A proposed new energy source: The “mixing energy” Of engine exhaust gas

SUMMARY

A proposed new energy source: The “mixing energy”

Of engine exhaust gas

                                                                                            By: 

                                                                                            Kumari Neha

                                                                                            PGDIE 42

                                                                                            A-42

One potential source of useful energy that seems to be unrecognized and overlooked is the “entropy of mixing” of engine exhaust gases with the atmosphere. In particular, exhaust gases from combustion engines typically contain carbon dioxide at concentrations 250 times greater than in the atmosphere and have an oxygen concentration that is typically less than one-tenth the oxygen concentration in the atmosphere. Fuel cell power sources can produce exhaust gases containing CO2 concentrations 1000 times greater than in the air. Currently, engine exhaust gases are simply ejected into the atmosphere where they spontaneously mix with the air, and the potential to do useful work is lost. Rather than discarding combustion gases into the atmosphere, it is possible—in principle—to exploit the entropy of mixing to produce useful work. However, the energy that could potentially be produced by mixing exhaust gases with air is at least an order of magnitude less than the energy released by combustion of the fuel. Exploiting this energy source is likely to prove difficult and expensive with current technology, but ultimately, this “mixing energy” could be an incremental energy source to be finessed from combustion engines, kilns, furnaces, and fuel cell power sources. This article proposes conceptual designs for “mixing engines” that could augment the power output of a diesel engine by as much as 3.0% or augment the power output of a fuel cell power plant by 3.5%.

Fuel cells offer a major efficiency advantage relative to combustion engines since they convert chemical energy of the fuel directly into electricity _and are therefore not “heat engines” subject to the Carnot efficiency limit. An engine designed to produce energy by mixing engine exhaust gases could take advantage of the relatively high exhaust gas temperatures _500–600 °C_ produced by engines and gas turbines. So far, this article has highlighted the potential for extracting useful work from a resource that is currently unused, unrecognized, and _literally_ discarded into the atmosphere. Whether the mixing energy of engine exhaust gases ever proves to be practical or cost-effective will likely depend on future advances in fuel cell technology. The solid oxide fuel cell _SOFC_ converts energy released by oxidation of hydrogen, natural gas, or other fuels directly into electricity. In its normal application, oxygen is reduced at the cathode, and hydrogen is oxidized at the anode to form water vapour. At the operating temperature of the SOFC, typically within the range of 500–800 °C, the electrolyte conducts oxide ions O−2 from the cathode to anode.

It would also be possible, in principle, to harness the energy of mixing of carbon dioxide in exhaust gases with the air in a specially adapted molten carbonate fuel cell MCFC, operated as a concentration cell. The MCFC has been developed to produce electrical power by oxidizing hydrogen or natural gas. The electrolyte consists of molten carbonate salts, held within a porous ceramic matrix, which transport carbonate ions from cathode to anode electrodes. The mixing of exhaust gases with air provides a potential source of energy that is currently unrecognized and untapped. The modest additional power output that could be achieved and technical difficulties likely to be encountered in extracting this energy would undermine the economic viability of exploiting this energy source with current technology. However, future technological developments, particularly relating to improvements in fuel cells, may enable the mixing energy of exhaust gas to be a practical incremental energy source from the combustion of fossil fuels in power generation plants, kilns, and furnaces.