Heat > Electricity conversion (Seebeck/Peltier effect)

[lifer]

I've been searching for informations about Peltier modules but I didn't find any evidence that those Peltier modules can be used in reverse (to convert heat into electricity).
Also, I didn't find anything about their conversion efficiency.

Is there any other device to convert heat to electricity (diy or commercialy available)? Thanks in advance for any clue.   

[hydrosun]

any peltier can be used in both directions, but some are optimized for one use. Higher efficiency peltiers for electricity production are still in the single digit efficiency range. The trick with using peltier technology is to get the highest temperature difference between th etwo sides without exceeding the maximum temperture that destroys the junction. There are some off the shelf products for sale, like the woodstove fan and pots that are filled with water an dput on a burner to recharge usb devices. So for small amounts of power and relative  high costs you can use this technology. But solar panels and batteries are a much less costly alternative.
Chris

[DamonHD]

When used to generate electricity this is known as Seebeck effect; I have a couple of devices sitting (unused, boo!) on my desk.

You need a big temperature differential to have a hope of getting much out of them.

Rgds

Damon

[lifer]

Actually, I was thinking about replacing the batteries with a big water tank, heated by an electric heater powered by a wind turbine. This way, you can store a large amount of energy without the "deep discharge" effect and those charging limitations of lead-acid batteries. You can heat the water as long as wind blows, no matter how small is the output power.

You can mount the Peltier module directly on the tank surface (or through some kind of radiator) and keep the other side in some kind of "ocean" (a large open water tank).
This way, you can (at least) maintain the ambient temperature on one side (20-30 Celsius) and a much higher temperature on the other side.
I just didn't find anything related to commercial Peltier modules efficiency.

All the Peltier I've seen seems to be adapted for CPU cooling (or stuff like that). Maybe a large surface (and suitable for radiator mounting) might be an advantage for thermal to electricity conversion. Do you know about something like this?

PS: Many people currently use a dump load anyway for their PV panles or wind turbine, so this might be a good chance to recover a part of that waste energy.

LE: @Damon: The temperature differential it's in 50-60 Celsius range.. so if you have 20-30 C on one side (the ambient temperature) I guess it's not that hard to have some water heated at 70-90 Celsius. But I have to review the specificatoins (performance curves) for smaller temperature differential (does the output power decrease very quick?).

[Mary B]

The efficiency is limited by the junctions in the device and their ability to withstand heat. I have a couple big 450 watt ones laying on my desk I was thinking of using for a dump load when I get around to building a small wind generator. Mount them in a super high efficiency cooler and keep my water and pop cold.

[lifer]

Yes, I was surprissed to read about that 138 Celsius limit for the "hottest" side of the Peltier module. In the datasheet, they claims that the limit is dictated by the melting point of the soldering (are they serious?!).. I thought that any semiconductor can withstand much more than that so I really don't get the true cause of that limitation.

@MaryAlana, have you "played" with the Peltier module (the reverse operation)? I'm curious about the conversion curve.. it's a linear or a logarithmic/exponential one?

[DamonHD]

For the sort of temperature I could get between hot water and air when I was considering using them as a part of a generative heat dump from solar thermal, I expected efficiency of ~2%, IIRC.

Roughly the equivalent of using 6% solar PV instead for the roof area concerned.

Rgds

Damon

[lifer]

I think that a most important factor is to properly insulate the two sides and to provide an efficient method (putting it on a radiator) to cool the "cold" side.
From the diagrams, seems like the output power decrease linear with the temperature differential.. still I don't understand if it's a voltage source or a current source (like PV panels). 

[joestue]

there is a website that properly explains operation both backwards and forwards for these things and i wish i could find it right now but to make it simple:

most efficient thermodynamic operation is at approximately half the maximum thermal difference possible, this is something i've understood for some time but its subject to various situation dependencies.

heat transferred via electrical means is proportional to current.
heat conducted to both sides is proportional to current SQUARED
heat conducted from hot to cold is proportional to thermal difference.

given those suppositions, electrical efficiency and thermal efficiency can be calculated for various chemistries and situations, but what it comes down to is the 5$ 40mm^2 packages cannot create a thermal temp difference greater than 60C. and that's only theoretical.

so end result is you need to calculate how much area of semiconductor you need to carry the heat.

for a specific application i slit a 40mm^2 package in thirds, and glued the third to a heat spreader and stuck it on top of a full package, ran both in series and ran them off 12vdc. that's one example. this attempt failed because i didn't properly seal up the package, and the water intrusion caused it to fail quickly, within a dozen hours.

for high temperature heat recovery something similar would probably have to be done.
buy high temperature expensive units and stack them on top of low efficiency cheap low temperature units.
but you have to calculate what the area ratios are to get the most power.

[lifer]

Quoted from here:

"For over a century thermocouples were made from metallic conductors and though many different metals were investigated, efficiencies rarely exceeded 3%. With the advent of semiconductors the efficiency of thermoelectric generators was greatly increased and by the 1950's, generator efficiencies had reached 5% [..]."

Well, that's kind of funny! "The advent of technology".. there's no hope, I guess.

@joestue, thank you very much for your informations but I think it's a dead end. Do you guys have any informations about some kind of turbine to convert the thermal energy into electricity (or at least into some mechanical form, to run a PMG or something)?

[Mary B]

Never bothered playing with one to generate electricity, solar panels are far more efficient.

[XeonPony]

using a closed loop gass turbine you could do some thing, using a refrigerent instead of steam, but it would get very complex, and expensive, as stated, Pv is cheaper easier and more reliable.

As for free TEC chips, water coolers, allot use a 50w tec in them

[lifer]

My main interest in thermal energy is for storage purpose only. It seems to be cheaper to have a water tank heated by a wind turbine then to extract the specific amount of energy of your needs.

Anyway, the PV cell converts light (not heat) into electricity so it's a different story.

[XeonPony]

yes we know exactly what you want to do, but we all so know from experiance it ain't going to happen in a uselfull way.

So there are your options.

Rankin cycle using R-290, Peltier, or PV and battery

I use the heat directly for comfert, then use the pv for power personaly.

and as I say from certain water coolers you can harvest a 50watt peltier, 2 NTC 10K thermisters, and if lucky a really nice toleroid transformer 110v to 24 volt center tapped, some rectifiers a 741 op amp, and a few other goodies.

[DamonHD]

1) Matching a wind turbine electrical output to a water heater input is surprisingly hard it seems, at least without some kind of MPPT or buck/boost or similar mechanism (at which point you might consider a battery of some sort to help with the matching), or a clever array of heaters to be switched in and out to match power and voltage.

2) You can easily capture and store heat or electricity with solar and convert back and forth (inefficiently or expensively or both).  Be clear that a kWh of low-grade heat energy has nothing like the capability (ability to do useful work) nor convenience of a kWh of electricity.

Rgds

Damon

[Mary B]

Damon how about a wide range dc-dc converter feeding an inverter for 120/240 volts to use with standard water heaters. Would have to be some hefty equipment but doable.

[DamonHD]

Yes, that's the sort of thing that might work I imagine.

But mainly I'm pointing out that it's distinctly non-trivial because of the the fact that the voltage changes along with the available power but the usual heating loads have fixed resistance.

As Kermit would say...

Rgds

Damon