Abstract
A sustainable energy system defines the pattern of energy generation and uses that can be sustained into the future. This study develops possible transition paths towards the future low carbon energy by proposing various energy flow scenarios using natural gas as a clean source of energy for micro-CHPs, and then renewable biogas/biomethane as a replacement fuel. In Part 1, mathematical modelling/optimization of the natural-gas based Distributed Energy Supply System (DESS), both at the building and overall energy supply network level was carried out for three types of micro-CHPs – SOFCs, Stirling Engines, ICEs – and for various operating strategies – cost-driven, primary energy-driven, CO2-emission-driven, with a novel cap on electricity export. In Part 2, Mathematical modelling/optimization at both levels was also developed for a retrofit energy supply network with biogas/biomethane generation from different feedstocks. The biogas utilization framework consisted of a novel approach suggested by this study for upgrading biogas in an existing natural gas processing plants. Optimization criteria included maximizing the NPV, the GHG reduction, the use of biogas/biomethane, and minimizing the use of natural gas. All optimization was carried out using the General Algebraic Modelling System (GAMS). Utilizing micro-CHPs on a natural gas-based energy network showed the primary energy consumption driven options achieving a 6-10% reduction of total primary energy use compared to the base case. When the system was optimized for cost, applying a cap on the electricity export did not change the selected optimal PGU technology/capacity, but affected some quantitative assessments significantly – primary energy consumption; or at least moderately – payback time. It was established that biogas/biomethane could replace 25% to 84% of the domestic natural gas demand for the UK, with GHG savings of 1.61 – 3.23 tonnes CO2e/year/household (42% to 85% reduction from the base case). Total capital spend ranged from £5,133 to £29,955/household with NPVs ranging from £7,036 to -£19,202 per household, propped up by government subsidies. Gasification had a greater potential than Anaerobic Digestion (AD). Also, centralized CHP heat was established as too expensive at current prices, and its use made no significant impact to the overall energy flow. Sensitivity for subsidies, CO2 price and capital costs revealed both several positive and negative effects.