| Free cooling is a passive cooling technology that uses natural sources of cold to improve indoor thermal comfort and reduce air conditioning energy consumption.However,due to the intermittent and unstable nature of the natural cold source and the limited cold storage capacity of the building envelope,the cooling effect of the building’s natural cooling technology is constrained.Therefore,in order to extend the natural cold source utilization time and increase its utilization rate,thermal energy storage technology is particularly important.As is seen to all that the thermal energy storage technology based on phase change material named latent heat storage has an unparalleled advantage over sensible-based thermal energy storage technology.Latent heat storage is distinguished from typical traditional heat storage materials with the high latent heat characteristics of the phase change material itself.Phase change material(PCM)has much higher energy storage potential because of its heat absorption and release capability during the phase transition process,which can store 5 to 14 times the heat per unit volume in comparison to ordinary materials.This means a much smaller volume of the material is needed to store the same amount of energy.Therefore,the latent heat storage systems accomplished by phase change material(PCM)has drawn much attention and been applied extensively in recent years.Another important feature for PCM is that the material temperature remains almost constant during the phase transition period.Nowadays,the most prevalent PCM applications in buildings is the integration of PCM into the construction elements of the building,such as walls,ceilings,wallboards,window glass,floor and solar chimney.Therefore,the latent heat storage is a unique choice to improve energy efficiency and building thermal comfort,which attracts more attention of many research scholars.Firstly,this paper proposes the use of cylindrical annuluses composed of phase change material(PCM),called as PCM-air heat exchangers(PAHE),for regulating supplied air temperatures for ventilated buildings.In actual climate conditions with fluctuating outdoor air temperatures,the force convection inside the PAHE tube enhances the heat transfer between the air and PCM,and thus both the sensible and latent heat transfer result in the attenuation of air temperature fluctuation amplitude and the shaving of peak cooling/heating loads.Both experiments and numerical simulations were performed to investigate the performance of PAHE in ambient environments with fluctuating temperatures.In the numerical simulations,the effective heat capacity method was adopted for simulating the effects of latent heat transfer of PCM.The numerical results of air temperature agree well with the measured ones.Then the explicit expression of the air temperature inside the tube of the PCM-air heat exchanger(PAHE)and the temperature of the inner wall surface of the tube is obtained,and the theoretical results are compared with the results of the simulation to verify the reliability of the theoretical derivation.Secondly,the effects of three main influencing factors of the PAHE,volumetric flow rate through the PAHE tube,melting temperature of PCM and external diameter of PAHE,on the performance of the EAHE,were numerically investigated.Maintenance of the stability of inner surface temperature of PAHE is crucial for maximizing the capability for reducing air temperature fluctuation amplitude.To achieve this purpose,the above three parameters should be appropriately adjusted.Finally,the PAHE cooling performance in a hot-summer and cold-winter region is evaluated.The simulation incorporated real meteorological data of both transitional seasons and summer(i.e.,from May to October)were carried out.The results indicate the applicability of PAHE in such climates.Finally,the energy storage heat transfer performance of PCM-air heat exchangers(EAHE)and earth-air heat exchangers at different depths is compared and analyzed.The feasibility of replacing EAHA with PCM-air heat exchanger from the perspective of cost and heat transfer performance is studied. |
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