This paper compares the performance of medium size CSP (Concentrating Solar Power) plants based on an ORC (Organic Rankine Cycle) power generation unit and using linear Fresnel collectors, thermal oil as heat transfer fluid and two-tank direct and thermocline energy storage systems. The comparative performance analysis was carried out by means of specifically developed simulation models and considering different values of solar multiple and thermal energy storage capacity. The results of the performance assessment demonstrate that if the CSP plant has to be optimized for the highest specific energy production, that is, for the highest solar energy conversion efficiency, two-tank energy storage systems show slightly higher performance than thermocline storage systems. However, the study also demonstrates that thermocline storage systems can be an interesting option to reduce the energy production cost of CSP plants. (C) 2015 Elsevier Ltd. All rights reserved.