If you want heat, why bother converting sunlight to electricity first? You lose 80%. Is it that much more expensive to use mirrors to concentrate sunlight and capture near-100% of the energy as heat?
Do the numbers. Making the top of the heliostat isn't what matters. What matters is making the inside of a pile of dirt that hot.
PV panels are dirt cheap and dead simple. They're also portable. Wire a bunch of panels to a heating element and dunk it in sand/water/etc.