1 23 Heat and Mass Transfer Wärme- und Stoffübertragung ISSN 0947-7411 Volume 51 Number 6 Heat Mass Transfer (2015) 51:875-887 DOI 10.1007/s00231-014-1465-3 Second law analysis and optimization of a parabolic trough receiver tube for direct steam generation H. C. Nolte, T. Bello-Ochende & J. P. Meyer