After a lot of research, I decided to get some evacuated tube hot water collectors. These tubes allow short wave IR and visible light to pass through the outer pyrex glass, to be converted to long wave IR (heat) on the selective absorber inner glass. This heats the air in the tube, which has a heat pipe to transfer the heat to a manifold at the top, where it heats the circulating water.
I was impressed with how well it works. My 24" parabolic coffie pot will boil the water in less than 1/2 hour.
I built a stainless steel tubular frame for the tubes so the angle could be adjusted for different times of year.