Hot Rocks

Brian mentioned in a recent thread at LP it would be good to hear more about Hot Rocks as a power source. While we hear a lot about nuclear and wind power solving our energy needs it seems that geothermal energy is decidedly unsexy. One of the basic problems with the electricity supply is the need for 24 hour base load power generation. Currently Hydro is the main contender for a renewable base load source, but in a dry continent its scope is limited. Biofuels are another possibility but have generally been seen to be too expensive.

Probably the most promising renewable base load solution is geothermal energy. The first geothermal electricity plant was constructed in 1904 in Italy. These plants used hydrothermal power, relying on underground aquifers venting steam or high-pressure hot water to the surface. There is some potential for this in Australia ans there is a limited amount of electricity currently generated by this method in Birdsville, Queensland.

There are other Australian schemes for harnessing Geothermal power which are currently being investigated. Over the past few years a number of Australian companies have listed on the stock exchange with their primary goal being to explore and tap hot dry rocks. These are based around the idea of injecting water into rocks kilometres below the surface and extracting the superheated high-pressure water coming back out to drive generators.

From the website of Petratherm:

HR geothermal energy has the potential to supply large-scale base-load electricity that is cost competitive with the fossil fuel market. Petratherm’s modelling indicates a conventional midsized power station (300 MWe) will produce power at a cost comparable to that of gas (approximately $40 MWh). By increasing plant size to 1000 MWe and/or implementing the latest developments in geothermal heat processing these costs may even undercut average coal- fired production costs, which are presently the cheapest form of electricity production.

There are several difficulties with this technology, the first being temperature. The rocks have to be hot, not just warm, but hot with the efficient temperature being greater than 220 degrees C. The second thing is drilling costs. In general the deeper you drill the hotter the rocks become, but typically this increases at around 17 – 30 deg. C/km. Currently drilling costs begin to increase dramatically past a depth of 3kms thus usually needing over 8kms of drilling to get the required temperature makes the cost prohibitive. In some places where there is a blanket of insulating sedimentary material above it, certain rocks may be much warmer close to the surface. Fortunately according to Geodynamics, Australia has ample amounts of this.

One cubic kilometre of hot granite at 250 deg. C has the stored energy equivalent of 40 million barrels of oil when the heat is extracted to a temperature of 150 deg. C.

Australia is known to have several thousand cubic kilometres of identified high heat producing granites and these have the potential to meet the total electricity demand of the country for hundreds of years….

…we have extremely large quantities of hot dry rocks within 3 – 5km of the surface, in favourable locations throughout Australia.

Most of the exploratory activity is currently being carried out in South Australia, although some exploration is also being carried out in some locations in New South Wales.

If the exploratory projects are able to generate the type of power at the low cost that is claimed, hot rock geothermal energy will form a significant part of the solution to CO2 emission in Australia. It does not emit CO2, doesn’t create hazardous waste and does provide baseload power. Hot rock geothermal does suffer from having large upfront capital costs in drilling, but has minimal ongoing costs once established as it doesn’t require fuel. Fortunately, due to experience from other industries, and a great deal of research worldwide this drilling cost has come down significantly and some of the potential sites are close to populated areas and established transmission lines. However, it is still an unproven technology and the difficulties may be greater than imagined, but the potential clean energy resource is vast.

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2 Responses to Hot Rocks

  1. […] To follow up on an earlier posting I did on this topic, it appears that Petratherm one of several companies trying to develop electricity source from hot subterranean rocks has has some good results recently, and is moving on to the first stages of drilling to produce a pilot power plant. […]

  2. […] Old news, but I didn’t read it at the time and given I’ve mentioned Geothermal energy in Australia before, I thought I’d link to it anyway. The Economist from nearly a month ago mentions some developments that have been making geothermal power at either extreme (low or high temperature) more usable. The low temperature part I think is particularly interesting. Not all geothermal activity is hot enough to bring water to the boil. The Chena hot springs, in Alaska, for example, are just right for bathers, at a porridge-like 43°C, but not much use for traditional geothermal power generation. Even within the spa’s wells, the water is only 74°C. Nonetheless, its owners, in conjunction with United Technologies, an engineering conglomerate, have worked out how to generate power from the tepid flow—the coldest ever used in a geothermal plant. […]

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