Advances in superyacht hybrid power systems

20 January 2015By Richard Boggs
Northern Lights’ hybrid power system’s batteries pass electricity to a power control system. A traction motor powers the prop.

While hybrid power systems haven’t yet met with widespread acceptance in the yachting industry, a new system from Northern Lights coupled with battery improvements may mean the next generation of yachts will be able to incorporate truly useful hybrid power technology.

Losing electrical power on a large yacht, even when off charter, can be much more than a temporary inconvenience. A steady flow of clean electricity at the correct voltage and frequency is of critical importance to modern communications and control systems. And an unplanned loss of electrical power may create the first link in a chain of seemingly unrelated equipment problems that might not manifest themselves until days, weeks or even months after power is restored.

An unplanned loss of electrical power may create the first link in a chain of seemingly unrelated equipment problems

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Electrical power is affected by all of the consumers on the distribution system. Starting and stopping heavy loads such as stabilizer pumps, air-conditioning compressors and some laundry equipment can cause large momentary changes in voltage and frequency to create what electrical engineers call ‘dirty power’. This is the source of many unexplained problems with a yacht’s electronic equipment.

Battery power

Electrical power generation and distribution on large yachts seems to be as much art as science. Even if a yacht is delivered with generator capacity perfectly matched to demand, normal alterations and refits often increase and occasionally even decrease power requirements to the point where one generator is marginally capable of handling the load, but placing a second unit online means both are significantly underloaded. Overloading a single generator is unacceptable at any time, but engineers and owners are beginning to understand that underloading can be equally costly.

Recent improvements in storage battery technology related to the development of hybrid power systems for the automotive industry have created a shadow market for marine hybrid propulsion systems. But to date, very few hybrid marine power systems have met with widespread acceptance, partially due to high initial cost and limited performance.

Very few hybrid marine power systems have met with widespread acceptance, partially due to high initial cost and limited performance

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The 2011 introduction of a marine version of the highly successful BAE Systems’ HybriDrive terrestrial drive system by Northern Lights, the Seattle-based manufacturer best known in the yacht market for its marine generator products, may provide a glimpse into the future of superyacht electrical power. How successful it will be for propulsion remains to be seen, but the technology it incorporates should have a far-reaching influence in future yacht designs.

Called the Northern Lights Hybrid Marine system, it utilizes a diesel engine to drive a generator that supplies electricity to a propulsion motor. If the power required to drive the yacht is less than that available, excess power is stored in a battery until needed.

That description is accurate as far as turning a propeller is concerned but ignores several features that might lead designers down a completely different path with regard to electrical power generation and distribution on large yachts.

The system is unique among its current competitors in that it does not transmit mechanical power from the engine to the propeller shaft. This means that the diesel engine and its attached generator can be mounted where and how they best fit.

Yacht owners developed the Bahamas and Caribbean’s first alternative power hybrid island – Over Yonder Cay

So far, none of this is much different than diesel-electric propulsion systems that have been used to drive ships for over a century and, at first glance, is identical to the system used to power World War II submarines.

As in those submarines, diesel engines produce electricity to power an electric drive motor and charge batteries. When the engines are shut down, energy stored in batteries is used to power the motors. What sets Northern Lights’ system apart from similar offerings, and should make it much more interesting to designers is that the generator produces alternating current (AC) that is rectified to direct current (DC) to charge the battery, while DC power from the battery is converted back to AC to power the motor.

At first glance, it is identical to the system used to power World War II submarines

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While this might sound like an energy wasting exercise, it is if we only consider propulsion. The potential to scale up the storage side of the system and extract electrical power for use on the yacht’s power grid more than offsets the losses inherent in a marine propulsion-only installation.

A large component of storage battery performance is based on the chemical reactions that either produce electricity under load or reverse to store electricity when undergoing a charge.

A battery’s suitability for use in a marine propulsion or power system is determined by it’s cost, and the battery’s performance. The latter’s factors include:

  • The amount of power that can be stored,
  • How quickly that power can be extracted and replaced,
  • How many times that cycle can be repeated, and
  • How large and heavy a battery is related to the amount of power it stores.

If we use the familiar lead-acid storage battery as a baseline measure of performance, the potential benefits offered by development of lightweight and high-power lithium-ion batteries become clear.

Breaking battery limits

A lead-acid battery will provide about 180 watts per kilogram. If the battery is discharged down to about half its capacity or depth of discharge, it might accept up to 800 recharges before its capacity drops too low to be useful. A storage battery’s useful life is considered finished when its capacity drops to 80 per cent of that of a new battery.

The latest lithium-ion battery technology will deliver about 2,400 watts per kilogram and has a useful life approaching 3,500 charge/discharge cycles.

Attempts to market hybrid power in the marine market seems to miss the areas which would benefit the most

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The heart of the HybriDrive system is a Nanophosphate (lithium iron phosphate) battery developed by Waltham, Massachusetts-based A123 Systems. The performance of any storage battery is related to the surface area of its active components: the plates and the electrolyte. That is why a large battery can supply a lot more energy than a small battery that uses the same chemistry.

When scientists discovered the means to ‘nano-ise’ the phosphate material used in the cathode (the positive plate) they were able to exponentially increase the active surface area and create a storage battery with a capacity and lifespan never thought possible just a few years ago. A123 Systems claims its latest Nanophosphate batteries have a lifespan of approximately 10,000 charge/discharge cycles.

Thanks to the efforts of automotive and utility power engineers, hybrid power systems are quickly becoming a robust and highly efficient means to supply electrical power on the road and in the home.

Attempts to market hybrid power in the marine market seems to miss the areas which would benefit the most: electrical power generation and distribution.

The same technology that keeps the lights on in Fairbanks should be used to reduce the costs and environmental impact of the yacht industry.

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