Experimental And Simulation Study On The Building Envelopes Containing Hydrated Inorganic Salt/Expanded Graphite Composite Phase Change Materials

Buildings will consume a large amount of resources and energy in their whole life cycle.Accelerating building energy conservation is of great significance to the realization of green and low-carbon development goals.The heat gain and heat loss of buildings are closely related to the thermal insulation performance of the building envelope.However,the building materials generally have the shortcoming of small heat capacity,which easily cause the large fluctuation of indoor temperature.By integrating the phase change material?PCM?into the building envelope,the large latent heat can be used to store and release the heat energy,which can make up for the poor heat storage capacity of common building materials,thus improving the indoor comfort and reducing the building energy consumption.Due to the high price and flammability of organic PCMs,integrating them into the building envelope will inevitably increase fire hazards and investment costs.In contrast,those inorganic hydrated salts with low price,non-flammable and phase change temperatures close to the comfortable temperature of the human body are more suitable for the building envelope.This paper focuses on the preparation of new phase change energy storage building materials and the design of phase change energy storage building envelope.The hydrated inorganic salt composite PCMs with stable performance and suitable phase change temperature were prepared and integrated into the building envelope.By means of experimental research and numerical simulation,the building envelope was optimized and its energy saving potential was evaluated.In this paper,CaCl₂·6H₂O/expanded graphite?EG?composite PCM with the small supercooling degree,large latent heat and stable thermal properties was prepared.The PCM panels were made by encapsulating the PCM in the PVC board.The experimental room model was built by combining the wall structure with the PCM panels,and the thermal performance of the room was evaluated.The results showed that the PCM panels could reduce indoor temperature fluctuation and improve room thermal comfort.Moreover,the thermal performance of the room was best when the PCM panels were moved from the outside position to the most inside one.Through numerical simulation,it was found that the effect of CaCl₂·6H₂O/EG on the room thermal performance was better than that of RT27/EG.Compared with the reference room,the energy saving rates of the two PCM rooms were 13.4and 12.7%respectively.Subsequently,a large scale room model was established to simulate and optimize the room containing CaCl₂·6H₂O/EG composite PCMs.The results showed that the thermal performance of the room could be improved more effectively by increasing the thickness of the PCM panel than by further increasing the thickness of the polystyrene foam boards.In addition,the PCM with different phase change temperatures should be selected for different building orientations,and the area with a large temperature difference between day and night was more suitable for the application of PCM.Through investigating the influence of the PCM panels on the thermal performance of different building structures,it was found that the energy saving rate of the concrete wall was the highest,the temperature time lag of brick wall was the largest,and the temperature decrement factor of the structural insulated panel was the smallest.In order to make the phase change energy storage building envelope suitable for both cooling and heating seasons,two kinds of PCMs with different phase change temperatures should be integrated into the building envelope.In this work,CaCl₂·6H₂O was mixed with different mass fractions of Mg?NO₃?₂·6H₂O to prepare a series of CaCl₂·6H₂O-Mg?NO₃?₂·6H₂O eutectic PCMs with different phase change temperatures.It was found that the latent heat and the phase change temperature of eutectic PCMs decreased withtheincreaseofMg?NO₃?₂·6H₂Ocontent.Subsequently,CaCl₂·6H₂O-8wt%Mg?NO₃?₂·6H₂O and CaCl₂·6H₂O-15wt%Mg?NO₃?₂·6H₂O with the suitable latent heat and phase change temperature were selected.They could be applied in cooling seasons and heating seasons respectively.CaCl₂·6H₂O-Mg?NO₃?₂·6H₂O/EG composite PCMs were prepared by adding the nucleating agent SrCl₂·6H₂O into the eutectic PCMs and combining it with EG.Two PCM panels with different phase change temperatures were obtained by using a vacuum bag to encapsulate the composite PCMs.The double-layer PCM panels were fabricated by wrapping two PCM panels with a high-temperature resistant flame retardant aluminum foil fiberglass cloth.And then it was bonded with the aluminum veneer to build the experimental room model.The thermal performance of the room in summer was investigated,and the reliability of the numerical model was verified.The results showed that the double-layer PCM panels could reduce the temperature fluctuation and improve the indoor comfort.In addition,the experimental results were in good agreement with the simulation results.The validated numerical model was used to simulate and optimize the room containing CaCl₂·6H₂O-Mg?NO₃?₂·6H₂O/EG composite PCMs with different phase change temperatures,and the energy saving potential of the room in the cooling and heating seasons was evaluated.The results showed that better energy saving potential could be achieved by placing the PCM indoors,in which the PCM with lower phase change temperature was installed on the south wall,while the PCM with higher phase change temperature was placed on the roof.The suitable PCM for the south wall was CaCl₂·6H₂O-15wt%Mg?NO₃?₂·6H₂O/EG,while the suitable PCM for the roof and the other three walls was CaCl₂·6H₂O-2wt%Mg?NO₃?₂·6H₂O/EG.The increase of the PCM layer thickness could reduce the energy demand of the room,but the suitable PCM of the roof and the south wall would not change with the change of the PCM layer thickness.The type of PCMs integrated in the roof and south wall of the room in Ningxia and Beijing was basically the same.For Shanghai,the optimal PCM integrated in the south wall was CaCl₂·6H₂O-12wt%Mg?NO₃?₂·6H₂O/EG.In order to further improve the utilization efficiency of PCMs,a new type of phase change energy storage building envelope was obtained by adding an air layer on the roof and constructing a dual-channel ventilated Trombe wall on the south wall.The optimal design of the phase change energy storage building envelope was carried out through numerical simulation,and the annual energy consumption of the room under different influence factors was comprehensively investigated.The results showed that the optimal PCMs for the roof and south wall were CaCl₂·6H₂O-2wt% Mg?NO₃?₂·6H₂O/EG and CaCl₂·6H₂O-8wt% Mg?NO₃?₂·6H₂O/EG,respectively.When the air layer of the room was ventilated and the south wall was coated with the solar absorbing coating,the room could save about 30%of energy consumption.Moreover,the energy consumption of the room increased as the thickness of the air layer on the south wall became larger.The air layers played a different role in the building envelope.The optimal value of the flow rate between the air layer 2,the air layer 3 and the room was 0.09 m³/s.To reduce the room energy consumption,the PCMs with large thermal conductivity should be installed on the south wall,while the PCMs with small thermal conductivity should be integrated on the roof and the other three walls.In general,the phase change energy storage building envelope with the ventilated cavity can be used for year-round thermal management of the room,which can reduce the room energy consumption in both cooling and heating seasons.Therefore,it has a certain application prospect in the field of building energy conservation.