A number of advanced thermal storage concepts and heat transfer fluids are being trialled to increase the efficiency of CSP plants.

By CSP Today staff writer

The industry is currently seeing a lot of experimentation with thermal energy storage R&D.

Initiatives include combining heat transport fluids in a power tower plant with ceramic themocline storage; using carbon dioxide as the heat transfer fluid and solid ceramics for storage and a heat storage module in which the heat transfer fluid flows through a solid storage medium.

For instance, in the US the Department of Energy (DOE) is trying to develop an advanced heat transfer fluid that can operate within a temperature range from 80ºC up to 500ºC. It has also awarded a grant to Symyx Technologies, the only company chosen to research advanced heat transfer fluids. In addition, DOE has chosen other organisations and academic institutions to develop improved methods of thermal energy storage.

A large solar thermal power facility consists of many miles of fluid-filled pipes arranged in large grids with reflective mirrors used to capture radiation from the sun. Solar radiation heats the fluid, which is used to produce steam necessary to power large electricity generation turbines.

Currently, organic, oil-based fluid in the pipes has a maximum temperature threshold of 400ºC, allowing for the production of electricity at approximately 15 cents per kilowatt hour. Symyx emphasises that its new heat transfer fluid, when used with other advanced technologies, could significantly decrease solar electricity cost to as low as five to seven cents per kilowatt hour.

Experimental role

Going forward, there are several areas which the industry needs to consider to make the most of such efforts.

Kelly Beninga, renewable energy director at engineering consultancy WorleyParsons, says it is vital firstly to “field test” the different thermal energy storage (TES) technologies. Secondly, the new technology must be cost competitive with the molten salt TES system. Thirdly, it must provide high thermal efficiency.

“Additionally, the O&M cost or requirement must be comparative to molten salt TES. Without proper testing, a competitive cost and better efficiency, it would be very difficult to convince others in switching from a relative mature TES to a newly developed technology,” Beninga said.

The heat is on

It is being recommended that CSP plants should adopt heat transfer fluids (HTF) that are able to withstand the higher temperatures used in modern power plants today. This would greatly increase efficiency and thus reduce the cost of electricity produced.

The current HTF used in a trough plant can attain a temperature of about 734ºF whereas molten salt can be heated to 1050ºF, which is the superheated steam temperature, according to Beninga.

“The industry and NREL have been conducting R&D regarding the use of a higher temperature heat transfer fluid,” he said.

Direct steam generation (DSG) in the solar thermal plants is also being considered in both the parabolic trough and power tower systems.

“DSG system would eliminate the use of HTF (oil or salt), hence reducing a part of the system cost. Until then, molten salt is probably the most desirable storage medium,” Beninga said.

Double advantage

A two-tank storage system, where the heat transfer fluid (HTF) also serves as storage medium, is considered to be an advanced thermal energy storage option for solar thermal power plants.

In a two-tank storage system, molten salt is being used as both HTF and storage medium. If HTF is used, it would require oil-to-salt and salt-back-to-oil heat exchangers for transferring energy to and from the solar field to the TES. If molten salt is being used as both, the oil-to-salt and salt-back-to-oil heat exchangers can be eliminated.

The addition of a TES would allow the plant to provide power during evening and cloudy periods when solar radiation is low. “It is crucial for the plant to provide secure, predictable and programmable power supply to the utilities. Generally speaking, the cost of a 1,000 MWht TES (approx 2 to 2.5 hours of storage) ranges between $65 million to $100 million depending upon factors such as design, medium use, location, etc,” Beninga said.

Lower costs on horizon

Significantly, the DOE is working on its goal of reducing the cost of CSP electricity from 13–16 cents per kilowatt-hour (kWh) with no storage to 8–11 cents/kWh with six hours of storage by 2015. The DOE has strong links with the CSP industry through its support for technological development, testing and evaluation of components, and resource assessment. The DOE is also updating and expanding its test facilities at Sandia and NREL in order to better evaluate CSP components and systems, and cut costs.

Developing storage that is proven and financeable is a technology development and commercialisation process, says Dan Kabel, president and CEO, Acciona Solar Power Inc.

“Until there is more experience, the financial institutions will not underwrite the risks of storage,” Kabel said. “The DOE loan guarantee might be an answer.”

Also, the dispatchability and reliability of storage are somewhat unknown and therefore its difficult to draft a PPA around it today. Plus, the real life cycle costs of storage are unknown.

Team effort required

Beninga says as the interest in this technology increases, it should be a collaborative effort to look at cost savings in terms of efficiency improvement, new technology and materials.

The inclusion or installation of TES at a solar thermal plant would increase as utilities are demanding a more stable, predictable power supply from power producers. Unstable power supply could be problematic to a transmission grid network. Lowering the cost of electricity cannot depend on a single improvement in one component of a power plant, Beninga says.

State cash available

Last year the DOE indicated that it expected to make 10–25 awards through competitive tender. With a minimum 20% cost share by the private sector for research and development phases and a minimum 50% private cost share for final demonstration phases, the total research investment in advanced solar technologies under this tender is expected to exceed $75 million.

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Contact: Sara Lloyd-Jones by email sara@csptoday.com