CA2081378A1

CA2081378A1 - Storage salt mixtures

Info

Publication number: CA2081378A1
Application number: CA002081378A
Authority: CA
Inventors: Gerd Hoermannsdoerfer
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-12-31
Filing date: 1991-12-31
Publication date: 1992-07-01
Also published as: KR920703757A; EP0531464A1; DE4042268A1; EP0531464B1; WO1992012217A1; BR9106245A; JPH05504787A; DE59107486D1

Abstract

(57) Abstract The invention relates to lalent heat storage agents for storing thermal energy, The proposal is for the use of mixtures or var-ious perchlorates or perchlorates-hydrates with other organic substances or inorganic salts or hydrate salts. As is known, pure lithium perchlorate trihydrate melts at a temperature of 94.3 °C and, with a value of 306 J/g, has a markedly high melting en-thalpy. Its density of 1.89 also gives the very high volume-specific figure of 578 J/cm3. Because, however, its melting point is close to the boiling point of water it is not of practicable use in upressurised systems in which water is the heat conveyor.
However, the various mixtures proposes by the invention have lower melting points in a broad temperature range, also with very considerable melting enthalpies which are almost the same or not very much lower than that of the pure initial sub-stances.

Description

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STORAGE SALT MIXTURES

The invention relates to phase change materials which comprise mixtures of perchlorates or perchlorate hydrates and other inorganic salts, or salt hydrates, or organic substances and to phase change material devices filled with mixtures of this type.
It is known that when a substance is heated it absorbs a specific amount of energy during the trans-ition from the solid state into the liquid state.
Corresponding heating curves therefore show, after an initial temperature increase, a temperature plateau at the level of the melting point, before a further tempera-ture increase occurs after complete melting of the su~stance. For many substances this process is reversible. During cooling, therefore, the substance remains at the solidification temperature for a corresponding period, while the heat previously absorbed during melting is given off again. Because of the fact that this energy of fusion may be about one hundred to two hundred times larger than the specific heat of the substance, it thus becomes possible to store a fairly large amount of energy in a narrow temperature range with a relatively small volume requirement.
Substances for the storage of energy by making use of the solid/liquid phase transition and vice versa are called phase change materials (PCMs). Generally, they should have as high an enthalpy of fusion as possible;
what matters as a rule is the volume-specific enthalpy of fusion (i.e. that relative to the volume), in order to provide the maximum storage capacity per unit volume of the available storage space. In addition, such PCMs must be stable with regard to cycling, i.e. the solid-liquid-solid phase transition must remain reversibly repro-ducible over long periods of time and must not be adversely affected by chemical reactions, separation, elimination of water of crystallisation or similar processes. Other important criteria may be the , ~ , .~. . :
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, 2~8~78 Wo 92/12217 - 2 - pc~r/DE 91/01014 solidification beha~iour (e.g. the formation of a metastable melt, the extent of a volume change during the phase transition or the form of crystallisation), and also compatibility with the construction materials, physiological acceptability and availability at an accep-table price. It has so far proved difficult to find PCMs which meet all of these crit0ria in a manner optimal for the particular application.
Many of the previously known PCMs were developed for application in the field of space heating (e.g. in conjunction with solar panels or heat pumps), and accoxd-ingly have melting points in the process-water range.
Besides inorganic substances, such as salt hydxates and salt mixtures, organic substances have also been proposed for applications of this type.
Similar application areas for PCMs e~ist in industrial processes, if waste heat which can be used in another way is to be stored, or if heat reserves must be held available to cope with load peaks.
PCMs can also be used for special applications such as automotive preheating systems, thermal control devices in satellites, hotplates for food, in medical technology, or heat protection systems for measurement electronics, for example in industrial processes, or for borehole probes in geophysics.
In the case of the application areas listed as examples there is still a xequirement for improved PCMs, due on the one hand to the inadequacy of the substances previously used and on the other hand to the striving for constant improvement. In particular, it is the necessity to accommodate as large an energy content as possible into a given ~olume which establishes the object of providing PCMs whose volume-specific storage capacity reaches the physical limits.
According to previous accepted teachings, both for organic and for inorganic substances, maximum energy densities of the enthalpy of fusion of up to about 300 J/cm3 are to be e~pected in the temperature range up , ~-. .. .
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to about 130C (cf. Dr. P. Kesselring, "Zur Energiedichte im Latentwarmespeicher ~ einige grundsatzliche Uberlegungen" ~On energy density in the PCM device - some principal considerations], VDI-Berichte No. 288, 1977).
The Applicant's own investigations showed that on purely physical grounds maximum values of the volume-specific enthalpy of fusion are possible which correspond, at any melting point, to about the value of the melting point of the substance in degrees Kelvin multiplied by the factor 2, so that for example a substance with the melting point 95C (corresponding to about 368K) could, in the most favourahle case, have an enthalpy of fusion of approximately 740 J/cm3.
Although a substance proposed in US Patent lS 4,057,101, i.e. lithium perchloxate trihydrate, still falls slightly short of this maximum value, said substance does have comparatively very high calorific values and also excellent melting and solidification behaviour. Among the salt hydrates it has very probably the best crystallisation properties. Its thermal conductivity is also unusually high.
The Applicant's own investigations on the DSC
apparatus on the pure substance showed, at a melting point of 94.3C, an enthalpy of fusion of 306 J/g and a specific heat of 1.5 J/g/K in the solid state and 1.98 J/g/K in the liquid state. By means of the density given in the literature of 1.89 g/cm3 a volume-specific enthalpy of fusion of 578 J/cm~ is thus calculated, and a specific heat relative to volume of 2.84 J/cm3/K in the solid state and 3.74 J/cm3/K in the liquid st2te.
Unfortunately the melting point of lithium-perchlorate trihydrate, at 94.2C, is very close to the boiling point of water, which makes it virtually unusable in unpressurised system~ in which, for example, water is used for the heat transfer medium~ Its melting tempera-ture is also too high for it to be used as a heat storage medium within preheating units for automotive cooling-water or heating~water circulation systems. Even in the ..

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wo 92/12217 - 4 - PC~/DE 91/01014 case of the application intended in said patent, as a heat sink for electronic subassemblies, a lower melting point would be desirable in a number of instances.
The object was therefore to provide phase change materials which have similarly positive properties as has lithium perchlorate trihydrate and whose melting points should also cover a lower temperature range as widely as possible.
The ob~ect of the invention is achieved by mi~ing of specific perchlorates or perchlorate hydrates with other inorganic salts or salt hydrates or with organic su~stances.
The invention provides a whole family of mixtures of this type which cover a wide melting point range and which have, at the same time, very high enthalpies of melting. For these mixtures having at least one component from the group of the perchlorates or perchlorate hydrates there are proposed as additional components other perchlorates, the chlorides, bromides, nitrates and hydroxides, or in each case their hydrates, these addi-tional components preferably being selected from those groups which contain the elements lithium, sodium, potassium, magnesium, calcium, stxontium or barium.
A further family, those mixtures are proposed which, in at least one component, comprise a perchlorate or perchlorate hydrate, and as an additional component of at least one admixed organic substance, the admixed organic substances being selected from those groups which cannot be oxidised by perchlorates.
It was found that the perchlorates or perchlorate hydrates form, with one another or with the other sub stances mentioned, binary, ternary and multinary systems with eutectic or invariant melting points, thus allowing the selection of a series of diferent melting points, depending on the mixture. It is particularly surprising that some of these mixtures have decidedly high enthalpies of melting which are not very much lower than, for example, that of pure lithium perchlorate trihydrate.

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An additional advantage of the proposed mixtures resides in the formation of a very fine structure of the solidified mixture, as a result of which the detrimenkal and obstructive formation of coarse crystals, as it is known of the pure substances, is reliably inhibited.
Although some of the perchlorates contained in the mixtures have, with regard to physiological acceptability, been classified as irritants, and as a rule also as detrimental to health on swallowingr no MAC
values have been established. For the technical applications envisaged, the classifications of the perchlorates are relatively favourable, because they may thus be able to replace other substances having considerably greater hazardous potential. It should lS however be borne in mind that there is a risk of e~plosion, in particular if anhydrous perchlorates are mixed with combustible materials. In the case of industrial applications, therefore, appropriate design safeguards are required to preclude reliably any contact with such materials.
On the other hand it is an advantage that perchlorates and perchlorate hydrates do not attack aluminium and its alloys, thus allowing the cost-effective construction of appropriate containers and the like from aluminium-containing materials.
On the basis of the investigations so far, the mixtures provided by the invention may, for the time being, be divided into various groups. The most important group is formed by those mixtures which comprise predominantly, in terms of molar fractions, lithium per~chlorate trihydrate, which mixtures fuse and solidify congruently and on the whole, apart from having different melting points, show virtually no difference from ~he behaviour of pure lithium perchlorate trihydrate, thus, even in the case o~ a coxre~pondingly eutectic mixture composition, uniformly undergo the phase transition at a constant temperature. They also have in common that they return from the liquid to the solid skate without " ..
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supercooling.
This group includes a eutectic mixture of 79 mol of lithium perchlorate trihy~rate with 21 mol of its own monohydrate which melts at 91.9C and whose enthalpy of fusion barely differs from that of the pure trihydrate.
In addition, eutectic mixtures of lithium perchlorate trihydrate and 9 mol% of sodium chloride or 14 mol% of sodium hromide belong to this group (see Figure 1 and Figure 2); the melting points of these mixtures were found to be 87.7C and 82.5~C respectively, and their enthalpies of melting are also surprisingly high.
Mixtures of lithium pexchlorate trihydra~e and potassium perchlorate, and of magnesium perchlorate hexahydrate, magnesium chloride hexahydrate, lithium nitrate and sodium nitrate may well also belong to this group. In the case wh~re lithium nitrate is admixed it is particularly surprising that it forms, without water of crystallisation, a eutectic with lithium perchlorate at about 80.6C (see Figure 3), although lithium nitrate itself normally melts at 29.9C as the trihydrate. In the case of this eutectic the volume-specific enthalpy of fusion, in spite of the depression of the melting point, is again virtually identical to that of pure lithium perchlorate trihydrate.
A second group is formed by those mixtures which also for the greater part consist of lithium perchlorate trihydrate, but which in previous in~estigations gave somewhat indifferent results. Those are admixtures of potassium chloride, or lithium chloride monohydrate. In both cases the curves obtained by the DSC equipment do not show good reproducibility and are subject to constant change. It is thought that in the case of the admixture o lithium chloride monohydrate, the extreme hygroscopicity of the substance, produces an excess of water in the mixture, which may he the cause of an indifferent melting behaviour. Although this mix~ure may possibly melt incongruently, the quasi-binary system it .

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foxms appears to have at least an invariant melting point at about 73.0C. Although the molar composition has so far not been determined exactly, a ~alue for the enthalpy of fusion was measured which, in relation to the weight, is virtually identical with that of pure lithium perchlorate trihydrate.
The third group comprises mixtures which have a considerable molar proportion of lithium perchlorate trihydrate, but which, because of the added component, melt incongruently or peritectically, some of these mixtures tending to exhibit the behaviour which was found by the Applicant and which has be~n called ~'dry melting"
(see EP ~pplication 90730008). Among these are in par-ticular admixtures of lithium hydroxide monohydra~e or strontium hydroxide octahydrate. During investigations in the DSC equipment, a melting point of 40.0C and an enthalpy of fusion of about 300 J/g were found for such a quasi-eutectic mixture of 2 mol of lithium perchlorate trihydrate and 1 mol of strontium hydroxide octahydrate.
2G In the case of a mixture of 38 mol% of lithium perchlorate trihydrate and 62 mol~ of lithium hydroxide monohydrate, the melting point was dete~mined as 66.8C
and the enthalpy of fusion as 288 ~/g.
Said mixture of lithium perchlorate trihydrate with stron~ium hydroxide octahydrate can for example be used in the field of heating of buildings, for example to store solar energy. When dimensioning the corresponding containers, a slightly increased expansion space, for example by means of elastic walls, should be taken into account for this mixture because it still shows a slight volume increase after solidification.
The next group includes those mixtures which consist of a smaller fraction of a perchlora~e hydrate and a predominant residual fraction or example of chlorides or bromides, hydroxides or nitrates, or the respective hydrate, of the elements lithium, sodium, potassium, magnesium, calcium, strontium or barium.

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2~18i378 A further group i5 essentially fo~ned by ~hose mixtures which consist of a usually fairly small fraction of potassium perchlorate, the remaining fraction consist-ing of selected chlorides or bromides, hydroxides or nitrates, or their respective hydrates, of the elemenks lithium, sodium, potassium, magnesium, calcium, strontium or barium.
Finally, according to the invention, there exists a separate group of those mixtures which are mixed from a proportion of at least one perchlorate or perchlorate hydrate and a further proportion of an organic substance, the organic substance being selected from that group which cannot be oxidised by perchlorates. The mixtures of this group are distinguished by a pronounced tendency to supercooling. They are therefore advantageously intended for those applications where heat is to be available, for example, on demand (by means of deliberate initiation of crystallisation). Typical examples from this group are mixtures of lithium perchlorate trihydrate and mannitol or pentaerythritol.
Of the various groups~ the use of the higher-melting mixtures having melting points from about 70 to about 92C may be advantageous, for example, for a storage heater using off-peak electricity, whilst those mixtures with melting points from about 40 to about 70C
appear appropriate in conjunction with solar panels.
The~e mixtures also offer advantages in the case of latent heat sinks for heat protection systems for the purpose of thermal protection of temperature-sensitive measurement electronics, for example in continuous furnaces or in borehole probes for the geophysical investigation of deep boreholes.
According to another aspect of the invention, those of the abovementioned mixtures with a melting point between 60 and 80C are proposed for use in auto-motive preheating units. The object in this case is to charge a PCM device by means of the engine cooling water while the engine is running, and to store this amount of ~ : .

~08~378 heat over several days with as little loss as possible in order, in the case of a cold start, either to allow the immediate operation of the car heating or to short~n the cold start phase of the engine, in order to reduce wear and emissions. Units of this type are currently being developed, the PCM barium hydroxide octahydrate being used experimentally in prototyp~s at present.
Barium hydroxide octahydrate has the dràwbacks that it is very toxic and that, with a melting point of 78C, it is slightly above the temperature of about 70C considered most suitahle. Particularly serious however is the fact that barium hydroxide octahydrate~ like all alkali metal hydroxides, reacts violently with aluminium and its alloys, generating heat and fission gases~ rapidly destroying components made of said light metals. Since not only the radiator but also in some cases the cylinder heads and the engine blocks in modern motor vehicles are made from aluminium or aluminium alloys, such subassemblies would be damaged in the event of barium hydroxide octahydrate discharging into the cooling water circuit due to a leak of the storage container~ A risk of this type is eliminated according to the invention by the use of aluminium-compatible mixtures based on perchlorates or perchlorate hydrates. The comparatively higher cost of mixtures of this type does however have to be taken into account.
The invention further opens up various possibilities here of using particular mixtures above or below the eutectic point, so as to define a melting range instead of a fixed melting point. As an example of an adaptation of this type, a mixture of lithium perchlorate trihydrate and sodium nitrate may be mentioned, which melts eutectically at 64.0C at a molar composition of 62 to 38 (see Figure 4). By reducing the proportion of sodium nitrate a melting range extending to higher temperatures is formed, which can be determined exactly by defining the proportions o the mixture. In this way it is possible to adapt exactly, for said application .

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Wo 92/12217 - 10 - PCT/DE 91/01014 example, the melting point range of the PCM to the temperature conditions in the cooling water circulation system of various engine types (for example diesel or petrol).
From the larqe number of mixtures provided by the invention, a mixture of strontium perchlorate and strontium hydroxide octahydrate again appears to be of interest for automotive application, because this mixture has a very favourable melting point of 71.2C.
The object of reducing the melting point of barium hydroxide octahydrate, in the case where this i5 used as an automotive PCM, is also achieved according to the invention in several ways which consist in admixing perchlorates or perchlorate hydrates~ As an example of an adaptation of this type, a mixture of 86 mol of barium hydroxide octahydrate and 14 mol of po~assium perchlorate is to be mentioned here, which mixture has a melting point of 76.5C and whose volume-specific enthalpy of fusion is even slightly higher than that of pure barium hydroxide octahydrate.
For an overview of the invention a table has been appended whose upper section lists data for various pure substances. Selected examples of mixtures according to the invention are compiled in the lower section. Fox the sake of simplicity the abbreviations in parentheses in the upper section have been used in the lower section.
The molar proportions of each mixture are given where they have already been determined. In the case of virtually eutectic mixture proportions an ~E" has been added in parenthesesO After the temperature stated for each melting point determined, a parenthetic indication of the melting behaviour observed is also given. The abbreviations and the symbol used are explained at the foot of the table.
The figures show, as an example, four phase diagrams, obtained from DSC melting curves, of the following quasi-binary systems:
Fig. 1 ~ithium perchlorate trihydrate/sodium chloride ` ' ' ~

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W0 Y2/12217 ~ PCT/DE 91/01014 Fig. 2 Lithium perchlorate trihydrate/sodium bromide Fig, 3 Lithium perchlorate trihydrate/lithium nitrate Fig. 4 ~ithium perchlorate trihydrate/sodium nitrate It can be clearly seen that these systems behave eutectically, that section of the curve which runs towards the anhydrous component in each case naturally being very steep.
Overall the invention provides a family of storage hydrate mixtures which allows the selection of a wide variety of melting points in the process water range, has very nigh volume specific enthalpies of melting and is stable with regard to cycling. In terms of its physiological acceptability it appears relatively acceptable. ~he main mixture components are available in large quantities at favourable costs. Most of the mix-tures proposed have no, or only a small, tendency to segregate. The solidification structure is advantageously finely crystalline, the volume change upon melting is minimal. A chemical attack of aluminium and its alloys either does not happen at all or, in the case of some of the mixtures proposed, is relatively mild. The vapour pressure of some of these mixtures is distinctly lowered.
This provides the conditions for improved performance and improved properties of phase change material devices.

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WO 92/12217~ 12 - PCTtDE 91/01014 TABLE

Data Substance Melting p~int Density Enthalpy of ~usion (~C) (g/cm3) (J/g)(J/cm3) 5 Ca(ClO4)2 x 6H~O (CPH) 71.9 125 --Ba(OH)2 x 8H2O (BHO) 78.0 2.16 272 588 Sr(OH)2 x 8H2O (SHO) 85.0 1.91 344 657 LiCl04 x 3H2O (LPT) 94.3 1.89 305 576 LiCl x H2O (LCM) 95 1.73 230 398 LiOH x H20 (LHM) 106.8 1.51 419 633 MgCl2 x 6H2O (MCH) 117 1.57 169 265 Mannitol (NA) 166 1.50 306 459 Pent~erythritol (PE) (T) 189 1.55 294 456 Mg(ClO4)2 x 6H2O (MPH) 193 1.98 --- ---LiNO3 (LN) 252.7 2.36 367 866 NaNO3 (NN) 306 2.26 172 389 ~aBr (NB) 741 3.20 224 717 KCl (KC) 772 1.98 342 677 NaCl (NC) 800 2.16 493 1065 Mixtures (mol/~ol) LPT/SHO 20/lO (E) 40,0 (*) 1.90 300 570 LPT/NA --/-- 48.1 (c,s) ---- --- ---LPT/PE --/-- 57.2 (c,s) ---- --~
LPT/CPH --/-- 60.9 (c) ---- --- ---LPT/NN 62/38 (E) 64.0 (c) 1.98 243 481 LPT/L~N 38/62 (E) 66.8 (*) 1.77 288 510 LPT/LCN 56/44 73.0 (i) 1.85 305 564 LPT/KC --/-- 73.6 (i) ---- --- --- :-BHO/KP 86/14 76.5 (i) 2.18 275 600 LPT/LN 70/30 (E) 80.6 (c) 1.96 296 580 LPT/MCH 90/10 82.3 (c) 1.85 274 507 LPT/NB 86/14 (E) 82.5 (c) 2.01 281 565 L~T/KC 90/10 85.2 (i) 1.89 267 505 LPT/NC 91/9 (E) 87.7 (c) 1.90 304 578 LPT/MPH 90/lO 90.0 (c) 1.91 264 504 LPT/LPM 79/21 (E) 91.9 (c) 1.94 294 570 LPT/KP 90/10 93.8 (c) 1.95 284 554 (c) ~ congruent melting (i) - incongruent melting (s) 3 supercooling (*) ~ "dry" melting (T) ~ transition point (E) ~ eutectic -: ~, '' ' '

Claims

Storage Salt Mixtures Patent Claims l. Use of a salt mixture for storing latent thermal energy, this salt mixture absorbing heat energy during the solid-liquid phase transition and/or giving off heat energy during the liquid-solid phase transition, charac-terised in that this salt mixture comprises one or more perchlorates or perchlorate hydrates in a molar fraction of between 5% and 95%.
2. Use of a salt mixture according to Claim l, characterised in that the perchlorates or perchlorate hydrates are selected from those groups which contain the elements lithium, sodium, potassium, magnesium, calcium, strontium and barium.
3. Use of a salt mixture according to Claim 1, characterised in that the perchlorate is potassium perchlorate and/or the perchlorate hydrate is lithium perchlorate trihydrate.
4. Use of a salt mixture according to one or more of the preceding claims, characterised in that the further admixed salts are selected from the group of the chlorides or chloride hydrates.
5. Use of a salt mixture according to Claim 4, characterised in that the chlorides or chloride hydrates are lithium chloride or lithium chloride monohydrate, or potassium chloride and/or preferably sodium chloride.
6. Use of a salt mixture according to one or more of the preceding claims, characterised in that the further admixed salts are selected from the group of the nitrates or nitrate hydrates.
7. Use of a salt mixture according to Claim 6, characterised in that the nitrates or nitrate hydrates are preferably lithium nitrate or lithium nitrate trihydrate, sodium nitrate or potassium nitrate.
8. Use of a salt mixture according to one or more of the preceding claims, characterised in that the further admixed salts are selected from the group of the hydroxides or hydroxide hydrates.
9. Use of a salt mixture according to Claim 8, characterised in that the hydroxide hydrates are prefer-ably lithium hydroxide monohydrate, sodium hydroxide monohydrate, potassium hydroxide monohydrate, strontium hydroxide octahydrate or barium hydroxide octahydrate.
10. Use of a salt mixture according to one or more of the preceding claims, characterised in that the second or at least one further salt of the admixed salts is also a perchlorate or a perchlorate hydrate.
11. Use of a salt mixture according to one or more of the preceding claims, characterised in that a small excess amount of water is added.
12. Use of a salt mixture according to one or more of the preceding claims, characterised in that at least one organic substance is admixed which is chosen from that group which cannot be oxidised by perchlorates.
13. Use of a mixture according to the above claim, characterised in that the organic substance is selected from the so-called sugar alcohols.
14. Phase change material device or phase change material device element, comprising a sealable container which is charged at least partially with an inorganic salt mixture for storing latent thermal energy, this mixture either being meltable by application of a higher temperature and resolidifying by applying a lower temperature, or a hydrate contained in this mixture eliminating water of crystallisation by application of a higher temperature and reabsorbing this eliminated water of crystallisation by application of a lower temperature, heat energy being absorbed either by the solid-liquid phase transition or by the elimination of water of crystallisation, and/or heat energy being given off either by the liquid-solid phase transition or by the readdition of water of crystallisation, characterised in that a mixture according to Claims 1 to 13 is used for storing latent thermal energy.
15. Phase change material device for motor vehicles, chargeable by the engine cooling water and having a flowpath for the engine cooling water and at least one storage medium area separated therefrom by a heat exchange surface, characterised in that the storage medium used is a mixture according to Claims 1 to 13 whose melting point is selected in the range between 55 and 85°C, preferably in the range between 60 and 80°C.
16. Heat protection system for the thermal protection of a measurement-function subassembly of, for example, measurement electronics and/or sensors, comprising a thermally insulating container, a heat shield or Dewar vessel, and one or more latent heat sinks incorporated therein, characterised in that the latent heat sinks are filled with a storage hydrate mixture according to Claims.
1 to 13.