The principle for the energetic use of EGS (enhanced geothermal system) reservoirs is based on the production of a fluid which, carries the geothermal heat, in a production well, the extraction of heat from the fluid on the surface with a heat exchanger, and the reinjection of the cooled fluid into the reservoir in an injection well. The extracted heat can be used for heat, power, or chill provision (Figure 6.1). The different technical aspects and constraints which are related to such plants are the subject of this chapter. The following sections are designed to give a general overview on the energetic utilization options and EGS plant design. The focus will be on typical EGS applications which use formation water as heat carrier, in a temperature range between 100 to about 200 • C.Other EGS plant concepts which are researched and might be realized at a few sites in the future are not discussed. Such futuristic concepts refer to EGS plants that can assess steam reservoirs and geo-pressured reservoirs or EGS plants using heat carriers other than formation water, such as CO 2 .
Utilization OptionsIn the following section, the different options for energetic use of EGS reservoirs are outlined with focus on the most important thermodynamic aspects. More details on thermodynamic aspects can be obtained from the literature, for example, Ç engel and Boles (2006) and Dinçer and Rosen (2007). The goal of this section is to characterize the different utilization options based on energy efficiency and exergy efficiency considerations.
Energetic ConsiderationsThe form of energy which can be supplied by any heat source mainly depends on the temperature level. For EGS plants, this is the temperature of the reservoir or, more precisely, the temperature of the produced geothermal fluid. Different energy Geothermal Energy Systems. Edited by Ernst Huenges