They are still cool. I just recently installed one :)
The vacuum tube ones are IMHO the best, but there are also ones that circulate a heat exchange fluid in panels for direct connection to a heat-pump. The advantage of the latter is that you get good efficiency out of them in the winter, but it also means that the heat-pump has to run all the time. My vacuum tubes work fine most of the year with only a small circulation pump running (and smaller systems with a thermo-siphon don’t even need that).
They are still cool. I just recently installed one :)
The vacuum tube ones are IMHO the best, but there are also ones that circulate a heat exchange fluid in panels for direct connection to a heat-pump. The advantage of the latter is that you get good efficiency out of them in the winter, but it also means that the heat-pump has to run all the time. My vacuum tubes work fine most of the year with only a small circulation pump running (and smaller systems with a thermo-siphon don’t even need that).