Interview by Rikki Stancich

Determined to drive down the cost of CSP, renewable energy technology developer Bell Independent Power Corp. has made thermal storage its priority. Leveraging its experience in nuclear energy, the company has invested 5 years into an intensive R&D program, with the objective of developing a cost-competitive single-tank molten salt storage system.

Bell IPC is currently under contract with the Tucson Electric Power utility to build a 5 MW CSP demonstration plant with a 40 MWht thermal storage facility in Tucson, Arizona. The project will demonstrate that thermal storage can be integrated into CSP projects at a fraction of the price of two-tank molten salt systems.

If all goes to plan, Bell IPC’s demonstration plant will pave the way for a 50MW CSP plant with an 800 MWht thermal storage facility. The 50 MW project, which is currently in the design and negotiation phase, has been costed out at $44 per kWht for storage facility.

CSP Today speaks to Joseph Bell Jr., President of Bell Independent Power Corp., and Jerry Walter, Bell IPC’s Chief Financial Officer, to learn more about the advantages of single-tank molten salt storage systems; why molten salt is the best thermal storage medium out there; and about the applications for thermal storage beyond solar energy.

CSP Today: Bell IPC has developed a molten salt storage system. What are the unique characteristics of this latest solution?

Joseph Bell Jr: It is a single tank system that works with the phase change ability of molten salt – we can withdraw energy until the molten salts are back in their solid state.

This contrasts with the 2-tank system (the only perfected system out there), which has a hot and cold tank (which is in fact still hot, as the salt has to be kept molten). By keeping the salts in a liquid state, you can’t use all the storage ability of the salt.

While the system works with the molten salt, the molten salt medium is different to other molten salts. Bell IPC uses a unique salt mixture which is different than the solar salt that the 2 tank system uses.

Not only does the single-tank system provide continued power generation of electricity during storms, cloud cover, or beyond sunset, it also utilizes the lower temperature residual, in that it keeps the [HTF] oil in the [parabolic trough] solar field warm. By contrast, the traditional method is to burn gas to keep the solar field from freezing.

Additionally, it warms the components in the power block from ambient temperature to around 400 degrees F, which enables a quicker start-up.

So, our single-tank system uses a different type of salt; uses less salt per unit of storage; and can utilize more of the stored energy than the 2-tank system.

CSP Today: Is the system designed specifically for application to parabolic trough plants, or can it be adapted to other CSP technologies?

Joseph Bell Jr.: The system is designed around the capability of parabolic troughs, but it also has the flexibility to be used for tower technology.

CSP Today: How does the system compare on cost?

Joseph Bell Jr.: Cost performance is our biggest advantage. The only commercially available thermal storage is the 2-tank molten salt system, which costs around US$130/kWht of storage. Ours, at the identical scale (800MWht) will cost around US$44/kWht.

We only have one tank, so you halve the tank size and use 65% of the salt used by the 2-tank system. Meanwhile, we are able to use more energy per unit of salt. The two-tank system can only take the salt down to a certain temperature, whereas ours can solidify. There is a lot of energy in the phase change and we can capture part of that energy.

Jerry Walter: There is also no need for the sophisticated pumps that are required to move the molten salt from one tank to the next. The parasitic load is minimal – almost zero. The 2-tank system requires pumps and immersion heaters to keep the salt in a molten state.

Of the electricity we produce, we put more on the grid and use less internally than a 2-tank system would allow.

CSP Today: Why did you opt to work with molten salt over other mediums such as graphite, ceramic or stone?

Joseph Bell Jr.: We have done a lot of work with those mediums. Stone is a lot cheaper, but its ability to store heat is lower.

Graphite and ceramic are more expensive mediums and both are only at the early stage of development, which makes it more expensive.

The angle that we looked at, at each engineering stage was cost, focusing on what offered the lowest cost /kWht of storage unit.

CSP Today: Bell IPC has built several prototypes and recently signed a PPA with Tucson Electric Power to off-take power from a 5MW plant with 1.5 hours storage. What is the progress on this latest prototype?

Joseph Bell Jr.: We have built four prototypes – the largest is 1MWht of thermal storage.

We have two simulators – The most advanced 800kWht storage.

We run this scale simulator on a monthly basis, which is 1/50th of the size of the commercial plant planned in Tucson, Arizona.

We partially designed our own solar field and power block and we have a PPA in place with TEP. The demonstration facility will be located at the University of Arizona’s Science and Technology Park.

CSP Today: Bell IPC is assessing how solar thermal storage could be extended to other industries. Which sectors are you currently looking at?

Joseph Bell, Jr.: We have developed this storage technology for solar, however, there are other potential applications beyond solar, such as for steel foundries or aluminum manufacturing – anything with a thermal load and a high energy need.

If a facility is working on a continuous 24-7 cycle then there is no advantage, but if a facility operates on two shifts and needs to keep the equipment warm in-between shifts then waste energy can be stored and re-used to heat the equipment ready for the next shift. The thermal heat storage solution enables a good fuel economy.

There has been some interest in this locally. We have been talking to aluminum manufacturers and we are currently working through the details of temperature and energy loads.

To respond to this article, please write to the editor:

Rikki Stancich: rstancich@csptoday.com