Compressed air energy storage (CAES) systems store energy by compressing air and storing it underground, such as in salt caverns. During periods of low energy demand, excess electricity is used to power air compressors. The compressed air is then stored. When energy demand is high, the compressed air is released, heated with natural gas, and used to power turbine generators to produce electricity. Two commercial CAES plants currently operate in Germany and Alabama. CAES systems provide a cost-effective way to store large amounts of energy and help utilities meet peak energy demands.

1. Compressed Air Energy Storage
CAES
Compressed Air
Energy Storage
(CAES) Plant uses Electricity to Compress Air which is Stored in Underground Reservoirs

2. Compressed Air Energy Storage (CAES) uses
surplus Energy to Compress Air which is then Stored in an
underground reservoir. The compression of the air generates
heat. The Air can be released to a combustor in a gas turbine to
generate Electricity.
Compressed air Energy storage (CAES) is a technique for
supplying electric power to meet peak load requirements of
electric utility systems. It incorporates a modified state-of-
the-art Gas Turbine and an Underground Reservoir that
may be an aquifer, a salt cavity or a mined hardrock cavern.

3. The compressor and turbine sections of the gas turbine would be
alternately coupled to a motor/generator for operation during
different time periods. During nighttime and weekend off-peak
periods, low-cost power would be used to compress air which
would be stored in the underground reservoir.
During the subsequent daytime peak-load periods the compressed
air would be withdrawn from storage, mixed with fuel, burned and
expanded through the turbines to generate peak power. This
concept reduces the consumption of petroleum fuels by more than
60 percent for conventional CAES systems when compared to
simple cycle gas turbines currently used to provide peaking power.

4. Some advanced CAES concepts do not require any petroleum fuels at all.
Studies have shown that the CAES concept is technically feasible and,
under certain conditions, economically viable. CAES systems offer several
advantages over Conventional systems used by utilities for meeting peak
load requirements.
Replacement of current gas
turbines in the United States
by CAES plants could result
in annual savings of more
than 100,000,000 barrels of
oil.

5. When electricity is needed, this compressed air is withdrawn,
heated with gas or oil, and run through an expansion turbine to drive a
generator. The compressed air can be stored in several types of
underground structures, including caverns in salt or rock formations,
aquifers, and depleted natural gas fields.
Typically the compressed
air in a CAES plant uses
about one third of
the premium fuel needed
to produce the same
amount of electricity as in
a conventional plant.

6. The fixed-price turnkey cost for this first-of-a-kind plant is
about $400/kW in constant 1988 dollars. The turbomachinery of the
CAES plant is like a combustion turbine, but the compressor and
the expander operate independently. In a combustion turbine, the air that
is used to drive the turbine is compressed just prior to combustion and
expansion and, as a result, the compressor and the expander must operate
at the same time and must have the same air mass flow rate.
A 290-MW CAES Plant has been in operation in Germany since the
early 1980s with 90% availability and 99% starting reliability. In the
U.S., the Alabama Electric Cooperative runs a CAES plant that stores
compressed air in a 19-million cubic foot cavern mined from a salt dome.
This 110-MW plant has a storage capacity of 26 h.

7. 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.

8. 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).

9. 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

10. 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.

11. Adiabatic Method
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.
Storage Options
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.

12. For example, the CAES plant in Germany requires 4 h of compression per
hour of generation. On the other hand, the Alabama plant requires 1.7 h of
compression for each hour of generation.
At 110-MW net output, the power ratio is 0.818 kW output for each
kilowatt input.
The heat rate
(LHV) is
4122 BTU/kWh
with natural gas
Fuel &
4089 BTU/kWh
with fuel oil.

13. Due to the storage option, a partial-load operation of the CAES plant is also
very flexible. For example, the heat rate of the expander increases only by 5%,
and the airflow decreases nearly linearly when the plant output is turned down
to 45% of full load.
However, CAES plants have
not reached commercial viability
beyond some prototypes.

14. Australia’s first compressed Air Energy Storage System gets development approval
The South Australian government
has awarded development approval
for a $30 million compressed air
energy storage system, in an
Australian first. The project is
being developed at the former
Angas Zinc Mine site
at Strathalbyn by Canadian
company Hydrostor, which will
provide 5MW of backup power for
up to 2 hours, with 10MWh of
storage capacity and will create
around 40 jobs during the
construction phase.

15. Hydrostor will re-purpose the former zinc mine site, using the structures
created during the operation of the mine, including a substantial series of
underground tunnels, to store pressurised air that can be used to generate
electricity on demand. “We’re excited to demonstrate the significant
benefits of adding our emission-free storage solution, creating jobs and
helping South Australia develop a stronger electricity grid at lower cost to
consumers,”
As the project is able to
provide fast response
services, Hydrostor intends
to use the demonstration
project to provide frequency
response and grid inertia
services.