Image: TU Vienna

TU Vien­na invents chem­i­cal heat storage

Stor­ing ener­gy over the long term is prob­a­bly the biggest unsolved prob­lem of the ener­gy tran­si­tion so far. A new type of chem­i­cal heat stor­age sys­tem has now been invent­ed at the Vien­na Uni­ver­si­ty of Tech­nol­o­gy that can be used to store large amounts of ener­gy in an envi­ron­men­tal­ly friend­ly way for a vir­tu­al­ly unlim­it­ed peri­od of time.

Heat is used to trig­ger a chem­i­cal reac­tion. This process pro­duces high-ener­gy chem­i­cal com­pounds that can be stored for months with­out any prob­lems or loss of ener­gy. If nec­es­sary, the chem­i­cal reac­tion can then be reversed, releas­ing the ener­gy again. For exam­ple, waste heat from indus­tri­al plants or solar heat can be stored in the sum­mer and used to heat build­ings through­out the win­ter. The chem­i­cal reac­tion and the sus­pen­sion reac­tor spe­cial­ly devel­oped for it have now been patented.

Store in sum­mer, use in winter

There are many meth­ods of stor­ing ener­gy, but they all have their draw­backs: You can charge bat­ter­ies, but their capac­i­ty is lim­it­ed. It is pos­si­ble to pro­duce hydro­gen with elec­tric pow­er, but it is dif­fi­cult to store in the long term. The new method devel­oped by TU Wien is based on a com­plete­ly dif­fer­ent prin­ci­ple — the con­ver­sion of ther­mal ener­gy into chem­i­cal ener­gy and back again.

“There are dif­fer­ent chem­i­cal reac­tions that can be used for this pur­pose. For exam­ple, we use boric acid, a sol­id mate­r­i­al that we mix with oil,” explains Prof. Franz Win­ter from the Insti­tute of Process Engi­neer­ing, Envi­ron­men­tal Engi­neer­ing and Tech­ni­cal Bio­sciences at TU Wien. “This oily sus­pen­sion goes into a reac­tor whose wall is heat­ed to a tem­per­a­ture between 70°C and 200°C.” Many process­es in indus­try take place in this tem­per­a­ture range, so this method is ide­al­ly suit­ed to uti­liz­ing waste heat from indus­tri­al plants that would oth­er­wise sim­ply be lost. But you can also achieve such tem­per­a­tures sim­ply by focus­ing sunlight.

The heat caus­es a chem­i­cal reac­tion — for exam­ple, boric acid is con­vert­ed into boric oxide, and water is released in the process. The oily boric oxide sus­pen­sion can then be stored in tanks. If water is then added to this sus­pen­sion again, the chem­i­cal reac­tion takes place in reverse and the stored heat is released again.

“This clos­es the cycle and the sus­pen­sion can be used one more time,” explains Franz Win­ter. “In the lab­o­ra­to­ry, we have shown that many charg­ing and dis­charg­ing process­es can be per­formed in this way with­out any problems.”

Many advan­tages at the same time

The tech­nol­o­gy has already been patent­ed, but now it will be stud­ied in more detail to deter­mine how it can best and most effi­cient­ly be applied. “Dif­fer­ent reac­tor sizes will be opti­mal for dif­fer­ent appli­ca­tions,” says Franz Win­ter. “You always have to see these reac­tors as part of an over­all sys­tem. Depend­ing on what heat quan­ti­ties occur at what tem­per­a­tures in an indus­tri­al plant, for exam­ple, and what oth­er ener­gy tech­nol­o­gy equip­ment already exists there, you have to adapt the process optimally.”

In addi­tion to boric acid, oth­er chem­i­cals can be used — salt hydrates have also been stud­ied. Boric acid and hydrat­ed salts com­bine sev­er­al advan­tages: they are inex­pen­sive and eas­i­ly avail­able, rel­a­tive­ly harm­less and sta­ble over many cycles, and can be stored for any length of time. The reac­tor tech­nol­o­gy can be scaled up to indus­tri­al scale. The oil used allows opti­mal heat trans­fer while pro­tect­ing the reac­tor dur­ing reac­tion and the solids dur­ing storage.

It is not pos­si­ble to give an exact effi­cien­cy of the process at this time — it will depend heav­i­ly on how the stor­age sys­tem is cou­pled with oth­er tech­nolo­gies. The great advan­tage is the long-term stor­age pos­si­bil­i­ty of heat quan­ti­ties that would oth­er­wise sim­ply be lost, and their demand-ori­ent­ed use.
“We now want to con­tin­ue inten­sive research on this tech­nol­o­gy, also togeth­er with indus­tri­al part­ners,” announces Franz Win­ter. “We are con­vinced that this inven­tion rep­re­sents an impor­tant step for­ward that will also find its way into indus­tri­al appli­ca­tions in the com­ing years.”