Energy storage in the form of thermochemical bonds can potentially achieve significantly higher-density energy storage than can be realised with molten salts, which translates into cost reductions that can meet the SunShot Initiative goals.

By Susan Kraemer

The US DOE’s SunShot Initiative is investigating a new kind of thermal energy storage material and process for storing solar thermal energy in its latest round of awards: Efficiently Leveraging Equilibrium Mechanisms for Engineering New Thermochemical Storage (ELEMENTS).

Two of the ELEMENTS' winning teams, Colorado School of Mines and Sandia National Laboratories, are specifically investigating the use of sand-like particles called perovskites in place of molten salts to store energy with thermochemical energy storage - thermal (heat) storage that is given a boost by a chemical reaction.

Less capital requirements

“Energy storage in the form of thermochemical bonds can potentially achieve significantly higher-density energy storage than can be realised with molten salts, which translates into cost reductions that can meet the SunShot Initiative goals” says Dr. Ranga Pitchumani, Chief Scientist and Director of the Concentrating Solar Power and Systems Integration programs for the SunShot Initiative.

“That is why we are investing in the chemical energy storage technology, as a possible option for CSP. There is less material, there's less capital equipment, and this leads to significant reduction in costs.”

The SunShot Initiative is working on reducing the capital costs of building storage for CSP to less than $15 per kilowatt hour (of built storage) so that CSP can produce electricity at a levelized cost of energy of 6 cents a kilowatt hour or less by 2020. Thermochemical storage is one path to this goal.

“An overall benefit from thermochemical storage in general is that both sensible and chemical energy can be harnessed for power production,” explains Elisa Prieto, who as Director of Strategy at Abengoa is responsible for CSP storage technology and is working in the U.S. with Colorado School of Mines on this project.

Abengoa is serving as a subcontractor and cost-share provider in the award for exploration of the perovskite oxide thermochemical storage cycle and will be monitoring the synthesis and characterisation for a better understanding of the processes involved.

The company will also follow and contribute, where possible, to the development by the other subcontractor to the Colorado School of Mines award, the National Renewable Energy Laboratory (NREL).

Why perovskites?

“Perovskite oxides have the potential to store higher quantities of energy to allow for higher temperature power cycles,” says Prieto. Perovskites-based thermochemical energy storage temperatures would cycle between a high of 900°C and a low of about 500°C.

“Molten salts to date have temperature limits typically less than 600° C,” adds Greg Jackson, head of the Department of Mechanical Engineering at Colorado School of Mines, who is leading the research.

“There are some people looking at higher temperature salts, but they have questions about their stability. They are very corrosive, and so the cost of the materials to just hold them at these scales is itself a challenge.”

Perovskites are an earth-abundant material with no corrosive issues at high temperatures. They can also store more energy per mass and per volume than molten salts or sand.

And although it is an academic issue - since Jackson doesn’t think the receiver design is capable of handling such temperatures and nor is it necessary for their purposes - he says "we are even comfortable with our perovskites going up to 1500° C.”

“You're talking about a ceramic material,” he explains. “The other storage material people are looking at is very cheap ceramics; sand, or various clays, that get hotter, but there's no chemical reaction that occurs, whereas perovskites can actually release some of their oxygen without actually destroying their crystalline properties.”

“That's what's beautiful about them. They have a really nice ability to release oxygen rapidly, and that release of oxygen; the endothermic reaction, actually brings more energy into the material.”

Challenges

There is some mechanical re-engineering required.

Rather than having molten salts flowing up and down the tower, there is actually now a stream of falling particles, like sand in an hourglass, in the receiver.

Previous SunShot research has already begun R&D design on high temperature particle receivers, but simply getting these particles up the tower will be “something of a challenge”, Jackson suspects.

“The disadvantage of the particles vis-à-vis say a fluid is you can pump a fluid,” he says. The temperature needs to kept constant at about 500°C as the particles must somehow be taken up the tower to be reheated by solar flux.

“However we do that, whether it be through a bucket design or some other way, that technology has not yet been fully thought out,” Jackson says.

“People have proposed bucket elevators, where you actually have an elevator that can maintain the particles at some temperature above room temperature.

Other than the question of how to transport the particles up the tower, the equipment needn't be that different. They need to have a tank design able to maintain a relatively low oxygen level, because once they take the oxygen out, it is exposed to air to release the energy. That has been easily engineered.

The heated particles are not used to drive the turbines, but rather to heat a more traditional power cycle fluid like steam. So the power block does not necessarily need to change although cycles that do not require bodies of water for cooling are preferred for installations in the desert.

“One thing that worries us is that while they really do well at releasing their oxygen the particles cannot sinter,” he says.

“At high temperatures and pressures, we must keep the particles from joining to each other. Because if they lose surface area, they'll really slow down the rate at which they release the oxygen and also take it back up to go again, so we want to make sure that they don't glom together when they are stored at 900°C heat.”

Assuming they can resolve these issues, chances are good that the chemical reaction will be reversible, which would mean the heat can be exchanged for as many cycles as needed.

“It's using oxygen and ceramics and so it's not like something that's really nasty where we would have to worry so much about impurities and so forth,” he points out. “So we're pretty confident that we can make it work.”

Commercial path

The focus of the project is on commercialising the new methodology and materials, which would be used together with earlier R&D on falling particle receivers and other components capable of handling much higher temperatures.

“This research project will not only develop internal know-how in perovskite oxides, but it will also further the development work for particle receivers and fluidised bed heat exchangers, which will support other projects underway,” Prieto explains.

“In order to truly progress into a commercialisation track, the research will be working on identifying and mitigating all risks that may come to pass.”

Within the scope of the two-year project, Abengoa will ultimately be performing a techno-economic analysis of the overall cycle to assess commercial feasibility.

Prieto does believe that the composition and structure of the perovskite particles will allow for a more predictable reaction - which is an advantage in any commercial application.

And this is where the DOE SunShot Initiative comes in.

“Ours is a very critical mission in that we are de-risking transformative technologies that are sometimes brilliant ideas and then handed off to the next stage of funding along a commercial pathway,” says Pitchumani.

To comment on this article, please write to the author, Susan Kraemer.