Operating electric grids with more intermittent renewable energy sources, wind and solar, do not come without challenges. Some examples:
In California with increasing amounts of solar behind the meter it has been recognized that the load curve is changing significantly. The new load curve, based on its shape called the “duck curve”, will set high demands on the system to ramp up and down.
In Germany the grid operators and the authorities realized that with growing amounts of residential solar the grid under certain circumstances could get unstable and in the worst case collapse. Concerned about over frequency, which can happen at sudden drops of load and/or over generation, Germany set 50.2 Hz as the upper frequency limit for residential solar. It may have made sense when the amounts of solar was small, but with 32 GW of solar now (2012) connected it results in that too much generation can be lost too fast, putting the whole system at risk.
Nevertheless, we also hear about success stories like Denmark getting 30 % of its electric consumption from wind.
A major reason Denmark can manage such a high amount of intermittent energy is strong electric connections with neighboring countries. Denmark has 8 connections with Sweden with a total capacity of 2640 MW, 3 connections with Norway with a total capacity of 1240 MW, and 8 connections with Germany with a total capacity of 2100 MW. Altogether it adds up to 5980 MW, which is equal to about 45 % of the installed power generation. 6 of the 19 interconnections are with HVDC, which makes the ties highly flexible and controllable.
On the average Denmark has adequate domestic supply. In 2011 it exported about 4 % of its net production, while in 2012 it imported about 15 % of its consumption, which was unusually high due to low prices of hydro power from Norway and Sweden.
However, more interesting is that in 2011 the accumulated import was over 30 % of the electric consumption and the accumulated export was also over 30 % of the electric consumption. For 2012 the corresponding numbers were over 45 % and over 31 % respectively. It indicates that the export and import of power is an integral part of the Danish electric system. The high amounts further indicate that the strong electric ties with the neighboring systems are a prerequisite for managing the system.
Generally the bigger the electric system the easier it is to balance intermittent renewable energy. Germany is “electrically” more than 15 times bigger than Denmark. In 2012 wind and solar was compared to Denmark “only” 13 % of gross electric energy generation. However, without support from neighboring systems, it would not have been possible.
Accumulated over 2012 Germany exported 22 TWh, which was close to 4 % of the gross electric generation. Most of the export was brown coal fired thermal power. Most of the import was nuclear power from the Czech Republic and France.
Even though wind and solar seasonally and daily supplements each other quite well, the intermittency creates major grid balancing challenges. The variations in power output are big. Solar reached a peak May 25 2012 with 22.4 GW, which that day was about 35% of all generated power. Wind reached its peak on January 5 with 23.4 GW, which that day was about 40 % of all generated power. In between there were times when the output of these sources were close to zero.
Due to the heavy subsidies to wind and solar these resources get priority to feed into the electric grid. The fluctuations from wind and solar have to be balanced against load by the conventional power plants ramping up and down. But there are limits how much and how fast these plants can respond. Added to the challenges for the German grid operators to keep their system stable are the transmissions constraints, especially mowing power from north, where most of the wind is, to the south where most of the load is.
Available data for 2012 indicates that only with the benefits of export and import to and from surrounding systems, it has been possible to maintain grid stability. On a monthly basis imports were about 8 % of the power produced, while exports were about 10 %. The daily fluctuations could be substantially bigger. In January there were days when imports and exports could reach remarkably high amounts, over 40 % of generated power!
At such high levels the reliability of the system is at risk, and this is in a year (2012), when “only” 13 % of the electricity came from wind and solar. Germany has a stated goal of 45 % renewable energy (predominantly wind and solar, but also hydro and biomass) by 2025. Even with strengthened electric ties with the neighboring systems and high amounts of imports and exports, more actions will be needed.
One of the actions would be to strengthen the domestic electric transmission system. There are also plans under discussion to invest 10 billion Euros in a German “Supergrid” with four HVDC corridors as its most significant additions.
More storage will help. However, realistically the possibilities to expand pumped hydro seem to be limited. Best opportunity may be to supplement on-site solar with battery systems.
There are efforts under way to develop new technologies for nuclear power plants and coal fired power plants to improve the flexibility of these plants to ramp up and down, but even so it is questionable if it can replace the need for more of the much more flexible gas turbines.
No doubt more flexibility on the load side will help balancing the system. Consequently, introducing price responsive demand as well as other means to expand DR (Demand Response) will be important.
On the supply side it will be increasingly important that all generating resources, including wind and solar, are dispatchable. To enable more intermittent renewable energy, there must also be a realization that for a grid operator tasked with keeping the system stable, there are moments when too much is too much!