Antifreeze should be added, car antifreeze will do the trick.
Choose the appropriate antifreeze based on the lowest temperature in your area (for example: -25°C).
Main functions: Firstly, to transmit heat energy, transfer the energy from the heat collecting plate to the water storage barrel (erected cylindrical insulated water barrel), then performing heat exchange in the water storage barrel to heat tap water; second, to prevent winter frost from damaging the circulation pipes, causing antifreeze leaks and preventing heat transfer.
(I tried it myself. I've been using it for almost three years and it still works normally.)