Synthesis/Design Optimization of Organic Rankine Cycles for Low Temperature Geothermal Sources with the HEATSEP Method

Organic Rankine cycles are now a mature technology for the conversion of low temperature geothermal sources into electricity. In this paper a synthesis/design optimization of binary cycle power plants with a pure thermodynamic objective is discussed. According to the HEATSEP method, a “basic” plant configuration is first defined including all the components that are strictly needed to realize the “concept” of the plant itself and are different from heat exchangers, while different matching between hot and cold thermal streams within the plant are allowed for the same conditions at the boundary of the heat transfer section. Different structural options for the heat transfer section can then be obtained using the same basic plant model. Two organic fluids (isobutane and R134a) and different brine input temperatures are considered here and both super- and subcritical solutions are taken into account. Although the optimization problem is quite simple, the results show that the objective function (exergy recovery efficiency) is a non-smooth surface due to the feasibility constraint applied to the undefined heat transfer section. This surface is drawn to show not only the optimum solutions but also the suboptimal ones, which could be of interest for further evaluations with objectives of different nature.