Sep 24, 2018

Smarter Sustainability

Pump Energy Index evaluation ensures efficiency improvements

The efficiency island is bound by the maximum and minimum impeller diameters and the iso efficiency lines. Maximizing the island helps achieve the greatest range of flow  and total head span.
The efficiency island is bound by the maximum and minimum impeller diameters and the iso efficiency lines. Maximizing the island helps achieve the greatest range of flow and total head span.

While the goal of the U.S. Department of Energy’s (DOE) Energy Conservation Standard for pumps is to improve the overall efficiency of pumps sold in the U.S. market by 2020, the DOE also wanted to encourage the use of variable speed controls in variable load systems to maximize energy savings. That is why the DOE Pump Energy Index (PEI) includes two measurement methods to rate the performance of pumps: one for constant load equipment classes (PEICL) and another for variable load (PEIVL). PEICL applies to pumps sold without variable speed controls, while PEIVL applies to pumps sold with variable speed controls. Pumps that meet the standard in either capacity are assigned a PEI value of 1.0 or below.

Variable-speed drives (VSDs) often are applied to existing systems to increase overall efficiencies. When a VSD is properly installed, pumps can work more efficiently. This extends product life, reduces energy consumption and decreases electrical system stress. 

However, by establishing two methods of pump efficiency indexes, there is some confusion in the industry regarding compliance because the variable load test method can mask the deficiencies of an inefficient PEICL pump when a motor and VSD are added based on nominal efficiencies. Even though putting a VSD on an inefficient pump achieves the PEIVL rating acceptable under the DOE test procedures, the non-compliant pump has not been upgraded for efficiency and would not meet the DOE’s standard for PEICL if tested on its own. 

In order to ensure hydronic HVAC and plumbing systems are being designed from the start with the most efficient pumps, engineers should look at both PEICL and PEIVL with key emphasis on PEICL to confirm that the pump they are specifying has a rating of 1.0 or less in accordance with DOE standards. 

The following example illustrates the improvements to pump efficiency after hydraulic redesign using the DOE’s PEICL calculation. 

DOE Equations

Continuous load PEICL = PERcl/PERstd

Variable load PEIVL = PERvl***/PERstd

PEICL Pump Example

Old design: Bell & Gossett Series 80SC split coupled inline pump, size 4 by 4 by 9.5 in.

1.02 = 14.24/14.00

Because the PEICL is above 1.0 it would not qualify for DOE minimum compliance.

New design: Bell & Gossett Series e-80SC split coupled inline pump, size 4 by 4 by 9.5 in. 

0.95 = 13.39/14.16

Because the PEICL is below 1.0 it would qualify for DOE minimum compliance.

The difference between these two pumps calculated for PEICL is approximately a 6% energy savings through the redesign.  

The Performance Energy Rating (PERSTD) is an average of the driver input power for a minimal compliant pump. It is calculated based on each pumps hydraulic curves, load points and a standard level C-value that is provided by DOE. This is why the denominator changes in the above equations. These load points are 75%, 100% and 110% of BEP flow for the specific pump corrected to nominal speed of rotation.

The PERCL is an average of the driver input power of an actual pump. It is calculated based on each pump’s hydraulic curves, load points and actual pump efficiency at those load points. This is why the numerator changes. These load points are 75%, 100% and 110% of BEP flow for the specific pump and motor corrected to nominal speed of rotation.

Applying PEIVL 

Just as the 80SC and redesigned e-80SC pumps calculated for PEICL realized a 6% energy savings when applying a VSD to the same two pumps and calculating PEIVL, an additional 6% increase in energy savings can be achieved as illustrated in the following calculation.

The PERVL is an average driver input power to VSD control. It is calculated based on each pump’s hydraulic curves, load points and actual pump efficiency at those load points. This is why the numerator changes. These load points start with a 20% of TDH control head and 25%, 50%, 75% and 100% BEP flow equally weighted for the specific pump, motor and VSD corrected to nominal speed of rotation.

Old design. Bell & Gossett Series 80SC split coupled inline pump, size 4 by 4 by 9.5 in.

0.50 = 6.99/14.00

New design. Bell & Gossett Series e-80SC split coupled inline pump, size 4 by 4 by 9.5 in.

0.47 = 6.64/14.16

Pursuit of Efficiency

The PEIVL calculation further demonstrates how a specifying engineer can maximize efficiency in a hydronic system and make informed decisions about using compliant pumps. With an HVAC system accounting for up to 50% of a commercial building’s energy use, designing efficient heating and cooling systems is critical to keep a project on budget, as well as meet sustainability goals. 

In addition to the two versions of the PEI, the DOE also established U.S. Standards for Pump Testing that are based on the Hydraulic Institute’s (HI) 40.6 Pump Efficiency Test Standard. The DOE required changes to this standard, so HI updated the standard (HI 40.6-2016) so it is now aligned with federal law. This pump test standard defines the testing methods to determine the efficiency of a bare pump and allow for the addition of electric motors and variable speed drives based on Nominal Efficiency Tables or actual wire to water testing.

The DOE standards for pump efficiency are just the starting point. Even greater energy efficiencies can be realized if designers continue to seek smarter ways to specify pumps and stay abreast of industry advances.

About the author

Mark Handzel is global vice president, product regulatory affairs & compliance, for Xylem Inc. Handzel can be reached at [email protected]

expand_less