Waste heat recovery systems

Waste heat recovery systems recover the thermal energy from the engine, it can be from the exhaust gas by converting it into electrical energy via a steam turbine, while the residual heat can further be used for ship services (such as hot water and steam). The system can consist of an exhaust gas boiler (or combined with oil fired boiler), a power turbine and/or a steam turbine with alternator. Redesigning the ship layout can efficiently accommodate the boilers on the ship to better fit these systems.

Waste heat recovery is well proven on-board ships, but the potential can be variable depending on the size, numbers, usage and efficiency of the engines on-board. Furthermore, these measures are usually not relevant for retrofitting, due to large costs and efforts related to redesign, steel work, extra weight, etc.

Organic Rankin Cycle (ORC) systems are a more novel technology that can be used to recover waste heat from the exhaust gases of the marine engines or cooling circuits into electrical power for onboard usage.

Applicability and assumptions

This technology can be applied to all ships regardless of size, age and type even though it seems to be a practical lower limit on the engine size of 10 MW at present. However, the installations tend to be on ships with a large electrical load to use the electricity generated from the heat recovery, otherwise it needs to be combined with batteries.

The effect is assumed to be constant, as the vessels operating at a high enough engine load when in operation for the power turbine/steam turbine to work efficiently. Note that in reality a slow steaming vessel initially designed for e.g. 80% engine load on main engine would not be able to utilize a power turbine/steam turbine.

The benefit from the measure can be twofold; in terms of reduced fuel consumption on either a main engine equipped with shaft generator, or on the traditional auxiliary engines. Some vessel types with high propulsion power and low electrical consumption can become self-supplied with electricity from waste heat recovery and even lead excessive energy to the propeller shaft via a PTI, thus reducing the load on main engines. The production of electricity from waste heat can reduce, or even eliminate, the number of auxiliary engines required to produce the required energy, thus also reduce the cost for service and maintenance in addition to the fuel saved.

Installation of ORC requires additional space and weight. Some solutions can be delivered in a container with 150kW in each which then needs to be scaled up with number of containers after the engine size. Some additional piping when installing such systems is required.

Many ships have been built to operate on HFO, a fuel requiring heating to achieve correct viscosity. Such ships might today operate on MGO instead due to the global regulation for SOx-emissions. These vessels are normally already equipped with exhaust gas boiler and a system to distribute heat in the engine room, these ships can then use the existing system to operate an organic rankine cycle WHR-unit instead of the previously used fuel heating system.

Cost of implementation

The installation cost for this measure is estimated to a range of $5,200,000 – $11,500,000 (USD), per ship from the smallest up to the largest installations. There are a lot of costs involved with installing such a system which are more or less independent of size, and a cost element which is modelled linearly with ship size.

There will be some annual maintenance needed, mainly for the boiler, power turbine and/or steam turbine, in order to keep up the performance of the WHR-system. This cost is estimated to around $20,000 (USD) per ship, independent of size, for waste heat recovery systems with steam turbine. For waste heat recovery systems with power turbine $10,000 (USD) is estimated as an annual maintenance cost, and for the combined power and steam turbine system $30,000 (USD) is estimated.

Reduction potential

The reduction potential is estimated to 2 – 6% of total fuel consumption. This is very much dependant of what kind of waste heat system is installed, how the recovered energy is managed as well as vessel type and operation.

High-temperature units such as steam generators, or low-temperature units working with ORC can utilize the waste heat and convert it to electricity with up to 6% of the mechanically produced main engine power.

Other References

1. Eide, M. S., et al. (2013) CO2 abatement potential towards 2050 for shipping, including alternative fuels
2. MAN Diesel & Turbo (2024) Waste heat recovery system (WHRS) for reduction of fuel consumption, emissions and EEDI
3. Ma, X., et al. (2012) Conceptual Design and Performance Analysis of an Exhaust Gas Waste Heat Recovery System for a 10000TEU Container Ship
4. Singh, D., et al. (2016) A review of waste heat recovery technologies for maritime applications
5. Climeon