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Abstract- The most important challenge caused by the transition
from fossil fuels to renewable resources is that most renewables are
intermittent and unpredictable in their nature. The use of these unfailing and
renewable resources requires to study on the substrates and new infrastructure
of the transmission and distribution networks. Nowadays special
attention has been paid to the use of the storage processes to improve the
power quality of these power plants. Energy storage has the
potential to meet the power fluctuation challenge of renewable resources and
enables large scale implementation of renewables. In this paper, we have
investigated to introduce a new optimized hybrid compressed air energy storage
method. The suggested hybrid system based on compressed air storage
facilitates the use of renewable energies by shaving and shifting load picks
and it improves reliability of energy storage. The supposed optimized model has
been demonstrated by simulation in TermoFlow software. The results of the
simulation revealed that the hybrid model has optimized the regenerating of
electrical energy with considering all crus and pons of it.


Keywords- Compressed Air Energy Storage (CAES) System, Gas
Turbine, Air Turbine, Optimized Hybrid Generation, On-Peak.

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1.      Introduction

The energy value chain that includes resource/fuel,
power plant, transmission, distribution and consumption respectively, always
has suffered from absence of storage. So that the energy storage at off-peak
hours and releasing it at on-peak hours have had effective contribution in
prosper of power marketing 1.

Generally low-cost energy from renewable sources
extracts at a time when consumer energy demand is low and rising consumption
during high-cost hours increases the average cost of production. Therefore, releasing
the renewable energy stored in peak of consumption can lead to reduced energy
costs and increase the reliability of the system and will reduce the switching
of power plants in peak hours.

The storage system plays the role of a large consumer
at off-pick hours and avoids the power plants leaving the circuit, which is
very useful in exploiting nuclear power plant networks. At on-peak hours,
stored energy is released and performs as an emergency power plant and will
prevent the entry of gas power plants into grid as an emergency power plant.

In addition, energy storages facilitate the use of
renewable resources by capturing the wrong time energy and making it available
when needed 2. Also, It helps to shift and shave load picks 2-4

The figure (1) depicts solar energy and wind power
generation curves compared with power demand of the grid.

(1): Solar energy and wind power generation curves compared with power demand
of grid 5.

Maximum output extracted power of solar and wind occur
in midnight and midday, respectively, while often at this time grids are off-
pick and confront by overvoltage problem, so energy storage approach is a
practical solution to avoid from this challenge.

2.      The Solutions of System Management

Demand smoothing and peak shaving of power consumption
curve always have been a real concern in the power system management. The
wide power grids that face with the problem of extreme volatility
(m0=Pmin/Pmax  very low load distortion
coefficient) use gas turbines and pumped storage projects for peak managing,
adaption and shifting the low load to over load of grid that the power is
generated by higher costs. The Gas turbines are utilized as emergency power
plants to improve system stability and power generation during periods of load
peak, because of the ability of fast starting and quick placement in circuit. Anyway,
gas turbines don’t require performance for adjustment of production cost in low
load (minimum cost) and over load (maximum cost),
also due to its low efficiency, the experts of electric power industry, have
been encouraged to investigate competitive methods. In this
regard, pumped storage power plants despite of their high capacity of the power
generation have provided the capability of power supplying
in the peak- load periods and overlap of it with off-pick periods. In the
other words, the excess power generated during periods of low cost generating
can be used for storage purposes and then it can be injected to grid again. All
the advantages of pumped storage methods will be achievable if the adequate
resources of water and the height difference between charge and discharge
reservoirs be available. The mentioned restrictions on the use of these methods
on flat areas are unsolvable weak-points compared with conventional gas units.

Nowadays, the problem of off-pick and on-pick
non-overlapping has been solved with by reforming the cycle of generation. In
one of these reforms the efficiency increases by cooling the inlet air. In this
cycle, at the off-peak hours the ice is produced within the reservoir by an
evaporative condensation system, and in the on-peak hours inlet air is cooled
6, 7. This idea comes from the usage of gas turbine at tropical areas,
especially in warm seasons that consumption of energy increases, utilization
of inlet air cooling systems increases capacity of gas turbines
while this increment of production is provided with lower specific fuel
consumption than the specific fuel consumption of gas turbine under normal
circumstances. Hence, it could be concluded that the cooled air produces more
inexpensive MWs (in per MWh of Electricity of Cooling System is consumed about
40% less fuel than conventional electricity) and contamination from 40% less
fuel than a normal power is less at least 40%. This is apart from the decrement
of pollutants such as NOX and CO due to reduction of combustion temperature
8. The Figure (2) shows that any increasing of temperature of inlet air,
reduces the turbine output power and the thermal efficiency of turbine in
comparison with the ISO† conditions (Temperature 15oc, Relative humidity 60%,
Absolute pressure at sea level 101.325kPa). So that, increase of 1 degree in
temperature of inlet air of compressor, reduces the power extracted from the
turbine about 1 percent 9.

Figure (2): The heating rate,
thermal efficiency and output power characteristic for different temperatures

The above proposed method is applicable and effective
only at warm areas and regions where temperature variations of warm and cold
seasons are significant 10. Meanwhile, this system increases the production
in range of the efficiency of turbine. This system is recommended for
compensating the losses of gas units in high temperatures and improving the
effective efficiency. Due to the limitations of above proposed methods,
providing an efficient method which be able to meet the cited limits, seems to
be necessary.

The Compressed Air Energy Storage system (CAES) has
taken into consideration in recent decades and a lot of investigations have
been in this issue. Particularly, the installation of Hunturf ‡ power plants in Germany using energy storage
techniques in format of compressed air form 11,12 and electricity generation
by releasing the stored air have led to further studies to apply this technology
in other places 13. Totally, the needs for electricity storage technologies
such as CAES are growing, because of increasing the penetration rate of
renewable energies in power grid and constraints of other parts of production
including the thermal power plants in the electricity production chain.
Bearing  in our mind that the use of
compressed air storage system in combination with a gas turbine power plant
like Huntorf power plant, because of separating the pick times, increases the
produced power in a single unit approximately 3 times more than conventional
gas -fired power plants 14, 15.

3.      Operational Researches On CAES plans

Among all storage methods, pumped-hydro storage system
(PHS) and CAES are more viable, because of their storage capacity. Nevertheless,
the investment cost of CAES cases are less than PHS in the same capacity.

The challenge of accessibility to water and specific
geographical and geological conditions in PHS plants has caused the lack of
development in comparison with CAES plants. The first air-storage gas turbine
power plant in the world at Huntorf has fulfilled the desire of having a
storage power plant in flat areas. This power plant has both common advantages
of peak load emergency gas turbines and pumped storage techniques. In comparison
with operation of conventional gas turbines, the air-storage gas turbines
separate the generation and consumption time. The CAES units operate on the
same principle as traditional pump storage plants. The material storage is air,
instead of water. Another advantage of this method of operation is that the
useful work of the plant, according to the compressor and type of storage, is
20% to 45% more than the pumping work applied. Meanwhile, the CAES also has the
advantage of having a considerably lower capital investment in contras with PHS
schemes 15. CAES units are capable to respond to the variety of loads,
because they are renitent against continuous changes of turning-off and
turning-on 14.

Currently, the CAES is used in two
places in the world, in Huntorf, Germany and Macintosh, Alabama, America.
Macintosh power plant has been reformed compared Huntorf, by placing a
recuperator for preheating the output air of reservoir using waste heat of the
turbine and increasing the efficiency.

The size and cost of turbines not only depend on the
operating hours, but also are dependent on the pressure and temperature of
storage tanks. Operation of CAES is based on storage in off-peak and
regeneration of electricity in on-peak hours. Investigation on optimizing of aforementioned
system indicates that the best economical operation happens in
storing pressure of 46 (bar) to 66 (bar). The Figure (3) shows the new CAES
plan of Alstom Company based on ET11NM class gas turbine, the process has been
planned so that the first stage turbine expands the outlet air of tank without
primary combustion, and ignition occurs in second stage. This design leads to
reduce the production of NOx gases due to lower pressure combustion 13 and
also applying the recuperator has improved the performance, like
Macintosh case 16.

(3): New CAES plan of Alstom Company.

In reference 17, it has examined the possibility of
setting up a Macintosh-like plant in California, also a similar study on a CAES
with capacity of 280Mw in Mississippi has been presented in 18. A hybrid system of CAES has been suggested in 19,
which its charging process at off-peak hours is provided by turbine exhausts,
in this system natural gas is used as fuel of combustion chamber and coal as
fuel of heater.

4.      Practical Patterns of CAES

Nowadays, there are three patterns for the use of
Compressed Air storage system. These three patterns are recognized and
practiced in format of three generations of CAES.

first generation of CAES

This technology has been proven scientifically and
behaves like a gas power plant, i.e. Huntorf station. The Figure (4)
illustrates the mode of action and components of the system.

Figure (4): The
scheme of the first generation of CAES systems.

second generation of CAES

The second generation of this technology is similar to
the first one, with considering some reforms such as heat rate reduction and
efficiency increase. The efficiency of the second generation is approximately
54% while it is 48% to 50% for the first generation. This generation is an
open cycle system and uses ordinary gas turbines. It
employs the outlet residuum air of turbine for preheating of air before
entering to expander 19, 20. The Figure (5) illustrates the mode
of action and components of the system.

(5): The scheme of the second generation of CAES systems

third generation of the CAES

The third generation of CAES is called Adiabatic
Compressed Air Energy Storage (ACAES), so that new investigations have focused
in this context 21-23. Any fuels are not used in conversion process of stored air to electric
power. Cooling the compressors and heating the stored air for power generation
is possible via thermal energy storage.

Overall, in this pattern, the efficiency of energy
interchange between off-peak and on-peak varies from 70% to 75%. Storage of
thermal energy is the main defect of this generation and the matter is still
under investigation. The latest manner of thermal energy storage of materials
is the using of high temperature molten salt which also used in solar energy
production. The Latest solution of storing thermal energy is use of high
temperature molten salt which is used in concentrating solar power generation.
The Figure (6) shows the configuration and system performance 19.

Figure (6):
The scheme of the third generation of CAES systems 19.

5.      Proposed-Optimized Hybrid Model

The proposed model has been composed of two sections,
including single shaft gas unit and CAES unit, which are assembled on a same
shaft. Both Systems have been designed for operating in series and parallel to
each other. Figure (7) illustrates the configuration and simple plan of
suggested model.

The proposed scheme has the capability of exploiting
in three different modes. This characteristic increases its maneuverability and
stable performance in comparison of both gas and air compressing units.

The simulation of hybrid model has been affected by
THERMOFLOW software for all three practical modes, separately.

(7): Scheme of the proposed hybrid model for CAES systems.

The Gas Cycle

In the gas cycle mode, the clutch of Air Turbine (AT)
is disconnected and gas turbine (GT) is connected to compressor No.1 via its
clutch. According to Figure (8), in this mode, system is exactly like a gas
unit. The generated power in this mode is 38790 kW.

Figure (8):
The simulation of the A Cycle Mode in Thermoflow

The Compressed-Air cycle

For achieving this mode, the GT must be disconnected
from compressor No.1 and also AT is connected to generator via the clutch.
Finally, the air compressed, is released and entered to AT step 1 and 2. Figure
(9) depicts the configuration of mentioned mode in Thermoflow software.

Figure (9):
The simulation of the A Cycle Mode in Thermoflow

According to Figure (9) this mode consisting 4
elements of source of air, low pressure AT, high pressure AT and generator,
left to right respectively.

It must be considered that with discharging the air
tank, both air density and pressure will be reduced.

The Figure (10) depicts the LP AT and HP AT generated
power vs. air density of tank for different air pressure of tank.

Density of Tank kg/s


(10): the LP AT and HP AT generated power vs. air density of tank for different
air pressure of tank (Mode B).

According to Figure (10), it is visible that with
decreasing the air pressure and air density of tank, the generated power will
be reduced. The analysis of this picture is important for designing suitable
CAES process.

In addition, from financial point of view, we can
compare the generated power with consumed power for compressing the air (Table

Table (1): Comparison of generated power
with consumed power for compressing the air (Mode 5.2).

Generated/ Consumed Power kW at 70

Air Density kg/s

Compressor Power
























The Hybrid Generation cycle

In the hybrid mode, all the elements have been
connected through the clutches. In other word, compressor No.1, GT, AT and
generator are assembled on a same shaft. (Figure 11)

Figure (11):
The simulation of the A Cycle Mode in Thermoflow

As could be considered in Figure (11), the order of
situating the components is different with Figure (7), but it never causes any
discrepancies in results.

If the inlet air be supplied by the compressor No.1,
AT will freewheel and mechanical power production, will take place only by the
GT. Situating of AT on the shaft reduces the efficiency of the process,
significantly. In the other way, if the inlet air be supplied by air storage
tank, the compressor will freewheel and stored air leads to rotate the AT and
after passing across combustion chamber, it forces the gas turbine to

(10): the LP AT and HP AT generated power vs. air density of tank for different
air pressure of tank (Mode C).

With depicting the generated power of HP AT and LP AT
vs. air density variation for different air pressures of tank in Figure (12),
it would be investigated that the best pressure for applying the hybrid mode is
75 bars and the proposed hybrid mode doesn’t work properly for lower pressures.

In the other words, the inlet air pressure of GT must
be 0.6895 to 1.72 bar and these pressures couldn’t be achieved in 75 bar
pressure of tank.

Table (2): Comparison of generated power
with consumed power for compressing the air (Mode C).

Generated/ Consumed Power kW at 70

Air Density kg/s

Compressor Power










According to tables (1) and (2) in 122.9 kg/s, the
consumed and generated power of mode C is a little more than mode B, because of
increasing the efficiency of mode C (97.67%) against efficiency of mode B

Operating in the hybrid cycle provides the possibility
of prompt mode-changing from gas cycle to compressed-air cycle and increases maneuverability
in selecting of the operational mode. In addition, it leads to uninterrupted
and more economical performance.

6.      Benefits of the proposed plan

The proposed system is capable to develop and act in
local grids that includes lighting loads have the most contribution in peak
load. In Black-Out situation of power system, the CAES
is capable to rotate and recover only with potential energy of compressed air
without any other external prime mover.

In startup, the gas cycle does not require to drivers,
e.g. diesels, electrical motors and SFC§. In the other words, the function of starter system
in turbo generators is preparing compressed air for combustion chamber in
turbines and overcoming the initial torque and inertia of the system. In
suggested plan, the storage tank is capable to produce air compressed in
appropriate pressure for initiation and even turning gear. In addition, using
the tank of compressed air in startup obviates the needs of
using generator rotor special design which it done in usage of SFC.

Use of this plan along renewable resources such as
wind farm in complementary mood, as hybrid model, leads to increase power
stability. In the other hand, time separation of
generation and consumption processes through the storage template,
causes more adaptable conditions between generation and consumption of wind
farms. Practically, CAES optimized hybrid model system, stores the generated
inexpensive and clean energy of wind farms and injects it to grid at peak
times, hence has significant effect on economic dispatch and levels the demand

7.      Conclusion

In this paper, after reviewing the generations of
CAES, a new conceptual model of CAES has been presented, briefly. The
implementation of this system not only reduces the pressure on the power system
during peak-load, but also provides frequency stability and
increases efficiency.

In addition, the proposed model improves stability and
the reliability of the associated Wind- Gas units by providing the favorable
conditions for exploitation of hybrid production of Gas plant with CAES
interface. If the CAES stay connected during operation of gas power plant, the
operation leads to freewheeling rotation in AT without any production. This
system is capable to provide the power demands as an energy storage technology
even if power production of renewable resources decreases suddenly.
Normally, the economic dispatch and calculating of full load period of each
mode (gas cycle, compressed air cycle and hybrid generation cycle) result in
increasing of efficiency and appropriate planning of system.

In this article the main priority is technology
demonstration from technical and power generation point of views, hence the
economical profit margin and operational constraints are not necessarily
discussed. Thus, economical studies are suggested for future studies.

International Organization for Standardization

‡ -Huntorf, Niedersachsen, northwest of
Bremen, Germany.

1- Starting
Frequency Converter

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