“Reduce the speed by half; reduce the water flow and the power consumption by half. Then run the pump twice as long?! What savings?”
– Common Industry Misconception
In the July 2004 issue of the Harvard Heart Letter, Harvard Medical School published its Calorie Count Table listing the calories burned by dozens of activities listed by category, time, and the weight of the individual performing the activity. The university’s studies show that a 125-pound person who runs for 30 minutes at a pace of 6 miles per hour would burn a total of 300 calories. That same individual walking at a 4-mph pace for 30 minutes would burn a total of 135 calories.
This information enables us to extrapolate that a 125-pound person who walks 4 mph for a total of 60 minutes would then cover a distance of 4 miles burning a total of 270 calories. A 33% reduction in speed covering a 25% increase in distance and 50% increase in time, resulting in a 10% reduction in energy (calories).
This information received additional support in the studies titled "Energy Expenditure of Walking and Running," published December of 2004 in Medicine & Science in Sports & Exercise, finding that over a distance of 1,600 meters (roughly 1 mile), men burn an average of 124 calories while running, and just 88 while walking. Women burned 105 calories running and 74 walking.
In the September 2009 article titled “Tested: Speed vs. Fuel Economy,” Consumer Reports conducted a seven-vehicle test comparing the fuel economy of each at speeds of 55, 65, and 75 mph. The results were as expected, the slower a vehicle moves, the less fuel it burns, even when covering the same distance. One of the vehicles included in the test was the Toyota Camry, where at a speed of 75 mph, the fuel efficiency was tested at 30 miles per gallon; at a speed of 65 mph, the fuel efficiency tested at 35 mpg. A reduction of speed by 13.3%, resulting in a 14.28% decrease in mpg. With the U.S. Department of Transportation estimating that Americans drive an average of 13,476 miles per year and a U.S. average gas price of $2.419 per gallon (AAA National Average Gas Price), this 5 mpg decrease in fuel consumption could result in a fuel savings of roughly $160.00 per year.
Though Consumer Reports fuel efficiency findings do not seem as significant as the data provided by Harvard regarding calories, both do illustrate a disproportionate savings in energy when compared to speed and distance. To understand the potential savings in power consumption utilizing a variable speed pump, one has to understand that the reduction in power consumed is also not proportionate to the reduction in speed (rpm).
One needs to consider that the variable speed pump does also utilize a different motor type, which even without a reduction in speed does consume less power to operate than that of a standard pump.
A VSP utilizes a permanent magnet motor. The technology in the motor design (more similar to that of a smart car) is one of the factors that has led to its increase in popularity, not just in the pool industry but in factories worldwide. The name actually describes the design. Permanent magnet motors use high-performance rotor magnets. These magnets create a magnetic field that is constant compared to squirrel cage induction motors (standard pool motor type) which actually create a secondary magnetic field. This secondary magnetic field generates wasted energy in the form of heat loss. Permanent magnet design eliminates this, resulting in an increase of up to 12% operating efficiency.
In a centrifugal pump, it is important to understand that manipulation of a single variable can cause changes to hydraulic parameters. The Laws of Affinity for pumps express the relationship between motor speed, flow rate, and energy consumption. Energy consumption, as in the examples above, drops at a nonlinear (disproportionate) rate to both pump speed and water flow. To calculate and understand the power consumption at a lower speed consider that the percentage decrease in rpm (x) will result in an energy consumption decrease of that percentage cubed (x3). Utilizing the formula provided, we can determine that a decrease in speed (rpm) by 50% will reduce the flow rate by 50%, but will reduce the power consumption to 12.5% of the original draw (an energy savings of 87.5%).
Note the following chart from the U.S. Department of Energy:
It is important to understand that the Laws of Affinity do also apply to centrifugal pumps that utilize a squirrel cage induction motor. Meaning, a dual-speed pump operating at low speed (1700 rpm), a reduction of speed by 50%, a reduction of flow (gpm) by 50%, will also result in a draw (power consumption) that is equal to 1/8th the original draw of that pump at full speed (3450 rpm). The waste heat losses, typical of the squirrel cage induction motor will still be present, however this does not affect the power savings of that motor when compared to that same motor operating at full speed.
It is also important to understand that a significant energy savings can still be achieved, utilizing those same Laws of Affinity by manipulating the size of the impeller of that same pump and operating at 3450 rpm. Reducing the size of the impeller would similarly reduce the power consumption by the cube of the impeller diameter change. For example: a 30% decrease in impeller size would result in a power consumption equal to 34.3% of the original -- an energy savings of 65.7%.
Another factor of the Affinity Laws to consider – if you are trying to calculate what this all equates to utilizing a manufacturer-provided flow chart – is the implications of a variable change on head pressure. This again is nonlinear, meaning the reduced water flow will create less resistance through the systems plumbing in a ratio disproportionate to the reduction in rpm, or change in impeller size.
Simply said, the head pressure (calculated by multiplying pounds per square inch x 2.31) will vary with the change in speed. For example, a 50% reduction in speed, will result in a 25% reduction in head pressure. This new total dynamic head calculation would also recognize a change in flow rate that is also disproportionate to the reduction in speed (or a change in impeller size).