A sub-system design comparison of renewable energy based multi-generation systems: A key review along with illustrative energetic and exergetic analyses of a geothermal energy based system

Abstract

In recent years it has gained a significance importance to decrease ecological footprint so to have a positive contribution to environmental quality. Renewable energy plays a significant role in future trends for efficiency and environmental issues. Efficiency of systems is to be increased to meet future demands in terms of sustainability with increased consumption. There is a need for new and innovative research studies on promising technologies, processes and strategies to have sustainable water and energy management in cities. This review paper investigates the use of energy sources in single or multiple forms with different system designs from simple to complex structures. The examined studies were presented in a tabulated form including energy and exergy efficiency values as well as the methods and tools used. The results indicated that the systems with the highest energy and exergy efficiency values had several useful outputs, such as hydrogen, fresh water, drying, heating and cooling etc. The maximum exergetic efficiency was determined to be 94% for a multiple energy sourced multi-generation system while the maximum energetic efficiency was calculated to be 111.3% for a solar assisted and heat pump integrated multi-generation system amongst the reviewed studies. To illustrate the obtained results from review, a geothermal energy based multi-generation system was considered. Some parametric studies were also undertaken to see the effects of geothermal water temperature and flow rates on efficiency values. For a single production case, they were determined to be 13.7% and 50%, respectively while they were obtained to be 98.6% and 67.7% for a multi-generation system, in which electricity, low grade hot water production, high temperature hot water production and hydrogen were useful outputs. However, it should be noted that there was a threshold about the number of products to have an optimum increase in the efficiency values.