The application of fuel cell technology within buildings
Fuel cells are electrochemical devices that convert the chemical energy in fuel into electrical energy directly, with-out 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 wide 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 notfinancially 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 com-mercial operation. However, fuel cell technology has made significant progress in recent years, with prices pre-dicted 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 hydrogenflows 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 andflow through an external circuit to form an electric current.
As oxygenflows 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 volts, 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 processesfirst transform chemical energy to heat through combustion and then convert heat to elec-tricity through some form of power cycle (e.g. 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 2 to 3 times greater.
Fuel cell systems
In addition to the fuel cell itself, the system comprises the following subsystems:
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 to power 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 the PEMFC.
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 subsystems) 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.
Fuel cell systems also offer the following potential benefits:
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 3–5 year intervals there is little on-site maintenance. The 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, CO2emissions 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, minimizing 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.
Phosphoric Acid Fuel Cells (PAFCs) are the dominant current technology for large stationary applications and have been available commercially for some time. 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 compat-ibility. SOFCs represent one of the most promising technologies for stationary applications. There are difficulties
26 Fuel Cells
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 Polymer Electrolyte 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, optimization of operating conditions and advances in manufacturing.
The market for fuel cells
The stationary applications market for fuel cells can be sectaries 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.
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 CO2emissions, and potential reduction in electricity transmissioncosts. However, the barriers of resistance to distributed gen-eration, high capital costs and competition from Stirling engines needs to be overcome.
Small generator sets and remote power– 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 willfind 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.
Costs
Fuel cell technologies are still significantly more expensive than the existing technologies typically and, dependent on the fuel cell type, between £3,000–£5,000/kW. To extend fuel cell application beyond niche markets, their cost needs to reduce significantly. The successful and widespread commercial application of fuel cells is dependent on the projected cost reductions indicated, with electricity generated from fuel cells being competitive with current centralized and distributed power generation.
Conclusions
Despite significant growth in recent years, 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 as 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 fuel cell market.
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