>nuclear energy
The solution needs to provide district heating for Helsinki. Most citizens and politicians don't want a nuclear plant inside the city. Also the green party loves to screw everyone over by getting in the way of nuclear every step of the way.
>renewable energy
That's difficult in this area. There is very little sunlight in the winter when heating is needed the most, so sunlight is out of the question. The geography is pretty flat, so hydro doesn't provide much. Unlike Iceland, there are no geothermal vents nearby. Wind is probably the best bet, though it's not a particularly windy area.