4 Electricity Generation and Supply
The contribution of renewables to the electricity generation mix in Scotland in the past decade has increased rapidly, and is higher than in the rest of the UK. It is set to continue increasing, with significant potential from onshore and offshore wind and from wave and tidal power.
Scotland is a net exporter of electricity, with net exports of between 14% and 24% of total generation in each year from 2000 to 2010.
Box 3: Energy Terminology
Discussions of electricity output of generating plant focus heavily on 3 key factors; the installed capacity of a generating plant, the capacity factor and the output. This section will explain the differences in these terms, and how they are linked.
Generating Output:
This is the actual output of electricity delivered by a generating plant.
Installed Capacity
This is the maximum power output at which an electricity generating plant can operate. Manufacturers generally measure the maximum, or rated, capacity of generating plant to produce electric power in megawatts (MW).
Capacity Factor[6]
This is the actual output delivered by a generating plant over a period of time, expressed as a proportion of the generating plants maximum installed capacity. How much was produced over a period of time, as a proportion of how much could have been produced.
Example Calculations:
Output is calculated by taking the installed capacity of the plant, multiplying by the number of hours in a year, and then by the capacity factor.
So, for a 5MW wind turbine, with a 40% capacity factor;
5MW x 365 days x 24 hours x 40% = 17,520 MWh
Capacity is calculated by taking the output as a proportion of the total potential output.
So, a 5MW rated wind turbine above could theoretically produce 43,800 megawatt-hours in a year. If the turbine actually produced, for example, 15,000 megawatt hours (MWh), then the capacity factor would be ~34% in that year.
The 100% renewable electricity target is defined in terms of the Scottish electricity consumption and is measured in terms of output (MWh). However, language around the target often refers to the estimated installed capacity required to deliver that level of output.
The forthcoming Electricity Generation Policy Statement will provide more information on a possible future generation mix in Scotland.
Electricity Generation by Source
Overall electricity generated in Scotland fell slightly in 2010 from 2009 levels.
While coal generation increased from 23.3% in 2009 to 29.5% in 2010, generation from nuclear fell from 32.5% to 30.6% as a consequence of maintenance outages. Gas generation also fell from 18.7% to 16.8%.
Overall renewables accounted for 19.1% of total generation in 2010. This was a fall from 20.8% in 2009. As noted in Box 1 the target for 100% renewables is based on gross consumption not total generation - this is further illustrated in Box 4. Over the same period, UK renewable generation as a proportion of total UK generation increased by 0.1% between 2009 and 2010, to 6.8%.
However, 2010 saw a continuation of the increases in the level of renewable deployed capacity seen since 2000 with the decrease in output occurring as a consequence of lower than usual levels of rainfall. Following high rainfall in the winters of 2007, 2008 and 2009, the rainfall levels experienced in 2010 were the lowest since 2003. This resulted in a reduction in natural flow hydro generation. Figure 4.1 and 4.2 show the total electricity generated in Scotland and the proportion generated by fuel type respectively - in figures 4.1 and 4.2 renewable generation is defined as 'Hydro Natural Flow' and 'Other Renewables' combined.
Figure 4.1: Electricity generated in Scotland, 2000 to 2010
Source: DECC, Energy Trends, December 2011, *Coal includes a small amount of non-renewable waste.
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Figure 4.2: Percentage of electricity generated by fuel, 2010
Source: DECC, Energy Trends, December 2011, *Coal includes a small amount of non-renewable waste.
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
DECC have now indicated that quarterly regional renewable electricity output figures will be published on a quarterly basis. This will allow a more up to date assessment of output activity than was previously possible. The Scottish Government is considering how best to disseminate these statistics and will advise users in due course.
Electricity generated, consumed and transferred
Scotland typically generates around 50,000 GWh of electricity, as shown in figure 4.3. As a result, Scotland is a net exporter of electricity and has been for a number of years. In 2010, net exports to England and Northern Ireland accounted for 20.8% of total generation.
Figure 4.3 shows two measures of electricity consumption - both measures are illustrated in more detail in Box 4:
- Total electricity consumption is calculated as total generation, minus generators' own use, losses, and exports. Total electricity consumption was 33,606 GWh in 2010.
- Gross electricity consumption measures total generation minus net exports. It is equivalent to total consumption plus generators' own use plus losses. Gross consumption was 39,515 GWh in 2010. Scotland's renewables targets use this measure; see Box 4.
Figure 4.3: Energy generated, consumed and transferred
Source: DECC, Energy Trends, December 2011,
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Box 4 - Deriving the 100% Electricity Target
Renewable Electricity
Installed renewable capacity has risen year on year since 2000 and as shown in Figure 4.4, there has been an increase from 1,400 MW in 2000 to 4,365 MW in 2010. Despite the global recession, 2010 saw an increase in renewable installed capacity of 545 MW, an annual increase of 14%.
Figure 4.4: Installed Capacity of renewable electricity in Scotland
Source: DECC, Energy Trends, December 2011,
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
As at December 2011, Scottish Renewables report that Scotland had 4.4 GW of installed renewable energy generation capacity, with additional 3.3 GW of capacity either in construction or already consented, the majority of which is expected from wind generation, particularly offshore. Considering pipeline projects in planning or scoping potentially takes this figure to a total of 28.8 GW (see figure 4.5) - a 600% increase from the level deployed in December 2010.
Figure 4.5: Renewable Capacity in Scotland at various stages of project planning
Source: Scottish Renewables (21 December 2011)
Generation of renewable electricity from non-hydro sources has grown year on year since the start of the century from a starting point of almost zero as shown in figure 4.6. Building upon a long established base of hydro generation, the growth in deployment thus far has been predominantly through onshore wind. However, as recognised in the Renewables Routemap and forthcoming Electricity Generation Policy Statement, Scotland has significant growth opportunities in offshore wind, wave and tidal technologies.
As shown in Box 4, Gross Electricity Consumption is defined as total generation excluding exports. In 2010, electricity generated from renewable resources delivered the equivalent of 24.1% of Scottish gross consumption. This was a decline on the previous year due to lower than usual levels of hydro generation.
The Scottish Government has set a target to deliver the equivalent of 100% of gross consumption through renewable sources in 2020 with an interim target of 31% in 2011.
Early indications from the published DECC regional quarterly generation statistics suggest that 2011 has not only seen a recovery in hydro generation above historical trends but increased deployment of onshore wind including the opening of Clyde which upon completion will temporarily be Europe's largest onshore wind farm - prior to the completion of the ongoing expansion of Whitelee which then return to being Europe's largest. As a result we are likely to see 2011 generation delivering a record volume of renewable electricity.
Quarterly data for Q1-Q3 of 2011 suggest that the output for the first three quarters of 2011 was already 94% of the 2010 figure and 83% of the previous record year in 2009. For more detail, please see the Statistical Note published at http://www.scotland.gov.uk/Resource/Doc/933/0124593.pdf. DECC's recent decision to routinely publish quarterly regional data provide an early indication of progress towards the output levels required to deliver the 31% interim target. However, it will not be possible to officially report against the target until the regional consumption data is published in December 2012. We will consider in due course how best to disseminate the quarterly figures to users and will consider options, including a short quarterly update published on the Scottish Government website.
Figure 4.6 below shows renewable generation output by technology on the left axis and the percentage of gross consumption on the right axis. The graph highlights the growth in wind output, particularly since 2004 such that in 2010, wind generated more electricity than hydro for the first time.
Figure 4.6: Electricity generated from renewables in Scotland
Source: DECC, Energy Trends, December 2011
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Box 5 - Renewable Output
The reduction in hydro output in 2010 led to a reversal in the growing trend of renewables output witnessed each year since 2003. This is likely to be temporary, with renewable output already expanding through 2011.
It is recognised that renewable generation is by its nature variable in the short-run. It is also important to highlight that the transition to low carbon generation is likely to be characterised by a variability around an upward trend in renewable output as developments are brought on line - hence the ongoing requirement for thermal and nuclear generation during the transition. It is important not to attribute this variability to intermittency and it would also wrong to confuse intermittency and variability with unreliability.
As noted in Box 3, at its simplest, the relationship between the capacity of generation and the output from that generation is dependent on the load factor or capacity factor - while there are technical differences for the purpose of this simple article the two are consider synonymous.
For traditional thermal generation, the load factor can be considered to be the intensity of use and is typically a consequence of economic decisions driven primarily by the price of fossil fuels and also the relative price of individual fossil fuels. E.g. the load factor for a gas plant will be higher during periods of low gas prices, particularly when gas is cheaper than coal, because it is more economic to dispatch at this point.
Recent load factors for thermal generation are estimated in DUKES to have been in the region of 40-70%. Generally speaking the load factor for a thermal plant is determined by a conscious decision taken on the basis of economic influences and is considered to be endogenous - derived internally. The relative role of Gas and Coal in the generation mix is generally influenced by the relative price of the fuel inputs.
Due to complications associated with shutting down and restarting nuclear generation, shutdowns are minimised where possible. The safety requirements that have to be met during the shutdown/restart process mean that it is economically inefficient to use nuclear generation in a flexible way. As such, according to DUKES[7], nuclear is generally considered to be a baseload option with load factors of 50-70%. As with thermal generation, nuclear load factors are also typically determined by conscious decisions.
In contrast, the long run capacity factors for renewable generation are not generally as a consequence of a conscious decisions, rather they are driven by weather patterns and are considered to be exogenous - derived externally. i.e. renewable generation will typically dispatch when the rain falls or the wind blows rather than because of an explicit decision to switch on the plant. This reliance on weather patterns provides the cause of the intermittency but is also the primary determinant of the load factor for renewable generation.
However, during the transition to a low carbon generation mix, human decisions are also likely to be critical to the determination of renewable output and short run load factors at particular points in time. As set out in the forthcoming Electricity Generation Policy Statement, one of the challenges to increased deployment of renewables will be distribution and transmission grid upgrades. The current grid can lead to constraints to the volume of generation, both traditional and renewable, that can be dispatched to the grid. The issue of constraint payments is explored further in Box 7.
The planned grid upgrades such as the 'bootstraps' and Beauly to Denny line will help address these constraints but at particular points in time there may be some variability in the output of renewable generation while these developments are taken forward. Whether due to genuine grid constraints or temporary closure of grid to allow for upgrading, as has been seen at Whitelee[8], the short-term impact will be a divergence between actual renewable output and measured load factors as compared with potential renewable output and renewable availability from the same capacity. Such effects will however be temporary.
Figure 4.7 below shows renewable electricity generation broken down by UK country and by energy source. Scotland is the main location for UK hydro generation accounting for almost 89% of installed capacity and 91% of UK hydro output. Scotland also accounts for 48% of UK wind output (49% of capacity). Overall, Scotland accounts for 37% of total UK renewable electricity output in 2010.
Figure 4.7: Generation of electricity from renewables in 2010
Source: DECC, Energy Trends, December 2011, 22.8 GWh of Microwind are not allocated regionally.
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Of the devolved administrations and English regions, Scotland has by far the greatest electricity generation from renewables per unit of economic activity as illustrated in Figure 4.8. GVA is used as a proxy for economic activity.
Figure 4.8: Renewable electricity generation per unit of GVA, 2010
Source: DECC, Energy Trends, September 2011
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
GVA is provisional gross value added in 2010 workplace based) from ONS, December 2011,
http://www.ons.gov.uk/ons/rel/regional-accounts/regional-gross-value-added--income-approach-/december-2011/index.html
Electricity from Fossil Fuels
Fossil fuel generation in Scotland tends to vary over time and is influenced by a wide range of factors including fossil fuel prices (absolute and relative) and the prevalence of renewable and nuclear generation. In 2010, Scotland had just over 5GW of installed thermal generating capacity, with the coal fired Longannet and Cockenzie power stations making up the majority of this capacity. The gas fired power station at Peterhead accounts for much of the remaining thermal capacity. In 2011, Cockenzie was granted consent to build a new Combined Cycle Gas Turbine (CCGT) power station.
In 2010, fossil fuels accounted for almost 49% of Scotland's electricity generation up from 45% in 2009. Coal and gas provide the main fossil fuels used for electricity generation with oil used to a lesser extent. 2010 saw a greater contribution from coal than any year since 2006. Gas production fell to the lowest level since at least the year 2000 (see figure 4.9).
Figure 4.9: Electricity generated from fossil fuels
Source: DECC, Energy Trends, December 2011, *coal includes a small quantity of non-renewable waste.
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Nuclear
DUKES[9] estimates that in 2010 there was just over 2GW of installed nuclear capacity in Scotland, consisting of Torness (1205MW) and Hunterston B (890 MW) nuclear generating stations. In 2010, 30.6% of electricity generated in Scotland came from the two Nuclear power stations. This represented a fall from 32.6% in 2009, largely due to planned maintenance outages.
The two nuclear power stations in Scotland are currently due to be decommissioned in 2023 (Torness) and 2016 (Hunterston B) although both may receive plant extensions if the appropriate health and safety requirements can be evidenced. There are no plans for more nuclear power stations in Scotland. Further detail on this situation can be found in the forthcoming Electricity Generation Policy Statement.
Figure 4.10: Nuclear electricity generation (GWh)
Source: DECC, Energy Trends, December 2011
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx
Combined Heat and Power
Combined heat and power (CHP) schemes capture heat from the electricity generation process that would otherwise have been wasted and uses the heat for productive purposes such as space heating. The policy on Combined Heat and Power is explored in more detail in the Electricity Policy Generation Statement. Table 4.1 sets out the current number of CHP schemes, their capacity and output.
Almost all Scottish CHP schemes are powered by fossil fuels. In 2010, Scottish CHP schemes generated 2,779GWh of electricity and 6,700 GWh of Heat.
Table 4.1: Combined Heat & Power Schemes, 2010
| Number
of
Schemes | Electrical
capacity
MWe* | Heat
capacity
MWth | Fuel
used*
GWh | Electricity
generated*
GWh | Heat
generated
GWh |
Scotland
UK Total | 95
1,568 | 529
5,989 | 2,772
10,524 | 13,573
110,323 | 2,779
26,083 | 6,700
47,815 |
Source: DECC, Energy Trends, September 2011
http://www.decc.gov.uk/en/content/cms/statistics/publications/trends/trends.aspx