http://www.physorg.com/news88683362.html

This article stimulates a thought I have once in a while about home heating. I realize that deep drilling is generally necessary to produce industrial volumes of steam, but I do wonder just how deep one must go to generate just 120 degree water that might be used in a home heat exchanger.

Bob Winners

Any temperature above local atmospheric ambient is useful using a heat pump. Has anyone done a depth vs installation cost vs efficiency trade study. Even shallow heat exchangers make economic sense in South Dakota.

A couple of thoughts:
SDMike, what you are describing is (from what I know) called a "geothermal heat pump". A heat exchange line (for fluid) is buried under 6+ feet of dirt, and supplies an electric heat pump. It is the same thing as a regular heat pump (which uses air), and is from what I understand the most economical method of heating and cooling a house. Supposedly the second best is a "gas assisted heat pump" (also called a dual fuel heat pump).

Bob Winners
Any depth involved with producing 120 degrees F would still be pretty far down. You would have to transfer the working heat to the surface which would involve a pump, which would increase your cost of operation. I doubt that you could break even, much less make a ROI within a reasonable amount of time. With the systems they are describing here, the water is so hot that it turns to steam and rises without any outside action. In theory the cost of raising the water to the surface would be extracted from the heating of the water.

Now these systems still are not 100 % efficient in converting steam to electricity, so the exhaust (waste) heat would probably be in the range of what you are talking about, and could be used potentially. I believe this is being done in Europe in one of the countries that utilize a lot of Geothermal power. I believe they pump hot water throughout the country (well, probably city) to houses that use it to heat their hot water.

Now my question: what are the large scale / long term impacts of prematurely "cooling" the earth? I realize that this seems trivial, but if you start dropping these systems in all over the place and have them running for a long time, what happens? The idea that the heat is "trapped" is an over simplification, it is still radiating out in all directions, and has an impact on activity at the surface or even atmospheric levels. Think about it, what occurs if this concept becomes a major power source, and is supplying 50+% of all power generation on the planet?

This might seem silly, but a lot of arguments take place today about CO2 emissions, and if you were to ask Henry Ford what he thought the impact of his car would have on the environment, he probably would have laughed at the idea that an invisible gas coming out the tail pipe would be the biggest point of contention in science and politics in 100 years.

I'd sure like to read the original MIT report. The press release just leave so many questions unanswered. How deep? Where? Is the rock dry? wet? permeable? What temperatures would be available? Would dissolved salts be a problem? What kind of heat sink is available at the surface? What Carnot efficiencies might be available? What volume of rock is available? Without this kind of information, it's impossible to evaluate the proposal.

Jim shea

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I may be 13, but this realy intrests me.