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Important aspects to consider in relation to the potential of renewable energy supply sources in meeting electricity demand at any given point in time are flexibility and fluctuations in power supply.

Flexibility

Electricity consumption fluctuates from second to second, from hour to hour and from day to day. Figure 13 shows a typical electricity demand pattern (load curve); for details, we refer to the original publication.

Figure 13 Typical pattern load curve (average day)

Load curve (average daily pattern)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Average Week Weekend GW GW hours Source: Entsoe, 2009.

As Figure 13 shows, during an average day hourly electricity consumption (capacity demand) at night is substantially lower than during the day.

If there is wind energy available this will be the first source to meet (partially) electricity demand since wind energy is supply driven (if it is there, it is there), has zero marginal costs and has priority access to the grid. The remaining electricity demand has to be met by demand-driven electricity generation capacity. The lowest level of this remaining demand is the level for which base load will provide the necessary production capacity, that is, the minimum level of demand which will always occur. During peak hours, additional electricity production capacity is needed that can respond quickly to a sometimes unexpected increase in electricity demand. In general, gas turbine power plants (single cycle) can rapidly respond to demand fluctuations and can even be turned on and off fairly easily, making them very flexible electricity production installations. In contrast, coal and nuclear power plants, most frequently used for providing base load capacity, are less flexible18_. Besides flexibility, so-called merit order is another factor on which the demand-driven electricity production source depends. The merit order in a well functioning electricity market is defined according to an ascending order of marginal operating costs of power plants at any point in time. An example of a merit order is shown in Figure 14.

18 _{Modern types of coal and nuclear power plants are somewhat more flexible than older ones,}

Figure 14 Example of a merit order (Denmark), illustrating which plants produce power at a given market price

The number of hours per year a demand-driven power plant operates depends on fuel prices, plant efficiency, CO2 prices, size and the speed with which production can be increased or decreased (flexibility). In the case of Combined Heat and Power plants (CHP), the heat production is also a factor affecting the number of load hours. In the merit order, plants with a must-run status (like e.g. wind farms, must-run CHP plants, waste burners combined with electricity generation) rank first19 _{while plants with low marginal costs, such as nuclear,} hydro run-of-river and lignite plants, rank second, followed by other sources of electricity generation. As for coal and gas-fired power plants, their exact ranking in the merit order will partly depend on the energy efficiency (and therefore the age) of the specific plant concerned. Older plants are less efficient than new ones of the same type, so they their ranking in the merit order falls as capacity expansion progresses over time (European Commission, 2008). During hours of low electricity demand, such night time, coal and nuclear facilities along with must-run operated plants are the main suppliers of electricity at the moment.

Integral and marginal demand

When analysing which power plants provide the necessary electricity to meet additional electricity demand from electric cars, it is important to distinguish between integral and marginal additional demand. In terms of the way in which integral additional demand is being met by different sources of

electricity generation, this extra demand will be met by the electricity mix at that specific moment in time. This means that wind energy will contribute to meeting this additional demand whenever available while base load capacity will meet this demand at night and gas-fired power plants during the day.

Marginal additional demand can only be met by demand-driven capacity,

based on the merit order. This means that wind energy, which is supply driven, will never be the marginal source, unless future situations occur in which the amount of wind energy becomes so substantial that it is switched off if production exceeds demand.

19 _{In fact, they lead to a downward shift of the load curve and therefore lower the net}

electricity demand, which has to be met by demand-driven generation capacity. Must-run production capacity can only be the marginal power plant when the amount of electricity generated by these plants exceeds the minimum base load demand.

From an integral perspective, wind energy is expected to increase its

contribution due to its rising share in the European electricity supply, its low marginal costs (no fuel costs), its must-run character and priority access to the grid. Gas-fired power stations mainly supply electricity during middle load and peak load hours. Table 5 shows the average load factor per power plant type and the prognoses for 2010-2030 based on simulations of the PRIMES model. The load factor indicates the amount of time a plant operates at its maximum capacity (1 = 100%). The table concurs with the statements made above and shows that nuclear and coal have the highest load factor, i.e., they are almost always operating at their maximum capacity (base load). Wind energy has a must-run status, but only has a load factor of about 0.20 to 0.25 because the wind is not always blowing (at full speed). For offshore wind, the load factor is higher, up to 0.4.

Table 5 Average electricity load factor

2000 2005 2010 2015 2020 2025 2030

1. Nuclear 0.75 0.80 0.83 0.84 0.84 0.92 0.93

2. Solid fuels plants 0.51 0.53 0.59 0.65 0.71 0.73 0.77

3. Hydro 0.46 - 0.48 0.48 0.55 0.52 0.54

4. Large gas plants 0.40 0.47 0.42 0.45 0.45 0.44 0.40

5. Biomass plants 0.37 0.32 0.34 0.33 0.33 0.34 0.34

6. Small gas & oil 0.28 0.26 0.24 0.26 0.30 0.32 0.33

7. Wind (on shore) 0.20 0.20 0.23 0.25 0.26 0.26 0.27

Source: European Commission, 2008.