Christopher Levesque

Renewable Energy: Geothermal

Introduction

Geothermal Energy is one of earth’s renewable energy sources, which include solar, wind, tidal, biofuel, biomass and hydro energy. Energy can be captured from the earth’s natural heat. The earth’s temperature increases as the depth increases from the surface. The earth has many layers, but all geothermal energy applications involve exploiting the crust, the upper most layer. High temperatures in the crust range from 300°F to 750°F. Scientist and engineers have been able to drill and excavate to depths where this heat can be used to generate electrical energy or to use the heat for thermal comfort. Technologies can be used at all different depths and temperatures for a variety of uses. Uses include heating homes and buildings, dehumidifying spaces and generating electrical energy for consumption. As these technologies further develop the use is starting to increase across the world. The attached hyperlink is a
video[1] explaining the current uses of geothermal energy and its advantages as a renewable resource.

Technologies

Geothermal energy and geothermal heat is commonly used interchangeably, but the technologies differ in the use of earth’s heat. Geothermal heat pumps or ground source heat pumps use the earth’s heat to either cool or heat a medium and transfer this medium through a heat exchanger, which is used to either heat or cool a space requiring climate control. Geothermal energy use the earth’s heat to heat water to either form steam to heat another medium to form a vapor. This vapor is then used to propel a turbine runner and generator to form electrical energy.

Geothermal Heat Pumps
A geothermal heat pump simply uses the earth’s ability to transfer heat to a medium, through an evaporator, compressor and condenser. There are a few functional modes[2] of a geothermal heat pump, which include:
  1. Cooling with a water heating function – particular use during the summer when one still desires a warm shower or for washing applications
  2. Passive cooling – particular use in areas that have high humidity, this function does not utilize the heat pump.
  3. Standard heating – particular for thermal comfort during cold temperatures
  4. Domestic hot water heating – particular for facilities that operate with the need for heated water (Dormitory)
Components of a geothermal heat pump system include:
  1. Heat Exchangers – open or closed looped systems, orientated vertically or horizontally either in the ground or in a body of water. Condensers and evaporators are also considered heat exchangers.
  2. Medium – liquid used to transfer heat, can either be a refrigerant such as glycol or water. Water has a superior heat transfer rating but is susceptible to freezing during cold temperatures.
  3. Circulators – pumps that allow the medium to pass through its particular process and overcome pipe and equipment friction loss.
  4. Compressor – device that allows the heat in evaporator to move to the condenser where the thermal heat gain can be passed to a heating fixture such as a hot water tank or radiator.
  5. Expansion Valve – a device that release the cold water from the condenser back to evaporator to be reheated.

Steam Turbines
A steam turbine is a electromechanically designed equipment that uses expansion of the heated water (steam) or other medium (vapor) to spin the turbine rotor and subsequently spin a generator in motion to form electrical energy. The use of the steam turbine is used in all the types geothermal energy plants, which include dry steam, flash steam and binary cycle. There are two modes in which a turbine can function. Impulse mode is used with high pressure steam, which passes through a nozzle and pushes against the rotor blades. Reaction mode is used with low pressure, which is directed through vanes or wicket gates and spread evenly across the rotor. The reaction mode allows the steam to be in constant contact with the rotor. The vanes or wicket gates can be rotated to increase the rate of steam to the rotor. As the rotor spins it also spins a generator which has been induced with a magnetic field to generate electricity. The electricity is then transformed as required and flows to the electric grid for consumption.
Dry_Steam.jpg
http://geothermal.marin.org/GEOpresentation/sld049.htm

Flash_Plant.jpg
http://geothermal.marin.org/GEOpresentation/sld054.htm

Binary_Plant.jpg
http://geothermal.marin.org/GEOpresentation/sld059.htm

Components of a steam turbine include
[3]:
  1. Steam Piping – Input lines to the turbine
  2. Condensed Steam Piping – Return lines to the either the earth or mechanical heat exchanger depending on the process
  3. Casing – the housing for the components of the turbine
  4. Rotor – a part that has blade surfaces in which the steam can push against to create a spinning motion
  5. Nozzle – used to direct the flow of steam directly at the rotor blades
  6. Vanes or Wicket Gates – used to direct flow in the path of the rotor blades
  7. Main Shaft – used to connect the rotor to the generator
  8. Generator – used to create electrical energy (alternating current) through the creation of a magnetic field
  9. Exciter – used to charge (direct current) the magnetic field of the generator to induce electrical energy creation.
  10. Governor – electric/mechanical equipment used to regulate the applied electrical load and the turbine nozzle or guide vanes/wicket gates.
  11. Switchgear – electrical device used to transfer electrical energy to the national grid for consumption.

See Also
http://en.wikipedia.org/wiki/Steam_turbine - Description of steam turbines, history and uses
http://www.tic.toshiba.com.au/product_brochures_and_reference_lists/110gst.pdf - Toshiba turbine design recommendations
http://newenergyandfuel.com/http:/newenergyandfuel/com/2007/10/23/the-real-massive-geothermal-generator/ - Article discussing the use of geothermal as the new fuel
http://en.wikipedia.org/wiki/Heat_exchanger - Description of heat exchangers
http://www.waterfurnace.com/ - Manufacturer of geothermal heat pumps





[1] http://geothermal.marin.org/video/vid_page.html
[2] http://www.dimplex.de/animationen/waermepumpe-passiv.php?lang=en
[3] ISO 14224:2006, Petroleum, petrochemical and natural gas industries – Collection and exchange of reliability and maintenance data for equipment