In a concentrating collector, the radiant energy is optically concentrated before absorption.

Light entering a large aperture area is reflected or refracted to a relatively small target or receiver (e.g. at the focus of a parabola), where it is transformed into heat energy that is then collected in a conventional way, as shown in figure 3.3.

The advantage of concentrating collectors is that thermal losses are lower when the heated area is reduced. This then allows the collector to heat fluid to a higher temperature with a higher efficiency. Concentrating collectors are therefore associated with high temperatures, mainly due to the reduced losses resulting from this concentration. These losses nevertheless still increase as the difference between the fluid and ambient temperatures rises, and the energy efficiency falls as a result (see fig 3.4).

Source CRES-TRASOL CD-Rom - Figure 3.3 Concentrating collector

 

Trough type concentrators heat a length of tube, whereas dish concentrators focus on a single receiver, and therefore have the smallest area and higher efficiency.

 

Figure 3.4 Typical efficiency of a trough type concentrating collector

 

The natural configuration for an evacuated tube collector (fig 3.4) is the glass tube, which provides the structural strength to withstand the pressure differences. They are hermetically sealed and contain getters to absorb any molecules that outgas into the vacuum. These tubes are excellent for operating temperatures up to the 120-150oC range.

 

Their basic feature is a vacuum between the transparent cover surface and the absorber surface. In this way there are no heat losses associated with conduction and convection between these surfaces.

 

Source CRES-TRASOL CD-Rom
Figure 3.5 Schematic view of an evacuated tube collector