Heat engines Subject

It is commonly expressed that a fuel cell is more efficient than a heat engine because it is not subject to Carnot Cycle limitations, or a fuel cell is more efficient because it is not subject to the second law of thermodynamics. These statements are misleading. A more suitable statement for... 

The eyewitnesses have tried to persuade us that the crematoria ovens of Auschwitz and Birkenau were satanic contraptions operating above and beyond the realm of physical laws,6 not ordinary cremation facilities subject to the same laws of chemistry, physics and heat engineering as all other such installations. Historians have chosen to trust blindly in these witnesses, and in the process have let themselves get carried away into making entirely erroneous claims.7... 

Unlike batteries, fuel cells do not store chemical energy. Reactants must be constantly resupplied, and products must be constantly removed from a fuel cell. In this respect, a fuel cell resembles an engine more than it does a battery. However, the fuel cell does not operate like a heat engine and therefore is not subject to the same kind of thermodynamic limitations in energy conversion (see the Chemistry in Action essay on p. 736). 

Initial concepts of temperature came from the physical sensation of the relative hotness or coldness of bodies. This sensation of warmth or cold is so subjective relative to our immediate prior exposure that it is difficult to use for anything but simple qualitative comparison. The need to assign a quantitative value to temperature leads to the definition of a temperature scale. The concept of fixed points of temperature arises from the observation that there exist some systems in nature that always exhibit the same temperatures. The scientific or thermodynamic definition of temperature comes from Kelvin, who defined the ratio of the thermodynamic or absolute temperatures of two systems as being equal to the ratio of the heat added to the heat rejected for a reversible heat engine operated between the systems. This unique temperature scale requires only one fixed point, the triple point of water, for its definition. 

To ensure energy saving, the position of unit operations with respect to Pinch must respect the rules of the Appropriate Placement. For example, a heat engine has to be placed either above or below the Pinch, but not across it. Distillation columns should not be placed across the Pinch. Contrary, a heat pump is optimally placed across the Pinch. This subject will be discussed in more detail in the Chapter 11. 

This means that the electrical work that is produced is only slightly less than the decrease in enthalpy in the reaction. Note that if we simply let the reaction occur without producing work, the quantity of heat, — AH, would be released. This could be used to heat a boiler which in turn could run a turbine. But this heat engine is subject to the Carnot restriction the electrical work that could be produced by a generator operated by a turbine would be... 

In 1894, the German physical chemist Wilhelm Ostwald came forward in the Zeitschriftfur Elektrochemie with the proposal to build devices for a direct oxidation of natural kinds of fuel with air oxygen by an electrochemical mechanism without heat production (the so-called cold combustion of fuels) (Fig. 17.1). He wrote In the future, the production of electrical energy will be electrochemical, and not subject to the limitations of the second law of thermodynamics. The conversion efficiency thus will be higher than in heat engines. This paper of Ostwald was basic and marked the beginning of huge research into fuel cells. 

In my experience, such arguments have even less appeal to geochemists. This is perhaps unfortunate, because approaching the subject from various points of view certainly aids understanding, and in fact the heat engine approach is very useful in the derivation of Equation (4.3), and also the kelvin temperature scale. 

The objective of this part of the program is to identify mechanisms of failure in structural ceramics subjected to mechanical, loads at elevated test temperatures. Of particular interest is the damage that accumulates in structural ceramics as a consequence of high temperature exposure to stresses normally present in heat engines. 

Fig. 15.2. For each mole of reactants converted into products, a certain amount of heat is converted into work. More heat is converted into work in the system which is subject to external pressure variations and which produces pressure-volume work [mode (b)]. The figure is drawn for an endothermic reaction for which the net quantity of heat flowing from the reservoir to the system (reactor tank and heat engine) is larger than...

Now, this heat energy could be converted into work, and hence electrical energy, using a heat engine and generator. However, even in a reversible system this conversion would be subject to the Carnot limit . The upper temperature is Tp, the temperature of the fuel cell, as this is the temperature at which the gases leave the cell. The lower temperature is the ambient temperature Ta. So, the extra electrical energy that could be available in a reversible system is... 

---