Hybrid-Systems Containing Wind Energy

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Overview

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.

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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

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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 device 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.

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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 energy 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.

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

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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.

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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 combination wind turbine/storage to cover the high-priority loads. Designing a system which covers these loads by RES-sources or storage means the system is generally based on wind energy with the diesel generator as additional or back-up energy source. This operation strategy assures, that a great amount of fuel can be safed, which is important for gaining indepence from high fuel prices and variability.

Using the value of the high-priority load P, the basic wind energy function $P={\frac {1}{2}}\cdot (\rho \cdot A\cdot v^{3})\cdot \eta \cdot c_{p}$ can be used for calculation of the rotor diameter of the wind turbine, provided that all other variables in the function are known.

Conditions at 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?

Optimization of a wind-diesel hybrid system is a complex procedure, because besides the covering of the loads the utilization of the diesel generator and the storage system must be considered: Diesel generators typically have a minimum load for operation (20%-40% of rated capacity) and their efficiency varies significantly with load level. Additionaly frequent on/off-switching of a generator causes severe wear and tear processes and should be avoided for that reason.

The storage systems consist of a battery bank, a bi-directional power electronic converter and a current limiting impedance. It can be considered as a management tool between fluctuating loads, wind energy production and the efficient utilization of the diesel engine. The charging-discharging rates of the battery technology chosen should be able to cope with these tasks in the system. Commonly used battery types are lead-acid and nickel-cadmium batteries with energy densities of the order of 0,05 kWh/litre and 0,1 kWh/litre.

Sizing of converters: Converters have a great influence on the overall system efficiency, because they have a certain amount of self-consumption and their efficiency varies with load: At two-thirds of its rated capacity the input-output efficiency of converters is typically about 87-95%, but during times of very low loads (which occur very often especially in domestic supply), the converter efficiency falls rapidly and can reach values under 50%. Thus converter size must be chosen carefully: System failure will be caused, if the converter is chosen to small and overloads occur. If the size of the converter is too big, it will operate on inefficient load levels during long time periods.

Two strategies are implemented to avoid the difficulties of converter-sizing: In certain systems the number of converters is equal to the number of loads assuring that the influence of the converter on the system performance only occurs, if the related load has to be covered. The second strategy creates to separate systems: One DC-System for lighting and other DC-applications and one AC-System, which is only used in case an AC-application has to be supplied.

For an example see: Project Profile "Hybrid Wind-Diesel System / Isolated Grid in Gao/ Mali" http://www.gtz.de/de/dokumente/en-windenergy-mali-profile-gao-2004.pdf

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Costs of Wind Hybrid System

One main disadvantage of any hybrid system are the high initial cost, caused by the number of necessary parts in the system and especially by the relatively high costs of energy storage systems and – in case of their integration – PV-Panels. In remote areas alternative solutions like grid extensions or dependence on sole diesel generation are expensive as well: While grid extension may result in exorbitant installation costs, pure diesel generation causes a very low initial investment but the Maintainance and Operation Costs are a very substantial factor due to fuel prices and consumption. As a result planning a wind hybrid system requires a comparison of overall costs containing initial installation costs and maintainance and operation costs. The basic calculations for these two values are explained here, while a special section of the article 'Economical feasibility of wind energy projects' explains how to carry out a cost benefit analysis in more detail.

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Installation Costs of Stand-alone Hybrid Systems

For estimation of the initial costs one has to add the installation costs of all components listed in the first section of this article. Kaldelli proposes cost functions for each of the components, which include size and capacity. In this manner costs of the components have to be calculated seperatly and can finally be summarized by a simple addition. A hybrid system consisting of a wind turbine, a PV-array, a diesel generator and a battery bank is considered. The cost functions of main parts and electronic devices are presented in the following.

For a small wind turbine (rated power x₀ < 100 kW) can be estimated using

$IC_{WT}=\left({\frac {a}{b+N_{0}^{x}}}\right)\cdot N_{0}$

where according Kaldelli the paramater can be chosen as follows for the european market: $a=8,7*10^{5}$ €/kW; $b=621$ ; $x=2.05$ ; $c=700$ €/kW. The price for PV panels can be approximated by

$IC_{PV}=\alpha \cdot P_{t}\cdot x\cdot N^{+}$

where $\alpha$ is a function adressing the scale effects, if a high number of PV-panels $x$ is used. For a small wind hybrid system $\alpha$ is near 1 decreasing with the size of the system. $N^{+}$ is the power of one PV-panel in kW_p.$P_{t}$ are the cost of a PV-array expressed in €/kW_p .

The proposed function for the costs of the diesel generator proposed by Kaldelli is:

$IC_{d}=\beta \cdot N_{d}$

where $\beta$ is the specific price of kW of capacity. Appropriate values are $\beta =150-250$ €/kW.

The costs of the battery bank are a main factor in initial cost estimation of a system. For lead-acid batteries Kaldelli provides the following relationship

$IC_{bat}=\delta \cdot Q_{Max}^{1-\omega }$

with $\delta =5.04$ €/Ah and $\omega =0,078$. The size of the electronic devices is aligned differently depending on their position in the system: Inverter size is related to the capacity x_p of the UPS. Both inverter and UPS costs are included in the function. Rectifier and charge controller are align to the size of RES-Sources x₀. Thus the complete costs of electronic devices can be estimated by:

$IC_{el}=\eta \cdot x_{p}^{1-\gamma }+B\cdot N_{0}$

where the parameters are set to the following values by Kaldelli $\eta =483$ €/kW, $\gamma =0,083$, $B=380$ €/kW. Finally Kaldelli introduces an open variable called "balance of the plant cost", which he describes as a fraction of the wind turbine cost by

$f\cdot IC_{WT}$

According to Kaldelli $f$ varies between 0,15 and 0,50. 0,3 is considered as a common value.

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Maintainance and Operation Costs of Wind Hybrid Systems

M&O costs can be divided into variable maintainance costs VC_n and fixed M&O costs FC_n. VC_n generally represents the costs to replace system components with a lifetime r_k smaller than the lifetime of the whole system. According to Kaldelli the system VC_n can be expressed by summing up all replacement costs during the lifetime of a hybrid system:

$VC_{n}=IC_{0}\cdot \sum _{k=1}^{k=k_{0}}r_{k}\left(\sum _{l=1}^{l=l_{k}}\left({\frac {1+h_{k}}{1+i}}\right)^{l\cdot n_{k}}\right)$

where r_k is the replacement coefficient indicating the fraction of initial costs IC₀, which is caused by the replacement of the system-part k. The second term in brackets introduces the annual change in prices of the component as well as the technological improvements into the calculation. The value 1+h_k is a meta-variable representing these changes. By this sum function the periodical necessity of replacement of different parts of the system is represented.

Compared to this, the fixed M&O costs can be calculated in an easier manner: The fraction of FC_n given by the annual capital costs can be expressed as fraction m of the initial capital investment IC₀ corrected by a term for the annual inflation rate g_m. Given a certain annual fuel consumption (assumed in the study of Kaldelli), fuel costs can also be considered as fixed M&O cost. Entirely the function for FC_n is given by the following expression:

$FC_{n}=m\cdot IC_{0}\cdot \left({\frac {1+g_{m}}{1+i}}+({\frac {1+g_{m}}{1+i}})^{2}+...+({\frac {1+g_{m}}{1+i}})^{n-1}+({\frac {1+g_{m}}{1+i}})^{n}\right)$

$+c_{t}\cdot M_{t}\cdot \left({\frac {1+e_{f}}{1+i}}+({\frac {1+e_{f}}{1+i}})^{2}+...+({\frac {1+e_{f}}{1+i}})^{n-1}+({\frac {1+e_{f}}{1+i}})^{n}\right)$

where M_f is the annual fuel consumption, c_f is the annual fuel price and e_f is the fuel price escalation rate.

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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%.

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Small Desalination Systems

Scarcity of potable water is often found in remote island situations, which at the same time often have a good wind potential. In some areas of water scarcity, water desalination contributes a considerable part to potable water supply. Implementing wind-based water desalination units in these areas is a cost-saving and emission-avoiding alternative to running the desalination units by fossil fuels. The techniques described as most efficient for water desalination, are reverse osmosis and mechanical vapour compression. Unfortunately variations and interruptions in power supply are very unfavourable conditions for water desalination plants. For this reason hybrid-systems – based on wind, but allowing a less fluctuating energy supply – combine the advantages of renewable fuel saving energy technologies with the necessary supply security for the efficient operation of small desalination plants. A very promising development of technology are floating systems, consisting of plattforms with an installed hybrid system and micro desalination plant, floating near the coast on the water surface and using the preferable offshore wind conditions.

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Water Pumping

There are many areas with scarce surface water availability but sufficient water resources in appropriate depth for pumping systems. In case of poor infrastructure for water supply, wind turbines can be used as electricity source for these systems. Common aims are providing water for domestic and community supply in remote locations, as well as cattle watering and irrigation. Agriculture still is the economic activity with the greatest water consumption (almost two thirds of worlwide water consumption). The government of India promoted the implementation of wind-pumping system very intensive. The high distribution of small systems has created a well established market for small and multi-bladed wind turbines in recent years.

Besides a shift to Photovoltaic-systems occured because water scarcity is more often correlated with sun radiance as with appropriate wind conditions. The price-reduction has decreased the prevailing barriers of installation costs of these systems. For the same reasons as mentioned related to desalination plants hybrid-systems have advantages in comparison to the sole use of wind or PV: The greater availability of hybrid systems is an important characteristic, because a reliable water supply not at least economically essential for the growth of plants, watering of cattle and not at least the sufficient supply of the inhabitants of remote areas.

Kaldellis (2010) refers to pilot system installed in Greece consisting of a 2 kW wind turbine, 610 $W_{p}$ PV-Panel and an lead-acid battery bank: The system provides 23 m³ water per day from a depth of 30 m.

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Further Information

• Wikipedia: Wind-diesel hybrid power system (contains a list of communities using wind-diesel systems)

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