How is Bitcoin a boon to renewables? Bitcoin consumes energy, and that energy is not being used for other things. Taxing one form of energy will just mean everything that uses energy becomes more expensive; Bitcoin fees would rise or mining operations would simply move to places without the tax.
The actual problem is that Bitcoin's incentive structure works against energy efficiency. Usually we would expect to see improvements in technology result in reductions in energy consumption. Instead, when a new hashing machine is built that computes the same number of hashes for less energy, the response from Bitcoin miners is to compute more hashes using the same amount of energy. That is what Bitcoin incentivizes, and that is why Bitcoin will always be an environmental catastrophe even if we only had renewable power sources (take a look at the environmental cost of producing a solar panel -- there is more to the environment than carbon emissions).