Kenji Nasako

Stress on a reaction vessel by the swelling of a hydrogen absorbing alloy

There is a possibility that hydrogen absorbing alloys will generate unexpected stress in an alloy bed and deform or destroy the vessel because the alloys expand when they absorb hydrogen. The amount of stress generated on the vessel surface by alloy swelling was measured with the object of elongating the life time of the reaction vessel in heat utilization systems that use hydrogen absorbing alloys. As a result, it was found that 1) localized stress is generated at the bottom of the vessel due to hydrogen ab-/desorption cycles with an alloy packing fraction of 50 vol %, and this stress not only increases with each cycle, but also continues to increase even after plastic deformation of the vessel, 2) stress accumulation depends on the amount of hydrogen ab-/desorption and on the initial packing fraction, and 3) the mechanism for stress accumulation can be estimated as a two-step process in which agglomeration between the hydride particles occurs when the packing fraction of hydride is higher than 61 vol % in the initial cycles (Step 1), and then fine powder generated by pulverization during the cycles falls in gaps at the bottom of the vessel and causes the hydride packing fraction at the bottom of the vessel to gradually increase (Step 2).

Intermittent heat transport using hydrogen absorbing alloys

Abstract Hydrogen absorbing alloys enable heat to be transported over long distances using hydrogen as the transport medium, with theoretically very little heat loss. This report describes the system structure of a newly proposed heat transport system and an analysis of heat transport efficiency, and offers proof that a high efficiency of about 60% can be maintained by the system regardless of distance. Analytical results of a comparison between our transport method and a conventional method using heated water for the long-distance transport of intermittent heat (such as solar heat and plant waste heat) clearly show that transport by hydrogen enables the use of a more compact pipeline diameter for long-distance transport, that the rise time for heat transport is very short, that small-scale (2 kW) heat can be transported over a long distance, and that transport by hydrogen is much more efficient than transport by heated water for large-scale (100 kW) heat transport at distances over 2 km.

Development of an F-class refrigeration system using hydrogen-absorbing alloys

Abstract This paper presents a new refrigeration system to generate refrigeration heat below −20 °C and a coefficient of performance (COP) over 0.4 by using driving heat in the vicinity of 150 °C which can be supplied by solar collectors. The performance of the system was achieved mainly by development of a quaternary La-Y-Ni-Mn alloy with appropriate performance in the vicinity of −20 °C with regard to both equilibrium and kinetic characteristics for generating refrigeration heat.

Relaxation of internal stress generated in hydrogen absorbing alloy vessels

Hydrogen absorbing alloys cause internal stress in reaction vessels due to the expansion that occurs when they absorb hydrogen. This stress is affected not only by the amount of reacting hydrogen but also by the ab/desorption cycles. In this paper, we analyze the reaction distribution in a reaction vessel through simulations and show that locally excess internal stress occurs near the heat media inlet. We also show that this excess stress is decreased by unifying the reaction ratio by exchanging heat between the heat media inlet and the outlet in the reaction vessel.

Development of hydrogen-absorbing rare earth-Ni alloys for a −20 °C refrigeration system

Abstract The equilibrium and reaction rate performance of pseudo-binary La 1-xYxNi5-yMny (x=0–0.4; y=0–0.2) alloys were investigated with regard to their use in a −20 °C refrigeration system with a 130–150 °C thermally driven heat source, such as solar heat. The substitution of Mn for Ni in LaNi5 improved the reaction rate, while substitution of Y for La raised the equilibrium hydrogen pressure to approximately 1 MPa at room temperature, which is ideal for a refrigeration system. As a consequence, a −20 °C refrigeration heat cycle with La0.6Y0.4Ni4.8Mn0.2 was designed successfully.

Characteristics of hydrogen-absorbing Zr-Mn alloys for heat utilization

Abstract The substitution of various metals into ZrMn2 to produce possible materials for heat utilization systems was studied in the temperature range 200–250 °C. As a result, vanadium was found to be the most effective element for decreasing the equilibrium hydrogen pressure of the ZrMn2 alloy, cobalt was found to be an effective element for reducing the plateau slope of the ZrMn2 alloy and the ternary Zr-Mn-V alloy, and vanadium was found to be an effective element for reducing the hysteresis of the ZrMn2 alloy. Results show that quaternary Zr-Mn-Co-V alloy has excellent equilibrium characteristics in the temperature range 200–250 °C for heat utilization systems.

Long-distance heat transport system using a hydrogen compressor

Abstract A heat transport system using hydrogen absorbing alloys offers several advantages, such as enabling the use of more compact long-distance transport pipes, maintaining a constant level of efficiency regardless of transport distance, and enabling rapid transport. On the other hand, the system supplies heat to a load through a heat pump with a 20 K difference. The system therefore cannot be used for heat loads that require a higher temperature difference. Then, in order to increase heat load applications, we have proposed a hybrid heat transport system that uses a hydrogen compressor. We also developed a system structure and achieved a heat pump with a 40 K difference by using a 0.2-kW experimental system. Furthermore, we focused on using the same structure on the heat source side for supplying various heat loads, and have proposed a basic concept for a wide-area heat utilization system that transports heat to various loads from centralized heat sources.

Development of hydrogen-absorbing alloy applications as environment conscious materials

As environment conscious materials, hydrogen-absorbing alloys have several potential applications. We are now developing new applications, which are cadmium free secondary battery, CO2 emission free portable fuel cell and chloro-fluorocarbon free refrigeration system. This paper introduces these new applications and presents new hydrogen-absorbing alloys which are suitable for these applications.