Can't help you with Cornwall, but I did a project on alternative power sources in agriculture in my final year at college in 1981, and there were commercial methane digesters available at that time, as well as wind turbines. Small scale hydro plants also have been available for many years. All these would be considered as "alternative". If it fits in with your own research, I would take a guess that hydro power could be the earliest, it certainly is where I live. Large scale estates were often the places that could afford it. Water power is now perceived as alternative, but it was once the norm, for powering mills etc.