Decarbonization gained prominence with the witnessed rise of temperature over recent years, particularly in the aftermath of the adoption of the Paris agreement for limiting the temperature increase within 2°C until 2050. Biogenic resources are explicitly indicated as carbon-neutral from Life Cycle Assessment perspective by the IPCC, shedding light on the carbon-neutral society by applying Biogenic Energy Carbon Capture for creating negative emissions. This article proposes a novel modeling approach by introducing carbon layers with specification on the principal carbon sources and sinks based upon an optimization algorithm, in order to solve the carbon loop issue in a highly interconnected energy system due to increasing penetration of biomass and carbon capture, use, and storage. This study contributes to quantifying biogenic and nonbiogenic carbon footprints, and optimizing the circular economy associated with a net-zero-emission society, in favor of policy-making for sustainable development in long terms.

Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.

Water based combi-storages are frequently used in Central Europe for storing heat for space heating and domestic hot water in one device. The stratification efficiency of direct combi-storage inlets has a decisive influence on the energetic efficiency of solar thermal systems and even more of heat pumps that are connected to combi-storages. However, there are currently no simple recommendations for the geometrical design of direct storage inlets together with scientifically supported values for maximum mass flow rates that can be used for charging without disturbing stratification in the storage. In this work, the effect of different geometrical designs and mass flow rates on stratification of the storage was studied for direct storage inlets with computational fluid dynamics. Selected experiments were validated with laboratory tests.

The substitution of fossil energy carriers by renewable hydrocarbons is a promising measure to reduce net CO2 emissions. The production of renewable hydrocarbons requires hydrogen and carbon. Although hydrogen is easily accessible, the finding of optimal carbon sources requires an understanding of natural and anthropogenic carbon flows. However, there is no overview on the entire carbon flows in Switzerland yet. Although the potential of carbon from biomass is well known, it is unknown whether CO2 is the limiting factor for the production of renewable synthetic fuels. In a comprehensive analysis, 57 carbon paths within Switzerland and across its borders are identified. The carbon flows are quantified for 2013 and illustrated in a Sankey diagram. The diagram is universal and may be applied to other countries in this form. From this analysis, optimal carbon sources for renewable fuels are found. The question is discussed of whether CO2 for synthetic renewable fuels may also originate from nonbiogenic sources. Optimal carbon sources are either biomass or CO2 from incineration plants or cement‐manufacturing plants. If all accessible CO2 from these plants was used for the production of CH4, 43 % of Switzerland's road transportation could be powered with only a very low net CO2 emission if the electric power originates from renewable sources. The limiting factor for the production of synthetic fuel is renewable electrical energy rather than CO2. We propose that the origin of CO2 does not affect the sustainability of the synthetic fuel but the origin of the electric power does.