A CO2 geothermal thermosiphon to preheat supply air for ventilation heat recovery systems in cold climates.

• The effects of three parameters were evaluated in terms of the system performance. • Factors affecting performance: filling ratio, cooling fluid flow rate and temperature. • 90 % of heat transfer resistance is between evaporator wall and ground. • Improving evaporator-ground heat transfer resistance most effective for performance. • Combining geothermal thermosiphons and heat/energy recovery ventilators. Geothermal thermosiphons (GTs) and Heat/Energy Recovery Ventilators (HRV/ERV) are two technologies, commonly used in buildings in cold climates to fulfill separate duties. GTs are primarily used to maintain the integrity of buildings/foundations built over permafrost, while HRV/ERV are used to meet indoor air quality requirements and reducing energy consumption within buildings. Operating separately, HRV/ERV is subject to frosting, which consumes energy, while the heat extracted by the GT is rejected in the environment. Such practice, plagued with a double penalty of inefficiency and energy waste, needs to be upgraded. In this paper, a new application is proposed where the heat extracted by GTs is no longer rejected to the atmosphere but is rather recovered to preheat the incoming fresh air of HRV/ERV units. An experimental GT test bench filled with CO 2 and fully instrumented was constructed to investigate the system's thermal performance. The effects of filling ratio, cooling fluid flow rate and temperature on GT operation were examined and discussed. The results indicated that the heat transfer resistance between the evaporator outer wall and the ground accounted for 90 % versus 10 % for the internal thermosiphon resistance, a fact which highlights the elevated overall thermal conductivity of this device. As a result, reducing the heat transfer resistance between the evaporator and the ground is most effective for improved heat transfer performance of the GT system. Furthermore, in the range of the selected tests conditions, heat transfer rate values per unit length of the GT ranged between 24.5 W/m and 61.6 W/m. [ABSTRACT FROM AUTHOR]