A cup of water with salt added doesn't get noticeably warm since there's just not much entropy difference in salt water vs fresh water. These days you can reverse the process at very low electricity cost with reverse osmosis.
https://spectrum.ieee.org/fervo-cape-station-geothermal
It's still small scale but it's up and running and the site has plenty more capacity that can be added on bit by bit.
Sol ("the Sun") is a ball of hydrogen so ludicrously massive that its own gravity compresses it to the point where nuclear fusion occurs. The fusion produces ludicrous amounts of energy and so in some sense the hydrogen is from our point of view very, very hot, however it's not on fire because fire needs Oxygen and there is none.
Given this, and given the apparent failure of Zap's scheme to scale, I don't think fusion has any significant chance of succeeding now.
It first lost to PV for daylight generation, then it fell back on storing the heat for use generating after sunset but cheap batteries made that obsolete too.