Waste heat utilisation
Basically, the possibilities for using (industrial) waste heat can be divided into three major areas. The most frequent use is plant or process internal use, as well as general internal use, since the advantages for the company are most directly noticeable here. A less frequently used form of use is decentralised or external use. Until recently, the connection to a district heating network was a precondition for the external use of waste heat. In the future, innovative companies will make the necessity of expensive and inflexible piping systems for district heating less and less important, and at the same time the economic efficiency of waste heat utilisation will become more and more important. The requirement to further reduce emission limits and increase the efficiency in the use of existing resources to fight climate change will force the industry to optimise existing processes across all sectors.
For the German industry, an older study (Schaefer 1995 / Source: Short study “Industrial waste heat” Fraunhofer ISI, Nov. 2013) puts the maximum technical usable potential of waste heat at around 45% of the energy input for industrial process heat. A more recent study (Pehnet et al. 2010 / Source: Short study “Industrial waste heat” Fraunhofer ISI, Nov. 2013), based on Sollesnes et al. 2009, estimates the technical-economic waste heat utilisation potential (temperature level above 60°C) in a first approximation at 18% of the final energy input of German industry. These two exemplary studies show that due to the overall small number of studies and the different definitions of term, region, segments, industry, etc., as well as different survey methods (data determination, calculation method, etc.), general estimates about the savings potential are difficult to make.
It is a fact that the latest generation of (thermal energy) storage systems makes it possible to use waste heat from room temperature up to 1300°C. The very high temperature in connection with an intelligent control of the heat output makes it possible to use the heat across industry and location boundaries, which was not possible in the past. For example, it is conceivable that the coke oven gas produced in a coking plant is burned by means of a burner instead of being flared into the atmosphere via the chimney, and that a mobile storage facility is heated to around 1000°C with it. Due to the high temperature level, there are several possible uses. It would be possible to generate electricity directly by means of a steam turbine or to preheat the air supply for the blast furnace. It would also be possible to use the heat decentrally, e.g. to heat a swimming pool. In many industries, the efficient use of their own process heat offers undreamed-of potential for optimising sustainability and reducing the use of primary energy.
A recurrent obstacle in the implementation of waste heat utilisation is the fact that waste heat is usually available in the form of (combustion) exhaust gases. Due to their often corrosive properties, waste gases make the use of heat exchangers necessary. These are then usually heavily loaded in daily operation. In addition, decentralised use of heat should not lead to any dependencies on the heat source and heat sink. Discontinuous availability also reduces, supposedly, usability. However, many of these (old) problems can now be solved through innovative heat storage systems. For example, there are storage systems that are insensitive to common combustion exhaust gases and therefore no longer require a separate heat exchanger. Also, the problem of dependence on processes and the discontinuous availability of waste heat can be eliminated by integrating a storage system. Due to these new possibilities, the area of decentralised use is becoming more and more the focus of companies. Also through the establishment of companies specialising in heat storage systems, waste heat utilisation is increasingly becoming an important aspect of efficient and ecological process design, without which a company will never achieve its maximum resource efficiency.