Why are Parabolic Dish Water Heaters Not in Residential Usage?

A parabolic dish requires an accurate solar tracking mechanism with two tracking axis, 2 motors and moving plumbing. A parabolic trough collector requires a least one tracking axis, motor and perhaps moving plumbing.

It is important for consumers to know that a solar tracking solution is rarely appropriate in a residential solar heating collector. This is certainly unnecessary for solar room heating and usually the same for solar hot water (for bathing temperatures). Parabolic dishes and troughs are ineffective in overcast conditions. When the sun is out, troughs can provide high temperatures; dishes even more so - but residences do not need this high temperature.

When one considers that a stationary collector-and-plumbing is less expensive than one on a pivoting frame, and then contrast the added cost of the frame, hinges, tracker motors, linkages, controller, mirrors and labor - it is more cost effective to install many simple collectors instead of fewer but more complicated collectors.

In the same way if you are considering the use of heliostats to reflect light onto stationary collectors (for solar room heating and wash water), it is generally more expensive to use solar tracking mirrors than to install more collectors or to use white paint (or install white materials) to scatter more sunlight to the existing collectors.

Evacuated tube collectors can consistently provide higher temperatures than flat plate collectors and their cost is dropping towards parity. Evacuated tube collectors are excellent for extreme cold weather conditions as the insulation is a near perfect vacuum. However, this insulation is problematic with snow:

• Evacuated tube collectors will not melt snow cover - they must be manually cleared of snow with brushes or defrosters such as air, water or alcohol spray.
  
• They must be positioned above snow drifts; above any rooftop snow.
• They should be tilted vertical to minimize snow accumulation; a hydrophobic coating such as hard wax helps to repel driven snow.
  
• It may be necessary at some latitudes to install a large bright-white reflective surface such as roof shingles, concrete, yard tiles or gravel to reflect adequate sunlight onto the vertical tubes. (Fresh snow works the best.)
• It may be best to change the tilt away from vertical every spring, depending on the latitude.