| Issues such as energy tensions and climate change are the key topics of global concern currently.Energy conservation and emission reduction are highly valued by governments and people from all circles of society.With the continuous rapid development of China’s national economy,people’s requirements for indoor thermal comfort are increasing day by day.At the same time,China’s urbanization construction continues to advance,and the proportion of China’s building energy consumption in total energy consumption has accelerated.Building energy conservation has become one of the key tasks of governments at all levels in China.This paper proposes a new building envelope,named as pipe-embedded ventilation roof with outer-layer shaped-stabilized phase change material(referred to as phase change ventilation roof).The shaped-stabilized phase change materials are laid on the outer surface of the concrete hollow slab.During the daytime,the latent heat storage ability of phase change material can be used to absorb the solar radiant,while the outdoor low-temperature air will be introduced through the cavity of the hollow slab at night to ventilate,for accelerating the heat release rate of the phase change material and bringing heat out of the room,thereby reducing the heat transfer from outside to the room through the roof and decreasing the air condition cooling load.This paper takes the roof structure as the research object.Firstly,it ignores the temperature variation along the airflow direction of the cavity and uses the thermal resistance and heat capacity network method(R-C method)to establish a simplified dynamic heat network model(RC model)of the phase change ventilation roof.Based on the early research results of the research group on the hollow slab RC model,the relevant thermal resistance and heat capacity parameters of the RC model of the phase change layer is determined by using the simulation results of the CFD model,and the accuracy and applicability of the RC model identification results are analyzed.Then considering the temperature change along the airflow direction of the cavity,the model of the heat exchange between the cavity airflow and the roof structure is established by the heat transfer unit number method(NTU method),and is coupled with the RC model to obtain the dynamic heat transfer model of the roof,which can be used to calculate the temperature of the vent outlet,the heat removed by the airflow in the cavity and the heat transfer through the roof.The dynamic heat transfer model of the phase change ventilation roof is embedded into the energy consumption simulation software TRNSYS,and the building model with phase change ventilation roof is established.And then the cooling load of the building during the air conditioning period is simulated,and the effects of the phase change temperature,the thickness of the phase change layer and the ventilation speed on the cooling load of the building during the air conditioning period are analyzed.Based on the accumulated cooling load during air conditioning period,t,the optimal design parameters of the phase change ventilation roof are derived: the thickness of the phase change layer is 30～50mm,of which the optimal phase transition temperature corresponding to 30～40mm is 36～38℃,and the optimal phase transition temperature corresponding to 50 mm is 35～37℃,and the ventilation speed range is 2.5m/s ～ 3.5m/s.The energy-saving effects of phase change materials and night ventilation technology on building air conditioning cooling load are analyzed.The accumulated cooling load of the building model with phase change ventilation roof is reduced by 19.2% compared with the building model with reference roof under non-ventilated condition;when the night ventilation speed is 3.0m/s,the accumulated cooling load of building is reduced by 22.9% compared with non-ventilated condition.Taking the phase change ventilation roof with the optimized structure as the research object,the summer thermal characteristics of the roof structure are studied.On a typical meteorological day in Wuhan,the simulation results of the phase change ventilation roof and the reference roof when the cavity is not ventilated are compared.The results show that the application of phase change materials significantly enhances the thermal performance of the roof.Compared with the reference roof,the time lag of the phase change ventilation roof is delayed by 4.5 hours,the decrement factor increases by 228.7%,the equivalent thermal resistance and the heat storage coefficient of the outer surface increase by 36.7% and 22.55%,respectively.The heat transferred into the room through the roof is reduced by 26.9%,and furtherly reduced by 7.3% after adopting night ventilation strategy.The load characteristics and influencing factors of the phase change ventilation roof in other climatic regions are simulated and analyzed.The optimum structure of the roof in hot summer and warm winter zone,cold zone and severe cold zone are obtained as follows: the suitable thickness of the phase change layer are 40～50mm,30～40mm and 30～40mm for three zones,respectively,the best phase transition temperature are 34～36℃,34～36℃ and 32～34℃,respectively,and the best ventilation speed all are 3～4m/s,corresponding to the building’s accumulated cooling load in air conditioning period are reduced by 684.87 k W·h,470.77 k W·h and 158.39 k W·h compared with that of the reference roof building respectively,and the number in Wuhan is 505.57 k W.Therefore,the cooling load saving effects of phase change ventilation roof in hot summer and warm winter zone and hot summer and cold winter zone are better than that in the other two climate zones. |
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