Pneumatic systems are notoriously inefficient. Granted, in recent years researchers and manufacturers alike have made great strides in increasing energy efficiency in fluid-power systems, to improve sustainability and save money. Such innovations range from no-leak systems and high-flow valves to low-friction cylinder seals and variable-speed compressor drives.
But such advancements are often incremental at best. Now, researchers at Vanderbilt University hope to double the efficiency of common pneumatic actuators by recovering and reusing energy that is currently wasted.
In a project termed Pneumatic Exhaust Gas Recovery, sponsored by the National Fluid Power Association, graduate student Tyler Gibson detailed the research in a recent webcast hosted by the Center for Compact and Efficient Fluid Power. The primary goal of this research is to develop a way to retrofit current pneumatic systems and improve their efficiency, and keep costs lower than electric alternatives.
The motivation of this research is quite simple, said Gibson. Pneumatic equipment consumes roughly 0.5 Quads (0.5 quadrillion BTUs) of energy annually in the U.S. and that correlates to $10 billion worth of energy per year. But the average efficiency of industrial pneumatics systems is only about 15%, so billions of dollars are wasted annually due to inefficiency. Raising efficiency from 15% to 30% would correlate to $1.5 billion in annual savings, according to a 2012 Dept. of Energy report. Thus, there is enormous room for improvement with significant resulting benefits.
Researchers are exploring several methods to boost pneumatic efficiency. The main effort to date involves exhaust gas recovery, with work headed by graduate Ph.D. candidate Joshua Cummins and undergraduate Christopher Nash. Pressurized air exiting a pneumatic cylinder—which typically vents to atmosphere—is instead routed to an accumulator where it is efficiently captured as strain energy, and then reused as necessary in other pneumatic processes.
In essence, a strain-energy accumulator (SEA) is a rubber accumulator housed in a rigid cylindrical container. The SEA attaches to the exhaust port of a pneumatic system and inflates like a balloon to store the still-pressurized exhaust gas.
In a basic two-cylinder system, high-pressure plant air would power the first cylinder, and its exhaust would flow to the SEA. The stored gas would then be ported as needed to a second cylinder to do more work. And, in fact, SEAs have been used to power secondary pneumatic systems at pressures lower than the primary actuators, said Gibson. In theory, another accumulator and a third cylinder could be powered by the second, and so on.
In addition, electrically controlled valves can meter flow from the accumulator so the secondary process uses only the required gas, and not completely exhaust each cycle. The rest would be saved for another actuation. (View a circuit at enfieldtech.com.)
Maximum energy efficiency would theoretically increase 33% using exhaust gas recovery. But even a 20% increase would result in significant savings in pneumatic power, said Gibson.
Such a system does create backpressure in the primary cylinder, explained Eric Barth, assistant professor of mechanical engineering at Vanderbilt and the project advisor. But pneumatic automation systems run at either too high a pressure, or are locally regulated down to a lower pressure to save energy. “Our system is an alternative to either of these where the system would be running on the correct differential pressure between the supply and the ‘backpressure’ of the accumulator. Tests have shown that our system can save more energy than the regulator method,” he said.
And an SEA does not affect the speed or force of the first cylinder. In a two-cylinder arrangement, the system can be sized for a wide range of operating pressures that depend on the specific process. Barth envisions many practical industrial applications. “Nearly any system with multiple pneumatic actuators could be retrofitted with this technology to save energy,” he said.
In one recent application, researchers at the University of Illinois at Urbana-Champaign demonstrated a pneumatic powered ankle-foot orthosis with an SEA that reportedly increased device efficiency by 25%. Further testing on various industrial systems is forthcoming, said Gibson.
A second, alternative approach to the SEA method is to dynamically convert exhaust to usable energy, although those efforts are mainly theoretical at this point, said Gibson. One proposed device is a pneumatic boost converter, which would convert low-pressure, high-flow exhaust energy into high-pressure, low-flow air that can be directly fed back into the supply line.
It is analogous to an electrical dc-to-dc boost converter, but instead of using an inductor, switch and diode that boosts voltage by storing energy in the inductor, the pneumatic equivalent would use an inertia (possibly a small piston), valve and check valve. They’re currently modeling the concept in Simulink and hope to draw conclusions soon.
A related idea being pursued is a dynamic Pneumatic Elastomeric Accumulator. A dPEA combines a boost converter and an accumulator, using the latter as an energy-storage element. It’s analogous to an electrical Ćuk converter. Researchers plan to build and test systems and validate models. If successful, they’ll install such converters on real-world equipment to gage actual improvements in pneumatic efficiency.
Ajay says
Hi, I was planning to do a similar project and got this site..am fascinated by ur experiments so cheers.
could you pls suggest me (in case u believe in educating the lesser and not trying to sell technology/product/service all the time..) 🙂
In my industry,we have hundreds of pneumatic cylinders whose exhaust is wasted in air… their supply is 7kg/cm2. can i collect all vents route through NRV’s and connect to a receiver tank with safety valve operating at 1 kg below 7 (6kg/cm2) so that the primary cylinders dont face the back-pr problem. then use the air receiver tank(accumulator) for other applications… the hundreds of cylinders are continuously operating every 5-10 minutes so i have a lot of air vents… my Air receiver tank would mostly be venting that extra 1 kg pr as my secondary use is not continuous.. Would you be kind enough to recommend ? In case more info is required please ask.