Next-generation energy efficiency methodology from DNV GL

09 Apr 2014
George Dimopoulos, senior researcher and project manager, launches the DNV GL position paper

George Dimopoulos, senior researcher and project manager, launches the DNV GL position paper

DNV GL has announced a new position paper, dealing with what it describes as a new approach to ship energy efficiency methodology, based on ‘exergy’ rather than just energy management.

The class society says it has revisited the basic and universal laws of thermodynamics, to develop a methodology based on exergy, sometimes called available energy, otherwise defined as a metric for describing the maximum useful energy that can be derived from a process, component or system. This, claims DNV GL, gives more accurate and consistent results, which can be surprisingly different from conventional energy management methods.

Using either onboard measurements, or DNV GL’s COSSMOS modelling software, energy losses throughout the ship can be quantified and ranked. This means that the process can be applied to design of newbuilds as well as improving the efficiency of ships in service. The methodology looks at all aspects of the ship, including the hull, propulsion power train, machinery and electrical systems, to identify where losses are occurring and pinpoint where improvements can be made. DNV GL says that its method answers a question frequently posed by ship managers and operators – ‘how can we identify where the largest amounts of useful and recoverable energy are being wasted on our ships?

As examples, DNV GL has applied its process to three different analyses: the design of a newbuild waste heat recovery system; new technology – i.e. a fuel pre-processing sub-system for the Viking Lady fuel cell; and main engine operation on an existing Aframax tanker.

In the WHR system analysis, the study looked at up to 70 separate components. “Through our exergy-based methodology, the true sources of useful energy losses were identified, revealing a picture far from self-evident. Subsequent optimisation in DNV COSSMOS yielded an increase in fuel savings that halved the payback time of the system,” said George Dimopoulos, senior researcher and project manager of the position paper. In this case, conventional analysis suggested that improvements of the cooling system could provide the best solution, but using the exergy-based methods greater improvements could be obtained through optimisation of the turbocharger to the WHR system, offering potential for a 1% fuel consumption improvement, and, because the optimised system was significantly more compact, allowing for extra cargo space, the payback period for the WHR system was halved.

The study of the Viking Lady fuel cell pre-processing system showed a potential for 50% per cent reduction in exergy losses, though this could be explained partly by the fact that this was a first-of-a-king, engineered to work, which it did successfully, rather than optimised for efficiency.

When the main engine of an aframax tanker was analysed using operating data in combination with COSSMOS modelling, the true sources of losses were identified with greater accuracy than a traditional energy analysis, said Mr Dimopoulos. “In fact, the standard energy analysis failed to identify the turbocharger as being the second largest contributor to exergy loss.”

DNV GL regards exergy as offering a ‘common currency’ for efficiency and losses, which provides a key to energy management that will work for all ships, and all onboard systems and components. It can identify operational issues as well as potential technical modifications, and should enhance potential savings through SEEMP by identifying energy loss ‘hot spots’. The process is part of a current Norwegian-fiunded joint industry project between DNV GL, BW Gas and ABB which is looking into energy efficiency on a BW Gas LNG carrier over a one-year sailing period.

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