Diffuser augmentation of wind turbines

Abstract

The major deterrent to widespread use of windpower is economic. One of the more promising advanced concepts for reducing significantly the cost per output KW-hr is the Diffuser-Augmented Wind Turbine (DAWT). The diffuser controls the expansion of turbine exhaust flow, producing a highly sub-atmospheric pressure at the turbine exit. The low stat~c pressure induces greater mass flow through the turbine vs~~a conventional turbine design of the same diameter. Thus the output power and disk loading of the DAWT is much larger than for an unshrouded turbine. The freely expanding exhaust air downstream of the diffuser is reenergized by mixing with the externa.l wind flow.Our wind tunnel investigation of models of two diffuser design concepts is directed toward unconventional, very short, cost-effective configurations. One approach uses the energetic external wind to prevent separation of the diffuser's internal boundary layer. Another method used high lift airfoil contours for the diffuser wall shape.Diffuser model tests have indicated almost a doubling of wind power extraction capability for DAWTs compared to conventional turbines. Economic studies of DAWTs have used these test data and recent (1975) cost projections of wind turbines with diameter. The specific power costs ($/kW) for a realistic DAWT configuration are found to be lower than conventional wind turbines for very large size rotors, above 50 meters diameter, and for rotor diameters less than about 20 meters. The cost-to-benefit assessment for intermediate size rotors is affected by the uncertainty band of cost for these rotor sizes.

Theoretical Analysis of Shrouded Horizontal Axis Wind Turbines

“…The current work complements the analysis first introduced by Foreman [1] and backed by Lawn [7], but derives the results in a rather more general way. However, due to the complexity of the force on a diffuser, a closed theory cannot be established, and the analysis needs to be augmented with empirical data.…”

“…A duct structure typically provides a diffuser section behind the rotor that produces a power augmentation of considerable magnitude (typically 1.5-to two-fold) for a given size rotor, as well as the dampening of gusts, reducing performance sensitivity to wind directionality and raising the level of axial velocity significantly [1].…”

“…An experimental approach for ducted wind turbines was undertaken by Lilly and Rainbird [3] in the 1950s, and in the 1970s, a significant number of experiments were carried out by Foreman [1,4] from the Grumman Aerospace department. However, conclusions from these experiments varied significantly.…”

“…However, conclusions from these experiments varied significantly. The Lilly and Rainbird study concluded that no performance improvements were conceivable when the performance is normalized by the exit area of the diffuser, whereas Foreman [1] confirmed a power increase by a factor of four compared to the same rotor operating as a conventional or bare wind turbine. These research works performed not only experiments, but also developed theoretical models to verify their results.…”

Small-scale wind energy portable turbine (SWEPT)

“…The low pressure region draws more wind and causes it to accelerate. This is the reason why a ducted wind turbine with a diffuser exhibits higher power output than a wind turbine without diffuser [40].…”