Waste Heat Recovery on Merchant Ships
Diesel engines are used to supply most of the energy requirements on ships, but more than 50% of the energy consumed is lost in the form of heat.
Conventionally, a part of this heat is recovered in the stack and is normally converted into steam which is subsequently used in the different processes on the ship.
Steam used on ships is the target of bad press. First, several pieces of machinery are regarded as pressure vessels and require regular inspection. Second, the steam must be produced at port using the auxiliary boilers that, because they are not very efficient, consume a substantial amount of fuel. Third, all the auxiliary equipment (pumps, relief valves, steam traps, etc.) require continuous maintenance.
Although steam has been (and still is used) in several industrial processes because of its many advantages, the tendency in recent years has been to replace it with thermal fluids such as oil.
However, thermal oil presents certain disadvantages where energy efficiency is concerned. Its specific heat is lower than that of water. Its temperature must be kept higher than water in a boiler set at 6 bars, which reduces its heat exchange capability. With thermal oil, one cannot take advantage of the latent heat in the phase change of steam into water and, furthermore, pumps must be used to circulate the fluid which invariably increases the ship’s house loads. And, this does not preclude the use of a source of supplemental heat when the ship is in port.
Is Such High Temperature Really Needed?
On a merchant ship, 90% of the heat necessary for auxiliary processes can be considered as low temperature heat (below 100 degrees Celsius). High temperature ‘clients’ are those which affect heavy fuel oil injection and its treatment (purifiers).
All the other ‘clients’ operate at low temperature: tank heating, domestic hot water, accommodations heating, lube oil purifiers, main engine heating when in shutdown mode, etc.
An often unexploited source of heat is from the cooling systems of the jackets. In addition to being easily usable, it is available when the ship is in port, thanks to the generators. Water has high specific heat (4.2 kJ/kg/C), almost two times more than thermal oil (2.2 kJ/kg/C). Also, water has a lower viscosity than thermal oil and therefore requires less power to circulate it. Bear in mind that using 10 kW of electrical power to operate a pump will cost between $10,000 and $15,000 in fuel per year. Electrical consumption is therefore an important factor to consider when choosing a circulation pump.
Even if the temperature of the cooling water is relatively low (70-85 degrees Celsius), it is sufficient for many processes.
Other important factors to consider include selecting the right heat exchangers and pumps, and properly calculating pressure losses. One does not select a waste heat recovery exchanger in the same way as a cooler.
In the end, the hot water loop recovers heat from main engine as well as that of the auxiliaries. It’s a simple system that does not require maintenance and is self-regulating.
Normally, more than 200 kW of heat can easily be produced when the ship is in port. This heat can be used to warm up domestic hot water, and keep the generators at the correct temperature and similarly the main engine when it is shut down. In some cases, this loop can be cascaded to preheat other systems that require a higher temperature. In so doing, the boiler is not used to the same extent.
A low-temperature recovery system can be easily installed. It does not require removal of the systems already in place. It’s an add-on to an existing installation. It allows for savings and reduces GHG emissions. And, depending on the ship, the returns on investment of less than 2 years are not rare.
During the energy audit, GHGES Marine Solutions evaluates this opportunity and many others.