A review on solar-assisted fourth- and fifth-generation district heating networks

Solar-assisted district heating is a key decarbonization pathway for urban heat supply, yet its integration within low-temperature networks remains technically demanding and context-dependent. This review examines how solar thermal collectors, seasonal thermal energy storage, and heat pumps interact in fourth-generation (4GDH) and fifth-generation (5GDHC) district heating and cooling architectures. In 4GDH, centralized solar fields with large-scale storage achieve solar fractions of 40–70%, heat pump SCOP of 3.5–5.0, and distribution losses of 5–15%. In 5GDHC, an ambient neutral ring at 15–18 °C enables decentralized heat pumps with SCOP of 4.0–6.0, at the cost of thermodynamic degradation and summer conflicts between solar feed-in and cooling demand. Performance ranges are consolidated in structured comparative tables. A synthesis of 11 case studies identifies three recurring configurations: high-solar-fraction borehole storage (feasible but costly); cost-optimized pit storage 4GDH (40–50% solar fraction at 55–60 €/MWh), and emerging 5GDHC networks with heating, cooling and PV or hybrid PV-thermal integration. Levelized heat costs range from 45 €/MWh for cost-optimal 4GDH configurations to over 200 €/MWh for high-solar-fraction systems. The review identifies storage sizing, control strategy, and system boundaries as key sources of performance variability and outlines open research priorities.