Combined cooling and power generation system using waste heat

Abstract

Economic development, productivity and competitiveness suffers or is stunted in many industries, in remote regions and many rural parts for want of small amounts of assured local electricity availability 24 x 7, either due to inadequate grid-coverage or an unreliable supply. A number of renewable energy schemes are being encouraged by central and state governments. Those schemes which are dependent on carbon-credits, or on continuation of presently available generous subsidies, or are secured through "political leverage" or "patronage", or rely on licensed imported technology, or without assured long-term off-take are all doubtful propositions commercially. Furthermore, small schemes do not appear to be attractive unless it can be demonstrated clearly that the LCOE (levellized cost of electricity) and the PP (payback-period) are attractive, irrespective of the conversion efficiency of heat into electricity. The lowtemperature ORC (Organic Rankine Cycle) scheme proposed herein meets all these criteria, is cost-effective and can be developed indigenously. The ORC plants are small, modular and scalable. They can be easily transported, assembled and commissioned at site rapidly, in small industrial units or incorporated into "micro-grids" for remote isolated areas. The heat-energy converter of the ORC plant is a hermetically sealed unit with very few moving parts. The technology is available now and proven. The plant requires no operator, the maintenance cost is negligible over long periods, and the unit can be operated and monitored remotely. The present chapter describes two ORC arrangements which were modelled for two types of heat sources in MATLAB software and Genetic Algorithm, which were used for parametric optimization to maximize the thermal efficiency for eight different working fluids. The novel objective of the study is to relate thermal efficiency and other parameters of the cycle with thermo physical properties of the working fluids for better selection. For more accurate results, isentropic efficiency of the scroll expander used was not fixed; instead a regression model was used to evaluate the efficiency with pressure ratio variation with fixed built in volume ratio. It was found that higher the critical pressure and smaller the chain length of the refrigerant, higher would be the thermal efficiency of the working fluid. For geothermal heat source, R11 had the maximum thermal efficiency of 14.9%. And for solar heat source, it was observed that Benzene had the maximum thermal efficiency of 9.6%. It was also observed that aromatic refrigerants like benzene and toluene are good for low temperature heat sources only. The average increment in thermal efficiency from basic ORC to recuperated ORC is 2.5% and 0.5% for geothermal and solar respectively which shows that recuperator is not a big advantage for low temperature heat sources. It was also found that the optimum pressure ratios for these two fluids were 8.23 and 5.03 which are very close to the peak of expander isentropic curve which signifies the importance of expander efficiency in performance analysis. © 2020 by Nova Science Publishers, Inc.