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| Fig. 1: SOFC Diagram. [1] (Source; Wikimedia Commons) |
Fuel cells have been around for a long time. Its a device that generates electricity through an electrochemical reaction. It has an anode, a cathode, and an electrolyte. Oxygen molecules are reduced to form oxide ions, then pass through an electrolyte to the anode, where the oxide ions react with a fuel, producing water, carbon dioxide (in some cases), and importantly, electrons. The electrons flow through a circuit and generate electricity (Fig. 1). This particular study will focus on Solid Oxide Fuel cells, which utilizes a solid ceramic electrolyte to bridge the cathode and anode. The electrolyte is typically made of yttria-stabilized zirconia (YSZ) or ceria-based ceramics. Unlike combustion-based systems, SOFCs produce power without burning fuel, which aims to reduce pollutants. One thing that is important about this high-temperature fuel cell is that no expensive Platinum catalyst is needed, which means cheaper nickel alloys can be used to facilitate the cell reactions. [1] This is a high-level analysis of Solid Oxide Fuel Cells, but the specific construction of SOFCs can vary, particularly the ceramic electrolyte and catalytic electrode, since it often involves proprietary technologies.
SOFCs differ from typical fuel cells in that they can operate at incredibly high temperatures (600-1000 degrees Celsius). [2] They also don't use expensive noble metals. These operating conditions require many things to align. Each component of the SOFC must have proper stability, conductivity, chemical compatibility with the other components, similar thermal expansion, dense electrolytes, and porous anodes and cathodes, high strength, fabricability, high temperature compatibility, and a low cost. [2] Commercially, Solid Oxide Fuel Cells are deployed in products, most famously Bloom Energy's Bloombox, and more recently, FuelCell Energy's SureSource. There are a variety of inputs that SOFCs can take to produce electricity. The main options are Hydrogen (H2), Natural Gas (CH4), Ammonia (NH3), Methanol (CH3OH), Biogas, Syngas, and Propane/hydrocarbons (C3H8). The renewable inputs are Ammonia, Biogas, and Hydrogen, which produce no harmful byproducts. [3] Natural gas, methanol, syngas, and hydrocarbons are fossil fuels which do produce byproducts for the environment. [3]
Though SOFCs have the option to be run on green fuels such as hydrogen and biogas, most of them utilize natural gas as an input. For clarity, data from Bloom Energys IPO prospectuses say that only 9% of their fuel cell servers run on biogas. Therefore, the efficiency of the servers must be calculated assuming, practically, that natural gas is the input. In terms of fuel cell types (PEM, AFC, PAFC, MCFC, SOFC), the SOFC carries an electrical efficiency (LHV) of 60%, which is a relatively high number when compared to the other types of fuel cells. [4] However, we need to dig a little deeper to understand the mechanics of the efficiency proposition that SOFCs supposedly have. Heat rate and efficiency percentage are two indicators for energy conversion efficiency, and they are inverses of each other. A lower heat rate typically indicates a higher efficiency. Chen et al. conducted a study analyzing two natural gas powered Bloom Energy facilities in Delaware over time and monitored their efficiency and heat rate. [5] They found the thermodynamic efficiencies shown in Table 1.
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| Table 1: Thermodynamic efficiency of gas-powered Bloom Energy facilities in Delaware reported by Chen et al. [5] |
These efficiency numbers are quite competitive with other fossil-fuel based power plants and nuclear power in the United States, but the study also points to the glaring problem of fuel cell degradation causing generating efficiency to decrease over time. [5] Lee et al. conducted a study on improving degradation on SOFCs, and reported that SOFC stacks fabricated using conventional diffusion-blocking layers showed degradation rates of approximately 0.5% per 1,000 hours. [6]
It's now important to compare the efficiency numbers found by Chen et al to some comparable base-load power sources, namely coal and combined-cycle natural gas. In order to derive the thermodynamic efficiencies of each source, we will work backwards from CO2 emissions data from Mletzko et al. [7,8]
| 2557 lbs MWh-1 2.2 lbs kg-1 × 3.6 × 109 J MWh-1 |
= | 3.229× 10-7 kg J-1 |
| 771 lbs MWh-1 2.2 lbs kg-1 × 3.6 × 109 J MWh-1 |
= | 9.735× 10-8 kg J-1 |
These are kilograms of CO2. Counting atomic weights, we find that burning one kg of coal releases 44/12 kg of CO2 into the air and that burning 1 kg of natural gas releases 44/16 kg of CO2 into the air. The Joules of energy produced per kg of fuel consumed are thus
| 1 3.229× 10-7 kg J-1 |
× | 44 12 |
= | 1.136 × 107 J kg-1 (of coal) |
| 1 9.375 × 10-8 kg J-1 |
× | 44 16 |
= | 2.825 × 107 J kg-1 (of NG) |
Finally, the energy content of coal is about 2.9 × 107 J kg-1 and the energy content of NG is about 5.5 × 107 J kg-1. The thermodynamic efficiencies are then
| ηcoal | = | 1.136 × 107 J kg-1 2.9 × 107 J kg-1 |
= | 0.392 (39.2%) |
| ηNG | = | 2.825 × 107 J kg-1 5.5 × 107 J kg-1 |
= | 0.514 (51.4%) |
Based on the calculations derived from fundamental thermodynamic principles, the efficiencies of coal and natural gas power plants were found to be 39.2% and 51.4%, respectively. These numbers align with established benchmarks for coal and combined-cycle natural gas plants, where coals inefficiency is highlighted by its lower energy conversion rate despite its high carbon emissions. In contrast, natural gas achieves greater efficiency and lower CO2 emissions due to its cleaner combustion and modern combined-cycle technology. When compared to these power sources, Bloom Energys SOFCs, with a calculated thermodynamic efficiency of approximately 45% (Table 1), are comparable to coal plants in the US. However, they significantly underperform compared to the highly efficient combined-cycle natural gas plants. These results underscore the importance of thermodynamic efficiency as a metric for evaluating energy technologies, providing a more nuanced understanding than emissions data alone. When combusting Natural Gas, other than CO2, the two main pollutants are NOx and SOx. NOx accounts for about 7% of greenhouse gas emissions, so mitigation would still prove beneficial for combating this. [2] Since SOFCs don't involve any combustion, they produce negligible amounts of byproducts that aren't CO2. A study by Saidi et al. compared emissions of a traditional power plant and SOFC system and it showed that SOx and NOx emissions were 0, while the fossil fueled plant generated 12,740 and 18,850 kg/1650 MWh respectively. [9] More recent numbers by Bloom Energy analyzing their sites also showed negligible amounts of NOx being produced as a byproduct of the electrolysis reaction. [10]
While SOFCs offer an interesting solution that promises to reduce GHG emissions significantly, fundamentally, we found through calculations that SOFCs are comparable in thermodynamic efficiency to fossil fuel sources like coal, and underperform baseload power sources such as combined-cycle natural gas plants. While SOFCs offer advantages in terms of fuel flexibility and localized power generation, their efficiency must improve to truly compete with modern fossil fuel technologies. One area in which SOFCs shine, however, is their ability to completely remove other air pollutants that would otherwise be seen when combusting natural gas, such as SOx and NOx.
© Sanjay Swamy. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Breeze, "Power Generation Technologies, 2nd Ed.," (Newnes, 2014), Ch. 7.
[2] A. Stambouli and E. Traversa, "Solid Oxide Fuel Cells (SOFCs): A Review of an Environmentally Clean and Efficient Source of Energy," Renew. Sustain. Energy Rev. 6, 433 (2002).
[3] Y. Bicer and F. Khalid, "Life Cycle Environmental Impact Comparison of Solid Oxide Fuel Cells Fueled by Natural Gas, Hydrogen, Ammonia and Methanol for Combined Heat and Power Generation," Int. J. Hydrog. Energy 45, 3670 (2020).
[4] "Comparison of Fuel Cell Technologies," U.S Department of Energy, April 2016.
[5] W.-M. Chen and H. Kim "Energy, Economic, and Social Impacts of a Clean Energy Economic Policy: Fuel Cells Deployment in Delaware," Energy Policy 144, 113617 (2020).
[6] S. Lee et al., "Highly Durable Solid Oxide Fuel Cells: Suppressing Chemical Degradation Via Rational Design of a Diffusion-Blocking Layer," J. Mater. Chem. A 6, 15083 (2018).
[7] "Addressing Emissions From Coal Use in Power Generation ," Pew Center on Global Climate Change, 2008.
[8] J. Mletzko, S. Ehlers, and A. Kather, "Comparison of Natural Gas Combined Cycle Power Plants with Post Combustion and Oxyfuel Technology at Different CO2 Capture Rates" Energy Procedia 86, 2 (2016).
[9] M. Saidi, F. Siavashi, and M. R Rahimpour, "Application of Solid Oxide Fuel Cell For Flare Gas Recovery as a New Approach; a Case Study for Asalouyeh Gas Processing Plant, Iran," J. Nat. Gas Sci. Eng. 17, 13 (2014).
[10] "Load Following Solid Oxide Fuel Cell," Bloom Energy, February 2024.
