Abstract
M.Ing. (Mechanical Engineering)
The use of renewable energy sources has attracted a great deal of attention over the past two
decades due to the increase in fossil fuel prices. The focus has been on the use of these
energy sources to produce electricity and hence replace the current energy sources used for
energy sources, such as coal and radioactive materials, used for electricity generation. These
renewable energy sources may however be implemented on a smaller scale to reduce the
electricity demand of private residences and industry. The solar air heater (SAH) is one such
application. An SAH is a simple and inexpensive device used for heating air by means of
solar energy. These devices may be used in a variety of applications such as space heating,
pre heating for industrial applications, drying of various materials and pre heating for solar
water desalinators.
The present research is aimed at evaluating the unique SAH design in terms of its thermal
efficiency and temperature distributions within the SAH. The specific SAH design presented
in this investigation has received little attention in literature. Numerical SAH models
available in literature rely on assumptions which make the models possibly unsuitable for use
on the proposed SAH. The SAH will be evaluated using a commercial CFD package based on
the finite volume method. The solar load model (SLM) was implemented to calculate the heat
input to the SAH. The surface to surface (S2S) radiation model was implemented to account
for radiative heat transfer within the SAH. Initially the Reynolds stress model (RSM)
turbulence model was employed to model the turbulence within the domain. The shear stress
transport (SST) k-ω turbulence model and Re-Normalisation Group (RNG) k-ε turbulence
models were also investigated as part of a turbulence sensitivity study. The solution provided
by the CFD investigation is shown to be mesh independent by means of a full mesh
sensitivity study.
The results obtained from the CFD models were validated experimentally. An SAH was
constructed and tested under actual operating conditions. Air temperature measurements were
made at the inlet and the outlet of both ducts of the SAH. Eight further temperature
measurements were made at various points throughout the SAH. The direct solar radiation
was measured by means of a pyrheliometer and the total(global) solar radiation was recorded
by means of a pyranometer.
The average conversion efficiency observed for the SAH ranged between 23% and 83% and
the average collector efficiency between 11% and 44%. The values vary from test to test and
are also dependant on the solar radiative input power used for the calculation. The thermal
efficiency values predicted by the CFD models showed good agreement with the
experimentally derived values. The CFD model over predicted the thermal efficiency by
between 9.01% and 6.75% of the measured quantity, depending on the solar radiative power
input used. The comparison of modelled temperatures in the SAH with the measured
temperatures showed good qualitative agreement but do not compare well quantitatively for
all points of comparison. The average error between predicted temperature and experimental
results for the various points of comparison is 9.165 K. The maximum and minimum...