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 consum ption of greenhouses. These systems consist of buried pipes circulatin g air for storing and removing heat from the soil. First, a transient fully three-dimensional heat transfer model resting on the coupled con servation 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 us ed 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 exponential ly with the pipe center-to-center distance and the pipe length. It inc reases with the air velocity and this effect is enhanced as the pipe c enter-to-center distance diminishes. Nevertheless, as a compromise bet ween cost and performance, it appears that an air blowing velocity of 4 m s(-1) is nearly optimal. As the moisture content of the soil incre ases, Q(v) augments but its effect becomes negligible for large pipe l engths and small blowing velocities. Adding side insulation improves t he performance of the SHESS but the beneficial effect of insulation un derneath the bottom pipe row is significant. Finally, burying pipes de eper underground allows more energy to be stored during the day but le ss is recovered at night through the ground surface and the overall pe rformance declines. (C) 1997 Elsevier Science Ltd.