Optimal solar thermal retrofit for geothermal power systems considering the lifetime brine degradation

Geothermal power plants usually suffer from performance deterioration during the operation due to the geothermal resource's degradation. Combining solar thermal energy to retrofit the system is considered an effective way to alleviate this problem. This study explores the off-design performance of organic Rankine cycle-based hybrid system throughout a 30-year life span, rather than a typical single year as the previous research. Then several interesting questions are consequently discussed: (1) Which year is most suitable for system retrofit? (2) How many times should the system be retrofitted? (3) How much is the difference between the 30-year-based results and the conventional single-year-based analysis? Results indicate that the single-year-based approach could greatly underestimate the Levelized cost of electricity (LCOE) by up to 18% and also significantly overestimate the increment in electricity generation (E_tot). Thus a life-span-based approach is highly recommended for geothermal-based hybrid plants. Besides, the retrofit year has huge impacts on both thermodynamic and economic performance, and the optimal retrofit should be carried out in 6th year. Moreover, this study reveals that multiple retrofits can indeed further increase the E_tot but are not cost-effective. Therefore, a single solar thermal retrofit is recommended for existing geothermal power systems rather than multiple retrofits.