Convection effects on stratification during charging of a hot water store

Abstract

In domestic hot water stores the preservation of hot water layers is essential for the effective operation. Such stores are operated intermittently: when hot water is taken from the store, cold water is fed into it, so that part of the water in the store is hot and part of it is cold. These two parts should not mix, because this would deteriorate the efficiency of the store. When mixing occurs between the hot and cold parts, the hot water temperature is decreased. Thus, the store has, to be heated to a temperature above the needed temperature from the very beginning. With higher temperature, heat losses are also larger. More heat has to be fed into the mixed store than into the stratified when the same amount of heat shall be taken out with the same amount of water. Consequently, the energetic store efficiency as the ratio of heat taken from the store to heat fed into the store is less when the store temperature is higher. Thus, the store with no mixing is the best store.