Tugs typically operate in or close to harbours. This makes it particularly important for them to reduce emissions. To make this possible, Wärtsilä has developed a range of new systems building on its strong competencies in hybrid propulsion technology.
The new designs are based on the recently introduced Wärtsilä HY power module, as well as on the company’s proven LNG technology. Among the benefits of the new designs are increased flexibility and efficiency.
Tugs run mostly on low load, but there is a need to access power instantaneously. The new designs have solutions for flexible power generation as well as energy storage by running purely on batteries at low loads, and on a combination of batteries and gen-sets in other modes.
New battery technology, together with innovations in hybrid drives, DC-grid solutions, shore charging, automation, and PTI (Power Take-in) motors have all been tested and successfully used in other Wärtsilä projects and vessel segments.
To increase redundancy, the key is to find the optimum combination of engines, gen-sets, thrusters, distribution, and energy storage in accordance with the actual operational profile for the vessel.
Wärtsilä conducted a study to evaluate nine alternative propulsion concepts for a 75TBP harbour tug, in order to find the best option with regards to fuel consumption, emissions, and running hours. While its comparison was based on a 75TBP harbour tug, the results were relevant and can be adjusted for other tugs and operational profiles. Its profile is based on 260 operational days (3120 hours) per year including harbour standby, which would be relevant for most operational areas.
The test included three diesel non-hybrid alternatives for comparison purposes, as well as two PTI options without batteries, where the electric motors were powered from the auxiliary gen-set.
Two battery-supported alternatives with PTI boosts were also tested, one with Fixed Pitch (FP) propellers and one with Controllable Pitch (CP) propellers. Full bollard pull power was then achieved by a combination of the main engines and the PTIs with power supplied from the batteries. In normal transit, and stand by, though, the tug can be run on battery power by the electric motors alone.
The electric machines can work both, as motor (PTI) and generator (PTO). The batteries will then be charged either from the PTOs or from a shore connection. To get sufficient power from the main engines to run both the propeller and the PTO with FP propellers, either the main engines have to be oversized or a heavy duty slipping clutch capable of keeping the engines and PTOs at nominal speed while the propeller runs at a lower speed have to be installed.
The other alternative with CP propellers allows the main engines to run at higher rpm by adjusting the pitch of the propeller to give more available power from the engine than needed by the propellers and thereby have sufficient power to charge the batteries. The electrical machine acts as a generator/PTO in this configuration.
Two options featuring pure Diesel-Electric versions with variable RPM (revolutions per minute) on the main engines and DC-grid distribution system were also tested. These were separated into two independent drive lines. One had fewer cylinders and the third gen-set was replaced by batteries.
The modes of operation were as follows:
Electric mode: Both engines were off, and all the power was produced by batteries. This mode applies to waiting time, “green” and low-power operations.
Hybrid mode: Batteries supported the engine with dynamic loads such as peak shaving and short time boosts.
Single generator mode: One engine provided the power with batteries providing a boost for load peaks.
Two generator modes: The batteries and engines provided power simultaneously, with batteries supporting the engines with dynamic loads.
The Diesel-Electric option with battery consumed considerably less fuel than the other options. The Diesel-Mechanic options with battery also performed well, whether combined with FP or CP propellers.
Battery charging/discharging is controlled by a DC-DC converter to provide the correct current and voltage, thereby prolonging the life of the battery. Frequency Converters and a modern Power Management System controls the dynamics, optimises performance, and eliminates blackouts.
Since the electrical motor has instant power availability from the batteries, it can respond rapidly to the variations in load, reducing fuel consumption and wear and tear.
1. Peak shaving
As the battery absorbs all major load variations and peaks, the engines will operate on a stable load, and can be used at a higher loading without any need to start up new gen-sets during transients or normal load variations.
During peak-shaving, optimising the electric power consumption during periods of high power demand can fluctuate the SoC of battery. However, in the long run, it moves towards the set charge point.
2. The Energy Management System (EMS)
The Energy Management System (EMS) controls the vessel’s power systems. It contains the functionality normally performed by a Power Management System (PMS) by starting, stopping, and controlling the configuration of the diesel generators. It controls the vessel’s energy flow, and the use and charging of the batteries while optimising the diesel engines’ fuel consumption.
Loads are quickly reduced and controlled by the EMS to avoid overloads or, worst case scenario, blackouts. The battery room is insulated to prevent external heat sources radiating heat into the room. There is also a water mist system for fire protection.
The Diesel-Electric option with FP propellers and batteries brought the highest increase in efficiency, cutting fuel use by 38% compared with traditional system with high speed engines. Running hours were reduced by 58% on the main engines with no need for utility gen-sets.
The Diesel-Mechanic option with CP propellers and battery also performed well, increasing efficiency by 27% compared with a traditional system with high speed engines.
In short, the benefits achievable with the Wärtsilä HY propulsion system are many.
Here’s a list.
Less installed power and less running hours. This configuration will be able to meet the maximum bollard pull (BP) with less installed power. This, together with less running hours, will decrease the maintenance cost.
Lower fuel consumption means lower emissions. Fuel consumption and emissions are significantly reduced through the battery system that handles variations in low load modes.
Optimum engine loading. It is possible to run the engines at optimum load in all modes, and with the batteries handling the peaks, the engines will operate more efficiently thus reducing fuel consumption and the maintenance requirement.
Flexibility in power generation. For the Diesel Electric Hybrid configuration, the operator is provided with flexibility to run the vessel in the most efficient way in all operational modes.
Connection for future power sources. By introducing the DC-BUS technology and batteries, the system is prepared for possible future upgrades and power sources, e.g. fuel cells.
Instant load taking. Due to the instant power available from the batteries, the response from the system improves the manoeuvrability of the vessel.
Authors: Joost van Eijnatten, Product Manager, Thrusters & PCS, Roald Myhre, Business Development Manager, Concept Development, Corinna Nones, Technical Sales Manager, Rune Waage, General Manager, Concept Development, mail: firstname.lastname@example.org,