Wind power need not be backed up by an equal amount of reserve power

A simplified two-step method for calculation of real fuel consumption and emissions under absence of dynamic input-output characteristics of thermal power plants is proposed in this paper. Estonian case study shows that the integration of considerable capacity of wind turbines would increase the fuel consumption and emissions of thermal stations about 8-10%, which will reduce the environmental effect of windmills substantially. There can be situations where probably no environmental gain can be achieved at all.

It is vitally important to continue the discussion about the ability of power systems
to integrate large amounts of wind power and to develop further the methods for the
calculation of emission reductions.

Wind power need not be backed up by an equal amount of reserve power

14.11.2007

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The collaboration within the International Energy Agency (IEA) Implementing Agreement for Wind Energy, coordinated by VTT, has resulted in the publication of the first state-of-the-art report assessing the international experience gained on the system impacts of wind power.

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9. Conclusions and discussion

High penetration of wind power has impacts that have to be managed through proper plant interconnection, integration, transmission planning, and system and market operations. Several issues that impact on the amount of wind power that can be integrated have been identified. Large balancing areas and aggregation benefits of large areas help in reducing the variability and forecast errors of wind power as well as help in pooling more cost effective balancing resources. System operation and working electricity markets at less than day-ahead time scales help reduce forecast errors of wind power. Transmission is the key to aggregation benefits, electricity markets and larger balancing areas.

Integration cost can be divided into different components arising from the increase in the operational balancing cost and grid expansion cost. The value of the capacity credit of wind power can also be stated. Integration costs of wind power need to be compared to something, like the production costs or market value of wind power, or integration cost of other production forms. It is important to note whether a market cost has been estimated or whether the results refer to technical cost for the power system. There is also benefit when adding wind power to power systems: it reduces the total operating costs and emissions as wind replaces fossil fuels. In this report only the cost component has been analysed. The case studies summarized are not easy to compare due to different methodology and data used, as well as different assumptions on the interconnection capacity available.

Wind generation may require system operators to carry additional operating reserves. Windνs variability cannot be treated in isolation from the load variability inherent in the system. From the investigated studies it follows that at wind penetrations of up to 20% of gross demand (energy), system operating cost increases arising from wind variability and uncertainty amounted to about 1-4 €/MWh. This is 10% or less of the wholesale value of the wind energy. The actual impact of adding wind generation in different balancing areas can vary depending on local factors. From a first review of methodology some important factors were identified to reduce integration costs, such as aggregating wind plant output over large geographical regions, larger balancing areas, and operating the power system closer to the delivery hour.

With current technology, wind power plants can be designed to meet industry expectations such as riding through voltage dips, supplying reactive power to the system, controlling terminal voltage, and participating in SCADA system operation with output and ramp rate control. Grid reinforcement may be needed for handling larger power flows and maintaining a stable voltage, and is commonly needed if new generation is installed in weak grids far from load centers. The cost of grid reinforcements due to wind power is therefore very dependent on where the wind power plants are located relative to load and grid infrastructure, and one must expect numbers to vary from country to country. The grid reinforcement costs from studies in this report vary from 50 €/kW to 160 €/kW. The costs are not continuous; there can be single very high cost reinforcements, and there can also be differences in how the costs are allocated to wind power. It is also important to note that grid reinforcements in general should be held up against the option of curtailing wind or altering operation of other generation.

Wind generation will also provide some additional load carrying capability to meet forecasted increases in system demand. This contribution can be up to 40% of installed capacity if wind power production at times of high load is high, and down to 5% in higher penetrations or if local wind characteristics correlate negatively with the system load profile. Aggregating larger areas benefits the capacity credit of wind power.

State-of-the-art best practices so far include (i) capturing the smoothed out variability of wind power production time series for the geographic diversity assumed and utilising wind forecasting best practice for the uncertainty of wind power production (ii) examining wind variation in combination with load variations, coupled with actual historic utility load and load forecasts (iii) capturing system characteristics and response through operational simulations and modelling and (iv) examining actual costs independent of tariff design structure.

For high penetration levels of wind power, the optimisation of the integrated system should be explored. Modifications to system configuration and operation practices to accommodate high wind penetration may be required. Not all current system operation techniques are designed to correctly incorporate the characteristics of wind generation and surely were not developed with that objective in mind. Increasing system flexibility through such means as transmission to neighbouring areas, demand side management and optimal use of storage (e.g. pumping hydro or thermal) will impact the amount of wind that can be integrated cost effectively. There is growing recognition of the need to assess wind power integration at the international level to identify the needs and benefits of interconnection of national power systems in achieving stated policy goals of accommodating higher levels of renewable energy penetration.

This state-of-the-art report presents a summary of only selected, recently finished studies. In the final report, due end of 2008, there will be more studies included from the participating countries. Classifying power systems and giving rough estimates for wind integration impact remains the task for the final report of IEA WIND Task 25. Wind integration has been studied to wind penetration levels of 10-20% of gross demand (up to 50% of peak load). What happens in larger penetration levels, where wind becomes dominating part of power system, is still unclear — the future power systems may also provide different options for flexibility in demand side that do not exist today. Furthermore, if solar power takes off like wind power has, it will need to be incorporated into integration studies in similar manner and in many regions this will help smoothing the variability of individual technologies.

Preface

A R&D Task titled "Design and Operation of Power Systems with Large Amounts of Wind Power" has been formed within the "IEA Implementing Agreement on the Co-operation in the Research, Development and Deployment of Wind Turbine Systems" (www.ieawind.org) as Task 25. This R&D task will collect and share information on the experience gained and the studies made on power system impacts of wind power, and review methodologies, tools and data used.

The following countries and institutes have been involved in the collaboration (TSO is Transmission System Operator):

• Denmark: Risφ National Laboratories; TSO Energinet.dk
• EWEA (European Wind Energy Association)
• Finland: VTT Technical Research Centre of Finland (Operating Agent)
• Germany: ISET; TSOs RWE and E.ON Netz
• Ireland: SEI; UCD; TSO Eirgrid
• Norway: SINTEF; Statkraft
• Netherlands: ECN
• Portugal: INETI; TSO REN
• Spain: University Castilla La Mancha
• Sweden: KTH
• UK: Centre for Distributed Generation & Sustainable Electrical Energy
• USA: NREL; UWIG.

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