The current system operates 20 zones with solenoid operated diaphragm valves. These 20 valves are 24Vdc 0.3 Amp.
do you know the model number / link to the valve? The first thing is to make sure these are indeed DC valves. Also, whether it’s latching or non-latching. If you can tell me more about the specifications of the valves, I can give you better advice.
I have found a bit more detail in this pdf http://www.tasseron.nl/sites/default/files/product_groups/downloads/BR.pdf but unfortunately it is Dutch. I will continue to search to see if I can find some more information.
Here is a link with general information for the valve http://www.brinkman.com/catalogue/onderd/onderd-kranen/onderd-kranen-bluera