What Actually Is Wire-to-Water Efficiency?

The term has been around for years, but that has not negated its importance.

By Ed Butts, PE, CPI

One of the first terms I heard during my first year as an employee in 1974 was “wire-to-water efficiency.” Although this phrase did not mean much to me at the time, I soon became familiar with it and its connotation.

As the need for energy efficiency rapidly increased during the 1970s, the importance of determining wire-to-water efficiency also gained monetary relevance. An efficient and effectively operated pumping plant could avoid unnecessary power costs on the order of $500 to $3000 per year depending on the size of the pump and type and extent of the operation.

I soon heard the term often replaced with “pumping plant efficiency” and “overall plant efficiency.” This month, as a fitting end to another year, let’s examine the term and how time may have changed it but not diminished its importance.

Derivation of Wire-to-Water Efficiency

The term “wire-to-water efficiency” was coined to define the progressive loss in operating efficiency through a water pump and electric motor combination, beginning with the incoming power supply (the wire element) to the ultimate delivered product (the water). Traditionally, the term was used to describe the combined efficiency from the individual efficiencies of the pump and electric motor of a pumping installation, while often ignoring the impact of the power transmission method and motor controller.

Before roughly 1970, wire-to-water efficiency of most pumping systems was seldom considered except for large installations where the efficiency of the pump and motor combination was deemed to be significantly important as a measure of operating cost. But beginning with the energy crisis of the 1970s, the resulting almost universal rise in electrical costs made determining wire-to-water efficiency critically important even for smaller pumping systems.

There are only three basic ways to lower the operating cost of a pumping plant: reduce the input power of the plant, reduce the operating hours, or lower the unit costs of energy used.

As the third will likely never happen, we must concentrate on manipulating the first and second options. And in recent years, the introduction of the variable frequency drive, improved pump technology and materials, and high efficiency motors have only added to the term’s importance.

For a vertical or submersible turbine well pump (Figure 1), the energy required to pump a given rate of water is fundamentally determined by the following:

• Instantaneous flow rate
• Pumping lift to wellhead
• Operating pressure or additional lift to discharge point
• Pump hydraulic and mechanical frictional losses (pump efficiency)
• Driver efficiency
• Power transmission losses (lineshaft or drop cable)
• Power supply and controller in some cases.

Energy requirements decrease as pumping lift and operating pressure decrease and pumping plant efficiency increases. To maximize pumping plant efficiency, the components should be matched to site-specific conditions of pumping lift, operating pressure, and flow rate. Essentially, it is a method to figure out how efficiently a system uses electricity to perform work, such as the transfer (pumping) of water.

In hydraulic terms, it is an instantaneous or snapshot indication of the ratio of the fundamental or water horsepower (WHP) needed to pump a given flow rate against a given total dynamic head (TDH) to the actual or input horsepower to the motor.

Although other forms of determining and assigning flow rate, head, water horsepower, and input horsepower exist, the flow rate will be measured in gallons per minute (GPM) and the TDH in feet for our purposes.

Input horsepower includes the negative impact (i.e., reductions) from the efficiency losses of the pump (brake horsepower), motor, and power transmission method, if any. The instantaneous water horsepower, brake horsepower, input horsepower and efficiency for electric and diesel driven pumping plants are determined using the following equations:

 

 

 

 

 

 

 

 

 

 

 

 

Horsepower units are generally converted to kilowatts (kW) by multiplying the horsepower by 0.746 for the purposes of direct comparison. It is vital for all equations that comparable power units be used to provide uniformity and accuracy. Values of efficiency are entered in decimal form for all three versions, so an efficiency of 90% would be entered as 0.90.

Pump and motor efficiency for new equipment will typically increase with a corresponding increase in flow rate and motor horsepower. Thus, the wire-to-water efficiency of a new pump and electric motor combination will generally vary from 15% to 25% for low capacity and fractional horsepower circulating pumps; 30% to 50% for fractional and low horsepower (0.5 to 5 HP) 4-inch submersible well pumps; 50% to 75% for mid-range (7.5 to 30 HP) submersible and vertical turbine pumping systems; 65% to 80% for 40 to 150 HP submersible and vertical turbine units; and 80% or greater for larger (greater than 175 HP) submersible and vertical turbine pumps and motors.

Submersible motors will generally display 5% to 10% lower efficiency HP than their vertical turbine pump counterpart. Typically, this means older electric well pumping systems will possess 40% to 70% overall efficiency, and diesel engine- powered well pumps will possess between 15% to 35% overall efficiency depending on the size, age, and operating condition of the engine.

Modern methods of computing wire-to-water efficiency for a deep-well pump considers the efficiency of the pump, motor, power transmission method (submersible cable or lineshaft), and motor controller at a series of operating points that simulate actual operating conditions.

In certain cases, such as when an engine-driven generator set (genset) is permanently used to drive the motor, the efficiency of the genset should also be considered. Once factored, this will develop a baseline instantaneous efficiency in kilowatts or energy intensity for volumetric measurements in kilowatt- hours per million gallons pumped (kWh/MG) or kWh per acre foot (325,800 gallons) for irrigation systems. The
value can then be directly compared to the other pumps in a pumping system to determine which has the highest potential for energy savings and optimization.

Upgrade to Pumping Plant Efficiency

The passage of time, the need for accuracy, and increasing energy costs irrespective of whether they use electrical power or fossil fuels has created the need for a modified energy efficiency term.

Although the wire-to-water efficiency term is still valid and often used for electrically driven pumping plants, many pumping plants, especially with large irrigation systems, are driven by internal combustion engines as well as accompanying gearheads, driveshafts, vee or cog belts, or other speed reduction/increasing devices.

For electric units, this may also include the added impact from variable speed drives and eddy-current or hydraulic couplings. Thus, the term has been largely modified to the currently preferred terms of “pumping plant efficiency” or “overall plant efficiency.” These terms accurately and simplistically describe the efficiency of the entire pumping plant including the driver, pump, power transmission method, controller, and power supply, if applicable.

Hydraulic losses of the system and pump are generally components of the total dynamic head and factored into that value while mechanical losses are included as power losses (Figure 2). The loss in hydraulic efficiency within the pump is reflected in the difference between the water and brake horsepower. Today, a new electric motor’s efficiency rating is determined by performance tests required by the U.S. Department of Energy to prove compliance to current motor efficiency regulations.

In the real world, the output the pump produces can experience losses at multiple points between the initial connection and the final flow of the pump. The efficiency loss associated with the pump and driver typically comprise the greatest efficiency and power loss while the losses associated with power transmission and control systems are often referred to by the less desirable term of parasitic losses.

These losses include the lineshaft for a vertical turbine pump, drop cable to a submersible pump, driveline, and right-angle gearhead or other gearbox (when used). Typically, the aggregate from these additional losses amount to less than 3% of the overall efficiency or system power loss, but they are important enough, particularly in deep-well applications, to be included.

Inclusion of the parasitic losses truly reflects the wire-to-water efficiency as it includes the loss of all components that contribute to the pumping of water as well as the horsepower losses. The sum of these losses is typically reflected in the watthour power consumption for electrically powered systems or fuel consumption for engine-driven systems.

What Affects Pumping Plant Efficiency?

The most common factors that impact pumping plant efficiency include losses from pump and motor normal frictional wear or misalignment (particularly at bearings and running surfaces), wear from cavitation and abrasives, corrosion, encrustation, incorrect flow and head operating points, inadequate or improper suction or inlet conditions, and air entrainment.

Two opposite actions, encrustation and corrosion, are electrochemical reactions that are often overlooked but can play a significant role in plant efficiency due to plugging of hydraulic passages or deterioration/loss of material. Beyond these obvious factors, several other factors can also impact the efficiency of a pumping plant. These include the following.

Power Quality

The power quality entering the variable frequency drive (VFD), controller, or motor affects not only motor efficiency but the overall system efficiency. This is now more important than ever, as harmonics are now more common on utility power supply lines and from VFDs.

Prior to installation, it is important to assess the quality of the power that will enter the motor, making sure voltage and current are within acceptable tolerances and that any harmonics comply with Institute of Electrical and Electronics Engineers (IEEE) 519 requirements. Standard allowable power quality variations can be found in National Electrical Manufacturers Association (NEMA) Standard MG-1.

Be sure to discuss any variations found outside the standards with the motor manufacturer as some problems may have relatively simple solutions. If power is to be continuously supplied by a generator, consult with the motor manufacturer to ascertain if any special power quality corrections to the plant efficiency or genset are required.

Electric Power Transmission Cable

The type of cable used between a VFD and the motor plays a critical role in reducing harmonics and electromagnetic interference (EMI). For long offsets and deep submersible well pump installations, these cables should be shielded and specifically rated for use with a VFD. Check with the VFD motor or cable manufacturer to ensure that cables are the recommended size and type for the system’s voltage and current limits.

Resist the temptation to use smaller or improperly shielded cables to save money since an improperly sized cable can damage the motor. The correct type and size of cable will always cost less than a replacement motor and will pay off in the long run.

The distance between the motor and drive matters as well. An excessive drop in voltage between the VFD and motor can result in a corresponding drop in motor efficiency. While it is common to locate a VFD in an electrical room with the motor in the well or pump room, it is wise to locate them as close to each other as possible.

Lastly, there is equipment that can aid in power quality when long cable distances are required. Refer to the earlier Engineering Your Business columns on VFDs for details. Also, discuss these options with the VFD and motor manufacturer if required.

System and Motor Grounding

Motor efficiency can also be impacted if the entire system is not grounded correctly. In addition to the NEC Code requirement for the motor being grounded to a true ground, the motor should also be bonded to the VFD to alleviate high frequency noise that can be caused by the VFD if not installed correctly. This can affect motor and VFD operation with a resultant drop in efficiency.

The motor should be bonded to the drive, using a braided type of grounding wire that is, at minimum, the same size as a single power lead and is routed within the same conduit as the power leads.

Engine System Efficiency

Although the efficiency of an internal combustion engine used to drive a pumping plant is low, it is typically substituted for the electric motor as the driver. The potential losses associated with this power source necessitate consideration of the subsystems as well. This means that all support systems such as combustion air, lubrication, cooling, exhaust, fuel, and electric systems and filters should be maintained at a high level to maintain engine efficiency.

Drive Efficiency over Entire Speed Range

The primary benefit of a variable speed drive (VSD) system is its ability to adjust pump output to fit demand. To deliver this benefit, it must be installed and adjusted to perform efficiently across the entire projected speed range. Every VSD has a unique efficiency curve based on its specific components and settings. Even drives from the same manufacturer can cause a motor to yield different efficiencies across its speed
range.

Using the affinity laws, variable speed makes it possible to move along the system curve to meet demand while optimizing power usage. Basically, with a VSD, when the system needs less power, it will consume less power.

Switching frequency of a VFD, however, can also impact efficiency. A higher switching frequency in the VFD can increase motor efficiency but can also lower VFD efficiency and induce greater cable losses.

System efficiency depends on multiple components that produce different outputs when they interact. To achieve optimal motor efficiency, a drive must also be set up to respond correctly to changes in load. A drive should be set up to monitor system demand and adjust motor speed, as necessary, to meet that demand using a feedback system. Otherwise, it will operate at a constant speed, producing a constant output with a system that is less efficient than a regular constant speed system without a VFD. When not used to adjust speed to the pump’s best efficiency point (BEP) or range, a drive only adds unneeded losses and costs to the system.

Well Efficiency

Although well efficiency is not generally considered to be a component of pumping plant efficiency, it can actually be a major element as additional head to deliver a given rate of water may be required. In addition, other harmful well functions such as sand, cascading water, or air entrainment can impact pump performance and reduce its efficiency.

To obtain the highest plant efficiency possible, well performance should be tracked and recorded as an element of a plant test, with well rehabilitation conducted when normal system operation is impacted.

Can Plant Efficiency Be Guaranteed?

The decision whether to perform factory testing on new equipment must be weighed against several factors. These include:

• If the application has critical and exacting design, operating, or installation conditions
• If the application is subject to a guaranteed operating condition or possible performance penalty
• The type of unit (vertical turbine or submersible)
• The potential operational cost savings associated with verification of the application used to evaluate selection during bidding
• The time element needed to delay shipment to conduct the test
• Whether the unit is large or complex enough to warrant the test
• The ultimate cost of the test.

By far, factory testing is required more often for a vertical turbine pump than a comparable submersible unit. I have seen numerous examples of engineer-specified factory testing where the added cost of the test was far greater than the potential energy savings associated with a potential performance deviation. This can make justification of the test superfluous.

Two types of factory tests are generally available: witnessed and non-witnessed tests. Witnessed tests require much more monitoring and oversight, which means they are typically much more expensive than non-witnessed tests. The tests can be performed with the actual motor to be used or a typical test motor of like or similar horsepower with known operating parameters.

In a perfect world, the efficiency of a pumping plant would be simple to guarantee, but there is no such thing as perfect material, casting, assembly, and end-products and no pumping system operates in a perfect situation or condition. That is why original equipment manufacturers (OEMs) have historically had to compare and include the individual efficiency of each component including pumps, drivers, and other system components.

Motor efficiency has traditionally been rated using the specific test criteria of published standards to use something reliable, repeatable, and accurate. Constant speed motors without VFDs use the testing criteria identified in IEEE 112 Test Method B. Variable speed motors with VFDs must meet the IEEE standards or IEC/TS 60034-2-3, which is a new standard released for the European Union.

Values from these tests can be used to directly correlate with a pump’s efficiency at set speeds without factoring in the efficiency lost due to power quality, harmonic distortion, switching frequency, and other factors that can impact overall system performance.

Utilities and others seeking pumping plant efficiency guarantees can now use the Department of Energy’s Pump Efficiency Index (PEI), which makes it possible to evaluate a pump/motor combination or a pump, motor, and drive control at multiple points along a pump curve. These added standards make it possible to compare one system’s efficiency directly against another. A system with a high-performance drive or a flatter efficiency curve can receive a higher PEI rating than one that primarily depends on an efficient pump.

One specific limitation to consider is the DOE standards are written around laboratory testing. Factors such as power quality and installed system design will all impact system efficiency. Utilities may offer rebates for systems that meet these higher standards or penalize those that do not.

Because of the variations in field installations, though, equipment manufacturers charged with penalties for systems that do not meet efficiency standards should have the right to perform verification tests, using published standards in controlled settings to get reliable and repeatable readings. It is unreasonable for OEMs to be held to values taken in the field that can fluctuate depending on the installation (particularly for deep-well VTPs), the time of day, and power supply and demand at the time of the measurement.

The bottom line is this: There are many elements to pumping system efficiency. Select and assemble each one correctly, and the final output will come far closer to matching the input. To summarize, factory tests are expensive and time-consuming. In my judgment, they should be conducted only if the advantages of performance and energy consumption verification are found to outweigh the disadvantages of additional cost and delay in shipment. In most cases, I have found a certified performance curve on a previously tested unit of an identical model to suffice for an actual factory test.

Determining Pumping Plant Efficiency

It must be noted that improving the pumping plant efficiency will not necessarily reduce the energy consumption costs as the actual benefit gained through improving efficiency is not with the overall costs but the power demand per unit costs. This means that although the cost to pump a million gallons of water may be lower, there may now be more millions of gallons of water delivered during a specific time period
as the revised system is now capable of delivering more water.

This necessitates careful examination of the existing pumping plant and its components versus the potential cost associated with rebuild or replacement. Although there are numerous methods for testing a pumping plant, a recommended method of testing an electric motor or engine-driven vertical turbine pumping plant (Figure 3a) and submersible pumping plant (Figure 3b) is included for the reader’s use. A form for testing and evaluating pumping plants is also included in Figure 4.

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This concludes this installment and another year of the Engineering Your Business column. This column has now been written and published for almost 25 years! As such, many of the technical innovations as well as regulatory, business, and engineering practices I discussed years ago have seen tremendous— but not necessarily or always favorable—changes since I began my career 50 years ago.

Thus, next year, we will be revisiting many of my older columns and updating the topics as well as examining many new technologies used in today’s fast-paced world. We will begin with an examination of the current buzzword, artificial intelligence (AI), and other newer technology such as robotics to help address the question: Should we be worried or not about this world-shattering technological and societal change?

With that, I wish to express my heartfelt season’s greetings and wishes for a successful 2025. Let’s make it the best year ever! So as always, until next month, work safe and smart.

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Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.