André Bardow

Power-to-What? : Environmental assessment of energy storage systems

A large variety of energy storage systems are currently investigated for using surplus power from intermittent renewable energy sources. Typically, these energy storage systems are compared based on their Power-to-Power reconversion efficiency. Such a comparison, however, is inappropriate for energy storage systems not providing electric power as output. We therefore present a systematic environmental comparison of energy storage systems providing different products. As potential products, we consider the reconversion to power but also mobility, heat, fuels and chemical feedstock. Using life cycle assessment, we determine the environmental impacts avoided by using 1 MW h of surplus electricity in the energy storage systems instead of producing the same product in a conventional process. Based on data for several countries including the United States, Brazil, Japan, Germany and the United Kingdom, our analysis determines the highest reduction of global warming and fossil depletion impact for using surplus power in heat pumps with hot water storage and battery electric vehicles. Third highest environmental benefits are achieved by electrical energy storage systems (pumped hydro storage, compressed air energy storage and redox flow batteries). Environmental benefits are also obtained if surplus power is used to produce hydrogen but the benefits are lower. Our environmental assessment of energy storage systems is complemented by determination of CO2 mitigation costs. The lowest CO2 mitigation costs are achieved by electrical energy storage systems.

Life cycle assessment of polyols for polyurethane production using CO2 as feedstock: insights from an industrial case study

Polyethercarbonate polyols from carbon dioxide (CO2) are starting to be synthesized on industrial scale. These polyols can be further processed into polyurethanes enabling CO2 to be utilized in large amounts. Utilization of CO2 as alternative carbon feedstock for polyols is motivated from the potential to reduce greenhouse gas (GHG) emissions and fossil resource depletion. This article presents a life cycle assessment for production of CO2-based polyethercarbonate polyols in a real industrial pilot plant. The considered cradle-to-gate system boundaries include polyol production and all upstream processes such as provision of energy and feedstocks. In particular, provision of CO2 from a lignite power plant equipped with a pilot plant for CO2 capture is considered. Production of polyols with 20 wt% CO2 in the polymer chains causes GHG emissions of 2.65–2.86 kg CO2-eq kg−1 and thus, does not act as GHG sink. However, compared to production of conventional polyether polyols, production of polyols with 20 wt% CO2 allows for GHG reductions of 11–19%. Relating GHG emission reductions to the amount of CO2 incorporated, up to three kg CO2-eq emissions can be avoided per kg CO2 utilized. The use of fossil resources can be reduced by 13–16%. The impacts reductions increase with further increasing the CO2 content in the polyols. All other investigated environmental impacts such as eutrophication, ionizing radiation, ozone depletion, particulate matter formation, photochemical oxidant formation, and terrestrial acidification are also lowered. Therefore, synthesis of polyethercarbonate polyols from CO2 is clearly favorable compared to conventional polyether polyols from an environmental point of view.

Life-cycle assessment of carbon dioxide capture and utilization: avoiding the pitfalls

Carbon dioxide (CO2) capture and utilization (CCU) aims at reducing both greenhouse-gas emissions and fossil-resource depletion. Assessment of these aims requires quantitative environmental evaluation. So far, evaluation of CCU is based on ad hoc criteria such as the amount of CO2 utilized, simplified CO2 balances or CO2 storage duration. Albeit these criteria may be useful for very early stages of potential research pathways, we show that they are insufficient as basis for decisions on implementations and that they may lead to even qualitatively wrong environmental evaluation of CCU. Therefore, a holistic evaluation using life-cycle assessment (LCA) is mandatory. However, the application of LCA to CCU is subject to methodological pitfalls: (i) utilized CO2 might intuitively be considered as negative GHG emissions; (ii) since CCU usually generates products both in the capture and in the utilization process, choices exist how to allocate emissions to the individual products and (iii) CO2 storage duration is not reflected in traditional LCA. To avoid the existing pitfalls, we provide a systematic framework for LCA of CCU in which (i) the utilized CO2 is correctly considered as regular feedstock with its own production emissions; (ii) recommendations for obtaining product-specific LCA results for CCU processes are given and (iii) the CO2 storage duration is incorporated into a time-resolved global warming metric. The developed framework is illustrated by simplified LCA of CO2 capture from the atmosphere and from coal power plants, and of CO2 utilization for methanol and polymer production. Overall, the presented framework allows for the sound environmental evaluation of CCU.

Calculating thermodynamic properties from fluctuations at small scales.

We show how density and energy fluctuations of small nonperiodic systems embedded in a reservoir can be used to determine macroscopic thermodynamic properties like the enthalpy density and the thermodynamic correction factor. For mixtures, the same formalism leads to a very convenient method to obtain so-called total correlation function integrals, also often referred to as Kirkwood-Buff integrals. Using finite size scaling, the properties obtained for small systems can be extrapolated to the macroscopic system limit provided that the system is sufficiently far from the critical point. As derived in our previous work (Chem. Phys. Lett. 2011, 504, 199-201), the finite size scaling is significant and depends on 1/L, where L is the length of the small system in one dimension. By considering a reservoir with an ensemble of embedded small systems, we can use the scaling arising from surface effects to determine properties for macroscopic systems by extrapolation. We demonstrate this method for the WCA and LJ fluids, as well a for a heterogeneous system, i.e., argon adsorbed in silicalite-1 zeolite.

Selecting CO2 Sources for CO2 Utilization by Environmental-Merit-Order Curves

Capture and utilization of CO2 as alternative carbon feedstock for fuels, chemicals, and materials aims at reducing greenhouse gas emissions and fossil resource use. For capture of CO2, a large variety of CO2 sources exists. Since they emit much more CO2 than the expected demand for CO2 utilization, the environmentally most favorable CO2 sources should be selected. For this purpose, we introduce the environmental-merit-order (EMO) curve to rank CO2 sources according to their environmental impacts over the available CO2 supply. To determine the environmental impacts of CO2 capture, compression and transport, we conducted a comprehensive literature study for the energy demands of CO2 supply, and constructed a database for CO2 sources in Europe. Mapping these CO2 sources reveals that CO2 transport distances are usually small. Thus, neglecting transport in a first step, we find that environmental impacts are minimized by capturing CO2 first from chemical plants and natural gas processing, then from paper mills, power plants, and iron and steel plants. In a second step, we computed regional EMO curves considering transport and country-specific impacts for energy supply. Building upon regional EMO curves, we identify favorable locations for CO2 utilization with lowest environmental impacts of CO2 supply, so-called CO2 oases.

Sensitivity coefficient-based uncertainty analysis for multi-functionality in LCA

PurposeIn LCA, a multi-functionality problem exists whenever the environmental impacts of a multi-functional process have to be allocated between its multiple functions. Methods for fixing this multi-functionality problem are controversially discussed because the methods include ambiguous choices. To study the influence of these choices, the ISO standard requires a sensitivity analysis. This work presents an analytical method for analyzing sensitivities and uncertainties of LCA results with respect to the choices made when a multi-functionality problem is fixed.MethodsThe existing matrix algebra for LCA is expanded by explicit equations for methods that fix multi-functionality problems: allocation and avoided burden. For allocation, choices exist between alternative allocation factors. The expanded equations allow calculating LCA results as a function of allocation factors. For avoided burden, choices exist in selecting an avoided burden process from multiple candidates. This choice is represented by so-called aggregation factors. For avoided burden, the expanded equations calculate LCA results as a function of aggregation factors. The expanded equations are used to derive sensitivity coefficients for LCA results with respect to allocation factors and aggregation factors. Based on the sensitivity coefficients, uncertainties due to fixing a multi-functionality problem by allocation or avoided burden are analytically propagated. The method is illustrated using a virtual numerical example.Results and discussionThe presented approach rigorously quantifies sensitivities of LCA results with respect to the choices made when multi-functionality problems are fixed with allocation and avoided burden. The uncertainties due to fixing multi-functionality problems are analytically propagated to uncertainties in LCA results using a first-order approximation. For uncertainties in allocation factors, the first-order approximation is exact if no loops of the allocated functional flows exist. The contribution of uncertainties due to fixing multi-functionality problems can be directly compared to the uncertainty contributions induced by uncertain process data or characterization factors. The presented method allows the computationally efficient study of uncertainties due to fixing multi-functionality problems and could be automated in software tools.ConclusionsThis work provides a systematic method for the sensitivity analysis required by the ISO standard in case choices between alternative allocation procedures exist. The resulting analytical approach includes contributions of uncertainties in process data, characterization factors, and—in extension to existing methods—uncertainties due to fixing multi-functionality problems in a unifying rigorous framework. Based on the uncertainty contributions, LCA practitioners can select fields for data refinement to decrease the overall uncertainty in LCA results.

Fick Diffusion Coefficients of Liquid Mixtures Directly Obtained From Equilibrium Molecular Dynamics

A methodology for computing Fick diffusivities directly from equilibrium molecular dynamics (MD) simulations is presented and validated for acetone-methanol and acetone-tetrachloromethane liquid mixtures. Fick diffusivities are obtained from Maxwell-Stefan (MS) diffusivities and the so-called thermodynamic factor. MS diffusivities describe the friction between different components, while the thermodynamic factor is the concentration derivative of the activity describing the deviation from ideal mixing behavior. It is important to note that all mutual diffusion experiments measure Fick diffusion coefficients, while molecular simulation provides MS diffusivities. The required thermodynamic factor to convert MS into Fick diffusivities and vice versa, however, is usually difficult to extract from both simulations and experiments leaving a gap between theory and application. Here, we employ our novel method to compute the thermodynamic factor from small-scale density fluctuations in equilibrium MD simulations [Chem. Phys. Lett.2011, 504, 199-201]. Previously, this method was developed and validated for molecules with single interaction sites only. In this work, we applied this method to acetone-methanol and acetone-tetrachloromethane liquid mixtures and show that the method also works well in these more complex systems. This provides the missing step to extract Fick diffusion coefficients directly from equilibrium MD simulations. The computed Fick diffusivities of acetone-methanol and acetone-tetrachloromethane mixtures are in excellent agreement with experimental values. The suggested framework thus provides an efficient route to model diffusion in liquids on the basis of a consistent molecular picture.

Simultaneous process and working fluid optimisation for Organic Rankine Cycles (ORC) using PC-SAFT

Abstract Organic Rankine Cycles (ORC) have a broad economically attractive application range since they can be tailored to fit specific conditions. The tailor-made design of ORC involves the selection of a suitable working fluid and the optimisation of the process itself. Today, these design steps are usually performed separately. In this paper, the holistic design of ORC processes is rigorously approached employing computer-aided molecular design (CAMD) to select a working fluid and to simultaneously optimise the corresponding process within a single mathematical problem. Herein, the strong molecular picture underlying the PC-SAFT equation of state is exploited using the recently developed continuous-molecular targeting (CoMT) approach. The effectiveness of the CoMT-CAMD approach is demonstrated for the optimisation of both working fluid and process parameters in a waste-heat ORC application.

Maxwell–Stefan Diffusivities in Binary Mixtures of Ionic Liquids with Dimethyl Sulfoxide (DMSO) and H2O

Ionic liquids (ILs) are promising solvents for applications ranging from CO2 capture to the pretreatment of biomass. However, slow diffusion often restricts their applicability. A thorough understanding of diffusion in ILs is therefore highly desirable. Previous research largely focused on self-diffusion in ILs. For practical applications, mutual diffusion is by far more important than self-diffusion. For describing mutual diffusion in multicomponent systems, the Maxwell-Stefan (MS) approach is commonly used. Unfortunately, it is difficult to obtain MS diffusivities from experiments, but they can be directly extracted from molecular dynamics (MD) simulations. In this work, MS diffusivities were computed in binary systems containing 1-alkyl-3-methylimidazolium chloride (C(n)mimCl, n = 2, 4, 8), water, and/or dimethyl sulfoxide (DMSO) using MD. The dependence of self- and MS diffusivities on mixture composition was investigated. Our results show the following: (1) For solutions of ILs in water and DMSO, self-diffusivities decrease strongly with increasing IL concentration. For DMSO-IL, a single exponential decay is observed. (2) In both water-IL and DMSO-IL, MS diffusivities vary by a factor of 10 within the concentration range which is, however, still significantly smaller than the variation of the self-diffusion coefficients. (3) The MS diffusivities of the IL are almost independent of the alkyl chain length. (4) ILs stay in a form of isolated ions in C(n)mimCl-H2O mixtures; however, dissociation into ions is much less observed in C(n)mimCl-DMSO systems. This has a large effect on the concentration dependence of MS diffusivities. (5) Recently, we proposed a new model for predicting the MS diffusivity at infinite dilution, that is, Đ(ij)(x(k-->)1) (Ind. Eng. Chem. Res. 2011, 50, 4776-4782). This quantity describes the friction between components i and j when both are infinitely diluted in component k. In contrast to earlier empirical models, our model is based on the linear response theory and the Onsager relations which allows a clear interpretation of the results. The key assumption in the model is that velocity cross-correlations are neglected. The present study clearly shows that velocity cross-correlation functions in ILs cannot be neglected and that the dissociation of ILs into ions has a very strong influence on diffusion.

Comparative LCA of multi-product processes with non-common products: a systematic approach applied to chlorine electrolysis technologies

PurposeMulti-product processes are one source of multi-functionality causing widely discussed methodological problems within life cycle assessment. A multi-functionality problem exists for comparative life cycle assessment (LCA) of multi-product processes with non-common products. This work develops a systematic workflow for fixing the multi-functionality problem caused by the non-common products. A novel technology for chlor-alkali electrolysis is analyzed and compared to the industrial standard technology to illustrate the approach and to benchmark the new technologys environmental impact.MethodsA matrix-based workflow for comparative LCA of multi-product systems is presented. Products are distinguished in main products and by-products based on the reason of process operation. We argue that only main products form the reference flows of the compared multi-product systems. Fixing the multi-functionality problem follows directly from the chosen reference flows. The framework suggests system expansion to fix the multi-functionality problem if non-common main products exist. Non-common by-products still cause a multi-functionality problem. These by-products are systematically identified and the multi-functionality problem is fixed with avoided burden and allocation. A case study applies the workflow for comparing environmental impacts of the standard chlorine electrolysis to a novel process using oxygen-depolarized cathodes. Three scenarios are derived and evaluated. The assessed impact categories are cumulative energy demand, global warming potential, acidification potential, photochemical ozone creation potential, eutrophication potential, and human toxicity potential.Results and discussionThe proposed workflow minimizes the methodological choices. The multi-functionality problem is systematically fixed based on the distinction between the main products and by-products. Inconsistent solutions are prevented by rigorous identification of unequal by-products within the compared systems. Selecting avoided burden processes or allocation factors is the remaining ambiguous choice common to the standard methods. The case study demonstrates the applicability of the workflow to comparative LCA of multi-product systems. The case study results show lower environmental impacts for the novel electrolysis technology in all practically relevant scenarios and impact categories.ConclusionsThe framework for comparative LCA of multi-product systems with non-common products adds systematic clarity to the general ISO standards. The approach reduces the subjective choices of LCA practitioners to the identification of reason of process operation. This reason is defined if the site-specific economic conditions are known. The matrix-based formulation allows identification of inconsistencies caused by multi-functionality. For the novel electrolysis technology, the results indicate significant potential for environmental impact reduction.