Fuel cell systems are in early stage of development. These systems enable the direct conversion of chemical energy of the fuel into electrical energy without intermittent conversion to mechanical energy. In general fuel cells are like batteries, the only difference being chemical energy (generally hydrogen, natural gas, etc) is stored outside the power generating device. Further, fuel cells are not subject to Carnot cycle limitations and electric efficiencies as high as 80% are possible. Typical efficiencies range in 20-60% (depending on fuel) and can reach to 80-90% in CHP mode. Since no combustion takes place, fuel cells are clean, quiet, and efficient.
Fuel cells can be divided into five different categories based on type of electrolyte and operating temperature. Below this list is a more comparative study on different fuel cell types.
- Polymer Electrolyte Membrane Fuel Cell (PEMFC): This type of fuel cells uses solid electrolyte and operates at low temperatures (~80 oC). These cells have high power density, quick start up time, and fast dynamic response. PEMFC are seen as potential candidates for auto applications, although residential systems are being developed. Hydrogen or reformate is used as a fuel. Efficiencies are in range of 40-55% with hydrogen as a fuel and 20-30% with other fuels like natural gas, propane, etc. These fuel cells are best suited for 1 kW to 100 kW applications.
- Alkaline Fuel Cells (AFC): This type of fuel cell uses aqueous KOH solution as the electrolyte. AFC have been widely used by NASA in their Gemini and subsequent space shuttle operations. AFCs have high efficiency in range of 50-70% and don't use precious noble metal catalysts. On the downside, these fuel cells require COx free hydrogen and oxygen, which is exceptionally expensive. Consequently, AFCs will find application only in niche markets.
- Phosphoric Acid Fuel Cell (PAFC): This type uses concentrated phosphoric acid as an electrolyte and operates in temperature range of 150-250 oC. Efficiencies are in range of 35-50%. In CHP mode, efficiencies as high as 80% are achievable. Today, more than 200 phosphoric acid fuel cells have been deployed worldwide. These 200 kW units are well proven for reliability and durability, although they are still relatively costly.
- Molten Carbonate Fuel Cell (MCFC): This type uses a carbonate-salt-impregnated ceramic matrix as an electrolyte and operates in temperature range of 650-700 oC. Due to their high temperature operation, these cells are best suited for large stationary applications. Typical electrical efficiency are in range of 40-55% with potential to reach 85% with cogeneration. Many MCFCs are currently undergoing real-world testing at several wastewater treatment facilities and other commercial and industrial settings.
- Solid Oxide Fuel Cell (SOFC): This type uses solid oxygen ion conduction as an electrolyte and operates in temperature range of 800 - 1000oC. SOFC makes excellent cogeneration devices for industrial applications where high temperature steam is required.
Table: Characteristics of Major Fuel Cell Types (Source: Energy Nexus Group)
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