STUDY OF A BATCH SOLAR WATER HEATER WITH INTEGRATED COLLECTOR STORAGE SYSTEM AND COMPOUND PARABOLIC REFLECTOR

Abstract

A batch heater is an integrated collector storage unit wherein the absorber serves the dual purpose of a collector and storage unit. The present experimental work is an integration of the older concept of batch water heating with the modern trends in solar water heating technologies i.e. incorporating a concentrator in the design. The concentrator used here is the Compound Parabolic Concentrator (CPC) which is a non-imaging device having wider acceptance angle. It requires only occasional tracking and thus can be best suited for household purposes. The concentrator i.e. the reflector, in this case, is supported on a wooden cradle which comprises the two parabolas of the compound parabolic concentrator. While Batch Solar Water Heater (BSWH) facilitates easy installation, operation, and maintenance besides providing cost reduction it operates at much lower daily collection efficiencies as compared to separate storage tank facilities. The focus of the study is therefore to obtain good heat retention and better long time performance estimates and in accordance, changes have been made in the design in addition to the use of CPC concentrator. In the present work, experimental studies have been carried out and mean collector efficiency is computed on the model with an air gap introduced in the side walls (arms of the CPC). Unlike conventional systems with a large number of smaller diameter tubes, here is a single larger diameter drum which serves both as an absorber and storage unit positioned at the focus of CPC. The system works on thermosyphon principle, unlike conventionally forced circulation domestic water heating systems. The system as a whole operates at a lower temperature which reduces the overall convective and radiative losses and increases useful heat gain. The parametric study of the theoretical design by EES hovering before and after solar noon is in good agreement with experimental results with the thermal efficiency of the collector as high as 38% obtained and with water temperatures varying from 40oC to 60oC depending upon the time of the year. The key aspect is its heat retention capability which can prolong high temperatures attained for longer hours after v

dusk. This model shows better performance compared to a similar model designed and developed but without an air gap. Heat loss tests performed on the collector on a 24-hour cycle period showed good long time performance estimates. Collector characterization parameters are obtained by performing thermal performance tests on the collector, under conditions meeting ASHRAE specifications for outdoor tests. The response time of collector computed and performance characteristic curve plotted to predict system response under any given conditions of solar insolation and ambient temperature. The economic figures of merit of the model are also obtained by f-chart design method and using EES software. The proposed model has low initial cost and better long time thermal performance estimates. The annual solar fraction for the model is 0.55 and the payback period is two years. Comparative studies with similar models show its competency based on collector efficiency and economic viability. Highlights  Passive system employed in comparison to the conventional active systems for household applications.  Introduction of a concentrator in domestic solar water heating system.  No usage of Transparent insulation material (TIM) or Phase change material (PCM), heat retention obtained by introducing an air gap, without adding to the cost of collector.  Better adaptability for metro cities like Delhi, where roof space is a major constraint for installation of flat plate collectors.  Economically viable