Investigation On The Effective Utilization Of Solar Energy By Non-transparent Envelope And Double Skin Facade

The contradiction between the comfort demand of the built environment and the energy consumption of the building is becoming increasingly intensified with economic development.How to reduce building energy consumption is an urgent problem to be solved while improving the comfort of the building environment.Solar energy is a feasible energy source for reducing building energy consumption because solar energy is inexhaustible and clean energy source in nature.Solar energy enters into a room through the opaque walls and the windows.Although the solar absorptance of the walls is lower than that of the windows,the wall area is usually at least 1.2 times larger than the window area because the window-wall ratio of the south wall is less than 0.45 by the requirement of the Chinese design standard,meaning that solar energy entering rooms through the opaque walls（i.e.,SER） can effectively reduce building heating energy consumption and it should be given as much attention as the solar energy entering rooms through the windows.The previous studies were performed under the combined influences of various factors such as solar radiation,outdoor air temperature,and the characteristics of the envelope structure.However,the SEW has not been analyzed clearly so far.Many previous studies have analyzed the solar energy entering the room through windows,but the above researches mainly focuse on the energy saving potential and the impact of thermal comfort.The air in the cavity flows from bottom to top under thermal pressure in the Double-skin fa?ade（DSF）.Photocatalytic oxidation（PCO） on the irradiated surfaces of semiconductor photocatalysts is regarded as a promising method for VOCs decomposition.This study presents an innovative design of an external building fa?ade system for the decomposition of the ambient air mixed with BTEX（air-BTEX）by PCO techniques,which can provide reaction surfaces and precise control of airflow velocity （i.e.,reaction time）.The hot summer and cold winter（HSCW）zone is one of the most economically developed regions in China.Traditionally,residential buildings in this region were not equipped with central heating systems.However,room heating in winter has been popularizing with the development of the economy and the improvement of the living standard in the HSCW zone,and building heating energy consumption is increased dramatically.Therefore,this study first conducts an in-depth study and analysis of the solar energy absorbed by the wall under the background of winter heating in hot summer and cold winter areas.The reduced heat loss of a solid wall caused by solar radiation is the solar energy actually absorbed by the solid wall（SEW）.The absorbed solar energy calculated by using the radiation absorption coefficient of the wall surface is thus not the SEW.This study analyzes the parameters that may affect the SEW,including the heat accumulation coefficient of the wall（S）,the heat-transfer coefficient of the south wall’s outer surface（h）,the radiation intensity（I）,the consecutive sunny days（D）,and the outdoor air temperature（T）.The results show that the number of consecutive sunshine days has little effect on the outer surface temperature of the south wall and the net heat flux,but the number of consecutive sunshine days has a significant effect on the actual solar energy absorbed by the wall（SER）.The SER decreases with increasing number of successive sunny days.The values will reach a steady state by the fourth day.The structure and material of the wall have a greater impact on SER.Vast amounts of heat from the wall with internal insulation are released directly into the atmosphere in the nighttime,which results in that the solar absorptance of a south-facing room through the opaque wall is less than 10%.For the wall with external insulation,the changes of SER are similar as the wall with internal insulation,solar absorptance of a south-facing room（ewall）for the wall with external insulation is less than that for the wall with internal insulation due to the effect of the thermal insulation layer.Solar energy absorbed by reinforced concrete wall is more than that by brick wall and self-insulating wall,especially on the first day of continuous sunlight.The difference reach to maximum value.On the first day of continuous sunlight,the ewall of the reinforced concrete wall is about 8%,which is twice the ewall of the self-insulating wall.The actual solar energy absorbed by the wall and the actual solar energy absorption（dissipation）rate of the wall have no changes for different outdoor temperatures,which indicates that the influence of outdoor temperature on the actual solar energy absorbed by the wall can be ignored.The SEW is different under different solar radiation intensities,but the ewall is basically the same.Therefore,the influence of solar radiation intensity on the ewall of the wall can be ignored.For the wall with internal or external thermal insulation,the ewall decreases with the increase of the convective heat transfer coefficient.It is difficult for solar energy to enter the wall because the outer side of the wall with external thermal insulation is constructed using thermal insulation material with large thermal resistance.Moreover,the ewall of the wall with external thermal insulation wall is hardly affected by the number of consecutive sunshine days,but for the wall with internal thermal insulation,the actual solar energy absorption rate in the first sunny day is about 75% higher than that in the stable sunny day.The absorption of solar energy of building walls can effectively reduce the heating energy consumption.However,it is inaccurate to directly calculated the SEW using the radiation absorption coefficient of the wall surface,because the SEW is primarily the reduced heat loss of a wall itself caused by solar radiation.A quantitative method of calculating the SEW has been proposed in the present study based on the CFD method.The numerical results indicate that ewall is significantly affected by three parameters,i.e.,S,h,and D.ewall is high when S is large or h is small or D is small.However,the radiation intensity（I）and the outdoor air temperature（T）have little impact on ewall.A prediction model between ewall and three influential factors（i.e.,S,h,D）is obtained by employing the multiple nonlinear regression analysis.The prediction model proposed in the present study can accurately calculate ewall for fast analysis of the beneficial effects of solar energy through solid walls on building energy consumption,and greatly reduce the computational costs.Moreover,the prediction model may also have a practical application of quickly and accurately determining the impact of wind speed and direction in winter on ewall for optimizing building layout.The ultimate purpose for analyzing the solar absorptance of a south-facing room（ewall）is to accurately estimate the effect of the SER on reducing building heating energy consumption and make efficient use of solar energy.The results show that the room solar absorptance of south facing room through the opaque wall can reach to 17 % in a day when the heat-transfer coefficient of the south wall outer surface is small（under calm wind or with high building density）.The reduction of the building heat loss index under SER,（35） qsun,accounts for 20% to 80% of the total（qsun） for the wall with internal thermal insulation and 10% to 45 % of qsun for the wall with external thermal insulation.The results also show that the SER is strongly affected by the heat-transfer coefficient of the wall’s outer surface,with the lower the coefficient having a more obvious the influence.The room solar absorptance（eroom）can nearly be doubled when the heat-transfer coefficient drops from 12 W/（m²·K）to 5 W/（m²·K）.Furthermore,it is of great significance to reduce building energy consumption by decreasing the ambient wind speed near the south wall in winter.This study proposes a novel double-skin fa?ade integrated with TiO₂ plates（T-DSF）for decomposing ambient air Benzene,Toluene,Ethylbenzene,and O-xylenes（BTEX） by utilizing the photocatalytic oxidation technique under natural sunlight.Volatile organic compound（VOC） emissions mostly come from consumer products,gasoline and vehicular traffic;and VOC pollution is becoming serious with the increasing number of fuel vehicles and the operation of heavy industries in cities.BTEX are volatile organic compounds（VOCs） of main concern.BTEX is extremely harmful to human health.This study presents an innovative design of an external building fa?ade system for the decomposition of the ambient air mixed with BTEX（air-BTEX） by PCO techniques,which can provide reaction surfaces and precise control of airflow velocity（i.e.,reaction time）.It is worth noting that the amount of BTEX processed over a period of time is related to the residence time on the TiO₂ surfaces and the decomposition efficiency,while the efficiency of decomposing BTEX is largely affected by the residence time around the TiO₂ surfaces.Therefore,to reach a maximum BTEX processing amount requires an optimal design of T-DSF with proper balance between the age of air and the decomposition efficiency.This paper presents the principles of the designed system and further demonstrates the photocatalytic decomposition performance of T-DSF by numerically evaluating the influences of the location of interior TiO₂ plates and the gap size between the TiO₂ plates and the glass shells.The mathematical relationship between the decomposition efficiency and the age of air was obtained according to the experimental data.Eighteen cases,i.e.,3 placement strategies of TiO₂ plates by 6 airflow gap sizes,were evaluated using a validated computational fluid dynamics model to design an optimal T-DSF.It is observed that the annual averaged volume of air with 20 ppb BTEX processed per day by the T-DSF,where the TiO₂ plates are staggered on the external and internal glasses with a 0.02 m gap size,is about 77 m³.The sensitivity analysis of solar radiation intensity indicates that this system has a stable performance for decomposing BTEX under different solar radiation intensity.Moreover,three kinds of particles with particle diameters of 2.5μm（small）,10μm（medium） and 20μm （large）were selected to analyze the effect of particulate matter from ambient air.It is found that particles will not affect normal photocatalytic oxidation progress because particles carried by the flow field are discharged directly from the system and do not to deposit on the TiO₂ plates.This study presents a promising and passive design that can effectively treat BTEX in the atmosphere by taking advantage of UV radiation in natural sunlight and solar-driven natural convection.