Heat exchange – Regenerator – Heat collector
Patent
1980-09-17
1983-02-01
Davis, Jr., Albert W.
Heat exchange
Regenerator
Heat collector
126436, 165111, 165 34, F28D 2100
Patent
active
043710280
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a heat storage device comprising a container which contains a storage material for storing and supplying heat during melting and solidification respectively, and means for pumping a heat carrying medium through a circuit in which it is brought into heat exchange contact with the storage material, of the kind described.
It is well known that many phase-change materials such as certain salt hydrates have a high heat of fusion that offers interesting possibilities of low temperature (less than 100.degree. Celsius) heat storage, for instance for solar heating systems or for off-peak charging of electric water heaters.
Such salt hydrates absorb large amounts of energy in the melting process which transforms their crystal structure into a liquid melt or solution. When the process is reversed and the hot melt is cooled to the melting point, it gradually solidifies through formation and growth of salt crystals, at the same time releasing the latent heat that was absorbed in the melting process.
Toxicity, flammability, chemical instability, and high price are factors that eliminate many salt hydrates from consideration. Among remaining candidates there are a few with particularly promising combinations of properties, for instance sodium sulfate decahydrate, sodium acetate and sodium thiosulfate pentahydrate. However, they are also like in having poor thermal conductivity in crystal form which tends to inhibit their discharge of the latent heat. This is the problem with which the present invention is concerned.
If a conventional heat exchanger is used, the heat discharging process causes a steadily growing layer of crystals to form on the heat exchanger walls, resulting in a rapidly decreasing heat transfer rate. This may be counteracted through the use of greatly increased heat transfer surfaces, which, however, makes the construction both voluminous and costly.
Another solution to the problem is the use of direct contact heat transfer by means of an immiscible oil, i.e. an oil which has no tendency to mix with the salt solution. The oil is pumped through nozzles located at the bottom of the salt hydrate container. Because of its low specific gravity the oil rises rapidly through the salt solution, absorbing or giving off heat in the passage. This heat exchange between oil and solution is highly effective.
One known device of this kind has a horizontal oil pipe near the container bottom. From downward pointed nozzles in the pipe the oil rises freely to the surface when the salt hydrate is in liquid form. To permit operation when the nozzles are blocked by salt crystals the far end of the horizontal pipe is connected with a vertical pipe extending above the surface of the salt hydrate, establishing and oil circulation route by-passing the nozzles and hence always open. It has the effect of conducting heat from the hot oil to the nozzles. From the vertical pipe the hot oil is directed to the salt hydrate surface where the melting action continues.
The time consumed in the melting of a solid crystal block may be considerable. If extra water is added to the salt hydrate, for instance to avoid incongruent melting, a constant operation of the pump will tend to keep the solidified salt hydrate in a slurry or mush form, a condition that can speed the melting process.
A worse condition occurs during discharge operations, i.e. when the storage is giving off heat to the circulating oil. This process goes on smoothly only as long as the temperature of the solution is above the melting point of the salt hydrate. When it sinks below that point, the solution suddenly appears filled with countless tiny crystals which start growing in size. Their appearance and growth are accompanied by a release of energy that tends to maintain a temperature slightly below the melting point. The rising oil droplets keep the smaller crystals suspended in the liquid while the larger ones sink slowly to the bottom. Here they gradually block the nozzles, thus drastically reducing the oil flow long before all the liquid
REFERENCES:
patent: 4127161 (1978-11-01), Clyne et al.
patent: 4219072 (1980-08-01), Barlow
Davis Jr. Albert W.
Effex Innovation A/S
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