Variable speed drives for pumps used in intensive pond culture systems

Authors: Brown, Travis; Tucker, Craig

Submitted to: The Catfish Journal
Publication Type: Trade Journal
Publication Acceptance Date: 5/12/2015
Publication Date: 6/1/2015
Citation: Brown, T.W., Tucker, C.S. 2015. Variable speed drives for pumps used in intensive pond culture systems. The Catfish Journal. 27:(3)10-11.

Interpretive Summary: Pumps are fairly efficient mechanisms for transferring mechanical energy to water to lift and pressurize water while overcoming energy losses during water conveyance. However, to function efficiently and reliably, they must be wisely selected and incorporated based on the specific application. The most important consideration is to select a pump whose maximum efficiency occurs in the range of flow rates needed for a particular application. In general pump design, pressure and discharge are inversely related; that is, high pressure pumps usually have low discharge rates and high discharge pumps usually have low pressure ratings. Variable-speed technology is ideally suited for use in the new, intensive, partitioned pond production systems. VFDs wired to the electric motors of pumps used in split-ponds or in-pond raceways allow farmers to adjust pump speed as needed with a slight turn of a knob (the potentiometer). As an example, when fish biomass is low, very little water flow is needed between the fish-culture basin and the waste-treatment lagoon. More water flow is required as fish grow, and maximum water flow is essential at the end of the production season when fish biomass is high. A VFD can be used to increase pump speed and water flow through the year as fish grow. In summary, variable speed pumping can be a great option for farmers and researchers. In the case of the example above, a farmer could reduce their electrical energy used to pump water in a split-pond by as much as 50% and save about $143 per acre on electrical energy use. However, the cost of a correct sized VFD to operate a 10-hp electric motor can be as high as $2,800 (each), not including a NEMA 4 rated electrical enclosure (~$300-1,000). In addition, other components are required to increase the useful life of VFDs (cooling fans, heaters, thermostats, filters, etc.) which increase costs even more. In reality, variable speed pumping is a great resource for researchers because they can optimize pump performance through field testing and trials, although, in many cases it may not be the best decision for farmers because of the added expense and risk of failure of VFDs.

Technical Abstract: Prior to about 2010, the only large pumps on most catfish farms were those associated with the water supply. Water from wells is usually pumped to the surface using single-speed, vertical, lineshaft turbine pumps powered by three phase, electric motors. Since 2010, several catfish farmers have built split ponds or in-pond raceways requiring water flow between a fish-containment area and a much larger waste-treatment area. Several different pump types have been used in the new, intensive systems. All are expensive, and the pumps used in partitioned pond systems run for much longer periods than those used to supply water. Farmers using partitioned systems should become familiar with pump performance and ways to reduce costs. Variable-speed technology has been around for many years but has only recently been adopted on commercial fish farms. Variable-frequency drives (VFDs) are generally mounted in an electrical enclosure near the electric motor to control and hold the motor at a specified speed. When used with a pump, a VFD can provide energy-efficient flow from a pump at varied flow rates. Significant energy savings can be achieved in a pumping system by minimizing the required pump speed, especially when pumps are in continuous or long-term service which is the case when used in partitioned ponds. For example, reducing a pump’s motor speed by only 10% can reduce the energy required by the pump’s motor by as much as 30%. Two scenarios have been illustrated from pump performance data collected through a series of field tests. First, non-variable pumping (steady rate) and second, variable speed pumping, where the pumping rate was adjusted throughout the culture period. In both cases a 10-hp high-speed screw pump was operated 12-hrs per day over a 240-day growing season in a 7-acre split-pond with electrical energy costs set at $0.14/kW-h. The total energy use for non-variable pumping would be 14,486 kW-h and cost $2,028 as compared to variable speed pumping which would require 7,358 kW-h and cost $1,030. It is obvious there would be greater costs associated with non-variable pumping due to more energy use. In reality, variable speed pumping is a great resource for researchers because they can optimize pump performance through field testing and trials, although, in many cases it may not be the best decision for farmers because of the added expense and risk of failure of VFDs.