Fluid power is the use of fluids to generate, transmit and control power. Fluid power is sub-divided into hydraulics, which uses a liquid (typically mineral oil or water) as the working medium, and pneumatics, which uses a gas (typically air) as the working medium. Fluid power is used in a wide range of industries, including manufacturing, transportation, aerospace, agricultural, construction, mining and forestry. Nearly all U.S. manufacturing plants rely on fluid power in their production of goods; and over half of all U.S. industrial products have fluid power critical components.[i] Fluid power has the advantages of flexibility and high power density that can be more widely exploited. Based on magnetic material properties, an electric motor can develop the same torque as a hydraulic motor of the same size operating at a pressure of 250 psi.[ii] Since hydraulic systems routinely operate at pressures of 3000 psi, such a hydraulic motor will have twelve times the torque of the same sized electric motor. If the shaft speeds are the same, the power ratio will also be twelve, which is why fluid power systems can have a much higher power density than electric systems. In a comparison of rotary and linear actuators for robots, it is found that hydraulic actuators have a power density that is up to one hundred times greater than for electric motors based on volume and five hundred times greater based on weight.[iii] Fluid power systems also have one to two orders of magnitude greater bandwidth than electromechanical systems with similar power ratings.[iv] Fluid power is essential for large equipment which require high-power and high- force. The use of fluid power is so prevalent that any improvement in efficiency will have an important impact on energy consumption. Further, the intrinsic bandwidth and power density advantages of fluid power has not been exploited on smaller scale, portable and self-powered systems.
Based on 2013 U.S. Census Bureau data, sales of fluid power components exceeded $22 billion and employed 71,000 people. Fluid power also has a significant downstream economic impact. Ten key industries that depend on fluid power are estimated to represent more than 23,900 companies in the United States, employing more than 874,000 people with an annual payroll of more than $54.4 billion. The substantial size of the market in terms of dollars and jobs indicates the potential for new and expanded businesses. However, more than corporate profits and jobs are at stake. Reducing energy consumption is directly related to reducing carbon dioxide emissions, the major cause of global warming. There are literally millions of devices in use around the world today ranging from industrial air compressors to large agricultural, mining and construction vehicles. As will be discussed in the next paragraph, reductions in the fuel consumption in current applications of fluid power can have a significant impact on U.S. energy use. These efficiency improvements will also facilitate the expansion of fluid power into adjacent market segments. Further, new compact fluid power systems will increase our quality of life by enabling human-scale, untethered systems such as the patient transfer device and the orthosis.
A groundbreaking report titled “Estimating the Impact (Energy, Emissions and Economics) of the U.S. Fluid Power Industry” was published by Oak Ridge National Laboratory (ORNL) in December 2012.[v] The report was a DOE-funded study to estimate the energy-specific footprint (consumption, emissions, efficiency) of currently deployed fluid power systems in the U.S.. The authors were Dr. Lonnie Love of ORNL and Eric Lanke and Pete Alles of the National Fluid Power Association. Thirty-one industrial partners provided input to the study. The report provides information that will have an impact on fluid power research for years to come. Some of the key findings include:
- Fluid power systems consume between 2.0 and 2.9 Quadrillion Btus (Quads) of energy per year and produce 310-380 million metric tons of CO2. The energy to operate fluid power systems is 2-3% of all of the energy consumed in the U.S..
- Mobile hydraulics consumes 0.4-1.3 Quads/year
- Industrial hydraulic equipment consumes approximately 1.1 Quads/year
- Pneumatic equipment consumes approximately 0.5 Quads/year
- Across all industries, fluid power efficiencies range from 9% to 60% with an average of 22%. The specific application of the fluid power system impacts its efficiency.
- The study provides an overview of the aggregate, sector and market energy usage for fluid power systems. It also offers suggestions for some potential areas of improvement.
The DOE study provides insights on the impact of fluid power on energy consumption in the U.S. For the first time, fluid power practitioners have an understanding of energy use and efficiency in aggregate and by sector.
[i] Esposito, A., 1997, Fluid Power with Applications, 4th edition. Prentice Hall, Upper Saddle River, New Jersey.
[ii] McCloy, D. J. and H. R. Martin, 1980, Control of Fluid Power: Analysis and Design, 2nd ed., Ellis Horwood, Chichester, pp. 12-13.
[iii] Kuribayashi, K. 1993, “Criteria for the Evaluation of New Actuators as Energy Converters”, Advanced Robotics, Vol. 7, No. 4, pp. 289-37.
[iv] Maskrey, R. H. and W. J. Thayer, 1978, “A Brief History of Electrohydraulic Servomechanisms,” Technical Report 141, Moog, Inc., East Aurora, NY.
[v] Love, L. J., E. Lanke and P. Alles, “Estimation of the Impact (Energy, Emissions and Economics) of the U.S. Fluid Power Industry,” Oak Ridge National Laboratory Report ORNL/TM-2011/14 December 2012.