A matter of control
Intelligent controls and a green agenda are changing RTG drives for the better, as Stevie Knight discovers
The common diesel generator isn't, by itself, particularly efficient at the kind of stop-start operations common at ports... and some think that the situation has worsened since the economic crisis.
Leaving equipment idling may well reduce the efficiency of traditional diesel generators even further, since these are usually run at a constant speed to supply the drive system and any auxiliaries, regardless of whether the unit is in operation or in standby mode.
But handling equipment covers a very broad spectrum of machines - and although the typical forklift has smaller loads, a smaller brake to acceleration range and may run on batteries, according to recent research by Conductix-Wampfler more than 50% of the total fossil fuel energy consumption at a typical port is caused by diesel-powered container handling rubber-tyred gantry cranes.
While there are more hybrid and energy-recovery cranes around, most still sit firmly on their diesel-driven generators. What has recently changed, however, is the "intelligence" of the controls, which allows for lower power, (a 6ltr engine replaced a 21ltr unit in one installation) to match the idling 'draw', that can nonetheless be ramped up for peak operations.
It is useful because many initiatives now come with a 'green' agenda. Barry Wiles of Avtron, for one, sees drive technology moving toward offering more green alternatives. "Ports are being pressured to reduce their carbon footprint," he says. "Future technologies will have to help achieve these goals."
Diesel-electric hybrid drives are now commonly used in rubber-tyred gantry cranes so when a container is lowered, braking energy is stored. However, Siemens has put both the energy recovery and a new drive control together on its Eco RTG, claiming a whopping 70% fuel saving.
Volker Banholzer of Siemens explains that in order to optimise the drive system, the Eco-RTG applies a mathematically-based Equivalent Consumption Minimization Strategy (ECMS) to determine the exact performance setpoint value in real time for both motor and ultra-capacitors.
It is a finely tuned act: the drive receives as much energy as possible from the ultra-capacitors while making sure that the stored energy is never completely used up. "During the entire operating cycle of the crane there is a balance between the amount of energy supplied from the ultra-capacitors and the amount of energy that is stored in them when braking and lowering," he says.
This means, for example, if the crane is in no-load mode and only the auxiliary systems are consuming energy, the system pulls it from the ultra-capacitors and the motor is switched off.
So-called "intelligent drives" are proving useful at taking into account other factors too. Avtron provides a crane drive which, besides a "quick and dirty" software set up, also offers advanced features you can modify to suit, like shaping the way the joystick responds. Safety features like brake monitors which allow the drives to watch the hoist motion are also increasingly to be found under the drive controls. This means should a brake fail or inadvertently be forced open, the drives will automatically go into a load-float condition and signal an alarm.
Liebherr, on the other hand, has a different approach. It manufactures its own hydrostatic (diesel-hydraulic) drives in which the diesel powers a gearing system that splits the different motions out to various hydraulic pistons, providing an extremely fast acceleration and smooth transition. Joachim Dobler of Liebherr explains that the hydrostatic systems have a very low "moment of inertia", approximately 20% to 25% of diesel-electrical systems. Impressively, one test shows a Liebherr crane attaining a top hoist speed in around 40 seconds, around a fifth of the time of a comparable linear DC machine.
Mr Dobler goes on to say that there are a few advantages to the hydraulic diesel mix over diesel-electric. One is that after the generation point, hydrostatic units are one synchronous system. This doesn't just add a general robustness, it avoids the power loss that you get through transiting to other energy modes (like the loss involved in the banks of electrical resistors, for example) and it also reduces maintenance and spares. Further, energy is easily recaptured from the crane's braking and lowering motions.
The hydraulic systems, says Mr Dobler, have a higher fuel efficiency due to a variable diesel output, whereas the electrical systems work on a fixed diesel RPM because there is a need for constant frequency to supply the generator.
Gottwald, who are firmly behind the diesel-electric mix, contests this point, saying that the continuous running of the diesel to supply the electricity generation does not necessarily make for higher fuel costs, since the generator runs smoothly at 1,800 rpm and acceleration loads on the engine are effectively bypassed. "On the other hand," points out Gottwald's Peter Klein, "hydraulic systems need constantly changing engine speeds to operate the pumps, which leads to higher fuel consumption" - despite the intelligent drive controls. Further, he says, an advantage over hydrostatic systems is that fewer components are arranged in series, leading to higher overall efficiency.
The Gottwald harbour cranes rely on electric power for the hoists and slewing gear motors, though interestingly, the units do use electrically-driven hydraulic pumps. Mr Klein says that this is more of a question of using the right kind of propulsion for the required motion - as the luffing, propping and travelling motions are linear (and not rotary motions) hydraulics are the most efficient option. And as for the turn of speed, there are now units that can reach 140m per minute, although the acceleration may be a little slower than the hydrostatic equivalents.
Still, says Mr Klein, the electrical option fares best in efficiency despite the hydraulic transfer, and they have one other particular advantage over the hydrostatic systems, in that these units "actually gain in efficiency when linked up directly to a mains supply", rather than lose out by the extra energy transition - bypassing the generator completely.
Further, energy recovered from reverse motions can be fed back to the grid and can be used to drive other port equipment or simply sold back to the utilities. All in all, he says that diesel-electric offers "the best opportunity for both fuel saving and green port initiatives for the future".
However, Dr Joachim Harder of Onestone Consulting Group says that "bigger than the impact of the downturn is the push to smooth out operations, firstly because this makes maximum use of the equipment, and it also lengthens service life". A worthwhile pursuit in this cash-strapped age. He finishes by saying that despite everything, "the smart operations are putting in upgrades and modernising existing equipment while they have a chance".
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