Composite energy geo-structures for building decarbonization

Energy geo-structures (EGs) integrate heat-exchange loops into geotechnical structures to provide heating and cooling using shallow geothermal energy. However, the contribution of standalone EGs to building thermal demand may decrease in large-scale applications and can be accompanied by seasonal ground thermal imbalance. To overcome the limitations of standalone EGs, a composite EG system integrating energy tunnels (ET), energy diaphragm walls (EDW), energy piles (EP), and boreholes (BHE) is proposed. A three-dimensional numerical model of a composite energy geo-structure system was established and evaluated against experimental or published benchmark data for the component subsystems. The model was used to investigate the short- and long-term thermal performance of the composite system. The results show that, despite localized reductions in the heat flux in length of EP and BHE, the composite EGs achieved a higher total heat exchange capacity—1.90, 2.70, 11.24, and 24.87 times greater than those of standalone ET, EDW, EP groups, and BHE arrays, respectively. Over the ten-year simulation, ET and EDW showed limited interannual variation, while the EP-BHE subsystem approached a repeatable seasonal response after the first complete annual cycle. Additionally, under the prescribed building-load conditions, circulating flow rate and soil thermal conductivity mainly altered the local ground-temperature field, whereas annual heat exchange was primarily determined by the imposed building demand and the COP-dependent ground-side load. Compared with coal-fired heating and electric air-conditioning under the same building load, the composite system reduced annual operational emissions by 49.80% (962.03 t CO 2 -eq), demonstrating its potential for efficient and low-carbon building thermal supply.