A direct numerical simulation of the three-dimensional unsteady flow in a 1:33 scale model of the the Manzanares solar chimney power plant in zero load condition (negligible pressure drop across turbine) was carried out. Instantaneous flow visualizations for the collector show transverse rolls near the collector inlet and longitudinal rolls about halfway into the collector. Based on earlier linear stability theory investigations for plane two- dimensional channel flow, the observed structures are attributed to a Rayleigh-Bénard- Poisuille instability. The present simulations also indicate separation from the bottom and top wall of the chimney inlet and downstream of the turbine mount. The buoyancy-driven flow in the chimney is fully turbulent.