Engineered Geothermal Systems: Energy Return on Energy Investment

Abstract

Energy Return On Investment (EROI) is an important figure of merit for assessing the viability of energy alternatives. Too often energy systems are compared using “efficiency” when EROI would be more appropriate. For geothermal electric power generation, EROI is determined by the electricity delivered to the consumer compared to the energy consumed to construct, operate, and decommission the facility. Critical factors in determining the EROI of Engineered Geothermal Systems (EGS1) are examined in this work, specifically the energy embodied into the system. Embodied energy includes the energy contained in the materials, as well as, that consumed in each stage of manufacturing from mining the raw materials to assembling the finished system. Also critical are the system boundaries and value of the energy – heat is not as valuable as electrical energy. The EROI of an EGS depends upon a number of factors that are currently unknown, for example what will be typical EGS well productivity, as well as, reservoir depth, temperature, and temperature decline rate. Thus the approach developed is to consider these factors as parameters. Since the energy needed to construct a geothermal well is a function of depth, results are provided as a function of depth. Parametric determination of EGS EROI is calculated using existing information on EGS and US Department of Energy (DOE) targets and is compared to the “minimum” EROI an energy production system should have to be an asset rather than a liability.