Fuel cells

Fuel cells are electrochemical devices that convert the chemical energy in fuel into electrical energy directly, without combustion, with high electrical efficiency and low pollutant emissions. They represent a new type of power generation technology that offers modularity, efficient operation across a range of load conditions, and opportunities for integration into co-generation systems. With the publication of the Energy White Paper in February this year, the government confirmed its commitment to the development of fuel cells as a key technology in the UK's future energy system, as the move is made away from a carbon based economy.

There are currently very few fuel cells available commercially, and those that are available are not financially viable. Demand has therefore been limited to niche applications, where the end user is willing to pay the premium for what they consider to be the associated key benefits. Indeed, the UK currently has only one fuel cell in regular commercial operation. However, fuel cell technology has made significant progress in recent years, with prices predicted to approach those of the principal competition in the near future.

Fuel cell technology
A fuel cell is composed of an anode (a negative electrode that repels electrons), an electrolyte membrane in the centre, and a cathode (a positive electrode that attracts electrons). As hydrogen flows into the cell on the anode side, a platinum coating on the anode facilitates the separation of the hydrogen gas into electrons and protons. The electrolyte membrane only allows the protons to pass through to the cathode side of the fuel cell. The electrons cannot pass through this membrane and flow through an external circuit to form an electric current.

As oxygen flows into the fuel cell cathode, another platinum coating helps the oxygen, protons, and electrons combine to produce pure water and heat.

The voltage from a single cell is about 0·7 V, just enough for a light bulb. However by stacking the cells, higher outputs are achieved, with the number of cells in the stack determining the total voltage, and the surface area of each cell determining the total current. Multiplying the two together yields the total electrical power generated.

In a fuel cell the conversion process from chemical energy to electricity is direct. In contrast, conventional energy conversion processes first transform chemical energy to heat through combustion and then convert heat to electricity through some form of power cycle (eg gas turbine or internal combustion engine) together with a generator.

The fuel cell is therefore not limited by the Carnot efficiency limits of an internal combustion engine in converting fuel to power, resulting in efficiencies two to three times greater.

Fuel cell systems
A schematic of a typical fuel system is shown in figure 1. In addition to the fuel cell itself, the system comprises the following sub-systems:

• A fuel processor – this allows the cell to operate with available hydrocarbon fuels, by cleaning the fuel and converting (or reforming) it as required.

• A power conditioner – this regulates the dc electricity output of the cell to meet the application, and powers the fuel cell auxiliary systems.

• An air management system – this delivers air at the required temperature, pressure and humidity to the fuel stack and fuel processor.

• A thermal management system – this heats or cools the various process streams entering and leaving the fuel cell and fuel processor, as required.

• A water management system – pure water is required for fuel processing in all fuel cell systems, and for dehumidification in proton exchange membrane fuel cells.

The overall electrical conversion efficiency of a fuel cell system (defined as the electrical power out divided by the chemical energy into the system, taking into account the individual efficiencies of the sub-systems) ranges from 35-55%. Taking into account the thermal energy available from the system, the overall or cogeneration efficiency is 75-90%.

Also, unlike most conventional generating systems (which operate most efficiently near full load, and then suffer declining efficiency as load decreases), fuel cell systems can maintain high efficiency at loads as low as 20% of full load. Other potential benefits include:

• At operating temperature, they respond quickly to load changes, the limiting factor usually being the response time of the auxiliary systems.

• They are modular and can be built in a wide range of outputs. This also allows them to be located close to the point of electricity use, facilitating cogeneration systems.

• Noise levels are comparable with residential or light commercial air conditioning systems.

• Commercially available systems are designed to operate unattended and manufactured as packaged units.

• Since the fuel cell stack has no moving parts, other than the replacement of the stack at three to five year intervals there is little on-site maintenance. Maintenance requirements are well established for the auxiliary system plant.

• Fuel cell stacks fuelled by hydrogen produce only water, therefore the fuel processor is the primary source of emissions, and these are significantly lower than emissions from conventional combustion systems.

• Since fuel cell technology generates 50% more electricity than the conventional equivalent without directly burning any fuel, CO₂ emissions are significantly reduced in the production of the source fuel.

• Potentially zero carbon emissions when using hydrogen produced from renewable energy sources.

• The facilitation of embedded generation, where electricity is generated close to the point of use, minimising transmission losses.

• The fast response times of fuel cells offer potential for use in ups systems, replacing batteries and standby generators.

Types of fuel cell
There are four main types of fuel cell technology that are applicable for building systems, classed in terms of the electrolyte they use. The chemical reactions involved in each cell are very different, and the main characteristics are summarised in table 1 together with those of conventional generating systems.

Phosphoric acid fuel cells (pafcs) are the dominant current technology for large stationary applications and have been available commercially for some time. The only working fuel cell installation in the UK, in Woking, uses a pafc, rated at 200 kW. There is less potential for pafc unit cost reduction than for some other fuel cell systems, and this technology may be superseded in time by the other technologies.

The solid oxide fuel cell (sofc) offers significant flexibility due to its large power range and wide fuel compatibility. SOFCs represent one of the most promising technologies for stationary applications. There are difficulties when operating at high temperatures with the stability of the materials, however, significant further development and cost reduction is anticipated with this type. The relative complexity of molten carbonate fuel cells (mcfcs)has tended to limit developments to large scale stationary applications, although the technology is still very much in the development stages.

The quick start-up times and size range make proton exchange membrane fuel cells (pemfcs) suitable for small to medium sized stationary applications. They have a high power density and can vary output quickly, making them well suited for transport applications as well as ups systems. The development efforts in the transport sector suggest there will continue to be substantial cost reductions over both the short and long term.

All four technologies remain the subject of extensive research and development programmes to reduce initial costs and improve reliability through improvements in materials, optimisation of operating conditions and advances in manufacturing. It is expected that all types will be commercially available in limited markets by 2006, and with mass market availability by 2010.

The market for fuel cells
The market for stationary applications for fuel cells can be summarised as follows:

Distributed generation/chp – for large scale applications, there are no drivers specifically advantageous to fuel cells, with economics (and specifically initial cost) therefore being the main consideration. So, until cost competitive and thoroughly proven and reliable fuel cells are available, their use is likely to be limited to niche applications such as environmentally sensitive areas from 2005. Wider commercialisation is likely closer to 2010, with high temperature cells (mcfcs and sofcs) being most suitable, although pemfcs may be preferable in specific areas, ie where hydrogen is available.

Domestic and small scale chp – the drivers for the use of fuel cells in this emerging market are better value for customers than separate gas and electricity purchase, reduction in domestic CO₂ emissions, and potential reduction in electricity transmission costs. However, the barriers of resistance to distributed generation and high capital costs need to be overcome. Commercialisation depends on cost reduction, and successful demonstration which is expected to begin in the next two to three years, leading to wider commercialisation through the gas and electricity utilities from around 2010. Systems based on sofcs and pemfcs are being developed for this application.

Small generator sets and remote power – particularly relevant to regions where a grid system is not available. The drivers for the use of fuel cells are high reliability, low noise and low refuelling frequencies, which cannot be met by existing technologies. Since cost is often not the primary consideration, fuel cells will find early markets in this sector. Existing pemfc systems are close to meeting the requirements in terms of cost, size and performance. Small sofcs have potential in this market, but require further development.

Cost comparison
Table 2 provides an indication of the capital and operating costs of different fuel cell types, together with comparative figures for the existing technologies. The projected figures for the fuel cell technologies are based on economies typically achieved through mass manufacture.

The projected costs for 2005 show the fuel cell technologies still being significantly more expensive than the existing technologies. To extend fuel cell application beyond niche markets, their cost needs to reduce significantly. The successful and wide-spread commercial application of fuel cells is dependent on the projected cost reductions indicated, with electricity generated from fuel cells being competitive with current centralised and distributed power generation.

Analysis of the market for fuel cells in developed countries have estimated that if the cost reductions are met, fuel cells could achieve up to 50% penetration of the global distributed energy market by 2020.

Typical current project costs
Table 3 gives an indication of the typical cost breakdown to be expected for the installation of a fuel cell system in the UK. This is based on information from the manufacturer and from economic evaluation/feasibility studies, since with only one working system installed to date in the UK there is no available accurate cost data. The unit is a standard commercially available pafc, complete with fuel processor, fuel stack and power conditioning system. The parameters are as follows:

• rating – 200 kW/235 kVA, 400 V, three phase;

• power generating efficiency – 40%;

• heat output – 204 kW, 60°C hot water;

• fuel, consumption – natural gas, 54 m³/h;

• external location;

• size – 5·5m x 3 m x 3 m;

• weight – 20 tonnes;

• noise level at full load – 62 dBA at 10 m.

The above illustrates the fact that the costs to supply, install and set to work a modestly sized fuel cell unit are prohibitively high, compared with incumbent generator technologies. Despite being the only commercially available unit, only 220 units have been sold worldwide, and so the full benefits of volume manufacture have not been realised, and a proportion of the costs associated with developing the unit is included within the unit cost.

Conclusions
Fuel cells are still at a relatively early stage of commercial development, with prohibitively high capital costs preventing them from competing with the incumbent technology in the market place. However, costs are forecast to reduce significantly over the next five years on the basis that the technology moves from niche applications, and into mass production.

However, in order for these projected cost reductions to be achieved, customers need to be convinced that the end product is not only cost competitive but also thoroughly proven, and government support represents a key part in achieving this.

The governments of Canada, USA, Japan and Germany have all been active in supporting development of the fuel cell sector through integrated strategies, however the UK has been slow in this respect, and support has to date been small in comparison. It is clear that without government intervention, fuel cell applications may struggle to reach the cost and performance requirements of the emerging market for alternative power generation technologies.

Mott Green Wall would like to thank element energy (ben.madden@element-energy.co.uk) for its assistance in the preparation of this cost model.