Performance Analysis of a Closed-Loop Climate Control System Using Underground Air Tunnel

A passive summer cooling technique that utilizes the underground soil temperature has application in climate control of residential as well as agricultural buildings. The soil temperature stays fairly constant at a depth of eight feet or more (references [1], [2]). Earlier studies [3, 4] have shown the usefulness of this technique for an open-loop system. However, the previous analyses in the literature did not evaluate the usefulness and limitations of this method for closed-loop air conditioning. In this study an analysis of the “Coefficient of Performance” (COP) of a closed-loop system, based on the above technique, in combination with a conventional air conditioner, has been done. In this system, the cooling needed to neutralize the heat gain of the conditioned space is provided by the air cooled in an underground air pipe in combination with an air conditioner. The underground air tunnel is used for hot parts of days and is off for cooler parts of days and nights. The analysis has been done by a computer model solution, using central finite difference method. When the system is on, the air temperature and the soil temperature are calculated. When the system is off, the heat is transferred within the soil and a new set of soil temperatures around the pipe are calculated for the next day. As the soil temperatures around the pipe increase, the COP of the system decreases. The COP is calculated for each hour until it decreases to the COP of an air conditioner. This shows us the length of time for which the underground cooling method will be useful. Since the knowledge of soil properties is very important, a computer model solution has been developed to predict the soil thermal properties by using an approximate analytic method based on simple temperature measurements.