1. Field of the Invention
This invention relates to a heat exchanging unit with a hydrogen adsorption alloy mainly composed of metal hydride, and more particularly to a heat exchanging unit having a high heat exchange efficiency which is difficult to reduce in spite of repeated uses when the unit is incorporated into a heat exchanger.
2. Prior Art
Heretofore, several arts have been developed in which hydrogen is adsorbed in a certain metal or alloy to be stored therein and transferred therefrom in the form of a metal hydride. These arts have been further applied to such practical use as purification of hydrogen, pressure rise, heat pump, air-conditioning system, etc.
In such case, since an exothermic reaction or an endothermic reaction is necessarily taking place at the time the metal hydride adsorbs or discharges hydrogen, it is possible to take advantage of such a property for a heat exchanger, heat pump, etc.
When it is a principal object to store or transfer hydrogen, delivery of hydrogen is not effectively performed without rapid delivery of heat between the metal hydride and the outside in view of the high thermal efficiency of the heat exchanger or efficient storage and transfer of hydrogen.
However, a thermal conductivity of hydrogen adsorption alloy itself in the form of particles is not high, and therefore several attempts have been proposed aiming at efficient deliver of heat.
According to one of the proposed attempts, in order to improve the hydrogen adsorption alloy itself, surfaces of the particles are plated with a dissimilar metal of high thermal conductivity as described later with reference to this invention.
According to another attempt, the structure of a heat exchanging unit is improved so that a hydrogen adsorption alloy in the form of particles is brought into contact with a heat transfer element as close as possible. For example, as shown in FIG. 19, a heat exchanger manufactured by Solar Turbines Incorporated is disclosed, wherein a heat pump for temperature rise is provided with a tube and fins outside as a heat transfer element. Fourteen copper tubes 8A are disposed in fins 9A of large diameter being 0.02 inch in thickness, and spaces formed between the fins at an interval of 0.15 inch (3.8 mm) are filled with a metal hydride 6A. FIG. 20 shows another heat exchanger for prototype heat pump disclosed by the same company, having six radial fins 9B disposed in a copper tube 8B of 1 inch (25.4 mm) in inner diameter. Numeral 18 is a filter in FIG. 20. These two drawings are shown in pages 67 and 72 of Metal Hydride/Chemical Heatpump Development Product. Phase 1, Final Report, BNL-51539 published by Brookhaven National Laboratory.
A further proposed attempt is one which utilizes compression molding. FIG. 21 shows a proposal already made by the applicant and disclosed in U.S. Pat. No. 4,609,038, wherein surfaces of particles of hydrogen adsorption alloy are coated with a dissimilar metal by plating and molded into a compact 6C. Apertures are then perforated through the compact to insert a heat exchanging pipe 8C therethrough, the ends of the pipe being respectively communicated with a supply port and an exhaust port for a heating or cooling medium. A modification of this proposal is also disclosed in the foregoing application, wherein particles of hydrogen adsorption alloy coated with a dissimilar metal by plating are infiltrated into a porous material of high thermal conductivity and this porous material is formed into a compact by compression molding.
In effect, in order to improve thermal efficiency of a heat exchanger using a hydrogen adsorption alloy, there have been proposed means for improving the hydrogen adsorption alloy itself, means for increasing contact areas between the alloy particles and heat transfer surfaces as much as possible (by Solar Turbines Incorporated), and a method for improving a compact of hydrogen adsorption alloy formed by compression molding (i.e., porous metal matrix hydrides) proposed by Prof. Ron Technion and further improved by applicant.
The foregoing proposals, however, have their respective problems to be solved.
In the first attempt of improving a hydrogen adsorption alloy itself to elevate thermal conductivity, there is a limit in distance within which heat can be transferred from a heat transfer surface, since the thermal conductivity is essentially low when the alloy is in the form of particles. By the same reason, sufficient improvement of thermal conductivity is not attained, either, even when a lot of fins are densely fitted for rapid delivery of heat in the second attempt of increasing the contact area.
In this connection, a filter is usually fitted for shielding the alloy from outside in order to prevent the alloy particles from floating and getting out, but since an apparent specific gravity of the alloy is small and there is no bonding strength among particles when the alloy is in the particle state, such shielding does not bring a stable holding of the alloy. That is, when the hydrogen adsorption alloy is repeatedly used, free particles are further micronized and collapsed by repetition of shrinkage and expansion leading eventually to the particles getting out and being scattered. In this way, when lots of fins are densely fitted to increase the heat transfer area, the thermal conductivity is declined rather than improved.
In the third attempt of molding the particles into a compact, thermal conductivity is indeed considerably improved as compared with the form of particles or powder, but a problem exists in how to make close contact between a heat-transfer element and a hydrogen adsorption alloy compact without maintaining a heat insulating boundary. For example, in the case of arranging a heat exchanging unit by forming a compact of alloy (formed by compression molding) as shown in FIG. 21 and inserting several heat transfer pipes (copper pipes) through inside of the compact, it is necessary to provide through holes for insertion of the heat-transfer pipes. Such through holes can be made directly on the compact after molding it. It is also possible to arrange preliminarily a mold suitable for formation of such holes. But in any case, a spacing is required between the compact and the heat transfer pipe, because without such spacing it is impossible to build up a heat exchanging unit by the insertion of pipes.
Thus, it is an essential requirement for the prior art to maintain a spacing, and this spacing negatively affects the heat transfer between the heat transfer element and the compact of hydrogen adsorption alloy.