The need is clear and well recognized for clean, safe, and reliable forms of energy that can provide prescribed levels of power consistently, and on demand. Yet, most forms of non-combustion electric generation have limitations that impact widespread use of the technology, especially as a primary source of electric power (i.e., base load power). Solar energy, for example, depends on the sun. Extended days of cloudy skies can severely limit the generation of electricity, and power availability is generally considered to be between 25 to 35%. Wind turbines are designed to turn kinetic energy into electricity. They too, depend on factors that cannot be controlled. In this case, the presence of wind and a certain minimum wind velocity are required. As a result, power availability is judged to be in the range of 30 to 35%. Geothermal sources require heat energy from underground geothermal fields, which mean they are restricted to certain geographic locations. Similarly, hydroelectric plants are confined to locations near major rivers and are also somewhat constrained by nature.
Thus, without adequate and consistent sun, wind, heat, and water flow, such sources of power
Generation are limited by the whims of nature and cannot be considered as reliable sources of base load power when and where needed. Fuel cell technology, on the other hand, has advanced to the point where it is now a viable challenger to combustion-based plants for growing numbers of base load power applications. Today, fuel cells are reaching their potential as the cleanest and most reliable sources of distributed power generation. With 95% power availability and electric power generation efficiency of 47%, they represent a viable means of producing Ultra-Clean power, reliably, consistently, and on demand. While the need to ensure the availability of hydrogen has been seen as a concern in the operation of fuel cells, Direct Fuel Cells (DFCs) developed by FuelCell Energy, Inc. are unaffected by such limitations because they use natural gas and biofuels (gases from food processing and wastewater treatment) as a source fuel. Furthermore, with system adjustments, these fuel cells can also operate with a wide range of alternate fuels, including ethanol and propane. Direct Fuel Cells have even been shown to generate clean power from diesel fuel and coal gas, fuels traditionally considered to be high pollution sources. DFCs internally reform hydrogen from the source fuels and emit dramatically reduced CO2 greenhouse gas compared to fossil fuel power plants, and only negligible amounts of pollutants, such as NOx and SOx.
WHAT IS ITS PURPOSE?
The purpose of a fuel cell is to generate electricity through an alternative, non-polluting electrochemical process. Output voltage is increased by adding fuel cells in series called stacking.
WHY IS IT IMPORTANT?
Fuel cell technology (FCT) is receiving attention to address the depletion of natural resources and global environmental concerns such as global warming and the greenhouse effect. FCT is also being proposed to move away from non-renewable natural resources such as fossil fuels and move towards renewable such as hydroelectric and solar power. Fuel cells also promise greater operating efficiency with lower emissions over conventional power sources used today.
WHAT ARE ITS APPLICATIONS?
Stationary (A.C. Applications)
- Power plants
- Home use
- Isolated rural areas
- Military applications
Enclosed Environments (D.C. Applications)
- Space Station
- Space vehicles (space shuttle)
- Underwater vehicles (submarine)
Motive Transportation (D.C. Applications)
- Personal Vehicles (ZEV's -Zero Emission Vehicles)
- Public Transportation
- Commercial and Military Vehicles
Principle of Operation:
A fuel cell is an energy conversion device that converts the chemical energy of a fuel directly into electricity without any intermediate thermal or mechanical processes.
Energy is released whenever a fuel reacts chemically with the oxygen in air. In an internal combustion engine, the reaction occurs combatively and the energy is released in the form of heat, some of which can be used to do useful work by pushing a piston. In a fuel cell, the reaction occurs electrochemically and the energy is released as a combination of low-voltage DC electrical energy and heat. The electrical energy can be used to do useful work directly while the heat is either wasted or used for other purposes.
In galvanic (or “voltaic”) cells, electrochemical reactions form the basis in which chemical energy is converted into electrical energy. A fuel cell of any type is a galvanic cell, as is a battery. In contrast, in electrolytic cells, electrical energy is converted into chemical energy, such as in an electrolyzer or electroplater.
A basic feature of fuel cells is that the electric current load determines the consumption rate of hydrogen and oxygen. In an actual systems application, a variety of electrical loads may be applied to the fuel cell. Comparison of Fuel Cells with Batteries:
Fuel cells and batteries are both galvanic cells and therefore have many similarities. Both fuel cells and batteries consist of an anode and a cathode in contact with an electrolyte. Both devices generate electrical energy by converting chemical energy from a high energy state to a lower energy state using an electrochemical reaction.
These reactions occur at the anode and cathode with electron transfer forced through an external load in order to complete the reaction. Individual cells of both batteries and fuel cells generate only small DC voltages, which are then combined in series to achieve substantial voltage and power capacities.
Fuel cells differ from batteries in the nature of their anode and cathode. In a battery, the anode and cathode are metals; zinc or lithium is typically used for the anode and metallic oxides for the cathode. In a fuel cell, the anode and cathode are composed of gases often in contact with a platinum catalyst to promote the power generating reaction. Hydrogen or a hydrogen-rich gas mixture is typically used as the anode and oxygen or air as the cathode.
Fuel cells also differ from batteries in the fundamental method in which the chemical reactants are stored. In a battery, the anode and cathode form an integral part of the battery structure and are consumed during use. Thus, assumed after which it must either be replaced or recharged, depending on the nature of the materials.
In a fuel cell, the chemical reactants are supplied from an external source so that its materials of construction are never consumed and do not need to be recharged. A fuel cell continues to operate as long as reactants are supplied and the reaction products are removed.
Comparison of Fuel Cells with Internal Combustion Engines:
Fuel cells and internal combustion engines share similarities of form. Both fuel cells and internal combustion engines use gaseous fuel, drawn from an external fuel storage system. Both systems use hydrogen-rich fuel. Fuel cells use pure hydrogen or a reformate gas mixture. Internal combustion engines typically use hydrogen-containing fossil fuels directly, although they could be configured to operate using pure hydrogen.
Both systems use compressed air as the oxidant; in a fuel cell engine the air is compressed by an external compressor. In an internal combustion engine, the air is compressed internally through piston action. Both systems require cooling, although engines operate at higher temperatures than fuel cells.
In some respects, fuel cells and internal combustion engines are fundamentally different. Fuel cell reacts with the fuel and oxidant electrochemically whereas internal combustion engines reacts the fuel and oxidant combatively. Internal combustion engines are mechanical devices that generate mechanical energy while fuel cells are solid state devices that generate electrical energy (although the systems used to support fuel cell operation are not solid state). Pollution is related to the fuel composition and the reaction temperature. Fuel cell engines operating on pure hydrogen produce no harmful emissions; those that operate on hydro-gen-rich reformate produce some harmful emissions depending on the nature of the process. Internal combustion engines operating on pure hydrogen can be designed to produce almost zero harmful emissions; those that run on conventional fuels produce significantly more pollution.
Fuel Cell System Efficiency:
Fuel cell system efficiency relates to the overall performance of a fuel cell powerplant. A fuel cell stack can only operate if provided with pressurized air and hydrogen and flushed with coolant. Practical fuel cell systems require additional equipment to regulate the gas and fluid streams, provide lubrication, operate auxiliary equipment, manage the electrical output and control the process. Some systems include reformers for fuel processing. All of this equipment introduces losses and reduces the total efficiency of the system from its theoretical ideal. In order to make meaningful efficiency comparisons between fuel cell and other power generating systems, each power-plant must be defined in a similar way.
When comparing a fuel cell power plant to an internal combustion engine for an automotive application, it is convenient to define each as a device that inputs fuel and air and delivers mechanical output power to a driveshaft. In either case, fuel is drawn from a tank in either gaseous or liquid form that has been stored after refining or other processing.
Both systems compress atmospheric air; the internal combustion engine uses piston action whereas the fuel cell power plant uses an external compressor. The internal combustion engine delivers mechanical power to the driveshaft directly while the fuel cell power plant uses an inverter and electric motor. Both systems reject waste heat to the ambient surroundings using a coolant pump, radiator and other heat management equipment. Both systems supply equal auxiliary vehicle loads.
The overall efficiency of an internal combustion engine is often quoted as between 15 and 25%. These values are representative of the output efficiency at the wheels of a vehicle; efficiencies at the output of the flywheel are more typically between 30 to 35% and even higher for diesel engines.
For a fuel cell power plant operating on pure hydrogen, the comparable efficiency breakdown at the output of the fly-wheel is roughly as follows:
Fuel cell efficiency: 40 to 50%
Air compression: 85% (uses 15% of gross power)
Inverter efficiency: 95%
Electric motor efficiency: 97%
Multiplying each of these values together yields an overall system efficiency of roughly 31 to 39%.
For a fuel cell system that operates using a reformer, these efficiencies are further reduced by 65 to 75% (depending on the type of reformer) for an overall system efficiency of roughly 20 to 29%.
More difficult to quantify is the effect of overall system weight. Fuel cell systems (including fuel storage) are heavier than internal combustion engine systems of comparable power and range, and therefore use more power on an ongoing basis. Batteries have electrochemical efficiencies comparable to fuel cells. When used as an automotive power plant, battery systems also require an inverter and electric motor, although they do not require air compression, complex cooling equipment or reformers. Batteries as a means of power storage are heavier than fuel cells although this is offset somewhat by the elimination of other components.
When stepping back further, the source of fuel becomes an essential component of the overall efficiency. With an internal combustion engine, this usually involves refining hydro-carbon fuels. With a fuel cell, this involves producing hydro-gen from fossil fuels or through water electrolysis, or the production of secondary fuels such as methanol for use with an on-board reformer. With a battery system, this involves a source of electrical power for charging.
Analysis of these factors is complex and depends on the source fuel, processing method, handling and transportation difficulty, and other factors such as the energy required to compress or liquefy the final fuel. In the end, these factors reduce the overall fuel cost although this cost may not take into account the cost associated with long-term damage of the environment.