Infinity Turbine's Cluster Mesh supercritical CO₂ turbine can drive a generator that lives outside the CO₂ loop entirely: a magnet-free switched reluctance generator (SRG), cooled by ordinary air and running at around 60,000 rpm.
The pairing plays to each machine's strengths. The turbine stays compact, sealed and efficient in dense supercritical CO₂. The generator stays in air, where windage is low, heat is easy to reject, and technicians can reach it. And because a switched reluctance machine has no permanent magnets, the generator needs no rare-earth materials at all.
Why supercritical CO₂, and why a cluster
Supercritical CO₂ packs a lot of power into a very small turbine. Above its critical point (31 °C, 73.8 bar), CO₂ is dense like a liquid but flows like a gas, so the turbomachinery for a given output is a fraction of the size of a steam turbine.
Small turbines spin fast. Instead of one large machine, Cluster Mesh uses many compact turbine modules working in parallel. That approach brings three practical benefits:
- Scalability. Capacity grows by adding modules rather than redesigning a larger machine.
- Redundancy. One module can be taken offline for service while the rest keep producing power.
- Manufacturability. Identical small units can be built and tested in volume.
The catch is that each module runs at tens of thousands of rpm, and the generator attached to it must keep up.
Why the generator moves outside the CO₂
A generator immersed in supercritical CO₂ pays a heavy price in windage. Drag on a spinning rotor scales roughly with fluid density and with the cube of speed, and supercritical CO₂ is around a hundred times denser than air or more. At high rpm, a submerged rotor can lose a meaningful share of the turbine's output just stirring the fluid around it.
Placing the generator outside the pressure boundary, in ambient air, removes that penalty and brings further advantages:
- Simple cooling. Stator heat is rejected to air through fins and ducts, with no need to manage heat inside a high-pressure casing.
- Lower insulation stress. Windings are not exposed to high-pressure CO₂, which can permeate insulation and cause damage when pressure drops.
- Serviceability. The generator and its electronics can be inspected or swapped without opening the CO₂ loop.
- Smaller pressure vessel. Only the turbine needs a high-pressure housing.
The trade-off is that power must cross the pressure boundary mechanically. The two usual options are a shaft through a dry gas seal, or a magnetic coupling acting through a sealed non-metallic containment wall. The best choice depends on module power, allowable leakage and coupling losses at speed. A dry gas seal keeps the whole drivetrain magnet-free; a magnetic coupling brings magnets back, though not necessarily rare-earth ones.
Why a switched reluctance generator
A switched reluctance machine is the simplest rotating electrical machine there is: a rotor of stacked steel laminations with salient teeth, and a stator carrying concentrated coils. The rotor has no magnets, no windings, no bars and no slip rings.
That simplicity is exactly what a high-speed, air-cooled generator needs:
- No rare earths. No samarium-cobalt or neodymium, so no exposure to a concentrated, price-volatile supply chain.
- No demagnetization. High temperature or a fault current cannot weaken the field, because there are no magnets to damage.
- No retention sleeve. A magnet rotor at high speed needs a carbon fibre or metal sleeve to hold it together. A steel lamination rotor does not.
- Heat where you can reach it. Nearly all losses occur in the stator, which sits against the air-cooled housing. The rotor runs comparatively cool.
- Fault tolerance. Each phase is electrically independent, so the machine can keep producing reduced power with a failed phase. A short circuit does not generate the heavy drag torque a magnet machine can.
Switched reluctance starter-generators were developed for aircraft engines for many of these same reasons: high speed, high temperature and a need to keep running after a fault.
How it makes power
A switched reluctance generator produces DC through its converter, stroke by stroke. Each phase works in a two-step cycle as the rotor teeth pass the stator poles:
- Excite. As a rotor tooth reaches alignment with a stator pole, the converter connects that phase to the DC bus and builds up magnetic flux.
- Generate. The switches open as the tooth pulls away. Because the magnetic path is opening up, the phase current is pushed back through diodes into the DC bus. More energy returns than was borrowed, and the difference is the turbine's work converted to electricity.
The controller sets output by adjusting just two numbers: the rotor angles at which each phase is switched on and off. Closed-loop control of those angles holds the DC bus voltage steady as turbine power varies.
The output is native DC, which suits modern facilities well. It can feed a DC distribution bus or battery storage directly, or pass through an inverter for AC loads and grid export. The machine needs a small DC source, such as a battery or charged capacitor, to start; once running, it sustains its own excitation.
Designing for 60,000 rpm
At 60,000 rpm, a switched reluctance generator is well within proven territory; commercial switched reluctance motors have run near 100,000 rpm. The design choices that make it work at this speed are well understood.
| Design issue | What happens at 60,000 rpm | Design response |
|---|---|---|
| Rotor stress | A 100 mm rotor sees a tip speed of about 314 m/s | Keep rotor diameter around 60–80 mm, or use high-strength cobalt-iron laminations; generous fillets at the tooth roots |
| Core loss | A 6/4 machine switches each phase about 4,000 times per second | Low pole counts and thin laminations (0.1–0.2 mm) |
| Flux build-up | Each excitation window lasts only tens of microseconds | Adequate DC bus voltage and few turns per coil |
| Switching | Fast, precise angle timing | Single-pulse control with silicon carbide power devices and a fast position sensor or sensorless estimator |
| Radial forces | Small air gap plus any eccentricity produces magnetic side pull | Account for it in bearing selection and rotordynamics |
| Windage | Salient rotor teeth churn the surrounding air | Fill or shroud the gaps between teeth for a smooth rotor surface |
Operating in air rather than CO₂ makes the windage line far easier to handle. In air, a lightly shrouded rotor has modest drag. Submerged in supercritical CO₂, the same rotor would be a serious loss.
The trade-offs
No generator choice is free, and the switched reluctance route has known costs:
- Torque ripple and noise. Power is produced in pulses, which creates torque ripple and an acoustic signature. Phase overlap, rotor shaping and control profiling reduce both; an enclosure handles the rest.
- Power density. A well-designed permanent magnet machine is typically smaller for the same output. Removing the CO₂ windage penalty and the magnet sleeve narrows that gap.
- Converter rating. Excitation energy flows back and forth every stroke, so the converter is rated somewhat above the net output. The DC link capacitor also sees significant ripple current.
- Seal or coupling losses. Taking power across the pressure boundary adds a seal or coupling that must be designed for speed, pressure and leakage.
For a modular, high-speed generator that must be robust, cool easily, and avoid rare earths, these trade-offs are often worth making.
Application: turning data center heat into power
Data centers reject enormous amounts of heat, and most of it is thrown away. A Cluster Mesh system can recover part of that energy, or heat from on-site generation, and convert it back into electricity close to the load.
The switched reluctance generator fits this setting well:
- DC-native output pairs naturally with the industry's move toward higher-voltage DC power distribution and battery-backed racks.
- Modular redundancy matches data center practice: modules can be serviced one at a time without stopping the plant.
- Air cooling uses the same kind of airflow and heat rejection infrastructure the facility already runs.
- Local generation helps where grid connections are constrained, as in many parts of the Netherlands, where Infinity Turbine is focusing this application.
Learn more
Keeping the generator out of the CO₂ and the magnets out of the generator gives a drivetrain that is simple, cool-running, serviceable and free of rare-earth supply risk. That makes it a strong candidate for modular sCO₂ power at data center scale.
To discuss Cluster Mesh for your site or project, contact Infinity Turbine at infinityturbine.com or greg@infinityturbine.com.