A Study Of Thermal Performance And Heat Recovery Efficiency For Exhaust Air Insulation Wall

Enhancing the thermal insulation performance of the wall and heat recovery from exhaust air are two key ways to reduce building energy efficiency. With exhaust air leakage through the wall, the exhaust air insulation wall is improving the thermal insulation performance of the wall, also heat recovery from exhaust air, then achieving the purpose of energy conservation. A novel building envelope, which is called exhaust air insulation wall, was proposed. And the thermal performance of the wall has been analyzed.Firstly, by the frequency domain finite difference method, the numerical heat transfer model has been carried out, then with the mode, the frequency domain performance of the wall has been analyzed, and the two issues include the velocity of exhaust air and the wall thickness, which affect the frequency domain performance, have been discussed. The results prove that the dynamic thermal performance of the wall has been improved with exhaust air leakage through the wall. Compared to the inner thermal insulation wall, the exhaust air insulation wall won a better dynamic thermal performance under the same wall thickness. Even the thickness of the wall is less than the inner thermal insulation wall, the dynamic thermal performance of the exhaust air insulation wall is still better.Secondly, in the practical engineering, the source to drive the exhaust air leakage through the wall comes from the positive pressure in the room. Combined with Darcy formula, it was found that increasing the thickness of the porous material reduced the exhaust air velocity when the pressure difference is certain. Moreover, the thickness of the porous material and the exhaust air velocity are two key issues which affect the thermal performance of the wall. Based on one-dimensional mathematical model, the formula of heat flux density is derived. With the formula, the variation law of thermal insulation performance of the wall was found under the condition of constant pressure difference, the results indicate that there is a unique critical thickness of porous materials, and under the thickness, the thermal insulation performance of the exhaust air heat insulation wall is the worst. Then, the issues which affect the critical thickness of porous materials have been analyzed.Moreover, in order to analyze the heat recovery efficiency, the heat recovery efficiency of the exhaust air heat insulation wall has been defined. An office room has been selected for analysis, the results indicate that the exhaust air heat insulation wall won a better heat recovery efficiency than the conventional exhaust heat recovery device. Especially, in summer conditions, the solar radiation is relatively large, the exhaust air heat insulation wall has a great advantage in terms of sensible heat efficiency. However, the wall is not as good as the conventional exhaust heat recovery device in terms of the total heat recovery efficiency. Then, the issues which affect the heat recovery efficiency of the wall have been analyzed.Finally, Based on the structure of the exhaust heat insulation wall, a novel structure, which is called bidirectional ventilation wall, was proposed. This structure combines with dynamic heat insulation technology. The numerical heat transfer model of the bidirectional ventilation wall have been carried out, the energy saving efficiency and Heat recovery efficiency of the wall was analyzed. The results indicate that the bidirectional ventilation wall provide better thermal performance, compared to the inner thermal insulation wall, the bidirectional ventilation wall reduces energy consumption by 36.7%. And compared to the typical exhaust air heat insulation wall, the bidirectional ventilation wall reduces energy consumption by 18.3%.Based on the above study, the exhaust air heat insulation wall provide a great thermal performance, and through improving and optimizing the structure of the wall, this advantage is more obvious.