Exergy analysis of the use of geothermal energy and carbon capture, transportation and storage in underground aquifers

Abstract

At the moment global climate change is one of the most prominent environmental and energy issues of our life time. Currently CO2 levels in the atmosphere stand at 387 ppm, almost 40% higher since the start of the industrial revolution and the highest for at least the last 650,000 years. About 96% of these carbon emissions are the result of using fossil fuels. Another problem is that the fossil fuel reserves will be exhausted within 200 years and the nuclear energy within 260 years. Something has to be done to stop the addiction of fossil fuels and carbon dioxide emissions! However investing in renewable energy is still expensive but high energy prices, CO2 emissions, rising temperatures, fossil fuels depletion and the demand to be less dependent on other countries makes it more and more attractive to invest in renewable energy, like solar energy, wind energy, biomass, geothermal energy, tidal power and hydro power. Not all renewable energy sources are suitable in the Netherland. Geothermal energy, still unknown by a lot of people, has a very good potential to succeed in the Netherlands. The average temperature of the Dutch subsurface at a depth of 2,000 meters is around 75 – 80 °C. This energy is very suitable to heat houses, afterwards the rest heat can be used to heat low heat demanding facilities like swimming pools and greenhouses. In spite of the good potential, geothermal energy is still used on a small time scale in the Netherlands. Hopefully DAP can make a change in this by making more people aware that geothermal energy has a great potential for the Netherlands. The Netherlands has a geothermal potential of 90,000 PJ in heat. The benefits of geothermal energy are that it is clean and available 24 hours a day, 365 days a year. Also geothermal power plants have average availabilities of 90% or higher, compared to 75% for coal power plants. The greenhouse gas emissions of the geothermal plants are only 91 gCO2/kWh, this is very low compared to other fuels. The annual Dutch CO2 emission is nearly 180 Mt CO2 at present, of which approximately 100 Mt CO2/year emitted by the energy and manufacturing industry. The biggest emitters are large point sources like the power generation sector and large energy-consuming industries like oil and gas processing, iron and steel, cement and chemicals production. CCS can lower the emissions from the large point sources by capturing the carbon dioxide. CCS is not a new technology, this proven technique is already used for nearly 100 years for industrial purposes or to increase oil or gas production. CCS technology can reduce carbon dioxide emissions from large industrial sources and coal-fired power stations by approximately 85 - 90% depending on the used type of capture technology. Also transportation of CO2 is not a new technology in the Netherland because there is already a pipeline of 85 km from the Shell refinery in Pernis to greenhouses in the Westland area. However, large-scale CCS will require a new transportation infrastructure to link sources and sinks. In densely populated countries such as the Netherlands this can become a considerable challenge, think of Barendrecht. Even though the Netherlands has a very good storage potential there is a spatial mismatch between CO2 sources and sinks. When a new energy source is discovered the first and most important thing to know is how much energy (quantity) can be extracted from this source. But what we really need to know is the work potential (quality) of this energy source, in our case a geothermal well beneath the TU Delft. Work potential is the amount of energy which can be extracted as useful work, this property is called exergy. The maximum available power from the DAP geothermal reservoir is 0.66 MW and is obtained at a flow rate of 180 m3. It is not smart to increase the production rate above 200 m3 because the losses are increasing faster than the extra gained exergy. The invested energy in materials and drilling are minor compared to the energy needed for capture and compression. Over 94% of the total energy demand is needed to capture the CO2 over a life-time of 30 years.