The U.S. Department of Energy has awarded $10 million through its SunShot Initiative to six research teams in order to develop a new generation of storage technologies that could be incorporated into concentrating solar power (CSP) stations.
Unlike technologies currently employed involving heat-transfer fluids or molten salt, the award recipients are tasked with developing techniques for storing solar energy collected from CSP plants in the form of chemical bonds for weeks or even months.
Existing forms of thermal storage involve either sensible or latent energy storage. Sensible energy storage requires large-volume, insulated tanks and is perhaps best exemplified by district heating systems, such as the solar thermal district heating system Arcon is building in Denmark. In a latent energy storage system, heat from the solar receiver is used to melt a material, such as the molten salt of the Solana CSP plant.
A third way to store energy is in chemical bonds. In carrying out a chemical reaction using the sun's energy, energy is stored in the formation of the bonds. Energy stored in this way is also stable and even transportable, much like fossil fuels. When electricity is desired, the bonds are broken, releasing the energy as heat, which in turn boils water to drive the turbines in the power block.
Ranga Pitchumani, program manager for CSP at SunShot, says one of the factors motivating the technology development awards is a desire to achieve significantly higher density energy storage than what is currently available with sensible or latent energy storage technologies.
‘One of the advantages of CSP is that in the process of converting sunlight into electricity, there is an intermediate step of turning it into heat, which can be stored very cost-effectively for later use,’ Pitchumani says.
The oft-stated goal of the SunShot initiative is to drive the cost of solar power down to the same as fossil-based generation sources on the grid without any subsidies by the year 2020. On a levelized cost of energy basis, the goal is to get it down to about $0.05 to $0.06/kWh.
Another goal is to bring down the cost of thermal energy storage. When SunShot began in 2011,Â thermal energy storage cost about $27/kWh. The cost goal is $15/kWh. By way of comparison, Pitchumani says the cheapest battery he has come across today is about $500/kWh. On the other hand, the low-density latent energy storage methods used in CSP plants require large containment structures and can only serve the power block for relatively short periods of time.
From a utility standpoint, instead of looking at thermal storage with a duration of hours, perhaps overnight, thermo-chemical storage raises the possibility of creating storage products that might be kept for several months. That presents an interesting option of seasonal storage or even generating solar thermal storage in one place and using it in another.
‘Can you generate a whole bunch of these products in summer and release the energy when needed in winter?’ Pitchumani asks. ‘Can you create product in the [direct normal irradiance (DNI)]-rich states of the country and take it to those that are DNI poor, and release the energy where it is needed in a thermal generation plant? These would be fantastic opportunities.’
Although the technologies covered in the latest CSP storage awards may be considered foundational, SunShot is an applied technology program. Pitchumani says his group was very conscious about the application and commercial developments of the proposals they considered.
‘The last thing I want to do is to fund projects that sit on the shelf,’ he says. ‘In every one of these awards, there is a close industrial partner that will be working with the awardee to make sure it has commercial pathway to implementation.’
Two of the winning teams – Colorado School of Mines and Sandia National Laboratories – are pursuing research based on sand-like particles called perovskites; two teams – Southern Research Institute and the University of Florida – are researching carbonate chemistry; Pacific Northwest National Laboratory is researching hydride chemistry; and the University of California, Los Angeles is researching ammonia chemistry.
In each case, Pitchumani says, the research projects are promising from the standpoint of producing practical thermal storage results while being based on abundant materials and that leverage the existing infrastructure of the chemical process industry. ‘We are hoping that these will lead to viable solutions for next-generation CSP plants,’ he says.
More information about the six winning teams and a thumbnail description of their projects can be found here.