Solar Heating with Seasonal Storage in Bracht: Construction, Initial Operation and Night-time Cooling against Stagnation
Abstract
The Bracht solar district heating system is a rural network that supplies an existing village predominantly with solar heat from an 11,640 m² collector field coupled to a seasonal 26,600 m³ pit thermal energy storage, targeting about 98% CO 2 reduction compared to the current situation. The construction challenges posed by groundwater ingress, constrained water supply for filling, and the lack of market-ready submerged temperature measurement were addressed through targeted engineering and a bespoke measurement setup. A comprehensive instrumentation package facilitates closed energy and mass balances, as well as model validation. The primary operational challenge pertains to elevated summer input, a consequence of updated 2030 irradiance statistics. This update leads to a risk of stagnation, particularly when considering limited storage capacity due to economic design of the system. Instead of installing a recooler, an evaluation of night cooling strategies was conducted that utilizes the collector field as a dissipative sink. The results of the simulations indicate that the potential for late-summer cooling is approximately 2–2.5 kWh per m² per night (equivalent to 17 – 25 MWh). Preliminary results indicate that predictive scheduling can eliminate the risk of stagnation while achieving full seasonal charge, positioning Bracht as a test bed for control strategies in highsolar rural district heating.
Keywords
Solar District Heating, Seasonal Storage, Stagnation Risk and Avoidance, Night Cooling