Theoretical Analysis of Shrouded Horizontal Axis Wind Turbines

Abstract

Numerous analytical studies for power augmentation systems can be found in the literature with the goal to improve the performance of wind turbines by increasing the energy density of the air at the rotor. All methods to date are only concerned with the effects of a diffuser as the power augmentation, and this work extends the semi-empirical shrouded wind turbine model introduced first by Foreman to incorporate a converging-diverging nozzle into the system. The analysis is based on assumptions and approximations of the conservation laws to calculate optimal power coefficients and power extraction, as well as augmentation ratios. It is revealed that the power enhancement is proportional to the mass stream rise produced by the nozzle diffuser-augmented wind turbine (NDAWT). Such mass flow rise can only be accomplished through two essential principles: the increase in the area ratios and/or by reducing the negative back pressure at the exit. The thrust coefficient for optimal power production of a conventional bare wind turbine is known to be 8/9, whereas the theoretical analysis of the NDAWT predicts an ideal thrust coefficient either lower or higher than 8/9 depending on the back pressure coefficient at which the shrouded turbine operates. Computed performance expectations demonstrate a good agreement with numerical and experimental results, and it is demonstrated that much larger power coefficients than for traditional wind turbines are achievable. Lastly, the developed model is very well suited for the preliminary design of a shrouded wind turbine where typically many trade-off studies need to be conducted inexpensively.

Characterization of aerodynamic performance of ducted wind turbines: A numerical study

“…A significant amount of literature on DWT is based on the combined use of theoretical, numerical, and experimental techniques . Due to its simplicity and the fact that the results can be obtained analytically, it clearly appears that the classical axial momentum theory (AMT) still remains popular . The main limitation of the AMT approach is that the detailed interaction between rotor blade and duct surface is neglected (see Figure ).…”

“…2-7 Due to its simplicity and the fact that the results can be obtained analytically, it clearly appears that the classical axial momentum theory (AMT) still remains popular. 8,9 The main limitation of the AMT approach is that the detailed interaction between rotor blade and duct surface is neglected (see Figure 1). As a consequence, the relation between the thrust force exerted on the duct and the rotor is linear.…”

Wind Micro-Turbine Networks for Urban Areas: Optimal Design and Power Scalability of Permanent Magnet Generators

“…Different one-dimensional (1D) theoretical models have been proposed in the literature in order to predict the power production of wind turbines, with particular reference to shrouded or diffuser-augmented ones [4,[24][25][26][27]. The turbine is seen as an actuator disc and the flow over the disk is supposed to be ideal (i.e., without a rotational velocity component and frictionless).…”

“…The aforementioned power coefficient expresses the relation between the obtainable mechanical power and the power contained in the wind passing a reference area (conventionally the rotor swept area) with a given wind velocity [4]. Ducted/Shrouded Horizontal Axis Wind Turbines (S-HAWT) or Diffuser-Augmented Wind Turbines (DAWT) allow for enhancing the power output with respect to conventional or bare wind turbines, for a given rotor size [24][25][26][27]. In fact, the presence of the diffuser can provide an increase in the mass flow rate through the turbine blades, which produces a significant augmentation of the power coefficient C P referred to the rotor swept area [26].…”

“…Ducted/Shrouded Horizontal Axis Wind Turbines (S-HAWT) or Diffuser-Augmented Wind Turbines (DAWT) allow for enhancing the power output with respect to conventional or bare wind turbines, for a given rotor size [24][25][26][27]. In fact, the presence of the diffuser can provide an increase in the mass flow rate through the turbine blades, which produces a significant augmentation of the power coefficient C P referred to the rotor swept area [26]. The aforementioned power coefficient expresses the relation between the obtainable mechanical power and the power contained in the wind passing a reference area (conventionally the rotor swept area) with a given wind velocity [4].…”