Design of Geothermal Energy Conversion Systems with a Life Cycle Assessment Perspective

Abstract

The use of enhanced geothermal systems (EGS) for the cogeneration of electricity and district heating has recently gained interest, and is expected to know an important development in the future. Major research questions on the design of the energy conversion system concern the increase of the efficiency in the usage of geothermal resources, as well as the increase of their economical profitability. The quantification and the minimization of the generated life-cycle environmental impacts is as well a key point for the public acceptance of geothermal energy and for the choice of both the conversion technologies and the building depth of EGS by the engineers. This paper presents a systematic methodology for the optimal design and configuration of geothermal systems considering environomic criteria. In a first time, the different components of the system superstructure, including conversion technologies, energy services demand profiles and exploitable deep geothermal resources are separately modeled using flowsheeting software. The conversion technologies for cogeneration include single and double flash systems, organic Rankine cycles with different designs and working fluids, and Kalina cycles. In a second step, the system is designed by integrating together the resources, technologies and demand profiles models, using process integration techniques. The overall configuration of the geothermal system is hence extracted from the superstructure. Finally, the performance of the integrated system is 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, which allows to calculate the environmental impacts representative of the system configuration, and not only an indicative value for the average technology. The life cycle inventory includes the construction phase of the geothermal systems, its operation, including the emissions from the resource or from the cycle, as well as the substituted heat and power to account for the system efficiency, and its decommissioning. The overall sequence is implemented in a multi-objective optimization framework to calculate the environomic optimal system configurations, choosing the objective functions in the different considered performance indicators. The methodology is used to determine the optimal ratio between heat and power production from an EGS constructed at a potential varying depth, considering the use of the different cogeneration cycles embedded in the superstructure. The multi-objective optimization is performed both for economic and environmental objectives. The effects of including the environmental dimension in the system design procedure are discussed.