Compressed air energy storage (CAES) is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods.
Since the 1870’s, CAES systems have been deployed to provide effective, on-demand energy for cities and industries. While many smaller applications exist, the first utility-scale CAES system was put in place in the 1970’s with over 290 MW nameplate capacity. CAES offers the potential for small-scale, on-site energy storage solutions as well as larger installations that can provide immense energy reserves for the grid.
How Compressed Air Energy Storage Works
Compressed air energy storage (CAES) plants are largely equivalent to pumped-hydro power plants in terms of their applications. But, instead of pumping water from a lower to an upper pond during periods of excess power, in a CAES plant, ambient air or another gas is compressed and stored under pressure in an underground cavern or container. When electricity is required, the pressurized air is heated and expanded in an expansion turbine driving a generator for power production.
The special thing about compressed air storage is that the air heats up strongly when being compressed from atmospheric pressure to a storage pressure of approx. 1,015 psia (70 bar). Standard multistage air compressors use inter- and after-coolers to reduce discharge temperatures to 300/350°F (149/177°C) and cavern injection air temperature reduced to 110/120°F (43/49°C). The heat of compression therefore is extracted during the compression process or removed by an intermediate cooler. The loss of this heat energy then has be compensated for during the expansion turbine power generation phase by heating the high pressure air in combustors using natural gas fuel, or alternatively using the heat of a combustion gas turbine exhaust in a recuperator to heat the incoming air before the expansion cycle. Alternatively the heat of compression can be thermally stored before entering the cavern and used for adiabatic expansion extracting heat from the thermal storage system.
Diabatic CAES Method
Two existing commercial scale CAES plants in Huntorf, Germany, and in McIntosh, Alabama, USA, as well as all the proposed designs foreseeable future are based on the diabatic method. In principle, these plants are essentially just conventional gas turbines, but where the compression of the combustion air is separated from and independent to the actual gas turbine process. This gives rise to the two main benefits of this method.
Because the compression stage normally uses up about 2/3 of the turbine capacity, the CAES turbine – unhindered by the compression work – can generate 3 times the output for the same natural gas input. This reduces the specific gas consumption and slashes the associated carbon dioxide emissions by around 40 to 60%, depending on whether the waste heat is used to warm up the air in a recuperator. The power-to-power efficiency is approx. 42% without and 55% with waste heat utilization.
Instead of compressing the air with valuable gas, lower cost excess energy can be used during off peak periods or excess renewable energy in excess of local energy demand.
The aforementioned plants both use single-shaft machines where the compressor-motor/ generator-gas turbine are both located on the same shaft and are coupled via a gear box. In other conceptual CAES plant designs, the motor-compressor unit and the turbine-generator unit will be mechanically decoupled. This makes it possible to expand the plant modularly with respect to the permissible input power and the output power. Using conventional gas turbine exhaust heat energy for the purposes of heating the high-pressure air before expansion in an air bottoming cycle allows for CAES plants of variable sizes based on cavern storage volume and pressure.
Much higher efficiencies of up to 70% can be achieved if the heat of compression is recovered and used to reheat the compressed air during turbine operations because there is no longer any need to burn extra natural gas to warm the decompressed air.
Independent of the selected method, very large volume storage sites are required because of the low storage density. Preferable locations are in artificially constructed salt caverns in deep salt formations. Salt caverns are characterized by several positive properties: high flexibility, no pressure losses within the storage repository, and no reaction with the oxygen in the air and the salt host rock. If no suitable salt formations are present, it is also possible to use natural aquifers – however, tests have to be carried out first to determine whether the oxygen reacts with the rock and with any microorganisms in the aquifer rock formation, which could lead to oxygen depletion or the blockage of the pore spaces in the reservoir. Depleted natural gas fields are also being investigated for compressed air storage; in addition to the depletion and blockage issues mentioned above, the mixing of residual hydrocarbons with compressed air will have to be considered.
CAES power plants are a realistic alternative to pumped-hydro power plants. The capex and opex for the already operating diabatic plants are competitive.