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Thread: "Hot Rock" Energy. Clean, renewable energy alternative for the future?

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    Senior Member Vanir's Avatar
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    Post "Hot Rock" energy. Clean, reliable & renewable energy for the Future?

    Hot Rock Energy, clean & reliable energy source for the future?
    http://hotrock.anu.edu.au/

    The Concept
    Hot Rock Energy is a vast, environmentally friendly, economically attractive energy source. As this diagram shows, the concept is very simple.

    Water is injected into a borehole and circulated through a "heat exchanger" of hot cracked rock several kilometres below the surface. The water is heated through contact with the rock and is then returned to the surface through another borehole where it is used to generate electricity. The water is then re-injected into the first borehole to be reheated and used again.

    The Hot Rock Energy system works with 2 closed circulation loops:


    1. The subsurface loop
    This loop circulates water down an injection borehole where it passes through the underground "heat exchanger" and is heated. The superheated water is then recovered by one or more production boreholes which return it under pressure to the surface.
    By keeping the water under pressure and preventing it turning to steam, any materials dissolved from the underground rock mass (such as silica or carbonates) are kept in solution and can be returned to the ground.
    At the surface, the superheated water is passed through a metal heat exchanger where most of the heat is removed. The now cooled water is then returned to the injection borehole where it is sent down again to recover more heat.

    2. The power station loop
    At the surface a second closed loop fluid system is used to transfer the heat into the power station and generate the electricity in a turbine.
    The fluid used in the power station loop can be water, but more usually a lower boiling point fluid is used. Organic fluids such as refrigerants and iso-pentane are often used.

    The Energy
    Because the heat used in the Hot Rock Energy process is eventually replaced by the Earth, this energy source has been classified as renewable energy by agencies such as the International Energy Agency and the Australian Greenhouse Office.

    The median global heat flow through Earth's surface is around 60 mWm-2. While this figure is small in comparison to mean global insolation (solar energy), which is around 1400 Wm-2, the temperature in Earth's crust nevertheless increases worldwide at an average rate of around 17 - 30oC/km. This means that temperatures high enough to produce energy are quite accessible in many places worldwide.
    As is discussed in the Economics section though, the cost of drilling is the biggest single cost involved in using Hot Rock Energy. It is therefore necessary at present to identify areas that are suitable for energy extraction at only moderate depths. Future developments using new technologies such as largely automated drilling and advanced drilling bit designs, may well be able to economically tap Hot Rock Energy almost anywhere on Earth.

    Building a Hot Rock Energy System
    Heat is extracted by pumping water through an engineered heat exchanger connecting two or more wells. This heat exchanger is a volume of hot dry rock with enhanced permeability. It is fabricated by hydraulic stimulation. This involves pumping high pressure water into the pre-existing fracture system that is present in all rocks to varying degrees. The high pressure water opens the stressed natural fractures and facilitates micro-slippage along them. When the water pressure is released, the fractures close once more but the slippage that occurred prevents them from mating perfectly again. The result is a million-fold permanent increase in permeability along the fracture systems and a heat exchanger that can be used to extract energy.

    In a typical system, an initial borehole is sunk into the hot rock mass and a hydraulic stimulation is performed. A three dimensional microseismic network deployed on the surface and in nearby wells is used to record "acoustic emissions" (i.e. small noises) caused by the slipping fractures. The network records the locations of the acoustic emissions while pumping continues over several weeks. In this way, the progress of the stimulation is monitored and the size and shape of the growing heat exchanger is mapped.
    A second well is then drilled into the margin of the heat exchanger 500 metre or more from the first well. Now water can be pumped through the underground heat exchanger and in superheated form it can be returned to the surface. There it can have its energy extracted before being reinjected to go around the loop again.

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    Senior Member Hengist Duval's Avatar
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    Does not work very well in Scandinavia, where Earth's crust is very thick and old. You need to drill very deep here to get reasonable heat for example running turbogenerator.

    Better way is to use local uranium deposits as raw material for nuclear fuel and use nuclear power instead(Full fuel cycle needed, achievable through domestic machining industry). Waste products could be buried deep to stabile base rock, like they are planning to do here in Finland.

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    Senior Member Sigel's Avatar
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    Great idea. It seems odd that we are all sitting on top of an enormous power station and it never gets used.
    A people which takes no pride in the noble achievements of remote ancestors
    will never achieve anything worthy to be remembered with pride by remote descendents.

    Lord Macauley

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    Geothermal energy has been in use since 1906 at Lardarello in Italy. There is also a geothermal power plant at "The Geysers" in California and near Mt. Keflavik in Iceland. Reykjavik is heated by geothermal energy and geothermal heat has been used in Boise, Idaho. Unfortunately, the places in the world where the geothermal gradient (the rate at which temperature increases downward) is high enough for geothermal wells of feasible depth to be used are few and often far from centers of energy-consuming population. Most of these areas are already being exploited and there is little likelihood that more can be found. Satellite data could reveal their presence, but nothing worth trying to use has been found. Much deeper geothermal wells could be used almost anywhere. However, the cost of sinking them is not competitive with other energy sources.

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    Quote Originally Posted by Egil Skallagrimsson
    Geothermal energy has been in use since 1906 at Lardarello in Italy. There is also a geothermal power plant at "The Geysers" in California and near Mt. Keflavik in Iceland. Reykjavik is heated by geothermal energy and geothermal heat has been used in Boise, Idaho. Unfortunately, the places in the world where the geothermal gradient (the rate at which temperature increases downward) is high enough for geothermal wells of feasible depth to be used are few and often far from centers of energy-consuming population. Most of these areas are already being exploited and there is little likelihood that more can be found. Satellite data could reveal their presence, but nothing worth trying to use has been found. Much deeper geothermal wells could be used almost anywhere. However, the cost of sinking them is not competitive with other energy sources.
    That is the problem. I saw a building program on TV where they give ideas about building homes, etc. This program came from Wisconson so it was cold there. They used a system of tunnels, not vertical but more horizontal from the basement into the earth. They claimed a 57 degree F. or so figure was involved and it was almost constant throughout the year. This meant that this air could be pumped into the house and heated in the winter (as opposed to very cold air outside) and in the summer it could just be pumped into the house instead of air conditioning (hot an damp summers).

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    I haven't seen the TV program, but the claim of 57º F. seems very dubious to me. "Earth temperature" as in caves, wine cellars, root cellars, etc. is usually in the 40's. However, even if the air temperature in the tunnels were as high as 57º F. as soon as it is pumped out for use in domestic heating or cooling, it will be replaced by colder or warmer air from outside. Soil and rock are both poor conductors of heat. The fresh air will attain the temperature of the walls of the tunnel only very slowly. However slowly air may be withdrawn from the tunnel, the air temperature in the tunnel will gradually approach that of the outside air. A far more efficient method of heating without using fossil fuel directly to heat the air in a house, but which requires a nearby body of water, is to use a heat pump, basically a reversed refrigerator which takes heat out of a body of water (which has a high heat capacity) and transfers it into the house. This does work. I studied physics under a young man from Lousiana who heated his home there by this method. Though the heat pump does, of course, use electrical energy, it is much more efficient than electric heating.

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    Corrigendum: The temperature of cold water from the tap after it has been allowed to run foir a few minutes should be close to earth temperature. Having measured that temperature here in central CT this morning, I find it to be about 52º F. , so the 57º F. temperature cited above is not unreasonable.

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    Also, a comment on heat pumps. If the outdoor temperature falls below the freezing point of water for long periods of time, i.e. days at a time, a heat pump will not operate efficiently. Apparently, that doesn't happen in Louisiana.

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    There are many deep caves in California and Oregon and they are chilly if you are down there more than an hour but the temperature is in the 50s (F).

    There is a type of pump like a refrigerator reversed in which fluid filled rods are allowed to expand, absorbing heat outside and then the fluid is compressed inside, yeilding the heat. This type of pump can be made to heat interiors even in the Arctic. In fact, Nazi writer Wilhelm Landig says the Germans experimented with this type of heating in one of their Canadian Arctic bases. It will work up to some minus figure in outside temperature.

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    Quote Originally Posted by Egil Skallagrimsson
    Also, a comment on heat pumps. If the outdoor temperature falls below the freezing point of water for long periods of time, i.e. days at a time, a heat pump will not operate efficiently. Apparently, that doesn't happen in Louisiana.
    True. I live in northern suburban section of New Orleans. Our coldest winters on average will peak in the low to mid 20's for a few days. Hard freezes are not regular but occassionaly happen but not for long. Mild freezes always last no more than a day (though more frequently.)

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