A parabolic dish/AMTEC solar thermal power system and its performance evaluation

Natural convection heat loss inevitably occurs in cavity-type receivers in high concentrating solar dishes, downward focusing systems and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. To investigate natural convection losses from cavity type receivers, an electrically heated model receiver, was tested at inclinations varying from -90 deg (cavity facing up) to 90 deg (cavity facing straight down), with test temperatures ranging from 450 to 650 deg C. Ratios of the aperture diameter to cavity diameter of 0.5, 0.6, 0.75, 0.85 and 1.0, were used. In addition to measurements of overall heat loss, the Synthetic Schlieren technique was used to visualize the flow pattern out of the cavity. Numerical modeling of the convection losses from the cavity was carried out for positive angles with the commercial computational fluid dynamics software package, Fluent 6.0. Good agreement was found between the numerical flow patterns at the aperture region with the schlieren images and between measured and predicted values for heat loss. Of the previously published work that has been reviewed, a model proposed by Clausing, A. M., 1981, An Analysis of Convective Losses from Cavity Solar Central Receivers, Sol. Energy 27 (4) pp. 295-300 shows the closest prediction to both numerical and experimental results for downward facing cavities despite its original use for bigger-scale central receivers.

Experimental Investigation of Natural Convection Heat Loss From a Model Solar Concentrator Cavity Receiver

Convection heat loss from cavity receiver in parabolic dish solar thermal power system: A review

Up to now, nearly all the experimental and theoretical work treating convective heat loss of cavity receivers had been effected for downward and sideward-facing receivers, as they are used in dish concentrators and solar power towers. With a test facility it was possible to show flow patterns and a temperature field in a hot cavity for a variety of parameter settings and over the whole range of tilt angles. Two types of geometries were examined : a spherical and a hemispherical cavity, both with an inner diameter of 400 mm, the aperture diameter ranging between 60 and 195 mm. Temperatures were varied between 573 K and 873 K. The dependence of losses on temperature, tilt angle, aperture-radius, geometry, ribs, and ventilation is reported. Special emphasis is invested to get an understanding of the air flow in an upward facing cavity. Based on the works of Clausing et al. (1987), and Stine and McDonald (1989), two algorithms are developed that allow the calculation of convective loss from upward-facing receivers. Over the whole range of tilt angles, calculated results are compared with measurements. Good agreement between the calculated and measured data is found.

Convective Heat Loss from Upward and Downward-Facing Cavity Solar Receivers: Measurements and Calculations

A cavity-receiver containing a tubular absorber for high-temperature thermochemical processing using concentrated solar energy

An experimental investigation of heat transfer by natural convection from a smooth, isothermal cubic cavity with a variety of side-facing aperatures is described in this paper. The study was motivated by the desire to predict the convective loss from large solar thermal-electric receivers and to understand the mechanisms which control this loss. Hence, emphasis is placed on the large Rayleigh number, Ra, regime with large ratios of the cavity wall temperature T{sub w} to the ambient temperature T{sub {infinity}}. A cryogenic wind tunnel with test section temperatures which are varied between 80 K and 310 K is used to facilitate deduction of the influences of the relevant parameters and to obtain large temperature ratios without masking the results by radiative heat transfer. A 0.4-m cubic cavity, which is mounted in the side wall of this tunnel, is used. The area of the aperture A{sub a} and its location are key variables in this study. The data which are presented cover the ranges: 1 < T{sub w}/T{sub {infinity}} < 3, L{sup 2}/18 {le} A{sub a} {le} L{sup 2}, and 3 {times} 10{sup 10}.

L. D. Lister

Papers

Natural Convection From Isothermal Cubical Cavities With a Variety of Side-Facing Apertures

Combined convection from isothermal cubical cavities with a variety of side-facing apertures