Patent Application
20190048242
The present invention relates to thermal storage media that change phase at a prescribed temperature and also to thermal storage packs, thermostatic vessels, and transport boxes using such media.
Commercial and other articles that need to be kept at a temperature for quality preservation have been conventionally kept at a range of temperature that is suitable for the articles during transport. For example, refrigerant cooled down to a required temperature is placed in a thermostatic box. The commercial article is then put into the thermostatic box so that the article can be kept at low temperature.
As another example, medicines and like articles need to be kept at 2° C. to 8° C. during transport. For this purpose, a thermal storage medium is needed that melts around 5° C. A current, commonly used thermal storage medium with a melting point of 5° C. is paraffin-based material, which is flammable. For this reason, studies are underway to develop a flame-retardant thermal storage medium that exhibits as much latent heat as paraffin, in order to replace paraffin-based material.
Clathrate hydrates, and semi-clathrate hydrates in particular, crystallize when an aqueous solution of their base compound is cooled below a temperature at which a hydrate is formed. The crystals will store thermal energy that may later be utilized as latent heat. The clathrate hydrate may therefore be used as a latent thermal storage medium or as a component of such a medium.
Substances worth a mention here are hydrates of quaternary ammonium salts, which are a typical example of semi-clathrate hydrates encaging a non-gaseous species as a guest compound. These hydrates form under normal pressure, give out a large amount of thermal energy (amount of stored heat) upon crystallization, and are unlike paraffin, inflammable. Therefore, hydrates of quaternary ammonium salts are easy to handle and therefore finding more and more applications than before in heat transport media and thermal storage tanks that are more efficient than ice thermal storage tanks used in buildings for air conditioning purposes.
Patent Literatures 1 and 2 have successfully lowered a congruent melting point by lowering the concentration of TBAB and also prepared a cold storage agent that has a desirable melting point by mixing a clathrate hydrate with a suitable amount of a substance whose melting point is lower than that of water as a melting point depressant.
Patent Literature 1: Japanese Unexamined Patent Application Publication, Tokukai, No. 2005-126728
Patent Literature 2: Japanese Unexamined Patent Application Publication, Tokukaihei, No. 11-264681
The cold storage agents disclosed in Patent Literatures 1 and 2 are, however, mixtures containing an aqueous solution that is in liquid phase at use temperatures to form a slurry of a clathrate hydrate. The mixed presence of an aqueous solution that is in liquid phase at use temperatures reduces latent heat, which will be explained next in reference to
The present invention, having been made in view of these issues, has an object to provide a flame-retardant thermal storage medium that has an effective temperature range lower than the effective temperature range of congruent-melting-point-concentration TBAB without reducing the latent heat of the congruent-melting-point-concentration TBAB and also to provide a thermal storage pack, thermostatic vessel, and transport box using such a medium.
To achieve this object, the present invention, in one aspect thereof, is directed to a thermal storage medium that changes phase at a prescribed temperature, the medium containing: water; TBAB at such a concentration with respect to the water as to give a congruent melting point of a semi-clathrate hydrate; and KCl dissolved in the water.
The thermal storage medium, thus containing water, TBAB, and KCl, can exhibit an effective temperature range lower than the effective temperature range of the congruent-melting-point-concentration TBAB while substantially maintaining the effective-temperature-range-sustaining time of the congruent-melting-point-concentration TBAB. In addition, the TBAB, used at such a concentration with respect to the water as to give a congruent melting point of a semi-clathrate hydrate, can preserve the amount of latent heat for a lower effective temperature range.
The present invention can provide a thermal storage medium that has an effective temperature range lower than the effective temperature range of congruent-melting-point-concentration TBAB while substantially maintaining the effective-temperature-range-sustaining time of the congruent-melting-point-concentration TBAB.
A description will be now given of embodiments of the present invention in reference to drawings.
A thermal storage medium in accordance with the present invention is a latent thermal storage medium that changes phase at a prescribed temperature and is composed primarily of water, tetra-n-butylammonium bromide (hereinafter, “TBAB”), and potassium chloride (hereinafter, “KCl”).
TBAB is a quaternary ammonium salt. The quaternary ammonium salt hydrate is a typical example of a semi-clathrate hydrate encaging a non-gaseous species as a guest compound, forms under normal pressure, and gives out a large amount of thermal energy (amount of stored heat) upon crystallization. Unlike paraffin, the quaternary ammonium salt hydrate is inflammable and hence easy to handle. By using TBAB which forms semi-clathrate hydrates of this nature, a large amount of latent heat energy becomes available for applications.
The present embodiment uses TBAB which forms such semi-clathrate hydrates. The TBAB semi-clathrate hydrate preferably has a concentration of 40.5 wt %±0.5 w %/o with respect to water. The molar ratio of KCl to TBAB is preferably more than or equal to 0.90.
Mixing KCl, which has a higher melting point than water, with water and TBAB of such a concentration with respect to the water as to give a congruent melting point of a semi-clathrate hydrate can suppress formation of slurry of a clathrate hydrate and provide an effective temperature range lower than the effective temperature range of the congruent-melting-point-concentration TBAB while substantially maintaining the effective-temperature-range-sustaining time of the congruent-melting-point-concentration TBAB, that is, preserving the amount of latent heat produced at melting point by the congruent-melting-point-concentration TBAB.
This composition can lower the onset temperature of melting of TBAB from 12° C. to about 4° C. and can also lower the maximum temperature to 8° C. or even further. The onset temperature of melting may be lowered by lowering the TBAB concentration. By doing so, however, the durability of the thermal storage medium is also lowered. In contrast, the thermal storage medium in accordance with the present embodiment contains a TBAB semi-clathrate hydrate at a concentration of 40.5 wt %±0.5 wt % with respect to water and substantially maintains the congruent melting point concentration. Therefore, the thermal storage medium in accordance with the present embodiment is less likely to lose its durability.
Method of Preparing Thermal Storage Medium
First, TBAB (40.0 grams (=0.124 mol) to 41.0 grams (=0.127 mol)), water (59.0 grams to 60.0 grams), and KCl (8.33 grams (=0.112 mol) to 9.48 grams (=0.127 mol)) are prepared which are ingredients of the thermal storage medium in accordance with an aspect of the present invention. Among these ingredients prepared, the water and TBAB are first mixed at room temperature. The KCl is then added and mixed with a resultant liquid mixture, which completes the preparation of the thermal storage medium. The order of adding and mixing the ingredients may be changed: the water and KCl may be mixed before the TBAB is added and mixed with a resultant water-KCl liquid mixture.
Next will be described measurement experimentation performed on thermal storage media. In the measurement experimentation, thermal storage media were prepared from different amounts of ingredients and subjected to (1) measurement of temperature changes, (2) differential scanning calorimetry (DSC), and (3) measurement of freezing point.
Table 1 shows the TBAB, water, and KCl contents of each thermal storage medium used in measurement (1) to (3).
The thermal storage medium in accordance with Comparative Example was prepared by mixing TBAB and water in such a conventional manner as to contain TBAB at the congruent melting point concentration (40.5 wt %). The thermal storage media in accordance with Examples 1 to 6 and 8 were prepared by mixing water and TBAB before adding and mixing KCl with a resultant water-TBAB liquid mixture. The thermal storage medium in accordance with Example 7 was prepared by mixing water and KCl before adding and mixing TBAB with a resultant water-KCl liquid mixture.
Temperature changes were measured of the thermal storage media in accordance with Comparative Example and Example 1.
Samples (50 grams each) of these prepared thermal storage media were put into respective plastic containers and frozen at −30° C. in a thermostatic chamber. After that, the environmental temperature was changed to 30° C., and changes in temperature of the thermal storage media were measured. Results are presented below. Note that the internal temperature of the thermostatic chamber was increased from −30° C. to 30° C. at a rate of 1° C./min and thereafter maintained at 30° C.
It is understood from these results that by adding KCl to a 40.5 wt % (=congruent melting point concentration) aqueous solution of TBAB in a molar ratio KCl:TBAB=1:1, the resultant thermal storage medium exhibits an effective temperature range lower than the effective temperature range of the congruent-melting-point-concentration TBAB while substantially maintaining the effective-temperature-range-sustaining time of the congruent-melting-point-concentration TBAB.
Specific heat is generally larger in liquid phase than in solid phase. For this reason, if Example 1 contained liquid phase, specific heat would be larger in Example 1 than in Comparative Example. Example 1 and Comparative Example would not exhibit the same temperature changes; temperature would increase more slowly in Example 1 than in Comparative Example.
Differential scanning calorimetry (DSC) was performed on the thermal storage media in accordance with Comparative Example and Examples 1 to 8.
Temperature conditions during differential scanning calorimetry were as follows. Temperature was decreased from 30° C. to −30° C. at 5° C./min, maintained at −30° C. for 5 minutes, and then increased from −30° C. to 30° C. at 5° C./min.
Extrapolated onset temperature of melting (melting point) is determined by extrapolating the temperature at which an endothermic peak starts toward a baseline on a DSC thermogram obtained by DSC. Latent heat is calculated from an area of an endothermic peak on a DSC thermogram obtained by DSC.
The following will discuss the extrapolated onset temperatures of melting (melting points) and latent heats of the thermal storage media in accordance with Comparative Example and Examples 1 to 8 in comparison with those of the thermal storage medium in accordance with Example 1.
Meanwhile, in Example 7, there appeared an exothermic peak, which was missing in Comparative Example and Examples 1 to 6, at or below −30° C. (=minimum temperature). The exothermic peak appeared due to solidification of a substance that melted at −14.8° C. The addition of KCl before TBAB presumably made the aqueous solution of KCl easier to freeze. To verify this presumption, the same measurement was performed as Example 7a with a change in minimum temperature from −30° C. to −40° C.
The temperature of a thermal storage medium in accordance with Example 7 was decreased from 30° C. to −40° C. at 5° C./min, maintained at −40° C. for 5 minutes, and then increased from −40° C. to 30° C. at 5° C./min. In other words, the minimum temperature was changed from −30° C. to −40° C.
Heat flow was measured in Example 8 with the minimum temperature being changed to −40° C. as in Example 7a.
These comparisons indicate that the thermal storage medium in accordance with Example 1, prepared by mixing KCl with a 40.5 wt % aqueous solution of TBAB in a molar ratio KCl:TBAB=1:1, is the most preferred. The comparisons also indicate that a thermal storage medium is obtainable that changes phase at 2° C. to 8° C. if the TBAB has a weight percentage of 40.5 wt %±0.5 with respect to the water and the KCl has a molar ratio of at least 0.90 with respect to the TBAB.
In addition, KCl is preferably added to the aqueous solution of TBAB in such an amount as not to exceed its saturation level. For example, when KCl was added to a 40.5 wt % aqueous solution of TBAB adjusted to 20° C. in a molar ratio KCl:TBAB=1.39:1 (i.e., 13 grams of KCl was added to 100 grams of a 40.5 wt % aqueous solution of TBAB), it was observed that the KCl dissolved completely. Meanwhile, when KCl was added to the same aqueous solution (40.5 wt % aqueous solution of TBAB adjusted to 20° C.) in a molar ratio KCl:TBAB=1.49:1 (i.e., 14 grams of KCl was added to 100 grams of a 40.5 wt % aqueous solution of TBAB), the KCL did not dissolve completely and produced some precipitate.
From these observations, it is concluded that under these conditions, KCl is preferably added in a molar ratio KCl:TBAB≤1.39:1. If more than this amount of KCl is mixed, the resultant thermal storage medium (or refrigerant) exhibits a decrease in effective latent heat per unit weight that corresponds to the mass of the KCl that precipitates without being dissolved.
The freezing point was measured of the thermal storage medium in accordance with Example 1.
A sample (50 grams) of the thermal storage medium in accordance with Example 1 were put into respective plastic containers, which were in turn placed in a 30° C. thermostatic chamber. The internal temperature of the thermostatic chamber was decreased from 30° C. to −30° C. at 1° C./min and thereafter maintained at −30° C.
The thermal storage media are applicable to thermostatic vessels.
The thermostatic vessel main body 110 houses the article S0 and the thermal storage pack 120 so that the thermal storage pack 120 can keep the precooled article S0 at low temperature. This structure enables the thermostatic vessel main body 110 to house the article S0 while maintaining the inside of the thermostatic vessel at 2° C. to 8° C. The structure further enables articles such as vaccines and like medicines that need to be kept at 2° 2 to 8° C. to be stored at a suitable temperature for a period of time, without damaging their effects.
A thermal insulation material 130 may also be included in the thermostatic vessel 100 either between the thermal storage pack 120 and the article S0 that needs to be kept cold or outside the thermal storage pack 120. The thermal insulation material 130, thus housed inside the thermostatic vessel 100, restrains the article S0 from being warmed up by heat dissipated by the thermal storage medium. That in turn enables the article S0 to be kept at a suitable temperature for an extended period of time.
The thermal storage media are applicable to transport boxes.
The transport box 200 may be composed of a thermally insulating substance. The thermostatic vessel 100, housed inside the transport box 200 composed of a thermally insulating substance, restrains the internal temperature of the transport box 200 from changing due to thermal conduction. That in turn enables the article S0 to be kept at a suitable temperature for a further extended period of time.
The transport box 200 may be composed of a sheet that blocks radiant heat. The thermostatic vessel 100, housed inside the transport box 200 composed of a radiant-heat-blocking sheet, restrains the internal temperature of the transport box 200 from changing due to radiant heat. That in turn enables the article S0 to be kept at a suitable temperature for a further extended period of time.
This international application claims priority to Japanese Patent Application No. 2016-021131 filed on Feb. 5, 2016, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-021131 | Feb 2016 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/003351 | 1/31/2017 | WO | 00 |
