Design and off-design performance of a low-temperature geothermal power plant

In this work, we investigate the effect of assumptions made in the design stage of a geothermal power plant on its real off-design performance. We focus on the influence of the fluctuating environmental conditions and the changing electricity price. The results are obtained using a two-step optimization methodology. In the first step, we calculate the optimal design of a geothermally-fed air-cooled ORC, corresponding to the assumed (and constant) values for the environmental temperature and the electricity price. The net present value is the optimization objective since it takes into account the thermodynamic performance, the economics and the time value of money. In reality, the designed power plant often operates in off-design conditions. The plant is already installed so the design is fixed, however the electricity prices and environmental conditions are different from the assumed values for which the power plant was designed. In the second step, we maximize the net electrical power output for the real environmental temperature at every moment. If we calculate the net present value in a post-processing step, obviously, this value is different than that of the design step and the difference might be significant. This emphasizes the importance of well-considered parameter assumptions in the design stage. The results show the sensitivity of the electrical power output towards changing environmental conditions for a given ORC design. Furthermore, we indicate the optimal ORC design accounting for its off-design operation. Real hourly data for the electricity prices and environmental conditions have been used.status: accepte