Ceramic fuel cells work by converting natural gas and renewable fuels like hydrogen into heat and power, and are particularly promising as a cleaner energy source because they increasingly address the need for higher efficiency energy production with relatively low greenhouse gas emissions.
Whereas batteries store a limited amount of electrical energy, fuel cells consume fuel and are consequently able to operate virtually continuously as long as the necessary flows of fuel and air are maintained.
There are many different types of fuel cells, ranging from those that operate at lower temperatures, which require a pure supply of hydrogen as a fuel and specialised equipment to those that operate at higher temperatures and have the added flexibility to operate on natural gas, without needing specialised equipment or new infrastructure. These higher temperature fuel cells, which use gas as a fuel and existing natural gas infrastructure, have been reported to operate more efficiently than the hydrogen-fuelled lower temperature counterparts.
Operating between 450 and 1,000ºC, Solid Oxide Fuel Cells (SOFC) are a prime example of high temperature fuel cells, which have electrical efficiency ranging up to 70 per cent. Using natural gas as a fuel, they can be used for all types of stationary power generation, from below 1 kilowatt (kW) to many megawatts (MW).
Using gas as a feedstock
Australian company Ceramic Fuel Cells (CFCL) has developed a SOFC system that can be connected into a regular natural gas network, making the fuel cell system more accessible since there is no need to convert hydrocarbon fuel sources into hydrogen gas. However, SOFCs can also be redesigned to use other fuel sources such as hydrogen, LPG, biogas, coal gas, ethanol, methane and other hydrocarbon fuels.
“CFCL has designed the fuel cells to operate on natural gas, primarily because the natural gas infrastructure already exists – rather than waiting for the “˜hydrogen economy’ to eventuate. We see the interaction as complimentary, as CFCL’s fuel cell will be integrated inside a future appliance, such as a boiler, that operates on the natural gas grid,”? says CFCL’s Trent Rowe.
Gas-fuelled SOFCs are more efficient when compared to other types of electricity generation such as coal-powered turbines. While the efficiency of coal-powered electricity generation has improved over time, there is still a considerable amount of waste heat generated. This is because of the indirect method used to harness the energy in coal, with system electricity efficiency ranging from 30 to 37 per cent. In contrast, SOFCs directly convert chemical energy into electricity, heat and water with 40 to 70 per cent electrical efficiency from energy input.
In addition, since the production of electricity is a direct process, SOFCs do not produce large quantities of greenhouse gases, nitrous oxides or sulphur oxides, emitting only steam and possibly low levels of carbon dioxide. The production of heat makes SOFCs ideal for domestic combined heat and power applications that not only produce heat for space heating and hot water, but also produce electricity which can be used around the house or fed back into the electricity grid.
Technical challenges
As evidenced by the considerable flexibility of the SOFC system, it is not unsurprising that the biggest challenge in developing the fuel cells relates to the complexity of the materials science behind the technology.
“Finding the exact materials for the system to function, and function with increasing reliability is the technical challenge. Over the years CFCL has developed ceramics, metal alloys and a number of other materials that are critical for the operation of the fuel cell,”? says Mr Rowe.
A prime example of the type of technical challenge has been matching the coefficient of thermal expansion between ceramics and metals in the stack to ensure that the materials do not bend or break when subjected to the high operating temperatures of ceramic fuel cells, up to 1,000ºC for SOFC systems.
“The company had previously developed metal-ceramic stacks but was limited by the available metal alloys and subsequently developed fuel cell stacks made chiefly from ceramic materials. This allowed the company to continue development of other parts of the system, such as power management and the safety system,”? said Mr Rowe.
However, since then, newer materials have been developed, enabling the company to resume its development of metal-ceramic stacks that now are capable of producing double the power in a smaller package size. The company has reported that advances in power density have enabled it to increase the output from each of its fuel cell stacks to 2 kW of electricity, reducing the unit’s cost per kW and saving up to three tonnes of carbon dioxide per year. A 2 kW unit provides ample power for the average household’s annual baseload requirements, as well as additional power for export to the grid.
The market for ceramic fuel cells
The major challenge in scaling up to the mass market requires coordination among all the different players in the gas generation and transmission industries.
“A disruptive technology like fuel cells requires coordination from every part of the business, from retail, through to generation management and financing. If today, CFCL had a product that met all the performance criteria, there would still be a delay as regulators and utilities considered how to connect, control and commercially package the technology for consumers,”? Mr Rowe says.
He adds that from a market readiness perspective there are many facets to consider. “As many utilities and network operators know, managing the network can be a challenging task at times. If we look into the future, network operators need to manage the variable generation technologies such as wind, solar, wave power. For example: in a localised area if 100,000 uncontrolled 1 kW fuel cells were installed in people’s houses, that would add 100 MW that the utility would need to consider. Then what about connection standards? Feed in tariffs? Government rebates?”?
“If we look at deployment in Australia we are faced with a unique problem where the cost of energy is very low. As an example, the US Department of Energy listed the price of electricity in Australia in 2004 as $US0.09 per kWh, whereas the cost in Germany is roughly double and Denmark is more than three times as expensive compared to Australia,”? Mr Rowe notes.
Despite the impact of low gas prices on immediate commercialisation within Australia, the Australian Government has supported the company with funding and export grants to technically develop the fuel cells. Moreover, the promotion of higher energy efficiency appliances may eventually shift the focus onto SOFCs in Australia, given their considerable potential when used as part of a combined heating and power system and as their ability to be used as part of cooling systems is further refined.
As Mr Rowe concludes “Fuel cells are not a silver bullet technology that will solve the world’s energy problems; they form part of a complete solution integrating with incumbent and future generation technologies.”?