The Organic Rankine Cycle, usually shortened to ORC, is a closed thermodynamic process that converts low, medium or high grade heat into mechanical power and, through a generator, into electricity. Instead of water it employs an organic working fluid whose boiling point lies below that of water at the same pressure. This simple substitution allows the cycle to stay in the vapour phase at temperatures as low as 70 °C, a region where a classical steam cycle would operate in vacuum and suffer from huge volume flow losses. Because the fluid is organic, the molecular mass is higher than that of water, so the turbine can run at moderate tip speeds and reach good aerodynamic efficiency even when the enthalpy drop per stage is small. Taken together, these features make the ORC a practical power block for heat sources that are considered too small, too cool or too intermittent for a conventional steam plant.
Table of Contents
Historical Background
The theoretical roots reach back to William John Macquorn Rankine who published the idealised vapour power cycle in 1859. The “organic” twist emerged almost a century later when, during the 1950s, the Israeli engineer Harry Zvi Tabor proposed the use of monochlorobenzene instead of water in small geothermal stations. Industrial interest remained modest until the oil crises of the 1970s forced governments to look at every available heat source. During the 1980s the Italian firm Turboden, today part of Mitsubishi Heavy Industries, commercialised the first packaged ORC skids for biomass CHP plants in Europe. The technology then spread to geothermal, waste heat recovery and, more recently, solar thermal and maritime propulsion. Each new application brought tighter safety rules, better heat exchanger alloys and a wider palette of working fluids, turning the ORC from a laboratory curiosity into a mainstream product offered by dozens of manufacturers on four continents.
How the Cycle Works
The ORC follows exactly the same four stages that every Rankine cycle performs, only the numbers on the temperature entropy diagram are shifted to the left.
Stage 1 – Pressurisation
A feed pump raises the pressure of the subcooled liquid coming from the condenser. Because the fluid compresses as a liquid, the pump work is tiny, typically 2 % to 4 % of the turbine output.
Stage 2 – Vaporisation
The high pressure liquid enters an evaporator where it absorbs heat from the external source. The heat exchange can be direct, through a tube bundle immersed in hot brine, or indirect, via a thermal oil loop. Sensible heat raises the liquid to saturation temperature; latent heat then turns it into dry or slightly superheated vapour.
Stage 3 – Expansion
The vapour expands through a turbine, producing shaft work. Most commercial machines use a single stage radial inflow turbine for powers below 2 MW and an axial machine for larger units. The expansion line on the T s diagram is aimed to end just above the saturation curve so that the last few percent of moisture do not erode the blades.
Stage 4 – Condensation
The low pressure vapour enters an air or water cooled condenser where it rejects heat to the ambient and returns to the liquid state. The cycle then repeats.
Because the working fluid is chosen so that its critical temperature lies only a few tens of degrees above the maximum heat source temperature, the cycle can operate entirely in the supercritical region when desired, eliminating the two phase plateau and raising the average temperature of heat addition. Such supercritical ORCs are now common in high temperature waste heat recovery where the hot gas may reach 400 °C.
Key Components
| Component | Function |
| Evaporator | Transfers heat from the source to the working fluid |
| Turbine | Converts enthalpy of vapour into mechanical torque |
| Generator | Turns torque into three phase electricity |
| Condenser | Rejects waste heat and condenses vapour |
| Feed pump | Returns liquid to the high pressure side |
| Regenerator | Optional internal heat exchanger that preheats liquid with turbine exhaust |
| Control valve | Modulates flow to keep turbine inlet temperature constant during source variations |
| Instrumentation | Pressure, temperature and flow sensors linked to a PLC for unattended operation |
Working Fluids
The fluid must be chemically stable at the maximum cycle temperature, non corrosive, compatible with elastomers, safe for humans and have a low ozone depletion and global warming potential. Early plants used CFCs and HCFCs, but modern units rely on one of the following families:
Siloxanes – MM, MDM and D4 are favoured in biomass and geothermal plants above 250 °C because they resist thermal cracking up to 350 °C.
Hydrocarbons – Cyclopentane and n pentane give excellent efficiency in high temperature waste heat recovery and are inexpensive, yet their flammability demands strict safety rules.
Refrigerants – HFC 245fa and HFO 1233zd are common in small skid mounted units below 200 °C. New low GWP refrigerants such as R1234ze are entering the market.
Alcohols – Ethanol and butanol are used in experimental solar plants because they are non toxic and easy to transport.
The choice is a compromise between thermodynamic performance, safety and cost. A fluid with a steep saturation dome gives a high enthalpy drop but may require a low condensing temperature that is hard to reach with warm cooling water. Conversely, a flat dome fluid fits air cooled condensers but yields lower power. Manufacturers therefore keep a portfolio of at least three fluids for each frame size.
Efficiency Considerations
The thermal efficiency of an ORC plant is the net electrical output divided by the thermal input measured at the evaporator boundary. For today’s commercial units the number lies between 8 % and 24 %, increasing with source temperature and plant size. The theoretical ceiling is set by the Carnot efficiency that corresponds to the temperature difference between the average heat addition and heat rejection. Real losses come from:
Pinch point in the evaporator – The minimum approach temperature is usually 5 K to 8 K.
Turbine inefficiency – Small radial turbines achieve 80 % isentropic efficiency while large axial turbines reach 88 %.
Pump and generator losses – Motor driven canned pumps absorb 5 % to 7 % of their hydraulic power; generators lose 2 % to 4 % in windage and copper.
Parasitic loads – Cooling fans, lube oil pumps and controls can consume 3 % to 6 % of gross output.
By adding a regenerator, a second pressure level or a supercritical cycle, designers can raise the net electrical efficiency by 2-4 percentage points, but the extra heat exchangers and valves must be paid for by higher electricity revenue.
Comparison with the Steam Rankine Cycle
| Parameter | Steam Rankine Cycle | Organic Rankine Cycle |
| Working fluid | Water | Organic fluid |
| Minimum practical temperature | 150 °C saturated steam | 70 °C |
| Turbine tip speed | High, 300 m s⁻¹ | Moderate, 200 m s⁻¹ |
| Part load behaviour | Poor, needs live steam throttling | Good, pump speed controls mass flow |
| Freeze protection | Required | Not required for most fluids |
| Maintenance | Water chemistry, deaerator | Fluid stability analysis, tightness test |
| Capital cost per kW | 1500 – 2500 USD | 2000 – 3500 USD |
Application Areas
Geothermal – Binary plants reinject 100 % of the brine, avoiding silica precipitation and environmental objections. Units from 300 kW to 50 MW operate in Turkey, Kenya, Germany and the United States.
Biomass CHP – Wood chips or agricultural residues feed a thermal oil boiler that heats an ORC skid. The condenser heat is delivered to district heating networks at 80 °C, giving total efficiencies above 90 %.
Industrial waste heat – Cement kilns, glass furnaces and steel reheat furnaces exhaust gas at 250 °C to 400 °C. An ORC produces electricity without interrupting the host process and pays back in 3 5 years where electricity prices exceed 0.10 USD kWh⁻¹.
Solar thermal – Small scale concentrated solar power plants up to 5 MW use linear Fresnel mirrors and toluene based ORCs to generate electricity after sunset thanks to thermal storage in molten salts.
Maritime – Cruise ships install ORCs on the jacket cooling water of large diesel engines, saving 5 % to 8 % of fuel and cutting CO₂ emissions.
Data centres – Supercritical CO₂ and R245fa cycles are being tested to turn 40 °C server heat into power that is fed back into the uninterruptible power supply.
Market Outlook
According to the latest industry reports, the global ORC market is expected to grow from 0.9 GW of installed capacity in 2023 to 2.2 GW by 2030, a compound annual growth rate of 13 %. Waste heat recovery will overtake geothermal as the largest segment, spurred by carbon taxes and the rise of power intensive industries in Asia Pacific. North America and Europe will remain technology hubs, but more than half of new shipments will be delivered to China, India and Southeast Asia where local assembly lines are being erected. The average unit size is drifting upward: whereas ten years ago 500 kW was considered large, today several manufacturers offer standard 10 MW blocks that can be paralleled to 50 MW.
Cost Drivers
The installed cost of an ORC plant is governed by three factors: heat exchanger area, turbine stage count and certification effort. Plate fin evaporators made from brazed aluminium keep the area small but require clean heat sources; shell and tube units made from 316 L stainless steel tolerate fouling yet cost twice as much. Radial inflow turbines carved from a single forged block are cheaper to machine than axial stages with dozens of blades, but they hit an efficiency wall above 3 MW. Finally, every new fluid family demands a fresh set of safety certificates under ASME, PED and Chinese GB standards, adding 0.2 MUSD to the price of the first unit. As production volumes rise, learning curve studies predict a cost reduction of 8 % for every doubling of cumulative output, bringing the capital cost below 1500 USD kW before 2030.
Advantages and Limitations
Advantages
Operates on low temperature heat that is otherwise wasted
Simple start stop procedure allows daily cycling
No need for water treatment or deaeration
Quiet operation, suitable for urban CHP plants
High cycle efficiency at partial load thanks to variable speed pump
Limitations
Working fluid is flammable or toxic in many cases
Higher purchase price per kilowatt than a steam turbine above 5 MW
Organic fluids degrade above 350 °C, capping the upper temperature
Heat exchangers are larger because of lower heat transfer coefficient
Limited vendor base for spare parts in some regions
Future Trends
The next decade will see three technological shifts. First, supercritical cycles using carbon dioxide as the working fluid will push the efficiency of high temperature waste heat recovery above 30 %. Second, printed circuit heat exchangers made from diffusion bonded plates will shrink the footprint by 40 % while resisting 600 bar pressure. Third, digital twins fed by live sensor data will predict the moment when fluid cracking or turbine blade fouling begins, allowing maintenance to be scheduled only when needed and raising plant availability above 98 %. Together these advances will turn the Organic Rankine Cycle from a niche recovery device into a mainstream power technology that harvests every usable calorie from an energy hungry world.
Conclusion
The Organic Rankine Cycle turns low-temperature heat that was once lost into reliable electricity. By matching an organic fluid to the available heat, the cycle reaches useful efficiency at temperatures far below those demanded by steam. Continuous improvements in turbine design, heat exchanger technology and fluid chemistry have pushed unit sizes from kilowatts to multi-megawatt blocks, while costs keep falling. As industries face rising power prices and tighter carbon rules, the ORC stands out as a proven, plug-and-play solution that converts waste heat into profit and lowers environmental impact at the same time.
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