Compressed Air Energy Storage (CAES)

Executive Summary

Compressed Air Energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... Storage (CAES) plants are largely equivalent to pumped-hydro powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. plants in terms of their applications, output and storage capacityThe rate at which equipment can either generate, convert or transfer energy. See also Power.. But, instead of pumping water from a lower to an upper pond during periods of excess power, in a CAES plant, ambient air is compressed and stored under pressure in an underground cavern. When electricity is required, the pressurized air is heated and expanded in an expansion turbine driving a generator for power production.

Discussion

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 dischargeThe process of extracting stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. from the storage system. 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 generationThe manner in which electricity is generated. The electricity that flows through California phase by heating the high pressure air in combustors using natural gasA mixture of hydrocarbon gases occurring with petroleum deposits, mainly combining methane with varying quantities of ethane, propane, butane and other gases. Used as a fuel and for manufacturing organic compounds. This type of gas generates about 38... fuel, or alternatively using the heat of a combustion gas turbine exhaust in a recuperator to heat the incoming air before the expansion cycleOne sequence of storage charging and discharging. Also known as charge-discharge cycle.. Alternatively the heat of compression can be thermally stored before entering the cavern and used for adiabaticOf, relating to, or being a reversible thermodynamic process that occurs without gain or loss of heat and without a change in entropy. See also diabatic(Physics) involving a transfer of heat. See also diabatic.(Physics) involving a transfer of heat. See also diabatic.. expansion extracting heat from the thermal storage system.

Diabatic CAES Method

The only two existing CAES plants in Huntorf, Germany, and in McIntosh, Alabama, USA, as well as all the new plants being planned in the 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 CO2emissions 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 wind energy that cannot meet the daily demand1. The rate at which electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by... cycle.

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 current planning of CAES plants, 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.

Adiabatic Method

A much higher efficiency 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 up the decompressed air. An international consortium headed by the German energy company RWE is currently working on the development of the necessary components and the heat storage. The pilot plant is scheduled to start operations in 2018. Thermal oil and molten salt storage is being investigated in the US.

Storage Options

Independent of the selected method, very large storages are required because of the low storage density. Preferable locations are in artificially constructed salt caverns in deep salt formations. Salt caverns are characterised by several positive properties: high flexibilityRobust responses to changing needs and opportunities that are enabled by use of a device or technique that is adaptable, versatile and, in some cases, transportable., no pressure losses within the storage, 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.

Areas of Application of CAES Plants

  • Price arbitrage
  • Balancing energy (supply &demand)
  • Higher utilization and greater integration of renewable energy
  • Ancillary services including, regulation, spinning reserve & MVAR generation
  • Stabilizing conventional generation (improving operating efficiency)
  • Provision of black-start services

Summary

CAES power plants are a realistic alternative to pumped-hydro power plants. The capex and opex for the already operating diabatic plants are competitive.