“…The two tanks can be set cascaded with a constant flow rate and all inputs and outputs of the tanks are controlled by valves to adjust their levels. 8,9 Another configuration of the double-tank hybrid system is one tank is set at a different height from the other tank either the upper tank is cylindrical as same as the bottom one, 10 or has a spherical shape. 11,12 Also, the two tanks can be set at the same height, connected by two valves, and the outputs of the tanks are controlled by valves to control the second tank.…”