Carnot cycle and

The COP in real refrigeratiou cycles is always less than for the ideal (Carnot) cycle and there is constant effort to achieve this ideal value. 

A fascinating point, especially to physical chemists, is the potential theoretical efficiency of fuel cells. Conventional combustion machines principally transfer energy from hot parts to cold parts of the machine and, thus, convert some of the energy to mechanical work. The theoretical efficiency is given by the so-called Carnot cycle and depends strongly on the temperature difference, see Fig. 13.3. In fuel cells, the maximum efficiency is given by the relation of the useable free reaction enthalpy G to the enthalpy H (AG = AH - T AS). For hydrogen-fuelled cells the reaction takes place as shown in Eq. (13.1a). With A//R = 241.8 kJ/mol and AGr = 228.5 under standard conditions (0 °C andp = 100 kPa) there is a theoretical efficiency of 95%. If the reaction results in condensed H20, the thermodynamic values are A//R = 285.8 kJ/ mol and AGR = 237.1 and the efficiency can then be calculated as 83%. 

A reversible isothermal heat-transfer process between the Carnot cycle and its surrounding thermal reservoirs is impossible to achieve... 

An absolute scale of temperature can be designed by reference to the Second Law of Thermodynamics, viz. the thermodynamic temperature scale, and is independent of any material property. This is based on the Carnot cycle and defines a temperature ratio as ... 

As an example, if the hot temperature is 1273 K (1000 °C) and the cold temperature 373 K (100 °C) then the efficiency is approximately 70%. In practice the operation of a real engine does not follow the Carnot cycle and the efficiency is considerably lower. For a medium sized motor car with an internal combustion engine the fuel efficiency is about 12%, much of the wasted 88% demanding water cooling. There are continuous improvements made in petrol and diesel engine technologies and in the fuels and projections suggest that thermal efficiencies a little over 50% will eventually be achieved. 

Cryogenic facilities are needed to support low-temperature operations. A major consumer is cold water of 5 °C for acrylonitrile absorption, with a duty of 7.7 MW. The power W needed to extract the heat duty Qj can be roughly estimated by assuming a reversible Carnot cycle and global efficiency of 0.6, by using the relation ... 

The conventional generation of electrical energy from a fuel requires the use of a heat engine which converts thermal energy to mechanical energy. All heat engines operate by the Carnot cycle, and their maximum efficiency is about 40-50% (for the modern gas-fired power stations, the efficiency is about 55%). 

In the table, the symbol AC is the output of the Carnot cycle. At PoTq there is no Carnot cycle and its output is 0 as in the third column. At high temperature 1300 K, there must be a Carnot cycle of significant output (fifth and seventh columns, values marked ), different for the two reactions, perfect CO and imperfect pressure-sensitive H2O. 

The classic definition of temperature is based upon thermodynamics. Any suitable relation, based on the laws of thermodynamics, can be used to describe temperature on a thermodynamic scale. The two most commonly used relations are the efficiency of the reversible engine (the Carnot cycle) and the intensity of blackbody radiation (Planck s Law) expressed mathematically by... 

The cycles O, O and OO are the Carnot Cycles and thus, from their definition and construction they are, imaginatively4 in principle, the infinite cycles in each of them the following criterion of an infinite cycle (see [14]) it is valid inevitably,... 

Let ns nse an ideal gas in a Carnot cycle and find the efficiency of the cycle by using ideal-gas properties in ennmerating the changes in the four steps of the cycle. Let us designate the intial state of step 1 with the subscript A, the initial state of step 2 with B, and so on. The high temperature at step 1, which is i on the thermodynamic temperatnre scale, will be Tj on the ideal-gas temperature scale. The low temperature of step 3 will be T2, corresponding to 02. The work and heat terms of a step will be designated with subscripts 1, 2, 3, or 4. 

The father of thermodynamics is Sadi Carnot. He wrote Reflections on the Motive Power of Fire in 1824. This was a discourse on heat, power, and engine efficiency. The Carnot engine, Carnot cycle, and Carnot equations are named after him. 

The plan of the remaining sections of this chapter is as follows. In Sec. 4.3, a h)q)o-thetical device called a Carnot engine is introduced and used to prove that the two physical statements of the second law (the Clausius statement and the Kelvin-Planck statement) are equivalent, in the sense that if one is true, so is the other. An expression is also derived for the efficiency of a Carnol engine for Ihe purpose of defining thermodynamic temperature. Section 4.4 combines Carnot cycles and the Kelvin-Planck statement to derive the existence... 

The Otto cycle is essentially the cycle describing the internal-combustion automobile engine. This is a four-stroke cycle, in contrast to the simpler two-stroke Carnot cycle and the various others, such as the Stirling and Brayton cycles, that operate on a single oscillation of the piston. The Otto cycle consists of an intake expansion, a compression, an expansion resulting from ignition... 

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