An efficiency model at design performance for counter-rotating turbines is developed and validated. Based on the efficiency equations, an off-design performance model for counter-rotating turbines is developed. Combined with a thermodynamic model for a solar chimney system and a solar radiation model, annual energy output of solar chimney systems is determined. Two counter-rotating turbines, one with inlet guide vanes, the other without, are compared to a single-runner system. The design and off-design performances are weighed against in three different solar chimney plant sizes. It is shown that the counter-rotating turbines without guide vanes have lower design efficiency and a higher off-design performance than a single-runner turbine. Based on the output torque versus power for various turbine layouts, advantageous operational conditions of counter-rotating turbines are demonstrated. (c) 2005 Elsevier Ltd. All rights reserved.