| The huge quantity of thermal energy is demanded for hot water in industrial productionand daily life. Therefore, heat pump technology is growing concern in the preparation of hotwater for energy saving and environmental advantages. However, the heat source of heatpump, such as, air energy, solar energy, geothermal energy, industrial waste energy and otherlow-temperature energy, is difficult to meet the actual demand because the supply of thermalenergy can not fulfill such a demand in terms of the mismatch both in time and intersity. Inview of the efficiency of the energy utilization and environment protection, a potential way isto use the heat pump with phase change thermal energy storage, which is considered to beimportant energy technology of thermal engineering applications, because it provides a highenergy storage density, small temperature fluctuations and is easy to control, especially itbridges the gap between energy generation and consumption.Energy storage heat exchanger for heat pump water heater with phase change thermalenergy storage is studied. Based on theory of heat transfer, fluid dynamics, thermodynamicsand theory of phase change energy storage, some researches are carried out in the following:(1) Firstly, the microscopic theory of solid-liquid phase change is introduced, the main factorsthat influence the phase change heat transfer process, namely, nucleation and crystal growth,is discoursed in detail. Further, the first law model of thermodynamics that simulation thephase change problems is discussed, the mathematical models and numerical methods forsolid-liquid phase change are established. Secondly, the second law of thermodynamic usedfor assessment the performance of heat exchanger with phase change energy storage isdiscussed, the quality of energy utilization of energy storage heat exchanger during energystorage and release is evaluated by exergy analysis. At last, the validation of numericalsimulation solid-liquid phase change and natural convection is rigouous verification, andexperimental data taken from the literatures are conducted to validate the model. Thenumerical results show a good agreement with the experimental ones.(2) In present work, the effects of different cavity volume fractions of phase change material(PCM) on fluid flow and heat transfer behavior in a latent thermal unit are studied numerically.The commercial Computational Fluid Dynamics (CFD) code is used for the numericalsolution based on transient nonlincer conjugate heat transfer. The volume expansion ratio, thetime of complete thermal storage, heat flux, liquid fraction, velocity and temperature fieldsare investigated for the range of PCM cavity volume fractions from35%to95%. It is notedthat a vortex (as a heat transfer enhancers) is present near the heating plate wall for the PCM cavity volume equal to85%. It is found that the volume expansion ratio decreases as PCMcavity volume fractions increasing, whereas the time for complete energy storage increases.Further, the correlations of the volume expansion ratio and the time of complete thermalstorage are developed as a function of PCM cavity volume fractions. The detailed knowledgeregarding interface heat transfer rate provides a deeper understanding the heat transfermechanisms.(3) The fluid flow and heat transfer in a plate-fin unit with a characteristic length of2mmused for rapid heat storage/release by paraffin (PCM) are investigated numerically. Transientsimulations are performed based on the finite volume method. The effect of temperaturedifferences on the fluid flow and heat transfer in the energy storage/release system is analyzed.It is found that temperature differences play a key role in the performances of energy storagewhen temperature differences are less than20℃. It is noted that part of not solidified PCMcan be observed clearly during energy release, and a vortex in the air region is formedremarkably at the moment of complete thermal energy release. The correlations are developedas a function of the associated variables. The obtained correlations are useful for futurecomponent design and system optimization.(4) The performance of energy storage/release of energy storage unit with various dimensionsis investigated, based on the application of the first law of thermodynamics and the secondlaw of exergy analysis. During energy storage process, small temperature difference and largewidth on PCM and fin is benefiting the natural convection formed; therefore, the heat transferis enhanced. Moreover, the better heat transfer performance is obtained for higher fin. Heatcollection is identical between high fin, large width on PCM and fin, and short fin, smallwidth on PCM and fin. During energy release process, the influence of dimensions on solidfraction is remarkable. However, the effect of temperature difference of energy release onsolid fraction is small, small temperature difference is beneficial for heat conduction.Furthermore, the obtained baseline dates are useful for energy storage unit structuraloptimization and system design. The exergy analysis shows that the larger of heat transferfluid (HTF) velocity, the smaller of exergy efficiency.(5) The plate-fin heat exchanger used for heat pumps phase change thermal storage is studiedby experiment, based on PCM and efficient heat exchanger. The interface temperaturechanges between plate heat exchanger and energy storage material, outlet temperature andflow in different HTF flow rates on heat pump phase change thermal storage device ofplate-fin are explored. The experimental results show that the temperature difference is smallin the same side of the plate at different cross-section by thermocouple measured, and there is a gentler section of the temperature measurement curve. The greater HTF flow rates, theshorter of exothermic under the same temperature drop of HTF. The flow rate of HTF shoulduse0.3m3/h when outlet temperature is40℃. Moreover, the high quality of using energy,namely, high exergy efficiency, is obtained for the flow rate0.3m3/h. The heat transfer rate ofenergy storage heat exchanger is increases as HTF flow rate increases. Further, the coefficientof performance (COP) of system reached5.0or above. Therefore, it presents good energyconservation and land saving due to high energy storage density. The results providedexperimental basis for the plate-fin heat exchanger used for heat pump phase change thermalstorage. The results have practical significance and reference value on plate-fin heatexchanger, which is widely used in heat pump phase change thermal storage device. |
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