Life Cycle Assessment of High Temperature Geothermal Energy Systems
Mathilde MARCHAND, Isabelle BLANC, Aline MARQUAND, Antoine BEYLOT, Sophie BEZELGUES-COURTADE, Hervé TRAINEAU
[MINES ParisTech, France]
European and French regulations state that 50% of the energy mix in the French Caribbean should be sourced from renewable energies by 2020. Because of the volcanic conditions of the French Caribbean islands, geothermal energy would seem to be a very favorable solution to reach this ambitious objective, as, unlike other renewable sources, it is continuous and weather independent. According to the Intergovernmental Panel on Climate Change (IPCC), geothermal energy source is recognized as a competitive energy source (with a carbon footprint around 50 gCO2eq/kWh over its lifetime) compared to conventional energies such as coal or oil (with a carbon footprint around 800 g CO2 eq/kWh). The IPCC make their overall environmental assessments of energy pathways using Life Cycle Assessment (LCA). LCA assesses the environmental and human health impacts throughout the life cycle stages of a product by providing a “cradle-to-grave” environmental profile. A LCA of an existing high temperature geothermal system is reported here with two objectives: quantifying the environmental impacts of a geothermal plant installed in the French Caribbean islands, and comparing and identifying technological alternatives which potentially reduce its environmental impacts. The geothermal power plant assessed in this study is Bouillante geothermal power plant located in the Guadeloupe island. Built in the 80s, Bouillante is a high temperature geothermal system (the reservoir temperature is around 250°C) which is representative, in terms of size, spatial and technological constraints, of future power plants to be developed in French overseas territories. Its medium size (15.75 MW) enables it to supply 6 to 7% of Guadeloupe’s annual electricity needs. It has two production units: UB1, a double flash technology (4.75 MW), and UB2, a simple flash technology (11 MW). The data inventory is mainly based on site-specific data, extracted from drilling reports: annual environmental and exploitation reports, and technical sheets completed with personal communication with experts. This power plant however presents some unusual design configurations related to the age of its construction: use of a sea water cooling system and absence of geothermal fluid reinjection. To model a configuration that fits better with current practices, two new scenarios based on alternative technologies are considered: a cooling tower or air dry cooling condensers. Three scenarios are assessed via a multicriteria approach using a selection of life cycle environmental indicators: climate change, water consumption, eutrophication, land use, ecotoxicity, primary energy demand, abiotic depletion, acidification and human toxicity. These environmental indicators are assessed at all phases of the plant life cycle: drilling, construction and installation of the surface equipment, operation, and end of life (decommissioning). First results show that greenhouse gases (GHG) are mostly generated at the operation step (around 90% of total GHG) and are mainly due to leakage of CO2 and CH4 emissions (a geothermal stream is composed of non-condensable gases fraction such as CO2, and CH4 which are emitted due to the decrease in pressure). Results range from 38 to 47 gCO2eq/kWh over the 3 scenarios. Primary energy demand is mainly due to the construction and installation phase (around 70% of total energy consumption) from background processes such as steel or copper production processes. The primary energy demand and GHG for the reference scenario and the cooling tower system alternatives are found to be lower than those for the aerocondenser cooling system scenario (for the same energy production). As an outcome of the study, we establish the development of a general parameterized LCA model developed for conventional geothermal systems with a temperature reservoir ranging from 230°C to 300°C. Results obtained from this model enable high temperature geothermal systems to be positioned from an environmental perspective in comparison with other energy systems, and also highlight the main drivers leading to the reduction of environmental impacts of future geothermal systems.
|        Topic: Environmental and Societal Aspects||Paper Number: 02028|