This wasn't as much for solar efficiency as for cooling a structure. From the article, Water Cooled Roof Under the wrong conditions, your attic can reach temp

This wasn't as much for solar efficiency as for cooling a structure. From the article, Water Cooled Roof Under the wrong conditions, your attic can reach temperatures as high as 150 degrees. Not only is this detrimental to standard asphalt roofing materials, but that heat is cooking your house! You waste hard earned energy cycling cool air in and throughout your home just to fight back against this suffocating blanket of heat. So in addition to carrying away heat from your panels, the system is carrying heat away from your attic as well. Once again, we’ve discovered an efficiency gain that which seems almost hidden by the primary features. So now, water heating equals panel cooling AND roof cooling, which of course means you require less energy for cooling your home.