Fuel cells and energy efficiency

Summary: Fuel cells generate electricity and heat by reacting a fuel such as hydrogen with an oxidant through electrolysis. Due to their high efficiencies, quiet operation, and ability to store energy, these devices have many applications, but have had limited uptake due to prohibitive costs.

Fuel cells are electrochemical cells that convert chemical energy into heat and electricity by reacting a fuel with an oxidant, typically through the process of electrolysis. One common example is the hydrogen-based fuel cell, which converts hydrogen and oxygen to heat, power, and water. Other fuel cells use other hydrocarbons or methanol as fuels. The idea for a fuel cell goes back to the first half of the 19th century, but it was not until the late 1950s that the devices were developed beyond the prototype stage, finding their first application in NASA’s Project Gemini. At that time, fuel cells for other applications were developed, but none took off on a large scale.

The structure of all fuel cells is common in principle: an electrolyte layer is sandwiched between two electrodes, a positively charged anode and a negative cathode. The two electrodes are formed by the fuel and the oxidant respectively, such that their direct mixing is prevented, and the only type of interaction is through the transmission of electric (ionic) charge through the electrolyte. A catalyst at the anode may oxidize the fuel, producing a positively charged ion and a negative electron. Hence, as electrons cannot pass through the electrolyte, during the process of electrolysis the anode is “used up” in the sense that it contributes positively charged ions to the electrolyte, which are then carried to the cathode. The electrons move the other way around the circuit, that is, through the wire connecting anode and cathode, before reaching the positive ions at the cathode, and there reacting with another chemical, typically oxygen, to form water or carbon dioxide. An individual fuel cell produces a relatively small voltage, such that these cells are typically combined into “stacks” depending upon the application, whereby connection in series and parallel can be used to provide higher voltage and current, respectively.

Based upon these basic principles of operation, various specific types of fuel cells have been developed. A common type of fuel cell is the polymer electrolyte membrane fuel cell (PEMFC), which uses a solid polymer as the electrolyte, runs on hydrogen and oxygen, and employs carbon electrodes with a platinum catalyst. The latter is required in order to separate the positive and negative ions issued from the anode, but this significantly adds to the system cost. The fast startup time from running at low temperatures, and the associated durability from less wear on components, are contrasted by disadvantages in terms of high costs and the need for hydrogen as a fuel. Alternative types of fuel cells, such as molten carbonate or solid oxide fuel cells (MCFC and SOFC, respectively), which operate at significantly higher temperatures, have therefore been developed. These devices operate in the temperature range of 500–1,000 degrees Celsius, which eliminates the need for a precious metal catalyst and thereby reduces cost. Another advantage of the higher temperature is that, while these fuel cells still use hydrogen as a fuel, they can internally reform other fuels to hydrogen, such that it does not need to be externally produced, which can further reduce costs. Typically, these higher temperature fuel cells have higher overall efficiencies; however, precisely because of the high temperatures, they have longer startup times and suffer from greater wear and tear on the components.

Researchers at the University of Michigan developed a fuel cell that is more efficient and has an enhanced performance. Their carbonate-superstructured solid fuel cell (CSSFC) directly uses hydrocarbon fuel and is an alternative to the slow oxide ion transfers of conventional fuel cells. They believe their fuel cells has the potential to be used in many commercial applications.

Depending upon the type of fuel cell, the electrical efficiency varies between around 30 and 80 percent. Fuel cells also produce heat, and if this heat can be utilized, the overall system efficiency will increase. Also, where the hydrogen and oxygen for the fuel cell have to be produced, this needs to be considered when evaluating efficiencies. Unlike natural gas, for example, hydrogen and oxygen cannot be found in pure form in nature, and hence energy has to be invested to produce or separate these substances. If the whole supply chain efficiency for fuel cells is considered, it is therefore much lower than the upper value of 80 percent.

Fuel cells have a wide range of applications, wherever heat and/or power is required, such as in vehicles to provide motive power, as mobile power devices, and as stationary CHP units for industrial or residential needs. Due to the lack of moving parts, fuel cells are highly efficient compared to other heat and power generating devices (CHP), but they are still relatively expensive compared to these other devices, which has limited their market penetration. Vehicle applications are not limited to road transport, but also include providing motive power for marine, aerospace, and industrial applications (such as fork-lift trucks), as well as light-duty vehicles (LDVs) and buses. Currently, there are no fuel-cell powered vehicles in mass production, as the high costs have confined their development to prototype vehicles. Due to the lack of moving parts and therefore the low maintenance required, their compactness, and high power-to-weight ratio, fuel cells are very suited to remote applications, such as in space exploration or submarine warfare. The very low noise of fuel cells in operation is advantageous for the latter application. Energy storage is another area where fuel cells can make a significant contribution: Because of their possible two-way operation, in generating heat and power in one direction, or converting (excess) electricity to chemical energy in the other, they have a key role to play in a future energy system dominated by fluctuating renewable electricity generation.

CHP applications for domestic use are generally more efficient than separate generation of heat (e.g., through a boiler) and electricity supply through the grid, which might have an overall efficiency of around 30–40 percent. However, there are other technologies for such micro-CHP applications in the range up to about 50 kilowatts of electrical power, which are significantly cheaper than fuel cells. Fuel cells have a heat to power ratio in the range of 0.3 to 1, which is much more favorable for domestic applications requiring more electricity than heat than other CHP devices such as internal combustion engines. This means that, technically at least, they are more suited to cogeneration in buildings with only modest heat loads. Given that, primarily due to better insulation through retrofit and new build measures, the residential heat demand is expected to reduce significantly in the coming decades, the fuel cell could be a very attractive technology for such residential applications, if the costs can be reduced substantially.

One further barrier to the widespread adoption of fuel cells based on hydrogen, or any other fuel that is not widely available, is the lack of supply and distribution infrastructure for this fuel. Unlike petrol (gasoline), diesel, or electricity, little or no infrastructure exists for the widespread distribution of hydrogen, so there is a “chicken and egg” situation in which the energy companies that would develop the infrastructure, alongside or integrated into conventional petrol stations, have little incentive to do so because the fleet of vehicles using this fuel is so small that it would not be economically feasible. On the other hand, the vehicle manufacturers have a similar disincentive to develop and produce fuel-cell powered vehicles on a large scale, exactly because there is a lack of infrastructure for this fuel. Neither side has so far been willing to take the substantial risk involved in such a venture; what is probably required is collaboration between both parties on a large scale, to guarantee a minimum capacity of vehicles and refueling points. There are some refueling stations for hydrogen, for example, in the United States or in European countries, but the number and total capacity is negligible compared to other conventional fuels. Notwithstanding these barriers, fuel cells are commercially available in many countries, such that applications in all areas are experiencing significant growth. Researchers at Michigan Tech are hoping to remove some of these barriers. Further future cost reductions combined with increasing costs for alternatives, such as fossil fuels, should enable these devices to become much more competitive over the coming years.

Bibliography

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