CPG systems – storing CO2 for geothermal energy production

In an article written for Rinnovabili,  Martina Leveni, PostDoc researcher at the Ohio State University, shares an interesting take on how CO2 can be used to utilise geothermal energy. We initially credited another publication and author with the text, but it is based on her article in Rinnovabili.

In the article she describes the so-called CO2 Plume Geothermal technology (CPG), which allows to store CO2 to produce energy from geothermal.  The US company TerraCOH has developed CPG technology capable of combining CO2 storage with geothermal energy. In this way it is possible to produce clean energy by exploiting the reduction of emissions.

Annual greenhouse gas emissions continue to rise, despite the Paris Agreements signed in 2015. Each year, more than 40 billion tons of carbon dioxide enter the atmosphere, slowing efforts against climate change, so a solution that can utilize these emissions to actually produce energy is rather interesting.

Two urgent challenges are to reduce emissions and at the same time integrate renewable energies. CO2 storage and geothermal energy production can combine these goals. In reality, using CO2 in geothermal plants is an innovative idea. Already in 2000 some geothermal systems exploited pressurized CO2, partly gaseous and partly liquid. The benefits were shallower wells and less heat losses. The US company TerraCOH has developed CPG technology capable of combining CO2 storage with geothermal energy. In this way it is possible to produce clean energy by exploiting the reduction of emissions.

CPG technology is different because it exploits not so much CO2 as a motive fluid, but the geothermal potential of its capture and storage.

The CCS ( Carbon Capture and Storage ) is the process of capturing carbon dioxide and subsequent storage . It serves above all to avoid greenhouse gas emissions from sectors where these emissions are difficult to eliminate.

For the capture there are different technologies divided into three groups: post-combustion, pre-combustion and oxy-combustion.

But how does CPG technology work?

CO2 plume geothermal CPG technology mainly uses carbon dioxide captured by thermal power plants. The CO2 is injected into saline aquifers that take it 1.5-5 km underground where it heats up thanks to geothermal heat. The extracted ‘hot’ CO2 sets a turbine in motion to produce electricity.

CO2 Plume Geothermal System – model (source: geg.ethz.ch)

Picture above shows a model of a CO2 Plume Geothermal System (CPG) from the Numerical Modeling project of CPG systems ETH in Zurich, Switzerland.

But why exactly carbon dioxide? Because compared to traditional fluids (such as water with a high salt concentration), it has many advantages:

• density varies more with temperature, so less energy is consumed with pumping
• thanks to a thermosiphon effect, the CO2 circulates more easily
• it has a low reactivity, therefore it reduces the scaling of wells and ducts
• it has a lower viscosity, so pressure losses are reduced into the reservoir

Let’s make a comparison. CO2 systems can produce power up to a reservoir depth of 1.5 km and more power than water systems up to 3.5 km. At reservoir depth of 5 km and a geothermal gradient of 50°C/km, systems with CPG technology produced between 0.7 MWe and 9 MWe more power.

The potential of this new technology is recognized, but for its effective implementation, adequate energy policies will be needed in a short time.

For the full article see link below, which includes also the underlying sources which provide further depth to this surely interesting research.

Sources: Leveni, M. “Combinare geotermia e stoccaggio della CO2: sistemi a emissioni negative” in Rinnovabili, Sambuceti, L. “CPG: stoccare CO2 per produrre energia geotermica” in Energycue

Note: We adapted this article, which we initially published May 17, 2021, as it referred to an article that did not correctly sourced and cited the original piece and author of the underlying content. We apologise for this.