Review of Past Geothermal Energy Return on Investment Analyses

Abstract

Energy Return On Investment (EROI) is an important figure of merit for assessing the viability of energy alternatives. EROI analyses of geothermal energy are either out of date or presented online with little supporting documentation. Often comparisons of energy systems inappropriately use “efficiency” when EROI would be more appropriate. For geothermal electric power generation, EROI is determined by the electric energy delivered to the consumer compared to the energy consumed to build, operate, and decommission the facility. Determination of EROI depends upon several factors. First is the input energy embodied into the system – the available energy needed to make, operate, and decommission a product or system. Two approaches that are used in determining embodied energy, Input/Output Analysis and Process Energy Analysis, are reviewed. Also, critical to determining EROI is the system boundaries and value of the energy – heat is not as valuable as electrical energy. The concept of closing the loop is a useful way of assessing if boundaries and the value of energy have been addressed adequately. The methodology and results of past geothermal EROI studies are reviewed and issues or problems conducting and interpreting EROI analyses are discussed. The validity of past geothermal EROI estimates is investigated by spot checking the major energy inputs into constructing a geothermal wellfield. A preliminary, update to EROI for geothermal power production is presented and applied to understanding future of geothermal development.