Compressorless Hydrogen Turbine Sets New Endurance Record

Turbines based on the conventional approach operate using the Brayton cycle, which is the same closed cycle involving compression, the addition of heat and expansion that A-level students study when learning about thermodynamics and heat engines. The gas is compressed, which increases both its pressure and temperature; afterwards, fuel is burned at approximately constant pressure in order to add further thermal energy; then the hot gas expands through a turbine stage, turning the pressure and thermal energy into mechanical work on the turbine shaft. The issue, from a thermodynamic point of view, is that the energy required for compression has to be taken from the turbine’s own output. In an ideal, frictionless environment, that energy would be recovered during the expansion process, but real compressors and turbines are a long way from being ideal, and at each stage of the blades there are losses due to friction, turbulence and heat being transferred to the casing. When all these losses are taken into account, about half of the turbine’s mechanical output can be lost before it even reaches a generator or a propeller.

The principle used in KIT’s compressorless hydrogen turbine - pressure-gain combustion - challenges that assumption. Rather than burning the fuel at approximately constant pressure, as is illustrated in the typical textbook diagram of the Brayton cycle, the KIT design burns it in such a way that the pressure rises as part of the combustion process. This can happen because a detonation wave is not a flame in the ordinary sense. A flame front advances through a fuel-air mixture at a few metres per second, the movement being caused by heat conduction from the burning layer into the unburned gas in front of it. In contrast, a detonation wave travels at supersonic speed, usually exceeding a thousand metres per second in the case of a hydrogen-air mixture, and is caused by a shock wave which compresses and ignites the gas in front of it almost instantly. The shock wave is mechanically carrying out the function that compressor blades would normally perform by squeezing the gas, the difference being that it uses the energy released by the combustion reaction rather than energy drawn from the turbine shaft.

Hydrogen is especially well suited to detonation combustion due to the speed of its reaction. The ignition delay time, which is the period between a fuel-air mixture reaching ignition conditions and actually starting to burn, is much shorter than that of hydrocarbon fuels like kerosene or methane. Since detonation waves depend on the reaction keeping up with the shock wave that sets them off, a fuel that reacts slowly is likely to produce detonations that decouple, weaken or fail completely. The fast chemical reaction of hydrogen together with its wide flammability range makes it much more forgiving, and this is one of the reasons why KIT selected it for the first working demonstration rather than using a conventional aviation fuel.

All of this involves considerable engineering difficulties. The detonation waves subject the combustion chamber walls to repeated and rapid pressure and temperature spikes that are much more severe than the relatively mild, continuous heat load found in a conventional combustor, which is the reason why the KIT chamber is water-cooled rather than using the air-film cooling methods usual in jet engines. In order to keep the detonation wave fixed in the chamber and prevent it drifting, weakening or exploding, it is necessary to have precise control of the fuel injection timing and the mixture ratio, which is why the team is still working with modest hydrogen flow rates of one to two grams per second rather than with industrial-scale amounts. Scaling up a detonation combustor while at the same time maintaining wave stability is one of the main unresolved problems in the field, and that is why a five-minute run should be regarded as a real achievement rather than just a small improvement.

For students who are studying the thermodynamics section of A-level physics or engineering programmes, the compressorless hydrogen turbine provides a useful example of where real losses occur in an idealised cycle. Although textbook diagrams of the Brayton cycle usually treat compression as a simple and loss-free stage, in reality it is one of the most costly parts of the entire machine to construct, operate and maintain. Not only does eliminating the compressor reduce weight and the number of components, but it also fundamentally alters the energy balance of the cycle. Whenever engineers are able to make pressure-gain combustion reliable and scalable, the efficiency improvements will be applicable in any situation where a gas turbine is used, whether that be for decentralised power generation using green hydrogen or, even further ahead, for aircraft propulsion.