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• A three-dimensional unsteady state mathematical model of the multi-field coupling of backfill body, tube and fracture was established. • A test bench was designed and built to verify the three-dimensional coupling model. • The effects of a single fracture position, aperture and fractured depth on the heat extraction performance of a full-size BFHEs unit in the fully dry or fully saturated conditions were investigated. • The combined effects of multiple fractures on the performance of a BFHES unit were discussed. Thermal hazards in deep mines can be mined synergistically with the deposit as associated geothermal energy resources. Backfill heat exchangers (BFHEs) as a combination of mine backfilling and ground heat exchanger technology are one of the effective methods to realize ore deposit-geothermal energy synergy mining. However, in the complex environment of deep mines, BFHEs will inevitably suffer from fracture damage during long-term heat storage/release cycle operation, which will affect their working performance. To address this problem, this paper established a scaled-down experimental setup for fractured BFHEs units, and used the experimental data to verify the accuracy of the established three-dimensional transient heat transfer mathematical model of the BFHEs unit. Numerical simulation methods were used to investigate the effects of the position, aperture and fractured depth of a single fracture on the heat extraction performance of the BFHEs unit under fully dry or fully saturated working conditions. Significant differences were found in the effect of different fracture positions on the heat extraction capacity of the BFHEs unit. The extraction of geothermal heat by the BFHEs unit was favored when it was in the middle of two tubes, while the opposite was true for other positions. The effect of fracture on the heat extraction capacity of the BFHEs unit increased linearly with fracture aperture. The effect of the fractured depth was relatively greater in the 1/2–3/4 H b range. The presence of groundwater significantly reduced the effect of the fracture. The effect of the fracture on the heat extraction capacity of the BFHEs unit was reduced by more than 80 % by changing the operating conditions from fully dry to fully saturated condition. The overall effect of a fracture on the heat extraction capacity of the BFHEs unit was not significant, with a 5 mm wide fracture having an effect of no more than 2.5 %, even under the fully dry condition. However, there was a combined effect of multiple fractures, and for a deposit size of 900 × 7 × 80 m, ten 2 mm apertures fractures would results in a maximum reduction of geothermal extraction from BFHEs by 8527 GJ in 10 years, which was equivalent to the heat load of a 13,753 m2 building for one winter in the cold climate of China. [ABSTRACT FROM AUTHOR]

• A new structural design method for earth-to-air heat exchangers in greenhouses is described. • The structural design method incorporates greenhouse heat balance model to better match greenhouse characteristics. • The system is more energy-efficient and stable in intermittent operation mode. • Application of an earth-air heat exchanger is a green technology for greenhouse sustainability. An earth-air heat exchanger (EAHE) is a type of environmental regulation equipment that is suitable for greenhouses. In the future, it could be a green technique for greenhouse energy supplementation. However, EAHEs lack structural design methods for application in greenhouses. Consequently, this paper presents a structural design method for an EAHE based on the architectural characteristics of greenhouses. Firstly, the EAHE design heat exchange amount was calculated according to the greenhouse heat and cold load, then the pipe length, diameter, air velocity, and burial depth were determined by combining the heat transfer efficiency and number of transfer units (NTU), and finally the final parameters were optimized and selected according to the energy loss and construction cost. The design method was validated in a greenhouse in northern China, and the heat transfer performance was analyzed in both intermittent and continuous operation modes. The results showed that the novel structural design method could guide the application of EAHE in a greenhouse and satisfy the energy demand of greenhouses. The measured average greenhouse temperature at night increased by 1.57 °C and 1.11 °C, respectively, under intermittent operation and continuous operation and decreased by 1.19 °C and 1.32 °C, respectively, during the day. The EAHE performance coefficients for heating and cooling conditions under intermittent operation were 21.49 and 18.15, respectively, significantly higher than those for continuous operation. [ABSTRACT FROM AUTHOR]