U.S. patent application number 14/371569 was filed with the patent office on 2015-01-01 for electricity generation device and method.
The applicant listed for this patent is Highview Enterprises Limited. Invention is credited to Stephen Gareth Brett, John Daniel Alastair Harris, Robert Morgan.
Application Number | 20150000280 14/371569 |
Document ID | / |
Family ID | 45813968 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150000280 |
Kind Code |
A1 |
Harris; John Daniel Alastair ;
et al. |
January 1, 2015 |
ELECTRICITY GENERATION DEVICE AND METHOD
Abstract
The present invention relates to electricity generation devices
and methods that use a cryogenic fluid such as liquid nitrogen or
liquid air and a source of low grade waste heat, and means of
increasing the efficiency of energy recovery from such devices by
combining Rankine and Brayton cycles.
Inventors: |
Harris; John Daniel Alastair;
(Feltham, GB) ; Morgan; Robert; (Shoreham-by-Sea,
GB) ; Brett; Stephen Gareth; (Reading, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Highview Enterprises Limited |
London |
|
GB |
|
|
Family ID: |
45813968 |
Appl. No.: |
14/371569 |
Filed: |
January 10, 2013 |
PCT Filed: |
January 10, 2013 |
PCT NO: |
PCT/GB2013/050037 |
371 Date: |
July 10, 2014 |
Current U.S.
Class: |
60/651 ;
60/671 |
Current CPC
Class: |
F02C 1/08 20130101; F02C
6/14 20130101; F02C 1/05 20130101; F05D 2220/60 20130101; F01K
25/103 20130101; F01K 7/025 20130101; F01K 25/00 20130101; F01K
7/22 20130101 |
Class at
Publication: |
60/651 ;
60/671 |
International
Class: |
F01K 25/00 20060101
F01K025/00; F01K 7/22 20060101 F01K007/22; F01K 25/10 20060101
F01K025/10; F01K 7/02 20060101 F01K007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2012 |
GB |
1200548.4 |
Claims
1. An energy generation device comprising: a storage tank for
storing a cryogenic fluid, a fluid pump for compressing cryogenic
fluid taken from the storage tank to a high pressure, an evaporator
for evaporating the high pressure cryogenic fluid, to provide a
high pressure gas, a first expansion turbine for expanding the high
pressure gas and extracting work from the high pressure gas; a
first reheater for reheating gas exhausted from the first expansion
turbine using ambient or waste heat; a second expansion turbine for
expanding working fluid exhausted from the first reheater and
extracting work from the working fluid exhausted from the first
reheater; wherein the second expansion turbine has an exhaust
outlet which is split into first and second paths such that the
working fluid exhausted from the second expansion turbine is
divided into first and second portions, wherein the first portion
of working fluid is directed along the first path to ambient
through a first exhaust, and the second portion of working fluid is
directed along the second path to an inlet of the evaporator such
that the second portion of working fluid exchanges thermal energy
with the high pressure cryogenic fluid within the evaporator; and a
first compressor for compressing the second portion of working
fluid after it has passed through the evaporator, wherein an
exhaust outlet of the compressor is connected with an exhaust
outlet of the first expansion turbine such that the second portion
of working fluid and the gas exhausted from the first expansion
turbine are combined and directed into the first reheater to be
reheated using the ambient or waste heat.
2. The device of claim 1 wherein at least one of the first and
second expansion turbines is used to drive a generator to produce
electricity.
3. The device of claim 1 wherein the fluid pump provides for
compressing the cryogenic fluid to a pressure of at least 50
bar.
4. The device of claim 1, further comprising a superheater for
heating high pressure working fluid output from the evaporator to a
high temperature using a source of heat from a co-located
process.
5. The device of claim 4, wherein the co-located source of heat is
at least one of the ambient environment, the atmosphere, the
ground, river, sea or lake water, and waste heat from a power
station or industrial plant.
6. The device of claim 1, wherein the first expansion turbine is
mounted on a same power shaft as the first compressor.
7. The device of claim 1, further comprising: a third expansion
turbine and a second reheater positioned between the evaporator and
the first expansion turbine, and a fourth expansion turbine and a
third reheater positioned between the first reheater and the second
expansion turbine.
8. The device of claim 7 wherein the first and third expansion
turbines are mounted on a same power shaft as the first
compressor.
9. The device of claim 1, further comprising a second compressor
for compressing the second portion of the working fluid after it
has passed through the evaporator a first time and directing the
second portion of the working fluid back through the evaporator a
second time before the second portion of the working fluid is
compressed by the first compressor.
10. The device of claim 9 further comprising: a third expansion
turbine and a second reheater positioned between the evaporator and
the first expansion turbine, and a fourth expansion turbine and a
third reheater positioned between the first reheater and the second
expansion turbine, wherein the first and third expansion turbines
are mounted on a same power shaft as the first and second
compressors.
11. A cryogenic energy storage system having a power recovery
component comprising the device of claim 1.
12. A method of generating energy comprising: storing a cryogenic
fluid in a storage tank; extracting the cryogenic fluid from the
storage tank and compressing the cryogenic fluid to a high pressure
using a fluid pump; evaporating the high pressure cryogenic fluid
in an evaporator to provide a high pressure gas; expanding the high
pressure gas using a first expansion turbine and extracting work
from the high pressure gas; reheating gas exhausted from the first
expansion turbine using a first reheater and ambient or waste heat;
expanding working fluid exhausted from the first reheater and
extracting work from the working fluid exhausted from the first
reheater using a second expansion turbine; wherein the second
expansion turbine has an exhaust outlet which is split into first
and second paths such that the working fluid exhausted from the
second expansion turbine is divided into first and second portions,
wherein the first portion of working fluid is directed along the
first path to ambient through a first exhaust, and the second
portion of working fluid is directed along the second path to an
inlet of the evaporator such that the second portion of working
fluid exchanges thermal energy with the high pressure cryogenic
fluid within the evaporator; and using a first compressor to
compress the second portion of working fluid after it has passed
through the evaporator, wherein an exhaust outlet of the compressor
is connected with an exhaust outlet of the first expansion turbine
such that the second portion of working fluid and the gas exhausted
from the first expansion turbine are combined and directed into the
first reheater to be reheated using the ambient or waste heat.
13. The method of claim 12 further comprising using at least one of
the first and second expansion turbines to drive a generator and
produce electricity.
14. The method of claim 12, further comprising using a superheater
and a source of heat from a co-located process to heat high
pressure working fluid output from the evaporator to a high
temperature.
15. The method of claim 12, further comprising: expanding the high
pressure gas using a third expansion turbine; reheating gas
exhausted from the third expansion turbine using a second reheater;
and expanding the working fluid exhausted from the first reheater
using a fourth expansion turbine; and reheating the gas exhausted
from the fourth expansion turbine using a third reheater.
16. The method of any of claim 12, further comprising using a
second compressor to compress the second portion of working fluid
after it has passed through the evaporator a first time and
directing the second portion of working fluid back through the
evaporator a second time before it is compressed by the first
compressor.
17. The method of claim 12, wherein the cryogenic fluid comprises
at least one of liquid nitrogen, liquid air and liquid natural
gas.
18. The device or method of claim 15 further comprising using a
second compressor to compress the second portion of working fluid
after it has passed through the evaporator a first time and
directing the second portion of working fluid back through the
evaporator a second time before it is compressed by the first
compressor.
19. The device of claim 4 further comprising: a third expansion
turbine and a second reheater positioned between the evaporator and
the first expansion turbine, and a fourth expansion turbine and a
third reheater positioned between the first reheater and the second
expansion turbine
20. The device of claim 4 further comprising a second compressor
for compressing the second portion of the working fluid after it
has passed through the evaporator a first time and directing the
second portion of the working fluid back through the evaporator a
second time before the second portion of the working fluid is
compressed by the first compressor.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to electricity generation
devices and methods that use a cryogenic fluid such as liquid
nitrogen or liquid air and a source of low grade waste heat, and
means of increasing the efficiency of energy recovery from such
devices.
BACKGROUND OF THE INVENTION
[0002] Electricity distribution networks (or grids) are often
supported by a fleet of diesel generators and open cycle gas
turbines that provide electricity during periods of high demand and
emergency events such as the unexpected failure of a power station.
Such generating assets, often referred to as peaking plant, burn
fossil fuels at low efficiency and can be a significant source of
atmospheric pollutants. The services provided by such peaking
plant, include, but are not limited to: [0003] balancing
differences in supply and demand at different times of the day and
at short notice; [0004] providing electricity required to power the
auxiliary equipment required for restart of a generating asset in
the event of total network failure (black-start support); [0005]
network reinforcement where parts of the electricity distribution
network have a shortfall in capacity during periods of high power
demand; [0006] injecting power into the network to support the
frequency of the grid when demand for electricity increases
rapidly.
[0007] In addition, the loss of power from the electricity
distribution network can result in significant economic loss to
some consumers, such as a data centre, or danger to personnel, for
example in the event of a power failure at a hospital. Such
applications often utilise diesel generators to provide standby
electricity in the event of an interruption to the supply of
electricity from the distribution network. Replacement of such
diesel powered generators with a zero emissions device that uses a
fuel from a sustainable source would be of benefit.
[0008] There is a need for a device that can provide a similar
service but that uses a fuel that produces low or preferably zero
atmospheric pollution that originates from a sustainable
source.
[0009] The present inventors have realised that there is potential
to generate electricity using the expansion of liquid air, liquid
nitrogen or cryogen to drive a turbine to generate electricity.
Such a device could provide a compact, reactive and environmentally
clean solution to the problems of balancing network supply with
demand.
[0010] WO 2007/096656 discloses a cryogenic energy storage system
which exploits the temperature and phase differential between low
temperature liquid air, liquid nitrogen or cryogen, and ambient
air, or waste heat, to store energy at periods of low demand and/or
excess production, allowing this stored energy to be released later
to generate electricity during periods of high demand and/or
constrained output. The system comprises a means for liquefying air
during periods of low electricity demand, a means for storing the
liquid air produced and an expansion turbine for expanding the
liquid air. The expansion turbine is connected to a generator to
generate electricity when required to meet shortfalls between
supply and demand.
[0011] GB1100569.1 develops the power recovery element of WO
2007/096656 and discloses a device, the cryogenset, and method for
the generation of zero emission electricity that uses a cryogenic
fluid and a source of low grade waste heat, and can be used to
provide load balancing and emergency support to an electricity
distribution network, or back up power to a critical consumer such
as a hospital or data centre. Referring to FIG. 1, the invention of
GB1100569.1 utilises a cryogenic fluid, such as liquid nitrogen or
liquid air, and a source of low grade waste heat 140 to power a
turbogenerator. The emissions from the device are either gaseous
nitrogen or gaseous air and present no environmental concerns. The
cryogenic fluid is manufactured in an industrial refrigeration or
air separation plant 100 using power from the grid 150 or from a
renewable source 160 and supplied by tanker or pipeline 110 to the
cryogenset 130 preferably via a storage tank 120.
[0012] A major constraint on the efficiency of such systems and
devices is the poor utilisation of the thermal energy released from
the cryogenic fluid during heating to ambient temperature. The
exhaust of the cryogenset is only a few degrees above the
temperature of the cryogenic fluid, and therefore significantly
below ambient temperature. Ideally, the exhaust from the process
would be much closer to ambient temperature due to more effective
recovery of work from the process.
SUMMARY OF THE INVENTION
[0013] The inventors have discovered that further work can be
extracted from the working fluid by the inclusion of additional
power recovery cycles to the main open Rankine cycle described in
GB1100569.1. The additional power recovery cycles combined with the
main Rankine cycle exploit the temperature differences between
ambient and the exhaust of the first Rankine cycle to extract more
work from the working fluid. Several cycles can be included to
progressively increase the final exhaust temperature to close to
ambient. However, the efficiency of each additional cycle
progressively reduces as the temperature difference between the hot
and cold parts of the cycle reduces, thereby lowering the Carnot
efficiency of each incremental additional cycle. In practice, one
or two additional cycles would be applied as the cost--benefit of
further cycles is marginal.
[0014] The approach is well known, and the so called `topping` or
`bottoming` cycle is often applied to power generation devices. For
example, a combined cycle gas turbine power station utilises an
open Brayton cycle combined with a closed steam Rankine bottoming
cycle. A cryogenset could be combined with a closed Rankine cycle
utilizing a working fluid that condenses in the operating range
between ambient and the temperature of the cryogen (see FIG. 2).
Alternatively, an open or closed Brayton cycle could be used (see
FIGS. 3a and b).
[0015] In most cases, different working fluids are used for the
main and bottoming cycle, such as air and steam in the case
mentioned above. The inventors have noticed that in the case of the
cryogenset, a single working fluid can be used for both the main
working fluid and bottoming cycle working fluid if an open Brayton
cycle (such as that shown in FIG. 3b) is used as the bottoming
cycle. This has the advantage of simplifying the design and
reducing cost, an essential feature in the target market of reserve
power. In the cycle of FIG. 3b, the inlet air to the Brayton
bottoming cycle may be first passed through an air purification
unit to remove water and carbon dioxide. Referring to FIG. 4,
simplification of the design is achieved by combining the main
working fluid and bottoming cycle working fluid at the inlet of the
first or second expansion turbine.
[0016] Accordingly, the present invention provides an energy
generation device comprising:
[0017] a storage tank for storing a cryogenic fluid,
[0018] a fluid pump for compressing cryogenic fluid taken from the
storage tank to a high pressure,
[0019] an evaporator for evaporating the high pressure cryogenic
fluid, to provide a high pressure gas,
[0020] a first expansion turbine for expanding the high pressure
gas and extracting work from the high pressure gas;
[0021] a first reheater for reheating the gas exhausted from the
first expansion turbine using ambient or waste heat;
[0022] a second expansion turbine for expanding the working fluid
exhausted from the first reheater and extracting work from the
working fluid exhausted from the first reheater; wherein
[0023] the second expansion turbine has an exhaust outlet which is
split into first and second paths such that the working fluid
exhausted from the second expansion turbine is divided into first
and second portions, wherein the first portion of working fluid is
directed along the first path to ambient through a first exhaust,
and the second portion of working fluid is directed along the
second path to an inlet of the evaporator such that the second
portion of working fluid exchanges thermal energy with the high
pressure cryogenic fluid within the evaporator; and
[0024] a first compressor for compressing the second portion of
working fluid after it has passed through the evaporator, wherein
an exhaust outlet of the compressor is connected with an exhaust
outlet of the first expansion turbine such that the second portion
of working fluid and the gas exhausted from the first expansion
turbine are combined and directed into the first reheater to be
reheated using the ambient or waste heat.
[0025] Consequently, the present invention combines a Rankine cycle
with a Brayton cycle.
[0026] The compressor is generally driven by an electric motor or
similar device. One or both of the expansion turbines could be used
to drive a generator to produce electricity from the rotational
energy produced by expansion turbines.
[0027] The cryogenic fluid acts as the working fluid within the
system.
[0028] With the arrangement of the present invention, the working
fluid in the two cycles is the same. In addition, the mass flow of
fluid through the second turbine is inherently greater than that
through the first turbine.
[0029] The fluid pump compresses the cryogenic fluid to a high
pressure of at least 50 bar and more typically over 100 bar.
[0030] The electricity generation device may further comprise a
superheater for heating the high pressure working fluid output from
the evaporator to a high temperature using a source of heat from a
co-located process. The co-located source of heat may be from the
ambient environment, from the atmosphere, the ground, river, sea or
lake water or from a source of waste heat such as a power station,
or industrial plant such as steel works or a data centre, or
similar source of low grade waste heat, e.g. cooling water from a
power station. The superheater may be positioned in the system
between the evaporator and the first expansion turbine.
[0031] The combined flow of fluid that passes through the reheater
is expanded to around ambient pressure in the second expansion
turbine. The second expansion turbine comprises a low pressure
turbine from which work is extracted.
[0032] The divided exhaust of the second low pressure expansion
turbine releases part of the flow to ambient through the first
exhaust and the remainder is circulated to the evaporator where the
low pressure working fluid exchanges thermal energy with the
cryogenic high pressure working fluid. The resulting low pressure
and low temperature gas is compressed in the compressor before
merging with the exhaust of the first high pressure expansion
turbine.
[0033] The reheater reheats the working fluid exhausted from the
first expansion turbine using a source of ambient or waste heat.
The peak cycle temperature is driven by the available heating
source. This can be from the ambient environment, from the
atmosphere, the ground, river, sea or lake water or from a
co-located process such as a power station, or industrial plant
such as steel works or a data centre, or similar source of low
grade waste heat.
[0034] The device may further comprise a third expansion turbine
and a second reheater positioned between the evaporator and the
first expansion turbine, and a fourth expansion turbine and a third
reheater positioned between the first reheater and the second
expansion turbine. In this case, the heated cryogen working fluid
is expanded through two high pressure stages and two low pressure
stages and the pressure is progressively reduced in the four
stages, with interstage re-heating between each expansion stage. In
this case, a superheater may be positioned between the evaporator
and the third expansion turbine.
[0035] The device may further comprise a second compressor for
compressing the second portion of working fluid after it has passed
through the evaporator a first time and directing the second
portion of working fluid back through the evaporator a second time
before it is further compressed by the first compressor. In this
arrangement, the second portion of the working fluid is first
cooled in the evaporator and then compressed by the second, low
pressure compressor before returning for further cooling in the
evaporator and compression in the first, high pressure compressor.
The additional compressor reduces the compressor work, by utilising
interstage cooling.
[0036] The high pressure turbine stage, or stages, may be mounted
on the same power shaft as the compressor, or compressors. This
arrangement has the advantage of both higher efficiency and reduced
cost, through reduced drive losses.
[0037] A device according to the present invention may be used as
the power recovery component of a cryogenic energy storage
system.
[0038] The device of the present invention has an improved
cost/benefit trade off relative to the cryogenset described in
GB1100569.1. Calculations by the inventors have indicated that a
20% or more improvement in specific work can be achieved for a less
than 4% increase in equipment cost. In the storage market, the
significant reduction in operating costs resulting from the
improved efficiency of the present invention will be favourable for
the modest increase in capital cost.
[0039] The present invention offers a number of significant
advantages over the devices shown in FIGS. 2, 3a and 3b. The
advantages include:
[0040] 1. Reduced part count and therefore cost. The present
inventors have calculated that the cost increase of the present
invention relative to the single cycle cryogenset of GB 1100569.1
is less than 4%, for a 20% improvement in efficiency. The designs
shown in FIGS. 2, 3a and 3b would be likely to incur cost increases
of 20 to 40% for similar efficacy improvements.
[0041] 2. Simplified fluid handling. The closed Rankine and Brayton
cycles require separate fluid loops which lead to an associated
complexity in preventing cross contamination of the fluids. The
system of the present invention does not suffer from such
complexity.
DESCRIPTION OF THE DRAWINGS
[0042] Embodiments of the present invention will now be described
with reference to the figures in which:
[0043] FIG. 1 shows the configuration of a cryogenset in relation
to co-located sources of waste heat and the cryogen delivery
options from a remote located air separation plant;
[0044] FIG. 2 shows a cryogenset with a Rankine bottoming
cycle;
[0045] FIG. 3a shows a cryogenset with a closed Brayton bottoming
cycle;
[0046] FIG. 3b shows a cryogenset with an open Brayton bottoming
cycle;
[0047] FIG. 4 shows a first embodiment of an electricity generation
device and method of the present invention with two turbine
stages;
[0048] FIG. 5 shows a temperature entropy diagram for the first
embodiment of the invention shown in FIG. 4;
[0049] FIG. 6 shows a second embodiment of an electricity
generation device and method of the present invention with four
turbine stages; and
[0050] FIG. 7 shows a third embodiment of an electricity generation
device and method of the present invention with four turbine stages
and two compressor stages.
DETAILED DESCRIPTION OF THE INVENTION
[0051] A first embodiment of the present invention, shown in FIG.
4, consists of a cryogenic tank 400 from which a cryogenic, or
working, fluid is transferred to a high pressure pump 410. The
cryogenic fluid is compressed to a high pressure, of at least 50
bar and more typically over 100 bar. The high pressure fluid is
then heated in a heat exchanger 420, referred to as an evaporator,
where thermal energy is transferred between the cryogenic working
fluid from the tank and low pressure working fluid in a Brayton
loop of the combined cycle. Further heat is optionally added from a
co-located source of waste heat 480, such as cooling water from a
power station, in the superheater 430. The resulting high pressure
fluid, which is now in the gaseous state if the pressure is below
the critical pressure or in a liquid state if the temperature and
pressure conditions are supercritical, is expanded through a first
high pressure turbine 440, from which work is extracted. The
exhaust from the high pressure turbine is combined with the
discharge from the Brayton cycle loop compressor 470 and reheated
using ambient or waste heat 480 in the reheater 450. The combined
flow (Rankine loop and Brayton loop) is expanded to around ambient
pressure in the low pressure turbine 460 from which work is
extracted. The exhaust of the turbine is then divided, part of the
flow is released to ambient through the exhaust 490 and the
remainder is circulated to the evaporator 420 where the low
pressure working fluid exchanges thermal energy with the cryogenic
high pressure working fluid. The steady-state flow from the exhaust
490 is equal to the mass flow from the tank. The portion of the
exhaust which is circulated to the evaporator 420 is a low pressure
and low temperature gas which is compressed in the compressor 470
before merging with the exhaust of the high pressure turbine 440.
The working fluid in the Rankine loop and the Brayton loop are
inherently the same.
[0052] The cycle is represented on a temperatureentropy diagram in
FIG. 5, where the state numbers shown on the diagram of FIG. 5
correspond to the numbered positions shown in hexagons in FIG.
4.
[0053] In a second, preferred embodiment of the invention, shown in
FIG. 6, two additional turbine stages are added to the process to
improve the work recovery from the working fluid. The inventors
have found a significantly improved performance is achieved by
expanding the heated cryogen working fluid through two high
pressure stages 441 and 442 and two low pressure stages 461 and 462
where the pressure is progressively reduced in four stages, with
interstage re-heating between each expansion stage. In this way,
the expansion process is closer to the ideal isothermal case. The
complete process is as follows: the cryogenic working fluid is
first transferred from a tank 400 to a high pressure pump 410. The
now high pressure fluid is then heated in an evaporator 420 where
thermal energy is exchanged with a low pressure fluid in a Brayton
loop. The high pressure warmed working fluid is then optionally
further heated by waste heat or ambient heat in a superheater 430.
The high pressure and high temperature working fluid is then
expanded in a high pressure turbine 441, reheated in a re-heater
451 and expanded in a further high pressure turbine 442. The
working fluid then combines with the high pressure feed from the
Brayton loop and is heated in a further re-heater 452. The fluid is
expanded in a low pressure turbine 461, is then re-heated in a
further re-heater 453 and finally expanded in a further low
pressure turbine stage 462. The exhaust of the final low pressure
turbine is divided into a re-circulation flow and an exhaust flow
490. The re-circulation flow is first cooled in the evaporator 420
and then compressed in a compressor 470 before combining with the
main working fluid flow upstream of the reheater 452. Typical
temperatures, pressures and mass flows at various points around the
cycle are shown in the following table (Table 1) and refer to the
numbered positions shown in hexagons in FIG. 6:
TABLE-US-00001 Process Temperatures, Pressure Temperature Mass Flow
Pressures and Flows Bar abs .degree. C. Kg/s 1 Storage Tank 5.0
-197 30 2 Evaporator Inlet 102 -193 30 3 Super-Heater Inlet 101.5
-57 30 4 Stage 1 Turbine Inlet 100 20 30 5 Stage 1 Turbine Outlet
31.6 -54 30 6 Stage 2 Turbine Inlet 31.6 20 30 7 Stage 2 Turbine
Outlet 10 -53.9 30 8 Stage 3 Turbine Inlet 10 20 93.83 9 Stage 3
Turbine Outlet 3.32 -51.26 93.83 10 Stage 4 Turbine Inlet 3.32 20
93.83 11 Stage 4 Turbine Outlet 1.1 -51.37 93.83 12 Compressor
Inlet 1 -186 63.83 13 Compressor Outlet 10 -96.03 63.83
[0054] It is noted that the conditions shown in table 1 refer to
one example of the invention, operating at relatively low pressure
(Stage 1 turbine inlet of 100 bar), in line with the operating
pressures of readily available turbo-machinery. Analysis by the
inventors has indicated better performance can be achieved at
higher peak working fluid pressures should such equipment be
available.
[0055] In another embodiment shown in FIG. 7, an additional
compressor stage is added to the system to reduce the compressor
work, by utilising interstage cooling. The low pressure Brayton
loop working fluid is cooled in the evaporator 420 and first
compressed by the low pressure compressor 472 before returning for
further cooling in the evaporator and compression in the high
pressure compressor 471. The compressor work is reduced by this
design but at the expense of the complexity of an additional
compressor stage.
[0056] In a further embodiment, which is not shown in the figures,
the high pressure turbine stage 440 or stages 441 and 442, are
mounted on the same power shaft as the compressor 470 or
compressors 471 and 472. The inventors have discovered the power
delivered by the high pressure turbine stages almost exactly
matches the compressor power requirement at the optimal operating
conditions. This embodiment has the advantage of both higher
efficiency and reduced cost, through reduced drive losses through
the deletion of the electric motor connected to the compressor and
reducing the power output and therefore size of the generator
connected to the turbines. Although the generator output is reduced
in this embodiment, the net output of the system remains the same
as the previous embodiment as the parasitic electrical load of the
compressor motor is removed through directly driving the compressor
by the high pressure turbines.
* * * * *