RE Creating power from Peltier devices?

Printer-friendly versionPDF version

Forums: 

Daniel,

Certainly it is possible to generate power from a peltier device. The
Voyager, Ulysses and Cassini deep space probes all use thermoelectric power
generators. They get about 6-7% efficiency. And there are a couple
companies make special TE power supplies, mostly for remote applications
where reliability is the key factor. Thermoelectric power generators and
coolers both work on the same basic principles, but the designs are really
different.

> -----Original Message-----
> From hcdmshaloam@eritter.net [mailtohcdmshaloam@eritter.net]
> Sent Wednesday, August 19, 1998 409 PM
> To news_group@seebeck.zts.com
> Subject Re Creating power from Peltier devices?
>

>
> Is it possible to create power from a peltier device? How much heat is
> too much (hot water, steam, direct flame)?

Today's commercial devices designed to be coolers all use essentially the
same material(s) to build the thermocouples. A few manufacturers have
devices designed to work up to 200 oC, but most of them use less expensive
solders, insulators, connecters and such and don't work that hot. Or not
for long anyway. Each manufacturer will quote you their 'recommended'
upper limit, so you should check with them. But 100 oC is probably OK for
most of them.

> How much power can be
> expected (e.g. if a device uses 12V/5A to keep a cooler going, will it
> also produce 12V and how much current if I put a 40 degree C dT on the
> device?

Your idea that the effects should be 'reciprocal' is actually quite right,
but not in the way you suggest. It is a little more complicated than that.

This will be pretty crude, but you can make an estimate of the power out.
But first, lets get a couple ideas straight. If you put (i.e. force) a
temperature difference of 40 oC, it will generate some open circuit
voltage Voc. The value Voc depends mainly on dT and the number
thermocouples in series. Heat is flowing in the hot side and out the cold
side. So, you can only keep the device in this state if you 1) supply heat
to the hot side and 2) have a heat sink to take heat out of the cold side.

To get power out, you put some load across the device. Just like getting
power out of any current source, the maximum power delivered to your load
occurs when the voltage across the load is 0.5 Voc. So, you get out half
of the open circuit voltage.

Now, let's run the thing as a cooler. A cooler means you have heat running
_in_ the cold side and _out_ the hot side. We call the amount of heat
flowing in the cold side the 'heat load'. The cooler has to be able pull
this much heat away, so this is called the 'cooling capacity'. TE
manufacturers generally quote you the Maximum possible cooling capacity
Qmax and the voltage Vmax and current Imax that you use to get Maximum
cooling. Imax times Vmax gives the power input required to achieve the
maximum cooling.

The peltier cooling effect itself is proportional to the current, so more
current is better. But more current means more Joule heating, which is
proportional to the current squared. So there is a trade off, a 'best'
current.

The point of my little discussion here is to illustrate that the output
voltage and current of a TE generator depend on dT, the temperature placed
across the TE device. On the other hand, the maximum cooling capacity (and
Imax and Vmax) depend hardly at all on the temperature difference. The
temperature difference you actually achieve with the cooler depends mainly
on the heat load.

So, take a TE device built for cooling. Look up the manufacturers specs
and estimate the dT you think you can achieve in power generation mode.
For our purposes you can take Tc=300 K

____TEG_ PROPERTIES_____ _COOLER_PROPERTIES_
(TEG voltage output at maximum power) = 0.5 * (dT/Tc) * Vmax
(TEG current output at maximum power) = 0.5 * (dT/Tc) * Imax
(maximum TEG power output) = 0.5 * (dT/Tc)^2 * Qmax
= 0.25 * (dT/Tc)^2 * Imax*Vmax

Some of the larger modules (more than 1" square) might get Qmax=20-30
Watts. So, with your dT=40 oC you'll get maybe 0.2-0.3 Watts out. The
efficiency might be about 2%. That may not sound like much, but with such
a small dT you really can't expect much.

> Are there any sources for cheap peltier devices I could
> experiment with?
Sure. Lots of them. Look on ZT Links http//www.zts.com/ztlinks and surf
some of the manufacturers. Obviously you should give a preference to the
advertisers . Some of them have special offers now and then. Also,
there are some distributors listed under 'Products'.

> Has anybody tried a system using cool spring water and a solar/thermal
> hot water system?
>
> I'm looking at options for a renewable energy system (solar heat) that
> could be enhanced with biomass fuels (e.g. wood).
>
> Thanks for any info or leads you have in this area -
>
> Daniel Reynolds
> AA0NI

Don't know this last stuff. The cost per watt of power produced might be
pretty high, particularly with such low grade heat.

Best of luck!

ZU,
Cronin

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
- Cronin B. Vining, ZT Services, Inc. +
+ 2203 Johns Circle, Auburn, AL 36830-7113 USA -
- Ph. +1.334.887.2404 FAX +1.334.887.2604 +
+ e-mail vining@zts.com http//www.zts.com -
-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
+ This message is from the "ztnews" mailing list. -
+ Your e-mail address is on the list. -
+ Subscribe, Unsubscribe, view past "ztnews"s at -
+ http//www.zts.com/ztnews -
+ -
+ WARNING A REPLY TO THIS MESSAGE GOES TO EVERYONE ON THE LIST -
+ "ztnews" is brought to you by ZT Services, Inc. -
+ For thermoelectric information check out http//www.zts.com/ -
+ Please consider sponsoring these information services. -
+ -
+ Sponsored in part by The International Thermoelectric Society -
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Opinions expressed are those of the authors only.