Organic Rankine, Kalina and sCO2 cycles: Science and thermodynamics

Why 9% efficiency can still be a thermodynamic success

Organic Rankine cycles swap water for a fluid that boils at lower temperature, so they can make power from heat at roughly 90-260 C where steam struggles. Conversion is low in absolute terms but a fair share of what thermodynamics allows; Kalina, supercritical CO2, thermoelectrics and Stirling engines are niche or emerging.

Grade of energy

DOE describes conventional ORC units taking heat at 320-500 F (about 160-260 C) and newer low-temperature units at 200-300 F (about 95-150 C), with some innovations working down to about 200 F (about 95 C). That covers cement cooler air, engine jacket water, geothermal brine and biomass thermal oil. On ships, scavenge air, jacket water and lube oil heat are too cool for steam and can only feed an ORC, according to Korlak's heat balance study. Supercritical CO₂ targets the other end: the STEP pilot runs a turbine inlet of 500 C.¹⁷¹¹

Exergy versus energy efficiency

DOE gives 15-20% heat-to-power conversion for conventional ORC and 8-9% for low-temperature units after parasitic loads. These look poor, but a 120 C source rejecting to 30 C has a Carnot limit of only about 23% (calculated), so 8-9% is roughly 35-40% of the theoretical maximum (calculated), comparable with steam cycles at higher temperature. Turboden reports turbine efficiencies up to 90% and good part-load behaviour. Thermoelectric modules convert directly with no moving parts but at low efficiency: the CEC-funded lead telluride modules exceeded 5%. A 30 kWe biomass Stirling prototype reached about 20% electrical efficiency in CHP mode.¹⁴⁹¹⁰

Media compatibility and corrosiveness

Most industrial ORCs keep the organic fluid away from dirty gas by using an intermediate thermal-oil loop; at Riyadh Cement six heat-recovery units on two kiln lines feed one thermal-oil circuit. This adds a heat-exchange step but protects the turbine loop from dust and acid. Organic fluids can be flammable or have high global warming potential, so fluid choice matters. The Kalina cycle uses an ammonia-water mixture that needs high-pressure, corrosion-resistant materials: in a diesel-exhaust study it ran at about 100 bar against about 10 bar for a siloxane ORC.²⁸

Variants compared^137891013

Sources

1. Waste Heat to Power, CHP Technology Fact Sheet Series (DOE/EE-2347) US Department of Energy, 2021
2. Turboden commissions 13MW waste heat power plant at Riyadh Cement CemNet (International Cement Review), 2026
3. Enhancing energy security and sustainability in cement plants through Turboden ORC technology Turboden (press review), 2026
4. Turboden ORC biomass cogeneration (Bini), presentation to IEA Bioenergy Task 33 (company figures, dated) Turboden / IEA Bioenergy, 2010
5. STEP Demo pilot plant achieves full operational conditions for Phase 1 of testing Southwest Research Institute, 2024
6. Heat recovery from Diesel engines: a thermodynamic comparison between Kalina and ORC cycles, Applied Thermal Engineering 30(2-3), 212-219 IAEA INIS record of journal article (Politecnico di Milano, University of Brescia), 2010
7. Advanced Thermo-Electric Generator System (ATEGS), CEC-500-2024-105 California Energy Commission, 2024
8. Power generation and cogeneration (small-scale biomass technologies) IEA Bioenergy Task 32, 2026
9. Comparative analysis of the heat balance results of the selected Tier III-compliant gas-fuelled two-stroke main engines (Korlak), Combustion Engines 193(2) Combustion Engines (journal), 2023
10. Exergy launches US operations to support rapid geothermal growth across North America (company figures) ThinkGeoEnergy, 2026