Theoretical and experimental studies of stratified thermocline storage of hot water

Abstract

A theoretical calculation of the degradation of heat in stratified thermocline storage has been carried out based on a conduction model. Since this neglects mixing, eddy currents and other degradation mechanisms, it provides an upper limit to the performance of a stratified thermocline storage tank. The calculation can be carried out for any selection of dimensions, temperatures, and choice of insulation. The results indicate that heat conduction through the insulation to the ambient can be a larger loss mechanism than conduction across the thermocline, except in large diameter tanks with very heavy insulation. With a properly designed tank (length/diameter > 10, diameter > 1.5 ft, insulation resistance > 20 hr ft 2 °F/B.t.u.) efficient storage of heat through a daily cycle should be routinely simple based on conduction. Experiments were carried out in static and dynamic modes. In the static experiments, a fixed thermocline was established, and temperatures were monitored at spatial intervals above and below the thermocline. Some mixing occurred during formation of the thermocline, which caused an initial broadening not present in the calculations. Aside from this, it was found that the spreading of the thermocline was only slightly faster than predicted by conduction theory. If a thinner wall tank had been available, agreement between experiment and theory probably would have been closer. Dynamic experiments were conducted with a moving thermocline (both upward and downward). The results indicate preservation of the initial thermocline was excellent at linear flow rates below about 0.2 ft/min. It is believed that stratified thermocline storage has been shown to be technically viable.