The efficient use of energy has never been as important as it is today. In industry, process heat takes up a large share of the energy consumption that industrial plants need to produce all kinds of goods. However, the question “How can we produce more energy efficiently?” can only be answered once it has been clarified what current processes and involved aggregates consume in terms of energy. Especially in the case of complex large-scale plants, such as the research autoclave at CFK Nord in Stade, the answer is not easy. Energy monitoring systems create transparency: they not only make energy consumption visible, but also allocate it to individual consumers.
The scene of the crime – energy-intensive curing and consolidation processes in the autoclave
High-performance structural components made of thermoset and thermoplastic fibre composites are manufactured under pressure and heat in an autoclave to achieve the necessary component properties. This can involve relative pressures of up to 10 bar and temperatures of up to 180°C for thermosets or 420°C for thermoplastics. Often the pressure is generated with an inert gas, such as nitrogen, to prevent fires from occurring during heating. The process control – the alternation of heating, holding phase to the desired target temperature and final cooling phase – is very individually designed. Once the cooling temperature is reached, the pressure is released and the inert gas is vented to the environment. In the autoclave process, a consolidated component with a fibre volume content of about 60 % and a very low air void content is created during curing.
Which main consumers are responsible for this energy-intensive manufacturing process? One consumer is suspected that sits in the dock in many thermal machines and plants: the electrical resistance heating. A single culprit? Investigations show that this need not always be the case.
The taking of evidence – collection of consumer-oriented energy data
The measuring system of a world-renowned German industrial company with a focus on automation and digitalisation is used to gather evidence. The basis is formed by communication-capable measuring modules. They are installed in the control cabinets of the individual consumers and can transmit the recorded energy values in real time to a higher-level monitoring system. In the case of the Stade research autoclave, six of these measuring modules are installed in different control cabinets. In addition to the electrical resistance heating (power cabinets, control cabinet and fans), they also measure the energy consumption of the two fans, the control cabinet and the inert gas generation.
The investigation result – main consumers identified
After the successful commissioning, the energy monitoring system is now being used in the NBank project LEITWERK (thermoset curing at 180°C) and in various EU projects (thermoplastic consolidation at 420°C). In the case of the curing process for a thermoset component, the result is surprising: in addition to the suspect, another sub-process comes into focus and is identified as the main consumer: It is the generation of the inert gas, which is used to build up the pressure in the autoclave. In terms of percentage, the generation of inert gas has an energy consumption comparable to that of electrical resistance heating. The fans are identified as auxiliary consumers. The control cabinet as a consumer is hardly noticeable.
In the case of the consolidation process (thermoplastic, 7 bar, 385°C), however, the energy consumption of the electrical resistance heating is dominant; all other consumers are subordinate to it. Summarised in one figure, this is 81% of the total energy consumption. These initial results show where there is potential for savings. Energy efficiency can be increased by using heatable tools in the autoclave to reduce the energy consumption for the heating process of mould tools,
- sensor systems to shorten the process time,
- alternative fluid systems with energy-efficient generation,
- Intermediate storage for the reuse of process heat,
- renewable energies for auxiliary consumers.
Initial estimates suggest a 50 % reduction in energy consumption in the autoclave process.
