Special Issue on “Technologies for Climate-Neutral Energy Systems”

1. Introduction

Climate-neutral economy aims to achieve net-zero greenhouse gas (GHG) emissions in all human activities, thus implying a paradigm shift that must be based on sustainability principles, leading towards an ecological transition integrating economic growth, social well-being, and environmental protection. This transition involves economic, environmental, and social dimensions, with energy systems being the cornerstone for change.

Energy decarbonization and sustainable industry has been identified as some of the transformations necessary for achieving the Sustainable Development Goals (SDGs) on the 2030 Agenda for Sustainable Development [1]. Despite its importance, the concept of sustainable energy development has changed over the past few decades to encompass the recently synthetized Energy Trilemma: energy security, energy equity, and environmental sustainability of energy systems [2].

A fully decarbonized energy system requires the development of a complex portfolio of solutions on energy supply and demand, with a fully integrated multi-vector grid, that allows operation with a high degree of affordability and flexibility. Moreover, several hard-to-abate sectors (i.e., long-distance freight transport, air travel, highly reliable electricity, and steel and cement industries) need tailor-made solutions to achieve neutrality [3].

The most promising, effective, and readily available options for achieving climate neutrality are energy efficiency, renewables, and electrification. These “no-regret” solutions present an enormous decarbonization potential, but they must be complemented with disruptive changes to achieve carbon-neutrality (i.e., small modular nuclear reactors, carbon capture use and storage, green hydrogen, and power-to-X technologies) [4].

This Special Issue is focused on collecting and highlighting novel technologies and applications on renewable energies and energy efficiency. On the one hand, increasing the use of renewable energy sources substitutes the use of fossil fuels in power, transportation, heating, and cooling systems while reducing GHG emissions. On the other hand, the promotion of energy efficiency is a cost-effective way to reduce the energy (and fossil fuels) used, increasing the resilience of energy systems based on renewables. To reach a net-zero system by 2050 (achieving the 1.5 °C goal), it is necessary to triple the renewable capacity and double energy intensity improvements by 2030, and the two main keys to achieving this are renewables and efficiency [5].

The market growth of the renewables industry is predominantly contributed to by solar PV installations for electricity, but the role of wind, biomass, and other renewable technologies, such as solar technologies, is crucial for reducing GHG emissions in the power, heat, and transportation sectors [6]. In this Special Issue, articles have been published on three ready-to-market technologies with high potential for supporting the decarbonization of fossil fuel-based energy sources; important challenges remain in unlocking their full techno-economic potential:

• Photovoltaic–Thermal systems (PVT), a form of solar technology, combine the generation of electricity and heat in one panel, and have important implications for non-domestic (i.e., hotels, hospitals, and offices) and residential buildings [7].
• Hydrothermal liquefaction (HTL) of biomass is one of the most versatile technologies for converting biomass feedstocks (particularly in the wet state) into biocrude oil [8].
• Biogas production based on anaerobic digestion of biomass and waste is an established technology for producing biogas that can be upgraded to biomethane and directly substitute natural gas [9].

Industrial decarbonization requires a mix of technologies and policies in order to achieve sustainability objectives without competitiveness losses. Important drivers such the use of direct or indirect (i.e., electrification or green hydrogen use) renewable sources, CCUS, material efficiency, or industrial symbiosis, must be balanced with specific solutions for industrial sectors (i.e., iron and steel, glass, and cement) [10]. However, industrial energy efficiency remains the “first fuel” for mitigating climate change [11]. Both a general and sectoral analysis of energy efficiency in industry with multiple practical solutions are presented in this Special Issue.

2. Biomass- and Solar-Based Renewable Technologies

Simón-Allué et al. [12] presented an experimental framework developed around their own patented air-based PVT collector, consisting of a high-quality photovoltaic laminate and a newly designed thermal absorber. It is important to note that two configurations were tested for one full year: the single panel and a system with 2.5 panels in series (to maximize output temperature). Their results show an almost constant electrical performance of 15–19%, and a very variable thermal performance (between 15 and 52% for the individual panel and between 11 and 35% for the system). Field operation presents average thermal and electrical efficiencies ranging between 16 and 20% with an electrical–thermal generation ratio close to 1:1.

Eladnani et al. [13] demonstrated that the catalytic HTL of seeds and shells of kurrajong (Brachichyton populneus) could produce a high-value liquid biocrude oil. In their experimental study, the optimal operational conditions (330 °C, 7–15 MPa, 10 min reaction time) for the HTL processes were analyzed. Additionally, the presence of Ni/Al₂O₃ as a catalyst and Fe as a hydrogen donor notably increased the conversion and high heating value of the products due to the enhanced hydrodeoxygenation of fatty acids into hydrocarbons.

Miana et al. [14] developed a complete computational fluid dynamic (CFD) model for simulating the anaerobic digestion of wastewater. The comprehensive model combined heat transfer, multiphasic flow, and biochemical reactions to obtain the methane production rate. The model was calibrated and validated with experimental data from a lab-scale patented non-conventional anaerobic digester for winery wastewater treatment. The results confirmed that the model increases the accuracy of CFD simulations of anaerobic digesters under real operation conditions.

3. Energy Efficiency and Decarbonization in Industry

Carmona-Martínez et al. [15] provided a review of currently available renewable energy technologies (regarding the 2030 scope) that will be useful in achieving the goal of decarbonizing energy-intensive industries (EIIs) by 2050. These renewable options have been classified into (a) renewable power for process electrification and (b) renewable heat. Renewable power includes, together with the introduction of new electrochemical processes, the electrification of heat (at low temperatures, mainly by heat pumps and boilers; at medium high temperatures, with electric arc, infrared, induction, dielectric, direct resistance, microwave, and electron beam heating). Renewable heat was analyzed in terms of the potential of biomass, geothermal, solar, and green hydrogen applications. This review also included a cross-sectoral analysis of the potential implementation of these technologies in different EIIs.

In contrast to the cross-sectoral review, two articles were focused on the energy efficiency potential of specific sectors in Italy: the pharma and glass industries. This analysis was based on the information contained on the mandatory energy audits of large enterprises and EIIs according to the national transposition of the European Energy Efficiency Directive. Bruni et al. [16] analyzed the energy consumption and potential energy efficiency measures (EEMs) in the pharma industry. Energy consumption is dominated by electricity (61%) that is used (70% of the final energy) in auxiliary services (i.e., heat and cold energy production, air handling units, water purification, air compression, and water pumping). Auxiliary systems predominantly impact on the Energy Performance Index, which is defined by the size of the plant instead of by production (like in EIIs). Most suggested EEMs regard cold and hot energy production, as well as on-site energy production (mainly PV and CHP systems). The payback time is less than 4 years for many EEMs, including both technical and managerial ones.

Cantini et al. [17] developed a detailed analysis of energy efficiency technological solutions in the glass industry. A comprehensive list of technological energy-saving opportunities was provided, dividing them according to both the process phases and the asset involved, and compared them with the proposed EEMs of more than 100 glass factories in Italy. The results of the energy audits show that auxiliary systems are most frequently considered for the improvement and revamping of pressure systems and engines. Instead, from a production process point of view, in the fusion and refining phase, most of the implemented interventions relate to furnaces and heat recovery systems. In the finishing phase, most of the solutions proposed deal with interventions on pressure systems, heat recovery systems, and engines. This information should be pivotal to increasing the EE in the glass sector.

Finally, two interesting technological solutions directly applied to the agri-food and steel industries are presented. Guillen-Angel and Julian [18] analyzed the technical feasibility of Solar Heat Industrial Process (SHIP) integration with heat storage to partially decarbonize different agri-food industries. A modeling approach based on TRNSYS was applied to an Italian spirits distillery, to a Spanish winery, and to a French charcuterie. This work demonstrated that solar heat has limited applicability in industries whose primary heat demand is in winter, and the profitability of its implementation strongly depends on the inter-relation between available irradiation, thermal demand, and available surface for collectors’ arrangement. Only in the distillery did the applied solution present a reasonable payback time of 8 years.

Arroyo et al. [19] developed a CFD model of an industrial test furnace in the steel sector in order to analyze the impact of different fuels (natural gas and/or blast furnace gas-BFG) in the combustion process and facilities. The direct use of BFG (a byproduct of the steelmaking process) is generally limited due to the low heating value, which induces low temperatures or combustion instability issues. The model was validated with high accuracy using industrial data. Several modifications to burners must be introduced in industrial facilities to ensure high rates of BFG combustion stability and correct heat transfer to the load.

4. Conclusions

In total, eight papers were finally accepted for publication and inclusion in this Special Issue. All of the articles were selected as “Feature Papers” by the editorial team of Processes, underlining the high quality of the works presented. Some important common highlights emerge from the analysis of the published papers:

• The development of technologies for climate-neutral energy systems needs strong international collaboration between academia, research/technology centers, and enterprises. In the current Special Issue, regarding the authors of the eight papers, there were thirteen different affiliations from six countries, of which five were private enterprises (38%), four were technology-research centers, and four were universities (31%).
• Innovative small and medium-sized enterprises (SMEs) are key players in the sector. A proportion of 80% of the private companies that have published in this Special Issue are SMEs.
• Modeling and simulation tools are crucial for developing technologies on an industrial scale. However, robust experimental data remains the basis for all subsequent research activities.
• There is no silver bullet for climate-neutrality, but multiple technologies must be integrated and contribute to achieve the most suitable techno-economic scenarios.
• Industrial energy efficiency requires a two-pronged approach to be effectively implemented. On the one hand, a plethora of cross-sectoral technologies can be applied to multiple auxiliary systems in the industry. On the other hand, specific solutions must be developed for hard-to-abate sectors (such as steel or glass industries).