LITERATURE REVIEW

Literature Review

Literature Review

Introduction

Using the freely available solar energy is an extremely attractive option for low end application like domesticwaterheating, especially in India which receives a mean daily solar radiation in the range of 5-7 kWh/m2 with more than 275 sunny days in a year. Solar waterheating is cost effective for India and can reduce India's demand for oil, gas and coal if pursued to meet the hotwater demand in industry and households. Solar hot water systems (SHWS) are devices that use solar energy to heat water. The glass cover of the Collector ensures that radiation is received by the absorber but very little is lost by it (since glass is opaque to the far infrared radiation emitted by a warm body, the principle of the greenhouse).

Fig. 1: Line diagram of a typical DSHWS

Fig. 2: Cut section of a typical flat plate collector used in DSHWS

The performance of the whole system depends on the choice of Materials, the important factors being the thermal conductance of the absorber tubes and fins, the efficacy of the bonding between tubes and fins, the quality of the insulation, and the corrosion resistance of the entire system, bearing in mind that some parts are exposed to water (of variable purity) at moderately high temperatures and most of the others have to withstand sun and humid air. It is relevant to point out here that water supply in India, even in urban areas, is far from free of suspended matter, even grit. Besides, it can, especially if it is taken from an open well or tube well, contain a significant proportion of dissolved salts. Use of this hard water leads to reduction in system performance due to the scales formations in the water passages. Such issues lead to the use of complicated designs having a heat transfer liquid flowing through the collectors which transfers the heat received to the water in the tank via a heat exchanger provided for this purpose. (Pohekar and Ramachandran 2004 1449-1460)

Overview of DSHWS technology evaluations and developments

Even though, use of solar energy for waterheating and other low end applications is centuries old, the SHWS in the present form was designed, developed and investigated. The basic unit of SHWS, the flat plate collector, was initially investigated by Hottel and Woertz , and Bliss . The performance of the collector in terms of design parameters related to, types of absorber plate were investigated by Mathur et al. and Patil ; number and type of glass covers was studied by Whillier ; Thickness and type of insulation was studied by Whillier and Saluja ; anti-reflective coating on glass cover was studied by Hsieh and Coldewey ; heat mirror coating on inner glass as an alternative for selective absorber was investigated by Winegarner ; spacing between absorber and inner glass and successive Glazings were studied by Nahar and Garg ; studies on coatings on absorber plates were taken up by Nahar and Garg . While some of these works concentrated on performance evaluation studies also, there ...