terça-feira, 26 de outubro de 2010

Large scale integration of wind energy into electricity grids

Wind power as a generation source has specific characteristics, which include variability and geographical distribution. These raise challenges for the integration of large amounts of wind power into electricity grids.
In order to integrate large amounts of wind power successfully, a number of issues need to be addressed, including design and operation of the power system, grid infrastructure issues and grid connection of wind power.

Variability of wind power

Wind power is often described as an “intermittent” energy source, and therefore unreliable. In fact, at power system level, wind energy does not start and stop at irregular intervals, so the term ”intermittent” is misleading. The output of aggregated wind capacity is variable, just as the power system itself is inherently variable.

Since wind power production is dependent on the wind, the output of a turbine and wind farm varies over time, under the influence of meteorological fluctuations. These variations occur on all time scales: by seconds, minutes, hours, days, months, seasons and years. Understanding and predicting these variations is essential for successfully integrating wind power into the power system and to use it most efficiently.

Electricity flows – both supply and demand – are inherently variable, as power systems are 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.

Changing weather makes people switch their heating, cooling and lighting on and off, millions of consumers expect instant power for TVs and computers. On the supply side, when a large power station, especially, if it is a nuclear reactor, goes offline, whether by accident or planned shutdown, it does so instantaneously, causing an immediate loss of many hundreds of megawatts. By contrast, wind energy does not suddenly trip off the system. Variations are smoother because there are hundreds or thousands of units rather than a few large power stations, making it easier for the system operator to predict and manage changes in supply. Especially in large, interconnected grids, there is little overall impact if the wind stops blowing in one particular place.

Predictability is key in managing wind power’s variability, and significant advances have been made in improving forecast¬ing methods. Today, wind power prediction is quite accurate for aggregated wind farms and large areas. Using increasingly sophisticated weather forecasts, wind power generation models and statistical analysis, it is possible to predict generation from five minute to hourly intervals over timescales up to 72 hours in advance, and for seasonal and annual periods. Using current tools, the forecast error for a single wind farm is between 10 and 20% of the power output for a forecast horizon of 36 hours. For regionally aggregated wind farms the forecast error is in the order of 10% for a day ahead and less than 5% for 1-4 hours in advance.

The effects of geographical distribution can also be signifi¬cant. Whereas a single wind farm can experience power swings from hour to hour of up to 60% of its capacity, monitoring by the German ISET research institute has shown that the maximum hourly variation across 350 MW of aggregated wind farms in Germany does not exceed 20%. Across a larger area, such as the Nordel system covering four countries (Finland, Sweden, Norway and Eastern Denmark), the greatest hourly variations would be less than 10%, according to studies. 3)


Design and operation of power systems

One of the most frequent misunderstandings occurring in the public discussion about integrating wind energy into the electricity network is that it is treated in isolation. An electricity system is in practice much like a massive bath tub, with hundreds of taps (power stations) providing the input and millions of plug holes (consumers) draining the output. The taps and plugs are opening and closing all the time. For the grid operators, the task is to make sure there is enough water in the bath to maintain system security. It is therefore the combined effects of all technologies, as well as the demand patterns, that matter.

Power systems have always had to deal with these sudden output variations from large power plants, and the proce¬dures put in place can be applied to deal with variations in wind power production as well. The issue is therefore not one of variability in itself, but how to predict, manage this variability, and what tools can be used to improve efficiency.

Wind power as a generation source has specific characteristics, which include variability and geographical distribution. These raise challenges for the integration of large amounts of wind power into electricity grids.

In order to integrate large amounts of wind power success¬fully, a number of issues need to be addressed, including design and operation of the power system, grid infrastructure issues and grid connection of wind power.

Experience has shown that the established control methods and system reserves available for dealing with variable demand and supply are more than adequate for coping with the additional variability from wind energy up to penetration levels of around 20%, depending of the nature of the system in question. This 20% figure is merely indicative, and the reality will vary widely from system to system. The more flexible a power system in terms of responding to variations both on the demand and the supply side, the easier the integration of variable generation sources such as wind energy. In practice, such flexible systems, which tend to have higher levels of hydro power and gas generation in their power mix, will find that significantly higher levels of wind power can be integrated without major system changes.

Within Europe, Denmark already gets 21% of its gross electricity demand from the wind, Spain almost 12%, Portugal 9%, Ireland 8% and Germany 7%. Some regions achieve much higher penetrations. In the western half of Denmark, for example, more than 100% of demand is sometimes met by wind power.

Grid operators in a number of European countries, including Spain and Portugal, have now introduced central control centres which can monitor and manage efficiently the entire national fleet of wind turbines.

The present levels of wind power connected to electricity systems already show that it is feasible to integrate the technology to a significant extent. Experience with almost 60 GW installed in Europe, for example, has shown where areas of high, medium and low penetration levels take place in different conditions, and which bottlenecks and challenges occur.

Another frequent misunderstanding concerning wind power relates to the amount of ‘back up’ generation capacity required, as the inherent variability of wind power needs to be balanced in a system.

Wind power does indeed have an impact on the other generation plants in a given power system, the magnitude of which will depend on the power system size, generation mix, load variations, demand size management and degree of grid interconnection. However, large power systems can take advantage of the natural diversity of variable sources, however. They have flexible mechanisms to follow the varying load and plant outages that cannot always be accurately predicted.

Studies and practice demonstrate that the need for addi¬tional reserve capacity with growing wind penetration very modest. Up to around 20% of wind power penetration, unpredicted imbalances can be countered with reserves existing in the system. Several national and regional studies indicate additional balancing costs in the order of 0 to 3 €/MWh for levels of wind power up to 20%. In Spain, with 12% of wind penetration, the cost of balancing power was assessed in 2007 at 1.4 €/MWh 4).

The additional balancing costs associated with large-scale wind integration tend to amount to less than 10% of wind power generation costs 5), depending on the power system flexibility, the accuracy of short-term forecasting and gate-closure times in the individual power market. The effect of this to the consumer power price is close to zero.

In order to reduce the extra costs of integrating high levels of wind, the flexibility of power systems is key. This can be achieved by a combination of flexible generation units, storage systems, flexibility on the demand side, interconnec¬tions with other power systems and more flexible rules in the power market.

Storage options

There is increasing interest in both large scale storage implemented at transmission level, and in smaller scale dedicated storage embedded in distribution networks. The range of storage technologies is potentially wide.

For large-scale storage, pumped hydro accumulation storage (PAC) is the most common and best known technology, which can also be done underground. Another technology option available for large scale is compressed air energy storage (CAES).

On a decentralised scale storage options include flywheels, batteries, possibly in combination with electric vehicles, fuel cells, electrolysis and super-capacitors. Furthermore, an attractive solution consists of the installation of heat boilers at selected combined heat and power locations (CHP) in order to increase the operational flexibility of these units.

However, it has to be pointed out that storage leads to energy losses, and is not necessarily an efficient option for managing wind farm output. If a country does not have favourable geographical conditions for hydro reservoirs, storage is not an attractive solution because of the poor economics at moderate wind power penetration levels (up to 20%). In any case, the use of storage to balance variations at wind plant level is neither necessary nor economic.

Grid infrastructure

The specific nature of wind power as a distributed and variable generation source requires specific infrastructure investments and the implementation of new technology and grid management concepts. High levels of wind energy in system can impact on grid stability, congestion management, transmission efficiency and transmission adequacy.

In many parts of the world, substantial upgrades of grid infrastructure will be required to allow for the levels of grid integration proposed in this report. Significant improvements can be achieved by network optimisation and other ‘soft’ measures, but an increase in transmission capacity and construction of new transmission lines will also be needed. At the same time, adequate and fair procedures for grid access for wind power need to be developed and implemented, even in areas where grid capacity is limited.

However, the expansion of wind power is not the only driver. Extensions and reinforcements are needed to accommodate whichever power generation technology is chosen to meet a rapidly growing electricity demand. 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 the present situation wind power is disadvantaged in relation to conventional sources, whose infrastructure has been largely developed under national vertically integrated monopolies which were able to finance grid network improvements through state subsidies and levies on electricity bills. But while a more liberalised market has closed off those options in some countries, numerous distortions continue to disadvantage renewable generators in the power market – from discriminatory connection charges to potential abuse of their dominant power by incumbent utilities.


Wind power’s contribution to system adequacy

The ‘capacity credit’ of wind energy expresses how much ‘conventional’ power generation capacity can be avoided or replaced by wind energy. For low wind energy penetration levels, the capacity credit will therefore be close to the average wind power production, which depends on the capacity factors on each individual site (normally 20-35% of rated capacity). With increasing penetration levels of wind power, its relative capacity credit will decrease, which means that a new wind plant on a system with high wind power penetration will replace less ‘conventional’ power than the first plants in the system.

Aggregated wind plants over larger geographical areas are best suited to take full advantage of the firm contribution of wind power in a power system.

Grid connection issues

Agrid code covers all material technical aspects relating to connections to, and the operation and use of, a country’s electricity transmission system. They lay down rules which 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, 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 assuming more and more power plant capabilities, i.e. assuming active control and delivering grid support services.

In response to increasing demands from the network operators, for example to stay connected to the system during a fault event, the most recent wind turbine designs have been substantially improved. The majority of MW-size 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 moving towards becoming wind energy power plants that can be actively controlled.

“In 2008, we installed 8,454 MW of wind capacity in Europe, making up 36% of power installations. That was more than any other power generating technology. There are good reasons for the wind sector’s top spot: the range of benefits it offers to European citizens is unmatched by any other energy source”.

Arthouros Zervos, President of the Global Wind Energy Council, March 19, 2009

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