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

["Evapotranspiration", "Solar power plants", "Air flow", "Dissertations -- Mechanical engineering"]

Westdyk, Dimitra ; Kroger, D. G.

Stellenbosch University

Others  :  https://scholar.sun.ac.za:443/bitstream/10019.1/50712/1/westdyk_evapotranspiration_2007.pdf

瑞士|英语

来源: Stellenbosch University

【 摘 要 】

ENGLISH ABSTRACT: The search for ways of utilizing solar energy for power generation in the arid areas ofthe world has led to the investigation of the feasibility of erecting a solar chimneypower plant for generating electricity. There is the added possibility of combining thispower generation with agricultural activities underneath the outer rim of the glasscollector. In order to investigate the influence of evapotranspiration on the propertiesof air flowing over vegetation growing under glass, an experimental solar tunnel wasbuilt. Air was drawn over the grass surface growing in the glass roofed tunnel and thesituation was investigated experimentally and analytically. The primary purpose ofthe study was to measure the average rate of evapotranspiration taking place, tomeasure the change in dry- and wetbulb temperatures of the air and hencedetermine the magnitude and direction of the change in air density occurring undervarious air inlet and weather conditions. This is necessary since the power output ofthe turbine in a solar chimney power plant is dependent on the volume flow rate of airthrough it, which is in turn dependent on the density of the air. The second was todetermine a value for the effective convective heat transfer coefficient between thegrass and the air flowing over it. The third was to use the inlet air state and thenapply the Penman-Monteith and the conservation equations to subsequent one meterlengths of the tunnel in order to predict the exit state of the air as well as the variationin the grass temperature along the tunnel. It was found that the maximum averagerate 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 grasstemperature increases along the tunnel length and is usually a few degrees higherthan the air drybulb temperature under most test conditions. The effective convectiveheat transfer coefficient was found to be between 30 W/m2K and 40 W/m2K for an airvelocity ranging from approximately 1.5 m/s to 2.5 m/s Tests show that for typicalhigh summer temperatures (above 35 °C) the outlet drybulb temperature of the air islargely governed by the relative humidity at the inlet: the outlet drybulb temperaturebeing lower than the inlet drybulb temperature for a relative humidity below about40 % and for higher values of relative humidity, the drybulb temperature at the outletis slightly higher by between 0 °C and about 3 °C. Since there is a simultaneousincrease in the wetbulb temperature due to evapotranspiration, the density of the airmay decrease slightly or increase slightly or remain the same. Latent heat transferaccounts for between 80 % and 90 % of the total heat transfer between the grassand the air. Predicted values of average rate of evapotranspiration, average grasstemperature and the exit state of the air were in good agreement with experimentallymeasured values and hence validate the use of this mathematical model. In theapplication to the solar chimney power plant analysis in another project [07 PR 1] itwas found that the annual output of the power plant would experience a reduction ofapproximately 30 % for a circular glass collector of 5000 m diameter with vegetationplanted radially 1000 m inward from the perimeter.