Improving compressed air energy storage efficiency via chemical reactions

Scientists from the Oregon State University, in the United States, have proposed to improve the efficiency of compressed air energy storage (CAES) by recovering the thermochemical heat produced by facilities relying on the technology.

“Conventional CAES has relatively low cost compared to all kinds of batteries and we would imagine that this technology should only improve that advantage,” the research's corresponding author, Nicholas AuYeung, told pv magazine. “Cost analysis has not been performed yet, but we are interested in doing a thorough techno-economic analysis.”

Described in the paper “Thermochemical heat recuperation for compressed air energy storage,” published in Energy Conversion and Management, the novel approach consists of applying a thermochemical energy storage (TCES) technique that stores energy in chemical bonds to recover the heat produced during air compression operations. “TCES systems based upon metal oxide redox reactions which can release oxygen under high partial pressures of oxygen are especially of interest, although any chemistry that can operate under high pressures could be considered,” the study reads. “These schemes typically take on the form of solid-gas reactions.”

The scientists proposed to use, in particular, resistance heating to decompose barium oxides in the charging step of CAES through three different strategies: Utilizing direct heat transfer through a reactive bed of TCES materials in a solid-gas reaction; indirect heat transfer between hot air and the TCES system; and a combination of direct and indirect heat transfer.

“We looked at TCES with packed beds filled with rocks and barium oxides,” AuYeung stated. “Our results showed a similar round-trip efficiency between beds with TCES and beds without because of the relatively low heat capacity and heat of reaction for the barium oxides.”

According to him, the proposed system configuration can ensure a 60% round-trip efficiency with a 20-hour storage time after charge. This compares to a round-trip efficiency of only between 40% and 50% in conventional CAES. “To better illustrate the potential of the concept, we came up with a hypothetical material with the same heat capacity as rocks but a thermochemical storage capacity three times that of barium oxides, and we looked at that hypothetical material in our model,” AuYeung further explained. “Results showed that a potential round-trip efficiency improvement of more than 5% can be obtained, as well as longer storage duration. Also, 45% less filler volume would be needed to achieve storage capacity similar to rock-filled beds.”

Looking forward, the research team is planning to investigate more materials. “There are non-oxygen chemistries such as hydrates and carbonates that have the hypothetical properties – high heat capacity, high heat of reaction – we looked at, but right now we haven’t identified one for a redox material that operates on oxygen swing,” AuYeung concluded.