A solar thermal system is formed by a collector absorbing solar energy and an insulated water tank where heat is stored. Two different solar systems exist: natural circulation and forced circulation.
In a natural circulation system the fluid flows through convection. The fluid heated in the solar panel expands and reaches the heat exchanger inside the tank. In it, the heat is released to water. Natural circulation system is simpler than the forced one. There is no electric power loss due to a circulation unit connected to the solar circuit. The tank is placed higher than the solar panels to which it is connected to. For aesthetic reasons an horizontal version is always preferred. The natural circulation, compared to the forced one, is more sensitive to pressure loss (flow resistance).
To solve this problem compact kits have a tank next to solar panels.