Wednesday, May 13, 2009
Huge and still not fully exploited geothermal potential
Energy under our feet, geothermal power, represents de facto unlimited clean, stable and reliable source of energy. Geothermal power can be the best answer and energy solution nowadays, but its bulky potential remains almost unused.
According to various scientific estimations, Earth accumulates 10E31 Joules of energy ("E" represents "mathematical exponent"). It would be enough to satisfy global energy demand for more than 10 billion years. That being said, Earth is about 6E24 kg. The specific heat of silica & iron (the two most common minerals) is .7 & .45 J/gk - average it to .55. That would mean 3E24 J for a 1 degree drop. 3600J is a watt-hour... so 2.1E19 J is a terawatt-year. That means it would take about 140,000 years of 1TW 'drain' to cool the entire (interior of) Earth about 1 degree. Even assuming that all human electricity was generated via geothermal energy, it would take somewhere in range of millions of years. This is enormous „heat of impact and compression released during the original formation of the Earth“, stemming from specific chemical and nuclear reactions in the center of the Earth, as well as from solar heat adsorbed by Earth surface.
Geothermal heat hidden underground is geological phenomena on a planetary scale, which means it is located under the whole surface, but in various depths. In practice, a few or more kilometers under the ground, there is very hot area, just at our disposal and ready to become effective and clean source of thermal energy. As picture on the left side shows, the principle of getting it up, above the surface, is very simple: We need to find geological anomalies, to drill some 2-5 km down, and to establish water circulation. Cold water is pumped down into reservoir and after it becomes hot it is drawn up above the Earth surface. Hot water turns into steam, which is used in steam turbines producing electric power. At the same time, hot water can find also the secondary utilization – it can be transported into other facilities like a fuel for heating systems.
Geothermal energy has attracted many countries all over the world since the Second World War. It was considered as an opportunity how to use Earth's energy for free, which does not need to be imported from anywhere abroad. On the other hand, due to technologies available at that time, it was able to exploit geothermal potential just in limited geographic areas, characterized by higher number of geological anomalies, so called tectonic plate boundaries. Also current technologies enable us to drill economic and time effective just only to three (max. five) kilometers deep, which means linking to those limited areas as well. The general potential of geothermal energy is huge, but at this time we can consume only its „low-hanging fruit“.
As was mentioned above, heat from the depth is brought on surface by water. Sometimes water circulation happens naturally, in some areas - where magma is closer to the surface - warm springs appear. If no natural water is available, one has to make reservoir and pump it manually. In general, the temperature of water depends on so called geothermal gradient. In practice, it means that in average with one kilometer of depth there is warmer space of some 25-30 Celsius degrees. Surely, this is dependent also on the geological structure, which means the proximity of tectonic plate boundaries. On the other hand, quantity and the quality of source of geothermal heat can be improved by drilling.
In fact, geothermal wells nowadays are rarely deeper than 3 kilometers. This fact limits the use and exploitation of geothermal potential just to some appropriate areas. Wells deeper than 3 kilometers become more expensive and in addition, the costs of „ultra deep drilling“ rise exponentially, not linearly. Just for imagination, drilling into 5-10 km represents tens of millions dollars in general.
Enhanced Geothermal Systems (EGS) offer an opportunity to drill wells deeper than 3-4 kilometers. They are also called Engineered Geothermal Systems, Hot Dry Rock or Fractured Rock Geothermal Systems. Even though EGS expands the use of geothermal energy, as well as linked to the „classical“ geothermal technology it finds application just in limited geographic areas, especially in United States or Island. The world is still waiting for appropriate drilling technology, which would allow to drill ultra deeply (8 and more kilometers) and time and cost effectively. At the same time, in other words, the world is also lacking the technology that would allow bringing geothermal power really anywhere, not just to limited numbers of areas.