💨Airborne Wind Energy Systems Unit 2 – Wind Energy Basics and Boundary Layer

Wind Energy Fundamentals

• Wind energy harnesses the kinetic energy of moving air to generate electricity
• Wind is caused by uneven heating of the Earth's surface, creating pressure differences and air movement
• Wind power is a renewable energy source that does not produce greenhouse gases or pollutants during operation
• The amount of energy available in the wind depends on factors such as wind speed, air density, and swept area of the turbine blades
• Wind energy has been used for centuries for tasks such as pumping water and grinding grain (windmills)
• Modern wind turbines convert wind energy into electrical energy using a generator
  • The generator is typically located in the nacelle at the top of the turbine tower
  • The generator is connected to the turbine blades through a gearbox that increases the rotational speed
• Wind energy is one of the fastest-growing renewable energy sources worldwide, with installed capacity increasing rapidly in recent years

Atmospheric Boundary Layer Basics

• The atmospheric boundary layer (ABL) is the lowest part of the atmosphere, directly influenced by the Earth's surface
• The ABL is characterized by turbulence, which is the chaotic and irregular motion of air
• The height of the ABL varies depending on factors such as surface roughness, temperature, and wind speed
  • Over land, the ABL can extend up to 1-2 km in height during the day and a few hundred meters at night
  • Over water, the ABL is typically thinner due to the smoother surface
• Wind speed and direction in the ABL are influenced by surface friction, which causes a wind shear profile
  • Wind speed generally increases with height above the surface, following a logarithmic profile
  • The rate of increase in wind speed with height is called the wind shear
• Turbulence in the ABL is generated by wind shear and thermal effects (convection)
  • Mechanical turbulence is caused by wind shear and surface roughness
  • Thermal turbulence is caused by heating and cooling of the surface, leading to convective mixing
• Understanding the characteristics of the ABL is crucial for wind energy applications, as it determines the available wind resource and affects the performance of wind turbines

Wind Resource Assessment

• Wind resource assessment involves measuring and analyzing wind speed, direction, and other meteorological data to determine the feasibility and potential of a wind energy project
• On-site measurements are typically conducted using meteorological masts (met masts) equipped with anemometers, wind vanes, and other sensors
  • Measurements are usually taken at multiple heights to characterize the wind shear profile
  • Data is collected for at least one year to capture seasonal variations in wind patterns
• Remote sensing techniques, such as LiDAR (Light Detection and Ranging) and SoDAR (Sonic Detection and Ranging), can also be used to measure wind speed and direction at various heights
• Numerical weather prediction (NWP) models and reanalysis datasets provide long-term wind data and can be used to supplement on-site measurements
• Wind data is analyzed using statistical methods to determine key parameters such as average wind speed, wind rose (directional distribution), and Weibull distribution parameters
• The wind resource is typically characterized by the annual mean wind speed and the wind power density (available power per unit area)
• Wind resource maps are created to visualize the spatial distribution of wind speed and power density over a region
• Accurate wind resource assessment is essential for selecting suitable sites for wind energy projects and estimating their energy production potential

Types of Wind Turbines

• Wind turbines are classified based on their axis of rotation: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs)
• HAWTs are the most common type of wind turbine used for utility-scale power generation
  • HAWTs have blades that rotate around a horizontal axis, parallel to the ground
  • The blades are typically three in number and are attached to a hub, which is connected to the generator through a shaft and gearbox
  • HAWTs require a yaw mechanism to orient the rotor towards the wind direction
• VAWTs have blades that rotate around a vertical axis, perpendicular to the ground
  • VAWTs can be further classified into Darrieus (egg-beater shaped) and Savonius (S-shaped) designs
  • VAWTs are omnidirectional and do not require a yaw mechanism to face the wind
  • VAWTs are less common than HAWTs and are typically used for smaller-scale applications or in urban environments
• Wind turbines can also be classified based on their size and power output
  • Small wind turbines (up to 100 kW) are used for residential, agricultural, or remote power applications
  • Medium-sized turbines (100 kW to 1 MW) are used for community-scale projects or distributed generation
  • Large wind turbines (1 MW and above) are used for utility-scale power generation in wind farms
• Offshore wind turbines are designed for installation in marine environments, with larger rotor diameters and higher power ratings compared to onshore turbines
• Airborne wind energy systems (AWES) are an emerging technology that uses tethered flying devices (kites, gliders, or drones) to harvest wind energy at higher altitudes

Aerodynamics of Wind Energy Systems

• The aerodynamics of wind turbines govern the conversion of wind energy into mechanical energy through the interaction of the blades with the air flow
• Wind turbine blades are designed as airfoils, which create a pressure difference between the upper and lower surfaces when exposed to a moving air stream
  • The pressure difference results in a lift force perpendicular to the air flow direction
  • The lift force causes the blades to rotate around the hub, driving the generator to produce electricity
• The amount of power that can be extracted from the wind depends on the swept area of the blades ($A$), the air density ($\rho$), and the cube of the wind speed ($v^3$), as given by the equation: $P = \frac{1}{2} \rho A v^3$
• The efficiency of a wind turbine in extracting energy from the wind is described by the power coefficient ($C_p$), which is the ratio of the actual power output to the theoretical maximum (Betz limit)
  • The Betz limit states that a maximum of 59.3% of the wind's kinetic energy can be converted into mechanical energy by an ideal wind turbine
  • In practice, wind turbines achieve power coefficients of 0.35-0.45 due to various losses and design limitations
• The tip speed ratio (TSR) is the ratio of the blade tip speed to the wind speed and is an important parameter in wind turbine design
  • High TSRs (6-8) are typically used for three-bladed HAWTs to maximize aerodynamic efficiency
  • Lower TSRs are used for VAWTs and turbines with a higher number of blades
• Wind turbine blades are twisted and tapered along their length to optimize the angle of attack (the angle between the blade chord and the relative wind direction) and maintain a constant TSR
• Blade pitch control is used to adjust the angle of attack in response to changing wind conditions, maximizing power output and limiting loads on the turbine structure

Power Generation and Conversion

• Wind turbines convert the mechanical energy of the rotating blades into electrical energy using a generator
• Most modern wind turbines use a doubly-fed induction generator (DFIG) or a permanent magnet synchronous generator (PMSG)
  • DFIGs have a wound rotor connected to the grid through a partial-scale power converter, allowing variable-speed operation and control of reactive power
  • PMSGs have a permanent magnet rotor and are connected to the grid through a full-scale power converter, providing a wider range of variable-speed operation
• The generator is coupled to the turbine rotor through a gearbox, which increases the rotational speed to match the generator requirements
  • Some turbines use direct-drive generators, which eliminate the need for a gearbox by using a low-speed, high-torque generator
• The power output of a wind turbine varies with wind speed, following a power curve
  • The cut-in speed is the minimum wind speed at which the turbine starts generating power (typically 3-4 m/s)
  • The rated speed is the wind speed at which the turbine reaches its rated power output (typically 12-15 m/s)
  • The cut-out speed is the maximum wind speed at which the turbine shuts down to prevent damage (typically 25-30 m/s)
• Wind turbines are equipped with power electronics and control systems to regulate the power output, maintain grid stability, and protect the components
  • The power converter converts the variable-frequency AC power from the generator into grid-compatible AC power
  • The controller monitors and adjusts various parameters such as blade pitch, generator torque, and power output based on wind conditions and grid requirements
• Wind farms consist of multiple wind turbines connected to a common electrical infrastructure, including transformers, switchgear, and transmission lines
  • The power output of individual turbines is aggregated and delivered to the grid as a single source
  • Wind farm control systems optimize the overall performance and minimize wake effects between turbines

Environmental and Social Impacts

• Wind energy has several environmental benefits compared to fossil fuel-based power generation, including reduced greenhouse gas emissions, air pollution, and water consumption
• However, wind energy development can also have negative environmental impacts, such as:
  • Noise pollution from turbine operation, which can affect nearby residents and wildlife
  • Visual impact on landscapes, particularly in scenic or culturally significant areas
  • Collision risk for birds and bats, which can be mitigated through careful siting and monitoring
  • Habitat fragmentation and disturbance, especially during the construction phase
• Wind energy projects can have social and economic impacts on local communities
  • Positive impacts include job creation, income for landowners hosting turbines, and increased tax revenue for local governments
  • Negative impacts may include the displacement of traditional land uses, the impact on property values, and the distribution of benefits and costs among community members
• Public acceptance of wind energy projects is crucial for their successful development and operation
  • Engaging local stakeholders, addressing concerns, and ensuring transparent decision-making processes can help build public support
  • Benefit-sharing mechanisms, such as community ownership or local content requirements, can promote a more equitable distribution of the benefits of wind energy projects
• Environmental and social impact assessments (ESIAs) are conducted to identify, assess, and mitigate the potential impacts of wind energy projects
  • ESIAs typically involve baseline studies, impact prediction, mitigation planning, and public consultation
  • Best practices and guidelines have been developed by industry associations and international organizations to ensure responsible and sustainable wind energy development

Future Trends in Wind Energy

• The wind energy industry is expected to continue its rapid growth in the coming decades, driven by the need to decarbonize the energy sector and meet global climate targets
• Offshore wind is a key growth area, with increasing investments in larger turbines, floating foundations, and grid infrastructure
  • Offshore wind resources are generally stronger and more consistent than onshore, allowing for higher capacity factors
  • Floating wind turbines can access deeper waters and expand the potential for offshore wind development
• Airborne wind energy systems (AWES) are an emerging technology that could unlock high-altitude wind resources and reduce the material and land use requirements of conventional wind turbines
  • AWES use tethered flying devices (kites, gliders, or drones) to harvest wind energy and transmit it to the ground through a cable
  • Challenges for AWES include the development of reliable control systems, the optimization of power generation and transmission, and the integration with the electrical grid
• Digitalization and advanced analytics are transforming the wind energy industry, enabling improved performance, reliability, and cost-effectiveness
  • Sensors, data acquisition systems, and IoT platforms allow for real-time monitoring and optimization of wind turbines and farms
  • Machine learning and artificial intelligence techniques can be used for predictive maintenance, power forecasting, and control optimization
• Grid integration and energy storage are becoming increasingly important as the share of wind energy in the power mix grows
  • Advanced power electronics and control systems are being developed to improve the grid compatibility and ancillary services provided by wind turbines
  • Energy storage technologies, such as batteries, pumped hydro, and hydrogen, can help balance the variability of wind power and provide flexibility to the grid
• Recycling and end-of-life management of wind turbine components, particularly blades, is an emerging challenge and opportunity for the industry
  • Developing circular economy solutions, such as blade recycling and repurposing, can reduce the environmental impact and improve the sustainability of wind energy
• Social acceptance and community engagement will remain critical factors for the successful deployment of wind energy projects
  • Innovative approaches, such as community ownership, participatory planning, and benefit-sharing, can help align wind energy development with local priorities and values