The potential benefits of LNG cold energy recovery via an organic Rankine cycle (ORC) unit have been theoretically evaluated for a container ship and a ferry, with promising fuel saving potential.
Wärtsilä’s patented cold energy recovery system has already demonstrated operational savings on the LNG-fuelled ferry Viking Grace by reducing the amount of electricity consumed in HVAC system cooling compressors, but a new study from Denmark looks beyond that to the potential of LNG cold energy for vessels with dual-fuel engines.
Enrico Baldasso from the Technical University of Denmark is lead author of a study published in the Journal of Cleaner Production that theoretically evaluates the potential benefits of LNG cold energy recovery for HVAC, for further cooling of engine scavenge air and for multiple uses via an ORC unit. ORC units use an organic working fluid with a low boiling point to recover low-grade heat, and they can be an efficient option when recovering energy from a cold source, particularly where the working fluid also has a low freezing point.
Baldasso and co-authors from Technical University of Denmark and Nanyang Technological University in Singapore evaluated the value of harnessing LNG cold energy on a container ship, with a high pressure LNG gas supply system connected to a two-stroke dual-fuel MAN 9S90ME engine, and a ferry with a low pressure LNG supply system connected to two Wärtsilä 7L46DF four-stroke engines.
The researchers found that the highest fuel savings were attained for the ferry: up to 2.37 percent for the ferry and 0.87 percent for the container ship. The higher potential of the ferry was due to a higher amount of cold energy being available when LNG is supplied to the engine at low pressure. The specific cooling effect that can be extracted from the LNG is 743.4 kJ/kg for a low pressure supply system and 482.1 kJ/kg for a high pressure supply system.
Main engine exhaust gases, jacket cooling water, seawater and an intermediate glycol-water system for HVAC cooling were considered as heat sources for the ORC unit. The maximum power output was obtained using butane as the working fluid for the seawater and glycol-water configurations, iso-pentane for jacket water and n-pentane for engine exhaust gases.
For both ships, the largest fuel savings could be achieved using engine exhaust gases has heat source for the ORC unit. The exhaust gases were available at 365.4oC on the ferry and 210oC on the container ship. Therefore, the greater savings for the ferry were not only due to a higher recovery of LNG cold energy, but connected to the higher temperature of the exhaust gases.
The results for using the ORC unit to absorb heat from the engine jacket cooling water were found to be comparable with the ones attainable using the concept installed on Viking Grace, and in the range from 0.5 to 1.15 percent.
The savings attainable when using the LNG cold energy to further cool engine scavenge air (to increase engine efficiency and reduce NOx emissions) were highly dependent on relative air humidity. High humidity meant that up to 39 percent of the heat was used to condense the water vapour rather than to cool the air in tropical conditions. The estimated fuel savings under ISO conditions (ambient temperature of 25oC and relative humidity of 30 percent) ranged from 0.41 to 0.44 percent.
The study authors note that an ORC system requires a large number of components to be installed, whereas the use of LNG cold energy for HVAC and scavenge air cooling require only the installation of a heat exchanger and perhaps an intermediate glycol loop for transferring the cold energy to where it is required. However, an ORC unit suffers only minor weather effects and the electricity can be used for a variety of onboard purposes.
The installation of an ORC unit on the exhaust line of a marine engine does, however, increase engine back pressure, resulting in a decrease of the engine performance and variation in available waste heat. In further research published in Applied Thermal Engineering, Baldasso and co-authors presented a methodology for determining an optimal design and evaluated it for an LNG-fuelled container ship.
Baldasso is also lead author on a study published in the Journal of Cleaner Production that presents a novel way to supply zero-emission power during port stays. The system combines thermal energy storage with a waste heat recovery system based on ORC technology, and it was evaluated for a hypothetical ferry requiring 1MW of auxiliary power. The results suggest that the proposed system could reduce CO2 emissions by eight percent and could be economically competitive with the installation of battery systems to supply power during port stays, especially for newbuilds.