الفهرس 
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Abstract
In view of the increasing concerns about wind energy exploitation as one of the promising alternatives of renewable energy systems, numerous efforts are directed to rotor blades design in order to achieve the best performance. Forces acting on rotor blades of horizontal axis wind turbine (HAWT), and hence the turbine power coefficient, are greatly affected by deviations in blade profile shape and surface roughness from design conditions. Both may change due to environmental impacts such as dust, sand, ice…etc. One of the methods proposed for restoring performance of blades suffering from such changes is to attach a winglet at the tip of the blades.
The present work examined the winglet effect on blade performance at different operating conditions. An experimental study on a model of HAWT was carried out at the project laboratory to investigate the winglet positive effect on power coefficient. Blades are tested in six modes namely; clean surface with and without winglet, rough surface with 1-mm and 3-mm height models and rough surface with 1-mm and 3-mm height models provided with winglet.
Primary variables were measured at different air speeds and blade angles of attack, and performance parameters were deduced. A remarkable improvement in power coefficient was generally obtained.
A winglet effect, WE, was investigated to show the effect of winglet on blade performance. It was shown that mounting winglets at the tips of the blades have positive and negative variations in power coefficient depending on operating conditions.
A restoration factor, RF, was introduced to indicate how much of the blade-power lost due to increased surface roughness could be restored when attaching a suitable winglet. Although RF varies greatly with operating conditions, with values ranging between zero% to 100%, its average positive effect was obvious. The favorable effect of the winglet extended to include improving the performance of blades even at the clean mode (designed). A power enhancement factor, PEF, was thus introduced and estimated. Calculated results showed an increase in power coefficient exceeding 20% at test conditions.