Optimization Study On A Metal Hydride Heat Pump System For Heat Upgrading By Numerical Simulation

Many kinds of metals or alloys can react reversibly with hydrogen, and the reaction products are commonly called metal hydrides (MH). The heat effect from this reaction can be used in a metal hydride heat pump (MHHP) for heat upgrading, heat amplification and refrigeration. The MHHP can be driven by thermal energy and is environmentally benign, and it shows good flexibility under varied operation conditions. For these reasons, the system has great potential in the application of waste heat recovery. In-depth studies were carried out for the heat upgrading system based on MH in this thesis and the main results are as follows.(1) A two-step procedure was proposed to select the pair of alloys for the MHHP system, in which the integrated performance of the system was highlighted. In the first step, all feasible pairs of alloys are obtained according to the driving force of reaction. A deduction routine is implemented based on the equilibrium pressures under given temperatures, through which a single stage system can be readily expanded into a two stage system. In the second step, the performance indices for the heat pumps adopting different pairs of alloys are calculated, then the results are integrated using a classical analytic hierarchy process (AHP). Finally the optimized pair with best integrated performance arises.(2) A 2-D model for the absorption/desorption processes in a MH reactor packed with LaNi5 was formulated. The model was solved using the COMSOL Multiphysics 3.5a package and validated against the literature data. The effect of heat transfer enhancement by Al foam or MH compacts was discussed for a tubular reactor. When adopting heat transfer enhancement, we can observe more uniform reaction in the bed, which is favorable for the system operation. The COP of the heat pump system is slightly reduced by around 3% due to more parasitic sensible heat loss. Meanwhile, the reaction rate is much improved and SHP increases greatly for a remarkable reduction in cycle time. In a word, the effect of heat transfer enhancement is basically positive. When the operation pressure varies in a wide range from 0.3 to 1.1MPa, the improvement in reaction rate by heat transfer enhancement is unanimously remarkable. However, such effect tends to attenuate if the external convective heat transfer gets poor.(3) The absorption/desorption in a MH reactor are typical multi-physical processes, and the transport processes often limit the actual reaction rate. Two characteristic parameters—heat transfer controlled reaction rate rh and mass transfer controlled reaction rate rm were defined, whose values can be obtained through simple algebraic calculations. By comparison of rh, rm and the intrinsic reaction rate rk, we can determine the controlling step of the absorption/desorption process and make a rough estimate about the actual reaction rate. The influences of many factors on the actual reaction rate, such as reactor configuration (tubular/disc/annulus disc), design parameters (bed thickness/diameter of filter tube) and operation parameters (fluid temperature/exerted pressure), were discussed by parameter analysis and numerical simulation, the comparison between both results showed qualitative agreement.(4) The model for the cyclic operation of MHHP system was developed based on the single reactor model. LaNi5 and LaNi4.7Al0.3 were respectively packed in the coupling reactors, and Al foam was used to enhance heat transfer in the bed. Quite a few realistic factors were considered in the model, including the mutual influence of coupling reactors, the distributed and heterogeneous physical properties, the flux continuity in between the bed and vessel wall, the local reaction during pre-heating/pre-cooling processes. The discretization and solution of the model equations were based on the classical finite volume method, and a procedure resembling the SIMPLE algorithm was implemented. However, due to the compressibility of hydrogen, the correction value of pressure was introduced into the correction of both density and velocity. The simulation results agreed well with the experimental data from the literature, thus the accuracy of the model was proved.(5) The changes in reacted fraction and temperature for the MH may affect the stress condition and cycle life of MH, thus collecting the information during repeated operation is crucial. In the thesis, numerical simulations were carried out for multiple working cycles of the heat pump. It was shown that the reaction and heat transfer do not proceed uniformly, yet local state generally repeats in a cyclic manner. Fast reaction was observed in the position close to the fluid inlet, while the reaction slows down in other places. For the reactor packed with LaNi5, obvious local reaction occurs during pre-heating/pre-cooling and the reacted fraction varies drastically by 0.8 in certain positions, which may result in stress intensification. On the contrary, the local reaction is rather insignificant for the reactor with LaNi4.7Al0.3, and the reacted fraction throughout the reactor varies in a moderate range.(6) Two new indices—average temperature rise and standard deviation of output were introduced together with COP and SHP to evaluate the performance of the MHHP, and sensitivity analysis about the various design and operation parameters were conducted. The main conclusions of the simulation are listed as follows. For COP, the transferred amount of hydrogen shows the most significant effect, whose increase would result in the increase of COP. The parameters related to the sensible heat loss, such as vessel wall material and operation temperature, come next. As sensible heat loss is increased, COP will be decreased. The rest parameters show little effect on COP. With the transferred amount of hydrogen fixed, SHP is largely determined by the cycle time. As cycle time is shortened, SHP would generally be improved. The trends of the last two indices are often similar. The latter would increase with the former, which implies a larger fluctuation in output.Based on the above mentioned contents, the working frame for the optimum design of a MHHP used in heat upgrading was formed. The investigations are helpful for a better understanding of the complicated nature of absorption/ desorption processes which integrate mutiple physics. The theoretical method proposed and the simulation conclusions are expected to be useful in the engineering practice of MHHP development.