Sustainability Factors for Doublets and Conventional Geothermal Systems

Abstract

Renewable capacity is defined as the surface heat discharge rate over the entire anomaly associated with the reservoir and it represents the outflow rate of heat in the natural (pre-production) state whereas the sustainable heat production capacity is the sum of renewable capacity and the maximum heat mining rate. Geothermal reserves are normally expressed in terms of MW capacity sustainable for the life of a project. The sustainability factor is used to define the ratio of the sustainable capacity to the renewable capacity. The literature reports that the sustainability factor is typically an order of magnitude higher than renewable capacity. This paper presents a comparitive discussion on the performances of a conventional type geothermal reservoir system and a sedimentary system used in a doublet. Lumped parameter models are used to model the pressure, temperature, and sustainability behavior of the systems. Simple quantitative expressions are given for assessment of the renewable and sustainable capacities, the heat recovery factor, and the sustainability factor. The renewable capacities of sedimentary systems typically used in doublet operations are lower than the renewable capacities of the conventional geothermal systems. Primarily because of this reason, comparison of the performances of the systems show that the sustainability factor for a doublet system is higher than for the conventional geothermal system.