The future of onboard power generation ?

Compared to marine diesel engines and gas turbines, fuel cells are a quiet, clean and efficient way of generating electricity onboard ships. But what exactly are fuel cells, how do they work and what about their marine applications?

Fuel cells generate electricity through an electrochemical process in which the energy stored in fuel is converted directly into DC electricity. Because electrical energy is generated without combustion of fuel, fuel cells have a number of advantages over conventional combustion-based power systems. First, they produce little pollutant to air and very smaller quantity of greenhouse gases. For pure hydrogen fuel cells, only heat and water are emitted. Because of the direct energy conversion from fuel to electricity, a fuel cell can operate at a much higher efficiency than internal combustion engines. For example, the efficiency of a typical diesel engine generator is about 30 to 35%, while that of fuel cells is as high as 60%. Fuel cells also have no moving parts, so they are quiet and reliable requiring minimal maintenance.

In principle, a fuel cell operates like a battery. Reactants in a battery are stored internally and, when used up, the battery must be either recharged or replaced. Unlike a battery, a fuel cell does not run down nor require recharging. It produces electricity and heat as long as fuel and an oxidizer are supplied.

A fuel cell generally consists of a fuel electrode (anode) and an oxidant electrode (cathode), electrolyte, external circuits and a casing. Its performance depends on a number of factors, including electrolyte composition, geometry, particularly the surface area of the anode and cathode, operating temperature and gas pressure.

The essential part of the system is the fuel cell “stack”. In this case, each cell is supplied with hydrogen and oxygen. Individual fuel cells are regarded as a unit element of the system with a voltage about 0.7 volts.

The number of fuel cells in the stack determines the total voltage, and the surface area of each cell quantifies the total current. Multiplying the

voltage by the current yields the total electrical power generated.

There are four primary types of fuel cell classified by the electrolyte used:

l Phosphoric Acid Fuel Cell (PAFC)

l Molten Carbonate Fuel Cell (MCFC)

l Solid Oxide Fuel Cell (SOFC)

l Proton Exchange Membrane Fuel Cell (PEMFC)

Fuel cells can run on pure hydrogen or any hydrogen-rich liquid or gas, such as gasoline, natural gas, liquid propane, petroleum distillates, ethanol and methanol.

The technical feasibility of using fuel cells for ship propulsion and auxiliary power has been demonstrated by successful application in navy vessels.

In air independent (AIP) submarines and submersibles the technology enables submarines to remain submerged up to five times longer than conventional diesel-electric equivalents and the silent operation characteristics and greater efficiency in fuel consumption are an advantage.

There are several studies on adaptation and integration of fuel cells into commercial marine operations. One is the Ship Service Fuel Cell Power System (SSFC) project by U.S. Office of Naval Research which aims to demonstrate that commercially developed fuel cell technology can be adapted to marine applications at reasonable cost. Another is the “Fuel Cell Technology for Ships ? FCSHIP” project designed to enable EU ship owners to utilise the benefits of this new technology and enable its providers to be competitive in the future market for maritime applications.

PEMFC is currently considered as the most promising option for marine applications. Another candidate technology for marine applications is MCFC because of its total intolerance to air contamination and carbon. A comparison of MCFC and PEMFC with conventional marine power systems is given in Table 1.

Benefits in application

l Efficiency: over the entire

operating range, fuel cells have a greater thermal efficiency compared to that of marine diesel engines and gas turbines. Fuel cells are electrochemical devices which have a more efficient conversion process.

l Operational characteristics and human factors: With no combustion and/or moving parts, fuel cells offer a quiet and reliable source of power. For vessels, such as cruise ships, private yacht and cabin cruiser low levels of noise and vibration are critical for comfort of passengers and crew.

l Environmental Impact: Low environmental impact (emission and noise) is one of the most significant characteristics of fuel cells. When fuelled with pure hydrogen, fuel cells emit only water. However, for marine applications, use of hydrocarbon fuels via a fuel reformer has to be considered since pure hydrogen is not practical for commercial ships. Nonetheless, fuel cells offer a significant reduction in CO, CO2, NOx and HC emissions.

Fuel cell costs

One of the main barriers to commercially viable fuel cells is cost. In principle, fuel cell generated electricity costs 3 to 10 times more than other methods. However, fuel cell operation has no lubricating oil cost and requires around 80-85% less maintenance than a diesel electric system. Also, system optimisation and the advancement of technology promises future cost reduction.

Electricity requirement on commercial ships is normally not less than 1500 kW. With a feasible capacity of fuel cell systems at about 250kW, the demand can be met by using multiple stacks. For main propulsion systems, most units are within the range of 4MW which is out reach of fuel cells? capability at present due to technical difficulties. Thus, development of fuel cells for marine application is aiming at auxiliary electricity generation fuelled with commercial marine fuel. It has to be able to supply power in the range of 250 kW- 1MW with a capability of using commercial diesel fuel. Current marine MCFC and SOFC within this power range are in prototype or demonstration stages and estimated to be available commercially by 2008 at about 3MW. As stated before, PEMFC, of a high power density, low stack cost and high efficiency, is the most potential candidate for marine power

systems for capacity larger than 500kW. Market studies have shown that a sizable commercially availability of PEMFC will appear by 2005. Although fuel cells are penetrating into marine market, large scale

commercial application is anticipated to be around the year 2015.