Russell: So the sterling engine puck will deliver well under 5 mA under the best con= ditions. Is that correct, or did I lose too many zeroes? Sent from my phone > On Jun 30, 2014, at 2:31, RussellMc wrote: >=20 > On 30 June 2014 20:55, William "Chops" Westfield wrote: >=20 > RM said: >=20 >>> small but steady temperature differential twixt down-there and surface >> may allow power generation. >>=20 >> I thought someone said that poor conduction of soil was a problem? You >> have a nice steady temperature difference because underground is isolate= d >> from heat sources. Once you start pumping heat down there, via heat pum= p >> 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.) >>=20 >> I was thinking of the large Coober Peddy underground spaces represented = by > end-of-production mines. The surface areas are large compared to many pip= e > systems and they can be 'rather deep'. >=20 > It is illegal to backfill a mine! - they MUST be left open. I believe thi= s > 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. >=20 > A lot of "thermal energy harvesting" seems to have a bigger problem keepi= ng >> the cool end cool, than providing heat for the hot end. Stirling engine= s, >> too. >=20 > Yes. Maintaining "delta-T" is essential. In high temperature above ambien= t > systems a modest rise in sink temperature does not hurt efficiency much a= nd > 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. >=20 > 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. >=20 >=20 > https://www.kickstarter.com/projects/epiphanylabs/epiphany-one-puck >=20 > Many unhappy backers and legal action possible >=20 >=20 > https://www.kickstarter.com/projects/epiphanylabs/epiphany-one-puck/comme= nts >=20 > 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 >=20 > Best practice at 6500C + gives > 50% Carnot in practice or say 30-40% > absolute. > I've seen over 50% absolute claimed. >=20 > As you get down round 100C it gets immensely hard to get high fractions o= f > Carnot Z. > At 100C 10% absolute would be superb and 5% would be good. >=20 > Compare this to a TMG or good Peltier - at 100C hot side and say 20C col= d > I've seen figures of 5% + - but it's very easy to get less. >=20 >=20 > 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 --=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 .