Wind Energy

Overview

Wind energy, or energy extracted from the wind, has long been used by humans in a variety of applications: in windmills, to move boats, to pump water, among other uses. However, although there are wind turbines to generate electricity that date back to the 1930s, research into the accelerated use of this technology as a means of generating electricity began in the 1970s, motivated by the substantial increase in oil prices that occurred at the time (Burton et al., 2001).

Currently, this type of energy is highly encouraged due to concerns about climate change. Wind energy has low CO2 emissions (restricted to manufacturing, installation and disposal processes), as well as applicability in several regions of the world, in addition to being, currently, quite competitive in financial terms.

The latest report from the International Renewable Energy Agency (IRENA, 2021) mentioned that the cost of wind energy continued to decline. Between 2010 and 2021, the cost of onshore wind energy fell by 68%, currently costing an average of US$ 0.033/kWh, while offshore wind energy currently costs an average of US$ 0.075/kWh. In comparison with other renewable energies, onshore wind energy currently has a lower cost per kWh than photovoltaic and hydroelectric energy, both with an average cost of US$ 0.048/kWh. Furthermore, investing in new wind farms is currently cheaper than investing in the less expensive fossil fuel option, as shown in Figure 1 (IRENA, 2021). The upward trend in fossil fuel prices has further favored the use of renewable energy sources. Offshore wind energy, on the other hand, has higher costs, especially due to the greater complexity of construction, and also of connecting the turbine to the electricity grid.

Figure 1: Cost of energy generated by Solar and Wind sources (IRENA, 2021)

The increase in the capacity and efficiency of wind turbines was mainly due to the increase in the diameter of the machines, in addition to improvements in performance and design optimization. In 2016, for example, commercial turbines with a capacity of 8 MW and 164 meters in diameter were already available (IRENA, 2016), and the largest wind turbine in the world, in the operational phase, has a rotor with a diameter of 222 meters. The Chinese company MingYang claims to be building a turbine with a rotor measuring 242 meters in diameter and a capacity of 16 MW of power, with the prototype expected to be installed in 2023. The manufacturer Vestas has a prototype measuring 236 meters in diameter and 15 MW of power.

At a national level, wind energy currently ranks as one of the most important sources of renewable generation. In 2021, Brazil had 21,161 MW (IRENA, 2022) of installed wind power capacity. For comparison purposes, Brazil has 109,426 MW of hydropower capacity and 13,055 MW of installed solar power capacity. The total renewable energy capacity in Brazil is 159,943 MW and therefore wind power represents approximately 13.2% of renewable energy production (IRENA, 2022). EPE provides more detailed data and, in the annual energy balance for 2022 (base year 2021), reported an increase in the share of wind power in relation to the total energy generated, to 10.6%, due to the water shortage that occurred during the year. In any case, wind power has considerable importance in the Brazilian energy matrix.

A disadvantage of wind power, compared to energy generated by fossil fuels, such as thermoelectric plants, is that it is an intermittent energy source that depends on variable wind conditions. In addition, energy production depends significantly on the potential of the region where the plants are installed. For this reason, in Brazil, the greatest potential for wind generation is concentrated in the Northeast region, with a presence also in the South region, as shown in Figure 2. Consequently, Brazilian wind power plants are mainly concentrated in these regions. The potential for offshore generation in Brazil is also notable, although it is still little explored. Offshore installation allows the use of larger rotors, since noise levels are less of a problem on the high seas.

Figure 2: Map of wind speeds in Brazil (Global Wind Atlas)

Technical Aspects

Essentially, the wind turbine is a mechanical device designed to extract kinetic energy present in the wind. The rotor blades of wind turbines have an aerodynamic profile, so that the incident flow will generate a lift force on the blades, which will keep the turbine turning. The lift force is the same responsible for, for example, keeping an airplane in the air when it is flying at high speed.

Therefore, the main performance parameter used to measure the efficiency of a wind turbine is the power coefficient, which relates the energy available in the wind that passes through the turbine rotor area, and the aerodynamic power extracted by the turbine. The theoretical maximum value of this coefficient is approximately 0.59 (derived from fluid mechanics relations), and the closer a wind turbine can get to this limit, the greater its efficiency.

Another important technical aspect in the operation of a wind turbine is the blade tip speed ratio, known as TSR (Tip Speed Ratio), and often represented by the Greek letter λ. This dimensionless parameter is a measure of the speed of the blade tip of the turbine, relative to the speed of the incident wind. The blade tip speed, in turn, depends on the design rotation speed and the diameter of the turbine. This parameter is essential to define the type of turbine to be installed in a given location, according to the average wind speeds at that location. Figure 3 shows some performance ranges for different types of wind turbines:

Figure 3: Typical wind turbine performance ranges (Alabdali et al., 2020)

As can be seen, the well-known horizontal axis wind turbines have the highest value for the power coefficient among the technologies presented in the figure and, therefore, are widely used today. Horizontal axis turbines are particularly efficient in regions with winds considered more stable, and with greater incidence, and currently, almost all large wind farms in the world use horizontal axis turbines.

However, for use in distributed generation mode, this type of turbine has some disadvantages, mainly regarding operation in conditions in which the wind speed is very unstable and difficult to predict, such as urban environments, full of obstacles. The noise level is also a negative point of horizontal axis wind turbines.

In this sense, vertical axis wind turbines, such as the Darrieus, based on the support; or Savonius, based on the drag force generated by the wind; they have greater adaptability to operate in more unstable wind conditions, although their efficiency level is lower, as shown in Figure 3. In addition, these turbines have lower noise levels and are easy to install, being a potential solution for use as a distributed generation source in urban environments.

Vertical axis wind turbines also require lower wind speeds to start operating, compared to horizontal axis turbines (Kumar et al., 2018). Although their use is not yet widespread, vertical axis wind turbines can be used in urban environments, as well as on roads with high vehicle flow. Currently, they are particularly used in urban environments in China (IRENA, 2016).

The challenging environment for the use of wind generation in urban environments is due to the greater difficulty in predicting wind conditions, in addition to the problem of lower efficiency presented by vertical axis wind turbines, or noise problems and low adaptability of horizontal axis wind turbines.

In conclusion, wind energy is quite relevant in the world, showing accelerated growth in recent years, a trend that has been repeated in Brazil. The increase in fossil fuel prices, combined with greater pressure to accelerate the energy transition, will make wind energy even more relevant on the global scene. Improvements in designs, combined with the reduction in costs of horizontal axis wind turbines, have made installations on land and at sea viable, where there is still unexplored potential and with a tendency to grow. Brazil also still has unexplored potential, although the electricity matrix is already mostly renewable. Finally, the installation of small vertical axis wind turbines can be an ally in distributed generation, contributing to the energy transition.

References

Alabdali, Q. A., Bajawi, A. M., Fatani, A. M., & Nahhas, A. M. (2020). Review of Recent Advances of Wind Energy. Sustainable Energy, 8(1), 12–19. Portuguese: https://doi.org/10.12691/rse-8-1-3

Burton, T., Sharpe, D., Jenkins, N., & Bossanyi, E. (2001). Wind Energy Handbook.

IRENA. (2016). Wind Energy Technology Brief.

IRENA. (2021). Renewable Energy Generation Costs in 2021.

IRENA. (2022). Renewable Capacity Statistics 2022.

Kumar, R., Raahemifar, K., & Fung, A. S. (2018). A critical review of vertical axis wind turbines for urban applications. In Renewable and Sustainable Energy Reviews (Vol. 89, pp. 281–291). Elsevier Ltda. https://doi.org/10.1016/j.rser.2018.03.033