Electronics > Power/Renewable Energy/EV's

Small scale, efficient steam generator

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elektrinis:
I have a theoretical problem on my hands. There is an industrial process, generating a byproduct in form of hot (25C) air with power of around 20kW. It's a real shame to see all that energy being wasted, very tempting to recover that in to electricity somehow.
I am obviously not going to build anything, just want to discuss this issue.

Now, currently the "industry standard" way of dealing with this is steam generators. Obviously, water will not work in this case, as temperature is only 25C. To pull this off, one would need a heat pump to up the temperature from 25C to above 100C to boil water and get enough steam to drive the turbine, or find other liquid, that boils at much lower temperatures, like a common r134 refrigerant. Now, you can't dump refrigerant to atmosphere, so you would end up with a closed loop. Which sounds good, as it would allow full energy recovery.
The more I think about this, the more closer I'm getting to a conventional heat pump, but with a turbine and alternator in place of expansion valve.

Again, obviously, if this were as easy, we would have many products by now, that do just that. So I am likely doing some false assumptions at a fundamental level.
I did find several patents on this matter (which does not mean much, but it's definitely not new idea), also a legend about a company offering such products in the '90s (that went bust). In is being referenced by some perpetuum mobile activists and I would certainly hate to end up in that mine field.
If there's someone here well familiar with heat pumps and steam turbines, I would love to hear some thoughts where my error is.

elektrinis:
Well, it seems it's not impossible:
http://www.fujielectric.com/company/research_development/theme/heatpump.html

I consider Fuji a credible source...

Gregg:
If usable power could be extracted from 25 degree C air, otherwise useless politicians and other nefarious groups that blow a lot of hot air could actually provide some value for their presence.
Steam, small, efficient: you can only achieve two out of the three.  Steam is a wonderful energy transfer medium but it requires a large temperature differential and is only efficient when exhausted to vacuum.  Steam powered ships are reasonably efficient because they have large bodies of water to cool condensers.  Nuclear power uses steam because of the high reactor temperatures but the cooling towers are immense to get reasonable efficiency.  The problem with steam is all of the ancillary equipment and associated maintenance needed.
There are volumes written on the thermodynamics of steam; look at some steam tables for comparisons of temperature differentials and steam pressures.
A sterling cycle engine can be made to rotate with 25C heat, but it still requires a temperature differential to work.

elektrinis:
Found a book on thermodynamics and gave it a good read. Now I get the complexity. Thanks for your input.

SeanB:
Is the hot air 25c above ambient, or is it 25C  above zero. Waste low grade heat can be used in either a Stirling engine ( reasonably efficient, though at 25c differential it will be poor) or a simple peltier thermal generator, which will give some power at abysmal efficiency, around 3% probably.  Will around 600W be useful power for this.

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