Thermal-energy stores for supplying domestic hot-water and space-heating

Factors influencing the ability of a conventional domestic hot-water tank to deliver domestic hot water rapidly have been culled, collated or assessed experimentally. It has been deduced that, with a horizontal-axis coiled, finned-type, heat exchanger--immersed in the traditional domestic hot-water tank--for extracting heat from the tank where and Ra are the mean Nusselt and Rayleigh numbers, respectively, for the freely-convecting flow in the tank. Among the design conclusions are: 1. (i) The optimal value of the ratio of height-to-diameter for the thermal store lies between 3 and 4. This is a compromise between achieving the improvements associated with (a) having a high degree of stratification (e.g. resulting from the use of a tall tank) and so facilitating rapid heat removal, and (b) minimising the surface area (and hence the rate of wild heat loss from the store). 2. (ii) Preferably, the walls of the thermal store should be made of a low-thermal-conductivity material and, provided mechanical integrity can be assured, only be of small thickness, thereby enhancing the degree of stratification achieved in the store. 3. (iii) An approximate value for the optimal thickness of thermal insulant applied to the tank, in order to minimise the wild heat losses through the tank's walls, can be calculated. Thus, for a typical insulant, it is recommended that a thickness of more than three times that traditionally used on domestic hot-water tanks be applied. 4. (iv) The use of a horizontal plate, located near the middle of the store, can lead to small increases in the rate of useful heat recovery from the store.