Fuel Cells: Standby Power's Future?

Today's integrated transmission system, sometimes called the "infinite busbar system," allows numerous utility power plants to be paralleled on line. Providing an enormous reservoir of power, this system has, historically, resulted in reasonably stable supplies that operate within set tolerances.

09/01/2001


Today's integrated transmission system, sometimes called the "infinite busbar system," allows numerous utility power plants to be paralleled on line. Providing an enormous reservoir of power, this system has, historically, resulted in reasonably stable supplies that operate within set tolerances.

However, supply stability is still affected by lightning strikes on overhead transmission lines, high-voltage system faults, grid high-voltage equipment switching, reactive load switching—power factor correction equipment and motors—and certain power electronic circuits. High-magnitude transient over-voltages are usually very short in duration—lasting milliseconds—the amount of energy generated depending on the surge source. Externally generated transients are suppressed by surge arresters installed at the point of utility intake, while internally generated transients are limited by surge suppressors between the power supply and the equipment requiring protection.

Segregation from the utility supply provides a solution to the problem of externally generated transients. The main alternative power source has traditionally been diesel-fueled standby generators, which protect against potential loss of the power supply. These are typically used in conjunction with uninterruptible power- supply (UPS) systems, which have the two-fold benefit of providing a clean and conditioned source of power during normal use, and also a battery backup.

UPS systems protect critical equipment by bridging the time delay between loss of main supply and the standby generator's start-up, in addition to filtering the utility supply.

Fuel Cells an Alternative

Fuel cells are probably the most exciting technology in development, and are poised to replace both conventional standby generation and UPS systems for ensuring both security and integrity of power supplies within commercial and even residential applications.

First developed in 1839 by Sir William Grove, fuel-cell technology was seriously pursued beginning in the 1960s—when all types of technologies were being pushed to the limit by the desire to conquer space. Fuel cells proved considerably safer than nuclear power and cheaper than solar power. Their early success in the Apollo missions spawned confidence and they are still used today in space shuttle missions.

Fuel cells are electrochemical devices (see Figure 1). They convert hydrogen and oxygen into water, a process that produces electricity. Hydrogen is fed into an anode catalyst with oxygen entering the cathode catalyst. The anode catalyst encourages the hydrogen atom to split into its constituent parts. The proton (ion) passes through the electrolyte and the electron takes an external route before recombining with the oxidant to produce water in an extremely efficient chemical process. The electron flow is harnessed in an external circuit and converted to an AC supply before being utilized.

There are various types of fuel cells but all operate on the same basic principle. The main difference is in the type of electrolyte, which is the primary governing influence on the performance and characteristics of the fuel cells and is either solid ceramic, liquid or solid polymer.

Fuel Issues

From an environmental point of view, this type of power generation is a zero-emission process (apart from water of course, if pure hydrogen is used). However, for commercial usage, hydrogen must first be extracted from a hydrocarbon fuel in a process utilizing a fuel reformer.

Typical fuels used in this way are natural gas, methanol, ethanol, diesel and gasoline; these fuels are reformed into hydrogen-rich gas, not pure hydrogen. The fuel reformer not only enables the use of existing fuels, but more importantly, negates the need for a brand-new fuel storage and transportation infrastructure.

Methods of fuel processing include endothermic steam reforming, where fuel is combined with steam at extremely high temperatures; hydrogen is separated through membranes. Another method is the partial oxidation reformer (POx), a process that produces CO 2 ; while this is not necessarily a desirable byproduct, it is certainly not as distasteful as conventional fossil fuel emissions.

Presently, when hydrogen is required in its pure form, production is achieved by combining methane (CH 4 ) with water (H 2 O) to produce hydrogen (H 2 ) and carbon dioxide (CO 2 ). Other production methods have been explored in an effort to achieve a solution that dispenses with the use of fossil fuels altogether, including:

  • Bacteria. Hydrogen is produced as a byproduct of the metabolic process of a single cell organism, cyanobacteria, which flourishes in either air or water and feeds on solar energy.

  • Photovoltaics and wind turbines. The energy produced is used to electrolyze water, thus producing hydrogen.

Hydrogen is used as the energy carrier in both of these methods; the end waste product is water allowing the process to continue in a never-ending cycle of energy production.

Suitable Applications

Conventional standby generators do what the name says—provide standby power, usually during utility power failure. They are not in continuous operation, mainly because of the noise and emissions.

In contrast, fuel cells are extremely quiet, produce almost negligible levels of emissions, are more efficient and have no moving parts. For these reasons they can be sited almost anywhere and used continuously and in tandem with the main power supply if the system is so designed.

Providing a separate distribution system to compliment the alternative power source eliminates the inconvenience of externally generated and potentially disastrous voltage transients. This means the connection of electronic equipment can be made directly, dispensing with the need for a separate UPS system in appropriate circumstances. However, if reactive loads are supplied from the alternative power source, the problem of internally generated transients must be addressed, and appropriate suppression and filtering systems employed.

An alternative to designing separate distribution systems within the building would be to connect the fuel cell in parallel with the utility power supply. Synchronizing equipment would have to be provided to ensure that voltage and frequency levels match. Also permission by the supply authority would have to be obtained.

Another advantage of having a site-based fuel cell is that heat is produced during the process. Putting this heat to use improves fuel cell efficiency from between 40% and 60%, on up to 85%. When utilized as a replacement for a standby generator, as well as a heat source, a payback period of as little as 3.5 years may be achievable.

Additionally, if a permanent natural gas connection is available, the need for expensive and bulky fuel storage tanks is eliminated.

Costs Determine the Future

Hydrogen is an abundant and renewable energy source. Fuel cells also make efficient use of existing fuel reserves. Fuel cells are an immediate solution to environmental impact that will become greener as time passes. Using fossil fuels in this way significantly reduces air emission levels of NOx, SOx, CO, hydrocarbons and particulates.

Opinions, however, are divided as to whether the development of fuel cells has progressed far enough for these devices to be considered a replacement for utility power. Currently, fuel cell cost approximately $3,000 per kW.

Using a fuel cell in lieu of a standby generator or UPS system can create a payback period of less than five years; for this reason alone, we should see an increase in the installed capacity of fuel cells over the next decade. However, only when costs fall below $1,500 per kW will we really see the fuel cell begin to compete with combustion generation—a reality when mass produced units hit the marketplace.

From Pure Power, Fall 2001.



Fuel Cells in Detail

Phosphoric Acid (PAFC)

Operates at temperatures up to 1,200°F, at 60% efficiency.

Uses waste heat, increasing overall efficiency up to 85%.

Considered the first generation, this type is the most developed and commercially available.

Used for transportation as well as stationary generation.

Proton Exchange (PEM)/Solid Polymer (SPFC)

Operates at temperatures up to 180°F and at 60% efficiency.

Uses waste heat, increasing overall efficiency up to 85%.

Viewed as the next generation; considered the most promising for economical and efficient power production.

Demonstrates high power density; can vary output to meet demand. Well-suited to applications where rapid start up and shift is required such as vehicles and large motors.

Used for stationary generation and transportation.

Alkaline (AFC)

Operates at temperatures up to 400°F, at 40% efficiency.

Used by NASA for the space programs.

Molten Carbonate (MC)

Operates at temperatures up to 400°F, at 45% efficiency.

Can run on coal-based fuels.

Units from 10 kW to 2 mW have been successfully tested using a variety of fuels.

Solid Oxide (SOFC)

Operates at temperatures up to 1,800°F, at 60% efficiency.

Suited to high-power applications, including industrial and large-scale generating stations.



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