WO2017178412A1 - Nitrate-based heat storage medium and solar-thermal power plant - Google Patents

Abstract

The invention relates to a nitrate-based heat storage medium, in particular a binary nitrate salt mixture. The invention proposes for the first time a heat storage medium for use in solar-thermal power plants that are operated with thermal oils as a heat transfer medium which can be produced on the basis of nitrate, in particular on the basis of a binary salt mixture of sodium and calcium nitrate and requires either no potassium nitrate at all or just a small amount of potassium nitrate. This is important in particular because, in countries in which solar-thermal power plants may become an important source of energy, potassium nitrate is primarily used for agriculture as a fertiliser for food production.

Description

description

Heat storage medium based on nitrate and solar thermal

Power plant

The invention relates to a heat storage medium based on nitrate, in particular a binary Nitratsalzgemisch and / or a heat storage medium based on a binary Nitratsalzgemisches and a solar thermal power plant in which such a salt mixture is provided as a heat storage medium.

Linienfokussierende conventional solar thermal power plant ^¬ plants (engl. Concentrating Solar Power, "CSP"), in particular of the type parabolic trough (engl. Parabolic trough) use, mainly to the prior art, a system based on biphenyl and biphenyl ether mixture (eutectic melting or Solidification temperature of 12 ° C, maximum operating temperature of approximately 395 ° C, eg Solutia Therminol® VP-1 ™ or Dow® Dowtherm A ™) as heat transfer fluid ("HTF") to absorb absorbed solar energy in

Receiver tubes in the form of heat via a heat exchanger to transfer a water-steam cycle with generator. In order to generate electricity during the night as well as Schlechtwetterperi ^¬ oden is ^¬ det during the daytime phase continu ously ^¬ a portion of the collected heat for heating a liquid salt mixture of about 290 ° C to about 390 ° C USAGE. This salt mixture used as thermal energy storage ("TES") is known by the name SolarSalt and is usually a mixture of 60 wt .-% sodium nitrate and 40 wt .-% potassium nitrate, with a non-eutectic melting point of 238 ° C, where the "cold" salt mixture at 290 ° C in the cold tank is heated up to 393 ° C via a special oil-salt heat exchanger against the hot heat transfer medium leaving the solar field Hot tank pumped and stored for night use. After sunset, this now hot salt mixture is again used to heat the heat transfer medium, thereby thermally discharged via oil-salt heat exchanger from about 390 ° C to about 290 ° C and pumped from the hot tank in the cold tank. The heat oil heated by these heat provided by the TES system is thus able to supply the turbine of the generator with steam during the night phase, thus maintaining the continuous generation of electricity through the generator. This TES system is thus essential for the solar thermal power plant technology, as it allows to provide energy in a simple manner according to the principle of "heat-to-power" for a later date and thereby save relatively inexpensively.

The costs for the known solar duty fluctuate since the nitrates are used not only for TES storage salt production, but mainly for fertilizer production. Apart from that, the low-cost nitrate salt mixtures also always entail the risk that chloride-contaminated salt mixtures are used which, when used in storage facilities, cause corrosive damage there.

Especially with potassium nitrate availability of Chlo ^¬ rid-poor potassium nitrate on an industrial scale is expensive because potassium nitrate is mainly derived from the naturally occurring potassium chloride by reaction with Chilean nitrate (sodium nitrate), which is a more or less strong contamination with chloride entails. It is known that Chlo ride ^¬ develop a significantly pronounced corrosion behavior towards steels in connection with nitrates at higher temperatures, which is why in solar thermal Kraftwerksan- were chlorides are largely avoided in salt mixtures.

As a result, a total cost reduction of solar thermal power plants will lead to lower cost of electricity ("LCOE"), which is necessary to achieve this ecologically compatible type of energy

Power generation, such as solar thermal power plant technology, to bring more and more economic in (socio-) compete with fossil Ener ^¬ gieerzeugung.

From US 7,588,694 Bl a heat transfer medium with low fuel melting point is known, comprising a mixture of four Nitra ^¬ th. Is present Kali ^¬ umnitrat as the main ingredient in the mix.

EP 0 049 761 A1 discloses a ternary mixture with crystalline calcium nitrate as heat transfer medium.

From US 2012/0056125 AI is known an extremely low melting heat transfer medium nitrate-based, at least a ternary mixture of four different nitrates, but at least potassium and lithium nitrate known.

From WO 2014044652 Al a heat transfer medium based on a double salt of calcium and potassium nitrate is known.

WO 2015043810 A1 discloses a heat storage medium based on a double salt of potassium calcium nitrate and a sodium nitrate, which can optionally be used in solar thermal power plants in the presence of water of crystallization.

A disadvantage of the previously known nitrate-based heat transfer and / or heat storage media is the high potassium content, which, as explained above, with cost-effective availability, entails the risk of corrosive damage to the installation. In CSP experts so far will focus on the lowest-melting formulation of a salt mixture into ^¬ particular with regard to the possible use as a heat transfer medium, so instead of conventionally verwende- th thermal oils laid.

Therefore, there is a need for further, previously unknown for the storage technology solar thermal power plant technology Nitrate-based salt mixtures are available which, owing to the risk of corrosion in the power plants, are as low-chloride and inexpensive as possible and nevertheless can be used as storage medium in solar thermal power plants, as described above, with regard to their liquidus temperature and / or their thermal stability.

The use of nitrite mixtures in combination or as a replacement of nitrate-based salt mixtures for Wärmespeicherme- is serving disadvantageous because nitrite mixtures are toxic and easily react un ^¬ ter air and / or oxygen to the respective Nitra ^¬ th, which with a change in the thermophysical properties and therefore can not be tolerated in solar power plant operation.

This object is achieved by the subject matter of the present ^¬ invention as disclosed in the description, the claims and the figures. The subject of the present invention is therefore a heat storage medium for solar power plants based on nitrate, characterized in that nitrates of the alkali and alkaline earth group are combined so that the smallest possible proportion of potassium nitrate results and still a liquidus temperature of 270 ° C, in particular 250 ° C, particularly preferably of 230 ° C is not exceeded.

General knowledge of the invention is that among the cations of the alkali and alkaline earth group technically and economically useful only a salt mixture with at least unge ^¬ approximately 5Mol% calcium nitrate, especially in admixture with Nat ^¬ nitrate is used as a heat storage medium. Additions of other nitrates from the group, such as cesium-barium-potassium-strontium-lithium cations, make sense, but are expensive. The other cations from the group are either unstable as nitrates in the relevant temperature range up to about 450 ° C, or otherwise as economically / technically useless, so for example toxic. Potassium nitrate arms or even potassium nitrate-free salt mixtures nitrate based as heat storage media are to unanswered shouted ^¬ ben, as is understood in the art from a continuous operating temperature of 565 ° C and above, as for used there heat transfer media, prevails in parabolic trough solar power plants. In the case of salt mixtures used only as a heat storage medium, this requirement does not apply; rather, it is possible to start from a continuous service temperature of 450 ° C., so that no stability of the salt mixture above this temperature is required. The maximum Ge ^{¬ use} temperature of thermal oils, which are used in the considered here solar thermal power plants as a heat transfer medium is approximately at 400 ° C to 450 ° C, so that a pure heat storage medium in these solar power plants, which are just operated with thermal oils, no stability above must have this temperature.

As "potassium nitrate-free" is in the present case a salt mixture is characterized loading which does not hold appreciable amounts of potassium nitrate ent ^¬, wherein in case of technical salts and salt mixtures usual impurities with potassium, so as salt mixtures with potassium amounts up to 0.5 mol% here also as " Potassium free ". The reason is that this technical see salt mixtures are used and no high-purity Sal ^¬ ze, as are common for example in medical technology.

According to a preferred embodiment of the invention corrosion-demanding substances such as hydroxides, carbonates, halides, halogens and / or sulfates in the salt mixture of the heat storage medium ^¬ killed, directly or indirectly, be avoided. As being "directly in Salzge ^¬ mixing" it is meant the addition of salts of these anions, whereas an "indirect presence" of these anions in the salt mixture is referred to when the possible reaction of the salt mixture carbonate for example, a hydroxide and / or carbonyl in air is likely and / or the contamination of the salt mixture by a manufacturing process of a Heat storage medium salt used, for example, a halide, such as a chloride comprises.

According to a preferred embodiment of the invention, the heat storage medium to more than 60 mol%, in particular more than 70 mol%, and particularly preferably more than 90 mol ~ 6 a binary salt mixture of calcium nitrate and sodium nitrate, calculated free of water of crystallization, but can be containing water of crystallization.

According to a preferred embodiment of the invention, the heat storage medium is a binary salt mixture of calcium and sodium nitrate, containing water of crystallization or free of water of crystallization. It can be provided that the salt mixture contained potassium amounts less than 40 mol%, in particular less than 30 mol%, particularly be less than 5 mol% ^¬ vorzugt.

It is preferred that the ratio of potassium to calcium ions in the salt mixture is 1 to 6 or greater, ie for example 1 to 7 to 1 to 999 or more, that is to say 999 in the analysis of the salt mixture used as the heat storage medium Calcium ions only a single potassium ion or even less potassium ions. In particular, it is not provided that more potassium than calcium ions are contained in the heat storage medium.

The corresponding% by weight results from the mol% over the molar masses of the respective compounds, in which the Gew% is indicated in grams. This results in the so-called mole fraction, which is usually between 0 and 1 and is still multiplied by 100 for the indication in percent. Figure 1 shows the liquidus of the binary salt mixture Ca (N03) 2 ^_ NaNÜ3. The binary, eutectic mixture with 53.6% by weight or 69 mol% NaNO ₃ and 46.4% by weight or 31 mol%

Ca (NO 3) 2 has a melting point of about 233 ° C and thus a very low solidification temperature. It is potassium nitrate-free and shows a relatively high specific heat capacity due to the high sodium nitrate content. This is particularly the case since the specific heat capacity in the liquid state at 400 ° C. of potassium nitrate, over sodium nitrate towards lithium nitrate in the alkali group increases sharply and sodium is known to be above potassium in the Periodic Table of the Elements (PSE) (LiNO ₃ : 2.04 J / (gK), NaN0 ₃ : 1.63 J / (gK), KN0 ₃ : 1.38 J / (gK)).

According to an advantageous embodiment of the invention, it is therefore envisaged to aim for a particularly high proportion of sodium nitrate in the salt mixture for the heat storage medium. Although the specific heat capacity of liquid calcium nitrate, the second component of the considered binary eutectic - following this trend - is lower than that of potassium nitrate (0.95 J / gK), the specific density is highest there. Advantageous also jewei ^¬ time volume changes of the alkali and alkaline earth prove in this course on melting. Thus, the volume increase of sodium nitrate during melting is + 10.7%, that of potassium nitrate + 3.3% and that of calcium nitrate approx. + 10%.

Since potassium nitrate in the solid state has the lowest specific gravity (2.11 g / cm ³ ), sodium nitrate medium (2.26 g / cm ³ ) and calcium nitrate the highest (2.46 g / cm ³ ), it follows that a liquid salt mixture as a heat storage medium, the According to a preferred embodiment of the invention, the binary salt mixture of calcium and sodium nitrate and / or it comprises a high proportion of sodium nitrate and a lower proportion of calcium nitrate, a leveling of volumetric energy storage density takes place, since the reduction of the specific total heat capacity by the calcium Part is partially compensated due to the higher density of calcium nitrate. So has the binary eutectic salt mixture of 53.55 wt .-% of sodium nitrate, and 46.45 wt .-% calcium nitrate in FLÜS ^¬ sigen condition of 290 ° C-390 ° C sufficient for use as a heat storage medium volumetric energy store cherkapazität in [kJ / m ³ K], is a binary salt mixture and thus easy and inexpensive to produce. For example, in United ^¬ equal to the conventional "solar salt" of 60 wt .-% of sodium nitrate and 40 wt .-% of potassium nitrate, which is currently introduced on the market as a heat storage medium for solar thermal power plants, operated with thermal oil, resulting in the here potassium-free or at least low-potassium salt mixture, that solar salt at 400 ° C has a specific heat capacity of 1.52 kJ / kg-K, at a density of 1820 kg / m ^3. This corresponds to a stored energy for the temperature range of 290-390 ° C. of about 277 MJ / m ^3. For the eutectic NaN03-Ca (NO _{3) 2} mixture results in the SpeI ^¬ assurance of 270 MJ / m ³ of sensible heat, which corresponds to about 97.5% of the solar larSalt levels. by Using a non-eutectic mixture with increased calcium nitrate content, which then has a comparable melting point with the solar salt, ie 238 ° C or higher, instead of 233 ° C, it is possible to use a virtually volumetrically identical spe icherfähige formulation of the binary salt mixture Ca (NC> 3) ₂ ^_ NaNO> 3 to preserver ^¬ th.

The dynamic viscosity is due to the calcium nitrate ge ^¬ slightly higher than that of the solar Salts such as Ge in Figure 2 shows ^¬ is. Figure 2 shows the dynamic viscosity of solar ^¬ salt and the Na-Ca / nitrate eutectic in the relevant temperature range of 290 ° C (about 9 mPa-s) to 390 ° C (about 4 mPa-s).

In absolute terms, the viscosity values differ accordingly le ^¬ diglich to five (290 ° C) or two mPa.s requires that for common salt pumps, for example of the type submersible pump, no change of a design, and / or a pump power.

The binary sodium nitrate-calcium nitrate salt mixture differs from SolarSalt mainly in the use of calcium nitrate instead of potassium nitrate. This component is mostly commercially available as a tetrahydrate of the chemical formula Ca (NO ₃ ) ₂ H ₂ O (so-called "Norway ^Salpeter ") and represents a relatively inexpensive material compared to potassium nitrate (about 50-60% of the price of KNO ₃ ). In the preparation of binary Na-Ca / nitrate melts, in

Premelterprozess, in which the two salt components are usually first and once melted together at about 300 ° C and pumped into the cold storage tank, expelled this crystalline water content of calcium nitrate. This represents an easily manageable process beginning at 110 ° C. and concluded at 250 ° -300 ° C. In FIG. 3, the expulsion behavior investigated by means of differential scanning calorimetry is correspondingly shown.

Figure 3 shows the heating behavior of the crystal water-containing mixture of calcium nitrate tetrahydrate and sodium nitrate, which yields the anhydrous eutectic when heated to 300 ° C, un ^¬ ter indicate the enthalpies (above), second melting of the unique dehydrated mixture with evidence of formation of the eutectic with melting point 230 ° C (center) and the Erstar ^¬ approximately behavior during cooling (below).

The pure expulsion of the crystal water from 50 ° C (after melting the calcium nitrate tetrahydrate) to 350 ° C therefore requires 392 Jg ^-1 , the heating of the crystal water-containing, binary mixture of NaNÜ ₃ and Ca (NO ₃ ) ₂ 'H ₂ O. from room temperature up to 400 ° C approx. 611 Jg ^-1 . From these enthalpy values, the necessary energy to the heating from Raumtempera- structure and expulsion of the crystal water content so beispielswei ^¬ se 16.9 wt .-% of the total formulation, ie 1203.4 kg give crys- tallhaltige mix 1000 kg crystal anhydrous salt mixture, a crystal-water-free tonne can be derived, eutectic mixture, for the first time and once, is necessary:

611 Jg ^-1 · 1,203,370 g = 736 MJ per ton of anhydrous Salzmi ^¬ research. With the conversion of Joule in the Wh arise

736 MJ / 3600 JW ^_1 h ^_1 = 205 kWh per ton. As premaster plants are operated on-site using natural gas, the energy required to produce one ton of the mixture containing water of crystallization in mmBTU natural gas units (1 mmBTU ~ 293,071 kWh) can be determined to be 205 kWh / 293,071 kWh / mmBTU = 0.7 mmBTU per anhydrous tonne.

With an assumed price of about 2:08 US- $ per mmBTU- unit as it stood in January 2016, this results in a cost of the necessary natural gas of about $ 50,000 for 34,500 Ton ^¬ nen potassium nitrate-free Na-Ca / nitrate storage salt mixture comprising an energy content of about 1400 MWhhermisch and an input / aT the Withdrawal of 100 Kelvin and thus can provide _e ktrisch about six hours under the power of a power plant block size of 100 MW i _e.

It is also possible and particularly advantageous to use anhydrous or partially dewatered calcium nitrate, since then the expulsion of the proportion of water of crystallization is eliminated or the effort is significantly reduced.

For example, it can be relevant that the used

Calcium nitrate component contains very little of ammonium nitrate impurity, since this tends to decompose on strong heating. For this reason, according to one embodiment of the invention, it is provided that the ammonium nitrate content of the NaNO 3 Ca (NO 3) ₂ melt is less than 0.5% by weight, preferably less than 0.3% by weight. It is also provided according to a further embodiment of the invention that the chloride content should be as low as possible in order to minimize the tendency to corrode to keep, according to an advantageous embodiment below 0.05 wt .-%.

It is also possible, starting from binary sodium nitrate, calcium nitrate mixtures, in particular close to the binary NAT riumnitrat calcium nitrate eutectic ternary and to use higher For ^¬ formulations by admixture with other alkali metal and alkaline earth metal nitrates, in particular lithium nitrate, the final properties of the Molten salt, eg the specific heat capacity, the volumetric energy storage density, the specific density, the dynamic viscosity and / or the costs to further optimize.

At the same time, it should be emphasized that in the context of the present invention, such mixtures based on the mixture of sodium nitrate calcium nitrate and optionally further additives are in focus of use as energy storage salts for oil-based

Parabolic trough power plants are in the range of about 280 ° C to 400 ° C and should not be used as heat transfer media. For example, the base mixture of sodium nitrate and anhydrous calcium nitrate, in particular close to the binary eutectic, that is in the range 60-75 mol% a 03 and 25-40 mol% Ca (03) 2 have, however, it can be used to optimize the egg ^¬ properties purposeful as a heat storage medium be said binary base mixture with alkali and Erdalkalinitraten, in particular lithium nitrate, potassium nitrate, rubidium nitrate, Stron ^¬ tiumnitrat and barium nitrate, to ternarise, quaternarize or quinitize. In the event that during operation of the heat storage medium tempera ^¬ ren below 400 ° C, such as maximum supply Injection ^¬ temperatures in the range below 350 ° C, in particular below ^¬ semi-300 ° C, bevorzugtesten at below 200 ° C, are used, for example, Magnesium, aluminum and / or zinc nitrate, which form decomposition-free dehydratable nitrates in these temperature ranges, added as additives of the binary Nitratsalzgemisches of calcium and Natriumnit ^¬ rat. Are exemplified in Figures 6-9 with the FactSage 6.1 ^¬ be calculated Liquidustemperaturproj ections ternary mixtures of the continuous base mixture NaN0 ₃ ~ Ca (NO _{3) 2} and the nitrates of lithium nitrate, potassium nitrate, cesium nitrate and Stron ^¬ tiumnitrat shown.

Figures 4 to 7 show calculated liquidus temperature projections of ternary mixtures of the base mixture

Ca (NC> 3) 2 ^_ NaNC> 3 and the nitrates lithium nitrate, potassium nitrate, cesium nitrate and strontium nitrate.

The nitrates of lithium nitrate, potassium nitrate, cesium nitrate and strontium nitrate are according to a preferred execution of the invention form to the Ca (N03) ₂ ^_ NaN03 salt mixture for the preparation of the heat storage medium in an amount Be ^¬ ranging from 0.001 to 40 mol%, in particular in a Amount of 0.01 to 20 mol%, and particularly preferably in an amount of 0.1 to 10 mol% added. Advantageous embodiments of the invention with respect to the respective concrete Salzmi ^¬ research on the basis of the binary salt mixture of calcium, sodium nitrate, will become apparent to the skilled person in an obvious manner from the results shown here diagrams. As a base mixture of Ca (N03) ₂ ^_ NaNÜ3 has a Salzmi ^¬ research from 30 to 95 mol% NaN03, in particular 60Mol% to 95Mol% and 5 to 70 mol% Ca (N0 _{3) 2,} in particular from 5 to 45 mol %, proven. Preferably, of course, the eutectic mixture is Ca (N03) 2 ^_ Na NO ₃ with 69 mol% sodium nitrate and 31 mol% ₃ NaNÜ kristallwas ^¬ serfreiem calcium nitrate Ca (N03) 2 with up to 10 mol% of

Nitrate additive, and this mixture for the responsible here expert solar thermal power plants obvious salt mixtures.

By the addition of nitrates provided according to an exemplary embodiment of the invention, as stated above, arise at least ternary, so three-component mixtures Salzmi ^¬ which preferably 60-75 mol% (43.5-61 wt .-%) a 03 so as ^¬ 25-40 mol% (39-56.5 wt .-%) Ca (N0 _{3) 2} and 0.001-10 mol%, so for example 6-22 wt .-%, of at least one additive from the group lithium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, magnesium nitrate, strontium nitrate and / or

Barium nitrate include. Characterized 280-400 ° C are thermiekraftwerksanlagen liquid, sensible unloading, loadable ther ^¬ mix energy storage salts for thermo oil-based solar preferably in the temperature range, in particular of the type parabolic trough available.

Solar thermal energy as an ecological form of energy production, which so far is the only technology that can store heat in the TES system in the form of heat that can be called later, is increasingly in competition with the agroindustry in terms of chloride-free potassium nitrate and thus socio-economic distress. Therefore, the change to ei ^¬ nem potassium nitrate outdoor heat storage medium for Thermoölba- catalyzed solar power plants should be encouraged. In particular in poorer, but sun-intensive parts of the world, especially Africa, which would in fact be predestined for the sustainable production of electricity by solar thermal power plants, the use of large quantities of fertilizers for energy storage may be difficult for the respective population to mediate instead of cultivating feed and food and cause unrest or even attacks on the power plant, especially in remote areas. Looking soberly from the economic point of view, it is always attractive for a provider of solar thermal power plants to save costs in order to gain a competitive advantage. Since the TES system or the required solar energy for several hours of energy storage in a CSP plant with conventional sizes (50-100 MWeektrisch) a two-digit, in extremely large systems (by 200-1000 MW _e i _e ctrisch) one three-digit million amount ^¬ accounts, savings in the range of a few percent are already significant and beneficial locking. A further advantage of a heat storage medium according to the present invention that calcium nitrate-containing salt mixtures ^¬ known corrosion inhibitors with respect. Steels are common.

Advantageously, the solidification behavior of the salt mixtures for the first time described here as heat accumulators for solar thermal power ^¬ works on thermal oil-based, firstly because of the freezing point is low and calcium nitrate itself known to act as a cooler and retarded solidification falls below the actual solidification temperature of the mixture for a time.

The thermal stability of the anhydrous, here for the first time as a heat storage medium for solar thermal power plants Ther ^¬ moölbasis described salt mixtures ° was measured by thermo gravimetric analysis at very low heating rates (1 K / min) of 290 ° C to 400 detected without loss of mass, and thus permits the use of the salt mixture over years or decades almost without refill and / or purification.

The invention is first a heat storage medium for use in solar thermal power plants, operated with thermal oil as a heat transfer medium proposed which is ^¬ adjustable nitrate-based, in particular on the basis of egg ^¬ nes binary salt mixture of sodium and calcium nitrate forth and either without potassium nitrate or manages with ge ^¬ ringer amount of potassium nitrate. This is particularly important because in those countries where solar thermal power plants can become an important source of energy, potassium nitrate is needed primarily for agriculture as fertilizer for the production of food.

Claims

claims 1. heat storage medium for solar power plant on a nitrate basis, characterized in that nitrates of alkali and alkaline earth alkaline group with at least 5 mol% calcium nitrate are combined so that a liquidus temperature of 270 ° C is not exceeded. 2. Heat storage medium according to claim 1, characterized in that it comprises less than 40 mol%, in particular less than 5 mol% potassium nitrate. 3. Heat storage medium according to one of the preceding claims 1 or 2, characterized in that more than 60 mol%, in particular more than 75 mol% of the salt mixture, a binä ^¬ res salt mixture of sodium and calcium nitrate is present. 4. Heat storage medium according to one of the preceding claims, characterized in that the proportion of sodium nitrate in the salt mixture is higher than that of the calcium nitrate. 5. Heat storage medium according to one of the preceding claims, characterized in that the heat storage medium is at least 90 mol% of a binary salt mixture of sodium and calcium nitrate, in particular that the heat storage medium is potassium-free. 6. Heat storage medium according to one of the preceding claims, characterized in that in the heat storage medium up to 10 mol% of at least one further nitrate, selected from the group of the following nitrates: lithium nitrate, potassium nitrate, Rubidium nitrate, cesium nitrate, magnesium nitrate, strontium ^¬ nitrate and / or barium nitrate is present. 7. Heat storage medium according to one of the preceding claims, characterized in that in the heat storage medium 30 mol% to 95 mol% of sodium nitrate and 5 mol% to 70 mol% calcium nitrate is present. 8. Heat storage medium according to one of the preceding claims, characterized in that in the heat storage medium 60 mol% to 75 mol% of sodium nitrate and 25 mol% to 40 mol% calcium nitrate is present. 9. Heat storage medium according to one of the preceding claims, characterized in that the ratio of potassium ions to calcium ions in the salt mixture is 1 to 6 or greater. 10. solar thermal power plant of the type parabolic trough, characterized in that a thermal oil is provided as a heat transfer medium, which is combined with a heat storage medium according to one of claims 1 to 8.