Science & Tech.


The use of an organic fluid allows for the achievement of high isentropic efficiency in the turbine, which would not be possible for such a low power output, if steam were the process medium. Economic and technological constraints often dictate a small optimal power output, if the primary energy source is fully sustainable, i.e., biomass, solar radiation, geothermal heat, or heat recovered from industrial processes. In all these cases ORC technology is effective and cost-competitive.
Besides the high isentropic efficiency of the turbine, other advantages of ORC technology include:

  • High overall conversion efficiency. For the same maximum temperature, the efficiency of an ORC plant can be equal to the one reachable in large steam power plants;
  • Maximization of the overall conversion efficiency for any required power output and heat source temperature by selecting the optimal working fluid;
  • Simplicity of configuration, i.e., small number of components. High efficiency can be reached with just a pump, a once-through boiler capable of producing superheated vapor, a single- or double-stage turbine, a regenerator (optional), and a condenser;
  • Low mechanical stress in the turbine, i.e., high reliability. The use of a heavy fluid implies a small specific work per turbine stage, therefore a low fluid velocity and low peripheral rotor speed. For many working fluids the expansion in the turbine does not cause condensation, as opposed to steam, therefore also the problem of blade erosion is avoided.
  • Low mechanical stress in the boiler. Organic fluids have saturation pressures far lower than water, therefore boilers for organic fluids require much thinner pipe walls and also, very importantly, do not require attending personnel per current regulations.

The main constraint for the ORC is represented by the thermal stability of the available working fluids which imposes a maximum cycle temperature that cannot exceed 400 oC and is often kept to a lower value. A higher temperature limit, closer to the limit of metallic construction materials (500-600 oC), as in steam power plants, would be desirable.