You could use an RV water pump, along with an automotive thermostat, positing the t-stat after the RV pump, when it heated up, the t-stat would open reducing the pressure and engaging the pump, it would shut off automatically at night when the tank cooled and the t-stat closed ( increasing pressure making the pump turn off )...It may require tho that you use an open loop system for this approach, if closed it may just pressurize and make the pump shut down. Also, these pumps are 12volt, they're not cheap tho, they can run between 200 - 300.00 ( here in Canada anyways )...
If your system is like the other posted in the link there :
Is this closed loop? If so you may want to use a fresh water intake and a dump valve ( along with the pressure relief valve ). A dump valve could be as simple as a car t-stat with a higher setting, just below the steam point, the fresh water fill valve ( a good idea in both closed and open systems, one to relief pressure by cooling down, the other to counteract evaporation ) could be as simple as a fill valve from a toilet ( keeping to simplicity ). Another option you may want to consider, if this is only going to 1 reservoir or 1 radiator is no pump. Position your collector lower than the other and let the heat migrate on its own ( at the moment i cant recall the proper term for this lol ).... you could easily make this a 0 energy ( not counting water from the tap ) deal.... just my thought's tho...