09 ChCHAPTER - 2 Literature Reviewapter 2

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energy and exergy analyses of different renewable energy systems
  15 CHAPTER - 2 Literature Review 2.1. Introduction  An extensive review of the literature has been done on energy and exergy analyses of different renewable energy systems in the present chapter. The literature review on the exergoeconomics has also been done as one of the objectives of the thesis is to evaluate the renewable energy systems on the basis of available literature on exergoeconomic. The main idea was to get thrust and scant area of exergy analysis of Renewable energy systems and have possible future direction of research. The literature review has been classified as under: a) Solar heating systems - It has been classified in two different categories as below: i. Solar air heater ii. Solar water heater b) Solar photovoltaic (SPV) systems c) Renewable energy cooking devices - It has also been classified in two different categories as below: i. Solar cookers ii. Biomass cook stoves d) Exergoeconomics 2.2. Solar Heating Systems This section deals with the literature survey on solar heating systems viz. solar air heater and solar water heater for useful applications.  16 2.2.1. Solar Air Heater For commercial applications, ability of the drier to process continuously is very important to dry the end products for their safe storage to maintain the quality and nutrient values of the product. Normally thermal storage systems are employed to store thermal energy, which includes sensible heat storage, chemical energy storage and latent heat storage. The solar drier is an energy efficient option in the drying processes [1]. Use of forced convection solar driers seems to be an advantage compared to traditional methods and improves the quality of the product considerably [2-4]. The common sensible heat storage materials used to store sensible heat are water, gravel bed, sand, clay, concrete, etc. for different applications [3-6]. In recent years, few authors [7-13] have studied different feature of solar collector systems using various approaches. For example, Kurtbas and Durmus [7] have studied the solar air heater for different heating purposes whereas, Luminosua and Farab [8] and Torres-Reyes et al, [9] have studied the optimal thermal energy conversion and design of a flat plate solar collector using exergy analysis. On the other hand, Bakos et al [10], Kaushik et al. [11], and Tyagi et al [12] have studied the optimum design of a parabolic trough collector (PTC) and gave some fruitful results, especially, the mass flow rate of the moving fluid and the concentration ratio of PTC collector. Ozturk and Demirel [13] experimentaly investigated the thermal performance of a solar air heater having its flow channel packed with Raschig rings based on the energy and exergy analyses. Average daily net energy and exergy efficiencies were found to be 17.51 and 0.91%, respectively. Also, the energy and exergy efficiencies  17 of the packed-bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Potdukhe and Thombre [14] designed, fabricated, simulated and also tested a solar dryer fitted with a novel design of absorber having inbuilt thermal storage capabilities. The length of operation of the solar air heater and the efficiency of the dryer were increased, and better quality of agricultural products in terms of colour value was obtained compared with open sun drying. MacPhee and Dincer [15] worked on thermodynamic analyses of the process of charging of an encapsulated ice thermal energy storage device (ITES) through heat transfer. The energy efficiencies are found to be more than 99%, whereas the thermal exergy efficiencies are found to vary between 40% and 93% for viable charging times. The results confirm the fact that energy analyses, and even thermal exergy analyses, may lead to some unrealistic efficiency values. Enibe [16] worked on the design, fabrication and performance evaluation of a passive solar powered air heating system based on exergy analysis. The system consists of a single-glazed flat plate solar collector integrated with a phase change material (PCM) heat storage system. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The spaces between the module pairs serve as the air heating channels, the channels being connected to common air inlet and discharge headers. The experiments were carried out under the climatic conditions of Nsukka (Nigeria) in the daytime with no-load conditions where the ambient temperature varied in the range of 19  – 41 °C, and a daily global irradiation varied in the range of 4.9  – 19.9 MJ/m 2 . Peak cumulative useful efficiency was found to be about 50% while peak temperature rise of the heated air was about 15 °C. The system has been found suitable for the use as a solar cabinet crop dryer for aromatic  18 herbs, medicinal plants and other crops, which do not require direct exposure to sunlight. Kurtbas and Durmus [17] designed a new solar air heater and evaluated it on the basis of exergy analysis. In their study they used five solar collectors with dimensions of 0.9x0.4 m and the flow line increased where it had narrowed and expanded geometrically in shape. These collectors were set to four different cases with dimensions of 1x2 m. Therefore, heating fluids exit the solar collector after at least 4.5 m displacement. According to the collector geometry, turbulence occurs in fluid flow and in this way heat transfer is increased. In this study they found that the efficiency of the collector enhances with the increase of mass flow rates due to an enhanced heat transfer to the air flow and also increase in efficiency depends on the surface geometry of the collector and extension of the air flow line. Collector efficiency, temperature difference of the air and pressure loss is the more important parameters in order to decrease the exergy loss.  Ajam et al. [18] worked on the optimization of the solar air heater based on the exergetic analysis. For this purpose, an integrated mathematical model of thermal and optical performance of the solar heater has been derived. The overall thermal loss coefficient and other heat transfer coefficients of the heater were assumed to be variable while deriving an equation for the exergy efficiency. Using the MATLAB toolbox the exergy efficiency equation has been maximized. After maximizing the exergy efficiency equation it has been compared with the thermal efficiency of the heater, which ultimately results in an extraordinary increase of the exergy efficiency according to the optimized parameters. They also concluded that the exergy analysis was a better method for design, development and optimization of solar air heaters due to the fact that exergy efficiency is a proportion to common quantities in solar
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