What are the advantage of a heat pipe solar collector versus u tube collectors?
Extreme high pressure-since only the manifold is pressurized, and heat pipes ends have a much smaller area than several meters of pipe, higher pressures are possible. Better drain back-draining the top manifold is easier than several meters of pipe in a u-shaped path. More resistance to corrosion and failure-less water in contact with metal and less volume pressurized mean less risk of failure. Less circulation pump load-path of water is only through the top manifold, not through several meters of pipe.
