Double Multiple Stream Tube Theory Research Articles

Development of a floating Vertical Axis Wind Turbine for the Mediterranean Sea

Differently from Horizontal Axis Wind Turbines (HAWTs), which are the reference technologies in the wind market for their reliability and maturity, Vertical Axis Wind Turbine (VAWT) applications are related to small-scale contexts, such as providing electricity in isolated areas or urban settings. Consequently, the capacity of VAWTs results significantly lower than the order of megawatts and does not exceed a few tens of kilowatts. A promising field of application for VAWTs is the floating offshore: among the main advantages there are an increased static stability, by placing the rotor-nacelle assembly (RNA) at the base of the VAWT and reduced operational and maintenance (O&M) costs. Moreover, the different wake dynamics allows to reduce the aerodynamic losses, allowing closer turbine installations. However, to be competitive with floating HAWTs, it is necessary to have numerical models for the analysis and simulation of multi-megawatt VAWTs. This paper aims to introduce a time domain model of a floating vertical axis wind turbine, developed within the Matlab-Simscape environment. The model comprises an aerodynamics module, based on Double Multiple Stream Tube theory while hydrodynamics is modelled using WEC-Sim. A case study, involving a Darrieus H-rotor VAWT tested in the Mediterranean Sea and supported by a semi-sub foundation, the OC4-DeepCwind, is introduced. The results obtained are compared with those from QBlade, an software developed by TU Berlin, demonstrating a good agreement between the two codes.

Double multiple stream tube theory coupled with dynamic stall and wake correction for aerodynamic investigation of vertical axis wind turbine

This study describes an effectively analytic methodology to investigate the aerodynamic performance of H vertical axis wind turbine (H-VAWT). An in-house code based on double multiple stream tube theory (DMST) coupled with dynamic stall and wake correction is implemented to estimate the power coefficient. Design optimization of airfoil shape is conducted to study the influences of the dynamic stall and turbulent wakes. Airfoil shape is universally investigated by using the Class/Shape function transformation method. The airfoil study shows that the upper curve tends to be less convex than the lower curve in order to extract more energy of the wind upstream and generate less drag of the blade downstream. The optimal results show that the power coefficient increases by 6.5% with the new airfoil shape.

An Analytical Methodology for Aerodynamic Analysis of Vertical Axis Wind Turbine

This study describes an effectively analytic methodology to investigate fully aerodynamic performance of H vertical axis wind turbine (H-VAWT). An in-house code based on double multiple streamtube theory (DMST) coupled with dynamic stall and wake correction is implemented to estimate power coefficient. A separated optimization of airfoil shape is first conducted to study the influences of dynamic stall and turbulent wakes. Airfoil shape is universally investigated by using Class/Shape function transformation method. The wind turbine solidity, tip speed ratio and pitch angle of the blade are then studied along with airfoil shape to optimize power coefficient. The airfoil study shows that upper curve tends to be less convex than lower cure in order to extract more energy of the wind in the upstream and generate less drag of the blade in the downstream. The optimal results show that the power coefficient increase by 6.5% with the new airfoil shape and 8.2% when considering solidity, pitch and airfoil together.

Performance analysis of the deflector integrated cross axis wind turbine

Wind turbines in the context of integration in an urban environment are considered as one of the most promising technologies for the efficient diffusion of renewable energy sources. However, situations such as low wind velocity, highly turbulent and skewed wind flow conditions are commonly encountered in this environment. This misaligned flow could affect the horizontal axis wind turbine performance. The vertical axis wind turbine (VAWT) is more suitable to operate under these unfavorable conditions. Nevertheless, the conventional VAWT has low efficiency and poor self-starting characteristics. Therefore, a novel design of deflector integrated cross axis wind turbine (CAWT) is proposed. Tests were carried out to harness wind energy from both the horizontal and vertical components of the skewed flow induced by the deflectors. A semi-empirical approach was also developed to analyse the performance of the CAWT. The aerodynamic behaviour of the CAWT was determined by combining the blade element momentum (BEM) model and double multiple streamtube theory (DMST). The approach showed notable agreement between the experimental and theoretical results (9.8%–14.2% difference). In conclusion, the findings from the experiment and the model showed that the concept has very good potential in the renewable energy industry.

Performance and dynamic characteristics of a multi stages vertical axis wind turbine

Vertical axis wind turbines (VAWTs) are used to convert wind energy to mechanical output or electricity. Vertical axis wind turbines are favorable at buildings as it can receive wind from any direction, have a design that can be integrated simply with building architecture and they have better response in turbulent wind flow which is common in urban areas. Using a calculation code based on the Double Multiple Stream Tube theory, symmetrical straight-bladed NACA0012 wind turbine performance was evaluated. The induction factor for both upwind and downwind zone is determined with the aid of a root-finding algorithm. This numerical analysis highlighted how turbine performance is strongly influenced by the wind speed (Reynolds number) and rotor solidity (turbine radius and blade chord length). Also a dimensional analysis is introduced and is to be considered in such a way to generalize the design for different turbine specifications. One of the qualities provided by dimensional analysis is that geometrically similar turbines will produce the same non-dimensional results. This allows one to make comparison between different sizes wind turbines in terms of power output and other related variables. One of the main problems affecting the turbine performance and dynamics is the torque ripple phenomena. So in this paper a turbine design configuration is introduced in order to decrease the turbine torque fluctuation. This design is carried out by constructing more similar turbine units (stages) on the vertical axis on top of each other with different orientation phase angles. The results showed that using even number of turbine assembly is better than odd number to avoid torque fluctuation and mechanical vibrations acting on the turbine. Also it is preferred to use four turbine stages as the eight stages will have no sensible effect on decreasing the torque fluctuation.

A new approach for optimization of Vertical Axis Wind Turbines

A new approach has been developed for the optimization of Vertical Axis Wind Turbines (VAWTs). The approach is derived from double multiple streamtube theory (DMST) and adopts the concept of “Representative streamtube” wherein the entire aerodynamic property of the VAWT is assumed to be represented by a single streamtube occupying a specific azimuthal location. Five input parameters namely; power, wind velocity, aspect ratio, air viscosity and air density and six output parameters that are minimally required for the construction of a straight bladed, constant pitch VAWT are considered in the study. The discrete data of lift and drag coefficients pertaining to specific values of Reynolds number and angle of attack are arranged in the order of decreasing lift to drag ratio and starting with the first set of coefficients, a check is made if it is eligible to become Representative streamtube for the particular problem. The check is done through a tri-directional “Demand Factor” test that seeks the compatibility of Reynolds number and angle of attack values of the aerofoil data under consideration. The first set which gains the eligibility to become “Representative streamtube” marks the solution for the particular problem.

Design and Experimental Analysis of an Exhaust Air Energy Recovery Wind Turbine Generator

A vertical axis wind turbine (VAWT) was positioned at the discharge outlet of a cooling tower electricity generator. To avoid a negative impact on the performance of the cooling tower and to optimize the turbine performance, the determination of the VAWT position in the discharge wind stream was conducted by experiment. The preferable VAWT position is where the higher wind velocity matches the positive torque area of the turbine rotation. With the proper matching among the VAWT configurations (blade number, airfoil type, operating tip-speed-ratio, etc.) and exhaust air profile, the turbine system was not only able to recover the wasted kinetic energy, it also reduced the fan motor power consumption by 4.5% and increased the cooling tower intake air flow-rate by 11%. The VAWT had a free running rotational speed of 479 rpm, power coefficient of 10.6%, and tip-speed-ratio of 1.88. The double multiple stream tube theory was used to explain the VAWT behavior in the non-uniform wind stream. For the actual size of a cooling tower with a 2.4 m outlet diameter and powered by a 7.5 kW fan motor, it was estimated that a system with two VAWTs (side-by-side) can generate 1 kW of power which is equivalent to 13% of energy recovery.