To develop a high-efficiency energy system, a novel seasonal thermal energy storage (STES) system based on a cluster of coaxial borehole heat exchangers is proposed. The system actively harvests and stores solar radiation, air thermal energy, and other climatic resources, achieving spatiotemporal energy transfer characterized by “storing summer heat for winter use and winter cold for summer use.” To address the high computational cost and low efficiency associated with traditional three-dimensional numerical models, a simplified two-dimensional numerical model in a cylindrical coordinate system is developed. The model reduces computational complexity by transforming the densely arranged coaxial borehole cluster into an equivalent concentric-ring structure. The accuracy of the three-dimensional model is validated through sandbox experiments. Based on this, a comparative analysis between the two-dimensional and three-dimensional models is conducted in terms of heat storage capacity, temperature distribution, and computational efficiency. The results demonstrate that the proposed two-dimensional model maintains high accuracy, with a maximum relative deviation of less than 7%, while improving computational efficiency by approximately 172.9%. The method is suitable for rapid simulation and optimization design of large-scale, densely arranged borehole heat exchanger systems.

With high energy consumption in buildings, the emissions of greenhouse gases are also increasing. It leads to some environmental problems. To realize resource conservation and environmental protection target, latent heat thermal energy storage systems (LHTES) are introduced into all kinds of buildings. A variety of air-LHTES and water-LHTES are analyzed in this study based on the heat transfer fluid medium adopted. The results of this study indicate that the air-LHTES uses the low-temperature ambient air to store cold during nighttime and releases cold during the daytime in summer vice versa in winter with auxiliary heat sources. The water-LHTES stores the cold and heat generated by various natural sources (solar energy, nighttime sky radiation, air conditioning condensate) through the water, and then releases the cold and heat to the buildings to reduce the energy consumption of the buildings. However, for some regions with extremely hot climate, the ambient temperature is still high during nighttime in summer. It is difficult to achieve cold storage of ambient air. Accordingly, other natural cold sources should be adopted for cooling in air-LHTES. Due to the cooling effect of nighttime sky radiation, water temperature in water-LHTES could be lower enough for cold storage. Thus, a combination system of water-LHTES and air-LHTES is recommended. In this system, cold storage is achieved by collecting low-temperature, and released by supplying cooling air. The proposed system can also achieve heat storage in winter by collecting solar energy, and release heat by supplying heating air.

Latent heat storage units are widely used in building heating systems due to its high energy storage density, whereas the practical performances of them are limited by the low thermal conductivities of phase change materials. In this paper, copper nanoparticles were added into paraffin to enhance the heat transfer rate of a latent heat storage unit using a coil heat exchanger. A three-dimensional numerical model was built to simulate the melting process of phase change material, and it was well validated against the experimental data. The simulation results showed that the nanoparticle-enhanced phase change material saved 19.6% of the total melting time consumed by the pure phase change material. In addition, the dispersion of nanoparticles significantly alleviated the temperature non-uniformity in the unit. Moreover, for the unit using nanoparticle-enhanced phase change material, the flow rate of heat transfer fluid was not recommended higher than 0.75 m³/h. The dispersion of nanoparticles could enlarge the optimum heat transfer fluid temperature range to 60-70 °C compared with that of pure phase change material (60-65 °C). Therefore, the application of nanoparticle-enhanced phase change material in the latent heat storage unit can significantly enhance heat transfer, and the proposed optimum inlet heat transfer fluid temperature range could contribute to higher energy efficiency.