Life cycle assessment of a novel strategy based on hydrothermal carbonization for nutrient and energy recovery from food waste.

In this work, a novel strategy for food waste valorization was evaluated from an environmental life-cycle perspective. A system based on acid-assisted hydrothermal carbonization of food waste combined with the exploitation of hydrochar by combustion and process water through nutrient recovery stage and subsequent anaerobic digestion, was assessed and compared with stand-alone anaerobic digestion as the reference system. This combination of processes aims to recover both nutrients in a stage of struvite precipitation from process water and energy through hydrochar and biogas combustion. Both systems were modeled in Aspen Plus® to identify and quantify their most relevant input and output flows and subsequently evaluate their environmental performance through the life cycle assessment methodology. The novel combined system was found to generally involve a more favorable environmental performance than the reference stand-alone configuration, which would be closely linked to the substitution of hydrochar for fossil fuels. In addition, the impacts associated with soil application of the struvite produced in the integrated process would also be reduced compared to the use of the digestate generated in the stand-alone anaerobic digestion process. Following these results and the evolving regulatory framework for biomass waste management, mainly in the field of nutrient recovery, combined process based on acid-assisted hydrothermal treatment plus nutrient recovery stage and anaerobic digestion is concluded to be a promising circular economy concept for food waste valorization.

Novel short-term national strategies to promote the use of renewable hydrogen in road transport: A life cycle assessment of passenger car fleets partially fuelled with hydrogen.

This work presents an energy analysis combined with a comparative environmental life cycle assessment (LCA) of eight different passenger car fleets that use renewable hydrogen and a conventional fuel (natural gas or gasoline) under the same total energy input and the same hydrogen-to-mixture energy ratio. The fleets under comparison involve vehicles that use the two fuels separately or in a mixture. Using Italy as an illustrative country, this research work aims to help policy-makers implement well-supported strategies to promote the use of hydrogen in road transport in the short term. The proposed strategies achieve a carbon footprint reduction between 7 % and 35 % with respect to their conventional fleet benchmark. Within the current context, the results suggest the energy and environmental suitability of using hydrogen blends as short-term solutions, involving vehicles that require minor modifications with respect to current compressed natural gas vehicles and gasoline vehicles, while paving the way for pure hydrogen mobility.

Hourly marginal electricity mixes and their relevance for assessing the environmental performance of installations with variable load or power.

The ongoing energy transition is causing rapid changes in the electricity system and, in consequence, the environmental impacts associated with electricity generation. In parallel, the daily variability of generation increases with higher shares of renewable energies. This affects the potential environmental impacts or benefits of devices with variable load or power, such as electric vehicles, storage systems or photovoltaic home systems. However, recent environmental assessments of the actual benefit of such systems are scarce, with existing assessments majorly using average grid mixes that are frequently outdated and disregard the dynamic nature of renewable generation. This article provides detailed hourly average and marginal electricity mixes for each month of the year, determined for Spain as an illustrative country with a diversified (renewable) power generation portfolio that experienced a rapid change in the last years. These are combined with specific life-cycle emission factors for each generation technology. Main drivers for the impacts of the marginal mix turn out to be natural gas plants and imports, but also pumped hydropower due to its comparably low storage efficiency. Applied to a hypothetical photovoltaic rooftop installation, the differences between environmental assessments on hourly and on annual basis are found to be surprisingly low when assuming that the generated electricity replaces the average grid mix, but substantial when considering the marginal generation mix (i.e., the generation technologies that respond to a change in demand at a given time). This highlights the importance of considering the dynamics of the electricity system and the corresponding marginal electricity mixes when optimizing flexible load or generation technologies under environmental aspects.

Social life cycle assessment of green methanol and benchmarking against conventional fossil methanol.

Green methanol could play a major role in decarbonising both the chemical and energy sectors. While techno-economic and environmental studies on green methanol following a life-cycle perspective are available, its social implications from a supply-chain standpoint remain largely unexplored. In order to fill this gap, this work presents the first social life cycle assessment of green methanol produced with CO2 directly captured from the air and hydrogen from wind power electrolysis. When compared to conventional methanol from natural gas, the results suggest an unfavourable performance of green methanol under negative social indicators (forced labour, women in the sectoral labour force, health expenditure, social responsibility promotion, and fair salary) due to the increased supply-chain complexity of the green system. In contrast, green methanol would outperform its conventional counterpart in terms of sectoral contribution to economic development, a positive social indicator which would benefit from the increase in working hours. Besides future consideration of a higher number of positive indicators and potential improvements in country- and sector-specific risk levels towards high-quality social and working conditions, an enhanced social life-cycle performance of green methanol requires technical improvements to reduce the high demand for energy inputs and equipment across its supply chain. Acknowledging decarbonisation as the actual driver of green methanol deployment, future social studies are suggested to focus on the comparison of renewable alternatives for its production and the effect of social regulations.

Energy-socio-economic-environmental modelling for the EU energy and post-COVID-19 transitions.

Relevant energy questions have arisen because of the COVID-19 pandemic. The pandemic shock leads to emissions' reductions consistent with the rates of decrease required to achieve the Paris Agreement goals. Those unforeseen drastic reductions in emissions are temporary as long as they do not involve structural changes. However, the COVID-19 consequences and the subsequent policy response will affect the economy for decades. Focusing on the EU, this discussion article argues how recovery plans are an opportunity to deepen the way towards a low-carbon economy, improving at the same time employment, health, and equity and the role of modelling tools. Long-term alignment with the low-carbon path and the development of a resilient transition towards renewable sources should guide instruments and policies, conditioning aid to energy-intensive sectors such as transport, tourism, and the automotive industry. However, the potential dangers of short-termism and carbon leakage persist. The current energy-socio-economic-environmental modelling tools are precious to widen the scope and deal with these complex problems. The scientific community has to assess disparate, non-equilibrium, and non-ordinary scenarios, such as sectors and countries lockdowns, drastic changes in consumption patterns, significant investments in renewable energies, and disruptive technologies and incorporate uncertainty analysis. All these instruments will evaluate the cost-effectiveness of decarbonization options and potential consequences on employment, income distribution, and vulnerability.

Life cycle sustainability assessment of synthetic fuels from date palm waste.

The use of biowaste feedstock is often suggested for sustainable production of synthetic fuels through gasification followed by the Fischer-Tropsch process. While the technical performance of this type of bioenergy system has significantly been investigated, comprehensive sustainability analyses are still required. The present study evaluates the life cycle sustainability performance of synthetic diesel and gasoline from Tunisian date palm waste, and compares it with that of conventional fossil fuels. Life cycle inventories are elaborated to subsequently characterise the performance of the synthetic biofuels under a set of 12 environmental, economic and social indicators. Both environmental and economic hotspots were found to be associated with the need for electricity and oxygen. Direct emissions to the air and the investment in the plant's power section were also found to significantly affect the environmental and economic performances, respectively. Potential social impacts were found to be mainly linked to the supply chain of equipment and infrastructure, while electricity arose as the most contributing operational element. Overall, the evaluated synthetic biofuels could be considered competitive with conventional fossil fuels and contribute to the achievement of sustainable development goals only if environmentally- and socially-friendly (renewable) electricity and oxygen sources are implemented and the scale and configuration of the plant are optimised.

Comparative Social Life Cycle Assessment of Two Biomass-to-Electricity Systems.

Biomass plays a fundamental role in numerous decarbonisation strategies that seek to mitigate the short- and long-term effects of climate change. Within this context, decision-makers' choices need to comprehensively consider potential sustainability effects associated with bioenergy systems. In particular, due to the lack of studies addressing the social sustainability of bioelectricity, the present work applies the Social Life Cycle Assessment (S-LCA) methodology to compare the social performance of two biomass-to-electricity systems located in Portugal based on either fluidised-bed or grate furnace technology. S-LCA involves a comprehensive approach for holistic evaluation and data interpretation of social aspects. Six social indicators were benchmarked: child labour, forced labour, gender wage gap, women in the sectoral labour force, health expenditure, and contribution to economic development. The results show that the implementation of fluidised-bed furnaces as a more efficient conversion technology could reduce by 15-19% the selected negative social impacts, except women in the sectoral labour force. When enlarging the interpretation to a sustainability perspective, the general suitability of the fluidised-bed furnace system would be further emphasised under environmental aspects while jointly providing valuable insights for informed decision-making and sustainability reporting.

Revisiting the role of steam methane reforming with CO2 capture and storage for long-term hydrogen production.

Road transport is associated with high greenhouse gas emissions due to its current dependence on fossil fuels. In this regard, the implementation of alternative fuels such as hydrogen is expected to play a key role in decarbonising the transport system. Nevertheless, attention should be paid to the suitability of hydrogen production pathways as low-carbon solutions. In this work, an energy systems optimisation model for the prospective assessment of a national hydrogen production mix was upgraded in order to unveil the potential role of grey hydrogen from steam methane reforming (SMR) and blue hydrogen from SMR with CO2 capture and storage (CCS) in satisfying the hydrogen demanded by fuel cell electric vehicles in Spain from 2020 to 2050. This was done by including CCS retrofit of SMR plants in the energy systems model, as a potential strategy within the scope of the European Hydrogen Strategy. Considering three hypothetical years for banning hydrogen from fossil-based plants without CCS (2030, 2035, and 2040), it was found that SMR could satisfy the whole demand for hydrogen for road transport in the short term (2020-2030), while being substituted by water electrolysis in the medium-to-long term (2030-2050). Furthermore, this trend was found to be associated with an appropriate prospective behaviour in terms of carbon footprint.

Life cycle assessment of volatile fatty acids production from protein- and carbohydrate-rich organic wastes.

Volatile fatty acids (VFAs) are platform molecules with numerous applications. They can be obtained by adjusting the operational conditions of anaerobic digestion to avoid methanogenesis while focusing on fermentative stages. There are gaps in the knowledge of how, from a life-cycle perspective, the fermentative process performs in VFAs production from waste, including environmental consequences of substituting common commodities in the current market. Mass and energy balances of VFAs production from protein-rich microalgal and carbohydrate-rich agro-industrial wastes were used herein as a key source of inventory data for life cycle assessment. Two waste treatment options were considered: (i) VFAs production (anaerobic fermentation) plus anaerobic digestion of the resulting waste after VFAs separation, and (ii) anaerobic digestion of the original waste for bioenergy. Several scenarios were formulated to evaluate their life-cycle performance. VFAs production generally shows a better environmental behaviour than conventional anaerobic digestion, principally due to the substitution of conventional chemicals.

Comparative life cycle sustainability assessment of renewable and conventional hydrogen.

Hydrogen is gaining interest as a strategic element towards a sustainable economy. In this sense, sound decision-making processes in the field of hydrogen energy require thorough analyses integrating economic, environmental and social indicators from a life-cycle perspective. For this purpose, Life Cycle Sustainability Assessment (LCSA) constitutes an appropriate methodology jointly handling indicators related to the three traditional dimensions of the sustainability concept. In this work, the sustainability performance of renewable hydrogen from both wind-powered electrolysis and biomass gasification was benchmarked against that of conventional hydrogen from steam methane reforming under a set of five life-cycle indicators: global warming, acidification, levelised cost, child labour, and health expenditure. The results led to identify the stage of driving-energy/biomass production as the main source of impact. When compared to conventional hydrogen, the life-cycle sustainability performance of renewable hydrogen was found to underperform under social and economic aspects. Nevertheless, the expected enhancement in process efficiency would significantly improve the future performance of renewable hydrogen in each of the three main sustainability dimensions.

Long-term production technology mix of alternative fuels for road transport: A focus on Spain.

Road transport is one of the main sources of greenhouse gas emissions due to the current dependence on fossil fuels such as diesel and gasoline. This situation needs to be changed through the retirement of fossil fuels and the implementation of alternative fuels and vehicles such as biofuels, battery electric vehicles, and fuel cell electric vehicles fuelled by hydrogen. Nevertheless, the environmental suitability of alternative fuels is conditioned by how they are produced. Through the case study of Spain, this article prospectively assesses - from a techno-economic and carbon footprint perspective- the production technology mix of alternative fuels from 2020 to 2050. The proposed energy systems optimisation model includes a large number of production technologies regarding biofuels (bioethanol, biodiesel, synthetic diesel/gasoline, and hydrotreated vegetable oil), electricity, and hydrogen. The combined study of these fuels provides a relevant framework to discuss the targets established for the road transport sector with a high level of detail not only regarding fuel type but also technology breakdown. The results show the relevance of second-generation biofuel production technologies in fulfilling the future biofuel demand. Regarding the extra electricity demand associated with the penetration of electric vehicles, the results suggest a key role of wind- and solar-based technologies in meeting such a need. Concerning hydrogen as an option to decarbonise the transport system, even though steam methane reforming is the most mature and cost-competitive production technology, hydrogen production would be satisfied through electrolysis in order to avoid relying on fossil resources as the main feedstock. Overall, this integrated approach to the long-term production technology mix of alternative fuels for road transport is expected to be relevant to a wide range of decision-makers willing to prospectively assess road transport systems from a technology perspective.

Prospective carbon footprint comparison of hydrogen options.

The Life Cycle Assessment methodology is often used to evaluate the environmental performance of hydrogen energy systems. However, even though hydrogen is usually seen as a strategic energy carrier for the future energy sector, there is a lack of case studies assessing its prospective life-cycle performance. In order to contribute to filling this gap, this work addresses a carbon footprint comparison of hydrogen options from a prospective standpoint. Four relevant hydrogen production pathways (steam methane reforming, grid-powered alkaline electrolysis, wind-powered alkaline electrolysis, and biomass gasification) under three time scenarios (reference, year 2030, and year 2050) are assessed, taking into account the expected evolution of key technical parameters such as efficiencies, lifespans, and the grid electricity mix. The results show a favourable carbon footprint of renewable hydrogen from biomass gasification and wind electrolysis, with a relatively steady near-zero carbon footprint. Despite the unfavourable carbon footprint results of conventional hydrogen from steam methane reforming and hydrogen from grid electrolysis, the latter is associated with a rapid trend towards a suitable long-term carbon footprint.

Sensitivity of operational and environmental benchmarks of retail stores to decision-makers' preferences through Data Envelopment Analysis.

Within the framework of multi-criteria decision analysis (MCDA), weighting methods are typically used to capture decision-makers' preferences. In this regard, the increasing use of the combined LCA (Life Cycle Assessment) + DEA (Data Envelopment Analysis) methodology as an MCDA tool requires an in-depth analysis of how the preferences of decision-makers could affect the outcomes of LCA + DEA studies. This work revisits a case study of 30 retail stores/supply chains located in Spain by applying alternative weighted DEA approaches to evaluate the influence of decision-makers' preferences (weights) on the final outcomes, with a focus on efficiency scores and operational and environmental benchmarks. The ultimate goal is to effectively capture the view of stakeholders when applying LCA + DEA for the sound, sustainability-oriented management of multiple similar entities. Different weight vectors are separately applied to three types of DEA elements: operational inputs, time terms, and divisions. Besides, preferences from three alternative standpoints are considered: company manager through direct rating, and environmental policy-maker and local community through AHP (analytic hierarchy process). A significant influence on efficiency scores and sustainability benchmarks was found when weighting decision-makers' preferences on operational inputs. Additionally, a moderate influence was observed when weighting divisions according to a policy-maker or local community perspective. Although the results are case-specific, they lead to the general recommendation to enrich LCA + DEA studies by following not only an equal-weight approach but also approaches that include the preferences of the stakeholders effectively involved in the study.

Sustainability-oriented efficiency of retail supply chains: A combination of Life Cycle Assessment and dynamic network Data Envelopment Analysis.

Assessing the efficiency of retail supply chains (RSCs) requires analytical tools that address the different activities involved in these chains. In this sense, dynamic network Data Envelopment Analysis (DEA) arises as a suitable method to evaluate the operational performance of RSCs over a period of time. However, its use for sustainability-oriented efficiency assessment constitutes a knowledge gap that limits its applicability for thorough decision-making processes, e.g. at the retail company level. This article fills this gap through the combination of Life Cycle Assessment (LCA) and dynamic network DEA. A novel five-step LCA + DEA approach is proposed and applied to a case study of 30 RSCs in Spain for the period 2015-2017. In this case, the supply chain structure involves three divisions: central distribution, operation of retail stores, and home delivery. Both overall- and term-efficiency scores were found to widely range from 0.38 to 1.00, with only 1 RSC deemed efficient. Regarding divisional efficiency, store operation was found to generally show significantly higher efficiency scores than the distribution divisions. The link between long distribution distances and low efficiency stresses the relevance of integrating a network perspective into the efficiency assessment. In addition to efficiency scores, the LCA + DEA approach enriches the assessment by providing environmental, operational and socio-economic benchmarks to further support the management of RSCs from a sustainability perspective.

Sustainability-oriented management of retail stores through the combination of life cycle assessment and dynamic data envelopment analysis.

A sound management of retail stores is a crucial aspect in the path towards a sustainable commercial sector, with a lack of research studies in the field of joint efficiency and sustainability assessment within this sector. In this sense, this work delves into the role of operational efficiency in the sustainability-oriented management of retail stores through the case study of 30 groceries in Spain over the period 2015-2017. With this purpose, and given the current knowledge gap in period-oriented sustainability benchmarking for management plans, for the first time a five-step methodological framework based on the combination of Life Cycle Assessment (LCA) and dynamic Data Envelopment Analysis (DEA) was proposed and applied to a case study within the service sector. The overall- and term-efficiency scores calculated through this method led to the general conclusion of a relatively good performance of the set of grocery stores over the evaluated period, which is associated with the centralised management strategy followed by the retail company. Furthermore, operational, socio-economic and environmental benchmarks were calculated as target values that could assist decision-makers at the retail company level in setting the path for a sustainable operation of the company's stores. Overall, the proposed period-oriented LCA + DEA method proved to be a feasible and valuable tool for sustainability management of retail stores, being preferred over the static (i.e., single term) alternative provided that time-series data are available at the company level.

Combined use of Data Envelopment Analysis and Life Cycle Assessment for operational and environmental benchmarking in the service sector: A case study of grocery stores.

Ensuring sustainable production patterns doing more and better with less is a key sustainable development goal. In this sense, the joint use of Life Cycle Assessment and Data Envelopment Analysis (i.e., the LCA+DEA methodology) arises as a quantitative tool for the eco-efficiency assessment of multiple similar entities. To date, the LCA+DEA methodology has been widely applied to case studies within the primary and secondary sectors. However, the applicability of this combined methodology to case studies within the tertiary (service) sector is still unexplored, which constitutes a current knowledge gap in this field. This work contributes to filling this gap by benchmarking the operational and environmental performance of a sample of 30 groceries located in Spain. All the evaluated groceries were found to involve relative efficiency scores above 0.60, with one third of the groceries deemed fully efficient. Average reductions of 3-26% in the consumption of operational inputs were calculated, leading to average reductions of 9% in the carbon footprint and 10% in the energy footprint. Furthermore, economic savings of up to 3% of the annual turnover were estimated. These results were further enriched through the application of a super-efficiency DEA model for a refined identification of the best-performers, as well as through the novel use of a specific model for the gradual operational and environmental benchmarking of the sample. Overall, a high applicability of the LCA+DEA methodology for eco-efficiency assessment within the service sector is concluded, facilitating the identification and quantification of sustainable operational patterns.

Robust eco-efficiency assessment of hydrogen from biomass gasification as an alternative to conventional hydrogen: A life-cycle study with and without external costs.

Hydrogen is a key product for the decarbonisation of the energy sector. Nevertheless, because of the high number of technical options available for hydrogen production, their suitability needs to be thoroughly evaluated from a life-cycle perspective. The standardised concept of eco-efficiency is suitable for this purpose since it relates, with a life-cycle perspective, the environmental performance of a product system to its value. Hence, this work benchmarks the eco-efficiency performance of renewable hydrogen produced through biomass gasification against conventional hydrogen from the steam reforming of natural gas. For the eco-efficiency assessment, the harmonised environmental indicators of global warming, acidification and cumulative non-renewable energy demand were individually used, while the product system value was based on the levelised cost of hydrogen with/without internalisation of the external socio-environmental costs associated with climate change and human health. On the one hand, when the environmental and economic performances are separately considered, hydrogen from biomass gasification performs significantly better than hydrogen from steam methane reforming under environmental aspects (e.g., greenhouse gas emissions saving of 98%), whereas the opposite conclusion was found from an economic standpoint (levelised cost of 3.59 € and 2.17 € per kilogramme of renewable and fossil hydrogen, respectively). On the other hand, when combining life-cycle environmental and economic indicators under the umbrella of the eco-efficiency assessment, it is concluded that the renewable hydrogen option outperforms the conventional one, which is further remarked when implementing socio-environmental externalities. In this regard, a relative eco-efficiency score above 14 was estimated for the renewable hydrogen option when benchmarked against conventional hydrogen.

Energy balance and life cycle assessment of a microalgae-based wastewater treatment plant: A focus on alternative biogas uses.

The techno-environmental performance of a medium-scale wastewater treatment system using high-rate algal ponds was evaluated through mass and energy balances and life cycle assessment. The system involves wastewater primary treatment, microalgae-based secondary treatment, thermal hydrolysis with steam explosion of microalgae, anaerobic co-digestion of pre-treated microalgal biomass and primary sludge, and biogas cogeneration. Furthermore, two scenarios based on alternative biogas uses were considered: (i) biogas for heat and electricity, and (ii) biogas for heat, electricity, and biomethane. Pumping wastewater to the primary settler arose as the main source of electricity consumption. When compared to conventional activated sludge plants, a large decrease in the energy consumption was observed for the secondary treatment. Moreover, a favourable life-cycle performance was generally found for the microalgae-based systems when displacing conventional energy products. Finally, the preference for a specific scenario on biogas use was found to be highly conditioned by the techno-environmental aspects prioritised by decision-makers.

Environmental impact efficiency of natural gas combined cycle power plants: A combined life cycle assessment and dynamic data envelopment analysis approach.

The energy sector is still dominated by the use of fossil resources. In particular, natural gas represents the third most consumed resource, being a significant source of electricity in many countries. Since electricity production in natural gas combined cycle (NGCC) plants provides some benefits with respect to other non-renewable technologies, it is often seen as a transitional solution towards a future low­carbon power generation system. However, given the environmental profile and operational variability of NGCC power plants, their eco-efficiency assessment is required. In this respect, this article uses a novel combined Life Cycle Assessment (LCA) and dynamic Data Envelopment Analysis (DEA) approach in order to estimate -over the period 2010-2015- the environmental impact efficiencies of 20 NGCC power plants located in Spain. A three-step LCA+DEA method is applied, which involves data acquisition, calculation of environmental impacts through LCA, and the novel estimation of environmental impact efficiency (overall- and term-efficiency scores) through dynamic DEA. Although only 1 out of 20 NGCC power plants is found to be environmentally efficient, all plants show a relatively good environmental performance with overall eco-efficiency scores above 60%. Regarding individual periods, 2011 was -on average- the year with the highest environmental impact efficiency (95%), accounting for 5 efficient NGCC plants. In this respect, a link between high number of operating hours and high environmental impact efficiency is observed. Finally, preliminary environmental benchmarks are presented as an additional outcome in order to further support decision-makers in the path towards eco-efficiency in NGCC power plants.

Life-cycle consequences of internalising socio-environmental externalities of power generation.

Current national energy sectors are generally unsustainable. Within this context, energy policy-makers face the need to move from economy- to sustainability-oriented schemes. Beyond the integration of the sustainability concept into energy policies through the implementation of techno-economic, environmental and/or social restrictions, other approaches propose the use of externalities -based on life-cycle emissions- to deeply take into account sustainability in the design of the future energy system. In this sense, this work evaluates the consequences of internalising socio-environmental externalities associated with power generation. Besides the calculation of external costs of power generation technologies and their implementation in an energy systems optimisation model for Spain, the life-cycle consequences of this internalisation are explored. This involves the prospective analysis of the evolution of the sustainability indicators on which the externalities are founded, i.e. climate change and human health. For the first time, this is done by endogenously integrating the life-cycle indicators into the energy systems optimisation model. The results show that the internalisation of externalities highly influences the evolution of the electricity production mix as well as the corresponding life-cycle profile, hastening the decarbonisation of the power generation system and thus leading to a significant decrease in life-cycle impacts. This effect is observed both when internalising only climate change externalities and when internalising additionally human health external costs.