2.2 Alternative Energy Systems
2.2.6 Fuel cell
2.2.6.6 Types of fuel cell
Five types of fuel cell can be distinguished depending on the electrolyte used, which can be either liquid (alkaline or acidic), polymer film, molten salt, or ceramic. As shown in Table 2.1, each type has advantages and disadvantages that make it appropriate for various applications.
Table 2. 1: Fuel cell technologies and features. (Albarbar & Alrweq, 2018)
Type
Working Temperature
(oC)
Advantages Disadvantages Applications
Alkaline 25 to 100
• developed technology
• No valuable metals
• Use pure hydrogen • Space
Proton Exchange Membrane
0 to 85
• Function at ambient temperature
• High-power density
• Need for humidification
• Sensitive to CO poisoning
• Distributed power
• Transportation
Phosphoric
Acid 170 to 220
• Mature
• Reformate intolerant
• Cannot start from ambient temperature
• Bulky
• Heavy-duty transportation
Molten
Carbonate 650
• Some fuel flexibility
• High-grade waste heat
• Fragile electrolyte matrix
• Electrode sintering
• Utilities
• Distributed power
Solid Oxide 800 to 1000
• Maximum fuel flexibility
• Highest cogeneration efficiency
• Exotic materials
• Sealing and cracking issues
• Utilities
• Distributed power
Alkaline Fuel Cell (AFC)
Alkaline fuel cells (AFC) (Figure 2.9) are amongst the first practical fuel cell, especially for space applications for on-board electricity and potable water for the crew. The AFC has some attractive characteristics, such as moderately high efficiency because of low internal resistance and high electrochemical activity, quick start-ups, few precious metal requirements and low corrosion. However, the AFC’s corrosive environment requires that it utilises some exotic materials, and the alkaline (potassium hydroxide solution) concentration must be firmly controlled as it has poor tolerance to deviations.
Fundamentally, the alkali is promptly killed by acidic gases, so both the approaching fuel and air need carbon dioxide clean-up. This restraints AFC application to those in which pure hydrogen is utilised as the fuel, since a fuel processor produces substantial amounts of carbon dioxide. The small amount of carbon dioxide in air (˜0.03%) can be taken care of using an alkaline trap upstream of the fuel cell and, subsequently, is not as much of an issue
Figure 2. 9: Alkaline Fuel Cell. (Abderezzak, 2018)
.
Phosphoric Acid Fuel Cell (PAFC)
Although Phosphoric Acid Fuel Cell (PAFC) (Figure 2.10) accepts carbon dioxide, the requirement for water to be available to support proton movement augments the system complexity. It is presently a developed technology, designed broadly for unmoving power applications, and units with power of around 200 kW are in the market and have shown 40,000 hours of operation. Unlike the AFC, the PAFC provide reliable operation with system efficiency of 40 % to 50 % even when functioning on low quality fuels. This fuel flexibility is empowered by higher operating temperature of about 200°C against 100°C for the AFC as this increases electro-catalyst resilience to impurities.
However, the PAFC still has a large weight and does not have a fast start-up since it requires heating up to 100°C prior to deliver a current. The PAFC is, however, appropriate for fixed power generation.
Molten carbonate Fuel Cell (MCFC)
Molten Carbonate Fuel Cell (MCFC) (Figure 2.11) functions at 650°C and utilises an electrolyte produced using lithium carbonate and molten potassium salts. Operation at high-temperature is perfect for unmoving applications since the heat by-product can be used for combined heat and power systems; it also permits fossil fuels to be reformed directly, and this decreases the size and complexity of the system. However, the MCFC experiences cathode corrosion and sealing issues initiated by the high-temperature its molten electrolyte. Similarly, thermal cycling is constrained since once the electrolyte becomes solid; it is likely to create cracks at the time of reheating. Further existing issues are anode sintering and elution of the oxidised nickel cathode into the electrolyte.
Figure 2. 11: Schematic of MCFC. (Barbir et al., 2016)
Solid Oxide Fuel Cell (SOFC)
In Solid Oxide Fuel Cell (SOFC) (Figure 2.12), the electrolyte is a solid oxide ceramic.
So as to prepare strong solid oxide ions, the cell must operate at temperatures as high as 1,000°C. This guarantees fast diffusion of gases into the permeable electrodes and subsequent electrode reaction and suppresses the need for external reforming.
Therefore, besides hydrogen and carbon monoxide fuels, the solid oxide fuel cell can also reform methane directly. Subsequently, this fuel cell has attractive specific power, and cogeneration efficiencies greater than 80 % might be feasible. Furthermore, the SOFC can be air-cooled, simplifying the cooling system, although the need to preheat air requires extra heat exchangers.
Figure 2. 12: Schematic of SOFC. (Barbir et al., 2016)
Elegant based on its conception, the SOFC, however, has essentially costly materials, for instance an electrolyte fabricated from zirconium dioxide stabilised with yttrium oxide, a strontium-doped lanthanum manganite cathode, and a nickel-doped stabilised zirconia anode. Furthermore, no cheap techniques for the fabrication of the unit exist.
Proton exchange Membrane Fuel Cell (PEMFC)
Proton exchange membrane fuel cells (PEMFC) (Figure 2.13) are the most promising fuel cell for transportation purposes. Unlike the PAFC, it has much greater power density; a typical PEMFC stacks can generate about 1 kW per litre. Potentially, it is more affordable and, since it utilises a thin solid polymer electrolyte sheet, it has few corrosions and sealing problems, furthermore, no issues related to electrolyte dilution because of water by-product. Because it can function at ambient temperature, the PEMFC can start up fast. However, it has two main drawbacks which are:
i. low efficiency and
ii. more strict purity demands.
The low efficiency results from the complication during the recovery of heat by- products, while the catalysed electrode’s resilience to impurities decreases considerably because of the temperature drop. For instance, while a PAFC functioning at 200°C can accept 1 % of carbon monoxide, the PEMFC, functioning at 80°C can only accept ˜0.01 %.
The membrane of a PEMFC demands constant humidification to keep a pressure of about 0.5 bars, as failure to achieve that can result to a huge rise in resistance.
design, yet available membranes are not able to last long when operating at higher temperatures and require further improvement. This type of fuel cell can operate using fuels others than hydrogen. For instance, in the direct methanol fuel cell (DMFC), a dilute methanol solution of 3 % is supplied into the anode, and a multistep procedure induces the liberation of protons and electrons while water and carbon dioxide are released. Since fuel processor is not needed, the unit is conceptually very attractive.
Nevertheless, the multistep procedure is not faster than the simpler hydrogen reaction, and this makes the direct methanol fuel cell stack to deliver less power and to require more catalyst.
Figure 2. 13: Layout of proton Exchange Membrane Fuel cell. (Yuan et al., 2019)
2.3 Power Quality Requirement for Grid-Connected Alternative Energy Systems