Seasonal thermal energy storage (STES) allows storing heat for long-term and thus promotes the shifting of waste heat resources from summer to winter to decarbonize the district heating (DH) systems. Despite being a promising solution for sustainable energy system, large-scale STES for urban regions is lacking due to the relatively high initial investment and extensive land use. To close the gap, this study assesses the potentials of using two naturally available structures for STES, namely valley and ground pit sites. Based on geographical information system (GIS) methods, the available locations are searched from digital elevation model and selected considering several criteria from land uses and construction difficulties. The costs of dams to impound the reservoir and the yielded storage capacities are then quantified to guide the choice of suitable sites. The assessment is conducted for the northern China where DH systems and significant seasonal differences of energy demand exist. In total, 2,273 valley sites and 75 ground pit sites are finally identified with the energy storage capacity of 15.2 billion GJ, which is much larger than the existing DH demand in northern China. The results also prove that 682 valley sites can be achieved with a dam cost lower than 20 CNY/m³. By conducting sensitivity analysis on the design dam wall height and elevations, the choices of available natural structures are expanded but practical issues about water pressures and constructions are also found. Furthermore, the identified sites are geographically mapped with nearest urban regions to reveal their roles in the DH systems. In general, 560 urban regions are found with potential STES units and most of them have STES storage capacities larger than their own DH demand. The novel planning methodology of this study and publicly available datasets create possibilities for the implementations of large-scale STES in urban DH systems.

Recovering the waste heat (WH) of a power plant can conserve energy and reduce emissions. Scholars have proposed utilizing the WH of power plants in a combined heat and water (CHW) system, which is considered an economical, energy-saving, and environment-friendly way to integrate water and heat supply into long-distance transportation in urban areas of northern China. However, to date, a detailed design of the case on the heat source side of the CHW has not been developed. Therefore, in this study, the heat source side of a CHW system was divided into two cases: a single-generator set and a double-generator set, and both cases were optimized. The parameters of a multi-effect desalination (MED) process were examined; the optimal number of evaporation stages during the MED process was 12, and the optimal heat source temperature during the first stage was 70 ℃. Then, by matching the extraction and exhaust steam flows, the WH of the exhaust steam in the heating season was finally utilized. Further, under each case optimal conditions, energy, exergy, and cost were analyzed. The results showed that the exergy efficiency in the heating season for each case was approximately 50%, whereas that in the non-heating season was approximately 3.5%. The economy and water quality of the single-generator case were better than those of the double-generator case. However, the absorption heat pump required in the single-generator case is difficult to realize because it operates under two working conditions.

Low-grade industrial waste heat could be a considerable potential energy source for district heating, on the condition that the heat from different industrial waste heat sources is integrated properly. This study considers a method for integrating low-grade industrial waste heat into a district heating system and focuses on how to improve the outlet temperature of heat-collecting water by optimizing the heat exchange flow for process integration. The pinch analysis concept is considered, and a newly developed thermal theory called entransy analysis is introduced. By using entransy dissipation to describe the energy quality loss during the heat integration, this study analyzes how heat exchange flows influence the final outlet water temperature and attempts to provide an efficient method to optimize the heat exchange flow. Finally, the effectiveness of the proposed methodology is demonstrated by testing it in a project involving the recovery of waste heat from a copper plant for district heating.

A ground heat exchanger system is applied as a way to improve the heat recovery ratio in a project using industrial waste heat recovery for district heating. In order to meet the requirements of industrial processes, the outlet water temperature of the system should be strictly controlled within a range, but the heat transfer power varies with the warming or cooling of the soil. An operation strategy named "block by block" is implemented and a corresponding model is established. In this strategy, the total area of the system is divided into several blocks. The water flows by a specific combination of blocks at any given time, ensuring the outlet water temperature is relatively stable. By simulation, it is concluded that the strategy is fit for the practice of industrial waste heat storage and extraction.