The final graph from CleanTechnica shows the contribution made by solar and wind in Germany over the week of May 21 thru May 27 of this year. The effect is to show how solar electricity is well timed to deal with the peak periods of demand, which occur in the afternoon and early evening.
The horizontal axis is time with the seven days marked off. The vertical axis is megawatts of electricity produced. At its maximum the country produces just short of 60,000 MW. At periods of lowest demand – in the middle of the night – demand falls to around 30,000 MW. The gray area represents the contribution of conventional fuels, which lumps together coal, gas and nuclear. The light blue portion is wind and the yellow portion is solar.
As you can see, solar works in excellent fashion meeting the demand at peak periods during the middle of the day. This is valuable because these peak periods are usually met with expensive natural gas. Nuclear and coal provide “base load” power, which means it pretty much runs steadily night and day. Nuclear is particularly adjusted to base load since it is complicated to rev reactors up and down, particularly on short notice.
The mid-range of power is called “load following.” This is done best by natural gas and some coal. Boilers can be taken on and off the grid of they can be revved up and down to match demand. Trying to adjust them too much is not good, however, since anything that requires boiling water takes time and causes wear and tear on equipment.
The best peaking power is natural gas turbines, which are essentially jet engines bolted to the ground. They can be started and stopped instantly. Gas turbines are extremely expensive to run, however, since gas is more expensive than coal and without boilers they do not make very efficient use of heat.
Of course this week was carefully selected because it was a season when the sun was bright and there were no cloudy days. In cloudy weather of during the winter months, solar contribution would be much smaller.
Solar has an important role to play in meeting peak needs. But it is not going to be able to replace nuclear or coal since they are both needed to run all day and all night. Germany has more solar capacity per capita than any other country in the world but it is unrealistic to think it can replace nuclear. It is much more likely to replace the gas used for meeting peak demand.