Possibilities in power management on board
What are the gensets doing right now? This is one of the most common questions in the world of modern yachting due to the rise of onboard automation systems. Engineers spend untold hours scratching their heads to find out whether a device is functioning normally or not.
Due to more stringent emission rules, higher energy demands and the improved possibilities for automation, generators and their switchboards are becoming smaller, more sensitive and increasingly complex. The philosophy of keeping things simple is not always kept in mind when setting up power management systems. Sometimes it even looks like systems are becoming more complex instead.
There are various possibilities when setting up a generator power management system. Piet Brouwer has worked out four options for you:
1. Twin gensets, each large enough to supply the full possible load, with a seamless switch over if required
2. Twin gensets, together large enough to supply the full load, and paralleling in case of increasing load
3. Two different gensets running at different levels or together at a maximum
4. Two small gensets within configuration 2 and 3, with a load managed by means of non-essential loads and heavy consumer requests, priority settings, priority shifts and night or day modes
All these options can of course also be implemented with three or more gensets, and in combination with an emergency generator and/or shore power unit if required.
The following is a description of these four options and their particular advantages and drawbacks.
The easiest solution is to take two large gensets, each large enough to supply the full load required by the vessel. While calculating the maximum load is quite easy, average load is another problem entirely, requiring knowledge of consumption with and without the presence of guests, and depending on season and on whether the boat is sailing, anchored or moored. Calculating the load on a yacht is not as easy as for a cargo vessel, which is mostly used in a consistent way to transport, charge and discharge cargo. Excessively large engines unfortunately result in yachts producing unacceptable levels of smoke and pollution.
A good alternative is Option 2.
After calculating maximum load, two gensets are installed, each providing half the maximum required power. In the baseline scenario, one genset will be in operation while the second is started up only in case of increased load. Once the second generator is in operation, it is synchronised and paralleled and the load sharing is controlled. Once the load decreases, one genset can again be turned off and the other maintained until the load increases again. While this is relatively simple, it raises a number of questions.
What if the second genset does not start despite an increased load? If so, the load will rise to a maximum and the breaker will trip or the diesel engine collapse, ultimately leading to a blackout – in other words, the exact opposite of why we need the gensets in the first place. To work around this, the concept of non-essential loads (NEL) is formulated, i.e. loads that are not essential for the manoeuvring and safe manning of the yacht. The galley and laundry are disconnected first, followed by boilers and air conditioning, which are also major consumers. The vessel will be safe, possibly at the cost of an upset chef, a stewardess with a big pile of laundry to do, and owners sweating in their stateroom. This is safe, but not exactly ideal.
And what about very low loads, e.g. while the boat is moored over winter with only one watch keeper on board? After a couple of weeks we would have the same overproduction of smoke and pollution.
This is why we invented Option 3.
Option 3 involves two different gensets, in a 70/30 or 60/40 configuration, for instance. It functions as follows: first, the system calculates the actual load and activates the appropriate genset. If the second one does not start, we activate the NEL tripping – as we saw above, this is not ideal, but keeps us firmly in control.
This can work for a while. However after a couple of incidents when the second genset starts up too slowly to supply sudden increases in the load or big peaks within the air conditioning system, the crew and guests will start managing the load by themselves. Finally, the stewardesses will need to check which genset is running to decide if they can start the tumble dryer or not, use the dishwasher or do the dishes by hand. Finally, an electrician will install a "cooking" button to parallel the two gensets beforehand when needed, and, of course they will start running in parallel for many years. With low loads and generators running in parallel we will be back to the smoke and pollution issue.
Then it is better to make the system more intelligent instead.
With this option the load should not be managed by the cook or stewardess, but by a new, fully automatic state-of-the-art system. This works as follows: two or three gensets of equal or unequal capacity are installed. When a large load is about to kick in, the system receives a heavy consumer (HC) request, checks the available load and responds with a go-ahead to the consumer if possible. If the load requires an extra genset, it is started and paralleled before the HC request is accepted. This is the ideal option – consumers wait for their turn, gensets start and stop as needed, and no more thinking is required in the laundry or galley. Everything can be used normally while power management is left to the system.
Though there is still the issue of how to avoid a new genset starting up four times a night, as people use the lavatory or wash their hands. And what happens when we need to heat up the boiler a little more, or wait with the air conditioning to achieve a certain temperature hysteresis? How do we avoid tumble drying laundry while cooking – can the dryer wait and start up automatically when there is spare capacity? This will lead to an Option 5, creating new questions, such as: why does the tumble dryer not start, why is the air conditioning delayed or why is the boiler sometimes hotter than at other times?
All these issues are just a normal part of power management. Finding the right moment for starting the next genset is another challenging adventure: 80% or 78%, or maybe 85%, for one, maybe two minutes, short fast peaks, slow long-term peaks – these are all questions that will keep us occupied for a while.
In the opinion of Piet Brouwer Electrotechnology, what we all need to do is keep things simple. And, even more importantly, we need to visualise what is going on. Why and when does the second genset start? Sometimes nobody will see the peak but the genset will already be up and running for another 15 minutes. Information for engineers and crews is key within these systems, and logging and trending is essential. This should not be too complex, with the users staying in control and having as much information as possible, allowing them to know what their gensets are doing at any given time.