Patent Abstract:
Systems and methods for generating electrical power using a solar power system comprising pressurized pipes for transporting liquid water. The pressurized pipes flow through solar collectors which concentrate sunlight on the water flowing through the pipes. The pressurization in the pipes allows for the water flowing therethrough to absorb large quantities of energy. The pressurized and heated water is then pumped to a heat exchanger where the thermal energy is released to produce steam for powering a steam turbine electrical generator. Thereafter, the water is returned to the solar collectors in a closed loop to repeat the process.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application No. 61/237,769, filed Aug. 28, 2009, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Large scale solar power plants utilizing concentrating solar power (“CSP”) technology have been in production for over twenty years. The Solar Electric Generating Systems (“SEGS”) facilities in the Mojave Desert of California are a well-known example of solar power plants using such CSP technology. Concentrating solar power utilizes solar collectors comprising large mirrors or lenses which concentrate solar energy upon an unpressurized pipe or tube that contains a heat transfer fluid. Typically, a synthetic oil having a high boiling point is used as the heat transfer fluid. For example, the SEGS facilities utilize Therminol® from Solutia, Inc. as the heat transfer fluid. 
     As the heat transfer fluid flows through the unpressurized pipe inside the solar collectors, it is heated by the incident sunlight. One or more pumps are situated along the pipe to pump the fluid through the solar collectors and towards a boiler or heat exchanger. There, the transfer fluid is used to heat water in the boiler to produce steam. The steam is then used for powering a conventional steam turbine to produce electricity. After the heat transfer fluid releases its thermal energy in the boiler/heat exchanger, the heat transfer fluid is pumped back to the solar collectors to be heated once again. 
     One disadvantage of the use of a synthetic heat transfer fluid is that the fluid has a relatively low energy density. For example, Therminol® has an energy density of approximately 2100 J/kg° C. whereas ordinary water has an energy density of approximately 4200 J/kg° C. This relatively low energy density for Therminol® means that it can carry relatively less thermal energy from the solar collectors to the heat exchanger than water. 
     Another disadvantage of synthetic heat transfer fluids is that they are often flammable. As a result, care must be taken in handling the fluids and they must be prevented from overheating. 
     For these and other reasons, a number of solar power systems have recently been developed to produce steam directly from water rather than using a synthetic heat transfer fluid. Such systems—dubbed Direct Solar Steam generation (“DISS”) or Direct Steam Generation (“DSG”)—distribute water through the unpressurized pipes in the solar collectors rather than distributing a synthetic heat transfer fluid. Because water has a much lower boiling point than a synthetic heat transfer fluid, the water will eventually turn to steam after being heated a sufficient amount. Thereafter, the steam is directed to a steam turbine for generating electricity. 
     Such DSG systems have their own drawbacks, however. First, the presence of steam in the pipes of the solar collectors reduces the efficiency of the collectors because steam has a significantly lower heat capacity than water. Thus, the steam can carry less thermal energy towards the turbine than can pressurized water. Second, the use of a two-phase (water/steam) flow within the pipes of the solar collectors creates a condition known as the Ledinegg Instability. This phenomenon results in a boiling front as the water moves through the pipes and flashes over to steam. To compensate for this instability, an undesirable pressure drop must be introduced into the system. Finally, DSG systems are more sensitive to variations in solar flux density and changes in atmospheric conditions because the systems will not function properly unless the water in the solar collectors is sufficiently heated to flash over to steam. Taken together, these drawbacks necessitate the use of larger, more expensive solar collectors to produce a given amount of electricity. Therefore, such DSG systems may have little or no cost savings in comparison to traditional CSP systems containing synthetic heat transfer fluid. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are systems and methods for generating electrical power using a solar power system comprising pressurized pipes for transporting liquid water. The pressurized pipes flow through solar collectors which concentrate sunlight on the water flowing through said pipes. Because the pipes inside the solar collectors are pressurized, the water flowing therethrough can be heated well above the ordinary boiling point of water (100° C.). Advantageously, the systems and methods described herein rely upon the superior heat transfer capabilities of water in comparison to synthetic heat transfer fluids. Furthermore, the lack of synthetic heat transfer fluid minimizes the added costs and safety concerns associated with the use of such fluids. 
     Finally, the pressurized pipes described herein prevent the water flowing therethrough from flashing over to steam when heated to a high temperature. Accordingly, the instabilities and unwanted pressure drops associated with two-phase (water/steam) flow are eliminated. Furthermore, the use of water rather than steam for transporting thermal energy takes advantage of water&#39;s superior energy carrying capacity in comparison to steam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a detailed view of a pressurized solar power system according to one embodiment of the present invention. 
         FIG. 2  shows a view of the embodiment of  FIG. 1  including the steam turbine and power generation portion of the system. 
         FIG. 3  shows a view of a second embodiment of a pressurized solar power system. 
         FIG. 4  shows the heat exchanger of  FIG. 3  and a plurality of thermal storage tanks for use with the embodiment shown in  FIG. 3 . 
         FIG. 5  shows a view of a third embodiment of a pressurized solar power system. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-4  show various embodiments and aspects of the present invention, with like reference numerals indicating like parts throughout the several views. 
       FIG. 1  shows a detailed view of a pressurized solar power system  100  in accordance with one embodiment of the present invention. A pressurized solar loop  1  comprising a hollow pipe or tube is present. A portion of the pressurized solar loop  1  is positioned within a solar collector receiver array (not pictured). The solar collector receiver array may comprise any suitable means of concentrating solar energy on the pressurized solar loop  1  including, but not limited to, parabolic troughs, parabolic dishes, compact linear Fresnel reflectors, linear Fresnel reflectors, compound parabolic collectors, two axis tracking systems that focus solar energy on a tower or other structure, and any other solar energy concentration system. 
     The pressurized solar loop  1  forms a closed loop and preferably contains water within the loop. Other suitable heat transfer fluids known to those skilled in the art may be used instead of water, however. A pressurizer  3  is attached to the pressurized solar loop  1  to pressurize the solar loop  1  above normal atmospheric pressure. 
     Preferably, pressurizer  3  is a steam bubble pressurizer comprising a large internal chamber where steam can form in the upper section of the chamber but cannot be released. As the water in the solar loop  1  is heated due to the concentrated sunlight directed towards solar loop  1 , a steam bubble will form in the upper portion of steam bubble pressurizer  3 . The steam bubble can also be formed by pre-heating the water in solar loop  1 . After forming, the steam bubble in the upper section of the pressurizer  3  keeps pressure on the water in the pressurized solar loop  1 . Advantageously, this pressure increases the boiling point of the water in the pressurized solar loop  1 , thus preventing the water from flashing over to steam. As solar energy increases the temperature of water circulating in solar loop  1 , the steam bubble in the pressurizer  3  increases in pressure thereby creating a self-regulating pressure control system. 
     As described above, the use of a single-phase (water only) pressurized solar loop  1  prevents Ledinegg Instability and unwanted pressure drop. Water also has an increased energy carrying capacity in comparison to steam. Thus, the pressurized water in pressurized solar loop  1  can carry more energy than a comparable DSG system with a two-phase (water/steam) energy transport mechanism. 
     One or more pumps  8  are present along the pressurized solar loop  1 . These pumps  8  act to circulate water through the solar collector receiver array and to the heat exchanger  4 . Control mechanisms known to those skilled in the art operate to control the pumps  8  and the flow rate of water flowing through pressurized solar loop  1 . 
     An auxiliary heating device  9  can be attached to pressurized solar loop  1 , preferably near the point where the pressurized solar loop  1  enters the heat exchanger  4 . One or more pumps  10  can be provided to pump water from the solar loop  1  into the auxiliary heating device  9 . The auxiliary heating device  9  can be used to heat the water in the solar loop  1  if there is insufficient solar energy to heat the water to an appropriate operating temperature such as on cloudy days or during the nighttime hours. 
     In some embodiments, an optional distillation unit  5 , condenser  6 , and water collector  7  can be connected to the pressurized solar loop  1 . The distillation unit  5  can use the hot water from the pressurized solar loop  1  to boil water to create steam. This steam can then be transferred to condenser  6  where it will be cooled and condensed into clean distilled water. Such distilled water can be collected in water collector  7 . The distilled water can later be used for any number of purposes including, but not limited to, providing makeup water for the heat exchanger  4  or the pressurized solar loop  1 . 
     After the water is heated in the portion of pressurized solar loop  1  that lies inside the solar collectors, the water travels to the heat exchanger  4 . The heat exchanger  4  preferably comprises a pressurized steam generator vessel  2  with liquid water in the lower portion of the steam generator vessel  2 . Preferably, the pressurized solar loop  1  will enter the lower portion of the steam generator vessel  2 . A sizeable length of solar loop  1  will be present within the lower portion of the steam generator vessel  2 , preferably in a coil, loop, or other configuration so as to expose a substantial surface area of the solar loop  1  to the water contained in the lower portion of heat exchanger  4 . The hot water contained in solar loop  1  will transfer its heat to the water in the bottom of heat exchanger  4  thus causing the water in the heat exchanger  4  to boil and produce steam. The steam generator vessel  2  of heat exchanger  4  preferably comprises suitable ports or openings for releasing steam and for introducing makeup water into the heat exchanger  4 . Preferably, the makeup water is cooler than the water present in the pressurized solar loop  1  so as to facilitate the transfer of thermal energy inside the heat exchanger  4 . As described in more detail below, cooling towers or other means for cooling water can be used to sufficiently cool water for use as makeup water. 
     After the hot water in the pressurized solar loop  1  transfers its thermal energy to create steam inside the heat exchanger  4 , the cooled water exits the heat exchanger  4  and returns to the solar collectors. In such a manner, the water inside pressurized solar loop  1  continuously circulates through solar loop  1 , absorbing thermal energy from the sunlight at the solar collectors and releasing thermal energy inside the heat exchanger  4 . 
     With reference to  FIGS. 1 and 2 , the steam produced inside heat exchanger  4  exits the steam generator vessel  2  and proceeds through steam piping  11  towards a steam turbine  16 . As known to those skilled in the art, the steam turbine  16  utilizes the energy contained in the steam to generate rotary motion. This motion, in turn, is used by generator  15  to produce electricity. 
     As shown in  FIG. 1 , an optional superheater  12  may be attached to steam piping  11  prior to entry into steam turbine  16 . The superheater  12  can be used to add additional heat to the steam from any external heat source  14  including, but not limited to, additional solar heating sources. An optional moisture separator  13  can also be attached to steam piping  11 . 
     Returning to  FIG. 2 , after powering the steam turbine  16 , the steam will exit the turbine  16  and enter a condenser  17  where it will be condensed back into water. The water then is transferred to a heat rejection device  18  such as a cooling tower. The cooled water will then flow back into the steam generator vessel  2  of heat exchanger  4 . One or more pumps  19  may act to pump the water back to the heat exchanger  4 . In such a manner, the water is ready to again be heated by the pressurized solar loop  1  to form steam inside the heat exchanger  4 . 
     As described above, the pressurized water in pressurized solar loop  1  allows for the water to absorb substantial energy and rise to a temperature well above 100° C. without flashing over to steam. Advantageously, this allows the pressurized solar power system  100  to carry more energy than a two-phase (water/steam) DSG system or a system using a synthetic heat transfer fluid in a non-pressurized solar loop. The enhanced efficiency of the pressurized solar power system  100  described herein also allows for the use of smaller and/or fewer solar collectors than in prior art systems. The efficiency of the pressurized solar power system  100  can be further increased by placing the steam turbine  16  and the heat exchanger  4  in the center of the array of solar collectors, thus reducing the length of piping between the solar collectors and the heat exchanger  4  as well as the length of piping  11  between the heat exchanger  4  and the steam turbine  16 . 
     Turning to  FIG. 3 , a second embodiment of a pressurized solar power system  200  is shown. The embodiment shown in  FIG. 3  is similar in many respects to the embodiment shown in  FIGS. 1-2 , with like reference numerals indicating like parts between the two embodiments. The pressurized solar power system  200  of  FIG. 3  generally comprises a pressurized solar loop  1  that preferably contains pressurized water. The pressurized water in solar loop  1  absorbs thermal energy from the concentrated solar energy produced by one or more solar collectors and transports said thermal energy to a heat exchanger  104 . 
     Heat exchanger  104  preferably comprises two vessels: a pressurized steam generator vessel  102  and a non-pressurized storage media vessel  101 . The storage media vessel  101  contains a substance suitable for storing and transporting thermal energy such as molten salt. The steam generator vessel  102  contains water in the lower portion of the vessel which, when heated sufficiently, will boil and produce steam in the upper portion of steam generator vessel  102 . 
     A portion of the pressurized solar loop  1  preferably enters the storage media vessel  101  near the lower end of the storage media vessel  101  and forms a coil, loop, or other shape to expose a substantial surface are of the solar loop  1  to the surrounding salt inside the storage media vessel  101 . The hot water in the pressurized solar loop  1  advantageously heats the molten salt contained in the storage media vessel  101 . In turn, the molten salt is in contact with the exterior portion of steam generator vessel  102  and transfers heat from the molten salt to the steam generator vessel  102 . This causes the water inside steam generator vessel  102  to heat up and eventually turn to steam. As described above with respect to  FIGS. 1 and 2 , the steam can be used to drive a steam turbine  16  and produce electrical energy at an electrical generator  15 . 
     Turning to  FIG. 4 , a plurality of thermal storage tanks  105   b - 105   e  are shown. One or more of such thermal storage tanks  105  may optionally be used in conjunction with the pressurized solar power system  200  of  FIG. 3 . Advantageously, the thermal storage tanks  105  can be used to store heat energy during the day for use during the night or on cloudy days. 
     The thermal storage tanks  105  preferably contain molten salt or any other substance suitable for storing heat including, but not limited to, eutectic salts, brines, and graphite. Each storage tank  105   b - 105   e  also has disposed therein a portion of a pressurized solar loop  1   b - 1   e . Similar to the pressurized solar loop  1  that heats the molten salt in the heat exchanger  104 , the pressurized solar loops  1   b - 1   e  are utilized to absorb solar energy as thermal energy, transport that thermal energy to a storage tank  105 , and heat the molten salt contained in the storage tank  105 . That is, each of the pressurized solar loops  1   b - 1   e  are connected at one end of the loop to one or more solar collectors and are connected at the other end of the loop to a storage tank  105 . In such a manner, solar energy can be absorbed during a sunny day, converted to thermal energy, and stored in a storage tank  105  for use during the night or on cloudy days. 
     As shown in  FIG. 4 , a storage media loop  103  travels from the storage media vessel  101  of heat exchanger  104  to the storage tanks  105 . The storage media loop  103  continues from the storage tanks  105  back to the storage media vessel  101 . One or more pumps  106  are present along the storage media loop  103  to pump the molten salt. On cloudy days or during the night, hot molten salt from the storage tanks  105  can be pumped into the storage media vessel  101  of heat exchanger  104  to produce steam in steam generator vessel  102 . As such, the pressurized solar power system  200  can continue to produce electricity even when there is little or no sunlight. 
     Returning to  FIG. 3 , an optional co-generation or combined cycle power generation aspect of the present invention is shown. Specifically, the pressurized solar power systems  100 ,  200  described herein may be used in conjunction with conventional power generation systems (such as natural gas or coal fired power generation plants) to supplement the power produced by the pressurized solar power system  100 ,  200 . As shown in  FIG. 3 , hydrocarbon fuel such as natural gas can be used with a conventional gas turbine  112  to power an electrical generator  111 . One or more heat recovery coils  113  can advantageously be used to recover waste heat from the gas turbine  112  to heat water in the pressurized solar loop  1 . Similarly, one or more heat recovery coils  114  may be used to pre-heat the water before it enters the steam generator vessel  102  of heat exchanger  104 . 
     Turning to  FIG. 5 , a third embodiment of a pressurized solar power system  300  is shown. The embodiment shown in  FIG. 5  is similar to the embodiment shown in  FIG. 3 , with like reference numerals indicating like parts between the two embodiments. The pressurized solar power system  300  comprises an array of solar collectors (solar array), a pressurized solar loop  1 , a heat exchanger  104 , a steam turbine  16 , and an electric generator  15 . The heat exchanger  104  comprises a steam generator vessel  102  and a storage media vessel  101  and functions in a manner similar to the heat exchanger  104  of  FIG. 3 . 
     The pressurized solar power system  300  in  FIG. 5  is shown operating in conjunction with a geothermal power source  301  and a natural gas source  311 . Hot water, steam, natural gas, and/or other carriers from the geothermal power source  301  are directed to a separation tank  302  where natural gas can be separated from the hot water generated by the geothermal power source  301 . The natural gas can be directed through pipe  305  to a natural gas pipeline or natural gas storage tank for suitable use, including as a fuel for a conventional gas turbine for use in combined cycle power operations. 
     After separating the natural gas from the hot water inside separation tank  302 , the hot water can be directed through pipe  303  to heat exchanger  104 . There, the hot water can supplement the thermal energy produced by the pressurized solar power system  300 . After the hot water from the geothermal source  301  has released much of its heat in heat exchanger  104 , the water can be injected into the ground through pipe  304 . 
     Advantageously, this injection of water into the ground can be used to bring natural gas to the surface from natural gas source  311 . A natural gas well  312  can collect the natural gas and transport it to a separation tank  313 . Any water mixed with the natural gas can be removed through pipe  314  and injected into the ground through pipe  304 . The recovered natural gas can be collected through pipe  305  and used in any suitable manner, including for combined cycle power operations. 
     Accordingly, while the invention has been described with reference to the structures and processes disclosed, it is not confined to the details set forth, but is intended to cover such modifications or changes as may fall within the scope of the following claims.

Technology Classification (CPC): 5