The traditional Thai sail windmill (TSW) has been a workable machine for centuries and is used mainly to pump water for agriculture, but it suffers from low operating efficiency in the order of 10%. For a wind turbine, pitch angle and wind statistics are significant parameters which affect efficiency and annual energy production (AEP). The optimum pitch angle depends on the local wind statistics. The pitch angle of the TSW is typically designed using rules of thumb,, which have been carried on from one generation to the next. Therefore, the optimum design features have not been systematically confirmed. The objective of the current study is to investigate the optimum pitch angle of a TSW by a systematic experiment in order to obtain the maximum AEP. The test model was built similarly to a TSW but scaled down 8 times and the rotor orientation was changed to downstream (DTSW model) to catch the wind more easily. The experiment was performed by the tow testing method. The Weibull function was used for the AEP estimation in the Nakhon Ratchasima Province of Thailand as a case study. The results showed that the optimum pitch angles of the 4-blade and 6-blade DTSW models were 5 and 10 degrees and the maximum efficiencies were 17% and 25% and produced AEPs of 20.4 kW-hr and 37.3 kW-hr, respectively.
