Combining geothermal energy with CO2 storage Feasibility study of low temperature geothermal electricity production using carbon dioxide as working and storage fluid

Abstract

Abstract One of the emerging solutions for today’s excess of carbon dioxide emissions, which is one of the major causes of global warming, is the geological storage of carbon dioxide in geothermal reservoirs as depleted gas and oil fields or saline aquifers. The carbon dioxide is captured and pumped into a reservoir which is closed when the reservoir is full. Using this method a certain amount of carbon dioxide is avoided from being emitted to the atmosphere and prevented from contributing to global warming. Another solution is through the usage of renewable energy sources such as geothermal energy. This green energy also uses geothermal reservoirs but in a different way as is the case with carbon dioxide storage. At depths varying from several hundreds of meters to several kilometers geothermal energy is used to provide heat. From a depth of 6 kilometers and deeper geothermal energy is used to produce electricity by means of superheated steam.

This study aims to use the best of two worlds by combining carbon dioxide storage and geothermal energy and integrate them in a system where carbon dioxide is used as working fluid for a geothermal system at a depth of 2 to 3 kilometers which produces electricity. However, before this can be realized a clear understanding of carbon dioxide and its behavior when exposed to geothermal pressure and temperature is required.

A clear result from this research is that carbon dioxide is suitable as working fluid for geothermal energy, mainly because of its low critical point, which lies at 73.8 bar and 304.1 K. Using average conditions in the Netherlands a depth of 718 meters is required to change carbon dioxide from liquid to supercritical. At supercritical phase carbon dioxide has a much lower viscosity than water at the same pressure and temperature which allows for a larger flow in a geothermal reservoir. Compared to water, carbon dioxide has a lower heat capacity and therefore requires more flow to extract the same amount of energy.

It is possible to produce geothermal electricity using carbon dioxide as working fluid at average conditions in the Netherlands. The chosen base reservoir depth is 2000 meter which allows for a temperature of 345 kelvin. This choice is based on the fact that it is deep enough to produce a decent amount of heat while it does not come near the depths for geothermal power production. A reservoir at 2000 meter depth allows for a geothermal electricity production of 0.49 MWe which can be maintained for at least 50 years using a distance of 1400 meter between the injection and recovery well. When more ideal properties are used such as a depth of 3000 meter or a temperature gradient of 50 degrees per kilometer the electricity production can rise to 4.95 MWe per doublet. If the reservoir is large enough multiple doublets could be placed which greatly enhances the overall power production.

A geothermal carbon dioxide system can be placed on an empty reservoir or on a reservoir which has already been filled with carbon dioxide as a result of a carbon storage project. This has no effect on the electricity output but determines to a large extent the costs of the system. If a reservoir still needs to be filled with carbon dioxide extra investment costs occur due to the need of injection wells, transport costs and compression costs. On the contrary, more revenues are earned by gaining carbon credits for the stored carbon dioxide.

At average conditions in the Netherlands it is not economically feasible to produce electricity using a geothermal system with carbon dioxide as working fluid if the reservoir still needs to filled with carbon dioxide. When the reservoir is already filled, the pay back period varies from 38 years and more. However, in more ideal circumstances, the pay back period reduces to 5 to 11 years. This reduction in pay back period is mainly caused by higher electricity sales but also by higher incomes from carbon credits due to carbon neutral electricity production.

This research shows that using carbon dioxide as geothermal working fluid is very promising. The production of a geothermal carbon dioxide system allows for electricity production without the need of going to large depths. It uses techniques which have been used for decades. The system is applicable at nearly every gas field below 800 meters. Depending on the reservoir the system can pay itself back as early as 5 years. And the system can contribute to combating climate change by the production of carbon neutral electricity.