Trade-offs of affordability, sustainability and security in the optimal design of multi-energy systems

The energy transition to local multi-energy systems has the potential to reduce the cost and carbon emissions of energy supply while increasing its reliability. However, cost, emissions, and security of energy supply have trade-offs that affect decision-making. This paper presents a multi-objective optimization model that quantifies the trade-offs among expected annual costs, life-cycle greenhouse gas emissions, and energy not served under single-failure events of local multi-energy systems. The developed model optimally sizes candidate conversion and storage technologies and operates them under nominal and failure modes. Results show that system planners can leverage the non-linearity in trade-offs for the best compromise among costs, emissions, and security of energy supply. Results for a Swiss neighborhood show that local multi-energy systems simultaneously reduce costs by 15%, emissions by 64%, and increase reliability by 80% over the business-as-usual way of meeting household electricity and heat demand. This best compromise optimal multi-energy system diversifies the heating supply by primarily utilizing heat pumps, supplemented by gas boilers and thermal storage, that mitigate grid supply interruptions and heat pump failures. While such heating systems reduce emissions significantly (64%), independence from gas-based heating will require heat pumps with reduced life-cycle impacts and high availability. In the electricity sector, the optimal choice of technologies to meet reliability targets differs for varying emission targets, i.e., CHP is favored for loose emission targets. In contrast, solar PV and batteries are favored for strict emission targets. Critically, maximizing reliability via multi-energy systems has a significant environmental impact in low-emission grids.