This type of solar water heater provides mains pressure preheating via a highly effective copper coil located within the solar collector tank

This solar collector heats water is the same as the non pressure solar collector. Rather than using the water in the tank directly, as the non-pressure does

This type use copper coil to work as heat exchanger. When water flows through the coil it absorbs heat from the water inside the solar collector tank. The solar collector tank is there for a thermal store