Geothermal Energy

A Hot Topic

Introduction

• http://www.youtube.com/watch?v=kjpp2MQffnw&NR=1

Basic Premise

• Geothermal power plants tap into heat stored in the Earth’s surface and use it to generate electricity, heat homes directly, or perform a number of other functions (space heating, roadway warming, etc).

History

• Prince Piero Ginori Conti tested the first geothermal generator on July 4, 1904 at the Larderello dry steam field in Italy.

• First US geothermal plant built in 1922 in CA by John Grant to power a resort hotel.

• The Geysers site (Grant’s original) now generates 750 MW of power/year.

Methods of Power Generation

• The most common types of geothermal energy use:– binary cycle generation– hot dry rock generation (EGS)– dry steam generation– flash steam generation

Binary Cycle Generation

• Hot water (<175°C) is brought up out of the Earth’s surface and passed through a heat exchanger before being returned to the underground reservoir. A second working/binary fluid with a very low boiling point (normally butane) is passed through the heat exchanger and is transformed into steam. The resulting steam is directed through a turbine, condensed, and cycled back through the heat exchanger.

Hot Dry Rock/ EGS Generation

• Pumps high pressure water down an injection well through hot rocks (>200°C) 3-5 km below the Earth’s surface. The superheated water then returns to the plant through a second well, and is converted to electricity via a steam turbine.

Dry Steam Generation

• Only steam, and no water, is brought up from underneath the Earth. The steam passes through a turbine, and is then condensed and injected back into the surface.

• Prince Conti took advantage of this technology with his plant.

Flash steam generation•Usually employed when water temperature exceeds 350⁰F (177⁰C)•Involves the extraction of superheated liquid from underneath Earth’s surface that is evaporated, or “flashed,” into steam in a series of pressurized containers•More “flashes” produce more energy: a dual-flash cycle produces 20%-30% more energy than a single-flash cycle for the same fluid flow rate•Excess fluid and re-condensed steam are recycled back under the Earth in order to replenish the underground water level•Flash steam geothermal plants and other geothermal plants produce small amounts of carbon dioxide, nitric oxide, and sulfur (about 2% of that produced in fossil fuel plants)

“Green”ness

• Releases 2%-5% of the greenhouse gases of a traditional, fossil fuel burning power plant.

• Harmful substances emitted include:– CO2– Nitric Oxide– Arsenic– Mercury

Disadvantages

• Geothermal fluid is corrosive• Steam is at a low temperature, which

produces less energy• Geothermal resources can cool over extended

periods of time (20-30 yrs.)• Introductions of water into previously dry

areas