By Dan Oancea - Twitter

Geothermal power is commercially produced in many countries around the world, including Iceland, New Zealand, Indonesia, U.S.A., Russia, China, Japan, France, Italy, Nicaragua, Costa Rica, and Mexico.  U.S.A. is the world’s largest geothermal producer;  Iceland covers 50% of its power needs from geothermal sources;  volcanic New Zealand derives about 7% of its power from geothermal power plants.

Australia also moves quickly towards a green future.  Despite 30,000 strong coal miners’ opposition, the country hopes to be able to cover as much as 10% of its actual power demand from geothermal sources by 2030.  South Australia’s Cooper Basin boasts the greatest potential.  Beverley uranium mine has already agreed to buy locally produced geothermal power.  Even though Australia is blessed with very hot rocks (200-300 degree Celsius) their location in the middle of the continent creates some challenges for power distribution.  At the same time, it creates a comfort zone for geothermal power producers (i.e. a sparse population means no vocal opposition to geothermal induced seismicity). We’ll pick this up later.

There is a mine that I like: Papua New Guinea’s Lihir gold mine.  The open pit mine lies on an island having the planned pit’s wall within 100 m of the coast line, and a final depth of 200 m below sea level.  And it’s not just that.  The open pit extracts gold ore exactly from the heart of an inactive volcano, a volcano that still hosts an underground magma chamber, which heats up the ground to 200 degree Celsius. Under these circumstances the ground has to be cooled down in order for people and machines to have access to the ore body.  The upside part of the story is that the company has just commissioned a new 20 MW geothermal power plant (a steam plant).   Nowadays, geothermal power covers 75% of its needs and will provide some $40 million in savings in 2007.  Not to mention that the company also plans to get a few more millions ($ of course) from the sale of its carbon credits (global warming means good business for companies able to produce without fully consuming their CO2 quotas).  Did I mention that the dormant volcano is frequently shaken by naturally occuring earthquakes?

Three main types of geothermal power plants can be recognized:

  • Dry Steam Plants – pressurized steam springs out of the ground and is used to rotate a turbine and produce electric power;
  • Flash Plants – use reservoir hot pressurized water which boils on its way to the surface (as a result of a drop in pressure); the resulting steam is subsequently used to produce power;
  • Binary Plants – use ground water not hot enough to boil, which is rushed through heat exchangers where a special fluid with a lower boiling temperature is thus boiled and the resulting steam would move the turbine.

There is another system marketed for individuals and small buildings: ground-source heat pumps generate small amounts of energy used for heating/cooling by circulating water through buried pipes located in a constant temperature layer of the ground.

Where does all that ground heat come from? It mostly comes from the decay of natural radioactive elements (e.g. uranium, thorium, potassium).  That is why granitic rocks, which contain more radioactive minerals (mostly radioactive potassium K-40) than any other rock, will be considered as potential ‘hot rocks’, i.e. good geothermal drill target. This is especially true if they are buried very deep. The deeper you go within the crust, the hotter it gets – the geothermal gradient theory.  There are some other places called ‘hot spots’ characterized by the fact that hot magma is closer to the surface; these places are radiating more geothermal heat than usual and are good geothermal targets too.  There are deep crustal faults that represent easy conduits for underground water circulation, water that seeps to the depths, heats up and then rise again, sometimes generating hot/warm springs.  Or there are plate boundaries, where a lot of tectonic and volcanic activity occurs.  And there are really hot rocks, geothermal springs, underground hot water reservoirs and geysers as an expression of an active or pretty much extinct volcanic process.  Thermal satellite pictures (heat flow maps), governmental repositories and a good knowledge of regional/local geology and geophysics would help you to identify the best drilling spots.

What to do with a hot rock?  Drill it (you only need a deep hole and a thermometer to figure out if it is really ’hot’), create ways for the water to flow through the host rock between adjacent drill holes, and pump down some pressurized water through the injection hole. The water absorbs heat and is subsequently pumped out from an adjacent hole (the production hole). Sounds pretty simple, and it is, especially the part about locating the geothermal target/system, drilling and casing the hole. The more complicated part of the operation is to create a constant flow of water between holes. The more or less permeable rock layer would then be considered to constitute a geothermal reservoir. This job needs all the engineering expertise that the oil industry got during decades of fighting similar problems in trying to enhance oil recovery.

Different techniques could be used as an aide in increasing the rock’s porosity (e.g. acidization – acid is injected in specific carbonate matrix host rock) and/or permeability (e.g. hydraulic fracturing – a pressurized fluid is injected into a portion of the well, having the purpose of creating and opening cracks and fractures).

 

 

 

 

 

To be continued ...