Hydraulic hybrid energy-recovery system in a port material handler raises efficiency by 30%.
Mobile material handlers are designed for demanding operations at ports, harbors and marine terminals, as they’re capable of unloading bulk materials like coal or stone or general cargo packaged in sacks or crates. They’re also suited for work in mines, scrapyards and other industrial settings.
Sennebogen, based in Straubing, Germany, recently introduced its 870-E handler, which is a sizable machine, to say the least. The body can stand as high as 30 feet off the ground in gantry versions, it weighs around 200,000 lb and boasts a safe working load of 8 tons at a maximum reach of nearly 80 feet.
What sets the 870-E apart from competing material-handling equipment is its sophisticated but straightforward hydraulic controls, and an innovative Green Hybrid system that reportedly cuts energy consumption by as much as 30%.
Hydraulic energy recovery
Green Hybrid is an energy-recovery system for large Sennebogen material-handling equipment. In principle, hydraulically lifting the machine boom creates an opportunity for capturing potential energy that, in conventional units, converts to waste heat when the arm lowers and flow is metered to tank. In contrast, this hybrid system stores energy when lowering the boom and subsequently reuses that energy to raise the boom on the next lift. In turn, that reduces power demand from the engine and substantially cuts fuel consumption, explained Andreas Ernst, strategic account manager based at Sennebogen LLC, Stanley, N.C.
The system on the 870-E is essentially an independent hydraulic circuit. It consists of a hydraulic cylinder on the boom connected to a bank of nitrogen-charged piston accumulators, plus the requisite valves to control flow. As the boom lowers, the cylinder retracts and forces pressurized fluid into the accumulators, which store energy by compressing gas. Initiating the next lift cycle releases the high-pressure fluid back to the cylinder to assist lifting, with minimal losses.
The boom on a conventional machine typically has two lift cylinders, explained Ernst. “We install a third hydraulic cylinder which is connected to the accumulators mounted in the rear of the machine,” he said. The main, pump-driven hydraulic circuit supplies the two outer cylinders, and the center Green Hybrid cylinder runs in tandem with the main machine hydraulics and augments the boom-up motion. Thus, all three cylinders supply power to lift the boom, he continued. Operation is transparent to the operator because the energy-saving circuit runs continuously.
The third cylinder is particularly beneficial when movement begins, said Ernst. While flow from the accumulators tapers off near the end of stroke, that’s rarely a concern, he notes. Usually operations involve only moderate up-and-down movements, say when unloading a barge. “In our world, hardly anybody uses the entire rod stroke,” he notes.
Cylinder size depends on the machine because Sennebogen uses the Green Hybrid on several models, said Ernst. (While he declined to provide specifics, from a cursory look at the 870-E on display at the recent Conexpo in Las Vegas, one can estimate that the two outer cylinders are about 5 in. in diameter and the Green Hybrid cylinder at around 10 in., with perhaps a 10 to 15-ft. stroke.)
Likewise, the number of accumulators, connected to standard gas vessels, varies by model from three to six accumulators, he said. Four are used on the 870-E. The quantities of oil and gas in the system can be adjusted as needed.
Also important, according to the company, is that the total gas volume in the accumulators is several times greater than the displaced volume in the cylinder. This means a relatively low degree of compression and pressure change in the gas tanks—regardless of cylinder-rod position.
According to basic thermodynamics, gases that are highly compressed heat up depending on their volume. Because the Green Hybrid system imparts only relatively minor compression of the gas, temperature and pressure fluctuations are not significant. Compared to systems that see large changes, the Green Hybrid system has a higher efficiency. Thus, most of the accumulated energy returns to the system to provide a relatively constant force as the cylinder strokes.
For safety, accumulator tanks mount in a protected location in the rear of the machine, surrounded by the counterweight and other heavy machine parts.
Why not use all three cylinders in the regeneration system? “That would complicate the machine, particularly how to control all three cylinders,” explained Ernst. In this design, the two outer cylinders are part of the load-sensing hydraulics and provide the necessary force and precision control of movements. The Green Hybrid cylinder merely provides supplemental force to aid in lifting.
Sennebogen engineers also considered regenerating from the swing function, as in some hybrid excavators. “We looked at the swing, but there’s not enough energy worth recovering,” noted Ernst. While the machine may constantly swing back and forth, it’s generally over relatively short spans. “As a material handler, our highest energy saving is in the boom, not in the swing,” he said.
The fundamental reason for going hybrid is to save energy, said Ernst. And as such, the results are impressive. The 870-E offers fuel savings of up to 30% versus non-hybrid models, and it streamlines the overall design, he noted. “Let’s look at it this way. If we would take out the third cylinder, in the middle, we would need to increase the size of cylinders on the outside. We would have to put in larger pumps. This machine has a 12-liter engine. Remove the Green Hybrid cylinder and we’d probably have to go with a 15-liter engine. A smaller engine obviously burns less fuel and needs a smaller cooling system.” There’s an entire cascade of benefits that combine to reduce energy consumption, thanks to the Green Hybrid system, he emphasized.
It’s also worth noting that the 870-E is built for durability and reliability in dirty, demanding, high-vibration operating environments. Thus, unlike designs by competing machine builders, Sennebogen’s “intelligent” hydraulics don’t rely on delicate electronics and controllers, said Ernst. And that helps prevent related failures and the associated downtime under harsh working conditions.
“Sennebogen’s philosophy is to build simple machines, so everything is hydraulic,” said Ernst. For instance, the machine uses hydraulic joysticks, not electronic joysticks connected by cables to run a computer and control the pumps.
Computers are sensitive in the environments where these machines work, like steel mills that are dusty and hot, he continued. “And in the industries we serve, we run a minimum of 2,000 hours a year and have some operations with 6,000 to 7,000 hours, which means 24/7. It’s not like a highway construction job site. Computerization of machines usually leads to a complex repair, at the end of the day.” A simple machine is easier to repair and get back up and running quickly, versus needing to troubleshoot the computers, relays, connectors, wiring harnesses and controllers, he added.
As a result, the unit relies on a load-sensing, LUDV-type system with hydraulic pilot-controlled work functions, as well as load-limit control. Load-sensing systems with downstream compensators, in general, experience low power losses because the pump can reduce both flow and pressure to match load requirements. It includes a valve control block teamed with a variable-displacement pump.
With LUDV technology, individual hydraulic functions can have different outputs, particularly where working conditions present challenging power-management requirements. Benefits of LUDV include high efficiency, better performance and controllability, and lower fuel consumption, when compared to constant-pressure circuits.
The 870-E’s circuit includes hydraulically controlled, swashplate-type, variable-displacement piston pumps, with individual regulation for each pump. They deliver load-pressure-independent flow on demand for simultaneous, independent control of various work functions. Maximum operating pressure of the system is 5,076 psi (350 bar).
The control system provides proportional, precision hydraulic actuation of movements via two hydraulic servo joysticks. Additional control functions are handled by switches and foot pedals.
According to the company, an all-hydraulic system is powerful, easy to operate and maintain, and it features only proven components, and the 870-E requires no special software or “black-box” components to troubleshoot the machine.
However, the machine is served by an advanced diagnostic and monitoring system called SENCON. It offers easy menu navigation, displays machine operating parameters, and the operator can easily make fine adjustments. For example, a load-moment warning with capacity utilization indicator provides an overload safeguard. In addition, the company’s SENtrack telematics system permits remote monitoring of machine location, operating status and performance metrics.
To protect workers and the environment, the hydraulic circuits include safety valves for secured emergency lowering of attachments in the event of an engine shutdown, or a tube fracture or hose burst, particularly on the lift and stick cylinders.
The slewing drive has a bent-axis hydraulic motor driving a compact planetary gear-set, with integrated brake valves. Slewing speed is variable up to 5 rpm. Several travel drives are available. For instance, a crawler-track undercarriage uses axial-piston hydraulic motors driving planetary gears. Top speed for the track drive is 1.8 mph (3 kph).
A dual microfiltration system is rated for 3 micron cleanliness; optional 60 micron pressure filters can be added for attachments. And users can opt to run on environmentally friendly bio-oil.
Bosch Rexroth supplies most of the hydraulic power and control components, said Ernst. Cylinders are custom manufactured to Sennebogen engineering specifications, he said, and are chrome or nickel plated or specially coated to resist corrosion in maritime environments.
The 870-E is not only efficient and durable, it’s also versatile. A number of boom and stick configurations let users match lift and reach requirements to their specific operations. The same holds for attachments. Various grapples and buckets include orange-peel grabs and clamshells, as well as electromagnet plates for scrap-handling, mobile shears and vacuum-lifting devices.
Likewise, the modular design lets the 870-E body easily mount to a number of different platforms. These include a mobile rubber-tired undercarriage with 4-point outrigger supports, crawler-track and rail versions, and even a fixed-mount underframe. Gantry and pylon superstructures are available for special port applications.
The 870-E is powered by a Cummins QSL 12-C350, Tier 4f engine that’s rated at 355 hp at 1,800 rpm. It includes an EcoMode for fuel economy in reduced load conditions. An optional electric drive is powered by a 250 kW, 400 V, 50/60 Hz motor. It can lower operating costs, eliminate fuel stops, and provide a quiet and virtually vibration-free work environment which, in turn, can lengthen the service life of hydraulic components.
For excellent operator visibility, particularly when unloading a ship hold, the cab can be hydraulically raised or extended. Some versions let the cab can extend more than 20 ft from the machine body and adjust height more than 30 ft. This helps ensure fast work cycles—especially valuable in terms of shorter berthing times when unloading ships. It also eases access, as the operator can comfortably enter the cab from the ground.