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benefit of such systems are scarce, with existing assessments majorly using average grid mixes that are frequently outdated and disregard the dynamic nature of renewable generation.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.

Life Cycle Assessment of a Vanadium Redox Flow Battery.

Batteries are one of the key technologies for flexible energy systems in the future. In particular, vanadium redox flow batteries (VRFB) are well suited to provide modular and scalable energy storage due to favorable characteristics such as long cycle life, easy scale-up, and good recyclability. However, there is a lack of detailed original studies on the potential environmental impacts of their production and operation. The present study fills this gap by providing a comprehensive life cycle assessment of a representative VRFB. Transparent and comprehensive inventory data are disclosed as a basis for further environmental studies. VRFBs are found to be promising regarding the assessed impact categories, especially at high energy-to-power (E/P) ratios. On the other hand, significant impacts are associated with the vanadium pentoxide production, which is why the origin and processing of the vanadium bearing ores are a key for further reducing the environmental impacts associated with the VRFB manufacturing. While the lower efficiency of the VRFB is a disadvantage in comparison to e.g. lithium-ion batteries (LIB), its recyclability is significantly higher. In this sense, the importance of taking a cradle-to-cradle life cycle perspective when comparing very different battery systems can be highlighted for further research on this topic.

Biomass pyrolysis for biochar or energy applications? A life cycle assessment.

The application of biochar as a soil amendment is a potential strategy for carbon sequestration. In this paper, a slow pyrolysis system for generating heat and biochar from lignocellulosic energy crops is simulated and its life-cycle performance compared with that of direct biomass combustion. The use of the char as biochar is also contrasted with alternative use options: cofiring in coal power plants, use as charcoal, and use as a fuel for heat generation. Additionally, the influence on the results of the long-term stability of the biochar in the soil, as well as of biochar effects on biomass yield, is evaluated. Negative greenhouse gas emissions are obtained for the biochar system, indicating a significant carbon abatement potential. However, this is achieved at the expense of lower energy efficiency and higher impacts in the other assessed categories when compared to direct biomass combustion. When comparing the different use options of the pyrolysis char, the most favorable result is obtained for char cofiring substituting fossil coal, even assuming high long-term stability of the char. Nevertheless, a high sensitivity to biomass yield increase is found for biochar systems. In this sense, biochar application to low-quality soils where high yield increases are expected would show a more favorable performance in terms of global warming.