I just read Project Hail Mary which suggests a use of solar thermal energy and I thought "ha too bad solar PV is so cheap in the real world this would almost certainly never happen" and yet, solar thermal energy plus heat pumps does seem extremely simple and cheap for this company

Does this become much trickier because they're trying to heat to 600c?

Pumping heat from 300K to 900K is not a big gain over heating -- the entire thing is premised on using extremely cheap intermittent electricity during the summer, and your savings are capped at 30%.

Solar water heaters are expensive, complicated, fragile, and generally immobile.

Solar panels and heating elements are cheap, simple and easily replaceable.

A solar water heater is just a black box with a tube running through it. Many houses here have one, because they're much cheaper than PV panels to generate similar kW heat. But a sibling comment pointed out they want to go up to 900 °K, you can't do that with those, of course.

I think in most places the economics have shifted to PV now.

Despite the simplicity of a solar water heater the even greater simplicity is often part of the argument for PV being better, but the higher grade of energy that you only optionally have to use for heat and relative price changes are the key driver

Read the article. He is talking about 600C heat. There aren't any heat pumps that

a) give you significant gain when you already have ΔT that large (because 900K / 300K is a big ratio)

b) have a working fluid that avoids decomposition at 900K

Ah that explains it, thanks, I missed that.

In addition to being at 600 C, this scheme also crucially revolves around keeping capex as low as possible. Resistive heaters have lower capex than heat pumps. In a system with low numbers of charge/discharge cycles the efficiency drops in importance compared to capex.