Full throttle into the energy transition
When Wolfgang Hartmann is asked about his contribution to combating carbon dioxide emissions, he points to a huge engine piston that is packed and ready for delivery at the outgoing goods department of the Neckarsulm plant. Lifting it would hardly be possible even with both hands, because the component weighs around 87 kilograms and measures 31 centimeters in diameter. By comparison, a piston for passenger car engines weighs just 200 grams. “This is a steel piston for large natural gas engines running in cogeneration power plants,” explains Hartmann, Head of Large Bore Pistons at Rheinmetall Automotive. At first glance, his response to the question may seem surprising, but it does make sense. In the current debate about green electricity from renewable sources such as wind, solar and hydro power, it is often forgotten that the environment-friendly generation of heat energy for heating systems and hot water supply also offers plenty of climate protection potential. In addition, the supply of electrical energy must also be ensured in times when alternative sources barely provide any electricity, for example when there is no wind on a cloudy winter’s day.
This makes modern gas-engine power plants, which are operated in combined heat and power generation, an essential part of the energy transition. The trick: the energy released by the combustion engines during operation is used twice. On the one hand, the crankshaft of the engines drives a generator that supplies electricity; on the other hand, the combustion heat is used to heat a water reservoir, which is then used to supply hot water and to heat buildings in the district heating network.
World’s most modern plant in Kiel
One of the world’s most modern power plants of this type is the coastal power plant K.I.E.L. (Kiels Intelligente Energie Lösung = Kiel’s Intelligent Energy Solution). The overall system consists of 20 gas engines in four blocks, an electrode boiler and a heat accumulator. The engines of the modular system can be run up to a rated output of 190 megawatts in less than five minutes. At the same time, a heat output of 192 megawatts is generated during operation – enough to supply the 70,000 customers of Kiel’s municipal utility with environment-friendly electricity and heat energy via the district heating network. Compared to the previous coal-fired power plant, nitrogen oxide emissions from the K.I.E.L. plant are reduced by up to 70 percent, and no sulfur is released during the combustion of natural gas.
The units used are 20-cylinder Jenbacher J920 FleXtra engines, each 8.4 meters long, 3.2 meters wide, 3.5 meters high and weighing 91 metric tons. In addition, there is a turbocharger module and generator and hence one complete unit is 16.8 meters long and weighs 176 metric tons. Rheinmetall Automotive is equipping the engines with pistons and a matching set of rings. Like the overall system, they are designed for particularly high efficiency. The extremely good energy conversion of the engines during combustion creates the basis for the plant’s high overall efficiency of 90 percent, half of which is thermal and half electrical. Compared with the separate generation of heat by gas boiler and electricity in the current EU mix, up to 7,800 metric tons of carbon dioxide can be saved annually per engine-generator unit, which, when extrapolated to the 20 units of the K.I.E.L., add up to 156,000 metric tons less carbon dioxide.
High pressures open up efficiency advantages
One component of the high efficiency of the engines used in Kiel is their combustion control. Unlike diesel engines, in which the air-fuel mixture ignites automatically due to the pressure in the combustion chamber, gas engines generally operate according to the spark-ignition principle – they require an external ignition source such as a spark plug to initiate combustion in the cylinder. During engine operation, the combustion and ignition parameters are set in such a way that maximum efficiency and thus energy exploitation of the fuel are achieved. This, however, increases the risk of knocking during combustion. Experts define knocking as uncontrolled combustion, resulting in pressure peaks that can damage engine components such as bearings, valves, cylinder heads and pistons. “The secret is to operate the engine as close as possible to the knocking threshold, but not to exceed it. To do this, combustion is set so that knocking occurs briefly, then the parameters are quickly readjusted,” says Hartmann.
The efficiency of the system is 90 percent – half of it electrical, the other half thermal
Pressure peaks can still occur, but they are much less harmful than with permanent knocking combustion and do not lead to any component damage if they are taken into account in the design of the engine components. For efficiency reasons, in the case of the power plant engines there is an additional engineering consideration, they are designed for particularly high ignition pressures of up to 250 bar, and the pressure spraying that can occur briefly during head control of the engine is correspondingly pronounced. “The ignition pressure level presented piston developers with a design challenge. After a detailed potential assessment, we decided on a two-part piston made of forged steel. The lower and upper halves of the component are bolted together,” says Hartmann, explaining the design of the piston.
Flexible output
The power plant engine by Innio Jenbacher is committed to longevity, simple installation and maintenance. It consists of standardized modules: Generator, engine and turbocharger. The engine concept ensures short downtimes; the complete engine unit can be exchanged as a module. The ease of maintenance is also apparent in the segmented camshaft. Each engine can provide 10.4 megawatts electrical power in less than 5 minutes when required. State-of-the-art model-based control technology ensures low emissions when starting the engine. Together with the integrated exhaust gas aftertreatment systems, it also minimizes nitrogen oxide emissions.
“In addition, the piston crown was adapted to gas operation and the specific requirements of the engine,” says Hartmann. The engine developers first calculated its shape by means of a simulation and then tested the piston on the test bench. This is because the optimum crown design plays a decisive role in determining whether combustion takes place homogeneously in the combustion chamber. “It is absolutely essential to avoid so-called hot spots, that is particularly hot areas on the component walls where fuel ignites uncontrollably and causes knocking.
Abundant flexibility
A special feature of the plant in Kiel is its high degree of flexibility, which allows the operating modes to be adapted to the specific electricity and heat requirements. In wintry weather, for example, the power plant delivers the required thermal energy and electricity to customers in Kiel. Excess electrical energy for which there is no local customer is sold on the electricity market. If good wind conditions in winter generate a lot of electricity from renewable sources, the capacities of K.I.E.L. can be reduced. Additional “green” electricity from the grid then heats the water for district heating in the electrode boiler. And in summer, when clouds and calm winds threaten to cause short-term bottlenecks in the supply of solar and wind energy, the K.I.E.L. intervenes at short notice, stabilizes the electricity grid and stores excess energy in the heat storage tank.
Idea ignites precedent
Other cities and municipalities have also recognized that innovative gas-engine power plants offer an environment-friendly and efficient alternative to traditional, frequently still coal-fired power plants. “We are experiencing an ever-increasing demand for pistons for power plant gas engines,” says Hartmann. For example, a new gas engine power plant is currently being built in Cologne. “Of course with pistons from Rheinmetall Automotive,” he adds.
