SunShot explores chemistry to cut CSP storage costs in ELEMENTS awards

The biggest advantage that Concentrated Solar Power (CSP) has is its thermal ability to store the sun’s energy by day to generate power when needed. And finding the optimum heat storage technologies is key to getting the costs of CSP down. One such promising new kind of storage is thermochemical.

Pacific Northwest National Laboratory (PNNL), one of SunShot ELEMENTS' awardees, will use their $2,906,415 to pair high and low temperature metal hydride beds to enable low pressure heat storage. Because metal hydrides do not freeze at the temperatur


By Susan Kraemer

There are two ways to potentially cut down the costs of thermal energy storage in CSP. One is to try a new storage material; the other is to try a new process.

In a new $10 million round of awards announced in June, the US Department of Energy SunShot Initiative is funding both methods. Researchers are being funded to try a variety of promising new thermal energy storage materials, along with a new process to store them: in actual chemical bonds.

Two of the winning teams - Colorado School of Mines and Sandia National Laboratories - will try thermochemical storage using sand-like particles called perovskites; two teams - Southern Research Institute and the University of Florida - will do it with carbonate chemistry; the University of California, Los Angeles is researching ammonia chemistry, and the Pacific Northwest National Laboratory will use metal hydrides.

Unlike the current state-of the-art CSP technology that uses latent energy storage (using the sun’s heat to melt a solid like molten salt) thermochemical energy storage works through chemical reactions that are driven by heat (thermo) like the chemical reactions familiar from chemistry class using a Bunsen Burner.

This funding round is called Efficiently Leveraging Equilibrium Mechanisms for Engineering New Thermochemical Storage (ELEMENTS).

“In a sense you are capturing the sun's energy in the chemical reaction and storing it in the chemical bond,” Dr Ranga Pitchumani, Chief Scientist and Director of the Concentrating Solar Power and Systems Integration programs for the SunShot Initiative tells CSP Today.

“The energy that the reaction needs - in order to take place - actually comes from the sun,” Pitchumani explains. “You can conduct a chemical reaction using the sun’s energy to convert the reactants into the products.”

“And you store the products for the time that you need. Then when you want to release the heat you conduct a reverse reaction, an exothermic reaction, to release the heat to generate the heated working fluid to the power plant. That is the cycle that it goes through.”

Greater energy density

“The reason we want to focus on thermochemical energy storage is that the energy density associated with storing the sun’s energy in chemical bonds via chemical reactions is that the energy density is extremely high,” he points out. “That means you can store the same amount of energy in a much smaller volume, which reduces your capital costs.”

The SunShot Initiative aims to reduce the capital cost of building energy storage to less than $15 a kilowatt hour, so that CSP will be able to produce power at a levelized cost of energy of 6 cents a kilowatt hour or less by 2020.

“In just the last three years, that cost has come down to 13 cents a kilowatt hour. That’s over the half-way mark in the cost reduction towards the eventual goal of 6 cents a kilowatt hour. And these are the unsubsidised numbers,” he notes.

“On an unsubsidised “apples to apples” comparison, PV at the utility scale is 11 cents a kilowatt hour today, with the difference that the CSP number of 13 cents today includes storage.”

Metal hydrides

One of the awardees; Pacific Northwest National Laboratory (PNNL) will use their $2,906,415 to pair high and low temperature metal hydride beds to enable low pressure heat storage.

Metal hydrides do not freeze at the temperatures they will get down to as their heat is extracted, so they will never need any energy to re-heat like molten salts, and they are known to achieve long cycle life, so PNNL expects that they can meet the 30 years lifetime target.

Senior Research Scientist Ewa Rönnebro tells CSP Today what led to PNNL’s choice of metal hydrides for thermochemical storage to achieve the SunShot cost target.

“The metal hydride materials we are working on have eight times higher energy density than molten salts, so our system can be eight times smaller,” she explains.

“We are using metals of low cost along with designing an engineered system that is very simple and straight forward. Since we are using a thermochemical energy storage based on reversible chemical reactions with a high enthalpy, we can reach close to 100% efficiencies.”

Her team is using a technology based on a dual-bed metal hydride system, which has a high-temperature metal hydride operating at 675°C to generate heat as well as a low-temperature hydride at room temperature that is used for hydrogen storage during sun hours until there is a need to produce electricity (such as during night time, a cloudy day, or during peak hours).

The two metal hydride powders will be contained in stainless steel tanks with a connection in between to allow for hydrogen transport between the tanks at ambient pressures.

“The metal hydrides operate at higher temperatures; 675°C, based on reversible thermochemical reactions unlike the molten salts which store latent heat at lower temperatures of about 500-550°C,” says Rönnebro. “Therefore we can attain 99% exergetic efficiency.

Built on earlier success

The award builds on the team’s earlier research, in which they were able to successfully demonstrate high temperature metal hydride storage and meet the targets for thermal energy storage, but at a much smaller scale - in a 10-kilogram bench-scale prototype for an ARPA-E award of $712,511 in 2011.

“The SunShot ELEMENTS program officially started June 1, 2014 and after one year we will have identified optimised compositions of the metal hydrides to be included in a 3 kWh bench-scale demonstration unit,” says Rönnebro.

Assuming they meet with success, then next they will build a larger scale 30-kilowatt unit, teaming with an advanced metallurgical powders manufacturer ADMA Products, as well as Butler Sun Solutions and the former Sandia solar thermochemical researcher Richard Diver of Diver Solar.

Together they will attempt to demonstrate low-cost, long-life metal hydride thermal energy storage at a large scale of 1,000 kg bed size that can deliver 240 kWh heat with exergetic efficiencies at 99%.”

Rönnebro expects her team to meet the SunShot storage target of $15 per kilowatt hour that will ultimately make possible CSP power production at a levelized cost of energy of 6 cents a kilowatt hour.

And hers is just one of the teams working on these new thermochemical storage ideas.

“We are very optimistic,” Pitchumani affirms in describing the SunShot Initiative. “We continue to invest heavily in CSP and are very optimistic that we will reach the SunShot goal.”

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