> the minimum output of variable sources like wind. If you have enough turbines the wind is always blowing somewhere, and the overall output of the entire fleet never drops below some predictable level.
Not at all true, but it doesn't need to be.
The energy in a fluid , such as air / wind, is proportional to the velocity SQUARED. In other words, if a 10 MPH wind has 100 units of energy, a 30 MPH wind has 625 units. A light breeze of 5 MPH, just 25 units. 40 MPH, 1600 units.
So suppose you build a turbine with a design speed of 25 MPH (625 units). You don't want it to fall apart in higher winds, so the blades, bearings etc need to be big and heavy enough to handle over 1,000 units. That means you'll have friction and other losses of about 25 units. Notice the loss is the same as 5 MPH of wind - you get zero energy production at 5 MPH. At 10 MPH, energy output is negligible. At much above the design speed, the force on the structure quickly becomes much higher than the 625 it's designed for, so the blades are rotated and such to work AGAINST the wind, to avoid having the turbine tower blown over or spin apart. These facts combine to mean turbines produce a useful amount of power only within a narrow range of wind speeds. Unfortunately, the rule power = velocity squared is a fundamental fact of physics. You can't change that by inventing a new type of battery chemistry or something.
If you look at a radar map of the US, you'll see one or two weather systems covering nearly a million square miles moving across the country. Missouri may be on the north end of a system while the southern wind of the system is in central Texas. That's pretty typical that the radar will show one or two systems for the whole country. So it's simply not true that the country as a whole always has "average" weather, that the wind is always 25 over much of the country. The fact is, a windy system will move across the country one week, then the next week heat wave will tour the country.
If you wanted to use wind as your "stable" primary energy source, you'd need a week of storage.
Fortunately not all energy needs to be a stable primary supply. If wind produces good power 10% of the time, you can reduce the use of natural gas generators 10% of the time. That's a good thing! If solar heating heats just your hot water, just 30% of the time, that's a lot of natural gas that doesn't need to be burned.
Since they are often idealists, it's not surprising that advocates of renewable energy always have their eye on renewables as a complete replacement for primary electrical generation, but it's sad because it means we've almost completely missed some great opportunities to make a big difference. Th syn is REALLY good at heating things up. If you've left water in your garden hose in the summer, you know making an effective solar water heater is dead simple - so simple most of us have done it on accident. Yet, most of us heat our water by burning fossil fuels. Why? Because we've ignored the obvious, simple, effective wins as we focus on the holy grail. We've spent tens of billions of dollars on solar electric and a workable solution is always five years and two billion dollars away. For half that money, we could have converted all homes to solar water heating AND mostly solved world hunger with the billions left over.