US20110259552A1

US20110259552A1 - Use of modified, low-viscosity sulfur as heat transfer and heat storage fluid

Info

Publication number: US20110259552A1
Application number: US13/062,363
Authority: US
Inventors: Christoph-Henrik Sterzel
Original assignee: Flagsol GmbH
Current assignee: Flagsol GmbH
Priority date: 2008-09-05
Filing date: 2009-07-08
Publication date: 2011-10-27
Also published as: EP2350224A1; CN102177216A; MA32761B1; WO2010025692A1

Abstract

The invention relates to the use of a low-viscosity sulfur as a cost-effective heat transfer and heat storage fluid, the viscosity thereof being greatly reduced by saturation with hydrogen sulfide. Alternatively, the reduction in viscosity is optionally obtained by adding sulfur chloride. The melting point can be reduced by adding phosphorous. The temperature range of the use is from 130° C. up to 700° C. The fluid is particularly suitable for solar thermal power plants.

Description

Liquids for transferring heat energy are used in various fields of industry. In internal combustion engines, mixtures of water and ethylene glycol convey the waste heat of combustion into the radiator. Similar mixtures convey the heat from solar roof collectors into heat stores. In the chemical industry, they convey the heat from electrically heated or fossil-heated heating systems to chemical reactors, or from these to cooling devices.
In accordance with the profile of requirements thereon, a multitude of liquids are used. The liquids should be liquid at room temperature or even lower temperatures and in particular have low viscosities. For relatively high use temperatures, water is no longer an option; its vapor pressure would be too great. Therefore, hydrocarbons are used up to 250° C., which usually consist of aromatic and aliphatic molecular moieties. In many cases, oligomeric siloxanes are also used.
A new challenge for heat carrier liquids is that of solar thermal power plants, which generate electrical energy on a large scale. Such power plants have been built with a cumulative installed power of about 400 megawatts to date. The solar radiation is focused by means of parabolic mirror troughs into the focus line of the mirrors. A metal tube located there is within a glass tube to prevent heat losses, the space between the concentric tubes being evacuated. A heat carrier liquid flows through the metal tube. According to the prior art, a mixture of diphenyl ether and diphenyl is used here.
The heat carrier is heated to a maximum of 400° C., and it is used to operate a steam boiler in which water is evaporated. This steam drives a turbine and this in turn drives the generator, as in a conventional power plant. Thus, overall efficiencies around 20 to 23% are achieved, based on the energy content of the solar radiation.
Both components of the heat carrier boil at 256° C. under standard pressure. The melting point of diphenyl is 70° C., that of diphenyl ether 28° C. The mixing of the two substances lowers the melting point to about 10° C. The mixture of the two components can be used up to a maximum of 400° C., decomposition occurs at higher temperatures. The vapor pressure at this temperature is about 10 bar, a pressure which can still be managed efficiently in industry.
In order to obtain higher efficiencies than 20 to 23%, higher steam input temperatures are needed. The efficiency of a steam turbine rises with the turbine input temperature. Modern fossil-fired power plants work with steam input temperatures up to 650° C. and thus achieve efficiencies around 45%. It would be entirely technically possible to heat the heat carrier liquid in the focus line of the mirrors to temperatures around 650° C. and hence likewise to achieve such high efficiencies; however, this is prevented by the limited thermal stability of the heat carrier liquids. There are obviously no organic substances which are capable of withstanding temperatures above 400° C. for a prolonged period; at least, none are known to date. There have therefore been attempts to switch to inorganic, more thermally stable liquids.
The possibility, known from nuclear technology, of using liquid sodium as a heat carrier liquid has been the subject of intense examination. However, practical use was opposed by the fact that sodium is quite expensive, that it has to be produced by electrolysis of sodium chloride with high energy expenditure, and that it reacts even with traces of water to evolve hydrogen, thus constituting a safety problem.
Another possibility would be that of using inorganic salt melts as the heat carrier liquid. Such salt melts are prior art in processes which work at high temperatures. With mixtures of potassium nitrate, potassium nitrite and the corresponding sodium salts, working temperatures up to 500° C. are attained. The fertilizer industry is capable of producing large amounts. However, salt melts are not used in solar thermal power plants because of two considerable disadvantages: being nitrates and nitrites, they have strongly oxidizing action at elevated temperatures on the metallic materials, preferably steels, as a result of which the upper use temperature thereof is limited to the approximately 500 degrees mentioned. Due to their crystalline melting point, the lowermost use temperature thereof is about 160° C.
The addition of nitrites or nitrates of alkaline earth metals achieves a further lowering in the melting point, but this is at the cost of an increase in the melt viscosity at low temperatures. In addition to an increase in the pump energy, however, this adversely affects the heat transfer.
There have likewise been studies of whether water is suitable as a heat carrier under appropriately high pressure. However, this is opposed by the extremely high vapor pressure of more than 300 bar, which would make the thousands of kilometers of pipelines in a large thermal solar power plant uneconomically expensive. Steam itself is unsuitable as a heat carrier due to its comparatively low thermal conductivity and the low heat capacity per unit volume compared to a liquid.
A further problem arises from the aim of also operating a solar thermal power plant during the night. For this purpose, considerable amounts of heat carrier liquid have to be stored in large thermally insulated tanks. If the intention is to store the heat content for a power plant with an electrical power around one gigawatt for thirteen to fourteen hours, this requires tank fillings of the order of magnitude of one hundred thousand cubic meters at 600° C. and an efficiency around 40% from the heat reservoir to the generator output. This means that the heat carrier must be very inexpensive, otherwise the capital cost of such a power plant will be uneconomically high. It also means that sufficient material of the heat carrier must be available, since supply on a large scale will require hundreds of one gigawatt units.
Thus, the solution to the question of economic supply with solar energy ultimately depends on a large scale on whether there is a heat carrier liquid which enables temperatures up to 650° C. in constant use, which has a minimum, economically manageable vapor pressure at this temperature, preferably up to ten bar, which does not oxidatively attack the ferrous materials used and which has a minimum melting point.
At first glance, these conditions are most likely to be met by elemental sulfur. Sulfur is available in sufficiently large amounts. There are very large, plentiful deposits of sulfur, and sulfur is obtained as waste in the desulfurization of fuels and natural gas. At present, there is no possible use for millions of tonnes of sulfur; it is stored in the form of large cast blocks or used to fill lake-sized soil formations.
The melting point of sulfur, at merely 120° C., is advantageously lower than the eutectic melting points of the currently used nitrate/nitrite mixtures. The melting point of sulfur, at 444° C., is within the correct range; decomposition is impossible. The vapor pressures are 2.1 bar at 500° C., 3.9 bar at 550° C., and 6.6 bar at 600° C. The vapor pressure at 650° C. is around 10 bar, a pressure which is still easily manageable by technical means. Above 650° C., the equilibrium vapor pressure of sulfur rises relatively steeply; at 700° C. it is 16.7 bar. Overall, the sulfur equilibrium vapor pressure is represented quite well by the equation log P (bar)=4.57579−3288.5/T (° K) (Thermodynamic Properties of Sulfur, James R. West, Ind. Eng. Chem., vol. 42, No. 4, 713 (1950)).
The density of liquid sulfur averages 1.6 kg/liter over wide temperature ranges, and the specific heat is around 1000 joules per kg and degree or around 1600 joules per liter and degree. It is thus below that of water at around 4000 joules per liter and degree, but above the specific heat of most customary organic heat carriers (substance data: Hans Günther Hirschberg, Handbuch Verfahrenstechnik and Anlagenbau [Handbook of process engineering and plant construction], page 166, Springer Verlag 1999, ISBN 3540606238).
However, elemental sulfur has a serious disadvantage for use as a heat carrier liquid: within the temperature range from about 160 to 230° C., the cyclic sulfur molecules polymerize with ring opening to give very long chains. While the viscosity above the melting range is around 7 mPas, it rises at 160° C. to 23 mPas, and attains maximum values around 100 000 mPas at temperatures in the range from 170 to 200° C. The polymerization of sulfur to give chain molecules thus causes an enormous rise in viscosity, which means that the sulfur is no longer pumpable within this temperature range, and is thus unsuitable for application.
It was the aim of the invention to find a heat carrier liquid based on sulfur, which does not exhibit the disadvantage outlined above, the high viscosity rise.
It is known from the scientific literature that small additions of hydrogen sulfide or halogens prevent the disadvantageous viscosity rise of sulfur. However, this finding has not been utilized to date in order to employ sulfur as a heat carrier and storage liquid.
Heat carrier and heat storage liquids according to the invention thus comprise, in addition to sulfur:

- a) hydrogen sulfide in such an amount that it forms, within the temperature range from 120 to 650° C., an equilibrium vapor pressure of 0.1 to 10 bar, preferably of 1 to 3 bar,
- b) optionally up to 10% by weight of a halogen, preferably chlorine,
- c) optionally up to 15% by weight of phosphorus.

Hydrogen sulfide causes the formation of shortened, low-viscosity sulfur chains with SH end groups or sulfane end groups. The chain length results from the concentration of terminators used (Topics in Current Chemistry, vol. 230, “Elemental Sulfur and Sulfur-Rich Compounds”, Springer, Heidelberg 2003, pages 92, 93).
Introduction of hydrogen sulfide into an unmodified sulfur melt over a period of 90 minutes and increasing the temperature from 125° C. to 190° C. completely prevented the rise in viscosity of the melt; 0.09 Pas was measured instead of 93 Pas. The proportion by weight of hydrogen sulfide which dissolves in the sulfur melt is 0.01 to 1%.
A comparatively small viscosity increase occurs within the temperature range from 250 to 350° C., but this is much less than for unmodified sulfur. The melting point is lowered only slightly to temperatures between 113 and 115° C.
Up to 370° C., the solubility of hydrogen sulfide in the sulfur melt increases owing to the formation of the SH-terminated polysulfanes. Between 300 and 370° C., approx. 0.2% by weight of hydrogen sulfide is absorbed at standard pressure (Wiewiorowski and Touro, J. Phys. Chem., 70, 234 (1966); R. Fanelli, “Solubility of Hydrogen Sulfide in Sulfur”, Ind. Eng. Chem. 41, 2031 -2033; Denis Yu Zezin et al. “The solubility of gold in hydrogen sulfide gas: An experimental study”, Geochemica et Cosmochimica Acta 71 (2007) 3070 - 3081).
This involves introducing the hydrogen sulfide into a stirred melt under standard pressure or elevated pressure within the temperature range from 150° C. to 370° C. over a period of 1 to 5 hours. The reaction of the hydrogen sulfide with the sulfur chains is apparently slow due to the low solubility, which requires the comparatively long reaction times under the customary laboratory conditions.
According to the invention, the hydrogen sulfide vapor pressure over the sulfur melt at 130° C. is 0.1 to 10 bar, preferably 1 to 3 bar. In the event of a temperature increase, this pressure rises only slightly or even falls because more hydrogen sulfide is converted to sulfane end groups at rising temperatures.
The inventive liquid is produced by introducing hydrogen sulfide into a sulfur melt within the temperature range from 250 to 350° C. until saturation, the ultimate vapor pressure of the hydrogen sulfide being 0.1 to 10 bar, preferably 1 to 3 bar. In industrial performance, the apparatuses known in chemical process technology can be used for this purpose, such as sparging stirrers or reaction mixing pumps, in which the melt is contacted with gaseous hydrogen sulfide under intensive shear with high surface area, in order to keep the time until saturation of the sulfur melt to a minimum, much shorter than described in the scientific literature.
Both batchwise operations such as stirred tanks and, preferably continuous operations such as stirred tank cascades, flow tubes or the combination of reaction mixing pumps with flow tubes or postreaction vessels can be used to produce the inventive liquids.
It is also possible to obtain the hydrogen sulfide by chemical reactions directly in the sulfur melt. For example, it is only necessary to add 0.01 to 2% by weight, preferably 0.1 to 0.5% by weight, of an alkali metal hydrogensulfide to the sulfur melt, in the simplest case sodium hydrogensulfide, which is commercially available in the form of flakes with a water content around 30% by weight.
When the melt is heated to temperatures between 300 and 400° C., hydrogen sulfide is released from the hydrogen-sulfides according to the overall equation
2 MeHS—>Me₂S+H₂S.
The hydrogen sulfide thus formed reduces the length of sulfur chains as a result of sulfane formation.
The alkali metal sulfide formed reacts with the excess sulfur to give, in the case of sodium, sodium pentasulfide, which is insoluble in the sulfur melt according to the known phase diagrams (D. Lindberg, R. Backman, M. Hupa, P. Chartrand, “Thermodynamic evolution and optimization of the Na-K-S system”, J. Chem. Therm. 38, p. 900-915 (2006)).
Na₂S+S_x—>Na₂S₅
The alkali metal polysulfides formed are insoluble in the sulfur melt; above their melting point, they form droplets in the melt; below the melting point, for instance at 260° C. in the case of Na₂S₅, they form black-brown flakes which are easy to remove from the sulfur melt by filtration at low temperatures and viscosities, for example in the temperature range from 130 to 200° C.
The hydrogen sulfide-containing sulfur melts produced by the different variants should be stored above their melting point in order to prevent outgassing of the hydrogen sulfide, caused by the crystallization of sulfur. The sulfane end groups are apparently particularly stable in the liquid state. In the phase transition from liquid to solid, they would disrupt the crystal lattice; therefore, the system thus avoids eliminating hydrogen sulfide on crystallization.
At higher temperatures, just below the boiling point of the sulfur and without backpressure, i.e. also already at quite a high sulfur vapor pressure, hydrogen sulfide evaporates as expected, as a result of which elevated viscosities are obtained again in the course of cooling. In order to avoid this, the heat carrier liquid should be used in a closed system of pipelines, pumps, control units and vessels. For reasons of operating reliability alone, all pipe connections, vessels and control units must be absolutely impervious; no hydrogen sulfide may escape.
It is also possible to lower the viscosity of the sulfur by the addition of halogens. The most active is chlorine which, introduced in the form of sulfur chlorides such as SCl₂or S₂Cl₂, reacts to form SCl end groups and thus lowers the chain length. When 0.75% chlorine is introduced into pure sulfur in the form of sulfur chlorides, the viscosity thereof within the temperature range from 150 to 320° C. is adjusted to values less than 0.2 Pas. The viscosity is thus lowered by a factor of 500 (Topics in Current Chemistry, vol. 230, “Elemental Sulfur and Sulfur-Rich Compounds”, Springer, Heidelberg 2003, pages 92, 93).
Halogens, preferably chlorine, are introduced as the end group by reaction of the sulfur melt with sulfur halides, preferably disulfur dichloride. Disulfur dichloride boils at 138° C. under standard pressure. It is mixed into the low-viscosity melt at 130° C. under ambient pressure, then the temperature is increased under the vapor pressure which develops to 250° C. within one to two hours.
However, the use of hydrogen sulfide to lower the sulfur viscosity is preferred because halogens at elevated temperatures can contribute to the corrosion of the metallic materials used.
In some cases, the melting point of sulfur can be lowered by the addition of phosphorus. The binary phase diagrams (Robert Fairman and Boris Ushkov, “Semiconducting Chalcogenide Glass”, Elsevier Academic Press 2004, ISBN 01275 21879, 9780 1275 21879) show that a sulfur melt which contains 7 to 10% by weight (also approximately 7 to 10 atom percent) of phosphorus crystallizes at 80° C.
Phosphorus can be introduced into the sulfur melt as an element, but also in the form of the sulfides, preferably as P₄S₁₀. In the sulfur melt, the phosphorus is always present in pentavalent form owing to the sulfur excess.
In the studies for the invention, it was found that phosphorus has crosslinking action on the sulfur melt and can therefore disadvantageously increase the viscosity. Therefore, in the application, a decision will be made as to whether the lower melting point or the lower viscosity is more important for the end use.
In the presence of phosphorus sulfides, the risk of corrosion by chlorine is lower. On penetration of moisture into the heat carrier, the phosphorus sulfides are hydrolyzed more rapidly than the sulfur-chlorine bond, as a result of which the chloride-induced corrosion is suppressed. It is obvious that the inventive liquids must nevertheless be protected from the ingress of moisture in production, storage, transport and use.
The variant of production of the hydrogen sulfide via the alkali metal hydrogensulfides cannot be used when the sulfur contains phosphorus. In this case, the melt reacts to form considerable amounts of solid substances, probably alkali metal salts of a thiophosphoric acid.
According to the Fairman reference, arsenic and silicon also lower the sulfur melting point. If the phase diagrams are examined experimentally, it is found that molar proportions of arsenic, introduced into the melt as arsenic trisulfide, increase the viscosity of the sulfur melt much more than phosphorus in equal molar proportions. For this reason and owing to its toxicity, arsenic is not an option as an additive for lowering the sulfur melting point. Silicon disulfide does not even dissolve in a sulfur melt under economically viable conditions, as a result of which silicon is likewise not an option for lowering the sulfur melting point.
The operation of plants at temperatures up to 700° C. with the inventive low-viscosity sulfur requires inexpensive materials which are stable against sulfidation at these temperatures. Specifically in recent times, steels have been developed for the power plant sector, which are suitable for this use. Such ferrous materials have a ferritic structure and are free of nickel, the sulfides of which form low-melting phases with iron.
The most effective alloy constituent is aluminum, which forms an impervious, passivating oxide layer on the material surface. Such older materials contain around 22% by weight of chromium and 6% by weight (11 atomic %) of aluminum. They were known by the name ‘Kanthal’. For the purpose of stabilization against sulfidation, it has been found that a high aluminum content is more important than a high chromium content.
With 8.5% by weight (16 atomic %) of aluminum, iron alloys are obtained with an expansion at room temperature by 20%, with 10% by weight (19 atomic %) of aluminum expansions by 10%, and with 13 to 14% by weight (24 to 25 atomic %) of aluminum expansions by 3 to 5%.
With aluminum contents greater than 12% by weight (approx. 23 atomic %), sulfidation is completely suppressed at 800° C. under a particular media composition (“Sulfidation/Oxidation Properties of Iron-Based Alloys Containing Niobium and Aluminum”, V. J. DeVan, H. S. Hsu, M. Howell, May 1989, Oak Ridge National Laboratory, Fossil Energy Materials Program, AA 15 10 10 0).
Improved iron alloys thus contain less chromium and more aluminum, as claimed, for example, in EP 0652 297.
Described therein are alloys of composition 12 to 18 atomic % of aluminum, 0.1 to 10 atomic % of chromium, 0.1 to 2 atomic % of niobium, 0.2 to 2 atomic % of silicon, 0.01 to 2% by weight of titanium and 0.1 to 5 atomic % of boron. Niobium, boron or titanium serve to cause separation of fine grains of iron aluminide (Fe₃Al ) and to bind carbon in the form of carbides, which results in an improved toughness with expansions above 3% and improved processability.
Iron alloys with even higher aluminum contents are more stable towards sulfur but are no longer processable under cold conditions. They are extruded or rolled at elevated temperatures. Such alloys, which are Fe₃Al -base alloys, contain 21 atomic % of aluminum, 2 atomic % of chromium and 0.5 atomic % of niobium, or 26 atomic % of aluminum, 4 atomic % of titanium and 2 atomic % of vanadium, or 26 atomic % of aluminum and 4 atomic % of niobium, or 28 atomic % of aluminum, 5 atomic % of chromium, 0.5 atomic % of niobium and 0.2 atomic % of carbon (EP 0455 752).
Chromium-containing iron aluminide Fe₃Al with 3-5 atomic % of chromium may have expansions of 20% at 100° C. (Oak Ridge National Laboratory). Alloys with 27 atomic % of aluminum, 9 to 10 atomic % of chromium and 0.5 to 1 atomic % of niobium or molybdenum attain expansions by 20 to 30% even at room temperature (EP 0455 752).
Molybdenum is a preferred alloy element because it counteracts thermal decomposition of the dissolved hydrogen sulfide or of the sulfane end groups to aluminum sulfide and hydrogen. Molybdenum catalyzes reaction of hydrogen with sulfur to give hydrogen sulfide.
The mechanical strength of iron alloys with a high aluminum content is sufficiently great up to temperatures of 700° C. for use with the inventive heat carrier liquids.
However, it is also possible to treat oxidation-resistant materials based on iron with aluminum vapor or liquid aluminum, which forms, on the surface, iron aluminides with high aluminum contents greater than 20 atomic %, which have excellent protection against sulfidation. Such coatings are already used in the process industry and produced on a commercial basis.
With heat carrier liquids according to the invention, it is possible to operate solar thermal power plants with efficiencies of fossil-fired power plants, and to operate them day and night, without interruption, by means of storage tanks of appropriate dimensions for the hot liquid. Owing to the high efficiency, the capital costs per kilowatt hour fall by a factor of nearly 1.5 compared to the prior art.
It is also readily possible and advantageous to produce the inventive liquids close to the site of use. Liquid sulfur is typically delivered by ship. When, for example, 0.5% by weight of hydrogen sulfide is mixed continuously into 100 000 tonnes of sulfur, there are 500 tonnes of hydrogen sulfide. The hydrogen sulfide need not be transported; it is likewise produced continuously on site. For this purpose, the chemical industry has developed elegant ambient-pressure processes by which molten sulfur and hydrogen can be used, under the action of catalysts, to produce just as much hydrogen sulfide as is required at that time (for example WO 2008/087086). In a subsequent stage, the hydrogen sulfide is compressed to the pressure needed for mixing into the sulfur melt. There is no need to store any great amount of hydrogen sulfide.
The hydrogen is likewise produced continuously as required on site, by the electrolysis of water; the electrical power required for that purpose is drawn from adjacent power plants.
Owing to the favorable weight ratio of hydrogen to sulfur in the hydrogen sulfide, only about 30 tonnes of hydrogen are required for 500 tonnes of hydrogen sulfide. Thus, for the production of 100 000 tonnes of the inventive liquid, by way of example, only about 30 tonnes of hydrogen are needed. This corresponds to an energy expenditure of 10 megawatts over the period of about 150 hours.
The possible disadvantage of the melting point around 116° C. without a phosphorus content to lower the melting point can be countered by design means with a low level of complexity by setting up the mirrors with a slight gradient and letting or sucking the heat carrier liquid out of the tubes just before sunset and storing it in the liquid state a few degrees above the melting point (for instance at 130° C.), in heat-insulated buffer tanks for operation the next day.
A particularly simple method is found to be the emptying of the part of the pipeline system to be cooled without a gradient in the construction, by forcing the sulfur out of the cooling pipelines briefly into the buffer tank by means of the sulfur vapor pressure from the high-temperature section. Excess sulfur vapor condenses therein and in the cooling pipeline sections.
This operation can be accomplished, for example, by temporarily opening a bypass to circumvent the pumps in the low-temperature section which convey the liquid against the vapor pressure and corresponding pressure-retaining valves. This does not give rise to any disadvantage for the operation of a plant which works with the inventive heat carrier and heat storage liquid. It is not necessary to completely empty the pipeline system when care is taken in the course of construction of the plant that there are no moving parts, such as pumps or valves, in the cooling pipeline sections. In this case, residues of sulfur can crystallize therein and be melted again without disadvantages.
Because the tanks for storage of the hot liquid must have an appropriately large volume and are additionally under the vapor pressure of the liquid, it is advantageous not to set up the tanks above ground, but to build them into the land surface. In this case, the liquid and vapor pressure can be absorbed by the masses of earth which surround the tank and the thermal insulation thereof.
However, the inventive heat carrier liquid is also suitable for all other fields of use of heat transfer and of heat storage in industry, which require an extremely wide temperature range of liquid phase and high temperatures.
Owing to its sulfur basis, it is the least expensive of all alternatives.

Claims

1. A method for transporting and storing thermal energy, said method comprising:

providing a liquid that includes sulfur modified with inorganic components, and

using said liquid for transport and storage of thermal energy.

2. The method of claim 1, wherein providing a liquid comprises providing a liquid comprises comprising hydrogen sulfide, wherein the hydrogen sulfide vapor pressure of the liquid in the use temperature range from 130 to 700° C. is 0.1 to 10 bar.

3. The method of claim 2, wherein providing a liquid comprising hydrogen sulfide comprises obtaining hydrogen sulfide by a chemical reaction in the presence of the sulfur melt.

4. The method of claim 1, wherein providing a liquid that includes sulfur modified by inorganic components comprises providing a liquid that includes up to 10 percent by weight of a halogen.

5. The method of claim 1, wherein providing a liquid that includes sulfur modified by inorganic components comprises providing a liquid having up to 15% by weight of phosphorus.

6. The method of claim 2, wherein the vapor pressure is between 1 bar and 3 bar.

7. The method of claim 4, further comprising selecting the halogen to be chlorine.

8. The method of claim 2, wherein providing a liquid that includes sulfur modified by inorganic components comprises providing a liquid having up to 15% by weight of phosphorus.

9. The method of claim 3, wherein providing a liquid that includes sulfur modified by inorganic components comprises providing a liquid having up to 15% by weight of phosphorus.

10. The method of claim 4, wherein providing a liquid that includes sulfur modified by inorganic components comprises providing a liquid having up to 15% by weight of phosphorus.

11. A composition of matter for transporting or storing thermal energy, said composition comprising a liquid that includes sulfur modified with inorganic components.

12. The composition of claim 11, wherein the liquid comprises hydrogen sulfide, wherein the hydrogen sulfide vapor pressure of the liquid in the use temperature range from 130 to 700° C. is 0.1 to 10 bar.

13. The composition of claim 11, wherein the sulfur is modified by up to 10 percent by weight of a halogen.

14. The composition of claim 13, wherein the halogen is chlorine.

15. The composition of claim 11, wherein the liquid includes up to 15% by weight of phosphorus.

16. The composition of claim 12, wherein the vapor pressure is between 1 and 3 bar.