Heat exchange – Regenerator – Checker brick structure
Patent
1981-04-02
1984-08-21
Davis, Jr., Albert W.
Heat exchange
Regenerator
Checker brick structure
165 10, 16510417, 165111, F28D 2100
Patent
active
044664788
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of forming heat exchange and mixing in a system for storing and taking out heat by melting and crystallization of one or more chemical compounds. The invention also relates to an apparatus for carrying out the method.
The utilization of the fusion heat in chemical compounds for storing energy affords large advantages, and a number of systems for this purpose are known from the literature. Compounds melting at temperatures slightly above room temperature (30.degree.-40.degree. C.) are particularly interesting for heating dwellings. In the use of solar heat or heat pump systems, substantially higher system efficiencies and storage densities are generally obtained than what would be the case if water, for example, were used as the heat storage medium.
Several easily obtainable salt hydrates, e.g. Na.sub.2 SO.sub.4.10H.sub.2 O (Glauber's salt) and CaCl.sub.2.6H.sub.2 O, have appealing characteristics in this connection. Salt hydrates are not one-component systems however, which results in the risk of forming undesired phases whereby the reversibility in the fusion-crystallization transitions can be lost, with deteriorated heat storage capacity as a result. The reversibility is most often connected with whether the material melts congruently or incongruently. With incongruent melting two phases of different density occur simultaneously, and the phase separation which thereby can occur generally leads to a complete crystallization transition not being able to take place.
Several methods have been used to prevent or inhibit this type of phase separation. Effective mixing is a method by which the systems can be made to work reversibly.
The same problem can also occur with component systems melting congruently. Concentration gradients can namely occur in the melt and result in a concentration increase of the salt in the bottom of the layer with the risk of forming undesired phases. The concentration gradients can be inhibited by suitably forming the heat exchangers in such a way that a self-convective agitation is achieved in the system. In some applications it would however be desirable with another type of mixing.
The heating power in charging and taking off energy in a heat-of-fusion store is determined by the melting and crystallization rate, which can most often be made completely independent of the heat transport to and from the store by suitable dimensioning of the heat exchanger. During melting, the transport of heat is limited mainly by convection in the liquid phase, and will thereby become more effective than during the crystallization, when heat transport is generally made more difficult by the heat exchanger being covered all the time by the solid phase formed. To reach a good rate of heat transfer when taking off heat, it is therefore required that either the heat exchanger surface is very large or that it can be kept free from salt crystals. The simplest way of providing large heat exchanger surfaces is to encapsulate the storage medium in smaller units. The encapsulating material itself can then constitute a heat exchanger. The method is expensive however, and therefore less suitable for large units which are adapted to storing heat during a long period of time.
Another method of ensuring large heat exchanger surface is to apply the principle of direct heat exchange liquid-liquid and use a liquid as heat transfer medium which is not miscible with the storing medium but can be brought into intimate contact with the latter even so. The principle was applied by Etherington as early as 1957 (T. L. Etherington: "Heating, Piping and Air Conditioning", p. 147, Dec. 1957). As a heat transfer medium he used a light mineral oil which in a dispersed form was pumped in from the bottom of a heat store comprising an aqueous solution of disodium hydrogen phosphate. At the same time as he obtained a large heat exchanger surface, he also achieved an effective mixing in the storage medium in this way. Later attempts with other salts have shown that the method has many advanta
REFERENCES:
patent: 4129014 (1978-12-01), Chubb
patent: 4263961 (1981-04-01), Morawetz et al.
patent: 4286650 (1981-09-01), Lindner
Carlsson Bo
Stymne Hans
Wettermark Gunnar
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