3. Parabolic Trough Collector Concepts

Figure 33 Enclosed collector concepts. (left) HelioTube (by Heliovis AG), (center) Airlight (by R. Presley), (right) Glasspoint

It can be observed that none of the collectors follow a specific design, but rather overflow to new approaches in order to adapt them to the enclosed aperture. The main curiosity of the concept focusses on the optical performance, since placing a layer in front of the collecting area cause additional optical effects. A general assumption on these collectors is based on a lower optical efficiency in comparison to the conventional collectors. The reflectance, transmittance and absorbance that are attached to the covering materials, can be the main reasons, as described for the maximal optical efficiency in Eq.(7).

6. 
   Category E: Fixed Focus
   

In a fix focus collector the main characteristic is featured by the stationary focal axis along the module, which implies alternative structures to those conventionally applied. In general a fix focus collector has the purpose to place the receivers on the focal axis and coaxially to it, the rotary axis of the parabolic concentrators.


Figure 34 a) Conventional version: The torque axis is placed at the torque unit of the collector (e.g. torque tube), which implies the movement of the receiver while tracking the sun. b) Fixed focus: The rotational axis coincides with the focal line of the parabola where the receivers are placed. In this case the reflector surface is the movable element. T stands for the torque and &_T for the torque angle from the middle plane as reference.