Science & Tech.


ORC power systems are useful for the conversion into mechanical or electrical power of several, mainly renewable, energy sources, like:

  • geothermal reservoirs
  • biomass combustion or gasification
  • concentrated solar radiation
  • industrial waste heat
  • waste heat from reciprocating engines and gas turbines

Geothermal energy is naturally present in the subsoil of the Earth, where the temperature increases on average 3.3 °C every 100 meters of depth. The energy is exploited by drilling wells into the subsurface – typically a few kilometers – and pumping the steam or hot brine to the surface. Sometimes the steam can be directly expanded into turbines, but if the steam is not clean enough, or if the geothermal fluid is pressurized water, the thermal energy present in the fluid can be converted into power by an ORC turbine, which can be combined with the production of heat used for district heating. Geothermal heat sources are typically available at low temperatures (100 – 200 °C).

Biomass is widely available from various agricultural and industrial processes, for example in the form of sawdust, bark, straw, bio-sludge, etc. It is most efficiently utilized in small, decentralized combined heat and power (CHP) plants. In biomass CHP plants the ORC system produces electricity and hot water from the thermal energy of the flue gases coming from the combustion of biomass. The hot water is used to fulfill local heat demand of, for example, wood drying processes or district heating. The maximum power output of biomass cogeneration plants is usually in the range of 6 – 10 MW thermal and 1 – 2 MW electrical.

Concentrating solar power is a technology whereby the radiation of the sun is concentrated by a solar collector and transferred to a fluid at high temperature. The thermal energy is converted into electricity in an efficient thermodynamic cycle. Two possibilities for the design of a solar ORC power plants in terms of the maximum temperature of the working fluid can be distinguished. Medium temperatures (> 250 °C)  entails a high conversion efficiency but an expensive engine and more costly collectors. Low temperature (< 150 °C) conversely leads to low efficiency with comparatively low-cost components.

In many industrial manufacturing processes significant amounts of medium to low- and medium-grade thermal energy (150 – 350 °C) is rejected to the atmosphere. The interest in industrial waste heat recovery increased in recent years due to high prices for energy and the need to reduce emissions. Potential waste heat sources for recovery with ORC power plants are found in the energy-intensive cement, glass, steel and oil & gas industry as well as in refineries and incineration plants. Currently installed ORC heat recovery systems have a power range of 1.5 – 4 MW.