Design an ORC that uses low-boiling organic fluids — R245fa, R134a, n-Pentane or Toluene — to convert 80–300 °C waste heat into electricity. Adjust heat-source and cooling temperatures, mass flow and the regenerator switch and see net power, cycle efficiency and the Carnot-efficiency ratio update in real time, for industrial waste heat, geothermal binary and biomass applications.
Parameters
Working fluid
Choose by source temperature; Tcrit and Tmax set the operating window
Heat source T_hot
°C
Exhaust gas, geothermal brine or thermal oil
Cooling T_cool
°C
Cooling water (air, sea or cooling tower)
Mass flow ṁ
kg/s
Pump efficiency η_p
%
Turbine efficiency η_t
%
Isentropic efficiency of the screw or radial expander
Regenerator (IHE)
Preheat the pump-discharge liquid with turbine exhaust (+12 % efficiency)
Heat from the red source enters the evaporator; the organic vapour drives the turbine and generator; the cold sink (blue) condenses the fluid; the pump re-pressurises it. With the regenerator ON, a dashed IHE loop appears between the turbine exhaust and the pump discharge.
T–s diagram (saturation dome and four state points)
ORC cycle efficiency and the Carnot upper bound (temperatures in K). f_fluid is the fraction of Carnot the real fluid can actually capture — typically 0.5–0.7 for organic fluids. Adding a regenerator (IHE) lifts efficiency by another ~12 %.
$$Q_{\text{in}} = \dot m \bigl(c_p\,\Delta T + h_{fg}\bigr), \qquad W_{\text{net}} = Q_{\text{in}} \cdot \eta_{\text{ORC}}$$
Heat input is the sum of sensible preheat (c_p·ΔT) and latent vaporisation (h_fg). Net power is heat input times cycle efficiency, equivalent to the electrical output after the pump load is subtracted.
I've never heard of an "Organic Rankine Cycle" before. How is it different from a normal Rankine steam turbine?
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Great question. The shape of the cycle — evaporator, turbine, condenser, pump — is exactly the same, but in an ORC the working fluid isn't water. It's a low-boiling refrigerant like R245fa or R134a, or a hydrocarbon like n-Pentane or Toluene. These fluids boil at much lower temperatures than water, so you can get useful vapour pressure out of a 100–300 °C heat source. That's why ORC is the go-to technology for low-temperature waste heat, geothermal and biomass power.
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So the fluid is chosen to bridge the temperature gap. If I switch the dropdown from R245fa to Toluene, the cycle barely runs unless I crank the heat-source temperature way up.
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Exactly — every fluid has its sweet-spot band. Toluene has a critical temperature of 319 °C, so it's for biomass or high-temperature waste heat at 200–350 °C. R245fa (Tcrit 154 °C) is the classic pick for 100–180 °C geothermal or industrial waste heat. R134a is best at even lower temperatures, 80–120 °C. So the first step of any ORC design is: look at the source temperature and choose the fluid. Pick the wrong one and either the cycle won't turn over or the fluid decomposes. The tool clamps the evaporator temperature to the fluid's Tmax for that reason.
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When I uncheck "Use regenerator", the efficiency drops noticeably. What exactly does that piece of equipment do?
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The regenerator — also called an IHE, internal heat exchanger — takes the still-hot superheated vapour leaving the turbine and uses it to preheat the cold liquid coming out of the pump. The evaporator then has less work to do, so for the same net output you need less heat in, and efficiency goes up by 10–15 %. Dry fluids like n-Pentane and Toluene leave the turbine with a lot of superheat, which makes the IHE especially effective. That's why almost every commercial geothermal or waste-heat ORC has one.
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A Carnot ratio of about 50 % sounds low. Why can't we extract more?
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That's actually the realistic ceiling for an ORC. Carnot is a theoretical limit you can never reach. Steam cycles can hit Carnot ratios of 0.6–0.7, but ORC fluids — with smaller latent heat and limited superheat — top out at 0.5–0.6. Even so, turning 150 °C waste heat into 12 % electricity is huge: heat that used to vent to atmosphere becomes revenue. Turboden and ORMAT have hundreds of these units running worldwide precisely because the "something from nothing" value beats absolute efficiency.
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So what's the biggest practical concern when designing one?
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Whether you can secure both the heat source and the cooling source year-round. Theoretical efficiency means nothing if the source temperature swings 50 °C between winter and summer, or if cooling water hits 40 °C in August — output can easily drop by half. Don't forget parasitic loads either: cooling-water pumps, lube oil and control systems steal another 5–15 % of net output. The W_net this tool shows is gross; the bankable figure is roughly 85–95 % of that.
Frequently Asked Questions
The cycle layout (evaporator → turbine → condenser → pump) is identical, but an ORC uses a low-boiling organic working fluid — refrigerants such as R245fa or R134a, or hydrocarbons such as n-Pentane or Toluene — instead of water. This lets it raise useful vapour pressure from 80–300 °C heat sources where steam is impractical, so ORCs are the workhorse for industrial waste heat, geothermal binary, biomass CHP and concentrated solar. Where steam needs more than 300 °C, an ORC can already deliver 8–12 % thermal efficiency at around 100 °C.
A dry fluid has a saturated-vapour line that slopes to the right on the T–s diagram (dT/ds > 0), so it remains superheated at the turbine exhaust. No liquid droplets form, which means there is no blade erosion and no separate superheater is needed. n-Pentane, Toluene and R245fa are close to dry/isentropic and are therefore widely used in ORC. Water, by contrast, is wet — isentropic expansion creates droplets, so steam Rankine plants must add superheat. At low-temperature heat sources where a dedicated superheater is impractical, choosing a dry fluid is a hard requirement for an ORC.
A regenerator uses the still-hot superheated vapour leaving the turbine to preheat the liquid leaving the pump. The evaporator duty drops, and overall efficiency typically improves by 10–15 %. This tool applies a factor of 1.12 to capture the effect. Because dry fluids retain a large amount of superheat at the turbine exit, IHEs are especially effective in ORC and are standard equipment for geothermal and waste-heat units. They are sometimes omitted in cascaded heat-recovery layouts where keeping the evaporator inlet temperature low is more important.
(1) Flue-gas heat recovery from cement, steel and glass plants (200–400 °C); (2) bottoming cycles for diesel gensets, gas engines and marine propulsion (150–500 °C); (3) medium-to-low temperature geothermal (binary plants in the 100–200 °C range where flash steam is unworkable); (4) wood-chip and agro-waste biomass CHP with a thermal-oil boiler at 200–300 °C; (5) small-to-medium concentrated solar power. Specialist vendors such as Turboden, ORMAT, Triogen and Enertime supply commercial units from tens of kW up to tens of MW.
Real-world Applications
Industrial waste-heat recovery: rotary cement kiln clinker-cooler exhaust (300–400 °C), glass-furnace stack gas (200–300 °C) and steel reheating-furnace flue gas — heat that used to be vented — are routinely converted into several MW of electricity. Payback depends on power price and operating hours; in Europe with feed-in support it is typically 5–8 years. Turboden alone has delivered more than 350 industrial waste-heat ORC units worldwide.
Geothermal binary plants: medium-to-low temperature geothermal resources at 100–200 °C are not economic with conventional flash steam. Binary plants instead transfer heat from the geothermal brine to a secondary organic loop (R245fa or n-Pentane) running an ORC. They are deployed across Japan's Kyushu and Tohoku regions, the western US (Nevada), Turkey and Indonesia. Because the brine is fully reinjected, environmental impact is small.
Biomass CHP: wood chips or agricultural residues fire a thermal-oil boiler at 200–300 °C, and the ORC produces both district-heating hot water and electricity. Hundreds of plants in the 500 kW – 2 MW range serve municipalities in Austria, Germany and northern Italy. Compared with steam, ORC plants can run unattended and at low pressure, which suits dispersed rural sites.
Internal-combustion engine bottoming cycles: large diesel gensets, marine engines and natural-gas engines reject useful heat in their exhaust (350–500 °C) and jacket water (80–100 °C). A bottoming ORC adds 5–10 % more electrical output to the engine's rated power and is already in commercial use. Cummins, Bosch and others are also developing truck-mounted ORC for long-haul heavy vehicles.
Common Misconceptions and Pitfalls
The biggest trap is quoting the Carnot efficiency in proposals. Carnot is the thermodynamic ceiling; a real ORC realises only 0.5–0.6 of it. In this tool, a 150 °C source and 25 °C cooling give a Carnot of about 25 %, but the real cycle returns around 12 %. Promising "20 % is achievable because Carnot is 25 %" will haunt you when the plant delivers half that on site. Use Carnot × 0.5 for early proposals and Carnot × 0.55–0.6 only for detailed designs with proven equipment.
The second pitfall is choosing a fluid on performance alone. R245fa balances performance and handling and dominated the market for years, but its GWP of 1030 is being phased out under European F-gas regulations. Low-GWP successors such as R1233zd(E), R1336mzz(Z) and other HFOs are taking over. For high-temperature ORCs, flammable hydrocarbons like Toluene also require explosion-proof zones and leak detection. Fluid selection has to weigh performance, regulation, safety, cost and chemical lifetime, and changing course takes years.
Third, do not evaluate the plant only at the design point. ORC output is extremely sensitive to source and sink temperatures. Push the cooling slider from 25 °C to 40 °C in this tool and net power falls sharply. In real installations, summer cooling-tower temperatures often cut output by 30 %, and reduced exhaust mass flow in winter can halve it. Annual generation typically lands at 60–80 % of nameplate, so economic models must use month-by-month temperature and flow profiles, not the rated point alone.
How to Use
Select working fluid (R245fa, R134a, n-Pentane, or Toluene) based on heat source temperature range and environmental regulations.
Input heat source temperature (e.g., 120°C from industrial exhaust), cooling sink temperature (e.g., 25°C ambient), and organic fluid mass flow rate in kg/s.
Set pump isentropic efficiency (typically 0.75–0.85 for small reciprocating pumps) and run simulation to obtain evaporator/condenser saturation temperatures, heat input, net power output, and cycle efficiency percentage.
Worked Example
Steel mill exhaust stream at 150°C supplies 5 kg/s to an ORC using R245fa with cooling at 20°C. Pump efficiency 0.80, evaporator pinch-point 10°C yields T_evap = 140°C. Condenser operates at T_cond = 30°C. Heat input Q_in = 680 kW, turbine expansion produces W_net = 142 kW, cycle efficiency = 20.9%, Carnot efficiency ratio = 78% (theoretical maximum 26.8% at these temperatures).
Practical Notes
R245fa dominates waste-heat recovery below 200°C; n-Pentane suits 150–250°C sources with higher power density but flammability requires secondary loop containment.