Energy and Exergy Analysis of a Seasonal Underground Cold Water Storage

Thermal behavior of long-term underground cold water reservoir was studied experimentally. The storage tank, which was considered in this study, is a cylindrical underground 450m3 water reservoir with a domed shape roof and equipped with wind towers (Baad-Gir). These historic reservoirs were used as a source of drinking cold water in hot arid central regions of Iran during hot and dry summer season. The cylindrical underground reservoir under study was filled with water in winter and remained intact till beginning of discharge cycle. The duration of the discharge period was from late April until mid-October when the demand for cold and fresh water was high. Temperature data were taken every ten days during the discharge cycle both in vertical and horizontal directions. Meanwhile, cold water was extracted from bottom of the tank on a daily basis at a rate corresponding to the regional inhabitants' water consumption. It was observed that stable thermal stratification was developed after charging the reservoir. The temperature of extracted water was in the range of 11.5-13.1 oC during the entire summer period whilst the outside ambient temperature was reached up to 42 oC. Energy and exergy analysis showed that about 80% of the cold energy stored in winter could be extracted on summer time at the desired temperature. Comparing the exergy content of the stratified reservoir with the fully mixed one, it was seen that the stable thermal stratification is responsible for preserving the quality of extracted energy. The measured temperature during the discharge cycle showed that the water content of the storage tank could be divided into two distinct regions, the bottom region with a linear thermal stratification and the upper one with an exponential thermal stratification. It is believed that the radiation heat exchange between the water surface and the storage ceiling, as well as the convective heat and mass transfer from the surface of water induced by airflow were primarily responsible for temperature profile change. However, the discharged water flow rate had a secondary effect on thermal stratification.