The research paper “Planning of the Grid Integration of Wind Energy in Germany Onshore and Offshore” (the dena Grid Study)1 examines the impact on the national grid and the power pool of an increase in the use of renewable energy sources (RES), and particularly wind, to generate electricity in the period 2003 to 2015. In a follow-on study (dena Grid Study II), developments resulting from a further increase in the contribution of renewables to the generation of electricity will be examined, the time horizon being 2025. The conclusions drawn by the dena Grid Study in respect of the impact of an increase in renewable energy sources on the regulation and reserve power of the entire power pool are analysed below.
The guaranteed capacity of renewable energy sources (capacity credits) describes how much of this capacity can be considered guaranteed for coverage of the maximum seasonal load. The analyses in the dena Grid Study assume that the supply of energy is 99% reliable. In Germany, the maximum annual load occurs during particularly cold periods. The capacity credits thus describe how much conventionally generated power is not needed as a reserve in the long term to guarantee the supply of electricity.
Diagram 1: Average increase in secured wind power capacity in the maximum annual load period. Source: dena Grid Study
Caption:
The amount of regulation and reserve power required when intermittent renewable energy sources (and particularly wind and solar radiation) are used is directly related to the accuracy of the forecast regarding the availability of the primary energy. The greater the difference between forecast and real electricity input, the greater the need for regulation and reserve power. If unexpected deviations in the input of renewable energies are to be compensated for in the short term, sufficient levels of positive and negative regulation and reserve power must be available at all times. A basic distinction is made between primary and secondary regulation and between minute and hourly reserves.
The availability and transmission of the regulation and reserve power required must be guaranteed even in critical periods. The location of new peak load power plants and when transmission capacities will be available therefore play an important role.
Primary regulation. Experiences with wind turbines in widely dispersed systems have shown that variations in the power output do not affect the primary control. This could, however, change once very large wind farms are in operation which might be disconnected after a failure in the grid or when the cut-off wind speed is reached. The dena Grid Study assumes that wind energy development would be accompanied by the installation of corresponding technology to compensate for such situations and that there is therefore no need for additional primary regulation.
Secondary regulation. The effect of wind power prediction errors on the level of secondary power to be maintained has not yet been conclusively resolved and was not examined in the dena Grid Study. It was assumed on the strength of past experience that the development of wind energy would not make additional secondary regulation necessary to correct deviations caused by an increase in inaccurate wind forecasting in the relevant period.
As wind energy is developed the need for positive and negative minute and hourly reserves will grow. The accuracy of wind forecasts is expected to develop as shown in Table 1.
| day ahead wind forecast | 4-hour wind forecast | |||||||
| Average | Standard deviation |
Min. | Max. | Average | Standard deviation |
Min. | Max. | |
| 2003 | -0.28% | 7.29% | -27.5% | 41.5% | 1.26% | 4.92% | -17.0% | 33.0% |
| 2015 | -0.32% | 5.91% | -23.5% | 29.5% | 0.97% | 3.89% | -14.0% | 24.3% |
Table 1: Day ahead and 4-hour wind forecasts in %
of the installed wind power capacity in 2003 and 2015.
Source: dena Grid Study
In 2003 an average of 1.2 GW and a maximum of 2.0 GW of positive reserve power had to be available one day ahead. By the year 2015 the average reserve power required will have increased to 3.2 GW (9% of the installed wind capacity, 4% of the estimated maximum annual load in 2015). A maximum of 7.0 GW of reserve power must be available one day ahead.
The following measures are necessary in respect of the power pool if the growing need for positive reserves is to be met:
As wind energy is developed, the need for negative reserves also increases. In 2003 an average of 0.75 GW and a maximum of 1.9 GW of negative reserves had to be available one day ahead. These figures will have risen by 2015 to an average of 2.8 GW (5% of the installed wind capacity, 3% of the annual maximum load) and a maximum of 5.5 GW. Negative reserves can be made available as follows:
Additional power station capacity is connected to the grid which can only be used in a limited fashion or not at all to maintain negative reserve levels:
Like positive reserve requirements, the call on positive reserves will grow from 2.1 TWh/a in 2003 (0.4% of annual electricity consumption) to 5.6 TWh/a (1% of annual electricity consumption). About 58% of this will be accounted for by the minute reserves and 42% by the hourly reserves. The pumped storage power stations, whose installed capacity remains constant in the same period, will contribute around 1.0 TWh/a of the additional 3.5 TWh/a called upon.
A summary of the positive and negative reserve levels to be maintained for the years 2003, 2010 and 2015 and the call on reserve energy associated with this is shown in Table 2.
| positive | negative | |||||
| average | max. | energy | average | max. | energy | |
| GW | GW | TWh/a | GW | GW | TWh/a | |
| 2003 | 1.2 | 2.0 | 2.1 | 0.8 | 1.9 | -0.6 |
| 2010 | 2.6 | 5.5 | 4.4 | 2.0 | 4.3 | -1.7 |
| 2015 | 3.2 | 7.0 | 5.6 | 2.8 | 5.5 | -2.3 |
Table 2: Average and maximum positive and negative regulation
and reserve power required in 2003, 2010 and 2015, and the call on
regulation and reserve power related to this.
Source: dena Grid Study
An increase in the contribution of renewable energy sources to electricity generation is not expected to result in a need for additional primary and secondary regulation before 2015 in the German electricity grid. By 2015, the need for positive and negative minute and hourly reserves will have increased to twice or four times that needed in the year 2003 to compensate for differences between forecasted and actual wind energy, which will still exist despite constant improvements in forecast accuracy. The additional reserve power can be provided by new peak load power stations (particularly gas turbines) and by an adaptation of existing pumped storage power stations. The scenarios set out in the dena Grid Study project an increase in the national installed peak load capacity of 1.7% to 3.4% between 2003 and 2015. According to the Study, approx. 30% of the additional reserve power required will be delivered by pumped storage power stations and 70% by fossil fuel-fired peak load power stations. The call on positive regulation power will thus amount to around 1% of the country’s electricity consumption in 2015.