Solar concentrators and concentration ratio

(a) Why and types

A concentrator uses mirrors or lenses to focus beam radiation from a large aperture onto a small absorber (receiver). The small receiver area gives low heat loss, so high temperatures (100–1500 °C) and good efficiency at high temperature are obtained, and less expensive absorber material is needed per unit of heat. Concentrators are used for process heat, industrial steam and solar thermal power plants.

Classification

1. By focus: line focus (parabolic trough, linear Fresnel) and point focus (parabolic dish, central receiver/heliostat field).
2. By optics: reflecting (mirrors) or refracting (Fresnel lens).
3. By tracking: single axis (trough) or two axis (dish, tower).

(b) Concentration ratio

$$C = \frac{A_a}{A_r} = \frac{W\,L}{\pi D L} = \frac{W}{\pi D} = \frac{3.0}{\pi (0.05)} = 19.1$$

If the aperture area is taken as the net opening (excluding the area shaded by the receiver), $(W - D)/\pi D = 18.8$. So, ideally, the receiver surface gets about 19 times the intensity falling on the aperture. This is a moderate value, suitable for medium-temperature work (about 150–300 °C); the ideal limit for a line-focus (2-D) concentrator is $1/\sin\theta_a$ (about 212 for a half acceptance angle of 0.27°).

Answer: $C \approx 19$.