For flexible power generation of biogas plants and effective heat utilization, efficient heat storage technologies will play a decisive role. For this purpose, a process is being developed in the ThermoFlex project, based on controlled operation of a secondary digester in thermophilic temperature ranges after a mesophilic main digester. This allows effective, flexible heat storage in the secondary digester for digester heating and external heat utilization at an acceptable temperature level without additional storage. The process biology of the secondary digester tolerates controlled temperature variations. An overview of the application potential and design of the ThermoFlex process is given. Batch and semicontinuous fermentation tests at various temperatures are described.

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Progressive global warming is one of the biggest challenges civilization is facing today. The establishment of a carbon dioxide (CO2)-neutral society based on sustainable value creation cycles is required to stop this development. The Integrated Cycles for Urban Biomass (ICU) concept is a new concept towards a CO2-neutral society. The integration of closed biomass cycles into residential buildings enable efficient resource utilization and avoid transport of biowaste. In this scenario, biowaste is degraded on-site into biogas that is converted into heat and electricity. The liquid fermentation residues are upgraded by nitrification processes (e.g., by a soiling®-process, EP3684909A1) to refined fertilizer, which can be used subsequently in house-internal gardens to produce fresh food for residents.Whereas this scenario sounds promising, comprehensive evaluations of produced amounts of biogas and food, saved CO2 and costs as well as social-cultural aspects are lacking. To assess these points, a feasibility study was performed, which estimated the material and energy flows based on simulations of the biogas process and food production.The calculations show that a residential complex with 100 persons can generate 21 % of the annual power (electrical and heat) consumption from the accumulated biowaste. The nitrogen (N) in the liquid fermentation residues enables the production of up to 6.3 t of fresh mass of lettuce per year in a 70 m2 professional hydroponic production area. The amount of produced lettuce corresponds to the amount of calories required to feed four persons for one year. Additionally, due to the reduction of biowaste transport and the in-house food and fertilizer production, 6 468 kg CO2-equivalent (CO2-eq) per year are saved compared to a conventional building. While the ICU concept is technically feasible, its costs are still 1.5 times higher than the revenues. However, the model predictions show that the ICU concept becomes economically feasible in case food prices further increase and ICU is implemented at larger scale, e.g.; at the district level. Finally, this study demonstrates that the ICU implementation can be a worthwile contribution towards a sustainable CO2-neutral society and enable to decrease the demand for agricultural land.

The integration of closed biomass cycles into residential buildings enables efficient resource utilization and avoids the transport of biowaste. In our scenario called Integrated Cycles for Urban Biomass (ICU), biowaste is degraded on-site into biogas that is converted into heat and electricity. Nitrification processes upgrade the liquid fermentation residues to refined fertilizer, which can be used subsequently in house-internal gardens to produce fresh food for residents. Our research aims to assess the ICU scenario regarding produced amounts of biogas and food, saved CO2 emissions and costs, and social–cultural aspects. Therefore, a model-based feasibility study was performed assuming a building with 100 residents. The calculations show that the ICU concept produces 21% of the annual power (electrical and heat) consumption from the accumulated biowaste and up to 7.6 t of the fresh mass of lettuce per year in a 70 m2 professional hydroponic production area. Furthermore, it saves 6468 kg CO2-equivalent (CO2-eq) per year. While the ICU concept is technically feasible, it becomes economically feasible for large-scale implementations and higher food prices. Overall, this study demonstrates that the ICU implementation can be a worthwhile contribution towards a sustainable CO2-neutral society and decrease the demand for agricultural land.

A process for a more efficient and flexible use of the produced heat in biogas plants is tested under real operational conditions. The process uses a mesophilic digester and a thermophilic secondary digester, and the heat demand of the plant is covered by using the heat stored in the secondary digester. The efficient heat use is achieved by a controlled variation of temperature in combination with a suitable process monitoring, without affecting the biogas production or disturbing the biological process. The concept was previously tested in ideal laboratory conditions. Here, the validation tests, in preparation for a planned large-scale trial, comprising one-year semicontinuous fermentation tests in the laboratory (close to real operational conditions) are presented. These real conditions are characterized by a high organic load of the reactors as well as by a more manifold substrate mix. During the experimental phase, it is determined that the maximum temperature, at high variation rates between −4 and +2 K per day, reaches 54 °C, allowing a stable process. These experiments confirm that the adaptability to temperature changes depends on the boundary conditions of the fermentation and case-specific experiments are required before significant changes are made to the temperature management of anaerobic digestion systems.