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5.4 Fuel Cells

Technology Overview

Like the previous technologies mentioned, fuel cells have had incarnations in existence for sometime. In the late nineteenth century, the principles for fuel cell operation were known. It was not until the mid-twentieth century that a commercial technology was made available. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy without combustion. The fuel cell uses external reactants of a hydrogen rich gas and oxygen to create DC power. On the surface, fuel cells are very similar to batteries in their use of chemical energy. Unlike batteries that are energy storage devices, fuel cells will produce DC power as long as reactants are supplied to the anode and cathode and thus are a generation technology. Figure 4 illustrates the fuel flow and fundamental operation of a fuel cell.

Figure 4. Basic Principle of a Fuel Cell [11].

There are many different types of fuel cells, characterized by the type of the electrolyte and/or catalyst used. The electrolyte used determines different operating conditions required such as heat and pressure [15]. The difference in electrolyte leads to some fuel cells being more suited to distributed generation applications than the others.

While there are many variants, there are five different types of fuel cells considered as potential for DG applications. Polymer electrolyte membrane (PEM), alkaline (AFC), and phosphoric acid fuel cell (PAFC) technologies are considered low temperature fuel cells and operate at about 80 o C. Figure 5 illustrates the various characteristics of these fuel cells appropriate for DG applications. While low temperature fuel cells are suitable in some applications where heat is undesirable, there is no option for co-generation. Co-generation requires high-grade heat that molten carbonate (MCFC) and solid oxide fuel cell (SOFC) technologies produce.

Figure 5. Fuel Cell Applications [16].

Total thermal efficiencies of waste heat conversion co-generation can reach 85%, and thus the SOFCs and MCFCs are much more efficient overall and are more enticing to raise the entire power grid efficiency [16]. The SOFC technology, though not as commercially developed as some other fuel cell technologies holds great potential in its "all solid" construction including the electrolyte [17]. Most other fuel cell types have liquid electrolytes and have problems associated with leaking, gas cross over and long-term installation reliability. In order for fuel cell DG technologies to be commercially viable, they must have at least a 10-year lifespan. The SOFC will be explored in detail as it has the potential for highest efficiency using co-generation with the benefit of all solid construction.

Technology Status

SOFC Ranked 4th

The technology for fuel cells has been around since 1839. NASA further developed the technology for the Gemini space project[18],[19]. Fuel cell technology remains in a developmental phase and only in the past ten years has development significantly increased. SOFCs in the form of tubular, planar, or monolithic design are currently being developed. These types are in small scale production by companies such as Acumentrics, Fuel Cell Technologies, Siemens Westinghouse, Fuel Cell Energy, Delphi, Cummins and GE Power systems [20].

The commercial systems seem to be reliable and there are some in various locations around the world. The technology status ranks last because there have been no real long term field tests to verify the validity of this technology. Though many have done commercial trials lasting years, fuel cells are relatively new when compared to technologies such as the internal combustion engine in real market and product applications.

Overall Efficiency

SOFC Ranked 1st

SOFCs have electrical efficiencies from 40-60% depending on fuel type used and load condition. While this is the highest efficiency among the technologies discussed, the reforming technology that extracts hydrogen from fuels such as natural gas or gasoline typically reduces the overall system efficiency leaving room for improvement. Adding co-generation can increase overall system efficiency to 85%.   Fuel cell technologies stand well above the other technologies in terms of overall efficiency.

Environmental And Noise Considerations

SOFC Ranked 1st

In principle, when pure hydrogen is used as the fuel source for a SOFC the only byproducts are water and heat [18]. However, the process to obtain pure hydrogen is anything but environmentally friendly. The current hydrogen initiative by President George W. Bush primarily involves distilling and/or producing hydrogen from coal, natural gas and other fossil fuels along with very high temperature nuclear reactors in a process called "water cracking" to produce hydrogen [22]. Fuel cells have the potential of being an environmentally friendly DG solution, but only if the fuel is processed from a clean source such as solar, wind or hydro. Fuel cells themselves are quiet, but the complete system, if it requires an air compressor, cooling, or cogeneration can cause unacceptable noise. Figure 4 exhibits the many components of a total fuel cell system. Because of the environmentally clean and quiet nature of fuel cells, they can be implemented in areas with strict air quality or noise level limitations such as hospitals or shopping malls [23].

Device Cost per Kw

Ranked 4th

Currently, overall costs of a SOFC are extremely high. When doing a cost analysis all aspects of power generation should be considered to be accurate. Current estimates for an entire system, including fuel and air supply, insulation, fuel cells stack, reformer and desulfurizer, piping, labor, depreciation, system control and power electronics is as high as $4000 per kW [20]. This is much higher than any other DG technology being considered in this analysis. The high cost is partially due to the current small-scale production of fuel cells and supporting technology. It also parlays the relatively new status of the technology. Figure 6 illustrates SOFC costs' the downward trend in the coming years as technological advances are made. The $4000/kW does not include any equipment necessary for cogeneration.

Figure 6. SOFC System Cost goals (100,000 unit production/year) [20].

Physical Size

Ranked 4th

A 2MW SOFC system can fit on less than one-tenth of an acre and if scalable, a 5MW unit will fit in an area of .25 acres [23]. The potential small sizes enable close placement to power needs and reducing the need for long power lines. While this does not seem excessive compared to the other technologies the current land requirement is significantly larger.

Intended Application

SOFC Ranked 3rd

Current SOFC development through the US DOE Solid State Energy Conversion Alliance (SECA) program uses 5kW fuel cell stacks and power electronics as a building block for larger SOFC systems. The feasibility of larger scale applications up to 20MW has been analyzed and predicted feasible [25]. Although expansion is possible, most current commercially viable installations are 200kW or less and the technology therefore receives a lower score. A photograph of an example 220kW system is in Figure 5. It is important to note however, that larger prototypes on the MW level have been developed. The SECA program modules are designed for extreme cost reduction illustrated in Figure 6. So, although SOFC systems are very capable of meeting the intended 5MW application, a reduced cost version is still being highly researched and developed.

Potential for Improvement

SOFC Ranked 1st

Although there are many shortcomings of fuel cells with regards to economics, size and up-stream pollution, the government has set high goals. According to the cooperating companies of the Solid State Energy Conversion Alliance, a cost goal of $400/kW for system costs by 2010 has been set [26]. If this goal is realized, the SOFC DG solution may become very widespread. As SOFC DG becomes more common, mass production will allow the initial capital costs to become more competitive to other technologies. Further, as the SOFC demand grows and the power levels are scaled for much larger installations, we expect the price of SOFC DG systems to drop below competing technologies.


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