The membrane electrode assembly is made up of anode and cathode electrodes and an electrolyte which is sandwiched between the two electrodes. The electrodes are permeable to gas so that the gases can make contact with the electrolyte. Points at which the gas, electrode, and electrolyte meet are points where an electrochemical reaction can occur which liberates an electron into or out of the electrode as an ion is transferred through the electrolyte. These points are called triple-phase boundaries . For example, with hydrogen electrochemistry and with proton-conducting electrolytes, the hydrogen splits, releasing two electrons into the anode while the hydrogen ions travel through the electrolyte. The ions then react with oxygen atoms and electrons at TPB locations in the cathode compartment to form water. A voltage difference between the anode and cathode is produced by the electrochemical reactions, which drives an electric currentthrough the external circuit. While the Nernst voltage can be realized when a fuel cell does no net work ,nft channel once electric current flows, additional voltage losses are realized. Figure 7 displays the ideal and actual responses of a fuel cell to increasing current demands.
Electrical energy is obtained from a fuel cell when a current is drawn, but the actual cell potential is lowered from its equilibrium potential because of irreversible losses due to various reasons. Several factors contribute to the irreversible losses in a practical fuel cell. The losses, which are generally called polarization or over potential, originate primarily from activation polarization, ohmic polarization, and gas concentration polarization. These losses result in a cell potential for a fuel cell that is less than its ideal potential.SOFC usually operate at high temperatures in the range of about 600-1000℃, which temperature range is well-suited to integrating bottoming cycles for additional power, heating or cooling production, which results in improved overall system efficiency when compared to an individual stand-alone system. An analysis of a hybrid system of microturbine and fuel cell as a prime mover of Combined Cooling, Heating, and Power plants was carried out by Saito et al.. They carried out electricity demand and consumption analyses of apartments, offices and hotels in Japan with the use of the hybrid system. They found that the annual fuel consumption dropped by 32%, 36% and 42% for the apartments, offices and hotels, respectively. One general method to recover the waste heat from the SOFC is the SOFC-GT system, in which SOFC is coupled with a Gas Turbine as the bottom cycle to increase the overall efficiency by recovering waste heat from SOFC exhaust.
The concept of SOFC-GT hybrid system was proposed for decades, and many researchers have studied the theoretical analysis of this hybrid system. Siemens Westinghouse Power Corporation developed the first hybrid power system, which integrated an SOFC stack with a gas turbine engine. The pressurized system generated 220kW of electrical power at a net electrical efficiency of 53.5% . Mueller et al. designed a theoretical solid oxide fuel cell-gas turbine hybrid system using a 60kW micro-gas turbine. Although using a gas turbine can recover waste heat from an SOFC, the exhaust gas from the gas turbine still has relatively high temperature that could be used in other bottoming cycles. If the waste heat from gas turbine can also be recovered by some method, then the energy conversion efficiency can increase further. Another method to recover the waste heat from the SOFC is the SOFC- Organic Rankine Cycle combined system. Using ORC as a bottoming cycle of the SOFC, the system can fully recover the waste heat from the SOFC. Some research has been conducted on the waste heat recovery of the exhaust from the SOFC by ORC. Akkaya and Sahin presented an energetic analysis for a combined power generation system consisting of an SOFC and an ORC. The results showed that the efficiency was increased by about 14-25% by recovering SOFC waste heat through ORC based on investigated design parameter conditions. And there existed an optimum value of fuel utilization factor maximizing the efficiency. Al-Sulaiman et al. proposed a cooling, heating and power production system based on the SOFC and ORC.
The energy analysis showed that at least a 22% gain in efficiency was achieved compared with the stand-alone SOFC system. Ghirardo et al. conducted a study on heat recovery for a 250kW SOFC onboard a ship. Using ORC could produce 35kW of electricity from the waste heat of 181 kW. The overall efficiency increased from 44% to 49% and the cost of energy dropped from 0.25$/kWh to 0.22$/kWh. In another study, Al-Sulaiman et al. analyzed CO2 emissions from the CCHP system. The study showed that the CO2 emissions per MWh are significantly less than that of the CO2 emissions per MWh of the electricity produced by the SOFC alone or the net electrical power of the system. In another study, Al-Sulaiman et al. studied the feasibility of using a CCHP plant based on ORC and solid oxide fuel cells. In their study, it was shown that there is 3–25% gain on exergy efficiency when compared with the power cycle only. In a different study, Al-Sulaiman et al. examined a CCHP system using a biomass combustor and an ORC. In their study, it was shown that the exergy efficiency of the CCHP system increases significantly to 27% as compared with the exergy efficiency of the electrical power case, which is around 11%. In SOFC-ORC combined systems, it is a novel idea to use the ORC waste heat for cooling purposes. Several studies were conducted in using absorption chiller as a bottoming cycle of the SOFC for cooling. Margalef and Samuelsen studied an integrated molten carbonate fuel cell and absorption chiller cogeneration system, showing that the overall electrical and cooling efficiency can achieve 71.7%. Furthermore, Silveira et al. examined a molten carbonate fuel cell cogeneration system integrated with absorption refrigeration which was applied to a dairy for electricity and cold water production. The results showed that the electrical efficiency of the system and the second law efficiency of the fuel cell unit were 49%and 46%, respectively. Leong analyzed an integrated natural gas fed solid oxide fuel cell with a zeolite/water adsorption chiller. The results show that the proposed cogeneration system can achieve a total efficiency of more than 77%. A steady state mathematical model was developed to simulate the effects of different SOFC operating conditions on an energy system incorporating SOFC and exhaust gas driven absorption chiller. The effect of fuel utilization factor on electrical, cooling, and total efficiency was investigated. Zink et al. studied an integrated solid oxide fuel cell absorption heating and cooling system for buildings, concluding that the combined system demonstrated great advantages in both technical and environmental aspects. An integrated SOFC and a double effect water/Lithium Bromide absorption chiller were presented by Yu et al.. The system performance was analyzed under different fuel utilization ratio, fuel flow ratio, and air inlet temperature.
Al-Sulaiman et al. studied the use of an SOFC integrated with both ORC and absorption chiller. In this study,hydroponic nft the waste heat from the ORC is used to produce cooling using a single-effect absorption chiller. The study shows that the maximum efficiency of the trigeneration plant is 74%, heating cogeneration is 71%, cooling cogeneration is 57% and net electricity is 46%. Asghari et al. studied dynamic integration of SOFC with AC and ORC. Figure 8 shows the schematic of configuration of this study. Results showed that the SOFC was capable of following the highly dynamic load with an average electrical efficiency of 46%. An average of 7% more power was produced through the ORC cycle with an average efficiency of 10%. The AC generated an average 125kW of cooling with an average Coefficient of Performance of 1.08. Mehpoya et al. investigated optimal design of solid oxide fuel cell-gas turbine power plant, Rankine steam cycle and ammonia-water absorption refrigeration. Results indicate that electrical efficiency of the combined system is 62.4% Lower Heating Value . Produced refrigeration and heat recovery are 101kW and 22.1kW respectively.The proposal system is based on the idea of gasifying the municipal waste, producing syngas serving as fuel for the trigeneration system. It is shown that the energy efficiency of such small tri-generation system is more than 83% with net power of 170kW and district energy of about 250W. Tian et al. proposed and investigated an integrated SOFC system, an ORC, and an ammonia water absorption chiller with a CO2 capture system is proposed and investigated. The results show that the net electrical efficiency and the exergy efficiency of the integrated system can reach 52.83% and 59.96%, respectively. The trigeneration efficiencies of the combined system without and with CO2 capture are 74.28% and 72.23%, respectively. During recent years there have been some research on integration of SOFC with adsorption technologies.The authors dynamically simulated the integrated system. Simulation results show the proposed system produces 4.35kW of electrical power, 2.448kW of exhaust heat power, and 1.348kW of cooling power. The energy efficiency of the system is 64.9% and the COP of the refrigeration system is 0.32. The implementation of SOFC technology in data centers has many advantages including fuel flexibility, lower emissions, higher production efficiency, and the production of high-quality exhaust heat for co-generation applications. These advantages could reduce the data center’s footprint through the synergistic integration of power and cooling to supply the data center. eBay installed 6MW of fuel cell from Bloom Energy as the primary power source for their data center in Utah while using the local utility grid for backup. In 2013, Apple installed 10MW capacity high temperature fuel cells to power their entire data center. Equinix started a project in 2017 to power 12 data centers in US with a total capacity of more than 37MW of Bloom fuel cell. Microsoft in collaboration with the National Fuel Cell Research Center demonstrated the concept of a rack of data center servers powered directly by the DC output of a fuel cell stack for the first time at the NFCRC in University of California, Irvine. Later, Microsoft started a new Advanced Energy Lab in Seattle, Washington to power 20 data center racks directly from DC power of SOFC systems. Direct highly efficient power generation at the rack level removes the need for complex power distribution systems, reduces costs significantly, and allows decentralized architecture. Having shown that the desired reliability is achievable through well-designed distributed fuel cell power systems, backup power sources are no longer required. National Renewable Energy Laboratory recently have started looking at rack level powered data centers using PEM fuel cells while producing hydrogen fuel for fuel cells by on site electrolysis using renewable resources. SOFCs are promising technology that can directly operate on variety of fuels and the high temperature operation allows internal reforming of carbonated gas. As an alternate to carbonated fuel and due to high pressure required for storage of H2, ammonia is getting more attention as a potential fuel for SOFC. Ammonia is carbon free, widely available, and has comparative price as hydrocarbons. Is it easily liquified and volumetric energy density of liquefied ammonia is higher than that of liquid hydrogen, which is useful in transport and storage. Anode and cathode half reactions of ammonia fed SOFC are presented by Equation 11to Equation 14 . The route of utilization of ammonia on SOFC is a two-step process . The ammonia first decomposes to hydrogen on the anode side , followed by electrochemical oxidation of generated hydrogen to form water . A few groups have reported the promising performance of ammonia-fueled SOFCs, however, there are very few studies looking into long term degradation effect of SOFC fueled by ammonia. Bernhard et al. tested the performance of ammonia as fuel for SOFC, their results showed that ammonia exhibits excellent performance as fuel for SOFC however, in their EIS measurements they observed significant increase in ohmic resistance. They found that operating at counter flow is more favorable to co-flow as less ohmic and diffusion resistance was measured. Occurrence of nickel nitrite, microscopic pores and particle enlargement and agglomerations was observed in micro-structure imaging. Hung et al. studied the effect of pressurized ammonia fed on anode supported SOFC. Their finding showed that pressurizing and increasing temperature enhances the performance of ammonia fed SOFC.