Renewable Electricity and the Grid

The Challenge of Variability

Godfrey Boyle

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136558740
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Integrating intermittent renewable energy sources like wind into electricity systems must be one of the most misunderstood issues in energy policy. This edited volume brings together a unique series of authoritative articles on the topic. There should be no excuse for misunderstanding from now on.
JIM SKEA, RESEARCH DIRECTOR, UK ENERGY RESEARCH CENTRE

The future design and operation of electric power systems with large injections of renewable energy generation is the subject of much debate, and some misunderstanding. This timely book, from a number of authors with expertise in the area, makes an important contribution to our understanding of this topic.
NICK JENKINS, PROFESSOR OF ENERGY SYSTEMS, UNIVERSITY OF MANCHESTER

We know the future will be different from the past. This book predicts how large proportions of renewable energy can be incorporated into electricity grids, without harm from the natural variability of these supplies. The chapter authors have different approaches and vision, yet the overall message is positive. Not only can we move to dominant use of renewable electricity, but we can do so utilizing many technological and efficiency improvements, with consumers benefiting from clean electricity at acceptable cost.
PROFESSOR JOHN TWIDELL, GENERAL EDITOR, WIND ENGINEERING

'Anyone interested in renewable electricity will find this book an important reference. It answers many of teh questions so often raised in public debates'
Sherkin Comment

Can renewable energy provide reliable power? Will it need extensive backup?

The energy available from wind, waves, tides and the sun varies in ways that may not match variations in energy demand. Assimilating these fluctuations can affect the operation and economics of electricity networks, markets and the output of other forms of generation. Is this a significant problem, or can these new sources be integrated into the grid system without the need for extensive backup or energy storage capacity?

This book examines the significance of the issue of variability of renewable electricity supplies, and presents technical and operational solutions to the problem of reconciling the differing patterns of supply and demand. Its chapters are authored by leading experts in the field, who aim to explain and quantify the impacts of variability in renewable energy, and in doing so, dispel many of the myths and misunderstandings surrounding the topic.

1

Variable Renewables and the Grid An Overview

Michael Laughton

Introduction

In all national electricity supply systems, the power demand varies over the course of a day; there is a rise and fall every 24 hours, with usually a nighttime minimum and a daily maximum. In order to assess the contribution that renewable or other sources of energy can make to electricity supply, the distinction between energy and power has to be kept clearly in mind. Whereas the commercial operation of each generation plant is measured against total energy delivered, in the UK the central grid control operated by the National Grid Electricity Transmission plc (National Grid), acting in its role as Great Britain System Operator (GBSO), has to ensure that the power generated (the rate of delivery of energy) balances the power demand at all times, otherwise the system fails.

Ensuring power supply security requires a deeper understanding of grid-related issues than those related to energy supply availability. Naturally varying renewable energy sources certainly provide secure quantities of energy when considered over, say, a year, but of themselves do not necessarily guarantee the secure delivery of power as and when needed. The significance of the separation of requirements for energy delivery and power delivery (which seems to escape many commentators and advocates in the energy field) gives rise to separate power supply-related questions, such as those concerning plant capacity, generation load factors, system capacity planning margins, probabilistic measures of system power supply security, and backup plant requirements.

These questions will be considered further in this chapter from the viewpoint of guaranteeing grid security of power supply. Although difficulties and constraints are highlighted, it is taken for granted that renewable energy forms an important component in future energy supplies for the electricity supply industry, the more so in the UK with increasing dependence upon imported gas and the future retirement of coal and nuclear stations. Problems raised, therefore, should be seen as problems to be solved – in some cases by more research, and in others by the development of technology.

Renewable Energy Source Variability

Renewable Energy Sources

Table 1.1 lists the main renewable energy sources used for electrical power generation, along with the distinctive types of development for energy conversion and extraction. Apart from the production of heat or chemicals, the generation of electrical energy is the main purpose.

Table 1.1 Renewable energy sources for electrical power generation

Renewable resource

Municipal solid waste (MSW)

Hydro: large scale small scale

Wind: onshore offshore

Biofuels: energy crops forestry wastes agricultural wastes

Wave: shoreline near shore offshore

Tidal: stream barrage

Solar: photovoltaic

Source: Tyndall Centre (2003)

From the viewpoint of a power system operator, some of the difficulties associated with renewable source variability affecting the delivery of electrical power are as follows:

• uncertainties in predictions of power available at any given time, leading to scheduling difficulties, although obviously the degrees of uncertainty vary with the length of forecasting horizon;
• magnitude of fluctuations in power output, where small fluctuations can be accommodated easily, but larger fluctuations require special countermeasures;
• speed of fluctuations, where slow changes in resource availability and, hence, power output are usually predictable, but fast changes are less so.

In addition, there are generating plant performance abilities to be considered, such as power conversion limits where generating plant can operate efficiently only within certain limits of energy availability.

Variability of Power Generation

The characteristics of the varying electrical power outputs obtained from these respective resources vary considerably.

Tidal energy captured either from tidal streams of water or by storage and subsequent release in barrages is the most predictable of variable renewable energy forms. Tidal cycles lasting just less than 12.5 hours each day allow generation on either the ebb tide or on both the ebb and flood tides. Generation on the ebb tide with additional pumping at high tide is a further option, as shown in Figure 1.1. Studies of a potential barrage in the Severn Estuary show generation is possible for five to six hours during spring tides and for about three hours during neap tides. Thus, a tidal barrage produces two totally predictable but intermittent blocks of energy each day, the size and timing of which follow the lunar cycle.

Note: A = filling; B = pumping; C = holding; D = generating; E = holding.

Source: Laughton (1990)

Figure 1.1 Tidal cycle and electricity generation periods for a barrage with additional pumping at high tide

Wave power is, at present, a relatively undeveloped and underemployed technology; therefore, without hard data the variability of power output from wave farms can only be surmised. It is known, however, that the variability depends upon both local and distant weather conditions. Wave power gives rise to further problems connected with plant limitations, apart from unpredictability. Figure 1.2 shows a typical probability relationship for wave power measured in kilowatts per metre (kW m⁻¹) of wave front for sea conditions in northern UK waters. Note the logarithmic scale. To design power take-off devices to capture the power in low-probability high-power waves would be too expensive; therefore, such devices are sized to cope with only a limited range of wave power levels that have a higher probability of occurrence. However, to withstand extreme conditions without being destroyed, the structure has to be designed to withstand such extreme events, regardless of their low probability.

Source: Select Committee on the European Communities (1988)

Figure 1.2 Typical annual variation in wave power levels

Photovoltaic (PV): the power and energy output of any PV array depends upon the irradiance, which, in turn, depends upon the time of day and the time of year, the maximum power generated, and the length of operation achieved in summer. Local weather conditions result in individual array power outputs with many spikes and troughs, although the overall daily power output from several arrays spaced across the country should follow approximately a bell-shaped curve centred on midday, with a spread depending upon the length of daylight. In the UK, midsummer irradiance could last, for example, from 05.00hrs to 21.00hrs, but with levels falling from 100 per cent at 12.00hrs to 70 per cent at 16.30hrs, and then steeply to less than 20 per cent by 18.00hrs. PV energy generation, therefore, is in the form of one block of energy each day during daylight hours with power levels achieved being both seasonally and weather dependent.

Wind power, like wave power, requires the generating plant to withstand extreme conditions without being destroyed. Wind turbines are currently designed to withstand maximum wind speeds of usually around 25 metres per second (m s⁻¹), at which level the turbines are switched off for protection. Figure 1.3 illustrates a typical power output characteristic for a wind generator showing output rising from a cut-in wind speed of about 4m s⁻¹ to 5m s⁻¹, to a maximum output at about 13m s⁻¹ to 14m s⁻¹ and a shut down speed at 25m s⁻¹.

Source: Boyle (2004)

Figure 1.3 Wind turbine output characteristics

Three sources of variability are apparent. First, there is zero output below cut-in wind speeds; second, between cut-in and maximum output, varying wind speeds can cause large changes in output, although these would tend to be smoothed out with many turbines covering a wide area; and, third, the turbine is switched off in storm conditions.

This last circumstance is illustrated in Figure 1.4, showing spot prices in the Danish electricity market during the first week of January 2005. Particularly strong winds during this time first of all produced ample supplies of wind power that sent the spot (marginal) prices to zero, followed by rapid rises in price as wind conditions strengthened beyond 25m s⁻¹ wind speed and many wind turbines were shut down. Such fast fluctuations in output may be anticipated but are difficult to predict accurately, both in degree and in time, without knowledge of the extent and progress of the particular storm circumstances.

Note: DKr = Danish Kroner.

Source: www.nordpool.com/nordpool/spot/index.html

Figure 1.4 Influence of storm conditions on spot electricity prices in Danish Kroners per megawatt hour in West Denmark during the first week of January 2005

More usually, the total wind power output from a number of wind farms across a region is subject to slowly occurring large fluctuations caused by the changing regional weather patterns. Figure 1.5 shows such variability at the end of April and beginning of May 2004 in the E.ON Netz system in North Germany (E.ON Netz, 2004). Although the changes in wind power output represent some 80 per cent of installed capacity, such variations are more easily predicted than the sudden storm disconnections. The problem faced by E.ON Netz is more to do with the measures that need to be taken to ensure that system frequency is controlled and power flows in a coordinated manner across the transmission network.

Source: E.ON Netz (2004)

Figure 1.5 Large fluctuations in wind output in the E.ON Netz network in Germany

Care should be taken in drawing parallels, however, between experiences in Germany and Denmark and the situation elsewhere, such as in the UK. Wind conditions over the whole British electricity supply system should be assumed to be different unless proved otherwise. Differences in latitude and longitude, the presence of oceans, as well as the area covered by the wind power generation industry make comparisons difficult. The British wind industry, for example, has a longer north–south footprint than in Denmark, while in Germany the wind farms have a strong east–west configuration. In addition, both Denmark and Germany operate a feed-in tariff system of support that allows wind generation to be determined entirely by the wind conditions and tariff levels – hence the uncontrolled Danish spot prices. Such support is common within the European Union (EU), but is not in the UK or in the Republic of Ir...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• List of Figures and Tables
• List of Contributors
• Preface
• Acknowledgements
• List of Acronyms and Abbreviations
• 1. Variable Renewables and the Grid: An Overview
• 2. Wind Power on the Grid
• 3. Renewable Resource Characteristics and Network Integration
• 4. The UK Energy Research Centre Review of the Costs and Impacts of Intermittency
• 5. Wind Power Forecasting
• 6. Flexibility of Fossil Fuel Plant in a Renewable Energy Scenario:Possible Implications for the UK
• 7. The Potential Contribution of Emergency Diesel Standby Generators in Dealing with the Variability of Renewable Energy Sources
• 8. Demand Flexibility, Micro-Combined Heat and Power and the ‘Informated’ Grid
• 9. A Renewable Electricity System for the UK
• 10. Reliable Power, Wind Variability and Offshore Grids in Europe
• 11. Planning for Variability in the Longer Term: The Challenge of a Truly Sustainable Energy System
• Index