Elaboration and Discussion of Simplified Parameterized Models for Carbon Footprint of Enhanced Geothermal Systems
Résumé
The development of "enhanced geothermal systems" (EGS), aiming at extracting energy from deep low-enthalpy reservoirs, is attracting attention as a promising solution for the development of the geothermal sector in new areas. For the promotion of such renewable energy (RE) based technology, it is important to assess its environmental performances accounting for all phases of the life of the plant, from its construction to its dismantling. Life Cycle Assessment (LCA) is a useful tool to perform a study with such perspective. However, the application of this cradle-to-grave approach is complex and time-demanding. To overcome such drawback of traditional LCAs, this study presents and discusses a methodology to generate simplified models for the estimate of life-cycle greenhouse gases (GHG) emissions of EGS, applicable to a large sample of configurations. An explicit detailed inventory of all input and outputs of materials and energy flows over the lifecycle of the system is generated, based on current EGS projects in central Europe. A parameterized model, called the reference model, is then established to estimate the life cycle GHG emissions from a set of parameters able to characterize current EGS configurations (size of the equipment and materials involved for example). Applying a Global Sensitivity Analysis (GSA) to this reference model allows identifying the key variables explaining most of the variability of GHG results over the considered range of input parameters. Simplified models are then established, enabling to estimate the GHG emissions of EGS as the only function of these key variables. Several simplified models are here proposed, depending on the level of reduction applied to the reference parameterized model and are compared. The results issued from our simplified models, thus avoiding the extensive application of the LCA methodology, are coherent with literature. GHG emissions of EGS are comparable to those of other RE technologies and significantly lower than those of fossil fuel-based plants. The level of simplification of the parameterized models is discussed according to the availability of input data and the level of simplicity required by the stakeholders of the EGS sector and by the decision makers. These new models contribute to the debate on the mitigation of the environmental effects caused by our current electricity mix by highlighting the environmental benefit of introducing a larger share of geothermal energy.
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