In 2016, engineers put a flared shroud around a wind-turbine rotor; wind-tunnel tests lifted its power output by 70–75% |
Wind turbines are usually designed around one simple idea: put the blades where the wind is strongest and let the rotor extract as much energy as possible. But in 2016, engineers at Lodz University of Technology in Poland tested a different approach. Instead of leaving the rotor exposed, they surrounded it with a specially shaped diffuser ending in a flared rim.According to the study published in the International Journal of Numerical Methods for Heat & Fluid Flow, the diffuser-augmented wind turbine (DAWT) was tested inside a wind tunnel and produced up to 70–75% more power than the same rotor without the shroud under the tested conditions. The result did not mean that simply adding a large ring to any wind turbine would automatically deliver three-quarters more electricity. The experiment involved a small model under controlled wind-tunnel conditions, but it showed how carefully shaped airflow could potentially make compact turbines more productive, particularly in areas with relatively low wind speeds.The idea behind the unusual shroudThey aimed to increase the amount of air flowing through the turbine’s working area. To achieve that goal, the rotor was placed in a diffuser, which in essence formed a short aerodynamic duct surrounding the rotor. The duct widened toward the rear and ended in a lip, or flange. This was a defining feature of the design. As air moved through the diffuser, it created a low-pressure zone behind the rotor.According to a study published in Renewable Energy, researchers compared the performance of a bare rotor with the same rotor fitted with a flanged diffuser, measuring factors such as rotor speed, torque and blade loading. The experiments showed that the diffuser changed the airflow reaching the rotor and affected blade behaviour. The concept was not entirely new; Japanese researchers had previously developed similar “wind-lens” designs to increase airflow through a turbine. In tests of a 3 kW diffuser-augmented wind turbine, the researchers examined blade behaviour at wind speeds of 6.9 to 11.6 metres per second.What happened inside the wind tunnel?The test performed in 2016 took place in a small subsonic wind tunnel of the Institute of Turbomachinery at Lodz University of Technology. During testing, the rotor was examined in both shrouded and non-shrouded configurations, and the power coefficient was plotted against tip-speed ratio. This gain was attributed to the increased flow through the diffuser, and the measurements showed how the pressure/velocity field near the flaring diffuser affected it.Other research has since examined how the geometry of the flange affects this effect. A 2017 study in the Alexandria Engineering Journal used computational fluid dynamics to investigate different flange angles. The researchers found that vortices forming behind the flange could create a lower-pressure region downstream of the diffuser, helping draw more air through the device. In the simulations, changing the flange geometry also altered the airflow through the diffuser and rotor region compared with the 15-degree baseline design. The findings suggested that the shroud was doing more than simply protecting the rotor. Its shape could actively influence the surrounding airflow and increase the amount of air passing through the rotor.
The test performed in 2016 took place in a small subsonic wind tunnel of the Institute of Turbomachinery at Lodz University of Technology. Image Credits: Wikimedia Commons.
A promising idea with practical limitsIn addition, the 2016 paper explored the application of the concept in small wind turbines. At that time, a 3 kW diffuser-augmented wind turbine was under development in the institute for use in conditions of low wind speeds. This turbine could have been useful in areas where conventional turbines are less effective at extracting energy from low-speed winds. By creating a pressure difference around the shroud, the diffuser could boost airflow into the rotor. Under the right conditions, the small rotor could still generate power. However, this design has certain disadvantages. The shroud increases the weight of the turbine as well as its dimensions. It also experiences higher aerodynamic forces due to its flared shape.Real-world turbines must cope with shifting winds, turbulence and structural stresses which cannot be replicated in laboratory conditions. Nonetheless, the test highlighted an interesting opportunity. Rather than simply scaling up turbine blades or increasing their speed, engineers can shape the airflow around the rotor. The 2016 lab experiment showed that a flared shroud could significantly increase airflow through the turbine.
