1. Geothermal Energy

2. Introduction The center of the Earth is very hot, approximately 5000 K. [1] More than 95 % of the Earth’s volume is warmer than 1000 K [4] The major part of the heat originates from radioactive decay (80 %), minor contributions come from tidal forces, and heat from the original formation of the Earth. [2] The continuous heat flow from the interior of the earth is 42 TW. [1]

3. Introduction This leads to an average heat flux of 0.1 MW/km 2. Through the crust the temperature gradient is 25 K/km. [3] Near tectonic plate boundaries and other hotspots these values are much higher. [3] [7]

6. Current use In 2005 more than 9 GW electricity was produced by geothermal power plants worldwide. [5] Additionally more than 28 GW were used for direct heating. [5] Geothermal energy provided 0.3 % of the worldwide energy consumption. [2] Almost all existing geothermal power plants are located in areas, where the geothermal activity is apparent. [1]

7. Wells The most important parts of a geothermal power plant is the wells. The wells are drilled into a natural collection of hot water, called a geothermal reservoir. Usually the wells are not deeper than 3000 m, but the limit for today's drilling technology is about 10000 m. [6] A geothermal reservoir can also be created by injecting water into the rock, this method is called hot dry rock or enhanced geothermal systems. [6]

8. Electricity production Converting geothermal energy into electricity is relatively simple. Hot water/steam, with a temperature of 150 – 200 °C, are led through a turbine, and then injected back into the rock. [6] [7]

9. Electricity production Binary geothermal plants can be used to produce electricity, when the water temperature is lower (100 – 150 °C). [1] The binary fluid has to vaporize at temperatures below 100 °C. [1] [7]

10. Heating Geothermal energy has been used for heating since ancient Roman times. Don’t require very hot water. [3] A effective way to use geothermal energy. [3] [7]

11. Heating Geothermal energy can be used to heat a single house or an entire city. [3] [7]

12. Advantages Geothermal energy is reliable, it will be available 24 hours every day. [1] Geothermal energy is renewable, as long as water is injected into the reservoir and the amount of water extracted is in balance with the capacity of the reservoir. [1] It produces very little pollution. [1] Highly scalable. [3]

13. Disadvantages Drilling is expensive. [3] The success of a drilling attempt can’t be guaranteed. [3] If water is injected into the Earth it can trigger earthquakes. [3]

14. References [1] A guide to geothermal energy and the environment, Alyssa Kagel, Diana Bates, Karl Gawell, Geothermal energy association, http://www.geo- energy.org/Publications/reports/Guide%20to%20Geothermal%20Energy%2 0and%20Environment.pdf [2] Wikipedia: Geothermal gradient, http://en.wikipedia.org/wiki/Geothermal_gradient [3] Wikipedia: Geothermal power, http://en.wikipedia.org/wiki/Geothermal_power [4] Earth's Interior, University of Nevada, Reno, http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html [5] Characteristics, development and utilization of geothermal resources, John W. Lund, Geo-Heat Center, Oregon Institute of Technology, http://geoheat.oit.edu/pdf/tp126.pdf [6] The future of geothermal energy, Massachusetts institute of technology, http://geothermal.inel.gov/publications/future_of_geothermal_energy.pdf [7] Introduction to geothermal energy slide show, Geothermal education office, http://www.geothermal.marin.org/GEOpresentation/index.htm