If we have dared to borrow from GDW's other lines of RPGs (Dark Conspiracy, 2300 AD, Cadillacs & Dinosaurs) why not steal, errr, creatively mimic one of the backbone technologies of Space: 1889, the Solar boiler, or in this case, solar concentrator.
The stage is set:
The Duke is sending out raiding parties for very odd items: satellite dishes and mounts ("Duke must have a link to the Mil-Gov ComNet"); mirrors of all kinds, even pieces of broken ones ("to defend against the Vampires, of course!"), Spotlights from the Broadway theaters ("He's gonna use them to hunt the dements/Mayor's troops/Dragons/Dragon Lords/River Vikings at night!", and rear-projection TV sets ("Hey, mebbe he's gonna show movies or sumpthin' to the Downtowners? I dunno...")
What the Duke IS gonna do is make as many mirror-lined satellite dish collectors that he can, and mount them on buildings that oversee rooftop gardens. By maneuvering the collectors, additional hours and concentrations of insolation may be applied to the plants struggling with the adverse growing conditions. Adding the Fresnel lenses from the spotlights and rear projection TVs will enable the users to concentrate the light even more, turning the parabolic collectors into giant magnifying glasses, which might, when multiple units are focused on a single target in a sort of solar furnace mode, might conveniently burst into flame or collapse from hyperthermia, or be blinded by the concentrated glare. Nice weapon for a sunny day?
This sort of focused sunlight can be made to power a boiler for a small steam engine, or to boil water to purify it, or to cook food without fuel or smoke, or....
Large solar furnaces can generate temperatures of up to 6330 F, but even small, crude ones may reach over 2000 F.
http://en.wikipedia.org/wiki/Solar_furnace
Thought just crossed my mind--what if a solar furnace caught the flash from a nuke on a clear day?