Electrochemical cell converts waste heat into electricity

November 18, 2014 by Tessa Evans, The Conversation
Diagram depicting how energy is generated as temperature varied. Credit: Gang Chen, CC BY

Picture a device that can produce electricity using nothing but the ambient heat around it. Thanks to research published in the Proceedings of the National Academy of Science today, this scenario is a step closer – a team from MIT has created an electrochemical cell which uses different temperatures to convert heat to electricity.

The cell only needs low-grade waste – less than 100C – to charge batteries, and is a significant step forward compared to similar devices which either require an external circuit for charging or high temperature heat sources (300C).

"It's a great idea to be able to recover useful electrical energy from waste heat," Anthony Vassallo, Delta Electricity Chair in Sustainable Energy Development at The University of Sydney, said.

At higher temperatures (60C), the cell (which is made of Prussian blue nanoparticles and ferrocyanide) was charged, and following cooling to 15C, the cell discharged energy. At lower temperatures the cell discharged more energy than was used to charge it, so converted heat to electricity.

The amount of heat energy generated is dependent on the temperature and the Carnot limit. The Carnot limit is the maximum absolute amount of heat energy that can be converted to useful electricity.

In cars, engine heat efficiency has reached around 20%, while the Carnot limit – the absolute efficiency which could be reached at that operating temperature – is 37%.

Credit: Tao Zero/Flickr, CC BY-NC-SA

This means that most heat energy conversion is based on high temperature, and low-grade heat conversion devices will never be able to achieve high conversion efficiencies.

This first prototype can only convert 2% to electricity, and, Professor Vassallo predicted, will have a Carnot limit of "less than 10%".

"While this will no doubt be improved, there are thermodynamic limits which basically say the maximum efficiency will always be low at the sort of temperatures these electrochemical cells could work at," he said.

When dealing with such low conversion efficiencies (generating watts rather than kilowatts), Damon Honnery – a research engineer at Monash University – said that "overcoming system losses can be a significant technical barrier".

But it's not all bad, according to Associate Professor Honnery: "There is a demand for low power sources. Lots of electrical systems require low power, and there could be niche uses for smaller devices where the energy density doesn't need to be so high."

On the road to application

The researchers want to try use the technology to harvest heat from the environment in remote areas. But as solar arrays already dominate the market, and operate more efficiently, it is unlikely heat conversion technology will supercede them any time soon.

And as the heat conversion battery needs two temperatures to operate, the battery would require fairly extreme fluctuations in in order to function outside the laboratory.

While this would be easy over long 24-hour cycles, rapid discharging is unlikely, so the amount of electricity generated over a day would be small.

Adam Best, a senior research scientist at CSIRO, said: "like all things in batteries, it's a materials science challenge. Can you get better materials which are able to convert this heat in a more efficient fashion?"

Dr Best suggested the technology may be better used in industrial facilities or in tandem with other energy systems to further enhance production.

Explore further: Electrochemical approach has potential to efficiently turn low-grade heat to electricity

More information: "Charging-free electrochemical system for harvesting low-grade thermal energy." PNAS 2014 ; published ahead of print November 17, 2014, DOI: 10.1073/pnas.1415097111

Related Stories

Yale engineer to build 'hot' solar cells

September 22, 2014

Associate professor of electrical engineering Minjoo Larry Lee has been awarded $2,540,000 to develop dual-junction solar cells that can operate efficiently at extreme temperatures above 750 degrees Fahrenheit. In addition ...

Towards more efficient solar cells

August 13, 2014

A layer of silicon nanocrystals and erbium ions may help solar cells to extract more energy from the ultraviolet (UV, high-energy) part of the solar spectrum. Experimental physicists from the FOM Foundation, the STW Technology ...

Clean power from waste heat

June 5, 2014

Siemens has developed a technology to use waste heat, which previously had gone unused, to generate electricity. The solution employs silicone oils, which have a lower enthalpy of vaporization than water, and is needed because ...

Recommended for you

Computer learns to recognize sounds by watching video

December 1, 2016

In recent years, computers have gotten remarkably good at recognizing speech and images: Think of the dictation software on most cellphones, or the algorithms that automatically identify people in photos posted to Facebook.

18 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Eikka
4.8 / 5 (5) Nov 18, 2014
In cars, engine heat efficiency has reached around 20%, while the Carnot limit – the absolute efficiency which could be reached at that operating temperature – is 37%.


Actually, many car engines, such as the Atkins cycle engine in a Toyota Prius, operate at 30% heat efficiency, and it is kept down from reaching higher by the fault of being so small, so the heat loss in proportion to engine power is greater.

The 20% figure comes after accounting for all the mechanical losses and auxillary loads to actually move the car - transmission, alternator, AC, rolling resistance, vacuum assist, power steering, lights, stereos, electric windows, computers and sensors... etc.

This is a kind of pet peevee of mine, because it's used by people to argue that modern cars are no more efficient than the T-Ford was, that there hasn't been any improvement over the 100 years we've had automobiles.

Well, true; and also a bald lie.

gkam
1 / 5 (7) Nov 18, 2014
Wasn't somebody complaining about exothermic batteries, and what to do with the waste heat?
Eikka
4.2 / 5 (5) Nov 18, 2014
Wasn't somebody complaining about exothermic batteries, and what to do with the waste heat?


Here's a riddle. A young engineer in an engineering company realized one day that ocean going cruise ships are warm on the inside, while the water they're moving in is cold, so he went to his supervisor to present an idea: what if you lined the bottom of the ship with thermoelectric plates to generate electricity for the ship?

The question is, why did the supervisor fire the guy?
gkam
1 / 5 (7) Nov 18, 2014
You are dodging my comments.
gkam
1 / 5 (7) Nov 18, 2014
Eikka, here is a real one, and simple at that. An insurance company runs on its computer, which occasionally gets cranky and unable to use. The only person who fixes it is the receptionist, who turns on the sprinklers outside.

What is going on?

That is typical of the little stuff I saw and taught every day to electricians and engineers. Not high math, but the natural understanding of flow and field effects and the specifics of how systems really work.

It is called Power Quality, and since I was doing it for our PG&E customers, I was hired by the Electric Power Research Institute to teach it to some of the 600 power companies in EPRI. Before I retired, the list was over 33,000 professionals in my long seminars.

Why do you insist on your accusations?
gkam
1 / 5 (6) Nov 18, 2014
I want to person who rated me with a one to answer the question:
An insurance company runs on its computer, which occasionally gets cranky and unable to use. The only person who fixes it is the receptionist, who turns on the sprinklers outside.

What is going on?
TheGhostofOtto1923
4.3 / 5 (3) Nov 18, 2014
I want to person who rated me with a one to answer the question:
An insurance company runs on its computer, which occasionally gets cranky and unable to use. The only person who fixes it is the receptionist, who turns on the sprinklers outside.

What is going on?
Well I would but youve chosen to put me on your ignore list.

Oh well.
It is called Power Quality, and since I was doing it for our PG&E customers, I was hired by the Electric Power Research Institute to teach it to some of the 600 power companies in EPRI. Before I retired, the list was over 33,000 professionals in my long seminars
So who put the powerpoint together for you and the rest of the team?
Eikka
4.3 / 5 (3) Nov 18, 2014
An insurance company runs on its computer, which occasionally gets cranky and unable to use.The only person who fixes it is the receptionist, who turns on the sprinklers outside.

What is going on?


It's hard to say because the question makes little grammatical sense. If an entire insurance company runs on its computer, I find it no surprise that it doesn't work - but what the computer is unable to use is a bit more mysterious. It reminds me of the popular internet meme that ends up in "Who was phone?"

But if you want an answer that is relevant to electricity, I'd say the house has a dodgy ground connection and the sprinklers make it better by wetting the ground.

You are dodging my comments.


I answered it quite sufficiently.

When you figure out why the young engineer in the riddle was sacked, you'll figure out why recovering 2% of the waste energy out of an inefficient battery is not a good solution to the problem.
Eikka
4.3 / 5 (3) Nov 18, 2014
That is typical of the little stuff I saw and taught every day to electricians and engineers. Not high math, but the natural understanding of flow and field effects and the specifics of how systems really work.


Glad you're finally coming out with the truth.

You were a consultant who wasn't responsible or expected to know any rigorous theory or have any wider understanding of power systems, but simply some specific practical advice on how the job is done in the field.

My father actually had a similiar job. Among his tasks was to explain to politicians why they can't simply add more wire to the end of a power line to add a new residential block to the zoning plan.

That's my other riddle: Why can't you?
docroc67
not rated yet Nov 18, 2014
Eikka - In answer to the cruise-ship conundrum, I think part of the answer is that the heat inside the ship is the result of energy being used explicitly to heat it and also waste heat from the engines, the kitchen, and other energy-using processes. SO part of the energy to be recovered by the thermoelectric plates is actually being fed iinto the system by burning oil or whatever is used to power the ship. In addition, in order for the temperature between the ship and the water not to reach equilibrium, the ship has to keep moving and this also uses energy.

As to the powerline puzzle, there are two issues: one is that adding wire doesn't increase the amount of energy available at the source, anymore than lengthening the hose at the gas pump increases the amount of gasoline in the tanks under the gas station available for pumping. The other issue is that, as you send electricity over longer and longer lengths of cable, you get more and more voltage drop.
Whydening Gyre
5 / 5 (1) Nov 18, 2014
It is called Power Quality, and since I was doing it for our PG&E customers, I was hired by the Electric Power Research Institute to teach it to some of the 600 power companies in EPRI. Before I retired, the list was over 33,000 professionals in my long seminars

So who put the powerpoint together for you and the rest of the team?

Wait, wait - I know this one.
The receptionist...
And the sprinklers are still runnin'...
Eikka
5 / 5 (1) Nov 19, 2014
SO part of the energy to be recovered by the thermoelectric plates is actually being fed iinto the system by burning oil or whatever is used to power the ship.


That is the ultimate concern, but, even if all the heat was coming off as waste and not needed, it still wouldn't make sense because the temperature difference is at most 30 degrees C and for all the thermoelectric plates that would cost millions, the price of the energy savings to the shipyard would be more expensive than the price of the energy even if the ships were running on kittens and puppies.

adding wire doesn't increase the amount of energy available at the source (...) you get more and more voltage drop.


All true - eventually - but that doesn't tell the local gov politician why they can't extend the cable another 300 yards when it's already several miles long. They can even show you on paper that there's plenty of power at the source and the voltage drop will be neglible.

Eikka
5 / 5 (1) Nov 19, 2014
Here's a hint to the cable conundrum: what happens when you short it?
docroc67
not rated yet Nov 19, 2014
Here's a hint to the cable conundrum: what happens when you short it?

Current approaches infinity (ie the limit of current as resistance approaches zero is infinity), and cable would melt down. But, as you add users onto the line, you are increasing the resistance, not decreasing it. SO I'm confused.
Eikka
5 / 5 (1) Nov 20, 2014
Current approaches infinity (ie the limit of current as resistance approaches zero is infinity), and cable would melt down. But, as you add users onto the line, you are increasing the resistance, not decreasing it. SO I'm confused.


The line has resistance the more the longer it gets.

The point is that the short-circuit current at the end of the line should be - by safety code - significantly higher than the peak current caused by all the users along the line. Otherwise the utility automated circuit breakers can't necessarily distinguish when there's an electric fire in a house at the end of the line, because it appears just the same as many people along the line using lots of power.

When the line was originally built, the same politicians gave the job to the lowest bidder, so the line was constructed to the specified distance with the minimum cable diameter allowed by the safety margins, and extending it would make it illegal.

Eikka
5 / 5 (1) Nov 20, 2014
In other words, suppose the utility company has designed the line for a potential of 1000 users with 3x25 Amps breakers in their box each. The utility therefore knows how much the maximum current draw on the line should be, so they can take the safety margin specified in the building code, and derive a simple equation that calculates the maximum length of the line based on cable material and diameter, and even things like ground leakage etc. if so desired - or the inverse of said calculation whichever was needed.

When I was younger, I helped my father translate a program that did just that onto a PC computer. The program was originally made in a variant of BASIC on a Sharp pocket computer like this:

http://upload.wik...1430.jpg
docroc67
not rated yet Nov 20, 2014
Eikka - I had never thought about the question of how the utility can distinguish between hi-load (but allowed) current use versus a disastrous short.
Eikka
5 / 5 (1) Nov 21, 2014
Eikka - I had never thought about the question of how the utility can distinguish between hi-load (but allowed) current use versus a disastrous short.


Well then, imagine the following scenario:

A property owner along the line decides to buy a bunch of solar panels and goes to the politicians complaining that the utility co. refuses to buy the electricity off of him, so the politicians make up a law that says they must, and they must pay a fixed feed-in tariff for it.

Meanwhile the utility co. engineers go, "God dammit, now we have current coming into the cable from somewhere other than through our circuit breakers and we can't know whether there's a short."

And then they have to sit down and figure something out to make it work, and the politicians aren't giving them any funding to do any of that. Guess what happens next?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.