2011.01.03
Grid Stability
Fluctuating supply from wind- and photovaltic power plants is a challenge for grid operation -
a threatening collapse is not an issue.
End of year 2010, in Germany there will be wind power plants installed with nominal power of approx. 28 GW and photovoltaic plants with a peak power of approx. 18 GW. Calculative a level is reached which compares to the base load, the continuous electricity demand to be observed in Germany. Theoretically, one may think, during periods of less electricity demand all thermal power plants, anyway if fired with coal, oil, natural gas or nuclear fuels, have to be shut down.
In a more detailed consideration, some incorrect assumptions immediately become obvious: It is not very likely that all wind power plants feed in at nominal power at the same time. Nominal power typically will be supplied from wind strength 6-7 bft. Country-wide weather conditions with such or even higher wind strengths are hardly realistic. The emergence of wind by time of day depends on many factors. In Germany, the probability for stronger wind is higher in the morning and late afternoon and early evening. In contrast, the period of less electricity demand occurs during night and early morning hours, especially on weekends. During winter half year there will be typically more wind - thus tending to result in a balance to less solar irradiation.
For photovoltaic plants feed-in is fairly better calculable: The position of the sun is predictable and maximum of irradiation occurs during noon time - the point of time where one of the peak demand of electricity can be observed. In consequence, building-integrated pv-systems contribute to a load removal on the grids, since decentral fed-in electricity reduces the need for electricity produced by far away power stations. The situation may become different if many large pv-systems are installed in rural regions with less electricity demand. Under these conditions, enhancement of the grid is necessary. Similar as for wind plants, peak power is supplied only a few hours per day under clear sky conditions. Furthermore there is a strong impact by orientation of pv-panels and seasonal effects. Again it will by less likely that all photovoltaic plants provide peak power to the grid in one moment. Weather conditions providing strong wind and clear skies at noon time are even more improbable.
Thus it becomes obvious that the scenario given in the introduction - simply accumulation of plants' key figures - does not represent reality. Additionally, German Renewable Energies Act (EEG) from 2009, §6 determines that plants larger than 100 kW have to provide for means of remote control by grid operators in order to prevent from overload conditions to grids. - This applies to all since then deployed wind power plants (typ. power: 2,000 kW) and de facto for pv-systems larger than 600-1,000 m²; although a legal loophole actually prevents them from this rule. Furthermore, a new draft to a low voltage guideline discusses the contribution of small power solar inverters to reactive power compensation in low voltage grids. A study recently published by Roland Berger consultants and Fraunhofer IWES shows increased feed-in capabilities from solar systems into low voltage grids when reactive power compensation is enabled - in some cases grid load may be twice or even higher.
It is obvious that a reconstruction of today's electricity supply systems to regenerative technologies will not be feasible without any reconstruction of grids. Storage systems will play an important role to balance fluctuating supply with actual demand (this will be another topic to discuss later). The sometimes evoked collapse of grids turns out to be a product of a lack of knowledge to the underlying principles.