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.

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:
       balancing differences in supply and demand at different times of the day and at short notice;   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);   network reinforcement where parts of the electricity distribution network have a shortfall in capacity during periods of high power demand;   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. 3   a  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. 3   b ) 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. 3   b , 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 ,  3   a  and  3   b . 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 ,  3   a  and  3   b  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. 3   a  shows a cryogenset with a closed Brayton bottoming cycle; 
           [0046]      FIG. 3   b  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 : 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Process Temperatures, 
                 Pressure 
                 Temperature 
                 Mass Flow 
               
               
                 Pressures and Flows 
                 Bar abs 
                 ° 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.