Improving photovoltaics efficiency by water cooling: Modelling and experimental approach
Introduction
The global energy demand is growing faster and the world energy consumption is expected to increase by 33% in the period 2010–2030 [1]. To build a low pollution power generation system, fossil fuels power plants fed by coal, oil and natural gas need to be limited. Therefore, being renewable energy sources (RES) derived from natural and available resources, characterized by low cost of operation and a minimal impact on the environment, are a viable way to generate clean electricity and heat. Compared to other RES, direct solar radiation has an enormous potential, especially in the tropical regions. Therefore, solar photovoltaic electricity generation is one of the most promising options to encounter the future energy needs. Nowadays, the installation of solar PV panels is increasing all over the world due to their capability of working under different operating conditions. Therefore, it is fundamental to consider that PV panels work at different climatic conditions. For this reason, several researchers have analysed the PV module behaviour and they have estimated that only 15–20% of the solar irradiation can be converted into electricity while the rest is wasted as heat [2]. This aspect constitutes a huge obstacle because the PV module efficiency decreases at a rate of 0.4–0.65% with a one-degree increment of the module temperature [3], [4], [5], [6]. Note that, as observed by Reddy et al. [7] during their investigations, the PV temperature can reach values higher than 80 °C if the PV system is installed in hot arid regions. So, there is a considerable interest on controlling the module temperature and, consequently, improving the PV cells performance. In literature, several studies concerning PV cooling and their classification can be found. The PV cooling systems can be grouped as: passive cooling technologies and active cooling methods. Passive cooling technologies do not require additional power to cool the PV module while an active cooling system is functioned by fans or pumps. Obviously, an active cooling system is much effective than a passive one especially when the PV system is in hot arid areas or in a desert. Natural circulation over and under the PV module is the simplest and widely adopted passive cooling method while the insertion of colourless and lucid silicon coat [8], microporous evaporation foils [9] or a thermoelectric cooling system based on the Peltier effects [10] are innovative and under investigation techniques. In general, a passive cooling system achieves a reduction of the PV module temperature in the range 6–20 °C while the improvement in the electrical efficiency is up to 15.5% [11]. As for the passive cooling technologies, active cooling methods have been largely investigated by several researchers. As an example, Krauter [12] proposed a water film flowing on the module front surface, utilizing multiple nozzles placed on the top of the module and a water circuit fed by a pump. The presence of the water film produced two important effects: a module temperature reduction of 22 °C and a decrease of the light reflection. In addition, the author estimates a potential 9% net gain in electricity production with the proposed system. Abdolzadeh and Ameri [13] developed a different cooling system, related to a PVPS (Photovoltaic Pumping Systems) application. Part of the pumped water is sprayed on the front of the cells achieving a strong temperature decrease and measuring a maximum increase of electrical energy yielded over the whole day of about 17%. A beneficial effect on light reflection is also observed. Krauter et al. [14] proposed and tested a system where the thermal mass of the PV module has been increased. An increment of the energy production up to 12% has been observed, but some problems linked to the weight of the structure have been also identified. Wilson [15] performed both experimental and numeric studies on a configuration in which water flows on the rear of the cell, and suggested the application of the system to a gravity fed circuit, that does not require a pump and allows reaching a zero-energy consumption system. The measured maximum cell temperature reduction was 32 °C while an energy output increment of 12.8% was also computed. Sai and Prudhvi [16] proposed a particular application of a frontal water film cooling system. Water is circulated in a closed loop, exchanging heat with the soil by undergrounded high conduction pipes. Also, several systems based on air cooling have been presented and tested (see, e.g., [17] and [18]) but they demonstrate lower efficiency than water based methods. To sum up, an active cooling method can improve the electrical efficiency up to 22% and reduce the PV module temperature up to 30 °C. Active or passive methods using water or air are cooling techniques but, in the last decade, several researchers proposed a different concept of PV cooling system: the so-called hybrid photovoltaic and thermal (PVT) collectors. The system simultaneously produces electrical and thermal energy: a way to improve the overall system performance. However, this kind of system requires a thermal user. As for passive and active cooling methods, also PVT systems have been deeply investigated. For a clear overview refer to, e.g., [18], [19], [20]. Based on the above-mentioned works, it is possible to claim that PV cooling is a hot research topic in which numerical and experimental investigations need to be done in order to develop energy and cost effective cooling methods applicable to both new and existing PV plants. For these reasons, in the present work, the mathematical models of the PV cell and of different cooling methods have been firstly built. Two different modelling approaches have been implemented: a steady-state model and a transient one. The first permits to evaluate the cooling potential of applied technology while the second one allows to simulate different management strategies for the cooling system varying the ON-OFF time cycles. After that, a simple cooling technique which does not require significant structural modification of the PV module but able to improve the system performance has been presented. Multiple results from the PV transient model equipped with the proposed cooling system are discussed. Finally, after the design and installation of a test rig, preliminary comparison between numerical and experimental results has been made. The rest of the paper is organised as follows: in Section 2 the steady-state and the dynamic model of the PV cell are presented while in Section 3 the results of the numerical investigations are outlined. In Section 4 an economical model and a case study application is presented, while in Section 5 the developed test rig and the experimental results are introduced. Conclusion remarks are given in Section 6.
Section snippets
PV steady-state and dynamic models
The PV steady-state model is presented in Section 2.1 while the water cooled steady-state model, the water film model and the spray cooling model are described in Section 2.2, 2.2.1 and 2.2.2, respectively. The transient model of PV panel is presented in Section 2.3 while the energy performance evaluations are summarized in Section 2.4.
Numerical simulations results
Multiple evaluations have been made with steady-state and transient models, both for the case of non-cooled module and for water film cooled module. Different conditions have been considered to investigate the sensitivity of the system to the different parameters.
The following Table 3, Table 4, Table 5 report temperature results for parametric variations on solar radiation G, wind velocity Vw and air temperature Tair. For each condition the non-cooled and cooled panel steady state temperatures
Economic evaluations
The economic convenience of a PV cooling system is evaluable by balancing economic revenues generated by the additional energy production and the additional installation and operation expenses related to the cooling system. Several factors impact on the overall convenience but the most important is the source of water. Water can be taken from a river, a well, a water main or a water tank. This has an impact on the installation cost (necessity to use a pump or not, to build a storage facility or
Experimental setup
An experimental activity has been conducted with the aim of investigating the behaviour of a real photovoltaic cooling system installation and compare it with the outcomes of the models described in the previous paragraphs of this paper.
The base elements of the experimental setup are the following:
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Photovoltaic panel XG60P produced by X GROUP, with an area of about 1.6 m2 and a nominal power of 220 W (Fig. 13)
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Panel support structure
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Water piping, nozzles and water collection system
In the first
Conclusions
The evaluations made in this study show that there is generally the possibility to realize a positive energy balance with a PV cooling system. The steady-state model allows to estimate the potential of a cooling system, while the transient model makes possible to optimize the work cycle of such a system.
The energy gain achievable with cooling is greater when the pump total head is low, and the weather conditions are such that the not cooled PV panel reaches high temperatures (low wind speed,
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