Open-Loop Direct Solar Hot Water Panels Systems
Here a standard electric tank is used in combination with a solar thermal collector. Although the standard electric tank is part of the system it is used only to store additional preheated water and serve as a back-up heating element.
An air vent is attached to the high point of the solar thermal collect to allow for purging air in the system initially. This vent can be manual or automatic.
As stated earlier this is an active system so a small circulator pump is required to move the water through the solar hot water system. But the pump is small and can often use as little as 10 watts, only requiring a small 10-watt PV module as a power source.
Another option is a thermostatically controlled AC pump in combination with a snap-switch which will cycle the water in your system to ensure that your water doesn’t get dangerously hot.
Below is a visual diagram of an open-loop system to aid your understanding.