Numerical simulation of soil heat exchanger-storage systems for greenhouses

A numerical study was conducted for the thermal behavior of soil heat exchanger-storage systems (SHESSs) aimed at reducing the energy consumption of greenhouses. These systems consists of buried pipes circulating air for storing and removing heat from the soil. First, a transient fully three-dimensional heat transfer model resting on the coupled conservation equations of energy for the soil and the circulating air is presented. The model is validated with experimental data taken from a SHESS installed in a commercial type greenhouse. Next, the model is used to examine the effect of various design and operating parameters on the performance of SHESSs. Results, indicate that the total amount of energy stored or recovered daily per volume Q_v decreases exponentially with the pipe center-to-center distance and the pipe length. It increases with the air velocity and this effect is enhanced as the pipe center-to-center distance diminishes. Nevertheless, as a compromise between cost and performance, it appears that an air blowing of 4 m s^±1 is nearly optimal. As the moisture content of the soil increases, Q_v augments but its effect becomes negligible for large pipe lengths and small blowing velocities. Adding side insulation improves the performance of the SHESS but the beneficial effect of insulation underneath the bottom pipe row is significant. Finally, burying pipes deeper underground allows more energy to be stored during the day but less is recovered at night through the ground surface and the overall performance declines.