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Dieser Artikel geht der Rolle der industriellen Überschusswärme bei der Transformation des Energiesystems nach und konzentriert sich dabei auf Österreichs Ziele der Klimaneutralität. Er beginnt mit einer Skizze des aktuellen Energiestatus in Österreich und betont die Notwendigkeit von Maßnahmen im Heizungssektor zur Reduzierung der Treibhausgasemissionen. Der Artikel untersucht dann das Konzept des industriellen Wärmeüberschusses, seine Quellen und die mit seiner Nutzung verbundenen Herausforderungen. Sie stellt zwei Methoden zur Bewertung überschüssiger Wärmepotenziale vor: Bottom-up- und Top-down-Ansätze. Der Artikel beleuchtet auch erfolgreiche Fallstudien wie das INXS-Projekt und das SANBA-Projekt, die das Potenzial überschüssiger Wärme in verschiedenen Branchen aufzeigen. Darüber hinaus werden die Temperaturniveaus überschüssiger Wärme und die zu ihrer Nutzung verfügbaren Technologien diskutiert. Der Artikel schließt mit der Betonung, wie wichtig es ist, überschüssige Wärme in das Energiesystem zu integrieren, um eine Trendwende im Wärmesektor zu erreichen.
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Abstract
In Austria, heat supply accounts for more than half of the country’s final energy consumption, with low-temperature heat below 100 °C for space heating and domestic hot water making up around 61% of that. Approximately 47% of total heat consumption is still covered by fossil energy sources. In the energy-intensive industrial sector in particular, previously unused excess heat is available for cascading use. This publication presents industrial excess-heat potentials identified through individual case studies and a nationwide survey. According to the comparison of figures, in an ideal scenario roughly 65% of private space heating consumption could be covered by excess heat. In addition to conventional district-heating systems, which represent the current state of the art, new options are emerging for distributing excess heat, including supra-national district-heating networks for longer transport distances. For challenges associated with predominantly low temperatures, mature and proven solutions exist through the combination of heat pumps and anergy networks. Increasing the integration of excess heat is absolutely essential in order to raise the efficiency of industry and further decarbonize the heating sector.
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1 Introduction
Since the Industrial Revolution, global average temperatures have risen significantly. The last decade was the warmest on record. Since anthropogenic greenhouse gas emissions are primarily responsible for this, the international community has set itself the goal of reducing them [1].
For Austria, the federal government has set a target in its coalition agreement for the years 2025 to 2029 to achieve climate neutrality by 2040. This goal means that Austria-wide greenhouse gas (GHG) emissions and their reduction through carbon sinks, as recorded in the national GHG inventory, will be balanced by 2040 at the latest [2].
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By 2030, Austria must reduce emissions in the areas of transport, buildings, waste, and agriculture (non-ETS) by 48% compared to 2005 levels. For the ETS sectors, i.e., industry and energy, the target is 62% [3].
2 The Current Energy Status in Austria
In order to achieve a successful long-term transition of the energy supply structure, the gas and, in particular, the heating sectors must also be included, in addition to the current strong focus on the electricity sector: In Austria, heating applications account for more than half of domestic final energy consumption (Fig. 1; [4]), with low-temperature heat < 100 °C for space heating and hot water accounting for approximately 61% (61.2 TWh) and the remainder accounted for by medium- and high-temperature heat (58.0 TWh) used in trade and industry.
Fig. 1
Final energy consumption in Austria and share of renewable energy divided between heat, electricity, and fuels in 2024 (own illustration and calculation, data source: Statistics Austria 2024) [5]
Today, around 47% (69.4 TWh) of total heat consumption is still generated from fossil energy sources. To achieve our climate targets, it is essential to implement measures in the heating sector. In addition to expanding renewable energy sources, the main focus is on reducing primary energy consumption by increasing energy efficiency [4, 6]. The cascading use of excess heat is very often overlooked in this context and no precise data is currently available on the share of excess heat in Austria’s heat supply.
The following two sections present the fundamentals and methodology for assessing excess heat. Section 5 shows examples of projects that were carried out at the Chair of Energy Network Technology, and the last section discusses possibilities for the cascading use of excess heat.
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3 Excess Heat from Industrial Processes
When providing medium- and high-temperature process heat in industries, excess heat inevitably arises as a by-product in process plants, in the form of wastewater (washing, dyeing, and cooling processes), hot products, and, to a lesser extent, in electric motor systems and ventilation and air conditioning systems. Excess heat refers to the losses of a process and can be reduced through efficiency measures, but cannot be completely avoided [7].
Although the excess heat can be used elsewhere in the process, temporarily stored, or fed into heating networks, it is often no longer used despite its high energy potential [8]. Reasons against the cascading use of excess heat often include the poor quality of the excess heat (exergy) for production processes, a lack of knowledge about the possibilities for increasing the excess heat quality, or a lack of economic feasibility, even outside the company.
According to a study from 2019, more excess heat is produced across the EU than is required to meet the heating needs of European buildings [9]. According to conservative estimates, excess heat could cover at least 25% of Europe’s district heating supply [10]. Excess heat utilization is already being implemented to a greater extent in some countries, and Austria is certainly one of them. Prominent examples include the use of excess heat from paper mills (Sappi Gratkorn, Zellstoff Pöls AG), steelworks (VA Donawitz, Marienhütte Graz), power plants, and waste incineration plants (Mellach power plant).
In order to promote the efficient use of excess heat at different temperature levels in a structured manner, the existing excess heat potentials must first be identified. There are certain challenges involved in tapping this potential, particularly with regard to identification and qualitative assessment, as well as the mismatch between the profiles of excess heat formation and its consumption [8]. In particular, for smaller industrial and commercial enterprises that are not included in EU-wide or national centralized online databases (e.g., the E‑PRTR), there is often no data available at all.
4 Methodology for Assessing Excess Heat Potentials
In a simplified view, any industrial plant can be regarded as a “black box” in which all incoming energy must also leave the system in some form, with endothermic and exothermic processes needing to be considered separately. The main challenge lies in the fact that heat flows leaving the system are often highly diluted or dispersed—meaning they occur at low temperature levels within large and difficult-to-capture volume streams—and that this excess heat is frequently tied to products that cannot easily transfer their heat to another medium [11].
In principle, excess heat potentials can be determined using a bottom-up or top-down approach. It is also possible to combine both approaches (Fig. 2). With the bottom-up approach, data from the company’s control system is known, or temperatures, mass flows, etc. are measured. A questionnaire survey is also possible. If it is not possible to get data from the company, a publication-based bottom-up analysis can be carried out using published data (environmental report, EMAS, European Union Emission Trading System EU ETS, key figures from BREF documents), also with the help of specific key figures, e.g., from the best reference documents BREF. Exothermic and endothermic processes must also be taken into consideration accordingly [12].
Fig. 2
Overview of the general methodological approach to determine excess heat potentials [12]
If no relevant database exists, top-down analysis is used. Here, key figures are derived from statistical data (energy requirements, employee numbers, turnover, etc.) or taken from literature and applied to the companies [12].
Classifications according to temperature categories, carrier media, and the type of potential (technical, physical, etc.) also need to be made. [11].
5 Excess Heat Potentials of Selected Companies and Austrian Industry
The following section describes examples of how excess heat was determined in completed projects.
5.1 Survey of Industrial Excess Heat Potentials in Austria
As part of the research and service study for the “Energy Transition 2050” program, the usable excess heat potentials of both energy-intensive and energy-extensive industries in Austria were assessed in the project INXS (Industrial excess heat—Assessment of industrial excess heat potentials in Austria) [13]. The project consortium with the Chair of Energy Network Technology (EVT) at Montanuniversität Leoben as the consortium leader, applied a predominantly bottom-up approach for the energy-intensive sectors and a top-down approach for the energy-extensive sectors.
In total, a technical excess heat potential of around 34 TWh was identified for all sectors examined, including gas compressor stations and wastewater treatment plants. The roughly 300 companies in the energy-intensive industry, which were assessed primarily using the bottom-up approach, account for a total technical potential of about 26 TWh. Using the statistical top-down method, a technical potential of around 4 TWh was determined for more than 1500 companies in the energy-extensive industry with more than 50 employees. The individual values can be viewed on the Austrian Heat Map [14]. In addition, the excess heat potential of over 600 wastewater treatment plants was evaluated at approximately 4 TWh (see Fig. 3; [15]).
Fig. 3
Results of the excess heat survey by various ÖNACE categories and temperature ranges [16]
As part of the SANBA (Smart Anergy Quarter Baden) project [17], the potential availability of excess heat from the NÖM dairy was assessed for supplying a future low-temperature heating and cooling (LTHC) grid serving a former military site in Baden near Vienna.
Since the dairy was a project partner, measurements could be carried out and data from internal data recording could be used. This made it possible to perform a classic bottom-up analysis. The analysis of the processes revealed five possibilities for extracting excess heat (see Fig. 4).
Fig. 4
Overview of the identified excess heat potentials [18] (CA compressed air)
The analysis of the NÖM dairy has shown that about 19% of the final energy used (gas and electricity) can be decoupled as low-temperature heat [18].
5.3 Excess Heat Potential at the Gmunden Dairy
As part of the CASCADE project (Geothermally powered cascade heating and cooling grids for industrial, commercial, and housing use) [19], research was conducted to determine whether the Gmunden dairy could be supplied with geothermal energy and what excess heat potential was available for internal heat recovery. In this case, too, the dairy was a project partner, which meant that a bottom-up analysis could be carried out using existing data and measurements (see Fig. 5).
Fig. 5
Excess heat potential of the Gmunden dairy; percentages represent the proportion of excess heat compared to total energy used and add up to 38.3% [20] (IW-Pre‑C ice water pre-cooler, WWTP waste water treatment plant, HTHP high temperature heat pump)
The analysis at the Gmunden dairy revealed a physical excess heat potential of 38.3% of the total energy used (electricity and natural gas). This means that more than a third of the energy used would be available for internal heat recovery or external use [20].
6 Conclusion und Outlook
Examples from the food industry have shown that more than 30% of the energy used can be recovered by utilizing excess heat. In the INXS study [15] on Austrian industry, for example, this figure was around 17 to 29% in the iron and steel sector, 10 to 35% in the non-metallic mineral products sector, and up to 40% in brick manufacturing.
The determined excess heat potential of 34 TWh can only be properly evaluated when related to consumption values. Comparing the energy consumption for space heating and hot water across all sectors (approx. 90 TWh, data from NEA, Statistik Austria [5]), around one third of this could be covered by excess heat (see Fig. 6). In comparison to the space heating consumption in the private sector (approx. 55 TWh), as much as 62% could be covered by excess heat [13].
Fig. 6
Comparison of the excess heat potential of Austrian industry with the heat requirements of the sectors space heating, hot water production, and heating for agriculture, services, and production [16]
This leaves the question of excess heat distribution, temperature levels, and availability over time.
In terms of temporal availability, it can be said that most industrial processes, especially in energy-intensive industries, run evenly throughout the year with only minor fluctuations in output. This also means a relatively steady supply of excess heat.
When it comes to distributing excess heat, i.e., transporting heat to consumers, there are already several decades of experience with the economical use of conventional district heating networks. However, clusters of industrial plants with great potential for excess heat are often located far away from larger urban areas with significant heating requirements.
To address this challenge, the Heat Highway research project [21] (for the Mur valley and the central region of Upper Austria) investigated how regions could be connected via supraregional district heating networks—similar to transmission grids for electrical energy. Steinegger et al. [22] show that it is possible to supply Graz with heat from the industrial regions of Mur and Mürz valley in Upper Styria via a supra-regional district heating connection.
The third issue to be discussed is the temperature level. As can be seen from the results of the excess heat projects, the majority of excess heat (81% or 28 TWh) is in the temperature class below 50 °C (Figs. 5 and 6). Excess heat at this temperature level can only be used for drying processes, air preheating, or for feeding into 5th generation heating networks, also known as ultra-low temperature district heating (ULTDH) [23] or anergy networks.
Use for internal heat recovery or feeding into conventional heating networks requires the temperature to be raised using heat pumps. Intensive research in the field of heat pumps has made it possible to achieve temperatures above 200 °C and generate steam [24]. When feeding excess heat into anergy networks, the temperature must then be raised to the appropriate level at the individual users’ sites using a heat pump—but with very little energy expenditure. Anergy networks also have the advantage that summer cooling loads from buildings can be absorbed in an integrated borehole thermal energy storage (BTES), which serves as seasonal storage and load balancing [17].
In order to overcome obstacles to the use of excess heat, technical, organizational, legal, and financial conditions must be improved. Technical conditions include the further development of state-of-the-art technology, while organizational conditions often simply involve knowledge of excess heat potential. One legal recommendation for action would be, for example, the mandatory disclosure of excess heat potential similar to the ETS emissions trading system. Financial conditions can be improved through subsidies such as investment grants, bank loans, ERP loans, or contracting [15].
Simulation results show that the use of industrial excess heat can also create positive economic added value in the form of GDP growth and additional employment in Austria [15].
The question of whether excess heat can make a relevant contribution to the energy transition can be answered with a resounding yes. In fact, it must be said that, in order to achieve a turnaround in the heating sector, it is absolutely necessary to integrate excess heat more strongly into the energy system. In doing so, it is essential to also take low-quality excess heat < 50 °C into account in the heating sector.
Funding
Open access funding provided by Montanuniversität Leoben.
Conflict of interest
A. Hammer, J. Steinegger, K. Pfleger-Schopf and T. Kienberger declare that they have no competing interests.
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