using alternative energy isnt really effecient when mining because mining requires alot of power to be able to mine a coin succesfully . using alternative energy isnt also helpful to the environment because your mining hardware will still blow polution or excess waste whenever you run it .
Clean energy actually is efficient enough to power miners. Plenty of them are looking into renewable sources. Technology in this sector is bound to improve and become cheaper in the coming years, too, so things are only going to get better from here.
Mining hardware only really generate heat, which isn't a pollutant. It could even be harnessed for other purposes.