New process discovered for chemically storing solar energy


July 14, 2011

An azobenzene-functionalized carbon nanotube molecule, which can store solar energy indefinitely (Image: Grossman/Kolpak)

An azobenzene-functionalized carbon nanotube molecule, which can store solar energy indefinitely (Image: Grossman/Kolpak)

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While solar panels are very useful at converting the sun's rays into electricity for immediate use, the storage of that energy for later use is ... well, it's still being figured out. The energy can be used to charge batteries, for instance, but that charge will wear off over time. Instead, scientists have been looking at thermo-chemical storage of solar energy. Last year, researchers from MIT discovered that the chemical fulvalene diruthenium was quite an effective storage medium. Unfortunately, the ruthenium element that it contains is rare and expensive. Now, however, one of those same scientists has created a new storage material that is cheaper, and is able to store much more energy.

The advantage of thermo-chemical storage in general is that the chemicals can be stored for long periods, without experiencing any energy loss. Suitable chemicals that don't contain the pricey ruthenium, however, have tended to degrade within just a few storage cycles.

MIT associate professor Jeffrey Grossman, who led the research last year, has now developed something better. He and postdoc Alexie Kolpak combined carbon nanotubes with the compound azobenzene, the result being a chemical that is less expensive than fulvalene diruthenium, and that has about 10,000 times the volumetric energy density - in other words, it can store more energy in less space.

Kolpak claims that its energy density is similar to that of a lithium-ion battery. By utilizing different methods of nanofabrication, it is also possible to independently control both how much energy can be stored, and how long it can be stored for.

The system works thanks to the azobenzene-functionalized carbon nanotube molecules, that change in structure when exposed to sunlight, and are capable of staying in that state indefinitely. When a stimulus such as a catalyst substance or a temperature change is applied, however, they revert to their previous form, releasing their stored energy as heat. That heat can be used directly in heating systems, or can be used to generate electricity. The molecules, meanwhile, are ready to be charged again.

"You've got a material that both converts and stores energy," said Grossman. "It's robust, it doesn't degrade, and it's cheap."

The MIT research was recently published in the journal Nano Letters.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

A true game changer if true, cheap and direct storage of solar would relegate the ICE to the history books. I\'ve never been one to use the words \"game changer\" but a cheap direct storage medium for solar would allow the transportation of energy from sunny desert regions to anywhere it\'s needed just like refined petro is now and the locomotives could just as easily be powered from the same source.

Michael Gene

The question that comes to mind is \" WHAT SURPLUS \" Power is produced during a few hours during the day time and the power is sent directly to the end users when demand during the day is high. Where does the surplus come from? It\'s not like you\'re producing excess Solar power at 2am in the morning when demand is low warranting storing the excess power. Is it me?

Matthew Jacobs

How much heat does it release and at what temperature? Does it get hot enough to be thermodynamically efficient at converting the heat into electricity without degrading the chemicals?


Also whats the efficiency of this material, what percentage of incident solar energy does it capture and at what frequencies. I imagine you would need to arrange this on flat sheets, and once one sheet gets saturated with energy move a fresh one into the sunlight. But the real question, whats the efficiency? Is it better or worse than already fairly lame PV?

So sick of these \'we can save the world with renewables\' stories promoting the CAGW agenda with no numbers attached!

How cheap, how heavy, how efficient?!


Cheap energy storage for Solar or any renewable energy source would be the catalyst that puts Solar panels on almost every roof in the US.

Cesar Arzayus Jr

The question in my mind is that having stored vast energy in chemical bond form how will the reverse process of using t5his energy by conversion to other forms be possible.In case lots of energy is generated ,how will controls be effected that the devise does not explode like a bomb?What sort of future technology need to be devloed to be able to use this stored energy?Does anybody have suggestion on this matter?

Shradhapati Singh

@Wombat56: You're asking the wrong people. Gizmag is a blog, not a scientific journal.

The questions you and other pose are all valid, and I share many of them. Perhaps the answers you seek are in the original published article in Nano Letters. Or better yet, contact the researchers directly to discuss your concerns. In fact, I just typed in the phrase "Jeffrey Grossman MIT" into google, and found a more detailed article directly from MIT News. I'm sure you can even do better than that.


IF this technology is inexpensive AND easy to store w/o risk of serious volatility AND is easily transported (ie. fluid pumped via pipes) THEN it might be a step forward in putting solar thermal in our homes. That would significantly reduce home heating loads... when the sun shines AND when a method is implemented to eliminate snow blockage.

This technology in no way will impact the ICE as commented previously.

Comparing to Li batteries is suggestive and misleading. This technology will in no way replace batteries... unless you are referring to battery powered sock heaters. Converting the heat to useable electricity is not efficient, not cheap and isn\'t conveniently portable.

Its interesting work that I hope we continue... but its uses may be limited to cold, sunny locations, like the high deserts of the American west where the day-night temperature delta is huge.

PS. I\'m interested because I live in the Colorado mountains. This could be a fun home project to cut my heating & domestic hot water bills.

Simons Engineering

Great find, now, let\'s get it out of the lab and into real world testing before some Chinese government sponsored factory grabs it and beats us to the punch.


You can buy very cheaply, hand warmers, which use a chemical crystal reaction( is it sodium thiosulphate?) that you recharge in a microwave oven, or hot water. Or: solar heating. Perhaps this could be scaled up.


I never understand why some are so negative about new developements or why they seem to want to maintain the status quo. The use of oil goes back to the 6th century BC but it wasn\'t until the early part of the last century that we discovered how to use it for transportation.There were no questions as to how efficient or not it was until recent times as it has become a problem and yet for some its still THE yardstick to measure all other suggestions by. If this same attitude had been adopted by early inventors of the ICE we would still be using horses for transport. Its time to move on and away from oil as a transport medium the facts are there,its inefficient,finite,polluting the very air we breath, screwing up the climate and we are warring over it.What better reason to consign it to history and embrace new ideas for solving these issues.


It\'s a phase change material. It stores energy from heat and can later release it. The twist here is that it can be made to hold that heat energy for a long time, and can be made to release it via different triggers.

Phase Change Materials have long been used in solar walls filled with black containers of the stuff. The sun heats and melts the PCM during the day then during the night it releases the stored heat and solidifies.

This new stuff could hold its heat until the coldest part of the night, or into the next day.

Gregg Eshelman

I often find the comments far more interesting than the news stories. There seem to be fewer negative couch-scientist ones on this subject thank goodness, [exception, see under WHAT SURPLUS above], but one or two muzzy brains are at work.....transporting charged [or changed] chemicals for instance would just use energy, and explosions? Nuff said! Thanks to dgate, Gregg Eshelman and alligator768 for introducing more positive thinking. This new approach could be very useful in stand-alone installations such as domestic energy saving, by just using the stored heat instead of converting to another form of energy for instance. Ian Colley.


\"Legacy inertia\" in the American nuclear industries prevents America from following the Chinese lead towards LFTR reactors, cheaper to build by a scale of X10, safer to operate, cheaper to fuel, with plentiful Thorium, they yield waste products safe for humanity after only 3 hundred years storage, produce no plutonium, (no Yucca Mountain expenses at all) and are a very safe, cheap and efficient source of heat for conversion to electrical energy. Likewise, electric bullet train networks and their associated electrically powered infrastructures,(LFTR sourced) soon to be daisy-chained Pan-Eurasia, and originating in communist China. Meanwhile an America, stuck on stupid, supports parasite nations in grand splendor to glean foreign oil imports from the likes of the OPEC countries, and Saudis. Small-time energy schemes like this one are important, But: Had the U.S.A. turned its back on G.Bush and the oil barons, turned away from being mercenaries for the Saudis, and invested the $650 billions U.S. spent on the Iraq fiasco, on conventional Solar/Thermal (proven, American technology) development of the South Western U.S.A., America would now have a huge surplus of very cheap electrical energy with which to compete against the Asian onslaught in the world market-place. No amount of Scientific discoveries in America can overcome the brutal stupidity demonstrated by the American War Machine. America must turn now, before it is to late, to reformulating the \"American Dream\" to a technologically progressive sustainable \"New\" dream for the American people or cease to exist as an independent country altogether. The time is now, the budget crisis just one of many symptoms, Pradigm shift is upon us as we speak!

Bruce Miller

liquid fluoride thorium reactors will be wonderful once they get invented, developed and approved for use.. probably will take 30 years just for the paperwork so dont hold your breath

Herm Perez

That indeed is fantastic. I gain greater confidence with such exciting disclosures and and certain that solar would erase all the apprehension that some might project. This further is unlimited energy without bothering intermittent price rise.

Asoor Shyam

I agree with other commenters about this being a possible game changer. I would ask the reporter of this story to come back here with more facts and contacts for individuals here to take advantage of his information. The level of interest on this possible breakthrough is very high.

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