Optimizing unitary pumping solutions

Unitary pumping solutions integrate variable speed drives (VSDs) to allow for pumps to self-optimize.

By Stephen Putnam, Grundfos Pumps Corp., Downers Grove, Ill. February 20, 2017
Learning Objectives
  • Learn where the unitary pumping solution is best suited.
  • Define the benefits and energy efficiencies that come with self-optimized pumps.

Pump manufacturers can offer engineers and building owners the ability to control equipment through onboard logic. These capabilities are being underused, however.

The market standard for a pump is to be driven by a third-party motor whose performance is dictated by a third-party variable speed drive (VSD) that was programmed by a third-party contractor. As capable as all of those involved in this process may be, this is not the best design to run pumping systems as efficiently as possible. As in the boiler and chiller market, it is the pump manufacturers know how best to use and control their products to achieve maximum utility while minimizing energy consumption. Pump manufacturers who design and produce their products are best equipped to control their pumps using the unitary pumping solution. 

What is a unitary pumping solution?

Electrically commutated motors (ECMs) originated in fans, as the technology was limited by low torque ratings in the fractional horsepower segment. But it did not take long for the ECM technology to spread into pumping systems, and this was the key that opened the door to incorporate integrated VSDs and ultimately "self-optimizing" logic onboard a pump.

ECM is driven by a permanently magnetized rotor, and the rotational force comes from alternating the polarity of the stator in the motor. There is an inherent risk of rotor lock in this design; the solution, engineers found, is to incorporate speed controls in the motor to protect against this threat. The solution proved to be valuable: With integrated speed controls, the energy-saving benefits VSDs offered to larger horsepower pumps become available in smaller horsepower pumps.

Once the market had integrated speed controls, a self-optimizing ability of a unitary pumping solution was the logical next step. Self-optimizing ability allows the pump to "rightsize" itself for the specific application, into which it is installed without end-user interaction.

All unitary pumping solutions have onboard controlling ability that allows for the pump to self-optimize its performance in which pump speed is dictated by an integrated VSD that is driven by an ECM. With the proper anatomy and performance capabilities, energy-saving rates of 80% to 90% can be achieved as compared with the market’s standard definition of a pump (wet-end and induction motor with no self-optimizing ability). For engineers, contractors, and end users looking for low-hanging fruit for energy savings, this is the most ripe. 

What is self-optimizing?

Before diving into depth on the economic argument for a unitary pumping solution, a basic engineering overview of pump curves and system curves is required.

Pump curve: The market-standard pump operates at a constant speed along its fixed pump curve. The impeller diameter inside of the volute is a fixed size, and the rotational speed at which the motor-and, consequently, the pump-operate is constant. Note that as flow (gallons per minute) increases, total dynamic head (TDH, commonly referred to as "head"), will decrease.

TDH = friction losses + elevation in system + discharge head

The reverse is true as well; if flow decreases, the head will increase. The only variable in the system is the flow, but the pump will only operate along its curve.

System curve: Every piping system has a system curve that denotes how much resistance is embedded (due to pipe friction, valves, fittings, elevation, and delivery pressure requirements) at any given flow. The system curve is static so long as there are no dynamic changes in the system that would change the system characteristics (pipe length, valves, or fittings added/taken away, etc.). Open systems (hot-water recirculation) have a fixed system curve as there is a fixed amount of piping, valves, and fittings. Closed systems (HVAC), have dynamic system curves. These dynamic systems are the result of heating zones opening and closing based on occupant demands. When all zones in an HVAC system are open, there is more length of pipe through which the product must be pumped as compared to one zone open, where there is less length of pipe through which the product must be pumped. Even though the pump is servicing the same system, the system curve changes depending on the number of zones that are demanding heat. Duty point:

The pump will always operate at the point of intersection of the pump curve and system curve. The duty point can migrate left and right along the pump curve.

Proportional pressure curve: Note that the pump curve and the system curves are inverses of one another. As flow may increase or decrease, the system curve adjusts itself along the pump curve at the new duty point. The performance is constrained by the pump and the system into which it is installed. The opportunity for self-optimizing ability comes in answering the question, "What does the end user need to do the job?"

Proportional pressure operation answers this question by attempting to mirror the system curve in that specific system. It will run at the necessary flow while eliminating the excess head produced by the fixed-speed pump, saving energy in the process (see Figure 1).

Self-optimization: The onboard algorithms will continually monitor the system, finding and operating on the most appropriate proportional pressure curve into the specific system into which it is installed. The end result is a pumping system that will continually rightsize itself down to the lowest head production while still doing the job, saving a significant amount of energy in the process. [subhead]

What are the benefits?

The greatest benefits of unitary pumping solutions depend on who you ask within the system design and operation:

Engineer: Cast one net, catch all of the fish. The ability to specify a minimal number of pumps for all of the applications found in a facility. Moreover, unitary pumping solutions provide the value of energy optimization to the end user.

End user: One energy-saving opportunity that is often overlooked is that unitary pumping solution systems will continually consume minimal energy while maintaining comfort. Additionally, there is a minimal amount of maintenance (if any) for these systems.

Contractor: The contractor does not have to size or select a pump with a great deal of specificity. Instead, a single pump can be installed in a vast array of applications. Among the largest opportunities for a unitary pumping solution is in hydronic heating applications. The application is more popular across the northern U.S., residentially, and is very common across the U.S. commercially. Often, there are circulating pumps employed in these systems, and the unitary solution has yet to penetrate the market in a significant way.

As an example, compare a market-standard fixed-speed circulator pump versus a unitary pumping solution. Both pumps run 5,000 hours/year and the end user has an energy cost of 9.5 cents/kWh. Most applications will have a variable load profile, with the most universally accepted sample load profile being the Blauer Engel load profile

Market standard

The market standard, through its fixed-speed operation, can only operate along its pump curve. Additionally, when the Blauer Engle load profile is applied, the pump will operate at different points along that fixed-speed pump curve for a certain number of hours during its annual operation. As the duty point migrates along the fixed-speed pump curve, each point will represent a specific amount of wattage consumption for that specific duty point. In reality, of course, the pump will operate at other points, but for the sake of this example, take each flow class as an average to simplify the calculation methodology.

The unitary pumping solution with an ECM and integrated VSD driven by the control logic allows the unitary pumping solution to run in a proportional pressure control scheme (gallons/minute and TDH have a direct relationship and more closely mirror the system curve). The proportional pressure operation will operate at the same flows (gallons/minute) as the fixed-speed operation, but allows the pump to generate less head (TDH) at the same flow rates. This allows for an opportunity to save substantial power, as the fixed-speed pump inherently produces excess head that is not vital to the system. Proportional pressure controls allow the pump to produce the head amount that is needed (as dictated by the system curve, Figure 2) but not overproduce by too much (it will produce even less head as the pump "self-optimizes" itself).

Simply by switching the pump to a VSD-driven ECM that allows for a proportional pressure control scheme, the end user will save 62% in energy consumption ($80.84/year in energy-consumption costs). This figure may seem minimal, but at larger facilities where there are numerous pump application opportunities to use these pump systems, the cost savings are quickly multiplied. 

Proportional pressure

The economic argument can now be brought back into the discussion to paint a complete picture for true cost of ownership from the end user’s perspective. In essence, the below line of thought is meant to represent the end user’s priorities.

The largest objection to a unitary pumping solution is almost always going to be cost. In the sample case study mentioned previously, assume the unitary pumping solution costs 150% of the market-standard single-speed pump. Carrying the 50% cost-premium assumption and knowing that the energy-consumption cost for the market standard and the unitary pumping solution is $129.54 and $48.70, respectively, reasonable end user questions are:

  • Assuming a first 15 years of operation, what is the cost of ownership for each?
  • What is the break-even point for the two technologies?
  • Over the first 15 years of operation, what is the cost-benefit of the unitary pumping solution as compared with the market standard?

Over the first 15 years of operation, the market standard will cost the end user $2,593 (initial costs + annual energy costs). The unitary pumping solution, however, will cost $1,705-saving the end user $888 over the 15-year period (no inflation rate). The break-even point between the two technologies is 4 years, so from years 4 to 15, the end user will begin saving on operational costs (see Figure 3). 


It can be easily argued that the savings figures in the previous section are very conservative, the reason being that Figure 3 shows a worst-case scenario for the proportional pressure operation. This assumes that the pump has not yet had a chance to self-optimize and rightsize itself down to the proper proportional pressure control curve. In reality, after a short period of run time, the pump will settle and run at a newer, lower head-proportional pressure curve in accordance with system characteristics.

As the pump continues to learn the specific system into which it is installed and rightsize itself, finding the lowest possible proportional pressure curve on which to operate and even more energy savings can be achieved. The self-optimization will still deliver the same flow as is needed for the system, but also will produce as close to optimal head in accordance with the system curve. This eliminates excess head and optimizes energy consumption in the process (see Figure 4).

As the unitary pumping solution’s self-optimizing ability continues to learn the system, it will operate at the lowest possible proportional pressure curve and still meet system head requirements. Through this process, energy consumption is further mitigated beyond that of a fixed proportional pressure curve. When compared to the market-standard fixed-speed pump, the unitary pumping solution will save 1,085 kWh/year (80%). The $103.08 savings to the end user in energy costs is achieved without any end-user interaction.

In reality, there are additional cost factors beyond that of the initial cost and lifetime energy costs. To paint the most accurate picture of the true cost of ownership for a pump versus a unitary pumping solution, maintenance costs and possible utility incentives must be taken into account. However, the true definition of lifecycle cost deals with additional factors (i.e., disposal, downtime, etc.) . In this example where it is assumed to be pumping water, only maintenance and energy costs will be taken into account. Disposal and downtime costs are not factors in this example, as those costs would be negligible.

Most market-standard pumps have three-piece construction, consisting of a third-party detached motor bolted to a bearing frame that is tied to the wet end (volute, impeller, etc.) of the pump. This design has a built-in maintenance cost, as the motor bearings may need to be lubricated and seals and impellers will wear and need to be replaced for the pump to maintain a 15-year useful life. In this example, an annual maintenance cost of $100/year is carried as this is the combination of total man hours for checking and potentially servicing the pump, as well as the material cost any replacement parts.

The circulator segment of the unitary pumping solution has wet rotor construction where the product being pumped (water, in this case) gets into the motor can and lubricates and cools the motor. The controller, VSD, ECM, and wet-end pump are all one integral unit and there are no seals to replace or motor bearings to be lubricated. It is maintenance-free construction. Consequently, the maintenance cost factor would apply to the traditional market-standard pump but not to the unitary pumping solution.

While many utilities across North America do not have dedicated pumping programs, most utilities do have some type of custom savings program. This acts as a catchall for the utilities to capture energy and power savings through technologies not specifically vetted through their existing program portfolio. If an end user, contractor, or engineer can prove energy and power savings, the utility will offer an incentive to install more energy-efficient equipment. Often, utilities pay out incentives based on kilowatt hours saved.

In this example, it is assumed that the utility will pay 8 cents/kWh saved in the first year of operation. The unitary pumping solution saves 1,085 kWh/year, so the incentive paid to the end user is $86.80, meant to bring the initial cost of the unitary pumping solution on par with the market standard. This may seem insignificant, but keep in mind that this would be for just one pump. Multiple pumps can be bundled together for a much larger incentive with higher kilowatt-hour savings.

When incorporating the maintenance costs for the market standard, the slope of the lifecycle cost curve increases. The true lifecycle cost would be $3,993. The unitary pumping solution has a lifecycle cost of $1,259, offering a $2,734 savings for the end user. The unitary pumping solution is more expensive initially, but the break-even point is after 1.5 years of service; for the duration of the life of the pump, it is saving the end user money.

Understandably, when unitary pumping solution are specified into projects, oftentimes they do not survive the value-engineering phase of a project. Looking at the total lifecycle costing aspect of a unitary pumping solution, however, can reprioritize the pumping system choice so that unitary pumping solutions are not value-engineered out of projects where specified. No matter if the unitary pumping solution comes in the form of a fractional horsepower circulator or a multiple-pump skidded system, the unitary pumping solution will be the most cost-effective option for the end user.

Enhance energy performance

The market-standard pumping system is a vinyl 12-song album, outdated and not reflective of today’s technology and capability. Engineers continue to specify and design these systems solely out of habit. Their true lifecycle cost far surpasses that of the unitary pumping solution because they typically require far more maintenance. Controlling these pumps requires more equipment and maintenance hours and saves less energy than the unitary pumping solution.

The unitary pumping solution brings the pumping market on par with the existing boiler and chiller market with onboard controls, integrated ECMs and drives, and wet-end pumps. Through their self-optimizing ability, they are able to optimize energy consumption while performing the work required by the end user. They can do this with minimal (if any) maintenance. The sum of all of these capabilities drive down break-even points and are much more cost-effective to employ as compared with today’s market standard. Unitary pumping solutions open opportunities to capture energy savings in the smaller hydraulic ranges that currently are being overlooked. Manufacturers, engineers, and contractors all work for the end user. All have a shared responsibility of designing, building, installing, and maintaining equipment that will meet end users’ needs and also make the most economic sense. The latter of these responsibilities is easily overlooked, but the economic case demonstrates the most value to end users.

-Stephen Putnam is business development manager at Grundfos Pumps Corp. in the energy/utility services group.