ECONOMIC OPTIMIZATION OF A COMBINED HEAT AND POWER PLANT: HEAT VS ELECTRICITY
This contribution presents the economical optimization of the parallel repartition between electric and heat production for geothermal application. The 350 m3/h flow of geothermal fluid, assimilated to liquid water at 185 °C, is then separated in two streams. Its reinjection temperature is fixed at 70 °C. An Organic Rankine Cycle (ORC) system is used to convert a part of geothermal energy into electricity. The refrigerant chosen is the R245fa. The different components of the ORC are sized in order to calculate the installation cost that depends on one characteristic dimension of each item (exchange surface for heat exchangers and power for the turbine and pumps). The operating cost is proportional to the installation cost. In this contribution, since we do not consider the detailed structural optimization of the District Heating Network (DHN), its investment cost is proportional to the supplied heat. The selling price of the electrical net power is a function of the recovered heat by the network. A Mixed Integer Non-Linear Programming (MINLP) optimization is performed using the GAMS® software. The problem is solved in order to determine the maximal profit of the global system. In this problem, the main optimization variables are the flow, temperature and pressure of refrigerant in ORC, the size and cost of equipment, the repartition between heat and electricity, and the structure of the DHN. Results show that it is preferable to produce electricity alone but this is dependent on the choice of the price of sale of heat by the owner. The sell price from which it is more profitable to produce and to sell the heat is determined for each case. The optimization for each case shows that it is not easy to predict the final results and it justifies the use of optimization.