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heat to electricity,what is the most efficent method? Answered

my plan is to build a generator that can be used in a house(prefrably installed into a new build chimney). I'm going to start by building a prototype and i need advice on what kind of devices are out there or i can manufacture.
also if possible could you tell me the output it is possible of? (eg enough to power a laptop)
any help is much apreciated :)


If you really want the most efficient conversion - Just guessing here but driving a stirling engine with the waste heat and using this to drive a generator might work for you - Getting a reasonable sized stirling engine may be a problem.

other indirect approaches could be - Used heat to drive a water pump to give you a decent head of water then use the water to drive a turbine to generate electricity.

In all honesty I would think this is more trouble than it's worth. Solar charging of battery packs is likely to be much better.

if you have a compressor and distill which can be actively used, try using an alcohol powered steam turbine, its my understanding it will be more efficient, but also flammable, so watch out, also try and reuse the alcohol as much as possible, and burn whatever cant be re-used to also power the turbines heat source.

otherwise sterling is my next best answer, but thats already been coverd.

Energy cannot be dstroyed - it is merely converted from one form to another.

That said, most energy today is "wasted" as heat. In other words, we generate heat and release it into the atmosphere where it dissipates.

I try to collect and "re-use" my heat as cheaply as possible. My waste hot bath water heats my plant-beds in my polytunnel by channeling it through pipes in the soil and then into a water butt. I have a hot-air vent which is waste and so my family get air-dried under it after a bath/shower, otherwise it gets vented into a cold room.

I'm presently trying to work out using the heat from my attic in the summer. Just now I'm thinking of using copper pipes against the black roof tiles to route water to pre-heat it up. However, the high cost of copper is negating this right now.

Either way, your endeavours are great and a fantastic way to fall off asleep at night as you dream of possiblilities of solving this issue creatively! Sleep well and Good luck!

Energy is conserved, but usable energy is limited and can be lost. In fact, the second law of thermodynamics guarantees that the usable energy in any thermal system will decrease over time. Why?

A thermal cycle requires a temperature difference in order to function. As energy flows from the hot bath to the cold bath, the total energy is conserved, but the temperature difference is reduced. Once the two baths are at the same temperature (and both contain lots of energy!), then no further work can be extracted.

Non-thermal energy conversion systems are not limited by temperature differences, but there is nearly always an equivalent parameter of the system which determines the conversion rate and efficiency, and which changes over the lifetime of the system.

Having said that, all of the efforts you describe are terrific! They make use of pre-existing temperature differences to extract work (or some sort of useful outcome) rather than letting the differences simply equilibrate.

thank you enormously, kelseymh, for a wonderful reply. You've given me a lot to read upon and this is what i love about the 'ible's community! thanks again! ;-)

You wrote, "I'm presently trying to work out using the heat from my attic in the summer. Just now I'm thinking of using copper pipes against the black roof tiles to route water to pre-heat it up. However, the high cost of copper is negating this right now."

This is a canonical solar-thermal installation (we see rooftop boxes with pipes coming out all over California :-). If you enclose the piping in a reflective lined box, it doesn't even have to be copper. You could use black-painted PVC to absorb the light and heat, and wrap the transfer pipes in foam insulation to reduce losses.

Another interesting option (but more pricey in terms of capital expense) would be to build a seasonal heat sink. If you live in an area with strong seasonal variations, you can put a large insulated water tank, or some other object with very high heat capacity, a meter or two below ground level (subsurface soil temperatures remain essentially constant year round, because of the extremely low conductivity). You can collect heat during the summer and store it in the sink, then extract it during the winter to provide heating.

What is the most efficient method? At present, that would be a steam turbine.

Check eBay for peltier cells. If connected to a battery, one side gets very hot and the other very cold. They can also be used in reverse (provided you have diodes to stop it going back the other way) if you heat one side and cool the other, you get electricity. It is a cheap solution and they usually come in 100w models at 12v = 8.3A. Laptops are about 18v @5A=90w, so at full power it should be enough to power a laptop. I wouldn't do it directly though, use it like a solar panel with which you charge a battery over time and then you can discharge large amounts at once. Alternatively, you could boil water if it is hot enough and use the steam to turn a small motor, and then rectify the AC waveform to get DC out. You can generate a lot of power this way, but it will be large, expensive, and fairly noisy. It is probably more efficient because the peltier cell needs cooling on the other side too, so you will need to attach a heatsink and fan or something to keep it cool. Maybe string 5 or so together for more power?

Peltier cells operating in thermoelectric mode are about 4% efficient, if you are lucky, and are destroyed above about 180C.


May I humbly suggest that you are thinking about this problem in a way that is slightly backwards.

There's a lot to explain,
but the most important thing to realize is this:

It is easier to extract work from high temperature heat than from low temperature heat.

For this reason you want to put high temperature heat into your engine first.   Then if there's any low-quality waste heat left over, and there always will be, you can use that to heat your house.

That is to say you should be thinking about how to build a power generator that can also heat your house, rather than thinking that you can stick some thing into your chimney, and thermodynamically "mop up" some work out of a waste heat stream.

The best instructables I have seen for making significant amounts of homemade heat and power are the gasifier 'ibles by this author:

I don't know if this is what you had in mind, but if you did something like that, a homemade power plant consisting of a gasifier plus an internal combustion engine, you would have plenty of power.