# Numerical Design and Laboratory Testing of Encapsulated PCM Panels for PCM-Air Heat Exchangers
## 1 Introduction  
Ventilation systems with PCM-Air heat exchangers can offer cooling with a low energy consumption as only a fan is used. They use the principle of thermal storage through latent heat to cool a space in climatic regions where the ambient air temperature is not low enough to cool the space during the day but cool enough to charge the PCM during the night. The concept has been studied extensively [1, 2, 3, 4, 5] and its performance in practice evaluated [6, 7, 8]. Phase Change Materials in PCM-Air heat exchangers are commonly encapsulated in panels and stacked into a channel; this is usually termed as a thermal battery (TB). However, available panels are for general purposes and not specifically designed for ventilation heat exchangers. As a consequence, they have low heat transfer efficiency. Most research to-date on PCM-Air heat exchangers focusses on the PCM itself with only few on the improvement of the heat transfer on the air-side. Typically, PCMs have low thermal conductivity; thus, improvements in the heat transfer between air and panel will increase the overall performance of the system and lead to a fast thermal response, fast charging period and a reduction in energy consumption. The most common techniques used to increase the heat transfer over a channel are to modify the channel surface to include obstructions in the form of ridges, fins, dimples and grooves. Figure 1 shows diagrammatically the obstructions discussed based on literature findings. [PERSON] [9] analysed sixteen ridge shapes with different geometries by varying pitch ratio through CFD simulations. The performance was analyzed using the Nusselt Number and friction loss and a strong relationship was shown with the cross-sectional rib shape. A boot shaped ridge presented the best results in terms of heat transfer, with a pressure drop similar to a square ridge for Reynolds number between 5000 and 50,000. [PERSON] et al. [10] introduced symmetric and staggered squared high ridges in channels on the bottom and the top of rectangular channel walls. The results, validated by experiments, showed that a larger blockage ratio corresponds to a higher heat transfer and friction factor. The symmetric arrangement and higher blockage increased \(Nu/Nu_{o}\) in the range of 6-7 and the staggered arrangement and higher blockage by 3.5. Conversely, a higher friction factor increases pressure drop thus increasing energy consumption as it demands more power from the fan. To reduce pressure drop, [PERSON] [11] suggested to break down the ribs and position them in various orientations. The surface temperature was recorded by a charged coupled device (CCD) video camera in order to obtain a detailed distribution of the heat transfer coefficient over the heat transfer surface. The study analyzed transverse integral and broken ribs and a V-shaped broken rib (at \(45^{\circ}\) and \(60^{\circ}\)) with three different pitch-to-height ratios. Results showed the highest \(Nu/Nu_{o}\) for transverse broken ribs with pitch-to-height of four variations in the range of \((3.2\)-\(2.4)\), followed by a transverse pitch-to-height of 8 and a V shaped rib for a \(Re=8900\)-\(36,000\). [PERSON] et al. [12] studied the effect of angled rib turbulators in a double-side wall cooling channel at \(Re=10,000\). Results obtained through simulations found an optimized value of Nusselt ratio of \(2.9\) at \(53.41^{\circ}\) for a simulated range from \(30^{\circ}\) to \(80^{\circ}\). Alternatively, protrusions and grooves can be combined. [PERSON] et al. [13] compared the heat transfer in a channel with a single-wall composed by one upstream rib and one downstream groove, with one upstream protrusion in three different positions in a channel and one downstream groove and three upstream protrusions in parallel followed by one groove.