JPS5941119B2 - Heat storage method using chemical reaction heat - Google Patents

Description

[Detailed Description of the Invention] Reflecting the global energy shortage situation, there is an increasing need to effectively utilize solar heat, which is said to be an inexhaustible source of clean energy, and to recover and utilize factory waste heat.

In order to utilize intermittent factory waste heat and solar heat, which is easily affected by the weather, as reliable energy sources, the development of highly efficient heat storage devices is essential.

Currently, the most commonly used heat storage materials are those that utilize sensible heat, such as water, rock, and concrete.

Let us consider the case of water, which has the highest specific heat.

Now, prepare 1 ton (lrn') of water and heat it to 40℃.
If heat is stored by raising the temperature, the amount of heat that can be obtained is about 40,000 days, which is insufficient even as a heat source for general household use.

For this reason, in solar houses, even on a small scale, it is necessary to prepare a heat storage water tank with a capacity of several tons, and the heat storage device occupies a huge volume.

Considering the housing situation in Japan, it is extremely difficult to install such a large-capacity heat storage water tank in each house, and this is one of the reasons that hinders widespread use of solar heat.

The capacity of the heat storage tank must be reduced by some method, and one solution is the so-called latent heat storage method.

This method uses the latent heat of melting released (or absorbed) by various inorganic water-use compounds and organic substances such as paraffin at their melting points to store heat. It is said that compared to the conventional type, at least several to ten times as much heat can be stored in the same volume, making it possible to make the heat storage device extremely compact.

However, inorganic water use compounds, which are considered relatively inexpensive, are generally accompanied by phenomena that are inconvenient for heat storage, such as overcooling and phase separation, which poses technical difficulties.

Further, although organic substances such as paraffin have few of the above-mentioned disadvantages, they are inherently flammable, which causes problems in terms of disaster prevention, and inevitably leads to high costs.

Therefore, there was a strong need for a change in the way of thinking about heat storage methods, and the present invention goes beyond the conventional physical phenomenon of sensible heat or latent heat of fusion, and actively utilizes chemical reaction heat to achieve low cost. The present invention provides a method that is safe and enables high heat storage density (heat storage amount per unit volume).

The method of the present invention utilizes the ion exchange reaction between the compounds and the accompanying exothermic or endothermic phenomenon for the purpose of heat storage in a combination of at least two types of inorganic salt compounds.

As a specific example, consider the reaction of the following equation.

Here, A and C represent cations, and B and D represent anions.

The progress method and reaction enthalpy of equation (1) can be estimated from already established thermodynamic data, but if equation (1) is a solid-solid reaction, the rate of progress is very slow. Therefore, it is often difficult to use it for heat storage purposes.

We investigated compounds AB and CD (AD and CB
When a small amount of liquid (e.g., water, alcohols, gelcols, or appropriate mixtures thereof) that has the ability to dissolve the compound is added to a mixture of 2000-200 It has been found that the operating temperature of the heat storage device can be set within a range suitable for the purpose.

This point will be explained in more detail.

Figure 1 shows compound AB and compound CD mixed in a container.
It is shown in a state where it is made into a powder and then sealed together with a small amount of liquid (water in this case).

Of course, AB and CD dissolve in water up to their solubility limits, so in this container, the compounds ABlCD, A
D and OB reach chemical equilibrium, and in water the ion A+
, Er, human and D- have reached their respective saturation concentrations.

At this time, the solubility product of compound AB is KAB-CA+)CB) Let [[] represent the concentration of ions.

Moreover, the solubility product of compound CD is shown by the following formula.

KOD-[C+][D-] Similarly, the compound after ion exchange reaction, AD and O
Assume that the solubility product of B can be expressed as KAD = (-A+) CD ) KOB = (0+) CB , ].

There are many possible exchange reactions represented by equation (1), but the chemical reactions used for heat storage can proceed in either the left or right direction depending on the temperature change of the system (this is a so-called reversible reaction). It is necessary.

The above reversible reaction was discovered as follows.

When the solubility product on the left side of equation (1), [KABxKoD], and that on the right side, [KADxKoB] are plotted against temperature (horizontal axis), as shown in Figure 2, at a certain temperature X, -(KAB xKOD) and [ A relationship in which the curves representing KADXKcB] intersect may be obtained.

Thermodynamically, the smaller the solubility product, the more stable the compound, so in the temperature range to the left of the dotted line in Figure 2, compounds AB and CD, and in the temperature range to the right, AD and O.
It can be seen that B exists preferentially.

Therefore, in the temperature range to the left of the dotted line in the figure, the reaction of equation (1) proceeds in the direction of the left side, and in the temperature range to the right, equation (1) proceeds in the direction of the right side.

Only such combinations of compounds can be used for the purpose of heat storage, and in FIG. 2, combinations of compounds that do not cross over the appropriate temperature range cannot be used for heat storage.

On the other hand, the standard free energy ΔH of the reaction of formula (1) can be calculated from the already established free energy of formation of each compound.

It goes without saying that a reaction in which ΔH is too small cannot be adopted, even for the original purpose of making the heat storage device more compact.

From the above-mentioned viewpoint, we have examined a large number of chemical reactions and have compiled the ones that are considered to be promising in Table 1.

The temperature corresponding to point X in FIG. 2 and the heat of reaction ΔH per mole are shown in the table.

(However, this is a value when water is used as the liquid.

) Now, the reaction of A1 in Table 1, 2KNOB +B a (OH) 2 ・8 H2O→
2KOH”H20+Ba(NO3)2+6H20(2)
For example, the reaction heat △H is 38.6 kcil/
mol, and the heat storage density is found to be 160 kCaI/l from the molar volume on the left side of equation (2).

The X point regarding equation (2) is 65°C.

This value is ``For example, Na 2820g ・5H2
s3v/l of molten latent heat type heat storage using 0 (melting point 48℃)
It is considerably larger than the previous model, and can be said to be an excellent method of storing heat.

Table 1 shows several equilibrium temperatures X.
All of these values are within the range suitable for air conditioning and hot water supply using solar heat.

Since this X point varies greatly depending on the type of liquid added as a solvent, it is possible to finely adjust the X point to the required temperature.

Furthermore, it is also possible to adopt a heat storage method in which two or more types of compounds undergo a similar exchange reaction, or in which heat of reaction is partially utilized and latent heat of fusion is partially utilized.

Therefore, the scope of application of the method of the present invention is not limited to the examples shown in Table 1.

In addition, the heat capacity Q of such a heat storage material utilizing reaction heat increases relatively slowly around the equilibrium temperature X, as shown by curve A in Fig. As such, it changes suddenly around its melting point M.

In the case of a sensible heat type heat storage material, as shown by curve C, there is a direct change in temperature.

For this reason, in the latent heat utilization type, unless a temperature higher than point M is supplied, the amount of heat storage will actually be lower than that of the sensible heat utilization type, whereas in the reaction heat utilization type (curve A), if the supplied heat is It has the feature of being able to store a certain amount of heat (much more than a sensible heat type) even if the temperature is below 1000 yen. It is considered convenient.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the state when compounds AB and CD are sealed together with water in a container, and Figure 2 is a diagram showing the solubility product, [K
ABXKoD] and (KADXKOD). Figure 3 is a diagram showing the temperature dependence of the heat capacity of reaction heat storage materials, latent heat storage materials, and sensible heat storage materials. It is a diagram.

Claims (1)

[Claims] 1. A heat storage method characterized by using at least two types of inorganic salts and a liquid saturated with them, and utilizing the heat of ion exchange reaction between the inorganic salts.