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Scientists work on sun-charged “heat battery”

Oct. 27, 2010
Courtesy of the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy
and World Science staff

Sci­en­tists are en­vi­sion­ing a new type of re­charge­a­ble bat­tery that would store heat ab­sorbed from the sun in­stead of elec­tri­cal charge.

The idea is a step clos­er to real­ity, they say, with a new study re­veal­ing how a mol­e­cule called ful­va­lene diruthe­nium works to store and re­lease heat on de­mand.

A YouTube video from MIT scientists describes their research into a new type of re­charge­a­ble bat­tery that would store heat ab­sorbed from the sun.


Al­though the chem­i­cal, dis­cov­ered in 1996, is pro­hib­i­tively ex­pen­sive, re­search­ers pre­dict the new un­der­stand­ing should make it pos­si­ble to find si­m­i­lar, cheaper ma­te­ri­als. The find­ings are de­scribed in a pa­per in the Oct. 20 is­sue of the re­search jour­nal Ange­wandte Che­mie.

The mol­e­cule, ful­va­lene di­ru­the­nium, un­der­goes a struc­tur­al change when it ab­sorbs sun­light, put­ting it in­to a higher-en­er­gy state where it can re­main sta­ble in­def­i­nite­ly. This state is anal­o­gous to that of a rub­ber band that’s stretched and then put some­where where it stays stretched out for any de­sired length of time.

In the case of ful­va­lene di­ru­the­nium, the mol­e­cule can snap back in­to its orig­i­nal shape trig­gered by a small ad­di­tion of heat or a sub­stance called a cat­a­lyst. In the pro­cess, it re­leases the heat that was orig­i­nally ab­sorbed. 

That’s a sim­pli­fied ver­sion of what hap­pen­s—but ac­tu­al­ly, the study re­vealed “there’s an in­ter­me­diate step that plays a ma­jor role” in the pro­cess, said Jef­frey Gross­man of the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy, who led the re­search. In this mid­dle step, the mol­e­cule forms a semi-sta­ble con­figura­t­ion part­way be­tween the two pre­vi­ously known states. “That was un­ex­pect­ed,” he said, but it helps ex­plain why the mol­e­cule is so sta­ble, why the pro­cess is easily re­vers­i­ble and al­so why sub­sti­tut­ing oth­er el­e­ments for ru­the­ni­um has­n’t worked yet.

In ef­fect, ex­plained Gross­man, this pro­cess makes it pos­si­ble to pro­duce a “re­charge­a­ble heat bat­ter­y” that can re­peat­edly store and re­lease heat from sun­light or oth­er sources. In prin­ci­ple, Gross­man said, a fu­el made from ful­va­lene di­ru­the­nium, when its stored heat is re­leased, “can get as hot as 200 de­grees C, plen­ty hot enough to heat your home, or even to run an en­gine to pro­duce elec­tricity.”

Com­pared to oth­er solar-en­er­gy ap­proaches, he said, “it takes many of the ad­van­tages of solar-thermal en­er­gy, but stores the heat in the form of a fu­el. It’s re­vers­i­ble, and it’s sta­ble over a long term. You can use it where you want, on de­mand. You could put the fu­el [out] in the sun, charge it up, then use the heat, and place the same fu­el back in the sun to recharge.”

Be­cause of its cost, this mol­e­cule “is the wrong ma­te­ri­al, but it shows it can be done,” he added.

The next step, he ex­plained, is to use a com­bina­t­ion of sim­ula­t­ion, think­ing, and databases of tens of mil­lions of known mol­e­cules to look for oth­er can­di­dates that have struc­tur­al si­m­i­lar­i­ties and might show the same be­hav­ior. “It’s my firm be­lief that as we un­der­stand what makes this ma­te­ri­al tick, we’ll find that there will be oth­er ma­te­ri­als” that will work the same way, Gross­man said.


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Scientists are envisioning a new type of rechargeable battery that will store heat obtained directly from the sun instead of electrical charge. The idea is a step closer to reality, they say, with a new study revealing how a molecule called fulvalene diruthenium works to store and release heat on demand. Although the chemical, discovered in 1996, is prohibitively expensive, researchers predict the new understanding should make it possible to find similar chemicals based on cheaper materials. The findings are described in a paper in the Oct. 20 issue of the research journal Angewandte Chemie. The molecule, fulvalene diruthenium, undergoes a structural change when it absorbs sunlight, putting it into a higher-energy state where it can remain stable indefinitely. This state is analogous to that of a rubber band that’s stretched and then put somewhere where it stays stretched out for any desired length of time. In the case of fulvalene diruthenium, the molecule can snap back into its original shape triggered by a small addition of heat or a substance called a catalyst. In the process, it releases the heat that was originally absorbed. That’s a simplified version of what happens—but actually, the study revealed “there’s an intermediate step that plays a major role” in the process, said Jeffrey Grossman of the Massachusetts Institute of Technology, one of the investigators in the study. In this intermediate step, the molecule forms a semi-stable configuration partway between the two previously known states. “That was unexpected,” he said. The two-step process helps explain why the molecule is so stable, why the process is easily reversible and also why substituting other elements for ruthenium hasn’t worked yet, he added. In effect, explained Grossman, this process makes it possible to produce a “rechargeable heat battery” that can repeatedly store and release heat from sunlight or other sources. In principle, Grossman said, a fuel made from fulvalene diruthenium, when its stored heat is released, “can get as hot as 200 degrees C, plenty hot enough to heat your home, or even to run an engine to produce electricity.” Compared to other solar-energy approaches, he said, “it takes many of the advantages of solar-thermal energy, but stores the heat in the form of a fuel. It’s reversible, and it’s stable over a long term. You can use it where you want, on demand. You could put the fuel [out] in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge.” Because of its cost, this molecule “is the wrong material, but it shows it can be done,” he added. The next step, he explained, is to use a combination of simulation, thinking, and databases of tens of millions of known molecules to look for other candidates that have structural similarities and might show the same behavior. “It’s my firm belief that as we understand what makes this material tick, we’ll find that there will be other materials” that will work the same way, Grossman said.