The ZellFlex project, funded under the German Federal Ministry of Education and Research (FKZ 03EN3001), investigated the integration of solar‑thermal (STA) generation, network losses, and local heat retention in district heating systems. The study combined detailed simulations with practical field data to evaluate how varying solar‑thermal areas, temperature set‑points, and storage configurations influence overall system performance.

In the solar‑thermal experiments, a 500 m² STA area was compared with a 1,000 m² configuration. Doubling the collector area roughly doubled the heat supplied to the network, confirming the linear scaling of solar output with collector surface. When the reference temperature curve was lowered by 5 K, the STA injection increased by 9–18 MWh a⁻¹ (≈ 4 %), and a 10 K reduction raised the injection by 19–38 MWh a⁻¹ (≈ 8 %). Across all curves, the STA contributed only 0.45–0.48 % of the total heat generation, indicating that solar input remains a small but valuable fraction of the overall supply. The simulations also quantified network losses and undersupply events, showing that higher solar penetration can reduce undersupply but also increases the risk of network over‑temperature if not properly managed.

Local heat retention was examined through a “cell” model that incorporated decentralized heat production, storage, and load‑management. The analysis revealed that by shifting heat loads from Saturday mornings to Friday evenings, the required central storage capacity could be reduced by roughly 0.25 kWh per kWh of load shift. For a 750 kWh daily load, this translates to a 3.8 % reduction in central storage volume, a modest but measurable benefit. The study also demonstrated that during summer months (May–September) the cell could operate autonomously for periods ranging from a few days to over three weeks, thanks to the combined effect of local storage and high solar output. However, the simulation acknowledged that fixed temperature requirements of consumers were not fully represented, potentially underestimating undersupply risks.

Demand‑driven regulation of the supply temperature was another key focus. By implementing a temperature‑pinch‑point strategy, the project reduced network losses and improved the distribution of heat. The simulations identified critical temperature points where the network was most vulnerable to undersupply, and showed that lowering the set‑point temperature could shift these points, thereby mitigating risk. The analysis also highlighted the importance of having sufficient measurement points; limited sensor coverage can obscure the true distribution of temperature pinch points.

Overall, the technical results demonstrate that solar‑thermal integration, when combined with strategic storage and demand‑side management, can enhance the resilience and efficiency of district heating networks. The modest gains in storage reduction and the ability to achieve partial autonomy during peak solar periods suggest that such hybrid systems are viable for future energy‑efficient urban heating strategies. The project’s findings provide a quantitative basis for policymakers and utilities to design district heating networks that balance renewable input, storage capacity, and consumer demand while minimizing network losses.