Transitioning from traditional to sustainable clean energy sources remains a major criterion for reaching the goals of the Paris Climate Agreement and successfully combatting global warming. From the myriads of renewable energy sources available today, wind energy remains one of the most promising ones. This is partly due to the high efficiency of harnessing and converting it to electrical energy while offering competitive market prices, compared to traditional energy sources. At the end of 2017, the installed wind capacity exceeded 539 GW worldwide with about 10% of the installation happening in 2017 alone. This is expected to reach an estimated 840 GW by 2022. Interestingly, as the installations keep rolling in, the price continues to drop to around $0,03/kWh in markets like Mexico, Canada, Morocco  making it more affordable for more people.
To increase renewable energy efficiency, many stakeholders are looking at improving the energy output generated by already installed and future technologies. Recently, Greenfish was commissioned by a large energy service provider to perform a study on vortex generators (VGs). This paper highlights the overall impact of VGs on typical wind farms from a technical point of view. First, we begin with an introduction on the need for VGs and the working principle, then we delve into some factors to consider in terms of maximizing VG efficiency. Subsequently, the estimated impact on Annual Energy Production (AEP), the environment and the turbines are analysed before we offer our conclusion. Find the results of our study below this video.
Blowing in the wind: What are vortex generators?
Wind turbine manufacturers are aiming to produce turbines that can generate maximum energy at minimum cost. However, a compromise is usually reached between aerodynamic and structural efficiency of the turbine blades, which influences the choice of materials and the manufacturing process that determines its thickness. Typically, to counter the high bending stress close to the base (root), the blade tends to be thicker than its aerodynamic optimum in this region. This results in a “stall” which reduces the blade efficiency.
Hence, just like the use of special coating on solar panels to reduce energy loss by reflection ,, various techniques are being employed to improve power generation in wind turbines. Some examples are pitch control, yaw optimization and more recently, vortex generators (VGs) have been used to improve the efficiency of wind turbines.
VGs are passive fin-like devices mounted on a turbine blade to change its surface airflow by creating small vortices which regulate boundary-layer airflow. This is accomplished when the blade is in motion in relation to the air. The VG generates small vortices by drawing in high energy air from the free stream into the boundary layer, thereby increasing the boundary layer’s energy. This results in high energy air sticking to the surface of the blade and increasing the attached flow for longer periods – hence, increasing the lift and ultimately, the AEP ,.
Figure 1: Typical installation section of VGs and their working principle 
Since the early 2000s, several studies have been undertaken to determine the VG’s optimal characteristics: blade placement location, materials, shape and size of VGs play a critical role in the performance of these devices. Although VGs are generally situated along the blade’s root, most studies show that an extensive computational fluid dynamics analysis is needed to evaluate the optimal placement location based on the blade profile . Some materials like epoxy-glass composites and thermoplastics possessing varying properties (strength, corrosion-free, stability) have been shown to be effective in weathering intense physical and chemical atmospheric conditions . The adhesives (Pliobond, acrylic foam tapes) that attach the VGs to the blade are also important as they need to ensure firm bonding in extreme climates.
Our study shows that the impact of the VGs on AEP is highly dependent on three major factors:
- Site Location: The power captured by the wind turbine is mainly dependent on the cube of the wind speed. This makes it mandatory that the atmospheric conditions onsite are measured accurately to evaluate the potentials of the vortex generators. Typically, most VGs can improve energy production of low-medium wind speeds between 3 m/s and 10 m/s. Hence, estimating the frequency of occurrence of this wind speed range and direction is critical in evaluating the average number of hours the VGs will be effectively utilised if deployed on a wind turbine.
- VG size: the dimensions of the VG have a pronounced effect on the amount of lift that can be generated. An increase in the trailing edge height results in improved lift but also creates a drag penalty which leads to a reduction in lift-drag ratio. On the other hand, extending the VG length leads to negative influences on both lift and drag coefficients while the correct increase in intra and inter spacings between adjacent VG pairs reduces the flow separation, thereby increasing the lift coefficient.
- Measurement Techniques: Prior to the recent methodology in calculating the AEP gains due to VGs, the inability to reproduce the exact site wind conditions on the same turbine prior to the VG installation, made it impossible to guarantee an AEP increase, if any. However, in using a combination of techniques – power density normalization, and proper filtering methods to eliminate other variation sources (uneven terrains, wake conditions) that affect power output, a cleaned data set is obtained. After completing the pre-processing steps, the 100-kW bin-wise power difference between the test and control turbines is computed. This is evaluated using site data PRE- & POST VG installation. The overall power difference created over the complete output power spectrum as shown in Figure 2. has made it possible to quantify how advantageous the VGs can be .
Figure 2: Graph showing bin-wise output power variation PRE- & POST VG installation 
Winds of change: Overall impact of VG technology:
The consensus in the wind energy industry is that VGs can increase a turbine’s AEP by up to 2.0% based on appropriate measurements and computations of the generated power pre- and post VG installation . To this end, using the same analytical method, the results from our study estimated a 1.71% improvement in AEP across 23+ wind farms in very different geographical locations using at least 3 years’ worth of hourly wind data which clearly corroborates the industry’s estimates.
Noise emission measurements conducted by TU Berlin additionally show that, for a typical wind turbine blade airfoil, the sound pressure level at high angles of attack measured by high fidelity microphones is considerably lower when VGs are employed on wind turbines . Because flow separation is a major source of noise in wind turbines, VGs help to delay stall, resulting in lower noise emissions close to the root region . This highlights a major advantage of VGs in reducing the environmental noise impact around the vicinity of wind farms.
Furthermore, extensive research performed by the CORE-Team & SmartBlade showed that VGs have an insignificant impact on the blades because the maximum induced VG weight is much less compared to blades’ design load limits. This was achieved after numerous simulations were carried out in accordance to IEC 61400-1 standards .
Seeds in the wind: VGs, the way forward?
For many years, there has been continuous monitoring of the Energy Return on Investment (ERoI) for conventional fossil fuel sources. The EROI is a metric used to compare very different fuels by estimating how much energy is required to make a fuel usable. It is simply expressed as:
The decline in the EROI of conventional energy sources (coal, conventional oil) indicates dwindling availability of cheap high-quality fossil fuels, thereby forcing us to switch to costlier sources for energy production. This explains the rapid rate of adoption of renewables in recent years. Currently, despite some drawbacks such as variability, wind energy appears to be the most promising renewable source due to the declining cost curves in extracting energy and its increasing ERoIs, making it competitive with the traditional sources. With that in mind, there has been a conscious effort to rapidly improve the development of efficient turbine technology. Hence, the implementation of solutions like optimised blade designs and pre-fabricated blade VGs by some turbine manufacturers (Vestas, Enercon, SiemensGamesa) have resulted in increased realisable energy produced from wind turbines with similar power ratings as older ones. From this perspective, VG can take on its full meaning and play a pivotal role in the energy transition.
Notwithstanding, there are still many existing wind farms that do not currently benefit from this technology mainly because the turbines were installed prior to industry-wide adoption of VGs. It is also important to note that aside from the benefits associated with VGs (increased AEP, noise reduction), there are some costs (purchasing, installation, downtime) incurred. Therefore, it is essential to perform in-depth financial analysis to evaluate parameters like revenue, RoI, and pay-back periods, that determine the profitability of such projects.
Hence, in a bid to accelerate energy transition and remain at the forefront of energy efficiency, it becomes paramount that simple yet relevant innovations like vortex generators should continue to be adopted, for with each passing minute, we are missing out on the opportunity to cater to our energy needs in a sustainable fashion.
Samuel Sopeju – Junior Consultant at Greenfish
Adrien Girard – Project Manager, Green Solutions at Greenfish
Quentin Lancrenon – Project Analyst, Green Solutions at Greenfish
 https://www.sciencedirect.com/science/article/pii/S0960148117306213: Quantifying the effect of vortex generator installation on wind power production: An academia-industry case study