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.

Influences of baffles on the rate of heat recovery via a finned-tube heat-exchanger immersed in a hot-water store

The heat transfer at the external wall of the heat exchanger occurs primarily by buoyancy-driven natural convection in the surrounding water. An analysis of the effects of the presence of a rectangular duct on this heat-transfer process is presented. Small improvements in the rate of heat recovery were obtained repeatedly when a horizontal plate was located in the middle of the store.

Heat-transfer correlations for an immersed finned heat-exchanger coil transferring heat from a hot-water store

Natural-convection heat transfers, to a finned-tube heat-exchanger coil immersed in a hot-water store, have been investigated. Cold water was passed through the pipe of the heat-exchanger in order to extract heat rapidly from the hot water in the store. Natural convection currents in the stored water were created by buoyancy forces, which were induced by the temperature gradients that developed as a result of the heat-extraction process. A heat-transfer correlation in terms of Nusselt and Rayleigh numbers has been deduced in order to predict the natural convection heat-transfer coefficient on the outside surface of the heat exchanger. This correlation, which is valid for heat entering the fins, to within an accuracy of better than 4%, is: Nu=0·280 Ra0·293 for 100