[remolacha]

An “old energy production source” can be cheaper now because the environmental damage it causes is an unpriced externality. A carbon tax solves that by giving a price to that environmental damage. Suddenly, fossil fuels only make sense in the rare cases where renewables cannot be used.

[andy9775]

That's one way of encouraging investment in green energy sources but may not be perfect. Again, it's a wait and see game. Further, for a firm starting up mining ops your cost isn't just the variable cost of electricity, you also have the fixed cost of the plant. If the cost of a coal plant + cost of electrical production/maintenance + carbon tax < cost of a solar plant + cost of electrical production/maintenance then clearly coal here is the winner.