JPS6221038B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a mixture for the storage of thermal energy using molten materials and a method for producing the same. With the increasing use of solar energy heating, there is an almost absolute need to store thermal energy to unwind the excess solar heat available during the day for use at night and on cloudy days. The use of heat-of-fusion materials for heat storage has recently gained increasing benefits due to their low cost and high heat of fusion per unit weight. It is desirable that such heat-of-fusion materials meet criteria of low cost, availability in large quantities, and ease of manufacture. Furthermore, they are preferably non-toxic, flame-resistant, non-combustible and non-corrosive. The cheapest materials for use are bulk chemicals based on sodium, potassium, magnesium, aluminum and iron compounds. Preferably the materials are in the form of salt-hydrates and eutectics thereof. Low cost compound types are limited to chlorides, nitrates, sulfates, phosphates and carbonates, while additives or modifiers can include borates, hydroxides and silicates.
Included among these are low cost salt-hydrates with high heat of fusion, low cost and minimal incompatibility due to undesirable properties.

[Table] In use, these materials are usually placed in a closed container with a nucleating agent, and the system allows the heat of fusion of the selected material to be used to "store" the cold heat. Continuous heating and cooling above and below the melting point is performed. The need for a nucleating agent is discussed in U.S. Patent No. 2,667,664 [inventor Maria Telkes,
Patent dated May 4, 1954]. As described in this patent, a suitable heterogeneous nucleating agent is a small amount of about 2 to about 3% borax (sodium tetraborate decahydrate). These nucleating agents provide the necessary seeds to initiate crystal formation and thereby avoid supercooling that can occur in a quiescent liquid solution. Other known nucleation techniques can be used to promote crystallization. Of course, crystallization is necessary to utilize the heat of fusion of the material. With supercooling, only the specific heat of the material is used. The specific heat of the material is much less than its heat of fusion, so nucleation is necessary. Sodium tetraborate decahydrate in combination with Na ₂ SO ₄ 10H ₂ O (nearly isomorphic nucleation agent)
When using, after the onset of crystal formation it is possible to obtain complete crystallization in the melt by turning the container or shaking it from time to time. Unfortunately, when used for the storage of thermal energy, it is not always convenient or possible in this case to shake the container. Another problem encountered in the use of heat-of-fusion materials is that after several cycles of heating and cooling, the liquid tends to separate from the salt-hydrate and form less hydrated salt crystals, or in this case an anhydrous salt, which is less effective. This resulted in a corresponding loss of heat of fusion. Stated differently, the melting of sodium sulfate decahydrate and a number of other salt-hydrates was found to be partially discordant. That is, during melting some anhydrous sodium sulfate remains undissolved in the crystallization water released during melting. Due to its high density, sodium sulfate sinks in saturated solutions. When this mixture solidifies again without mechanical mixing or agitation, the dissolved sodium sulfate will combine with the water of crystallization, but these heavy sodium sulfate crystals at or near the bottom of the container will only re-solidify with water molecules in the immediate vicinity. Combine to form solid sodium sulfate decahydrate crystals.
This solid layer prevents further recombination of the remaining water of crystallization and the remaining sodium sulfate. Due to this effect, when solidified without stirring or without additives, molten sodium sulfate decahydrate forms three distinct layers - white anhydrous sodium sulfate crystals somewhat embedded in the sodium sulfate decahydrate crystals. , a relatively large middle layer of translucent sodium sulfate decahydrate crystals, and a top layer of liquid saturated solution. The heat of fusion required to melt this salt is 108 BTU per pound which can be released again if the salt is homogenized during solidification by stirring or by suitable additives. During cooling (without homogenization or stirring), less heat is released. This is because part of the precipitate cannot recover its water of crystallization. In this case some saturated solution remains depending on the solubility of the salt as the mixture is cooled. Salt-hydrate separation and precipitation must be prevented. For several years, thickening agents have been included as additives in thermal storage mixtures for the purpose of creating gels that do not precipitate salt-hydrates with continued heating/cooling cycles. A number of different thickeners, including wood shavings, wood pulp, sawdust, various types of cellulose mixtures, and organic materials sold under the trade name "METHOCELL", starches and organic alginates Medications were tried. Inorganic thickening agents such as silica gel, diatomaceous earth, and other finely divided silica products have also been used. Although many of these materials served their purpose quite well, they were limited in repetition rate. Some organic materials are gradually hydrolyzed or degraded by bacteria or by enzymatic action. In many cases, such effects can be prevented or delayed by adding small amounts of formaldehyde or other suitable reagents. Wood shavings, wood pulp, etc. were not found to last long enough. The silica gel formed in the mixture itself proved to be an obstacle to filling the container with the mixture as it thickened too quickly. Salt-hydrate eutectics are used to improve the freezing point of various hydrates. Many of the eutectics are based on low cost compounds such as sodium chloride, ammonium chloride, potassium chloride and other known types. Since most eutectic materials tend to melt incongruously in parts, they also require nucleating agents as well as homogenizing or thickening agents. Homogenizing agents prevent precipitation of dense anhydrous compounds. It is therefore an object of the present invention to eliminate many of the disadvantages of prior art heat of fusion mixtures. Another object of the invention is to provide an improved heat of fusion material that has a reduced tendency for water and salt-hydrate to separate during solidification and melting. It is a further object of the present invention to provide an improved method for producing heat-of-fusion mixtures that melt in such a way that compatible materials melt. According to a preferred embodiment of the invention, a method for the storage of thermal energy utilizing the heat of fusion of a material includes a salt-hydrate, a nucleating agent and a homogenizing agent, the homogenizing agent being It is a clay-type substance that exhibits denaturation (thixotropy). This viscosity-type material consists of particles that are lath-like in appearance. Attapulgiyaite type clay is preferably used. As used herein, "lath-like" refers to crystal grains having a shape similar to "lath" used in architectural terminology. In other words, a thin and elongated plate-like shape,
For example, it refers to a crystal structure shaped like a single board of Venetian blinds. This crystal structure is described, for example, in "Clay Mineralogy" by Ralph E. Grim (McGraw-Hill Book Company),
Published in 1968]. It is shown in FIGS. 6 to 10 of FIG. As mentioned above, a preferred method of producing a mixture for the storage of thermal energy is to form a starting mixture by mixing water and a clay-type material exhibiting thixotropy due to its lath-like particles; The starting mixture is mixed with a salt to form a hydrate. When the composition is prepared in this way, it provides a stable colloidal suspension exhibiting thixotropy that envelops all of the salt-hydrate crystals, allowing the crystals to fall to the bottom of the container and This prevents the heat of fusion from decreasing. In short, such a mixture prevents the salt-hydrate from melting inharmoniously, so that the salt-hydrate does not separate from the solution. The mixtures of the invention act as homogenizing or thickening agents for salt-hydrate materials used for thermal energy storage. Thickening agents result from the partially discordant melting of salt crystals during successive heating and cooling cycles that typically occur in such materials when used for thermal energy storage. Prevent the aqueous solution from separating the salt crystals. Salt-hydrates that can be used for thermal energy storage include those described herein above. In particular, these preferred salt-hydrates, which should have a heat of fusion greater than 50 BTU per pound, include calcium chloride hexahydrate, soda carbonate decahydrate, disodium phosphate dodecahydrate, and calcium nitrate tetrahydrate. sodium sulfate decahydrate, and sodium thiosulfate pentahydrate.
These are chosen because of their relatively high heat of fusion (greater than 50 BTU per pound) and low cost, as discussed above. Mixtures containing such salt-hydrates, if necessary together with suitable nucleating agents of known type, may be formulated to prevent the solution from supercooling instead of crystallization during phase cooling. do. Preferably, salt-hydrates with a heat of fusion greater than 100 BTU per pound are used (see table above). According to the present invention, a novel homogenizing agent is used to prevent inharmonious melting of the salt-hydrate during successive heating and cooling cycles. The homogenizing agent according to the invention is a clay-type material exhibiting thixotropy, the particles of which are lath-like in structure. Any known nucleating agent or device can be used in this mixture. Such nucleating agents include those disclosed in the Telkes patent, supra, and for sodium sulfate decahydrate, typically contain from about 2% to about 5%, preferably from about 2% to about 5%, based on the weight of the total salt. A small amount of a heterogeneous nucleating agent such as sodium tetraborate decahydrate is included in the amount of about 3% to about 4%. Other nucleation agents and the inventor's pending U.S. patent application filed December 11, 1974.
The device for nuclear generation described and claimed in No. 531674 can be used. The thixotropic or homogenizing agent used in the mixture should be relatively inexpensive. As is known, thixotropic agents form highly fluid suspensions with water (or other solvents) during agitation or agitation of a mixture. On the other hand, when the mixture is stationary, it usually thickens after a short time to form a gel. According to the invention, clay-type thixotropic agents are used whose particles are lath-like in their structure and which give a high degree of colloidal stability in the presence of salt solutions or other electrolytes. This clay suitable for use according to the invention is known as attapulgite, polygorskite or sepiolite. Such clays are hereinafter referred to as attapulgiaite.
It is called molded clay. One such attapulgite-type viscosity is Berkeley Springs West Virginia, West Virginia.
Floridin Compagnie
Product name “Min-U” manufactured by
- Gel200". This particular material is a calcium silicate hydrate finer than 200 mesh. Attapulgiyaite clay has the chemical formula (OH ₂ ) ₄ (OH) ₂ Mg ₅ Si ₈ O ₂₀ ·4H ₂ O. According to this theoretical formula, attapulgite is a hydrated magnesium silicate or, more specifically, a hydrated magnesium aluminum silicate, since aluminum ions can replace magnesium and silicon ions in the crystal structure. . Thus, actual chemical analysis typically indicates the presence of aluminum oxide (Al ₂ O ₃ ).
Typical chemical analysis values for this clay are as follows. Oxide attapulgite SiO ₂ 57.85 Al ₂ O ₃ 7.89 Fe ₂ O ₃ 2.82 FeO … MgO 13.44 CaO 0.30 K ₂ O 0.08 Na ₂ O 0.53 TiO … H ₂ O− H ₂ O+ 16.95 Total 99.86 Structurally, attapulgite has a long axis Consists of a double chain of silicon and oxygen tetrahedra parallel to . This double chain is linked by spaced layers of magnesium atoms in a sextuple bond. In other words, the chain forms a network of strips that tie together along the edges. Each crystal unit contains eight water molecules. Attapulgiyaite clays can be thought of as bundles of lath-shaped units connected at their long edges. Because of this unique structure, ternary chain, attapulgite clay cannot expand like clays such as montmorillonite clay, which are sheet- or plate-like. Furthermore, Si-O
Cleavage parallel to the 110 plane along the −Si bond gives the particles a lath-like appearance. When dispersing clay in water, this lath-shaped unit tends to separate into smaller bundles by cleavage along these edges where the laths connect together. The degree of splitting is a function of the amount of work that goes into the splitting. Individual laths may be separated, but they tend to remain in bundles not unlike a stack of brushwood or a pile of grass. It is this tendency to form a Chigusa mountain-type structure, and it is believed that this structure gives the attapulgite clay its unusual properties that make it particularly suitable for use with heat-of-fusion mixtures. Chigusa's mountain-shaped structure maintains the surface support of the crystals. What gives attapulgite such high adsorption is its unusually high surface area. This large surface area, along with its tendency to bundle, gives attapulgiyaite its properties. The surface area of commercial grade attapulgiaite varies up to 2110 m ² /g. Attapulgite has its own weight of 200
Can absorb up to % water. In particular, it is the surface area that attracts water molecules and in turn allows the clay to remain colloidal even in the presence of electrolytes. Lath-like clays have a number of advantageous rheological properties that give them particular utility according to the present invention. When the lath-like crystals dissociate to form a random lattice, they trap liquid and increase the viscosity of the system. They can thicken both fresh and salt water. The suspension is thixotropic and has a high viscosity even at low concentrations. In aqueous systems, the viscosity of suspensions depends on additives and dispersants.
Alternatively, in a non-aqueous medium, it can be improved with a surfactant. A typical attapulgia truss has a length of about 1 micron, a width of about 0.01 micron, and a thickness of about 500-100 Angstroms. In other words, the length to thickness ratio of the lath is about 1000, while the length to width ratio of the lath is about 100. As mentioned above, attapulgite clay in colloidal suspension is chemically generally unaffected by salts. They do not clump. numerous other electrolytes,
These salt-hydrates, which are used in particular as heat-of-fusion materials, also have little influence. According to a preferred method of the invention, the material remains stable and stable over many cycles of heating and cooling (melting and crystallization) so that the heat of fusion of the material can be used to store cold energy. A heat-of-fusion material is produced that melts harmoniously. As a first step in the process, water is vigorously mixed with the attapulgite clay and thixotropic agent to form a starting mixture. It is often desirable to let the starting mixture stand for some time and mix repeatedly at intervals. A nucleating agent such as borax in microcrystalline form is then added to this mixture, thoroughly stirred and the resulting product is mixed with the required amount of salt-hydrate, e.g. sodium sulfate decahydrate. Mix. If necessary, the drug can be dispensed with using a nuclear generating device such as that described in the inventor's pending US patent application Ser. No. 531,674 filed December 11, 1974. The resulting mixture can be easily flushed from the tank or mixing vessel with agitation into a desired storage tube or other storage system. Seal the filled storage container. The mixture begins to gel rather quickly after the stirring is stopped. Containers in known thermal and/or cold storage systems can be used here. As mentioned above, they are capable of multiple cycles of heating and cooling with coordinated melting of the salt-hydrate to achieve effective generation and use of the high heat of fusion of the salt-hydrate. EXAMPLE According to the present invention, 56 parts by weight of water, attapulgite clay (Min-U-Gel 200) were prepared as described above.
7 to 10 parts by weight, 3 parts by weight of borax, 10 parts by weight of sodium sulfate
A thermal storage mixture was produced having a composition of 44 parts by weight of hydrate. This mixture was subjected to more than 100 consecutive heating and cooling cycles. This corresponds to more than 5 years of use without any obvious water separation. All of the heat of fusion of the salt-hydrate was used, making this mixture a highly effective heat storage material. In addition to the above examples of the present invention, the following experiment was conducted as a control. (a) The salt-hydrate compositions used in the examples with other thickening agents were tested and the mixtures were subjected to alternating heating and cooling cycles. In one experiment the additive was bentonite. This clay was used up to 10% by weight. After 8 cycles, liquid separation could be observed. (b) In another experiment, 10% by weight of asbestos-pulp
It was used in amounts up to. After 5 cycles some segregation could be observed. (c) In still other cases, clays without lath-like particles were tested in amounts up to 10% by weight.
Although the clay was an excellent thickening agent, it could not prevent segregation to a certain extent after four cycles. (d) In further tests, close to 100 cycles could be obtained with the use of some more expensive organic agents based on kelp extracts, but their use did not allow the material to remain in the molten state for longer periods. If held, it was unreliable. For storing refrigeration, a eutectic mixture can be used according to the invention with the same thixotropic additive. Salt-
Eutectic mixtures of hydrates have lower freezing points than typical salt-hydrates and therefore can store refrigeration at sufficiently low temperatures to be effective in air conditioning applications. In all cases, dispersants, wetting agents, etc. can be suitably used with the salt-hydrate or salt-hydrate eutectic mixture. Such use reduces the amount of thixotropic agent required and can often reduce costs considerably. A grade of attapulgite clay available from Floridin Co.
Additives to Min-U-Gel 200 can be dispersed using tetrasodium pyrophosphate (TSPP), an inexpensive and readily available material. This dispersant is 2 to 3 times the amount of Min-U-Gel used.
Used in an amount of % by weight. The recommended method is to dissolve TSPP in the required amount of water and then
By adding U-Gel and stirring rapidly for a short period of time. Dispersion occurs rapidly and maximum viscosity is quickly achieved without the need to allow hours of mixing and standing to obtain the same results without the use of TSPP. Regarding eutectic mixtures, the table below lists some of the salts used with sodium sulfate decahydrate to produce thickened mixtures within the scope of the present invention. The table shows the molar ratios used. A eutectic mixture that can be used with the concentrated sodium sulfate decahydrate mixture contains the following compounds:

TABLE In general, it is preferred to use about 92% to about 95% of the salt-hydrate or eutectic mixture with about 5% to about 8% of the thixotropic agent. The nucleating agent can be either borax or a nucleating device such as that described in the inventor's pending US patent application Ser. No. 531,674 filed December 11, 1974. It is understood that the thixotropic agents used in the present invention can be used with most other salt-hydrates, and that sodium sulfate decahydrate and others specifically mentioned herein are used by way of example only and It should also be appreciated that this is given to enumerate preferred embodiments. Thus, the use of attapulgite-type clays as homogenizing agents in conjunction with salt-hydrates and their eutectic mixtures for use as heat-of-fusion materials in the storage of solar energy has been described. moreover,
A unique method for forming such a mixture is described. Numerous embodiments can be made from the inventive concept, and numerous modifications can be made in the embodiments hereinbefore described. Accordingly, all of the materials described herein are for illustrative purposes only.
It is to be understood that these are for illustrative purposes only and are to be construed in a limited sense. Various improvements which may themselves be readily suggestible to those skilled in the art are included within the scope of the claims to the extent permitted by the prior art.

Claims (1)

Claims: 1. The heat of fusion of a composition comprising a salt-hydrate having a heat of fusion greater than 50 BTU per pound, a nucleating agent, and a thixotropic agent consisting of an attapulgite-type clay material having lath-like particles. Mixture for storage of utilized thermal energy. 2. The composition of claim 1, wherein the salt-hydrate is a low-melting salt-hydrate upon cold storage. 3. The composition of claim 1, wherein the mixture comprises about 56 parts by weight of water, about 7 to 10 parts by weight of attapulgite clay, about 3 parts by weight of borax, and about 44 parts by weight of sodium sulfate decahydrate. 4. The composition of claim 1, wherein the lath-like particles are about 1 micron long, 0.01 micron wide, and 50-100 Angstroms thick. 5. The composition of claim 1, wherein the lath-like particles have a length-to-thickness ratio of about 1000. 6. The composition of claim 1, wherein the lath-like particles have a length-to-width ratio of about 100. 7. The composition of claim 1, wherein the salt-hydrate has a heat of fusion greater than 100 BTU per pound. 8 Water is mixed with an attapulgite-type clay material having lath-like particles that exhibit thixotropy when dispersed in water to form a starting mixture, and the starting mixture is mixed with a nucleating agent and a salt to produce a
A method of producing a mixture for thermal energy storage comprising retaining a salt-hydrate having a heat of fusion greater than 50 BTU. 9. The method of claim 8, wherein the dispersant is first dissolved in the water prior to mixing the water with the clay-type material. 10. The method of claim 8, wherein the salt-hydrate has a heat of fusion greater than 100 BTU per pound.