Efficiency at Maximum Power of the Low-Dissipation Hybrid Electrochemical–Otto Cycle

David Diskin,Leonid Tartakovsky

doi:10.3390/en13153961

Abstract

A novel analytical method was developed for analysis of efficiency at maximum power of a hybrid cycle combining electrochemical and Otto engines. The analysis is based on the low-dissipation model, which relates energy dissipation with energy transfer rate. Efficiency at maximum power of a hybrid engine operating between two reservoirs of chemical potentials is evaluated. The engine is composed of an electrochemical device that transforms chemical potential to electrical work of an Otto engine that uses the heat generated in the electrochemical device and its exhaust effluent for mechanical work production. The results show that efficiency at maximum power of the hybrid cycle is identical to the efficiency at maximum power of an electrochemical engine alone; however, the power is the product of the electrochemical engine power and the compression ratio of the Otto engine. Partial mass transition by the electrochemical device from the high to the low chemical potential is also examined. In the latter case, heat is generated both in the electrochemical device and the Otto engine, and the efficiency at maximum power is a function of the compression ratio. An analysis performed using the developed method shows, for the first time, that, in terms of a maximal power, at some conditions, Otto cycle can provide better performance that the hybrid cycle. On the other hand, an efficiency comparison at maximum power with the separate Otto-cycle and chemical engine results in some advantages of the hybrid cycle.

Highlights

According to equilibrium thermodynamics, the maximal efficiency of a cycle operating between two reservoirs at thermodynamic potential PH and PL (PH > PL ) is the Carnot efficiency, ηC = 1 − PL /PH .finite size engines that work at equilibrium conditions cannot produce finite power
We begin the discussion with the case of full conversion of the high-potential carrier in the electrochemical engine
The first question is whether the hybrid cycle could provide better performance than Otto cycle and what level of hybridization is needed for achieving maximum power

Summary

Introduction

To the best of our knowledge, the combination of chemical engine and Otto cycle was not analyzed by the low-dissipation approach for finite power. This combination is called a “hybrid cycle” hereafter. Hybrid cycles that combine electrochemical engine (fuel cell) and a bottoming heat engine have been studied and developed since 1990s [9,10]. Research on an ICE in combination with a SOFC was initiated as well [14,15] In these studies, integration of engine experiment results with a basic fuel cell model were performed. When a hybrid cycle involving a fuel cell and an ICE is considered, usage of fuel reforming in combination with waste heat recovery known as Thermochemical. Hydrogen-rich fuel cell exhaust gases combust and expand in the ICE to produce mechanical work

Low-Dissipation of the Hybrid

Low-Dissipation Model for Partial Conversion

Results and Discussion

Conclusions