>> For example it's not clear to me at all that the best artificial >> way >> to collect solar energy over a large field is with solar panels. Any method of collection solar input at the earth's surface has a maximum possible output of only 4 or 5 times what is achievable now. ie 20% versus 100%. Say 4 times as much would be "nice", but that's not so vastly much more that it would make all the difference, especially so if the cost was say twice as high per area. >> efficiencies of 10-18%, there is a lot of room to do better. A >> bunch >> of mirrors for concentrating the radiation and a custom designed >> heat >> engine for the task should be able to do considerably better. The >> theoretical maximum high temperature for the heat engine is the >> temperature at the surface of the sun, and the low is ambient air. >> The carnot efficiency should be pretty good. Even if only half of >> that is actually realized, it should still beat solar cells >> handily. Carnot efficiency is (Thot-TCold) / Thot (= deltaT/Thot). You don't need to get anywhere near solar radiation temperature to get respectable efficiencies. call Tcold 300k (27Cish) for nice round figures. At Thot = 1200 K =~ 930C you get 75% Carnot efficiency. That's enough to start with. Even at say 1000C collector temperatures you have very nasty material problems. > Now you have a few choices for heat engine. Stirlings have NOT > caught on > well have they? Could it be the pricing? Or is it that to approach > high > efficiencies they need working fluids that are caustic and expensive > to > contain? Sort of, no, sort of. Stirlings are doing very well thank you in niche applications. (cf eg www.whispergen.com * ). Costs are higher partially because volumes are low and applications tend to be specialist. WHEN someone gets some of the more major problems sorted out the Stirling engine will rule. I think Philips said something like a $US6 billion development effort was needed and a payback period after success of under a year was anticipated. Finding anyone who believes this enough to put up the $US6B(now more) is the trick. Greatest problems with Stirling engines is sealing a moving piston with little or no lubrication at high temperatures and very large pressures using a has that loves to diffuse through steel. Lower temperature lower pressure Stirling engines using air as the working fluid work just fine. but energy output per volume is "modest" making them highly unsuited to portable/mobile applications. So people start to do thinks to increase energy density. And the fun begins. First swap the working 'fluid" to Helium or better still Hydrogen. Helium will hold 5+ times as much energy as air all things being equal and hydrogen about 14 times as much (!). Both also have other advantages over air BUT are better escape artists. Hydrogen especially will escape through anything given time. At high temperatures and pressures it will happily diffuse through steel. Small gaps in rotating seals etc are even easier. Diffusion is not only not good for containment, it leads to 'hydrogen embrittlement" of metals. Needless to say, high temperature, high pressure Hydrogen is an exciting product to work with. Serious Stirling engine'ites try to run at pressures of 2000 - 4000 psi (15 - 30 MPa) and temperatures as high as materials so far won't let them. > Is there a LOW cost heat engine? One that can be made from existing > hardware > for example? Absolutely. You can make a Stirling engine that runs on under 1 degree C differential out of plastic and cardboard. Carnot efficiency is "poor". You can also use eg an old (or new) Briggs and Stratton bottom end as the basis for a modest medium temperature engine. Stirling engines are not at all hard to make if you don't want a high energy density. Something akin to the old Lister single lunger run for ever diesels is eminently doable. *AND*, because it is external combustion, it will run on about anything that you can get to burn. Here's one that was designed to run on rice husks! It's a 4.5 kW/6 HP Stirling using air at 5 atmospheres. It was envisaged in the early 1980s. Some hundreds were built but some secondary problems and at that stage falling oil prices killed them. (The main problems related to linkage problems which were not the typical problems associated with high performance Stirling engines)., This one is HUGE - a substantial reduction in size would be possible for the same power level. http://www.stirling-tech.com/images/Madras4.htm Information from the inventor here (1998 message) http://listserv.repp.org/pipermail/stoves/1998-November/004743.html Their text: ... sub-contracted by the Asia Foundation to Sunpower, Inc. in Athens, Ohio. The engine was completed in the first year of the project and by 1982 a prototype rice husk fuelled Stirling engine had been designed, built and demonstrated at Sunpower's facility in Ohio. The engine had a bore of 300 mm, displacement of 7 litres and was designed to operate at 5 bar air pressure at 600 rpm. There are many enthusiasts worldwide building various engines. Sunpower are still going and have built many wonderful and interesting products, but their main income comes from Stirling cryo coolers. A most interesting company. http://www.sunpower.com/ read "The sun, the Stirling engine, and the drive to save the world" for a good picture of Sunpower and Stirling ins and outs. > Steam engines are a possibility, but the boilers are unsafe and > valving > modifications can be difficult. I personally have seen a 5hp Brigs > and > Stratton gas engine that was modified by replacing the cam shaft to > run as a > steam engine. The Stirling engine was originally invented by a Scottish clergyman whose brother was a mine owner, in an attempt o make a safer engine than the steam engines which at that stage tended to be rather dangerous devices. > And there are other ideas.... Such as /internal/ boiler > steam > engines: All good stuff. But the basic cycles are inferior to the Carnot cycle. A good eg Rankine or Otto etc engine will outperform a bad Carnot, but sooner or later Carnot based cycles (eg Stirling) will win out due to the inherent thermodynamic superiority. it just may take 'a while' :-). >> ... I've always been intrigued by the idea of taking >> water and grabbing CO2 out of the air to make a carbon based fuel, >> like methane or methanol for example. I think the real problem is >> that there is so little carbon in the air, that concentrating it >> will >> be inefficient. The greatest problem is that if you "build a fuel" you must put energy in from somewhere. it comes down to whether you have a suitably cheap source of energy to "put into your fuel". In this case breaking the C and O bonds is going to take energy which you will get back in due course when you burn it back to CO2. RM -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist