Design, build and test a passive thermal system for a loft : a roof solar chimney application for South African weather conditions

[摘要] ENGLISH: The design, construction and testing of a passive thermal system, a roof solarchimney, for a loft is considered. Unlike conventional solar chimneys the solarcollector is constructed from corrugated iron roof sheets with the aim that it can beintegrated into existing buildings at a lower cost or used in low cost housingdevelopments. The main objective of the study was to determine the feasibility ofsuch low-cost design to regulate thermal conditions in a loft, that is heating the loft during winter and enhancing natural ventilation during summer, by carrying out anexperimental and analytical study. The results obtained from the experimental study showed that for winter the solar chimney, having a channel width, depth and length of 0.7 m, 0.1 m and 1.8 m respectively and with a peal solar radiation of 850 W/m², heated the room air 5°C higher than the ambient temperature during the hottest periods of the day, which is only marginally better than a loft with conventional roof insulation. At night, it was found that reverse airflow occurred through the chimney, cooling the loft down toambient temperature, due to radiation heat loss from the roof collector to the nightsky. For summer operation, the experimental data showed that the chimney was ableto maintain the loft at ambient temperature and the analytical study found that thechimney was able to enhance natural ventilation effectively, reaching air exchange rate of 6.6 per hour for the 4.6 m³ volume space. It was also found that the chimney's performance dropped rapidly and significantly during periods of low solar radiation and at night. A sensitivity analysis illustrated that for both summer and winter operation, the size, tilt angle and absorptivity of the roof collector greatly effected the efficiency and mass flow rates of the system, agreeing well with other literature. These results prove that this low cost solar chimney cooling design was feasible to enhance natural ventilation mainly during hot summer conditions with high solar radiation. Compared to a loft with only conventional roof insulation, the chimney did not perform effectively during the winter to heat the loft up, meaning that winter operation for this specific design is not feasible. Possible improvements to the design include using construction materials with higher thermal capacities to retain heatenergy and ensure continued operation during periods of low solar radiation, as wellas using selective absorber coatings on the collector surface. It is recommended thatfurther work on the project include the integration of these improvements into thepresent design and to use the findings obtained from the sensitivity analysis toimprove system efficiencies. CFD analysis of the test-rig will be insightful as anadditional means to validate and compare with the analytical and experimental dataobtained in this report. With the continuation of these studies, this low-cost solar chimney design can be optimised, validated on a commercial scale and built into existing and new housing developments. Incorporating such a passive thermal device will aid homeowners in air regulation and thermal comfort of their living space as well as saving on energy requirements.

[发布机构] Stellenbosch University