Analysis and practice show that north-south mounted, east-west tracked parabolic trough collectors have the best annual performance profile. Hence this is a wide spread practice in PTC plants. This mounting gives the highest collector energy yield and least losses since the one-axis tracked collector has distributed daily profile with acceptable cosine losses in winter and early and late day times.
I agree with Dr. Louy Qoaider and Dr. A.K. Mukerjee that N–S axis horizontal E–W tracking is the best orientation. I add some references for further details:
Ouagued, M., Khellaf, A., & Loukarfi, L. (2013). Estimation of the temperature, heat gain and heat loss by solar parabolic trough collector under Algerian climate using different thermal oils. Energy Conversion and Management, 75, 191-201.
Kalogirou, S. A. (2004). Solar thermal collectors and applications. Progress in energy and combustion science, 30(3), 231-295.
Boukelia T.E, Mecibah M.S. Estimation of direct solar irradiance intercepted by solar
concentrator in different modes of tracking (Case study: Algeria). International Journal of Ambient Energy, in press. http://dx.doi.org/10.1080/01430750.2013.864587