The Heat Engine

Fundamentally, there are two possible ways to integrate a heat engine exhaust. In Fig. 6.31 the process is represented as a heat sink and heat source separated hy the pinch. Integration of the heat engine across the pinch as shown in Fig. 6.31a is coimterproductive. The process still requires QHmm, and the heat engine performs no... 

Now let us take a closer look at the two most commonly used heat engines (steam and gas turbines) to see whether they achieve this efficiency in practice. To make a quantitative assessment of any combined heat and power scheme, the grand composite curve should be used and the heat engine exhaust treated like any other utility. 

Carnot s cycle A hypothetical scheme for an ideal heat machine. Shows that the maximum efficiency for the conversion of heat into work depends only on the two temperatures between which the heat engine works, and not at all on the nature of the substance employed. 

The maximum possible efficiency at which a heat engine can work is defined by the Carnot efficiency equation E = (T2—Tl)/T2, where E is the efficiency of the heat engine, T1 is the temperature of the cold... 

The purpose of a heat engine is to remove heat Q, from a thermal reservoir at a higher absolute temperature T, extract useful work W and reject heat to a second thermal reservoir at a lower absolute temperature T. The device used to obtain the useful work is the heat engine. 

The laboratory area had no ventilation, and the system used elsewhere in the building was unable to handle the extra load. As a result, a separate system had to be installed. At the laboratory operator s insistence, this was to be a system -with no recirculation of ah . There was considerable resistance from management due to the higher cost for both installation and operation. To make matters more difficult, the heating engineer had never seen a need for such a system on previous jobs. One argument finally settled the situation. It was pointed out that fragrance evaluations would often have to be performed as part of product evaluation. This would be difficult if much of the air were recirculated. 

Mechanical Work. All cells exhibit motile and contractile properties. The remarkable thing about these activities of cells is that they are based on the direct coupling of chemical to mechanical action, in contrast to the heat engines that we have developed to perform our work for us. The mechanisms by which this coupling of chemical to mechanical processes takes place is not well understood, but the hydrolysis of adenosine triphosphate is known to be an important part of the molecular pathway. Although thermodynamic studies cannot provide information about the molecular steps involved, any mechanism that is proposed must be consistent with thermodynamic data [4]. 

Assume that the working fluid flows through the heat engine in a steady-state fashion. The rates of heat rejection and addition of the heat engine are... 

The power output (P) of the heat engine according to the first law of thermodynamics is... 

The second law of thermodynamics requires that gdotn/Tw = fidotL/Tc The efficiency (ly) of the heat engine is i =p/edotH... 

The power versus efficiency characteristics of the endoreversible Carnot heat engine is a parabolic curve. The endoreversible heat engine is a simple model, which considers the external heat-transfer irreversibility between the heat engine and its surrounding heat reservoirs only. 

By taking into account the rate of heat transfer associated with the endoreversible cycle, the upper bound of the power output of the cycle can be found. This bound provides a practical basis for a real power plant design. The industrial view is that the heat engine efficiency is secondary to the power output in power plants whose worth is constrained by economic considerations. 

Determine the maximum power output of the cycle. Find the heat-transfer added, heat transfer removed, heat transfer surface area of the low-temperature side heat exchanger between the heat engine and the heat sink, and efficiency of the cycle at the maximum power output condition. 

The system efficiency r)syst of a hydrogen fuelled combined fuel cell-heat cycle is plotted over the cell temperature lie in Figure 2.14 on the right side. The exergetic efficiency of the fuel cell fc is varied between 0.7 and 1 and the exergetic efficiency of the heat engine he is constant at 0.7. 

Fig. 2.17 The influence of the heat engine design on the system efficiency )Syst of SOFC-heat engine hybrid cycles.

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