Energy harvesters transform waste into electricity

May 16, 2011

Billions of dollars lost each year as waste heat from industrial processes can be converted into electricity with a technology being developed at the Department of Energy's Oak Ridge National Laboratory.

The thermal waste converter actively cools electronic devices, , computers and large waste heat-producing systems while , according to Scott Hunter, who leads the development team. The potential for energy savings is enormous.

"In the United States, more than 50 percent of the energy generated annually from all sources is lost as waste heat," Hunter said, "so this actually presents us with a great opportunity to save industry money through increased process efficiencies and reduced while reducing ."

Initially, Hunter envisions the technology being used for cooling high-performance , thereby helping to solve an enormous problem facing manufacturers of petaflop-scale computers. These mega machines generate massive amounts of heat that must be removed, and the more efficient the process the better. Turning some of that heat into is an added bonus.

Hunter's technology uses cantilever structures that are about 1 millimeter square in size. About 1,000 of these energy converters can be attached to a 1-inch square surface such as a computer chip, concentrated photovoltaic cell or other devices that generate heat. Although the amount of electricity each device can generate is small – 1 to 10 milliwatts per device – many arrays of these devices can be used to generate sizable amounts of electricity that can power remote sensor systems or assist in the active cooling of the heat generating device, reducing cooling demands.

The underlying concept, pyroelectricity, is based on the use of pyroelectric materials, some of which have been known for centuries. First attempts to use this technology to generate electricity began several decades ago, but these studies have been plagued by low thermal to electricity conversion efficiencies – from about 1 to 5 percent.

This is also the case for techniques using thermoelectric, piezoelectric and conventional pyroelectric platforms. However, using arrays of cantilevered energy converters that feature fast response and cycle times, Hunter's team expects to achieve efficiencies of 10 to 30 percent – depending on the temperature of the waste heat generator – in an inexpensive platform that can be fabricated using standard semiconductor manufacturing technology.

"The fast rate of exchange in the temperature across the pyroelectric material is the key to the energy conversion efficiency and high electrical power generation," Hunter said, adding that ORNL's energy scavenger technology is able to generate electrical energy from thermal waste streams with temperature gradients of just a few degrees up to several hundred degrees.

The device is based on an harvesting system that features a micro-electro-mechanical, or MEMS, pyroelectric capacitor structure that when heated and cooled causes current to flow in alternate directions, which can be used to generate electricity. In this configuration, cantilevers are attached to an anchor that is affixed to a generator substrate. As this substrate becomes hot, the cantilever also heats and bends because of the bi-material effect, similar in principle to the bimetal switch used in room and oven thermostats.

"The tip of the hot cantilever comes into contact with a cold surface, the heat sink, where it rapidly loses its heat, causing the cantilever to move back and make contact with the hot surface," Hunter said. "The cantilever then cools and cycles back to the cold heat sink.

"The cantilever continues to oscillate between the heat source and sink as long as the temperature difference is maintained between the hot and cold surfaces."

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User comments : 22

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Scottingham
not rated yet May 16, 2011
If my math is right, which is probably isn't...these would be upwards to 10KW per square meter. That's pretty awesome considering how much waste heat there is to go around!
Eikka
4.5 / 5 (4) May 16, 2011
Unfortunately they still can't break Carnot's limits. If you have a computer chip that has to operate below 75 C, like a processor, then you can't get 30% efficiency no matter how much you want to.

If you run the chip to the maximum temperature limits, and you have 20 degrees air for your coolant, then the maximum theoretical efficiency you can get is 15% for a perfect ideal device.

Theirs is not.

Ideally you'd have the chip run at much lower temperatures to increase its lifetime and decrease the probability of random errors, which means even smaller margins of energy recovery.

Eikka
4.5 / 5 (2) May 16, 2011
The problem is that waste heat is invariably so low in temperature that recovering it becomes hard.

To actually get 30% efficiency, you would need to have the waste temperature at 250-300 C minimum for any plausible recovery system to work.

There are not many things that can be 250 C hot while in operation. Car exhausts come to mind, but it's not trivial to build such a heat exhanger that cools the escaping gasses and leaves the heat for the device to utilize at a high temperature.
Scottingham
2 / 5 (1) May 16, 2011
Eikka, you think too small. Picture this in use at the power plant level...Coal, Gas, Nuclear, they would all benefit from this process.
Caliban
3 / 5 (1) May 16, 2011
If the (lifecycle) manufacturing, installation, and "external" costs for the devices are substantially lower than the net electrical production, then the level of efficiency isn't really at issue.
jscroft
2 / 5 (2) May 16, 2011
Line a heat exchanger with these things, and you have ELIMINATED the turbine from every Carnot engine ever invented. It means we can generate electrical power with nothing but heat and PIPES... literally no rotating machinery required.

This changes everything.
kaasinees
2.5 / 5 (2) May 16, 2011
literally no rotating machinery required.


it still requires cold water pumped to the heatsink.
spacer
1 / 5 (3) May 16, 2011
The Canadian invention of GRAVITY CONTROL is the most efficient way to generate power at the lowest cost and non-polluting. It is based on the technology of the Flying Saucer. After I got the patent I offered it to Nasa , so that it could be applied to the Shuttles, which could then fly at very low cost anywhere. Mr. Borden rejected it, it would make the Heavy Lifter obsolete. So now I can offer it for the generation of power. These spheres under a Flying Saucer are the Propulsion Units (PU), they can lift a 10 or 100 ton weight off the ground and beyond using very little power.
These PUs can also lift a 100 ton weight in two silos 1000 feet up using the technology. When the weight comes down, it can generate many megawatts of power at 1 cent per Kilowatt or less.
A Power Station would cost a fraction of a Nuclear plant and can be built anywhere, where we have gravity.
Look at One Terminal Capacitor Joseph Hiddink
RealScience
not rated yet May 17, 2011
Eikka is right, most waste heat is waste because it is low temperature. And things that extract energy from heat differences work by impeding the flow of heat to harness it, which makes that which you are cooling run hotter, so I am skeptical for computer chips.

Even where the heat is truly waste this would have to be lower cost than a traditional heat engine operating from the same temperature difference, or one would just use a traditional heat engine.

MEMS chips currently cost ~$10/in2 and packaging another $10/in2. If it is 10 mW/cantilever, that is 10 W/in2 = 100 kWh/year or 2-year payback. At 1 mW/cantilever it is 20-year payback.

So this is actually reasonably plausible where there is a high density of truly waste heat (exhaust gases in a car).

However big power plants already have several stages of heat recovery, so I'd be much more skeptical there as the heat is much lower grade.
Beard
not rated yet May 17, 2011
Hey spacer, where you that guy on the Discovery channel who builds flying saucers in his garage?
mjesfahani
not rated yet May 17, 2011
It's a very good idea.
antialias
5 / 5 (2) May 17, 2011
Eikka has got it right with the Carnot process. You can't fool entropy. This statement in the article:
In the United States, more than 50 percent of the energy generated annually from all sources is lost as waste heat

is oversimlified because much of that lost heat is due to those limitations in the carnot cycle. Only a very small percentage of that is open to exploitation by these pyroelectric devices.

Still: One shouldn't knock it. There are certainly viable applications for it out there.
jscroft
1 / 5 (1) May 17, 2011
literally no rotating machinery required.


it still requires cold water pumped to the heatsink.


Yes, good point, although you could make a case for using a gravity potential--like an existing hydroelectric dam--to eliminate that component. Or maybe some kind of entrainment scheme. But in any case, a coolant pump is a far cry, maintenance- and cost-wise, from a power turbine!

Remember, this stuff is just coming out of the lab. Looking down the road, though, this is an open path to high-efficiency exploitation of temp gradients without large moving machinery. If you can get close enough to Carnot efficiency that whatever you leave on the table is more than covered by the cost savings of not having to build and maintain a power turbine, it's a winner.
Newbeak
not rated yet May 17, 2011
The larger heat sources (power plants,auto exhaust,etc) could run Cyclone Technologies WHE.They claim diesel engine efficiencies.
jscroft
1 / 5 (1) May 18, 2011
@Newbeak: True, but at heart the WHE is a rotary engine, and there are a lot of moving parts in a rotary engine. From an applications perspective, Hunter's device is solid state. Even if its efficiency is lower in converting waste heat, there's a good bet that savings due to the form factor will more than account for the difference.
kaasinees
not rated yet May 18, 2011
@Newbeak: True, but at heart the WHE is a rotary engine, and there are a lot of moving parts in a rotary engine. From an applications perspective, Hunter's device is solid state. Even if its efficiency is lower in converting waste heat, there's a good bet that savings due to the form factor will more than account for the difference.

The weight of the engine in the car probably undo's all the energy it could get from the heat.
Although such a heat engine probably does have some useful applications in none-moving objects.
Eikka
5 / 5 (1) May 21, 2011
Eikka, you think too small. Picture this in use at the power plant level...Coal, Gas, Nuclear, they would all benefit from this process.


At the power plant level, the waste heat comes out at well below 100 C because they need to turn the steam back to liquid water, and the lower the temperature they get, the stronger the resulting vacuum at the cold end of the steam turbine.

So the heat recovery device is left with a minimal temperature difference, and thus will operate at very poor efficiency anyways.
Eikka
5 / 5 (1) May 21, 2011
In essence, if you put a heat recovery device at the other end of a power plant's steam turbine, getting the recovery efficiency up means raising the temperature of the turbine output, which decreases the turbine's thermodynamic efficiency.

So what you gain in heat recovery is lost in the plant's turbine efficiency, negating most of the gains. The difference is slightly on the positive side, but it's still too low to justify the costs of installing the waste heat recovery system.

Unless of course you can buy the whole thing for a dollar, in which case it would make sense to increase the total efficiency of the plant by .5%
El_Nose
not rated yet May 23, 2011
A lot of people seem to be educated on this --

would it sound feasible to place these on the back of solar panals on a solar farm for an increse in energy generation??
Placing them on the back would not obstruct the light that needs to hit the panel and since the solar panal gets really warm anyway this would be an added boon.

1000 per square inch generating 1 watt of energy. Solar panals are typically 1 square meter = 1550 square inches.
Using the provided numbers this is about 1 ~ 15 watts per panal of additional electricity generated.
Newbeak
not rated yet May 23, 2011
@Newbeak: True, but at heart the WHE is a rotary engine, and there are a lot of moving parts in a rotary engine. From an applications perspective, Hunter's device is solid state. Even if its efficiency is lower in converting waste heat, there's a good bet that savings due to the form factor will more than account for the difference.

No,actually it has very few moving parts,and it can be very compact.Cyclone has prototypes the size of grass trimmer engines.For automotive applications,it could conceivably be fitted into the exhaust manifold,and provide all auxiliary power for the vehicle: http://www.cyclon...whe.html
jscroft
1 / 5 (1) May 23, 2011
In essence, if you put a heat recovery device at the other end of a power plant's steam turbine, getting the recovery efficiency up means raising the temperature of the turbine output, which decreases the turbine's thermodynamic efficiency....

Unless of course you can buy the whole thing for a dollar, in which case it would make sense to increase the total efficiency of the plant by .5%


I think the best application of this idea--well, its descendants--will be to REPLACE the turbine.
Newbeak
not rated yet May 24, 2011
A lot of people seem to be educated on this --

would it sound feasible to place these on the back of solar panals on a solar farm for an increse in energy generation??
Placing them on the back would not obstruct the light that needs to hit the panel and since the solar panal gets really warm anyway this would be an added boon.

1000 per square inch generating 1 watt of energy. Solar panals are typically 1 square meter = 1550 square inches.
Using the provided numbers this is about 1 ~ 15 watts per panal of additional electricity generated.

See: http://www.physor...ney.html