# Wind Turbines
Harvesting wind energy is one of the oldest forms of energy production in human history. Even without an electric generator it can be used to drive pumps or mills directly. This chapter will explain the core principles of wind energy and highlight the advantages as well as the disadvantages.
# Turbine Types
The two main types of turbines are distinct by their working principles. Drag type turbines use the fact that certain bodies or geometries have different drag when moving against the wind and are comonly reffered to as "Savonius turbines". Lift type turbines use the lift effect on airfoils in moving liquids, much like an airplane.
According to Betz's Law, the maximum amount of energy that can be extracted from the wind is around
# Drag Runner
Drag type turbines are always Vertikal Axis Wind Turbines (VAWT), where the rotational axis is vertical to the wind.
The mathematical equation to describe the force on the scoops is given by
A Savonius rotor's rotation speed can not exceed the wind velocity, so the Tip Speed Ratio (TSR) defined as
# Lift Runner
Lift running turbines use airfoils as blade geometry and are mostly divided into VAWT and Horizontal Axis Wind Turbines (HAWT). There are other hybrids and experimental types of turbines, but those are not commonly used. Most lift types have a much higher TSR (
Horizontal turbines are most commonly seen in commercial wind energy production due to there high power efficiency (
|High Power coefficient||Yaw system to follow wind needed|
|No pulsating torque on drive train||High noise generation due to high tip speed|
|Well understood and developed||Complex blade design and production|
Vertical axis wind turbines have a power coefficient of
|No yaw mechanism needed||Medium Power coefficient|
|Low noise profile||High normal forces on axis|
|Simple blade geometry||Oscillating torque on generator|
Electric generators can be classified into radial flux machines and axial flux machines. The main difference is the flow direction of magnetic flux, where it is either parallel or radial to the rotaional axis. The common synchronous and asynchronous (induction) machines belong to the radial flux machines and are mainly used in large turbines. One of the main challenges for larger wind turbines is the connection to the grid. While DC-Sources can be connected to the grid when their voltage is equal to the grid voltage, AC-Sources have to be in synch with the grid's voltage frequency and amplitude . For small scale, especially off-grid turbines, axial flux generators are more common as they can be fairly easily constructed without high level machinery, especially for low-rpm usecases.
The distinctive property of induction generators is the slip between rotor and stator fields. In generator operation, the turbine moves the rotor above the synchronous speed. Because of the two magnetic fields running against each other, this type of generator can introduce high reactive power loads in the system.
The synchronous speed
The synchronous speed for a four pole generator on grid frequenzy of
# Field Coil Excitation
Synchronous generators can be build with a much higher pole count than asynchronous generators and are used for gearless setups which reduces the maintance requirements significantly. If they are directly coupled to the grid, they need to be synchronized first and cannot run with different speeds. In fact, when the force on the turbine increases and the generator comes out of sync with the grid, the system can be damage due to balancing currents. Most turbines have a rectifier and a power inverter to connect to the grid.
# Permanent Magnet Excitation
Exitation with permanent magnets reduces the complexity and therefor maintance. For this reason, PM generators are used mostly on offshore wind farms. There is also no energy needed for exitation so they have slightly better efficieny. Considering the large amount of rare earths needed for permanent magnets though, this effect is negligible .
The axial flux or axial gap generators are a special type of synchronous PM generators not widely found in larger applications. It can however be found frequently for small scale DIY turbines. Their flat structure and easy-to-wind coils makes them very suited for small workshops. They provide a fairly good efficiency and common materials such as wood, steel and epoxy can be used. Since the rotor becomes wider with increasing pole count, rotational inertia and centrifugal forces are increased as well, though higher pole count mostly means lower rpm needed.
Depending on the generator used and to what load (machine, grid, battery, ...) the sytem will be connected, different electronic components are necessary. Since this OER is aimed at makers and for off-grid (or local tiny-grid) use cases, high power AC/DC-DC/AC setups, frequency synchronizing elements for direct coupled wind turbines or storage units like pumped hydroelectric energy storage are not discussed. Most common standalone systems consist of a source, a sink and a storage unit.
As a storage unit, mostly batteries are used. While it is possible to connect a turbine directly to some types of batteries using a rectifier and a charge controller like Hugh Piggott suggested, it has some serious drawbacks.
Whenever there is an increase in voltage or the battery is full and no load is connected, current will be diverted to the dump load and dissipated. If the voltage of the turbine is much higher than the battery's, it's not suitable to charge the battery. The same happens when the turbine voltage is below charging voltage. To mitigate these flaws, a rectifier and a DC/DC-Converter can be used to either step-down or step-up the voltage needed to efficiently charge the battery. This is mostly done inside a charge controller which can take several parameters into account like optimal current at given generator voltage. Modern batteries like Lithium-Ion require a proper Battery Management System to work safely.
The disadvantage of this design is that all components need to be able to take the full maximum power taken by the load or given by the turbine.
One of the most famous DIY-projects is Hugh Piggott's (opens new window) 2F-Windturbine. His books (opens new window) provide many details as well as basic principles on how to design, build and run small off-grid turbines.
Another seriously low-tech wind turbine is the design provided as open source by Daniel Connel (opens new window). It is a VAWT as a combination of lift and drag type and can be constructed for under 100€ in material cost.
 !Original: RottweilerVector: Cmglee, CC BY-SA 3.0, Link (opens new window)
 By Vector by רונאלדיניו המלך, Original by J Doug McLean, CC BY-SA 4.0, Link (opens new window)
 E. Hau, Windkraftanlagen: Grundlagen, Technik, Einsatz, Wirtschaftlichkeit. Springer-Verlag Berlin Heidelberg, 2008
 S. Heier, Windkraftanlagen: Systemauslegung, Netzintegration und Regelung. 6. Auflage, Springer Vieweg Verlag Wiesbaden, 2018, DOI: https://doi.org/10.1007/978-3-8348-2104-1
 By Funkjoker23 - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=17634829
 Wikipedia contributors. (2020, October 19). Rare-earth element. In Wikipedia, The Free Encyclopedia. Retrieved 07:59, November 6, 2020, Linkhttps://en.wikipedia.org/w/index.php?title=Rare-earth_element&oldid=984305200)