Heat transfer/storage fluids and systems that utilize such fluids

Abstract

Heat transfer/storage fluids that are resistant to oxidation in air at elevated temperatures, and systems that utilize such heat transfer/storage fluids, for example, as part of a concentrating solar power (CSP) system or other electricity-generating systems. The heat transfer/storage fluid is a molten chloride solution comprising two or more chlorides selected from the group consisting of CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, NaCl, and KCl.

Claims

1. A system comprising containment equipment and a heat transfer/storage fluid that is transported or stored within the containment equipment and exposed to air and temperatures of greater than 550° C. within the system, the heat transfer/storage fluid being a molten salt solution of calcium chloride, sodium chloride, and barium chloride or calcium chloride, sodium chloride, barium chloride, and potassium chloride, and wherein the molten salt solution has a liquidus temperature of about 450° C. or less.

2. The system of claim 1, wherein the system is a heat transfer system or a thermal energy storage system or part of an electricity-generating system and the heat transfer/storage fluid is a heat transfer fluid or a thermal energy storage fluid.

3. The system of claim 2, wherein the electricity-generating system is a concentrating solar power system or a nuclear power system or a fossil-fuel-based power system or a hydrothermal power system.

4. The system of claim 1, wherein the heat transfer/storage fluid is in contact with a containment material of the containment equipment, and the containment material is an iron-containing material or a nickel-containing material or a chromium-containing material or an aluminum-containing material or a silicon-containing material.

5. The system of claim 1, wherein the heat transfer/storage fluid is in contact with a containment material of the containment equipment, and the containment material is an oxide-bearing material, wherein the oxide-bearing material is selected from the group consisting of one or more of aluminum oxide, silicon oxide, chromium oxide, magnesium oxide, calcium oxide, nickel oxide, cobalt oxide, manganese oxide, titanium oxide, zirconium oxide, phosphorus oxide, and yttrium oxide.

6. The system of claim 1, wherein the heat transfer/storage fluid is in contact with a corrosion-resistant containment material of the containment equipment, and the containment material exhibits a recession rate of less than 30 microns per year while in contact with the heat transfer/storage fluid in air at a temperature of at least 550° C.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a graph depicting an X-ray diffraction pattern from a CaCl.sub.2—NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) salt solidified after exposure to air for 50 hours at 750° C.

(2) FIG. 2 is a graph depicting corrosion kinetics of a nickel specimen with a thickening NiO surface scale during exposure to a NiO-saturated CaCl.sub.2—NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) liquid in air at 750° C. Extrapolation yielded an annual Ni recession of about 13 micrometers.

DETAILED DESCRIPTION OF THE INVENTION

(3) The following describes heat transfer/storage fluids (as nonlimiting examples, heat transfer fluids and/or thermal energy storage fluids) that are resistant to oxidation in air at temperatures up to and exceeding 750° C., and further describes solid containment materials that are resistant to corrosion when contacted by such heat transfer/storage fluids in air at temperatures up to and exceeding 750° C. The heat transfer/storage fluids are molten chloride solutions comprising two or more chlorides selected from the group consisting of CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, NaCl, and KCl, and are intended to be capable for use in high-temperature heat transfer and thermal energy storage (TES) systems, and the solid containment materials are intended to be capable for use in thermal energy transport and TES equipment, such as, as nonlimiting examples, pipes, valves, pumps, and TES tanks of concentrating solar power (CSP) and other electricity-generating (nuclear power, fossil-fuel-based power, hydrothermal power) systems. As such, the heat transfer/storage fluids and containment materials are capable of use in robust CSP systems (and other electricity-generating systems) that may be air (leak) tolerant at temperatures exceeding 550° C., so as to reduce if not eliminate the need for costly hermetically-sealed thermal energy transport and TES equipment (as nonlimiting examples, pipes, valves, pumps, and tanks) and/or extensive molten salt chemical monitoring and/or gettering agents for removing oxygen-bearing species, hydroxyl-bearing species, and water from the molten salt.

(4) A key factor for determining the suitability of a heat transfer/storage fluid to meet the above criteria is the relative thermodynamic stability of the heat transfer/storage fluid to oxidation, which may be described for molten chloride solutions by the following reactions:
{MgCl.sub.2}+½O.sub.2(g)=MgO(s)+Cl.sub.2(g)  (1)
2{KCl}+½O.sub.2(g)=K.sub.2O(s)+Cl.sub.2(g)  (2)
{CaCl.sub.2}+½O.sub.2(g)=CaO(s)+Cl.sub.2(g)  (3)
2{NaCl}+½O.sub.2(g)=Na.sub.2O(s)+Cl.sub.2(g)  (4)
where { } refers to species dissolved in a chloride liquid solution. The standard Gibbs free energy change ΔG°.sub.rxn) at a temperature of 750° C. for reaction (1) is negative (−44.3 kJ/mol), whereas ΔG°.sub.rxn values at 750° C. for reactions (2), (3), and (4) are quite positive (+457.3, +143.5, +356.3 kJ/mol, respectively); that is, MgCl.sub.2 oxidation is much more strongly favored than the oxidation of CaCl.sub.2, KCl, and NaCl. Using activity data for MgCl.sub.2 in a MgCl.sub.2—KCl melt (32 mol % MgCl.sub.2, 68 mol % KCl; T.sub.eut=426° C.), and assuming unit activity for MgO(s), the equilibrium partial pressure ratio, p[Cl.sub.2]/(p[O.sub.2]).sup.1/2, for reaction (1) at 750° C. is found to be 3.0 (where p[Cl.sub.2] refers to the partial pressure of Cl.sub.2(g) and p[O.sub.2] refers to the partial pressure of O.sub.2(g)). Hence, reaction (1) will be favored at 750° C. (MgCl.sub.2 should react to form MgO) in air (p[O.sub.2]=0.21 atm) unless the chlorine partial pressure exceeds 1.4 atm. Using activity data for CaCl.sub.2 in a CaCl.sub.2—NaCl melt (53 mol % CaCl.sub.2, 47 mol % NaCl; T.sub.eut=504° C.) with unit activity for CaO(s), the equilibrium ratio, p[Cl.sub.2]/(p[O.sub.2]).sup.1/2, for reaction (3) at 750° C. is found to be 2.3×10.sup.−8. Hence, reaction (3) will not be favored in air once the local chlorine partial pressure exceeds 1×10.sup.−8 atm (10 ppb); that is, the extent of CaCl.sub.2 oxidation (to generate such an extremely low Cl.sub.2 partial pressure) in this CaCl.sub.2—NaCl melt in air at 750° C. should be negligible. Similar thermodynamic calculations indicate that KCl, NaCl, and BaCl.sub.2 are even less prone to oxidation. FIG. 1 is a graph depicting an X-ray diffraction pattern from a CaCl.sub.2-NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) salt solidified after exposure to air for 50 hours at 750° C. and confirms that, unlike molten MgCl.sub.2—KCl, the CaCl.sub.2—NaCl liquid is resistant to forming solid oxides when exposed to air at 750° C., which is consistent with the thermodynamic calculations above.

(5) Regarding salt evaporation rates, using available activity data for CaCl.sub.2—NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) and MgCl.sub.2—KCl (32 mol % MgCl.sub.2, 68 mol % KCl) melts, equilibrium vapor pressures of CaCl.sub.2 and NaCl over such a CaCl.sub.2—NaCl melt at 750° C. (2.3×10.sup.−8 and 3.4×10.sup.−5 atm, respectively) were calculated to be lower than the equilibrium vapor pressures of MgCl.sub.2 and KCl over such a MgCl.sub.2-KCl melt at 750° C. (7.3×10.sup.−6 atm and 6.6×10.sup.−5 atm, respectively). On this basis, it was concluded that the rate of evaporation of a CaCl.sub.2—NaCl-based melt at 750° C. will be lower than for a MgCl.sub.2—KCl—NaCl melt at 750° C. Because BaCl.sub.2(l) has a lower vapor pressure than CaCl.sub.2(l) at 750° C., the rate of evaporation of a BaCl.sub.2—NaCl-based melt at 750° C. will also be lower than for a MgCl.sub.2—KCl—NaCl melt at 750° C., and likely lower than the rate of evaporation of a CaCl.sub.2—NaCl-based melt at 750° C. On this basis, molten chloride solutions comprising two or more chlorides selected from the group consisting of CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, NaCl, and KCl, including but not limited to molten CaCl.sub.2-NaCl—BaCl.sub.2 and CaCl.sub.2—NaCl—BaCl.sub.2—KCl salt solutions, are concluded to be suitable for use in heat transfer and TES systems operating at high-temperatures, such as at least 550° C. and above.

(6) Other considerations for heat transfer/storage fluids that might be considered suitable for use in high-temperature heat transfer and TES systems include melting temperature and cost. Regarding salt melting points, CaCl.sub.2—NaCl—BaCl.sub.2 and CaCl.sub.2—NaCl—BaCl.sub.2—KCl salts have been reported with liquidus temperatures of 421 to 450° C., and molten chloride solutions comprising two or more of calcium chloride, strontium chloride, barium chloride, sodium chloride, and potassium chloride are believed to have liquidus temperatures of about 510° C. or less. Regarding cost, the cost per kWh of a CaCl.sub.2—NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) salt is much less expensive than a NaNO.sub.3-KNO.sub.3 (64 mol % NaNO.sub.3, 36 mol % KNO.sub.3) solar salt used at temperatures below 600° C., and it is anticipated that low-melting molten chloride solutions, comprising two or more chlorides selected from the group consisting of CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, NaCl, and KCl, will remain less expensive to use than NaNO.sub.3-KNO.sub.3 solar salt at operating temperatures up to and exceeding 550° C. Analyses based on commodity salt prices indicate that the cost per kWh of low-melting CaCl.sub.2—NaCl—BaCl.sub.2 and CaCl.sub.2—NaCl—BaCl.sub.2—KCl salts are within a factor of 1.5 of a MgCl.sub.2—KCl—NaCl (40 mol % MgCl.sub.2, 40 mol % KCl, 20 mol % NaCl) salt.

(7) Materials suitable for transporting and containing molten chloride solutions, comprising two or more chlorides selected from the group consisting of CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, NaCl, and KCl, should be resistant to oxidation and reactive dissolution in such salts at temperatures up to and exceeding 550° C. in air. Consider the following reactions:
NiO(s)+{CaCl.sub.2}={NiCl.sub.2}+CaO(s)  (5)
FeO(s)+{CaCl.sub.2}={FeCl.sub.2}+CaO(s)  (6)
⅓Cr.sub.2O.sub.3(s)+{CaCl.sub.2}=⅔{CrCl.sub.3}+CaO(s)  (7)
⅓Al.sub.2O.sub.3(s)+{CaCl.sub.2}=⅔{AlCl.sub.3}+CaO(s)  (8)
ΔG°.sub.rxn values for reactions (5), (6), (7), and (8) at 750° C. are +137.7 kJ/mol, +131.0 kJ/mol, +215.8 kJ/mol, and +258.9 kJ/mol respectively. Assuming unit activities for NiO(s), FeO(s), CaO(s), Cr.sub.2O.sub.3(s), and Al.sub.2O.sub.3(s), and using activity data for CaCl.sub.2 in a CaCl.sub.2—NaCl (53 mol % CaCl.sub.2, 47 mol % NaCl) melt, the calculated equilibrium (saturation) activities for {NiCl.sub.2}, {FeCl.sub.2}, {CrCl.sub.3}, and {AlCl.sub.3}, in this salt at 750° C. are only 4.4×10.sup.−8, 9.7×10.sup.−8, 9.7×10.sup.−18, and 4.9×10.sup.−21, respectively. Hence, after extremely limited amounts of reaction, FeO and/or NiO and/or Cr.sub.2O.sub.3 and/or Al.sub.2O.sub.3that form as a result of oxidation of an iron-containing or nickel-containing or chromium-containing or aluminum-containing alloy (as nonlimiting examples, a stainless steel or Ni-based superalloy) should be thermodynamically stable when contacted by a CaCl.sub.2—NaCl-based liquid at 750° C. while exposed to air (or another oxygen-containing gas). After this saturation point, it is expected that the corrosion of FeO-forming or NiO-forming or Cr.sub.2O.sub.3-forming or Al.sub.2O.sub.3-forming metals in such liquids should shift to slow growth of the FeO or NiO or Cr.sub.2O.sub.3 or Al.sub.2O.sub.3layers on the metal surfaces. FIG. 2 demonstrates a slow parabolic growth of a NiO scale on nickel during exposure to a NiO-saturated CaCl.sub.2—NaCl liquid in air at 750° C. On the basis of this data, the projected nickel recession rate is about 13 μm/year at 750° C.

(8) Other candidates for containment materials may form oxides or may contain or entirely consist of oxides, that are resistant to corrosion when contacted by a CaCl.sub.2—NaCl-bearing liquid at 750° C. while exposed to air (or another oxygen-containing gas). Nonlimiting examples include materials that, as a result of oxidation, bear oxides of one or more of silicon oxide, magnesium oxide, calcium oxide, cobalt oxide, manganese oxide, titanium oxide, zirconium oxide, phosphorus oxide, and yttrium oxide. Notable examples include silicon-containing materials that form silicon oxide when subjected to oxidation conditions. Nonlimiting examples also include ceramic materials that contain or entirely consist of of one or more of aluminum oxide, chromium oxide, silicon oxide, magnesium oxide, calcium oxide, nickel oxide, iron oxide, cobalt oxide, manganese oxide, titanium oxide, zirconium oxide, phosphorus oxide, and yttrium oxide. Notable examples include silicon-containing materials that form silicon oxide when subjected to oxidation conditions.

(9) From the above, it was concluded that CaCl.sub.2-NaCl-based liquids are suitable as air-stable, low-melting (e.g., liquidus temperatures of 510° C. or less, and in some cases 450° C. or less), and inexpensive heat transfer/storage fluids for use as heat transfer fluids and/or thermal energy storage fluids in heat transfer and TES systems, and their use in combination with corrosion-resistant materials used as transport and storage (containment) equipment provides for robust, air (leak)-tolerant CSP systems at temperatures of at least 550° C. The use of such heat transfer/storage fluids and corrosion-resistant materials can significantly simplify the design, operation, and cost of high-temperature CSP systems by avoiding the additional costs associated with hermetic sealing of pipes and TES tanks and with extensive monitoring of the molten salt chemistry and with active gettering of oxygen-bearing species, hydroxyl-bearing species, and water from the heat transfer/storage fluid as it flows and is held at high temperatures in pipes and TES tanks. Such air-stable, low-melting chloride-based fluids are therefore believed to be highly attractive as heat transfer fluids and as fluids for thermal energy storage for a wide variety of electricity-producing systems, including but not limited to fossil-energy-based power plants, nuclear power plants, hydrothermal power plants, and solar energy power plants.

(10) While the invention has been described in terms of particular embodiments and investigations, it should be apparent that alternatives could be adopted by one skilled in the art. For example, the heat transfer/storage fluids can be used in combination with various types of transport and containment equipment used in a wide variety of industries and applications, and appropriate materials could be substituted for those noted. As such, it should be understood that the above detailed description is intended to describe the particular embodiments and certain but not necessarily all features and aspects thereof, and to identify certain but not necessarily all alternatives to the embodiments and their described features and aspects. Accordingly, it should be understood that the invention is not necessarily limited to any embodiment described herein, and the phraseology and terminology employed above are for the purpose of describing the disclosed embodiments and investigations and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.