Research On Solar-Air Complementary Heating Systems And Their Energy Storage Characteristics

With the ever-increasing global energy consumption,the focus of energy utilization has shifted towards the development and usage of renewable energy sources.Among the various renewable energy sources,solar energy stands out due to its inexhaustible,inexpensive,safe,pollution-free,and transport-free characteristics.However,the utilization of solar energy is limited by the season and weather conditions,and the low-density heat flow,which restricts the application of various direct solar heat utilization systems.Firstly,to address these limitations,heat pump systems have been developed that can efficiently increase the temperature and quality of low-temperature,low-density heat sources.Heat pumps have been shown to be highly efficient when operated at high evaporation temperatures.Therefore,the solar collector heat,as a low-temperature heat source,can be utilized by solar heat pump technology.Furthermore,a significant amount of research has been dedicated to the use of latent heat storage methods to achieve the storage and utilization of solar energy.The development of phase change thermal storage materials and their integration with heat pump systems will enhance the applicability of heat pumps in various scenarios.To evaluate the performance and applicability of a complementary solar-air source heat pump heating system with heat storage,experimental studies are carried out.The results provide valuable insights into the performance of the system and offer a reference for the application prospects of complementary multi-energy heating systems.The present study proposes a novel multifunctional heat pump system that combines a solar heat pump with an air source heat pump,forming a solar-air source heat pump.The system is designed to operate in three different modes,namely,combined cooling and heating,domestic water heating,and space heating.A comparative analysis of the performance of the solar heat pump and the air source heat pump in the different modes was conducted.Subsequently,switching logic for the various modes was developed,and the system performance at varying operating temperatures was evaluated.The results reveal that the performance of the system is influenced by the operating temperature,with a decrease in COP observed with an increase in operating temperature.In combined cooling and heating mode,the average COP of the system can reach up to 6.0.In the SHP mode,the average COP for domestic water heating and space heating is 3.94 and 4.25,respectively,which is superior to that of the air source heat pump under similar operating conditions.It is recommended to switch domestic water heating from SHP to ASHP only when solar irradiance is less than 300 W m-2.In the second aspect of this study,microencapsulated phase change materials(MPCMs)were synthesized using an in-situ polymerization technique.To enhance the thermophysical properties and photothermal conversion performance of MPCM slurries,multi-walled carbon nanotubes(MWCNTs)were incorporated and the microscopic morphology and thermophysical parameters of MWCNT-MPCM slurries were investigated.Specifically,the thermal conductivity,viscosity,and photothermal conversion properties of the slurry were analyzed.Results indicated that the synthesized MPCM possessed a high latent heat of phase change up to 135.92 k J/kg.Moreover,the MPCM was found to be homogeneously dispersed in water,and its thermal conductivity increased with the temperature,reaching a maximum value of 0.6W/m-°C when MWCNTs were introduced.Meanwhile,the viscosity increased with the addition of MPCM when 0.5% of MWCNTs were introduced,and the viscosity of the 20%MPCM slurry exceeded 3000 m Pa-s.Importantly,it was observed that the peak temperature of the mixture could reach 60°C under 1 solar irradiation when the MWCNT concentration was0.5%.The MWCNT-MPCM slurry demonstrated high photothermal conversion efficiency without sacrificing other desirable thermophysical properties.Thirdly,graphene/paraffin/EVA PCMs(Gr Pr E)were fabricated through a process involving melting,mixing and ultrasonic dispersion.Graphene was added in mass fractions of0.05%,0.1%,0.25% and 0.5% to achieve enhanced thermal conductivity.The physical properties of Gr Pr E were characterized using SEM,DSC and Hot-Disk analysis.In order to evaluate the thermal efficiency and performance of Gr Pr E in energy storage applications,a thermal storage tester was set up for experimental testing.The results indicate that Gr Pr E exhibited good dispersion and stability,with no significant changes in temperature or latent heat of phase change.Furthermore,the thermal conductivity of the Gr Pr E increased with increasing graphene mass fraction.In thermal storage tests,the heat transfer rate of the graphene-added material was found to be improved as compared to the unadded material.The optimal graphene addition ratio was determined through careful analysis.Again,the present study investigated the thermophysical properties of paraffin/EVA/graphene nanocomposites as phase change materials(PCMs)through molecular dynamics(MD)simulations.The paraffin PCMs,prepared experimentally,were embedded in an EVA matrix with or without graphene,and five molecular models with graphene content ranging from 0 to 7.0 wt.% were constructed.The results indicated that the microscopic distribution and structure of the three components affected the corresponding thermodynamic properties at different temperatures.The non-equilibrium MD simulations demonstrated that the incorporation of graphene enhanced the heat transfer in the composite.However,when the graphene content exceeded 7.0 wt.%,the complex interactions between EVA and graphene hindered further improvements due to the disordered PCM crystal structure.The equilibrium MD simulations showed that the molecular diffusion coefficient increased and then decreased with increasing graphene content,thus affecting molecular mobility and phonon transport.A novel phase change filled bed,which is coupled with a heat pump system for efficient heat supply,is presented in this chapter.A block diagram of the system is included,detailing the solar collector heat storage and the coupled heat pump operation for building heating and domestic hot water.The impact of solar irradiation on the storage process is investigated,with faster solar heat storage rates observed at lower irradiation levels,achieving a storage completion time of 3.4 hours on average at 900 W/m2.The relationship between the heat collection area and the heat storage rate is also examined,highlighting the need to consider cost-benefit analysis when increasing the heat collection area.The temperature evolution pattern inside the phase change filled bed is investigated,and the impact of paraffin content and nanomaterials on the heat storage rate is analyzed.The influence of porosity on the flow thermal properties and heat storage of the filled bed is also studied.A longer total time for heat storage completion is observed for a porosity of 0.61,and a larger porosity is found to affect the phase change at the edge of the filled bed location.The coupling characteristics of the heat storage unit and the heat pump at different heat pump extraction temperatures are investigated,and the effect of different flow rates on the variation of outlet water temperature in the extraction condition is analyzed.Finally,a porous polydimethylsiloxane(PDMS)sponge containing phase change microcapsules(MPCM)was fabricated using the sacrificial template method to address the thermal insulation requirements of the heat storage unit.The PDMS sponge was combined with MPCM to create a porous thermal insulation material(PPM)that possessed phase change properties.Microscopic analysis revealed that PPM exhibited a rich three-dimensional pore structure,and the MPCM particles were uniformly distributed within the PDMS crosslinker.At an MPCM content of 40%,the latent heat of phase change of PPM was found to be 44.38 k J/kg.Furthermore,PPM demonstrated greater mechanical compliance,flexibility and hydrophobicity when compared to pure PDMS.Thermal insulation tests demonstrated that PPM increased the temperature difference by up to 9.4°C relative to pure PDMS.Moreover,the incorporation of MPCM imparted excellent thermal protection and dynamic temperature regulation properties to PPM.