Critical evaluation of heat transfer coefficients applicable to solar chimney power plant collectors

A solar chimney power plant consists of a translucent collector which heats the air near the ground and guides it into the base of a chimney at its centre. The buoyant air rises in the chimney, and electricity is generated through one or more turbines in or near the base of the chimney. Two comprehensive studies are relevant, namely those of Pretorius and Kroger [6] and Bernardes et al. [2]. Pretorius introduced refinements, like wind over the collector roof and chimney top, and controlled plant output according to specific demand patterns. The analysis of Bernardes, on the other hand, presents good agreement with data measured at a small scale plant near Manzanares in Spain in the 1980s. To calculate the heat transfer and pressure drop of the air stream in the collector, Bernardes models it as developing flow between two circular, parallel, flat plates. Pretorius. assumes that the collector roof height varies inversely with distance from the chimney centre, according to a power law, such that the velocity through the collector typically remains essentially constant. He shows that the flow between the ground surface and the collector roof then quickly becomes fully developed. The paper compares the methods used to calculate the heat fluxes in the collector, and their effects on solar chimney performance. The heat transfer equations for forced and natural convection of both models are compared and evaluated based on corresponding parameters. Reasons for the discrepancies between the predictions of the two models are given. The Pretorius correlations were introduced in the Bernardes code to run comparative simulations. In general the Pretorius model produces higher heat transfer coefficients and higher heat fluxes for both the roof and for the ground surfaces. The two approaches lead to very similar air temperature rises in the collector and thus, similar produced power.