Nuclear Production of Hydrogen, Fourth Information Exchange ...

HEAT PUMP CYCLE BY HYDROGEN-ABSORBING ALLOYS TO ASSIST HTGR IN PRODUCING HYDROGEN
Then, the A hydride of MaH X is heated from T L to T M , and the Alloy B shown by Mb is heated from
T M to T H . Consequently, the hydrogen equilibrium pressure of MaH X rises to p A,M , and that of Mb rises
to p B,H . Since it is designed that p A,M is higher than p B,H , hydrogen is transferred from Alloy A to Alloy B.
Heat is supplied to MaH X in order to desorb hydrogen from Alloy A. At the same time, heat is extracted
in hydrogenating Mb to MbH X . In this condition, heat is flowed from the low temperature of T M to the
higher temperature of T H . Then the B bed remained T H during hydrogenating. The second stage is the
reversal of the first stage. After the second stage, Alloys A and B are at the same initial condition of the
first stage. Thus, the absorption-desorption cycle is continued, and the heat pump cycle is completed.
The entropy change of hydrogen-absorbing alloy is almost constant regardless of different alloys.
This means that the intercept along the vertical axis in Figure 5 is almost the same regardless of
different alloys. In addition, the enthalpy change that means the slope of the van’t Hoff plot in
Figure 5 decreases with the partial displacement of V by Fe. Therefore, two alloys comprised in the
heat pump can be found.
Experiment of high-temperature heat pump
Temperature enhancement during H 2 absorption
An alloy of ZrV 1.9 Fe 0.1 was manufactured from metal ingots of Zr, V and Fe in an Ar-arc melting
furnace, and the alloy was crushed and screened between 12 to 32 mesh in an Ar globe box, because
the alloy is flammable in air. The alloy particles of 5.012 kg were packed in a dual cylindrical vessel
made of SUS-316. After evacuated by a diffusion pump at temperature of 600°C, the alloy bed is cooled
or heated to a specified temperature of T 0 by the outside electric furnace. After sufficient time,
electricity input to furnace was stopped. At the same time, hydrogen was supplied under a constant
flow rate dented by W. Temperature was measured in several positions in the bed, and it was found
that the temperature was almost uniform in the bed.
Figure 6 shows several experimental results of variations of temperature in the inside of the
ZrV 1.9 Fe 0.1 particle bed with hydrogenating. T is a local temperature, q m is the average hydrogen absorbed
amount in the bed, and q 0 is the saturated absorbed amount after sufficiently long time has elapsed.
The experimental temperatures shown by solid lines increased without any external heating during
hydrogenating.
Figure 6: Temperature enhance factor with hydrogen absorption amount
Figure 7 shows the maximum temperature increase defined by T max -T 0 that is plotted as a
function of the inlet temperature of T 0 and the hydrogen flow rate of W. Heat generated during
hydrogenating of ZrV 1.9 Fe 0.1 is consumed for heating hydrogen introduced to the bed and heat leakage
412 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010