Characterization of Pumps for Irrigation in Central California: Potential Energy Savings

Abstract

The annual agricultural electric pumping usage in California is around 10 million MWh and most of it occurs in the Sacramento and San Joaquin Valleys, wher e the majority of agriculture is located (Burt et al., 2003). Pumping costs are often higher than they should be for two reasons: more water is pumped than is necessary, and/or the pumping plant operates inefficiently (either the pump itself is inefficient, or the total dynamic head is greater than needed). Ideally, new electric overall pumping plant efficienci es (OPPE) should be at leas t 70 percent (for greater than 25 kilowatts) and every new pumping plant should be tested to verify/determine the starting OPPE. Current practices in the California agricultural irri gation market do not typically guarantee a new OPPE, nor are verification tests performed by the pump vendor or others. Pumps that are initially efficien t can become inefficient through pump wear, changes in groundwater conditions, and changes in the irrigation system (Hanson, 1988). Options for improving OPPE include adjusting impellers, repairing or replacing worn pu mps, replacing mismatched pumps, and converting to energy efficient electric motors (Hanson, 2002). Variable frequency drives, while not improving the OPPE, reduce the input kW by only produc ing the flow and pressure combin ation that is required at the moment. Pumping plants should be evaluated every several years to determine th e status of the pump and possible reasons for poor efficiency. Evalua ting a pumping plant requires a pump test, during which capacity (flow rate), lift, discharg e pressure and input horsepower are measur ed. Electric utilities such as Pacific Gas and Electric Company have provided such eval uations for over 70 years in California to minimize energy consumption in the irrigation sector. Additional programs have been sponsored by the California Energy Commission (Burt and Howes, 2005). Though pump repair or replacement can substantially improve performa nce, energy savings will also depend on management and the design of the irrigati on system. To reduce electrical energy use, the kilowatt-hours must decrease because of fewer kilo watts (kW) or less operati ng time, or both. If the new/repaired pump produces a higher flow rate than before, the hours of operation must be reduced to deliver the same volume; operating the same number of hours can use just as much electricity as before. Irrigation pumps are typica lly overdesigned to cope with the worst working conditions (normally peak demands, and low groundwater levels) but this means that in normal operation the pump will use more kW than necessary for a desired flow rate. In such cases, installing variable frequency drives (VFDs) allows pumps to run at slower speeds in cases of lower demand (pressure or flow rate), requiring less kW – even though the impeller/bowl efficiency may be lower than at the maximum design flow rate and pressure. High efficiency motors should save 3- 5% of their operating co st, although some motors labeled as being “high efficiency” a ppear to have lower efficiency th an standard motors (Burt et al. 2008). The points above are well known in concept. However, information from large da tasets of actual pump performance is difficult to obtain. This paper desc ribes an analysis of ove r 15,000 electric irrigation pump tests in Central California.