It should now be clear that accommodating significant amounts of wind capacity on the electricity system is not likely to pose any major operational challenges, and this view has been confirmed by the GB system operator, National Grid Company. It is also the conclusion of a comprehensive report on this issue
commissioned by the Carbon Trust and DTI25. At higher wind penetrations, the capacity value of wind is indeed reduced, and this does lead to additional balancing requirements. However, this represents a cost rather than a barrier, as
additional reserve requirements will lead to an increase in systems costs – this is explained further in Chapter 4.
On an operational level, wind power has one distinct advantage when compared to large centralised plant. Faults at conventional plant can cause a large instantaneous loss of supply Figure 9:Wind farm forecast (+ 1 hour) Vs actual output, Ireland 2004 (data provided by Garrad Hassan)24
0 10 20 30 40 50 60 70 80 90 100
1 May 2004 2 May 2004 3 May 2004 4 May 2004 5 May 2004 6 May 2004 7 May 2004
Date P o w er (% o f capacity) Actual Forecast
that must be dealt with using a full range of balancing services. In contrast, combined wind output does not drop from the system in the same way, even under extreme weather conditions (too much, or no wind). Variations in wind output are smoother, making it easier for the system operator to manage changes in supply as they appear within the overall system. There is often some confusion between the additional reserve capacity needed for wind and the ‘plant margin’ – the extra capacity that any electricity system needs, over and above the likely peak demand. It is sometimes implied that an extra plant margin is needed to provide the additional reserve capacity to cover for wind, but this is also misleading. Analysis of the effect of integrating 20% wind output shows that although the apparent plant margin is higher, this is simply because the capacity factor of wind plant is lower. In reality, some
conventional plant will have been displaced (because of the capacity value of wind),
meaning the higher plant margin consists solely of wind plant, because of its lower capacity factor. The additional reserve capacity required to integrate wind energy will therefore be provided by the remaining thermal plant. This issue is explained in more detail in Annex B.
Future reserve options
As already stated, the additional reserve requirements related to the variability of wind could be provided by increasing the use of storage and more emphasis on demand
management. These are further explained below: • Demand management:There is scope for a
considerable expansion in demand management services, with the possibility of domestic and commercial appliances such as refrigerators being able (with the appropriate technology installed in them) to respond to a drop in frequency by temporarily switching themselves off, without damaging the food inside.
• Storage:In the longer term there is the possibility of much greater use of storage, although at present this is seen as an
expensive solution. The UK already has several pumped storage plants, but future storage solutions could rely on developing new large- scale ‘battery’ technologies, or compressed air energy storage.
These options may become more attractive as the percentage of intermittent renewables on the national grid increases and as technologies improve. A large increase in electricity prices would also provide a big incentive, particularly for storage. They both offer low or lower-carbon alternatives to increased use of reserves (which, as shown below, may come from inefficient plant), although in reality all available options will be utilised to some degree.
3.6 Displacement of fuel use and
emissions
As nuclear plant currently provides the primary base load of electricity supply, wind generation is likely to displace coal and gas-fired plant This is illustrated well by Figure 8, which shows how coal generation is the primary load-following
(marginal) plant. It is therefore reasonable to assume that wind power output will mainly displace coal, at least in the short to medium term. In the longer term, with greater reliance on gas-fired plant, significant wind penetration, and any increase in the price of gas relative to coal, wind may also begin to displace gas, but this will depend heavily on the actual fuel mix in the future and the extent of demand management and storage options.
It should be noted that the plant displaced by wind, and the plant needed to meet additional reserve requirements are not necessarily the same types of plant. Plant used to provide additional reserve requirements might be the same type as displaced plant, but not necessarily.
As discussed, some additional reserve may be required by the system operator when wind power penetration becomes significant. Running plants at reduced output is less efficient and so a small amount of additional fuel is used for this purpose. However, the additional fuel
requirement will be far less than the total amount of fuel displaced by the wind generated electricity. When wind produces 20% of total output, it is estimated that the emissions savings from wind will be reduced by a little over 1%, meaning that 99% of the emissions from the displaced fuel will be saved17.
3.7 Limits on wind capacity
The capacity of an electricity system to absorb wind generation is determined more by economics than by absolute technical or practical constraints. As the percentage of wind generating capacity rises, so do the technical and network reinforcement issues that will require resolution. All of these are to some degree influenced by the technologies available at the time, and future technological innovations make the determination of long-term absolute limits unreliable – for example electricity storage developments could make higher wind
penetrations possible.
The most obvious practical constraint on wind capacity occurs when peak wind output exceeds the lowest period of demand on the grid system (ie. a windy summer night), allowing for the requirement for some base load plant to continue operating. At this point excess wind capacity will need to be curtailed, and this has an economic cost for wind plant. Technical constraints include the ability of wind plant to respond to system faults, and this is related both to the type of wind turbine technology used and to the dispersal of wind generation on the
network. Improvements in turbine technology and network reinforcement are both possible solutions, again with possible cost implications. It is generally considered that up to 20% wind capacity penetration is possible on a large electricity network without posing any serious technical or practical problems. Indeed, there is no absolute technical limit to UK wind capacity – instead the issue is an economic one, with higher penetrations leading to increased unit costs. The following statement from National Grid Company confirms this: