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Analysis of a water-trickle solar collector

University of British Columbia

A theoretical study of the performance of water-trickle collectors was carried out in this thesis, where both the flat-plate and corrugated-plate water-trickle collectors have been considered. In order to predict -the performance of water-trickle collectors, the different modes of heat loss from the collectors must be taken into account. These modes of heat loss are radiation, convection, and evaporation-condensation. A theoretical model for water-trickle collectors has been developed previously. In this model, the heat loss due to evaporation-condensation is predicted by assuming evaporation is by means of molecular diffusion of water vapor through a stagnant air layer. In the new model developed, the evaporation-condensation loss is predicted by means of the heat-mass transfer analogy [equation not included]. By evaluating this mode of heat loss in a different way, the sensitivity of the results based on the assumption of the previous model is tested. At an average plate temperature of 50°C or higher, the heat loss due to evaporation-condensation was found to account for approximately 50 percent or more of the total loss. This result is also predicted by the previous model. Thus both methods can be used for evaluating evaporative heat loss from water-trickle collectors. The heat-mass transfer analogy was then extended to corrugated- plate water-trickle collectors. The heat loss due to evaporation-condensation was evaluated and was found to be less than losses from flat-plate water-trickle collectors. A new procedure was developed in the present work to evaluate (τα), the fraction of incident energy that is ultimately absorbed by a corrugated water-trickle collector. This procedure was derived based on performing radiative heat balances over surfaces inside a water-trickle collector. Based on this procedure, values of (τα) for corrugated surfaces were obtained and were found to be higher than values for flat surfaces. This result is reasonable since there is a larger area exposed to radiative heat input on a corrugated surface than on a flat surface. Expressions for the temperature distribution along the corrugated cross-section of the absorbing surface of a water-trickle collector were developed. These expressions were derived based on standard heat transfer correlations for fins. With these expressions, temperature profiles of the cross-section of a corrugated water-trickle collector was obtained by using radiation data available for Vancouver. With these temperature profiles, an expression for the rate of useful heat gain of a corrugated water-trickle collector was obtained. An attempt has been made to derive standard collector factors such as F' and F[sub R], which, if successful, would simplify future effort in obtaining values of the useful heat gain of a corrugated water-trickle collector extensively. The expressions derived in this thesis, however, are complicated and are determined to be of little practical significance. The reason being that extensive computations are usually involved in obtaining values for these factors and such efforts are seldom worthwhile.

Text

2010-03-06

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http://hdl.handle.net/2429/21616

Mechanical Engineering

University of British Columbia

Graduate