Waste Heat Recuperation
The main principal of a waste heat recovery system is to recover the thermal energy available in the exhaust gasses of a marine engine. Most of this thermal energy is converted in electrical energy while the remaining heat is used for ship services where heat is required e.g.( hot water supply, steam supply). The main parts of a waste heat recovery system are the exhaust gad boiler and the turbine that converts the thermal energy into electrical energy.
Waste heat recovery is a proven technology for decades onboard ships. It can be applied on all ships with a minimum engize size of 10MW. It is difficult to install a waste heat recovery system as a retrofit measure. This due to large costs and efforts related to redesign, steel work, extra weight, etc.
The improvement on the total efficieny of the ship is assumed to be constant, as the vessels operating at a high enough engine load when in operation for the turbine to work efficiently.
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. For simplicity the estimated reduction potential is here given as efficiency gain on the main engine, taking into account this twofold benefit possibility. The reduction potential is estimated at 3% to 8% of main engine fuel consumption.
Hybrid propulsion systems is the umbrella term for propulsion systems that have at least 1 liquid fuelled engine and 1 electric engine. Mechanic and electric power work together in the propulsion train, optimising the propulsion efficiency for ships with a flexible power demand.
The combination of mechanical power assures the ship a broad operational capability, providing the right amount of power and torque to the propeller in each operation mode. These operational modes are:
-combination (mechanical + PTO)
-Booster (maximum power to the propellor)
Whereas a diesel-mechanic propulsion system is often designed according to its maximum power demand, which, for example, is fitted for a tanker or cargo vessel according to the most hours of the operation profile, a hybrid propulsion plant is better prepared for changes in operation during the vessel’s trip or even the vessel’s lifetime.
The specific fuel oil consumption and emissions from an internal combustion engine highly depend on the engine load. Typically, engines are calibrated for optimum performance at high loads. For ship types that experience large load variations during operation, the addition of electro motors may allow the engines to operate optimally with respect to fuel oil consumption and/or emissions. An example of this is dynamic positioning (DP) vessels often experiencing high transient power demands while operating frequently at unfavourable low engine loads.
There are several different types of shaft generators in common use on ships. The simplest type is a shaft generator connected to the main engine by a gearbox with a fixed gear ratio. To obtain constant frequency electric power the main engine must operate at constant RPM, which requires the use of a controllable pitch propeller (CPP). This is well suited for medium-speed diesel engines, which are normally fitted with a CPP.
A shaft generator powered by the main engine operating at constant RPM cannot operate in parallel with some ship service applications. The reason for this is that main engine RPM will vary more than the diesel generator’s RPM, particularly when the ship is pitching in waves, plus the larger size of the main engine means it accelerates slower than smaller diesel generators, making it hard to hold constant frequency and load sharing between the two generators.
Frequent operation at part load conditions with this type of shaft generator can actually raise annual fuel consumption, even though the main engine has lower SFOC than a ship service diesel generator(SSDG). The increase in main engine SFOC caused by suboptimum propeller setting offsets the savings in SFOC for generating power. Whether there is a fuel savings or fuel increase very much depends on the specific circumstances of the vessel and its service.
Alternative shaft generators are available that have either variable ratio gears or frequency control. Both of these types can work with a fixed pitch propeller over a range of RPM (usually 75 to 100 percent RPM), alleviating some of the issues with the constant gear ratio shaft generator. However, these shaft generators are more expensive and less efficient so the savings in fuel compared to using a SSDG is unclear for these types as well.
It is most likely savings will be found from installing a shaft generator if it is possible to substitute one shaft generator for one SSDG. If this can be done there will be savings from the reduced installation cost of the shaft generator compared to the SSDG and, similarly, from the reduced maintenance costs of the shaft generator. However, depending on the specifics of a project, the payback can be many years, if at all.
Cold ironing or Alternate marine power (AMP), as the name suggests, refers to the usage of other power supply sources to feed power to the ship. Such AMP is used when the ship is halting at a port so that the engines of the ship (working on diesel) do not need to be used unnecessarily. This in turn reduces the emissions by the ships by a great margin.
The process of cold ironing is quite simple. When a ship is at berth power is supplied to them by cables from shore. At present there are four different variations in the AMP that is provided from the port to a ship or a tanker. The same can be listed as follows:
- 11000 Volts of AC (Alternate Current)
- 6600 Volts of AC
- 660 Volts of AC
- 440 Volts of AC
There are some disadvantages to cold ironing. One of the most important is cost, first there is an investment needed onshore and secondly also the ship needs to be prepared for cold ironing. Also the reduction in pollution occurs only when the ship is stationary. This is beneficial for local pollutants that are not emitted to the atmosphere but for GHG emissions this is highly dependend on the GHG intensity of the onshore grid.
Fuel cells are electrochemicals cells that convert chemical energy into electrical energy and this with a high effciciency of up to 60%. This is due to the direct conversion of fuel to electrical energy. Fuel cells also have lower noise and vibration emissions than conventional engines. Fuel cells can only be seen as a renewable energy sources onboard a ship if the fuel used for the fuel cell is renewable e.g. (green hydrogen, renewable methanol,…)
The three most promising fuel cell technologies for maritime use are SOFC (Solid Oxide Fuel Cell), PEMFC (Proton-exchange membrane fuel cell) and HT-PEMFC (High Temperatur PEMFC). In addition to pure hydrogen, fuel reformers enable the use of fuels such as natural gas, methanol and low-flashpoint diesel. When fuels other then pure hydrogen are used a fuel reformer converts the original fuel into hydrogen rich fuel for use in the fuel cells.
Mass production, which is expected to occur beyond 2022, should allow production costs to reach a competitive level. Development projects are underway, and the most promising project for maritime fuel cells, e4ships (figure below), is aiming for a market launch in 2022. With increased production, the impact of material costs will become a dominant factor in fuel cell prices. Maintenance and operational costs will reach a competitive level after fuel cell durability reaches the same level as the longevity of combustion engines.
The international regulation for the design and construction of maritime fuel cell applications is currently under development at the IMO as part of the International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code). Existing class rules form the basis of special permits. The current international regulatory framework is geared towards combustion engines. Apart from some class rules, there is no binding international regulatory framework for maritime fuel cell applications.
Only small maritime fuel cell applications with an electrical power output of up to 100 kW are currently in operation. Current research and development work aims to make maritime fuel cell systems marketable and scalable from 2022. It should be noted that the lifetime of fuel cell systems and reformer units has not yet been shown to be satisfactory.
-Shaft generators for low speed main engines ( MAN Diesel and Turbo)
-Hybrid Engines ( MAN Diesel and Turbo)
-A review of fuel cell systems for maritime applications (van Biert et al.) 2016