Assessing generating capacity of Rwanda geothermal fields from green field data only

Abstract

"Geothermal resource assessment for green fields" is the evaluation of the expected potential of supplied geothermal electricity that might become available for exploitation of a given reservoir. Different methods of assessment, such as counting volcanoes, surface thermal flux, surface CO2 flux and the stored heat method known as the Monte Carlo method are applied in the present report using the available data for Rwanda. This approach could be considered as level 0 to obtain a first course estimate of Rwanda’s generating capacity. The assessment is presented for two cases, the overall country and the promising Karisimbi volcano geothermal field. The results range from 26 to 345 MWe. In the case of the evaluation of the potential as a whole, very different results are found, from approximately 100 MWe (by counting the volcanoes) to 26 MWe (by considering that the heat is transferred by conduction only); a recovery factor of 1% is applied for the electricity conversion capacity. In the case of the Karisimbi volcanic field, results differ from 17 MWe for the surface CO2 flux to 80±40 MWe for the surface thermal flux and finally 345 MWe for the Monte Carlo volumetric method. Those results represent the best guess of accessible energy and what can be turned into electricity at the present stage of geothermal development in Rwanda. An average generating capacity from the four methods studied is approximately 120 MWe, of which 50 MWe can be considered as a reasonable initial target for geothermal generation in Rwanda in say the next 5-6 years. However, this estimation should not be regarded as proven until drilling and additional surveys can confirm the accessibility of the resource and the reservoir properties are determined. Furthermore, the protection of the whole National Volcanoes Park must be considered prior to any development of the Karisimbi field.