This study aimed to using geothermal energy in heating and cooling of agricultural structures. The greenhouse is taken here as a case study. Geothermal energy system was analysed and restructured as three sub models: soil temperature model, ETAHE model and greenhouse model. Soil temperature model was developed using previous researches and adjusted to suit Egyptian conditions. It was validated against two sets of data. The results show good agreement with measurements in both cases. It was found that the root mean squares of deviations of the first set at 1.5 and 2 m depths were 1.93, 1.85 °C respectively and for the second set at 2, 3 and 4 m depths were 2.65, 1.65 and 0.39 °C respectively. This soil temperature model was used as a component of ETAHE model. Similarly an earth to air heat exchanger (ETAHE) model was developed. Its results were validated against the results of three other studies. The current model gave good agreement with these studies. It can be suitably used to predict the thermal performance of ETAHE system. Using ETAHE model, a parametric analysis was carried out to investigate the effect of pipe diameter, pipe length, pipe material and air velocity inside the pipe on the earth tube inlet air temperature, the efficiency of ETAHE system and pressure drop under Egyptian conditions during both cooling and heating seasons. The results illustrate that the optimal values of pipe length used as inputs to design an ETAHE should be greater than 30 m and don’t exceed 90 to 150 m. The optimum diameter was found to be in the range of 0.10 m to 0.30 cm. The diminution of air velocity causes an increase of thermal efficiency and a diminution of pressure losses. Plastic or metallic materials of pipe lead to very similar energy performances. The developed greenhouse model is used to predict the heating and cooling loads for a typical gable even span greenhouse of 256 m2 floor area was considered, to be 42.91 kWh and 170.39 kWh respectively. Experimentation with the model has shown that, to minimize the installation cost of ETAHE system for heating and cooling greenhouse under consideration, it is better to use smaller pipe diameters (from 0.10 to 0.30 m), because pipe diameter larger than this range leads to a little improvement in performance of ETAHE system and increases the installation cost. It also preferable to use a smaller air velocity which can be ranged from 5 to 15 m/s, because the air velocity less than 5 m/s required longer pipes and this leads to increasing the costs and the air velocity larger than 15 m/s required high fan power and reduces the efficiency of the system. The results indicated that, using of an ETAHE system for heating greenhouse was more efficient and low cost compared to using it for cooling in all cases of pipe diameters and air velocities. In case of using it for cooling, the remaining cooling requirements could be obtained by other cooling systems, e.g. evaporative cooling systems (fan-pad, fog/mist and roof evaporative cooling systems). |
