On the Determination of the Average Temperature in a Horizontal Storage Tank and Its Use in Monitoring the Performance of a Solar Thermosyphon System

The mass weighted average storage temperature is a useful parameter for evaluating the performance of thermal storage systems. It is commonly used in system simulations since detailed temperature data in the storage medium are not known (or even considered). However, it is rarely used in the evaluation of operating systems because of the difficulty in determining it, especially in potentially multidimensional storage configurations, like a horizontal tank. This paper examines the issue of the evaluation of the average storage temperature, specifically from the point of view of reproducibility and how much data is required for a confident calculation. Unfortunately, this issue was not completely resolved and was shown to be strongly dependent on the locations of the measurements. Two examples are presented that demonstrate the usefulness of knowing the average storage temperature. The examples also give a quantitative demonstration of the effect of withdrawal flow rate on the amount of thermal energy that can be withdrawn from a given system. A reduction in flow rate by a factor of four increased the delivered energy by 70 percent.