Abstract:
The present invention is a method for storing solar collected heat and using the stored heat to produce power, comprising the steps of diverting solar heated fluid at a temperature greater than a predetermined power plant block (PPB) nominal temperature from a heat transfer circuit; transferring heat from the diverted solar heated fluid to a portion of a liquid water storage medium maintained at a temperature significantly less than the predetermined PPB nominal temperature and thermally storing the heated portion; and transferring heat from the heated liquid water storage medium to a fluid during periods of decreased solar radiation levels, whereby to produce power by means of said heated heat transfer fluid. 
     The present invention is also directed to a solar derived thermal storage system, comprising a hot water storage medium (HWSM), a cold water storage medium (CWSM), conduit means interconnecting the HWSM and said CWSM, and a storage medium heat exchanger in heat exchanger relation with the conduit means and with solar heated fluid, for heating thermally storable water flowing from said CWSM to said HWSM when the solar radiation is above a nominal value which establishes a predetermined power plant block (PPB) nominal temperature.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of solar based power generation. More particularly, the invention relates to a solar derived thermal storage system for effectively utilizing the stored solar energy during periods of decreased solar influx. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fossil fuel power plants suffer from several drawbacks, including decreasing supplies of fuel and environmental pollution. Governments and research institutes have therefore been considering for decades power generation methods based on alternative energy resources which neither cause environmental pollution nor are dependent on decreasing fossil fuel resources. One of the most significant alternative energy resources is solar energy due to its ubiquitous availability and environmental advantages. 
         [0003]    Solar thermal energy plants employ collectors to concentrate sunlight onto a receiver and to thereby heat a heat transfer fluid passing through the receiver to a sufficiently high temperature to produce power. The intensity of solar energy varies, being unavailable at night and progressively increasing from sunrise until noon and then decreasing until sunset when it is once again unavailable. Furthermore, its intensity depends on the seasons and also varies according to the cloud level during the day. As a result, a thermal storage system is needed to permit the production of power during periods of reduced solar influx. 
         [0004]    One possible thermal storage medium is oil. Thermal oil can reach high temperatures and therefore can transfer heat to a motive fluid of a power producing thermodynamic cycle. However, oil is costly and often has a low heat capacity, often requiring large pressure vessels to store therein a sufficiently large volume of oil in order to transfer a required amount of heat to produce power. 
         [0005]    U.S. Pat. No. 4,171,617 for example discloses a thermal storage medium in the form of molten salt. Molten salt is non-flammable, and can achieve a higher temperature than oil. However, the maintenance of such molten salt thermal storage systems are involved and in addition molten salt can crystallize so that maintenance system for such systems are costly. 
         [0006]    Another prior art thermal storage medium is pressurized steam of high heat capacity for storage in a small accumulator when a portion thereof condenses, and flashes back to steam when its pressure is subsequently lowered. [“Sunny Outlook: Can Sunshine Provide All U.S. Electricity?”, Scientific American, Sep. 19, 2007] However, the accumulator must be able to withstand very high pressures and is therefore expensive to manufacture. In order to effectively store the required volume of pressurized steam, a large number of accumulators need to be employed. 
         [0007]    The present invention provides a solar derived thermal storage system based on an inexpensive storage medium that can store solar energy for use during periods of decreased solar influx. 
         [0008]    In addition, the present invention provides a solar derived thermal storage system based on a storage medium that can be heated to a sufficiently high temperature which allows heat to be transferred to a motive fluid and to thereby produce power at a relatively high thermal efficiency, yet has a relatively low cost of maintenance. 
         [0009]    Furthermore, the present invention provides a water based solar derived thermal storage system by which solar energy can be stored at a relatively low pressure. 
         [0010]    Additionally, the present invention provides a solar derived thermal storage system that is relatively simple in construction and low in cost. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is a method for storing solar collected heat and using the stored heat to produce power, comprising the steps of diverting solar heated fluid at a temperature greater than a predetermined power plant block (PPB) nominal temperature from a heat transfer circuit; transferring heat from said diverted solar heated fluid to a portion of a liquid water storage medium maintained at a temperature significantly less than said predetermined PPB nominal temperature and thermally storing said heated portion; and transferring heat from said heated liquid water storage medium to a fluid during periods of decreased solar radiation levels, whereby to produce power by means of said heated heat transfer fluid. 
         [0012]    The present invention is also directed to a solar derived thermal storage system, comprising a hot water storage medium (HWSM), a cold water storage medium (CWSM), conduit means interconnecting said HWSM and said CWSM, and a storage medium heat exchanger in heat exchanger relation with said conduit means and with solar heated fluid, for heating thermally storable water flowing from said CWSM to said HWSM when the solar radiation is above a nominal value which establishes a predetermined power plant block (PPB) nominal temperature. 
         [0013]    The storage medium heat exchanger can further comprise a preheater for preheating fluid by means of said heated thermally storable water flowing from said HWSM to said CWSM when said solar radiation decreases below a nominal level. 
         [0014]    In one embodiment, said preheater preheats solar heated fluid with heat from said heated thermally storable water from said HWSM when said solar radiation decreases below the nominal level. 
         [0015]    In another embodiment, said preheater preheats motive fluid of said PPB with heat from said heated thermally storable water from said HWSM when said solar radiation decreases below the nominal level. 
         [0016]    In further embodiments, by operating the PPB at a further suitable predetermined PPB nominal temperature lower than the previously mentioned PPB nominal temperature, a further heat exchanger can be included in the system so that heat from said heated thermally storable water can supply heat to the motive fluid of the PPB not only to preheat it but also to provide at least some of the heat needed for vaporizing or boiling the motive fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the drawings: 
           [0018]      FIG. 1  is a schematic illustration of a thermal storage system implemented in a solar based power plant according to one embodiment of the present invention; 
           [0019]      FIG. 1A  is a schematic illustration of a thermal storage system implemented in a solar based power plant according to another embodiment of the present invention; 
           [0020]      FIG. 1B  is a schematic illustration of an example of an embodiment of the power plant block (PPB) according to the present invention; 
           [0021]      FIG. 2  is a schematic illustration of a thermal storage system implemented in a solar based power plant according to further embodiment of the present invention; and 
           [0022]      FIG. 2A  is a diagram showing the thermal characteristics of a system operating according to the embodiment of the present invention described with reference to  FIG. 2 . 
       
    
    
       [0023]    Similar reference numerals refer to similar components of the system. 
       DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates a power producing system  25 A provided with solar derived and water-based thermal storage system  10 A, which transfers heat to power plant block (PPB)  20  during period of decreased solar influx, according to one embodiment of the present invention. 
         [0025]    Power producing system  25 A comprises a solar collector and concentrator unit (SCC)  30  through which a heat transfer fluid, e.g. synthetic oil, etc. passes and is heated. Power producing system  25 A can be optimized to have a relatively high thermal efficiency when the heat transfer fluid is heated to a predetermined PPB nominal temperature, e.g. ranging from about 300-400° C. SCC  30  may be any collector, or array of collectors, well known to those skilled in the art including e.g. mirrors, reflectors, glass transmitters, tracking systems such as solar heliostat collectors in a field for providing heat to a central receiver at e.g. the top of a solar tower, parabolic troughs, Fresnel reflectors, and a unit without a concentrator. The solar heated heat transfer fluid is circulated in a closed heat transfer circuit  11 , such as by means of a pump (not shown), to main heat exchanger  80 , where the solar heated fluid transfers heat to motive fluid delivering thermal energy via power block circuit  21  to PPB  20 , and is then returned to SCC  30 . PPB  20  may be based on any thermodynamic cycle, or combination of thermodynamic cycles, well known to those skilled in the art. Non-limiting examples of such thermodynamic cycles include steam turbine cycle operating on e.g. a Rankine cycle, a combined steam/organic motive fluid cycle (see  FIG. 2B ), where the organic motive fluid cycle can be the bottoming cycle to the steam turbine cycle, both operating on e.g. Rankine cycles, etc. 
         [0026]    Thermal storage system  10 A, which is schematically indicated by a dashed frame, is in periodic fluid communication with heat transfer circuit  11 , and is adapted to store heat from solar energy collected by SCC  30  which is not transferred to PPB  20 , thus allowing the stored heat to be utilized by PPB  20  during periods of decreased solar influx. 
         [0027]    Thermal storage system  10 A has two tanks  40 A and  40 B which are interconnected by conduit means or line  51 , and heat storage medium heat exchanger  50  in heat exchanger relation with conduit means or line  51 . The volume of tanks  40 A and  40 B and the volume of water contained in each of the tanks is sufficiently large to ensure that the water storage medium will be retained in a liquid phase despite fluctuations in temperature and pressure. To provide some indication of water volumes needed for storage tanks  40 A and  40 B, the following example is given. For summer time conditions in Eilat, Israel during the month of July, e, g., storage tanks  40 A and  40 B will have volumes of between about 60-85 m 3  per MW produced when using operation level of about an 80% (an 80% operation level from the peak solar radiation level available during the month of July) for PPB  20  and the heat transfer fluid heat circulated in a closed heat transfer circuit  11 . Similar volumes will be needed in East Mesa, Calif., U.S.A. during the summer month of July. Of course, different design considerations could change these volumes. The liquid contained in hot water tank  40 A can be maintained at a temperature ranging from about 230-240° C. and a pressure ranging from about 30-40 bar. The temperature and pressure of liquid contained in cold water tank  40 B will be significantly less than that of the water contained in hot water tank  40 A. 
         [0028]    Conduit means or line  51  comprises apparatus for delivering water, upon demand, either from tank  40 A to tank  40 B or from tank  40 B to tank  40 A. Such apparatus may comprise a first conduit or line and a first transfer pump operatively connected to the first conduit or line for delivering water in a first direction between tanks  40 A and  40 B, and a second transfer pump operatively connected to the second conduit or line for delivering water in a second direction opposite to the first direction. Such an alternative can be used to preheat the heat transfer fluid supplied to SCC ( 30 ). 
         [0029]    Thermal storage system  10 A also comprises preheater  60  for preheating motive fluid condensate flowing in power block circuit  21  upstream to main heat exchanger  80 . An additional conduit or line  41  isolated from conduit means or line  51  extends from hot water tank  40 A to cold water tank  40 B while passing through preheater  60 . A pump operatively connected to conduit or line  41  delivers hot water upon demand to preheater  60 . If so desired, a thermal storage system having a single water tank can be used (see  FIG. 1A ). In the example shown in  FIG. 1A , single water tank  40 C contains hot water stored in the upper portion of the tank and cold water present in the lower portion of the tank using e.g. a thermocline between the hot upper portion ( 40 D) and cold lower portion ( 40 E) of tank  40 C. 
         [0030]    Accordingly, thermal storage system  10 A has two operational modes:
       (a) a storing mode by which heat produced by collected solar energy is stored in tank  40 A during periods when excess solar heat is available; and   (b) a release mode by which previously stored heat is released and transferred to the motive fluid flowing in power block circuit  21 .       
 
         [0033]    During the storing mode, such as during periods of 100% sunshine, the flow rate of heat transfer fluid in circuit  11  is increased and a portion of the heat transfer fluid is diverted from circuit  11  to heat storage medium heat exchanger  50  by opening valve  61 . The increased flow rate of heat transfer fluid in circuit  11  can be achieved during this storing mode e.g. by operating heat transfer fluid pump  55  which supplied heat transfer fluid from heat transfer fluid tank  57 , The remaining heat transfer fluid, or all of the heat transfer fluid upon termination of the standby mode, circulates within circuit  11  and transfers solar collected heat to the motive fluid circulating in power block circuit  21  by means of main heat exchanger or vaporizer or boiler  80 , backflow to storage medium heat exchanger  50  being prevented by means of valve  31 , which may be a check valve. 
         [0034]    Prior to opening valve  61 , valve  24  is closed and valve  53  is opened and then cold water is delivered to heat storage medium heat exchanger  50 , by activating e.g. pump  73 . The cold water storage medium is heated by the diverted heat transfer fluid and the temperature of the cold water storage medium is increased. The mass flow rate of the diverted heat transfer fluid being delivered to heat storage medium heat exchanger  50  can be regulated by valve  61  so that the instantaneous temperature of the heat transfer fluid exiting SCC  30  remains substantially constant. The heat depleted heat transfer fluid then flows to the inlet of SCC  30 . After the storage medium is sufficiently heated, pump  73  is deactivated and the hot storage medium is held in standby in hot storage tank  40 A for use during the release mode. 
         [0035]    During the release mode, such as during periods of 50% sunshine, e.g. during the time period near sunset and thereafter, or the time period near sunrise and thereafter, or during cloudy periods of weather, the solar radiation level reaching SCC is reduced. If stored solar energy were not transferred to the motive fluid flowing in power block circuit  21 , the motive fluid would not be sufficiently heated and PPB  20  would not be able to operate at its nominal thermal efficiency. To ensure continued operation at nominal thermal efficiency of PPB  20 , motive fluid is now supplied to preheater  60  by opening valve  24  while verifying that valve  61  and valve  53  are closed and heat transfer pump  55  is not operating. Pump  81  operatively connected to power block circuit  21  consequently supplies condensed motive fluid exiting PPB  20  via conduit or line  15  to preheater  60 . Hot storage medium is now delivered to preheater  60  via conduit or line  41 , such as by opening valve  75  and activating pump  71 , the condensed motive fluid will now be sufficiently heated by hot water storage medium supplied from hot storage tank  40 A so that when being subsequently delivered to main heat exchanger  80  and additionally heated by the heat transfer fluid, it will achieve substantially the same temperature as when the temperature of the heat transfer fluid exiting SCC is substantially equal to the predetermined PPB nominal temperature. During this release mode of operation, heat transfer flow in circuit  11  remains constant. 
         [0036]    Valves  24 ,  75 ,  61  and  53  as well as pumps  71 ,  73  and  55  may be manually operated. Alternatively, they may be automatically operated in conjunction with a controller and sensors for sensing the instantaneous temperature of the heat transfer fluid exiting SCC  30  or a sensor for sensing the solar radiation level. 
         [0037]    Reference is now made to  FIG. 2  wherein  25 B refers to a power producing system operating in accordance with a further embodiment of the present invention. This embodiment operates basically in a manner similar to the operation of the embodiment described with reference to  FIG. 1 . However, in the present embodiment, the nominal operating temperature of PPB  20  is set at a lower temperature than that of the embodiment described with reference to  FIG. 1  so that more heat from heat storage tank  40 A can be used to heat the motive fluid of the PPB during stand-by mode. Consequently, during stand-by mode, heat is supplied from heat storage tank  40 A not only to preheater  60  but also to secondary boiler  80 B for transferring heat to preheated motive fluid of PPB  20  and boiling some of the motive fluid producing steam. The motive fluid is then supplied to primary boiler  80 A wherein further heat from the solar heated heat transfer medium is transferred to the motive fluid for boiling the remainder of the motive fluid (see e.g.  FIG. 2A  showing also superheating if used in an example where only 60% solar radiation is available). The produced steam is then supplied to PPB  20  for producing power. 
         [0038]    Thus, in accordance with the present invention, by choosing an operation level for PPB  20  and the heat transfer fluid heat circulated in a closed heat transfer circuit  11 , solar heat collected during the storing mode and stored by the cost-effective relatively low temperature water based thermal storage system of the present invention described herein, can be utilized at a different period of time in the release mode for producing power using PPB  20  when less than 100% of the peak solar radiation is available. Usually, the stored heat can be used for providing pre-heat and also vaporization or boiling of the motive fluid of PPB  20 . 
         [0039]    While the above description refers to a water-based heat storage system having e.g. two tanks or one tank, a further embodiment of the present invention can also include a thermal oil heat storage system together with the water based heat storage system. In this alternative, the low temperature operation of the system can be supplied by heat stored in water-based heat storage system whereas the higher temperature operation of the system (e.g. portion of boiling and/or superheating of the motive fluid) can be supplied by heat stored in the thermal oil heat storage system. 
         [0040]    In addition, while the above describes stored heat being transferred to the thermal heat transfer fluid or the water motive fluid of the PPB, alternatively, when a combined cycle water/organic motive fluid PPB is used, according to an embodiment of the present invention, stored heat can be transferred to the organic motive fluid for preheating e.g. the organic motive fluid. In such an alternative, good heat-source heat-sink of the stored heat and the organic motive fluid being preheated can be achieved. 
         [0041]    Furthermore, while the above description describes the use of solar heated fluid in a solar heated heat transfer circuit, in an alternative, working fluid of the PPB, e.g. water, can be circulated through the solar collector and concentrator unit (SCC)  30  and supplied directly to the PPB for power production. 
         [0042]    Moreover, even though the concept of the present invention is a water based storage system for storing heated produced by solar radiation and using the stored heat for producing power during periods of reduced solar radiation during the time period near sunset and thereafter, or the time period near sunrise and thereafter, or during cloudy periods of weather, the systems described herein can be used in other circumstances as well. E.g. if need be, the stored heat can be used to operate organic cycle portion of the PPB during night hour when no solar radiation is present. 
         [0043]    In addition, while the embodiments of the present invention refer to a system operating at substantially constant temperature, alternatively, according to the present invention, a system can be designed to operate where the temperature of the heat transfer fluid exiting the SCC changes. In such a case, the water-based thermal storage system described above can be designed to operate in series during storing mode. In release mode, several operational modes can be used. One example of the options can be the use of the stored heat for operating only the organic power plant of the PPB. 
         [0044]    While some examples of some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be carried out with many modifications, variations and adaptations, and with the use of numerous equivalents or alternative solutions that are within the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.