Organic Rankine Cycle
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Energy Conversion. Vol. 8, pp. 85-90. Fergamon Pre~s, 1968. Printed in Great Britain
TheSupercriticalThermodynamicPowerCycle E, G, FEHERt
Thermodynamic power cycles most commonly used for closed cycle engines today are the Rankine Cycle and the recuperated Brayton Cycle. Both are characterized by two constant pressure and two isentropic processes. The Rankine Cycle operates mainly in the saturated region of its working fluid whereas the Brayton Cycle processes are located entirely in the superheat or gas region.
The simple Rankine Cycle is inherently efficient. Heat is added and rejected isothermally and therefore the ideal cycle can achieve over 90 per cent of Carnot efficiency between the same temperatures. Pressure rise in the cycle is accomplished by pumping a liquid, which is an efficient process requiring small energy input. The ratio of net work output to gross work in the cycle is large. Among the limitations of the cycle are the following:
(i) The temperature range of the cycle is severely limited by the nature of the working fluid. Adding superheat in an attempt to circumvent this will depart the cycle from isothermal heat addition. Increasing the temperature range without superheat leads to excessive moisture content in the turbines, resulting in blade erosion.
(ii) Simple recuperator cannot be employed to recover heat from the turbine exhaust.
(iii) Expansion ratio of the cycle is usually large, requiring in some cases more than 30 turbine stages,
The recuperated Brayton Cycle adds heat at constant pressure over a temperature range. The temperature level is independent of the pressure level. No blade erosion occurs in the turbine. The pressure ratio is low, therefore one or two turbine stages are adequate. A simple re- cuperator can recover much of the turbine exhaust heat. Some of the limitations of the cycle are:
(i) The compression process requires large energy input, therefore the net work to gross work ratio is small.
(ii) The cycle is very sensitive to compressor efficiency and pressure drop.
(iii) Heat transfer surfaces are large for pressure levels that are typical for current Brayton engines.
A thermodynamic power cycle has been devised which avoids most of the problems of these cycles and yet retains many of their advantages. This cycle operates entirely above the critical pressure of its working fluid; it is the Supercritical Cycle.
t Astropower Laboratory Missileand Space SystemsDivision, Douglas Aircraft Co., Inc., 2121 Campus Drive, Newport Beach, California.
2. Description of the Cycle
For the thermodynamic analysis of the Supercritical Cycle, the properties of its working fluid are represented in Figs. 1 and 2. A pure substance (such as water or carbon dioxide) is shown on a temperature-entropy diagram and on an enthalpy-entropy diagram. Also
Fig. 1. Temperature-entropy diagram for the supercritieal
Fig. 2. Enthalpy-entropy diagram for the supercritical cycle.
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