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Background

A solar chimney, or thermal chimney, is a form of passive ventilation that can be applied to a structure. It uses the principles of heat transfer and fluid mechanics to naturally ventilate a structure without the need of an outside source of electricity. This makes the solar chimney a promising alternative to forced air in both the developed and the developing world.

Indoor air quality is a health issue that plagues the developing world. In many regions the ambient temperature of a household can far exceed a comfortable living temperature. In other families prepare their food and water indoors and the living space is laden with smoke [1]. The implementation of a solar chimney in an appropriate climate can help to increase the indoor air quality of a home and subsequently improve the health and living conditions of residents.

Principles

A solar chimney takes advantage of the fact that as the temperature of air changes, the density of air changes as well. The chimney is heated during daylight hours due to sunlight exposure. This in turn heats the air contained inside the chimney, creating a temperature difference between the air in the chimney and the air in the dwelling. Since the the density of air varies with temperature, there will be a density difference between the air within the dwelling and the air within the solar chimney. The difference in density creates a pressure difference and drives the air from inside the dwelling into the solar chimney, and the air in the solar chimney to the exterior. This process exchanges the air inside the dwelling, providing air exchange and a breeze for occupants. This increases indoor air quality and comfort.

The pressure difference created by a difference in density can be modelled using equation 1 below [2].

Solarchimney pressuredifferenceequation1.png (1)

To determine the effect of the solar chimney on ventilation we must find the velocity. The equation for velocity due the density difference is shown below [2].

Solarchimney velocityequation 1.png (2)

Once the velocity is determined, the volumetric flow rate of the air can be found using equation 3 below [2].

Solarchimney volumetricflowrate 1.png (3)

Construction

Finite Element Analysis

A finite element analysis was performed using THERM 6.2 software. THERM is a software that is mainly used as a modelling program for glazing systems in buildings. THERM is the benchmarking program used by the NFRC (National Fenestration Rating Council) to model energy and solar properties of fenestration assemblies and was created by Lawrence Berkeley National Laboratory. The software is used by many leading glazing manufacturers in order to test their glazing system performance. The software is freeware and as such can be used by anyone without purchasing a license.

With knowledge of the software it is possible to use the features of therm to estimate the thermal profile of a system by assigning appropriate boundary conditions and defining material properties. A model of the solar chimney described in the construction section was created using THERM. The results of the analysis as well as the thermal profile of the chimney are shown below. The findings from the therm program are used in the performance results section along with the theory described in the principles section in order to estimate the velocity and volumetric flow rate of the air leaving the home.

Performance Results

Conclusion

References

[1] Indoor air pollution in developing countries: a major environmental and public health challenge, Nigel Bruce, Rogelio Perez-Padilla and Rachel Albalak. http://www.who.int/docstore/bulletin/pdf/2000/issue9/bul0711.pdf

[2] Air Flow and Velocities due to Natural Draft, Engineering Toolbox, http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html.

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