Siemens recently introduced an Organic Rankine Cycle (ORC) module for industrial plants or power stations for the conversion of waste heat to electricity. In an ORC system, the working medium drives a turbine, and then cools and reverts to its initial liquid state. Because waste heat produced in industrial plants or power stations often does not have enough energy to drive a turbine with steam, the Siemens ORC module system employs silicone oils, which have a lower enthalpy of vaporization than water. The Siemens system can generate electricity from waste heat of only about 300 ˚C.
|The new Siemens Organic Rankine Cycle (ORC) module. Click to enlarge.|
Conventional power plants usually convert only about 50% of fuel energy into electricity, and most of the waste heat is released through a cooling tower, noted Siemens. A great deal of waste heat is produced in other industries as well, such as chemicals, glass-making, paper-making and steel production. This waste heat is often used to pre-heat other substances or, if that is not possible and if the waste heat is not hot enough to drive a conventional steam turbine, the valuable energy is wasted.
The Organic Rankine Cycle (ORC) module is derived from the Rankine Cycle, a closed loop used in steam-driven heat engines. In this case, however, organic silicone oils are used as the work medium.
The oil absorbs the waste heat energy by way of a heat exchanger. It turns to vapor and drives a turbine before being completely liquefied again in a condenser and pumped back to the vaporizer. The heat released in the cooling process is also recovered to pre-heat the oil.
The ORC module has an output of up to two megawatts; variants with higher output ratings are expected to come on line in the medium term. The heart of this module is the proven SST-060 steam turbine, which has already been installed successfully more than 850 times. The silicone oil employed in this module is chlorine-free and non-toxic.