A cascade organic Rankine cycle (ORC) system utilizing solar energy and liquefied natural gas (LNG) for thermal power generation is proposed. Energy from solar collectors drives the evaporation of working fluid in the top cycle (I). The heat released by cycle I facilitates the evaporation of working fluid in the bottom cycle (II). LNG is the cold source of cycle II. The system is optimized with respect to a new index, i.e., the equivalent efficiency, which denotes the ratio of the extra electricity produced by adding collectors and the received solar irradiation. Its advantages are clarified in comparison with existing indices like thermal efficiency, cold energy efficiency, and exergetic efficiency. The effects of working fluids on the system performance are investigated. Flat plate collectors (FPC) and evacuated tube collectors (ETC) are exemplified. It is shown that the equivalent efficiency is the most suitable index to embody the effectiveness of combination of solar collectors and LNG. A maximum equivalent efficiency of 5.99% can be obtained on the use of isopentane/R125 and ETC. The volume ratio for each expander in the cascade ORC is much smaller than that in single-stage ORC, thereby resulting in easier design and manufacture of the expanders. (C) 2016 Elsevier Ltd. All rights reserved.