Best way to turn hot air into kinetic or electric energy

[mntash]

So I know that you can use energy recovery units (heat plate, heat pipe and energy recovery wheels) to reclaim the heat in air, but it only puts it back into the air (like in winter conditions).

What if you wanted to turn the heat in air into kinetic or electric energy?

Hot air turning fans (mini-turbines?) and charging batteries?

Just thought we could kick around some ideas... thanks!

 

[MintJulep]

Stirling engine.

 

[zdas04]

How much hot air are we talking about here? How many BTUs per unit time? If you are considering a low-intensity source like the attic vent on a building I think you'd be hard pressed to turn that into useful quantites of kinetic energy.

One thing that I've been thinking of is using the heat of compression in a compressor (pull 250-300F gas off upstream of the cooler) to make steam that can drive a turbine. The numbers work, but even with a really big compressor you don't get a lot of kW out the back end. This sort of combined cycle can increase your system efficiency by 5-10% which is nothing to sneer at, but it isn't huge.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[willard3]

Turbochargers do this specifically, ie, take a high temp/high pressure gas stream and convert it to mechanical energy to run a compressor.

No reason it couldn't run a generator.

 

[mntash]

Hi everyone and thank you ALL for your replies, it is really appreciated.

As far as heat output, we are probably looking at 60,000-100,000 BTU/h coming out of a space approximately 3-4 sq. ft. Not exactly sure of the CFM of hot air as of yet.

Thanks!

 

[moltenmetal]

100,000 BTU/hr isn't enough to bother recovering, unfortunately. The payback on whatever capital you spend will be infinite.

 

[IRstuff]

Generally, the conversion efficiencies are what kills most apparently plausible concepts. While 100 kBTU/hr seems large, being equivalent to 6 large stovetop burners, the "quality" is low, being dispersed in a gigantic volume of air.

The efficiency of most thermoelectric conversions are incredibly poor, and brute-force, to boot.

TTFN

FAQ731-376

 

[CH5OH]

a typical heat exchanger starts "doing its job" with delta T>10°C
an ericson cycle (closed gassturbine loop) would need a much bigger delta T (>500°C)
a stirling engine would probably work on paper, however imagine a teacup stirling model engine.
and multiply kw "your project"/kw model ratio with the surface area of the displacer piston
or the surface area of the heat exchanger.

recovery wheel heat recovery efficiency is around 95%
cost savings = 0.95 x cost fuel/ kW x kW "your project"
rotating energy heat recovery efficiency is around 30%
cost savings = 0.25 x cost electricity/ kW x kW "your project"
(if you manage to recover more than 1/4 of the waste energy into electricity, have a run to the patent bureau)

 

[moltenmetal]

100,000 BTU/hr is modestly more than what my home furnace puts out. Does that put it in perspective?

 

[zdas04]

My on-demand water heater in my home is 165,000 BTU/hr.

 

[mntash]

So 100,000 BTU/hr isn't much, but what if this were multiplied by Qty (20), or even Qty. (100). This "project" has the possibility to pull 100,000 BTU/h times 100... perhaps several hundred in some cases.

 

[dcasto]

A turbo charger running a generator will not work. It will back pressure on the engine and you will get less power from the engine.

A 100,000 BTU steam has less than 20 KW or $3.00 US per hour in electricity or $1.00 in natural gas.