Research On Building Energy Saving Effect Of Active-passive Coupling System Based On Composite Phase Change Materials Containing Hydrated Inorganic Salts

The ever-increasing building energy consumption makes building energy saving particularly urgent.Introducing phase change materials?PCMs?that can absorb or release a large amount of latent heat during phase transition into the building is an effective way to achieve building energy conservation.In buildings,PCMs can not only be integrated into the envelope structure to absorb or release heat by relying on temperature fluctuations to achieve passive energy saving,but also can be introduced into HVAC systems to achieve active energy saving.Most researches on the application of phase change materials in building energy conservation focus on the organic phase change materials represented by paraffin.Although organic phase change materials have the advantages of suitable phase change temperature,no supercooling and no phase separation,their flammability will threaten the safety of buildings.In contrast,hydrated inorganic salts that have the advantages of low cost and non-flammability are more suitable for buildings.This paper focuses on the application of hydrated inorganic salts in building energy conservation.First,the phase change temperature of the hydrated inorganic salt is adjusted to make it more suitable for buildings;the obtained inorganic phase change material is compounded with two kinds of porous carriers with different thermal conductivities to prepare two kinds of composite phase change materials;the above two composite phase change materials are applied to the envelope structure and the ventilation system respectively,and the energy saving effect is improved by combining passive and active methods.Then,according to the application of the radiant floor system,two kinds of composite phase change materials with different phase change temperatures based on inorganic hydrated salts are developed;a double-layer radiant floor is constructed and its energy-saving effect is evaluated in winter and summer climates.Finally,the phase change envelope structure,the phase change ventilation system and the double-layer phase change radiation floor based on hydrated inorganic salts are integrated into a large-sized room,and the thermal characteristics of the test room with active-passive energy-saving system are studied by numerical method.According to the application of PCMs in building envelopes,floors and ventilation systems,this paper focuses on the development of composite PCMs based on hydrated inorganic salts with suitable phase change temperature and thermal conductivity.The application performance and the energy saving effect of inorganic hydrated salt PCMs are evaluated by experimental investigation and numerical simulation.In order to broaden the application range of inorganic PCMs in buildings,a ternary mixture PCM of CaCl₂·6H₂O-NH₄Cl-SrCl₂·6H₂O with adjustable phase change temperature is developed.The thermal properties of the ternary mixture are characterized by DSC,cooling curves,XRD,FT-IR spectrum and melting-solidification cycling experiment.It is found that NH₄Cl can decrease the phase change temperature of CaCl₂·6H₂O.The CaCl₂·6H₂O-NH₄Cl mixture exhibits a phase change temperature of 23.5°C?TM24?or 20.8°C?TM20?when the content of NH₄Cl is 4 wt%or 8 wt%.The cooling curves show that adding 1 wt%SrCl₂·6H₂O can almost eliminate the supercooling of the ternary mixture.The XRD and FT-IR results suggest that there is only physical mixing between the three salts.After 200 cycles of melting-solidification experiments,the maximum deviations of the phase change temperature and the latent heat are 2.0%and 2.6%,respectively,indicating the thermal reliability of the ternary mixture is good.Based on the research status of phase change walls and phase change ventilation systems,the TM24 mixture PCM with a phase change temperature of 23.5°C is compounded with expanded perlite?EP?and expanded graphite?EG?,respectively.The obtained two composite PCMs are prepared into two inorganic composite phase change panels with different thermal conductivities.The TM24/EP composite panels are integrated in the envelope structure,and the TM24/EG composite panels are installed in the ventilation system simultaneously to experimentally study their energy saving effects.Furthermore,a numerical model of the phase change ventilation system is constructed.The influence of the inlet air flow rate and the thermophysical properties of PCM are systematically studied under the continuous supply of fresh air to optimize the overall thermal performance of the ventilation system.The experimental results show that the TM24/EP composite PCM exhibits a thermal conductivity of 0.1170 W/?m·K?,making it suitable for thermal insulation of envelopes;while the thermal conductivity of TM24/EG composite PCM is as high as 9.720 W/?m·K?,which helps to improve the heat exchange efficiency between PCM and flowing air in the ventilation system.Compared with the room equipped with only TM24/EP composite phase change panels,the room containing a phase change envelope and a phase change ventilation system exhibits a reduced indoor temperature fluctuation?22.29-28.69°C?,a longer temperature lag?1.6 h?and a higher frequency of thermal comfort?78.3%?.The simulation results show that the outlet temperature fluctuation of the ventilation system reduces as the inlet air flow rate decreases or the panels'thickness increases.Moreover,narrowing the phase change temperature range of the PCM in the outlet channel can further reduce the maximum outlet temperature and improve the PCM utilization rate.In addition,when the inlet air flow rate is 11.47 kg/h and the panels'thickness is 12 mm,the optimal thermal conductivity of the phase change panels is 13.0 W/?m·K?.In this case,the outlet temperature fluctuation of the ventilation system is 22.5-27.9°C.For the application of PCMs in radiant floor,the TM20 mixture PCM developed above is compounded with EG to prepare a TM20/EG composite PCM with a phase change temperature of 20.2°C.On the other hand,a Na₂HPO₄·12H₂O-Na₂Si O₃·5H₂O/EG composite PCM with a phase change temperature of 31.3°C is developed.Then,the above two kinds of composite PCMs with different phase change temperature are simultaneously introduced into the radiant floor as energy storage materials for the heat storage layer and the cold storage layer,respectively.A double-layer inorganic phase change radiant floor with both cooling and heating functions is constructed and its thermal characteristics are experimentally studied.In winter climate,the room equipped with a radiant floor containing an upper thermal storage layer and a lower cold storage layer,exhibits a thermal comfort duration of 2.2 times that of a reference room using pebbles as the energy storage material in the floor.In summer climate,the radiant floor containing an upper cold storage layer and a lower heat storage layer gives the room a thermal comfort duration of 8.1 h,which is 1.7 times that of the reference room.Based on the peak-valley electricity price,it is found that appropriately increasing the thermal conductivity of the composite PCMs can improve the energy storage efficiency of the radiant floor.Furthermore,the double-layer phase change radiant floor can save power costs by shifting the peak load.At last,the thermal characteristics of a large-scale test room integrating a phase-change envelope,a phase-change ventilation cavity,and a double-layer phase-change radiant floor is numerically studied.Based on the PMV-PPD human thermal comfort evaluation and the annual energy consumption,the indoor temperature regulation and energy-saving effects of the passive-active coupling energy-saving system are investigated by adjusting the thicknesses of the phase change layers,the thermal conductivities of the PCMs,and the location of the heat source in the radiant floor.The simulation results show that,compared with the reference room only installed with radiant floor using sand and gravel as the energy storage material,the phase change room not only has lower annual energy consumption,but also has a high indoor level II thermal comfort rate of 79.87%,with an increase of 16.58%.Moreover,appropriately increasing the thickness of the phase change layers or increasing the thermal conductivity of the PCMs can further optimize the regulation effect of the radiant floor on the indoor thermal environment.In addition,moving up or down the heat source is not conducive to maximizing the utilization of PCM in the radiant floor.Compared with the reference room,the passive-active coupling energy-saving phase change system can save 9.77%of the energy consumption of the PCM room while maintaining a similar indoor thermal comfort rate.