Utilising waste heat continues to drive engine efficiency

Even with current conventional two-stroke propulsion power plant, approximately 50 per cent of the energy content of the fuel is lost, mainly to heat, without being used for mechanical work.

“By supplementing the ship’s main propulsion plant with a waste heat recovery system (WHRS), the fuel can be utilised more efficiently, because less energy is lost in the exhaust gas flow,” Markus Virtasalo of ABB explains. “As a further environmentally-beneficial consequence, the amount of CO2 emissions in relation to the engine’s mechanical power output can be decreased.

“Through the WHRS, the recovered energy, which typically amounts to about ten per cent of the main propulsion’s shaft power, is converted back for mechanical work,” Virtasalo adds. “When the WHRS is provided with a propeller shaft generator/motor, a further saving is gained by improving the main engine’s loading condition at various points within the ship’s operating profile. In addition, energy recovered from the main engine exhaust can be converted to mechanical work and added back to the propeller shaft as well.”

A WHRS is a combination of equipment installed on board to assist the ship’s main propulsion machinery recover a part of the energy contained in the fuel that cannot be efficiently utilised by the main engine. Without the WHRS, that energy would be lost as heat into the atmosphere and sea water.

“The mechanical efficiency of the main engine is close to 50 per cent,” Virtasalo continues. “The rest of the energy contained in the fuel consumed by the engine is not converted into shaft power, but is lost, mainly to heat and friction. The WHRS is designed to recover as much energy from these losses as is economically viable.”

Recovery of the waste heat begins in the exhaust gas boiler. Compared with conventional exhaust gas boilers, the WHRS’ dual pressure exhaust gas boiler is designed to efficiently generate steam with characteristics that make it suitable for electricity generation.

“For optimum efficiency, steam is generated at two pressure levels - high and low,” Virtasalo says. “Both high and low pressure steam flows are then led through the ship’s steam piping system to a condensing steam turbine, which is connected to a generator. The turbine will then convert the thermal energy of the steam into mechanical energy to run the generator. When the thermal energy has been used, steam will exit from the turbine and condense in the sea water-cooled vacuum condenser attached below the steam turbine. This condensate water is collected into a deaerating feed water tank and pumped back into the exhaust gas boiler. On its way there, the condensate will recover heat from the main engine jacket, cooling water and/or the main engine, scavenging air by flowing through the respective heat exchangers. This part of the process is called feed water heating. The entire circulation process of the steam and condensate water is closed, and the quality of steam/condensate is monitored.”

Energy is also mechanically recovered from the main engine exhaust gas flow. Part of the main exhaust gas flow is diverted into a power turbine, which is connected to a generator. This part of the process runs the power turbine, which is similar to the turbine side of a main engine turbocharger, and thereby complements the steam turbine’s generating capacity.

The steam turbine and the power turbine can be installed in two different configurations. They can either be on the same bed frame with one common generator, or on separate bed frames with dedicated generators. The choice between the two options can be made on the basis of the ship’s engine room layout, as well as what is technically the optimum and most feasible approach. In all configurations the turbines are connected to the generator through a reduction gear. With the common generator configuration, the power turbine and generator connection are also provided with a special freewheeling clutch, enabling automatic engagement/disengagement depending on operating conditions.

On ships with two main engines, a configuration with two power turbines, one for each main engine, can be considered. In special cases, a WHRS with only a steam turbine and generator or only a power turbine and generator, can be provided, but with consequentially a lower heat recovery capability.

The WHRS can be applied to any propulsion plant with sufficient power output to make the investment economically viable. “There is a clear economy of scale here, and the bigger the main engine output, the more waste heat can be recovered,” Virtasalo says. “The power level above which the WHRS becomes economical depends on the price of fuel, as well as required payback time, and should be validated by making detailed calculations as to system efficiency. As an indication, however, given various parameters prevailing at the beginning of 2012, ABB estimates it would be economically feasible to use WHRS on board container ships with main propulsion machinery with a mechanical output of 20MW or more.”

Another consideration, which determines the economic viability of the WHRS, is the operating profile of the propulsion plant. Ships with a relatively stable operating profile, especially with higher propulsion loads, have the biggest potential for savings. The more the vessel has a high-load operation, the shorter the payback time for the WHRS will be. The WHRS is not run in port or manoeuvring situations, so the smaller these are as a portion of a ship’s overall operating profile, the greater the economic potential of the WHRS.

To date, WHRS have typically been installed on deep sea container vessels and very large crude oil carriers (VLCCs) equipped with a two-stroke engine propulsion plant.

The WHRS will function only when the main engine load is above a certain limit. That limit depends on the system design for each project, but is typically about 40 per cent of the main engine MCR for an ABB WHRS. The propeller shaft generator/motor is functional from any low load, the main engine can run up to 100 per cent of the main engine MCR and the shaft generator/motor can be optimised to give 100 per cent output power at a specified main engine load, for example 80 per cent of the main engine MCR. Optimising specifications during the design phase allows for maximum flexibility in the recovery and utilisation of waste energy during the ship’s operation.