Development of a geothermal-flash organic Rankine cycle-based combined system with solar heat upgrade

Abstract

In this paper, a new concept of clean energy harvesting from solar energy source is explored in which a fluctuating nature of solar energy source is enhanced with a geothermal energy source. Due to solar radiation's diffuse and fluctuating character, an expensive thermal storage system is required in a solar-only system. On the other hand, due to the relatively low-temperature level of geothermal brine, the flash Rankine power plant has low energy efficiency. The method used in the present study is system integration in which solar thermal energy enhances brine temperature and superheats steam for improved power generation efficiency. Synergistically, the geothermal power cycle eliminates the need for a solar thermal energy storage system, and thus, it reduces capital costs. A geothermal flash Rankine cycle is considered, which is cascaded with an organic Rankine cycle and heat recovery unit for cogeneration. Three types of organic Rankine cycles are investigated with various working fluids for regular, retrograde and transcritical configurations. The transcritical configuration, operating with Perfluorocarbon (C3F8), is found the best working fluid. The maximum exergy destruction in the organic Rankine cycle takes place in the turbine with 48%, whereas at the source side heat exchanger, this is 30%. After flashing, the vapour quality influences power generation efficiency, which shows a maximum vapour quality of around 10% for the case studied. Nevertheless, the maximization of cogenerated exergy rate happens at higher flashing vapour quality than the maximization of exergy efficiency for overall cogeneration application. The exergy efficiency for cogeneration is better during nighttime, for instance, 68% versus noontime 40%, because with solar energy input, there is large exergy destruction at solar collectors. However, during the peak solar radiation at noontime, the cogenerated exergy is more than double that of nighttime, showing an enhanced power generation. This concludes that the proposed integration is effective and creates a higher revenue stream potential than the two technologies working independently.