Technology

While Sener is holding the details of its next generation thermal storage system close to its chest, Sierra Mercedes, vice president at SENER’s San Francisco office, talks CSP Today through the medium that promises to be the lynchpin of HELSOLAR’s heat storage advantage.

By Rikki Stancich in Paris

Earlier this month, engineering company SENER was awarded a contract by the U.S. Department of Energy (DOE) for the development of a next generation thermal storage system.

SENER is working with GrafTech International Ltd. and the University of California at Berkeley, to develop the HELSOLAR (High Efficiency Thermal Storage System for Solar Plants) storage system, which proposes to develop a high temperature solid storage concept based on the utilization of graphite.

According to Sener, this concept will allow the implementation of energy storage capabilities across a wide range of CSP technologies more cost-efficiently than existing storage systems.

While it is too early for Sener to comment on the storage system itself, CSP Today interviews Mercedes Sierra, vice president and head of business development for concentrating solar power at SENER’s San Francisco office, to learn more about the storage medium that will be used.

CSP Today: Could you please explain more about the properties of the proposed graphite material as a thermal heat storage medium?

Mercedes Sierra: Graphite in general is a material with very high specific heat coefficient, high density, low thermal expansion and able to withstand extremely high temperatures, what makes it an ideal candidate to be used for thermal storage systems.

CSP Today: What is the operating temperature range of graphite?

Mercedes Sierra: Graphite can operate up to 4,000ºC, but we are going to use it quite below that limit, we will stay below 1,000ºC.

CSP Today: What are the cost advantages of the proposed graphite material?

Mercedes Sierra: At the end of the day, it is not so much the cost of the raw material itself, but what you can do with it. Even if the basic cost per kilogram is higher as compared for instance to molten salt, the very good thermal properties plus the very low operational and maintenance costs have led us to think that graphite may be a good option for reducing the price of solar thermal energy in the future and we really appreciate the support provided by the DOE to push this investigation forward.

CSP Today: What is the availability of graphite compared to salt?

Sierra Mercedes: Graphite is made from carbon, which is one of the most common elements in nature. One could think that salt is also a pretty common substance, but there are only a few suppliers able to provide the high purity nitrates required to operate the thermal storage systems with good performances over the plant lifetime what has led to a significant  price increase in recent times. Thermal storage is a key element for solar energy dispachability and we need to identify different alternatives.

CSP Today: How easily does it handle - is it environmentally benign; does it require any special equipment?

Sierra Mercedes: Graphite is a non-toxic, environmental friendly material. It does not require specific precautions others than those derived from the high temperatures reached when storing a significant amount of energy. Besides, being a solid material, it does not have the freezing problems we need to fight with when dealing with molten salt.

CSP Today: What is the current and anticipated level of demand for the material – what other industries use it?

Sierra Mercedes: Graphite, and carbon in general under its different forms, has a wide range of applications that go from nuclear power plants to aircraft structures, sophisticated sportive material or human body implants. I would say it is difficult to find an industry for which carbon does not play a significant role (even in jewelry, since we all know that diamonds are just another form of carbon).  

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Rikki Stancich: rstancich@gmail.com