Another approach is to increase the ratio between aperture and absorber area. This ratio is called geometrical concentration ratio. CPCs (Compound Parabolic Concentrators) are applied, because these optical concentrators have no focal point. Thus tracking can be avoided in combination with concentration ratios lower than two.

Figure 2 shows a stationary, non-evacuated CPC-type collector without a vacuum and with a concentration ratio of 1.5. It is made up of riser tubes centred in symmetrical CPC valleys with two asymmetrical CPC valleys for the headers. The absorber is V-shaped (inverted V). The collector has only two inlet/outlet connections and has a minimum back insulation of 3 cm. There

is enough distance between the glass and the top of the absorber for the collector to be provided with anti-convective barriers such as a Teflon® foil.

The research institute INETI (Lisbon, Portugal) was involved in the development. The collector is available on the market and has an operating temperature level of 110°C, e. g. suitable to operate an absorption cooling machine.