Organic Rankine Cycle

  The Turboden turbogenerator operates according to the Organic Rankine Cycle (ORC) concept. The Organic Rankine Cycle (ORC) is similar to the cycle of a conventional steam turbine, except for the fluid that drives the turbine, which is a high molecular mass organic fluid. The selected working fluids allow to exploit efficiently low temperature heat sources to produce electricity in a wide range of power outputs (from few kW up to 3 MW electric power per unit).

The Cycle
The organic working fluid is vaporised by application of a heat source in the evaporator. The organic fluid vapour expands in the turbine and is then condensed using a flow of water in a shell-and-tube heat exchanger (alternatively, ambient air can be used for cooling). The condensate is pumped back to the evaporator thus closing the thermodynamic cycle. Heating and cooling sources are not directly in contact with the working fluid nor with the turbine. For high temperature applications (e.g. Combined Heat and Power biomass-powered plants, high temperature thermal oil is used as a heat carrier and a a regenerator is added, to further improve the cycle performance.

Advantages
Key technical benefits include:
a. high cycle efficiency
b.  very high turbine efficiency (up to 85 percent)
c. low mechanical stress of the turbine, due to the low peripheral speed
d. low RPM of the turbine allowing the direct drive of the electric generator without reduction gear
e.  no erosion of blades, due to the absence of moisture in the vapour nozzles
f.  long life
g.  no operator required

The system also has practical advantages, such as simple start-stop procedures, quiet operation, minimum maintenance requirements, good part load performance.

Applications
Typical applications are:
a. low enthalpy geothermal plants, up to 3 MW electric per unit
b.  combined Heat and Power (CHP) biomass powered plants, in the range 400 to 1500 kW electric
c. heat recovery applications, in the range 400 to 1500 kW electric
d. solar applications

Modularity
The ORC turbo-generator (up to about 800 kWel) consists of a single skid-mounted assembly, containing all the equipment required for the turbo-generator to be operated (i.e. heat exchangers, piping, working fluid feed pump, turbine, electric generator, control and switch-gear). Larger units are composed of multiple modules, pre-assembled at the factory. Hence the units are easy to transport and to install, and they are easy to interface with the hot and cold sources on site.

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