By Tom Blansett, Technical Director, International Fluid Power Society
Fluid power engineers and users seem to understand that energy efficiency is an admirable goal. But maybe it doesn’t get the attention it deserves. Let’s look at efficiency in terms of dollars and cents to add a bit of perspective. In 2018 energy usage data supplied by the U.S. Energy Information Administration shows that 101.3 Quadrillion BTUs (Quads) of energy were consumed in the U.S., with industrial hydraulic systems accounting for approximately 1.1 Quads. With the average cost of commercial electricity in the U.S. in 2018 being 10.67 ¢/kWh, 1 Quad costs about $31.2 billion.
Studies reveal that efficiencies of industrial hydraulic systems range from <9% to 60% efficient, and average efficiency was 22%. So on average, the cost of wasted energy is $24.3 billion
. Improving efficiency of industrial hydraulics by just an additional 10% would result in savings of approximately $3 billion per year.
Efficiency is the ratio of output power to input power. For example, if a hydraulic system can transmit 100 hp out for some work process, but because of inefficiencies in the system it requires 125 hp of input from a prime mover, then the hydraulic system is 80% efficient.
The reality is that there are many sources of inefficiency – or energy loss — in a typical industrial hydraulic system. Causes include:
- Inefficient electric motors.
- Differences in pump design and type that influence energy losses.
- Fluid selection. Oil viscosity that is too low or high can result in added frictional losses.
- Conductors and connectors not optimally sized.
- Throttling losses in valves.
- Excessive leakage in actuators.
- Friction in cylinders.
- Oversized reservoirs that may require supplemental heating in colder environments.
- Pressure controls inadequately set.
- Pumps sized too large for demand.
- Lack of temperature control of fluid.
- An overall faulty design.
The good news is there are many opportunities to improve efficiency and garner substantial energy and cost savings. Start with an understanding of the system operation and by reviewing schematics. Ask yourself, what areas are inefficient in their use of energy during operation? The key is analyzing the design’s required output power versus input power.
Log system operating parameters such as pressure, temperature and speed. Identify potential areas that are of concern, like oversized electric motors, frequent fluid replacement and improper plumbing. A large heat exchanger or other cooling mechanism needed to remove heat provides a telltale sign that there are numerous sources of inefficiencies generating excess heat. With a bit of sleuthing, you can recommend potential solutions to help minimize energy losses. Here are eight areas worthy of consideration.
Reservoirs. Beware of rules of thumb that result in sizing the reservoir too large. Standard practice is three times the pump flow. But the primary reason given is to remove heat, which is a myth. The reservoir is not an effective heat sink and requires an extremely large size to dissipate typical heat, unless duty cycle is low.
Also evaluate the use of alternate materials to increase heat transfer where possible. Aluminum has almost 5× greater heat dissipation than carbon steel and is almost 8× more effective than stainless steel.
If heaters are required at startups due to cold temperatures, then oversized reservoirs waste energy as it takes approximately 1 W to raise a gallon of oil 1° F in an hour. The best solution is to work with the supplier/OEM to ensure the reservoir is correctly sized for your system.
Prime movers. Review the current electric-motor design and decide if the return on investment for energy savings would justify replacing the current unit with a premium high-efficiency motor. And it may be better for overall operation of the system to retrofit with a variable-speed drive. Except for applications where the pump-motor combination runs continuously at a constant speed, a VFD can often quickly pay for itself with sizable energy savings.
The Energy Independence and Security Act of 2007 (EISA) raised to a premium level the mandatory minimum nominal full-load efficiency for general-purpose motors rated below 1,000 V and up to 200 hp. For example, the previous standard efficiency for a 10 hp motor was 86.7% and the new minimum efficiency level is 92.2%. Minimum efficiency for motors greater than 200 hp is 96%. However, keep in mind that a 1% increase in efficiency for a 100 hp motor will result in more cost savings than a 9% increase in efficiency of a 10 hp motor.
The flow source (pump). Depending on the design and operation, efficiency can vary significantly from one type of pump to another. Analyze the overall efficiency of the pump and the control method used, and evaluate your ROI to determine if it is justified to change to a more-efficient system.
Analyze dwell times, if any exist, and determine how to minimize energy use during this time. Among the possibilities: add an accumulator, unload the pump, select a variable-speed drive, or upgrade the pump controls. Using RMS power calculations helps determine if the prime mover can be downsized, which will reduce overall energy consumption during dwell times. Also, analyze pump displacement and determine if it is properly sized for the energy demand.
Evaluate the fluid. Determine fluid requirements, for example the viscosity, based on the application and system components. Two areas that primarily impact pumps and motors are volumetric efficiency and hydromechanical efficiency. There is a viscosity range where fluid friction, mechanical friction and volumetric losses are minimized and optimal for hydraulic system performance. This is the viscosity range where the hydraulic system will operate most efficiently — the highest ratio of output power to input power.
A study published in Machinery Lubrication magazine showed that by using high Viscosity Index fluids, typical cost savings per vane pump in mobile hydraulic systems was approximately $400 per year. Mobil Corp. conducted a study on a typical ISO VG 46 fluid versus a high VI fluid at the same viscosity grade and showed efficiency improvements of 3 to 6% due to the fluid alone.
In-plant savings would be typically less as the temperature of fluids used in industrial systems is much more regulated and consistent, although during cold start-ups or high temperature operations, savings would be realized in industrial systems through high VI fluids.
Fluid conductor sizing. Pressure drop caused by frictional losses in fluid conductors is a significant source of wasted energy in a typical hydraulic system. Designers attempt to balance pressure drop against the cost of conductors and, in most cases, reduce the size of conductors to lower initial system cost without regard to total operating cost due to wasted energy.
Let’s look at the physics. Flow in straight pipe tends to be streamlined. However, abrupt changes in cross sections upset the laminar flow and transitions to turbulent flow. Frictional losses increase exponentially, which increases pressure drop, causes heating of the fluid and results in undesirable energy losses.
So circuit designers should avoid extra fittings, sharp bends and undersized inner diameters of conductors — especially for pump inlets to avoid cavitation. In the pump inlet line, it is recommended that a straight length of at least 10 times the inner diameter of the fluid conductor be established directly prior to the pump inlet to allow for a transition back to laminar flow and minimize potential cavitation.
Undersized valves can also lead to increased energy loss due to throttling effects and friction (heat). Improperly sized pressure controls can have excessive pressure override which also results in energy waste and reduces efficiency.
Contamination control. To ensure a high-performing and reliable hydraulic system, engineers should determine the target cleanliness level, evaluate the contamination control system, and sample fluid regularly and look for evidence of oxidation.
Proper filtration and contamination control is essential for energy efficient systems. Improper contamination control leads to many consequences, none of them good. It results in increased component wear, with more internal leakage and wasted energy. It changes fluid viscosity which impacts efficiency as described previously.
Heat is also a contaminant, as it accelerates degradation (oxidation) of the fluid and impacts system efficiency. Lack of contamination control can result in valves partially opening and increasing throttling losses. Likewise, excessive air is another contaminant in the fluid, and it can affect heat transfer rates and damage pumps.
Actuator selection. Evaluate seal choice because internal friction results in energy loss. Different materials, types and designs can have a noticeable impact on dynamic seal performance and efficiency. Also evaluate the condition of internal seals, as damaged seals permit leakage which generates heat and wastes energy.
Determine if any side-loading is present on either cylinders or motors as this will require additional force to overcome, which is energy inefficient, plus it results in internal wear which increases leakage.
For cylinders with sizeable strokes and subject to side loads, consider adding a stop tube. It provides better internal rod support and lessens forces on seals and bearings.
Five Key Areas
In summary, here are five key areas engineers should evaluate to help improve system efficiency:
- Analyze existing system and calculate actual output power versus input power (electric motor horsepower).
- Evaluate pump controls and the type of pump being used to determine if a more energy-efficient solution is possible.
- Look for the presence of a heat exchanger, and analyze its size compared to a typical unit based on size of the system. A cooler rated at higher than 30% of overall system horsepower could be indicator of energy inefficiencies that should be addressed.
- Sample the fluid and look for signs of oxidation.
- Determine the recommended contamination level for the system and evaluate if contamination controls are adequate to meet that benchmark.
Tom Blansett is a Certified Fluid Power Specialist, Connector and Conductor Specialist, and Accredited Instructor with the International Fluid Power Society.
For more information on assessing your hydraulic system efficiency, listen to Tom Blansett’s interview on NFPA's Fluid Power Forum podcast.
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