In our previous study overpotential resistances of nickel/metal-hydride and lithium-ion batteries were measured to estimate the battery temperature rises during rapid charge and discharge cycles by using our battery thermal model. However, the cell impedance Z͑͒ measured by ac impedance meter did not agree with those induced by charge/discharge characteristics. Therefore, Z͑͒ values were again measured by the method of Takano et al. ͓J. Electrochem. Soc., 147, 922 ͑2000͔͒, who measured Z͑͒ of lithium-ion batteries by Laplace transformation of both signals of the voltage-step input and its current response. This method has been extended here to Laplace transformation of current-step or current-pulse input and its voltage response to measure Z͑͒ for any charge/discharge current of nickel/metal-hydride or lithium-ion battery. Nearly the same Z͑͒ was obtained by the three different methods ͑voltage-step, current-step, and current-pulse inputs͒, and the measured Z͑͒ did not depend on either the charge/discharge current or the state of charge/charge input. Moreover, the Z͑͒ measured by the current-pulse method, which includes the Warburg impedance at low frequency, approaches the overpotential resistance that can provide a good estimate of the battery temperature rise in our battery thermal model.
In order to properly understand the power generation performance of polymer electrolyte fuel cells (PEFCs), it is necessary to have accurate data on water management, such as the diffusion coefficient of water through the membrane electrode assembly (MEA) and gas diffusion layer (GDL), electro-osmotic coefficient through MEA, and power loss data such as the activation and resistance overpotentials. In this study we measured these data with the aim of analyzing our experimental results from PEFC power generation tests done using our two-dimensional simulation code. Our code simultaneously solves mass, charge, and energy conservation equations, and the equivalent electric-circuit for PEFC to obtain numerical distributions of hydrogen/oxygen concentrations, cell potential, current density, and gas/cell-component temperatures. The current density distributions calculated with our simulation code were compared with the distribution measured using a segmented electrode cell. The distributions measured under various operating conditions agreed well with the calculated ones, demonstrating that our code is reliable. The concentration overpotential through GDL was also estimated with the parallel fine-pore model, but the estimated concentration overpotential was very small. Also, the cathode flooding is discussed with the calculated distribution of saturation degree along the channel flow, in comparison with experimental stability.
Keywords: PEFC (polymer electrolyte fuel cell), membrane properties measurement, overpotential measurement, current distribution analysisIn order to grasp properly the power generation performances of PEFC (Polymer Electrolyte Fuel Cell), it is necessary to know the water management data, such as diffusion coefficient of water vapor through MEA (Membrane Electrode Assembly) and GDL (Gas Diffusion Layer), and electro-osmotic coefficient of MEA, and to know power loss data, such as activation and resistance overpotentials. In this study we have measured these data to analyze our experimental results of PEFC power generation tests by our two-dimensional simulation code. These data were adopted in our simulation code.It is desirable to of develop the simulation code of PEFC power generation performance. By developing the simulation code of PEFC some experiments might be taken place by numerical simulations and could save the developing time and money. Therefore we have made a simulation code as a useful supporting tool of to develop PEFC. Our code considers simultaneously the mass, charge and energy conservation equations, and the equivalent electriccircuit for PEFC (Fig. 1) to get numerical distributions of hydrogen/oxygen concentrations, current density, cell potential, and gas/cell-component temperatures along gas flow. In this study current distributions of PEFC under a wide operating conditions have been calculated and compared with experimental distributions by our segmented electrode cell. The measured distributions under various operating conditions agreed well as shown in Fig. 2 with the calculated ones showing that our code is verified experimentally.The current distributions shown in Fig. 2 were measured at an operating condition of low dew point temperature, medium O 2 utilization ratio and co-flow type. In this condition the membrane resistance is high at cell entrance due to the dry anode and cathode gases, and at the middle of the cell the membrane is humidified by the generated water, keeping the same resistance along flow direction until the cell outlet. The concentration overpotential through GDL was also estimated by the parallel fine pores model, however the estimated concentration overpotential was very small to be neglected in the power generating performance of PEFC.
Impedance spectroscopy of Z(ω) is often used in the electrochemical field to analyze electrode reactions and to calculate transient responses. Our previous study measured the overpotential resistance for our thermal behavior model to calculate the temperature rise of Nickel/metal-hydride battery or Lithium-ion battery during charge and discharge cycles. However, the Z(ω) measured by AC impedance meter did not agreed with the ones induced by charge/discharge characteristics. Therefore, we focus on the impedance measurement method by Takano et al, who obtained Z(ω) for Lithium-ion battery at wide frequency region by the Laplace transformation of both signals of the voltage-step input and its current response. We have extended this method to the Laplace transformation of current-step or current-pulse input signal and its voltage response signal to get Z(ω) for any charge/discharge current of Nickel/metal-hydride battery or Lithium-ion battery. We can get almost the same Z(ω) by the three different methods, and the measured Z(ω) does not depend on both charge/discharge current and the state of charge or the charge input. Moreover, Z(ω) including Warlbulg impedance at low frequencies gets near the overpotential resistance that can estimate well the battery temperature rise in our battery thermal behavior model.
SUMMARYThe power generating efficiency of solid oxide fuel cell (SOFC) and gas turbine combined cycle is fairly high. However, the exhaust gas temperature of the combined cycle is still high, about 300 °C. Thus, it should be recovered for energy saving, for example, by absorption chiller. The energy demand for refrigeration cooling is recently increasing year by year in Japan. We propose here a cogeneration system by series connection of SOFC, gas turbine and LiBr absorption chiller to convert the exhaust heat to the cooling heat. As a result of cycle analysis of the combined system with 500-kW-class SOFC, the bottoming single-effect absorption chiller can produce a refrigerating capacity of about 120 kW, and the double-effect absorption chiller can produce a little higher refrigerating capacity of about 130 kW without any additional fuel. But the doubleeffect absorption chiller became more expensive and complex than the single-effect chiller.
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