Solar water heaters definitely have the capacity to boil water, if they are made right (use a high temp transfer fluid, vacuum-insulated tubes, proper solar condensers, etc). It’d be difficult to get them to do it on a small scale without a heat pump of some sort in the mix, but even inefficient solar water heater setups need to be installed with a regular water heater after them mostly to down-regulate the temp as they get far far far too hot for practical use directly.

The greenhouse barrels are just for passive radiant heating, but if you use enough of them in the proper places, and insulate well, you can use them to make a passively-heated greenhouse that doesn't freeze at all, even up in the frigid northern climates (Canada, midwest US, Siberia, etc.)