Experimental demonstration of enhanced solar energy utilization in flat PV (photovoltaic) modules cooled by heat spreaders in conjunction with cotton wick structures
Introduction
Solar Energy is the most important source of renewable energy which is utilized by a variety of different technologies. The primary technologies that uses solar energy include photovoltaics, concentrating solar power, solar heating and solar cooling systems. A solar cell or photovoltaic (PV) cell is a device that converts solar energy into electricity by the photovoltaic effect. Technologies for the development of solar cells are rapidly emerging to threaten the domination of crystalline silicon (c-Si) technology in the near future. Mono-crystalline modules are free from lattice impurities and defects. Hence, they operate with the highest efficiency but expensive to manufacture. On the other hand, multi-crystalline silicon are lower in its material quality and hence cheaper than the mono-crystalline silicon. The shortage and high prices of these crystalline silicon have made the PV manufacturers to seek for alternative raw materials such as metallurgical-grade silicon (mg-Si). In thin film technology, thin film of solar cells were made from a variety of materials like amorphous silicon (a-Si), Copper–Indium–Diselenide (CIS), Copper–Gallium–Diselenide (CGS), Copper–Indium–Gallium–Diselenide (CIGS) and Cadmium Telluride (CdTe). Though the manufacturing cost of thin film technology is lower than that of crystalline silicon, the major challenge in the development of this technology is to increase its efficiency and to overcome its initial performance degradation. Techniques have also been developed to produce multi-junction solar cells based on Gallium Arsenide (GaAs) and GaInP/GaInAs/Ge (Gallium Indium Phosphide, Gallium Indium Arsenide on a Germanium substrate). These multi junction solar cells exhibited a maximum efficiency of 30–40%. Newer PV technologies were also reported which include organic, dye sensitized, quantum well solar cells and nanostructured materials for solar energy conversion [1].
Non concentrated or flat solar PV systems are ideal for remote rural areas where other kind of electric power sources are either impractical or unavailable to provide electrical power for lighting, appliances and other applications. Further, the cost of extending an electric power line in remote locations may not be affordable. Hence, under these circumstances, it is more cost effective to install stand alone PV system [2].
In general, PV module converts only less than 20% of the incoming solar radiation into electricity. Thus, more than 50% of the incident solar energy is converted into heat resulting in undesirable short-term and long-term losses in PV modules. The increase in cell temperature, decreases in electrical power yield and the efficiency of the PV module are some of the common problems that are referred as short-term losses. On the other hand, long-term loss is performance degradation and it is dependent on environmental factors like temperature, water ingress and ultraviolet intensity [3]. The permanent structural damage caused by the development of thermal stress due to excessive heating of PV module at elevated operating temperature is known as thermal degradation of the module. Life time and reliability may be adversely affected due to hostile weather conditions such as high ambient temperature [4]. The variation of energy conversion efficiency with temperature depends on type of module such as crystalline silicon or thin film. For a crystalline silicon module, the thumb rule is that the conversion efficiency decreases by 0.5% for every 1 °C increase in the module operating temperature [5]. However, thin film type has lower negative temperature coefficient compared to crystalline silicon. For thin film technology, the drop in efficiency of PV modules made of amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) is in the order of 0.21%, 0.25% and 0.32–0.36% for 1 °C rise in module temperature [6].
Though the electricity yield and efficiency of PV module can be improved by cooling the solar cells with a fluid stream like air or water, a better option will be to re-use the heat energy extracted by the coolant. This led to the evolvement of PV/T hybrid solar technology. Among these technologies, air and water based types are relatively mature technologies while refrigerant and heat-pipe based PV/T systems are still at exploratory level [7]. Other techniques like sun tracking and cooling systems are also often used to increase the electrical power output of PV modules and to decrease its investments costs [8]. In the technique of sun tracking, the module surface is aligned with the direction of sunrays to maximize the reception of sun's direct radiation. PV modules usually track the solar radiation by either single or two axis tracking system. Single axis tracking system tracks the sun from east to west on a daily basis or north to south on a yearly basis. On the other hand, a two axes tracking system tracks the sun both from east to west on a daily basis and from north to south on an annual basis. For example, Mousazadeh et al. [9] demonstrated a 24.5% increase in PV electrical power output with a single axis tracking system when compared with a fixed PV module. Similarly, Abdallah [10] showed the electrical power output could be increased up to 43.87% with a two axes tracking as compared with the fixed PV module. McColl et al. [11] evaluated the means of increasing electrical power output of PV module using sun tracking and cooling techniques. They recommended sun tracking technique could be utilized during the cooler months while a cooling configuration could be utilized during the warmer and hotter months for increasing electrical power output of a PV modules that are operating in middle-east climatic conditions prevail in the location of Abu Dhabi, United Arab Emirates.
The thermal regulation technique used for solar PV modules can be classified as either a passive technique or an active technique. Passive technique requires no direct application of electrical power whereas active technique requires external electrical power. Active thermal regulation methods for the control of temperature of PV module often employs (i) spraying of water on the top surface of the panel [12], [13], [14] (ii) jet impingement cooling [15] and (iii) passing air or water through channels or ducts [16], [17], [18], [19], [20], [21], [22]. The various passive thermal regulation methods adopted for the control of temperature of PV module include (i) immersion of PV module in dielectric medium [23] (ii) submerged water cooling [24], [25] (iii) air flow induced by buoyancy [26], [27], [28] (iv) wind-driven roof top turbine ventilator [29] (v) heat dissipater/heat sink [30] (vi) phase change materials (PCM) [31], [32], [33] (vii) evaporative cooling technique [34] (viii) cotton wick cooling [35] and (ix) expansion of stored gas to spray water [36]. Table 1 shows the summary of the review of the previous research works on cooling of PV modules.
From the literature review, it is learnt that a lot of research efforts were made in the development of cooling system for PV module and hence it is an important research topic. The factors like minimum operating cost, high reliability due to minimum risk of operation and eco-friendliness favour the use of passive thermal regulation techniques for PV modules [37]. In our previous work, a simple passive cooling with cotton wick structures was proposed [35] to control the temperature of PV module. Therefore, in the present work, a cooling system with aluminium heat spreader in combination with cotton wick structures is proposed to surmount the problem of heating of heat spreaders during high ambient conditions. With this research motivation, experimental investigations were performed at Tiruchirappalli, India (78.6°E & 10.8°N) with a 25Wp PV module.
Section snippets
Experimental setup
The experimental setup consisted of two flat PV modules of same peak power. One PV test module is used as reference module while the other PV test module is equipped with cooling system which comprised indigenously fabricated aluminium heat spreaders, cotton wicks and headers is used to investigate the effect of cooling of PV. A digital multi-metre is used for measuring the current and voltage which are used to find the electrical power output of the module. K type thermocouples with a six
Data reduction
The simplest explicit equation that links the operating temperature of a PV module (Tm) with the ambient temperature (Ta) and incident solar irradiation (G(t)) is given by
In the above linear expression, k is a dimensional parameter known as the Ross coefficient whose value depends on the type of mounting structure and the air gap behind the PV modules [38], [39]. The Ross coefficient was estimated with the help of the following equation obtained by the method of regression analysis.
Results and discussion
In this section, discussions on the results of the confirmatory test, thermal and electrical characteristics of cooled and un-cooled PV modules are presented.
Conclusions
A simple passive cooling system with heat spreader and cotton wick structures was developed for controlling the temperature of flat PV module. The following were the conclusions drawn from the present work.
- (i)
Preliminary tests were conducted to test the proper functioning of PV panel and the capillary action of wick. The proper working of PV module was validated using Ross coefficient while the wettability of cotton wick structures was validated with red tinted water by visual inspection.
- (ii)
The
Acknowledgements
The authors would like to thank the Center for Technology Development and Transfer (CTDT), Anna University, Chennai, India for funding this research work (Lr.No.AU/ROT/BIT/R&D/YFP/MECH/2013-14/001) under Research Support Scheme for young faculty members. Authors also thank the editor and anonymous reviewers for their valuable comments to improve the quality of the manuscript.
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