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Organic Rankine Cycle (ORC)

The Organic Rankine Cycle (ORC) works using a thermodynamics cycle, transforming heat into work or energy (thermo meaning heat). Today it is widely used  in biomass or geothermal applications popping up in a backyard or farm near you! According to Wikipedia, there are over 250 power plants worldwide (and counting) using this method.

75-kilowatt Organic Rankine Cycle power plant originally used at Kuopio (Finland)

“ORC power plant” by Tommi Nummelin – 75- kW Organic Rankine Cycle power plant turbogenerator originally used at Kuopio (Finland) ORC plant during 1986 – 1990. The turbo generator is designed by Lappeenranta University of Technology. Its nominal speed is 27 000 rpm. The generator is fully hermetic; there are only flanged connections and no rotating shaft seals. Bearings are lubricated by the liquid process fluid.

The ORC is named after William John Macquorn Rankine (July 5, 1820 – December 24, 1872). Rankine was a Scottish engineer and physicist (among many other interests) who was a founding contributor to the sciences of thermodynamics. His impact was world wide, proof being his manuals and hundreds of papers and notes on sciences being used for many decades after their publications. The first prototype of an Organic Rankine Cycle generator, however, was developed and exhibited in 1961 by solar engineers Harry Zvi Tabor and Lucien Bronicki.

ORC systems work by vaporizing an organic fluid with a molecular mass higher than water and a liquid-vapor phase change (boiling point) that’s at a lower temperature then water-steam. An analogy can be made with a turbogenerator creating water steam that turns a turbine that turns the steam into rotational movement then into electrical energy though an electric generator.

Using an organic fluid with above described characteristics – as apposed to water – has a few different positive benefits. For starters it creates a slower rotation of the turbine, which leads to less wear on any moving parts of the ORC. It also works at a lower pressure which leads to less erosion too!

Full ORC Diagram - source (http://www.bios-bioenergy.at/en/electricity-from-biomass/orc-process.html)

Full ORC Diagram – source (bios-bioenergy.at)

Organic Sources and Usages

What kind of sources can the ORC use? Given the characteristics of the organic fluid given above, sources can be from biomass combustion, industrial waste heat capture, geothermal heat, solar ponds

 

  • Waste heat recovery, as found in industrial or farming applications such as power plants, organic fermentation, ovens, furnaces, condensations, vehicles, compressors, etc.
  • Biomass power plants are a great investment because of their low erosion, low input fuel requirements. For example, if a farm has a lot of dung it’s basically a free fuel source.
  • Geothermal plants can work with a geothermal heat source of 50°C to 350°C, but to maximize efficiency of this technology one should hope for a heat source of greater then 100°C, but also depends on the ambient temperature (heat sink temperature).
  • Solar ponds or heaters can very effectively heat up the organic thermal oil which virtually leads to near zero maintenance costs and running costs (no fuel required), and can essentially last for a very long time due to the benefits of the ORC turbine listed above.

 

The Organic Rankine Cycle

  1. A heat source heats thermal oil to a high temperature, typically about 300°C, in a closed circuit;
  2. The hot thermal oil is drawn to and from the ORC module in closed circuit. In the ORC it evaporates (isentropic) the organic working fluid of the ORC in a suitable heat exchanger system (pre-heater and evaporator);
  3. Organic vapour expands in the turbine, producing mechanical energy, further transformed into electric energy through a generator;
  4. The organic vapour is then cooled by a fluid in a closed circuit and condensed (isobaric) back to thermal oil. The water warms up at about 80 – 90°C and it is used for different applications requiring heat;
  5. The condensed organic fluid (thermal oil) is pumped back into the regenerator to close the circuit and restart the cycle.
The Organic Rankine Cycle (ORC) Diagram. source (forcefieldenerty.com)

The Organic Rankine Cycle (ORC) Diagram. source (forcefieldenerty.com)

Efficiency

As always, one hopes to have 100% efficiency, but as one would expect working in thermodynamics, much is lost to heat. Turboden ORC claims about 20% is transformed into electrical energy and about 78% is lost due to heat (the final 2% lost in thermal isolation, radiance, and generator losses).

Another cause for loss of efficiency in the transformation of heat into energy is the path that the hot thermal oil takes in the process, through heat exchangers and into/out of pools.

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