Reducing the cost of energy from Offshore Wind Turbines: An experimental study of very thick flatback airfoils

March 11, 2020
"Distant Wind Farm" by Lee-Anne Inglis

The use of Offshore Wind Turbines as a clean and renewable source of energy is well known. The ability to harness the power of wind that is produced out at sea means there is an unlimited and non-polluting source that has the additional benefits of being able to support much larger unit capacities and sizes (than onshore wind farms), and with lower visual and noise impact.

A key design feature of the turbines are the turbine blades. Like airplane wings, wind turbine blades consist by a series of airfoil profiles. However, wind turbine blades have specific requirements. As wind turbines grow larger (see Figure 1) to reduce the levelized cost of energy, their blades grow more slender and require significantly thicker airfoils. A high lift-to-drag ratio is essential in designing an efficient turbine blade, as is flow control.

A recently awarded project, led by Dr Marinos Manolesos will use state-of-the-art measurement techniques to examine a very thick airfoil and the possibility to dynamically control its performance.

Using the £1.2million wind tunnel facility, Dr Manolesos, aims to reduce the cost of energy from Offshore Wind Turbines through novel design and innovation. The Swansea University wind tunnel is specifically designed to test airfoil models under static and dynamic conditions, using force, pressure and velocity measurements.

Airfoils such as those used on larger turbines have reduced maximum lift and are very prone to flow separation. In addition to this reduced performance, this also reduces turbine life span. One solution to the challenges caused by the use of very thick traditional airfoils is the use of flatback airfoils.

It is known that flatback profiles have blunt trailing edge (TE) and provide higher lift and reduced sensitivity to tripping. However, there is limited research on this area, especially with regards to (a) how blunt the TE can be before it is actually detrimental and (b) the possibility for dynamic flow control.

Figure 1: Expected Growth in Offshore Turbine Size Globally. Ryan Wiser et al, Reducing Wind Energy Costs through Increased Turbine Size: Is the Sky the Limit? Berkeley Lab study shows significant potential for further turbine scaling, Nov 2016

Objectives

The objectives of this project are:

  1. To investigate the aerodynamic performance of an airfoil with very thick TE (20% chord) at high Reynolds number (Re ≈ 2M)
  2. To investigate the unsteady wake characteristics of the same airfoil with the use of combined unsteady pressure and velocity measurements using microphones and hot wires
  3. To examine the possibility of dynamic control of airfoil performance

Working With Vestas

Working with Vestas, the world’s largest wind turbine manufacturer, Dr Manolesos, will conduct the world’s first wind tunnel test of a flatback airfoil with a TE thicker than 17.5%.

Initially, a 36% thick traditional airfoil model (provided by Vestas Technology UK) will be tested and the results will be compared against proprietary data (also provided by Vestas) from different Wind Tunnels. Then a modified airfoil with 20% thick trailing edge will be tested (see Figure 2). The wing will be equipped with state-of-the-art microphone sensors (Knowles FG-23629-P16) both along the chord of the profile (to study transition) and along the span of the TE (to study the wake frequencies and its 3Dimensionality).

Following the flatback tests, a number of dynamic control tests will follow, using a suitable flap device (see Figure 2). This will be the first time the flap device is tested (a) on an airfoil with a TE of this thickness and (b) in pitching conditions.

Figure 2: A traditional 36% thick airfoil (dark blue solid line) will be transformed to an unconventional airfoil with 20% thick TE by using two wedges (green dashed lines). A pitching flap (solid gold line) will be tested to examine the dynamic variations of the loads on the airfoil. Microphones are indicated with black circles.

Findings

This research allows for the possibility and suitability of the innovative flap device for dynamic control to be tested for the first time. Initial findings will be presented at the Wind Energy Science Conference 2021 (WESC) from the European Academy of Wind Energy (EAWE) on 25 – 28 May 2021 at Leibniz University Hannover.

For more information visit Dr Marinos Manolesos or email: Marinos.Manolesos@Swansea.ac.uk

Teeside Offshore Wind Farm by aul Howzey
“Teeside Offshore Wind Farm” by Paul Howzey

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