4.3 Fuel cell technology
The concept of the 'hydrogen society' has long been speculated about. There is no doubt that promise of a high efficiency; pollution-free energy source makes hydrogen an attractive fuel source for the future. The apparent problem is the implementation of a global infrastructure to meet the demand of the transport and power generation industries. For the marine industry in particular, the competitive advantage shipping has above other means of transport, in addition to economies of scale, lies in its ability to utilise cheap, low-grade fuel, which would otherwise be an unwanted waste product of refining crude oil.
However, there is at present, a huge drive towards the development of hydrogen as a fuel source, particularly in the automotive industry, which appears to be pushing the concept. Indeed, some leading car manufacturers have announced that Fuel Cell Electric Vehicles (FCEV) will be commercially available by 2004 [22]. If this does indicate the long-term future of the fuel industry, there is no doubt that the marine industry will be forced to adopt the same technology.
Fuel cells are essentially electrolytic cells whereby hydrogen rich gas and an oxidant recombined to produce water and electrical energy. The main types of fuel cells are as follows [23] :
・Proton exchange membrane fuel cells (PEMFC) ;
・Solid oxide fuel cells (SOFC) ;
・Molten carbonate fuel cells (MCFC) ;
・Alkaline fuel cells (AFC).
The PEMFC is the preferred type at present and its construction essentially comprises of a number of individual cells coupled together in a similar way to a conventional battery. The fuel and oxidant are fed into the cell as gaseous reactants under low pressure (typically 2-3 bar, although increased pressure improves efficiency) and are recombined to yield electrical current and water, as shown in Figure 5.
This principle is common to all types of fuel cell ; however, they differ in the characteristics of their electrolyte composition, operating temperature or catalyst. The alkaline fuel cell has a unique disadvantage in that the anode is consumed during the reaction ; therefore the through life cost is expensive although the energy storage density is excellent.
Fuel cells are not just restricted to running on pure hydrogen, methanol and reformat fuels have also been utilised. To this end, reforming technology is under development so that conventional fuels such as marine gas oil can be reformed directly prior to use in the fuel cell. In the shorter term, this option is far more realistic as it requires minimal change to the global bunkering infrastructure, unlike the implications of moving to hydrogen fuelling.
4.4 Gas turbines
For naval vessels gas turbine when compared to steam turbine plant had a high level of installed power, reduced fuel consumption, reduced on-board maintenance, better availability, reduced manning requirements and improved machinery space working conditions. For marine applications between 2-10 MW the gas turbine competes directly with medium to high-speed diesel engines. Low to medium speed diesel engines can burn poor quality and much cheaper fuels and are used where size and power density is not critical and in these cases gas turbines would not be considered. The specific fuel consumption of a simple gas turbine cycle is poor compared to a diesel engine and gets worse the lower the power rating. A recuperated gas turbine has a specific fuel consumption comparable to a diesel engine at full load but is still significantly worse at light load operation. A well-designed combines cycle gas turbine will have significantly better specific fuel consumption than a diesel engine through exhaust gas heat recovery. 95% of the fuel energy not converted into output shaft power is held within the high-grade high temperature exhaust whereas with a diesel engine a large proportion of waste heat appears in low-grade sources such as cooling water/oil and lubricating oil [24].
For the first time modern gas turbines such as the Rolls Royce WR-21 are in a strong position to challenge the markets traditionally dominated by medium speed diesel engines, exploiting their compact, high power characteristics. The inherent reliability of the gas turbine has long been recognised, and this could be one of its major advantages when competing for new markets, in particular the fast transportation sector.
Gas turbines have been the prime mover of choice for naval vessels for many years due to their high power-to-weight/volume ratio and rapid start-up time. However, traditional disadvantages of lower thermal efficiency and the inability to burn residual fuel oils, which have previously hindered their acceptance in merchant vessels, are under revision. Currently, the most common gas turbines operate on a simple cycle with full load efficiencies around 35%, although the part load efficiency is significantly less.
Advanced cycle gas turbines can offer higher full-load efficiency and superior part-load performance. The intercooled and recuperated WR-21 gas turbine has a projected efficiency of 42% across 80% of the load range [25]. The advanced cycle gas turbine is ideally suited to power generation when utilising a high-speed alternator and this may become common on ships with fully integrated electric propulsion.
The use of ceramic materials in gas turbines has been the subject of considerable interest for some time. The maximum combustion temperature achievable limits the cycle efficiency of the gas turbine ; higher temperatures would give higher efficiency. The maximum temperature is limited by the physical properties of the materials used in the hot section of the engine, which generally have a melting point of around 1300℃. Ceramic materials with superior high temperature strength and durability allow significantly higher maximum cycle temperatures, which can lead to 20% improvements in thermal efficiency and 40% increase in power output.
4.4.1 Improvements to the Simple Cycle
(a) Combined Cycle
Higher cycle efficiency can be achieved with a combined gas and steam cycle, feeding the hot exhaust from the gas turbine into a Heat Recovery Steam Generator (HRSG) that will then supply a steam turbine. State-of-the-art efficiency on land based, large-scale combined cycle systems have been reported to reach 58% [11]. Although this combined system has a high first cost and is complex and heavy, the fuel savings possible could make it an attractive option for certain vessels such as large cruise ships. The whole-system efficiency can be very high, especially when residual heat is used for cabin heating and galley services.
(b) Humid Air Turbine (HAT)
The HAT concept can be used to economically increase the efficiency of gas turbine machines. Air from the compressor is humidified rapidly within a column before it reaches the gas turbine combustion chamber. The introduction of water to the cycle in this way increases the turbine mass flow rate providing greater specific power output and overall cycle efficiency. Humidification is achieved by utilising waste heat from the exhaust gas and the amount of water added depends mainly on operating conditions and combustion flame stability [26]. Testing of the HAT cycle has been conducted using an intercooled aeroderivative turbine but significant modifications to the combustor and to the turbine gas dynamics, cooling, and materials were required.
