Energy 'Scavenger' Could Turn Waste Heat From Devices Like Refrigerators Into Electricity (sciencemag.org) 101
"Scientists have known for nearly 200 years that certain materials can convert heat to electricity..." reports Science, describing research into an intriguing new approach:
Refrigerators, boilers, and even lightbulbs continually dump heat into their surroundings. This "waste heat" could — in theory — be turned into electricity, as it is sometimes done with power plants, automobile engines, and other high-heat sources. The problem: These "low-grade" sources give off too little heat for current technology to do the conversion well.
Now, researchers have created a device that uses liquids to efficiently convert low-grade heat to electricity. The advance might one day power energy-scavenging devices that can light up sensors and lights and even charge batteries... Thermocells are good at converting small temperature differences into electricity, but they typically produce only tiny currents... This thermocell generated five times more power for the same electrode area than previous versions, materials physicist Jun Zhou and his colleagues at the Huazhong University of Science and Technology report this week in Science.
It also more than doubled the efficiency needed to make a viable commercial device. A paperback book-size module of 20 thermocells could run LED lights, power a fan, and charge a mobile phone, the team found.
Now, researchers have created a device that uses liquids to efficiently convert low-grade heat to electricity. The advance might one day power energy-scavenging devices that can light up sensors and lights and even charge batteries... Thermocells are good at converting small temperature differences into electricity, but they typically produce only tiny currents... This thermocell generated five times more power for the same electrode area than previous versions, materials physicist Jun Zhou and his colleagues at the Huazhong University of Science and Technology report this week in Science.
It also more than doubled the efficiency needed to make a viable commercial device. A paperback book-size module of 20 thermocells could run LED lights, power a fan, and charge a mobile phone, the team found.
diminishing returns (Score:5, Insightful)
This kind of implementation is limited by available source heat, heat exchange efficiency, and the efficiency of the device itself. You can't get to perpetual motion from here.
Sterling Engine? (Score:2)
Sterling Engine is heat to mechanical work but you can convert that to electricity. And if you were planning to use it to power a fridge then you want the mechanical work to run the compressor.
Re:Sterling Engine? (Score:4, Informative)
Saying any heat engine converts heat to mechanical work or electricity is an imprecision that can lead you to imagine things that are not possible. What they do is convert a heat gradient to mechanical work or electricity. If you don't have a temperature difference between the hot reservoir and the cold reservoir, the engine can't work. And in the course of doing that work, the engine will equalize the temperature difference. Entropy will not be denied.
Re: (Score:2)
Sadly, you are wrong. It's true that some sort of gradient is needed but heat _is_ being converted into work. Where do you think the work energy is coming from?
Re: (Score:3)
No, he's correct. "Heat" is really the kinetic energy of particles flying (or vibrating) around in random directions. When there's no gradient, because they're flying randomly equally in all directions, the net kinetic energy is zero.
Re: (Score:2)
Re: (Score:2)
a simple demonstration would be sufficient
A Sterling engine is one demonstration. Your refrigerator is another. The engine in your car is yet another. None of them violate the 2nd law of thermodynamics.
Also, TFA is silly. You can indeed generate electricity from the "waste" heat from a refrigerator, but only by making the refrigerator less efficient by far more than the electricity extracted.
And there is no way to "fix" this problem. The limitation is a fundamental law of the universe.
In summary: TANSTAAFL.
Re: (Score:2)
Re: (Score:2)
Re: (Score:2)
That's what I said.
Work is not necessarily a volume change, and you can use the energy in a temperature gradient to do work. Energy is defined as "the ability to do work."
Re: (Score:1)
Saying any heat engine converts heat to mechanical work or electricity is an imprecision that can lead you to imagine things that are not possible.
What they do is convert a heat gradient to mechanical work or electricity. If you don't have a temperature difference between the hot reservoir and the cold reservoir, the engine can't work. And in the course of doing that work, the engine will equalize the temperature difference. Entropy will not be denied.
If you're going to be pedantic, you should be fully pedantic :P
The heat gradient refers to one system, and this would imply multiple systems are always isolated and don't have an effect on each other.
There is a heat gradient in the fridge caused by its compressor turning electricity into work.
Most of that heat gradient is in the form of heat transfer, keeping the inside cold and thus the hot outside it. The rest of the heat comes from the inefficiencies in the compressor.
That system is not the one being mo
Re:diminishing returns (Score:5, Interesting)
The main problem is economic. Solar cells and batteries are cheap and most places you have waste heat you have light too. Scavenging energy from your fridge is pointless, the cost will never be lower than just using the mains power that is right there anyway.
There are a few places this sort of thing makes sense but it will never be mainstream.
Re:diminishing returns (Score:5, Insightful)
It translates into a more efficient refrigerator or freezer. This could offset the huge hit in efficiency people take when they get french door units or in-door water dispensers. The most efficient and cheapest refrigeration for your home is a simple chest freezer. It's not the most convenient though, when all your pizzas are stacked on top of everything else. So we take compromises to efficiency. But we would be stupid if we aren't ready to take some of that lost efficiency back.
Re:diminishing returns (Score:5, Insightful)
It doesn't. A fridge or freezer is the last place you'd want to use something like this. The point of a fridge is to move heat, so you want to get it away from the radiator as efficiently as possible. Making it go through some device to do work can only decrease net efficiency.
You might use a waste heat generator somewhere where you've got some unavoidable production of heat as a byproduct of doing something else, like making light, but it's generally not useful even there because typically you again need to get that waste heat away as efficiently as possible. You wouldn't want to replace the heat sink and fan on your CPU with what is effectively an insulating blanket, for example.
Where these things are useful is when you've got a source of heat and you need a bit of electricity to run something very small. Like a solar cell in a calculator, except with heat instead of light.
Re: (Score:2)
Making it go through some device to do work can only decrease net efficiency.
1. place compressor in an insulated box, let it run fairly hot because it runs only periodically
2. setup device to slowly leak heat from insulated box into the environment
3. profit
Main disadvantage is cost. And it's entirely probably that the amount of energy you save isn't worth the cost, but that's not the same as "decrease net efficiency".
Re: diminishing returns (Score:3)
Re: (Score:2)
not necessarily true. you don't want a hot compressor motor to transfer heat to your working fluid. but that can be avoided with careful selection of materials, like stainless steel or nylon couplings instead of copper (or BeCu alloy).
Talk about re inventing the wheel... (Score:1)
No. This is EXACTLY what a heat pump does in order to provide heat to a house/room. And copper is used to better transfer heat throughout the system.
The nice thing is that these things are super efficient.
Re: Talk about re inventing the wheel... (Score:2)
A heat pump is not a single compoent, it is composed of multiple parts. The heat travels through piping and a radiator not the motor housing. The motor itself creates additional heat as a byproduct of friction and electrical resistance, which is normally wasted. Why are you being willfully ignorant here? There is some fundamental problem with your argument if you can't look at a typical implementation of a refrigerator and discuss it.
Re: (Score:2)
The hot refrigerant travels from the compressor through piping and a condenser coil, (and after expansion, the cold refrigerant travels through an evaporator coil and piping back to the compressor) but the motor housing is usually enclosed within the refrigerant circuit, to prevent leaks.
Still the GP was wrong. Most of what was written as "the heat of the motor" on a heat pump, seems to be rather the useful work of the motor. Motors an
Re: (Score:2)
None-the-less, most compressors are hermetic or semi-hermetic - that is, the motor is sealed inside the refrigerant circuit - to prevent leaks. Refrigerant is often purposefully used to cool the motor (Modern efficiency standards may cause manufacturers to avoid that and rather try to direct as much motor heat to the air as possible.)
Re: (Score:2)
Re: (Score:2)
Actually my point was that a hot motor is less efficient than a colder one because the winding resistance is higher and losses are I^2 R.
Note that this is "necessarily true" at least with our current understanding of physics. A hot compressor WILL be less efficient than a cold one.
Re: (Score:2)
FTFY
Re: (Score:3)
The compressor is already in an insulated box. Literally. Usually the box is round so it might be more accurate to call it a can, but the compressor in a refrigerator (and in some other devices like RV air conditioners) is sealed into a can and suspended by springs to reduce noise. This is why you should not transport refrigerators lying down. Sure you can let them sit upright after transport and the liquid refrigerant will settle and you can run them, but the springs in the compressor vessel are only desig
Re: (Score:3)
If it's hermetically sealed (most of them are) it is often referred to in the trade as a can. But it's not always insulated.
If the springs are such a problem, why wouldn't the manufacturer ship the units with springs locked down?
Re: (Score:2)
If the springs are such a problem, why wouldn't the manufacturer ship the units with springs locked down?
It's not a problem in initial shipping, because the units are shipped upright. Being able to [un]lock the springs would require some [un]locking mechanism which would cost money. It's only a problem when people ship their own units, and don't have enough truck (or brains, depending) to ship them in the proper orientation like the pros do.
Re: (Score:2)
The compressor motor falls into part 2, places where you unavoidably produce heat. If an electric motor is efficiently designed it shouldn't produce much heat, and making it run hot will shorten its life. It's very rarely (i.e. never) worth that tradeoff to try and increase efficiency.
If you had some kind of fuel powered fridge and no access to electricity it might be worth it to produce a bit of electricity to run an LED fridge light or something. You'd probably still be better off with a little generator
Re: (Score:1)
Re: (Score:2)
Lol. Content free Slashdot posting at it's best.
Re: (Score:1)
Indeed, I would think it feasible to have some sort of switched approach, where during cold months heat is retained within the building, and in warmer months it's exhausted. If it's harvested along the way, bonus.
DCs would benefit from anything that can divert a significant amount of heat from the energy-intensive cooling loops.
Re: (Score:2)
It does seem odd that people don't install their refrigerators and freezers with the radiator outside. Again, it's probably just not worth the effort.
Re: (Score:2)
It does seem odd that people don't install their refrigerators and freezers with the radiator outside. Again, it's probably just not worth the effort.
In the winter it's good to keep that energy inside the home in many climates. Dumping it into the hot water system makes sense year-round.
Of course, it the costs to put together a higher efficiency system are greater than the costs to just install another photo-electric panel on the roof, it might not make as much sense.
Re: (Score:2)
This could offset the huge hit in efficiency people take when they get french door units or in-door water dispensers.
That "huge" hit isn't even remotely at big as you think. Sure it is compared to a chest freezer, but it isn't appreciably different compared to any other fridge design where the biggest impact on efficiency will be the details in the design rather than the style of doors or if it has a water dispenser.
so you believe in perpetual motion? (Score:3)
THE point of a refrigerator is to use energy to create a temperature gradient (difference) between the inside of the fridge and the outside.
And when you use special materials to "get energy out of heat", you are basically doing the reverse, you're using a temperature gradient to get energy.
In the end, when you go from A to B and then from B to A, you are left with less energy than you started with, due to inefficiencies in both conversions.
This is the basis of all perpetual energy debunking - showing that y
Re: (Score:2)
The gradient you are working to create is the temperature difference between the inside of the fridge and the outside.
The gradient between the outside of the fridge and the room temperature is an undesirable side effect that we are already working to try to limit as much as possible. It is the later gradient, and not the former, that they are proposing removing energy from. I don't see any violation of conservation of energy here, and it's quite possible that some energy could be removed from that second gr
Re: (Score:3)
Re: (Score:2)
Houses used to have pantries before fridges were common. You can build a house that has central HVAC which includes a pantry or at least cooling for a fridge, but you will still need something separate for a freezer.
Re: (Score:2)
Not in the summer.
Re:diminishing returns (Score:5, Funny)
You can't get to perpetual motion from here.
Sure you can -- just make sure you're first plugged into your neighbors outlet and it'll stay in motion for free!
Re: (Score:2)
Yes but that's not perpetual motion. The perpetual motion will come form your neighbours who will keep coming back to your house to tell you to stop stealing their electricity, and you who keeps plugging back into your neighbours.
Re: (Score:2)
No, but it could be used to increase the efficiency of existing machines. For a very considerable expense, so I don't know how commercially viable it would be. Upfront costs are always first in the consumer's mind - the running costs not so much, and energy is cheap.
Refrigerators? (Score:2)
How about cooling towers?
Re: (Score:1)
Do you know the Muffin Fan? (Score:2)
Do you know the Muffin Fan?
The Muffin Fan?
The Muffin Fan!
Do you know the muffin fan?
Who lives on Drury Lane?
Re: (Score:2)
Heat is down at the infra-red end of the spectrum, Low frequency = low energy, Much better to concentrate on harvesting the higher frequencies, i.e. what existing solar cells are already doing.
Re: (Score:2)
Heat != electromagnetic radiation, of any frequency.
Re: (Score:2)
Radiant heat is.
Re: (Score:2)
I see you don't understand the != operator.
Re: (Score:2)
So we should be trying to harvest those powerful ultra-violet rays, right?
Re: (Score:3)
Light is significantly easier to convert into usable energy than low-gradient heat. High-gradient heat is fairly easy to covert. Carnot's theorem limits the efficiency of heat-to-work conversion, whereas radiant energy (light) is not subject to this. Radiant heat is a photon, of course, but heat doesn't radiate fast enough and tends to radiate frequencies that are less efficient to convert via PV (solar) technology. This device and other TEGs convert non-radiant heat which is bouncing around in matter a
Re: (Score:3)
The only useful place I can think of to install this is on a catalytic converter. They need to stay hot to function, and they are really hot. Cars are using ever-increasing amounts of electricity to power accessory systems.
Re: (Score:2)
Well, this should not be installed on the catalytic converter then, since you need it to stay hot. But maybe in the tailpipe downstream.
Or maybe to help keep solar cells cool, while harvesting a little energy from their heat.
Re: (Score:2)
Because it has to do work with the heat, it will slow down the thermal transfer by decreasing delta-T. Right now the catalyst is highly cooled by being in open, moving air.
Re: (Score:2)
Why would taking energy out of a system slow down cooling? Shouldn't it have the opposite effect?
Effect on efficiency of the heat source? (Score:3)
Would this make the refrigerator have to work harder since the heat isn't dissipating?
Yes, it probably would. (Score:5, Insightful)
You are correct. Short of a device like the refrigerator not working in an optimal fashion, producing more heat than is necessary period, you're going to be spending more in electricity creating the heat than you can scavenge from it.
For example, with an air conditioner, you could instead make the device more efficient by instead putting the money/materials for this into making the condenser bigger, which increases the efficiency of the heat dispersion, reducing the electricity needed for a given amount of cooling.
Where this would make more sense, I think, is where you have a heat differential you can't otherwise get rid of - something like the exhaust from a hot water heater or gas furnace.
Or you could, you know, have a system where the gas is first burned to fuel a Stirling engine or Brayton cycle turbine, THEN heat the water with the waste heat.
Re: (Score:2)
What I wonder is if this technology can run backwards so that you input electricity and it moves heat. If so, how would the efficiency stack up yo existing cooling technology. Peltier coolers, for example, are really neat, but they require way too much power. If this is more efficient than thermocouples and it can run backwards, maybe it could entirely replace compressor/condenser based cooling. That would be interesting. It's probably doubtful.
Also, on collecting waste heat from refrigerators, it would obv
Re: (Score:2)
Cheaper refrigerators might still have a condenser coil on the back.
I could be wrong, but none of them that I know of use the skin of the refrigerator to radiate heat - that would not help any efficiency ratings.
Re: (Score:2)
The dedicated freezer that I have definitely radiates head through the sides. My older refrigerator has coils on the back. Not sure what criteria leads to them being set up one way or another.
Re: Yes, it probably would. (Score:2)
Using the waste heat to heat water is something a few hvac units do now. I used to work at a hvac manufacturer that happened to sell water heaters and we sold a closed system that could do both, but the complexity of the damn thing made it a non-starter for most hvac guys and plumbers didn't care for it in the other direction either. It also wasn't very efficient at the consumer scale so they dumped it and only sale it for commercial purposes where the giant units have a crap ton more heat to get rid of.
Re: (Score:2)
But the efficiency of the boiler or furnace would not be any problem. The cold water coming in is much colder than the heating water or return air, so the flue gas temperature would not go up, it'd probably go down, which is a
Re: (Score:2)
I doubt this would be economically useful, but if you use it to convert heat in the condenser coil to make electricity, you would be helping the process, not hindering it.
Re: (Score:3)
If it is impeding the heat flow sure, but who says it has to? Just place it on the side of the pipe before it heads to the devices radiator where the heat is highest anyway. There it shouldn't be able to effect the efficiency of the device (fridge, AC, etc), though it would require it's own separate radiator. This setup would limit it's energy production and may render it impractical though depending on its efficiency and cost. I think I read an article a while back about a company planning to do someth
Re:Effect on efficiency of the heat source? (Score:4, Insightful)
Thermodynamics.
Re: (Score:2)
Disagree. The device is removing heat from the system to convert it to electricity. Removing heat shouldn't make cooling less efficient, if anything, the reverse should be true.
Re: (Score:2)
That's not how thermodynamics works. The device is not "removing heat". It's exploiting the potential energy represented by a temperature gradient.
The most efficient way to move heat from something like a hot radiator is to dump it into a cold sink via the most efficient combination of conduction, radiation and convection you can. If you stick some other component into that system that does work, it must impede the heat flow in order to do that work.
Thermodynamics works much like electricity in many ways. T
Re: (Score:2)
To be pedantic, it is using heat from that temperature gradient to make electricity and removing that energy from the system.
Again, this is not doing work, it is creating electricity from the flow of heat. And who says it has to impede that heat flow? It could be attached upst
Re: (Score:2)
Thing is, for this to work it has to BE a cold sink by definition. Who are you to say that it's a worse cold sink than room air (which isn't actually that great a cold sink). This wouldn't work if it didn't cause the thing it was taking heat from to cool down, that's where it gets it's energy.
Re: (Score:2)
Again, thermodynamics. If you this thing into the existing heat path, it's cold side is going to be at best the same temperature as the existing radiator. If you install it somewhere colder, why didn't you just put the radiator there in the first place?
Re: (Score:2)
Sigh. A refrigerant compressor sends the super-heated gas to a condenser to get rid of the heat. The compressor would first send the gas through the new device and then the now cooler gas would go through a condenser. The compressor would not be effected in any way, you just get some electricity from heat you were rejecting into the environment anyway.
Some home ACs send the heated gas through a water-heater first, free hot water.
Extracting energy from fridge waste heat? (Score:3)
Extracting energy from the waste heat of a refrigerator?? A refrigerator is a heat pump, with the efficiency being highly dependent on the temperature of the heat exchanger (condenser) that's sitting at the back. A high-efficiency refrigerator will generally have a large heat exchanger that is only a few degrees above the air temperature. My fridge's heat exchanger is currently 27 C whereas the room temperature (away from the fridge) is 21 C. If you want to reduce the power usage by the fridge's compressor, you'll need a larger heat exchanger (no space for that) or use forced convection (which comes with its own power consumption and maintenance issues).
If you want to extract electricity from that 6 C temperature difference without raising the temperature on the refrigerator side, then guess what: you'll need a large heat exchanger or forced convection at the low-temperature side. It will not produce more electricity than how much you would have saved in the compressor by attaching the large heat exchanger directly to the fridge.
Re: (Score:2)
A refrigerator is a very poor example, yes. I suppose you could save some of the waste heat from the motor. You could run the hot side as a two-stage process though: First bring the hot gas down a few degrees in the energy reclaimer, then the rest of the way in a standard heat exchanger dumping to air.
Just imagine what this creative energy (Score:2)
If nuclear powered electricity were single digit pennies per kWh, imagine what we could do if we weren't so hyper focused on efficiency to cope with scarcity of electricity...
Stupid idea (Score:4, Interesting)
It's like extracting fertilizer from the rat droppings in a sewage treatment plant...
Re:Stupid idea (Score:4, Funny)
That's a crappy analogy and you know it.
Half the year, that heat is not wasted (Score:1)
In the Winter, the windows are closed and the "waste" heat from appliances subtly reduces the need to use dedicated heaters. The rest of the year, it's just cheaper to open a window and reject the heat, and even when it works against the A/C it's negligible. I've often thought that the best way to make a fridge more efficient would be to have a cold air intake for the Winter. That seems like a very clear winner in some climates, but I've never heard of it being done for home users. As others have pointe
Re: (Score:2)
You have weird thermometers at your place, mine are in celsius.
Re: (Score:2)
Re: (Score:2)
On my last bill, it was 42MiB-centimeter per Kilocelsius-hour.
Re: (Score:2)
The whole purpose of radiant heat is to heat your skin directly, rather than heat the air. So, yes, of course, if you move away from it you will feel colder.
But if you could keep the room warm using a 600 watt convector, you could keep it warm using a 1,000 watt radiant heater. The only difference would be if you were turning the heater on and off it could take the radiant heater longer to bring the space up to temperature.
Or you could... (Score:3)
Use the waste heat to generate electricity to power the device that is generating the waste heat, thereby increasing the efficiency of said device by some infinitesimal amount.
Here is an excellent engineering problem. You have two power supply designs, one is 95% efficient, one is 96% efficient. Does that extra efficiency matter, and why?
From an energy use standpoint, it is really a draw. Don’t spend a lot of money chasing the 1% increase in efficiency, it is unlikely to every pay off.
But, there is another really good reason to go with the 96% efficient design. It generates 20% less heat. That means a smaller heatsink and more compact design. Which may or may not matter.
Its not that simple (Score:2)
First, you have to get the government to repeal the laws of thermodynamics...
Re: (Score:2)
You're not under the impression that they're proposing complete recovery of all the energy put into a device, are you?
I mean, if you're able to capture 30% of the energy otherwise lost as waste heat, that's still worth it. It's not like anything under 100% is pointless.
Re: (Score:1)
Well, since it seems American culture and politics are very much on the "anything less than 100% effective is a waste of time" bandwagon, why not science as well?
And on a more relevant note, a brief interjection to lampoon those ridiculous devices where you freeze an ice pack in your freezer then drop it in a box with a fan blowing on it, and get "air conditioning for pennies a day!"
And on a more relevant note yet, there are already quite a few examples of capturing minute amounts of waste heat (or waste
electricity heat electricity (Score:2)
But can the air conditioner / refrigerator convert waste heat into enough electricity to reduce its electric draw and reduce power requirements?
Perpetual motion cooler. (Score:2)
So attach this device to run the fan(s) and I have cooling for almost nothing.
Cold shortage (Score:2)
Reminds me of a guy who said we have a cold shortage.
Think about how much energy is spent pumping heat out via Air conditioning.
So you have an AC unit (or fridge), spending energy to try and move the heat.
Then this electrical generator creates electriciy by impediting the transfer of heat.
Entropy.
Every time someone thinks of these great ideas, they never do a thermodynamic analysis.
Can I...? (Score:2)
Can I put this next to my balls and charge a phone in my pocket?
Useless in cold countries (Score:2)
Re: (Score:2)
Only half the year.
Five times zero still equals zero (Score:2)
Thermodynamics alarm just went off (Score:3)
I glanced over the summary. Are you kidding me? Once energy has been turned to heat, it is at its lowest level. The general idea of energy efficient products is to do useful work, and not dump heat. So the way to make a refrigerator more efficient is to improve insulation, and put the heat exchanger pipes somewhere cool, like bury them in the ground. Trying to extract power out of the pipes at the back of the fridge is just silly.
On a larger scale, gleaning power from small temperature gradients can work. It might work as a renewable power source. It is already done in power stations, and large ocean-going vessels, to squeeze the last drop out of the temperature differential.