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Stellenbosch University (2007)

Evapotranspiration effects on air flowing over grass in a small glass roofed tunnel

Westdyk D

Titre : Evapotranspiration effects on air flowing over grass in a small glass roofed tunnel

Auteur : Westdyk D

Université de soutenance : Stellenbosch University

Grade : MScEng 2007

The search for ways of utilizing solar energy for power generation in the arid areas of the world has led to the investigation of the feasibility of erecting a "solar chimney" power plant for generating electricity. There is the added possibility of combining this power generation with agricultural activities underneath the outer rim of the glass collector. In order to investigate the influence of evapotranspiration on the properties of air flowing over vegetation growing under glass, an experimental solar tunnel was built. Air was drawn over the grass surface growing in the glass roofed tunnel and the situation was investigated experimentally and analytically. The primary purpose of the study was to measure the average rate of evapotranspiration taking place, to measure the change in dry- and wetbulb temperatures of the air and hence determine the magnitude and direction of the change in air density occurring under various air inlet and weather conditions. This is necessary since the power output of the turbine in a solar chimney power plant is dependent on the volume flow rate of air through it, which is in turn dependent on the density of the air. The second was to determine a value for the effective convective heat transfer coefficient between the grass and the air flowing over it. The third was to use the inlet air state and then apply the Penman-Monteith and the conservation equations to subsequent one meter lengths of the tunnel in order to predict the exit state of the air as well as the variation in the grass temperature along the tunnel. It was found that the maximum average rate of evapotranspiration from the grass occurs at the solar noon on a cloudless, windless summer day and is about 0.76 kg/m2h at the experimental site. The grass temperature increases along the tunnel length and is usually a few degrees higher than the air drybulb temperature under most test conditions. The effective convective heat transfer coefficient was found to be between 30 W/m2K and 40 W/m2K for an air velocity ranging from approximately 1.5 m/s to 2.5 m/s Tests show that for typical high summer temperatures (above 35 °C) the outlet drybulb temperature of the air is largely governed by the relative humidity at the inlet : the outlet drybulb temperature being lower than the inlet drybulb temperature for a relative humidity below about 40 % and for higher values of relative humidity, the drybulb temperature at the outlet is slightly higher by between 0 °C and about 3 °C. Since there is a simultaneous increase in the wetbulb temperature due to evapotranspiration, the density of the air may decrease slightly or increase slightly or remain the same. Latent heat transfer accounts for between 80 % and 90 % of the total heat transfer between the grass and the air. Predicted values of average rate of evapotranspiration, average grass temperature and the exit state of the air were in good agreement with experimentally measured values and hence validate the use of this mathematical model. In the application to the solar chimney power plant analysis in another project [07 PR 1] it was found that the annual output of the power plant would experience a reduction of approximately 30 % for a circular glass collector of 5000 m diameter with vegetation planted radially 1000 m inward from the perimeter.

Présentation (NRF)

Page publiée le 18 octobre 2017