Review of Challenges in Heat Exchanger Network Development for Electrified Industrial Energy Systems

Abstract

Shifting towards electrified industrial energy systems is pivotal for meeting global decarbonization objectives, especially since process heat is a significant contributor to greenhouse gas emissions in the industrial sector. This review examines the changing role of heat exchanger networks (HENs) within electrified process industries, where electricity-driven technologies, including electric heaters, steam boilers, heat pumps, mechanical vapour recompression, and organic Rankine cycles, are increasingly supplanting traditional fossil-fuel-based utilities. The analysis identifies key challenges associated with multi-utility integration, multi-pinch configurations, and low-grade heat utilisation that influence HEN design, retrofitting, and optimisation efforts. A comparative evaluation of various methodological frameworks, including mathematical programming, insights-based methods, and hybrid approaches, is presented, highlighting their relevance to the specific constraints and opportunities of electrified systems. Case studies from the chemicals, food processing, and cement sectors demonstrate the practicality and advantages of employing electrified heat exchanger networks (HENs), particularly in terms of energy efficiency, emissions reduction, and enhanced operational flexibility. The review concludes that effective strategies for the design of HENs are crucial in industrial electrification, facilitating increases in efficiency, reductions in emissions, and improvements in economic feasibility, especially when they are integrated with renewable energy sources and advanced control systems. Future initiatives must focus on harmonising technical advances with system-level resilience and economic sustainability considerations.

1. Introduction

The magnitude of the climate crisis now demands the complete decarbonization of the economy rather than merely reducing emissions. This means not just improving the efficiency of fossil fuel use but eliminating it entirely. Achieving this goal requires transitioning to zero-carbon energy, primarily through the electrification of sectors that have traditionally relied on non-electric energy sources, and rethinking the role of energy efficiency in decarbonising society [1]. The industry sector generated 9.0 Gt of CO₂ in 2022, which is about 25% of the global emissions, according to the International Energy Agency (IEA) [2]. This amount must be reduced by an ambitious 7.0 Gt of CO₂ by 2030 according to the Net Zero Emissions by 2050 Scenario (NZE Scenario) [3]. In 2022, the heat process accounted for approximately two-thirds of the industrial sector’s greenhouse gas emissions. Achieving a climate-neutral industry necessitates a transition in processes of heat supply towards carbon-neutral energy sources. Since the industrial sector primarily relies on fossil fuels, and renewables are not yet widely used in industry, these emissions can be significant contributors to energy consumption. For example, according to Eurostat, the industry in the European Union (EU) was responsible for 25.6% of the total energy consumption in 2022 [4], and primary relied on electricity and natural gas. In contrast, renewables and biofuels accounted for 11%, as illustrated in Figure 1.

A detailed analysis of the technological potential of hydrogen and electricity as alternative sources for the process of heat generation has identified the primary barriers to their adoption and outlined the necessary actions for effective policy frameworks [5]. The electrification of the chemical industry necessitates the identification of well-defined targets for chemical processes that can significantly influence the energy–carbon nexus [6]. Prioritising such processes is crucial for maximising the impact of these interventions on energy efficiency and carbon reduction. By focusing on these key areas, the industry can accelerate the transition towards sustainable and low-carbon production methods, and thereby address the dual challenges of reducing greenhouse gas emissions and optimising energy use [7]. The impact of industry electrification within the framework of various 100% renewable energy system scenarios can be considered by different key stakeholders [8]. For example, one study examined how the role of electrification may vary depending on the specific context. The findings indicate that the direct electrification of industrial heat generation is generally preferable to alternatives, such as shifting to hydrogen-based process systems, despite the latter’s potential to offer greater flexibility. This preference for direct electrification highlights its superior efficiency and effectiveness in contributing to decarbonization efforts within industrial processes [9].

The difference between fossil-fuel-based and electricity-based industrial heat supply is presented in

Table 1. Comparison of fossil-fuel-based and electricity-based heat supply in industry.

Aspect	Fossil-Fuel-Based Heat Supply	Electricity-Based Heat Supply
Energy Source	Combustion of coal, natural gas, oil	Electricity from grid or renewable sources (e.g., wind, solar, etc.)
Emissions	High (CO2, NOx, SOx emissions) [10]	Very low if sourced from renewables, varies with grid mix [11]
Efficiency	30–60% due to combustion losses [16,17]	90% and higher with technologies like resistance or induction heating [24]
Heat Transfer Method	Direct flame or hot gases [25]	Electric resistance [26], induction [27], microwave [28], or heat pumps [29]
Temperature range	High temperatures up to 2000 °C [30]	Some technologies limited in max temperature [31]; high-temperature options emerging
Infrastructure compatibility	Widely compatible with existing systems	May require retrofitting or new equipment [32]
Operational flexibility	Slower response times, good for base load [33]	Fast response, better for dynamic operation and smart control [34]
Fuel supply chain	Requires transport, storage, and handling of fuels [35]	Simpler logistics, just grid or onsite electricity [36]
Air pollutants	Produces particulates, NOx, SOx, etc. [10]	Virtually none at point of use [11]
Decarbonization potential	Limited, unless using carbon capture and storage [37]	High, especially with clean electricity [38]
Maintenance requirements	Higher due to moving parts and combustion residues [18]	Lower for many electric systems [20,21]
Initial investment	Generally lower upfront for existing technologies [14]	Higher initial cost with possible long-term savings [19]

. Fossil fuel-based heating relies on burning coal, oil, or natural gas in boilers and furnaces, which leads to significant greenhouse gas emissions and air pollution [10]. In contrast, electricity-based heating systems, including electric heaters and heat pumps, can utilize a mix of energy sources, especially renewables like wind, solar, and hydro [11]. While electric heating is typically more efficient, its environmental impact is influenced by the carbon intensity of the electricity grid [12], and renewables may impact the intensity of local grid emissions [13]. Transitioning to electric heating, particularly with heat pumps, is essential for decarbonizing the industrial sector and reducing its reliance on fossil fuels, and could pave the way for a cleaner and more sustainable energy future. Fossil-fuel-based heating systems, such as gas boilers and oil furnaces, have relatively low initial capital costs due to their established technologies and straightforward installation [14]. However, they face higher operational costs over time due to fluctuating fuel prices [15], their lower thermal efficiency [16,17], and the need for increased maintenance [18]. In comparison, electricity-based systems, especially heat pumps, demand a higher upfront investment due to their complex installation and advanced components [19]. Nevertheless, they offer significantly lower operating costs and a reduced need for maintenance [20,21]. Despite the higher initial expenditure, electricity-based heating systems provide greater long-term economic and environmental benefits, particularly in regions with low-carbon electricity grids [22]. However, it is important to quantify the embodied emissions of network assets in relation to the operational grid over time, accounting for the CO₂ per assets cost, functional unit, energy source, and operational period [23].

Process industries, where the target product requires chemical transformations, remain the primary energy consumers. These industries are chemical, petrochemical, pulp and paper, food and beverages, etc. The production system of these industries includes chemical reactors, product and by-product separators, and a heat exchanger network (HEN) that supplies and recovers energy from the main processes and utilities (see Figure 2). HENs play a crucial role in the energy use of process systems through heat recovery and supplying thermal utility for heating and cooling purposes; the main perspectives on these networks are described in [39]. Negative emission technologies exhibit varying levels of permanence and durability in their carbon dioxide removal capabilities, which may present significant challenges. However, established principles and process integration techniques can be effectively adapted to address the emerging complexities associated with planning carbon dioxide removal portfolios [40].

Heat exchanger network synthesis [41] and retrofitting [42] are long-term objectives that have been developed by many researchers [43]. These objectives are mainly related to fossil-fuel-based industries with specific utility system features [44] and HEN development approaches [45]. Environmental challenges are changing the paradigm for industrial energy systems, and one of the main challenges is electrifying fossil-fuel-based utilities by replacing them with renewable energies.

Modified approaches for HEN synthesis and retrofitting include the application of new heating and cooling strategies, heat recovery, and heat engine applications, as well as trade-offs between the capital expenditure (CAPEX) and operational cost (OPEX). The chemical process, utilising an electrified thermal utility and a heat exchanger network, is illustrated in Figure 3.

The diagram in Figure 3 depicts a simple chemical process integrated with an electrified utility system, highlighting electricity’s role in facilitating thermal and mechanical operations within a sustainable industrial framework. Central to this system is an electrified boiler that utilises electrical energy, symbolised by the transmission tower, to produce high-pressure steam (illustrated by red lines). This steam is conveyed through a network of heat exchangers, where it transfers thermal energy to various process streams and thereby supports endothermic chemical reactions or assists in meeting heating requirements. Upon releasing its thermal energy, the steam is condensed (represented by blue lines) and converted back into water, which is then pumped through feedwater preheaters, which completes the closed-loop system. The utility network substitutes traditional fossil-fuelled boilers with electric heating solutions, promoting decarbonization and operational flexibility [46]. Additionally, the presence of electrically driven compressors and pumps underscores the transition from fossil-fuel-dependent mechanical processes to electrification. The interconnected loops and control valves signify an optimised heat exchanger network that maximises energy recovery, minimises emissions, and provides precise temperature control throughout the processes, all of which are reliant on a centralised electric grid.

2. Challenges in Heat Exchanger Network Developments

2.1. Multiple Utility and Driving Forces

The electrification of industrial energy systems presumes the use of different types of utility heat transfer equipment such as electric furnaces [47], electric heaters [48], plasma heaters [49], electric steam boilers [50], etc. This means that the targeting procedure must be modified from that used for fossil-fuel-based energy systems [51]. The main principle is keeping the hot utility colder and the cold utility hotter, but there is no need to cascade the heat from the high-temperature utility to lower levels, which makes cogeneration useless [44].

However, this problem becomes untrivial when considering different types of equipment for process stream heating. For instance, in the case of steam heaters, the heat transfer area of heat exchangers can be reduced when the steam temperature is higher [52]. However, this may increase the equipment cost due to the pressure and temperature levels of the vessel being higher as a result. The problem with energy capital trade-off may be reduced when using the direct heating of process streams with electric heaters, electric furnaces, etc. The diagrams in Figure 4 illustrate this issue.

Steam expansion is ineffective for electricity generation; the electricity originally obtained from cogeneration plants should be replaced by electricity exported from the grid or produced by renewable energy sources [53]. Electricity-based heating systems offer greater flexibility in HENs compared to fossil-fuel-based systems. This increased flexibility can significantly reduce the design complexities associated with HENs under electrification. Modifying the industrial utility system also affects the development strategy for utility heat exchangers in the overall network context, and the number of units, unit type, and temperature level should be defined. Site-wide energy systems can be engineered based on the thermal demands and specific types of utilities required at the process level. In industrial complexes with multiple units or processes, a centralised utility system is typically employed in non-electrified environments to generate and distribute steam to individual plants [54].

2.2. Multi-Pinch Problem

The utilisation of different temperature levels for utility heat transfer in HENs creates multiple pinch points, complicating the design and optimisation process. In conventional systems, this challenge is typically addressed by dividing the HEN into distinct design regions [55], each of which is balanced using appropriate utilities [56], such as low- or medium-pressure steam or process furnaces, to meet the remaining heat demands. Electric heaters can be employed in any design region in electrified processes, offering greater flexibility. However, the selection of heating units must be carefully tailored to the specific process conditions. Additionally, when integrating different design regions that are separated by utility pinches, the number and configuration of electric heaters, steam boilers, and furnaces must be optimized to ensure efficient energy utilization and minimize operational costs. The targeting procedure demonstrates the temperature profile for heating cold streams but different heaters can be used for different process streams at the same temperature level, as presented in Figure 5. Further, an additional degree of freedom exists when using electric heaters, since they can be used at any temperatures that do not violate the pinch rules [57], which creates additional variables when developing HENs with mathematical programming [58].

When addressing high-dimensional problems in the synthesis of HENs, achieving near-globally optimal solutions is paramount. This objective can be effectively pursued through a recently proposed two-step approach that integrates mixed-integer linear programming (MILP) and mixed-integer nonlinear programming (MINLP) techniques [59]. This novel methodology capitalises on the decomposable structural characteristics inherent to HENs, enabling the independent synthesis of multiple HENs in the initial step [60]. Subsequently, a compact and computationally efficient MINLP model is formulated in the second step, which facilitates the cost-optimal design of HENs within each plant. Such approaches streamline the synthesis process and enhance the scalability and feasibility of solving complex HEN synthesis problems and HEN process design in industrial applications [61].

2.3. The Use of Heat Pumps, Mechanical Vapour Recompression and Organic Rankine Cycle

The emerging paradigm of industrial energy systems, characterised by a high reliance on electricity, necessitates the integration of various heat engines to enhance energy integration and optimise the utilisation of low-grade heat, which is often dissipated into the environment [62]. This consideration is increasingly essential in integrating renewable energy sources into industry [63], where efficient energy management and waste heat recovery are critical for maximising overall system efficiency and sustainability [64]. As renewable energies and CO₂ capture continue to play a larger role [65], effectively harnessing and repurposing low-grade heat becomes essential to achieving a more resilient and energy-efficient industrial infrastructure [66].

2.3.1. Heat Pumps

This increasingly critical concept necessitates the development of an expanded suite of process analysis tools to thoroughly understand the complex interactions between heat recovery and process electrification, such as the integration of heat pumps [67]. This development can be carried out with exergy pinch analysis tools and methodologies to establish lower-bound work targets by precisely balancing process heat recovery with heat pumping strategies [68]. These advanced tools enable a more accurate assessment of the thermodynamic efficiency of industrial processes, providing a robust framework for optimising energy use and enhancing the sustainability of electrified industrial systems. The heat load flexibility of the process plant is crucial for the efficient integration of heat pumps, as it may seriously affect the system’s economic performance; this is confirmed by the increase in net present value (NPV) of 19.3% obtained in a specific case study [69]. The economic efficiency of the integration of a heat pump into the industrial process can also be improved by varying the temperature of the refrigerant in condensers and reboilers, which affects the HEN [70]. Dynamic pinch analysis targeting improves heat pump integration into batch process plants, resulting in up to 33% less equipment size and decreased operating costs [71]. An optimal heat-integrated water network with a heat pump can be developed by creating the superstructure with mathematical programming [72]. The cost of energy inputs plays a significant role in determining the choice between a heat pump and a gas boiler, and thus impacts the overall system efficiency as well as the marginal cost of the final solution [73]. The use of heat pumps within industrial systems was extensively summarised by Bobbo et al. [74], in which the essential role of heat exchangers was underlined and described in different industrial environments.

Distillation remains the predominant and most widely utilised method for splitting components in condensable mixtures and requires enormous amounts of thermal energy. Heat pumps are vital for electrifying distillation systems [75], which use around 30% of the heat energy obtained through chemical processes via utility heat exchangers. Heat pumps could be a part of a heat recovery system that utilises the heat of condensers to process stream heating. The electrification of distillation columns has been investigated intensively over the last decades [76], which has delivered research insights and practical recommendations. Exergy-based optimisation enables an improved heat load distribution, which can significantly enhance the internal efficiency of HENs and further facilitate the conceptual design of heat pump-assisted distillation. It was applied in the azeotropic distillation of a multi-component mixture [77], natural gas liquid processing [78], methanol-water distillation [79], biofuel recovery [80], and other applications. For instance, applying heat pump-assisted distillation for light hydrocarbon separation generates an extensive HEN and results in a 60% reduction in thermal energy consumption [81].

2.3.2. Mechanical Vapour Recompression

Some processes can be completely electrified by selecting alternatives other than mechanical vapour recompression (MVR) and heat pumps [82]. Vapour recompression heat pumps play a supplementary role in decarbonising distillation processes by enhancing their efficiency and facilitating the electrification of process units. The use of intermediate heating methods, such as intermediate reboilers and feed evaporators, can further optimise the performance of these heat pumps [83]. By lowering the temperature of the heat source, these methods contribute to reduced payback for investments, which makes them more economically viable. This may improve energy efficiency and support the transition towards sustainable and low-carbon distillation processes. Modifying milk production with MVR and thermal vapour recompression (TVR) resulted in primary energy savings of 13.7–41.6% and emissions savings of 14.5–47.3%. Further, such changes in energy and environmental benefits result in complications in HEN retrofitting [32]. MVR was used for an ethane-ethylene distillation column with optimised process parameters; as a result, the refrigerant consumption for overhead vapour condensation was reduced by 34.29% compared to the base case, and the overall utility consumption was reduced by more than 50% [84]. The final design requires both the involvement of new heat exchangers and the use of existing ones, which initiates a new optimisation problem for the generated heat recovery network. The investigation of the use of MVR for milk production, alongside the application of triple-effect fouling film evaporators, reveals the necessity of several control loops that also generate some limitations of the specific process [85]. Vapour recompression is mostly used in the food industry due to the low-temperature processes that are common in this industry, and efficiency can be confirmed from both the technical and economic sides. Different vapour recompression systems used in the food industry were summarised, and the configuration of heat transfer equipment was demonstrated [86]. However, MVR can also be used to intensify different processes, e.g., biodiesel production, which results in the electrification of the thermal utility, the extension of the water cooler network, and the need to reassemble the main process flow diagram (PFD) [87]. The application of this method in sulfuric acid waste treatment demonstrated improved energy efficiency due to recovering the latent heat of vaporization, but the exergy efficiency was still low, with high exergy destruction being contributed by compressor [88]. This confirms the opportunities for process improvements at both the unit and system levels, especially if such technology is part of a large industrial facility. The optimisation of heat-integrated distillation sequences can be performed by stochastic optimisation that employs MVR as the main vehicle of electrification [89]. The generated flowsheets for five-component separation include the heat exchanger networks of condensers and evaporators with varying total annual costs, where the detailed design of the system is challenging due to the simultaneous condensation and evaporation in the same vessel. Both network optimisation and process intensification are vital to achieving CAPEX reduction. Intermittent mechanical vapour recompression is also considered to reduce the primary energy target, and a case study demonstrated that 73.86% less energy was used compared to the single-effect evaporation [90]. The synergistic effect of MVR coupled with HEN optimisation may reduce the thermal energy targets and cut the electricity requirements, which are crucial when integrating renewable electricity into industrial processes [83]. The batch processes require a completely different approach for their process integration and optimisation [91]. However, MVR can be applied with heat steam storage technology [92]. The experimental results confirmed that the batch evaporation process consumes a small amount of electricity on a specific schedule. This issue should be additionally addressed when integrating heat pumps into the industrial environment. The control strategy becomes crucial, as it allows for the integration of thermal energy storage (TES) into industrial facilities and improves the flexibility of electrified batch processes [93].

2.3.3. Organic Rankine Cycle

Electrified industrial energy systems assume that heating and cooling facilities are powered by electricity from the grid, and such systems require a significant grid extension to be implemented. In this scope, industrial waste heat can be used for electricity generation to reduce energy export from the grid. The organic Rankine cycle (ORC) is a long-term developing technology which can be used for these purposes. However, it faces many challenges when applied on an industrial scale [94]. The energy targets of industrial clusters can be additionally investigated to find the maximum energy utilisation by an ORC, as shown in Figure 6.

The integration of the ORC generates a new problem for HEN development, especially when considering big industrial sites. Hipólito-Valencia et al. demonstrated the complexity of the superstructure (see Figure 7) and problem-solving using illustrative case studies that include plants with two hot and cold streams [95]. A real industrial environment has many more process streams that increase the task complexity, e.g., a polymer plant’s specific pyrolysis and gas separation units have more than 120 streams, of which more than 60 are cold ones [96].

The utilisation of industrial low-grade waste heat with a reversible high-temperature heat pump and ORC may generate both electricity and useful heat, but the HEN’s performance should be optimised as well as that of the individual heat exchangers [97]. The dynamic performance of the ORC for the utilisation of industrial waste heat is crucial to maintaining the pinch point while condensing the working media, which seriously affects the heat exchanger’s performance and requires a modelling approach [98]. A hybrid system can utilise industrial waste heat, incorporating a solid oxide fuel cell, a gas turbine, and reverse osmosis for power and water production. The ORC can be used as a subsystem to utilise the waste heat generated by the system. The process flowsheet includes eight heat exchangers that have the potential for performance optimisation despite the proven economic efficiency of the system, which may generate an additional reduction in operating costs of up to 20 million euros [99]. The prediction of the transport properties of hydrocarbon working fluids is important to defining the proper sizing of all ORC components, especially heat exchangers [100]. The availability of waste heat sources is often inconsistent, which adds more variables when integrating the ORC and can result in an additional problem in proving the techno-economic viability of heat transfer equipment [101].

Low-temperature process heat, often considered a by-product and typically discarded through cooling systems [102], represents a significant and underutilised energy resource within industrial sectors. Instead of being yielded as waste, this thermal energy can be effectively repurposed for a variety of applications, including district heating networks [103,104], ORC systems [105,106], and comprehensive total-site heat integration strategies [107,108]. This repurposing enhances energy efficiency and promotes sustainability within industrial processes.

A critical enabler of this transition from waste heat to valuable resource is the implementation of heat pumps to elevate low-grade thermal energy to more usable temperature levels, which thereby expands the potential applications of this previously neglected energy source [109]. Heat pumps have emerged as a foundational technology that is essential for optimising energy consumption, electrifying process heat, decreasing operational costs, and minimising carbon emissions throughout industrial clusters by facilitating temperature lifting and addressing discrepancies between energy supply and demand. The reconfiguration of the power distribution system acts as a vital mechanism for service restoration by optimizing the system’s topology through the appropriate adjustment of designated tie and sectionalizing switches. In parallel, the reconfiguration of the district heating system presents an effective strategy for addressing disruptions by optimally adjusting the heating supply topology with specifically targeted tie and sectionalizing valves within the district heating [110]. This challenge can be further addressed through risk-averse decentralized energy management strategies, which are designed to mitigate the adverse impacts of uncertainties associated with renewable energy sources and fluctuating energy prices [111].

The systematic harnessing of this “lost” energy creates a new capacity for sustainable and circular energy use, marking a pivotal advancement in achieving resilience and sustainability in industrial operations [112]. Furthermore, integrating low-temperature process heat into energy systems fosters a decentralised energy approach, reducing reliance on fossil fuels and promoting the incorporation of renewable energy sources [113]. In addition to district heating and ORC systems, low-temperature heat recovery can support various applications such as agricultural heating, greenhouse operations, and residential heating solutions. The versatility of reusing this energy showcases its potential to alleviate pressure on existing energy infrastructures while providing economic benefits to industries that adopt these innovative solutions.

As industries seek to future-proof their operations against climate change and resource scarcity, leveraging low-temperature process heat through advanced heat pump technologies aligns with broader sustainability goals [114]. This transformative approach supports compliance with regulatory standards for emissions reductions, enhancing overall operational efficiency and economic viability, and paving the way for a more sustainable industrial landscape. Maximising the utility of low-temperature process heat is not merely an option, it is becoming an imperative for industries that are striving to thrive in an increasingly environmentally conscious world.

2.4. The Role of Energy Efficiency

The electrification of industrial processes will significantly increase the demand for renewable energy in order to achieve decarbonization on the supply side. This shift will necessitate substantial investments in energy generation infrastructure and grid expansion, which must ensure a reasonable return on investment. To alleviate the associated economic burden, the concept of energy recovery in process systems should be revisited. In traditional process systems, heat recovery targets are determined by balancing the energy costs against the investment required for heat transfer areas (Figure 8a) [115]. The use of renewable energy may shift the optimal point to a higher ΔT_min (Figure 8b) due to recent trends in renewable energy prices, which have surpassed those of fossil fuels, according to Net Zero Emission by 2050 [116]. However, when implementing electrified industrial utility systems, grid expansion and the potential need for new energy generation facilities must be considered in optimising such trade-offs. The selection of the optimal minimum temperature difference (ΔT_min) may involve a balance between grid extension and HEN investment (Figure 8c).

When addressing such challenges, reducing the primary energy target is essential to achieving the economic feasibility of industrial energy systems. Enhanced heat recovery will likely play a dominant role, potentially supplemented by heat engines; however, optimisation remains necessary to ensure both technical and economic viability. In this context, maximising heat recovery leads to minimising driving forces to their lowest possible values and imposes specific demands on heat transfer equipment. The temperature lift is critical for the efficiency and economics of industrial heat pumps. Increased temperature lift usually leads to a lower COP, resulting in higher energy consumption and operational costs for the same heat output. This inefficiency can diminish the benefits of electrification, especially when high-temperature heat is needed. Moreover, larger temperature lifts often require advanced compressor technologies or specialized refrigerants, which can raise CAPEX. To mitigate this, careful system integration can reduce the required temperature lift, improving both the CAPEX and OPEX. Various types of high-efficiency heat exchangers can be integrated into industrial environments [117], although additional operational challenges may be encountered on the supply and demand sides in their implementation [118]. These include issues related to temperature and pressure levels [119], installation space, pressure drop [120], fouling [121], exposure to aggressive media [122], the use of specialised materials [123], and reliability concerns [124].

2.5. Heat Transfer Enhancement

In electrified industrial energy systems, optimising the heat transfer is crucial for improving thermal efficiency, process control, and energy utilisation. This involves the application of high-heat-flux electric heating techniques, such as resistance, induction, the use of microwaves, and arc heating, which facilitate rapid, localised thermal input with minimal energy losses. Key improvements are achieved by increasing the surface area through the implementation of fins or microchannels [125], which enhances convective heat transfer via forced fluid circulation [126], and incorporating phase-change materials for effective thermal storage and load balancing. The use of advanced materials and coatings that possess high thermal conductivity or emissivity further improves heat exchange efficiency. Additionally, tuning electromagnetic fields enhances the energy absorption in induction and microwave heating systems [127]. The integration of smart sensors and sophisticated control algorithms allows for real-time adjustments, which ensures accurate thermal management [128]. Collectively, these strategies contribute to the development of compact, responsive, and energy-efficient heating solutions that align with the goals of decarbonized industrial applications.

Enhancing heat transfer is crucial to the effective performance of heat exchangers, electric heaters, and steam boilers, particularly in addressing operational challenges and minimising design CAPEX [129]. In tube heat exchangers, the incorporation of helical grooves has significantly improved heat transfer efficiency by inducing turbulence and increasing the effective surface area. However, the optimal configuration of the helix angle remains to be established [130]. Investigations into helical and segmental baffles are crucial to enhancing the performance of electric heaters, as the heat transfer coefficient significantly increases when optimal inclined angles for the baffles are identified [48]. Further advancements on the shell side of tube heat exchangers can be achieved through the implementation of Savonius-shaped baffles [131], the optimisation of circular baffles [132], and the use of spiral perforated baffles within coiled elastic copper tubes [133].

The twisted elliptical tube heat exchanger has improved the overall performance of heat recovery processes, as evidenced in the context of MVR, although further precise application investigation is warranted [134]. Integrating a fully twisted inner pipe combined with conical rings enhances the performance of heat exchangers, with configurations featuring six full twists and conical rings demonstrating substantial improvements in both flow directions [135]. Innovative designs that mechanically influence and alter the boundary layer can further intensify heat transfer by incorporating devices such as wide, thin, and elongated vibrating turbulators made of latex [136]. Cross-flow designs are employed for gas heating and cooling applications, with a focus on increasing the low heat transfer coefficients within gas flows [137].

The approach to temperature management is crucial for developing reliable solutions, which can be achieved through high-efficiency heat exchangers. Convectively enhanced radiant orifice panel heat exchangers improve the thermal performance of systems by combining forced convection with radiant panels, although this method is primarily applicable to air systems [138]. For high-temperature applications, maximising the heat utilisation hinges on the proper design of heat transfer equipment [139]. The integration of phase-change materials in heat recovery systems can reduce the overall energy demand; however, effective strategies for heat transfer enhancement in shell-and-tube and plate designs must be established [140]. Techniques such as additive manufacturing [141], advanced computational fluid dynamics (CFD) analysis [142], and machine learning algorithms [143] can be leveraged to augment the heat transfer across various heat recovery units.

The fouling phenomenon poses a significant threat to the operational efficacy of heat exchangers, particularly electric heaters, necessitating the exploration of various mitigation strategies [121]. Recent proposals have suggested the efficacy of micro- and nano-scale surface modifications, as well as the application of electric currents, in inhibiting microbial fouling [144]. Additionally, the variability inherent in supply-side operations presents challenges to heat exchanger networks (HENs), requiring investigations into network stability and the implementation of advanced regulatory strategies [145]. An active control method that employs micro cuboid vortex generators has been proposed, demonstrating a performance coefficient enhancement of up to 7.01% [146].

2.6. CAPEX and OPEX Assessments

As industries increasingly transition to electrified energy systems, a comprehensive understanding of CAPEX and OPEX becomes crucial for making informed investment decisions. Recent studies have provided valuable insights into how the cost structures of energy systems evolve during this shift, highlighting both new opportunities and complexities [147]. CAPEX is frequently observed between heat recovery systems and traditional fossil-fuel-based configurations [148]. This indicates that electrification does not necessarily entail a higher upfront investment for integrating new energy technologies [149]. Industries can thus invest in electrified solutions without significant financial penalties compared to conventional systems.

However, the cost structures for utility exchangers exhibit substantial variability. Electric heaters [150], steam boilers [151], and electric furnaces [152] carry distinct cost profiles that differ considerably from their fossil-fuel-based equivalents. These variations affect the initial investment costs and have implications for the overall design and feasibility of energy systems, which necessitates careful consideration during the planning stages [153]. On the OPEX front, the volatility of electricity prices plays a critical role in determining operational expenditures. In electrified systems, the operational costs are increasingly influenced by fluctuations in dynamic energy markets [154]. Therefore, strategies such as flexibility in energy consumption, the implementation of smart control systems, and tariff optimisation become essential for enhancing cost efficiency and mitigating financial risks associated with energy price volatility.

The incorporation of heat pumps in industrial settings has shown substantial potential for OPEX and lowering emissions, particularly in the replacement of fossil-fuel-based heating systems. Industrial HPs exhibit high COPs, which typically range from 2.5 to over 5. This signifies a decreased energy consumption per unit of heat produced [155]. The enhanced efficiency of HPs not only lowers fuel costs but also improves the overall economic viability of energy systems, especially when they are powered by low-cost or surplus electricity, including that generated from renewable sources. Furthermore, HPs negate direct combustion emissions, which results in a significant reduction in CO₂ emissions and facilitates adherence to increasingly stringent environmental regulations [156]. Empirical studies conducted across various sectors, including food processing, chemicals, and paper manufacturing, have demonstrated that implementing HPs can lead to OPEX reductions of 20–50% and a decrease in carbon emissions ranging from 30% to 80%, although this contingent upon the temperature lift and the source of electricity used [74] (Odeh & Behnia, 2009). As the availability of decarbonized electricity increases, HPs are emerging as a scalable and cost-effective solution for providing clean industrial heat, which makes them a vital component in the transition toward sustainable industrial operations.

Successfully balancing CAPEX and OPEX in this evolving landscape requires a nuanced and multifaceted approach. Such an approach must integrate economic considerations, technological advancements, and environmental frameworks [157]. To obtain a resilient and future-ready energy system, industries must carefully evaluate and adapt to the changing dynamics of energy costs [158]. Moreover, as markets evolve and the demand for electrification increases, there is an opportunity for businesses to invest in emerging technologies that enhance their energy efficiency and sustainability. For instance, utilising demand response strategies, energy storage solutions, and innovative grid technologies can further optimise CAPEX and OPEX [159]. By thoroughly analysing the interplay between CAPEX and OPEX and adopting forward-thinking strategies, industries can position themselves not only for compliance with future regulations but also to obtain competitive advantages in an increasingly electrified world. This proactive stance is essential for fostering sustainable growth and ensuring long-term operational resilience in evolving energy market dynamics.

The economic assessment of electrified heat systems and HENs within industrial contexts typically employs quantitative metrics such as the payback period, the net present value (NPV), and sensitivity analysis to evaluate their financial feasibility across varying cost scenarios. The payback periods associated with electrification initiatives, including, for instance, the integration of heat pumps, generally fall within the range of 3–7 years, which is influenced by reductions in OPEX and improvements in energy efficiency [160]. Sensitivity analyses are conducted to examine the effects of variations in critical parameters, such as CAPEX, electricity prices, and carbon costs, on the overall economic viability of projects [161]. By systematically adjusting these variables, these analyses facilitate the identification of economic break-even points and evaluate the robustness of investment decisions under conditions of uncertainty. Collectively, these methodologies provide a thorough framework for assessing the cost-effectiveness and risk profiles of electrification strategies, and thereby aid in informed decision-making during the transition to low-carbon industrial energy systems.

2.7. Equipment Challenges

The transition to electrified heating in industrial processes entails many technical and operational challenges, particularly concerning the design and integration of heat exchanger networks. Some process streams are well-suited for direct electric heating, utilising technologies such as electric immersion heaters [162], resistance heaters [163], or induction coils [164]. The successful integration of these technologies often necessitates considerable upgrades to existing electrical infrastructure. This includes enhancements to power distribution systems, advancements in control mechanisms, and the implementation of robust safety measures to accommodate the increase in electrical load and maintain operational safety. It is crucial to note that not all process streams can be electrified effectively. For instance, high-viscosity fluids, streams containing reactive components, or sensitive product lines may necessitate continued steam heating due to safety concerns, quality control mandates, or compatibility issues. Consequently, steam heating networks must often remain operational, which results in hybrid systems that can complicate the overall system design and increase maintenance requirements [165,166].

Electrified heating solutions can lead to elevated wall temperatures within equipment [167]. This increase may exacerbate fouling and material degradation, particularly in streams that are known to be prone to fouling [168]. The existing literature extensively documents these challenges, underlining the necessity of meticulous thermal management practices and proper material selection to ensure long-term operational reliability [121]. Effective strategies may include using advanced coatings or self-cleaning technologies that help mitigate fouling rates. While electrification offers significant energy and environmental advantages, it necessitates fundamentally reevaluating equipment design and operational strategies to maximise its potential benefits. This involves not only adapting existing infrastructure but also exploring new designs for heat exchanger networks that can optimise heat transfer and ensure the efficient integration of electric and steam heating solutions. Additionally, implementing advanced control strategies [169] and predictive maintenance [170] can enhance the efficiency of hybrid systems. Leveraging data analytics and real-time monitoring can facilitate better decision-making and operational adjustments, which ultimately leads to improved energy utilisation and reduced operational costs [171].

2.8. Energy Storage Involvement

Energy storage is crucial in facilitating the decarbonization of industrial energy systems, particularly in enhancing the electrification of HENs and the supply of process heat. By decoupling the supply of energy from its demand, energy storage solutions, such as TES [172], batteries [173], and phase-change materials (PCM) [174], enable load shifting, peak shaving, and the integration of variable renewable energy sources. TES systems, which include sensible heat storage (e.g., molten salts, packed beds) and latent heat storage (e.g., PCMs), are particularly beneficial for industrial processes that require medium- to high-temperature heat [175]. They enhance the flexibility and efficiency of electrified HENs by allowing the storage of excess electricity as thermal energy during periods of low demand for subsequent use in heating operations [176]. The incorporation of storage in electrified HENs supports dynamic operations, alleviates grid constraints, and increases resilience while simultaneously reducing operational costs and emissions [177]. Therefore, the integration of advanced energy storage technologies with intelligent thermal network design is vital for achieving sustainability and reliability within industrial energy systems, particularly as electrification and renewable energy shares increase [178].

HENs can effectively be adjusted to the variability of renewable energy sources and fluctuations in electricity prices through the implementation of advanced control strategies, multi-period optimization, and the integration of energy storage systems. Transitioning from static to dynamic operational paradigms enables HENs to respond to temporal changes in electricity market signals and the availability of renewable energy generation [179]. TES plays a critical role in decoupling heat production from the demand, permitting excess renewable electricity to be converted into thermal energy during periods of low prices or high availability, and this energy can then be utilized later to ensure consistent process operations [180]. Predictive control systems that incorporate forecasts of electricity pricing [181] and renewable generation [182] allow for real-time adjustments to heat load distribution, stream matching, and the scheduling of electric heating technologies. The incorporation of hybrid utility configurations and demand-side management strategies further enhances the system flexibility, enabling the temporary substitution of alternative heat sources during peak grid demand or when the renewable energy is insufficient [183,184]. These adaptive capabilities, bolstered by multi-objective optimisation, enable HENs to maintain both economic viability and environmental performance while enhancing their responsiveness to grid conditions in electrified industrial systems. There are some good examples of energy storage research in the brewery [185], food [186], chemical [187], steel [188], and other industries, but the synergy effects of different solutions should further analysed.

3. Rethinking of Heat Exchanger Networks for Electrified Industrial Energy Systems

The new paradigm of the electrified industry presents the prospective role of HENs within an industrial energy system, highlighting the significance of renewable energy integration and electrification. This framework utilises renewable energy sources, including solar and wind, to supply the power grid and industrial processes. The HEN functions as a central hub for efficient heat recovery and redistribution, linking various thermal network components, as illustrated in Figure 9. This includes the provision of process heating and integrating technologies such as MVR, heat pumps, and ORCs, which enhance energy conversion and facilitate thermal recycling.

Furthermore, the system accommodates low-temperature heat distribution for district heating systems and applications, such as water and space heating, which expands its utility beyond industrial operations. Incorporating thermal storage units boosts operational flexibility while cooling towers are employed to manage excess heat. This interconnected strategy enables significant energy savings, reductions in greenhouse gas emissions, and overall system optimisation, promoting sustainable industrial practices.

3.1. Interpretation and Development of HEN for Industrial Energy Systems

The existing interpretation of HENs within grid diagrams necessitates a comprehensive re-evaluation of this concept. Traditionally, HENs are conceptualised as simplified conduits for thermal integration [189], as presented in Figure 10. However, this oversimplification may obscure critical elements such as energy transfer pathways, pinch point considerations, and the system’s operational flexibility. A more nuanced representation of HENs should elucidate distinctions among direct heat exchange processes, utility integrations, and intermediate thermal storage systems. This modified framework would ensure that the grid diagrams accurately reflect the intricate thermal and operational interdependencies that are critical to efficient thermal energy management.

There is a pressing need for a structured methodology to create grid diagrams that incorporate contemporary and varied thermal technologies. Inclusions in this approach should include the following:

• Heat pumps, which should be clearly delineated into input and output streams, along with annotations of their COP, which should aptly represent the efficiency of these systems;
• The illustration of MVR systems must encompass both thermal and mechanical flow metrics to reflect their dual operational characteristics;
• A thorough integration of ORCs should capture the nuances of waste heat recovery mechanisms and the associated power generation cycles;
• Energy storage, utilising both the process waste heat and process heating under varying energy supply and waste heat demand sides;
• Steam boilers, electric heaters/furnaces, plasma heaters, and microwave heaters are also available. These components should be represented with comprehensive inflows that illustrate their fuel or electric inputs juxtaposed against their thermal output. It is essential to distinctly differentiate the roles of utility heat and process heat.

This methodological framework must prioritise consistency in symbols, clear flow directional indicators, and a representation of energy quality (temperature levels), which will ultimately enable comprehensive comparability across diverse systems and configurations.

The representation of equipment types within grid diagrams should be significantly improved to clearly differentiate their functional roles in heating, cooling, recovery, or conversion alongside their energy sources (thermal, electrical, or renewable) and thermodynamic properties (temperature lift and heat duty). Implementing standardised icons and colour coding, complemented by a legend for reference, would mitigate ambiguity and facilitate the automated analysis of thermal systems. The interpretation of HENs within grid diagrams was previously implemented in different software and using different tools that support different process system engineering simulation environments.

Integrating a synthesised overview of the current system design state into grid diagrams is crucial for illuminating key operational attributes, identifying inefficiencies, and recognising opportunities for improvement. This should include the following analytical items that represent the equipment listed above:

• The identification of underutilised heat sources or sinks to enhance energy efficiency;
• The visualisation of energy recirculation loops to enable the assessment of their efficiency and performance;
• The assessment of dependencies on external energy utilities, which will allow for enhanced resilience and sustainability;
• The highlighting of modular or scalable components to facilitate adaptability in the face of changing energy demands or technological advancements.

Such synthesis supports refined design decisions and enhances operational strategies, particularly as facilities pivot towards low-carbon or electrified heating solutions. Incorporating these advanced methodologies and representational strategies will ultimately contribute to new trends and updates in HEN development and representation, contributing to engineering practice.

3.2. Revisiting the Approaches for Heat Exchanger Network Design and Retrofit in the Age of Industrial Electrification

The accelerating shift towards electrification in industrial processes has prompted a critical reassessment of established design and retrofit methodologies for HENs. As enhanced electrification introduces new variables, such as the incorporation of electric heating and cooling methods, fluctuations in heat demand, decarbonization imperatives, and integrating renewable energy, selecting an appropriate methodological framework becomes essential. The three principal paradigms for HEN development and retrofitting include mathematical programming (MP), insights-based approaches, and hybrid methods [190]. Each presents unique capabilities and inherent trade-offs that are crucial for navigating the complexities of modern electrified environments.

During the preliminary targeting stage of HEN synthesis, linear programming (LP) is frequently employed. LP models facilitate the rapid and scalable estimation of minimum energy requirements (MERs), utility loads, and initial stream pairings using linear objective functions and constraints. Their computational efficiency allows for the assessment of multiple scenarios or the identification of pinch points within the network; however, LP’s inability to account for nonlinear behaviour limits its accuracy in reflecting actual process conditions. In solving the objective function, LP usually includes the following process steps:

• Define stream data (supply/demand temperatures, heat capacity flows);
• Calculate composite curves and determine pinch points;
• Set up the LP model with constraints for energy balance and capacity limits;
• Minimize the total utility cost (objective function);
• Solve to obtain MER targets (minimum heating/cooling utilities).

Nonlinear programming (NLP) advances the design process by incorporating the thermodynamic and physical characteristics of heat exchangers, such as temperature-dependent heat capacities and nonlinear cost-area relationships. NLP has proven particularly valuable in optimizing the heat load distribution across exchangers after the network’s architecture has been established. The process steps of the NLP procedure include the following:

• Assume or predefine a network topology (e.g., matches);
• Formulate the energy balance, temperature approach, and exchanger area constraints;
• Define the nonlinear objective, e.g., minimize the total cost or total exchanger area;
• Use numerical solvers (e.g., IPOPT [191], SNOPT [192]) to find the optimal heat load distribution.

Mixed-integer nonlinear programming (MINLP) further refines NLP by integrating binary variables, which enables the simultaneous optimization of both the network structure (e.g., the presence of stream matches) and the intricate parameters of heat exchangers [193]. This approach is effective for both new and retrofitted HEN designs, facilitating the cooperative determination of the system’s topology and operational conditions. Despite its high accuracy and comprehensive nature, MINLP models demand significant computational resources and may encounter convergence difficulties, particularly in large-scale industrial applications. The workflow for MINLP procedure presumes the following:

• Define a superstructure of all feasible stream matches;
• Introduce binary variables to model the presence/absence of each match;
• Formulate the nonlinear heat balance, temperature constraints, and cost models;
• Define the objective, e.g., minimize the total annualized cost (CAPEX + OPEX);
• Solve with global MINLP solvers (e.g., DICOPT [194], BARON [195]).

Multi-objective optimization (MOO) methodologies are increasingly being utilized to reconcile competing design objectives, such as the reduction of carbon emissions alongside capital expenditures. MOO frameworks yield Pareto-optimal solutions, assisting decision-makers in navigating trade-offs between economic viability and environmental outcomes. For instance, in a case study involving crude oil distillation, a multi-period, multi-objective optimization strategy was implemented to balance the utility costs and CO₂ emissions across various operational contexts, which thereby promoted more sustainable decision-making [196]. The MOO procedure may be implemented utilizing the following steps:

• Define all relevant objectives (e.g., minimize CO₂ emissions, utility cost, or area);
• Choose a multi-objective optimization strategy (e.g., scalarization, Pareto-based);
• Formulate necessary constraints (same as in NLP or MINLP);
• Generate a solution set (Pareto front) that represents trade-offs;
• Use decision-making criteria (e.g., economic value, policy targets) to select the final design.

Mathematical programming techniques have proven to be robust optimisation methodologies that enable the systematic synthesis and retrofitting of HENs amidst intricate constraints [197]. These methods typically employ formulations such as mixed-integer nonlinear programming (MINLP) [198] or mixed-integer linear programming (MILP) [199] to account for both thermodynamic viability and economic feasibility. In the context of industrial electrification, MP approaches can be augmented to consider the integration of electric heating and cooling systems, variable demand response mechanisms, and fluctuating electricity pricing models. Recent studies have focused on optimising networks that include electrified technologies like MVR systems [200] and electric boilers [201], which has enabled a holistic approach to network optimisation. A heat integration superstructure that facilitates multiple thermodynamic pathways for streams may undergo changes in pressure and temperature. The superstructure will be linked to a HEN superstructure, which allows for the integration of hot and cold streams identified within the work and heat integration framework [202]. MP approaches excel in tackling large-scale, complex problems with multi-objective optimisation, integrating diverse techno-economic, environmental, and operational constraints [203]. They are also highly compatible with automation and support scenario analysis. However, these methods face challenges, such as high computational costs and a lack of transparency, which makes their interpretation difficult for end users and impedes decision-making. The differences between the MP methodologies are demonstrated in

Table 2. Comparison of MP methodologies.

Aspect	LP	NLP	MINLP	MOO
Focus	Utility targeting	Detailed heat load optimization	Structure + parameter optimization	Trade-off analysis between multiple goals
Variables	Continuous (linear)	Continuous (nonlinear)	Mixed (binary + continuous)	Continuous/mixed, multi-objective
Topology decision	Not included	Fixed	Optimized	May be optimized
Objective function	Single (cost or utility)	Single (cost or area)	Single (cost, often annualized)	Multiple (cost, emissions, etc.)
Complexity	Low	Medium	High	High (especially with Pareto analysis)
Solver requirements	Simple LP solvers	Nonlinear solvers	MINLP solvers	MOO or evolutionary algorithms
Application	Early design targeting	Detailed retrofit optimization	Full design with discrete choices	Sustainable and policy-driven decisions

.

To address dynamic variables such as fluctuating electricity prices and the intermittent availability of variable renewable energy, HEN optimization models have evolved from static representations into multi-period or dynamic frameworks. These advanced formulations divide the time horizon into discrete intervals, facilitating the direct incorporation of time-dependent variables, such as electricity costs, heat demand, and renewable energy supply, into both the objective function and the associated constraints. MINLP and MOO methodologies are particularly well-suited to this approach, as they concurrently optimize network configurations, operational parameters, and energy storage strategies in response to temporal changes. The dynamic integration of these models allows for cost-optimal operation by enabling adjustments in energy consumption during periods of low electricity prices or heightened renewable availability. They assess trade-offs between economic performance and environmental objectives. These models can integrate forecasts, stochastic elements, or scenario-based analyses to bolster resilience amidst uncertainty, and thereby facilitate the flexible and decarbonized operation of electrified industrial energy systems.

Insights-based methodologies leverage thermodynamic principles, graphical analysis tools (such as composite curves and grid diagrams, etc.) [204], and heuristic strategies to guide the design of HENs and retrofit endeavours [205]. In the electrification context, these approaches prove invaluable for promptly identifying retrofitting opportunities and visualising heat surpluses and deficits, which makes them particularly useful during preliminary evaluations and design phases [206]. With the growing utilisation of electrified technologies, graphical methods have evolved to accommodate non-conventional heat sources and sinks, which has facilitated the intuitive examination of electrification prospects and potential shifts in pinch points due to electric heating or cooling strategies. Insights-based approaches offer enhanced transparency and interpretability, which makes them particularly beneficial for users during the initial design stages and applicable to small- to medium-scale systems due to their minimal computational requirements. However, these methods often result in suboptimal outcomes when compared to MP approaches, and they may encounter challenges in scaling larger or more constrained systems [207].

Hybrid techniques encapsulate the advantages of MP and insights-based methods, establishing a balance between computational accuracy and conceptual clarity [208]. These methods typically employ heuristic or graphical insights to define boundaries or structural frameworks for optimisation problems, and thereby reduce the complexity of system design and improve problem solvability [209]. Within electrified systems, hybrid approaches can effectively pre-screen retrofit options or decarbonization strategies using pinch analysis followed by detailed MP-based optimisation [210]. Moreover, emerging hybrid frameworks are increasingly integrating dynamic simulations and multi-period optimisation strategies, which are vital in contexts where electrification introduces operational variability, such as time-of-use electricity pricing or intermittent renewable energy sources. Hybrid methods provide an improved balance between achieving optimal solutions and ensuring user interpretability [157], which makes them adaptable to multi-objective or multi-period optimisation scenarios while they remain scalable to accommodate the evolving landscape of electrification technologies. However, their effective implementation necessitates the meticulous integration of various methodologies and may require iterative processes throughout different design stages to effectively refine the outcomes [211].

4. Case Studies in Different Industries

As global industries strive for decarbonization and enhanced operational efficiency, integrating advanced energy systems has become a strategic necessity. Among the most significant advancements are HENs and the electrification of thermal utilities, which facilitate considerable energy savings, emissions reductions, and process performance improvements. Several case studies illustrate how various sectors utilise HEN optimisation and electrified thermal systems to achieve sustainability targets while preserving or enhancing profitability.

4.1. Oil and Gas

Electricity-based heating systems offer greater flexibility in HEN design compared to fossil-fuel-based systems, thereby significantly simplifying the design complexities associated with electrification. This is exemplified by several case studies of heat recovery systems, where capital costs and design complexities are minimised by effectively integrating the characteristics of the electrified energy supply into the overall system design [24]. The HEN of the gas separation plant in Thailand was retrofitted to reduce the temperature approach by applying two NLP optimisation steps, which resulted in environmental and financial benefits [212]. A novel optimisation approach was proposed to efficiently achieve solutions in a multi-objective context. The case study focuses on a real industrial-scale crude oil distillation preheat configuration. The findings demonstrate that prioritising carbon compensation for the waste heat recovery option can substantially reduce carbon emissions and transform the energy distribution within systems [196]. Another effective application of a hydrogen plant with carbon dioxide capture was investigated [213]. A multi-period graph-theoretical methodology was introduced to identify the optimal retrofit strategy, taking into account economic and environmental considerations [214].

4.2. Food Processing

A highly efficient, fully electric milk evaporation system was developed through the strategic integration and selection of heat pump and MVR technologies. In this context, the study presents an effective process integration and electrification method, which resulted in a 32% OPEX reduction and 82% lower emissions [82]. Integrating decentralised heat pumps that facilitate heat exchange between process streams and electric heaters yields a modest reduction in energy consumption of approximately 56% in the milk industry, and profitability was proved with energy price forecasts from 2020 [160].

4.3. Chemical Industry

The reconfiguration for electrification and the reduction of carbon emissions in conventional chemical processes was examined, with a particular emphasis on the indirect electrification of processes and the direct electrification of utilities. The research utilised a case study of the advanced coal-to-ethylene glycol process and obtained an energy efficiency improvement of about 50%, as well as reduced carbon dioxide emissions [215]. To decarbonise the petrochemical industry, a utility system was modelled to supply electricity and heat to an olefins plant located in the Port of Rotterdam. Mathematical optimisation techniques were employed to determine the optimal hourly operations of the plant in response to fluctuating energy prices. The analysis reveals that the cost-optimal utility system comprised electric boilers, integrated TES, and technologies for the on-site production, storage, and utilisation of hydrogen [161]. The seasonal availability of renewables is crucial for electrified industrial systems. Another case study in the chemical industry explored the adjustment of stream temperature and HEN synthesis for combining high-temperature co-electrolysis and the Fischer–Tropsch process for carbon capture, reducing the electricity targets and increasing plant revenue [216]. The integration of electrification with the inherent storage capacity of the steam network has led to a potential average increase in profits for industrial facilities with a large steam network. The reduction in emissions was estimated at 18 tCO₂/d, and, additionally, a potential decrease in blow-off steam of 0.24 t/h was achieved [217].

4.4. Others

The model-based approach for the indirect integration of multiple parallel ORCs was introduced in [218]. The comprehensive model encompasses the extraction of waste heat and power generation. A modified superstructure was proposed to facilitate heat extraction modelling, which streamlines the development of HENs and minimises the computation time. The application of ORC solutions for cement factories may achieve the 43% self-coverage of power demand or grid supply, increasing the profitability of the existing production facility [219]. In the cement industry, the electrification of clinker and calcination processes also represents a viable decarbonization strategy [220]. Integrating MVR and bottom flash technologies with the ORC system enhances the recovery and reutilization of low-temperature waste heat in ethyl acetate/cyclohexane separation. The energy efficiency of the process was improved by 17%, and the total annual cost was reduced by 8% [221]. MVR technology is widely used in distillation processes. An exergy-based optimisation method that facilitates enhanced heat load distribution was introduced for acetone–water mixture distillation, resulting in significant improvements in internal efficiency. This optimisation further supports the conceptual design of heat pump-assisted distillation, which utilises MVR to supply heat loads to the intermediate heat exchangers [222]. The concurrent implementation of HEN retrofitting and MVR in a calcium chloride evaporation facility reduced carbon dioxide emissions by 19 ktCO₂/y and achieved a 51% decrease in steam consumption. This significant reduction in steam usage presents opportunities for CAPEX savings when transitioning the process to electric steam boilers [223].

In addition to the insights gained from these case studies, it is beneficial to incorporate TES systems, especially when they are integrated with electrified utilities, as illustrated in reference [224]. This combination enhances the efficiency and reliability of energy supply and allows for the better management of energy consumption during peak demand periods. By leveraging TES, facilities can store excess energy generated during off-peak times and utilise it when demand increases, which effectively reduces their reliance on grid energy and leads to a more sustainable energy strategy. The summary of key performance metrics including energy savings, emissions reduction, and cost impacts is presented in

Table 3. Key performance metrics of the considered case studies.

Industry/ Process	Technology/ Strategy	Energy Savings	Emissions Reduction	Cost Impact	References
Milk evaporation	Heat pump + MVR integration	~56%	82%	32% OPEX reduction	[82,160]
Gas separation	HEN retrofit with NLP optimization	Not quantified	Not quantified	Financial benefits reported	[212]
Crude oil distillation	Multi-objective HEN optimization + waste heat recovery	Not quantified	Substantial	Improved energy distribution, cost-effective	[196]
Hydrogen production (SMR + CO2 Capture)	Electrified heat pumps + electric heaters	Moderate	Not specified	Profitable under 2020 price scenarios	[213]
Coal-to-ethylene glycol	Process electrification + renewables integration	~50% efficiency improvement	Reduced CO2	Not specified	[215]
Olefins plant	Electrification + TES + hydrogen utility	Dynamic optimization	Low-carbon utility system	Optimized operational costs	[161]
Oil refining (crude unit)	Multi-period retrofit planning with electrification	Not specified	Environmental gains noted	Cost-effective retrofitting	[214]
Syngas and Fischer-Tropsch	HEN + CO2 recycling	Electricity reduction	Increased revenue via CO2 use	Revenue boost and energy efficiency	[216]
Steam network (Total Site)	Electrification + TES + flexibility modelling	Not specified	18 tCO2/day	Reduced blow-off steam, higher profitability	[217]
Cement	Parallel ORCs + HEN optimization	43% power self-coverage	Not specified	Increased profitability	[218,219]
Cement (Clinker/calcination)	Electrification via microwave technology	Not quantified	High potential	Under review	[220]
Ethyl acetate separation	MVR + ORC + pressure-swing distillation	17% energy efficiency gain	Not specified	8% reduction in total annual cost	[221]
Acetone-water distillation	MVR + exergy optimization for heat pump integration	Improved internal efficiency	Not specified	Conceptual design improvement	[222]
Calcium chloride production	HEN retrofit + MVR	51% steam reduction	19 ktCO2/year	CAPEX savings potential via electric boilers	[223]

.

The adoption of HENs in electrified industrial settings is hindered by several practical challenges that limit their widespread implementation despite their capabilities for enhancing energy efficiency and reducing emissions. The retrofitting of existing thermal systems with electrified HENs often encounters numerous technical issues, such as compatibility with existing infrastructure, spatial limitations, and the necessity for comprehensive system redesign, especially when incorporating technologies like heat pumps or electric boilers [212,213].

Skill mismatches in the workforce pose a substantial barrier, as personnel trained in traditional fossil-fuel systems need specialized education in electrified process control and energy integration. Compounding these challenges are regulatory and market-related obstacles, including varying carbon pricing and electricity tariff frameworks that fail to promote electrification, as well as inadequate access to low-carbon electricity supplies [214]. Economic concerns, particularly regarding high capital investments and uncertain returns on investment, highlight the necessity for supportive financial tools and risk-sharing arrangements [160]. Integrating electrified systems with dynamic energy markets and intermittent renewable energy sources introduces additional operational complexities which demand advanced control strategies and real-time optimization techniques [217]. Addressing these challenges is crucial for enabling the scalable and resilient implementation of electrified HENs in strategies aimed at industrial decarbonization.

5. Conclusions

The electrification of industrial energy systems marks a critical advancement in the quest for deep decarbonization, necessitating the reconfiguration of HENs as both a technical imperative and an avenue for innovation. This review systematically explores the complex challenges and emerging strategies in designing, optimising, and retrofitting HENs within electrified contexts. Significant technological shifts, such as the substitution of fossil-fuel-based utilities with electric boilers, heaters, and advanced heat engines, are reshaping utility configurations, heat recovery methodologies, and the balance between capital and operating expenditures.

A particularly transformative element is the integration of utilities that operate across multiple temperature levels, including heat pumps, MVR, and ORCs. These innovations introduce new thermodynamic and economic complexities, particularly within multi-pinch network architectures. Concurrently, the electrification process has broadened the potential for low-grade heat recovery, driving the adoption of advanced heat transfer enhancement techniques and the investigation of novel materials and geometries in heat exchanger design.

This evolving landscape necessitates a re-evaluation of traditional HEN methodologies. Mathematical programming techniques provide accuracy and robustness for large-scale, multi-objective optimisation problems, while insights-based and hybrid methodologies facilitate conceptual clarity and computational efficiency. Future research efforts must emphasise the integration of dynamic system behaviours, variable energy pricing, and the variability in renewable electricity within HEN optimisation frameworks.

In addition to technical design considerations, aligning economic and environmental performance metrics is essential. Assessments of CAPEX and OPEX, fluctuations in energy pricing, and the integration of thermal energy storage will be crucial in justifying investments in electrified HENs. Moreover, graphical and simulation tools should be updated to accurately represent the complexities of modern electrified utility systems.

The advancement of next-generation HENs for electrified industrial processes is crucial to achieving climate objectives, enhancing energy efficiency, and improving operational resilience. Industrial HENs can evolve from fossil-fuel-dependent frameworks to flexible, intelligent, and sustainable energy networks through the combination of methodological innovation, digital integration, and system-level thinking.