In certain solar concentrator applications, not only the position of the sun is predetermined relative to the position of the absorber, e.g., due to the local latitude; the tilting of the aperture of the concentrator can also be limited to restrictions, such as shading or the covering glass. In the example shown in Fig. 11, the building integrated photo voltaic module (BiPV) may require the concentrator aperture to be also parallel to the wall, in order to minimize the shading between concentrators. By searching among the flowlines within the ideal concentrator and , as shown in Fig. 12, we can meet such a requirement by limiting the aperture to be parallel to the absorber. A simple binary search routine using starting points for flowlines is shown in Fig. 12. In this process, the tilting of aperture , and so on, is compared with the angle of and the program stops when the angle difference is within the tolerance of the design. This results in the concentrator shown in Fig. 13. By constructing an array of such concentrators, not only the relevant etendue at the aperture (the seasonal angle variation of the sun in this case, according to the full area of the wall) is fully used, but also the ideal concentration law of is also satisfied. The flowline in this case provided another degree of freedom to the ideal concentrator design by allowing the tilting angle of the aperture also to be flexible. Such a result cannot be achieved by simply tilting the conventional CEC19 or adding a secondary concentrator to the symmetric concentrator.13 The detailed ray tracing can be found in Ref. 20.