| With dwindling fossil fuel reserves and growing greenhouse gas effect, the futureenergy is bound to be dominated by solar energy, wind energy, ocean energy andgeothermal and other renewable energy. However, generally the supply of renewableenergy is always not continuous, and the low energy density is another limitation.Therefore, in order to meet the energy mismatch between supply and demandcontradictions, and to increase efficiency in the use of renewable energy, we mustvigorously develop advanced energy storage technology.The magnesium/magnesium hydride thermochemical heat storage systemstudied in this paper is a great potential for regenerative system. The hydrogen contentof magnesium hydride is7.6wt%, of all the binary metal hydrides, it has the highestenergy storage density of about2814KJ/Kg MgH2. In addition, the magnesium is rich,low price, low toxicity, so it has been more extensively studied.In this paper, based on the magnesium/magnesium hydride system, atwo-dimensional mathematical model of the heat storage and releasing processes wasestablished to study the non-steady-state heat and mass transfer processes. It mainlydiscussed the influences of the reactor wall temperature, hydrogen pressure and metalfoam on the rate of reaction. Simultaneously an experimental system of themagnesium/magnesium hydride system was built to study effect of the reactor walltemperature, hydrogen pressure, and magnesium powder particle size on the rate ofreaction. Finally the decomposition of magnesium hydride was studied by thesimultaneous thermal analyzer. The main conclusions are summarized as follows:1. Add metal foam can effectively increase the effective thermal conductivity of thereactor bed, and the smaller the porosity is, the greater the effective thermal conductivity. Heat transfer capability of the reactor bed is a key factor affectingthe performance of the reaction. Adding metal foam with a porosity of0.92, theheat transfer capacity of the reaction bed is greatly enhanced and the reaction timeis shortened by40%, the heat power has got an increase of60%.2. Add metal foam will reduce the total heat capacity of the system, and for thecooling fluid at different temperatures, in order to achieve a highest heat releasingpower, the porosity of the metal foam should be appropriated chosen.3. For the exothermic process, there is an optimum wall temperature, making theheat release rate to achieve the fastest. Wall temperature is too high or too low willmake the deviation from the theoretical reaction temperature of the bed, therebydecreasing the reaction rate. During exothermic process, the equivalent thermalconductivity of the reactor bed is not always the larger the better. Theimprovements of the heat capacity of the reaction bed should be combined withthe boundary temperature and the ultimate aim is to ensure that the reactiontemperature of the bed could be approached to the best value as close as possible.4. During the magnesium hydride decomposition process, the heat transfer capabilityof the reaction bed determines the reaction rate, the closer to the outer wall, themore rapidly the reaction rate will be. The entire decomposition reaction movesfrom the outer wall to the center. In certain extent, enhancing the thermalconductivity and the wall temperature will accelerate the reaction rate, but beyonda certain limit, the effect of improving the reaction rate is no longer apparent.5. In the experiment of hydrogen absorbing, there is an optimal temperature of thereactor wall to ensure the system to get the fastest reaction rate, and at differenthydrogen pressures, the optimum wall temperature is not the same. The reactionrate is faster if the particle size of the magnesium powder is smaller, and thecorresponding degree of reaction is also greater. The decomposition rate of themagnesium hydride is enhanced with the decomposition temperature. |
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