Energy from the Earth

Geothermal energy comes from the structure of the earth and its interior heat source and circulation.

There is a continual flow of heat energy outwards towards the surface

Surface Manifestations Include:

• Volcanoes
• Hot springs
• Geysers

Conventional Uses of Geothermal Energy:

• A Tourist Attraction - come sit in our hot springs with people you don't know
• As a direct source of space heating
• As an instant steam generating facility to spin a turbine

There are production sites in New Zealand, Iceland and, of course, California

Averaged over the earths surface, the heat energy flow is 0.06 Watts per square meter (500 times less than incoming solar energy flux)

Of course, there are local concentrations of geothermal energy that are quite high (e.g. yellowstone).

On average though, it is clear that to get large scale electrical energy generation from geothermal we will be withdrawing energy more rapidly than it is replenished. Hence we are essentially mining geothermal energy in the same way that we mine fossil fuels.

Example:

• At the Geysers project in Northern California, heat is withdrawn at a rate which is 80 times greater than it is being replenished
• Hence, Geothermal is NOT a renewable energy source

Existing Production Sites:

• The Geysers --> 1365 MW capacity in 1985
• Total US Capacity in 1990 was 2800 MW metered at a rate of 4 --6 cents per KWH

The main problem with geothermal, of course, is lack of easily accessible surface sites. Turbines on Old Faithful would probably not be well received.

Geothermal using natural steam sources is quite efficient, almost 100%. Only real source of loss is turbine friction.

Note that Geothermal does pollute the environment with small amounts of sulfur and carbon emission but to date, the tapped sources are natural so this would happen anyway

Categories of Geothermal Resources:

• Hot Water Reservoirs: geothermally heated underground water. US has large reserves. Heat engine driven electricity is possible. Also very viable for space heating needs.

• Natural Steam Reservoirs --> Instant steam but they are very rare

• Geopressurized Reservoirs: Brine (water) completely saturated with natural gas and under lots of pressure from the weight of the overlying land.

• Normal Geothermal Gradient: Even in dry rock the typical termal gradient is 30 degrees C per kilometer. Drill holes of 20,000 feet are acheivable and at that depth the temperature is 190 C. The potential of this resource is enormous but so far no technology exists to extract energy in a commercially useable way.

• Hot Dry Rock: Same as the previous but this exploits about 5% of the US land area in which the thermal gradient exceeds 40 degrees C per kilometer (so you don't have to drill as deep).

• Molten Magma: Extremely high temperature (2000 C) but how do use?

Note: Although the efficiency of geothermally heated steam powered turbines is quite high, only a small fraction of the available thermal heat goes into producing steam

Example -- The Geysers Geothermal Site:

• Covers an area of 70 square kilometers
• Heat is recovered from the top 2.0 kilometer of crust
• In this region the temperature is 240 degrees C
• The mean annual surface temperature is 15 degrees C
• The specific heat (s.h.) of the rock is 2.5 Joules per cubic centimeter per degree C

  Volume of rock = 70 km² x 2 km = 140 km³

  Heat content (Q) = V*s.h.*Temp difference

  Q = 140 km³ * 10¹⁵ cm³/km³* 2.5 J/(cm³C) * 225C = 8x10¹⁹ Joules

• Overall 2 % of the total available thermal energy in this region heats water for steam
• How many years can this source provide power for electricity generation at the rate of 2000 MW every year?

• Total Capacity required is 2000 Mw per year/0.02 = 100,000 MW per year
• 1 Joule/second = 1 Watt
• 100,000 MW * yr = 10⁵ MW *yr * 10⁶ W/MW 3.15 x 10⁷ secs/yr =
  
  3.15 x 10¹⁸ W secs = 3.15 x 10¹⁸ Joules each yr

• hence the lifetime is:

  8x10¹⁹Joules / 3.15 x 10¹⁸ Joules/yr = 26 years

• This shows that we are mining the resource!

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