High efficiency pressurized solar water heater

Working principles:
Copper coil Preheated solar water heater provide mains pressure via a high efficiency copper coil heat exchanger which located within the tank.
1. For water in the main tank, principle is the same as the non-pressure solar collector:
All-glass evacuated tubes absorb and transfer sun energy into heat energy, then heat water inside vacuum tubes. Depend on the different density between the hot water and cold water; a cycle can be created in the tube which hot water ascending and cold water descending status. Keeping natural circulation like this, the water in the tank becomes hotter and hotter.
2. for water in the copper coil:
When cold water flows through the coil it absorbs heat from the water inside the pre-heating tank. And the water we actually use is the water in the copper coil, it means it is the fresh water (there is no pollution in the water).