Solar chimney turbine characteristics T.W. von Backstr€ om * , A.J. Gannon Department of Mechanical Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 17 January 2003; received in revised form 5 August 2003; accepted 7 August 2003 Abstract A typical layout of a solar chimney power plant has a single axial turbine with radial inflow through inlet guide vanes at the base of the chimney. Turbine efficiency depends on the turbine blade row and turbine diffuser loss coef- ficients. The paper presents analytical equations in terms of turbine flow and load coefficient and degree of reaction, to express the influence of each coefficient on turbine efficiency. It finds analytical solutions for optimum degree of re- action, maximum turbine efficiency for required power and maximum efficiency for constrained turbine size. Char- acteristics measured on a 720 mm diameter turbine model confirm the validity of the analytical model. Application to a proposed large solar chimney plant indicates that a peak turbine total-to-total efficiency of around 90% is attainable, but not necessarily over the full range of plant operating points. Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction The turbine is one of the main sub-systems of a solar chimney power plant. Other systems are the solar col- lector, the chimney and the generator (Fig. 1). Air he- ated in the solar collector surrounding the chimney enters it radially through passages between the pillars supporting the chimney. The pillars may be airfoil- shaped and arranged along non-radial chord lines, to act as inlet guide vanes (Fig. 1). The typical solar chimney turbine is of the axial flow type. It has characteristics between those of wind turbines and gas turbines: it has more blades than the typical 2 or 3 of wind turbines, but not as many as gas turbines; the rotor blades are ad- justable, like those of wind turbines, but, as in gas tur- bines, the flow is enclosed, and the solar chimney turbine may have radial inflow inlet guide vanes. The main function of the turbine is the efficient conversion of fluid power to shaft power. A secondary function of solar chimney turbines is flow and output power control by adjustment of its blade angles. Solar chimney literature has little to say about factors affecting efficiency of the turbines, but merely assumes various fixed values of efficiency, e.g., 83% (Haaf et al., 1983), 40–80% (Mullett, 1987), 80% (Schlaich, 1995), 77.0, 78.3 and 80.1% (Pasumarthi and Sherif, 1998) and 80% (Von Backstr€ om and Gannon, 2000). We could find no papers dealing with the operating characteristics of solar chimney turbines. The objective of this paper is to develop a simple model for the prediction of solar chimney turbine efficiency and operating characteristics, to help in solar chimney power plant design optimisation. 2. Assumptions • The turbine is of the axial type, with radial inflow inlet guide vanes (stator blades). • The analysis proceeds as if the stator blade row also has an axial through-flow direction. Note that the definition of load coefficient is w ¼ DH =ð 1 2 U 2 Þ where DH ¼ H 1  H 2 . 3. Turbine efficiency Noting that DH loss is the difference between the real and ideal (no-loss) stagnation enthalpy drops across the turbine, the turbine efficiency is: * Corresponding author. Tel.: +27-21-808-4267; fax: +27-21- 808-4958. E-mail address: twvb@ing.sun.ac.za (T.W. von Backstr€ om). 0038-092X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2003.08.009 Solar Energy 76 (2004) 235–241 www.elsevier.com/locate/solener