Here’s how it works. Mirrors called “heliostats” track the Sun and focus sunlight onto a central “power tower”. This energy is stored in molten salt as heat, warming the salt to 565°C. This energy storage has an efficiency up to 93 %. To produce electricity, the hot salt is pumped into a generator, where the heat is transferred to steam which drives a turbine. Once the salt is cooled to 290°C (still warm enough to be molten), it returns to the tank to be reheated.
The Sun doesn’t shine at night, but this is not a problem for a solar thermal plant because it has a store of energy ready to go at any time. CST (concentrating solar thermal) can produce power around the clock. The ZCA2020 report describes it as “better-than-baseload” because it is more flexible. CST works well with wind power, because the stored solar energy can be used when there is not enough wind.
As the cheapest form of renewable energy, wind can provide a generous portion of our electricity. Because the wind isn’t blowing all the time, wind farms average only 30% of their capacity. At least half of the electricity produced (ie. 15% of capacity) is expected to be as reliable as “baseload”.
Finally, the Plan includes more than enough backup biomass capacity to fill the gaps created by worst-case weather. The hydro and biomass backups are required for just 2% of demand.
The report modeled the ZCA2020 grid, based on real-world insolation and wind speed. They assumed a demand 40% higher than today (accounting for increased energy efficiency and electrification of transport and heating). The modeling confirmed the proposed portfolio of solar, wind, hydro, and biomass would indeed supply demand.