This heater uses PVC instead of copper. PVC has a much lower thermal conductivity than PVC does, however there are a few ways to over come this weakness.
One way to I increased the efficiency was to use old florescent light bulbs as sleeves over the PVC pipe to act as a double glazing. This helps insulate the PVC and increases the efficiency of the heater. I also insulated the PVC connections to help shield them from the intense heat of sunlight.
The other way I increased efficient was increasing the surface area. Since PVC is cheap it did not cost much to run more pipes that would be exposed to the sun.
I also installed an automatic electric backup heater for $30. It is a bucket heater that turns on at 80 degrees and off at 110. Click on the link above to see more about that.