Experimental and numerical study of a large-scale water pit thermal energy storage: Quantifying the impact of progressive lid insulation degradation

Seasonal water pit thermal energy storage (PTES) plays a vital role in addressing fluctuations and intermittency of solar energy, ensuring a stable and reliable heating supply. Existing research often assumed constant lid insulation for PTES, neglecting the degradation caused by moisture and thermal damage. This leads to an overly optimistic estimation of PTES thermal efficiency. To address this gap, this study investigated the long-term performance of a large-scale PTES located in Marstal, Denmark, explicitly considering insulation degradation of the top lid for the first time. A monitoring dataset (2013–2017) was processed to characterize operational dynamics of the PTES. Then, an improved TRNSYS model (Type 1301) capable of dynamically adjusting thermal resistance of top lid during multi-year simulations was developed and integrated with Type 1536 to simulate thermal storage in a truncated rectangular pyramid configuration. Subsequently, a monitoring data-driven inverse parameter fitting method was developed to determine the year-by-year lid insulation degradation patterns. Finally, the long-term performance of the PTES was comparatively analyzed for the measurements and two simulation cases. Results revealed a significant increase in thermal conductivity of top lid from 0.065 W·m⁻¹·K⁻¹ in 2014 to 0.29 W·m⁻¹·K⁻¹ in 2017. The optimized case using dynamic insulation properties of top lid demonstrated better agreement with monitoring results compared to the reference case assuming constant thermal conductivity of 0.06 W·m⁻¹·K⁻¹ (50 % degradation). Specifically in 2017, the reference case significantly deviated from the measurements due to the liner breakage and subsequent water seeped into the insulation. The annual discharged energy deviations in 2017 for the optimized and reference cases were 5.09 % and 45.54 %, respectively; annual heat loss deviations were − 3.13 % and − 55.52 %, respectively. The storage efficiency of reference case consistently simulated higher values, ranging approximately from around 65 % in 2014 to 75 % in 2017. In contrast, the measurement and the optimized case displayed similar trends, with efficiencies ranging from around 60 % to 65 % from 2014 to 2016 but decreasing significantly to 40 % in 2017. A sensitivity analysis showed that the water content of soil and the lid insulation materials had great impact on the performance of PTES. This study determined the lid insulation degradation patterns based on a historical dataset, enabling enhanced accuracy in simulating the performance of the Marstal PTES.