Heat in the industrial sector

The economic potential for increasing efficiency has already largely been exhausted in many conventional industrial processes. Furthermore, all of the technical potential needs to be tapped in the future. Above all, it is important to reduce primary heat demand by optimising or converting processes and preventing heat loss, e.g. through optimal insulation, because: heat that does not need to be produced in the first place has the best carbon footprint. Beyond that, “unavoidable” waste heat can be utilised in a variety of ways. For example, factories and businesses can use it directly for preheating processes, to heat premises and to heat water. Alternatively, it can also be fed into a local or district heating network and thus made available to external parties, for example a local business, or to heat buildings in urban districts.

Converting to electricity is also possible. If the temperature of the waste heat is too low to utilise, it can be increased using heat pumps, for example. Waste heat utilisation offers great potential for climate protection since nearly half of the energy used to generate heat is still currently being lost as dissipated heat. A recent analysis of potential by the North Rhine-Westphalia Agency for Nature, Environment and Consumer Protection assumes 88 to 96 terawatt-hours of technically available waste heat per year in NRW, of which around half (44 to 48 terawatt-hours) is reckoned to be technically usable – this would equate to a CO₂ reduction of up to 13 million tonnes per year. When developing the much-discussed prospective hydrogen infrastructure, further waste heat sources can be expected, particularly from electrolysers.

The upside is that there are no additional conversion losses when using renewable heat. The downside, however, is that renewable sources of heat at temperature levels that are technically and economically usable for industrial purposes are only available to a limited extent in NRW. 

In this country, temperatures of up to about 250°C can be generated using solar thermal power with what are known as non-concentrating collectors, which use both direct and diffuse solar radiation. However, this type of heat production is extremely volatile and depends on the time of day. If combined with a heat accumulator, however, solar heat can offer sound support to generate renewable heat for low temperature applications such as drying and heating processes, e.g. in the food industry.

The use of deep geothermal energy, also known as deep geothermics, involves drill depths of over 400 metres. The temperatures that can potentially be achieved are heavily dependent on the location and the quality of the subsoil. In NRW, large segments of the subsoil have yet to be sufficiently investigated, but this area is presumed to have great potential. With geothermics, it is possible to achieve temperatures of up to 180°C at depths of several kilometres. One great advantage is its base load operation capability, which means that it is not subject to fluctuations due to the time of day, time of year or the weather. For this reason, it is suitable for providing a reliable supply for industrial processes at lower to middle temperature ranges (e.g. drying processes in the paper industry) and can also be combined with heat pumps if necessary to boost the temperature level.

The opportunities for converting electricity generated by renewable resources into heat are manifold and are amalgamated under the term “Power-to-Heat”. If heat is produced using electricity, no direct greenhouse gas emissions arise. A key prerequisite for becoming neutral along the entire efficiency chain, however, is that electricity is produced sustainably, e.g. from sun, wind or water, and that the proportion of renewable energy in the electricity mix on the grid is increased considerably.

Factories can operate electrode boilers for instance, which convert electricity into hot water or steam. These systems can reach maximum temperatures of several hundred degrees. Until now, however, process steam has mainly been produced using fossil sources, since natural gas is still a cheaper fuel than electricity. Moreover, electrification using the electricity mix currently available is not yet advisable from a climate protection perspective. Nevertheless, there are potential advantages: the level of efficiency when using electricity in a boiler is high and electrode boilers can easily be integrated into systems at relatively low investment costs.

Heating technologies that use conduction (by means of electrical resistance), induction or electrical arcing, for instance, offer further options for generating heat from electricity with a view to heating or even smelting glass, aluminium and steel.

In the future, industrial companies could potentially even use surpluses from volatile renewable energy production to generate the heat they need in a flexible and cost-effective way, thus making a contribution to storage and grid stabilisation.