“ORCs are the unrivalled technical solution for generating electricity from low-medium temperature heat sources of limited capacity”
cit. Organic Rankine Cycle(ORC) Power Systems – E.Macchi, M.Astolfi
Traditionally, in large-scale power generation, steam cycles are the preferred solution for heat recovery and external combustion of solid and heavy fuels, while internal combustion engines are the usual choice for clean and standard liquid or gas fuels.
Still there is a large variety of energy sources, with limited temperature and/or thermal power available, for which gas and steam cycles are not a convenient choice, Organic Rankine Cycles have proven to be the most technically and economically valuablesolution available, offering higher performance, flexibility, and lower capital costs.
Organic Rankine Cycle (ORC) systems are used for power production from low to medium temperature heat sources in the range of 80 to 350 °C and for small-medium applications at any temperature level. This technology allows for exploitation of low-grade heat that otherwise would be wasted.
The working principle of an Organic Rankine Cycle power plant is similar to the most widely used process for power generation, the Clausius-Rankine Cycle. The main difference is the use of organic substances instead of water (steam) as working fluid. The organic working fluid has a lower boiling point and a higher vapour pressure than water and is therefore able to use low temperature heat sources to produce electricity. The organic fluid is chosen to best fit the heat source according to their differing thermodynamic properties, thus obtaining higher efficiencies of both cyle and expander.
The main components of an Organic Rankine Cycle power plant design are:
- 1. THE TURBINEit’s the key component of the entire ORC power plant, which determines the ORC system performance. It expands the working fluid producing mechanical energy that is converted into electricity by a generator coupled with the turbine shaft.
- 2. THE HEAT EXCHANGERSThe working fluid flows through the heat exchangers, extracting the heat from the heat source. Shell and tube heat exchangers are usually applied but they can vary geometry and configuration depending on the energy source and the total thermal input.
- 3. THE CONDENSERWith the direct air to fluid heat exchanger, the organic fluid is cooled and liquefied before entering the pump. The use of air eliminates the requirement for water to treatment and make up. It is possible to use also a water cooled condenser.
- 4. THE FEED PUMP Brings the organic fluid from the condensation pressure to the maximum pressure of the Organic Rankine Cycle. The pump is usually driven by an electric motor at variable rotating speed.
By converting thermal energy to electricity at low temperature, ORCs are suitable for a wide range of applications, with plant size between 100 kW ad 50 MWe including:
Low-medium enthalpy geothermal sources exploitation (90 – 260 °C)
Biomass
Power generation from waste heat in industrial processes
Power generation increasing efficiency of systems based on diesel and gas turbines and reciprocating engines
Organic Rankine cycle usually uses low boiling point organic working fluids, such as atmospheric ethane, n-Pentane and Isopentane, etc. the medium and low-temperature geothermal fluid is used to exchange heat with low boiling point organic working fluid to make the latter evaporate and generate steam with high pressure to drive steam turbine to do power generation.
The main components of an Organic Rankine Cycle power plant design are:
- The turbine
- The heat exchangers
- The condenser
- The feed pump
What is organic Rankine cycle? Organic Rankine Cycles (ORCs) are a technology suitable for the exploitation of many different energy sources and are particularly suitable for medium–low temperature heat sources and/or for small available thermal power. Organic Rankine cycle (ORC) development started in the 1850s and followed the development of steam engines.
How does the Rankine cycle work? The heat is supplied to a closed loop, which typically uses water as working fluid. The Rankine Cycle based on water provides approximately 85% of worldwide electricity production. The Organic Rankine Cycle’s principle is based on a turbogenerator working as a conventional steam turbine to transform thermal energy into mechanical energy.
When did the Organic Rankine cycle start?Organic Rankine cycle (ORC) development started in the 1850s and followed the development of steam engines.
What is the efficiency of a Rankine cycle? Organic Rankine cycles, in general, have low thermodynamic efficiency due to limited temperature differences between the heat source and heat sink streams. Therefore, the efficiency of the overall cycle is highly sensitive to the efficiency of the expansion machine.