The Limitations of Wind & Solar Generation Explained to a 5th Grader

Many people visualize a power generation future dominated by wind and solar farms. Beyond the weedy details, wind and solar do seem to offer environmental benefits so it’s easy to understand why this concept is popular. The problem is that electrical systems can’t be dominated by wind and solar generation. This isn’t a matter of an “antiquated” grid, it’s a matter of hard science…explained below.

Before we get into some of the “science”, let’s cover 3 terms that are important to our conversation:

1.??????“Generators” create energy in the form of electricity that’s carried by wires and related equipment to homes & businesses where it’s then used to light rooms, provide air conditioning, refrigerate food, run computers, etc.

2.??????The “grid” consists of the wires and related equipment that carry the electric energy from generators to homes & businesses. The grid includes (among other things) the wires supported by utility poles located along road shoulders seen nearly everywhere.

3.??????“Loads” are the lights, air conditioners, refrigerators, computers, etc. that require electricity to operate.

So, we have generators that create electric energy, the grid that carries electric energy to where it’s needed and loads that use the electric energy to do something useful. This is almost all of the foundational information we need to discuss the “science” mentioned above….but there is one more important concept: electricity must be generated at the moment it’s needed.

For example, at the instant that a light switch is turned on, generators supplying electricity to the grid sense the new load and respond to provide the electric energy required. Of course the generation capacity feeding “the grid” is massive, so a small load addition like a few lights isn’t even measurable at the point where the generator connects to the grid….but when thousands of people turn their lights on at the same time, the load (burden) placed on the generators is certainly measurable.

The burden that generators sense when new loads are added is like the burden that a lawn mower senses as it is pushed into taller grass. Most of the time, the power of the mower is much greater than cutting strength needed to cut grass….but as the grass becomes thicker and taller, the mower engine will struggle to continue cutting. In fact, if a running mower is rapidly pushed into very thick grass, the additional cutting strength requirement may be enough to cause the mower engine to slow or even stall. Anyone that’s mowed a thick lawn has experienced a mower stalling under such circumstances….and likely has figured out that if the mower handle is quickly pushed down to lift the mower blades above the grass before the stall happens, the mower will recover its speed and force. This happens because the burden (load) of grass cutting has been removed. In an electrical system, this is very much how “load shedding” works: if loads are stressing the capability of the generators supplying them energy, some of the load has to be turned off so that the generator can continue to function properly for the remaining loads.

Obviously, there are many differences between a “mowing system” and an “electrical system” but the mowing analogy above is pretty good for understanding how generators respond to load additions, particularly bigger loads added suddenly.

Let’s complicate things a little. In the real world, many generators feed electricity into the grid. Continuing our mower analogy, imagine that a crew of 20 people all have mowers and are working together to mow a large pasture. The mowers all have identical capabilities, and all the grass is essentially the same height and thickness. In this scenario, all the mowing could be done in a coordinated fashion at maximum efficiency. This would be like an ideal electrical system where all the loads and all the generation were carefully matched and without variation. With circumstances like this, the system and its operation could be meticulously planned….but of course nothing like this actually exists.

In the real world, electrical loads are turned on and off at random times and, as mentioned in the first paragraph above, the output of electrical generators must adjust to these load variations immediately as they occur. To put this in terms of our analogy, the grass height and thickness is not uniform so each crew member has to adjust their mowing technique and each lawn mower must likewise adjust its output according to the variation in grass height and thickness as the mower is pushed/pulled over that grass.

Now lets think about what would happen if the power of each lawn mower were to vary randomly while being pushed through grass that is randomly thick and tall. How would that influence the way mowing would be done? It’s pretty obvious that mowing would become much more difficult, even frustrating, particularly if the mower power were to randomly decline just when it was being pushed into thicker grass.

This analogy fits the case where wind and/or solar power is being used exclusively to supply power to the grid. Remembering again that the amount of power that the grid requires at any moment must be generated at that exact moment, it’s clear that a generating system that cannot be reliably adjusted in step with random load changes will fail. Even in this basic analysis we can easily see that random, intermittent generation is unable to meet the needs of variable/random loading. Fortunately, our existing electrical system is not configured like this.

The electrical system as presently configured has a mix of dispatchable (adjustable) generation and randomly variable (intermittent wind and solar) generation. This mix is significantly dominated by dispatchable generation. So long as the capacity of the intermittent generation is small with respect to the capacity of the dispatchable generation, the dispatchable generation can adjust its output to counterbalance the randomness of the intermittent generation. The ability to match the capacity of all generation sources to the total load (which varies continuously) is an absolute requirement of electrical systems. It is not a “flaw” or some indication that the existing grid is antiquated in any way, it is a characteristic dictated by science.

As the capacity of intermittent generation is permitted (encouraged) to grow and as dispatchable generation generating plants are shutdown….the capacity of the intermittent generation becomes an ever bigger percentage of the overall generation portfolio supplying the grid. At some point this mix of generation can tilt too heavy toward intermittent generation. The result: dispatchable generation will be unable to counterbalance the randomness of both intermittent generation and load. When this happens, the grid will protect itself by disconnecting load.

Let’s go back to the crew of 20 mowers. This time, let’s say 10 are traditional (dispatchable) mowers and 10 have randomly variable (intermittent) mowers. Let’s also say that all the mowing the entire crew must do has to be done in the same time frame that would be done by 20 dispatchable mowers. With this requirement, we’re holding the mowing crew to the same high standard required of an electrical generating system: immediate on-time delivery.

Since we don’t (and can’t) know the range of “randomness” of the 10 intermittent mowers, it’s hard to know how the 10 dispatchable mowers will need to perform to ensure that all the mowing is done on time. In the best case, the intermittent mowers could perform just as well as the dispatchable mowers and no extra work for any member of the crew would be needed. In the worst case, it’s possible that none of the intermittent mowers would operate at all, leaving the 10 dispatchable mowers to do all the mowing required.

This analogy isn’t perfect for an electrical system consisting of 10 dispatchable generators and 10 intermittent generators. Fact is, 10 dispatchable generators could not do the work of 20 dispatchable generators if the 10 intermittent generators failed to generate electricity at all. Instead, in the electrical case of this analogy, 20 dispatchable generators would need to be matched to the 10 intermittent generators. With the fleet of generation configured in this way, the dispatchable generation would be able to cover the requirement for 20 fully working generators should the intermittent generation fail or be unavailable. As such, the intermittent generation is completely “extra”: if it works, fine…if it doesn’t work, fine. Either way, the generation requirement will be met.

Some will argue that the “extra” generation is still valuable because it can, often significantly over time, replace dispatchable generation which generally derives its capability from burning coal or natural gas. No doubt reducing the emissions that come from burning fuels is a good thing….but these “extra” generation sources are not free of their own environmental costs: materials that must be mined, equipment that must be manufactured and real estate must be allocated. Needless to say, calculating the true net benefit of “extra” generation is complicated.

Although the mower analogy for electric power isn’t perfect, the takeaway is still valid: it is not possible to power the entire grid using intermittent wind and solar generation. In fact, it’s not possible to even power the majority of the grid using intermittent generation. There are two reasons for this:

1.??????There must be enough dispatchable generation capacity feeding the grid power to adjust to the highs and lows of intermittent/random generation capacity. This is necessary for system stability.

2.??????There must be enough dispatchable generation capacity to replace all of the intermittent/random generation capacity that will not be available when the wind stops blowing and the sun stops shining.

In summary, the amount of wind and solar generation that can be permitted on the grid is governed by the capacity and controllability of the dispatchable generation already installed and supplying the grid. This is a simple fact. So, we cannot continue to add wind and solar generation without also adding dispatchable generation. Battery storage could contribute to dispatchable generation, but as things stand today, the amount of battery storage required for grid stability is simply too expensive. In addition, the manufacturing of batteries comes at a very high environmental cost and that cost may actually exceed the environmental benefit.

Electricity is the key to modern civilization. We literally depend on it every minute of every day. Existing electrical systems are quite strong and resilient thanks to a very focused emphasis on generation capacity and system reliability. Let’s not lose that focus.