Long-term storage of chilled water in cisterns in hot,arid regions

Seasonal storage of chilled water in a large underground reservoir was studied by considering a two-dimensional heat flow in water and the soil surrounding the cistern, and by employing a finite-difference technique for the numerical solution of the energy equations. The cistern considered in the analysis was similar to those which have been employed in the hot, arid region of Iran for centuries for the storage of cold potable winter water for summer use. These cisterns were filled with cold water in winter and water removal did not start until mid-spring. During the water storage and usage, there was always a wind-induced airflow over the water surface to maintain an evaporation rate from the water surface. In these cisterns the water removal was from the bottom at such small flow rates that the thermal stratification in the cistern was not disturbed.It was found that while the temperature of water at the surface was high and followed the ambient air temperature closely, the temperature of the water removed from the bottom was very low, although it increased gradually toward the end of the storage season. During the summer months the temperature of water removed from the bottom of the cistern was below 10°C. This temperature was found to be a function of the size of the cistern, the weather conditions, the initial temperature of water in the cistern, the soil properties, and the year of operation of the cistern.The system analyzed here finds applications in the hot, arid regions of the developing countries for the storage of cold winter water for drinking, for food preservation, or for other agricultural and industrial applications. It can also be used for air conditioning of buildings.