| Abstract |
Geothermal energy production has one of the lowest life cycle CO2 emission factors (~ 15 g-CO2/kWh) for plants producing electricity (Hondo, 2005). This life cycle factor includes indirect emissions from plant construction as well as ongoing operations such as additional well drilling and equipment replacement. Not included in this calculation are direct emissions from geothermal power production. Geothermal flash plants emit CO2 at rates related to the reservoir concentration. Binary plants are typically assumed to have zero emissions of high global warming potential gases because of their closed loop system design. However, binary plants use working fluids to drive the turbine generators and leakage of these working fluids have not yet been quantified. To assess the working fluid emissions from geothermal power production using an Organic Rankine Cycle, reported isobutane emissions were accessed for 3 California binary plants from the Great Basin Unified and Imperial County Air Pollution Control Districts as well as the California Air Resource Board. Each binary plant used in this study has been operating for more than 25 years. From the data, two types of isobutane emissions were accounted for over a 15 month period: ‘normal’ equipment leakage and equipment breakdown emissions. This preliminary analysis of working fluid emissions from these plants suggest that if a binary plant has equipment failures, the majority of the isobutane releases can be due to these events. The results indicate that a well maintained binary plant releases approximately 0.13 g-Isobutane/kWh where as a plant with equipment failures releases approximately 1 g-Isobutane/kWh. Assuming a greenhouse warming potential of 3 for isobutane, these emission values become 0.4 and 3 g-CO2e/kWh (where CO2e is the CO2 equivalent effect). Binary power plant with significant equipment issues can emit significantly more GHG via equipment breakdown that is emitted through normal leakage of valves and seals. However, even for binary plants that isobutane have fugitive emissions, these emission do not significant increase the life cycle CO2 emission factor. This limited data set suggests that when direct CO2e fugitive emission are add to the indirect CO2 emissions, the total CO2 emissions from binary power plants are still low and equivalent or less than nuclear, as well as other renewables power production such as wind and solar. |