The Rankine Cycle

The Rankine Cycle, which approximates to the most efficient theoretical process (the Carnot Vapour Cycle), is the ideal cycle for vapour power plants and is still the design basis of most new installations.  A working fluid, almost always water, is vaporised and superheated by an external heat source (combustion of coal or gas, or through reaction of nuclear fuel) before entering a multi-stage turbine that extracts the velocity and pressure energy from the gas.  This energy is converted into rotational energy by action of the turbine blades, which is then transferred to a generator shaft in order to convert the energy into electricity.

Axial turbines are the predominant type used in power generation, and they spin at very high speeds (in excess of 30,000rpm) due to the large mass flow rates of steam in the system. Energy extraction of any turbine is governed primarily by the difference in the inlet and outlet pressures, and the larger the difference between the two, the higher the energy extracted.  However, the quality of the vapour decreases as more and more energy is removed through the stages of the turbine, which in turn causes vapour to condense into droplets.  If droplets are allowed to form within a turbine, they impinge upon the blades at an enormous relative velocity and result in erosion of the turbine material.  For this reason, steam is superheated as much as possible before the turbine inlet, and the outlet pressure from the turbine is raised to a point where the gas exits the turbine at a near-saturated condition.  In general, the high pressure steam is re-heated between turbine stages to keep it dry and boost the energy output of the machine.  Once the vapour leaves the turbine, it is condensed and sub-cooled by heat rejection to atmosphere before being pumped to the boiler at high pressure.

The Organic Rankine Cycle (ORC) is identical to the process described above, except that an organic working fluid is used in the place of water.  Organic working fluids used in such cycles have varied physical and chemical properties, but it is their low boiling points (lower than that of water) that make them suitable for use with low temperature heat sources.