WFSV hybrids come closer

06 Jul 2017
Will the WSFVs of the future run on batteries?

Will the WSFVs of the future run on batteries?

“When you look at the considerations for WFSVs,” said Andrew Eydt of PBES, “Human safety as well as operational concerns are top, and what you want for both is redundancy that’s key.”

However, the high speed transits are fairly long, so the batteries can get another chance at a top up and there’s plenty of time to absorb it without demanding more from the engine capacity.

This is often followed by hours of relatively short transits between towers and extended periods of loitering around the windfarms on standby. “This low-speed loitering presents a very inefficient loads cycle for the engines,” pointed out Mr Eydt. However, while the ‘pushing on’ element of engaging with the towers is a typically low engine load scenario, there are typically sudden changes as the waters surge across the foundations - here, it seems that batteries’ ability to respond in milliseconds might yield important, hitherto unexplored, advantages.

Of course, there’s the unpredictability of the operational pattern which is where hybrids again win: even if the onsite manoeuvring continues for an extended periods, it’s possible to cycle energy storage and engines alternately. 

All this makes a very good case for batteries or does it?

What has put many operators off is the physical footprint of the energy solutions and those unpredictable operational matters make WSFV designs particularly weight conscious - and operators wary.

However, battery technology has shaped up a great deal in just a few years, doubling output for the same weight, and also halving in price.

Alone, this is impressive but it might not yet be a game changer for the WFSV market. However, PBES has worked out a novel trick or two to answer the limitations of faster craft: Mr Eydt explained part of it is down to choosing the right lithium composition for the job: at the moment, a Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC) chemistry gives the best combination of safety, energy storage and power density for the marine environment.  But there’s also how you handle it: “When all the energy is coming from batteries alone demands on the system can be very high - so cooling systems are essential too for the battery life. Liquid cooling will allow discharge rates as high as 3C, without damaging the battery,” he said. 

However, like it or not, cells age anyway. Cell degradation can lose the system a couple of percent a year in capacity “which acts like compound interest” added Mr Eydt, eating away at the bottom line. The problem has been that battery suppliers think in terms of getting a good, long 10 year life from the cells, and because of this they try to make sure there’s enough power left at the end of the decade for full operation. Adding it up, “the losses on a 10 year system at 2% a year can amount to 20% of the battery capacity”.

The solution, says Mr Eydt, is almost the reverse. Consider the cells as consumables, and swap them out after five years leaving the rest of the system architecture in place. This way, cell capacity loss is greatly reduced and the battery banks can afford to be quite a bit smaller. By drawing all these elements together, PBES believes it can drop the system size by 30%. So, fitting the ‘base case’ vessel with a pair of 600kW engines, backed up by a relatively modest 390kWh battery would give the operator the best of all worlds.

And, he added, using PBES’ ‘CellSwap’ means that you can invest for a five year interval. This re-coring also means that the operator will benefit from significantly improved cells.

There are some limitations: while the re-coring process works for all PBES batteries, the smaller five year system only makes economic sense if the duty cycle of the vessel requires the energy storage solution to be run at higher discharge rates.

There is one further element to consider: the entrance of larger hotel or W2W vessels. These W2W vessels have power generation capacity to spare and could in fact be optimized by energy storage themselves. This might also mean WFSVs can use them to pick up a charge on site, furthering the case for full electric operation.

The ability to do this will, say PBES, speed up the acceptance of hybridised WSFVs across the board.

However, if the sums add up, the WFSV will have instant, spinning reserve at its disposal for peaks in demand for pushing on, or operation of cranes and other equipment. There are also the advantages of decreased fuel consumption, air pollution, noise and maintenance costs.

The case needs developing - but it seems that there's already "interest" says Mr Eydt.

By Stevie Knight