Solar chimney turbine performance
Thesis (PhD (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2002.
This project investigates the performance of solar chimney power plant turbines. A solar chimney power plant consists of a tall chimney surrounded by a transparent deck or solar collector. The sun heats the air in the collector through the greenhouse effect. A turbine extracts energy from the hot air rising up the chimney. An investigation of the requirements and operation of such turbines is needed. Correct matching of the turbine to the plant requires the determination of the turbine operational range and other requirements. An air-standard cycle analysis is extended to include component and system losses. Simple steady-state and transient collector models are added to take into account the coupling effect of the collector air temperature rise and mass flow rate on the turbine operation. The predicted turbine operational range for a representative day shows that the expected pressure drop in a full-scale solar chimney turbine is significantly higher than has previously been predicted. A turbine design method is developed and used to design a turbine for the representative day. The methods can easily be extended to include more operating points for a full year of operation. A turbine layout is suggested that uses the chimney support pillars as inlet guide vanes (IGVs). These introduce pre-whirl to the turbine and reduce the amount of exit whirl thus decreasing the kinetic energy at the turbine exit. Non-radial inlet guide vanes add to the torsional stiffness of the chimney base. A matrix throughflow method is used to design the radial to axial duct between the IGVs and rotor. The turbine blade profiles are simulated using a surface-vortex method. This is coupled to an optimisation scheme that minimises both the chord length and maximum flow velocity of the profile to reduce blade drag. An experimental program investigates the performance of the turbine. Volume flow, pressure drop, torque and speed are measured on a scale model turbine to map the turbine performance over a wide range. The velocity and pressure profiles are measured at two design points to investigate the flow through the turbine in more detail. These are compared to the design predictions and used to improve the design method. The experiments show that the design of a solar chimney turbine with a total-to-total efficiency of 85 % - 90 % and total-to-static efficiency of 75 % - 80 % is possible. Analysis of the experimental results shows that the turbine efficiency can be improved.