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). Hybrid system are very often used for stand-alone applications at remote sites. For this reason the article focusses on stand-alone hybrid systems containing storage or diesel-backup.

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 remote 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 using renewable energy (RES) (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.

By storing surplus energy and operating as an additional energy source, when production from the RES-source is low, the independence of the hybrid system is increased. If a diesel generator is part of the system, the storage will allow a more efficient management of the given generation units, avoiding emissions by a more efficient utilization of the diesel generator: Frequent shut-down and restart procedures as well as very unefficient generator loads can be avoided. In this manner, depence on the availability of fuel and thus on fuel price variability is reduced.

Regulation by a storage devices improves the quality of the supplied power, because variations in the frequency of the current can be minimized and voltage control is available. The degree of this improvement clearly depends on the size of the storage device and the adjustment of the whole system.

It must be mentioned, that a storage device significantly raises the initial costs of the hybrid system. Desregarding the type of storage which is chosen (pumped hydro, batteries, flywheels...) the environmental impacts have to be considered.

Losses during charging and discharging-processes lower the efficiency of the whole system reducing the positive effect of avoided generator utilization.

System electronic devices

AC/DC rectifier: 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.

DC/DC charge controller: 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.

DC/AC inverter: 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.

Wind-Diesel hybrid systems

The combination of a diesel generator and a wind turbine in a hybrid system a very common and frequently used in remote areas. The following description includes considerations about system design and sizing of the components. Central questions of system design

are explained by the example of the wind-diesel hybrid system.

Components

Since an equally distributed power supply by RES-sources is essential for the efficiency of a hybrid system, the wind turbine generator should be applicable for maximal power point tracking (MPPT). During times of low wind speed the tip speed ratio of the wind turbine must be adjusted by controlling the electromagnetic torque. Thus the wind turbine should be supplied with a modern power electronic control. For wind velocities higher than the rated wind speed of the turbine, pitch-control is an important tool to keep currents and voltages within safety limits. These control mechanisms influence the sizing of the system, because the capacity of controllers and inverters could be decreased including a lower capacity overhead needed to protect the system components against overloads.

The wind generator can either be a syncronous or an induction generator. Often used modern generator types are permanent magnet syncronous generators (PMSG) or doubly-fed induction generators (DFIG). In case of induction generator a source for excitation, either by excitation capacitators or by grid-connection must be available. In modern stand-alone hybrid-systems a direct-driven version of PMSG is the prevailing choice of wind turbine, because it does not require an external DC current for excitation. Problems to keep frequency at 50 Hz during low wind velocities are avoided by modern construction concepts with a large number of poles.

The diesel generator should be supplied with a syncronous generator. A first order modell with a single time constant can be chosen. The single time constant describes the ratio between fuel consumption and mechanical torque production. The action of the speed governor is controlled by an integral controller gain.

System design and sizing

Load assessment: If the planned hybrid-system is the only source of energy for a village or community, it will be important to categorize the loads which have to be supplied in the village. Medical centers are an example for high-priority loads, while economic and agricultural loads can be labeled as medium-priority loads. Finally domestic supply can be considered as a low priority load in most cases under the assumption, that electricity is a scarce resource. By categorizing the prevailing loads and collecting information about the distribution of these loads during the day the project developer creates the base for choosing the size of the system components.

Resource assessment: The distribution of the wind velocities at the proposed sites has to be analyzed.

Sizing of the generation units: The essential question to be answered concerns the ability of the hybrid system to cover the high-priority loads. The basic wind energy function $P={\frac {1}{2}}\cdot (\rho \cdot A\cdot v^{3})\cdot \eta \cdot c_{p}$ the Rotor diameter must be calculated using the essential load as P. The site must be checked: is it possible to install such a wind turbine on this site at a height with an appropriate wind speed?

Sizing of energy storage depends on wind variability, should be chosen to cover for minimum the essential loads , providing at the same time enough power for optimization of the diesel engines load.

Sizing of electronic devices

siehe PV bei Kaldelli

Operation

Two basic operational strategies

The management of the output reduces the utilization of diesel generators: Only if the essential loads cannot be fulfilled by the wind turbine, the diesel generator is started

Applications

T/C Stations

Stations for telecommunication in remote areas have to be supplied with power during long time periods. Extension of the electricity grid in most cases is a big financial effort, while the supply by sole diesel generators causes additional fuel- and maintainance-costs permanently. Small hybrid systems can be used to reduce fuel-consumption: A small wind turbine may be placed on the relay-mast of the T/C station, avoiding the additional installation costs of a turbine tower. As load variations of a T/C stations are rather low but a steady supply is needed, hybrid systems combining different RES-sources are preferable for this application. A battery storage for system back-up is necessary.

If the T/C station supply should be provided mainly by RES-sources, larger wind turbines on separate towers have to be installed. The inclusion of a PV-System results in reduced variations in RES-output and allows the reduction of the necessary storage size. A well designed hybrid system minimizes the fuel costs of the station supply. Several examples have shown the efficiency of fuel savings gained by the application of hybrid systems: Kaldelli refers to a system installed for remote T/C stations in Kenya consisting of a 7,5 kW turbine, sealed batteries and an inverter, which reduced fuel consumption for remote T/C stations by 70-95%.

External links

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