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

Westdyk, Dimitra (2007-03)

Thesis (MScIng) -- University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: 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/m2 h 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/m2 K and 40 W/m2 K 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.

AFRIKAANSE OPSOMMING: Die soeke na metodes om son energie te benut in die droe dele van die wereld het gelei tot die ondersoek na die uitvoerbaarheid om 'n sontoring kragstasie op te rig. Daar bestaan die moontlikheid om kragopwekking te kombineer met die bedryf van landbou onder die buitenste dele van die glas kollektor van die sontoring. Ten einde die invloed van evapotranspirasie op die eienskappe van lug wat oar plantegroei vloei te ondersoek, is 'n eksperimentele glasdak tonnel gebou. Lug is oar die gras oppervlakte wat in die tonnel geplant is, getrek en die opset is eksperimenteel en analities ondersoek. Die hoofdoel van die ondersoek was om onder verskillende inlaat- en weerstoestande die tempo van evapotranspirasie te meet, die verandering in droebol en natbol temperatuur te meet en daarvolgens die verandering in die lugdigtheid te bepaal. Die krag uitset van die sontoring turbien is afhanklik van die lug vloeitempo daardeur en dit word bepaal deur die lugdigtheid. Die tweede doelwit was om die waarde van die effektiewe konvektiewe warmteoordrag koeffisient tussen die gras en die lug te bepaal. Die derde doelwit was om die Penman-Monteith vergelyking tesame met die energie en massa behoud vergelykings op agtereenvolgende een meter lengtes van die tonnel toe te pas om sodoende die uitlaat toestand van die lug sowel as die verandering in gemiddelde gras temperatuur te voorspel. Daar is gevind dat die maksimum gemiddelde tempo van evapotranspirasie plaasvind wanneer die son op sy hoogste is op 'n windstil, wolklose somersdag en die waarde daarvan is naastenby 0. 76 kg/m2 u vir die bepaalde toetsomgewing. Die gras temperatuur neem al langs die tonnel toe en is gewoonlik 'n paar grade hoer as die lug temperatuur onder die meeste toets toestande. Verder is daar gevind dat die waarde van die effektiewe warmteoordrag koeffisient tussen die gras en die lug vir 'n lugvloei snelheid van 1.5 mis tot 2.5 m/s tussen omtrent 30 W/m2 K en 40 W/m2 K gele het. Toetsresultate toon dat vir tipiese hoe somer temperature (bokant 35 °C) die uitlaat droebol temperatuur deur die relatiewe vogtigheid van die lug by die inlaat bepaal word: die droebol temperatuur by die uitlaat is laer as by die inlaat vir 'n relatiewe vogtigheid laer as 40 % en vir hoer waardes van relatiewe vogtigheid sal die uitlaat droebol temperatuur effens toeneem of dieselfde bly. Aangesien die natbol temperatuur toeneem a.g.v. evapotranspirasie sal die lugdigtheid of effens afneem, of dieselfde bly of effens toeneem. Latent warmte oordrag is verantwoordelik vir tussen 80 % en 90 % van die totale warmte oordrag tussen die gras en die lug. Deur gemete en voorspelde waardes met mekaar te vergelyk word afgelei dat die wiskundige model die uitlaat toestand van die lug, die gemiddelde gras temperatuur en die gemiddelde tempo van evapotranspirasie binne eksperimentele limiete, voorspel. Wanneer hierdie model toegepas word in 'n ander projek [07 PR 1] op die sontoring kragstasie analise word gevind dat die netto jaarlikse kraguitset met 30 % verminder in die geval van 'n 5000 m diameter glas kollektor wat 'n afstand van 1000 m radiaal binnetoe vanaf die buiterand van die kollektor beplant word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50712
This item appears in the following collections: