A very useful trick with relays or solenoid valves to reduce the dissipation in them is to "fire" them from a capacitor held at the nominal coil voltage, and then to hold them at something like 1/2 supply or even less. Obviously, since dissipation is proportional to V^2 you save a lot of wasted heat. We use micro-valves for medical instruments which are made of Teflon, and cannot take the heat of their own trigger coils if run permanently without the trick I describe.

And don't forget the flywheel diode....

Steve