It’s a question we ask with or without an Energy Storage System (ESS), but which has plagued us for years & is always hard to answer; How many generators do we need & at what size or capacity?
Superyachts, unlike many commercial vessels have loads which are incredibly variable depending on the owner, crew & countless other factors which should be identified & realistically examined prior to answering this long standing question.
3 Original Schools of Thought:
Over the years there have been 3 common schools of thought, although other options have been seen. The original school of thought was to have 2 generators, both sized to take 100% of the maximum hotel load. We did this for years, but found the approach to be lacking in efficiency as the generator ran below their load capacity for most of its life span, creating soot, smoke & all kinds of other mechanical issues which can affect the lifespan of the unit.
This eventually progressed to a second school of thought based on 3 smaller generators, so we could keep one online most of the time & when the load demand increased, a second generator would come online to take the extra load while the third was on standby. This approach was better, but not ideal as we still had the possibility of under-loading the generator & have no real load control system in place. Another disadvantage of this system was having an often noisy generator kicking in & out, often effecting the guests’ quiet enjoyment of the yacht.
The 3rd progression was load banks, a large water heater, in which we controlled the load of the generator by trying to heat the ocean water that the heater pulled in. This is better for the generators as it keeps them running at a more stable & efficient load capacity, however this system is wasteful both in terms of lost energy (heat) & in the additional cost the owner incurred in fuel to create this heat, which is ultimately discharged over the side. In effect trying to heat the ocean, a bit of an environmental disaster.
Whats different with an ESS?
In recent years, a new & promising school of thought has emerged thanks to the arrival of Energy Storage System (ESS), which allows an engineer to accurately control the load of the generator while providing a source of uninterruptable power to the vessel. The ESS allows the generator to run at the most efficient point while capturing the majority of the energy created for use throughout the boat, meaning very small energy losses.
An ESS system can be used in many different ways with new ones emerging as the technology changes, but the three main ones I will focus on are:
· Peak Shaving:
§ In Peak Shaving the ESS takes up the small surges in power that would in most cases overload the running generator or would start a second generator.
· Load Leveling:
§ In Load Leveling we control the generators load & set it at the most efficient point, the difference in power between what the generator is producing & what the vessel is using is made up by the ESS. In most cases this will be a slow charging of the ESS, when the ESS is fully charged the generator turns off & allows the ESS to discharge until a generator is needed to start the cycle over again.
§ In the ESS discharge mode the vessel is completely quiet & no generator is running, this is known as “Quite Time”, & is a huge benefit to most owners.
· Uninterruptable Power Supply (UPS)
§ This is a backup supply option where the ESS is in standby mode so it can seamlessly take over the load of the ship if the generator fails. In many yachts the ESS is standing by for the shore power connection as well.
The Bottom Line:
As we have seen there are many of options for generators & ESS, but what does this really mean for the size of the generators, & how many do we need?
Sizing:
On the sizing front the equation becomes much easier when integrating an ESS as we need to calculate the vessels base load & maximum load. If we have a system that can load level & peak-shave, then as long as the generator is larger than the base load the only difference will be in recharge time of the ESS. A bigger generator will recharge the ESS faster & a smaller one will recharge it slower. The key is to size the generator above the base load here & have an ESS large enough to take the extra loads.
Quantity:
As to the question of how many generators we can have, with an ESS system most yachts can drop back to 2 generators, while actually increasing reliability.
Benefits:
For docking situations, the ESS will generally supply the power to the thrusters so you could safely & efficiently get away with one generator. With peak shaving & load leveling running a second generator for hotel load is minimized or even eliminated depending on the size of the ESS system making for a quieter, less costly experience for the owner.
The additional benefits of having the whole ship backed up by a large UPS, are almost infinite from a safety, reliability, fuel consumption & guest comfort standpoint. Imagine sitting at the dock with your owner & the shore power connection fails- what happens? Usually, the ship goes dark & the crew clamber to get the boat online again before the owner gets too upset. With a UPS system, this could never happen as the ESS would simply take over- without even a flicker of the lights!
For new build yachts & retrofits the addition of a ESS for the Hotel switchboard is an amazing addition that will pay for its self in increased reliability, increased safety, lower fuel consumption, increased comfort. & less guessing about the proper size of the generators as the ESS system takes up all of the discrepancy’s in the calculations.
The Bottom Line:
There are many other benefits to adding a ESS on to a ship, this blog cannot begin to address all of the other aspects or even delve into the bigger details of the points I have mentioned here already. Just remember, always size above the base load. Please contact me if you are looking for someone to help you navigate this exciting new technology or to discuss this blog in further detail!