A short technical report on Compressed Air Energy Storage, a type of energy storage best suited for for intermittent renewable energy electricity production.

1. Compressed air
Energy Storage
Introduction:
With increasing power demand and
consumption, the need for storage system
which is cost efficient, reliable, feasible
and environmental friendly is of highest
urgency today. While pumped hydro
storage, batteries and other storage
technologies have some advantages, only
compressed air energy storage has the
potential of pumped hydro but with lower
cost and less geographic. Compressed air
energy storage works like a battery which
temporarily stores energy in the form of
compressed air which is driven
electrically. Air is pumped into large
storage tanks or any naturally occurring
underground formations aquifers. It has
gained special status recently as a means
of addressing the intermitting problems
associated with wind turbine electrical
generators. Air is compressed to about
100bar which 35% more pressure is than a
car tyre. During low-cost off-peak load
periods, a motor consumes power to
compress and store air in the underground
salt caverns. Later, during peak load
periods, the process is reversed; the
compressed air is returned to the surface;
this air is used to burn natural gas in the
combustion chambers. The resulting
combustion gas is then expanded in the 2-
stage gas turbine to spin the generator and
produce electricity.
Components of CAES:
A CAES plant mainly consists of
compressor train, motor-generator unit, gas
turbine and underground compressed air
storage. The compressor-motor/ generator-
gas turbine are both located on the same
shaft and are coupled by a gear box.
Types of CAES:
The air can be stored in three ways
1. Diabatic
2. Adiabatically
3. Isothermal
The air may be pressurized diabatically by
pumping air into dry cavern.
Adiabatically, by storing heat generated
during compression which is usually
stored in heat accumulators.
Diabatic CAES:
Diabatic CAES do not use the heat emitted
during the compression process. Therefore,
in Diabatic Plants additional fuel is used
during expansion process to reheat the
compressed air. Diabatic CAES plant lose
heat energy from the compression. Heat
need to be regenerated before the
compressed air is expanded into the gas
turbine. In a diabatic process, 2/3 of the
turbine capacity is used in the compression
stage. The CAES turbine can generate 3
times the output for the same natural gas
input. This results in reduction of
consumption of specific gas and reduces
the associated CO2 emissions by around 40
to 60%. The power-to-power efficiency is
approximately 42% without the use of
recupertors, and 55% with the use of them.
Both the current plants use single-shaft

2. machines.
Specification of Diabatic CAES
Adiabatic:
Efficiency of up to 70% can be achieved if
the heat due to compression is recovered
and reused to heat the compressed air
during turbine operations because there is
no longer any need to burn extra natural
gas to warm up the decompressed air.
When the air is compressed, most heat is
captured in a heat storage facility instead
of being released into the surrounding.
During the discharge cycle, the heat stored
in the heat storage facility releases its
energy into the compressed air. Therefore,
no gas combustion is necessary to heat the
compressed air and thus eliminating the
CO2 into the atmosphere. Adiabatic CAES
use a separate thermal energy storage
system like a recuperator to store heat
energy generated during compression
cycle. During expansion or generation
cycle, the stored heat energy is used to
reheat the air which is then expanded in a
sliding pressure air turbine. This enhances
the cycle efficiency because there is no
fuel used utilized to reheat the air resulting
in lower carbon dioxide emissions by the
system. Using exhaust heat energy by
conventional gas turbine for heating the
high-pressure air before expansion in an
air bottoming cycle allows for CAES
plants of different sizes based on cavern
storage volume and pressure.
Adiabatic plant
Wind integrated CAES:
One of the central applications for
CAES is for the storage of wind energy
during abundance and generation onto
the grid during times of shortfalls in
wind output. Such wind balancing
applications require not only large-
scale, long duration storage but a quick
response times and siting availability.
The capital cost of adding incremental
amount of storage capacity can be
much lower than for other comparable
storage technologies. CAES consumes
Power range Upto 100 MW
Energy range 100 MWh to 10
GWh
Discharge Time Upto 10 hours
Life 30 years
Reaction time Few minutes
Efficiency Approximately
55%
CAPEX Energy Approximately
50-150 $/kWh
CAPEX Power 400-1200 $/kW

3. significantly lesser fuel than
conventional gas turbine per unit of
energy delivered. Greenhouse effect
emissions from wind/CAES systems
can be quite low.
Storage:
Artificially constructed salt caverns in
deep salt formations are highly preferred.
High flexibility, low pressure losses within
the storage, no reaction with the oxygen in
the air are some desirable characteristics of
salt caverns. Natural aquifers are another
alternating option; however, it must be
taken care that rock and microorganisms
do not react with oxygen. Otherwise, it
could lead to oxygen depletion or the
blockage of the pores spaces in the
reservoir. Depleted natural gas fields are
also one of the futuristic options for
compressed air storage. Mixing of
Residual hydrocarbons with compressed
air is also one other challenges to be
addressed.
Variants of CAES:
AA CAES:
When AA CAES is operated at the
expansion mode by integrating a thermal
Energy storage system, compressed air
energy is converted into electrical power
output without a combustion process
involved. The main benefit of AA CAES
is zero carbon emissions. Heat exchanger
is used to cool airflow through
compressors and heat input airflow to each
turbine. The overall efficiency of AA
CAES is higher than that of the
conventional CAES.
Small Scale CAES
A small scale CAES facility can use over
ground cylinders with suitable dimensions
as a storage facility. Stored facility can
either be on site compression facility or
delivered as prefilled high-pressure air
cylinders. Turbines in these CAES need to
have high efficiency, fast response and low
maintenance.
LAES
A variant of CAES, using an electrical
machine to drive an air liquefier and the
resultant liquid air is stored in insulated
tank at atmospheric pressure. During
discharge, the liquid air is released and
pumped to high pressure, and then
vaporized and heated to ambient
temperature. This resultant high pressure
gaseous air is used to drive the turbine and
generate electricity.
Commercial CAES plants:
Huntorf plant:
The world’s first utility-scale CAES plant
commissioned by Brown Boveri was
installed at Huntorf, Germany in 1978. It
was designed to meet the peak demand
while maintaining a constant capacity
factor in the nuclear power industry.
Thereafter, its functionality has been
buffering against intermittence of wind
energy production in Northern Germany. It
has 2 salt caverns operating at a high
pressure between 4.8Mpa and 6.6Mpa. The
plant
runs
in a
daily
cycle
Technology Self-
discharge
Life
time
Cycling
times
Discharge
efficiency
Round trip
efficiency
Large
CAES
Small 20-
40yrs
8000-
12000
70-80% 42%,
54%,70%(based
on current
plants)
Small
CAES
Very
small
23+yrs Tested
30000
75-90% -
LAES Small 20-
40yrs
- - 55-80%

4. with 8 hours of compressed air charging
and 2 hours of expansion operation at a
rated power of 290 MW. It has excellent
performance statics with 90% availability
and 99% starting reliability. The round-trip
efficiency of this plant is about 42%.
Huntorf plant
Mcintosh Plant:
Started in 1991, the 110 MW plant can
deliver continuous power output for up to
26 hours. It has a single salt dome cavern
to store compressed air in the range of
4.5Mpa to 7.4Mpa. The McIntosh facility
deploys a heat recuperator to reuse part of
heat energy from the exhaust of gas
turbines thereby reducing fuel
consumption by 22-25% and improves
cycle efficiency from 42% to 54%. The
average running reliability is 96.8% and
99.5%, average starting reliability is 91.2%
and 92.1% during generation and
compression cycles respectively.
Specification and comparison of CAES:
Technology Energy
density(Wh/L)
Power
density(W/L)
Specific
Energy(Wh/kg)
Power
rating
Rated
capacity
Large CAES 2-6 0.5-2 30-60 110 &
290 MW
580 & 2860
MWh
Small CAES 2-6 0.5-2 140 0.003-
3MW
0.002-
0.01MWh
LAES 4 times than
CAES
- 214 0.3-
2.5MW
2.5MWh
Technology Storage
duration
Discharge
time
Power
capital cost
Energy
Capital cost
Operation and
maintenance
cost
Large CAES Hours-
months
1-24+ hours 400-
1000$/kW
2-120$/kWh 0.003$kWh
Small CAES Hours-
months
Up to 1 hour 500-
1500$/kW
200-
250$/kWh
Very low
LAES - 1-12 hours 900-
2000$/kW
260-
530$/kWh
-

5. Number of cycles per year
Comparison:
CAES power plants are a realistic
alternative to pumped-hydro power plants.
The Capital expenditure and operating
expenditure for operating diabatic plants
are competitive. Just as pumped storage,
the power can be released almost
instantaneously. A merit over pumped
storage is that the visible impact on the
landscape is low. The efficiency of the
320MW plant in Germany, Huntorf is
about 42% and that in Mcintosh is 54%.
They are 20% less than the efficiency of
pumped storage plants. Employ
compressed air in gas fired power plants
for regeneration, which limits efficiency
and creates emissions.
Pumped hydroelectric storage is high
technical maturity storage technology with
an installed total capacity of 127-129 GW
in 2012 and represents around 99% of
worldwide bulk storage capacity.
The capital cost of adding
incremental amount of storage
capacity can be much lower than for
other comparable storage
technologies. CAES consumes
significantly lesser fuel than
conventional gas turbine per unit of
energy delivered. Greenhouse effect
emissions from wind/CAES systems
can be quite low. The demand for
flexible balancing power to maintain
grid stability shows strong growth.
Wind and solar are highly weather
dependent and show an intermittency
between zero and 85 % of the
maximum installed capacity. If the
electricity grid is to remain stable,
these fluctuations must be balanced.
2 salt caverns in
Huntorf operating at a
high pressure between
4.8Mpa and 6.6Mpa.
The plant runs in
daily cycle with 8
hours of compressed
air charging and 2
hours of expansion
operation at a rated
power of 290 MW.
The McIntosh facility
deploys a heat
recuperator to reuse
part of heat energy
from the exhaust of
gas turbines thereby
reducing fuel
consumption by 22-
25% and improves
cycle efficiency from
42% to 54%.
It has excellent
performance statics
with 90% availability
and 99% starting
reliability. The round-
trip efficiency of this
plant is about 42%.
The average running
reliability is 96.8% and
99.5%, average starting
reliability is 91.2% and
92.1% during
generation and
compression cycles
respectively

6. Capital cost of CAES in comparision to other
storage
Technical maturity
Challenges:
Output
Turbine operation
Compressor operation
290MW(<3hrs)
60MW(<12hrs)
Air flow rates
Turbine operation
Compressor operation
Air mass flow ratio
in/out
417kg/s
108kg/s
1/4
Number of cavers 2
Air cavern
volumes(single)
Total volume of
cavern
1,40,000m3
1.70.000m3
3,10,000m3
Cavern location top
and bottom
top
bottom
650m
800m
Maximum diameter 60m
Well spacing 220m
Cavern pressures
Minimum permissible 1 bar
20bar
Minimum
operation(exceptional)
Minimum operation
(regular)
Maximum permissible
and operational
43bar
70bar
Maximum pressure
reduction rate
15 bar/h
Huntorf plant specifications
The air that is heated up during
compression process must be cooled down
to the ambient temperature before it can be
stored in the cavern. This cold air must be
reheated for discharge of the storage
facility since it cools strongly when
expanding in a turbine for power
generation. This results in a considerable
decrease of the efficiency. The air is stored
and used daily and not for a long term.
This results in pressure fluctuation which
has consequences on the size and design of
the cavern. Also, humidity can lead to
more corrosion of the underground bore-
hole equipment, the cavern heads, pipes
and fittings. Turbine must cope with the
considerable fluctuations in pressures. 1
million m3
of salt cavern would cost less
than a 1000 m3
cylinder. Adiabatic
systems require large thermal storage
which are massive entailing to high costs
and large parasitic loads.
Developments:
Solarelectric ltd has developed two highly
efficient forms of Compressed air energy
storage known as the TES CAES and
CCGT CAES. They are capable of
compressing the air to 70 bar and also
store heat.
TES CAES licensed from TES CAES
technology limited is adiabatic and
involves no burning of gas. They have

7. efficiency of 60-70% and also are zero
emission plants.
CCGT is a diabatic and burns gas during
regeneration. It has efficiency of 55-60%
but is cheaper to build and be retro fitted to
existing power stations.
ADELE
Adele places extremely heavy demands on
the equipment used: Cyclical stresses,
temperatures of over 600o
C and pressure
of up to 100 bar. Current plans
• In latest 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 standardly with respect to the
permissible input power and the
output power.
• Currently, there have been several
works in the development of
Hybrid Compressed Air Battery is
developed by a UK based
company- Energetix group. The
power range is between 2kW and a
few MW. The adaptation of Scroll
expander technology has led to
high expansion efficiency in
Hybrid CAB. The system uses pre-
prepared compressed air. The
hybrid connection to
supercapacitor allows fast
responses CAES can address the
vast market disruption and sytem
risk caused by the mass build-up of
renewable energy.Store electric
believes it can achieve a levelized
cost of electricity of £100/MWh
cheaper than gas fired peaking
plants and levelized cost of storage
of 68£/MWh cheaper than hydro.
The university of Chester, UK is building
a test facility with a capacity of 200kW.
5 caverns located in Cheshire capable of
20MW for 50 hours are planned to be
operational soon. Also, 500MW
commercial facility in Cheshire is planned
to be operational in 2 years.
Siemens, PwC, GE are all part of this
project. The European network
Transmission service operators Electricity
set up by European commission has
included projects in Cheshire as part of
their 10 year development plan and has
officially recognized it as important
infrastructure at a continental scale.
Areas of application and economics:
• Balancing Energy (supply and
demand)
• Higher utilization and greater
integration with renewable energy
• . Stabilizing conventional
• For customers on dynamic rates
CAES allows energy arbitrage
opportunities.
• Energy storage technologies are
currently not uniformly deployed
but will create jobs in
manufacturing and installation as
the technology market penetration
expands.
• In addition, through energy grid
stabilization and smoothening, the
technology is expected to support
economic growth objectives.
Conclusion:
During surplus energy, on a windy day, the
power can be stored and then fed into the
grid during the calm. If this succeeds on a
large scale, the integration of conventional
power plants with renewable resources can

8. be optimized. Though storage technologies
may not be a solution to all the problems,
they could considerably gain importance in
tomorrow’s electricity market.
• References:
http://wrap.warwick.ac.uk/65595
/1/WRAP_Luo_1-s2.0-
S1876610214034547-main.pdf
• http://energystorage.org/compres
sed-air-energy-storage-caes
• https://www.power-
technology.com/features/featurec
ould-air-be-the-next-big-battery-
breakthrough-5864457/
• http://www.climatetechwiki.org/t
echnology/jiqweb-caes
• http://citeseerx.ist.psu.edu/viewd
oc/download?doi=10.1.1.374.7597
&rep=rep1&type=pdf
• https://www.youtube.com/watch?
v=K4yJx5yTzO4
• https://www.youtube.com/watch?
v=Bj2jTm0PtWw