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PO-D solar water heater is not designed for mains pressure. The maximum pressure rating is 0.4 bar, which is equivalent to 4m of head. For this reason the system should be supplied with water by a small gravity feed tank located no more than 4m higher than the collector automatic filling valve is available which can provide the tank with automatic water top-up with pressure from a mains water supply.
PO-D solar water heaters are suitable for open flow configuration. It could also be used as closed loop, supplying heat via a heat exchanger, but a circulation pump would be required as mains pressure pre-heated water is required, the SPHE-C solar collector may be a better choice.
A circulation pump is not normally required.
The only reason you would need a pump is if:
1. The mains cold head pressure is insufficient for the cold water to reach the solar water heater or feed tank a small pump controlled by a water level switch in the feed tank can solve the problem.
2. The height from the solar water heater to the hot water tap is not enough to provide pressure hot water. In this case a small pressure pump with built in flow switch on the hot line would provide higher pressure hot water(may need a thermostatic mixing valve prior to pump as some pumps cannot handle temperatures above 60?C).
3. The system was used in a closed loop-see the section above.

Freezing is a concern for any solar water heater, but with the PO-D, mild sub-zero temperature is not a major problem. The solar tubes are very well insulated (vacuum), as is the tank (5 polyurethane). Furthermore, the tank is full of warm or hot water. It would take several days of weather day and night to freeze the water. Also ice floats and so would not form in the evacuation tube, but on the surface of the water in the tank.
As long as there is some sunlight during the day, the water temperature would increase and preventing freezing at night. If you have sub-zero temperatures day and night and virtual sunshine, then it might be necessary to empty the system of water (including the water in tubes) for such periods each year. Glycol can be used in the system to prevent freezing, but such the system could have to be a closed circulation loop. In such case it would be better to use the sun power solar water heater.
The advantage of solar tubes is that they insulate the inner tube form heat loss. This means once heat is absorbed, it is transferred to the water in the manifold, and not lost to the out environment. Because the water is in direct contact with the inner glass tube the heat transfers very well. The performance of the tube itself can reach 80%, but because of the heat losses of the tank, the total system efficiency is around 70-75%(depends on ambient temperatures)