What is a microturbine?

How a microturbine works

A microturbine runs a recuperated Brayton cycle: compress air, heat it, expand it through a turbine, and recover what is left in the exhaust. The sequence below follows the air and gas path in order, from the intake filter to the terminals.

1. 1

   Air intake and compression

   Ambient air is drawn in through a filter and enters a centrifugal compressor mounted on the main shaft. The compressor raises the air pressure and, as a consequence of that compression, its temperature. Everything downstream depends on getting a steady, clean, pressurised air supply at this stage, which is why air filtration is the main consumable on a microturbine at a dusty site.

2. 2

   Recuperator

   The compressed air passes through a recuperator before it reaches the combustor. A recuperator is a heat exchanger that puts the hot exhaust leaving the turbine into thermal contact with the compressed air leaving the compressor, which is already hot from compression and still much cooler than the exhaust. Heat crosses from one stream to the other, so the air arriving at the combustor is already hot and less fuel is needed to reach firing temperature. That single component is the difference between a small turbine with poor electrical efficiency and one that is worth running as a prime power source. An unrecuperated microturbine of this size would waste most of its fuel energy out of the exhaust.

3. 3

   Combustion

   Fuel is injected into the preheated air in a continuous-flow combustor and burns steadily. Combustion is continuous, with none of the intermittent explosions of a piston engine, at lower peak pressure and with more complete burn. That is the reason for the low particulate figures, and it is also why the combustor can accept a wide span of liquid fuels with no change to the rest of the machine.

4. 4

   Turbine expansion

   The hot, high-pressure gas expands through the turbine wheel and spins it. Part of the work done by that expansion drives the compressor on the same shaft; the remainder is the useful output of the engine. Exhaust gas leaving the turbine remains hot, and it is routed back through the recuperator before it goes to atmosphere.

5. 5

   High-speed generator

   The generator rotor sits on the same shaft as the compressor and turbine, so there is no gearbox and no belt drive. The shaft turns at very high speed: the MTP40 operates across 50,000 to 120,000 RPM and the MTG12 across 65,000 to 135,000 RPM. Spinning the generator directly at turbine speed is what allows a machine this small to produce useful electrical power.

6. 6

   Power electronics

   A generator turning at those speeds produces AC at a high frequency that varies with shaft speed, which no ordinary load can use. Power electronics rectify that output to DC and then invert it back to clean AC at 50 Hz or 60 Hz. This stage decouples shaft speed from output frequency, which is why the engine can change speed to follow load while the voltage and frequency at the terminals stay stable.

Air bearings, and why they matter

The shaft in a Bladon microturbine runs on air bearings. Rotation drags air into a converging gap between the shaft and the bearing surface, and the pressure generated in that film lifts the shaft clear. Once the machine is at speed the shaft is supported by air, with no oil-lubricated journal and no rolling elements.

The consequences are practical. There is no lubricating oil in the engine, so no oil changes, no oil to degrade at temperature and no oil to leak or dispose of. There is no bearing wear at speed, because nothing touches. There is no liquid coolant circuit on the MTG12, which is air cooled. Dust and heat shorten oil life and force shorter oil-change intervals at desert and roadside sites. Neither applies here, because there is no oil.

Air bearings are a demanding technology and it is worth being clear about where the difficulty lies. At start and shutdown the shaft is below the speed needed to build the air film, so it is briefly in contact with the bearing surface. Surviving that contact for thousands of cycles is a matter of coating chemistry and of bearing geometry held to fine tolerance. It is the reason air bearing turbines took decades to reach the field, and it is solved: Bladon's units are running on them today.

Shaft speed is high and it varies with load. The MTP40 operates across 50,000 to 120,000 RPM, and the MTG12 across 65,000 to 135,000 RPM. There is no single flat running speed, because the power electronics decouple shaft speed from output frequency.

What microturbines are used for

• Off-grid and bad-grid telecom towers

  Tower sites with no grid, or a grid that fails daily, need firm power at a cost that scales across thousands of locations. Service visits, not fuel alone, dominate the bill.

• Defence forward operating bases

  Deployed power where the logistics chain is the constraint. Fewer service visits and multiple usable fuels reduce the number of movements a site depends on.

• Oil and gas remote monitoring

  Wellhead instrumentation, cathodic protection and pipeline telemetry at sites hours from the nearest depot, often in high ambient temperatures and heavy dust.

• Critical infrastructure backup

  Emergency services masts, water treatment, rural broadband exchanges and other loads where an outage has consequences beyond the site itself.

• EV charging support

  Charging points ahead of a grid reinforcement, where a quiet, low-particulate generator can be sited close to people and vehicles.

Are microturbines expensive?

Capital cost per unit is higher than a comparable diesel genset. That is a real difference and it does not disappear over the life of the machine; it is recovered, or it is not, through what the site spends after commissioning.

Three conditions decide the answer. Sites with a high cost per service visit, whether from distance, access or security escort, benefit from moving to one service visit per year. Sites with fuel theft exposure benefit from running kerosene or paraffin, which have little black-market value. Sites under emissions or noise limits benefit from the particulate and noise figures. Where none of those apply, for example low run-hour standby behind a reliable grid, a diesel usually remains the cheaper answer.

The honest way to settle it is to model your own site: your fuel price, your visit cost, your run hours and your theft rate.