Fig. 5.4 Temperature/entropy-diagram (7>-diagram) of a closed (left, (a)) and an open cycle (right, (b)) (the arrows refer to the direction the cycles performed)
- If the working medium is part of an "inexhaustible" reservoir (e.g. ambient air) and its final state is different from the initial state, the process is referred to as an "open cycle" (Fig. 5.4); yet, strictly speaking, such a process is also closed since the last state change takes place outside of the actual process, namely within the "inexhaustible" reservoir.
In the following such cycles are illustrated by means of temperature/entropy-diagrams. These representations offer the advantage, that both isothermal (i.e. constant temperatures) as well as isentropic (i.e. constant entropy) state changes can be represented as straight lines (Fig. 5.4 (a)) /5-2/.
- Within the Carnot cycle the entire exergy is extracted from the supplied heat so that its full working capacity becomes useful. This cycle consists of isentropic compression/decompression (i.e. performance of pressure change work) and isothermal heat supply and dissipation. The Carnot cycle is an ideal comparative process; however, mainly the isentropic compression/expansion cannot be put into practice (Fig. 5.5 (a)).
- The Ericson cycle represents the first technical approach to an ideal Carnot cycle; isobaric compression and expansion substitute isentropic compression/decompression. Within this cycle addition and evacuation of heat is supported by internal heat transmission (Fig. 5.5 (b)).
- The Stirling cycle is similar to the Ericson cycle. However, compression/decompression is isochore (i.e. density remains constant) (Fig. 5.5 (c)).
The Joule cycle is composed of isentropic compression, isobaric heat addition (combustion), isentropic expansion and isobaric heat dissipation (Fig. 5.5 (d)).
- The Clausius-Rankine cycle (steam power cycle/two phase cycle) makes use of the phase transformation of matters. Such phase transformations correspond to isothermal heat addition and large additions of specific volume. Their technical application is easy (isotropic compression/decompression, isothermal heat addition and dissipation). This is why such processes were first technically applied (Fig. 5.5 (e), (f)).
For current industrial applications Joule and Rankine cycles are most commonly applied.
- For the Joule cycle the working medium "ambient air" is aspirated and compressed prior to adding heat. Heat can either be added by caloric devices or internal combustion (e.g. by combustion of natural gas). For solar applications heat is transferred directly from the absorber to the working medium of the energy conversion process. The volumetric absorber itself has a very large surface to benefit both heat transfer and radiation absorption. Since pressurised air is used as working medium such an absorber must be of closed design. Indirect heat addition, for instance by means of a heat transfer medium, is disadvantageous since the working medium air only has a poor thermal conductivity and thus requires large surfaces for heat transmission.
- The Clausius-Rankine cycle, by contrast, requires a phase change medium to allow for isothermal heat addition. In most cases water is applied, but there are also processes using organic working media for low-temperature applications (so-called Organic Rankine Cycles (ORC)). At the beginning, the liquid working medium is highly pressurised and undergoes a phase change while heat is added. The now gaseous material is subsequently expanded, possibly after further heat has been added. Afterwards condensation is performed under low pressure while heat is dissipated.
All above-mentioned cycles have in common that heat is first applied to increase the volume flow of a gaseous working medium. Subsequently, during its expansion, this volume flow performs mechanical work in pressure engines, which can either be designed as oscillating machines of varying working volume (i.e. reciprocating engines) or as machines with stationary flow (i.e. turbo-machines or turbines). For large-scale power plants dealing with large volume flows almost exclusively turbo-engines are applied.
Turbines are referred to as turbo-engines which first transform the potential energy of a flowing working medium into kinetic energy and afterwards into mechanical energy of the rotating turbine shaft. The medium flows through the turbine either axially or radially, causing it to rotate. A stator whose blades form nozzles causes the working medium to first expand and at the same time accelerates the rotor. Inside the rotor coupled to the turbine shaft the kinetic energy of the working medium is subsequently converted into shaft torque. The combination of rotor and stator is referred to as turbine stage; for instance, in large turbines up to sixty subsequent stages are implemented. Inevitable friction, inconvertible kinetic energy at the turbine exit and so-called gap leakages are considered to measure the efficiency of a turbine; current steam turbines reach efficiencies above 40 %, while those of gas turbines even exceed 55 %.
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