JP2008297504A - Heat accumulation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the following problem: when a copper-made heat accumulation vessel or heat exchange part is used for storing or heat exchanging with heat accumulation material comprising, by weight, 97 to 99.9% sodium acetate trihydride and 0.02 to 3% sodium pyrophosphate, the corrosion inhibition of copper is difficult. <P>SOLUTION: The heat accumulation material comprising, by weight, 97 to 99.9% sodium acetate trihydride and 0.02 to 3% sodium pyrophosphate is admixed with 0.05 to 0.2% sodium hydroxide. Thus, when the copper-made heat accumulation vessel or heat exchange part is used, the corrosion of copper can be inhibited. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a heat storage system that has high corrosion resistance and can use heat with high efficiency.

The latent heat storage material used in the heat storage system has been put to practical use by taking advantage of the fact that the heat extraction temperature is stable because the heat storage density is higher than the sensible heat storage material and the phase change temperature is constant. . In particular, sodium acetate trihydrate has been well studied as a latent heat storage material for hot water supply. However, in conventional heat exchangers, the heat transfer tubes and fins are often made of copper, and the latent heat storage material using hydrated salt is highly corrosive, so the heat transfer tubes and fins gradually corrode. As a result, there is a problem that the heat exchange part is damaged and the life of the heat storage system is shortened. Conventionally, a heat storage system in which an epoxy resin coating is pretreated on the surfaces of the heat transfer tubes and fins is used as disclosed in Patent Document 1. In addition, as disclosed in Patent Document 2, in particular, when the material of the heat transfer tube and the fin is copper, it is possible to form a film mainly composed of copper (I) oxide by heating the surface. I have been.
JP 2003-232595 A Japanese Patent Publication No. 4-63149

However, since the configuration of Patent Document 1 as a conventional example performs pretreatment of epoxy resin coating, there is a problem of increasing thermal resistance. Although the configuration of Patent Document 2 is intended to form an oxide film composed mainly of copper oxide _{(I) (Cu 2 O)} , it may progress corrosion if insufficient oxide film produced amount is there. In any of the methods, it is not easy to form a film having an appropriate thickness that suppresses corrosion of the heat transfer tubes or fins and also has good thermal conductivity.

  An object of the present invention is to provide a heat storage system that solves the above-described problems.

  In order to solve the above-described problem, in the heat storage system of the present invention, the heat storage material contains sodium acetate trihydrate 97 to 99.9% by weight, sodium pyrophosphate 0.02 to 3% by weight, and the heat storage material In order to store or dissipate heat to the heat storage material, and to store or dissipate heat to the heat storage material, a heat medium that exchanges heat with the heat storage material is circulated, and at least a part of the material of the heat storage unit is copper, and the heat storage The material contains 0.05 to 0.2% by weight of sodium hydroxide.

  More preferably, the heat storage container is provided with fins for increasing the heat transfer coefficient, and the material of the fins is copper.

  According to the present invention, it is possible to form a film having an appropriate thickness that suppresses corrosion of heat transfer tubes or fins and has good thermal conductivity by a simple method.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment)
FIG. 1 is a configuration diagram of a heat storage system according to the present embodiment. The heat storage container 2 is filled with a latent heat storage material 3. And the thermal storage part 1 is comprised by laminating | stacking the thermal storage container 2. As shown in FIG.
Also, through the heat transfer wall 6 of the heat storage container 2, one side is a first heat exchanging unit 4 that performs heat exchange between the heat medium during heat storage and the latent heat storage material 3, and the other is a heat medium during heat dissipation. A second heat exchanging unit 5 is provided for performing heat exchange. Furthermore, a heat pump cycle including a compressor 10, a radiator 11, an expansion valve 12, and an evaporator 13 is provided as a heat source unit for heating the heat medium.

  In the present invention, hot water is used as the heat medium. In addition, carbon dioxide suitable for hot water supply is used as a refrigerant for the heat pump cycle.

Next, operation | movement of the thermal storage system concerning this Embodiment is demonstrated. At the time of heat storage, the water flows in the flow direction 8 at the time of heat storage, and heat exchange with the refrigerant having a high temperature and a high pressure is performed in the radiator 11, and the heated water is from above the first heat exchange unit 4. After flowing in and heating the latent heat storage material 3, it flows out from the lower side and operates as a cycle in which heat exchange with the refrigerant is performed again. Moreover, at the time of heat dissipation, water flows in the flow direction 9 at the time of heat dissipation, water flows in from the lower side of the second heat exchange unit 5, absorbs heat from the latent heat storage material 3, flows out from the upper side, Provided for use.
(Investigation of copper corrosion control methods)
First, a method for measuring the corrosion rate of copper and the amount of copper oxide film produced of the present invention will be described. 1000 cc of a latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate was prepared in a 1500 cc container. Sodium pyrophosphate is added as a supercooling inhibitor for sodium acetate trihydrate. When the added sodium pyrophosphate was allowed to stand for a long time without stirring, the content of the upper part of the container was sometimes reduced to 0.02% by weight, but it was confirmed that the heat storage performance was not affected. Further, when the content of sodium pyrophosphate is increased, the content of sodium acetate trihydrate is decreased and the amount of stored heat is decreased. The adjusted latent heat storage material was melted by heating at 80 degrees, and the solution was subdivided into polypropylene containers by 20 cc. A sample in which a copper foil having a width of 8 mm, a height of 70 mm, and a thickness of 0.05 mm was immersed in this polypropylene container was prepared, and left in a constant temperature bath at 80 ° C. for a predetermined period. After a predetermined period of time, the copper foil was taken out from the sample, and the elution amount of copper ions was measured by analyzing the solution by ICP emission spectroscopic analysis (VISTA-RL manufactured by VARIAN), and the corrosion rate was calculated from the result. Regarding the amount of copper oxide coating produced, the DSCV (Double Sweep Cyclic Voltammetry) method described in the reference (Shigekichi Nakayama et al., “Voltammetric Analysis on the Growth of Copper Oxide Oxide Coating” Materials and Environment, 51 (2002), 566). It was measured by.

Next, copper oxide film production will be described. Simultaneously with the corrosion of copper, two types of copper oxide films, that is, copper oxide (I) (Cu ₂ O) or copper oxide (II) (CuO) are usually formed on the surface. Which is produced depends on the humidity and impurity gas concentration in the air if exposed to the air, and the components in the liquid if immersed in the liquid. The properties of both oxide films are different, and it is necessary to accurately analyze in advance which oxide film is produced in the latent heat storage material of the present invention.

Table 1 shows a comparison between a copper oxide film of a latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate and a latent heat storage material consisting only of sodium acetate trihydrate. A sample was prepared by the above-described method and left in a constant temperature bath at 80 ° C. for 4 days, and then the copper foil oxide film was analyzed by the DSCV method. From Table 1, when only sodium acetate trihydrate is used, copper (II) oxide (CuO) is the main component, and when sodium pyrophosphate is added, only copper oxide (I) (Cu ₂ O) is formed. I understand that.

FIG. 2 is a comparison of the amount of elution between a latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate and a latent heat storage material containing only sodium acetate trihydrate. As described above, it is desirable to add sodium pyrophosphate as a supercooling inhibitor. However, it can be seen that the amount of elution increases when sodium pyrophosphate is added. When sodium pyrophosphate is added, only copper oxide (I) (Cu ₂ O) is formed as described above, and the reason why the amount of elution increases is that copper oxide sufficient to prevent elution This is probably because (I) or (II) a film is not formed. That is, it is considered that sodium pyrophosphate (chemical formula: Na ₂ H ₂ P ₂ O ₇ ) dissociates in the solution and reacts with copper (II) oxide (CuO) as shown in the following formula.

^{_{2CuO + P 2 O 7 -4 +}} 2H 2 O = 4OH - + Cu 2 P 2 O 7
FIG. 3 shows changes over time in the amount of copper oxide (I) (Cu ₂ O) coating produced by a latent heat storage material composed of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate. A sample was prepared under the above-mentioned conditions, and after being left for a predetermined period, the amount of copper oxide (I) (Cu ₂ O) film produced was measured by DSCV method. Although the amount of film formation continues to increase in the initial stage, the amount of film formation becomes almost constant after the 30th day.

FIG. 4 shows the relationship between the amount of copper (I) oxide (Cu ₂ O) film produced and the corrosion rate of copper. When the amount of copper (I) oxide (Cu ₂ O) coating is small, the corrosion rate is high and the corrosion proceeds vigorously. On the other hand, when a copper (I) oxide (Cu ₂ O) film is formed in an amount of 1 (W / 1000 (kgm 2)) or more, the corrosion rate is reduced to 1/10 or less, and the corrosion does not proceed. In other words, the amount of copper oxide (I) (Cu ₂ O) coating produced greatly affects the amount of copper corrosion, and corrosion does not proceed if the copper oxide (I) (Cu ₂ O) coating is sufficiently formed.

As described above, as shown in Table 1 and FIGS. 3 and 4, the latent heat storage material comprising 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate is a copper (I) oxide (Cu ₂ O) coating. And does not decisively advance copper corrosion. However, as will be described later, the elution amount of copper can be suppressed by further adding sodium hydroxide to this system.

  Fig. 5 shows a comparison of the elution amount when only copper foil is immersed in a latent heat storage material consisting of sodium acetate trihydrate and sodium pyrophosphate and when copper oxide (II) (CuO) is immersed in copper foil. Indicates. A copper oxide (II) (CuO) film is sufficiently formed on the surface in advance using an oxidation treatment agent (Emplate MB-438A and 438B (Meltex Co., Ltd.)) on the copper foil (5 (W / 1000 (kgm2)). )) More) However, as shown in FIG. 5, the elution amount did not decrease, but rather showed an increasing tendency. That is, it can be seen that a latent heat storage material composed of sodium acetate trihydrate and sodium pyrophosphate cannot suppress the amount of copper corrosion even if a copper (II) oxide (CuO) film is formed.

As described above, in order to suppress the amount of corrosion when copper is used in the latent heat storage material composed of sodium acetate trihydrate and sodium pyrophosphate, the copper (I) oxide (Cu ₂ O) coating is It is valid. And it is thought that the elution amount of copper can be further suppressed by rapidly forming a copper (I) oxide (Cu ₂ O) film.

On the other hand, hydrogen ions (H +) and hydroxide ions (OH−) are often involved in the corrosion reaction of metals in aqueous solutions, and the pH of the solution must be considered. FIG. 6 is a potential-pH diagram of Cu—H ₂ O. However, the environment in which copper is immersed in the present invention is not an aqueous solution consisting only of water (H ₂ O), but an aqueous solution in which sodium acetate trihydrate is melted, and therefore it can be said that the environment completely coincides with the tendency of FIG. Absent. However, since sodium acetate trihydrate is a hydrated salt, it is considered to show a tendency close to that of water alone.

FIG. 7 shows the relationship between the amount of sodium hydroxide (NaOH) added to the latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate and the pH. The pH rises as the amount of sodium hydroxide (NaOH) added increases, and the pH becomes substantially constant when 0.4 wt% or more is added. On the other hand, when pH exceeds 13 from FIG. 6, it will be considered that it enters into the stable region of CuO ₂ ²⁻ which is an ion, and a copper (I) oxide (Cu ₂ O) film is not stably formed. From the above examination, pH = 13 was considered as a standard, and the amount of sodium hydroxide (NaOH) added was determined.
(Example)
EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded.
Example 1
A latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate was prepared, and 0.05% by weight of sodium hydroxide (NaOH) was added. A sample in which copper foil was immersed was prepared in the same manner as described above, and the corrosion amount of copper was measured after a predetermined period. The measurement results are shown in Table 2.
(Example 2)
A latent heat storage material similar to that in Example 1 was prepared, and 0.10% by weight of sodium hydroxide (NaOH) was added. Others were the same as in Example 1. The measurement results are shown in Table 2.
Example 3
A latent heat storage material similar to that in Example 1 was prepared, and 0.20% by weight of sodium hydroxide (NaOH) was added. Others were performed in the same manner as in the comparative example. The measurement results are shown in Table 2.
(Comparative Example 1)
A latent heat storage material comprising 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate was prepared. This test is for confirming the elution amount when sodium hydroxide is not added. Others are the same as in the first embodiment. The measurement results are shown in Table 2 (Comparative Example 2).
A latent heat storage material similar to that of Comparative Example 1 was prepared, and 0.01% by weight of sodium hydroxide (NaOH) was added. This test is to confirm that there is no effect when the amount of sodium hydroxide (NaOH) added is small. Others were performed in the same manner as in Example 1. The measurement results are shown in Table 2.
(Comparative Example 3)
A latent heat storage material similar to that in Comparative Example 1 was prepared, and 1% by weight of sodium hydroxide (NaOH) was added. This test is for confirming that the amount of elution increases conversely when pH> 13. Others were the same as in Example 1. The measurement results are shown in Table 2.

  From Table 2, the latent heat storage material comprising sodium acetate trihydrate and sodium pyrophosphate as in the present invention is 0.05% by weight or more and 0.20% by weight or less, more preferably 0.10% by weight. If added in an amount of not more than%, the elution amount is reduced by the effects described above, and the elution amount of copper can be reduced as compared with the case where sodium hydroxide is not added. Moreover, since the configuration of the present invention does not require pretreatment such as surface treatment unlike the conventional example, the cost of the heat storage system can be reduced.

  Further, as described above, it is considered effective to use other alkalis such as KOH as well as NaOH for adjusting the pH. However, since the heat storage system of the present invention is a system that reversibly uses sodium acetate trihydrate, it is most appropriate to use NaOH that generates sodium ions.

  The heat storage system according to the present invention can be developed for household water heater applications, but is not necessarily limited thereto, and can also be deployed for household heating applications, industrial waste heat storage, and the like.

The block diagram of the thermal storage system in embodiment of this invention Comparison of the amount of elution of a latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate and a latent heat storage material consisting only of sodium acetate trihydrate The figure showing the time-dependent change of the copper oxide (I) (Cu2O) film production amount of the latent heat storage material which consists of sodium acetate trihydrate 99 weight% and sodium pyrophosphate 1 weight%. The figure showing copper oxide (I) (Cu ₂ O) film production amount and copper corrosion rate Figure comparing the amount of elution when only copper foil is immersed in a latent heat storage material consisting of sodium acetate trihydrate and sodium pyrophosphate, and when copper oxide (II) (CuO) is immersed in copper foil. Potential-pH diagram of Cu-H ₂ O The figure showing the relationship between the amount of sodium hydroxide (NaOH) added to the latent heat storage material consisting of 99% by weight of sodium acetate trihydrate and 1% by weight of sodium pyrophosphate and pH

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heat storage part 2 Thermal storage container 3 Latent heat storage material 4 1st heat exchange part 5 2nd heat exchange part 6 Heat transfer wall 7 Fin 8 Flow direction at the time of heat storage 9 Flow direction at the time of heat dissipation 10 Compressor 11 Radiator 12 Expansion Valve 13 Evaporator

Claims (1)

  A heat storage material containing sodium acetate trihydrate 97 to 99.9% by weight and sodium pyrophosphate 0.02 to 3% by weight, and a heat storage unit comprising a heat storage container for storing the heat storage material, and storing heat in the heat storage material Alternatively, in order to dissipate heat, a heat medium that exchanges heat with the heat storage material is circulated, the heat storage part of the heat medium is provided, and the heat storage container is provided with fins for increasing the heat transfer rate, and the heat storage At least a part of the material of the container and the fin is copper, and 0.05 to 0.2% by weight of sodium hydroxide is contained with respect to the total weight of the heat storage material.

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