Managing risk as batteries break new ground
The drive for energy efficiency – in particular the harnessing of otherwise wasted energy from a ship or its environment - is causing the marine industry to look more closely at energy storage. Bill Thomson examines the latest guidance from Lloyd’s Register.
Light, waves and tides can all offer ‘free’ energy, while ship machinery such as hot exhaust gases or free-running crane motors can easily generate electricity. Because other industries – notably road vehicles – are enjoying advances in hybrid power technology, batteries and energy management systems are evolving to make full use of the potential of these energy sources.
As with any onboard equipment, marine batteries have to be proven to be as safe as possible, as well as fit for purpose and dependable in often harsh surroundings. So class societies are looking ever more closely at battery technology. Lloyd’s Register of Shipping (LR) considers itself to be in the forefront of marine hybrid developments, having classed some 17 hybrid vessels including tugs for Kotug and Svitzer, and the Scandlines and CalMac ferries. It has recently introduced guidance covering the use of batteries onboard.
LR’s guidance covers various battery types, with their differing chemical, electrical and mechanical properties, from the design and manufacture of individual cells, considering aspects such as ventilation, fire safety, maintenance and electrical performance. It takes an overall view of possible hazards associated with large battery installations, without taking a specific view of any particular chemistry.
According to Louise Dunsby, LR’s lead electro-technical specialist, technological advances have prompted increased interest in battery and hybrid power. “LR is involved in a wide range of projects which aim to make batteries efficient, stable and commercially viable,” she says. “LR's work in the laboratory, and increasingly on the water, is helping shipowners save fuel and increase efficiency, while reducing emissions. Battery installations also give significant reductions in noise and vibration compared to traditional fuel-based power systems."
Among the various battery types, lead-acid is an old, well-established technology offering relatively low energy density but is a low-cost, proved option that still finds applications onboard. Other aqueous battery types include nickel-cadmium, nickel-metal hydride, nickel-zinc and silver-oxide, all of which are alkaline cells, avoiding potential hazards associated with acid electrolytes. Nickel-zinc and nickel-cadmium batteries have a low energy density, the latter having particular advantages of durability and good low temperature performance, making it potentially suitable for marine applications, while the silver-oxide and nickel-metal hydride types offer a higher energy density but the former does not operate well at low temperatures and comes at a high lifetime cost, the latter suffering relatively poor charge retention and is susceptible to thermal imbalance.
Most of these disadvantages are overcome in the later lithium-ion (Li-ion) batteries, which are non-aqueous, but use flammable electrolytes. So they possess certain inherent hazards such as thermal runaway, particularly if over-charged, so Li-ion installations need sophisticated protection and monitoring circuitry.
LR’s guidance aims to mitigate hazards so that risks associated with battery installations are no greater than those of a conventional power system. Batteries therefore have to be protected against mechanical damage, and the chemical reactions involved in battery operation have to be considered to ensure that there is adequate ventilation and protection against any possible risk of corrosion.
Electrical short circuits are a potential problem with any battery type, although the hazards are greater with some than with others, so the battery design has to account for protection against thermal runaway – if one cell fails, then temperature rises of adjacent cells must be limited. Interrupting devices such as uses, circuit breakers, thermostats and other positive temperature coefficient devices must be considered. The charging and discharge circuitry has to be carefully designed to protect against over-voltage or under-voltage, and to avoid unacceptable temperature rise.
Fire is possibly the greatest potential hazard associated with battery banks, particularly those of Li-ion type. So as well as protective devices which guard against any cell reaching a dangerous temperature, the battery compartment must be protected by a fire extinguishing system of an appropriate type having regard to the cell chemistry and risks of production of hazardous or toxic gases. The battery space must also be insulated to guard against the temperature being raised by fire or overheating in other parts of the vessel.
Other important considerations, according to the LR guidelines, include the human factor – operators must understand the battery management system and know the appropriate action in case of failures or alarms. Also, the maintenance process should be fully documented.
Although LR does not consider it appropriate to issue prescriptive rules - because of the variety of different cell chemistries as well as the rapid advances in technology - the class society believes it essential to be involved from the initial design process, in order to adopt a risk-based appraisal of the battery installation as part of the whole ship. Under the Ship Rules, battery installations should comply with the general requirements for electrical and control systems, while any associated software will be subject to the relevant conformity assessments. In general, large battery installations will be covered by LR’s provisional rules for DC distribution systems.
Most marine battery installations have formed part of the ship’s auxiliary system, though many are capable of powering the ship alone for a limited period. But all-battery ferries are gaining a foothold. Just about all of the large commercial vessel battery banks have made use of Li-ion technology, which appears to currently offer the best balance of energy density, battery life, and through-life cost, and which, with appropriate protective circuitry, has proved to meet acceptable levels of risk.
LR’s Louise Dunsby says it is important to be aware of technological advances: “As soon as the world starts to take an interest in a technology, lots of interesting research starts happening, and if this research achieves a power dense energy source, then the scope is revolutionary. The impact on the maritime sector could be immense.”
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