HEAT ENGINES & FUEL CELLS

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HEAT ENGINES AND FUEL CELLS
by Daniel C. Sweeney, Ph.D

Within the overall alternative energy scene there is a curious neglect of what are known as heat engines which today constitute the overwhelming majority of mechanical power sources in all settings—transportation, stationary electrical power generation, and portable devices such as power tools. The general assumption seems to be that fuel cells combined with electrical motors are going to take over from the internal and external combustion engine, and that by this means we will manage to save ourselves from global warming and the depletion of fossil fuel reserves.

Four years ago when I began to write about energy there was no bigger proponent of fuel cells than myself and no more staunch believer of the above thesis. I was absolutely convinced that fuel cells were on the verge of happening in a very big way and that they would decisively displace conventional heat engines within a few decades. I decided then on the basis of my admittedly limited knowledge but boundless enthusiasm for these devices that if I were to have any relevance as a reporter and commentator on new energy technologies I would have to educate myself thoroughly on the subject of fuel cells.

I did, and I reluctantly reached the conclusion that fuel cells were unlikely to move out of the niche markets they currently occupy within any time span that would be acceptable to knowledgeable investors. How I reached this conclusion will be the subject of future postings. Suffice it to say that the current limitations and liabilities of fuel cells hinge upon a multitude of physical factors—that is, there is not merely one or two deficiencies that have to be overcome. The chemistry and materials technology is simply not there to permit the manufacturing of rugged, reliable units at any reasonable price now or any time soon. Whether fuel cells can be brought to a level of practicality that would give them a fighting chance in the marketplace is more difficult to say. My guess is that if a sufficiently enormous research budget were made available one might place even odds upon the outcome---in other words, there’d be roughly a fifty percent chance of success. The problem is that past failures are going to ensure that private investment for that kind of research will not be forthcoming which throws the whole burden of development back on the universities and national labs. Unfortunately, research budgets for the latter are not going up nor will they any time in the mid term. Hence unless some researcher comes up with an entirely new kind of fuel cell that neatly bypasses all of the problems of existing designs, we’re not likely to see many fuel cells on the market.

Therefore in the ensuing two or three decades, heat engines, that is internal and external combustion engines, will continue to produce almost all of the mechanical power in the world. Very likely they will be doing so with a dwindling supply of fuel. Will this lead to improvements in efficiency? And are significant improvements even possible?

I will explore these issues in detail in following posts and will examine various designs fairly exhaustively. But I’d like to make a few introductory remarks here.

Heat engines, excepting the jet aircraft engine, utilize heat, generally but not always from combustion, to raise the temperatures of gases which are then permitted to expand in confined spaces bounded by moving parts. The moving parts, known as actuators, are pushed or pulled by the motions of the fluid. Examples of such actuators would be the pistons in a reciprocating engine or the blades in a turbine.

Heat engines are divided into two broad groupings, external combustion engines and internal combustion engines. External combustion engines use fuel or some other heat source to raise the temperature of an internal working fluid such as steam, hot air, helium, etc. In an internal combustion engine, on the other hand, the working fluid consists of the products of the combustion of the fuel, namely the gas and vapor that are the fuel’s waste products.

Heat engines of recognizably modern form date back to the early eighteenth century when steam power was first harnessed to drive industrial machinery. In the nearly three hundred years since, a large number of distinct designs have been developed, only a few of which have found widespread acceptance.

Because of the tremendous design diversity within the broad category of heat engines, it is very difficult to generalize about them. Some heat engines are highly efficient while others exhibit very poor efficiency. Some put out tremendous amounts of power per unit of mass or volume, others don’t. Different designs vary greatly in their power curves, that is, the relation of power output to rotational speed, in their tendency to noise and vibration, their generation of waste heat—the behaviors of the different designs and different optimizations of the same designs are so diverse that one almost despairs of providing an overview of the subject.

In this light, I find it remarkable that so many commentators on alternative energy tend to view heat engines, especially internal combustion engines, as fairly equivalent to one another. Advocates of fuel cells, particularly, like to cite the relative inefficiency of heat engines as compared to fuel cells, and to point out—correctly, in fact—that all heat engines are subject to what are known as Carnot limits after the French mathematician who developed the relevant formulae, whereas fuel cells are not bound by similar constraints. One finds uncritical repetition of these statements everywhere on the Web to the point where they virtually preclude discussion of the prospects of heat engines in the mid term.

The truth is that in practical terms heat engines can be made just as efficient as fuel cells. Moreover, they can be made much smaller and lighter per a given a power output. Furthermore, the operating life of well designed heat engines is incomparably greater than that of any fuel cell made to date. In addition, they can be manufactured far more easily and less expensively than fuel cells. And finally their mechanical, chemical, and thermodynamic attributes are better understood than is the case with fuel cells.

If one had to wager as to which technology could better address the energy problems of the present, the heat engine would be the better bet. In future posts I’ll explain why.