Types of waste heat from ships and ways to utilise it (2)

For a long period of time in the future, the burning of fuel in internal combustion engines will remain the main way for ships to obtain propulsion power and electrical energy. The ship industry is facing the dual challenges of rising operating costs due to fuel shortage and price increases and ecological and environmental problems caused by fuel burning. Waste heat utilization technology has been a common concern of all mankind.

The main sources of waste heat in ships: diesel engine exhaust, boiler smoke, charge air, cylinder liner cooling water, etc. The exhaust temperature of low-speed two-stroke diesel engine is around 320℃～350℃, and the exhaust temperature of medium-speed four-stroke diesel engine is around 350～380℃; the exhaust temperature of auxiliary boiler is around 150～250℃; the temperature of pressurized air is around 120～150℃, sometimes even up to 200℃; the temperature of diesel engine cooling water is around 80～90℃, and the effective use of this part of waste heat is The effective use of this part of waste heat is the key to reducing the cost of ship transportation and the energy saving and emission reduction of ships.

At present, the ways of using waste heat in ships include heat exchange through heat exchangers, for oil heating and heat preservation, heating seawater distillation to make fresh water, cabin heating and hot water required for daily life, etc.; using exhaust gas turbocharger, power turbine, exhaust gas boiler, steam turbine generator, waste heat power generation and other devices to generate power; cooling through compression or absorption refrigeration devices, for ship cold storage and air conditioning.

Power turbine: The power turbine is a different device from the exhaust turbine in a turbocharger for recovering waste heat from the ship's main engine. The power turbine bypasses a portion of the flow directly from the exhaust of the main engine, driving the turbine to produce shaft work, which drives the generator to generate electricity to ensure the ship's power supply or to output useful work to the outside through the shaft drive. When the main engine is at low speed and low load, the exhaust gas energy is not high, so the exhaust gas bypass valve will be closed and only the turbocharger will be used to produce work without going through the power turbine.

Composite exhaust gas turbocharger: The exhaust gas power turbine and exhaust gas turbocharger working in series is called a composite exhaust gas turbocharger. In some diesel engines with higher boost, the exhaust gas energy in addition to driving the exhaust gas turbocharger, there is still excess energy for driving the low-pressure exhaust gas power turbine, which is connected to the engine output shaft through the gear transmission and hydraulic coupling. In this way, the exhaust gas turbocharger achieves the purpose of boosting pressure, while the exhaust gas power turbine transforms the exhaust gas energy directly into power for the crankshaft. Composite exhaust gas turbocharger can make full use of exhaust gas energy, so that the power performance, economic performance greatly improved, but the structure is complex, costly and technically difficult.

ABB in the late 1970s based on power turbine technology first proposed a large ship diesel engine exhaust residual energy recovery technology, the technology can be used alone, but also with the exhaust gas boiler generated steam driven turbine to form a combined power recovery system. According to ABB, the power turbine alone can recover up to 5% of the main engine's power, and in combination with a turbine, up to nearly 10% of the main engine's power.

Exhaust gas boilers: Exhaust gas boilers are boilers that produce steam by heating water with the heat of the flue gases from the diesel engine. The exhaust stream from the main engine passes through the boiler and heats hot water into superheated steam, providing the steam needed for turbomachinery, turbine generators, heating oil and everyday life (bath water, kitchen steamer, etc.).

On diesel ships, the exhaust gas boiler is installed on the diesel engine exhaust pipe at the top of the engine room and is only put into operation when the main engine enters rated working conditions, with its evaporation amount depending on the power issued by the diesel engine. When the ship is under normal navigation, the load of the main engine is high (more than 75% of the rated power) and the exhaust gas flow is stable, which is high quality thermal energy. The best way to convert the steam heat recovered from the exhaust heat is to use the turbine generator to generate electricity, in addition to heating and miscellaneous domestic use.