Organic Rankine Cycle

The Organic Rankine Cycle ( ORC for short ) is a method of operation of steam turbines with a working fluid other than water vapor. The name of the method goes back to William John Macquorn Rankine, a Scottish- British physicist and engineer. As the working medium of organic liquids can be used with a low evaporation temperature.

The method is especially used when the available temperature difference between the heat source and sink is too low for the operation of a steam turbine driven by water. This is the case especially in power generation using geothermal energy, combined heat and power and in solar power plants and ocean thermal power plants. The expansion machines (turbine, screw expander, steam engine / Hubkolbenexpander ) are typically operated with silicone oil, refrigerant or combustible gas.

Working media

Starting from the Ts diagram of three different fluid classes are distinguished by the form of the saturated vapor curve:

• The saturated steam curve "dry" media is increasing; in the majority are high 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; this includes R11 and R12,

Isentropic and " dry" media promise a number of thermodynamic advantages in their use.

Possible working fluids are:

Temperature ( 1 atm )

Heat ( 1 atm )

Saturated steam curve

At about

A further improvement in efficiency is possible by the use of mixtures. Carried out in subcritical curves both the evaporation and the condensation not isothermal; the cooling curve of the heat carrier can be followed with much lower temperature differences; so that the irreversibilities reduce in heat transfer.

Recently, synthetic working fluids are being developed for the ORC process. These are adjusted in their material properties to the specific temperature and pressure characteristics of the ORC process. Such a new synthetic working medium silicon base with the label GL160 is free especially environmentally friendly chlorine and fluorine, and for this reason. Synthetic media work higher thermodynamic efficiencies are achieved than would be possible with bulk chemicals that are accidentally fitted into existing thermodynamic gradient.

Functional Description

The Organic Rankine Cycle is like - based on the individual components - the classical Clausius- Rankine cycle. The main differences are in the process parameters pressure and temperature - both are well below the values ​​that prevail in steam power plants - and in the deviation of the evaporation and the condensation from the isothermal curve.

The selection criteria for the most appropriate working fluid resulting 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 is for the actual heat source far greater importance than in conventional thermal power plants.

Basically, many of the developed process optimization method of the classic steam process can also be transferred to the ORC process. Some approaches, such as reheat, bring but because of the other thermodynamic properties of the working media only limited or no benefits. Others, such as the expansion of supercritical process ( Critical Point ), are hardly realistic to implement with water. The use of a recuperator is meaningful only with "dry" media.

Through the use of organic working fluid, however, occur several new technical issues to the fore. Turbines are usually special turbines, as the working medium strong water differs ( molar mass, lower specific heat capacity), the instruments of labor are partly aggressive, so that the surfaces of the turbines and the heat exchanger coated or must otherwise be protected against corrosion, the seal of the circuits is more complex than in water, in some cases, be difficult to realize.

Recently, steam engines / Hubkolbenexpander be developed and used for ORC processes. These offer advantages over turbines in the conventional relative power plants small performance classes as well as the media used some advantages. These machines are quasi modern steam engines, as they work on the same basic principle and thus represent the "Comeback of the steam engine " dar.