As you can see in that picture the bob wall has been in filled and those ducts installed, I have seen one picture from when it was a substation. The power system was a joint venture between several of the mines (You'll notice the substation at Crofty is a similar vintage) and the tram company, with the original power station being the tram depot that was where Mcdonalds now is. As the power plant became unable to cope with the demand it was decided to build a new power station, with two sites shortlisted (Hayle and Par), which led to the construction of Hayle Power Station, which was subsequently rebuilt in the 1930's as electricity became more of a household commodity. There are still a few of the very old substations in use including South Crofty and Lanner Primary, which was originally erected to supply Tresavean for its reopening.