Why is using energy "bad for the ecology" in and of itself. Or "bad for sustainability". Sustaining what?
Is this some kind of malthusian scientific case for genocide you're presenting? (all human activity, eating and breathing especially, consumes energy, it's called being alive)
Why are you presenting these vague, unscientific assumptions to be in any way relevant to Bitcoin's hashing security
Assume, for a moment, that you/I/They forked a bTN and a BTR. These stand for Bitcoin generated only by Nuclear power, and only by Renewable (solar/wind/geothermal/hydro), respectively.
The free market then quickly establishes if anyone cares. I think they don't...
Here in Northern Europe you are considered as "bad person" if you use energy without a cause. It's been a trend for a while now. And if such fork existed, some people would by it just because saving environment is cool. My guess is that "green fork" could be 10%-20% of BTC's value.