Compressed air energy storage (CAES) can be used to store excess electricity for later use.  In times of power demand, the air is sent to a turbine where it is expanded to produce electricity.  A major challenge for traditional CAES is reaching high round-trip efficiency.  Compression of air increases its temperature, which is typically rejected as waste heat and unrecovered in conventional CAES.  Upon discharge of the air to produce power, the air is commonly heated via combustion of natural gas to increase the enthalpy of the turbine feed.  Though this increases the turbine output, this introduces fossil energy to the process and reduces the overall efficiency.  To get around these problems, there have been two common approaches towards reaching higher thermal efficiency:

1)    Pursuing isothermal, or near-isothermal compression, where the air is compressed reversibly to minimize temperature rise, thus avoiding these thermal losses.

2)    Addition of post-compression cooling and pre-expansion heating via thermal storage, in most cases with multiple stages of each.  This is known as ‘Adiabatic’ or ‘Advanced Adiabatic’ Compressed Air Energy Storage (A-CAES or AA-CAES).

The first option (isothermal compression) is technically difficult to achieve and has the economic risk of higher capital costs due to greater equipment requirements.  Though the second option has been explored using both sensible and latent heat storage, thermochemical energy storage methods have not been investigated.  Thermochemical energy storage (TCES) offers inherently greater energy density than sensible or latent heat thermal storage methods by virtue of storing energy in chemical bonds.  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 than can operate under high pressures could be considered. These schemes typically take on the form of solid-gas reactions.  In such a scheme, hot air would heat a bed of solid particles which undergo a decomposition reaction.  The air is then stored at pressure like in conventional CAES schemes.  In a times of power demand, the cold air then flows back through the bed where the reverse reaction occurs thus reheating the air (albeit at slightly lower temperature).  If pressure drop is sufficiently low, direct-contact heat transfer is possible between the high-pressure air and the TCES bed; the bed itself is the heat exchanger.  Compared to other thermal storage strategies, smaller vessel sizes for a given energy storage capacity translate to lower capital costs.  Depending on the desired turbine inlet temperature, such a concept should also enable the elimination or significant reduction of fossil fuel inputs for temperature recuperation prior to turbine in hybrid CAES systems.   

Presented here will be a preliminary feasibility study exploring various process schemes, impacts on efficiency, and potential TCES chemistries. 


Thermochemical Heat Recuperation for Compressed Air Energy Storage

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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