Assessment the performance of combination of variable speed and constant speed pumps on energy management of low pressure irrigation systems

To supply discharge in some low pressure irrigation systems, a certain head is required and water is pumped to a tank at a constant height. This height is determined based on peak water requirement. Since the constant speed pump is used in this process, water and energy consumption efficiency are acceptable in only a small range of pressure and discharge. So when we need less discharge or less height, the extra discharge has returned from tank to the suction sump and also height has been provided more. It causes energy losses and reduction on system efficiency. The use of variable speed pumps, which are more flexible in operation conditions, energy loss can be largely avoided. Hence, variable speed and constant speed pumps can be combined so as to meet the real water and energy requirement of system. In this study, regarding to development of low pressure irrigation, hydraulic aspects of the proposed method, how to combine pumps, selection of required revolution of variable speed pumps and economic benefits of variable speed pump replacement in operation, is investigated in a practical problem. Results show that combined pumps in dynamic operation condition, had controlled entry power to the system and had reduced energy losses. Also the efficiency of pumping system increase in high discharges and overall efficiency of pumps decrease in low discharges.