Optimal decarbonization strategies for existing districts considering energy systems and retrofits

Abstract

Integrated energy system planning at the district level can contribute towards the sustainable transformation of the building sector by unlocking solutions beyond individual buildings. This is particularly true for existing districts, whose older buildings have a low energy performance and for which measures to reduce the energy demand and ensure a low-emission energy supply must be implemented. In the urban context, district heating networks (DHN) are a promising way of doing the latter, especially with carbon capture and storage (CCS) on the horizon. However, investment decisions for both types of measures – energy supply and demand reduction – must consider individual buildings as part of district-scale considerations as building-level demand-side interventions affect energy demand patterns and densities. These can in turn affect energy supply decisions at the district-level.This study presents a comprehensive methodology for determining optimal decarbonization strategies for existing districts while considering building-level energy supply and retrofitting investment decisions and the expansion of existing DHN. We do so by extending the MANGOret (Multi-stAge eNerGy Optimization - retrofitting) optimization framework for the long-term investment planning of building-level multi-energy systems and envelope retrofits. In addition, the study presents an approach for modeling CCS as an emission-reduction technology for the waste incinerator supplying the marginal expansion of the district heating network.The developed optimization model considers a long-term time horizon with multiple investment stages, allowing it to represent economic and technological developments over the model time horizon. It incorporates a multi-objective perspective, capturing the trade-offs between the total system costs and emissions. The model is applied to two existing case study neighborhoods in the city of Chur, Switzerland. The long-term energy system design and operation for the two neighborhood typologies – mixed-use and residential – are analyzed as part of the DHN expansion investment decision.The results show that retrofitting is the main cost driver of any decarbonization strategy. Therefore the choice and the size of the energy systems offer better leverage for reducing emissions with a moderate increase in costs. In dense inner-city neighborhoods with high heating demands, district heating is the cost-optimal heating choice. Together with a low-emission district heating source such as a waste incinerator with CCS or biomass, it also becomes the CO2-optimal choice. Furthermore, the case studies demonstrate that a combination of heat pumps, hot water thermal storage, and solar PV is not only the CO2-optimal but also the cost-optimal decentralized heating option if no DHN is available. This highlights the attractiveness of energy-efficient heating systems already today.