Ny utformning för gårdsbaserad biogasproduktion : En utvärdering ur material- och energisynpunkt

Sammanfattning: Renewable energy and sustainable use of natural resources are two important aspects of sustainable development. Biogas production contributes to this since the gas is considered to be a renewable and non-fossil fuel. In addition, the production of biogas results in a nutrient- rich substance, called digestate. Biogas is produced at large-scale or at farm-level. However, large-scale production is the predominant approach, which means that most biogas techniques available are adapted to large biogas plants. For that reason, there is potential for development of farm-level biogas plants. On behalf of Gunnar Bech, chairman of the innovation group Innovationsverket in Gamleby, a new design of farm-based biogas plants has been studied. The purpose was to examine whether the design has potential to be more optimal than existing farm-based biogas plants with comparable size and capacity. The study was focusing on material consumption and energy efficiency. Initially, an extensive literature review was carried out. The biology of the biogas process and important components of existing farm-based biogas plants are described in the review. Moreover, a model of the new design was constructed. This model comprised of six small digesters, each with a volume of 200 m3, which were placed in a building. The roof of the building was used for a solar power system with the aim to contribute to the heating requirement of the biogas plant. An external heat source in form of a pellets boiler covered the remaining heat demand. Based on this model, the thickness of insulation and the solar power system were dimensioned. Besides, the annual heating demand was calculated. All calculations were performed using the program Matlab and they were based on assumptions and simplifications. In order to evaluate the potential of the new design, similar calculations were made for a reference model, which represented an existing farm-based biogas plant with the same size and capacity. The results showed that the new design required more material compared to the reference model. Depending on the circumstances inside the digesters and the building, the new design required between 170-300 m3 more insulation than the reference model. Furthermore, the results indicated that it is advantageous from an energy point of view to place several small digesters inside an insulated building, provided that solar panels are installed on the roof. This was because the annual contribution from the external heat source decreased due to the solar panels. The results showed that the annual heat generated from the external heat source was 50-70 MWh lower compared to the reference model. However, the study showed that it was not optimal from an energy perspective to mount solar panels on the whole roof surface if the excess heat could not be utilized. On the basis of the results, the conclusion of the study was that the new design has potential to be a sustainable solution for farm-scale biogas production. The results indicated that the emissions of greenhouse gases were greater for the new design during the first years of operation compared to the reference model. On the other hand, the total amount of emissions was lower in a long-term perspective. After about 7 years, the new formation was better from an emission point of view compared to existing farm-based biogas plants. However, more studies are required to determine whether the design can be implemented or not.