Operating Power Systems based on Wind and Solar

The power output of a weather dependent renewable system, such as a solar or wind power system, will vary depending on the current weather conditions. On sunny or windy days, the power output will be high, while on cloudy or calm days, the power output will be low. Why is this important? Well, an electrical power system is a sensitive real time system which means that power generation and power demand always have to be balanced. In other words, the power generation from various sources such as thermal, hydro, wind and solar have to match the power consumption at any given time. If the power generation is smaller than the power demand, the system will become unstable. An imbalance the other way around also puts safe operation at risk.

In a conventional power system, it is easy to keep the balance, you just turn on the different fully controllable power plants. Let’s look at a infeed pattern with very little renewable power, a January week from 2015 (all data from smard.de):

Wind and solar are visible in blue and yellow, but do not play a significant role. The different kind of controllable power plants were turned on when needed. Life was simple.

Now we look at the power infeed mid January 2023 – 8 years later:

I accept, it is a windy week. At the same time, the change within only 8 years is gigantic. There are two challenges – just remember that the power generation always has to balance the load:

• When the wind dies down after the sun set, you have to fire on a lot of other generation – so there is the question of sufficient capacity
• The rate of change will be a challenge, as your controllable power plants need to react very quickly when the wind dies or the sun goes down - so system operation also becomes an issue of power plant response time.

Today the power generation of wind and sun in Germany is still below the power demand and the strong European system supports stable operation. But it will not take long, and we will see days when we will have more renewable electricity than we have load. So, the challenge will not only be producing enough electricity, but also producing not too much - or rather using it at the right time.

Here the difference between energy and power is very important. For non-engineers it is explained in this article. Also the basics of flexibility you can find here.