Power and Efficiency Characteristics of a Hybrid Electrochemical-ICE Cycle

Power and efficiency characteristics of a hybrid cycle combining an electrochemical device (Fuel-Cell) and an internal combustion engine (ICE) were analyzed using the low-dissipation model. The low-dissipation model links energy dissipation with the energy transfer rate through the cycle. In the considered cycle, the electrochemical device transforms chemical potential of the fuel to electrical work, and the ICE uses the heat rejected by the electrochemical device and its exhaust effluent for mechanical work production. The cycle efficiency was calculated as a function of the hybridization level. The latter is defined as the electrical work fraction in the total cycle work. The results of the study show that the cycle efficiency is growing with the electrical work fraction increase. On the other hand, maximum power of the cycle is attained at an intermediate hybridization level. Moreover, power to weight ratio and power density of the cycle have maxima at different hybridization level. Cycle cooling losses are modeled as heat leak to the ambient that depends on the temperature and the duration of the cycle. Cooling losses are found to be the most influential parameter in optimization of the hybridization level for maximum power. In the extreme case of zero cooling losses, maximum power could be attained with ICE operation alone without the electrochemical reaction. The latter finding might be of interest for aerial propulsion systems. However, if efficiency is more important - for example for ground propulsion systems - the hybrid cycle is beneficial.