Shipboard energy losses identified

May 01 2014


DNV GL has unveiled a novel approach to overcome challenges of assessing on board energy efficiency in a consistent manner. As a result, priorities for improvement can be determined accurately.

In a new report recently presented in London, DNV GL said that it had answered the question - How can a ship manager identify the biggest sources of useful energy that are currently being wasted on their ships?

“Ship operations and environmental legislation have become more complex and it has become increasingly difficult to assess or even define efficiency with consistency and accuracy,” said Rune Torhaug, director, strategic research & innovation, DNV GL. “We have therefore revisited the basic and universal laws of thermodynamics to develop a methodology based on exergy, sometimes called available energy, which is a metric for describing the maximum useful energy that can be derived from a process, component or system.”

The methodology can be adjusted to suit newbuilds still in the design phase, or operating ships and it is designed  to help managers make the most out of their Ship Energy Efficiency Management Plans (SEEMP).

Using both on board measurements and the DNV GL modelling suite COSSMOS, energy losses throughout the ship, including hull, propulsion power train, machinery and electrical systems, are quantified and ranked. Even  difficult-to-capture processes, such as throttling and fluid mixing, can be incorporated.

The report includes an analysis of a waste heat recovery system. These complex systems can easily contain 70 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, DNV GL senior researcher and project manager of this position paper.

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

Aframax analysis

Both energy and exergy analyses were conducted in order to assess the performance of the main engine of an existing Aframax. The analyses were based on actual operational data, as recorded in the main engine periodic  reports submitted by the crew to the vessel operators.

The study’s goal was to highlight the significant increase of useful information, the improved insights into the system and its processes, plus the accurate mapping of the sources of useful energy losses that can beachieved through DNV GL’s exergy-based methodology.

The ship was fitted with a 2-stroke slowspeed marine diesel engine delivering 11,300 kW at 97 rev/min at its maximum continuous rating. For this study, the engine was considered as a sub-system, comprising the turbocharger (compressor, turbine), charge air cooler, main engine cylinders/combustion block and exhaust gas economiser.

The data available mainly consisted of pressures and temperatures at different locations in the fuel, air, water, steam and exhaust gas path, plus fuel flow rate. In addition, using the turbocharger performance maps, DNV GL said that it was able to accurately estimate the air and exhaust mass flows.

 

 Finally, DNV COSSMOS was used for the thermophysical property calculations, as well as the derivation of the exergy and energy rates. The performance report obtained was for a sailing condition in which the engine operated at 80% of its nominal load.

First, an energy-based analysis was performed. From this it was observed that the energy efficiency of the main engine sub-system was 48.1%. The heat losses (charge air, jacket, lube-oil coolers and radiation) amounted to  26.6% of the fuel energy, while another 25.1% of the fuel energy left through the exhaust gasses. Finally, 647 kW of heat was recovered and converted to service steam in the exhaust gas economiser.

This led to an overall system efficiency of 51.5% based on the results, the greatest sources of losses were the charge air cooler, jacket cooler and the exhaust gasses. Therefore, the attention and efforts of the operators should be focused towards accurate monitoring and improvements in these sources, the class society said.

Picture change

However, the picture changed significantly when exergy analysis was applied, as shown in Fig 2. The first important difference between the two approaches was the significant increase in information gained through the better decomposition of losses and exergy destruction per component. This led to improved mapping and better understanding of energy/exergy flow in the system.

Taking into account the exergy input of all resources used (fuel, air and cooling water), the exergy efficiency of the main engine sub-system was 44.9%. With respect to cooling and heat losses, the exergy-based figure amounted to only 1.9% of the total exergy input, since these are low grade heat losses that have limited potential for useful energy extraction. In addition, another 2.2% of the total exergy input was destroyed in the charge air cooler.

Thus, a significantly different picture was reached, with 4.1% (exergy losses and destruction) compared with the 26.6% of losses derived through 1st law energy analysis.

The exergy flow of the exhaust gasses leaving the engine was 10.4% of the total exergy input, compared with the energy-based 25.1%. Furthermore, through the exergy analysis, the engine turbocharger was identified as an important source of exergy destruction, with 6.9% of the total exergy input; this figure cannot be determined by 1st law energy analysis. Finally, the largest source of exergy destruction was the combustion process, with a contribution of 33.9%.

This comparison of energy and exergy analysis revealed important disparities and distinctions. When using exergy analysis, the mapping of losses was different and more detailed. From exergy analysis DNV GL identified the major sources of exergy destruction in the system - the combustion process contributed 77.9% of the total exergy destruction, turbocharger 15.7%, charger air cooler 5.1% and exhaust economiser 1.2%.

The total exergy destruction amounted to 43.5% of the total exergy input to the system, while losses to the environment (cooling and exhaust) were only 10.8%. Finally, the overall exergy efficiency of the system (power and steam production) was 45.9%.



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