# Heat gradient -> Energy - How?

## Main Question or Discussion Point

I'm looking for the most efficient (practical, not theoretical) way to turn a heat gradient of unknown measure (at least roughly from -10°F through 100°F) into energy.

Probably some expert arround? Any engineers here working on something similar?

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brewnog
Gold Member
Heat IS energy. What type of energy do you want the output of this device to be?

current. i thought that was obvious. "turning something into energy" usually implies obtaining electricity from it (i thought)

Hey ManDay, may help to give a little more detail. it's pretty general at this point, you have obviously been thinking about your situation but all others are cold on it ... so you have a temp gradient (which at one point is a higher energy state than the lower temp point) presumably in a homogeneous medium?

in fact the situation is just as open as it could be. the medium? whatever you choose! the components? whatever like!

just mind that it has to be exactly at 2:36PM on a Sunday. :lol: Just kidding. Sorry as I can't present you with any further detail. As I said I'm /just/ looking for a most efficient tequnique to obtain electric energy (or even any sort of usuable energy, it all could be transformed to electricity at last) from a heat gradient.

to put it a more figurativly way: what device will level an uneven dispersion of heat to equilibirium and extract energy from it?

(please forgive me, i really dont know how to phrase it any better)

ps: welcome to the forums ted_s

brewnog
Gold Member
Well, a thermocouple converts a thermal gradient into a potential difference. A peltier device will do the same thing. A steam turbine converts heat to mechanical power, which can power a generator to produce electricity. A diesel generating set does a similar thing.

Any good?

russ_watters
Mentor
Is this homework? As said already, this is far to general to provide much of a meaningful answer.

Well, a thermocouple converts a thermal gradient into a potential difference. A peltier device will do the same thing. A steam turbine converts heat to mechanical power, which can power a generator to produce electricity. A diesel generating set does a similar thing.

Any good?
The steam turbine is not really what I was thinking of. That's why I'm asking for the most efficient way. I'm sure there is something which is far more advanced than a steam turbine, isnt there?

A thermocouple isn't really good to extract energy from a heat-source, heh? But maybe it could be tweaked so it creates a higher current from the heat than used for measurement. I dont know. In other words: What could be the largest yield from thermoelecticity?

No, this is not any homework. Just a matter of interest to me.

russ_watters
Mentor
I'm sure there is something which is far more advanced than a steam turbine, isnt there?
Not really, no. That's why they are still used in power plants.
A thermocouple isn't really good to extract energy from a heat-source, heh? But maybe it could be tweaked so it creates a higher current from the heat than used for measurement. I dont know. In other words: What could be the largest yield from thermoelecticity?
Yes, that is a pretty inefficient way to generate electricity. Something like 10%.

I think from your enthusiasm, what you want is another perpetual motion machine...

Not really, no. That's why they are still used in power plants.
Lets approach this from a different site: As you cannot use a steamengine on a heat gradient from 0°C to 20°C I'm looking for something different.

Mapes
Homework Helper
Gold Member
As you cannot use a steamengine on a heat gradient from 0°C to 20°C I'm looking for something different.
You could if your working fluid boiled between 0°C and 20°C.

The steam turbine is not really what I was thinking of.
Then please BE MORE SPECIFIC: are you looking for something solid state? Miniaturizable? Sexier than an old-fashioned steam engine?

How can I be more specificif I don't know anything about it myself? Seriously, I'd love to make it a bit easier for you - of course I would - but what exactly are you looking for?

Differently, another time: Be creative and invent something which can obtain energy (electricity) from the difference of temperatures at a winter night and winter day.

Does this help?

Mech_Engineer
Gold Member
Differently, another time: Be creative and invent something which can obtain energy (electricity) from the difference of temperatures at a winter night and winter day.
I think you need to first of all stop using the words "energy" and "electricity" as if they are interchangeable. While electricity can be defined as a form of energy, the reverse is not true. You want to use a small temperture gradient to generate electricity (I'm assuming to use in your home).

You have a tough problem on your hands- you want to use a temperature difference that occurs between two different times. 20 degrees is not a very big difference to start with, and the only way to get any kind of usable energy out of it between two different times would probably be to do something with thermal expansion/contraction. If you had a huge spring kind of like a wound watch spring, it would contract at night, and expand during the day. So if you used a system of gears to use that physical expansion/contraction to run a generator, you could in theory extract some electricity from it.

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Redbelly98
Staff Emeritus
Homework Helper
I'm looking for the most efficient (practical, not theoretical) way to turn a heat gradient of unknown measure (at least roughly from -10°F through 100°F) into energy.
Thermoelectric coolers (TEC's) that use the Seebeck effect can do this. I don't know what sort of efficiency they get, but the fact that they are not in widespread is probably an indication.

http://en.wikipedia.org/wiki/Thermoelectric_effect

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russ_watters
Mentor
Lets approach this from a different site: As you cannot use a steamengine on a heat gradient from 0°C to 20°C I'm looking for something different.
Well, there's always a stirling engine. It's more efficient than a steam engine anyway.

Something you need to know about heat engines, though: efficiency depends on the temperature gradient, so a 0-20C temperature gradient will be very difficult to make an efficient heat engine from.

russ_watters
Mentor
Does this help?
No, it doesn't. Unless you have a specific application in mind, you are "merely" asking us to invent a new way to generate energy to help solve the world's energy issues. Don't you think if I knew how to do that, I'd already have patented it and become filthy rich? People have been working on that general problem for centuries and most of us discussing it with you studied the problem for years in college. It isn't something that can be accomplished in an internet forum.

The title of your thread is essentially the definition of/purpose of thermodynamics. You may as well have asked us to teach you thermodynamics (in a 1 paragraph post).

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Mech_Engineer
Gold Member
The fact that the temperature gradient is the fluctuation of air temperature between night and day makes the problem monumentally more difficult. It is much more difficult when compared to say two bodies of water with different tempeatures.

for small temprature gradients you could use a heat engine based on the STIRLING CYCLE as russ watters says. with a small temp gradient efficiency would be awful. you can get small demo models that sit on a cup of hot water and work, also if you sit them on a cup of ice, they will work also, in reverse.but they're just for fun. the cycle is also used in refrigeration

when you're looking for the maximum efficieny possible i believe your looking at the CARNOT EFFICIENCY iirc its stg like 1-(TL/Th) with T's in KELVIN.
this will tell you the maximum theoretical efficiency for a heat engine. the real one WILL be less.[friction entropy etc]

basically if your looking for something to work between 0-20 degrees celcius the max efficiency you can hope for is about 7%, not much good.

the best way to maximise your efficiency is to maximise your temprature, power plants, auto manufacturers would love to be able to run at thousands of degree's C. but steel looses its integrety at a couple of hundred.

stewartcs
Seebeck voltages are normally extremely small and thus not useful at all for power generation.

Thermocouples, for example, usually have amplifier circuits to make the Seebeck voltage useable.

CS

russ_watters
Mentor
The fact that the temperature gradient is the fluctuation of air temperature between night and day makes the problem monumentally more difficult. It is much more difficult when compared to say two bodies of water with different tempeatures.
Heh - he slipped that one by me. I missed it. Yeah, that makes the problem extremely difficult - probably not worth doing, as such a criteria really is pointless. You'd need a medium to store the heat from day to night, but if you are collecting heat during the day, why aren't you using a solar collector? If you need a heat sink, why not use the ground or a river?