Seasonal thermal energy storage (STES) can effectively mitigate the supply and demand imbalance of solar energy between winter and summer. Large-scale water pit thermal storage systems require efficient and accurate computational simulations to avoid investment waste. This study proposes a simplified numerical analysis method and establishes a cylindrical underground pit with a total volume of 11304 m³ to describe the operation of a STES system. The model establishes a one-dimensional heat transfer model for the water body and a two-dimensional heat transfer model for the soil, separately solving for the water and the soil temperature field. The two models are connected through the temperature boundary at the pool wall to simulate the entire system. To comprehensively verify the accuracy of the numerical simulation model, validation was conducted under standby, charging, and discharging modes. The results indicate that the developed model has good accuracy and reliability. Under the standby mode, the temperature error of the five water layers in the sandbox test is less than 10%, with the highest accuracy in the middle and lower-middle water layers, with an average absolute error of 1.75% and 1.24%, respectively. Under the charging mode, the average relative error is 1.57%, and the average temperature error is 0.44 ℃. Under the discharging mode, the average relative error is 0.46%, and the average temperature error is 0.24 ℃.

The use of large-scale water-pit thermal energy storage (PTES) systems can increase the share of renewable energy sources in district heating systems. Currently, the available models for PTES systems are mostly based on numerical models that are not amenable to fast calculation, lacking accurate analytical models. Accordingly, this study proposes a analytical model suitable for PTES. The model solves for the water and soil domains separately and then couples them through the pit sidewall, bottom temperature, and boundary heat fluxes. For water domain heat transfer analysis, a "three-zone" model is proposed, dividing the cross-section into central, transition, and edge zones. Compared to simple one-dimensional models, this "three-zone" model considers horizontal water flow, thereby providing a more accurate dynamic simulation of water temperature. In the heat transfer analysis of the soil domain, the finite cylindrical source model used in the field of ground-source heat pumps was improved to make it suitable for modeling PTES. Furthermore, the semi-analytical model was validated using 10 years of measured data from a 60000 m³ PTES in Denmark. The average temperature errors at the top, upper-middle, middle, lower-middle, and bottom positions were 0.233%, 0.44%, 0.445%, 0.316%, and 1.27%, respectively, all less than 1.5%, indicating that the model exhibits high accuracy and reliability.

To realize the goal of net zero energy building (NZEB), the integration of renewable energy and novel design of buildings is needed. The paths of energy demand reduction and additional energy supply with renewables are separated. In this study, those two are merged into one integration. The concept is based on the combination of photovoltaic, thermoelectric modules, energy storage and control algorithms. Five types of building envelope systems, namely PV+TE (S1), Grid+TE (S2), PV+Grid+TE (S3), PV+Battery+TE (S4) and PV+Grid+Battery+TE (S5) are studied, from aspects of energy, economic and environmental (E³) performance. The new envelope systems can achieve thermal load reduction while providing additional cooling/heating supply, which can promote advance of NZEBs. It is found that there is a typical optimum setting of thermal energy load for each one of them with minimum annual power consumption. Except for the S1 system, the rest can realize negative accumulated power consumption in a year-round operation, which means the thermal load of building envelope could be zero. The uniform annual cost for S1 to S5 under interest rate of 0.04 are 19.78, 14.77, 23.83, 60.53, 64.94 ＄/m², respectively. The S5 system has the highest environmental effect with 3.04 t/m² reduction of CO₂ over 30 years of operation.