Yanru Li

Solar radiation reflective coating material on building envelopes: Heat transfer analysis and cooling energy saving

Heat transfer through building envelopes constitutes the dominant part of indoor cooling load in summer. Coating building external walls with high reflectivity materials proves to be an effective way to reduce heat gains from solar radiation and save cooling energy consumption accordingly. In this paper, the transient heat transfer model of building external envelopes is established and validated through experiment, to investigate the thermal performance of building walls coated with retro-reflective materials. Moreover, taking an office building in Chengdu as an illustrative example, the cooling energy saving potential of such retro-reflective material coated building is evaluated in summer. The experiment results show that for the building box with retro-reflective coating materials (ru2009=u20090.59), the average indoor air temperature is about 2.4℃ lower than the reference box without coating materials, resulted from decreasing heat absorption of solar radiation for external walls. Furthermore, the illustrative example in Chengdu shows the cooling load can be reduced by about 9.1u2009W/m2, with such retro-reflective coating materials for building external walls, saving 15.2% electricity consumption in a whole summer. The incremental investment for coating can be paid back by 9.1 years for the studied case. Moreover, economic analysis and comparison indicate that such coating material is more applicable to southern cities in China, since the payback period is shorter due to more cooling energy saving for those with hot summer. This work can provide guidance for practical building envelope thermal design.

Research on thermal performance improvement of lightweight buildings by integrating with phase change material under different climate conditions

The application of a lightweight building is limited because its indoor temperature is highly affected by the outdoor environment due to the low thermal inertia of the lightweight envelope. In order to improve the thermal performance, this study focuses on lightweight buildings integrated with phase change materials. EnergyPlus software with building model validated by experimental data is used to analyze the thermal performance improvement of lightweight buildings by integrating with phase change materials under typical weather conditions in five climate zones in China. The results show that phase change materials can reduce temperature fluctuations through improving the heat stability of the lightweight envelope. Phase change materials can effectively improve the indoor thermal environment throughout the whole year in a temperate zone. Also, phase change materials can improve the indoor thermal environment of lightweight buildings significantly in the transition seasons, but it also reduces the hours of the indoor air temperature within the range 18°C (64.4°F) to 26°C (78.8°F) during cold winters and hot summers. For the lightweight buildings with air conditioning, due to the addition phase change material, heating loads can be reduced by 40–70%, while cooling loads increase slightly in the summers with the higher air temperature.

Heat Storage and Release Characteristics of Composite Phase Change Wall under Different Intermittent Heating Conditions

During intermittent heating, the heat storage and release of building envelopes are caused by indoor thermal environment variations. The heat storage and release process of the building envelope inside can be used to reduce heat loss in intermittent cycles and maintain the indoor thermal environment for occupant comfort while saving energy by efficiency in the operation. In this research, the building envelope was integrated with phase change materials, and the heat transfer model was established and verified to analyze the heat storage and release processes of composite phase change wall (composite-PCW) under two typical intermittent heating conditions. Effective heat storage utilization rate η and heat loss rate ε were used to evaluate the wall’s heat storage and release characteristics and energy efficiency, and the composite-PCW was compared with a solid brick wall. The results show that (1) the η of the composite-PCW was 75.07% in the long intermittent condition; (2) the thermal processes were relatively stable in the last three cycles of the short intermittent condition, and the η was greater than 70%; and (3) the ε of the composite-PCW was smaller under the two intermittent conditions compared with the solid brick wall and was reduced by more than 36%.