A few months ago, Pratt & Whitney announced a new version of its Geared Turbofan, the Advantage. More powerful but also more efficient and with a one-percent better fuel consumption compared to the base GTF, the Advantage will enter service on the Airbus A320neo-family in 2024. But there is another step to come in the efficiency of the GTF. Behind the scenes, P&W’s German partner MTU Aero Engines has been working on a revolutionary program called WET, short for Water Enhanced Turbofan. MTU's WET concept further improves GTF efficiency. MTU has been closely involved in the updates that are now part of the Advantage program, notably the high-pressure compressor (HPC) and the low-pressure turbine (LPT) to improve the airflow through the engine. Striving to offer solutions capable of reaching the goals of the 2015 Paris Climate treaty to limit global warming, MTU has been pushing ahead for new solutions. One of these is WET, a revolutionary concept to reduce climate impact while at the same time improving efficiency. During last November’s Dubai Airshow, AirInsight discussed WET with Fabian Donus, propulsion director at MTU Aero Engines. “WET stands for Water Enhanced Turbofan. The idea behind the concept is to bring down climate impact and at the same time, bring down the energy demand over the mission. The problem of many revolutionary concepts is that they either impact climate or energy demand. If you look at hydrogen, for example, the climate impact can be reduced significantly but the energy demand goes up due to the large volume of the hydrogen tank. Especially in times where the availability of green energy is limited, energy demands are crucial. One solution is to stick with a gas turbine and to feature it with something which could which can reduce climate impact. That's the WET concept.” WET wants to use the excess energy that typically is lost because it's exiting the engine. “If you stand behind an engine, it's very hot. And that's not good because it's unused energy. And if you just could use the energy and keep it in the system, you would significantly increase the efficiency of the system. And this is what the WET does. We use the excess heat to boil water. The generated steam is then injected into the combustion chamber. So it's still a gas turbine, but it just uses a higher amount of the energy which is stored in the fuel compared to a conventional system.” Increased thermal efficiency and 80 percent fewer NOx emissions Bringing the excess heat back into the system significantly increases thermal efficiency, says Donus, at the same time, helping to decrease specific fuel consumption (SFC). “It is not only the effect of recuperating the excess heat but also the use of water as a fluid. Water has a higher specific heat capacity compared to air, which provides additional power in the turbine and therefore has a positive effect on SFC. Of course, we add weight and complexity to the system by implementing new components. Also, drag will be increased as the heat exchangers produce pressure losses. But current simulations, taking all of these positive and negative effects into account, predict that the WET can achieve an up to ten percent lower fuel burn and therefore also CO2 than a conventional gas turbine of same technology level.” “The main focus of WET is the reduction of the climate impact. In aircraft, this is not only caused by carbon dioxide emissions but results also from the emission of nitrogen oxides (NOx) as well as the formation of condensation trails, which are mainly induced by the emission of particulate matter. The WET concept attacks all of these emissions at the same time.” The current version of the P&W Geared Turbofan is in service since January 2016. (Pratt & Whitney) Fabian Donus continues: “We bring down NOx emissions by 80 percent and more by injecting the hot steam in the combustion chamber. Typically, throughout the combustion chamber, you have regions where the temperature is much higher than the average temperature. These temperature peaks drive NOx emissions significantly. The injected steam caps these peaks and you end up with a much more homogeneous temperature profile. The average temperature however remains high which is important for high thermal efficiency. The 80 percent is a real number, already validated on stationary gas turbines, and we don't see any reason why it shouldn't work on the WET.” The idea of injecting water goes back as far as the Daimler Benz piston engines on the Luftwaffe’s Messerschmitt fighter planes in the Second World War. Pratt & Whitney used it on the JT-3C on the Boeing 707-120 in 1957. BMW repeated the trick in the 1980s on their turbo engines in Formula One motor racing. “In these cases, the focus was of course not the reduction of NOx but rather to drive power for a specific amount of time. However, the technical feasibility has been demonstrated many times. There have also been airplanes which used water injection in gas turbines to take off, for instance,” says Donus. The water is already in the system Carrying water to inject it into the engines contravenes an aircraft designer’s obsessiveness for reducing weight where possible. “You don't want to carry the water of course. There are big amounts going into the combustion chamber so you can't carry it all the way. That wouldn't trade-off because it's too heavy.” So where does the water come from? Fabian Donus: “We simply recycle it. What we do is we take an initial amount of water in a tank to start the system. And then, if you inject the water into the combustion chamber, the exhaust gas still contains this water. Before it exits the system, we have to recover it. The idea is to condensate the water out of the exhaust gas. In order to do so, we need to cool down this hot fluid. The heat we gain is used to boil the water again before we inject the steam.” “In the end, it is a loop. We inject steam into the combustor, then cool down the exhaust gas, condensate the water, collect it, boil it using the heat we gained through cooling down the exhaust gas, and finally inject it again. Of course, we will not be able to condensate all the water out of the exhaust gas. This would require ideal processes. But we don’t have to since the kerosene also contains water. We can use that as a second source. As long as the amount of water contained in the kerosene is higher than the amount which cannot be recovered we are on the safe side.” Water injection was already tried on the Daimler Benz engine in the Messerschmitt BF-109G, the famous fighter aircraft of the Luftwaffe, but only as a means to increase power. Moreover, condensation is the enabler to attack the third effect on the climate, the formation of condensation trails. “The process of condensation typically starts at so-called nuclei, which, in the exhaust of a gas turbine, are particles generated throughout the combustion of kerosene. Water droplets will grow around these particles. That's basically the same effect as when contrails behind the aircraft are being formed. So we use that exact same effect but try to do it within the aircraft system. The water droplets containing the particles are subsequently collected and then, before boiling, filtered out. This will reduce the total amount of particles emitted significantly. As these particles, in combination with the water content of the surrounding atmosphere, are the main driver of the formation of contrails, we expect major benefits.” More benefits from using hydrogen There could be more benefits from using hydrogen as a fuel in the WET concept, Donus points out. “Hydrogen is very cold, you need to take it in a cryogenic phase. This is beneficial since we need to cool down the exhaust gas to very low temperatures. With a very cold fluid, it is much easier to accomplish that. That would bring down the size of the components because the more temperature delta you have, the smaller you can design your components and the lighter they are.” Also regarding climate impact, the use of hydrogen could have additional benefits. Compared to kerosene hydrogen doesn’t even carry any carbon and therefore no additional particles would be produced during combustion says Donus. MTU, therefore, expects a further reduction in climate impact from contrail formation. Integrating the WET system into the airframe still needs further investigation. This is the GTF on an Embraer E190-E2. (Richard Schuurman) The challenge is to bring it onto an aircraft The big challenge, according to Fabian Donus, is to bring the WET concept onto an aircraft. The raw technological ingredients all exist. “All the different components are already industrialized: water condensers, heat exchangers, the injection systems. But bringing all these components together in an aircraft, that's the key challenge. The integration in the aircraft is crucial. We are looking at different concepts on how we could integrate such a system. Some have let's say more impact on the aircraft, some less. We have to understand which one is best in total.” If the concept is confirmed, when does Donus expect the WET technology to be ready for service? “Definitely not before 2035. It depends on how difficult that system would look be in the end, some customers have to buy it. It's a much more complex system than we have today. It's still a gas turbine but it will be much more complicated and you need time to mature the system. This is the first step and we are currently checking the feasibility and coming up with an optimized architecture. That's key.”
