Abstract:
A method of liquid air energy storage is provided. This method includes liquefying and storing air to form a stored liquid air during a first period of time; during a second period of time, introducing a compressed air stream into a cryogenic system, wherein the cryogenic system comprises at least one cold compressor, and at least one heat exchanger. The method includes producing a first exhaust stream and a second exhaust stream. The method also includes vaporizing at least part of the stored liquid air stream in the heat exchanger, thereby producing a first high pressure compressed air stream, then combining the first high pressure compressed air stream, the first exhaust stream and the second exhaust stream to form a combined exhaust stream, heating the combined exhaust stream, then expanding the heated combined exhaust stream in an expansion turbine to produce power.

Description:
BACKGROUND 
       [0001]    Electric power demand varies greatly during the day. The consumption that generates the demand is at the highest in the day time when industrial activities are also at the highest level. The demand is reduced to its minimum during the night time when less power is needed for lighting and when other industrial activities are also at the lowest level. The wide variations of daily power demand is also influenced by low consumption during weekends when less power is needed due to reduced industrial or business activities. Seasonal effects such as high load for air conditioning during the hot summer seasons or high heating loads in cold winter months also have additional impacts on the levels of power peaks or power off-peaks. This wide fluctuation is well known and utility companies must cope with it by providing spare capacity on the power grid to accommodate higher demand periods, and by having equipment configuration such that power blocks or generating units are capable of being taken off line when demand drops. 
         [0002]    Coal power plants and nuclear power plants, typically low cost fuel power plants, are relatively slow in capacity adjustment and load adaptation due to heavy equipment&#39;s inertia or safety constraints. For this reason, they are usually operated as base load plants to satisfy the core of the requirement. 
         [0003]    When power demand increases, backup peak shaving gas turbines, operated on relatively costly natural gas, can be started to keep up with the demand. It is obvious that the backup equipment must be able to get on stream very quickly because the demand peaks can occur quite rapidly and usually, instantaneously. It is also clear that, mainly because of economic reasons, the backup equipment cost must be minimized since they are only needed for short durations and not permanently. Gas turbines for base load plants are usually equipped with combined cycle to maximize the overall cycle efficiency. Steam boiler and condensing steam turbines of the combined cycle are high cost items and require relatively long time, in a matter of several hours, to be fully on stream. Because of those shortcomings, peak shaving gas turbines are simple cycle, not equipped with combined cycle, to yield the lowest investment cost per kW installed. Therefore the efficiency of the peak shaving gas turbine must be compromised. 
         [0004]    The fluctuations of the power demand can be smoothed out by providing an energy storage system: power is stored during the low demand periods and disbursed back to the grid during the high demand periods. A typical example of this setup is a hydraulic water pumping system: the surplus of power during the off-peak periods can be used to drive water pumps to send water from a low basin reservoir to a reservoir located at a higher elevation. When power demand increases, this water is returned to the low elevation reservoir by sending it to hydraulic turbines to generate supplemental electric power. The ramp-up is quite fast for this system. However, this setup is of course not applicable to most power plants since it requires an expensive infrastructure using high and low elevation reservoirs along with multiple large hydraulic turbines. In addition to the high global investment cost, the recovery, defined as the ratio of electricity output over the electricity input, is only in the range of about 60% due to the fact that the reservoirs are likely to be at remote locations such that transmission line losses can be quite high and the efficiencies of the pumps and hydraulic turbines are in the range of only about 70%. Therefore an efficient and economical process of storing energy is desirable to address the issue of power demand fluctuations. 
         [0005]    More and more power generation plants are being built with combustion gas turbine technology. Because of environmental issues, coal based power plants with gasification technology (IGCC Integrated Gasification Combined Cycle) are being built or selected for several projects. In the regions of the world where natural gas is available at relatively low cost, combined cycle natural gas power plant for base load operation is the technology of choice. Gas turbine concept by itself is not very efficient since about 50% of the turbine&#39;s power is wasted to compress the air for the combustion and expansion. However, the gas turbine cycle efficiency is improved significantly by adding a steam combined cycle on the turbine&#39;s exhaust gas: the waste heat of the exhaust gas is used to heat and vaporized water to form high pressure steam which is then expanded in steam turbines to generate additional power. The combined cycle concept is widely used today in the power generation industry. However, because of the complexity and the high cost of the multiple pressure heat recovery steam generation system (HRSG) and the steam turbines, and the heavy infrastructure of the very large cooling tower for the steam condensing circuit, the steam combined cycle can only be justified economically for plants larger than about 50 MW or even 100 MW. Plant size can be smaller in case of cogeneration when clients are available to purchase steam produced by the facility and to partially pay for the cost of the steam system. Because of this economic constraint, many small plants are operated based on a simple cycle concept, i.e. no combined cycle, with significant penalty on the cycle efficiency. Gas turbine vendors are implementing several improvements to the gas turbine technology in order to reduce the impact of poor efficiency such as increasing pressure ratio thus reducing exhaust temperature, or improving turbine&#39;s blade heat resistance to accommodate higher inlet temperature or using recuperated gas turbine approach. However those changes only result in smaller incremental improvement to the process efficiency. Therefore another approach less costly than the steam combined cycle capable of improving the efficiency of the gas turbine power generation system is highly desirable especially for the small and medium size plant application. 
         [0006]    When power demand increases, backup peak shaving gas turbines, operated on relatively costly natural gas, can be started to keep up with the demand. It is obvious that the backup equipment must be able to get on stream very quickly because the demand peaks can occur quite rapidly and usually, instantaneously. It is also clear that, mainly because of economic reasons, the backup equipment cost must be minimized since they are only needed for short durations and not permanently. Gas turbines for base load plants are usually equipped with combined cycle to maximize the overall cycle efficiency. Steam boiler and condensing steam turbines of the combined cycle are high cost items and require relatively long time, in a matter of several hours, to be fully on stream. Because of those shortcomings, peak shaving gas turbines are simple cycle, not equipped with combined cycle, to yield the lowest investment cost per kW installed. Therefore the efficiency of the peak shaving gas turbine must be compromised. 
         [0007]    Atmospheric air is a potential candidate for the medium used for energy storage. For example, air can be compressed during off-peak periods to higher pressure and stored in large underground cavern created by solution mining. During peak load periods, pressurized air of the storage can be heated by combusting natural gas to high temperature then expanded in gas turbine for power recovery. The efficiency of the power recovery depends upon the type of compression used to compress the air: adiabatic, diabatic or isothermal. This concept is simple but, similar to the water pumping scheme, requires important capital expenditure for the infrastructure. Site locations in case of mining solution are usually very remote. 
         [0008]    To minimize the storage size and the associated cost of compressed air system, air can be liquefied by cryogenic technique and stored economically in large quantity in conventional storage tank. This air, in liquid form, can be vaporized and transformed into gaseous form to restore the compressed air needed for power generation. This technique is promising because it facilitates the compress air energy storage approach without the high cost associated with the underground cavern at remote locations. A facility for air liquefaction can be easily deployed near the main users like large cities. The technology of air liquefier and cryogenic storage are very well known and can be implemented quickly and reliably. However, several technical issues must be resolved before this approach can be used economically. 
         [0009]    An object of this invention is to provide a technique of using liquid air to store energy. Liquefaction of air requires energy input, the specific power required to liquefy the air is about 0.5 kWh/Nm3. The liquefaction power can be improved slightly at the expense of higher investment cost for the equipment. This energy input must be recovered efficiently in the vaporization step otherwise the overall process efficiency will suffer. Therefore it is desirable to provide an efficient process for liquid air vaporization. 
         [0010]    Considering that the liquefaction is an energy intensive process, it is advantageous to avoid this liquefaction during the peak load periods where power cost is at the premium. Therefore liquefaction during off-peak periods, for example at night time, will maximize the cost effectiveness of the concept. Power consumption for equipment such as compressors in the vaporization step must be kept at a minimum. 
         [0011]    One potential technique of reducing power consumption of equipment is to utilize the cold or refrigeration supplied by cryogenic liquid of the cold compression process. Cold compression reduces the power consumption of the compressor significantly because the inlet temperature of the compressor is at very low level, usually in the range of −180° C. to −60° C. However, the main penalty of the cold compression is that the heat generated by the compression, even quite low at cryogenic level, must be evacuated at that cryogenic temperature level such that the required refrigeration will adversely effect the overall power consumption. In case the source of refrigeration available for the heat removal is a low cost cryogenic liquid produced inexpensively during off-peaks then cold compression becomes quite attractive. 
         [0012]    This invention relates to an improved technique of using liquid air as the energy storage medium. Liquid air produced and stored in off-peak periods can be restored to compressed air under high pressure by an efficient vaporization process assisted with cold compression technology. The compressed air is then heated and expanded in a compressed air combined cycle to generate additional power in peak periods and to improve the efficiency of the gas turbine without a costly steam combined cycle. 
         [0013]    The use of this invention can extend the concept of combined cycle to medium and small power gas turbine power generating units without the high cost and slow response of the traditional steam turbine combined cycle. 
       SUMMARY 
       [0014]    A method of liquid air energy storage is described. This method includes liquefying and storing air to form a stored liquid air during a first period of time. The method also includes during a second period of time, introducing a compressed air stream ( 108 ) into a cryogenic system, wherein the cryogenic system comprises at least one cold compressor, and at least one heat exchanger. The method includes cooling the compressed air stream ( 108 ) within the heat exchanger producing a first cooled compressed air stream and a second cooled compressed air stream. The method includes further cooling at least a portion of the first cooled compressed air stream, thereby producing a further cooled compressed air ( 115 ). 
         [0015]    In various combinations, the current method may include removing the further cooled compressed air ( 115 ) and compressing it in either one cold compressor ( 124 ) or two cold compressors in series ( 124 ,  126 ); or the current method may include removing the second cold compressed air stream ( 109 ) and compressing it in either one cold compressor ( 110 ) or two cold compressors in series ( 110 ,  123 ). Hence, in various embodiments, there may be between one cold compressor and four cold compressors in any possible combination. Illustrative embodiments follow. 
         [0016]    The method includes performing at least one of steps e) and f). Step e) includes compressing at least a portion of the further cooled compressed air by a first cold compressor ( 124 ), or compressing at least a portion of the further cooled compressed air by a first cold compressor ( 124 ) thereby producing a first cold compressor exhaust stream ( 121 ), cooling the first cold compressor exhaust stream in the heat exchanger ( 106 ), thereby producing a first intermediate cooled compressor exhaust stream ( 125 ); introducing the first intermediate cooled compressor exhaust stream into a third cold compressor ( 126 ), in series with the first cold compressor, thereby producing a first exhaust stream ( 121 ). Step f) includes compressing at least a portion of the second cooled compressed air ( 109 ) by a second cold compressor ( 110 ), or compressing at least a portion of the second cooled compressed air ( 109 ) by a second cold compressor ( 110 ) thereby producing a second cold compressor exhaust stream ( 111 ), warming the second cold compressor exhaust stream in the heat exchanger, thereby producing an intermediate warmed compressor exhaust stream ( 112 ); introducing the second warmed compressor exhaust stream into a fourth cold compressor ( 113 ), in series with the second cold compressor, thereby producing a second exhaust stream ( 114 ). The method also includes vaporizing at least part of the stored liquid air stream ( 105 ) in the heat exchanger ( 106 ), thereby producing a first high pressure compressed air stream ( 107 ). The method includes combining the first high pressure compressed air stream ( 107 ), the first exhaust stream ( 121 ) and the second exhaust stream ( 114 ) to form a combined exhaust stream ( 122 ), heating the combined exhaust stream, then expanding the heated combined exhaust stream in an expansion turbine ( 603 ) to produce power. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0017]      FIG. 1  illustrates the efficient vaporization of liquid air, in accordance with one embodiment of the present invention. 
           [0018]      FIG. 2  illustrates another arrangement of the cold compressor in the vaporizer block, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 3  illustrates another embodiment of the vaporizer block wherein the cold compression is performed in two compressors in series, in accordance with one embodiment of the present invention. 
           [0020]      FIG. 3   a  illustrates another embodiment of the present invention. 
           [0021]      FIG. 3   b  illustrates another embodiment of the present invention. 
           [0022]      FIG. 4  illustrates a gas turbine with combined cycle running on compressed air instead of steam. 
           [0023]      FIG. 5  illustrates a liquefaction plant to liquefy air, in accordance with one embodiment of the present invention. 
           [0024]      FIG. 6  illustrates the operation during the peak loads, in accordance with one embodiment of the present invention. 
           [0025]      FIG. 7  illustrates one improvement of the base scheme of  FIG. 6 , in accordance with one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    Illustrative embodiments of the invention are described below. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
         [0027]    It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0028]    As used herein, the term “cold compression” means the method of mechanically raising the pressure of a gas stream that is lower in temperature than the ambient level feeds to the cryogenic separation system and returned to the system at a sub ambient temperature or about ambient temperature. The compressor performing the cold compression is called “cold compressor” and can be a single-staged or a multi-staged device. 
         [0029]    The efficient vaporization of liquid air is described in  FIG. 1 . Liquid air produced in off-peak periods is stored in tank  101 , which is usually at about atmospheric pressure. During peak loads, liquid air stream  102  is withdrawn from tank  101  to pump  103  and pump  104  to form high pressure liquid air stream  105 . Stream  105  is vaporized and warmed in heat exchanger  106  to form first high pressure compressed air stream  107 , which may be at about 56 bar. A compressed air stream  108  with a molar flow of about 4.3 times higher than the liquid air stream  102 , at about 10.7 bar and about ambient temperature, is fed to exchanger  106  to be cooled down by the refrigeration provided by the vaporizing liquid air stream  105 . Prior to the dew point of air, a second cooled compressed air stream  109  of the cooled air stream  108  is extracted from exchanger  106 , which may be at about −137° C. and fed to second cold compressor  110  which boosted its pressure to about 13 bar to form second cold compressor exhaust stream  111 . Because of compression heat, the temperature of  111  is about −128° C. Stream  111  is then further warmed in exchanger  106  to yield intermediate warmed compressor exhaust stream  112  at about −85° C., which is the compressed by a fourth cold compressor  113  to yield second exhaust stream  114  at about 56 bar and at ambient temperature. 
         [0030]    The remaining portion of the cooled stream  108  is further cooled in exchanger  106  to form further cooled compressed air stream  115  which may be partially condensed. The liquid fraction of stream  115  is quite small at about 6%. After a phase separation in separator  116 , stream  115  at about −164° C. is separated in to a gaseous stream  117  and a liquid stream  118 . Liquid  118  is mixed with the liquid stream  119  of pump  103  to yield stream  120 , which is then pumped to 56 bar by pump  104  to form liquid stream  105 . Stream  117  is compressed by a first cold compressor  124  to 56 bar to form first cold compressor exhaust stream  121  which is then warmed and mixed with the first high pressure compressed air stream from the high pressure liquid air stream  107  to yield exhaust stream  122  at about 56 bar. Therefore, from a liquid air stream  102  and a medium pressure air stream  108  at about 10.7 bar, and with a flow about 4 times larger, the combined stream (about 5 times larger than liquid air flow) is compressed to 56 bar with minimal power input from the 3 cold compressors. It is useful to note that the total power of the cold compressors represents only 34% of the energy required to compress the combined stream from 10.7 bar to 56 bar. It can be seen from this numerical example that the energy contained in the liquid can be restored efficiently by producing a much higher flow gas stream a pressure with approximately a pressure ratio of 5. 
         [0031]    To simplify the next description, the system of pumps, cold compressors, exchangers etc. are grouped in a “vaporizer block”  123  as shown in  FIG. 1 .  FIG. 2  describes another arrangement of the cold compressor in the vaporizer block  123 . In the interest of clarity, the element numbers are consistent with those used in  FIG. 1 . In this case, if the final pressure of the vaporized liquid air is high enough (about 200 bar abs), the discharge flow  121  of first cold compressor  124  can be at about ambient and there is no need to send it through exchanger  106 . Stream  121  rejoins the vaporized liquid air  107  to form the combined exhaust stream  122 . 
         [0032]      FIG. 3  is another embodiment of the vaporizer block  123 . In the interest of clarity, the common element numbers are consistent with those used in  FIG. 1 . In this case, the cold compression of stream  117  is performed in 2 compressors in series. The discharge flow  121  of first cold compressor  124  is cooled in exchanger  106  to form first intermediate cooled compressor exhaust stream  125  which is then compressed further in a third cold compressor  126  to the final pressure. Prior to the dew point of air, a portion  112  of the cooled air stream  108  is extracted from exchanger  106  and fed to second cold compressor  110  which boosted its pressure to the final pressure exhaust stream  114 . First exhaust stream  127  at the outlet of third cold compressor  126  rejoins the warmed vaporized liquid air and stream  114  to form the combined exhaust stream  122 . 
         [0033]    The embodiment described in  FIG. 3   a  can be used at lower pressure level than the other embodiments. Cooled air  117  is cold compressed in a multi-staged compressor  124 . Stream  126  is extracted at an interstage of compressor  124 , warmed in exchanger  126  and further compressed in compressor  125  to the required pressure. Feed air  108  can be at about 6.5 bar, after cooling it is compressed to about 83 bar in compressor  124  with side stream  126  extracted at about 42 bar. 
         [0034]    In another embodiment described in  FIG. 3   b , the vaporized liquid can be separated from the compressed air cycle and extracted from the system as a pressurized product. Indeed, in many applications such as back-up vaporization, or peak shaving vaporization wherein the liquid product is vaporized to complement a gaseous production or to continue supplying gaseous product in case of outages. In this embodiment, the liquid  102  from storage tank  101  can be of liquid oxygen, liquid nitrogen or some other cryogenic liquids. This liquid is sent to exchanger  106  for vaporization as in other embodiments, however, this vaporization circuit is kept separated from the compressed air circuit such that the vaporized liquid product can be extracted from the system to supply a gas demand. Such compressed air circuit is then sustained by a closed loop air circuit driven by the recycle compressor as previously described. It is also feasible to replace air of the closed loop circuit with another gas such as nitrogen, argon, helium, or a mixture of gases for example. As such the refrigeration contained in the cryogenic liquid can be recovered and integrated into the energy storage concept even though the molecules of the stored gas are not subjected to the power recovery by expansion. 
         [0035]      FIG. 4  describes a gas turbine with combined cycle running on compressed air instead of steam. In the interest of clarity, the common element numbers are consistent with those used in  FIG. 1 . In this case, atmospheric air  401  is compressed by the compressor  402  of the gas turbine to form a pressurized air  403 , which is then mixed with fuel  404  and combusted in combustion chamber  405  to produce hot gas  406 . Stream  406  at about 1200° C. and at a pressure of about 17 bar is then expanded in turbine  407 . The net power produced by this gas turbine is used to drive a generator  408 . The exhaust gas  409  of turbine  407  at about atmospheric pressure and 580° C. is sent to a heat recovery exchanger  410 . A closed loop of compressed air is circulated by an isothermal compressor  411 . Compression heat can be removed by, for example, cooling water or air cooling. Air stream  412  of about 50-60 bar from compressor  411  is first heated in heat exchanger  413  to yield heated air  414  which is then further heated in heat exchanger  410 , be recovering the heat from the exhaust  409  of the gas turbine, to produce a hot compressed air stream  416 . Stream  416  can then be expanded in a hot gas expander  417  to a pressure of about 10-11 bar to recover the energy. Expander  417  can drive a generator  418  to produce power. Exhaust  419  of expander  417  is sent to exchanger  413  wherein it exchanges heat with stream  412  to yield a returned compressed air stream  108 , which is then re-compressed by compressor  411  to complete the loop. 
         [0036]    The thermal efficiency of the compressed air combined cycle as described in  FIG. 4  is marginally better (about 1-2%) than the simple cycle gas turbine. The efficiency of this compressed air combined cycle is much lower than the steam combined cycle by about 10%. Of course, the cost of the steam combined cycle is much higher. However, the compressed air combined cycle has a distinct advantage over the steam combine cycle because it can be improved significantly by integrating with a vaporizer block running with liquid air. This improvement is described in the following paragraphs. 
         [0037]    As mentioned above, power during off-peaks is abundant and can be quite inexpensive. This power can be used to drive a liquefaction plant to liquefy air.  FIG. 5  shows such an arrangement. In the interest of clarity, the common element numbers are consistent with those used in  FIG. 1  and  FIG. 4 . In this case, during off-peaks, the gas turbine&#39;s power is not needed and it can be shutdown (equipment shown in dotted line). The isothermal compressor  411  of the compressed air combined cycle can be used as recycle compressor for the liquefaction plant. Atmospheric air  501  is compressed by compressor  502  and cleaned in the adsorber  503  for water and CO2 removal. Clean and dry air  504  from adsorber  503  is admitted into the recycle loop and further compressed by compressor  411  to yield the high pressure air stream  505 . Stream  505  is sent to a liquefier unit  506 , which can be of traditional design and equipped with turbo expander-compressor machinery, to produce a liquid air stream  507 . Liquid air  507  is stored in storage tank  101 . 
         [0038]      FIG. 6  describes the operation during the peak loads. In the interest of clarity, the common element numbers are consistent with those used in  FIG. 1  and  FIG. 4 . In this case, liquid air  102  (produced in off-peak periods) from storage tank  101  is sent to vaporizer block  123  wherein it is combined with the returned stream  108  at 10.7 bar to yield the final compressed air stream  122  at about 56 bar, as described above. The final compressed air stream is then heated in exchangers  413  and  410  to form the hot compressed air stream  416  which is expanded in hot gas expander  417  to produce additional power. A portion  601  of the expanded gas stream  419  of expander  417 , with a flow essentially the same as the liquid air flow  102 , is reheated in exchanger  410  against the expander  407 &#39;s exhaust gas to form a second hot gas stream  602  which is then expanded in a second hot gas expander  603  to produce additional power. The remaining portion  604  of stream  419  is cooled in exchanger  413  and returned to the vaporizer block  123  to complete the loop. The isothermal compressor  411  needed during the off-peak liquefaction or during the normal combined cycle mode can be shut down during the peak loads. 
         [0039]    It can be seen that by vaporizing liquid air via the vaporizer block, additional air flow can be generated efficiently and can be used to recover the waste heat of the gas turbine&#39;s exhaust stream and to drive additional hot gas expanders for producing more power. This operation can be performed very quickly to respond to the demand changes of the grid. Since only low power input is needed for the cold compressors of the vaporizer block during peak loads, and none for the liquid production, a large fraction of this increase in power production can be fed to the power grid to boost the electricity output. And this without the high cost and slow response of the steam combined cycle. Such arrangement will complement nicely the simple cycle gas turbine setup. 
         [0040]    The embodiment of  FIG. 7  is an improvement of the base scheme of  FIG. 6 . In the interest of clarity, the common element numbers are consistent with those used in  FIGS. 1 ,  4 , and  6 . In this case, the final compressed air stream  122  from the vaporizer block  123  can be divided into 2 portions. Portion  701  is sent to a first combined cycle section wherein it is heated in exchangers  702  and  703  to yield a first hot gas stream  704  which is expanded in expander  705  to recover the power. Exhaust  706  of expander  705  is sent to exchanger  702  for heat recovery and yields a first portion  707  of the returned stream  108 . Portion  708  of the final compressed air stream  122  is sent to a second combined cycle, in series with the first combine cycle. Hot exhaust gas  409  of gas turbine  407  is cooled in exchanger  703  after exchanging heat with the first combined cycle. Its temperature is still high and its heat content can be further recovered in the second combined cycle. Stream  708  is heated in exchangers  413  and  410  to yield a second hot gas stream  416  which is expanded in hot gas expander  417  for power recovery. Exhaust gas  419  of expander  417  is divided into 2 portions: the first portion  601 , with a flow essentially the same as the liquid air flow  102 , is heated in exchanger  410  to yield a third hot gas stream  602 , which is expanded in expander  603  for power recovery. The second portion  604  of stream  419  is sent to exchanger  413  for heat recovery and constitutes the second portion  709  of the returned stream  108 . Hence, the heat exchangers that are indirectly exchanging heat with the gas turbine exhaust ( 703 ,  410 ) are in series on the hot side, and they are in parallel on the air side. The heat exchangers ( 702 ,  413 ) are in parallel. This embodiment can be used to extract more heat from the exhaust gas of the gas turbine and to further increase the power generation of the system. 
         [0041]    It can be seen from the above description of all three embodiments of the  FIGS. 1 ,  2  and  3  that almost the totality of heat provided for the vaporization of the liquid air comes from the heat of compression of the cold compressors. The heat input of the liquid air pumps is rather small and only represents about 3% of the total power or heat input. The embodiments of the invention all share this vaporization by cold compression&#39;s heat as the common feature, which differentiates this invention from the prior art. 
         [0042]    It should be noted that while the invention has been described in several different embodiments, it is obvious that some additional embodiments can be developed or added by the persons skilled in the art or familiar with the technology to further improve the invention without departing from the scope of this disclosure. For example, a portion of the compressed air from the compressed air combined cycle loop can be injected into the gas turbine and heated by the combustion of air and fuel to form a hot gas then expanded in the gas turbine to generate power.