Energy Generation Through Wind Power Systems
Because winds are primarily caused by uneven heating effects of the sun, wind energy is considered to be an indirect form of solar energy and is therefore renewable.
The primary cause of winds is the uneven heating of the earth's surface by the sun, which depends on latitude, time of day, and the distribution of land and large bodies of water, particularly the oceans. Another cause of winds is fluid friction between the atmosphere and the earth's surface, which allows the earth to drag the atmosphere around, producing turbulence. Horizontal components of wind velocities are normally much greater than the vertical velocity components.
The kinetic energy of the wind, and therefore the wind's power-generating potential, is proportional to the cube of wind velocity. Because winds are primarily caused by uneven heating effects of the sun, wind energy is considered to be an indirect form of solar energy and is therefore renewable.
Wind power is the use of airflow through turbines to provide energy to turn electric generators. A small wind turbine is a wind turbine that can be installed on properties as small as one acre in areas with sustained winds to create electricity. Small wind turbines typically have three propeller-like blades around a rotor connected to a shaft that spins a generator (see Figure 1). The two types of wind turbine systems are grid-connected wind turbine systems and off-grid (stand-alone) wind turbine systems.
Figure 1. Small wind turbines can be installed on properties that are one acre or larger. Image courtesy of Energy.gov
Grid-Connected Wind Turbine Systems
Although small wind turbines are typically off-grid systems, they can also be connected to a utility's electrical distribution system (grid). These are called grid-connected wind turbine systems. To work effectively, a small wind turbine that is connected to the grid requires an average annual wind speed of about 10 mph to 15 mph.
Grid-connected wind turbines are only allowed to operate when the utility grid is online. During power outages, the wind turbine is required to shut down due to safety concerns from islanding. Islanding is a condition in which a generator continues to power a location when electrical grid power is not present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered.
A grid-connected wind turbine project requires working with the utility to make the interconnection. Utilities have developed interconnection standards for the equipment and special meters that need to be installed at the service. Also, an electrical inspector must sign off on the system before the utility will allow connection to the grid. The inspector will require that all electrical work be completed by a licensed electrician.
Off-Grid (Stand-Alone) Wind Turbine Systems.
Small wind turbines that are not connected to the grid are called off-grid wind turbine systems, also known as stand-alone wind turbine systems. Off-grid wind systems can be installed to gain energy independence from the utility. However, a homeowner should be comfortable with uncertain power production due to fluctuations in wind speed. Off-grid wind turbine systems can be combined with solar PV systems to create a more reliable hybrid electric system. Wind and solar PV energy generation, along with battery storage, can offer enhanced improvements to an off-grid system.
Off-grid wind turbine systems are typically smaller and less expensive than grid-connected systems. Small wind turbines that are off-grid systems require annual maintenance. Annual maintenance usually requires that a person climb up the wind turbine tower. However, small wind turbines with tilt towers can be lowered to the ground for maintenance.
The kinetic energy of the wind is converted to electrical energy using a wind turbine. There are primarily two types of wind turbines, each being characterized by the orientation of the axis or shaft.
A horizontal axis wind turbine (HAWT) typically consists of a set of three blades mounted to a horizontal shaft that is connected to an electrical generator. This traditional "windmill"-style turbine is used in a variety of applications, from 5-MW wind farms to 100-kW residential applications.
A vertical axis wind turbine (VAWT) resembles an "eggbeater" and typically consists of three blades mounted to a vertical shaft. VAWTs are primarily used in small-scale applications and are less common than HAWTs. A vertical axis wind turbine is a design of small wind turbine that does not require exact wind orientation and can still operate in unfavorable wind conditions. Unlike a traditional wind turbine on a horizontal axis, a vertical axis wind turbine does not have to track the wind to produce electricity. Some vertical axis wind turbines can also have solar panels embedded in their housings, which increases the energy output while using the same square footage of space (see Figure 2).
Figure 2. A vertical axis wind turbine does not require exact wind orientation and can operate in unfavorable wind conditions. Some units have solar panels embedded on top of their housing.
Purchasing Wind Energy Systems
To purchase a wind energy system, it is important to know the necessary tower height, the power required from the turbine, the installation cost, and the cost to maintain the system. There may be grants or incentives available to defer some costs. A homeowner should also purchase wind insurance for liability and damage to equipment.
The average height of a small wind turbine is about 80′, which is about twice the height of a residential telephone pole. However, small wind turbines can range in height from 30′ to 140′. The output needed to power a dwelling can range from 2 kW to 10 kW. A large, grid-connected system can range from $10,000 to $70,000, while the purchase and installation of an off-grid small wind turbine (less than 1 kW) generally cost $4,000 to $9,000. The ROI for a small wind turbine ranges from 6 years to 30 years. The ROI is based on the energy use of the dwelling, average wind speeds, and the turbine's height above ground.
Less than 1% of all small wind turbines are used in urban applications due to zoning restrictions and poor wind quality in densely built environments. Wind resource information can be found through the National Renewable Energy Laboratory (NREL), local airport wind data, and state guidelines through the DOE's Office of Energy Efficiency and Renewable Energy. There are incentives for the purchase of wind turbines and for the sale of excess electricity. The Public Utility Regulatory Policies Act of 1978 (PURPA) is a federal regulation that requires utilities to connect with and purchase power from small wind energy systems.