New
Compressed Air Energy Storage Concept Can Improve the Profitability of Existing
Simple Cycle, Combined Cycle, Wind Energy, and Landfill Gas Combustion
Turbine-based Power Plants
Dr Michael Nakhamkin, Ronald H. Wolk, Sep van der Linden,
Ron Hall, and Dale Bradshaw
Introduction
While
combustion turbine manufacturers are steadily but slowly increasing Combustion
Turbine (CT) efficiency by increasing turbine firing temperatures and improving
the efficiencies of major components, there are a number of other approaches
including various thermal cycle modifications that can reduce the cost of
electricity.
One
such approach is the Compressed Air Energy Storage (CAES) power plant where air
is compressed using less expensive off-peak electricity and stored in the
underground air storage cavern. It is later released for the power generation
during peak demand hours. The first CAES plant in the US, the 110 MW Alabama
Electric Cooperative plant at McIntosh, AL has been in successful operation
since 1991. The application of this storage principle is particularly important
for utilization of the renewable energy sources like wind energy, which could be
produced at night and during other off-peak hours when electric power demand and
prices are both low. The integration of the wind and CAES plants will improves
economics of both plants - it allows selling the wind energy at peak price and
it improve CAES plant economics by driving compressors with wind energy (when
there is no demand) having the lowest costs. The
same principles can also be applied to landfill gas generation facilities.
A
second approach is to reduce the power consumption by the combustion turbine
compressor that is directly driven by the combustion turbine expander and takes
over 50% of its power. There are
number power plant concepts like the CHAT and HAT thermal cycles that achieve
these objectives by using the humidified air as a working fluid, thus reducing
the specific air consumption and parasitic power consumption by compressors.
Unfortunately these concepts didn't
findhaven't
yet found attractive
market
applications to justify required combustion turbine modifications by turbine
manufacturers. This paper presents the novel Compressed Air Energy Storage
(CAES-CT) concepts that utilize the aforementioned two approaches and which are
differentiated from the conventional CAES plants as follows:
1. The CAES-CT concepts utilize the existing reserve capacities of combustion turbine CT and Combined Cycle (CC) plants by injecting the stored air (similar to Steam Injection, without CT/CC any modifications.
2. CAES-CT concept avoids a) the complications and costs associated with development of highly customized specific CAES turbomachinery trains with reheat and recuperation; b) the costs associated with new projects development like site permits, licenses, etc.
3. The CAES-CT concept could be easily applied to meet a variety of the CAES plant power and storage requirements (without specific turbomachinery customization development efforts) by achieving required storage capacities through combining CTs of various capacities with various numbers of units.
4. The existing CTs as a rule present a state-of-the art proven technology with the highest performance characteristics and the lowest possible NOx emissions
5. The stored compressed air could be easily humidified before injection into CT/CC plants (with associated efficiency improvements and storage cost reductions) without the aforementioned complications associated with modifications of CTs.
The
paper also will demonstrate advantages of the integration of the renewable
energy sources with the CAES-CT concepts - large (100-300 MW) CAES plants with
the underground energy storage in salt, hard rock and aquifer geological
formations and small (5-15 MW) CAES plants with the air storage in the man-made
storage vessels including the buried high pressure piping.
Novel
Concepts Based on Use of Existing reserves of Combustion Turbine/Combined Cycle
Power Plants (CAES-CT and CAES-CC)
For the overwhelming majority of electric power
customers (in the USA and abroad) power demands reach their peak during summer,
when high ambient temperatures reduce the power output of combustion turbines
and combined cycle plants to the minimum. The simplified explanation for reduced
power production is that lower inlet air density, a result of the high ambient
temperature, reduces mass flow through a CT with a respective reduction of the
power produced. Similar situation exists at high elevations. Thus GE Frame 7FA
CT nominally rated at approximately 174 MW at ISO conditions (59 °F with 60% relative humidity), will produce
maximum power of approximately 191 MW, when the ambient temperature is 0 °F, but will produce only approximately 150 MW
at 95 °F. The significant power loss by a combustion turbine during
high ambient temperature periods (when the price for replacement electricity is
at its highest) requires a power generation company to install additional units
to meet summer peak demands. Power losses for a combined cycle power plant
during high ambient temperature operations are similar to those for a combustion
turbine. Thus CT/CC plants operate for the most of the time with significantly
lower than design capacities, which take place at low ambient temperatures.
Therefore existing capacity reserves of CT/CC plants could be utilized for
producing additional peak capacities via injection of the stored compressed air.
This approach allows development of the CAES plants by adding only the air
compression and storage facilities and avoiding highly customized and expensive
turboexpander trains. This method was invented by Dr. Michael Nakhamkin of ESPC,
as described in U.S. Patent #5,934,063
(Reference 1) and in Reference 2.
Figures 1a and 1b (including humidification)
illustrate schematic diagrams and simplified heat and mass balances of the CAES-CT
concept (with operating parameters), where the power of the GE 7FA-combustion
turbine, operating at 95 °F
ambient temperature, is augmented using compressed air stored in an underground
compressed air storage system. The major components of the CAES-CT plant are as
follows:
·
A
commercial combustion turbine with the provision to inject the externally
supplied compressed air at any point upstream of the combustor. Engineering and
mechanical aspects of the air injection for CAES-CT plants are similar to steam
injection for power augmentation, which is a standard CT option provided by a
number of OEMs;
·
An
auxiliary compressor train - a standard centrifugal compressor system
consisting of commercially available off-the-shelf standard compressor modules
and sized for the incremental airflow (and not for a full airflow) to be stored
and later injected into a CT/CC plant;
·
Compressed
air storage, which could be underground storage in salt, hard rock or aquifer
geological formations or above ground storage in various pressure vessels;
Figure 1a.
The CAES-CT Concept Based on The GE 7FA CT
This CAES-CT plant has three modes of operation:
·
A conventional combustion turbine operation,
where the CT is disconnected from the compressed air storage;
·
A CAES mode of operation, where during peak periods the CT's compressor
discharge air flow is complemented by the compressed air flow from the storage
and injected upstream of the combustors. The compressed air from the storage
could be optionally preheated in the recuperator and humidified. The storage is
charged with the compressed air by the auxiliary motor-driven compressor during
off-peak hours utilizing renewable resources or available nuclear or coal
capacities.
Figure 1b
CAES-CT Concept with Humidification based on The GE 7 FA CT
Figures 1 a and 1b illustrate that the additional
compressed air flows from the compressed air storage of 116 lbs/sec and 58
lbs/sec (with further humidification), respectively, being injected upstream of
the combustors will increase the combustion turbine power output from 150 MW to
190 MW. The performance characteristics presented in Figures 1a and 1b should be
considered as estimates only because the maximum amount of injected air, at any
given ambient temperature, could be restricted, also, by a number of external
limitations, like the electric generator maximum capacity and electrical system
restrictions, etc.
The additional CAES plant capacity of 40 MW achieved
by only one CT demonstrates that practically any required CAES plant capacity
could be achieved by combining capacity reserves of various number of existing
CT/CC plants. In addition to the power increase, the CAES-CT plant's heat
rate, characterizing the fuel consumption in BTU per kWh produced, is
significantly reduced to that typical for a CAES plant levels of approximately
4000 Btu/kWh for the CAES-CT plant and of 6200 Btu/kWh for cases with
humidification. In spite of a
higher heat rate for the case with humidification it is important to remember
that based on the Figures 1a and 1b the humidification concept reduces the
underground storage by a factor of approximately two (2) (as compared to the dry
air concept) with corresponding cost and schedule savings. The operating cost of
the CAES-CT plant in addition of the fuel, requires off-peak energy for the
compressed air storage recharging with the compressed air. The fuel and energy
related cost of electricity (COE) (without O&M costs) produced is calculated
as COE, $/kWh = [Heat rate, BTU/kWh) x (cost of fuel, $/BTU) + (the off-peak
energy for the storage recharging, kWh) x (cost of off-peak energy, $/kWh)]/
(total kWh produced in the power augmentation mode of operations). The CAES-CT
concept could be similarly applied to CC plants with the difference that the CT
is replaced by a CC. The CAES-CC plant operations are similar to the CAES-CT
plant, the only difference is that in the CAES and power augmentation modes, the
increased power of the CT will be complemented by an additional power produced
by a steam turbine of the bottoming cycle due to additional the CT exhaust flow.
ESPC
jointly with a consulting engineering company had performed the conceptual
engineering and cost estimates for the CAES-CT concept based on GE Frame 7FA,
with the compressed air storage sized for continuous six (6) hours operation
with the incremental (CAES) power of approximately 40 MW.
The overall plant has been optimized based on developed by ESPC program
including concurrent optimization of parameters, performance and economics of
the compressed air storage in a salt dome (with assumed characteristics), the
compressor train and other equipment involved including the compressed air
charging costs. The resulting storage requires approximately 3.8 million cubic
feet (with depth of approximately 1000 feet and the maximum minus minimum
pressure difference of 150 p.s.i.a.) with an estimated construction cost of $4
million (these data are based on prorating of actual parameters and costs of the
underground storage in the salt dome for 110 MW CAES plant). The compressor
train has been sized for two hours of compression for each hour of peak power
generation at 95 °F,
i.e. for half of the supplementary flow from the cavern (58 lbs/sec). Estimated
total incremental cost for equipment and systems required for the conversion of
the Frame 7FA combustion turbine into the CAES-CT plant with aforementioned
operating requirements is approximately $8.5 million dollars, which is
approximately $215/kW (40 MW additional power at 95 °F
ambient temperature). This compares favorably with the approximately $600/kW
specific cost for a turnkey installation of a large capacity CAES plant and it
is, also, significantly lower than turnkey specific costs of a CT.
Above Ground Air Storage Facilities
Figure 2. Conceptual arrangement of the Sub-Surface Compressed Air Storage
The
development of the SSCAES plant is a joint effort of EPRI and ESPC with support
from manufacturers and covered by the US Patent # 5,845,479 (see Reference 3)
and described in EPRI's report (Reference 4).
This section of the paper presents the latest performance and economics
of small 8-12 MW CAES plants with the compressed air stored in the subsurface
high-pressure vessels/piping. The engineering of the SSCAES plant was based on
the Rolls Royce Allison (RRA) combustor/expander/electric generator (CEG)
package. As typical for any CAES plant, the power generation heat rate is
expected to be approximately 4000-5000 Btu/kWh. This does not include the power
required for air compression, which is obtained from other sources (like wind
energy, coal or nuclear plants) during off-peak hours, when electricity is
inexpensive and available from other sources.
The
SSCAES plant utilizes off-peak energy to drive motor-driven boost compressors to
compress air and store the high-pressure air for expansion during peak demand
periods. It could be practically located at any site regardless of the
availability of specific geological formations required for a conventional CAES
plant.
Wind
Power Plants Integrated with Compressed Air Storage
One of the key economic issues that disadvantages
wind power is that it is intermittent and therefore non-dispatchable. Since
capacity credits cannot be obtained, wind power is at a significant economic
disadvantage relative to other forms of generation. Another disadvantage is that
in various areas of the United States and other areas of the world, as well, the
velocity of the local winds and therefore the amount of power that can be
generated does not peak at the same time that power demand peaks. If the wind
typically blows during the late evening hours, wind power will only have a value
equivalent to base load power in many areas, on the order of $15/MWh or less.
One approach to offsetting these deficiencies is to
combine wind power with a CAES Plants and for small capacities with SSCAES
plants. For wind energy integrated with SSCAES plant, as shown in Figure 3,
electricity produced by wind turbines during periods of low demand is used to
compress air that is stored in a pipe network of the type shown previously in
Figure 2. Optimization studies indicated that the compressed air should be
boosted by a system of compressors to the pressure of approximately 1500 -
2000 p.s.i.a and stored in the underground HP piping. When wind power is available and can be marketed, that power
is supplied directly to the grid. At those times when wind power can be
generated, but there is no market for the power, it is used to drive a
compressor that injects air into a Sub-Surface pipeline network. That air is
withdrawn during peak demand times and fed to a natural gas fired expander to
deliver a total of 15 MW to the system. A heat recovery system is used to
preheat the pressurized air by exchange against the hot expander exhaust. As
shown in Figure 3, the humidification of the injected air reduces the amount of
air that is required to be stored. This significantly reduces the investment
required for both the air storage system and the compressor. The additional cost
of the systems for humidification is small in comparison to these cost savings.
The detailed cost and performance data are presented in the References 4 and 5.
Specific costs are estimated as $550/kW for the SSCAES with the air
humidification for peak power generation and arbitrary selected the four-hour
period for peak power generation of and eight hours for charging the pipe
network system. The heat rate is 6030 Btu/kWh (without accounting for the
off-peak energy provided by wind). It should be noted that there is a 40 %
reduction in the amount of natural gas burned to produce the peak power in the
case with humidification as compared to a CT. This translates into comparable
reductions in NOx and CO emissions. These economics can be markedly improved by
integration of the wind-generated plant with the SSCAES-CT plant and with large
capacity CAES-CT (with underground storage) due to significantly reduced costs
associated with utilization of existing CT plants.
Figure
3. Heat and Mass Balance Diagram of an Integrated Plant with Wind Turbines,
Compressed Air Storage, Air Humidification System and a Rolls-Royce Allison
501-K Turboexpander
Conclusions
Novel
CAES-CT plant concepts have the following characteristics:
·
The CAES-CT
concept allows the CAES plant advantages to be utilized in the most cost
effective and practical manner by utilizing existing power capacities of CT/CC
plants without expenditures and complications associated with development of
customized reheat turbomachinery trains;
·
The air
injection into a CT is mechanically similar to the steam injection for the power
augmentation. The special
compressor train for the storage charging could be provided by a number of OEMs
and there are a number of companies capable of construction of underground
storage facilities.
SSCAES
plant is based on the small capacity modified CTs with the compressed air
storage in the man-made buried high-pressure piping. Specific costs of these
plants of approximately $550/kW are competitive with other storage options.
These CAES-CT and SSCAES
plant concepts are particularly effective when integrated with renewable energy
sources: they allow collecting and storing the renewable energy whenever it is
available and releasing it whenever the energy is needed at premium prices.
References
US
Patent # 5,934,063 "Method of Operating a Combustion Turbine
Power Plant at Full Power having Compressed Air Storage"
Dr.
M. Nakhamkin, R. Wolk - "Compressed Air Inflates gas Turbine Output",
Power Engineering, March 1999.
US
Patent # 5,845,479 "Method
for Providing Emergency Reserve Power Using Storage Techniques for Electrical
Systems Applications"
CAES
-Plant Cycles for Substation Applications, EPRI Report PA 8068-01, October 1997
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