Earth-to-air heat exchanger for air conditioning in a Colombian tropical weather
In a recent theoretical study, a passive cooling system using an earth-to-air heat exchanger (EAHE) was modelled for the Colombian tropical weather.
The earth-to-air heat exchanger (EAHE) is a passive cooling technology which takes advantage of the fact that at a certain depth, the ground temperature is almost constant throughout the year, allowing it to be used as a heat sink. EAHE consists of a simple system of buried pipes through which the ambient air circulates. Part of the thermal air energy is transferred to the ground, then, the outlet air from the EAHE can be directly used for space cooling or as pre-cooled air in an HVAC system. The performance of the EAHE depends on many parameters such as the design of the heat exchanger, as well as local atmospheric conditions (e.g. solar radiation, ambient air temperature and relative humidity).
Several studies in various locations have evaluated the performance of EAHEs and demonstrated that they can provide excellent thermal comfort and indoor air quality, with low energy consumption.
In a recent study, the performance of an EAHE in terms of ambient air-cooling capacity was simulated for the Colombian tropical weather. The model of an EAHE system was designed to improve thermal comfort in a 169m2 laboratory with an occupancy capacity of 26 people, using the weather conditions and soil characteristics of the city of Bucaramanga, Colombia.
In the model, the EAHE was buried at a depth of 1.5 m, where the temperature would remain almost constant at 22.3 °C. Regarding technical aspects, the authors found that an EAHE with a length of 252 m (7 pipes in parallel) and a pipe diameter of 8 inches (0.2 m) gave the best results. Furthermore, an air mass flow rate of 1560 kg/h allowed sufficient air exchanges per hour without reducing the EAHE performance while still meeting the calculated thermal loads. With these dimensions and operating conditions, the EAHE delivered air at temperatures between 20.9°C and 24.1°C, approximately 3°C below the ambient temperature and within the established thermal comfort conditions.
The weekly maximum cooling COP was around 160 and the daily COP variations ranged from 0.91 to 160. The authors found that variations in COP depended on soil temperature. During the day, from 9 am to 8 pm, the ambient air temperature was higher than the soil temperature with the air releasing heat into the soil, allowing the EAHE to provide cooling. During the night however, from 8 pm to 9 am, the soil temperature was higher than the ambient air temperature and heat flowed from the soil to the air. The authors suggested installing low power consumption fans (0.12 kW for the motor) in each pipe to vary the operation mode.
For the calculated specifications, the project had an internal rate of return (IRR) of 23% and a payback period of six years, taking into account interest rates in Columbia. Therefore, the authors concluded that the project was financially viable. EAHE could also be implemented in tropical areas with similar atmospheric and soil conditions.
- More studies on earth-to-air heat exchanger for cooling are available for download in FRIDOC:
Numerical evaluation of the thermal performance of a near-surface earth-to-air heat exchanger with short-grass ground cover: A parametric study.
Peñaloza Peña, S.A.; Jaramillo Ibarra, J.E. Potential Applicability of Earth to Air Heat Exchanger for Cooling in a Colombian Tropical Weather. Buildings 2021, 11, 219. https://doi.org/10.3390/buildings11060219