Organic Rankine Cycle

from Wikipedia, the free encyclopedia
ORC with regenerator

The Organic Rankine Cycle ( ORC abbreviation ) is a method for operating steam turbines with a working medium other than water vapor . The name of the process goes back to William John Macquorn Rankine , a Scottish- British physicist and engineer in the 19th century. Organic liquids with a low evaporation temperature are used as the working medium .

The process is mainly used when the available temperature gradient between the heat source and the heat sink is too low for the operation of a steam-driven turbine . This is particularly the case with electricity generation with the help of geothermal energy , combined heat and power, as well as solar and marine thermal power plants . The expansion machines (turbine, screw expanders , steam engine / reciprocating piston expanders) are typically operated with silicone oil , refrigerant or flammable gas.

Working media

Based on the Ts diagram , three different fluid classes are distinguished according to the shape of the saturated steam curve :

  • The saturated steam curve of "dry" media is increasing; the majority are higher molecular weight substances such as R113
  • "Wet" media such as water have a falling saturated steam curve
  • " Isentropic " media have an almost vertical saturated steam curve; these include R11 and R12

"Dry" and isentropic media promise a number of thermodynamic advantages when used.

Possible working media are:

medium molar mass Critical point Boiling temperature
at normal pressure 		Heat of evaporation
at normal pressure 		Slope of

Saturated steam curve

decomposition

at approx.

Ammonia (NH 3 ) 17th 405.3 K 11.33 MPa 239.7 K 1347 kJ / kg negative 750 K
Ethanol (C 2 H 5 OH) 46.07 516.25K 6.38 MPa 351.15K 845 kJ / kg negative
water 18th 647.0 K 22.06 MPa 373.0 K 2256 kJ / kg negative .
n -butane C 4 H 10 58.1 425.2 K 3.80 MPa 272.6 K 383.8 kJ / kg . .
n -pentane C 5 H 12 72.2 469.8 K 3.37 MPa 309.2K 357.2 kJ / kg . .
C 6 H 6 ( benzene ) 78.14 562.2 K 4.90 MPa 353.0 K 438.7 kJ / kg positive 600 K
C 7 H 8 ( toluene ) 92.1 591.8 K 4.10 MPa 383.6 K 362.5 kJ / kg positive .
R134a (HFC-134a) 102 374.2 K 4.06 MPa 248.0 K 215.5 kJ / kg Isentrop 450 K
C 8 H 10 106.1 616.2 K 3.50 MPa 411.0 K 339.9 kJ / kg positive .
R12 121 385.0 K 4.13 MPa 243.2 K 166.1 kJ / kg Isentrop 450 K
HFC-245fa 134.1 430.7 K 3.64 MPa 288.4 K 208.5 kJ / kg . 520 K
HFC-245 approx 134.1 451.6 K 3.86 MPa 298.2 K 217.8 kJ / kg . .
R11 (CFC-11) 137 471.0 K 4.41 MPa 296.2 K 178.8 kJ / kg Isentrop 420 K
HFE-245fa 150 444.0 K 3.73 MPa . . . .
HFC-236fa 152 403.8 k 3.18 MPa 272.0 K 168.8 kJ / kg . .
R123 152.9 456.9 K 3.70 MPa 301.0 K 171.5 kJ / kg positive .
CFC-114 170.9 418.9 K 3.26 MPa 276.7 K 136.2 kJ / kg . .
R113 187 487.3 K 3.41 MPa 320.4 K 143.9 kJ / kg positive 450 K
n -perfluoropentane C 5 F 12 288 420.6 K 2.05 MPa 302.4 K 87.8 kJ / kg . .

A further improvement in efficiency is possible through the use of mixtures. In subcritical processes, both evaporation and condensation do not take place isothermally ; the cooling curve of the heat transfer medium can be followed with significantly lower temperature differences ; this reduces the irreversibility of heat transfer.

Synthetic working media have recently been developed for the ORC process. The material properties of these are adapted to the special temperature and pressure properties of the ORC cycle. Such a new synthetic working medium based on silicone with the designation GL160 is free of chlorine and fluorine and therefore particularly environmentally friendly. With synthetic working media, higher thermodynamic efficiencies are achieved than would be possible with bulk chemicals that are randomly fitted into existing thermodynamic gradients.

Function description

ORC flow diagram.JPG
Evaporator of the geothermal power plant in Landau . With hot deep water, isopentane is preheated at the bottom and evaporated at the top in order to then operate a turbine . The evaporator is practically a heat exchanger in which the heat from the water is transferred to the pentane .
Turbine generator set of the Landau geothermal power plant . In the middle the turbine. The isopentane vapor that the evaporator has generated in the background flows in from above. On the right in the white tin box is the generator. On the left, the steam flows up to the dry cooler, where it is cooled and liquefied again.

The Organic Rankine Cycle is similar - in terms of the individual components - to the classic Clausius-Rankine cycle . The main differences lie in the process parameters pressure and temperature - both are far below the values ​​found in steam power plants - and in the deviation of the evaporation and condensation from the isothermal curve.

The selection criteria for a suitable working medium result from the temperature and the cooling curve of the available heat source. Due to the strong influence of thermodynamic losses on the overall efficiency at low process temperatures, the selection of the optimized process for the specific heat source is much more important than with conventional thermal power plants.

In principle, many of the traditional steam processes developed for process optimization can also be transferred to the ORC process. However, some approaches, such as reheating , bring only limited or no advantages at all because of the other thermodynamic properties of the working media. Others, such as expansion as a supercritical process ( critical point ), can hardly be realistically implemented with water. The use of a recuperator also only makes sense with "dry" media.

However, with the use of organic working media, various new technical issues come to the fore. Turbines are mostly special turbines because the working medium differs greatly from water ( molar mass , lower specific heat capacity), the working media are sometimes aggressive, so that the surfaces of the turbines and the heat exchanger have to be coated or otherwise protected against corrosion, the sealing of the circuits is more complex than with water, in some cases difficult to implement.

Recently, steam engines / reciprocating expanders have been developed and used for ORC processes. Compared to turbines, these offer a number of advantages in relation to conventional power plants, as well as in terms of the media used.

See also

Web links

Individual evidence

  1. ↑ Information on the ORC process. (PDF; 148 kB) uni-bayreuth.de
  2. ↑ ORC technology. GMK, accessed December 18, 2015 .
  3. ↑ Silke Köhler, Ali Saadat: Possibilities and perspectives of geothermal power generation . STR00 / 23, Geothermal Report 00-1, GeoForschungsZentrum Potsdam
  4. ↑ K. Gawlik, V. Hassani: Advanced binary cycles: optimum working fluids . 20-23. September 1998. Geothermal Resources Council 1998 Annual Meeting, San Diego, California.
  5. ↑ Paola Bombarda, Ennio Macchi: Optimum cycles for geothermal power plants . Proceedings World Geothermal Congress 2000. Kyushu - Tohoku, Japan, May 28 - June 10, 2000.