Constructing a working supercritical carbon (sCO₂) plant has eluded engineers in the US, Europe and Japan for decades. But in December, the China National Nuclear Corporation (CNNC) switched on a pair of grid-connected 15MW sCO₂ turbines, part of a waste heat recovery system at a Guizhou steel plant.

Dubbed ‘Chaotan One’, the new turbines replace steam and have apparently been responsible for an 85% jump in energy generation efficiency, from a 50% footprint reduction.

Small and power-dense, sCO₂ plants could become very interesting for naval architects in the coming years. Though not many steam turbines are in use on merchant vessels anymore, LNG carriers still operate simple-cycle turbines powered by boil-off gas, with low efficiencies that are claimed to be around 35-40%.

This has prompted discussion on what to retrofit on these high-CapEx and otherwise highly technologically advanced vessels to bring them up to modern efficiency standards.

But with the recent technological barrier having been broken that could lead to a step-change in the way this is done, almost certainly carrying major implications for ship design.

LNG dual-fuel engines are hard to retrofit: a much smaller sCO₂ plant, the bulk of which comprises modular pipes, seems a plausible alternative.

Compared with these steam plants, which top out at around 35% thermal efficiency, an sCO₂ turbine has a thermal efficiency of around 45-50%.

For a century, steam turbines have converted heat into energy. The principle remains largely unchanged whether this heat originates from burning coal and gas, from under the earth’s crust, from mirror-concentrated solar power, or from nuclear fission.

Steam turbines have already powered some of the fastest large vessels in history, including the coal-powered liner SS United States and the 70,000tonne aircraft carrier USS Enterprise, powered by nuclear steam turbines.

In a conventional steam powerplant, water is heated until it reaches its boiling point. The rapid thermal expansion generated by this phase-change is then used to drive a turbine, which in modern iterations is connected to magnets and coils which convert this rotation into an electrical current.

The steam passes through a condenser to return the water to a liquid phase, and the Rankine cycle begins again. Over the more than a century of steam turbine development, this process has been enhanced by systems that pressurise and heat the working fluid to greater extremes.

More ‘stages’ – rings of turbine rotors – have been added to eke more power and efficiency from the steam as it depressurises.

At 3,210psi and 374°C, water reaches the point of supercriticality – a fourth order of matter, neither fully liquid nor fully gas, but with properties of each. Substances seen on the roiling surface of Jupiter, for example, are supercritical, unable to fully phase-change due to pressure from the planet’s extraordinary gravity.

It is from this that supercritical and ultra-supercritical steam powerplants get their name. CO₂, though, reaches criticality at just 30.98°C and 1,070psi. Flowing like a gas, but compressing like a liquid, supercritical CO₂ (sCO₂) fluid density is 50% higher than that of steam and can put enough mass through the blades to generate equivalent power to steam in a turbine package of fewer stages, and one tenth of the diameter and, critically, lighter in weight.

In an sCO₂ turbine system, the working fluid would be compressed before entering a recuperator, which recovers heat from the hot turbine exhaust – reaching around 300°C, unlike the 60°C seen in a conventional steam turbine system.

From there, it goes to a heat exchanger and is heated by whatever source is available. Then, it drives the turbine, expanding rapidly and spinning the rotors, before being recirculated to begin the process again.

Smaller blade diameter could result in less mechanical stress on the blades, though the implications of a turbine spinning at much higher RPMs than a conventional steam turbine are yet to be studied.

Lower temperatures throughout the system are likely to result in fewer liability issues, though, the low critical temperature of 31°C will make it possible to scavenge heat at lower temperature ranges – the 400-500°C present in engine exhaust gases would certainly qualify.

Supercritical CO₂ turbines are very attractive to proponents of small modular nuclear reactors (SMRs) – under consideration by some in maritime – where a small reactor-turbine system could generate vast power in a package a fraction of the size of a medium-speed engine.

It is interesting to imagine what such vessels might be capable of, generating additional power from turbines a fraction of the size and weight.