eli5: Why do we not run heavily on solar energy as a society or at least in sunny places?
In: 8
Modern power grids depend on more than just energy availability. Here is one example. There is at least one more that is equally, if not more, complex.
I’ll be talking about frequency stability and [inertia.](https://www.nrel.gov/news/program/2020/inertia-and-the-power-grid-a-guide-without-the-spin.html)
Modern power grids are designed kind of like a [bucket of water.](https://energycentral.com/c/um/back-basics-simple-analogy-visualize-evolution-modern-grid) When the energy going into the bucket and the energy coming out of the bucket are balanced, the water level is stagnant (this is good.) The larger the bucket you have, the longer it takes for the water level to change when there is a disturbance (like when the garden hose filling the bucket cuts off or if some dirt plugs one of the holes that’s draining water.)
Power grids are designed to operate in a very narrow band of frequencies (I cango into why but that’s a whole topic in itself.) The frequency of a power grid is literally the speed that all synchronous machines will move at. When the frequency of a grid decreases, it’s because all the synchronous machines are slowing down, because the energy consumers are taking energy out of the bucket faster than it can be replaced by the generators. This “amount of available energy in spinning synchronous machines” is called “[inertia](https://en.wikipedia.org/wiki/Inertial_response)” and is exactly the same idea as a baseball having inertia when you throw it. Large conventional generators (natural gas, coal, nuclear, hydroelectric) are synchronous machines and have a large amount of mass for the amount of energy they produce, meaning they have a high inertia per unit of power. This means that having lots of these generators gives you a larger bucket to handle discrepancies between energy producers and consumers. If a generator forces offline for whatever reason and you have a large bucket, you’re less likely to violate those narrow frequency bands compared to if you had a small bucket.
So now we come to wind and solar (specifically photovoltaics, or solar panels.) Solar units have zero inertia, because there are no moving parts. They operate on solid state electronics, so they contribute nothing to the bucket size. Similarly, wind units are very light, so they *can* provide some inertia to the system, but are usually connected in such a way that they can generate a bit more energy by not being synchronous (if you’re curious to look further, [here](https://www.site.uottawa.ca/~rhabash/ELG4126WindGenerators.pdf) is a good non-eli5 explanation)
There are types of solar power generators that are synchronous, but they’re way less efficient and way more expensive, specifically solar thermal generators. They use a conventional turbine, but instead of getting their heat from burning a fuel, they get it from using mirrors to concentrate sunlight, usually to a central tower.
Power grids were built around this idea of high inertia, so while it is possible to mitigate concerns of low inertia generators (such as [synchronous condensers](https://en.wikipedia.org/wiki/Synchronous_condenser), which are basically generators without an energy source,) they’re expensive and are currently used in very limited scenarios.
There is also an idea of what’s called “synthetic inertia” or “virtual inertia” that wind and solar units can provide, but that is another pretty complicated topic that isn’t without its own caveats.
The other topic I referenced at the beginning, if you want to look into it, is voltage stability. The key concept to look into is “[dynamic reactive capability curves](https://en.wikipedia.org/wiki/Capability_curve).”
Edit: links
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