Solar tower system containing molten chloride salts

Abstract

A solar tower system is disclosed in which the heat transfer media is a molten salt at a temperature greater than 650? C. 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 whose compositions are described herein. The molten salt preferably is MgCl.sub.2KCl.

Claims

1. An improved solar tower system of the type having absorption tubes, a storage tank and a heat exchanger all of which contain a molten salt heat transfer media having a temperature greater than 650? C. and the molten salt is in contact with interior surfaces of the absorption tubes, a storage tank and a heat exchanger, wherein the improvement comprises the molten salt being a chloride salt at a temperature greater than 650? C. up to 800? C., at least one of the absorption tubes, the storage tank and the heat exchanger being made of an alloy that contains in weight percent 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.

2. An improved solar tower system of claim 1 wherein at least one of the absorption tubes, the storage tank and the heat exchanger being made of an alloy that contains in weight percent 30% to 42% nickel, 20% to 32% chromium, at least one of 0.2% to 1.0% columbium, 0.2% to 4.0% tantalum, and 0.05% to 1.0% vanadium, up to 0.2% carbon, about 0.05% to 0.50% nitrogen, 0.001% to 0.02% boron, up to 0.2% titanium and the balance being iron plus impurities.

3. The improved solar tower system of claim 1 wherein at least one of the absorption tubes, the storage tank and the heat exchanger is made of an alloy that contains in weight percent about 37% nickel, about 25% chromium, about 3% cobalt, about 1% molybdenum, about 0.5% tungsten, about 0.7% columbium, about 0.7% manganese, about 0.6% silicon, about 0.2% nitrogen, about 0.1% aluminum, about 0.05% carbon, about 0.004% boron and the balance being iron plus impurities.

4. An improved solar tower system of claim 1 wherein absorption tubes, a storage tank and a heat exchanger contain a molten salt heat transfer media having a temperature greater than 650? C. and at least one of the absorption tubes, the storage tank and the heat exchanger have a corrosion rate <60 ?m at 850? C. in molten chloride salts without corrosion inhibitors.

5. The improved solar tower system of claim 4 wherein the alloy has corrosion rate <60 ?m at 850? C. in molten chloride salts with Mg as corrosion inhibitor.

6. The improved solar tower system of claim 4 wherein the alloy has corrosion rate <60 ?m at 850? C. in molten chloride salts with Zr as corrosion inhibitor.

7. An improved solar tower system of the type having absorption tubes, a storage tank and a heat exchanger all of which contain a molten salt heat transfer media having a temperature greater than 650? C. and the molten salt is in contact with interior surfaces of the absorption tubes, a storage tank and a heat exchanger, wherein the improvement comprises the molten salt being a chloride salt, at least one corrosion inhibitor in the molten salt or on at least one of the interior surfaces of the absorption tubes, the storage tank and the heat exchanger and at least one of the absorption tubes, the storage tank and the heat exchanger being made of an alloy that contains in weight percent 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.

8. An improved solar tower system of claim 7 wherein at least one of the absorption tubes, the storage tank and the heat exchanger is made of an alloy that contains in weight percent about 22% chromium, about 14% tungsten, about 2% molybdenum, up to 3% iron, up to 5% cobalt, about 0.5% manganese, about 0.4% silicon, up to 0.5% niobium, about 0.3% aluminum, up to 0.1% titanium, about 0.1% carbon, about 0.02% lanthanum, up to about 0.015% boron and the balance being nickel plus impurities.

9. The improved solar tower system of claim 8 wherein the molten salt heat transfer media has a temperature greater than 800? C.

10. An improved solar tower system of the type having absorption tubes, a storage tank and a heat exchanger all of which contain a molten salt heat transfer media having a temperature greater than 650? C. and the molten salt is in contact with interior surfaces of the absorption tubes, a storage tank and a heat exchanger, wherein the improvement comprises the molten salt being a chloride salt, at least one corrosion inhibitor in the molten salt or on at least one of the interior surfaces of the absorption tubes, the storage tank and the heat exchanger and at least one of the absorption tubes, the storage tank and the heat exchanger being made of an alloy that contains in weight percent 18% to 20% chromium, 18% to 20% cobalt, 3.0% to 3.5% aluminum, 7% to 8% molybdenum, 0.4% to 0.8% tantalum, 0.4% to 0.6% titanium, 0.1% to 0.4% manganese, up to 0.3% tungsten, up to 1.5% iron, 0.04 to 0.2% silicon, 0.08% to 0.12% carbon, up to 0.015% phosphorus, up to 0.015% sulfur, 0.002% to 0.006% boron, 0.001% to 0.025% yttrium, 0.01% to 0.05% zirconium and the balance being nickel plus impurities.

11. The improved solar tower system of claim 10 wherein the alloy contains in weight percent about 19% chromium, about 19% cobalt, about 3.25% aluminum, about 7.5% molybdenum, about 0.5% tantalum, about 0.56% titanium, about 0.2% manganese, about 0.05% tungsten, about 1.0% iron, about 0.14% silicon, about 0.10% carbon, less than 0.002% phosphorus, less than 0.002% sulfur, about 0.002% boron, about 0.007% yttrium, about 0.02% zirconium and the balance being nickel plus impurities.

12. The improved solar tower system of claim 10 wherein the molten salt heat transfer media has a temperature greater than 800? C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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.

(2) FIG. 2 is an isometric view of a typical molten salt, solar absorption panel.

(3) FIG. 3 is a block diagram of a heating system in which a solar tower system can be used.

(4) FIG. 4 is a graph of the corrosion rates for 230? alloy and 233? alloy tested in a NaClKClMgCl.sub.2 salt composition at 850? C. for 100 hours.

(5) 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 NaClKClMgCl.sub.2 salt composition at 850? C. for 100 hours.

(6) 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 NaClKClMgCl.sub.2 salt composition at 850? C. for 100 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) 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.

(8) 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.

(9) 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.

(10) 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.sub.2KCl molten salt. Other suitable salts may include halides composed of LiCl, NaCl, KCl, MgCl.sub.2 or CaCl.sub.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.sub.2 or CaBr.sub.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.sub.2 or CaX.sub.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.sub.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.

(11) 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.

(12) 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 NaClKClMgCl.sub.2 or in NaClKClMgCl.sub.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 NaClKClMgCl.sub.2 and three were tested in NaClKClMgCl.sub.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.

(13) TABLE-US-00001 TABLE 1 Test # Alloy Salt Composition Surface Treatment Inhibitor emp. (? C.) 2.1 Haynes 282 NaClKClMgCl.sub.2* 120 grit N/A 850 2.2 Haynes 244 NaClKClMgCl.sub.2* 120 grit N/A 850 2.3 Haynes 233 NaClKClMgCl.sub.2* 120 grit N/A 850 2.4 Haynes HR120 NaClKClMgCl.sub.2* 120 grit N/A 750 2.5 Haynes 282 NaClKClMgCl.sub.2* 120 grit 1.15 mol % Mg 850 2.6 Haynes 244 NaClKClMgCl.sub.2* 120 grit 1.15 mol % Mg 850 2.7 Haynes 233 NaClKClMgCl.sub.2* 120 grit 1.15 mol % Mg 850 2.8 Haynes HR120 NaClKClMgCl.sub.2* 120 grit 1.15 mol % Mg 750 2.9 Haynes 230 NaClKClMgCl.sub.2* 120 grit ZrCl.sub.4/ZrCl.sub.3 buffer 850 2.10 Haynes 230 NaClKClMgCl.sub.2* 120 grit 1.15 mol % MgZn** 850 2.11 Haynes 230 NaClKClMgCl.sub.2* Sputtered Zr N/A 850 2.12 Haynes 230 NaClKClMgCl.sub.2* Mg-based 11D N/A 850 2.13 Haynes 230 NaClKClMgCl.sub.2* Sputtered Zr 1.15 mol % Mg 850 2.14 Haynes 230 NaClKClMgCl.sub.2* Mg-based 11D 1.15 mol % Mg 850 2.15 Haynes 242 NaClKClMgCl.sub.2* 120 grit N/A 850 2.16 Haynes 242 NaClKClMgCl.sub.2* 120 grit 1.15 mol % Mg 850 2.17 Haynes 230 NaClKClMgCl.sub.2* MgO N/A 850 2.18 Haynes 230 NaClKClMgCl.sub.2* MgO 1.15 mol % Mg 850 2.19 Haynes 230 NaClKClMgCl.sub.2* aluminide 1P N/A 850 2.20 Haynes 230 NaClKClMgCl.sub.2* aluminide 1P 1.15 mol % Mg 850 2.21 Haynes 230 NaClKClMgCl.sub.2* aluminide 2D N/A 850 2.22 Haynes 230 NaClKClMgCl.sub.2* aluminide 2D 1.15 mol % Mg 850 *ICL Dehydrated Carnallite (300278-8-3), 1-6 wt % H.sub.2O **71 at % Mg - 29 at % Zn (m.p. 347? C.)

(14) 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 <15 microns/year in the presence of magnesium. Other reducing metals could be used in place of magnesium.

(15) 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.

(16) 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.

(17) 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.

(18) TABLE-US-00002 TABLE 2 Oxidation Resistance Metal Loss Avg. Met. Aff. Max. Met. Aff. Alloy mils (?m) mils, (?m) mils, (?m) 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.

(19) 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

(20) 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's cost is minimized by utilizing autogenously welded and bead worked tubes and the storage tank's cost is minimized with using HR-120? alloy explosion clad layer on lower cost stainless steel material.

(21) 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 KClNaClMgCl.sub.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).

(22) 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.

(23) 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.

(24) TABLE-US-00003 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 >3 up to 3.5

(25) 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.

(26) 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.