Power conservation and management is an integral part of operations on board ships. Nowadays, ship engineers are specifically advised to inculcate best power saving practices while carrying out on various on board operations.
Power management on ships comprises of two main aspects:
a. Automatic Power Management Systems: Using automation to conserve power.
b. Using best practices and management guidelines to reduce power consumption.
Most modern day ships today are built with provisions for periodically unattended machinery spaces (PUMS). On such vessels, automatic power management system (PMS) plays a role, which is of utmost vitality. Not only does the PMS does away with manual synchronisation of generators, it efficiently regulates the number of generators on the busbar according to the changing load.
Some of the major functions performed by the PMS are as follows:
- Cutting in and out of the generators according to increase and decrease of load.
- Gradually loading and unloading of generator alternator sets, so as to minimise thermal and frictional stresses.
- Performing load sharing operations among the generators symmetrically or asymmetrically (depending on auto/manually set parameters).
Diesel generators are the primary components of the PMS. All generators have a maker’s specific minimum, maximum load criteria, and optimum load criteria. When the generators are synced with the ship’s PMS, engineers have the option of changing the minimum and maximum point beyond which, the generator cannot be loaded. This is to prevent various stresses on the physical components of the generator.
The loading and unloading of power from the alternator of the generator is driven by time lag functions, which often means, that a sudden spike in the load cannot be compensated by the PMS. A hardwired preferential trip, then, becomes, an essential requirement of the system in order to prevent sudden blackout.
Some ships are also fitted with a shaft motor, which not only compensates for a sudden drop in load, but also, minimises shaft torque on engines with a long propulsion shaft. Another advancement in technology has been in the form of a combined Shaft Motor/Generator set which is regulated by the PMS.
When generator sets are run in parallel, including, shaft generators, diesel generators and/or steam driven turbine generator, the PMS almost completely regulates the load on each component. Generally, in case of generators with equal load capacity, the load on the bus bar is distributed symmetrically on the alternators. However, different kinds of power generating machines, having different maximum an optimum load, the PMS distributes asymmetrically.
For efficient fuel consumption, it is always desirable to run the minimum number of generators, each at a load that is optimum. For instance, one generator running at 30% load may be more fuel efficient than 2 running at 15% and, conversely, one generator running at 70% may consume more fuel than 2 running at 35% load each. Thus, performance evaluation of generators according to their maximum and optimum rated capacity must be carried out regularly.
At the start of each voyage, marine engineers must discuss the power management plan and consider various factors like, number of reefers onboard, use of stabilisers during the voyage, maintenance to be carried out on any generator during the voyage to determine which and how many generators to run. An unexpected breakdown in the generators may require cutting down on the power consumption. Let’s take a look at a few factors which would help in smart reduction of consumption of power.
1. Reefers– Container ships, also, designed to carry reefers, will, of course consume a higher power with the increase in the number of live reefers onboard. Stowage plans must be checked so that reefers requiring ventilation would be carried on open decks. Where placed in cargo holds, efficient usage of reefer cooling water system is a much more economic way than using heavy inlet and exhaust fans for cargo hold ventilation. Thus, it is imperative the fresh water cooling system for reefers, which includes fresh water and sea water pumps, expansion tank and pipelines are kept in good working condition.
2. Ballast pumps– Most ballast pumps are heavy duty pumps which consume a lot of power. Ballast plans should be formulated with the aim of using ballast pumps only when required. Filling of tanks, where practical, must be carried out by gravity. Similarly, use of ejectors only while final stripping of tanks and not continuously while deballasting, reduces usage of pumps and eventually power.
3. Fuel Transfer pumps- Usage of service steam effectively to heat the fuel in storage tanks is an important power reduction factor. Fuel to be transferred must be kept at the temperature mentioned in the fuel specification document. Low temperatures of fuel result in frequent tripping of the pumps, not to mention, prolonged running of the pumps to transfer the same amount of fuel.
4. Air compressors- Any air leaks in the start air or service and working air must be repaired as soon as detected to prevent continuous running of compressors and to prevent frequent loading/unloading of compressors. Running hours of the compressors must be looked at closely and planned maintenance on the compressors must be carried out according to maker’s specification.
5. Fresh water- Most ships today use hydrophore tanks to pump fresh water for domestic and other purposes. These tanks must be topped up frequently with air, so as to minimise frequent running of hydrophore pumps to achieve the set pressure in the tank.
6. Central Cooling water system- Care must be taken when establishing the number of sea water, high temperature and low temperature pumps which are running. Sometimes, additional pumps may start resulting in higher power consumption.
7. Engine room ventilation- Ventilation fans are also large consumers. Engine room pressure and temperature must be carefully evaluated so as to run only the required number of fans. Where fan motors are dual speed or of a variable frequency type, selection of lower speeds, where practical, go a long way in reducing power consumption.
8. Lights– A simple, yet largely unpractised factor is switching off lights which are not in use. Cargo hold lights, steering gear room lights, deck lights should be switched on only when in use. This practice will go a long way in curtailing power consumption.
The above clearly shows how important planning a voyage is in order to minimise consumption of power. Considering the number of ship operations carried out on board ships, close coordination among deck and engine departments is absolutely essential, perhaps more important than the PMS itself.
Over to you..
What according to you are the most important points for an efficient power management on board ships.
Let’s know in the comments below.
Image credits: © Carabay – Fotolia.com
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