Improving thermal energy storage tanks with a concrete layer

Engineering researchers at the University of Arkansas came up with a thermal energy storage system that performs as a viable alternative to other currently available methods used to store energy collected from solar panels. Use of the newly developed design could increase annual energy production while significantly decreasing production costs of concentrated solar power plants, while ensuring longer operation without disastrous breakdowns.

“The most efficient, conventional method of storing energy from solar collectors satisfies the U.S. Department of Energy’s goal for system efficiency”, said Panneer Selvam, professor of civil engineering. “But there are problems associated with this method. Filler material used in the conventional method stresses and degrades the walls of storage tanks. This creates inefficiencies that aren’t calculated and, more importantly, could lead to catastrophic rupture of a tank.”

Current energy storage methods rely on molten salts, oils or beds of packed rock as media to conduct and maintain heat inside thermal energy storage tanks. Although these methods are found to be efficient, they are either expensive or are hazardous to storage tanks. For an example, the most efficient and least expensive method of packed rock use as media leads to thermal ratcheting – an event where stress during the thermal cycling where expansion and contraction causes the tank walls to break.

Selvam and doctoral student Matt Strasser came up with a solution by designing a structured thermocline system that uses parallel concrete plates instead of packed rock inside a single storage tank. Thermocline systems are units with distinct boundaries separating layers that have different temperatures.

The plates were made from a special mixture of concrete developed by Micah Hale, University of Arkansas associate professor of civil engineering. The mixture has survived temperatures of up to 600°C (1,112°F). The storage process takes heat, collected in solar panels, and then transfers the heat through steel pipes into the concrete, which absorbs the heat and stores it until it can be transferred to a generator.

Modeling results showed the concrete plates conducted heat with an efficiency of 93.9%, which is higher than the Department of Energy’s goal and only slightly less than the efficiency of the packed-bed method. Tests also confirmed that the concrete layers conducted heat without causing damage to materials used for storage.

In addition, energy storage using the concrete method cost only $0.78 per kilowatt-hour, far below the Department of Energy’s goal of achieving thermal energy storage at a cost of $15 per kilowatt-hour.

“Our work demonstrates that concrete is comparable to the packed-bed thermocline system in terms of energy efficiency”, said Selvam, who also directs the university’s Computational Mechanics Laboratory. “But the real benefit of the concrete layers is that they do not cost a lot to produce compared to other media, and they have the unique ability to conduct and store heat without damaging tanks. This factor alone will increase production and decrease operating expenses for concentrated solar power plants.”