Photovoltaic/Thermal (PV/T) systems: Status and future prospects
Introduction
The global demand for energy has increased due to the growth in population and improved economic situation in most parts of the world. While energy demand is growing, its primary sources, namely fossil fuels, are beginning to be exhausted due to over-consumption. The man activities resulting from energy consumption and the pollution caused have led to rapidly changing weather conditions, such as global warming, the melting of ice at the north and south poles, and damage to the ozone layer [1]. Environmental pollution and global warming problems can be reduced in the future by using renewable energies such as solar energy, specifically solar energy based on photovoltaic (PV) technology. PV is used to convert solar energy (light) to electrical DC energy. The smallest unit is called a solar cell and these are connected in series and parallel to produce a PV module. PV modules are also connected in series and parallel to produce PV arrays. Indeed, PV power plants shares have increased around the world, and many countries are now planning to increase the proportion of alternative energies in electricity generation. For example, the EU has decided to develop a plan to increase the share of renewables in energy production to a minimum of 30% by 2030, with this proportion rising to 100% in 2050 [2]. For countries still lagging behind in the establishment of power plants that operate using renewable energies, for example, Gulf Cooperation Council (GCC) countries, all the states in the region preparedness plans for the construction and production of energy from renewable sources. This development comes from the increased understanding and awareness of the decision-makers in these countries about the importance of changing the GCC power profile [3]. Modern statistics showed a very significant increase in the use of PV between 2004 and 2014, as the electrical energy produced by PV increased from 3.7 to 7 GW between 2004 and 2007. In contrast, this figure grew from 7 to 40 GW between 2008 and 2011. During one decade (2004–2014), the share of PV increased from 3.7 to 177 GW [4].
PV has attracted great interest from researchers, manufacturers, and decision-makers as a source of clean power generation due to its economic and environmental benefits [5]. There is great potential for the usage of PV plants with high efficiency in several areas around the world due to the high intensity of solar radiation in these regions. Ref. [6] indicates that building a station in the Sahara can supply the Mediterranean area, North Africa, and Europe with electricity. So, the whole expectations suggested increasing the demand for PV to compensate the shortfall in power supply nowadays, especially in remote and distant areas. Therefore, the use of solar energy will reduce fossil fuel consumption. PV is more attractive, thanks to its many promising advantages, such as the lack of serious maintenance and low operating costs, long life, and a reduction of CO2 emissions, resulting in a clean environment for future generations.
The electrical power generation capacity of a power station run on PV will vary depending on several factors, such as the site of the PV plant, meteorological variables, the solar energy technology itself, and the power station capacity. PV technology still faces many significant challenges, such as its sporadic productivity and the uncertainty in the solar PV is the most obvious. Fluctuations in output are mainly due to changes in the solar radiation intensity received by the solar panels [7], [8]. PV plants integration in the existing electricity grid has increased the technical issues, and the most important is the stability of equipped power, directly or indirectly. This issue arises because of the continuous changes in the solar radiation intensity and temperature with time, which disrupts the reliability of the network. Some of the fluctuations in the productivity of PV can be due to seasonal or environmental reasons. Large differences in weather conditions increase the uncertainty of the energy generation systems of PV. Therefore, the energy storage systems of PV are necessary to avert fluctuations in electrical power production [9], [10].
Many studies [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30] have shown that the performance of PV systems is linked to the geographical contrast, both at the city or remote areas levels. Differences in geographic location have a direct impact on the intensity of solar radiation in addition to changes in wind speed, humidity, dust, and air pollution deposits on the PV panel. Each of these variables cause low productivity and performance fluctuation in PV [11], [12], [13].
Refs. [14], [15] studied the impact of climate conditions on PV performance. Ref. [14] found that PV is heavily influenced by exposure to intense sunlight and high temperatures over a long period. Ref. [15] clarified the insignificant wind impact on the system temperature during the period tested. The increase in air temperatures caused a significant voltage drop and an insignificant increase in current value leading to a significant reduction in power. Refs. [[16], [17]] studied the effect of solar radiation on the performance of PV in Oman. The study showed that despite the adverse effects of rising temperatures in the area studied, the high intensity of solar radiation in the region for a long time during the day, especially during the peak period of use, makes this technology promising and economically efficient.
Ref. [18] investigated the effect of relative humidity on the productivity of PV panels. Relative humidity affects PV panels alongside other weather variables. The study showed the great effect on the performance of the PV panel as the voltage, current, and power dropped with increasing relative humidity. The researchers concluded that PV panels have a medium efficiency in a high relative humidity atmosphere.
The conversion efficiency of solar radiation in PV panel into electricity, ranging from 12% to 18% currently, and up to 80% of solar radiation either reflected or turn into heat [19]. As mentioned earlier, increasing the temperature of the PV panel reduces the performance. Every increase of 10 °C caused a decrease in PV panel efficiency of 5% [20].
Several researchers [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31] have suggested the removal of this heat using cooling fluids such as air and water to pull the heat from the PV panel and used it in other applications. In this case, the PV panel is called a hybrid PV/T solar collector. In addition to increasing the electricity produced from the PV panel, this hybrid PV/T solar collector system can heat air or water or both at the same time. Photovoltaic efficiency depends mainly on the solar radiation and temperature as the performance if PV/T includes the electrical and thermal efficiencies of the PV panel. Thermal efficiency usually includes the useful thermal profit ratio while the electrical efficiency shows the percentage of the advantages taken by the system from the solar radiation to produce electricity during a specified period.
Ref. [29] indicated that the use of building integrated PV thermal (BIPVT) is much more efficient than the building integrated PV (BIPV) system. The efficiency of the energy produced by the BIPVT system is 17–20% higher than similar standards in the BIPV system. Ref. [30] took advantage of the heat extracted from a BIPVT solar heating system to increase the efficiency of a solar distiller production.
This paper aims to review the most significant developments in the PV/T research area. The study will focus on the type of fluid used and its effect on the thermal and electrical efficiency of the system. In this study, and in order to uniquely for many high valued publications in this field, ultra-modern studies related to PV/T systems will be reviewed in terms of changes in the energy production and the system's electrical, thermal, and total efficiency. PV/T has the potential to be the future of PV technology and dispense with fossil fuels as an energy source.
This paper contains the following sections and subsections. PV/T principle and performance; thermal studies for PV/T systems, which is divided into: air, water, water and air cooled PV/T systems, using PCM and heat pipes with PV/T systems; electrical studies for PV/T systems; critical review and conclusions.
Section snippets
PV/T principles and performance
In general, PV modules are used in different system configurations: standalone, grid connected, hybrid and tracking systems. Usually, a PV system contains a PV module/array, charge controller and maximum power point tracking MPPT, battery (optional), and inverter, as shown in Fig. 1.
PV has received gradually more and been used in various applications. A large number of researchers have investigated PV design and how to improve efficiency and increase the applications that it can be used with.
Thermal studies for PV/T systems
Hybrid PV/T systems convert solar radiation into electrical and thermal energies at the same time. The basic form of this scheme consists of the open solar collector with a plate surface equipped with PV cells surface. The PV cells absorb sunlight and benefit from a part of this radiation by producing electricity, while the remaining portion is transferred to the cooling fluid (air or liquid) flowing through the collector. This hot fluid can be used at low or medium heat applications such as
Electrical studies of PV/T systems
In this section, the electrical side view of the PV/T is discussed and criticized in term of different parameters and key findings as shown in Table 6.
Tripanagnostopoulos et al. [114] investigated a PV/T water cooling system in Greece. The system was experimentally tested with and without glazing. The PV/T system's thermal conductivity was enhanced using aluminum reflectors. LCA, EPBT, and LCA were calculated. The authors claimed that the system EPBT is 0.8 years compared to 2.9 years for PV
Critical review
The research conducted for PV/T in the last four decades has been extensive. Research thus far has been successful in validating the importance of such studies in the effective adoption of PV/T as a reliable means of harnessing solar energy. However, the PV/T system is still under development and has many points and gaps that require intensive future studies. These points represent challenges to researchers and need to be overcome to establish effective PV/T design in term of technical and
Conclusions
The PV/T system converts solar energy into electricity and heat, respectively. In this review, a number of PV/T systems developed over the last four decades have been discussed and summarized with more emphasis on the electrical side view. The efficiencies (thermal and electrical) of the PV/T collector using different heat transfer fluids and designs, their advantages, limitations, applications, and scope for future research were discussed.
The thermal side views were classified into; air
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