Thermal performance of non air-conditioned buildings with vaulted roofs in comparison with flat roofs

Vaulted or domed roofs have been frequently adopted by builders and architects throughout the Middle East and other hot dry areas. However, the thermal performance of such buildings under hot dry climatic conditions has rarely been quantitatively studied. In this paper, a detailed finite element model for the investigation of the thermal performance of non air-conditioned buildings with vaulted roofs (VR) is suggested based on two-dimensional unsteady heat transfer in such roofs and solar geometry. This model allows a comparison of the thermal performance of non air-conditioned buildings with a VR and a flat roof (FR) under different climatic conditions. Results obtained by numerical calculation show that, irrespective of building type the VRs are applied to, buildings with a VR have lower indoor temperatures as compared to those with a FR. The reason is that such roofs dissipate more heat than a FR does by convection and thermal radiation at night due to the enlarged curved surfaces. This implies that such roof forms are suitable for buildings located in hot dry regions but not for those located in hot humid areas, and reasonably explains why curved roofs have been extensively adopted by builders and architects in the hot dry areas in the past. However, with the decrease in the half rim angle of a VR, the difference of indoor thermal condition between a VR and a FR building becomes small and insignificant. Results also indicate that the indoor air temperature is slightly influenced by the half rim angle _θ0${\theta }_0$ and the orientation _φv${\phi }_v$ of the VR. To be effective to create a favorable thermal condition inside buildings with a VR under hot dry climatic conditions, the half rim angle of a VR should be θ₀>50°, instead of _θ0<50^°${\theta }_0<50^{°}$, which is the optimal half rim angle of a VR of air-conditioned buildings, as found by the present authors in a previous study.     

植被屋面隔热性能对比测试分析

在夏季自然气候条件下对植被屋面和无植被对比屋的面隔热性能进行了测试,通过分析测试数据,获得了屋面内表面温度分布特征、温度波的衰减性、屋面内表面与室外空气温度的关联性以及植被屋面各构造层的衰减性等相关隔热性能参数,并对其进行了对比评价分析,为其在实际工程应用提供了实验依据。     

某绿色建筑屋顶隔热构造及热工性能分析

结合《绿色建筑评价标准》中对屋顶隔热节能的基本要求,对某公司综合楼的屋顶隔热构造模式进行了详细的分析和研究,并系统计算了不同构造模式的热工性能参数,提出了绿色建筑屋顶隔热的基本应用模式,以促进绿色建筑发展。     

Biophysical properties and thermal performance of an intensive green roof

Green roofs have been increasingly enlisted to alleviate urban environmental problems associated with urban heat island effect and stormwater quantity and quality. Most studies focus on extensive green roofs, with inadequate assessment of the complex intensive type, subtropical region, and thermal insulation effect. This study examines the physical properties, biological processes, and thermal insulation performance of an intensive green roof through four seasons. An experimental woodland installed on a Hong Kong building rooftop was equipped with environmental sensors to monitor microclimatic and soil parameters. The excellent thermal performance of the intensive green roof is verified. Even though our site has a 100 cm thick soil to support tree growth, we found that a thin soil layer of 10 cm is sufficient to reduce heat penetration into building. Seasonal weather variations notably control transpiration and associated cooling effect. The tree canopy reduces solar radiation reaching the soil surface, but the trapped air increases air temperature near the soil surface. The substrate operates an effective heat sink to dampen temperature fluctuations. In winter, the subtropical green roof triggers notable heat loss from the substrate into the ambient air, and draws heat upwards from warmer indoor air to increase energy consumption to warm indoor air. This finding deviates from temperate latitude studies. The results offer hints to optimize the design and thermal performance of intensive green roofs.     

Build-up and performance test of a novel solar thermal roof for heat pump operation

Global increase in energy demand and fossil fuel prices loaded ever-increasing pressure on identifying and implementing new means to utilise clean and efficient energy resources. Due to the environmental benefits, technical and economic possibilities of Solar-Assisted Heat Pump Systems, there has been a growing interest for such hybrid systems with a variety of system configurations for various climates. International Energy Agency Task 44 of the Solar Heating and Cooling Programme has recently started working on finding methods to most effectively use solar heat pump systems for residential use. In the present study, a novel solar thermal roof collector was developed by primarily exploiting components and techniques widely available on the market and coupled with a commercial heat pump unit. The proposed indirect series Solar-assisted Heat Pump system was experimentally tested and system performance was investigated. Yet, the analysis based on indoor and outdoor testing predominantly focuses on the solar thermal roof collector. A detailed thermal model was developed to describe the system operation. Also, a computer model was set up by using Engineering Equation Solver to carry out the numerical computations of the governing equations. Analyses show that the difference in water temperature could reach up to 18°C while maximum thermal efficiency found to be 26%. Data processing of the series covering the test period represents that Coefficient Performance of the heat pump (COP_HP) and overall system (COP_SYS) averages were attained as COP_HP?=?3.01 and COP_SYS?=?2.29, respectively. An economic analysis points a minimum payback period of about three years for the system.     

Modeling of energy performance of a house with three configurations of building-integrated photovoltaic/thermal systems

A theoretical and experimental study of energy performance of three different open loop air heating building-integrated photovoltaic/thermal (BIPV/T) systems that utilize recovered heat for home heating is presented. The configurations are: Configuration 1: base case of unglazed BIPV with airflow under it; Configuration 2: addition of 1.5 m vertical glazed solar air collector in series with Configuration 1; Configuration 3: addition of a glazing over the PV. The model developed has been verified against experimental data from a solar research house for Configuration 1. Obtained relationships for BIPV/T system exiting air temperature as function of solar irradiance and air speed in PV cavity may be used for developing fan airflow control strategies to achieve desired outlet air temperature suitable for different applications. For Configuration 1, preheated air is suitable for HVAC system and domestic hot water (DHW) preheating. Higher outlet air temperatures of the PV cavity suitable for DHW might be achieved by utilizing Configurations 2 or 3. With Configuration 2, significant outlet air temperatures are achieved in winter along with enhanced thermal efficiency making it suitable for coupling with a rockbed heat storage. Finally, Configuration 3 significantly reduces electricity production and may lead to excessively high PV panel temperatures.     

The effect of roof systems on seismic performance of reinforced concrete frame‐bent main building of large‐scale thermal power plant

The roof systems of turbine hall in large‐scale thermal power plant are often simulated by approximate rigid beam model. Wenchuan earthquake again revealed the frequent destruction of these roof systems. In this study, three kinds of general roof systems as space grid, trapezoid steel roof truss and reinforced concrete (RC) mansard roof truss were considered. Also, the numerical model of RC frame‐bent main building in large‐scale thermal power plant was established. Seismic performance of main building was compared with approximate rigid beam model, and the damage mechanism of roof systems was also investigated. The results indicate that the response calculation of frame part by rigid beam model is feasible, and roof systems give most contribution to the response of bent part. After considering roof systems, overall collaborative performance is enhanced, but bent part is still unable to become a second defence part after shear walls fail. The torsional irregularity of roof systems is very serious, and more attentions should be paid to the bearing shear failure of space grid and components strength failure of RC roof systems. Copyright © 2014 John Wiley & Sons, Ltd.     

A heat pipe photovoltaic/thermal (PV/T) hybrid system and its performance evaluation

Building-integrated photovoltaic/thermal (BIPV/T) system has been considered as an attractive technology for building integration. The main part of a BIPV/T system is PV/T collector. In order to solve the non-uniform cooling of solar PV cells and control the operating temperature of solar PV cells conveniently, a heat pipe photovoltaic/thermal (PV/T) hybrid system (collector) has been proposed and described by selecting a wick heat pipe to absorb isothermally the excessive heat from solar PV cells. A theoretical model in terms of heat transfer process analysis in PV module panel and introducing the effectiveness-number of transfer unit (?-NTU) method in heat exchanger design was developed to predict the overall thermal-electrical conversion performances of the heat pipe PV/T system. A detailed parametric investigation by varying relevant parameters, i.e., inlet water temperature, water mass flow rate, packing factor of solar cell and heat loss coefficient has been carried out on the basis of the first and second laws of thermodynamics. Results show that the overall thermal, electrical and exergy efficiencies of the heat pipe PV/T hybrid system corresponding to 63.65%, 8.45% and 10.26%, respectively can be achieved under the operating conditions presented in this paper. The varying range of operating temperature for solar cell on the absorber plate is less than 2.5 °C. The heat pipe PV/T hybrid system is viable and exhibits the potential and competitiveness over the other conventional BIPV/T systems.