Modellering och simulering av ett robust och resilient biogassystem för ökad försörjningstrygghet

Abstract: Within the Swedish defense there is a clear target when it comes to strengthening the country’s emergency preparedness. In connection to this, there are demands when it comes to the security of supply of fuel and electricity. This project has mainly focused on ensuring the supply of biogas from the local biogas plant in Uppsala (Kungsängens gård) during different levels of electricity shortage. In order to ensure the supply of biogas, different additional energy systems and components have been examined. The studied energy systems and components were: intermittent electricity production, energy storages and electricity production on site. The intermittent electricity production originated from a solar power plant (corresponding to 250 kWp) and a wind turbine (corresponding to 700 kW at most). Since intermittent electricity production is unpredictable and variable, a battery (corresponding to 800 kWh) was used. A fuel cell, using hydrogen as fuel, as well as a biogas engine, using raw biogas as fuel, were used separately to supply the remaining power load of the biogas plant. The hydrogen was produced via PEM electrolysis and was stored in a tank at 70 MPa. In addition, biogas was stored in liquid form by producing liquified biogas (LBG) from the biomethane production on site and storing the LBG in a tank. The LBG could then be converted to CBG (compressed biogas) whenever the biomethane production was insufficient. The energy systems and components were modelled and simulated using MATLAB as well as MATLAB Simulink in three different modes: normal operation (when the power supply from the grid is sufficient), island operation (when the power must be supplied via electricity production on site) and shortage situation (during a power outage or during power and capacity shortages). In addition, since the power need of the biogas plant depended on the amount of upgraded biogas being produced, three different cases depending on the fuel consumption of biogas buses and garbage trucks were modelled and simulated in island operation. The results revealed that the most reliable energy system in island operation was the one with a biogas engine, since most of the power need over a year was fulfilled as well as the (upgraded) biogas need (for the buses and the garbage trucks). The energy system containing a fuel cell was the most efficient in a shortage situation where only a small power need is fulfilled. Both systems could be useful in normal operation since they could supply some power during peak loads as well as contributing with heat production and oxygen production (in the case with an electrolyzer). In addition, the capacities of the fuel cell as well as the biogas engine in the island operation and the shortage situation differed vastly, which should be taken into account when determining what kind of system that is the most suitable and when. Furthermore, a brief economic analysis revealed that the investment as well as the operating costs for the studied systems and components are quite high. However, these investments must be put in relation to what that can be achieved in order to improve the resilience of the biogas plant and to ensure that fuel and electricity production is possible during times of crisis.