Energy and exergy analysis of a hybrid organic cycle power plant

Organic Rankine cycles are highly recommended technology to generate electricity from low temperature sources such as geothermal sources. The performance of the cycle mostly depends on operation conditions, installed equipment and selection of working fluid. This paper presents a comparative energy and exergy analysis of a geothermal-biomass organic Rankine cycle for three working fluids: R113, R245fa, and R600a. In addition, analyzed organic Rankine cycle is compared with the Rankine cycle that runs on Serbian lignite from the aspect of equivalent CO2 emission and necessary primary energy. The analyzed system is an organic Rankine cycle power plant with two heat sources, geothermal water from Vranjska Banja and wood biomass. The results revealed the highest thermal and exergy efficiency with R113, 14.53%, and 41.64%, respectively, as well as the turbine power output of 4.21 MW with 73.20 kW pump power input. Exergy analysis showed the highest exergy destruction rate in the heat exchanger with flue gases made by wood combustion (80%-89%) and the highest obtained exergy efficiency of 41.64% in the case of R113. If the power plant used Serbian lignite as a fuel for the same thermal power input, it would need between 133-169 kt per year of lignite, which would result in CO2 emission of around 63-80 kt per year.