Thermal utilization system using hydrogen absorbing alloys

Abstract: In order to make a thermal utilization system operable in a stable and efficient condition regardless the variation of the operating heat source used, the system is provided with three hydrogen absorbing alloys (MH₁, MH₂, MH₃) having different pressure-temperature characteristics, wherein absorption/desorption of hydrogen gas is performed in two steps, i.e. the hydrogen gas desorbed from a first alloy (MH₁) is absorbed by a second alloy (MH₂) at a predetermined pressure, and is absorbed by the third alloy (MH₃) at a higher pressure after it is desorbed from the second alloy (MH₂) in a case when the temperature of the operating heat source (18) is low, while the hydrogen gas desorbed from the first alloy (MH₁) is directly transported from the first alloy to the third alloy (MH₃) to be absorbed there when the temperature of the operating heat source (18) is high. Alternatively, two hydrogen absorbing alloys having different pressure-­ temperature characteristics are used in such a way that, when the temperature of the operating heat source (18) is low, the hydrogen gas desorbed from the first alloy (MH₁) is pumped to the second alloy (MH₂) to be absorbed there, but the gas is directly transported from the first alloy (MH₁) to the second alloy (MH₂) to be absorbed there when the temperature of the operating heat source (18) is high.

Description: This invention relates to thermal utilization systems such as a heat-pump type refrigeration apparatus using hydrogen absorbing or adsorbing alloys. This type of thermal utilization systems has been reported in, for example Japanese Patent Publication 58-19955, Japanese Patent Publication No. 61-20205, in which two kinds of hydrogen absorbing alloys are utilized in such a way that one of them is heated at an operating temperature of 130°C∼150°C to dissociate the hydrogen, which is transported to the other alloy and absorbed by this alloy exothermically, and endothermical reaction heat at returning the hydrogen provide refrigeration. However, these conventional systems have difficulty in that they require high operating temperatures in the range of 130°C∼150°C, which is too high for such thermal system as utilizes a cogeneration system, factory waste heat, or solar heat as an operating heat source, so that desirable operating efficiency cannot be gained. Other prior art such as Japanese Patent Publications Nos. 62-1188 and 62-1189 disclose similar refrigeration systems, which, however, have difficulty in that alloys must be heated so as to make the operating pressure of hydrogen, required for the absorption of the hydrogen dissociated from one alloy to the other, to exceed 20 atm. this implies that the container must be designed pressure-proof, resulting in a structural complexity of the container and accompanying sensible heat loss. Prior art documents EP-A-0001496 and FR-A-2393247 disclose similar multi-stage thermal cycles and EP-A-0055855 discloses a similar form of compressor operation. The invention is intended to overcome these disadvantages. Namely, a primary object of the invention is to provide a thermal utilization system using hydrogen absorbing alloys that may operate in a stable condition with an improved high operating efficiency regardless the kind of the operating heat source used. Another object of the invention is to provide a thermal utilization system using hydrogen absorbing alloys that may operate with an improved high operating efficiency under a reduced operation hydrogen pressure and with simple pressure-proof containers, thereby reducing the sensible heat loss therefrom. In order to attain these objects, the thermal utilization system according to the invention as defined in claim 1 has a first feature that it employs alternatively, depending on the temperature of the available operating heat source, a thermal cycle that utilizes three hydrogen absorbing alloys having different pressure-temperature characteristics, or a thermal cycle utilizing two of said hydrogen absorbing alloys except the one having intermediate characteristic. When the temperature of the operating heat source is low, three hydrogen absorbing alloys are used, in which hydrogen disorbed or dissociated from the first alloy is absorbed by the third alloy in two steps: namely the hydrogen is first absorbed by the second alloy under a intermediate pressure and then disorbed from the second alloy and given off to the third alloy under a higher pressure in the similar way. On the other hand when the temperature is high, the hydrogen is disorbed from the first and given to the third alloy directly and absorbed there. In this way, regardless the temperature of the operating heat source, efficient operation of the thermal utilization system is possible. The thermal utilization system of this invention has a second feature that it comprises a first alloy that disorbs hydrogen as it is heated by an operating heat source, a third hydrogen-absorbing alloy that absorbs said hydrogen, and a hydrogen pump for pumping the hydrogen pressure from said first alloy to said third alloy when the temperature of the operating heat source is low. When the temperature of the operating heat source is low, the hydrogen is pressurized by the hydrogen pump to ensure the absorption by the third alloy, while when the temperature of the operating heat source is high, the hydrogen is instead transported directly to the third alloy, not through the pump. This enables efficient operation of the thermal utilization system regardless the possible change in temperature of the operating heat source. The thermal utilization system of this invention has a third feature that the operating hydrogen pressure needed for the first and third hydrogen absorbing alloys are not more than 20 atm in said regeneration process and that said alloys have the characteristic that the minimum logarithmic pressure difference, required for the transportation of at least 0.6 weight % of hydrogen reversibly, is equal to or less than 0.8. In other words, the invention uses hydrogen absorbing alloys for which equilibrium hydrogen absorption pressure is not more than 20 atm near the upper limit of the absorption plateau at an ordinary room temperature, so that the thickness, and hence the heat capacity, of the container required for the pressure is not great. With the minimum logarithmic pressure difference of not more than 0.8, it is possible to lower the heating temperature, thereby improving the reversibility of the process and the efficiency for recovering "cold heat", which is defined as the heat extracted into the thermal system working as a refrigerator or a cold temperature source. For the above thermal utilization system the first alloy is preferably either: a hydrogen absorbing alloy having MgZn₂-type Laves phase structure and containing at least Zr, Mn, and Co; a hydrogen absorbing alloy having AB₅-type hexagonal structure and containing at least rare-earth metals (such as La, Ce, Nd, Pr, Sm and a mixture of these metals) and Ni, and at least one of the first transition metals, Al, and Sn; a hydrogen absorbing alloy having AB₅-type hexagonal structure and containing at least Ca, Ni, and at least one of the first transition metals, Al, Sn, and rare-earth metals; a hydrogen absorbing alloy having AB-type cubic structure and containing at least Ti, Fe, Ni; or a mixture of these alloys. On the other hand the second alloy is preferably either: a hydrogen absorbing alloy having AB₅-type hexagonal structure and containing at least rare-earth metals (such as La, Ce, Nd, Pr, Sm and a mixture of these metals) and Ni, and at least selected one of the first transition metals, Al, and Sn; a hydrogen absorbing alloy having MgCu₂-type Laves phase structure and containing at least Ti, Cr, and Mn; a mixture of these alloys; or said hydrogen absorbing alloy having AB₅-type hexagonal structure and containing at least Ce. Figs. 1 through 6 are given for the explanation of an embodiment of the invention; in particular, Fig. 1 is a graphical representation of the pressure-temperature characteristics of the thermal utilization system using three kinds of alloys, illustrating the principle of its operation; Fig. 2 is a graphical representation of the pressure-temperature characteristics of the thermal utilization system using two kinds of alloys, illustrating the principle of its operation; Fig. 3A and B illustrate continuous operation of the thermal utilization system using the three kinds of alloys; Fig. 4A and B illustrates continuous operation of the thermal utilization cycle using the two kinds of alloys; Fig. 5 shows the piping used for the first embodiment; and Fig. 6 is a graphical representation of the efficiency of the system in terms of the intensity of the solar energy used as an operating heat source. The concept of the embodiment is now explained before the details of the embodiment. As understood from the pressure-temperature characteristics shown in Figs. 1 and 2, three kinds of hydrogen absorbing alloys that are operable at high, low, and intermediate temperature, respectively, are provided. The high-temperature alloy (which will be hereinafter referred to as MH₁) may be a rare-earth metal Ni alloy such as a La rich Mm-Ni-Al system. The intermediate temperature alloy (which will be hereinafter referred to as MH₂) may be a rare-earth metal-Ni alloy such as a Mm-Ni-Al system. The low temperature alloy (which will be hereinafter referred to as MH₃) may be a rare-earth metal Ni alloy. These hydrogen absorbing alloys MH₁, MH₂, and MH₃ all participate in the operation of the thermal utilization cycle when the temperature of the operating heat source is low, say 90°C. When the temperature of the operating heat source is relatively high, say 130°C, the alloys other than MH₂ participate in the cycle, as seen in Fig. 1 and Fig. 2. Namely, as the alloy MH₁ heated by the operating heat source to 90°C and MH₂ is cooled by cooling water of 20°C, MH₁ disorbs the so far absorbed hydrogen to dissociated MH₂ (first regeneration process). Next, as MH₂ is heated by the operating heat source to 90°C and MH₃ is cooled by cooling water of 20°C, the hydrogen disorbed therefrom is transported to dissociated MH₃ (second regeneration process). Then the hydrogen is transported from MH₃ to MH₁ to recover cold heat at about -20°C through the endothermic desorption of hydrogen by MH₃ (cold heat generation process). On the other hand, in Fig. 2, as MH₁ is heated by the operating heat source to 130°C, while MH₃ is cooled by cooling water of 20°C, MH₁ disorbs the so far absorbed hydrogen to dissociated MH₃ (regeneration process). Thereafter the hydrogen is transported from MH₃ to MH₁ to gain cold heat at about -20°C through the endothermic desorption in MH₃ (cold heat generation process). Continuous operation of the thermal cycle is now described with reference to Figs. 3A and B, Figs. 4A and B. The containers 1 through 6 are filled with MH₁, MH₂, MH₃, MH₁, MH₂, MH₃, respectively, and are each thermally connected with either an operating heat source, a cooling water serving as a cooling source, or a refrigeration load. They are connected with each other so as to interchange hydrogen depending on required conditions. The case when the temperature of the operating heat source is relatively low (90°C) is illustrated in Figs. 3A and B. In Fig. 3A, hydrogen gas is simultaneously transported from the container 2 to 3, from the container 4 to 5, from the container 6 to 1, and through the endothermic desorption of gas by MH₃ in the container 6 cold heat of about -20°C is gained, which is provided to the refrigeration load. At the end of this process hydrogen gas is simultaneously transported from the container 1 to 2, from the container 3 to 4, and from the container 5 to 6 as shown in Fig. 3B, which provides the refrigeration load with refrigeration heat of about -20°C generated by the endothermic reaction with MH₃ in the container 3. These processes shown in Figs. 3A and 3B will be repeated alternately. When the temperature of the operating heat source is relatively high (130°C and above), hydrogen gas is transported simultaneously from the container 3 to 1, from the container 4 to 6, as shown in Fig. 4A. Next, hydrogen gas is simultaneously transported from the container 1 to 3, from the container 6 to 4, as shown in Fig. 4B. These procedures shown in Figs. 4A and 4B will be also repeated alternately. In this way either one thermal cycle using the three alloys or the other using the two alloys is selectively operated depending on the level of the temperature of the operating heat source, so that the temperature is measured by a temperature sensor or a pyrheliometer if the solar heat is used. The temperature at which the cycle is switched from one to the other may be set at 130°C, say. Referring to Fig. 5 details of the embodiment is now described. Containers 1 through 6 are filled with MH₁, MH₂, MH₃, MH₁, MH₂, MH₃, respectively. The container 1 and container 6 are connected with each other with a first and a second hydrogen gas pipes 7 and 8, respectively. The first pipe 7 is also connected with other containers 2 through 5. The pipes 7 between adjacent containers are provided with respective valves 9, 10, 11, 12, and 13. The second pipe 8 has a valve 14, and valves 15 and 16 on the opposite side of the valve 14, respectively. The containers 1 through 6 each have heat exchangers 17 for heating the alloys therein and for getting reaction heat (hot and cold heat). The containers 1, 2, 4, and 5 are connected in such a way that their heat exchangers 17 are selectively connected with the operating heat source 18 or with the 20°C cold heat source 19 by means of switching valves 20. The containers 3 and 6 are connected in such a way that their heat exchangers 17 are selectively connected with the cold heat source 19 and the refrigeration load 21 by means of switching valves 20. The pipes between the respective heat exchangers 17 and the heat source 18, and between the cold heat source 19 and the refrigeration load 21 are provided for forced circulation of the heat media, which may be carried out by a pump and the like. As mentioned above, a temperature sensor or pyrheliometer is used to measure the temperature level of the operating heat source 18, which level is compared in a control unit (not shown) with the predetermined temperature of 130°C, say, or with predetermined solar energy intensity. If the level is higher than the predetermined level, the control unit selects the thermal cycle using the two alloys MH₁ and MH₃, but selects the thermal cycle using the three alloys if the measured level is lower than the predetermined level. The valves 9 through 16 and the switching valves 20 are adapted to be either opened or shut accordingly to maintain the cycle selected. The operation of the embodiment is now described. If the control units finds the measured temperature level of the operating heat source to be lower than the predetermined level, it controls the switching valves 20 in such a way that the heat exchangers 17 of the containers 1, 3, and 5 are connected with the cold temperature source 19, the heat exchangers 17 of the containers 2 and 4 are connected with the heat source 18, and the heat exchanger 17 of the container 6 are connected with the refrigeration load 21, and controls the valves 14, 10, and 12 to open so that the containers 1 and 6 are connected, containers 2 and 3 are connected, and the containers 4 and 5 are connected. This permits the container 6 to provide the refrigeration load 21 with cold heat of about -20°C generated in the endothermic hydrogen desorption that has taken place in the container. At the end of this process the control unit controls the switching valves 20 so that the heat exchangers 17 of containers 1 and 5 are connected with the heat source 18, the heat exchangers 17 of containers 2, 4, and 6 are connected with the cold heat source 19, and the heat exchanger 17 of the container 3 is connected with the refrigeration load 21, and controls the valves 9, 11, and 13 to open or shut so that the containers 1 and 2 are connected, the containers 3 and 4 are connected, and the containers 5 and 6 are connected. In this case the container 3 provides the refrigeration load 21 with cold heat. Through such control of the switching valves 20 and the valves 9 to 16, these alternating operations may be repeated using three pairs of hydrogen absorbing alloys. On the other hand if the control unit finds the temperature level of the operating thermal source 18 higher than the predetermined level, it controls the switching valves 20 so that the heat exchangers 17 of the containers 1 and 6 are connected with the cold temperature source 19, the heat exchanger 17 of the container 4 is connected with the heat source 18, and the heat exchanger 17 of the container 3 is connected with the refrigeration load 21, and controls the valves 15 and 16 to open so that the containers 1 and 3 are connected, containers 4 and 6 are connected. This permits the container 3 to provide the refrigeration load 21 with cold heat of about -20°C generated in the endothermic hydrogen desorption that has taken place in the container. At the end of this process the control unit controls the switching valves 20 so that the heat exchangers 17 of containers 3 and 4 are connected with the cold heat source 19, the heat exchanger 17 of container 1 is connected with the heat source 18, and the heat exchanger 17 of container 6 is connected with the refrigeration load 21. In this way the hydrogen gas transported into the container 4 is transported back to the container 4, giving the refrigeration load the cold heat generated in the container 6 in the endothermic desorption reaction. Thus, through such control of the switching valves 20 and the valves 15 and 16, these alternating operations may be repeated using two pairs of hydrogen absorbing alloys MH₁ and MH₃. Fig 6 shows overall efficiencies of the thermal cycles using a solar energy heater as the operating heat source 18, for various degrees of solar energy intensity and in the case of cycles using three alloys and two alloys. As is apparent from this Figure, the two-alloy thermal cycle is operable with high efficiency for the solar intensity above 600 Kcal/hr·m², but otherwise operable with only limited efficiency. On the other hand the three-alloy thermal cycle is operable even for weak solar intensity with desirable efficiency, but under intense solar energy it suffers from a great heat loss due to its structural complexity. Therefore, it is possible to attain an extended and stable operation of the thermal cycles in a day-time by selectively using the three- and the two-alloy thermal cycles depending on the intensity of the solar energy (see the dotted curve in Fig 6). It should be understood that, although the embodiment above has been described for the case in connection with an automatic selection of the thermal cycles, manual selection or timer-controlled selection may be alternatively employed. Table below compares the performance of the thermal utilisation system using these alloys with that of a conventional system.

Claims: A thermal utilisation system comprising a first multistage thermal cycle using three kinds of hydrogen adsorbing alloys each having different pressure-temperature characteristics for the adsorption and desorption of hydrogen, a second single stage thermal cycle using two of said three alloys other than the one having the intermediate characteristic, a heat source for operating he cycles and control means for determining the temperature of the heat source and for operating the first cycle if the temperature of the heat source is lower than a predetermined level, and for operating the second cycle if the temperature of the heat source is higher than the predetermined level.

Inventors: FURUKAWA AKIO, HIRO NAOKI - -, MOROTO MASAKAZU, NASAKO KENJI, SAITO TOSHIHIKO, YONESAKI TAKAHIRO, YONEZU IKUO, FURUKAWA, AKIO, HIRO, NAOKI, 204, 1-5-13, MOROTO, MASAKAZU, NASAKO, KENJI, SAITO, TOSHIHIKO, YONESAKI, TAKAHIRO, YONEZU, IKUO
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1990
Main classification: F25B17
IPC codes: F02P17, F25B17