Efficiency of heat engines

This remarkable result shows that the efficiency of a Carnot engine is simply related to the ratio of the two absolute temperatures used in the cycle. In normal applications in a power plant, the cold temperature is around room temperature T = 300 K while the hot temperature in a power plant is around T = fiOO K, and thus has an efficiency of 0.5, or 50 percent. This is approximately the maximum efficiency of a typical power plant. The heated steam in a power plant is used to drive a turbine and some such arrangement is used in most heat engines. A Carnot engine operating between 600 K and 300 K must be inefficient, only approximately 50 percent of the heat being converted to work, or the second law of thermodynamics would be violated. The actual efficiency of heat engines must be lower than the Carnot efficiency because they use different thermodynamic cycles and the processes are not reversible. 

This equation was originally arrived at by the French Engineer Sadi Carnot in 1824 during his investigation on the efficiency of heat engines. In terms of the heat and the entropy changes in the system, the second law may be expressed as follows ... 

It should be remembered that the actual efficiencies of heat engines and fuel cells are substantially below their... 

From the discussion of heat engines, the second law of thermodynamics states that it is impossible to achieve heat, taken from a reservoir, and convert it into work without simultaneous delivery of heat from the higher temperature to the lower temperature (Lord Kelvin). It also states that some work should be converted to heat in order to make heat flow from a lower to a higher temperature (Principle of Clausius). These statements acknowledge that the efficiency of heat engines could never be 100% and that heat flow from high temperatures to low temperatures is not totally spontaneous. Simply, the second law states that natural processes occur spontaneously toward the direction in which less available work can be used. 

Note that the enthalpy of combustion is 283.0kjmol an unrelated fact. The product of a Carnot efficiency of, say, 50% and the combustion enthalpy is 143.5 kW smol (cf 534.5). The potential superiority of the fuel cell is evident. Referring the efficiency of heat engines to the fuel... 

We may stress at this point that the conception of the role played by irreversible processes developed here is quite different from that in classical thermodynamics. In the latter, irreversible changes appear only as undesirable effects which reduce the efficiency of heat engines and which one must attempt to eliminate. On the other hand thermodynamic coupling, enables us to predict results such as separations and syntheses, which would be quite impossible to derive in the absence of a consideration of irreversible changes. 

Stated as an abstraction and generalization of engineering observations on the efficiency of heat engines. We start the discussion by presenting a nonmathematical qualitative summary of the arguments on efficiency. Then we define entropy and state the second law. Section 13.5 applies the definition to calculate entropy changes and to predict spontaneity of processes. 

Gutkowics-Krusin, D., Procaccia, I., Ross, J. (1978) On the efficiency of rate processes. Power and efficiency of heat engines. /. Chem. Phys., Vol. 69, pp 3898-3906. 

We have said that entropy is a state function but we must justify this statement before proceeding. Traditionally this was done from a consideration of the efficiency of heat engines—a consideration which was a major preoccupation with the pioneers of thermodynamics. As chemists, we shall allow ourselves a short cut by considering only a perfect gas. 

A unique feature of heat engines is that some heat must be given off to the surroundings when they do work. With the piston in the up position, no further work can be done if we do not cool the cylinder back to T]. The cooling process removes some of the thermal energy that could otherwise be converted to work and thereby places a limit on the efficiency of heat engines. 

Using the equation in the Chemistiy in Action entitled The Efficiency of Heat Engines in Chapter 18 ... 

The efficiency of an OTEC system in corrverting heat stored in the warm surface water of the tropical oceans into mechanical work has a theoretical limif called the Carnot efficiency (named after Sadi Camof a French engineer who first defined the thermodyrrarrric restrictions on the efficiency of heat engines), where... 

French physicist, who first worked as a miiitary engineer. He then turned to scientific research and in 1824 published his analysis of the efficiency of heat engines. The key to this analysis is the thermodynamic Carnot cycle. He died at an early age of cholera. 

In 1865, building on the work of Kelvin, Sadi Carnot, and others who studied the efficiency of heat engines, Rudolf Clausius postulated that the entropy change of a system undergoing an infinitesimal reversible process was the heat absorbed (absolute temperature T (in kelvins) of the system ... 

We can use the second law of thermodynamics to establish precise theoretical limits on the efficiency of heat engines. 

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