Hybrid-Systems Containing Wind Energy

The term wind hybrid system describes any combination of wind energy with one or more additional sources of electricity generation (e.g. biomass, solar or a generator using fossil fuels). The combination of renewable energy technologies allows a more balanced electricity supply during day/night and seasonal changes: At most sites wind speed is low, when the sun is shining and reaches higher values on cloudy days. Thus the amount of energy generated by wind energy reaches its maximum in the winter months, while the output of PV-cells is significantly higher in the summer. Other important examples are Wind-Diesel systems often used in rural areas. A diesel generator will be used as backup, if the electricity demand can not be covered by the installed wind turbines. Regulation and conversion of the available energy sources is a central issue planning a wind hybrid system. Many hybrid systems are uses as stand-alone off-grid applications.

Description of a wind hybrid system

Wind hybrid-systems generally consist of generating units, storage facilities and system electronic devices:

• one or more wind converters of x₀ kW
• one or more other electricity generation options either either using renewable energy (e.g. Photovoltaic Panels) of fossil energy sources (e.g. diesel generator)
• an energy storage device
• an AC/DC rectifier of x_r kW in case the energy storage installation operates on DC current (e.g. all types of batteries)
• a charge controller of x_c kW
• a Uninterruptible Power Supply '(UPS) of x_p kW in order to guarantee high quality AC electricity generation
• a DC/AC inverter of x_p kW

Energy storage

Hybrid systems contain an energy storage device to store the surplus energy during times of high energy production, which can be used for supply when production from renewable sources is low (e.g. no wind). For this reason, the size of the device is often described by the period of time in hours h₀ the average load can be covered using the storage as the sole source of energy_. Other important characteristics are the overall efficiency of the storage device (determined by the loss of energy during the charge and discharge-process), the output voltage U_b and the maximum permitted discharge.

Lead-acid batteries today are the most common technology solution used in hybrid energy systems. There are several alternatives like flywheels, pumped hydro storage, hydraulic storage and fuel cells.

System electronic devices

In case the energy storage device consists of batteries, the three-phase AC current generated by a wind turbine has to be converted in a DC current for charging. This task is achieved by an AC/DC rectifier of a nominal power x_r corresponding to the rated power of the wind turbine x₀.

The AC/DC rectifier connects the generating units with the DC/DC charge controller of a rated power of x_c charging the battery system with a charging voltage U_cc. Besides the charge controller distributes the incoming energy between the charging process and other DC loads which have to be covered within the hole hybrid system. This description is valid for systems using batteries as energy storage device. For a storage fed by an AC current (e.g. pumped hydro storage) the output of the generating units certainly does not have to be converted. Nevertheless in this case a controlling unit is needed for distribution of energy between storage and system loads.

The stored in the batteries has to be reconverted into AC current before it can be used to supply a load. Thus a DC/AC inverter has to be included.

Ancillary equipment in hybrid systems

Batteries can be the most expensive part of an off-grid system, because their working life is limited and they may have to be exchanged several times during a off-grid system is in operation. In most cases, batteries with lead-acid technology are used and it is important to select high-quality cells whenever possible.

To prevent the batteries from over-charging or over-discharging, electric controllers are needed. Many wind turbines for off-grid applications are already delivered with a matching controller. Inverters change the DC current generated by wind turbines into the AC current.

External links

• Wikipedia: Wind-diesel hybrid power system (contains a list of communities using wind-diesel systems), retrieved 17.6.2011 [[2]]