Creating electricity from heat (without steam)

[Tommyboyblitz]

I have a heat source in the form of a waste oil heater for a small workshop, i get around 200 degrees C from it and it warms the workshop up nicely.

What i am wondering is is there a way i could produce enough electricty to run a small water pump from the excess heat? Something like a 12v pump or enough electricity to charge a battery which will power the pump on demand?

Ive looked at peltiers (i have no experience with these)but don't think they could give me the sort of power i would need

Idealy it would be compact but id like to try anything.

Cheers

 

[Simon Bridge]

Don't like thermoelectric effect devices?
How about a heat engine - say, a Sterling engine?

 

[Tommyboyblitz]

Don't like thermoelectric effect devices?
How about a heat engine - say, a Sterling engine?

Its not i don't like them, i just don't know anything about them and the ones I've seen power led's etc?

 

[sophiecentaur]

How much actual thermal Power is available? The maximum possible efficiency of any heat engine (including peltier) depends upon the (absolute scale) temperature difference between the heat source and heat sink.
η = 1- T_cold / T_hot
You would need to provide a cold sink that's kept significantly cooler than 200C and, of course, the hot side must be in good contact with the stove, with enough thermal power actually getting to it without cooling the stove down.
I have a wood burning stove with a peltier driven fan that sits on the top. The peltier unit is about 6 square cms in area. The hot sink is a foot, in contact with the stove top and the 'cold sink' is just the top set of fins, which the fan cools by drawing in air from the room so the temperature difference can't be many degrees. It is really feeble and just about manages to stir air out into the room, rather than just convecting up to the ceiling. (Looks cool though) It's obviously a poor cold sink and you could improve on it.
Basically, you don't get anything for nothing so a good design for a heat operated generator would require a fair energy input in order to produce appreciable energy output. You're into 'Combined Heat and Power' systems. A small fan (better than mine) or a very low power pump would be a reasonable expectation (and a good fun proj) but charging a battery etc. etc. would call for more than just 'harvesting' (that's the word of the moment) would be another matter, I think.

 

[Simon Bridge]

Not knowing about something is easy to fix.
The main issue is exactly as sophie points out above...

 

[Tommyboyblitz]

How much actual thermal Power is available? The maximum possible efficiency of any heat engine (including peltier) depends upon the (absolute scale) temperature difference between the heat source and heat sink.
η = 1- T_cold / T_hot
You would need to provide a cold sink that's kept significantly cooler than 200C and, of course, the hot side must be in good contact with the stove, with enough thermal power actually getting to it without cooling the stove down.
I have a wood burning stove with a peltier driven fan that sits on the top. The peltier unit is about 6 square cms in area. The hot sink is a foot, in contact with the stove top and the 'cold sink' is just the top set of fins, which the fan cools by drawing in air from the room so the temperature difference can't be many degrees. It is really feeble and just about manages to stir air out into the room, rather than just convecting up to the ceiling. (Looks cool though) It's obviously a poor cold sink and you could improve on it.
Basically, you don't get anything for nothing so a good design for a heat operated generator would require a fair energy input in order to produce appreciable energy output. You're into 'Combined Heat and Power' systems. A small fan (better than mine) or a very low power pump would be a reasonable expectation (and a good fun proj) but charging a battery etc. etc. would call for more than just 'harvesting' (that's the word of the moment) would be another matter, I think.

Thanks for the reply it has been helpful

The burner generates a lot of heat, at its base where the main burner is its up over 500C and i could probably get it a lot hotter by adjusting the air and fuel flow. The part which spreads the heat is an old 15kg butane gas cylinder which heats up to well over 200C. I used 200C as a base point as the tempreture varies a fair amount. I use it to boil a kettle which takes longer than if i was to boil the same kettle over a gas stove. I couldn't really give any numbers of figures.

The peltiers don't sound like they could be enough to provide enough power for a pump but will look into them. Cooling the other side of them would be a problem as everything around the burner gets warm/hot.

Also been looking at a sterling engine which looks like it would be fun to make, but maybe unreliable.

Not knowing about something is easy to fix.
The main issue is exactly as sophie points out above...

Well this is why i made the topic, i have looked into sterling engines and peltiers. Iwas wondering if thete was anything else i could try.

The problem is theory is allgood until trying to put into practice. I don't want to buy stuff that won't work or doesn't do the job.

At the moment i would probably be better off using a solar panel on the roof to charge up a couple of batteries to run pumps and more.

 

[Tommyboyblitz]

Sorry if my spelling and grammer is poor. Typing on the phone

 

[sophiecentaur]

Thanks for the reply it has been helpful

The burner generates a lot of heat, at its base where the main burner is its up over 500C and i could probably get it a lot hotter by adjusting the air and fuel flow. The part which spreads the heat is an old 15kg butane gas cylinder which heats up to well over 200C. I used 200C as a base point as the tempreture varies a fair amount. I use it to boil a kettle which takes longer than if i was to boil the same kettle over a gas stove. I couldn't really give any numbers of figures.

The peltiers don't sound like they could be enough to provide enough power for a pump but will look into them. Cooling the other side of them would be a problem as everything around the burner gets warm/hot.

Also been looking at a sterling engine which looks like it would be fun to make, but maybe unreliable.
Well this is why i made the topic, i have looked into sterling engines and peltiers. Iwas wondering if thete was anything else i could try.

The problem is theory is allgood until trying to put into practice. I don't want to buy stuff that won't work or doesn't do the job.

At the moment i would probably be better off using a solar panel on the roof to charge up a couple of batteries to run pumps and more.

I am keeping this at the level of a proper Engineering Project because a less stringent approach will surely leave you either with something that just doesn't do the job or a vastly overdone system that could cost you hundreds of Dollars or Pounds. The road of experience is littered with failed projects that could have been predicted to fail even before they were started. So the folowing is very important.
You would need to specify the actual Power required and the energy (kWh) needed per day i.e what job your electricity supply needs to do. What counts is not just the temperature but the Power available to maintain that temperatures of the hot and cold sinks. That requires the fuel input to be sufficient, the cooling to cope with all the waste heat and keep the cold sink at the required temperature. Cooling could require a good circulation of water and an efficient radiator (vehicle?), otherwise the temperature difference you are actually getting could be very modest (a lot less than you expect).
If you want to estimate how much energy is available from your heater, you could put a kettle of water on top of the stove and see how fast its temperature is raised. That will tell you whether you're talking Watts, Tens of Watts, Hundreds of Watts of kiloWatts. A fun experiment which only requires a kettle, a thermometer and a timer and which will give some concrete ideas about what you could hope to achieve.
Peltier is far better than you might imagine because it delivers Actual Electrical Power, which you can use for Anything. A stirling engine would need an alternator before it's any use to you. You can look up the spec of some Peltier Junctions and that will tell you the area you need (current) and the number of junctions in series that you need for your required Voltage.

On your other suggestion for using Solar Power, there is much more information available about PV systems and you would certainly be able to predict the performance of any system you might choose. Last year I installed a PV system (12V) with a nominally 40W panel, a 14Ahr battery and a regulator into my remote shed. It provides power for inside lights, outside security lights and also charges sundry other 12V stuff that I have. If your water pump could use a header tank, it could go at full whack in full sun and the Energy storage would, effectively, be in the stored water - much cheaper than a massive battery.

Read as many 'green' sites as you can for ideas (but watch out for the Green BS merchants).

PS. Stirling engine needs just the same quality of Cold Sink as Peltier - the same rules apply, aamof.