Energy From the Heated 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
• Earthquakes
• Cascadia at Risk
• Hot springs
• Geysers

These surface manifestations reveal that a network of hot water/steam is able to penetrate through various faults or weak spots in the upper part of the earth's crust. This usually requires the interaction of a natural aquifer and a relatively nearby heat source. An important component of this dynamic is that the aquifer does not leak "downwards" as its confined by impermeable rocks. If these conditions are met, then geothermal energy is avialable.

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 (Geysers Project):

The average heat flux incidence from the interior of the earth on to the earth's crust is somewhat difficult to measure. Different values exist, for instance, between oceanic and continental crust.

Recent scientific measurements suggest a global heat flux equivalent to 31 Terrawatts of Power.

Averaged over the earth's surface, this equates 60 KW per square kilometer of heat flux (or 60 mW/m²), but as shown in the figure below, there is considerable varation around this.

By comparison, averaged over the earths surface, the heat energy flow is 0.06 Watts per square meter which is 500 times less than the 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• At the Geysers project in Northern California, heat is withdrawn at a rate which is 80 times greater than it is being replenished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 to use?

Various methods of heat extraction are different depending on the depth one has to reach in order to extract the steam/heat.

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-years/0.02 = 100,000 MW -years
• 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! The lifetime at the Geysers is currently being expanded as water is pumped back from the surface down to where the heat is. But, of course, there is an energy cost associated with this.

Despite these potential limitations, Geothermal energy conversion into electricity remains a viable form of renewable energy. It has two specific advantages:

• low levelized costs (almost competitive to wind)
• Its mostly greenhouse gas emission free

Not suprisingly, the worldwide distribution of this resource is highly clustered to specific hotspots, particularly when you are limiting your depth to about 6 km. The regions shaded in blow all have potential geothermal resource located below 6 km. Areas that are denoted by red-yellow have the highest potential commercial yields.

The situation for the US is shown below. Not suprisingly, most of the geothermal sites are located in the Western US, specifically Nevada. The Geysers project in California is the red area in northern California.

Closer to home, the Columbia basin area of Washington state has a fairly large area of potentially accessible geothermal.

On this map

• the red dots (which are rare) are the best sites with well temperatures greater than 50 degrees C.
• the more populous blue dots are between 20 and 50 degrees C.

The bottom line for geothermal seems fairly clear. To date, the US has very low production in this resource but reasonably high potential. As the cost of coal fired electricity continues to escalate, and if wind capacity ramps up more slowly than expected and the LNG plan fails to materialize, then Western US based geothermal power plants may become more prevalant.

Right now, however, it does not look like geothermal energy will be an important component of the overall mix of technologies used in the US to product electricty.