I presume you mean trough collectors. There are other collector shapes that might be more suitable. You will not get as much concentration as a parabolic but they collect from a large area of the sky so even if it is cloudy, they will collect some heat. One is called the cusp collector and gets light from a full 180 degrees and another one is a trough shaped like a seashell. For a link to a great pdf about your different choices, Search for "Tapas Kumar Mallick Chapter 1 Photovoltaic Concentrators and Building Integrated Photovoltaics" and it is a 4.8 megabyte pdf. I am sure it will help you make a suitable choice. (the link was too long to past in here). I have sent this pdf to several people in the
last year and they sounded pretty enthusiastic (but no firm results back yet).
I think it is really worth finding out how wide the "average" cloud cover defuse the "average" solar influx in different areas because this has a huge bearing on what collector works best. Case in point, Calgary gets far better sun in winter than Victoria. We get a gray rainy monsoon in winter and the sun might be gone for weeks on end. Calgary gets sunny subzero weather for much of the winter but lots of people I know say it is preferable to the crap weather we get in winter here in lovely Victoria.