JPH0215598B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat storage material. More specifically, the present invention relates to a heat storage material that reduces the degree of supercooling during solidification and exhibits stable performance against long-term thermal cycles. Water and crushed stone have traditionally been used as heat storage materials, but because these have low heat storage densities (1 cal/g.deg or less), they require a fairly large heat storage device for practical use. Furthermore, as the heat is dissipated, the temperature inside the heat storage device gradually decreases, so it is technically quite difficult to obtain stable thermal energy. In response, research and development on applying the latent heat during melting and solidification of substances to heat storage has become active in recent years. A feature of such latent heat type heat storage materials is that they can stably absorb and release thermal energy at a constant temperature that matches the melting temperature of the material at a high heat storage density of several tens of cal/g. By the way, with recent advances in solar heat utilization technology and waste heat recovery technology, heat storage at relatively low temperatures of 50 to 60°C has been attracting attention as a heat source for hot water supply and space heating. Organic substances and inorganic hydrates such as paraffin wax and higher fatty acids are attracting attention as latent heat storage materials for storing heat at such low temperatures. Although organic materials as latent heat storage materials have good stability during melting and solidification, they have problems in absorbing and releasing heat because the material itself has poor thermal conductivity. Furthermore, since the specific gravity is small, the heat storage device becomes relatively large. On the other hand, inorganic hydrates have about twice the thermal conductivity as organic heat storage materials, and have a high specific gravity of about 1.5 to 2.0, so the heat storage device can also be made smaller.
However, inorganic hydrates generally exhibit a so-called supercooling phenomenon in which the solidification initiation temperature becomes lower than the melting temperature. Such a phenomenon, when an inorganic hydrate is used as a heat storage material, significantly impairs the characteristic of the latent heat type heat storage material that it stably absorbs and releases thermal energy at a constant temperature. Sodium acetate trihydrate CH ₃ COONa.3H ₂ O is
It has a melting temperature of 58°C and a high latent heat amount of 51 cal/g (measured by differential scanning calorimeter), so it is very promising as a latent heat storage material for hot water supply, space heating, and even air conditioning as a constant temperature. However, a supercooling phenomenon is also observed in the case of this hydrate. That is, once melted sodium acetate trihydrate does not solidify even if it is left at room temperature around 15°C. This is because the solidification initiation temperature of sodium acetate trihydrate is approximately -21°C, which results in supercooling corresponding to a temperature difference of approximately 80°C. Therefore, it cannot absorb and release heat at 58° C. smoothly, so it cannot be used alone as a heat storage material. The present inventor conducted various studies in search of a nucleating agent capable of reducing the degree of supercooling of sodium acetate trihydrate, and found that a specific sodium salt of phosphoric acid is very effective. Therefore, the present invention relates to a latent heat type heat storage material that reduces the degree of supercooling, and this heat storage material contains monosodium dihydrogen phosphate, trisodium phosphate, or trisodium phosphate as a nucleating agent in sodium acetate trihydrate. A mixture of these is added. These sodium salts of phosphoric acid may be hydrates such as monosodium dihydrogen phosphate dihydrate or trisodium phosphate dodecahydrate. The degree of supercooling reduction also depends on the addition ratio of the nucleating agent used, but adding too much nucleating agent will not only not produce the expected effect, but also cause deterioration of the material. Generally, about 0.1 to 20% by weight of sodium acetate trihydrate,
Preferably, it is used in a proportion of about 0.5 to 10% by weight. The degree of supercooling reduction is indicated by the difference ΔTsc between the melting temperature Tm and the solidification start temperature Tm' of the heat storage material. The value can be significantly reduced. In addition, as a result, the time required to return to the melting temperature is shortened, and stable performance is exhibited over a long period of time in thermal cycle tests, making it possible to perform a more efficient heat storage function. Adding monosodium dihydrogen phosphate or trisodium phosphate alone to sodium acetate trihydrate in this manner is effective in reducing the degree of supercooling, but adding these as nucleating agents The heat storage material has a relatively low environmental temperature at a high temperature that can reproduce the nucleation effect (the maximum heating temperature at which the material can be solidified with good reproducibility even if the operation of cooling and solidifying the material heated and melted at this temperature is repeated). trisodium phosphate
Although the dodecahydrate shows a temperature of 92°C, the maximum temperature for monosodium dihydrogen phosphate dihydrate is only 82°C. Generally, when a heat storage material is used for various purposes, it is important to increase the maximum heating temperature at which solidification can be reproduced by even 1°C or 2°C in order to improve the reliability of the heat storage material. The reason for this is that if the heat storage material is heated above that temperature, the nucleating ability of the nucleating agent will be lost, so the actual operating environment temperature of the heat storage material must be kept strictly below that temperature. It will happen. However, in order to keep the operating environment temperature below a certain temperature, complicated system control is required, which is a very difficult problem in reality. For this reason, during actual use, there is always a risk that the heat storage material will be exposed to a temperature environment higher than the maximum heating temperature at which solidification can be reproduced, and that its nucleating ability will be lost. Therefore, if the maximum heating temperature could be increased even a little, the reliability of heat storage materials used in a relatively narrow temperature range could be considerably improved, but monosodium dihydrogen phosphate and trisodium phosphate By adding both sodium as a nucleating agent in a ratio of about 10 to 40 mol %: about 90 to 60 mol %, a maximum reproducible nucleation heating temperature of 94° C. can be obtained. Next, the effects of the present invention will be explained with reference to Examples. Example 1 A predetermined proportion of monosodium dihydrogen phosphate dihydrate was added to sodium acetate trihydrate, and their ΔTsc values were measured according to the following method. Place 10 g of sodium acetate trihydrate in a 20 ml glass container, add a predetermined amount of nucleating agent to it,
Seal tightly. Place this in a constant temperature bath, and the upper limit temperature is 80.
℃, within the range of lower limit temperature 20℃, temperature increase rate 1
Temperature at which it melts when heated at °C/min (Tm)
After raising the temperature of the mixture above, the mixture was cooled at a cooling rate of 1°C/min, and a thermal cycle test was repeated to supercool and solidify to a certain temperature (Tm'), and the temperature at that time was The change was measured with a thermocouple, and the degree of supercooling ΔTsc (Tm - Tm') was investigated. The results obtained are shown in Table 1 below.

[Table] The effect of preventing supercooling must be demonstrated stably over a long period of time. Table 1 above
Although the result is the value at the 20th cycle, the effect remains unchanged from the first time and remains stable even after 50 cycles. When the addition ratio is 20% by weight or more, the melting temperature as a heat storage material is significantly lower than the original melting temperature (Tm) of sodium acetate trihydrate,
The result is that the required temperature range of 50 to 60°C is not satisfied. Based on these results, the upper limit of the addition ratio is about 20% by weight. This series of trends is the same when anhydrous monosodium dihydrogen phosphate is used. Incidentally, the temperature change over time in the thermal cycle test in case No. 3 is shown in the graph of FIG. Example 2 When 1% by weight of trisodium phosphate dodecahydrate was added to sodium acetate trihydrate and the value at the 20th cycle was measured, ΔTsc = 5deg, Tm
= 58℃. Example 3 A total of 3 mol% of monosodium dihydrogen phosphate dihydrate and trisodium phosphate dodecahydrate were added as nucleating agents to 10 g of sodium acetate trihydrate in a glass container with a capacity of 20 ml. Take it and seal it tightly. This is immersed in a constant temperature bath at 80°C for 4 hours to completely melt the sample, and then left at room temperature. The sample undergoes supercooling and remains in a liquid state even after reaching room temperature.
A trace amount of sodium acetate trihydrate was added to this supercooled melt to solidify it. After performing the solidification treatment as described above, each sample was immersed in a constant temperature bath at a predetermined temperature for 2 hours to heat and melt, and the melted sample was left at room temperature to observe its solidification state. The test results, which were repeated five times, were evaluated according to the following criteria. ○: Solidified with good reproducibility in all cases △: Nucleation ability was lost in some cases ×: No solidification at all

[Table] From the above results, when trisodium phosphate dodecahydrate is used within the range of approximately 60 to 90 mol% in the nucleating agent mixture, it solidifies with good reproducibility even at an environmental temperature of 94°C. I understand. This tendency can be seen when the amount of nucleating agent added is approximately
This is true at 0.1% by weight or more, but if it exceeds about 20% by weight, the melting temperature of the heat storage material will be 50°C or lower, compared to the original melting temperature of sodium acetate trihydrate, which is 58°C, and it will not meet the purpose. The heat storage temperature range falls outside of the specified heat storage temperature range. Note that the reproducibility of the environmental temperature as described above can also be obtained when anhydrous monosodium dihydrogen phosphate and trisodium phosphate are used. As described above, adding a nucleating agent to sodium acetate trihydrate can effectively prevent supercooling, but if water is coexisting in the system in addition to the nucleating agent, stable melting and solidification may not be achieved. It has been found that a more stable heat storage effect can be carried out over and over again. FIG. 2 is a two-component system equilibrium diagram of CH ₃ COONa-H ₂ O. Here, line C-B-D-F: CH ₃ COONa・3H ₂ O
Area where two phases of solid phase and aqueous solution exist Line A-B-D-E: Area where two phases of CH ₃ COONa solid phase and aqueous solution exist Line F-D-E: CH ₃ COONa・3H ₂ O
_{A region} where _a solid phase of The presence of K-D, that is, the precipitation of anhydrous CH ₃ COONa, reduces the efficiency of heat transfer in the heat exchange of the heat storage device, and also causes sludge to accumulate in the heat storage device, thereby destroying the original properties of the heat storage material. There is a risk that not only the performance but also the performance of the heat storage device will be deteriorated. In the present invention, such anhydrous CH ₃ COONa
If performance deterioration is observed due to precipitation of , it is effective to add water in addition to the nucleating agent and adjust the composition to be near point B in the two-component system equilibrium diagram. The amount of water added for this purpose is
Approximately 10% by weight or less based on CH ₃ COONa・3H ₂ O,
Preferably it is about 2-5% by weight. Addition of water in proportions greater than this not only causes the melting temperature to be significantly lower than that of the original material, but also causes a decrease in the amount of latent heat. Addition of water within this range does not impede the nucleating action of the nucleating agent, and an effective melting/solidification cycle can still be carried out. However, on the contrary, anhydrous
Reducing the water content of the heat storage material by adding CH ₃ COONa or the like is not preferable because it increases the precipitation of anhydride. Further examples of such aspects will be added. Example 4 0.38 g of water is further added to the material of Example 1, No. 4. This material corresponds to the composition of a 58% by weight aqueous solution of sodium acetate, and upon solidification, no precipitation of anhydrous sodium acetate was observed, and ΔTsc
Solidified at =5deg. Furthermore, it was confirmed that there was no change in solidification behavior even after 50 thermal cycle tests, and stable melting and solidification were repeated.

[Brief explanation of drawings]

FIG. 1 is a graph showing the temperature change over time in the thermal cycle test in case No. 3 of Example 1. Moreover, FIG. 2 is a two-component system equilibrium diagram of CH ₃ COONa-H ₂ O. Explanation of symbols, L: liquid phase, S ₁ : solid phase of CH ₃ COONa, S ₂ : solid phase of CH ₃ COONa.3H ₂ O.

Claims (1)

[Claims] 1. A heat storage material obtained by adding monosodium dihydrogen phosphate, trisodium phosphate, or a mixture thereof as a nucleating agent to sodium acetate trihydrate. 2. The heat storage material according to claim 1, wherein the sodium salt of phosphoric acid is a hydrate. 3 The nucleating agent for sodium acetate trihydrate is approximately
The heat storage material according to claim 1, wherein the heat storage material is added in a proportion of 0.1 to 20% by weight. 4 Approximately more water is added to sodium acetate trihydrate.
The heat storage material according to claim 1, wherein the heat storage material is added in a proportion of 10% by weight or less. 5. The heat storage material according to claim 1, which is used for heating purposes.