Techno-economic analysis of the partial evaporation organic Rankine cycle systems for geothermal applications

The organic Rankine cycle technology is an efficient and cost-effective way to utilise low-grade heat for power generation. Especially when the investment costs for the extraction of the heat source are high, it is of crucial importance to increase the power production in order to maximize the revenues and thus ensure a reasonable pay-back time. Besides the classical sub-critical cycles, various cycle architectures have been investigated to improve the performance of organic Rankine cycle systems. Among these, the partial evaporation organic Rankine cycle architecture is considered particularly promising because it provides a better match with the heat source temperature profile during heat exchange, resulting in a greater utilization of the available energy of the heat source. Previous studies have reported an increase in exergy efficiency of the system by up to 60 % when considering partial evaporation instead of complete evaporation of the organic fluid, with the greatest advantages occurring for low temperature heat sources. The use of partial evaporation also results in a more voluminous and costly organic Rankine cycle system. However, there are no previous analyses that address the economic feasibility of partial evaporation organic Rankine cycle systems. This paper presents a technoeconomic analysis of partial evaporation cycles, with particular focus on low-grade geothermal heat sources. Partial evaporation and sub-critical organic Rankine cycle systems were optimized for different heat source conditions and their techno-economic performances were compared. The results suggest that partial evaporation cycle systems are able to provide an increase in net power output between 20 % and 60 % compared with sub-critical cycles for the considered heat source temperatures, but also require higher specific investment costs (up to 75 %) due to the increase in heat exchange area and number of stages of the expander. Nevertheless, due to the small contribution of the power system to the cost of the whole plant and due to the high equivalent full load hours, the results of this paper indicate that adopting partial evaporation can reduce the levelized cost of electricity up to 12 % for geothermal plants.