Experimental Investigation of Thermal Performance of Photovoltaic Thermal (PVT) Systems
Abstract
The phenomenon of photovoltaic systems is based on the principals of semiconductor physics and they operate with a semiconductor element, such as silicon. Photovoltaic cells can generate electricity only when they receive a certain amount of photon energy and thus they convert only a fraction of the solar irradiance, which is received from the sun in the form of electromagnetic radiation in the electromagnetic spectrum, into electrical energy. The remaining radiation is stored as heat in photovoltaic systems, causing some irreversibilities in the system.
In general, the experimental setup, the accumulated heat, which reduces the efficiency of the photovoltaic systems, is aimed to be removed from the system and turned it into useful energy. By employing some heat transfer enhancement systems, the photovoltaic cell temperature decreased to the range of 40-60 °C, the temperature range at which a photovoltaic system runs optimal, whereby an approximate improvement of 20% in electrical efficiency of the PV system achieved. Aluminum and copper cylindrical fins or some refrigerant fluids used as heat transfer enhancement elements in the systems.
In this operating conditions, the electrical efficiency of the system decreases to around 6.5% down from the nominal electrical efficiency of 12% under optimal operating temperature. The fin surface temperature and ambient temperature of the control volume decreased in direct proportion to the air velocity. At about 5 m/s air velocity, the fins bodies and ambient air were cooled down by about 50%, accordingly, the electrical efficiency decreased from 12% to only 9.5%.
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Introduction
In parallel with the developing and increasingly diversified industry, as well as the seek for comfort, the need for energy is increasing rapidly. This need can no longer be met by relying on fossil fuels even if conventional energy producing systems are used effectively and environmental pollution is disregarded. Due to increased energy load and the adverse effects of fossil fuels on environment, a quest for new and renewable clean energy sources are sought [1,2]. Renewable energy sources have been seen as a solution to the disadvantages caused by fossil fuels and solar energy has made its first place with its high potential and large geographical availability.
Surveying the literature, one can easily see that there is an abundance of experimental study on the thermal analysis of the photovoltaic system. Most of these studies focus on removing the radiation-induced heat accumulation, which decreases the yield from the photovoltaic system, with appropriate cooling systems in order to maintain the electrical output at fair levels as well as obtaining a utilizable thermal energy source.
In the literature, there are many complex and detailed mathematical models for thermal performance analysis. However, in order for the performance analysis of such systems should easily be calculated and be compared to similar systems; generally, a performance analysis based on the first law of Thermodynamics is sufficient. In thermodynamic analysis of the Photovoltaic thermal (PVT) systems, the system is regarded to be a continuous flow control volume (open system). Mass and energy transfers from the boundaries of the control volume are calculated using conservation equations. Atypical control volume set for the experimental systems studied in the literature is given in Fig. 1.