|VoxSolaris: The Voice of the Sun|
Geothermal energy exists on a truly vast scale as the bulk of the Earth, beneath a crust of some 30 to 50Km is white hot. In terms of total energy reserves it is equivelent to something like 10 to 20 billion trillion barrels of oil. If we could extract enough of it to meet all our energy demands we would still be doing so when the Sun turns into a red giant and cooks us to death in four or five billion years time. But extraction is more than a little challenging. Drilling a hole anywhere on the Earth's surface will provide a temperature gradient of some 20 to 30°C per km of depth but this approach is not remotely economic. You would need dig down some 10km or so to get a temperature suitable for a steam turbine and in order to extract any real quantity of energy you would have to do extensive tunneling at that depth to get a decent heat transfer area. At this rate you might be looking at a red sun before you saw a black bank balance.
Fortunately, there are many places where nature has done most of the hard work for us. Water passes through cracks and porous layers in the Earth's crust, and if it passes though hot enough rock it can emerge as hot water or stream from what as a hydrothermal vent. Such vents are concentrated along tectonic plate boundaries and areas of thin crust known as hot-spots. There are several types of hydrothermal vents including the well behaved hot springs exploited since ancient times for hot baths, noxious fumaroles and oscillating geysers. These differences depend on the flow rate, the temperature and the chemicals encountered en-route.
Deep Sea Hydrothermal Vents
Modern day geothermal power stations primarily exploit the higher temperature vents to produce electricity but the opportunities to do this economically are not in great abundance. The total install base worldwide is around 12GW. Exploitation of lower temperature vents for heating purposes fares better with an install base or around 30GW. These numbers are orders of magnitude shy of a global solution but so far the story has all been land based. By far the largest and most powerful hydrothermal vents lay where the tectonic boundaries run under the sea.
Although difficult to discover there are several hundred substantial undersea hydrothermal vents known to be active. The temperatures vary considerably from around 60°C to upwards of 450°C as does the mix of minerals dissolved in the vent fluid (note that due to the pressure most if not all the fluid is still liquid). The vents are of commercial interest because of their energy potential and as a source of minerals which precipitate out as the vent fluids cool in the surrounding water. The precipitation process forms SMS (sea floor massive sulphide) deposits around the vent which contain inter alia, cadmium, lead, copper, iron and zinc but also trace quantities of silver, platinum and gold. There have been successful attempts to mine SMS deposits using robot submarines but only as experiments. There are as of yet, no commercial mining operations. Of the known vents, only around 100 are large and hot enough to be suitable for electricity production. And as a rule, these are among the deepest (2 to 3,000 meters and sometimes more). Not surprisingly, none are currently being exploited in this way as the engineering challenges are formidable.
Most serious proposals begin with a ship that will anchor itself above the vent site. This is necessary because hydrothermal vents can have short lifespans with many becoming inactive after as little as a year. It is important to be able to 'chase' them. It is also necessary to have a platform of considerable size to process and dispatch the large quantities of minerals harvested. The energy produced, in the form of electricity, is a stranded resource. You can drop hundreds of kilometers of cable to bring it ashore but you will lose money on the deal if the vent expires too soon. To be of use, this energy must be converted into something that can be transported. It can be used in processes such as high temperature electrolysis to extract metal from the sulphides or it can be used in low temperature electrolysis to extract hydrogen (and hence ammonia) from water.
Electricity production would be achieved by directing a hose or funnel into the hottest part of the vent's outflow and sucking the hot fluids through a heat exchanger which would drive either a steam turbine or stirling engine. However proposals differ on which part of this should be done where. For our part we are not keen on proposals that have a long funnel from seabed to ship. It is simply too long. To counter shear forces it would have to be made very sturdy. At lengths of up to 3km, perhaps more, would be cumbersome to store, drop and then reel back in. It would also make it difficult to stem thermal losses. Instead, we think the only physical link between the ship and the vent should be a HVDC line with all electricity generation taking place at the vent site. At that pressure we would go for a stirling engine rather than a steam turbine because the latter would be larger and much more vulnerable to hull implosion. With a stirling engine the external pressure confers an advantage in that the working fluids can be at high pressure which dramatically increases the power produced.
We are not going to pretend any of this would be easy. The mining of SMS deposits is going to come a lot quicker than electricity production. Both require a series of 'firsts' but there is a difference between prototypes that cost millions (mining robots) and prototypes that cost billions. But as far as we can assess, all the proposals we have looked at have one thing in common. In spite of the daunting challenge and the daring scale, the idea appears to be viable economically. The big vents offer very big rewards. The output energies are in the order of tens of gigawatts. It is not out of the question to garner some 15 ro 20 percent of this as electricity. As ambitious as the idea is, it never pays to bet against the calculator. The total potential is thought to be in the region of 100 to 150GW. An order of magnitude above the current geothermal install base.