AU2013304154A1

AU2013304154A1 - Method for improving nitrate salt compositions used as heat transfer medium or heat storage medium

Info

Publication number: AU2013304154A1
Application number: AU2013304154A
Authority: AU
Inventors: Katharina Federsel; Florian Garlichs; Michael LADENBERGER; Michael Lutz; Stephan Maurer; Kerstin Schierle-Arndt; Johan Ter Maat; Jurgen Wortmann
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2012-08-17
Filing date: 2013-08-09
Publication date: 2015-03-05
Also published as: CN104583358A; IL237047A0; AU2013304154A8; CL2015000387A1; EP2885369A1; MA37935A1; ZA201501768B; WO2014026915A1

Abstract

Disclosed is a method for maintaining or extending the long-term operating temperature range of a heat transfer medium and/or heat storage medium containing a nitrate salt composition selected from among the group consisting of alkali metal nitrate and alkaline earth metal nitrate and, optionally, alkali metal nitrite and alkaline earth metal nitrite. Said method is characterized in that the entire nitrate salt composition or a portion thereof is brought in contact with an additive composed of a combination of elemental oxygen and nitrogen oxides.

Description

I Method for improving nitrate salt compositions used as heat transfer medium or heat storage medium Description 5 The present invention relates to a method of maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium as defined in the claims, a corresponding process system as defined in the claims, the use of an additive for maintaining or widening the long-term operating temperature range of a heat transfer medium 0 and/or heat storage medium as defined in the claims and also a method of generating electric energy in a solar thermal power station as defined in the claims. Heat transfer media or heat storage media based on inorganic solids, in particular salts, are known both in chemical technology and in power station technology. They are generally used at 5 high temperatures, for example above 1000C, thus above the boiling point of water at atmospheric pressure. For example, salt bath reactors are used at temperatures of from about 200 to 500*C in chemical plants for the industrial production of various chemicals. 0 Heat transfer media are media which are heated by an energy source, for example the sun in solar thermal power stations, and transport the heat comprised therein over a particular distance. They can then transfer this heat to another medium, for example water or a gas, preferably via heat exchangers, with this other medium then being able, for example, to drive a 5 turbine. Heat transfer media can also be used in chemical process technology to heat or cool reactors (for example salt bath reactors) to the desired temperature. However, heat transfer media can also transfer the heat comprised therein to another medium (for example a salt melt) present in a reservoir and thus pass on the heat for storage. However, 0 heat transfer media can themselves also be introduced into a reservoir and remain there. They are then themselves both heat transfer media and heat storage media. Heat stores comprise heat storage media, usually materials compositions, for example the mixtures according to the invention, which can store heat for a particular time. Heat stores for 5 fluid, preferably liquid, heat storage media are usually formed by a solid vessel which is preferably insulated against loss of heat. A still relatively recent field of application for heat transfer media or heat storage media are solar thermal power stations for generating electric energy. 0 An example of a solar thermal power station is shown schematically in figure 1.

In figure 1, the numerals have the following meanings: I Incoming solar radiation 2 Receiver 5 3 Stream of a heated heat transfer medium 4 Stream of a cold heat transfer medium 5a Hot part of a heat storage system 5b Cold part of a heat storage system 6 Stream of a hot heat transfer medium from the heat storage system 0 7 Stream of a cooled heat transfer medium into the heat storage system 8 Heat exchanger (heat transfer medium/steam) 9 Steam stream 10 Condensate stream 11 Turbine with generator and cooling system 5 12 Current of electric energy 13 Waste heat In a solar thermal power station, focused solar radiation (1) heats a heat transfer medium, usually in a receiver system (2) which usually comprises a combination of tubular "receivers". 0 The heat transfer medium generally flows, usually driven by pumps, firstly into a heat storage system (5a), flows from there via line (6) on to a heat exchanger (8) where it gives off its heat to water and thus generates steam (9) which drives a turbine (11) which finally, as in a conventional electric power station, drives a generator for generating electric energy. In the generation of electric energy (12), the steam loses heat (13) and then generally flows back as 5 condensate (10) into the heat exchanger (8). The cooled heat transfer medium generally flows from the heat exchanger (8) back via the cold region (5b) of a heat storage system to the receiver system (2) in which it is reheated by solar radiation and a circuit is formed. The storage system can comprise a hot tank (5a) and a cold tank (5b), for example as two 0 separate vessels. An alternative construction of a suitable storage system is, for example, a layer store having a hot region (Sa) and a cold region (5b), for example in a vessel. Further details regarding solar thermal power stations are described, for example, in Bild der 5 Wissenschaft, 3, 2009, pages 82 to 99, and also below. Three types of solar thermal power stations are particularly important at present: the parabolic trough power station, the Fresnel power station and the tower power station. 0 In the parabolic trough power station, the solar radiation is focused via parabolic mirror troughs on the focal line of the mirrors. There, there is a tube (usually referred to as "receiver") filled with a heat transfer medium. The heat transfer medium is heated by the solar radiation and flows to 3 the heat exchanger where, as described above, it transfers its heat for steam generation. The parabolic trough-tube system can reach a length of over 100 kilometers in present-day solar thermal power stations. 5 In the Fresnel power station, the solar radiation is focused onto a focal line by generally flat mirrors. At the focal line there is a tube (usually referred to as "receiver") through which a heat transfer medium flows. In contrast to the parabolic trough power station, the mirror and the tube are not moved together to follow the position of the sun, but instead the setting of the mirrors is offered relative to the fixed tube. The setting of the mirrors follows the position of the sun so that 0 the fixed tube is always located on the focal line of the mirrors. In Fresnel power stations, too, molten salt can be used as heat transfer medium. Salt Fresnel power stations are at present largely still in development. Steam generation or the generation of electric energy in the salt Fresnel power station occurs in a manner analogous to the parabolic trough power station. 5 In the case of the solar thermal tower power station (hereinafter also referred to as tower power station), a tower is encircled by mirrors, in the technical field also referred to as "heliostats", which radiate the solar radiation in a focused manner onto a central receiver in the upper part of the tower. In the receiver, which is usually made up of bundles of tubes, a heat transfer medium is heated and this produces, via heat exchangers, steam for generating electric energy in a .0 manner analogous to the parabolic trough power station or Fresnel power station. Heat transfer media or heat storage media based on inorganic salts have been known for a long time. They are usually used at high temperatures at which water is gaseous, i.e. usually at 100*C and more. Known heat transfer media or heat storage media which can be used at relatively high temperatures are compositions comprising alkali metal nitrates and/or alkaline earth metal nitrates, optionally in admixture with alkali metal nitrites and/or alkaline earth metal nitrites. 0 Examples are the products of Coastal Chemical Company LLC Hitec @ Solar Salt (potassium nitrate: sodium nitrate 40% by weight: 60% by weight), Hitec @ (eutectic mixture of potassium nitrate, sodium nitrate and sodium nitrite). The nitrate salt mixtures or the mixtures of nitrate and nitrite salts can be used at relatively high 5 long-term operating temperatures without decomposing. In principle, such mixtures which have a relatively low melting point or relatively high decomposition temperatures can be produced by the combination of nitrate salts, usually those of the alkali metals lithium, sodium, potassium, optionally together with nitrite salts, usually 0 those of the alkali metals lithium, sodium, potassium or of the alkaline earth metal calcium. In the following, the term alkali metal refers to lithium, sodium, potassium, rubidium, cesium, 4 preferably lithium, sodium, potassium, particularly preferably sodium, potassium, unless expressly indicated otherwise. In the following, the term alkaline earth metal refers to beryllium, magnesium, calcium, 5 strontium, barium, preferably calcium, strontium, barium, particularly preferably calcium and barium, unless expressly indicated otherwise. It is still an objective to develop a heat transfer medium or heat storage medium which becomes solid (solidifies) at a relatively low temperature, thus has a low melting point, but has a high 10 maximum long-term operating temperature (analogous to a high decomposition temperature). For the present purposes, the maximum long-term operating temperature is the highest operating temperature for the heat transfer medium or heat storage medium at which the properties of the medium, for example viscosity, melting point, corrosion behavior, do not I5 change significantly compared to the initial value over a long period of time, in general from 10 to 30 years. Preference is given to using mixtures of sodium nitrate or potassium nitrate at relatively high temperatures. A routine long-term operating temperature range is from 290 to 565*C. Such 20 mixtures have a relatively high melting point. Mixtures of alkali metal nitrate and alkali metal nitrite usually have a lower melting point than the abovementioned nitrate mixtures, but also a lower decomposition temperature. Mixtures of alkali metal nitrate and alkali metal nitrite are usually employed in the temperature range from 150*C ?5 to 450 0 C. However, it is desirable, in particular for use in power stations for generating electric energy, e.g. solar thermal power stations, to increase the temperature of the heat transfer medium to far above 400*C, for example to far above 500*C, on arrival in the heat exchanger of the steam 0 generator (known as steam inlet temperature) since the efficiency of the steam turbine is then increased. It is thus desirable to increase the thermal stability of heat transfer media in long-term operation to, for example, more than about 565"C. '5 The chemical and physical properties of nitrate salt mixtures and nitrate/nitrite salt mixtures and thus, for example, their long-term operating temperature range in solar thermal power stations can change in an adverse manner in a number of ways. 0 For example, when the abovementioned mixtures are subjected, in particular over a prolonged period of time, to comparatively high temperatures, for example above 565*C in the case of nitrate salt mixtures and above 450*C in the case of nitrate/nitrite salt mixtures, they generally 5 decompose into various degradation products. This generally results in a decrease in the maximum long-term operating temperatures to below an economically and/or technically acceptable value and/or an increase in the melting point to 5 above an economically and/or technically acceptable value. Furthermore, the decomposition of the mixtures mentioned usually also results in an increase in their corrosiveness. Furthermore, the chemical and physical properties of nitrate salt mixtures and nitrate/nitrite salt mixtures and thus, for example, their long-term operating temperature range in solar thermal 0 power stations can change in an adverse manner as a result of uptake of traces or even relatively large amounts of water or carbon dioxide, for example due to a leak in the heat transfer medium/steam heat exchanger or as a result of open operation in which the heat transfer media or heat storage media are in contact with the atmospheric moisture of the outside air. 5 The properties of the nitrate salt mixtures or nitrate/nitrite salt mixtures can in this way deteriorate to such an extent that they become unsuitable as heat transfer medium or heat storage medium and generally have to be replaced by fresh mixtures, which in the case of the huge amounts comprised in, for example, the piping and storage system of a solar thermal 0 power station having multihour thermal stores is technically and economically disadvantageous or virtually impossible. It was an object of the present invention to discover a method which avoids or reverses the deterioration of a heat transfer medium or heat storage mediums based on a nitrate salt mixture 5 or a nitrate/nitrite salt mixture or widens the long-term operating temperature range of such mixtures. A further object of the present invention was to discover a method which makes a nitrite salt comprising heat transfer medium or heat storage medium suitable for higher long-term 0 operating temperatures. We have accordingly found the method, process system, use and method of generating electric energy defined in the claims. 5 For rationality reasons, the nitrate salt compositions defined in the description and in the claims, in particular their preferred and particularly preferred embodiment, will hereinafter also be referred to as "nitrate salt compositions of the invention/according to the invention". The nitrate salt composition of the invention is selected from the group consisting of alkali metal 0 nitrate and alkaline earth metal nitrate and optionally alkali metal nitrite and alkaline earth metal nitrite.

6 A very useful embodiment of the nitrate salt composition of the invention comprises, as significant constituents, an alkali metal nitrate or an alkaline earth metal nitrate or a mixture of alkali metal nitrate and alkaline earth metal nitrate and in each case optionally an alkali metal nitrite or alkaline earth metal nitrite. 5 The alkali metal nitrate here is a nitrate of the metals lithium, sodium, potassium, rubidium or cesium, preferably lithium, sodium, potassium, particularly preferably sodium, potassium, generally described as MetNO 3 , where Met represents the above-described alkali metals, which is preferably virtually water-free, particularly preferably free of water of crystallization, where the 10 term alkali metal nitrate encompasses both a single nitrate and mixtures of the nitrates of these metals, for example potassium nitrate plus sodium nitrate. The alkaline earth metal nitrate here is a nitrate of the metals magnesium, calcium, strontium, barium, preferably calcium, strontium, barium, particularly preferably calcium and barium, 5 generally described as Met(NO3)2, where Met represents the above-described alkaline earth metals, which is preferably virtually water-free, particularly preferably free of water of crystallization, where the term alkaline earth metal nitrate encompasses both a single nitrate and mixtures of the nitrates of these metals, for example calcium nitrate plus magnesium nitrate. .0 The alkali metal nitrite here is a nitrite of the alkali metals lithium, sodium, potassium, rubidium and cesium, preferably lithium, sodium, potassium, particularly preferably sodium, potassium, generally described as MetNO 2 , where Met represents the above-described alkali metals, which is preferably virtually water-free, particularly preferably free of water of crystallization. The alkali 25 metal nitrite can be present as a single compound or as a mixture of various alkali metal nitrites, for example sodium nitrite plus potassium nitrite. The alkaline earth metal nitrite here is a nitrite of the metals magnesium, calcium, strontium, barium, preferably calcium, strontium, barium, particularly preferably calcium and barium, 0 generally described as Met(N0 2

)

2 , where Met represents the above-described alkaline earth metals, which is preferably virtually water-free, particularly preferably free of water of crystallization, where the term alkaline earth metal nitrite encompasses both a single nitrite and mixtures of the nitrites of these metals, for example calcium nitrite plus magnesium nitrite. 5 Preference is given to the following nitrate salt compositions according to the invention: nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate and/or alkaline earth metal nitrate and in each case optionally an alkali metal nitrite and/or alkaline earth metal nitrite; 0 nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate selected from among sodium nitrate and potassium nitrate and in each case 7 optionally an alkali metal nitrite and/or alkaline earth metal nitrite; nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate and optionally an alkali metal nitrite; 5 nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate and optionally an alkali metal nitrite selected from among sodium nitrite and potassium nitrite; 0 nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate selected from among sodium nitrate and potassium nitrate and in each case optionally an alkali metal nitrite selected from among sodium nitrite and potassium nitrite and/or an alkaline earth metal nitrite selected from among calcium nitrite and barium nitrite; 5 nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate and/or alkaline earth metal nitrate; nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate selected from among sodium nitrate and potassium nitrate and/or an alkaline 0 earth metal nitrate selected from among calcium nitrate and barium nitrate; nitrate salt composition according to the invention comprising, as significant constituent, an alkali metal nitrate; 5 nitrate salt composition according to the invention comprising, as significant constituents, an alkali metal nitrate selected from among sodium nitrate and potassium nitrate. Further very useful nitrate salt compositions according to the invention comprising, as significant constituents, an alkali metal nitrate selected from among sodium nitrate and potassium nitrate 0 are, for example, the following: potassium nitrate in an amount in the range from 20 to 55% by weight and sodium nitrate in an amount in the range from 45 to 80% by weight, in each case based on the mixture; 5 potassium nitrate in an amount in the range from 35 to 45% by weight, preferably 40% by weight, and sodium nitrate in an amount in the range from 55 to 65% by weight, preferably 60% by weight, in each case based on the mixture. 0 Further very useful nitrate salt compositions according to the invention comprising, as significant constituents, an alkali metal nitrate and optionally an alkali metal nitrite selected from among sodium nitrite and potassium nitrite are, for example, the following: 8 potassium nitrate in an amount in the range from 30 to 70% by weight, preferably from 50 to 60% by weight, and sodium nitrate in an amount in the range from 3 to 30% by weight, preferably from 5 to 10% by weight, and sodium nitrite in an amount in the range from 20 to 5 60% by weight, preferably from 35 to 45% by weight, in each case based on the mixture. A mixture of potassium nitrate, sodium nitrate and sodium nitrite is also commercially available as the product Hitec @from Coastal Chemical Company LLC. 0 Further very useful nitrate salt compositions according to the invention comprising, as essential constituents, an alkali metal nitrate and optionally an alkaline earth metal nitrate are, for example, the following: potassium nitrate in an amount in the range from 30 to 50% by weight, preferably from 35 to 5 45% by weight, and sodium nitrate in an amount in the range from 5 to 30% by weight, preferably from 10 to 20% by weight, and calcium nitrate in an amount in the range from 20 to 63% by weight, preferably from 35 to 45% by weight, in each case based on the mixture. Apart from the abovementioned significant components, the nitrate salt composition of the 0 invention can comprise traces of further constituents, for example oxides, chlorides, sulfates, carbonates, hydroxides, silicates of the alkali metals and/or alkaline earth metals, silicon dioxide, iron oxide, aluminum oxide or water. The sum of these constituents is generally not more than 1% by weight, based on the nitrate salt composition of the invention. 5 The sum of all constituents of the nitrate salt composition of the invention is in each case 100% by weight. The nitrate salt composition of the invention goes over into the molten and usually pumpable form at a temperature above about 100-300*C, depending, inter alia, on the nitrite content and 0 the ratio of the cations forming the mixture. The nitrate salt composition of the invention, preferably in molten form, for example as pumpable liquid, is used as heat transfer medium and/or heat storage medium, preferably in power stations for generating heat and/or electric energy, in chemical process technology, for 5 example in salt bath reactors, and in metal hardening plants. Examples of power stations for generating heat and/or electric energy are solar thermal power stations such as parabolic trough power stations, Fresnel power stations, tower power stations. 0 In a very useful embodiment, the nitrate salt compositions of the invention, preferably in the molten state, for example as pumpable liquid, are used both as heat transfer medium and as heat storage medium in the solar thermal power stations, for example in parabolic trough power 9 stations, tower power stations or Fresnel power stations. In a further very useful embodiment, the nitrate salt compositions of the invention, preferably in the molten state, for example as pumpable liquid, are used either as heat transfer medium or as 5 heat storage medium in the solar thermal power stations, for example parabolic trough power stations, tower power stations, Fresnel power stations. For example, the nitrate salt compositions of the invention, preferably in the molten state, for example as pumpable liquid, are used in tower power stations as heat transfer medium and/or 0 as heat storage medium, particularly preferably as heat transfer medium. When the nitrate salt compositions of the invention, preferably in the molten state, for example as pumpable liquid, are used as heat transfer medium in solar thermal power stations, for example parabolic trough power stations, tower power stations, Fresnel power stations, the heat 5 transfer media are passed through tubes heated by solar radiation. They usually convey the heat arising there to a heat store or to the heat exchanger of the steam heater of a power station. The heat store comprises, in one variant, a plurality of, usually two, large vessels, generally a 0 cold vessel and a hot vessel (also referred to as "two-tank store"). The inventive nitrate salt composition, preferably in the molten state, for example as pumpable liquid, is usually taken from the cold vessel of the solar plant and heated in the solar field of a parabolic trough plant or a tower receiver. The hot molten salt mixture which has been heated in this way is usually introduced into the heated vessel and stored there until there is demand for generating electric 5 energy. Another variant of a heat store of the "thermoclinic store" comprises a tank in which the heat storage medium is stored in layers at different temperatures. This variant is also referred to as "layer store". When storage is carried out, material is taken from the cold region of the store. 0 The material is heated and fed back into the hot region of the store for storage. The thermoclinic store is thus used in a manner largely analogous to a two-tank store. The hot nitrate salt compositions of the invention in the molten state, for example as pumpable liquid, is usually taken from the hot tank or the hot region of the layer store and pumped to the 5 steam generator of a steam power station. The steam produced there, which is at a pressure of above 100 bar, generally drives a turbine and a generator feeds electric energy to the electricity grid. At the heat exchanger (salt/steam), the nitrate salt composition of the invention in the molten 0 state, for example as pumpable liquid, is generally cooled to about 290*C and usually conveyed back into the cold tank or the cold part of the layer store. When heat is transferred from the tubes heated by solar radiation to the store or to the steam generator, the nitrate salt 10 composition of the invention in the molten form acts as heat transfer medium. Introduced into the heat storage vessel, the same nitrate salt composition of the invention acts as heat storage medium, for example to make it possible for electric energy to be generated according to demand. 5 However, the nitrate salt composition of the invention, preferably in molten form, is also used as heat transfer medium and/or heat storage medium, preferably heat transfer medium, in chemical process technology, for example for heating reaction apparatuses of chemical production plants, where a very high heat flow generally has to be transferred at very high 0 temperatures with a small range of variation. Examples are salt bath reactors. Examples of the production plants mentioned are acrylic acid plants or plants for producing melamine. The nitrate salt composition of the invention is brought into contact with an additive. 5 The nitrate salt composition of the invention is here generally present in liquid, pumpable, in general molten, form. The additive, in the following also referred to as "additive according to the invention" is a combination of elemental oxygen and nitrogen oxides, preferably nitrogen monoxide. 0 The elemental oxygen can also be provided in the presence of nitrogen, for example in the form of air and/or in the presence of noble gases. Which nitrogen oxides are present depends on the boundary conditions such as pressure, 5 temperature, presence or absence of oxygen. Examples of nitrogen oxides are dinitrogen monoxide, nitrogen monoxide, nitrogen dioxide and dinitrogen tetroxide. The molar ratios of the components forming the additive according to the invention are generally not critical. 0 The molar ratio of elemental oxygen to nitrogen oxides is usually in the range from 1:10 to 10:1. For example, elemental oxygen (02) is combined with nitrogen monoxide (NO) in a molar ratio of 1:2, which corresponds to two equivalents of nitrogen dioxide (NO 2 ). The elemental oxygen can, for example, also be used in an excess over the nitrogen oxide. 5 The contacting of the nitrate salt composition of the invention with the additive according to the invention usually takes place at the pressure prevailing at the place where the additive is added, for example at a pressure in the range from I to 30 bar (abs). ) For example, the pressure at the place where the additive is added in large heat storage tanks of a solar thermal power station is a few mbar above atmospheric pressure, and the pressure in the central receiver of a solar thermal power station, for example a tower power station, is 11 usually 30 bar. The contacting of the additive according to the invention with the nitrate salt composition of the invention is generally effected by introducing the additive according to the invention under or 5 above the surface of the nitrate salt composition of the invention which is usually present in liquid, pumpable, in general molten, form. The contacting of the nitrate salt composition of the invention with the additive according to the invention generally takes place in a suitable apparatus. This can be a vessel and/or a pipe 10 through which the nitrate composition of the invention flows or is at rest therein or a subvolume of a vessel or pipe. For example, in solar thermal power stations, the additive according to the invention can be introduced into a vessel, for example a tank, which comprises the nitrate salt composition of the 5 invention. For example, in solar thermal power stations having a heat store comprising two tanks, viz. a relatively hot tank and a colder tank, the additive according to the invention is introduced into the hotter tank, preferably under the surface of the nitrate salt composition according to the ?0 invention which is present therein. A very useful embodiment of this is shown by way of example in figure 2 and is described below. .5 In figure 2, the numerals have the following meanings. I Hot tank 2 Cold tank 3 Introduction of an additive according to the invention 0 Figure 2 shows a two-tank storage system into which an additive (3) according to the invention, for example oxygen and nitrogen monoxide, is introduced under the surface of the nitrate salt composition according to the invention, for example a mixture of sodium nitrate and potassium nitrate in molten form, in the hotter tank 1. 5 In a heat store which comprises only one tank (also referred to as layer store), a gaseous additive can be introduced only with difficulty under the surface of the heat storage medium. In that case, rising gas bubbles would bring about convection of the heat storage system and the temperature layering of the store would be impaired. 0 A solution to this problem is to introduce the additive according to the invention onto the surface of the heat storage medium or into a feed stream of the heat transfer medium according to the invention to the store, for example into the hot region of the store.

12 A very useful embodiment of a one-tank heat store (also referred to as layer store) with addition of the additive according to the invention, for example oxygen and nitrogen monoxide, into the feed stream into the hot region of the heat storage system is shown by way of example in 5 figure 3 and is described below. In figure 3, the numerals have the following meanings. I Layer store 0 2 Receiver 3 Stream of a heated heat transfer medium according to the invention 4 Stream of a cold heat transfer medium according to the invention 5a Hot region 5b Cold region 5 6 Introduction of an additive according to the invention Heated heat transfer medium (3) according to the invention flows from a solar receiver (2) into the hot region (5a) of the store (1). A cold region (5b) is located, for example, beneath the hot region (5a). An additive (6) according to the invention, for example oxygen and nitrogen :0 monoxide, preferably finely dispersed by conventional means, is introduced into the stream (3). During operation of a heat storage system, operation results in a change in the storage temperature between a maximum value and the minimum value. The materials (heat storage medium and gases above it) and the storage system usually expand to a different degree as a 5 result. These effects can lead to high subatmospheric or superatmospheric pressures in the storage system which are outside the permissible pressure range. These undesirable pressure effects can be controlled by breathing of the store using a suitable gas, for example air and/or nitrogen. If the atmosphere of the vessel of the heat storage system comprises an additive which comprises, for example, nitrogen dioxide (NO 2 ), nitrogen monoxide (NO) or mixtures 0 thereof, nitrous gases can thus be released into the environment. A solution to this problem is shown by way of example in figure 4 and is described below. In figure 4, the numerals have the following meanings. 5 1 Heat storage system 5 Gas buffer system 6 Nitrogen oxide separator and/or remover 0 During operation, the heat storage system (1) requires breathing via the gas space. For this purpose, gases can be released into the environment via a nitrogen oxide separator and/or 13 remover (6), for example a DeNOx catalyst and/or a condenser, in case of superatmospheric pressure. Should subatmospheric pressure occur in the storage system (1), a suitable breathing gas, for example air or nitrogen, can be introduced by conventional means. In addition, a gas buffer system (5) can be used to effect temporary storage (buffering) of the amounts of gas 5 given off from the heat store during heating, in order to introduce them back into the storage system on cooling so as to avoid subatmospheric pressure. As a result of this measure, the amount of gases introduced into the heat storage system, preferably via the nitrogen oxide separator and/or remover (6), for example DeNOx catalyst and/or condenser, is effectively reduced. 0 An alternative to a gas buffer system is maintenance of the pressure in the storage system by removal or introduction of liquid heat storage medium according to the invention into a separate equalization tank or from a separate equalization tank. The removal and introduction is preferably carried out from or into the cold region of the heat storage system. Excess amounts of gas, e.g. nitrogen oxides, in the heat storage system can also arise as a result of 5 decomposition of the heat storage medium. These excess amounts of gas can be conveyed by the heat transfer medium into the relatively cold equalization tank in such a way that the amount of excess nitrogen oxides is reduced. The remaining gas can then be fed to a nitrogen oxide separator and/or remover, for example DeNOx catalyst and/or condenser. 0 The above-described introductions of the additive according to the invention into heat storage systems generally lead, thanks to the pressure maintenance systems outlined above, to no significant pressure increase in the gas space above the surface of the heat storage medium in the heat storage system. The gauge pressure in the gas space is generally in the range from 0 to 0.01 bar. 5 In a further embodiment of the invention, the additive according to the invention can be introduced into a vessel which is connected in parallel to the main amount of the nitrate salt composition according to the invention in molten form, for example a mixture of sodium nitrate and potassium nitrate in molten form, and into which a partial amount of the nitrate salt 0 composition according to the invention is introduced and taken from, either discontinuously or preferably continuously. The introduction of the additive according to the invention into a vessel connected in parallel to the main stream of the flowing nitrate salt composition according to the invention has the 5 advantage that, regardless of the respective operating pressure of the main stream, a different, advantageously higher, pressure and/or a different temperature can be selected in the vessel connected in parallel, which usually results in a faster reaction and therefore a higher degree of regeneration of the nitrate salt mixture according to the invention. 0 For example, in this embodiment, it is possible for the introduction of the additive according to the invention to be carried out at a relatively low temperature, for example 250 to 350*C, and for the thus-treated nitrate salt mixture according to the invention to then be introduced into the 14 generally hotter heat transfer circuit. Very useful embodiments of the above-described "parallel vessel embodiment" of the invention are described below by way of example for a solar thermal power station and are shown 5 schematically in figure 5. Here, figure 5a shows the introduction into the heat storage system figure 5b shows the introduction into the stream of the heated heat transfer medium 0 figure 5c shows the introduction into the stream of a cold heat transfer medium. In figure 5, the numerals have the following meanings. I Heat storage system 5 2 Receiver system 3 Stream of a heated heat transfer medium according to the invention 4 Stream of a cold heat transfer medium according to the invention 5a Hot region of the heat storage system 5b Cold region of the heat storage system 0 6 Introduction of an additive according to the invention 7 Taking off of a substream of the heat transfer medium according to the invention 8 Recirculation of the substream of the heat transfer medium according to the invention 9 External reaction vessel 5 Three variants showing how contacting of the nitrate salt mixture of the invention with an additive according to the invention can be configured for a solar thermal power station (see figure 1) are outlined by way of example in figure 5. All the variants have a receiver system (2) which exchanges a heat transfer/storage medium with a heat storage system (1) via the lines (3) and (4). The heat storage system (1) has a hot region (5a) and a cold region (5b). In the one 0 variant (figure 5a), the substream is, by way of example, taken from a middle temperature region of the heat storage system. Taking it from a hot or cold region of the storage system is likewise possible. In the second variant (figure 5b), the substream is taken from the heated main stream (3) of the heat transfer medium. In the third variant (figure 5c), it is taken from the cold main stream (4) of heat transfer medium. 5 The branching-off of the substream of the nitrate salt composition of the invention is carried out, for example, by pumping. After the substream has been taken off, it is contacted with the additive according to the invention in a separate reaction vessel. The reaction vessel can be set by conventional means to a different, preferably higher pressure and/or an altered temperature O compared to the offtake temperature in order to achieve, for example, a higher degree of regeneration of the nitrate salt mixture of the invention.

15 In solar thermal power stations of the tower power station type, the heat transfer medium is usually subjected to particularly high thermal stress, i.e. rapid temperature changes at very high temperature (for example 580*C) and very high heat flow densities. At the same time, the heat transfer medium is generally placed under a high pressure (for example 30 bar), for example in 5 order to reach the central receiver arranged at a great height (for example 100m) so as to prevent outgassing in the central receiver and achieve a particularly high flow velocity through the tubes of the central receiver. For example, the additive according to the invention, preferably nitrogen monoxide and oxygen, 0 can advantageously be introduced under high pressure in solar thermal power stations of the tower power station type. A method of introducing additive according to the invention under high pressure is indicated by way of example below. The method is, for example, suitable for use in a solar tower power 5 station and is shown schematically in figure 6. In figure 6, the numerals have the following meanings. 2 Central receiver !0 5 Heat storage system 5a Hot region of the heat storage system 5b Cold region of the heat storage system 6 Introduction of additive according to the invention 7 Introduction of further amounts of additive according to the invention 5 8 Recovered gaseous additive according to the invention 9 Pump with pressure increase 10 Gas separator 11 Pressure reduction pump 12 Mechanical shaft coupling 0 13 Removal of inert gases 14 Offgas stream of inert gases In figure 6, heat transfer medium according to the invention is conveyed under high pressure (for example 30 bar) by means of the pressure-increasing pump (9) from the cold region (5b) of 5 a heat storage system (5) to a receiver system (2), for example the central receiver of a tower power station. The additive according to the invention, for example finely divided by conventional means, is introduced (6) into this stream under superatmospheric pressure. In the central receiver system (2), the heat transfer medium according to the invention is heated and recirculated hot to the hot region (5a) of the heat storage system (5). Since the heat storage 0 system generally cannot withstand a high pressure, the pressure of the heat storage medium is, for example, greatly reduced by means of a pressure reduction pump (11) with recovery of energy so as to be able to be fed back. The energy liberated in the pressure reduction pump 16 can, for example, be passed on by means of mechanical shaft coupling (12) to the pressure increasing pump (9). The pump leakage in the pumps (9) and (11) can, for example, be compensated by a separate pump (not shown). After reduction of the pressure in the pump (11), part of the unconsumed additive according to the invention usually goes over into the gas 5 phase. This unconsumed gaseous additive according to the invention is, for example, separated off (8) in a gas separator (10) and can be introduced into the additive feed stream (to 6). Consumed additive can, for example, be replaced via feed stream (7). The amount of the additive according to the invention which is brought into contact with the 0 nitrate salt composition of the invention depends on the technical problem to be solved and can be determined by a person skilled in the art using conventional methods for determining the composition of the nitrate salt composition which is to be brought into contact with the additive according to the invention. 5 Examples of these methods are analytical methods such as determination of the basicity, determination of the nitrite and/or nitrate content of the nitrate salt composition which is brought into contact with the additive according to the invention. In a useful embodiment, for example well-suited to solar thermal power stations, the basicity of 0 the nitrate salt composition according to the invention which is to be brought into contact with the additive according to the invention, preferably oxygen and nitrogen monoxide, is determined, for example, by acid-based titration or potentiometrically. This determination can be carried out in-line, on-line or off-line. On the basis of the basicity value determined in this way, the amount of the additive according to the invention is determined and introduced, leading to 5 complete neutrality of the nitrate salt composition according to the invention, but preferably to a small residual basicity, as defined below, in the nitrate salt composition according to the invention. For the present purposes, the basicity (alkalinity) is the specific amount of acid equivalents 0 which an aqueous solution of a salt melt can take up until it reaches pH neutrality. The sensor parameter "alkalinity" can be measured in-line, on-line or off-line. The target value of "alkalinity" should be 0.001-5%, preferably 0.005-1% and particularly preferably 0.01-0.5%. Instead of measuring the alkalinity by means of titration, a substitute sensor parameter can also be employed after appropriate calibration. Substituted parameters can be: density, optical 5 parameters (spectrum), etc. If the additive is used in a substoichiometric amount, offgas treatment, for example using a nitrogen oxide separator and/or remover, for example DeNOx catalyst and/or condenser, may be able to be dispensed with. 0 In another embodiment, it is possible, for example in the case of high-temperature plants such as solar thermal tower power stations, to deliberately use the additive according to the invention 17 in a superstoichiometric amount. The present patent application also provides a process system as defined in the claims. 5 For the purposes of the present invention, such a system is made up of vessels, for example reservoirs such as tanks, in particular heat storage tanks, and/or apparatuses, for example apparatuses for pumping fluids (for example salt melts), e.g. pumps, which are connected by pipes and effect transport and/or storage of thermal energy by means of heat transfer media or heat storage media, for example the primary circuit for heat transfer media and/or heat storage 0 media in solar thermal power stations. Examples of such pipes are those which are located on the focal line of the parabolic trough mirrors or Fresnel mirrors in solar thermal power stations and/or which form the receiver tubes or receiver tube bundles in solar thermal tower power stations and/or those which, for example 5 in solar thermal power stations, connect particular apparatuses to one another without having the function of collecting solar radiation. A further example of a process system as defined in the claims is salt bath reactors of chemical process technology and systems formed by connecting them, which in each case comprise the 0 nitrate salt composition of the invention. All or part of the latter is brought into contact with an additive as defined herein. The present patent application also provides for the use of an additive as defined in the claims for maintaining or widening the long-term operating temperature range of a heat transfer 5 medium and/or heat storage medium comprising a nitrate salt composition as defined in the claims. For the present purposes, an additive is that which has been described in more detail above and is also described herein as additive according to the invention, including all preferred 0 embodiments. A nitrate salt composition is, for the present purposes, that which has been described in more detail above and is also referred to herein as nitrate salt composition of the invention/according to the invention, including all preferred embodiments. The abovementioned use preferably relates to a heat transfer medium and/or heat storage 5 medium in a) power stations for generating heat and/or electricity, particularly preferably solar thermal power stations, in particular those of the parabolic trough power station, Fresnel power station or tower power station type, b) in chemical process technology, particularly preferably salt bath reactors, or c) in metal hardening plants. 0 The present patent application also provides a method of generating electric energy in a solar thermal power station using a nitrate salt composition, as defined in the claims, as heat transfer medium and/or heat storage medium, where all or part of the nitrate salt composition is brought 18 into contact with an additive as defined in the claims. For the present purposes, an additive is what has been described in more detail above and is also described herein as additive according to the invention, including all preferred 5 embodiments. A nitrate salt composition is, for the present purposes, that which has been described in more detail above and is also referred to herein as nitrate salt composition of the invention/according to the invention, including all preferred embodiments. The abovementioned method preferably relates to a heat transfer medium and/or heat storage medium in solar thermal power stations of the parabolic trough power station, Fresnel power 10 station or tower power station type. The present application also provides for the use of an additive according to the invention for reducing or eliminating the corrosiveness of a nitrate salt mixture according to the invention. Here, an additive is what has been described in more detail above and has also been described '5 as additive according to the invention, including all preferred embodiments. Here, a nitrate salt composition is what has been described in more detail above and also has been described as nitrate salt composition according to the invention, including all preferred embodiments. The corrosiveness usually relates to iron-comprising materials, preferably materials composed ?0 of steel, and usually at temperatures in the range from 290 to 600*C, with the nitrate salt composition according to the invention usually being present in molten, preferably pumpable, form. The abovementioned materials are usually used in pipes or vessels, for example storage 5 vessels such as tanks, or other apparatuses, for example apparatuses for conveying fluids (for example salt melts), e.g. pumps. Examples of such pipes are those which are present in solar thermal power stations in the focal line of parabolic trough mirrors or Fresnel mirrors and/or which form the receiver tubes or 0 bundles of receiver tubes in solar thermal tower power stations and/or those which connect, for example, particular apparatuses in solar thermal power stations without having a solar radiation collection function. A further example of apparatuses in which the abovementioned materials are used are salt bath 5 reactors of chemical process engineering and their connections which in each case come into contact with the nitrite salt compositions according to the invention. Examples 0 Example 1: 500 g of a salt mixture composed of 60% by weight of sodium nitrate and 40% by weight of potassium nitrate were placed together with 8 g of sodium hydroxide in a stirred stainless steel 19 apparatus at 300*C. Over a period of two hours, 21.5 g of nitrogen monoxide (NO) were introduced together with 10 ! of air under the surface of the melt. After cooling, a sample of this salt was dissolved in water and analyzed, giving a hydroxide content below the detection limit (< 0.1 g/100 g). 5 It was thus able to be shown that sodium hydroxide as possible decomposition product in the nitrate salt composition according to the invention was removed by introduction of NO together with air, as a result of which the long-term stability of the melts increases. 10 Example 2: 500 g of a salt mixture composed of 60% by weight of sodium nitrate and 40% by weight of potassium nitrate were placed together with 5 g of sodium oxide/peroxide (80:20) in a stirred stainless steel apparatus at 300*C. Over a period of one hour, 18 g of nitrogen dioxide ("NO 2 ") together with 5 1 of air were introduced under the surface of the melt. After cooling, a sample of 5 this salt was dissolved in water and analyzed, giving a hydroxide content below the detection limit (< 0.1 g/100 g). It was thus able to be shown that sodium hydroxide as possible decomposition product in the nitrate salt composition according to the invention was removed by introduction of NO 2 together 0 with air, as a result of which the long-term stability of the melts increases. Example 3: 500 g of a salt mixture composed of 60% by weight of sodium nitrate and 40% by weight of potassium nitrate were admixed with 5 g of sodium carbonate (corresponds to 0.11% by mass 5 of carbon) and heated to 300*C in a stirred stainless steel apparatus. 6 g of nitrogen monoxide (NO) mixed with 5 I of air were subsequently introduced into the melt over a period of one hour. Analysis of a sample of the melt dissolved in water after the end of the experiment indicated a total carbon content of 0.031% by mass. 0 It was thus able to be shown that nitrogen monoxide together with air largely removes sodium carbonate as possible decomposition product from the nitrate salt composition according to the invention, which increases the long-term stability of the salt mixtures. Example 4: 5 500 g of a salt mixture composed of 60% by weight of sodium nitrate and 40% by weight of potassium nitrate were mixed with 5 g of potassium superoxide and heated to 300*C in a stirred stainless steel reactor. 9.8 g of nitrogen monoxide (NO) mixed with 5 I of air were subsequently introduced into the melt over a period of one hour. After cooling, a sample of this salt was dissolved in water and analyzed, giving hydroxide contents and nitrite contents below the 0 detection limit (< 0.1 and < 0.5 g/100 g, respectively). It was thus able to be shown that potassium superoxide as possible decomposition product in 20 the nitrate salt composition according to the invention was removed by introduction of NO together with air, as a result of which the long-term stability of the melts increases.

Claims

1. A method of maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium comprising a nitrate salt composition 5 selected from the group consisting of alkali metal nitrate and alkaline earth metal nitrate and optionally alkali metal nitrite and alkaline earth metal nitrite, wherein all or part of the nitrate salt composition is brought into contact with an additive composed of a combination of elemental oxygen and nitrogen oxides. 0

2. The method according to claim 1, wherein the heat transfer medium and/or heat storage medium is used in power stations for generating heat and/or electric energy, in chemical process technology or in metal hardening plants.

3. The method according to claim 1 or 2, wherein the power stations for generating heat 5 and/or electric energy are solar thermal power stations.

4. The method according to claim 3, wherein the solar thermal power stations are of the parabolic trough power station, Fresnel power station or tower power station type. 0 5. The method according to any of claims 1 to 4, wherein the contacting of the heat transfer medium with the additive occurs in a reservoir and/or in the main stream and/or in a reaction space which comprises a partial amount of the heat transfer medium and is arranged in parallel to the main stream of the heat transfer medium.

5

6. The method according to any of claims 1 to 5, wherein an amount of the additive which leads to complete neutralization of the nitrate salt composition of the invention or setting of a residual basicity in the nitrate salt composition of the invention is selected.

7. A process system in which pipes and vessels and/or apparatuses are connected and in 0 which a heat transfer medium and/or heat storage medium comprising the nitrate salt composition defined in any of claims I to 6 is present, wherein all or part of the nitrate salt composition is brought into contact with an additive as defined in any of claims 1 to 6.

8. The process system according to claim 7 which is a constituent of power stations for 5 generating heat and/or electric energy, plants of chemical process technology or metal hardening plants.

9. The process system according to claim 8, wherein the plants for generating heat and/or electric energy are solar thermal power stations. 0 22

10. The use of an additive as defined in any of claims I to 6 for maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium comprising a nitrate salt composition as defined in any of claims I to 6. 5

11. A method of generating electric energy in a solar thermal power station using a nitrate salt composition as defined in any of claims 1 to 6 as heat transfer medium and/or heat storage medium, wherein all or part of the nitrate salt composition is brought into contact with an additive as defined in any of claims 1 to 6. 10

12. The use of an additive as defined in claim 1 for reducing or eliminating the corrosiveness of a nitrate salt mixture as defined in claim 1.