Power plant utilizing compressed air energy storage and saturation

An improved power plant employing a combination of compressed air storage and saturation (simultaneous heating and humidification) of compressed air is disclosed. The power plant includes a combustor which provides hot gas for driving a turbine. The turbine is used in conjunction with a generator to generate electrical power. The power from the turbine is accessible by a compressor system during low power demand periods. The compressor system is used to compress air which is stored in an air storage chamber. The compressed air from the air storage chamber is used by the combustor during high power demand periods, while the compressor system is shut down, to provide compressed combustion gas to the turbine. To enhance the efficiency of the plant, while further lowering the capital cost of the plant, a saturator is positioned between the storage chamber and the combustor. The saturator receives compressed air from the storage chamber and simultaneously heats and humidifies it. The resultant heated and humidified compressed air is then conveyed to the combustor, typically after further heating by a recuperator.

BRIEF DESCRIPTION OF THE INVENTION 
This invention relates generally to an improved power plant. More 
particularly, this invention relates to a method and apparatus for 
enhancing the operation of a power plant by utilizing a combination of 
compressed air energy storage and saturation (simultaneous heating and 
humidification) of compressed air with water vapor. 
BACKGROUND OF THE INVENTION 
The power output demand on a power plant grid system varies greatly during 
the course of a day or week. During intermediate and high demand periods, 
typically between 7:00 a.m. and 11:00 p.m. on weekdays, the value of 
electric power is comparatively high. In contrast, during the low demand 
periods, typically on weekends and between 11:00 p.m. and 7:00 a.m. on 
weekdays, the value is relatively low. Thus, for the low demand periods, 
it would be highly advantageous to find an efficacious way to (1) store 
the mechanical, thermal, and/or electrical output of an individual power 
plant, or (2) store the electrical output produced by other power plants 
on the grid. The stored power could then be economically used during high 
demand periods. 
One approach to storing power generated during low demand periods involves 
the operation of compressors during these periods. The compressors produce 
compressed air which possesses mechanical and thermal energy which can be 
stored. The compressed air from storage may be utilized by the power plant 
at a later time while the compressors are shut down. While this approach 
realizes certain benefits, there are still some deficiencies associated 
with it. 
First, the capital cost and operating costs of compressors are high. 
Another issue relates to the practical requirement of cooling the 
compressed air before storage and then heating the compressed air after it 
is removed from storage. This heating is generally accomplished through 
recuperation and combustion of a carbonaceous fuel, which is expensive and 
results in the emission of pollutants. Prior art compressed air storage 
plants, even those with recuperators, do not utilize the exhaust thermal 
energy as efficiently as possible. The amount of carbonaceous fuel 
consumption, and hence emissions, can be reduced through a more efficient 
use of exhaust thermal energy generated in the power plant. 
These problems associated with compressed air storage have precluded the 
use of compressed air storage in fuel processing power plants (i.e., power 
plants with a major fuel processing system, such as a coal gasification 
power plant). There are a number of problems associated with fuel 
processing power plants which could be solved through proper utilization 
of a compressed air energy storage facility. One problem associated with 
fuel processing power plants relates to the high capital cost associated 
with fuel processing equipment. It would be advantageous to eliminate the 
fuel processing equipment associated with providing power to the 
compressor during high demand periods. Another issue with fuel processing 
power plants relates to altering the power output during the course of a 
day to address high demand and low demand periods. It would be 
advantageous to operate such a power plant such that it approaches a 
steady state condition. 
OBJECTS AND SUMMARY OF THE INVENTION 
Thus it is a general object of the present invention to provide an 
apparatus and method for utilizing the combination of compressed air 
energy storage and air saturation in a power plant. 
It is a related object of the present invention to incorporate a compressed 
air energy storage feature in fuel processing power plants in order to 
reduce their specific cost and improve their operating flexibility. 
It is another related object of the present invention to more efficiently 
utilize compressed air energy storage designs by incorporating a 
saturator. 
It is another object of the present invention to utilize the combination of 
compressed air energy storage and air saturation to reduce the capital and 
generation costs of power plants. 
It is yet another object of the present invention to provide a power plant 
which operates in a more balanced manner throughout high demand and low 
demand periods. 
It is another object of the present invention to realize high power output 
without an increase in the combustion of carbonaceous fuels. 
It is a related object of the present invention to provide a power plant 
which reduces the emission of pollutants. 
It is yet another object of the present invention to provide a power plant 
which efficiently recycles exhaust thermal energy and all other available 
thermal energies. 
It is another object of the present invention to provide a power plant with 
a saturator which uses thermal energy from a number of sources. 
It is another object of the present invention to provide a power plant with 
less compressor mass flow for a given power output. 
These and other objects are obtained by a method and apparatus for 
producing power in accordance with the present invention. The power plant 
includes a combustor which provides hot gases for driving a turbine. The 
turbine is used in conjunction with a generator to generate electrical 
power. The power from the turbine is accessible by a compressor system, 
typically utilized during low power demand periods. The compressor system 
is used to compress air some of which is stored in an air storage chamber. 
The compressed air from the air storage chamber is used by the combustor 
during high power demand periods to provide compressed combustion gas to 
the turbine. To enhance the efficiency of the plant, while further 
lowering the capital cost of the plant, a saturator is positioned between 
the storage chamber and the combustor. The saturator receives compressed 
air from the storage chamber and simultaneously heats and humidifies it. 
The resultant heated and humidified compressed air is then conveyed to the 
combustor, typically after further heating by a recuperator.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, wherein like components are designated by like 
reference numerals in the various figures, attention is initially directed 
to FIG. 1. FIG. 1 depicts a power plant 20 in accordance with the prior 
art. More particularly, FIG. 1 depicts a power plant with a fuel 
processing system. In accordance with the prior art, the power plant 20 
includes a turbine assembly 22 with a high pressure turbine 24 and a high 
pressure combustor 26. The turbine assembly 22 may also include a low 
pressure turbine 28 and a low pressure combustor 30. The combustors 26, 30 
are fed by a fuel processing system 32, for instance, a coal gasification 
system. 
The turbine assembly drives a generator 34. In turn, the generator 34 is 
coupled to grid 36 and shaft 37. Shaft 37 continuously drives a compressor 
system 40. Compressor system 40 includes a low pressure compressor 42 and 
a high pressure compressor 44. Preferably, low pressure compressor 42 is 
coupled to an intercooler 46 to remove some of the thermal energy of 
compression. The continuous output of high pressure compressor 44 is 
preferably coupled to aftercooler 48 which removes additional thermal 
energy from the resultant continuous compressed air stream. In accordance 
with prior art techniques, the resultant compressed air stream, flowing 
continuously and directly from the compressor system, may be conveyed to a 
saturator 60 and recuperator 70 before being fed to combustor 26. It 
should be noted that the saturator 60 is more effective if used in 
conjunction with aftercooler 48. The overall benefit of the saturator is 
marginal in the prior art because the aftercooler 48 removes thermal 
energy from the compressed air stream exiting the compressor system 40. 
Turning to FIG. 2, a compressed air energy storage (CAES) power plant 21, 
in accordance with the prior art, is depicted. During low power demand 
periods, energy may be drawn from the grid 36. This energy may be utilized 
by motor 38 to drive the compressor system 40. 
The compressed air stream produced by compressor system 40 contains 
mechanical and thermal energy. The stream is processed through aftercooler 
48, which withdraws most of its thermal energy. This is required so that 
the air will be cold enough to be compatible with a practical air storage 
chamber. The cold air stream is conveyed to air storage chamber 52. Thus, 
the air storage chamber 52 serves to store the mechanical energy of the 
compressed air. This energy may be utilized when the compressor system 40 
is shut down at times of high power demand. The energy may be utilized in 
conjunction with the fuel fed to the turbine assembly 22. Specifically, 
the compressed air from storage chamber 52 is conveyed to combustor 26 
through the appropriate configuration of the valves 54, as is known in the 
art. 
In accordance with the present invention, the prior art power plants of 
FIGS. 1 and 2 are enhanced by utilizing a combination of air storage and 
saturation. More particularly, the fuel processing power plant of FIG. 1 
is modified to include an air storage chamber, in addition to other 
complementary elements, and the CAES power plant of FIG. 2 is modified to 
include a saturator, in addition to other complementary elements. 
The combination air storage and saturation power plant of the present 
invention yields a number of advantages. As to be more fully described 
herein, this configuration, in conjunction with fuel processing equipment, 
enables a balanced and continuous operation of a power plant while meeting 
variable power demands. In addition, the apparatus and method of the 
present invention more fully exploits thermal energy sources of the power 
plant. This allows smaller fuel processing equipment and compressors; 
thus, the capital costs of the power plant may be reduced. 
By conveying the pressurized air stream from the air storage chamber to the 
saturator, the turbine assembly of the present invention receives a heated 
and humidified air stream with a greater mass flow and greater thermal 
energy. The higher mass flow and higher thermal energy provided by the 
saturator reduce the amount of energy needed for compression and thus the 
fuel required to provide the compression. Thus, the teaching of the 
present invention reduces fuel consumption and the emissions which result 
from fuel consumption. 
Having disclosed the general concept and advantages of the present 
invention, attention turns to FIG. 3 which depicts a specific embodiment 
of an enhanced fuel processing power plant 20A, in accordance with the 
present invention. 
As to be more fully described herein, the fuel processing power plant 20A 
of the present invention contains a combination of air storage, fuel 
processing, and saturation. As used herein, the term saturation refers to 
the simultaneous heating and humidification of air. 
The power plant 20A includes a turbine assembly 22 which may run 
continuously. During low-demand time periods, the turbine assembly 22 may 
produce more power than is required by grid 36. In these periods, some or 
all of the power of the turbine assembly 22 is applied to motor 38 rather 
than to grid 36. Motor 38 drives a compressor system 40. The thermal 
energy of the compressed air is removed by heating water in the 
intercooler 46 and aftercooler 48. Some of the heated water from the 
intercooler 46 and aftercooler 48 is conveyed to hot water storage tank 
56. Cooling tower 50 may also be provided to cool some of the water for 
reuse in intercooler 46 and aftercooler 48. 
Some of the compressed air stream produced by compressor system 40 is 
conveyed through open valve 54B to air storage chamber 52, while the 
remainder goes directly to the saturator 60 through open valve 54A. The 
compressor system 40 is preferably sized to compress more air per unit 
time, while it is on, compared to that which is consumed per unit time by 
the turbine assembly 22. Over the full cycle of a day or week, the air 
storage charging and withdrawal are in balance. Thus, the air storage 
chamber 52 serves to store the mechanical energy of the compressed air 
(and the small amount of thermal energy not removed by aftercooler 48), 
while the hot water tank 56 stores much of the thermal energy of 
compression. These sources of energy may now be profitably utilized in 
accordance with the current invention. Most significantly, the mechanical 
energy within the air storage chamber 52 may be utilized at time periods 
of high power demand in conjunction with the fuel fed to the turbine 
assembly 22. 
To improve the capital cost and overall heat rate of power plant 20A, in 
accordance with the invention, the air storage chamber 52 is coupled to a 
saturator 60. Specifically, the cold, compressed air from air storage 
chamber 52 is conveyed through open valve 54A to saturator 60, where it is 
converted to a heated and humidified compressed air stream. Preferably, 
the heated and humidified compressed air stream is then conveyed to the 
recuperator 70, where it is further heated. The resultant heated and 
humidified compressed air stream is then conveyed to the high pressure 
combustor 26 of gas turbine assembly 22, as is known in the art. 
The saturator 60 is of the type which is known in the art. In accordance 
with the invention, the saturator 60 receives hot water from a number of 
sources. First, the saturator 60 receives hot water from fuel processing 
system 32. In this embodiment of the invention, the fuel processing 
system's thermal energy is transferred to water rather than steam. The 
pressurized hot water produced by the fuel processing system is fed to the 
saturator 60 where it is used to heat and humidify the pressurized air 
stream. 
The saturator 60 is also preferably fed by hot water from the storage tank 
56. The pressurized hot water storage tank 56 accumulates pressurized hot 
water during operation of the compressor system 40. The water from the hot 
water storage tank 56 is used to heat the fuel and then is combined with 
some of the drain flow from the saturator 60 and fed to flue-gas water 
heater 58, where it is further heated by the exhaust thermal energy from 
turbine assembly 22. 
Thus, the saturator of the present invention effectively utilizes exhaust 
thermal energy from the fuel processing system 32, compressor system 40, 
and turbine assembly 22, and in so doing, it improves the plant 
efficiency. 
By conveying the pressurized air stream from the air storage chamber 52 to 
the saturator 60, the turbine assembly 22 receives a heated and humidified 
air stream with a greater mass flow and thermal energy. As a result of 
this increased mass flow, the amount of air required by compressor system 
40 is reduced. Consequently, smaller compressors may be used, and less 
power will be consumed while driving the compressors. The higher thermal 
energy of the compressed air stream provides more efficient operation of 
the power plant. The teaching of the present invention reduces fuel 
consumption and the pollutants which result from fuel consumption. 
Moreover, it enables use of a smaller and lower capital cost fuel 
processing system. 
The fuel processing system 32, for instance coal gasification, typically 
has large thermal flows (usually originating from cooling the fuel prior 
to its clean-up process). A further advantage of the invention is that it 
makes better use of this thermal energy in the form of hot water. Since 
hot water is used, rather than steam, the capital cost of the fuel 
processing power plant is reduced. 
Hot water preferably enters saturator 60 at the top, while the tepid water 
is mainly removed from the bottom of the saturator 60, where it is 
returned to flue-gas water heater 58 and reheated. Some of the water 
leaving the saturator 60 at various locations is recirculated to the fuel 
processing system 32 for cooling purposes. 
Preferably, the air which leaves the saturator 60 is conveyed through a 
recuperator 70 in which the heated and humidified pressurized air stream 
is further heated before it is fed to combustor 26 of the turbine assembly 
22. Recuperator 70 receives thermal energy from the exhaust gas of turbine 
assembly 22. The remaining thermal energy of the exhaust gas is conveyed 
to flue-gas water heater 58. 
The operation of the power plant 20A of FIG. 3 has been described in a 
continuous mode. In the continuous mode, the fuel processing system 32, 
the turbine assembly 22, and the saturator 60 are always operating. During 
low demand periods, the power from the generator 34 is used to drive the 
compressor system 40. During high demand periods, the compressor system 40 
is shut down, and the generator power goes to the grid 36, thus meeting 
the variable power demands. In the continuous mode, a balanced power 
plant, heretofore unknown in the art, is realizable. The compressor system 
40 is sized so that its power demand is equal to the turbine assembly 22 
output. The compressor system 40 is turned on for just enough duration 
during the low demand periods in the daily or weekly cycle to provide all 
of the compressed air required to continuously operate the turbine 
assembly 22. 
Other modes of operation are possible as well. For instance, during the low 
demand periods, if the compressor system 40 mass flow rate and on-time 
period is configured so that the generator system does not have enough 
power to run the compressor system 40 by itself, additional power can be 
drawn from the grid 36. If, during the low demand time periods, there is 
an extremely cheap or low polluting source of power available from the 
grid 36, it may be preferable to shut down the turbine assembly 22 and use 
the power from the grid 36 for the motor 38. 
Proper electrical connections between the generator 34, the grid 36, and 
the motor 38, are realized through standard switching techniques. Instead 
of electrical connections between motor 38 and generator 34, a single 
motor-generator may be used, connected to the compressor system 40 and 
turbine system 22 by mechanical clutches. 
Turning to FIG. 4, a more detailed description of an embodiment of the 
present invention is provided. The power plant 20AA of FIG. 4 is 
conceptually identical to the power plant of FIG. 3; like components are 
designated by like reference numerals. The primary differences between the 
two embodiments are described herein. 
First, the compressor system 40A includes low pressure compressor 42A, 
intermediate compressors 42B, 42C and a high pressure compressors 44A. A 
number of intercoolers 46A, 46B, and 46C are preferably provided. 
Another difference between the two embodiments relates to the details 
disclosed in relation to a fuel processing system 32. Fuel processing 
system 32 is a gasification system of the type known in the art; it may 
include a hydrolysis reactor 102 coupled to a reactor feed preheater 104. 
The gasification system 32 may also include a high pressure steam 
generator 106 and a low pressure steam generator 110. The gasification 
system 32 may also include a number of air saturator/water heaters 108A, 
108B, and 108C. Vapor liquid separators 112A and 112B are also utilized in 
accordance with prior art techniques. 
An important aspect of the embodiment of FIG. 4 is the utilization of a 
number of saturators 60A, 60B, 60C, 60D, and 60E. Saturator 60D receives 
hot water directly from gasification system 32 through a mixer 63. 
Saturators 60C, 60B, and 60A receive hot water through splitters 61C, 61B, 
and 61A. Saturator 60E receives hot water directly from water heater 68. 
Preferable temperatures (T), pressures (P), and mass flows (M) are 
indicated in FIG. 4. Temperatures are in Fahrenheit, pressures are in 
pounds per square inch, and mass flows are in pounds per second. 
Turning now to FIG. 5, an alternate embodiment of the present invention 
with a combination of compressed air storage and saturation is disclosed. 
More particularly, the method and apparatus of the present invention is 
applied to a CAES power plant 21A. In accordance with the present 
invention, the efficiency of the compressed-air energy storage plant of 
the prior art is enhanced by utilizing a saturator 60 between the air 
storage chamber 52 and the recuperator 70. In contrast to the prior art, 
the use of a saturator 60 in the present invention is highly effective 
since in the prior art the aftercooler 48 was already necessary to remove 
most of the thermal energy of compression for practical air storage 
compatibility. 
Specifically, with the present invention, during periods of high demand, 
the cold compressed air from the air storage chamber 52 is conveyed to a 
saturator 60 where it is converted to a heated and humidified compressed 
air stream. The heated and saturated compressed air stream is then 
conveyed to the recuperator 70 for further heating and then to a combustor 
of the turbine assembly 22. The hot water for the saturator 60 comes from 
the storage tank 56 and the return flow of the saturator 60 after further 
heating in the flue-gas water heater 58. 
By conveying the pressurized air stream from the air storage chamber 52 to 
the saturator 60, the turbine assembly 22 of the present invention 
receives a heated and humidified air stream with greater mass flow and 
greater thermal energy than is obtained in prior art compressed-air energy 
storage plants. As a result of this greater mass flow, the amount 
compression required by the compressor system 40 may be reduced. 
Consequently, smaller compressors may be used, and less power will be 
consumed while driving the compressors. Thus, less energy is required to 
be drawn from the grid 36 in order to drive the compressor system 40. In 
usual American practice, the grid power is derived from burning fossil 
fuel, so the overall fossil fuel consumption would be reduced by the 
invention. Consequently, the teaching of the present invention also 
reduces the pollutants which result from fossil fuel consumption. 
Moreover, it enables use of a smaller and less costly compression system. 
The combustor 26 is fed by ordinary premium fuel (e.g., distillate, natural 
gas) and humid, heated, high pressure air from recuperator 70. The 
recuperator 70 draws humidified, heated, high pressure air from the 
saturator 60. The saturator 60 draws cold, dry, high pressure air from the 
air-storage chamber 52. The turbine assembly 22 is coupled to a generator 
34 which provides power to grid 36 during high demand periods. 
During low demand periods, power from grid 36 may be used by motor 38 to 
drive compressor system 40. The cooled, compressed air produced by the 
compressor system 40 is conveyed to air cavity 52. In accordance with 
prior art techniques, the compressed air may be utilized by turbine 
assembly 22 at a later time. However, to enhance this subsequent use, in 
accordance with the invention, a saturator 60 is utilized to heat and 
humidify the air which leaves the air cavity 52. In one embodiment, this 
heated and humidified air may then be conveyed to the high pressure 
combustor of the turbine assembly 22. The recuperator 70 may be 
incorporated between the saturator 60 and the combuster 26 for improved 
efficiency of operation. As previously indicated, this results in a number 
of benefits. 
The saturator 60 is of the type which is known in the art. In accordance 
with the invention, the saturator 60 receives thermal energy from a flue 
gas water heater 58 which obtains thermal energy from a number of sources. 
The flue gas water heater 58 is fed by hot water storage tank 56. As 
previously discussed, the hot water storage tank 56 accumulates thermal 
energy during operation of the compressor system 40. The water from the 
hot water storage tank 56 is fed to flue gas water heater 58, where it is 
combined with tepid water draining from the saturator 60. The thermal 
energy source for the flue-gas water heater 58 is obtained from the 
exhaust thermal energy of gas turbine assembly 22. Thus, the saturator 60 
of the present invention efficiently utilizes exhaust thermal energy from 
the compressor system 40 and from the turbine assembly 22. 
Other configurations for feeding the various hot water flows to the 
saturator are also feasible. 
During the high demand time period, pump 62 operates, and the flue-gas 
water heater 58 receives hot water from hot water storage tank 56. The hot 
water from the hot water storage tank 56 may be conveyed through gas fuel 
heater 59. The saturator 60 receives the pressurized air from air storage 
chamber 52, as the saturator valve 54A is open, and the compressor valve 
54B is closed. Hot water preferably enters saturator 60 at the top, while 
the tepid water is removed from the bottom of the saturator 60, where it 
is returned to flue-gas water heater 58 and reheated. 
The air which leaves the saturator 60 may be conveyed through a recuperator 
70 which further heats the pressurized air stream before it is fed to 
combustor 26 of the turbine assembly 22. Recuperator 70 receives exhaust 
gas from turbine assembly 22. The remainder of the thermal energy of the 
exhaust gas is conveyed to flue gas water heater 58. Conversely, during 
low demand periods, the air storage chamber 52 receives pressurized air, 
while compressor valve 54B is open, and saturator valve 54A is closed. 
One skilled in the art will recognize that many alternate embodiments of 
the present invention are feasible. The fuel processing system 32 of FIG. 
3 need not be a coal gasification system. Other fuel processing techniques 
such as integrated liquefaction and the gasification of other fuels are 
also feasible, for instance, gasification of heavy oil, coke, oil shale, 
or tar. In addition, the combustors and fuel processor elements do not 
have to be discrete elements; rather, they can be integrated into a single 
system, such as a fluidized bed, as is known in the art. Also, the 
combustors may be replaced by externally heated or fired heat exchangers, 
as is known in the art. 
The foregoing descriptions of specific embodiments of the present invention 
have been presented for purposes of illustration and description. They are 
not intended to be exhaustive or to limit the invention to the precise 
forms disclosed, and obviously many modifications and variations are 
possible in light of the above teaching. The embodiments were chosen and 
described in order to best explain the principles of the invention and its 
practical application, to thereby enable others skilled in the art to best 
utilize the invention and various embodiments with various modifications 
as are suited to the particular use contemplated. It is intended that the 
scope of the invention be defined by the claims appended hereto and their 
equivalents.