5) It is known that the electrical efficiency of the PV module is a function of temperature given by [22] ( 6) where is the reference efficiency of the PV module and is cell temperature.
Exergy analysis is a method that uses the conservation of the second law of thermodynamics for the analysis, design and improvement of energy (and other) systems. It is a useful method in achieving the goal of more efficient energy use as it enables us to determine the location, type and magnitude of wastes and losses. Exergy analysis can indicate whether or not and by how much a design of the energy system can be made efficient by reducing the inefficiencies in the system [23]- [27].
However, in order to evaluate the efficiency of the system more precisely, the exergy efficiency must be calculated using exergies for thermal and PV (electrical). Assuming that the effects due to the potential and kinetic energy changes are insignificant, the general exergy balance can be expressed as following [28]: where T a , T o and T s are ambien, outlet and sky temperatures (T s =5777 K). A, N and I are area of PV, number of PV and irradiations, respectively.
The exergy efficiency of a PVT water collector is defined as ratio of net output exergy to net input exergy. It can be calculated as following: (11) The exergy loss of PVT water collector includes six term; (i) by optical losses in PVT collector surface, (ii) exergy destruction due to the temperature difference between the sun and PVT collector surface, (iii) exergy destruction due to heat transfer from the PVT surface to the working fluid at finite temperature difference, (iv) by heat loss from PVT collector to surrounding, (v) exergy destruction due to pressure drop in flow pipes and (vi) electrical exergy destruction [29].

STUDIES CONDUCTED ON PVT WATER COLLECTORS AN OVERVIEW
Zondag et al.
[30] evaluated seven different designs of PVT water collector. The designs were categorised to sheet and tube, channel, free flow and two absorbers. Although the sheet and tube design is indicated 2% less in efficiency than the channel design, it is the easiest design for manufacturing process. Thus, this design is the most promising compared to other designs. Several studies on the performances (energy or exergy) analysis of PVT water collectors were reported as shown in Table 1

CONCLUSION
Based on the present review the following conclusions can be drawn: From studies conducted of PVT water collector in 2011 to 2017, the energy and exergy efficiency of PVT water collector ranges from 28.5% to 85% and 6.8% to 14%, respectively. From energy analysis, PV and thermal efficiency of PVT water collector ranges from 1.5% to 28% and 23% to 75%, respectively. The development of PVT water collectors is interesting and very promising area of research. Today, PVT water collectors using in solar water heater.