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Abstract With the increase of energy consumption and wastes generation due to human activities, anaerobic digestion (AD), a technology which turns wastes into bio-energy, is receiving more and more attention in the world. It is well known that there are at least three stages involved in anaerobic digestion, i.e., hydrolysis, acidification, and methanogenesis. Until now, however, the advances in enhancing acidification and methanogenesis have not been reviewed. This review provides a comprehensive overview of the methods reported to enhance each step involved in anaerobic digestion. More important, enzymes are the key to anaerobic digestion, and the strategies for improving enzyme activity are summarized. As electron transfer has been reported to play an important role in anaerobic digestion, the research progress of the approaches for the acceleration of direct interspecies electron transfer in methane production is also introduced. In addition, the recent advances in increasing the reduction of carbon dioxide to methane, which has been widely observed in methanogenesis step, are reviewed. Furthermore, the techno-economic assessment of anaerobic digestion is made, and the key points for future studies are proposed.

The United Nations’ Sustainable Development Goals (SDGs) provide a guideline for humanity to respond to an array of pressing challenges. Due to our increasing need for energy supply and more stringent standards for environmental quality, having access to affordable and clean energy has been the foremost pursuit of SDG 7. Development in renewables represents a way to achieve this goal. Here, we establish a Footprint-Driver-Scenario (FDS) framework for accounting for the renewable energy footprint of 189 global economies based on a global multi-regional input − output (MRIO) model and identifying the major drivers behind based on the logarithmic mean Divisia index (LMDI) in 1990–2015, and projecting the national renewable energy footprint by 2030 based on the Shared Socioeconomic Pathways (SSPs) scenarios. We find that total and per capita renewable energy footprint varies substantially between nations. The improvement in energy efficiency (SDG 7.3) and decline in footprint-to-energy ratio contribute to the reduction of renewable energy footprint, as opposed to the per capita GDP, population, share of renewable energy in energy mix (SDG 7.2) and proportion of population with access to electricity (SDG 7.1), all of which lead to footprint increase considerably. Despite the great progress in SDGs 7.1–7.3 by 2030, the expected goals still cannot be fully reached in any of the SSP scenarios. Our research findings can assist policy makers in better understanding the critical role of renewable energy in achieving SDG 7. The FDS framework can be potentially applied to a wide range of SDGs at the global, national and sub-national scales.

Abstract Integrating low-carbon technologies (e.g. heat pumps, photovoltaic systems) in buildings influences the stability of the low-voltage grid, which therefore often requires to be reinforced. This article proposes a techno-economic methodology to identify the reinforcements needed to maintain grid stability at the lowest life-cycle cost. Novel contributions include the consideration of three-phase connection of low-carbon technologies as a reinforcement option and the fact that we study to what extent grid reinforcements can mitigate voltage unbalance issues. Additionally, to reduce computing time, a dummy island approach is used, whereby one feeder is modelled in detail and the remainder of the distribution island is represented by an aggregated load. Finally, random repetitions are proposed, to consider uncertainties related to building properties, occupants and the location of low-carbon technologies in the feeders. The methodology is applied to investigate the integration of heat pumps and photovoltaic systems in typical Belgian rural and urban grids. For the rural grid, heat pumps may lead to significant reinforcement costs (up to 1230 €/dwelling), mainly due to voltage stability problems. For the urban grid, heat pump and photovoltaic integration causes low reinforcement cost (

Abstract Many organizations in the world are interested in waste management problems and their potential solutions. In order to solve these problems, a Japanese venture company has developed an innovative thermal decomposer for organic wastes called ERCM (Earth-Resource-Ceramic-Machine). The ERCM reactor employs electron injected air to promote the thermal decomposition reaction, while the effect of electron injection into air has not yet been clarified. An experimental work was performed using a fixed bed reactor to explore the effects of different parameters of electron injection into air, the reaction temperature and different feedstock on the syngas generation. The main purpose of this study is to clarify the phenomena occurring in the ERCM reactor where a direct current electric field is produced in the flame reaction zone to enhance the thermal decomposition of wastes. In this regard, a mathematical model for simulating the thermal decomposition of solid waste in the presence of an electric field have been developed. The equations of aero-thermochemistry are coupled to the balance equations for densities of charged species, and the Poisson equation for the electrical potential is solved. The model was validated by the experimental data and showed a good agreement. The results showed that the electric field significantly improves the stabilization of the flame. From the release behavior of CO and CO2, it is noted that the electron injection would affect the char combustion process significantly. Finally the effect of the flame reaction zone generated by the field induced ion wind on the thermal decomposition was investigated.

Abstract With hydrogen becoming more and more important as energy carrier, there is a need for high capacity storage technologies preferably operating at low pressures. Chemical storage in metal hydrides is promising for that purpose, but they require thermal management for hydrogen release and storage, respectively. To overcome this challenge, it is beneficial to store the heat needed for hydrogen release during hydrogen storage in the storage system keeping the additional effort to provide that heat low. In this work, the experimental proof of concept of an adiabatic storage reactor is presented. Magnesium hydride and magnesium hydroxide have been used for hydrogen storage and thermochemical heat storage, respectively. A prototype reactor has been developed and experimentally investigated. It was found that the operating temperature of the materials can be adjusted with the gas pressure in a way to establish a temperature gradient from the MgH2 to the Mg(OH)2 and vice versa. Hydrogen storage and release is enhanced by the thermochemical heating/cooling. A pressure of 9 bar is sufficient to store hydrogen with a capacity of 20.8 gH2 L-1 based on the two materials only, without the steel vessel or insulation. In the heat storage compartment, 300 °C have been reached at 9.75 bar during heat release which is high enough to drive the MgH2 dehydrogenation.

Abstract PV-blind embedded double skin facade (PVB-DSF) is a promising facade system for building energy efficiency. This paper developed a coupled thermal-electrical-optical model for analyzing, evaluating and optimizing the system performance. The ray-tracing method, radiosity method and net radiation method are used for the optical model. The single diode RP-model and Lambert-W function are adopted in the electrical model. The airflow network and energy balance equations are coupled for system thermal model. A complex simulation algorithm is proposed for the thermal-electrical-optical model solution. A series of experiments were implemented for model verification. The comparisons between simulation and measurement data show a good agreement. Specifically, the relative mean square error (RMSE) for simulation result in optical model is 2.02 W/m2 in sunny day and 5.21 W/m2 in cloudy day; 2.24 V for output voltage and 1.47 W for output power; 0.67 °C, 0.41 °C and 2.17 °C for external, internal glass pane and PV-blind. On top of that, the model is used as a tool for understanding the system performance under different configurations and position of PV-blind, solar cell efficiency and airflow rate. PV-blind angle and width under different spacing settings are optimized for balancing system energy performance and indoor daylighting comfort level. This study offers a useful simulation tool and a deeper understanding of PVB-DSF, which is beneficial for the design, control, optimization and evaluation of this effective glazing facade and can contribute to building energy efficiency.

Abstract This study aimed to investigate the applicability of the solar heating system in different geographical regions with different meteorological conditions, of which parabolic trough solar collectors (PTCs) were operated with the absorption heat pumps (AHP) and oil/water heat exchanger (OWHE) at medium and low operating temperature respectively. The heat transfer model for PTCs was constructed with a lumped parameter method and validated by experimental results. The thermal performance of the system was evaluated by the index of primary energy ratio (PER). The results showed that on overcast days with low direct normal irradiance (DNI), the operation of the PTC + AHP/OWHE system was not cost-effective. On cloudy days with high DNI lasting for a short period (e.g. 2 h), the operation of the PTC + OWHE system was better than that of the PTC + AHP system as the latter needed more preheating energy before the system operation. While on sunny days with high DNI lasting for a longer period (e.g. 8 h), the PTC + AHP system was suggested to be operated as it owned higher PER values. Besides, the solar energy distributions in China could be divided into four categories, i.e. rich (S1), relatively rich (S2), available (S3) and absent (S4). In S1 region the PTC + AHP system was suggested to be operated. In S4 region the PTC + AHP/OWHE system was not suggested to be operated. While in S2 and S3 region, whether the operation of the PTC + AHP/OWHE system was suggested depending on the meteorological conditions. With these findings the developing strategy of the solar heating system in different geographical regions can be devised and the operating strategy on a specific day with different weather conditions can be developed, which is helpful to guide the application of solar energy and improve the energy structure in domestic heating.

Abstract A kinetic model based on the elementary reaction velocity theory is useful for investigating the kinetic rules of chemical reactions under subcritical and supercritical conditions. This model can be used to determine the stochastic relations between the hydrolysis products and conversion time in subcritical water based on the molecular collision theory. First, hydrolysis reaction experiments using rapeseed oil were conducted, and the collected data were used to verify the effectiveness of this stochastic kinetic model. The results showed that the kurtosis of this model was 1.11922, skewness was −1.49277 and mean error of the system was 0.81499, which was relatively small. Meanwhile, the adjustment coefficient (Adj. R2) was 0.98923, which indicated that this model is highly significant, that is, it can accurately depict the microscopic stochastic process of the rapeseed oil hydrolysis reaction. Furthermore, it verifies that the stochastic theory and molecular collision theory are feasible under subcritical conditions, which is a breakthrough in the field of the kinetic study of high-temperature and high-pressure reaction systems. Furthermore, the model reveals the changing rules of the reaction order and reaction velocity of the rapeseed oil hydrolysis reaction, which provides an important reference for parameter optimization of industrial reactor design.

Abstract The correlation between effectiveness of multiple operating parameters is a complicated issue and significant to the safe operation of solid oxide fuel cell/gas turbine (SOFC/GT) hybrid system. This paper presents a numerical analysis on biogas-fueled SOFC/GT hybrid system with a recirculation process using combustor exhaust gas. With consideration of safety constraints for critical components (fuel cell thermal crack, reformer carbon deposition, turbine blade overheat), the interaction mechanism of recirculation ratio, steam/carbon ratio and fuel/air ratio is studied from the perspective of thermodynamic analysis. Results show that the recirculation process could increase the electrical efficiency of system from 58.18% to 62.8%. However, for the safety consideration of SOFC, the acceptable recirculation ratio should be controlled between 0.17 and 0.32. Meanwhile, there exists a minimum point of turbine inlet temperature at the recirculation ratio of 0.3. With recirculation ratio switched from 0.1 to 0.3, impact of steam/carbon ratio variation on SOFC temperature gradient shrinks from 52.5% to 17.9% of the reference value. On the other hand, effect of fuel/air ratio variation on SOFC temperature gradient is promoted from 20.8% to 44.3% of the reference value, due to that oxidation reaction of biogas becomes a dominant factor. Based on these results, a parametric comparison is carried out to quantitatively describe the impact of recirculation process on the effectiveness of other parameters. Short discussion is conducted on parameter selection to acquire higher system efficiency and sufficient safety range, in which case the total output power of 176.8 kW and electrical efficiency of 61.78% could be achieved.