Abstract:
A solar power generation system includes a fluid housing, a solar collector, and a heating system. The fluid housing contains a heat transfer medium. The solar collector concentrates solar energy onto the heat transfer medium. The heating system includes at least one gas tank containing a gas and fluidically connected to a first catalyst. The first catalyst is configured to catalyze gas from the gas tank to create hot gas. The heating system also includes a first hot gas pipe fluidically connected to the first catalyst and positioned with respect to the fluid housing such that hot gas flowing through the first hot gas pipe comes into thermal contact with the heat transfer medium within the fluid housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to the following co-pending application filed on the same day as this application: “CATALYZED HOT GAS HEATING SYSTEM FOR PIPES”by inventors A. Little and A. Zillmer (U.S. patent application Ser. No. 12/319,893. 
     BACKGROUND 
     The present invention relates to thermal energy collecting systems, and in particular, to heating a molten storage medium used in thermal energy collecting systems. 
     Throughout the world there is an increasing demand for energy, which is typically provided by fossil fuels such as petroleum and coal. Additionally, due to scarcity and adverse environmental effects of fossil fuels, cleaner, renewable energy sources are becoming more desirable. As technology advances, alternative fuel sources are becoming practical to replace, or at least augment, conventional power plants to meet worldwide energy demand in a clean manner. In particular, solar energy is freely available and is becoming more feasible, especially in the form of concentrated solar power, which allows for energy storage and can be scaled for commercial production. 
     Concentrated solar power generation systems typically comprise solar collectors that focus solar rays onto a heat transfer medium such as a molten salt. For example, solar power towers use an array of thousands of heliostats to concentrate energy on an elevated central receiver through which molten salt flows inside of numerous pipes. In solar trough systems, molten salt flows through extended lengths of piping which are shrouded by solar collecting troughs that concentrate energy along lengths of the pipes. Heat from the solar energy is transferred to the molten salt and then through a heat exchanger to another medium, such as air or water, which is then used to generate mechanical energy that is ultimately converted to electrical power. Molten salt efficiently stores heat from the solar energy for extended periods of time such that electrical power can be generated at night or during other periods of low solar collection. 
     Molten salts can solidify if cooled below a certain temperature. Consequently, pipes and tanks holding the molten salt are typically wrapped in electrical trace heating elements (electrical resistance wires). Electrical trace heating can, however, be relatively expensive, increasing total cost of power production. Moreover, electrical trace heating can be prone to failure, causing the entire solar power generation system to require shut-down for maintenance. There is, therefore, a need for improved heating of pipes and tanks for the heat transfer medium in a solar power generation system. 
     SUMMARY 
     According to the present invention, a solar power generation system includes a fluid housing, a solar collector, and a heating system. The fluid housing contains a heat transfer medium. The solar collector concentrates solar energy onto the heat transfer medium. The heating system includes at least one gas tank containing a gas and fluidically connected to a first catalyst. The first catalyst is configured to catalyze gas from the gas tank to create hot gas. The heating system also includes a first hot gas pipe fluidically connected to the first catalyst and positioned with respect to the fluid housing such that hot gas flowing through the first hot gas pipe comes into thermal contact with the heat transfer medium within the fluid housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a concentrated solar power generation system having a heating system of the present invention. 
         FIG. 2  is a schematic diagram of a first embodiment of a portion of the heating system of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a second embodiment of a portion of the heating system of  FIG. 1 . 
         FIG. 4  is a schematic diagram of a third embodiment of a portion of the heating system of  FIG. 1 . 
         FIG. 5  is a schematic diagram of fourth embodiment of a portion of the heating system of  FIG. 1 . 
         FIG. 6  is a schematic diagram of a fifth embodiment of a portion of the heating system of  FIG. 1 . 
         FIG. 7A  is a sectional view of a first embodiment of a pipe heating zone along section  7 A- 7 A of  FIG. 2 . 
         FIG. 7B  is a sectional view of a second embodiment of the pipe heating zone along section  7 B- 7 B of  FIG. 2 . 
         FIG. 7C  is a sectional view of a third embodiment of the pipe heating zone along section  7 C- 7 C of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     In general, the present invention includes a heating system for heating a heat transfer medium in a concentrated solar power generation system. The heating system includes catalysts positioned near various parts of the solar power generation system that can contain the heat transfer medium. A blend of fuel and air is blown across the catalysts, reacts, and creates heat which is then transferred to the various parts and ultimately to the heat transfer medium. 
       FIG. 1  shows a schematic diagram of concentrated solar power generation system  10  having heating system  12  of the present invention. In the embodiment shown, power generation system  10  comprises a power tower system having solar collector system  14 , central receiver  16 , tower  18 , cold storage tank  20 , hot storage tank  22 , heat exchanger  24 , generator  26 , pumps  28 A,  28 B and  28 C, and pipes  30 A,  30 B,  30 C and  30 D. In other embodiments, power generation system  10  may comprise a beam down solar power generation system or a parabolic trough solar power generation system. Solar collector system  14  and central receiver  16  impart heat from the sun into a molten heat transfer medium contained in storage tanks  20  and  22  such that thermal energy can be converted to electrical energy using heat exchanger  24  and conversion system  26 . 
     Solar collector system  14  comprises an array of sun-tracking mirrors, or heliostats, that concentrate solar rays at central receiver  16  to heat a heat transfer medium located within pipes  30 A- 30 D. In one embodiment, approximately 8,500 heliostats, each having a having surface area of about 42 m 2  (square meters) to about 150 m 2 , are arranged concentrically around a tower, having a height of approximately 170 meters, to cover an area of approximately 1 to 2 square mile (˜2.59 to ˜5.18 square kilometers). The heat transfer medium typically comprises molten salt that is maintained in a molten state between approximately 500° F. (˜260.0° C.) and 1200° F. (˜648.9° C.) such that it remains liquid. Through pipe  30 A, pump  28 A directs cool heat transfer medium from cold storage tank  20  into a plurality of tubes within central receiver  16  whereby heat from the concentrated solar rays is imparted into the heat transfer medium. Through pipe  30 B, pump  28 B directs the heated heat transfer medium from receiver  16  to hot storage tank  22  where it is stored in a state ready for producing power with heat exchanger  24 . When power is desired to be produced, heated heat transfer medium is routed through pipe  30 C by pump  28 C from hot storage tank  22  to heat exchanger  24  where heat is input into conversion system  26 . Conversion system  26  may comprise any conventional system that converts thermal energy to mechanical energy, such as Brayton cycle or Rankine cycle systems. In the embodiment shown, conversion system  26  comprises a steam turbine generator having first stage expander  32 A, second stage expander  32 B, generator  34  and condenser  36 . Water within heat exchanger  24  is heated by the molten heat transfer medium to produce steam that turns first and second stage expanders  32 A and  32 B. Expanders  32 A and  32 B rotate a shaft to drive generator  34  to convert mechanical energy to electrical energy. Heat exchanger  24  therefore removes heat from the heat transfer medium before the heat transfer medium is returned to cold storage tank  20  through pipe  30 D. Although solar power generation system  10  is shown using three pumps to move molten salt through pipes  30 A- 30 D, more or fewer pumps can be used. For example, in various embodiments, the height of tower  18  provides enough pressure to move the molten salt into hot storage tank  22  such that pump  28 B is not needed. 
     The use of a heat transfer medium such as molten salt allows power generation system  10  to efficiently store thermal energy in salt contained in hot storage tank  22  such that electrical power can be generated at times when solar collector system  14  is operating below peak. Thus, power generation system  10  can be run 24 hours a day at low power production or at higher production levels for shorter intervals. In various embodiments, the molten salt can be salts composed of alkaline earth fluorides and alkali metal fluorides, and combinations thereof. Suitable elements of the molten salt include: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), Francium (Fr), Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), Radium (Ra), and Fluorine (F). Examples of suitable fluoride molten salts include, but are not limited to: FLiNaK, FLiBe, FLiNaBe, FLiKBe, and combinations thereof, as is described in greater detail in U.S. Pat. App. No. 2008/0000231 to Litwin et al. In other embodiments, other suitable heat transfer media may be used. 
     Salts, however, need to be maintained at elevated temperatures to remain in a molten state such that the salt can flow between components of power generation system  10  using pipes  30 A- 30 D and pumps  28 A- 28 C. Thus, heating system  12  is provided throughout power generation system  10  to maintain the salt at elevated temperatures. Heating system  12  includes fuel tank  38 , compressed gas tank  40 , gas supply pipe  42 , catalysts  44 A- 44 F, and pipe heating zones  46 A- 46 D. Heating system  12  also includes elements (not shown in  FIG. 1 ) inside of cold storage tank  20  and hot storage tank  22 . Fuel tank  38  can hold a compressed, combustible gas such as hydrogen or methane. Compressed gas tank  40  can hold compressed ordinary air, with atmospheric levels of oxygen and nitrogen. Fuel from fuel tank  38  can be blended with air from compressed gas tank  40  at levels that will not combust under ordinary conditions. This blend of fuel and air is then supplied to various locations in power generation system  10  via gas supply pipe  42  and blown across catalysts  44 A- 44 F. The catalyst material used for catalysts  44 A- 44 F can include a noble metal such as platinum, palladium, rhodium, or other suitable catalyst materials. In one embodiment, catalysts  44 A- 44 F can include a chamber containing a plurality of relatively small pellets (not shown). The small pellets can comprise a suitable catalyst material deposited on a parent material such as alumina (also known as aluminum oxide). As the blend of fuel and air passes across the small pellets, the fuel reacts with the oxygen and is combusted, which heats the product of the reaction and any gases that do not react, such as nitrogen and any remaining oxygen. Thus, catalyzed hot gas is created for use at cold storage tank  20 , hot storage tank  22 , and each of pipe heating zones  46 A- 46 D to maintain the molten salt at a particular temperature. Pipe heating zones  46 A- 46 D provide heat to portions of pipes  30 A- 30 D, respectively. In the illustrated embodiment, pipe heating zones  46 A- 46 D provide heat to substantially an entire length of pipe where the molten salt flows. Only relatively small gaps of pipe exist without any pipe heating. 
     Heating system  12  can be used to heat the molten salt in a variety of circumstances. For example, when heat exchanger  24  extracts heat out of the molten salt, the molten salt may drop near or below a minimum desired temperature. Heating system  12  can be used to maintain the desired temperature until the molten salt is delivered back to central receiver  16  to be heated by solar rays. Similarly, during periods of limited sun exposure, such as nighttime, temperature of the molten salt throughout most or all of power generation system  10  can drop near or below a minimum desired temperature. Heating system  12  can be used to maintain the desired temperature until adequate sun exposure returns. In certain circumstances, it may be desirable to allow the molten salt to solidify over night instead of continuously heating it. In that case, heating system  12  can be used to re-melt the salt each morning. Alternatively, cold storage tank  20  and hot storage tank  22  can be continually heated over night while only pipes  30 A- 30 D are allowed to cool below the desired temperature. Heating system  12  can also be used to melt salt any time it becomes necessary, such as during an initial start-up of power generation system  10 . 
     In each of the above heating examples, different areas of power generation system  10  can require different amounts of heat. Heating system  12  can use a set of valves or regulators to vary the amount of heat applied to each area by varying the amount of fuel and air delivered to each catalyst  44 A- 44 F. For example, heating system  12  can supply a relatively large quantity of fuel and air to catalysts  44 D,  44 E, and  44 A when salt is relatively cold in pipe  30 D, cold storage tank  20 , and pipe  30 A, while supplying little or no fuel and air to catalysts  44 B,  44 F, and  44 C when salt is relatively hot in pipe  30 B, hot storage tank  22 , and pipe  30 C. Temperature sensors can be placed throughout power generation system  10  to provide temperature information to help determine where heat is needed. In other embodiments, heating system  12  can include more or less catalysts depending on needs of power generation system  10 . 
     Catalysts  44 A- 44 F can be located at or near their respective areas of heating in order to reduce an amount of time it takes the catalyzed hot gas to reach its intended target. In one embodiment, fuel and air in gas supply pipe  42  can be mixed with a ratio that has little or no chance of combusting without a catalyst. This allows fuel and air to be piped relatively long distances through gas supply pipe  42  with little to no risk of fire or explosion even if gas supply pipe  42  is breached. 
       FIG. 2  is a schematic diagram of a first embodiment of a portion of heating system  12 .  FIG. 2  shows that portion of heating system  12  including catalyst  44 A and pipe heating zone  46 A for heating pipe  30 A. Although  FIG. 2  illustrates only one portion of heating system  12 , pipes  30 B- 30 D (shown in  FIG. 1 ) can be heated by catalysts  44 B- 44 D and pipe heating zones  46 B- 46 D in a similar manner. In the first embodiment, valve  48  blends air from compressed gas tank  40  with fuel from fuel tank  38  to create a desired ratio of fuel to air. In one embodiment, valve  48  can be a small servo valve. In another embodiment, valve  48  could be a more complex combination of regulators. Operation of valve  48  can be controlled by a controller connected to temperature sensors located throughout heating system  12 . The blend of fuel and air is passed over catalyst  44 A where it reacts and creates a catalyzed hot gas. The catalyzed hot gas is then passed through pipe heating zone  46 A to heat pipe  30 A (not shown in  FIG. 2 ) and is ultimately exhausted to the atmosphere. 
       FIG. 3  is a schematic diagram of a second embodiment of a portion of heating system  12 . The second embodiment of heating system  12  is similar to the first embodiment of heating system  12  except for the addition of gas heat exchanger  50 . In the second embodiment, the blend of fuel and air is passed through gas heat exchanger  50  prior to entering catalyst  44 A. Catalyzed hot gas from catalyst  44 A is piped through pipe heating zone  46 A and then through gas heat exchanger  50  prior to exhausting to atmosphere. The catalyzed hot gas leaving pipe cools as it passes through pipe heating zone  46 A but is still warm relative to the blend of fuel and air prior to entering catalyst  44 A. Consequently, gas heat exchanger  50  can transfer heat from the catalyzed hot gas to noncatalyzed fuel and air prior to the catalyzed hot gas being exhausted to the atmosphere. In certain applications, heating the blend of fuel and air prior to catalyzing can increase efficiency of that catalytic process. As with the first embodiment, pipes  30 B- 30 D can also be heated as described in the second embodiment. 
       FIG. 4  is a schematic diagram of a third embodiment of a portion of heating system  12 . The third embodiment of heating system  12  is similar to the first embodiment of heating-system  12  except that fuel tank  38  and compressed gas tank  40  are replaced with Tridyne tank  52 . Tridyne is a gas that includes various mixtures of inert gas and relatively small fractions of fuel and oxidizer. Tridyne is non-reactive under ordinary conditions but becomes reactive upon exposure to a catalyst. The fuel used for Tridyne can be hydrogen, methane, ethane, or a mixture thereof. The oxidizer used for Tridyne can be air, oxygen, or oxygen diflouride, or a mixture thereof. The inert gas for Tridyne can be nitrogen, helium, argon, xenon, krypton, or a mixture thereof. The catalyst used for catalysts  44 A- 44 F can include any suitable catalyst material such as those described with respect to  FIG. 1 . Composition and use of Tridyne is further described in U.S. Pat. No. 3,779,009—CATALYTIC METHOD OF PRODUCING HIGH TEMPERATURE GASES by Joseph Friedman, which is herein incorporated by reference. 
     Because Tridyne is substantially non-reactive under ordinary conditions, it can be stored in a single tank without fear of explosion. Using a single tank of Tridyne allows heating system  12  to be further simplified. Additionally, ordinary air may contain substances that can be harmful to power generation system  10  under certain applications. Use of Tridyne, such as a blend including nitrogen, hydrogen, and oxygen, can reduce exposure to contaminants found in ordinary air. As with the first and second embodiments, pipes  30 B- 30 D can also be heated as described in the third embodiment. 
       FIG. 5  is a schematic diagram of a fourth embodiment of a portion of heating system  12 .  FIG. 5  shows that portion of heating system  12  including catalyst  44 E for heating cold storage tank  20 . Although  FIG. 5  illustrates only one portion of heating system  12 , hot storage tank  22  can be heated by catalyst  44 F in a similar manner. In the fifth embodiment, valve  48  blends air from compressed gas tank  40  with fuel from fuel tank  38  to create a desired ratio of fuel to air. The blend of fuel and air is passed over catalyst  44 E, through tank inlet pipe  52 , and into cold storage tank  20 . As the catalyzed hot gas enters cold storage tank  20 , it flows through tank heat exchanger  54 . In the illustrated embodiment, tank heat exchanger  54  is a tube that winds through cold storage tank  20 . In other embodiments, other suitable heat exchangers can be used so long as they allow heat transfer from the catalyzed hot gas to the salt while preventing the catalyzed hot gas from mixing with the salt. The catalyzed hot gas eventually exits cold storage tank  20  via vent  56 . In another embodiment, gas heat exchanger  50  can be used to recover heat from catalyzed hot gas vented from cold storage tank  20  in a manner similar to that described with respect to  FIG. 3 . 
       FIG. 6  is a schematic diagram of a fifth embodiment of a portion of heating system  12 . The fifth embodiment is similar to the fourth embodiment except that tank heat exchanger  54  is replaced with gas distribution manifold  58 . Gas distribution manifold  58  blows catalyzed hot gas through orifices  60  into direct contact with the salt of cold storage tank  20 . When the salt is originally placed into the tank, it can be solid granules of salt which the catalyzed hot gas can flow over and through. After the salt is heated, it can be molten salt which the catalyzed hot gas can bubble through. The catalyzed hot gas eventually exits cold storage tank  20  via vent  56 . 
     Catalyzing hydrogen or methane with ordinary air creates catalyzed hot gas that typically will not react with molten salt or otherwise adversely effect power generation system  10 . Other heat transfer media may, however, require careful selection of fuel in fuel tank  38  and gas in compressed gas tank  40  in order to prevent the catalyzed hot gas from negatively reacting with the heat transfer media. In an alternative embodiment, Tridyne can be catalyzed for heating cold storage tank  20 . Use of Tridyne can be particularly beneficial when power generation system  10  uses a heat transfer medium that can be harmed by contacting substances in ordinary air. In another embodiment, gas heat exchanger  50  can be used to recover heat from catalyzed hot gas vented from cold storage tank  20  in a manner similar to that described with respect to  FIG. 3 . As with the fourth embodiment, hot storage tank  22  can also be heated as described in the fifth embodiment. 
       FIG. 7A  is a sectional view of a first embodiment of pipe heating zone  46 A along section  7 A- 7 A of  FIG. 2 . In the first embodiment, pipe heating zone  46 A includes hot gas pipes  62 A- 62 D and insulation  64 . Hot gas pipes  62 A- 62 D are relatively small tubes physically adjacent to an exterior surface of pipe  30 A. In one embodiment, hot gas pipes  62 A- 62 D can be made of stainless steel. Catalyzed hot gas flows through hot gas pipes  62 A- 62 D to transfer heat to salt in pipe  30 A. In the illustrated embodiment, hot gas pipes  62 A- 62 D run parallel to pipe  30 A and are spaced substantially symmetrically around pipe  30 A. Hot gas pipe  62 A is on an opposite side of pipe  30 A from hot gas pipe  62 C while hot gas pipe  62 B is on an opposite side of pipe  30 A from hot gas pipe  62 D. In an alternative embodiment, hot gas pipes  62 A- 62 D can spiral around pipe  30 A. In yet another alternative embodiment, the number of hot gas pipes can be fewer than four to reduce cost or can be greater than four to increase surface area of contact between the hot gas pipes and pipe  30 A. Insulation  64  is a layer of thermally insulating materials covering hot gas pipes  62 A- 62 D and pipe  30 A. Insulation  64  reduces heat loss from hot gas pipes  62 A- 62 D to the atmosphere so that more heat can be transferred to salt in pipe  30 A. 
     Shoe  65  is physically adjacent to hot gas pipe  62 A and to pipe  30 A for increasing heat conduction between the pipes. In the illustrated embodiment, shoe  65  is between portions of hot gas pipe  62 A and pipe  30 A, but a portion of hot gas pipe  62 A is also directly adjacent to pipe  30 A. In another embodiment, shoe  65  can be a larger cradle, physically separating hot gas pipe  62 A from pipe  30 A while still facilitation heat transfer. Shoe  65  can be made from stainless steel, copper, or other suitable heat conducting materials. Pipe heating zones  46 B- 46 D can also configured as described in this first embodiment. 
       FIG. 7B  is a sectional view of a second embodiment of pipe heating zone  46 A along section  7 B- 7 B of  FIG. 2 . The second embodiment is similar to the first embodiment except that hot gas pipes  62 A- 62 D are replaced with heating passage  66 . Heating passage  66  includes a passage outer wall  68  spaced concentrically with pipe  30 A. Catalyzed hot gas flows through annular region  70  between an outer surface of pipe  30 A and an inner surface of passage outer wall  68 . Pipe heating zones  46 B- 46 D can also configured as described in this second embodiment. 
       FIG. 7C  is a sectional view of a third embodiment of pipe heating zone  46 A along section  7 C- 7 C of  FIG. 2 . The third embodiment is similar to the first embodiment except that hot gas pipes  62 A- 62 D are omitted. Instead, catalyzed hot gas flows through pipe  30 A. The catalyzed hot gas heats salt in pipe  30 A through direct contact and is eventually vented to the atmosphere while the molten salt is retained in pipe  30 A. This method can benefit from using gases selected so as to avoid adversely reacting with the heat transfer medium. This method can also be used to heat pipe  30 A when it is empty, to control temperature changes during startup or shutdown procedures. The methods of heating pipe heating zone  46 A described with respect to  FIGS. 7A ,  7 B, and  7 C can also be used to heat pipe heating zones  46 B- 46 D. 
     Although the invention has been described using molten salt as the heat transfer medium, this invention is not limited to heating molten salt. The systems and methods describe above can be used to heat virtually any heat transfer media suitable for use in a concentrated solar power generation system. 
     It will be recognized that the present invention provides numerous benefits and advantages. For example, heating with catalyzed hot gas as in the current invention has a higher conversion efficiency (conversion of fuel to heat) than heating with electrical traces. This is because for electric heating, energy in the fuel must first be converted into electricity and then converted from electricity to heat. Catalyzed hot gas has one step of converting the fuel to heat. This increase in conversion efficiency can be a cost savings. 
     Additionally, heating with catalyzed hot gas can be relatively reliable. Electrical trace heating is typically more prone to failure than pipes and catalysts. Electrical traces can burn out or be stuck on. Furthermore, in the event of a loss of electrical power, a catalyzed hot gas heating system can continue to operate while an electrical trace heating system can fail. 
     Moreover, heating with catalyzed hot gas can be better for the environment. Electricity created by burning fossil fuels at high temperatures, for example, often creates various pollutants such as nitrogen oxide. Catalyzing hydrogen or methane can be a relatively clean combustion process, creating byproducts of mostly water and carbon dioxide. Because hydrogen and methane catalyze at a relatively low temperature, little or no nitrogen oxide is produced. 
     Although the present invention has been described with reference to particular embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention as claimed. For example, heating pipes with catalyzed hot gas as described above need not be limited to heating molten salt in a solar power generation system. These methods may be used to heat fluid pipes in other industrial process systems that are compatible with these methods.