Sustainable energy strategies for a University district in Padova: multi-objective optimization of multi-energy systems and networks

Integration of energy generation and consumption is one of the most effective ways to reduce energy system-related waste, costs and emissions in cities. This thesis considers a university district consisting of 32 buildings spatially located in a geographical area of 422000 m^2 in the city of Padova, Italy. The electricity demands of all buildings are currently met by withdrawing electricity from several points of delivery in the distribution grid. A portion of the district, accounting for 31% of the total heating demand, is connected to a centralized thermal power generation system via a local district heating network, while the heating demand of the remaining buildings is met by small autonomous gas boilers. The goal is to explore effective and sustainable solutions for the district’s energy development to reduce cost, environmental impact and primary energy consumption. To this end, three new configurations of the Multi-Energy System (MES) of the district are proposed. The first one considers the installation of new energy conversion units (photovoltaic, gas-fired cogeneration internal combustion engines and heat pumps) and new thermal and electric storage units taking into account the actual availability of space for new assets (e.g., roof for photovoltaic, technical rooms, etc.). The second and third ones include also new branches of the electric and heating networks with increasing levels of integration to better match the highly fragmented energy demands, while better exploiting the limited availability of space. Results show that the installation of gas-fired cogeneration engines leads to significant benefits compared to the existing MES in both economic (up to -12.3% of overall annual costs) and energy (up to -10.2% of primary energy consumption) terms; these benefits increase as the level of integration increases. On the other hand, the limited availability of space for photovoltaics results in increased CO2 emissions compared to the existing MES when only the economic objective (total cost minimization) is considered. However, solutions with high average heat-to-power ratios in gas-fired cogeneration engines enable a significant reduction in CO2 emissions with an acceptable economic penalty (-23.9% of CO2 emissions and +8.4% of total cost compared to the least costly solution, with costs still lower than those of the existing MES). Additional analyses include an operation optimization to evaluate the effectiveness of introducing trigeneration plants (including absorption chillers powered by the waste heat from cogeneration engines). Results indicate that, compared to solutions with cogeneration engines only, the inclusion of trigeneration plants is beneficial both in terms of overall costs (up to -1.3%) and primary energy consumption (up to -2.7%) but penalizes CO2 emissions (up to 2.3% higher). Finally, a sensitivity analysis on energy carrier costs considers the influence of volatile economic conditions on optimization results, showing that at current electricity-to-gas cost ratios, the investment safety margin is solid for gas costs higher than or equal to 100 €/MWh while it requires more accurate assessments for gas costs close to 50 €/MWh. The results emerging from the optimization and the complementary analyses performed provide a comprehensive perspective on the district MES, supporting the formulation of realistic technical solutions as well as practical recommendations for future energy developments of the university district.