Convective heat loss from a bladed solar receiver

Alternative structures have been proposed to improve light trapping and reduce radiative heat losses from central tower receivers. Bladed, star-shaped, and spiked receivers are candidates with a potential to improve light trapping and reduce thermal emissions. Of interest is the bladed receiver concept due to its ease of optical performance tuning and potential application to a diversity of receivers from central tower (surround field) to billboard (polar field) types. Although fractal-like receivers improve optical performance, a possible increase of convective heat losses due to extended surface effects could overshadow these improvements. Quantifying convective heat loss from solar receivers is a challenging task due to the nonlinearity of mixed convection, variable wind conditions and turbulence. In this study, we conducted a thorough computational fluid dynamics analysis of convective heat losses from a bladed receiver and show that the blades may actually decrease convective heat transfer coefficients (i.e. heat transfer rate per unit area per unit temperature) in comparison to a flat receiver. The simulations were first validated against wind tunnel experiments. Convective heat transfer coefficients are reported as a function of receiver orientation (pitch angle), wind speed, wall temperature, blade length to spacing ratio, and blade number. It was shown that the convective heat loss significantly decreases after a characteristic pitch angle of 45°. Furthermore, convective heat loss decreased linearly after a characteristic blade number, which was nine for the investigated configuration. The effect of the blade ratio changed with the receiver orientation, being marginal for shallow pitch angles but becoming significant for angles greater than the characteristic pitch angle of 45°. For example, it was shown that, by increasing the blade number, convective heat losses can be reduced up to ~ 57% even while keeping the same optical properties, which are assumed to be constant for a given bladed aspect ratio and pitch angle.