diverse range of quay infrastructures. In line with their
different terminals within the same harbour. Throughout its
or avoided completely. In this way, the already high efficiency
machines drop to zero and noise pollution is minimized. In
gear units and return it to the harbour mains.
sources, this further boosts the crane’s life cycle assessment. In
becomes an integral part of an end-to-end green energy chain.
external power. Retrofitted packages consist of the slip ring
to the crane’s voltage. This option is of particular interest to
Gottwald cranes installed use external power. A further such
electricity supply. Individual customers have reported cost savings
generator because regenerative hydroelectricity is very cheap.
constant speed of 1800 min-1 (at a frequency of 60Hz). This
voltage of up to 690 V. Since on-board power generation is the
mobile handling machines is steadily increasing.
drive technology to the crane’s internal power circuit.
resistors by converting it to heat. In contrast to previous
owners of mobile harbour cranes. Gottwald’s customers
inspection to assess the specific site conditions at the terminal.
The time required for a return on this investment is directly
dependent on future developments of the price of diesel fuel and
the energy-saving potential of the methods of working of the
crane and the operating hours.
In view of the demands for economical and, at the same time,
green drive technologies, it is becoming more important to
minimize the amount of energy dissipated. Indeed, what terminal
operators are looking for is technology to improve efficiency still
more, including that of electric drive systems. In other words,
technology that harnesses virtually all the energy recovered onboard
the crane and makes it available to the crane’s power
circuits. Energy should, therefore, only be dissipated when the
capacity of the energy storage system has been exceeded.
CHOICE OF THE ‘RIGHT’ STORAGE TECHNOLOGY
The emphasis now is on the search for suitable storage systems.
Mobile harbour cranes tend to be used worldwide in tough
terminal conditions for constantly changing applications. Key
characteristics of the operation of these machines in handling
containers, general cargo, bulk materials and heavy project
cargoes are these:
- differing hoisting and lowering heights;
- load-dependent hoisting and luffing actions (empty vs. full containers or grabs); and
- slewing angles from 45° to 180°.
Since their market launch around 50 years ago, mobile
harbour cranes have extended their fields of application to
include many more activities. The choice of the drive system,
which includes the short-term energy storage system, requires
the operating and fringe conditions and constraints typical for
the broad field of application of a mobile harbour crane to be
taken into account together with the investment costs for the
initial purchase or upgrade.
EXAMINATION AND ASSESSMENT OF POTENTIALLY APPLICABLE
Taking into account the above constraints, Gottwald conducted
tests on electrochemical, electrostatic and mechanical methods
for storing energy. The focus during this phase was not so much
on energy density as on power density and the realistically
achievable number of charging and discharging cycles in systems
already available on the market. The operation of mobile
harbour cranes, in particular the rapid load-changes in
conjunction with discontinuous lowering, hoisting and slewing
motions and the accompanying acceleration and deceleration
processes, requires such a system to absorb power rapidly and
return it to the system just as quickly. The way in which mobile
harbour cranes work is in stark contrast to that of Gottwald
AGVs, for example, which are equipped with a power output of
around 250kW and typically have short acceleration and
deceleration phases with long periods of continuous travel in
between. For a comparison of the advantages and
disadvantages of electrochemical, electrostatic and mechanical
energy storage media in mobile harbour crane applications,
please see the table on 73.
While batteries may have high energy density, a serious
drawback is their low power density. They also do not respond
positively to high cycle rates and deep discharge. After all, large
mobile harbour cranes require high power reserves and this can
only be achieved with a large number of battery packs, which
necessitates considerable space requirements. Due to their
sophisticated technology, broad range of applications and their
charge and discharge times, batteries tend to be preferred for
lower drive outputs — such as for Gottwald AGVs — where
they are the ideal source of energy and help terminals close to
centres of population to reduce their exhaust gas and noise
pollution in a sustainable manner.
Flywheels that store rotational energy and convert that to
electricity, while revolving at up to 25,000rpm in a vacuum,
typically have relatively high energy and power density. While
their typical charge and discharge times can be up to 60 s, they
are not only unreliable under tough working conditions, they are
also relatively expensive and require complicated ancillary
equipment such as a vacuum pump, cooling system and safety
In contrast to this, there are double-layer capacitors, or
ultracaps, which may only offer low energy density but they have
high power density, which is what mobile harbour crane
operation needs. In addition, these capacitors have a high
efficiency rating because the energy is stored as electricity and
does not have to be converted. As a result, double-layer
capacitors with typical maximum charge and discharge times of
30 s for this particular application have specifications that are
many times more favourable than those of conventional
capacitors. They are also designed for use in very tough
environments and can cope with very high numbers of duty
cycles. Ultracaps are sold by a number of established
manufacturers under such brand names as Goldcaps, Supercaps
As a result, Gottwald decided on double-layer capacitors for
its mobile harbour cranes because they best meet the key
requirements for high power density and guaranteed high
number of duty cycles. They also provide the best specifications
for applications in a changeable, rough environment. The well documented
restrictions on use in geographical areas where the
ambient temperatures are high can be easily countered by
installing relatively small, low-consumption cooling systems.
Indeed, the use of double-layer capacitors made by Maxwell
Technologies, which have been in use recently in a Gottwald
prototype crane have a service life of around a million recharge
cycles at an ambient temperature of 25°C. Even increased
operation, which involves a greater number of work cycles,
hardly influences their storage capability but only results in an
increase in internal resistance and, correspondingly, working
temperature, which has to be cooled down. Storage capacity is
not expected to diminish until shortly before the end of the
capacitors’ service life.
TYPICAL GOTTWALD CONTAINER AND GENERAL CARGO CRANE
AS A PROTOTYPE
For its trial period with double-layer capacitors, Gottwald
selected a typical container and general cargo crane, a
G HMK 6407, Model 6, Generation 5, with a maximum lifting
capacity of 100 t, hoisting speeds of up to 90 m/min and installed
diesel engine power of 895kW.
As is usual with these cranes, all the rotary motions of the
420 t machine are powered by electric drives. This two-rope
crane is equipped with a 300kW hoist and two slewing gear
units (90kW each), all of which are powered by DC motors.
Longitudinal motions such as luffing, propping and braking use
open-circuit hydraulic transmission. The main hydraulic
transmission unit, which supplies the luffing gear, ancillary drives
and lifting gear attachments, such as a spreader, is also electrically
driven via a three-phase motor which is fed directly from the
Thanks to its typically varied load spectrum, the prototype
crane provides the ideal conditions for the upgrade and the
many tests: during its typical annual operating hours, approx.
4,000, the machine loads and unloads both full and empty
containers. On top of this, it is used to unload comparatively
light loads such as fruit pallets from the vessels’ holds and these
cargoes, due to their nature, have to be handled very fast and
very carefully and then transferred within the terminal to their
intermediate storage bays.
CONVERSION AND EQUIPMENT LEVEL OF THE PROTOTYPE
MOBILE HARBOUR CRANE
In the first stage, the G HMK 6407 was fitted with dynamic
brake resistors, while the state-of-the-art brake resistors fitted
as standard were retained and continue to be used. The
conversion job involved modifying the control system, adding a
further converter to convert from 3-phase current to DC to
enable the supply to the resistors to be regulated, and rewiring
of the existing brake resistors. Even these initial steps were
sufficient, according to carefully conducted tests, to achieve
considerable fuel savings. The next conversion stage meant
integrating a total of six electrostatic double-layer capacitors in
series with a total energy rating of 2,000kWs, which was enough
to store all the energy recovered on-board the crane.
Integrating the ultracaps in the crane’s on-board electrical
system did not necessitate any modifications at all to the existing
diesel engine and generator circuitry, which means that, in an
emergency, crane operation can continue as in the past without
the new ultracaps.
The compact, weatherproof box, which houses the six
ultracaps, and the associated cooling system were mounted on
the roof of the superstructure where there is not only ample
space for the equipment and good access for maintenance but
also from here the cable lengths to the existing electrical system
Inside the crane’s electrics container, an additional inverter
for converting three-phase to direct current, an ultracap
discharger and a PLC controller to coordinate the power of the
ultracaps have been installed. The purpose of the ultracap power
coordinator is to allocate the energy recovered from the crane
motions to the ultracaps or, if their capacity has been exceeded,
to the dynamic brake resistors.
The comparatively compact space requirements for the
ultracaps and ancillary equipment underlines a further decisive
advantage of electric systems over purely hydraulic drive
systems. It is true that closed circuit hydraulic transmissions
permit a form of energy recovery. However, where
electric systems for recovering energy using
ultracaps are wear and friction free and with a
particularly high efficiency rating, hydrostatic drive
systems use wear-prone hydropneumatic processes
with comparatively low mechanical efficiency ratings
and are tied to equipment which is very heavy and takes
up large amounts of space.
SUCCESSFUL TEST RUN WITH G HMK 6407
During the test phase with the G HMK 6407, which
immediately followed the upgrade of the crane to
dynamic brake resistors and the installation of the new ultracaps,
it was demonstrated with the load spectrum described that fuel
savings in the double-digit percentage range can be achieved. As
intended, the excess energy was only dissipated in the dynamic
braking resistors when the ultracaps were full to capacity.
The good test results using the prototype mobile harbour
crane also confirmed earlier tests carried out by Gottwald on a
smaller four-rope grab crane, a Generation 4, HMK 170 EG
fitted with dynamic brake resistors.
Depending on the work schedule of the prototype G HMK
6407 mobile harbour crane, Gottwald intends to continue or
repeat its test series and to monitor this, the first mobile
harbour crane to be equipped with hybrid technology.
Operators of two-rope cranes like the G HMK 6407 tested
and who have comparable load cycles and annual handling rates
of around 4,000 hours can sustainably reduce their operating
costs by converting to hybrid drives by Gottwald and, at the
same time, switch over to a cleaner energy source. This new
drive technology is expected to be of particular interest to
those operating mobile harbour cranes for intensive container
handling or professional bulk handling, where they are run as
4-rope grab versions equipped with two hoists, and often run up
6,000 hours a year.
Throughout the ports and terminals industry, calls for energyefficient
technologies in handling and infrastructure are becoming
louder with a view to exploiting energy-saving potentials.
Gottwald is responding to these needs by developing marketready
green drive technologies and making them available to
ports and terminals as standard or for retrofitting to achieve
sustainable reductions in fuel consumption and emissions. Using
hybrid drives, it is possible to achieve fuel savings in the doubledigit
percentage range with mobile harbour cranes. New hybrid
drives for mobile harbour cranes make a significant contribution
to the sustainable economical and environmentally aware
operation of handling machines and, as a result, improve the
operators’ competitive standing.
The immediate future in this field will be dedicated to
evaluating the experience and test results gained with hybrid
technology from the G HMK 6407 pilot crane and preparing the
systems for market launch at competitive prices. The fact that
there will be economies of scale here with regard to the
purchase of the components needed, which will be passed on to
crane operators, will be a decisive selling point. Decisions on
retrofitting significant numbers of the existing fleet of cranes will
be subject to both the investment costs and the type and
intensity of the work for which each crane is used.
The company also aims in the medium term to respond to an
increased and more frequent use of double-layer capacitors on
each crane by equipping its cranes with smaller on-board diesel
engines. Using downsized engines in this way would generate
not only ecological benefits but also still more economical
advantages in the shape of lower operating costs. Furthermore,
the use of smaller diesel engines may make it possible to use a
range of engines which are produced and sold in larger
production runs and are, as a result, cheaper to buy, which would
have a beneficial effect on the overall costs of the cranes.
The more volatile the fuel price is over time and the more
governments pass new legislation to influence the use of fuels, as
a regulatory instrument to increase the price or to encourage
environmental awareness, the faster sustainable hybrid drive
systems for mobile harbour cranes will take hold. The
acceptance and sustainable operation of hybrid drive systems in
mobile harbour cranes will increase as the price of fuels
increases — either due to market forces or as a result of
regulatory measures applied to protect the environment.
Even if Gottwald has now made a decisive step towards
green, energy-efficient drive technologies by developing this
hybrid drive using double-layer capacitors, the company, a
subsidiary of Demag Cranes AG, is still only at the beginning of
developing drive technologies which will sustainably improve the
contributions made by ports and terminals to environmental
protection. As a consequence, Gottwald will keep a close watch
on the development of diesel engines on the one hand and
energy storage systems on the other and fully exploit
improvements in drive technology in cranes and vehicles in line
with customers’ needs.