On the influence of the storage wall thickness on the parasitic near-wall flow and exergy destruction in stratified thermal energy storages

Near-wall parasitic convective flow is a known phenomenon in stratified thermal energy storages (STES) leading to stratification degradation and exergy destruction. While many features of convective flow in STES have been studied previously, the influence of the storage wall thickness on the near-wall flow, heat transfer and exergy destruction has not yet been addressed. Temperature, heat flux density and optical velocity measurements were carried out in a model experiment for STES at different wall thicknesses. It is found that at a thicker wall, the stratification degradation and the exergy destruction is faster than at a lower wall thickness, caused by an enhanced heat transfer between the storage wall and storage fluid. As time proceeds, the temperature distribution in the storage tank and exergy efficiencies converge for both wall thicknesses. In the flow field, this is reflected in higher maximum velocities at the thicker wall. Furthermore, velocity gradients are larger in this case because of thinner boundary layers. Transient flow fluctuations appear to be stronger at the thicker wall as shown by higher amplitudes and broader spectra of the velocity oscillations. Additionally, the occurrence of maxima in the spectra of the fluctuations at the thicker wall which do not appear at the thinner wall may hint at differences in the transition to turbulence caused by the different wall thicknesses. • Higher parasitic heat transfer in stratified thermal storages with thicker walls. • Thermal stratification and exergy degrade faster at thicker storage walls. • Parasitic near-wall flow is faster at thicker storage walls. • Stronger velocity fluctuations in near-wall flow at thicker walls. • Storage wall thickness influences transition of near-wall flow to turbulence.