In an era of rising fuel prices, terminal operators can cut operating costs by using hybrid technology in their mobile harbour cranes (MHC) and by retrofitting older cranes with newer drives.

Traditionally, MHC customers have been able to specify a diesel engine or an electric motor for their main power source. Their decision is often determined by factors such as the availability of a shore side power supply, the local cost of diesel, whether the crane is required to move between berths and by environmental considerations.

Although electric drives are becoming more fashionable, some markets that don't necessarily have mains electricity supply in place on the quay are also now asking for low-emissions cargo handling.

One solution is to use a hybrid power supply.

This is particularly suitable for MHCs, since they use a lot of energy during hoisting and much lower amounts when setting a load down. With a hybrid drive, peak power, which is needed during hoisting a load, can be recovered during lowering. This results in decreased fuel consumption and a reduction in exhaust emissions, particularly CO₂.

However, until only very recently, no such solution was available for MHCs. Now, both Gottwald and Liebherr are offering their own systems to the market.

The technology is not that complicated. It has long been possible to return the energy recovered from deceleration and braking motions in diesel-electric drive technology to a crane’s internal power circuit. However, if none of the consumers connected to that circuit required the energy, the excess is dissipated in brake resistors by converting it to heat.

Gottwald says that, in contrast to previous solutions, whereby resistors could only be switched on/off in relatively large steps, dynamic brake resistors it has developed dissipate exactly the amount of the additional energy, thereby improving a machine’s overall energy efficiency rating.

According to marketing manager Peter Klein, terminal operators are looking for a technology that harnesses virtually all the energy recovered onboard the crane and makes it available to the crane’s power circuits. Energy should only be dissipated when the capacity of the energy storage system has been exceeded, he insists.

Savings accruing from combining a diesel-generator and a short-term electrostatic energy storage system, which is what Gottwald has done, will very much depend on the deployment conditions, the applications the cranes are used for and the number of annual operating hours, as well as on the individual circumstances of the operators, says Mr Klein.

“With a Model 6 crane (G HMK 6407 variant) running approximately 4,000 operating hours annually, handling both full and empty containers, as well as comparably light pallets, we have measured fuel savings in specific operation modes of up to 23.2%.”

To get around restrictions on use in geographical areas where the ambient temperatures are high, Gottwald has installed relatively small, low-consumption cooling systems. Indeed, the use of double-layer capacitors ensure a service life of more than two million recharge cycles at an ambient temperature of 25°C.

In the case of Liebherr's hybrid Pactronic solution, marketing manager Joachim Dobler claims it also offers a 30% increase in handling performance, which reduces the total cost of ownership.

Pactronic, which stands for “Power by Accumulator and Electronics”, is characterised by an energy storage device, which is added to the drive system as a secondary energy source. Charging of the accumulator is done by regenerating the reverse power while lowering the load and using the surplus power of the primary energy source.

Hoisting, as well as lowering speeds, are increased substantially, resulting in higher turnover. In addition, the crane’s fuel consumption is significantly reduced. This is achieved by fully utilising the reverse energy and surplus power within the system.

Compared with either a conventional diesel engine or electric drive, Pactronic's combined hybrid drive leads to a reduction in fuel consumption (litre/ton) in the range of 30% for the former and needs around 30% less energy (kW/ton) than the latter, depending on the operation.

Mr Dobler also observes that, because the Pactronic energy storage system is a hydraulic pressure accumulator, it offers several advantages compared with batteries or capacitors.

“It's proven technology requiring no maintenance, just a visible inspection every 10 years. In addition, high temperatures do not adversely affect the system, which is also additionally 100% recyclable,” he says.

In contrast, he adds, a hybrid drive system with the same performance based on batteries or capacitors has a realistic operational life of just four to five years, during which it produces tons of hazardous waste, while the Pactronic drive system is designed for a lifetime equal to that of the crane it is powering.

According to Mr Klein, return on investment on the Gottwald solution depends on the application (container, general cargo, bulk), the crane driver (experienced, not experienced) and the number of operation hours per year to name just a few parameters.

“It is also directly dependent on future developments in the price of diesel fuel,” he says.

As for the Pactronic drive system, this adds an extra 15% to a crane's cost price compared with a conventional Liebherr MHC, but return on investment is faster, says Mr Dobler.

“Three Liebherr Pactronic MHCs can achieve the same annual turnover as four identical machines equipped with a conventional diesel engine or an electric motor.

"For the same annual turnover, the customer is therefore faced with a lower initial investment, while the total cost of ownership is reduced by approximately 20% per annum thanks to less fuel/energy, less maintenance, fewer personnel expenses and better depreciation. Operating costs also decrease by 10%-15% comparing a hybrid crane with a conventional one.”