Can Hydraulic Systems be Energy Efficient?

In modern day vernacular, the phrase energy efficiency is commonly associated with environmentally friendly or “green” values, especially as they pertain to natural resources. Efficiency is about taking advantage of various energy sources while limiting the waste associated with the processes that use or convert that energy.

When talking about hydraulic efficiency, we consider factors such as the machine requirements and fluid power of hydraulic systems like motors and pumps. Generally speaking, hydraulics is not an efficient technology, because a substantial amount of power is lost in the process of converting energy. In that sense, hydraulic efficiency can be questioned as a bit of an oxymoronic term. However, hydraulic efficiency can also be defined as it pertains to the progress being made to research different materials and designs and subsequently manufacture new systems with increasingly enhanced energy-efficient features.

What is hydraulic efficiency?

There are a few different categories of efficiency to consider when discussing hydraulic pumps and motors: Volumetric efficiency, hydraulic efficiency, mechanical efficiency, and overall efficiency.

Volumetric efficiency is the actual flow produced by a pump at a certain pressure divided by the theoretical flow. Theoretical flow is the predominate category used to determine a hydraulic pump’s condition in terms of internal leakage, either by design or through wear and/or damage. While mechanical and hydraulic efficiency are sometimes grouped into a single category (hydromechanical efficiency), mechanical friction involves energy losses that occur among mechanical seals, the bearing frame, and stuffing box while hydraulic efficiency takes into account factors such as liquid friction and other losses that occur within the volute (diffuser) and impeller. Pressure losses and friction losses among those various components can negatively impact a system’s hydromechanical, or mechanical/hydraulic, efficiency.

A motor or pump’s overall efficiency is the product, as opposed to the average, of its volumetric, hydraulic, and mechanical efficiency. Simply put, overall efficiency is how well the machine can do its job of converting energy from one form to another.

That job of converting energy is virtually impossible to accomplish without losing at least some energy (or heat load) along the way. For example, hydraulic machines convert electrical energy to mechanical energy. Then, the mechanical energy must be converted into hydraulic energy, and finally, the hydraulic energy is converted back into mechanical energy. At each step, the machine’s process results in partial waste, and that waste is measured and designated as a percentage in terms of a system’s efficiency. In other words, you can produce a certain amount of usable hydraulic energy, but a fraction proportionate to the machine’s efficiency will be wasted as pure heat along the way.

Waste certainly tends to be higher in aged machines with older components, which means pumps and motors become less efficient over their lifetime. Hydraulic systems, however, tend to be accompanied with substantial leakage by design, simply as a byproduct of thoroughly lubricating the many components that are used to comprise the machine. Additionally, the leakage is worse for certain motors if they are operated outside their optimal torque and speed curve.

Taking into consideration the high clearances and larger amount of moving parts associated with hydraulic units, you can already experience a loss of heat energy associated with the pressure and flow of the leakage. Add to it the extensive fluid friction present among the pumps, valves and piping, which causes hydraulic systems to lose power, and you may once again wonder if hydraulic efficiency is not necessarily a myth, but a questionable reality.

Why use hydraulics at all?

There is a two-pronged answer this question. First, hydraulics have natural advantages over electric motors and pumps because of weight and capabilities. Secondly, if system efficiency is viewed as a continually evolving goal rather than an established status, you can consider how manageable variables, including proper system design, right-sizing components, and modern technology, can augment the energy-efficiency of hydraulic machines.

Going back to the first consideration, hydraulics have a size and weight advantage over electric motors, which helps save energy. You can size a hydraulic pump’s motor to coincide with the average load the system tends to carry, as opposed to the system’s peak load, as is required for electromechanical systems. In general, hydraulic units are powered by a significantly smaller motor that weighs less than the motor used for electromechanical models. Additionally, with the proper valves, hydraulic units also have the ability to vary torque and speed and reverse direction, leading to a wider range of uses and motive capabilities. In comparison, electric motors would need incredibly sophisticated electronic controls to possess the same features.

From components to overall design

Advances in technology, in terms of both research and what materials are manufactured, is going to be geared toward helping hydraulic units function in a more energy-efficient manner. Because hydraulic units are relied upon for a myriad of commercial and industrial operations across a range of industries, there is a continual effort to make designs and component technology more sophisticated, productive and cheaper.

When it comes to investing big money in exchange for a noticeable difference in quality, the pump is an especially important component to consider. For instance, a high-quality piston pump can be around 95% efficient, a significant amount more than old gear pumps. It also creates 80% less waste, which reduces cooling requirements. In short, piston pumps are currently among the most efficient pump designs available on the market.

Certain hydraulic pump features have been introduced that address some of the critical control issues that detract from the efficiency of fluid power systems. One such design feature is pressure compensation, which allows you to set the pump to a specific standby pressure. This means the pump can reduce its displacement to correspond with pressure drops or downstream flow, and will automatically generate the designated flow at a particular psi. If you want to be able to use lower psi (pounds of force per square inch) for a particular operation, you can use a load-sensing pump. Thanks to an additional compensator placed downstream of the metering valve, the pump can gauge load pressure and compare that with compensator pressure, only providing the flow and pressure needed by the actuator and circuit. This efficient management of pressure and flow results in a relatively low waste by-product. 

Your hydraulic machine’s overall efficiency will also be impacted by the hydraulic actuators you select. When it comes to the optimum hydraulic motor, you should consider a radial piston motor or axial piston motor, which have about 95%and 90% efficiency, respectively. Not surprisingly, you will pay less for vane motors, gear and orbital motors, but they are consequently rated lower in terms of energy efficiency, as well. That is the main consideration when it comes to hydraulic motors.

Manufacturers are also introducing other more energy-efficient components for modern hydraulic systems, including valves, filters, seals, and fluid, all of which help negate the detrimental effects of fluid friction.

The next step in achieving energy efficiency requires improving the method of electrohydraulic motion control, so multiple actuators work together in a smooth, coordinated manner to boost a hydraulic machine’s speed and productivity. Advancements in control technology, including features such as pressure, axis controllers, transducers and variable-frequency motor drives (VFDs), optimize dynamic performance and factor into overall efficiency. A VFD, which most modern variable-speed hydraulic pump controls rely upon, can precisely control pump speed and will start only if or when the circuit requires flow.

What is the bottom line?

Hydraulic systems may face greater challenges and requirements than their electric counterparts when it comes to owning a reputation as efficient. In light of modern technological advances, however, energy efficiency certainly is a viable goal. Further research into the design of hydraulic systems and the components they include will progress the efficiency of this technology, benefiting the companies and organizations that employ it and increasing the appeal and affordability of hydraulics as the power source of pumps and motors.