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| Fig. 1: Temperature requirements of various industrial processes and district heating. [4] (Image Source: A. Mukunda) |
Globally there are two main categories of greenhouse gas emissions from heating. Residential and commercial heating and cooking (primarily space and water heating) and industrial processes that require heat. The heating and cooking in buildings are estimated to account for roughly 6% of global greenhouse gas emissions. [1] Heating in industrial processes are estimated to account for approximately 8-10% of global emissions (industrial processes account for roughly 22 percent of global CO2 emissions and roughly 40% of these emissions come from heating). [2] This 10% of global emissions from industrial heat applications are considered to be among the hardest to decarbonize because they cannot easily be displaced by renewable energy technologies like wind and solar.
The IEA estimates that global industrial heat energy demand will grow from about 85 exajoules in 2017 to roughly 115 exajoules by 2040 with growth in demand for medium and low heat applications being slightly larger than growth for high heat applications. [3] Estimates of exactly what currently accounts for the bulk of industrial heating requirements vary. Roughly speaking however, 25% of industrial heating requirements are estimated to be for low temperature applications (below 100°C), 25% are estimated to be for medium temperature (100°C-400°C), while the remaining 50% are estimated to be for high temperature applications (> 400°C). [3] Fig. 1 shows typical examples of required supply temperatures for various processes as well as typical supply temperatures for district heating (a system for distributing heat to residential and commercial buildings within approximately 100 kilometers of a powerplant). Currently some of the most important processes represented on the graph are the production of steel, cement, aluminum, petrochemicals, and glass. In the future processes like the production of Hydrogen using either electrolyzers (low temperature) or steam methane reforming (high temperature), as well as direct air carbon capture, and desalination are expected to grow. [4]
Traditional nuclear reactors such as the light-water moderated reactors in use throughout the United States have a maximum coolant (steam) outlet temperature of about 300°C; Helium and CO2 gas cooled reactors can produce steam up to 540°C, and high-temperature gas cooled, graphite moderator reactors can produce process heat up to 950°C. [5] Fig. 1 shows the cutoffs for supplied outlet temperatures for various types of reactors.
In most nuclear reactors much of the heat that is produced is vented into the environment. When this heat is used for industrial processes or district heating, it is referred to as cogeneration of heat and power. This may seem like a new concept, but cogeneration actually has a long history in nuclear reactors with the first nuclear cogeneration plant being located at Calder Hall in the UK and operating from 1953 to 2003. [4] Low and high pressure steam was used for reprocessing, building heat, and other industrial processes while the plant also generated electricity. More recently, XEnergy and Dow Chemical in the United states are currently constructing a high temperature gas cooled reactor where waste heat will be used in the manufacture of plastics. Cogeneration for district heating (where hot steam/water is pumped from the reactor to homes and offices) has also been used in many places. The Agesta reactor outside Stockholm, provided heat to suburbs of the city for 10 years. Many other sites primarily in Russia and former Soviet states also used or have used district heating for years. More recently, the Haiyang reactor in Shandong China is projected to provide heating for the entire city of Haiyang with a population greater than 300,000. [4]
The upshot of cogeneration is that while traditional nuclear reactors are only able to convert roughly 35% of the energy released from nuclear fission into electricity, cogeneration is able to make use of far more of the released energy. One study on high temperature helium cooled reactors with waste heat being used to produce Hydrogen showed an increase in efficiency to 52.4%, while historically, nuclear reactors used in district heating systems achieved overall efficiency up to 80%. [4,6] To put this in perspective, consider the following calculation. Globally, nuclear reactors currently have a capacity of 413 GW of electricity production. For a rough estimate assume that none of these currently, use cogeneration, that they all run at 100% capacity factor and have a 35% efficiency. This means that they produce approximately 13 exajoules of energy per year. If they could all be used for cogeneration and reach 80% efficiency 100% of the time this would mean they could supply an additional 16.7 exajoules of heat energy for low and medium temperature industrial uses (under 350°C). This is enough to cover about 78% of the world's roughly 21 exajoules (21 × 1018 J) in demand for low temperature heat for industrial uses. Of course this theoretical limit is not achievable especially when only considering district heating. Many nuclear plants are located too far from population or industrial centers or in climates where district heating is not useful most or all of the year (heat distribution also has an upfront investment cost that could be prohibitive in locations that already have pre-existing infrastructure for decentralized heat generation). However, the opportunity for colocating new industrial facilities, especially ammonia, petrochemical, and hydrogen synthesis plants with gas cooled and high temperature gas cooled reactors (especially small modular reactors) as well as colocating desalination, direct air carbon capture, plastics recycling, methanol production, or bleaching/drying/sterilization with traditional nuclear reactors should all be explored as the cost of emissions from traditional heat sources increases over time.
© Amar Mukunda. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Krishnan et al., "The Net-Zero Transition: What It Would Cost, What It Could Bring," McKinsey Global Institute, January 2002.
[2] S. J.Friedman, Z.Fan and K. Tang, "Low-Carbon Heat Solutions For Heavy Industry: Sources, Options, and Costs Today," Columbia School of International and Public Affairs, October 2019.
[3] "World Energy Outlook 2017," International Energy Agency, 2017.
[4] "Nuclear Cogeneration: Civil Nuclear Energy in a Low-Carbon Future," The Royal Society, October 2020.
[5] J. Kupitz and M. Podest, "Nuclear Heat Applications: World Oveview," IAEA Bull. 26, No. 4, 18 (December 1984).
[6] J. Jerzedjewski and M. Hanuszkiewicz-Drapala, "Analysis of the Efficiency of a High-Temperature Gas-Cooled Nuclear Reactor Cogeneration System Generating Heat for the Sulfur-Iodine Cycle," J. Energy Resour. Technol. 140, 112001 (2018).