Defining Optimal Configurations of Geothermal Systems Using Process Design and Process Integration Techniques

The use of geothermal resources for the polygeneration of energy services has recently gained interest and is expected to know an important development in the future. Major research questions concern the increase of the efficiency in the usage of geothermal resources, as well as the increase of their economical profitability and the minimization of the generated life-cycle environmental impacts. This can be achieved by applying process design and process integration techniques to the overall geothermal system, formed by the subsystems of resources harvesting, energy conversion systems and demand. This paper presents a systematic methodology for the optimal design of geothermal systems configurations. In a first step, the different components of the system superstructure are separately modeled using a flowsheeting software. The superstructure includes not only the potential conversion technologies, but as well the different types of potential resources and the demand profiles for the district heating and cooling. It covers a wide pannel of conventional resources and technologies like deep and shallow aquifers, heat pumps, flash systems or organic Rankine cycles, as well as emerging resources and technologies, like Enhanced Geothermal Systems, Kalina cycles, supercritical cycles or organic Rankine cycles using fluid mixtures. Then, resources, technologies and demand profiles models are integrated together using process integration techniques. The resolution of the mixed integer linear programming problem extracts the configuration of the geothermal system from the superstructure. The state variables of the final configuration are then used to define the size of the equipment in the system. Based on these results, the economic, energetic and environmental performances of the integrated system are finally calculated. Performance indicators considered include energy and exergy efficiency, investment costs, operating costs and profitability. Life cycle impact assessment is as well performed by fully integrating the life cycle inventory in the process design framework. To account for the seasonal variation of the demand in energy services, a multi-period optimization model of the integrated system is proposed. The calculation sequence is implemented in a multi-objective optimization framework to calculate the optimal system configurations with their associated technologies, resources and process operating conditions, choosing the objective functions in the different considered performance indicators. This results into the thermo-economic Pareto Frontier that includes the set of the most attractive configurations of the system. The methodology is illustrated by an application to a case study of a city of Switzerland. The results are discussed, as well as their practical implications in terms of technological orientations to be favored in the development of geothermal energy.