This formula takes energy consumption at each operating point into consideration.
by Trygve Dahl
January 11, 2018

As global energy consumption continues to rise, greater burdens are placed on power generating infrastructure and the total cost of that energy. The pump industry reacted to these external market forces as manufacturers within the industry have innovated with new products and control systems.

That’s the backstory behind a new, more precise and reliable methodology for measuring pump efficiency: true weighted efficiency (TWE).

For those in the pumping industry, the external forces driving change begin at the U.S. Department of Energy (DOE). The agency found that pumps in industrial applications, such as heating, ventilation and air conditioning (HVAC); municipal water; chemical process and refinery applications account for as much as 20 percent of all energy consumed in the U.S., and their results launched a worldwide regulatory movement to reduce energy use.7

Similarly, smart fiscal practices within the pump industry provided motivation to look beyond the pump to the whole pump system. As the industry began to appreciate that life cycle operating costs should be a litmus test to evaluate pumping choices, we learned that 75 percent of lifetime expenses can be attributed to maintenance and energy costs.

Thus, for regulatory and competitive reasons, the spotlight has fixed on pump or system efficiency. Efficiency, the numerical ratio between 0 and 100 percent, tells us which pump system consumes less energy at a single point. However, pumps operate at different condition points for different periods of time. How is pump efficiency measured in this case?

TWE answers this question because it represents a true measure of overall pump efficiency for a specified pump working at many different operating points. TWE offers an easy, accurate and universally recognized methodology to compare energy efficiency. With broad and consistent use, this innovative metric will provide greater transparency of energy efficiency in the industry.

Background

Most of us understand the significance and complexity of making optimal pumping selections that meet essential functional operating requirements.1, 2, 3

The process involves many factors including pump type, configuration, system design and operating control strategy.4, 5

Life cycle costing (LCC)—calculating the lifetime cost to purchase, install, operate, maintain and then dispose of equipment—has become a more common practice.6

For pumps operating continuously or for a significant portion of time, energy consumption represents the largest single cost. The energy consumption for a single operating condition with a constant input power, is calculated as follows:

Equation 1

Most pumping systems, however, have a load profile defined by multiple operating conditions. These operating points occur at different head and flow conditions for a specified duration of time. The total system energy consumption for a representative load profile and four operating points is calculated as follows:

Equation 2

While calculating the total energy consumption over four different operating points is more exact than comparing pumps based on a single duty point efficiency, this calculation is rarely performed.
In summary, pump efficiency remains the preferred measure, but no standard method exists for calculating it across multiple operating conditions.

Change Unfolds

Recently, various efficiency metrics designed to compare pumps with multiple operating conditions have emerged. The Ecodesign requirements for water pumps, EU Regulation No 547/2012, ushered in a minimum efficiency index (MEI) that facilitates comparisons at the best efficiency flow rate and at 75 percent and 100 percent of the best efficiency point (BEP) as well. The pump energy index (PEI), which applies to constant and variable speed pumps with multiple modes of operation, came about when the DOE introduced legislation requiring specific types of pumps sold in the U.S. to meet a minimum PEI benchmark. However, neither of these metrics qualify as efficiency values, nor can they compare pumps using an actual load case and system curve.

An interesting approach known as the part load efficiency value (PLEV) was recently unveiled. The PLEV is a weighted average efficiency value inspired by the integrated part load value (IPLV), applicable to chillers and published in the Air-Conditioning, Heating and Refrigeration Institute (AHRI) Standard 550/590. The PLEV equation applied to pumps is:8Equation 3

Point A corresponds to 100 percent of the duty flow operating 1 percent of the time. Point B represents 75 percent of the duty flow, 42 percent of the time. Point C is 50 percent of the duty flow, 45 percent of the time. Point D is 25 percent of the duty flow, 12 percent of the time. PLEV is thus a time-weighted average of the efficiency values, EffyA, EffyB, EffyC, and EffyD, at each operating point.

While a solid step forward, this formula only weights the pump efficiency based the proportion of operating time at each point. Since energy consumption is based on both time and power at each operating point, an improved weighting method is needed.

How TWE Is More Accurate

TWE weights the energy (power and time) at each operating point and therefore more accurately tracks multi-point energy consumption between various pumps using familiar and easily obtained efficiency values. This approach can optimize either pump energy consumption using pump efficiency values or total input energy consumption based on wire-to-water efficiency values.

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