It is quite remarkable how far the process of generating electricity has come in the past 130 years, since the invention of the electric generator and the incandescent light bulb enabled Thomas Edison set up the first-ever electric company.
Supplying electric lighting to the offices of New York’s Wall Street, his business provided local generation for local customers – a model that was replicated in many other towns and cities in many parts of the world. There were no transmission lines in those days, just small, coal-fired steam generators providing power on a local system.
Since then the system has grown in complexity and scale. An extra level of complexity was added when, in the 1980s, the process of liberalizing the electric power industry began in many industrialised countries. Gradually this has separated the generators, the transmission system operators (TSOs) who run the high-voltage grids, the distribution service operators (DSOs) who manage the delivery of consumer-voltage power to customers, and the power sellers.
Many power companies are both generators and retailers of electricity. Electricity is more of a will o’ the wisp than a commodity. It can’t be ‘held’ somewhere in a storage tank or reservoir. It is generated, and used, in a second. Transmission system operators (TSOs) – who have little control over demand – have the remarkable task of matching supply to meet whatever demand might be at any moment of the day, or night, during winter and summer alike.
TSOs are like conductors of the ‘orchestra’ of power supply, bringing in one section of the orchestra and quietening another; or are like controllers of traffic at a busy and complex intersection. What they are doing is matching supply to demand – without ever being absolutely certain what demand will be. Of course, consumption patterns have been recorded and analysed, so the TSOs know broadly what to expect, and when. Forecasting of demand has become a sophisticated business, especially as it also underlies the prices at which electricity is bought and sold.
Large scale coal and nuclear plants are usually 500- 1,000 MW or more, and have little flexibility in their output – regardless of the demand. Hydro is supremely controllable, constantly ‘willing’ to release the vast energy of water that wants to obey the law of gravity, but with the supply of water to its turbines fully adjustable. Gas-fired generation can also offer a quick response to match demand.
It is the peaks of power demand that are trickiest for TSOs to handle. In order to maintain voltage within the system they may have to call on every available resource, especially if neighbouring operators have no spare power that can be brought in via the ‘interconnectors’ between different grid systems. This can mean bringing some plant out of semi-retirement, or calling on diesel generator sets. This ‘peaking’ or ‘peaker’ power can often be most among the most polluting, and expensive to run. In open markets it can also command a high price.
Dealing with wind farms’ variable power output
Power output from an individual wind turbine or wind farm varies over time, depending on the weather conditions. The fact that wind output varies is not itself a problem, provided that good information is available in advance to predict how much power wind farms will be producing at any given time.
Predictability, by means of accurate forecasting, is an essential tool to the successful integration of wind power into the electric power system.
Extensive evaluation and modelling is carried out before a wind farm is built. That provides a great deal of information about what these wind energy plants will deliver on a seasonal or monthly basis. This is reinforced once wind farms are in operation.
In countries with ‘priority access’ for renewable energy producers, such as Germany and Spain, TSOs manage the grid as a whole to ensure that the system can always accept the maximum output from the wind energy plants in windy periods, yet maintain the power supply to power consumers during less windy periods. TSOs can now be supplied not only with seasonal/monthly expectations, but a combination of good meteorological forecasting and sophisticated software enables them also to have reliable hour-by-hour forecasts of available wind power. These are of particular value for their complex task.
While there can be variations between forecast and actual, this is of course also the case on the demand side. Any power system is influenced by a large number of planned and unplanned factors, but they have been designed to cope effectively with these variations through their configuration, control systems and interconnection.
On the demand side, changes in weather makes people switch on or off their heating, cooling and lighting, and millions of consumers expect instant power for hair dryers, washing machines and TVs – sometimes all at the same time such as during a popular TV programme.
On the supply side, also, there are variations, and not only from renewable energy sources. When a large power station suddenly shuts down, whether by accident or for maintenance, this causes the immediate loss of many hundreds of megawatts of capacity.
With wind energy, which is produced by hundreds or thousands of individual wind turbines rather than a few large power stations, such sudden drops should never occur, and significant variations should be forecast and planned for. This makes it easier for system operators to predict and manage the changes in supply.
Grid operators in a number of European countries, including Spain, Denmark and Portugal, have now introduced central control centres that can monitor and manage efficiently the entire national fleet of wind turbines. Thanks to a combination of technology, forecasting and TSO expertise, in some parts of Europe wind power is routinely providing 30% of the electricity supply, and at times this goes far higher.
In western Denmark, and in parts of Germany, wind has at times provided more than 100% of demand, meaning that ‘spare’ electricity generated from wind has been available to export to neighbouring grid systems. In addition, large, interconnected grids lessen the overall impact if the wind stops blowing in one particular place.
The idea of new electric ‘superhighways’, such as the ‘supergrid’ proposed to tap into the vast offshore wind resource in northern Europe and link these countries’ electricity grids with the entire continent, would dramatically help to smooth out the variable output of the individual wind turbines.
A frequent misunderstanding concerning wind power relates to the amount of ‘back up’ generation capacity required to balance the variability of wind power in a system. The additional balancing costs associated with large-scale wind integration tend to amount to less than 10% of wind power generation costs, depending on the power system flexibility, the accuracy of short-term forecasting and functioning of the individual power market. The effect of this on the consumer power price is close to zero.
New transmission lines?
Unless electricity is generated on-site, from solar, wind, biogas or small hydro, and is then used on-site, some kind of transport is needed – either of fuel (coal, gas, LNG, diesel, biomass, nuclear fuel), or of waste products (fly-ash, spent nuclear fuel), or of the electricity itself.
Some forms of power generation (large hydro, geothermal) have little flexibility in their siting because of the nature of the resource. Coal power is more flexible – industry and power generation have to some extent grown up around coal resources, but more often the coal itself is transported by ship, rail or truck. Natural gas has to be piped – sometimes vast distances across continents. Increasingly it is pressurized and then shipped in containers in liquid form as LNG. Nuclear fuel and waste need especially secure transport. All these forms of transport and processing add to the cost of those fuels, and to the amount of carbon emitted in getting their energy from the ground and to the electric plug.
Wind power can be developed in a very wide range of locations, and at many scales – from one or two wind turbines to hundreds. Optimizing the siting can make a big difference to the power output from a wind turbine, and this is greatly magnified over a turbine’s operating lifetime of 20+ years.
However, as pointed out earlier in this book, up to now the rate of deployment of wind power by country has largely been dependent on political issues rather than resource criteria.
In Germany, for instance, a favourable political climate has led to the large-scale deployment of wind power, while its wind resource (especially in the centre and south of the country) is lower than in some other European countries with much less installed capacity.
So while wind power can work effectively in many locations, it is true that some regions have an excellent resource that is located away from the towns and cities needing power (in fact, truly windy locations have historically been avoided as places to live).
This might be true on a small scale (western Texas has excellent wind, but its cities are further east and south) or a larger one (Xinjiang’s excellent wind is many miles from China’s eastern seaboard cities). And then there is the case of connecting large-scale offshore wind with electricity users.
Connecting those resources to the locations where power is needed does require new lines – just as new lines would be needed for large hydro, or pipelines for gas.
A further factor to bear in mind is that the power lines built decades ago in some parts of the world badly need upgrading to cope with the demands and size of today’s power sector, so investment in grid upgrades is needed in any case. The IEA estimates that by 2030, over 1.8 trillion USD will have to be invested in transmission and distribution networks in the OECD alone.
In some parts of the world – such as Europe – grid upgrades are also a prerequisite for the operation of the market without conflicting with legislation allowing competition.
Yet with all this talk of power grids and centralized generation, it is worth noting that a high-voltage grid is not always the answer. In many parts of both the developed and developing world, the cost of building transmission lines to reach every part of the countries is simply prohibitive. There are thousands of communities that the grid may never reach.
Occasionally high-voltage lines even pass overhead, busily on their way from a large hydro plant to a big city – but there are no substations and distribution networks to deliver usable power to the people who live beneath. Here wind power, sometimes in combination with other renewables, can work on a smaller scale, off-grid, or on so-called mini-grids that serve a pocket of need.
The right quality of power
Another aspect to bear in mind is that the performance of, and output from, wind turbines harmonizes with the grid’s requirements and does not create any disturbances to the system. This is ensured by means of so-called ‘grid codes’ that lay down the parameters within which wind turbines must operate.
Grid codes cover the technical aspects relating to the operation and use of a country’s electricity transmission system. They also lay down rules that define the ways in which generating stations connecting to the system must operate in order to maintain grid stability.
Technical requirements within grid codes vary from system to system, but the typical requirements for generators normally concern tolerance (i.e. minimum and maximum voltage and frequency limits), control of active and reactive power, protective devices and power quality. Specific requirements for wind power generation are changing as penetration increases and as wind power is starting to function more like other large power plants.
Earlier generations of wind turbines were unable to respond if there was a fault on the network, and could even aggravate the situation. Today, however, modern wind turbines contribute substantially to the stability of the grid. The majority of wind turbines being installed today are capable of meeting the most severe grid code requirements, with advanced features including fault-ride-through capability.
This enables them to assist in keeping the power system stable when disruptions occur. Modern wind farms are wind energy power plants that can be actively controlled and provide grid support services.
www.gwec.net/fileadmin/documents/Publications/GWEO%202010%20final.pdf
