| Abstract |
Geothermal energy has a vast potential for energy production. It is clean and a renewable source of energy. If we cool off a wellbore over time by extracting heat, energy from underneath will bring the temperature back to initial value if we stop heat extraction. Heat from the underground is a stable source of energy basically without seasonal variations, and wells can be drilled with little environmental footprints. Compared to other renewable sources such as solar energy, wind energy and wave energy, geothermal energy have less variation over time, and is therefore a more stable source. Shallower geothermal wells can be drilled relatively inexpensive, but for deeper wells in the order of several thousand meters, drilling cost may render the project uneconomical. In many countries a high number of abandoned oil and gas wells can be used to produce geothermal energy at a low cost. A typical application of geothermal energy consist of a deep well with two tubes installed, one to bring cold water down to the bottom of the well, and another pipe to bring hot water back to surface, where the heat is extracted to for example heat a building. The system can be viewed as a counter-current heat exchanger. One problem is that during the convective water transport up to the wellhead, the water is cooled off. Surface temperature is therefore considerable lower than the bottomhole temperature. |