Solarthermic energie (aka solar boiler)

[der Papst]

I'm trying to create something that heats water, using solar energy - i don't like the way solar panel -> electric boiler -> storage -> steam engine, i also didn't find an electric boiler that works properly.
I've created a new entity with type=storage-tank and a control.lua which heats the water inside (controlled by game.darkness)
This approach works really nice, the only problem is: The offshore pump replaces the hot water consumes by steam engines with cold water, even in the night. This causes the temperature to fall and the power of steam engines falls too.

The solution would be a small pump between offshore pump and storage, only working when the water in the storage is >98°C...
But small pumps can only be controlled by the amount of liquid in storage tanks, not by it's temperature.

How can i modify storage tank's control.lua to only accept water from the offshore pump when the temp is >98°C?


Edit: I also tried giving the solar heater the type=pump, but I didn't figure out how to change pumping_speed from control.lua.
This would be the best solution, so how can I set pumping_speed?

Edit2: active=true/false seems to work, now i have to find the right balance between pumping speed and storage size.