Simulation and Experiment for a Design of a Solid Oxide Fuel Cell – Homogeneous Charge Compression Ignition Engine Hybrid System

Abstract

A solid oxide fuel cell (SOFC) hybrid system is a system that combines an SOFC with an additional power generation device to increase the efficiency of the system. Thus far, the SOFC–gas turbine hybrid system has been primarily investigated for SOFC hybrid systems. However, the current power generation capacity of an SOFC is less than several MWs

for this generation capacity, an internal combustion engine (ICE) is generally more efficient and economical than a gas turbine. Focusing on this point, recently, the concept of an SOFC–ICE hybrid system was proposed. To successfully combust the SOFC anode off-gas, which includes a large amount of diluents (H2O and CO2), the homogeneous charge compression ignition (HCCI) method was selected instead of spark ignition as the combustion strategy of the ICE in the hybrid system. Although the concept of an SOFC–HCCI engine hybrid system has been proposed, the researches on this hybrid system have been very limited. Therefore, this dissertation aims to fully investigate the operation characteristics of the SOFC–HCCI engine hybrid system by conducting experiments and simulations. In the first step of the investigation of the hybrid system, an experimental study of HCCI engine operation fuelled by SOFC anode off-gas was conducted. For the HCCI engine experiments, a single-cylinder HCCI engine and experimental equipment for emulating SOFC anode off-gas were constructed. Various HCCI engine experiments were performed while varying several control parameters, e.g., the fuel utilization factor of an SOFC, which primarily affects the composition and flow rate of the engine intake gas. The experiments indicated that the HCCI combustion was achieved even with highly diluted gas when the intake temperature was sufficient. In addition, the results indicated that HCCI engine operation fuelled by SOFC anode off-gas yields a significant amount of power (w/ 25-30% gross indicated efficiency) and produces significantly low NOx emissions (< 5 ppm @ O2 15%) under stable HCCI combustion (< 5% COV IMEPg, which is the coefficient of variance of the gross indicated mean effective pressure). Considering that the experiment was performed using a small single-cylinder engine, these experimental results reveal that the use of an HCCI engine as the bottoming cycle in an SOFC hybrid system is feasible. However, HCCI engine operation was not always stable in all experimental conditions. Engine operation with an exceedingly low engine load (IMEPg < 1.8 bar) should be avoided as it decreases the stability of engine operation. In addition, an engine intake gas with excessive dilution (fuel molar fraction < 0.125) should be avoided to decrease the amount of unburned CO emission and maintain a CO combustion efficiency higher than 90%. In the second step of the investigation of the hybrid system, the operation of the SOFC and the entire hybrid system was analysed. The impacts of the HCCI engine operation on the system, especially on the SOFC, were investigated by integrating the experimental results of the HCCI engine with simulation models of the other system components. Steady-state simulation models were constructed by utilizing MATLAB from Mathworks together with the Cantera thermodynamic tool box and GRI 3.0 mechanism. A direct internal reforming planar-type SOFC was selected for the SOFC in the hybrid system, and the SOFC was modelled using a one-dimensional model to investigate the coupling effects of direct internal reforming and electrochemical reactions. The system analysis indicated that the SOFC in the SOFC–HCCI engine hybrid system utilized SOFC anode inlet gas at a low temperature (e.g., 750 ~ 800 K) and low reforming rate (e.g., 30 ~ 40%). Therefore, the operating temperature at the entrance part of the SOFC is relatively low (e.g., 900 K)

thus, an anode-supported-type SOFC that can reduce the ohmic loss of the electrolyte at this relatively low temperature is preferable for the SOFC in the hybrid system. An analysis of the entire system was conducted while varying several control parameters, e.g., the fuel utilization factor of the SOFC. The design point of the operation was determined by considering not only the performance but also the stability of operation. At the design point, the SOFC generates 4.97 kW of power, and the HCCI engine generates 0.93 kW of power while emitting ~ 1 ppm NOx and ~ 1300 ppm CO. The system efficiency was calculated as 58.5% at this design point. In the third step of the investigation of the hybrid system, an experimental study of the entire SOFC–HCCI engine hybrid system was conducted to directly verify the feasibility of system operation by an experiment. The experiment of the first 200-hour continuous operation of the SOFC-HCCI engine hybrid system was conducted. The operating point of the 200-hour continuous operation was determined based on the previously mentioned system analysis. The experiment verified the feasibility of the hybrid system. Particularly, the experiment indicated that the pressure oscillation caused by the HCCI engine did not significantly affect the SOFC as it was cancelled a lot when it reaches the SOFC anode due to the flow path between the engine intake and the SOFC. However, in the experiment, electric heaters and a burner were utilized to increase the temperature of the anode and cathode inlet gas for the stability of SOFC operation due to the over-expected amount of heat loss from the system. Therefore, from the experiment of entire system, it was also confirmed that reducing the heat loss is essential for achieving more successful operation of the system. In the fourth step of the investigation of the hybrid system, methods to resolve major three problems of the system operation (CO emissions, a large amount of heat loss, and the limitation of increasing fuel utilization factor of the SOFC), which were confirmed from previously mentioned investigations, were studied. For this, simulations of the entire system were performed. Three methods resolving these problems (additional catalytic oxidizer, heat recovery from the engine coolant, and increasing the compression ratio of the engine) were introduced

the effects of each method were analysed. Based on these analyses, the optimized design of the SOFC–HCCI engine hybrid system was determined. With the optimized system design, the system efficiency was estimated as ~ 63.8%, and the pollutant emissions are expected to be maintained at near-zero for the 5-kW class power generation system. Considering that the size of the system is relatively small, these results can be regarded as very promising. In the last step of the research on the hybrid system, simulation study on the system performance was conducted while increasing the system size whereas maintaining the determined design of system. For scaling up the system, the number of modularized SOFC stacks was increased, and the size of a cylinder of HCCI engine was enlarged. Simulations indicated that as the system size increased, the surface to volume ratio decreased and the engine heat loss decreased

thus, the efficiencies of HCCI engine, SOFC, and the entire system were increased. Therefore, it was confirmed that the scale-up of the system helps to increase the system efficiency. Especially, it was estimated that when the system is scaled up to 100-kW, the efficiency of ~ 67.4% can be achieved. This result can be regarded as a very promising result that shows the possibility of the SOFC-HCCI engine hybrid system being used as ultra-high efficiency power generation systems.