CSP World

Engineering researchers at the University of Arkansas have developed a thermal energy storage system that will work as a viable alternative to current methods. They claim that incorporating this design into the operation of a concentrated solar power plant will dramatically increase annual energy production while significantly decreasing production costs.

The project was included in SunShot's 2008 "Advanced Heat Transfer Fluids and Novel Thermal Storage Concepts for CSP Generation Awards" and receive $770,000 from US Department of Energy.

Packed rock is currently the most efficient and least expensive method, but leads to thermal “ratcheting,” which is the stress caused to tank walls because of the expansion and contraction of storage tanks due to thermal cycling, they argues.

“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.”

As an alternative to conventional methods, Selvam and doctoral student Matt Strasser designed and tested a structured thermocline system that uses parallel concrete plates instead of packed rock inside a single storage tank. The plates were made from a special mixture of concrete developed by Micah Hale, associate professor of civil engineering. The mixture has survived temperatures of up to 600 degrees Celsius, or 1,112 degrees Fahrenheit.

Modeling results showed the concrete plates conducted heat with an efficiency of 93.9 percent, 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,” Selvam said. “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.”

SunShot's project desciption:

High Temperature Concrete for Thermal Energy Storage for Solar Power

The University of Arkansas, under the Thermal Storage FOA, is developing a novel concrete material that can withstand operating temperatures of 500°C or more and is measuring the concrete properties.

Approach

They are evaluating the concrete's performance with regard to temperature, heating rate, and heating/cooling cycles. The focus is not only on developing high-temperature concrete that can operate beyond 500°C, but also has superior material properties that can reduce the cost of the concrete.

The university is developing novel techniques of construction to increase the rate of heat transfer from the heat transfer fluid (HTF) to concrete and back, as well as reduce the difference in thermal expansion coefficient between the concrete and the reinforcement material. Other novel techniques that transfer the heat directly from the fluid to concrete without a pipe are also being investigated.

The cost of different methods of thermal energy storage (TES) is being evaluated and compared. The goals of the project are to reduce the cost of thermal energy storage from $25/kWth using concrete to the 2020 goal of costs below $15/kWhth and achieve a round trip efficiency >93%.

Innovation

The use of concrete as a TES medium could allow for higher-temperature storage. There is an attractive simplicity to solid-state, sensible heat storage. However, the use of concrete also brings challenges, including material compatibility with HTFs, durability after extensive thermal cycling, and partial charge/discharge issues. In addition, it is likely that "specialty concrete" is necessary for TES applications, which could raise the cost.

Abengoa evaluated concrete as part of their TES award (GO18156) and determined that a concrete TES system (based on a DLR configuration) is not competitive with baseline storage configurations (2-tank molten-salt) and actually results in a significant increase in levelized cost of energy (~10%). By project end, the University of Arkansas intends to have developed a concrete to be used as a TES medium that can achieve the above cost goals.