Abstract:
A thermal storage and transfer method for use in both indirect and direct heating solar power plants that involves the use of nitrogen gas as a thermal storage medium for heat transfer in circumstances where little or no solar radiation is available to produce the thermal energy needed to convert water into steam and generate electricity.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/754,766, filed Jan. 21, 2013. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to thermal storage systems used in concentrated solar power plants and in particular to a nitrogen-based thermal storage medium. 
       BACKGROUND OF THE INVENTION 
       [0003]    The global urgency to reduce carbon dioxide emissions and global warming has spurred the attention on alternative sources of energy. Conventional power plants using petroleum based products such a coal, gas, oil are being overlooked in favor of non-polluting alternatives. The design &amp; construction of new nuclear power plants is going through additional scrutiny after the Japanese Fukushima Daiichi nuclear power plant accident in 2011. Solar and wind power have started to be considered as viable alternatives to nuclear, coal and gas fired power generation. Large scale solar and wind generation units are under construction or have been commissioned. Increased investments have energized innovations in the design of wind and solar power plants. Better performing materials at reduced costs have appeared in the market. Improved designs with inexpensive components have led to power plants with improved performance at lower cost. The trend continues with encouraging results each passing year. 
         [0004]    Solar Power plants which form a major constituent of the alternative energy portfolio can be the Photo Voltaic type (“PV”) or the Concentrated Solar Plant type (“CSP”). 
         [0005]    In a Photovoltaic solar power plant electricity is generated by converting solar radiation into electricity using semiconductors that exhibit the Photovoltaic effect. The PV solar plants comprises of a number of solar panels. Each panel includes a number of solar cells containing a photovoltaic material. Advances in material and manufacturing techniques have led to a substantial decrease in the cost of solar panels. 
         [0006]    CSP use a large number of mirrors or lenses to concentrate sunlight onto a small area. The heat from the concentrated solar energy is used to directly or indirectly convert high pressure water to steam. The high pressure steam drives a steam turbine-generator unit which produces electricity. 
         [0007]    In a Tower type CSP plant the mirrors focus the sun&#39;s energy to the boiler tubes mounted on top of a tower. High pressure water flows inside the boiler tubes. The concentrated energy from the sun converts high pressure water into steam inside the boiler tubes. This is the mechanism for direct heating of high pressure water to steam. 
         [0008]    In a parabolic mirror CSP plant the parabolic mirrors focus the sun&#39;s energy on to a heat transfer fluid (“heat transfer fluid”) flowing through a central receiver pipe. The concentrated solar energy heats the heat transfer fluid. The energy from the heat transfer fluid is used to convert high pressure water to steam in separate large heat exchangers. This is the mechanism for indirect heating of high pressure water to steam. 
         [0009]    The concentrated solar power plants operate during daytime. The power plants are essentially shut down at nights. The performance of the plant is often affected during cloud cover. Electricity from other power plants is required to offset the loss of output from solar power plants at night time or during cloud cover. 
         [0010]    The loss of electrical power generation at night time in a solar power plant is often circumvented using thermal storage. During daytime, part of the captured solar energy is used to heat a thermal storage material to elevated temperatures and then stored in large insulated containers. At night, the energy from the heated thermal storage material is used to directly or indirectly heat high pressure water to steam. The high pressure steam drives a steam turbine generator unit producing electricity. 
         [0011]    Molten salt has been the thermal storage material of choice. During daytime, salt is heated to elevated temperatures with the solar energy. The molten salt at elevated temperatures is stored in large insulated containers. At night time, the energy from the molten salt is used to directly or indirectly heat high pressure water to steam. The high pressure steam drives a steam turbine generator unit producing electricity. 
         [0012]    Thermal storage requires very large quantities of molten salt. Procurement and transportation of molten salt to the solar power plants at remote locations is a challenge. Salt is a major component of the fertilizer industry and salt producers are often reluctant to divert years of production quotas to solar power plants. Molten salt freezes at 550° F. Freezing in pipe lines, valves and associated components at nights or during cold days results in a whole new genre of problems. To circumvent the freezing problem the entire molten salt containers, heat exchangers and pipelines have to be insulated. This leads to additional costs and risks. Irregularities in the composition of salt alter the melting temperature and thermal properties, thereby affecting the design of the thermal storage in its entirety. A need, therefore, exists in the art to develop an effective thermal storage system that does not suffer from the problems associated with molten salt or other substrates used by those in the art. 
       SUMMARY OF THE INVENTION 
       [0013]    According to the present invention, the foregoing and other objects and advantages are obtained by a thermal storage and transfer method for use in a direct heating solar power plant, comprising of nitrogen passing through a section of the boiler tubes receiving thermal energy transferred from focused solar radiation available during daylight; storing heated nitrogen in one or more containers connected to the boiler tubes; and at night time routing the heated nitrogen from the one or more containers to a Nitrogen Water Steam Heat Exchanger (“NWSHX”) wherein the thermal energy from the heated nitrogen is used to convert pressurized water to steam. In the NWSHX the heated nitrogen flows inside an array of tubes and the high pressure water and steam flow outside the tubes. 
         [0014]    According to another aspect of the invention, in addition to the above noted invention the additional thermal storage and transfer method further comprises transferring thermal energy from the sun to heat only nitrogen flowing through the boiler tubes, using the heat in a portion of the heated nitrogen to convert water to steam in a heat exchanger. The pressurized steam flows through a steam turbine generator producing electricity. In such aspect of the invention, the boiler located on top of the solar tower has to be designed to heat only nitrogen as opposed to heating nitrogen and water thereby simplifying the boiler design. 
         [0015]    The remaining nitrogen is stored in one or more containers connected to the boiler tubes; and at night time routing the heated nitrogen from the one or more containers to NWSHX wherein the thermal energy from the heated nitrogen is used to convert pressurized water to steam. 
         [0016]    According to another aspect of the present invention, there is also a thermal storage and transfer method for use in an indirect heating solar power plant comprising of heating the heat transfer fluid by solar energy in the solar field, routing the heated heat transfer fluid into a nitrogen heat exchanger wherein the heat from the heat transfer fluid is used to heat the nitrogen, storing heated nitrogen in one or more containers and at night time routing the heated, nitrogen from the one or more containers into a NWSHX, wherein the heat from the heated nitrogen is used to convert water to steam. 
         [0017]    According to another aspect of the present invention, there is also a thermal storage and transfer method for use in an indirect heating solar power plant comprising of heat transfer fluid and nitrogen by solar energy in the solar field. The heated heat transfer fluid is routed to a heat exchanger wherein the heat from the heat transfer fluid is used to heat high pressure water to steam. The heated nitrogen is routed and stored in one or more containers, at night time the heated nitrogen is routed to a NWSHX wherein the heat from the nitrogen is used to convert water to steam. Nitrogen can be heated to substantially higher temperatures than heat transfer fluid requiring smaller quantities of nitrogen and higher efficiency. 
         [0018]    According to another aspect of the present invention there is also a thermal storage and transfer method for use in an indirect heating solar power plant comprising of heating only nitrogen by solar energy in the solar field. A portion of the heated nitrogen is routed to a NWSHX wherein the heat from the heated nitrogen is used to heat high pressure water to steam. The remaining heated nitrogen is routed and stored in one or more containers. At night time the heated nitrogen from the storage containers is routed to a NWSHX wherein the heat from the nitrogen is used to convert high pressure water to steam. Nitrogen can be heated to substantially higher temperatures than heat transfer fluid, requiring smaller quantities of nitrogen and higher efficiency. 
         [0019]    According to another aspect of the invention, the thermal storage and transfer method further comprises using pressurized nitrogen so to reduce the storage volume of nitrogen and reduce the number of storage containers. 
         [0020]    According to another aspect of the invention, the heat exchanger used for transferring heat from heated nitrogen to convert water to steam for day and night time operation can be two separate heat exchangers, a single heat exchanger or two heat exchangers built into a single unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention will become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only, in the accompanying drawings wherein: 
           [0022]      FIG. 1  is a schematic detailing the day/night operation of a direct heating solar power plant according to one embodiment of the invention. The daytime operation is denoted by bold lines whereas the night operation is denoted by dashed lines. 
           [0023]      FIG. 2  is a schematic detailing the day/night operation of a direct heating solar power plant according to another embodiment of the invention. The daytime operation is denoted by bold lines whereas the night operation is denoted by dashed lines. 
           [0024]      FIG. 3  is a schematic detailing the day/night operation of a indirect heating solar power plant according to another embodiment of the invention. The daytime operation is denoted by bold lines whereas the night operation is denoted by dashed lines. 
           [0025]      FIG. 4  is a schematic detailing the day/night operation of an indirect heating solar power plant according to another embodiment of the invention. The daytime operation is denoted by bold lines whereas the night operation is denoted by dashed lines. 
           [0026]      FIG. 5  is a schematic detailing the day/night operation of an indirect heating solar power plant according to another embodiment of the invention. The daytime operation is denoted by bold lines whereas the night operation is denoted by dashed lines. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to the drawings wherein like or similar references indicate like or similar elements throughout the several views, there is shown in  FIG. 1  a schematic detailing the day/night time operation of a direct heating solar power plant, generally identified by reference numeral  10 . Boiler  30  is located on the top of a solar tower. During daytime, focused solar radiation is directed to the exterior surface of a section of the boiler  30  boiler tubes  40  that carry high pressure water. The concentrated solar energy converts water to steam that flows through a steam turbine  50  and generator  60  unit that generates electricity. The low pressure steam from the steam turbine is condensed in condenser  70 . The resulting condensate is pumped in a condensate pump  20  and routed back to boiler tubes  40 . 
         [0028]    Cold nitrogen from one or more cold storage tanks  80 , is pumped through a nitrogen pump  90  and routed to a section of the boiler tubes  100 . Concentrated solar energy heats the nitrogen flowing inside the boiler tubes. The heated nitrogen is stored in one or more nitrogen storage tanks  110 . 
         [0029]    During night time (or otherwise low light conditions) the heated nitrogen from one or more hot nitrogen storage tanks  110  is routed to a nitrogen water steam heat exchanger  120  wherein the heat from the heated nitrogen is used to convert pressurized water to steam. The pressurized steam generates electricity in the steam turbine  50  and generator  60  unit. The low pressure steam from the steam turbine is condensed in condenser  70 . The resulting condensate is pumped in a condensate pump  20  and routed back to boiler tubes  40  or the nitrogen water steam heat exchanger  120 . Meanwhile, cold nitrogen gas returns to the one or more cold nitrogen tanks  80 . 
         [0030]    Advantageously, the problems associated with the use of molten salt can be offset with the use of nitrogen as the thermal storage material. The advantages with nitrogen include the fact that it is easily available since 80% of the earth&#39;s atmosphere is nitrogen. There is no need to transport nitrogen to remote solar power plant locations. An inexpensive and readily available nitrogen extraction unit can be installed right at the solar power plant site at a low cost to extract nitrogen from the atmosphere Additionally, the thermal properties of nitrogen are stable over a wide range of pressures and temperatures. For purposes of this invention, nitrogen gas can be compressed and stored in cylindrical pressure vessels, the type of which are well known in the art. Compressing or pressurizing nitrogen leads to smaller storage volume and therefore smaller storage tanks. Commonly, solar power plants are located in deserts or areas with very low population density. The solar field occupies a large tract of land. The containers containing the heated nitrogen can be placed above ground or underground beneath the solar field. Underground storage eliminates the need for additional space to store the nitrogen filled pressure vessels (if space is a problem). Additionally, nitrogen does not solidify and remains in a non-combustible gaseous state in the temperatures and pressure ranges typically encountered in concentrated solar power plants. The problem of “freezing salt” is completely eliminated. Lastly, leaks in heat exchangers, valves, piping do not cause a safety hazard. If there is a leak, then nitrogen escapes to the atmosphere where it came from. 
         [0031]    As noted in  FIG. 1 , during direct heating daytime operation, the Nitrogen Water Steam Heat Exchanger  120  is idle. At night time the boiler on top of the solar tower is idle. 
         [0032]    There is shown in  FIG. 2  a schematic detailing the day/night time operation of a direct heating solar power plant according to another embodiment of the invention, generally identified by reference numeral  210 . Boiler  340  is located on the top of a solar tower. During daytime, focused solar energy is directed to the exterior surface of the boiler tubes  280  that contain nitrogen pressurized by a nitrogen pump  270 . The heated nitrogen exiting the boiler tubes  280  enters a nitrogen water steam heat exchanger  230  wherein the heat from the heated nitrogen is used to convert high pressure water to steam. The high pressure steam generates electricity in a steam turbine  240  and generator  250  unit. The low pressure steam from the steam turbine is condensed in condenser  260 . The resulting condensate is pumped in a condensate pump  220  and routed back to the nitrogen water steam heat exchanger  230 . 
         [0033]    Nitrogen from the one or more cold nitrogen storage tanks  310  is pumped by a nitrogen pump  320  and routed to the boiler tubes  290 . The concentrated energy from the sun heats the nitrogen. The heated nitrogen flows into one or more hot nitrogen containers  300 . 
         [0034]    During night time (or in a low light condition) the heated nitrogen from one or more hot nitrogen tanks  300  is routed to a nitrogen water steam heat exchanger  330  wherein the heat from the heated nitrogen is used to convert pressurized water to steam. The pressurized steam generates electricity in the steam turbine  240  and generator  250  unit. The low pressure steam from the steam turbine is condensed in condenser  260 . The resulting condensate is pumped in a condensate pump  220  and routed back to the nitrogen water steam heat exchanger  330 . Meanwhile, cold nitrogen gas is routed back to the one or more cold nitrogen storage tanks  310 . 
         [0035]    While not depicted, nitrogen water steam heat exchanger  330  and nitrogen water steam heat exchanger  230  can operate as a single heat exchanger. 
         [0036]    There is shown in  FIG. 3  a schematic detailing the day/night time operation of an indirect heating solar power plant, generally identified by reference numeral  410 . In indirect heating, miles and miles of parabolic mirrors in a solar field focus solar energy on a central receiver pipe that carries a heat transfer fluid, typically a heavy oil, which is heated to high temperatures. 
         [0037]    The heat transfer fluid is heated to high temperature in the solar field  470 . A portion of the heat transfer fluid enters the heat transfer fluid water steam heat exchanger  430  wherein the heat from the heat transfer fluid is used to convert high pressure water to steam. The high pressure steam generates electricity in a steam turbine  440  and generator  450  unit. The low pressure steam from the steam turbine is condensed in condenser  460 . The resulting condensate is pumped in a condensate pump  420  and routed back to the heat transfer fluid water steam heat exchanger  430 . 
         [0038]    Nitrogen from the one or more cold nitrogen storage tanks  500  is pumped by a nitrogen pump  490  and routed to the heat transfer fluid nitrogen heat exchanger  480  wherein the heat from the heat transfer fluid is used to heat the nitrogen. The heated nitrogen flows into one or more containers  510 . 
         [0039]    During night time (or in a low light condition) the heated nitrogen from one or more hot nitrogen tanks  510  is routed to a nitrogen water steam heat exchanger  520  wherein the heat from the heated nitrogen is used to convert pressurized water to steam. The pressurized steam generates electricity in the steam turbine  440  and generator  450  unit. The low pressure steam from the steam turbine is condensed in condenser  460 . The resulting condensate is pumped in a condensate pump  420  and routed back to the nitrogen water steam heat exchanger heat exchanger  520 . Meanwhile, cold nitrogen gas returns to the one or more cold nitrogen storage tanks  500 . 
         [0040]    There is shown in  FIG. 4  a schematic detailing the day/night time operation of an indirect heating solar power plant according to another embodiment of the invention, generally identified by reference numeral  610 . Nitrogen and the heat transfer fluid are heated to high temperature in the solar field  670 . 
         [0041]    The heat transfer fluid enters the heat transfer fluid water steam heat exchanger  630  wherein the heat from the heat transfer fluid is used to convert high pressure water to steam. The high pressure steam generates electricity in a steam turbine  640  and generator  650  unit. The low pressure steam from the steam turbine is condensed in condenser  660 . The resulting condensate is pumped in a condensate pump  620  and routed back to the heat transfer fluid water steam heat exchanger  630 . Meanwhile, cold heat transfer fluid returns to solar field  670  from the heat transfer fluid water steam heat exchanger  630 . 
         [0042]    Nitrogen from the one or more cold nitrogen storage tanks  690  is pumped by a nitrogen pump  680  and routed to the solar field  670  where it is heated to high temperature by the concentrated solar energy. The heated nitrogen flows into one or more containers  700 . 
         [0043]    During night time (or in a low light condition) the heated nitrogen from one or more containers  700  is routed to a nitrogen water steam heat exchanger  710  wherein the heat from the heated nitrogen is used to convert pressurized water to steam. The pressurized steam generates electricity in the steam turbine  640  and generator  650  unit. The low pressure steam from the steam turbine is condensed in condenser  660 . The resulting condensate is pumped in a condensate pump  620  and routed back to the nitrogen water steam heat exchanger heat exchanger  710 . Meanwhile, cold nitrogen returns to the one or more cold nitrogen storage tanks  690  from the nitrogen water steam heat exchanger  710 . 
         [0044]    There is shown in  FIG. 5  a schematic detailing the day/night time operation of an indirect heating solar power plant according to another embodiment of the invention, generally identified by reference numeral  810 . Only Nitrogen is heated to high temperature in the solar field  870 . 
         [0045]    Heated nitrogen enters the nitrogen water steam heat exchanger  830  wherein the heat from the heated nitrogen is used to convert high pressure water to steam. The high pressure steam generates electricity in a steam turbine  840  and generator  850  unit. The low pressure steam from the steam turbine is condensed in condenser  860 . The resulting condensate is pumped in a condensate pump  820  and routed back to the nitrogen water steam heat exchanger  830 . Meanwhile cold nitrogen returns to the solar field  870  from the nitrogen water steam heat exchanger  830 . 
         [0046]    Nitrogen from the one or more cold nitrogen storage tanks  890  is pumped by a nitrogen pump  880  and routed to the solar field  870  where it is heated to a high temperature by the concentrated solar energy. The heated nitrogen flows into one or more containers  900 . 
         [0047]    During night time (or in a low light condition) the heated nitrogen from one or more containers  900  is routed to a nitrogen water steam heat exchanger  910  wherein the heat from the heated nitrogen is used to convert pressurized water to steam. The pressurized steam generates electricity in the steam turbine  840  and generator  850  unit. The low pressure steam from the steam turbine is condensed in condenser  860 . The resulting condensate is pumped in a condensate pump  820  and routed back to the nitrogen water steam heat exchanger heat exchanger  910 . Meanwhile, cold nitrogen gas returns to the one or more cold nitrogen storage tank  890  from the nitrogen water steam heat exchanger  910 . 
         [0048]    While not depicted, the nitrogen water steam heat exchanger  830  and the nitrogen water steam heat exchanger  910  can be combined, in another embodiment, into a single unit.