Performance study of an isolated small scale Trombe wall with partially evacuated air gap

Abstract

The Trombe wall is a passive solar heating system that aims to mitigate heating load. However, its efficiency is impeded by significant heat loss through the glazing. To address this issue, a novel technique has been developed that involves partial evacuation of the space between the storage wall and the glazing. This approach, which has already found successful application in double-paned windows and solar collectors, is examined in this study to evaluate its impact on the performance of an isolated Trombe wall. An effectiveness criterion based on the ability of a system to preserve its stored thermal energy is defined and used for assessing the performance of a vacuumed system relative to a non-vacuumed one. An experimental test cell, composed of a solid concrete wall serving as a thermal storage and a glazing separated from the wall by an air gap, facing south is constructed in Kirkuk, Iraq. The wall is insulated at all its sides except the side facing the glass. The gap has been sealed meticulously to maintain a vacuum. Due to unrepeatability of the testing conditions experimentally, a numerical method and a computer code is also developed for simulation of the system. Measurement results obtained from the test cell under normal atmospheric pressure and a gage pressure of −0.3 bar of the air gap are used to validate the numerical method. The code is then used to simulate the performance of the system under the same ambient conditions but at different vacuum pressures of the air gap. Results obtained from the numerical tests show that partial evacuation of the air gap can be an efficient way to enhance the performance of the Trombe wall and the absolute pressure of 0.1 bar results in the most significant increase in the effectiveness of the studied model.