By Ulrich Decher
The impact of intermittent electricity on the power grid is sometimes difficult to understand or explain. Electricity is not a product you can see or touch and is often treated like any other consumer product. Electricity is different from other usual products, however, and I thought that a good way to explain it is to compare it to a game of musical chairs with slightly modified rules.
Here is the game we all know. The game involves players, chairs, and music. When the music stops, the players must scramble to find an empty chair. There is usually one less chair than the number of players. The one left standing loses the round.
Here is the modified game. The number of players is equal to the number of chairs, but the chairs must always be covered with a pillow for the player to sit down on when the music stops. The pillows come in two colors. One color is blue and the other is green.
You may see where I am going with this modified game. The chairs represent the demand for electricity. The pillows represent the supply. The blue pillows represent dispatchable power sources and the green pillows represent the intermittent sources. Dispatchable power sources are the conventional power sources, and some of them are judged as renewable. Intermittent sources are the wind and solar sources and are considered the major renewable sources, but are only available sometimes.
We can think of the combination of chairs and pillows as representing the balanced electricity grid. It must always be balanced to meet the expectation that electricity is always available. This balancing does not occur naturally. There are grid operators who make sure that the balance is exact, so that we don’t have blackout situations that can be life threatening. Therefore, a pillow must always cover a chair to prevent blackouts.
The blue and the green pillows have different characteristics. If a blue pillow is not needed to meet the demand, it can always be placed in a closet until it is needed later. In other words, dispatchable power sources can always be turned off or on as needed.
The green pillows, however, disappear when they are not available. They cannot be placed in the closet. If the green pillows (the intermittent power sources) are unable to meet the demands, then the supply of blue pillows is increased to cover the empty chairs.
The chairs are always covered with either blue or green pillows, in different variations. When the green pillows are not able to meet the demands to cover the chairs, the blue pillows take their place; the converse is not true, however. When the blue pillows (dispatchable power sources) are not available, the green pillows are unable to fulfill the supply for the chairs, because these intermittent power sources cannot be turned on when needed.
Does this game work? Yes, but only if the number of blue pillows (on the chairs or the ones put to the side for later) is equal to the number of chairs. The fact that green pillows (intermittent power sources) can sometimes cover the chairs does not reduce the need for the number of blue pillows to balance the grid. This means that the intermittent sources can only cover a fraction of the required electricity.
The way that small nations can claim a large contribution from intermittent power sources (Denmark, for example) is because they can play the game with other countries in a connected grid through imports and exports. This is like borrowing pillows from a neighboring musical chair party. This doesn’t change the conclusion that blue pillows must always be available to cover the lack of green pillows when viewed across the larger grid (all the parties).
The only other way to improve the game is to store the intermittent energy. However, the amount of electricity generated by a typical grid is huge. There is no known mechanical, chemical, potential, or kinetic energy device that can economically store the electricity generated by a grid, even for a day.
So, what is the benefit of the intermittent sources? Yes, they are emissions-free during power generation, but so are other sources (hydro and nuclear for example). Some of the intermittent power plants are not emissions-free. The Ivanpah solar plant in California, for example, uses so much natural gas to heat its boilers at startup that it exceeds the California carbon emission restriction for power plants.
The game of exchanging with other countries works, but only if the larger grid can cover the unavailability of the intermittent sources. From the viewpoint of the larger grid, its contribution is limited.
Some people claim that all we need is political will to achieve the goal of 100-percent renewable electric power generation. But political will is not sufficient to achieve something. The goal must also be technically feasible. For example, when we sent astronauts to the moon, we certainly needed political will to make that happen. However, before President Kennedy made the speech that committed the United States to that goal, he asked his technical advisers (those with experience in rocket propulsion) if it was technically feasible. They said “yes.”
Today, if we asked technical advisers (experienced in electricity generation) if relying mainly on intermittent power sources is feasible, the answer would certainly be “no.” If this is the case, maybe we should stop playing a game of musical chairs with our electricity production. Is anyone asking?
Ulrich Decher holds a Ph.D. in nuclear engineering. He is a member of the ANS Fuel Cycle & Waste Management and the Operations & Power Divisions. He is also a past contributor to the Cafe.