1 Prediction of Evaporation Rate in a Solar Dryer for Sewage Sludge

Efficient solar drying requires that the drying rate, as a function of the environment and the control, is quantitatively known. A solar drying installation for waste-water sludge in Füssen, Germany, designed by Thermo-System ® and the University of Hohenheim, has been used to collect data for the establishment of a drying-rate function. In this solar dryer, wet sludge is uniformly spread over a concrete floor under a greenhouse-like transparent cover. The sludge is intermittently mixed by means of a autonomous robot (electric mole®), while the air under the cover is ventilated (horizontally) and mixed (vertically) by electric fans. Data of evaporation rate, environmental conditions and control operations were collected over three drying cycles. Evaporation rate via sludge sampling and via vapour balance across the structure compared favourably, justifying the use of the hourly vapour-balance data to develop linear regression and non-linear neural network (NN) models to predict the evaporation rate. The most important predictors of evaporation turn out to be (1) outdoor solar radiation (2) outdoor air temperature, and (3) the ventilation flux. The dry solids content of the sludge is next in importance, but could not be quantified with confidence over the domain, for lack of sufficient data. Air-mixing is an order of magnitude less effective than ventilation. The experimental design did not include different rates of sludge mixing