Investigation into discontinuous low temperature waste heat utilisation from a renewable power plant in rural India for absorption refrigeration

This research focusses on utilising low temperature waste heat from a rural renewable power plant for absorption refrigeration. It forms part of a collaborative "Bridging the Urban Rural Divide" (BURD) research group across the United Kingdom and India investigating rural sustainable development through the provision of renewable electricity. The group is tasked with improving the educational environment and healthcare of a 45 household community (which is part of a larger village) in West Bengal, India. Working in collaboration with the Indian Institute of Technology Bombay as part of this thesis, a projected daily electrical demand for the community of 55 kW∙h per day was calculated, providing: lighting, fans and an electrical device charging station. To allow in excess of the daily electrical demand as well as for system ancillaries at 12 kW∙h, solar trackers at 14 kW∙h and 7 kW∙h for hydrogen production, a power plant producing 90 kW∙h was specified. This included daily electricity production of 70 kW∙h during the daytime from solar via a 10 kW concentrated photovoltaic (CPV) system and 20 kW∙h in the evening from a 5 kW biogas and hydrogen internal combustion engine electrical generator (genset). The biogas is produced from anaerobic digestion of food waste and aquatic weeds, and the hydrogen is produced from the electrolysis of water in an electrolyser powered by excess solar power. An energy and exergy analysis identified the daily quantity and quality of recoverable waste heat sources at 25°C. These are the CPV with an energetic value of 109 kW∙h and an exergetic value of 32 kW∙h at 60°C and the genset radiator with an energetic value of 32 kW∙h and an exergetic value of 5 kW∙h at 80°C. The exhaust heat from the genset has been allocated for other uses and, though calculated, is outside the scope of this research. The thesis then focusses on using these low temperature waste heat sources for absorption refrigeration. The working fluids selected are acetone and zinc bromide as these had been proven in the literature to operate at temperatures below those of the expected waste heat sources without the need for rectification (the process of separating two fluid vapours from each other). Due to the local climate with high ambient temperatures, averaging 24°C to 35°C, and the relatively low waste heat source temperatures, a number of configurations of absorption refrigerator were investigated to achieve lower, and therefore more versatile, evaporator temperatures. Some of these involve utilising some of the cooling produced from either or both of the heat sources to cool the absorber and condenser. The findings were that the most energy effective way of providing low evaporator temperatures was to use a small (2%) difference in weak and strong solution concentrations and not use a proportion of the cooling generated for the absorber or condenser. By operating two independent refrigerators powered by each heat source independently, the solution concentrations could be optimised to provide the lowest possible evaporator temperatures at a given ambient temperature. At the 25°C reference ambient temperature used for the energy and exergy analysis, the CPV waste heat can provide 33.4 kW∙h of continuous cooling per day at 6°C and the genset radiator 6.3 kW∙h at 0°C. This cooling energy collectively is sufficient to replace 12.7 kW∙h of electricity that would have been used to power a vapour compression refrigerator to provide the same amount of cooling, which is equal to 22% of the electrical power provided to the village. The genset waste heat source used for absorption refrigeration can provide cooling for food and medicine storage equivalent to 6 to 8 domestic refrigerators. The CPV waste heat source can provide space cooling for a room in a health centre for 6 to 9 hours per day. The investigations within this thesis highlighted the need for intelligent control systems to optimise the availability and temperatures of the refrigerators during unfavourable ambient conditions.