On 30 June 2014 20:55, William "Chops" Westfield wrote: RM said: > > small but steady temperature differential twixt down-there and surface > may allow power generation. > > I thought someone said that poor conduction of soil was a problem? You > have a nice steady temperature difference because underground is isolated > from heat sources. Once you start pumping heat down there, via heat pump > or thermoelectric devices or whatever, you start heating up the "local" > part of underground, and if conduction is poor (which is must be, or > underground would be the same temperature as above-ground, essentially), = it > rapidly approaches the surface temperature and your scheme stops working > very well. (Presumably, this is why ground source heat pumps don't work = as > well as people think.) > > I was thinking of the large Coober Peddy underground spaces represented b= y end-of-production mines. The surface areas are large compared to many pipe systems and they can be 'rather deep'. It is illegal to backfill a mine! - they MUST be left open. I believe this relates to the risk of buying people alive inadvertently (or vertently). This is in exchange for the existence of many many thousands of open vertical mine shafts - a real hazard. Falling into one would probably be fatal in most cases. A lot of "thermal energy harvesting" seems to have a bigger problem keeping > the cool end cool, than providing heat for the hot end. Stirling engines= , > too. > Yes. Maintaining "delta-T" is essential. In high temperature above ambient systems a modest rise in sink temperature does not hurt efficiency much and C/Watt rating of heat sinks can be realistic. In energy harvesting systems with delta T under or much under 100C sink temperature rises can have major implications. This was (just) one of the problems faced by the ill fated and obviously doomed aborning Kickstarter Epiphany 1 hockeyt puck Stirling engine power generator. https://www.kickstarter.com/projects/epiphanylabs/epiphany-one-puck Many unhappy backers and legal action possible https://www.kickstarter.com/projects/epiphanylabs/epiphany-one-puck/comment= s Carnot (max theoretical efficiencies with cold side =3D 27C ~=3D 300k are 66% at 900 K / 627C 50% at 600K / 327C 25% at 400K / 127C 20% at 373K / 100C 15% at 353K / 80C 9% at 333 K / 60C 4% at 313K / 40C Best practice at 6500C + gives > 50% Carnot in practice or say 30-40% absolute. I've seen over 50% absolute claimed. As you get down round 100C it gets immensely hard to get high fractions of Carnot Z. At 100C 10% absolute would be superb and 5% would be good. Compare this to a TMG or good Peltier - at 100C hot side and say 20C cold I've seen figures of 5% + - but it's very easy to get less. Russell --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .