  District heating networks are considered a key factor for enabling
emission-free heat supply, while many existing networks still heavily rely on
fossil fuels. With district heating network pipes easily exceeding a lifetime
of 30 years, there is a growing potential to retrofit the heat producers of
existing networks to enable low-emission heat supply. Today, the heat producer
retrofit for district heating networks usually focuses on simplified
approaches, where the non-linear nature of the design problem is relaxed or not
considered at all. Some approaches take non-linearities into account but use
optimization routines that are either not scalable to large problems or are not
reliable in obtaining an optimal solution, such as parameter optimization and
sensitivity studies. This paper presents an automated design approach, to
decarbonize existing heating networks through optimal producer retrofit and
ultimately enabling 4th generation operation. The approach uses
multi-objective, mathematical optimization to balance CO2 emissions and network
costs, by assessing different CO2 prices, and is based on a detailed physical
model. The optimizer is given the freedom to choose the producer types, their
capacities, and for each period, their supplied heat and supply temperature. A
non-linear heat transport model accurately accounts for heat and momentum
losses throughout the network, and ensures the feasibility of the proposed
design and operation. The multi-period formulation incorporates temporal
changes in heat demand and environmental conditions throughout the year. By
formulating a continuous problem and using adjoint-based optimization, the
automated approach remains scalable towards large scale applications. The
design approach was assessed on a medium-sized 3rd generation DHN case and was
able to optimally retrofit the heat producers.
