Innovative power distribution brings flexibility, more efficiency and energy storage capability to a growing variety of ships. For ABB’s Onboard DC Grid, orders and interesting projects are mounting
Diesel-electric propulsion continues to be superior in vessel efficiency terms to conventional plant for specific ship types, but varying[ds_preview] operational demands do not always make optimal use of AC’s need for generators to run at constant rpm.
This was the starting point for the development of Onboard DC Grid, the radically different power distribution solution enabled by developments in DC protection methods, and first proposed by ABB in 2011. In early 2016, despite shipping’s sustained slump, ABB has quietly been securing orders in high numbers and across a wide range of vessel types.
Operating at a nominal voltage of 1,000 V, Onboard DC Grid allows power sources/power levels to match vessel needs, integrating battery power/energy storage as one of those sources. Rather than being locked at a specific frequency (usually 60 Hz on ships), each power source and consumer is an AC or DC »island« controlled and optimized independently, allowing Onboard DC Grid to combine smart DC distribution with the advantage of AC components.
When a marine engine is operated at constant speed fuel consumption is typically minimized at around 85% load. In general, diesel electric solutions have always involved variable propulsion drive/propeller speeds. DC Grid, in addition, allows generators to be run optimally anywhere on the engine’s power curve, offering a further means of fuel consumption optimization.
The first Onboard DC Grid installation on the platform supply vessel »Dina Star« in 2013 saw owner Myklebusthaug Offshore confirming fuel savings in low load conditions of up to 27 %. When undertaking dynamic positioning in challenging weather conditions, 14 % fuel savings were achieved.
Even when oil prices nosedived, Onboard DC Grid orders have powered on. John Lindtjorn, ABB Product Manager Onboard DC grid and Energy Storage, says that this is because the technology offers owners more than fuel savings and improved dynamic response. Heading the list of benefits, he says, is the way DC Grid comes into its own in terms of energy storage. But it also brings more space for payload, fewer components, reduced weight, easier cable installation, lower maintenance and more effective use of shore power.
For the 40m carbon fibre Seasight ferry, delivered by shipyard Brødrene to owner The Fjords in June, ABB supplied a light weight variation of the Onboard DC Grid. »Variable speed engines and shaft generators will naturally benefit the ferry market because they will help lower engine fuel consumption and emissions. However, in this case the main driver for DC Grid has been the addition of energy storage, which can be fully integrated.«
An innovative offshore vessel, the Cefront Technology concept for a ship-ship oil cargo transfer vessel (CTV), currently being built by COSCO Nantong and COSCO GuangZhou, will also be equipped with the system. Onboard DC Grid will allow the ship’s four 3,600 kW ABB generators to operate at variable and optimum speeds, with a DC Grid-compatible 350 kWh battery used for energy storage, back up, enhanced dynamic support and peak shaving.
»The approach simplifies the process of exploiting stored energy as a source for power in a way that is functionally integrated with the other sources on board. An integrated power and energy management system (PEMS) lets each of the power sources play to its own strengths, thereby coaxing the most out of the system overall«, comments Lindtjorn.
»I see clear benefits for applying Onboard DC Grid in combination with a battery in the coastal tanker segment«, Lindtjorn adds. One entry point for DC Grid might be variable speed shaft generators, and owners can then consider the advantages of using DC Grid as a more efficient way of handling pumps, loading and unloading. In addition, response time can be improved when maneuvering, while there is potential to improve propulsion efficiency when a battery pack in installed.
One of the obstacles to the uptake of shore-power as an option for ships in port, according to Lindtjorn, has been that limited power ratings alongside force vessels to run onboard engines to cover peak onboard loads. »Storing energy in a battery can overcome this issue. The ship can draw the current steadily from shore and adapt to peak loads using the battery.«
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