Optimal configuration, design and control of a binary geothermal combined heat-and-power plant

In this paper, a two-step design optimization framework is developed for four low-temperature geothermal combined heat-and-power plant configurations. The economic comparison, including off-design performance, has not been done before. The optimization tool is applied for an existing district heating system and for geothermal and meteorological conditions which are based on the Belgian situation. It is concluded that a combined heat-and-power plant results in an economically profitable project (net present value of 3.46 MEUR), whereas the stand-alone electrical power plant does not (net present value of -3.65 MEUR). Furthermore, the design for the series set-up is optimal, and the best connections during operation are the series and parallel connections for low and high heat demands, respectively. Also, a less detailed (high-level) control optimization model is developed for this series set-up, based on the part-load operating maps which are calculated from the detailed two-step optimization model results. The calculation time is much faster ((similar to)milliseconds) and the errors on the total revenues are smaller than 0.1%. The goal of this high-level model is to optimize the amounts of heat and electricity to produce, so that the plant can be used as a flexibility tool in energy markets driven by price signals for heat and electricity.