Flexibility for Residual Load

We looked in the articles at two possible infeed patterns (high solar, Dunkelflaute) that could occur in 2037. The following wind and solar PV generation capacity might be installed in Germany in 2037:

Consequently, the power infeed could look like this on a day with Dunkelflaute (at night with no wind):

We also know that the power system is a sensitive real time system which means that power generation and power demand (load) always have to be balanced. The red curve shows the load on a November day in 2037 and it is quite clear that there is a gap in power generation.

We said before that we need flexibility of the power system (generation and load) to balance the load at all times. Consequently, we have to analyze the flexibility needs as a basis to find solutions. The concept of Residual load is very helpful in a renewables-based power system: Residual load is calculated by?subtracting hourly onshore wind, offshore wind, solar PV and run-off-river hydro generation from hourly demand data.?[DIW Berlin, 2013, Residual Load, Renewable Surplus Generation and Storage Requirements in Germany]

For our analysis we calculate the residual load with the very simple formula:

Residual Load = Load (red line) -Solar PV -Wind onshore -Wind offshore

On the left-hand side, you see the picture where I visualized the residual load for two points in time: 2:00 hours and 18:00 hours. On the right-hand side, you can see the residual load as a curve.

The residual load needs to be covered, so that we can supply all customers. Now two parameters are important here:

Maximum Residual Load: It is at about 18:00 hours and in this example about 105 GW (green arrow) – for comparison peak load today is about 85 GW. This needs to be covered by controllable power plants and/or the load needs to be reduced where economically and technically feasible.

There are more parameters that are of interest, we will look at the residual load in detail:

Power Ramp Rate: The black lines indicate the power ramp rate of the residual load. This rate is the rate of change of the load. In the morning from 4:00 hours to 9:00 hours when the load goes up and the wind dies down so that the power ramp rate is about 10 GW per hour. That means that in the future power plants must be able to react fast, much faster than is required today. In this case you have to switch on the equivalent of 10 large nuclear power plants or large coal fired power plants (just for comparison) every hour for 5 hours. On top of that comes a further increase in the afternoon from 14:00 to 18:00 hours. In the evening the load decreases with a slightly lower power ramp rate.

The orange circle shows another interesting phenomenon: the load increases around noon, but also the solar PV increases, and so the residual load is relatively stable. As the solar peak often coincides with the lunchtime peak load, this has a supporting effect on the system.

There is a further question that you cannot see when you analyse only one day: How often does this happen? One day? Several days in a row? Several weeks or even months in a row? That defines the amount of storage that you need. More about this later.

When designing the system flexibility of the demand that can contribute to reduce the need for generation capacity needs to be considered.

Summary:

In a period with Dunkelflaute you need to cover the residual load. The maximum residual load, the power ramp rate as well as the frequency of occurrence need to be analysed and are the basis for designing the power system.

Please note that I analyzed just one day that I picked by chance. It is not an extraordinary or extreme day, just a day that could happen in 14 years to come. Next week we will look at a day with generation surplus.