Recovered waste heat can give significant fuel savings

Much of the energy a ship produces are lost in the form of exhaust gases. But if you took better advantage of the waste heat from the exhaust gases, the ship would be able to save fuel and the environmental impact would be reduced.

Fredrik Ahlgren. Photo: Margareta Dahlqvist

Fredrik Ahlgren at the Kalmar Maritime Academy, Linnaeus University, shows in his licentiate thesis how a cruise ship can save 1372 kg of fuel per day, equivalent to 5.9% of the ship's average fuel consumption, by converting part of the waste heat from the exhaust gases into electrical energy.

Why have you chosen this particular area of ​​research?
- I have a great interest in new technology and a strong interest in environmental issues. As I have previously worked at sea, this research area was natural for me, and this is a way for me to feel that I can make a difference, Fredrik Ahlgren says.

The studies are based on one year of machinery data from the passenger ship M/S Birka, operating between Stockholm and Mariehamn. The cruise ship was chosen
because of its complex energy system and operating conditions, as it not only transports cargo from A to B, but must also maintain safety, comfort, recreation areas and restaurants for the passengers. This means that its energy system is generally more complex than an ordinary cargo ship.

Compared to other studies in the area, this study comprises a more thorough view of one single unit and its energy system over an extensive timeframe. The collected data have been run in various simulations to find out the best way to convert waste heat into energy in an organic Rankine cycle, ORC. A Rankine cycle uses waste heat to evaporate liquid. The steam is then expanded in a turbine to generate energy. The fluid is then condensed to a liquid phase again before it is pumped in return to the boiler in a closed system.

One fifth of the ship's electricity consumption
The simulations on the M/S Birka shows that the electricity generated by an ORC could be equivalent of up to one fifth of the ship's entire electricity consumption. But although the research shows how to save energy, Fredrik Ahlgren believe it will take time before this kind of energy efficiency will become a reality.

- Unfortunately, I think that oil prices will have to go up significantly before it gets interesting. As long as an investment need to be paid off in a few years, a long-term profitable investment will probably not be interesting.

By actually being on board the ship and gathering data from machinery systems in operation, and by interviewing the crew, Fredrik Ahlgren has combined theoretical and academic theories and models with on-board best practice and working experience. A bridge has been built between theoretical simulations and data logged on board the ship and the main contribution of the combined results is a better and more thorough understanding of how energy efficiency can be optimised from an operational point of view. theoretical framework to the data measured on board.

What will be your next step?
- I stand somewhat at a crossroad right now. Either I continue with cruise ships and expand my studies to more ships and technical solutions, or take a wider perspective and look at the role of technology in combination with humans who operates the ships. Does energy efficiency really lead to lower energy usage? Fredrik Ahlgren says.

According to the IMO, the UN International Maritime Organization, the shipping is expected to increase by 200-300 percent by the year of 2050. With that in mind, it is of utmost importance for shipping to reduce its climate impact.

How do you think your research can contribute to a better climate?
- Above all, we need to evaluate how existing technical solutions are working within a vessel's system today. Any research that leads to improved use of available technology provides at least a theoretical possibility to reduce climate impact, Fredrik Ahlgren says.