Patent Publication Number: US-2020291505-A1

Title: Solar tower system containing molten chloride salts

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/572,059 filed Oct. 13, 2017, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to solar cell towers which absorb heat from sunlight and transfer that heat for use in generating electricity using molten salt as the heat transport fluid. 
     2. Description of the Related Art 
     Surfaces of many materials will be heated when exposed to sunlight over a period of time. The art has developed systems to capture this heat for use in generating electricity or for heating buildings and other environments. One type of system known as a solar tower system has a series of heat absorption tubes or receivers that are exposed to sunlight and heated by that sunlight. The heat absorption tubes contain a heat transfer media which is directed from the heat absorption tubes to heat exchangers. There is a storage tank in the system which contains the heat transfer media. Molten sodium-potassium nitrate salt has been used as a heat transfer media in such solar tower systems. In those systems the sodium-potassium nitrate salt is heated to about 565° C. 
     U.S. Pat. No. 5,862,800 discloses a solar tower system which contains sodium-potassium nitrate salt at a temperature of about 565° C. The patent teaches that 625 alloy should be used in this system because that alloy when at a temperature of 605° C. has excellent resistance to corrosion from molten sodium-potassium nitrate salt, high resistance to chloride stress corrosion cracking due to either impurities in the molten salt or externally derived chlorides from the atmosphere or thermal insulation, a low coefficient of thermal expansion, good thermal conductivity, excellent creep and yield strengths and outstanding mechanical and thermal fatigue resistance. 
     304 and 316 austenitic stainless steels and Incoloy® 800 nickel-iron-chromium alloy have also been used for the receiver in the sodium-potassium nitrate salt solar tower systems. These alloys possesses high coefficients of thermal expansion, low yield and creep strengths, low thermal conductivities, low thermal fatigue properties but are susceptible to chloride stress corrosion cracking. 
     Alloys that are used in solar tower systems should be resistant to the molten salt&#39;s strong corrosion properties, resistant to chloride stress corrosion cracking, economically fabricated, weldable, acceptable to the ASME Boiler and Pressure Vessel Code and able to withstand the severe thermal strains caused by the through wall and across diameter temperature gradients. These strains, which are directly proportional to the material&#39;s thermal expansion coefficient, set the receiver&#39;s size by restricting the absorbed solar flux to a value determined by the material&#39;s allowable fatigue strain level for the imposed number of daily sun and cloud cover cycles over the receiver&#39;s lifetime. 
     There is currently a need for solar tower systems that can operate at higher temperatures from 650° C. up to as high as 1000° C. Such a system must have a salt media that is in a molten state at these high temperatures. The absorption tubes, heat exchangers and storage tanks in such a system must be made from a material, preferably a metal alloy, that is corrosion resistant to the molten salt at temperatures of between 650° C. and 1000° C. The alloys must also possesses high coefficients of thermal expansion, low yield and creep strengths, low thermal conductivities and low thermal fatigue at these high temperatures. 
     Although sodium-potassium nitrate salts have been used in solar tower systems operating at a temperature of about 565° C., these salts are not suitable for use at higher temperatures, particularly temperatures as high as 800° C. to 1000° C. For these applications one needs a salt that has a much higher freezing temperature than sodium-potassium nitrate salts. 
     While there are a number of known alloys which are sold for use in high temperature applications little is known about the corrosion resistance of these alloys when exposed to molten salt at higher temperatures from 650° C. up to as high as 1000° C. Although one skilled in the art might expect that any alloy which has been used in other high temperature applications of about 565° C. could be used in a molten salt solar tower system operating at temperatures of temperatures from 650° C. up to as high as 1000° C., we have found that this is not true. Many of them do not have both the corrosion resistance and the mechanical properties needed for molten salt solar tower system operating at temperatures from 650° C. up to as high as 1000° C. Only certain alloy compositions disclosed here are suitable for such systems. 
     SUMMARY OF THE INVENTION 
     We provide a solar tower system in which the heat transfer media is a molten salt at a temperature greater than 650° C. and the components that carry or hold the molten salt are made from commercially available alloys made by Haynes International and sold under the designations HR-120® alloy, 230® alloy and 233™ alloy. The nominal composition and compositions of alloys within the technical specifications for these Haynes alloys are provided below. These alloys have the desired corrosion resistance and the mechanical properties and can be used for some or all of these absorption tubes, heat exchanges and storage. Preferably the molten salt is MgCl 2 —KCl molten salt. 
     In alternative embodiments in which the molten salt is heated to a temperature above 800° C., HR-120 alloy is used only for the storage tank and 230 alloy or 233™ alloy is used for the receivers and other components that carry the molten salt. 
     The components that are made from 230® alloy or 233™ alloy could be coated with zirconium or magnesium to improve corrosion resistance. 
     We may add magnesium to the molten salt because magnesium will act as a corrosion inhibitor. Preferably 1.15 mol % magnesium is used. 
     Other objects and advantages of this solar cell system will become apparent from a description of certain present preferred embodiments shown in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a solar tower system known in the prior art which can be modified in accordance with the present invention to use molten salt at temperatures from 650° C. to as high as 800° C. to 1000° C. as the heat transfer media. 
         FIG. 2  is an isometric view of a typical molten salt, solar absorption panel. 
         FIG. 3  is a block diagram of a heating system in which a solar tower system can be used. 
         FIG. 4  is a graph of the corrosion rates for 230® alloy and 233™ alloy tested in a NaCl—KCl—MgCl 2  salt composition at 850° C. for 100 hours. 
         FIG. 5  is a graph, similar to  FIG. 4  of the corrosion rates for 230® alloy. HR-120® alloy, 244® alloy and 282® alloy tested in a NaCl—KCl—MgCl 2  salt composition at 850° C. for 100 hours. 
         FIG. 6  is a graph, similar to  FIG. 4  of the corrosion rates for 230® alloy, 233™ alloy and HR-120 alloy tested in a NaCl—KCl—MgCl 2  salt composition at 850° C. for 100 hours. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 and 3  a solar cell system of the type disclosed in U.S. Pat. No. 5,862,800 has a solar central cylindrical receiver  1  which is surrounded by a field of heliostats  2 . The receiver  1  is mounted on a tower  3  to provide the most efficient focal point height. The receiver  1  is made up of molten salt solar absorption panels  10 . The sun  50  provides solar rays  51  which shine on heliostats  2 . The solar rays  51  are reflected by the heliostats  2  to the solar central cylindrical receiver  1 . The molten salt solar absorption panels  10  are heated by the solar rays. The hot molten salt inside the panel tubes  4  transports the heat to heat exchangers which may use the thermal energy for process heat or to generate electricity. 
     A typical molten salt solar absorption panel  10  shown in  FIG. 2  has absorption tubes  4  which can be of seamless, welded or welded and drawn construction and headers  5 . The molten salt flow enters or exits the solar absorption panel  10  from or into conduits  9  through its headers  5 . In the embodiment shown in  FIG. 1  the receiver  1  is composed of multiple panels  10  arranged in two circuits, each with eight panels, having a serpentine flow path and forming a polyhedral, cylindrical surface. 
     In our solar tower system the molten salt heat transfer media is heated to a temperature greater than 650° C. up to as high as 1000° C. Referring to  FIG. 3  heated molten salt is conveyed from the absorption tubes  4  in the receivers  10  to heat exchangers  12  and then returned to the receivers  19  through conduits  9 . A storage tank  14  for the molten salt is provided in the system. 
     We have found that molten chloride salts are better candidates for use in molten salt solar tower systems that operate temperatures from 650° C. up to as high as 1000° C. In particular we prefer to provide MgCl 2 —KCl molten salt. Other suitable salts may include halides composed of LiCl, NaCl, KCl, MgCl 2  or CaCl 2 , as individual entities or as binary, ternary, quaternary or quinary mixtures, which are at least partially molten in the temperature range 300° C.-1000° C. One may also use molten halides composed of LiBr, NaBr, KBr, MgBr 2  or CaBr 12 , as individual entities or as binary, ternary, quaternary or quinary mixtures, which are at least partially molten in the temperature range 300° C.-1000° C. Another suitable salt may be molten halides composed of LiX, NaX, KX, MgX 2  or CaX 2  (where X can be Cl or Br), as individual entities or as mixtures, which are at least partially molten in the temperature range 300° C.-1000° C. Molten halides composed of LiF, NaF, KF or BeF 2 , as individual entities or as binary, ternary or quaternary mixtures, which are at least partially molten in the temperature range 300° C.-1000° C. may also be used. 
     The alloys that have been used in solar cells that operate at temperature below 600° C. do not have the corrosion resistance and the mechanical properties that are needed for absorption tubes, heat exchangers and storage tanks that contain molten chloride salts at temperatures from 650° C. up to as high as 1000° C. However, we have found that Haynes HR-120® alloy, 230® alloy and 233™ alloy have the desired corrosion resistance and the mechanical properties. They can be used for some or all of these absorption tubes, heat exchanges, conduits and storage tanks. 
     Corrosion tests were conducted on Haynes HR-120® alloy, 230® alloy, 233™ alloy, 244® alloy and 282® alloy to determine their suitability for use in our solar tower system. Three coupons of each of the alloys were tested for corrosion resistance in molten NaCl—KCl—MgCl 2  or in NaCl—KCl—MgCl 2  combined with 1.5 mol % magnesium which acts a corrosion inhibitor. 230® alloy, 233™ alloy, 244® alloy and 282® alloy were tested at 850° C. HR-120® alloy was tested at 750° C. Six coupons of 230® alloy were coated with zirconium and another six coupons of 230® alloy were coated with magnesium. Three of each of the coated coupons were tested in molten NaCl—KCl—MgCl 2  and three were tested in NaCl—KCl—MgCl 2  combined with 1.5 mol % magnesium. Table 1 lists each of the tests. The tests were repeated on HR-120® alloy and 230® alloy. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Test # 
                 Alloy 
                 Salt Composition 
                 Surface Treatment 
                 Inhibitor 
                 emp. (° C.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 2.1 
                 Haynes 282 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 N/A 
                 850 
               
               
                 2.2 
                 Haynes 244 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 N/A 
                 850 
               
               
                 2.3 
                 Haynes 233 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 N/A 
                 850 
               
               
                 2.4 
                 Haynes HR120 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 N/A 
                 750 
               
               
                 2.5 
                 Haynes 282 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % Mg 
                 850 
               
               
                 2.6 
                 Haynes 244 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % Mg 
                 850 
               
               
                 2.7 
                 Haynes 233 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % Mg 
                 850 
               
               
                 2.8 
                 Haynes HR120 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % Mg 
                 750 
               
               
                 2.9 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 ZrCl 4 /ZrCl 3  buffer 
                 850 
               
               
                 2.10 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % MgZn** 
                 850 
               
               
                 2.11 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 Sputtered Zr 
                 N/A 
                 850 
               
               
                 2.12 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 Mg-based 11D 
                 N/A 
                 850 
               
               
                 2.13 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 Sputtered Zr 
                 1.15 mol % Mg 
                 850 
               
               
                 2.14 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 Mg-based 11D 
                 1.15 mol % Mg 
                 850 
               
               
                 2.15 
                 Haynes 242 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 N/A 
                 850 
               
               
                 2.16 
                 Haynes 242 
                 NaCl—KCl—MgCl 2 * 
                 120 grit 
                 1.15 mol % Mg 
                 850 
               
               
                 2.17 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 MgO 
                 N/A 
                 850 
               
               
                 2.18 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 MgO 
                 1.15 mol % Mg 
                 850 
               
               
                 2.19 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 aluminide 1P 
                 N/A 
                 850 
               
               
                 2.20 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 aluminide 1P 
                 1.15 mol % Mg 
                 850 
               
               
                 2.21 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 aluminide 2D 
                 N/A 
                 850 
               
               
                 2.22 
                 Haynes 230 
                 NaCl—KCl—MgCl 2 * 
                 aluminide 2D 
                 1.15 mol % Mg 
                 850 
               
               
                   
               
               
                 *ICL Dehydrated Carnallite (300278-8-3), 1-6 wt % H 2 O 
               
               
                 **71 at % Mg - 29 at % Zn (m.p. 347° C.) 
               
            
           
         
       
     
     The results of the corrosion tests are reported in  FIGS. 4, 5 and 6 . The average of each set of the three coupons tested during the first tests is shown as a square. The average of each set of the three coupons tested during the second tests is shown as a diamond. The standard deviation for each test is shown by the whiskers extending from each point. The data shows that 233™ alloy, 230® alloy when used with a magnesium inhibitor or coated with zirconium and/or magnesium and HR-120® exhibit low corrosion rates (50-100 microns/year) at 850° C. The corrosion resistance of 233™ alloy and the corrosion resistance of HR-120® alloy were reduced to &lt;15 microns/year in the presence of magnesium. Other reducing metals could be used in place of magnesium. 
     Haynes 230® alloy can be used when coated with magnesium or when the molten salt contains magnesium. Only in the presence of an active reducing metal like magnesium could the corrosion rate be reduced to below 15 microns/year. 
     As molten chloride solar tower systems operate at higher operating temperatures than molten nitrate solar tower system, the oxidation properties of the alloys are equally important along with the corrosion and mechanical properties of the receiver tubes and tanks. The oxidation properties are required since the receiver tubes and tanks are exposed to air on the outside of the tubes and exterior sides of the tanks. As seen below the oxidation properties of these alloys are significantly better than currently used stainless steel tank material. 
     Oxidation data at 1800° F. in flowing air for 1008 h (cycled weekly) for HR-120® alloy, 230® alloy, 233™ alloy, Inconel 800HT®, 304 stainless steel and 316 stainless steel are given Table 2 below. Accordingly to the manufacturer Alloys 800, 800H and 800HT have the same nickel, chromium and iron contents and generally display similar corrosion resistance. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Oxidation Resistance 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Metal Loss 
                 Avg. Met. Aff. 
                 Max. Met. Aff. 
               
               
                   
                 Alloy 
                   
                 mils (μm) 
                 mils, (μm) 
                 mils, (μm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 233 
                 0.0 
                 (0) 
                 0.4 
                 (8) 
                 0.5 
                 (13) 
               
               
                   
                 230 
                 0.2 
                 (5) 
                 1.5 
                 (38) 
                 1.8 
                 (46) 
               
               
                   
                 HR-120 
                 0.4 
                 (10) 
                 2.1 
                 (53) 
                 2.7 
                 (69) 
               
               
                   
                 800HT 
                 0.5 
                 (13) 
                 4.1 
                 (104) 
                 4.7 
                 (119) 
               
               
                   
                 304SS 
                 5.5 
                 (140) 
                 8.1 
                 (206) 
                 9.5 
                 (241) 
               
               
                   
                 316SS 
                 12.3 
                 (312) 
                 14.2 
                 (361) 
                 14.8 
                 (376) 
               
               
                   
                   
               
               
                   
                 Metal Loss = (A-B)/2 
               
               
                   
                 Avg. Internal Penetration = C 
               
               
                   
                 Max. Internal Penetration = D 
               
               
                   
                 Avg. Metal Affected = Metal Loss + Avg. Internal Pen. 
               
               
                   
                 Max. Metal Affected = Metal Loss + Max. Internal Pen. 
               
            
           
         
       
     
     230® alloy, 233™ alloy, and HR-120® alloy also have the desired mechanical properties for use in absorption tubes, heat exchangers and storage tanks that contain molten chloride salts at temperatures from 650° C. up to as high as 1000° C. These properties are:
         Creep Rupture Strength (1700° F./10 ksi)—Transverse
           233™ Alloy=523 hours   230® Alloy=121 hours   HR-120® Alloy=25 hours   
           Creep Rupture Strength @ 1400° F./15 ksi (Plate/bar)
           230® Alloy=8200 hours   HR-120® Alloy=200 hours   304 stainless steel=10 hours   316 stainless steel=100 hours   
           (RT %) Thermal Stability of Alloys 1000 hours/1400° F.
           230® Alloy=33%   HR-120® Alloy=24%   233™ Alloy=16.5%   
           LCF Properties of Alloys (Cycles to Failure)
           760° C./Strain Range=1%; R=−1.0
               HR-120® Alloy=2220   230® Alloy=1097   
               870° C./Strain Range=1%; R=−1.0   HR-120® Alloy=1284   230® Alloy=228   
               

     230® alloy and 233™ alloy retain their mechanical properties over the working range of 350-1000° C. when in contact with molten chlorides while HR-120 alloy retains mechanical properties over the working temperature range of 350-800° C. All three alloys can be used as storage tank material. Since the storage tank operates at lower temperatures than the receiver tubes, the use of low cost HR120® alloy as construction material for tank with adequate strength optimizes the capital cost of the plant. For concentrating solar plants operating up to 800° C., HR-120°, 230®, and 233™ alloys can also be used for all components that carry or hold the molten salt. HR120® alloy should only be used as material of construction for thermal storage tanks in concentrating solar power plants operating above 800° C. The receiver&#39;s cost is minimized by utilizing autogenously welded and bead worked tubes and the storage tank&#39;s cost is minimized with using HR-120 alloy explosion clad layer on lower cost stainless steel material. 
     It is therefore surprising that 233™ alloy and HR-120® alloy whose composition is not very different to that of the commercial alloys mentioned above, gave corrosion rates in molten KCl—NaCl—MgCl 2  about 10 times lower than that of Haynes H-230® used without magnesium as a coating or in the molten salt and about 30-40 times lower than that observed for Haynes NS-163® alloy and Incoloy® 800H alloy. Specifically, 233™ alloy and HR-120® alloy showed corrosion of 50-60 microns/year instead of 500-700 microns/year for 230® alloy and 2000-3000 microns/year for NS-163® alloy and Incoloy® 800H alloy (all tested for 100 hours at 850° C., static conditions). In the presence of Mg, both 233™ alloy and HR-120® alloy also demonstrated very low corrosion (NMT 10 microns/year). 
     The nominal composition of Haynes 230 alloy is 22% chromium, 14% tungsten, 2% molybdenum, 5% or less cobalt, 3% or less iron, 0.5% manganese, 0.4% silicon, 0.5% or less niobium, 0.3% aluminum, 0.1% titanium, 0.1% carbon, 0.015% or less boron, 0.02% lanthanum, the balance 57% being nickel plus impurities. The 230® alloy coupons tested had this composition. Alloy compositions that contain elements within the following ranges in weight percent are expected to have the same properties described herein for 230® alloy: 20% to 24% chromium, 13% to 15% tungsten, 1% to 3% molybdenum, up to 3% iron, up to 5% cobalt, 0.3% to 1.0% manganese, 0.25 to 0.75% silicon, 0.2 to 0.5% aluminum, 0.5% to 0.15% carbon, 0.005% to 0.05% lanthanum, up to 0.1% titanium, up to 0.5% niobium, up to 0.015% boron, up to 0.03% phosphorous, up to 0.015% sulfur and the balance being nickel plus impurities. 
     European Patent No. EP 2 971 205 B1 covers and contains technical information about Haynes 233™ alloy. The nominal composition of this alloy is 19% chromium, 19% cobalt, 7.5% molybdenum, 0.5% titanium, 3.3% aluminum, 1.5%, or less iron, 0.4% or less manganese, 0.20% or less silicon, 0.10% carbon, 0.004% boron, 0.5% lanthanum. 0.3% or less tungsten, 0.025% or less vanadium, 0.3% zirconium, the balance 48% being nickel plus impurities. The 233™ alloy coupons tested had this composition. The patent teaches that the composition of alloys which have been discovered to possess the properties of 233™ alloy may contain: 15 to 20 wt. % chromium (Cr), 9.5 to 20 wt. % cobalt (Co), 7.25 to 10 wt. % molybdenum (Mo), 2.72 to 3.89 wt. % aluminum (Al), silicon (Si) present up to 0.6 wt. %, and carbon (C) present up to 0.15 wt. %. Titanium is present at a minimum level of 0.02 wt. %, but a level greater than 0.2% is preferred. Niobium (Nb) may be also present to provide strengthening, but is not necessary to achieve the desired properties. An overabundance of Ti and/or Nb may increase the propensity of an alloy for strain-age cracking. Titanium should be limited to no more than 0.75 wt. %, and niobium to no more than 1 wt. %. The broadest range, intermediates range and narrow range for the major elements for alloys having the properties of 233™ alloy are listed in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 233 ™ Alloy Major Element Ranges (in wt. %) 
               
            
           
           
               
               
               
               
            
               
                 Element 
                 Broad range 
                 Intermediate range 
                 Narrow range 
               
               
                   
               
               
                 Ni 
                 Balance 
                 Balance 
                 Balance 
               
               
                 Cr 
                 15 to 20 
                 16 to 20 
                 18 to 20 
               
               
                 Co 
                 9.5 to 20 
                 15 to 20 
                 18 to 20 
               
               
                 Mo 
                 7.25 to 10 
                 7.25 to 9.75 
                 7.25 to 8.25 
               
               
                 Al 
                 2.72 to 3.89 
                 2.9 to 3.7 
                 &gt;3 up to 3.5 
               
               
                   
               
            
           
         
       
     
     Haynes HR-120 alloy is the commercial version of the alloy compositions disclosed in U.S. Pat. No. 4,981,647. This is an iron-nickel-chromium alloy having a nominal composition in weight percent of 33% iron, 37% nickel, 25% chromium, 3% or less cobalt, 1% or less molybdenum, 0.5 or less tungsten, 0.7% manganese, 0.6% silicon, 0.7% columbium, 0.1% aluminum, 0.05% carbon, 0.02% nitrogen, 0.004% boron, 0.5% or less copper and 0.2% or less titanium. The patent for this alloy teaches that a composition falling within these ranges in weight percent will have the desired properties: 25% to 45% nickel, 12% to 32% chromium, 0.1% to 2.0% columbium, up to 4.0% tantalum, up to 1.0% vanadium, up to 2.0% manganese, up to 1.0% aluminum, up to 5% molybdenum, up to 5% tungsten, up to 0.2% titanium, up to 2% zirconium, up to 5% cobalt, up to 0.1% yttrium, up to 0.1% lanthanum, up to 0.1% cesium, up to 0.1% other rare earth metals, up to about 0.20% carbon, up to 3% silicon, about 0.05% to 0.50% nitrogen, up to 0.02% boron and the balance being iron plus impurities. 
     Although we have shown and described present preferred embodiments of our solar tower system, it should be distinctly understood that our invention is not limited thereto but may be variously embodied within the scope of the following claims.