The Rankine cycle is a closed thermodynamic sequence that pressurises liquid, boils it, expands the vapour through a turbine and condenses it back to liquid. When the fluid is water the cycle is simply called the Rankine cycle. When the fluid is an organic substance such as pentane, toluene, R245fa, MM siloxane or any refrigerant chosen for its low boiling point, the same sequence becomes the Organic Rankine Cycle. This single substitution moves the entire temperature entropy diagram to the left, allowing practical power generation from heat sources as cool as seventy degrees Celsius, a feat that a water based cycle cannot reach without operating in a deep vacuum.
Table of Contents
Historical Paths
William John Macquorn Rankine published the ideal vapour power cycle in 1859 and by 1900 every coal fired power station on earth was using it. The organic variant waited until the 1950s when Harry Zvi Tabor proposed monochlorobenzene for small geothermal wells. Steam dominated the twentieth century because coal and nuclear plants offered hundreds of megawatts at 500 °C, a region where water is unbeatable. The oil shocks of the 1970s, the rise of biomass combined heat and power in Europe and the global hunt for waste heat gave the Organic Rankine Cycle its opening. Today both cycles coexist: steam handles the very large and the very hot, ORC captures the small, the cool and the distributed.
Working Fluid: Water vs Organic Substances
Water is cheap, non toxic, non flammable and carries enormous latent heat. Its critical point lies at 374 °C and 221 bar, so supercritical steam plants can reach 45 % net efficiency. The downside is that at 100 °C the saturation pressure is only 1 bar absolute, dropping to 0.12 bar at 50 °C. Such vacuum demands huge volumetric flow, last stage blades several metres long and condensers operating at 0.05 bar with corresponding air in leakage risk.
Organic fluids trade away high critical temperature for high molecular mass and steep saturation curves. Pentane boils at 36 °C under atmospheric pressure, R245fa at 15 °C, MM siloxane at 191 °C. High molecular mass reduces the enthalpy drop per kilogram but also lowers the speed of sound, letting small radial turbines run at 15 000 rpm and still reach 80 % isentropic efficiency. The organic fluid can stay dry throughout the expansion, eliminating moisture erosion and the need for superheat.
Temperature and Pressure Windows
A typical steam turbine inlet sits at 300 bar and 600 °C for ultra supercritical plants, or 60 bar and 480 °C for subcritical units. Condenser pressure is 0.03 bar to 0.08 bar, corresponding to condensing temperatures of 24 °C to 41 °C.
Commercial Organic Rankine Cycle units accept heat source temperatures from 70 °C to 350 °C. Evaporating pressure ranges from 0.5 bar for low temperature geothermal brine to 30 bar for biomass plants using toluene. Condensing pressure stays above 0.1 bar in most cases, allowing air cooled condensers without vacuum pumps.
Component Differences
TableCopy
| Component | Rankine Cycle (Steam) | Organic Rankine Cycle (ORC) |
| Boiler | Water tube furnace, 200 bar safety valves | Plate fin or shell and tube evaporator, 30 bar |
| Turbine | Multi stage axial, last stage titanium blades | Single or double stage radial, aluminium or steel |
| Generator | Two pole or four pole, 3 000 rpm or 1 500 rpm | High speed permanent magnet, 15 000 rpm with gearbox |
| Condenser | Surface type, 0.05 bar, huge steam space | Brazed aluminium, 0.1 bar, compact |
| Feed pump | Multi stage centrifugal, 30 MW motor | Canned magnetic drive, 15 kW motor |
| Water chemistry | Deaerator, phosphate dosing, blowdown | Fluid stability test, acidity check, tightness monitor |
Efficiency Comparison
The Carnot ceiling for a steam plant operating between 600 °C and 30 °C is 65 %; real stations achieve 40 % to 45 %. An Organic Rankine Cycle working between 150 °C and 40 °C faces a Carnot limit of 26 % and delivers 15 % to 18 % net. Below 150 °C the steam cycle collapses because the condenser would need to operate at 0.05 bar, whereas the ORC still offers 10 % efficiency at 100 °C heat source. In other words, water wins above 250 °C, organics win below 200 °C, and in the overlapping band the choice depends on size, safety and cost.
Part Load Behaviour
Steam turbines control load by throttling live steam, a process that drops pressure without changing temperature and hurts efficiency. Below 40 % load the turbine enters the Wilson line and moisture appears in the last stages.
Organic Rankine Cycle plants modulate mass flow by varying pump speed while keeping turbine inlet temperature constant. Because the fluid stays superheated throughout the expansion, efficiency falls only gently down to 20 % load. Daily start stop is therefore routine for biomass and waste heat recovery ORCs, whereas base load coal steam plants avoid it.
Maintenance and Safety
Steam circuits suffer from dissolved oxygen, silica deposits and chloride cracking; they need continuous chemistry labs and scheduled outages for blade cleaning.
Organic circuits are sealed; the main ageing mechanism is thermal cracking of the fluid, monitored by acidity and colour tests. There is no risk of cavitation in the pump because the fluid is far below its critical pressure. However many organic fluids are flammable, so ATEX rated electrical equipment and gas detection are mandatory. Siloxanes are non flammable but costly, hydrocarbons are cheap but need nitrogen blanketing.
Cost Structure
A 500 MW supercritical steam block costs around 1 200 USD per kilowatt installed, dominated by the boiler, turbine hall and cooling tower.
A 1 MW Organic Rankine Cycle skid costs 2 000 to 3 000 USD per kilowatt, the evaporator and the radial turbine being the largest items. The price gap narrows when the heat source is below 200 °C because the steam plant would need a vacuum condenser, a deaerator and a huge last stage, items that are absent in the ORC.
Application Map
Steam Rankine Cycle dominates
- Coal fired power stations above 300 MW
- Nuclear reactors above 500 MW
- Combined cycle gas turbines where exhaust is 600 °C
- Concentrated solar power towers above 250 °C
Organic Rankine Cycle dominates
- Geothermal binary plants below 180 °C
- Biomass CHP below 5 MW
- Industrial waste heat recovery from cement, glass, steel
- Flare gas elimination on oil fields
- Maritime slow speed engine jacket cooling at 80 °C
- Small scale concentrated solar power with linear Fresnel reflectors
Market Share and Growth
Global installed steam capacity exceeds 2 TW, whereas ORC capacity stands at 3 GW. Yet the annual growth rate of new steam additions is below 1 %, while ORC shipments grow at 13 % per year. Drivers are carbon taxes, renewable heat incentives and the rise of distributed generation. Turboden, Ormat, Exergy, Enertime and more than forty smaller suppliers now offer standard skids from 100 kW to 50 MW.
Future Outlook
Supercritical carbon dioxide cycles may challenge both water and organics in the 300 °C to 700 °C bracket, but below 250 °C the Organic Rankine Cycle will keep its niche. Research focuses on low global warming potential fluids such as HFO 1233zd, compact printed circuit heat exchangers that cut footprint by forty percent and magnetic bearing turbo generators that remove lubricating oil. Digital twins fed by live sensor data will predict fluid degradation and turbine fouling, pushing plant availability above ninety eight percent. Water will remain king above 250 °C, yet for every calorie cooler than that, the Organic Rankine Cycle is poised to become the default technology for turning wasted heat into useful power.
Conclusion
Water and organic fluids obey the same thermodynamic laws but sit at opposite ends of the temperature spectrum. Steam delivers unmatched efficiency above 250 °C and scales to gigawatts; organics bring practical efficiency below 200 °C, tolerate daily cycling and fit inside a shipping container. Choosing between the two is no longer a philosophical debate—it is a straightforward question of heat source temperature, plant size and economic value of electricity.
Ready to Secure Your Operation?
👉 Get Custom Organic Rankine Cycle
👉 Get Free Consultation
👉 Get Free Quote on Organic Rankine Cycle
