Performance comparison of two water pit thermal energy storage (PTES) systems using energy, exergy, and stratification indicators

Water pit thermal energy storage systems have been demonstrated in Denmark and have proven effective in increasing the solar thermal fractions of district heating systems and in covering the mismatch between heat demand and production. This study analyzed five years of measurement data for two PTES systems in Denmark, namely Marstal and Dronninglund. Their efficiency was assessed using energy, exergy, and a seasonal efficiency indicator. The degree of stratification was investigated using the MIX number, the stratification coefficient, and a newly-introduced indicator, exergy destruction. Exergy destruction was shown to be a promising indicator for assessing stratification since it can be used to compare PTES systems with different heat losses, providing a quantitative evaluation of the amount of mixing. In addition, the seasonal efficiency was found to be suitable for estimating the long-term efficiency of combined seasonal and short-term storage systems. The storage in Dronninglund had 92% energy and 73% exergy efficiency, while Marstal had 63% energy and 48% exergy efficiency. All stratification and efficiency indicators showed that the storage in Dronninglund performed better overall than the one in Marstal.