Utilizing solar power using parabolic trough collectors for energy is considered most proven and lowest cost for large-scale solar power technology. So far commercial plants used oil as the primary heated fluid and steam was produced in a secondary heat exchanger. This seem to be a very inefficient process due to the need of extra heat exchangers and extra losses incurred while heat is transferred from oil to steam. The reason oil is used as the primary heated fluid is partially due to the reluctance of the designer to deal with the behavior of two-phase, water steam, in parallel pipes owing to the possible uneven flow distribution and instability related problems. Analysis of a system of two parallel pipes with common inlet and outlet manifolds that undergoes a process of heating and evaporation shows that multiple steady state solutions for the flow distribution in the two pipes may be obtained. A simplified stability analysis backed by new experimental results allows the determination of the actual physical solutions that take place. Design considerations are discussed and suggestions for optimal operation are included. (C) 2004 Elsevier Ltd. All rights reserved.