| Night ventilation is an efficient solution to improve the indoor thermal comfort and save the energy consumption of air conditioning in buildings.However,due to the intermittent of natural cold source and the less thermal capacity of structure elements,the cooling effect is not enough.Recently,more and more attention has been focused on the latent heat thermal energy storage(LHTES)technologies using phase change materials(PCMs),which have the advantages of high storage capacity and nearly isothermal nature of the storage process.In this perspective,PCMs can be integrated into mechanical ventilation systems to store outdoor cold at night and release it for indoor cooling during the daytime,which is regarded as a solution to improve the efficiency of free cooling.However,more fan electricity energy will be consumed by the ventilation system using PCMs over a conventional ventilation system without PCMs.Literature review shows that previous works were mainly focused on the thermal comfort enhancement in the buildings without active air conditioning,but the energy saving evaluation and optimization for the coupled operation of air conditioner(AC)and the ventilation system using PCMs have seldom been carried out.Moreover,the researches on the cooling energy contributions of PCMs and structure elements have little been reported.Also PCMs can be integrated into air conditioning systems to narrow the gap between the peak and off-peak loads of electricity demand.The existing air conditioning systems using PCMs,however,did not take advantage of natural cold source,which can further improve energy saving effects.This paper presents the performance analysis for the coupled operation of AC and the ventilation system using PCMs from two aspects respectively,and investigates the effects of outdoor environment,indoor environment and the design parameters of the ventilation system.One of the two aspects is using the ventilation system using PCMs to cool the outdoor air,and the other is to cool the recirculating air.Then,this paper proposes and experimentally studies a novel air conditioning system using PCMs which has double cold charging sources and six operating modes.Firstly,a computational heat transfer model of LHTES unit is developed based on the enthalpy method,and validated using experimental data.A good agreement between the predicted and experimental results can be found.The mean relative errors of outlet air temperature for the melting and freezing processes are found to be 1.8%and 3.1%,respectively.Then,the energy saving potential of the ventilation system using PCMs against a conventional ventilation system without PCMs used for fresh air supply for occupants in air conditioned office buildings during summer is quantitatively evaluated.The effects of climatic condition,thermophysical properties of PCM,along with construction and operation parameters of the system are discussed.In general,the higher the diurnal temperature range or the cooling demand of fresh air,the more the potential cooling energy supply and net electricity saving can be obtained by using the ventilation system with PCMs.However,the higher the diurnal temperature range or the lower the cooling demand of fresh air,the higher the cooling energy supply ratio and electricity saving ratio.With increasing the melting temperature of PCM,PCM slab thickness and charging air flow rate,the net electricity saving increases at first and then it decreases.Then,a genetic algorithm is applied to optimize the above three design parameters for maximizing the net electricity saving.Moreover,sensitivity analysis of net electricity saving to the three variables is carried out.It is found that both of the saving-optimal solution and the sensitivity of each variable are not unique and depend on the climate.PCM melting temperature and PCM slab thickness are the most sensitive variables,but the effect of cold charging air flow is not very sensitive and obvious.In general,the higher the cooling load of fresh air,the larger the impact of PCM melting temperature,but the smaller the effect of PCM slab thickness.Secondly,a dynamic building environment simulation model is developed based on fundamental heat balance principles.The present predicted indoor temperature and the results from the building simulation tool DeST-c are in good accordant with each other,with the maximum and mean relative errors of 3.9%and 1.4%,respectively.Then,a computational model for the coupled operation of AC and the ventilation system using PCMs is built to calculate the indoor temperature and energy consumption of buildings.The model consists of the building environment simulation model,the heat transfer model of LHTES unit and the performance curve of AC,Simulations are carried out to analyze the space cooling and energy saving effects of the ventilation system using PCMs in a typical air conditioned office room during summer.The indoor temperature,melted fraction of PCMs,energy provided by AC and PCMs for space cooling,the number of starts of AC,along with the runtime and electricity energy consumptions of AC and fan for the room with the ventilation system using PCMs are calculated and compared to those for a base case without night ventilation(NV)and a case with conventional NV system.The impacts of PCM melting temperature,air change per hour(ACH)at night,indoor temperature set point and climatic condition are discussed.Furthermore,to better understand the dynamic thermal behaviors of the room and the systems,a representative day is chosen to analyze.In general,the higher the melting temperature,the earlier the rapid decrease of indoor temperature at night.With the increase of ACH at night,the overall energy provided for space cooling decreases,but the overall electricity energy consumption decreases at first and then it increases.The higher the indoor temperature set point,the higher the cooling energy contributions of PCMs and structure elements,and therefore the more the potential energy saving can be obtained by using the ventilation system with PCMs.In Beijing,with changing the indoor temperature set point from 24 0C to 28 0C,the electricity saving ratio by using the ventilation system with PCMs over the base case without night ventilation increases from 16.9%to 50.8%,while that against the conventional NV system increases from 9.2%to 33.6%.The ranking orders of time lag to switch on AC,reduction ratio of the number of starts of AC,and electricity saving ratio in six typical cities are the same.The order from highest to lowest is Harbin,Shenyang,Beijing,Zhengzhou,Shanghai and Changsha.In addition,numerical simulations are carried out to investigate the space cooling and energy saving effects of the ventilation system using multiple PCMs in a typical air conditioned office room during summer for the climate of Harbin.The melted fraction of PCMs,energy provided by AC and PCMs for space cooling,and overall electricity energy consumptions are calculated and compared with those of the system employing a single PCM.The effects of multiple PCM's melting temperatures and fractions,and the charging air flow rate are analyzed.The results show that the system using multiple PCMs appears to be more energy-efficient than the system employing a single PCM only when the charging air flow rate is low.The electricity saving ratio is found to be about 5%.Finally,this paper proposes a novel air conditioning system using PCMs which has double cold charging sources and six operating modes.A prototype system was developed and tested in an enthalpy difference laboratory to investigate the system performance and operation characteristics.Experimental results demonstrated that the proposed air conditioning system can stably work.Under the outdoor temperature of 15?,cold charging from outdoor air appears to be more energy-efficient than that from vapour compression refrigeration when the stored cold is less than 5.8 MJ,but it takes more time to charge the cold. |
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