There are two distinct types of geothermal energy, in the following paragraphs I will address both but focus mainly on ground source heat pumps as these systems have a much greater availability for people. The energy comes from the heat stored in the Earth. There are two main sources of this heat.
The sources are as follows:
1. Radioactivity. Earth has a very high level of natural radioactivity and isotopes
of materials that will decay. These include U-238, U-235, Th -232 and Kr -40. As
these materials decay, they release heat which warms Earth's interior.
2. Heat of formation. In forming the Earth, large numbers of smaller bodies collided,
and the heat was trapped in the interior of the planet. Over billions of years this
heat will slowly be dissipated, but at Earth's core, the average temperature is still
over 6000K.
When we look at Earth's interior, the majority of it is actually fluid, and can flow. Since the core is the hottest, we get convection currents, this allows the heat to flow from the center to the outer layers.
The crusts consists of a very thin layer, only about 5- 100 km of the surface is actually solid, we refer to this as Earth's crust. Because of the convection currents under the surface, the crust will crack and in turn moves around (it is the regions where we have these cracks in the crust that are far more prone to Earthquakes).
There are two main types of cracks
1. Spreading Zones. this is where the pieces of crust
are being forced apart as the warm material below rises to the surface.
2. Subduction Zones. These are areas where one
piece of crust in sliding underneath another. These are regions with high volcanic
activity, and strong Earthquakes as well (the volcanoes in these areas are
referred to as cone volcanoes and are very explosive in their eruptions).
The crust is also not uniformly thick. In some places where the crust is very thin, sources of heat are quite near the surface. At these locations you can get the formation of shield type volcanoes that exist over what we refer to as 'hotspots' on the Earth's crust. Hawaii is the best example of this type of situation.
Areas where geothermal energy is available can usually be divided into 3
main temperature ranges
Low temperature - 90 degrees or less.
Medium temperature - 90 -150.
High temperature - 150 degrees or greater.
When we look at the heat sources there tend to be two types either wet or dry.
In a dry source, the rock below ground is quite hot, and we can pass water across it to warm it up. This heated warm is then used either for heating, or for electricity generation. In this type of situation the only thing being removed from the ground is the heat. (you can also do this with a wet source).
The other type is a wet source where the underground heating is combined with a reservoir of natural ground water. Here, water naturally collects near a source of heat. This water can reach temperatures of a few hundred degrees Celcius, and with the high pressures under the Earth, it remains liquid.
For low to medium heat sources we find that the water comes to the surface as either a liquid or liquid and some low pressure steam. For the high temperature sources, the water will quickly change to high pressure steam. This type is more useful for large scale electricity generation. When looking at this type of energy resource, the set up costs are very high. In addition tapping into some of these types of reserves can be an extremely dangerous process.
Geothermal energy alone is one of the renewables that does release carbon dioxide, but it is still significantly less than the burning of fossil fuels. It also produces a variety of other chemical fumes. One problem is the release of hydrogen sulfide - which smells like rotten eggs, yet it is not a dangerous gas, it does not cause acid rain. These pollutants can be strictly controlled if the water pumped through the ground is kept contained.
With the wet sources, you can cause ground subsidence if you remove the water at a faster rate than it is replaces.
- Average ground temperatures increase by about 15 to 30 degrees Celsius per 1000 metres depth globally.
- On average, the temperature at a depth of around 500 metres is somewhere between 7 and 15 degrees Celsius.
- For an example residential wells can reach several hundred meters, oil wells a few thousand meters.
The deepest hole ever bored was in Russia reaching 12000 meters.
- Ground source heat pumps fall entirely in the low-temperature scale.
- Heat pumps rely on the energy stored in the ground.
- They use a series of pipes to transfer a liquid medium that will (depending on season)
either release heat into the ground or capture it.
- The liquid medium is most likely distilled water or some type of water/antifreeze mixture.
- The system is know as a heat pump which works by promoting the evaporation and
condensation of a refrigerant to move heat from one place to another (same principal
as an air conditioner or refrigerator).
- It uses a heat exchanger to transfer heat from the liquid medium in the ground
loop to (depending on season )heat and evaporate refrigerants, thus changing them
to a gaseous state.
- A compressor is then used to increase the pressure and raise the temperature
at which the refrigerant condenses.
- A condenser then will give up heat to a hot water tank, or home heating system
which then distributes the energy throughout the home.
- An open vertical system works on an open system.
- It uses two wells that are drilled vertically into an existing underground aquifer.
- Water is drawn up through the first well and depending on the season the water is
used to cool the house or heat it up.
- The water is then released back into the aquifer via the second well.
- A typical home will need between 20,000 to 30,000 litres of water a day.
- There are governmental standards that need to be considered.
- During the winter theses systems are generally more efficient then others.
- Although they commonly suffer from corrosion and thus have efficiency losses of 25 to 30 percent.
- Because of the well depth a more powerful pump is necessary increasing electricity consumption.
- This is a system that utilizes a closed loop.
- It is used in applications where space is an issue and hence a vertical positioning
of the pipes will be necessary.
- The depth that the pipes are inserted commonly range from 15 to 100 meters.
- requires a piece of land only about 3 meters by 3 meters.
- This configuration is commonly a more expensive option (3 to 4 times).
- The liquid medium is never lost it will remain in the system indefinitely.
- This is a variation of the closed vertical system except that the pipes are positioned horizontally.
- Horizontally laid pipes are generally less expensive to install but they need a large area of open land.
- This varies considerably but approximately 15 meters by 15 meters of land would
be sufficient for a residential home.
- The liquid medium is never lost it will remain in the system indefinitely.
- Pipes typically buried between 1.5 and 3 meters underground.
- Additionally the loops can be placed in a lake or pond where the water will
serve the same purpose as the ground.
- Water needs to be 2 - 2.5 meters deep.
- Lake installations require government permission.
- can have environmental problems.
Another way geothermal heat is used is for the direct heating of buildings using a heat pump. In this case we are taking advantage of the fact that it is a different temperature below ground as compared to above ground.
This system can be used to heat in the winter and cool in the summer by using the temperature differential. In colder climates like Canada you generally still have to supplement with other forms of heat, as there is a limit to how much you can move the temperature up or down.
Using the earth for energy is an idea that has evolved to utilize technology in an ever increasing fashion. Industrial generation utilizing the hot steam although limited in it location does produce few pollutants and aids in weaning our population off fossil fuels. The real potential I feel is with the ground source heat pumps, with reductions commonly reaching 70%, this is an ideal and economical way to use less fuel, and pollute less.