Britta Groh Physics 106Spring 2010

 Almost 85% of the energy used in the US in 2007 was generated by fossil fuels  Fossil fuel technologies are no longer improving (in terms of efficiency) much if at all. These technologies have likely reached their performance limits, though government still spends far more on them.

 Power extracted from heat stored in the earth  Originates from the original formation of the planet from radioactive decay of minerals and from solar energy absorbed at the surface.  Geothermal energy is the most efficient renewable energy alternative and is improving the fastest. (wind is second)  Scientists estimate that the US hot dry rock resources could supply all of the US’s primary energy needs for at least 30,000 years  Geothermal energy is considered renewable because the heat is continually replaced. The water that is removed is put right back into the ground after its heat is used.

1. Direct geothermal energy: In areas where hot springs or geothermal reservoirs are near the Earth's surface, hot water can be piped in directly to heat homes or office buildings. Geothermal water is pumped through a heat exchanger, which transfers the heat from the water into the building's heating system. The used water is injected back down a well into the reservoir to be reheated and used again.

2. Geothermal heat pump: Fluid circulates through a series of pipes (called a loop) under the ground or beneath the water of a pond or lake and into a building. An electric compressor and heat exchanger pull the heat from the pipes and send it via a duct system throughout the building. The pipes draw heat away from the house and carry it to the ground or water outside, where it is absorbed.

3. Geothermal power plant: hot water and steam from deep underground can be piped through underground wells and used to generate electricity in a power plant. The three types of geothermal power plants are: 1. Dry steam plants 2. Flash steam plants 3. Binary cycle plants

 Dry steam plants are the simplest and oldest design. They use geothermal steam of 150°C or more to turn turbines.

 Flash steam power plants pull deep, high pressure hot water into lower pressure tanks and use the resulting flashed steam to drive turbines.  They require fluid temperatures of at least 180°C  This is the most common type of plant in operation today.

 Binary cycle power plants are the most recent development and can accept fluid temperatures as low as 57°C.  The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water.  This causes the secondary fluid to flash to vapor, which then drives the turbines.  This is the most common type of geothermal electricity plant being built today.

 It is cost effective, reliable, sustainable, and environmentally friendly, but has been historically limited to areas near tectonic plate boundaries.  Unlike wind and solar resources, which are dependent upon weather, geothermal resources are available 24/7  Geothermal plants release a fraction of the carbon dioxide produced by fossil fuel plants, and create very little nitrous oxide or sulfur gasses.  Because the energy is generated right near the plant, it saves on processing and transportation costs compared to other types of fuel.  However, a home geothermal energy pump can cut energy bills by 30 to 40 percent and will pay for itself within 5 to 10 years

 The initial costs of geothermal energy are high. Wells can cost $1 to $4 million each to drill.  Has historically been limited to areas near tectonic plate boundaries where high temperature geothermal resources are available near the surface.  The fluids drawn form deep within the earth carry a mixture of gases like carbon dioxide, hydrogen sulfide, methane, and ammonia, which contribute to global warming and acid rain.  This is a small fraction of the emission intensity of conventional fossil fuel plants.  Plants the experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce exhaust.  Geothermal plants could theoretically inject these gasses back into the earth, as a form of carbon capture and storage.

 We are the world leader in online capacity of geothermal energy.  The US generates an average of 16 billion kilowatt hours of geothermal power per year, about 0.37% of the electricity consumed in the US (comparable to burning 25 million barrels of oil, or six million short tons of coal.)

 In 2007 geothermal energy provided about 66 % of primary energy in Iceland.  The entire city of Reykjavik is heated with pumped from hot springs and geothermal wells. It is considered one of the cleanest cities in the world.  Renewable energy provides 100% of their electricity production, with 70% from hydropower and 30% from geothermal power.