Using nano-enhanced phase change materials is a widespread passive method to improve the melting performance, and also the storage capacity of the thermal energy storage units. In this study, the effects of CuO nanoparticles ð0 4 1:5%Þ and new proposed stair ns on the efciency improvement of latent heat thermal energy storage units are investigated. The stair ns are arranged in both upward and downward directions from the heated walls and the stair ratio is in the range of 0:67 b=c 4:0. One of the vertical walls of the PCM enclosure is subject to uniform temperature and the other three walls are insulted. The numerical results show that by adding nanoparticles with volume concentration of 4 ¼ 1:5% for b/c ¼ 0.67 to the ow, the energy storage capacity is enhanced by 9.1% compared to the pure PCM with downward ns. The maximum energy storage capacity of 474.1 kJ is achieved by using descending stair ns with b/c ¼ 4.0 and 4 ¼ 1:5% which is much higher compared to the cases without nano additives. Besides, the melting performance is signicantly improved by adding the nanoparticles. In fact, nanoparticles improve the thermal conductivity of the uid and also act as a heat sink to absorb the heat from the ns. The downward ns with larger stair ratios (b/c ¼ 4.0) perform signicantly better than the upwards ones which is because of the free convection effects and the recirculations ows on the upper face of these ns.

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