Flat plate collectors - run fluid through a black tube, basically.
Since the system can't be turned off, it has to be drained. Just one hour of excessive temperature chemically converts the glycol into glycolic acid, which erodes the pump and copper pipes, necessitating glycol replacement at considerable expense.
Even when properly drained, a thin film of glycol remains and degrades into glycolic acid, damaging the components and lowering the heat-transfer capacity.
An evacuated tube system captures the heat in a thermos-like glass cavity, increasing efficiency and allowing hot water generation in sub-zero conditions.
The system can't be drained in over-temperature conditions, so it has to have a radiator to waste excess heat not being used on very hot days. This means complexity - an extra pump and tubing and controls.
http://www.suntracsolar.com/downloads/SunTrac_Care_and_Maintenance.pdf
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