# Comparison of Heat Transfer Enhancement Techniques in Latent Heat Storage
## 3 Results and Discussion
### Automotive Heat Exchangers and Polymer Based Capillary Scale Tube Bundles  
This entire subsection is dedicated to particularly interesting systems as they are in principle extreme variations of finned and simple tube bundles. On one hand, Automotive HEX (AHEX), conceived from the idea of finned tubes for gas-to-liquid heat exchange, are mass produced on a wide variety of configurations usually completely made out of braced aluminum and seek to maximize the heat transfer surface. On the other hand, without additional material, very large heat transfer surfaces can be achieved by driving the diameter of a tube bundle to the capillary scale (below 5 mm). Additionally, the consequent thin walls produced at this scale minimize the influence of the material conductivity, opening the door to the utilization of polymers for a wide range of applications when temperature levels allow. These systems will be finally referred to as capillary tube bundles (CTB). Table 2 gathers the notation and representative schemes of the studies treated in this section. All additional data required for the calculation of NHTPC are presented in Table A1. The outer volumes of the units V\({}_{TOT}\), including the container tank surrounding the HEX without considering any insulation, were for the most part explicitly reported in the different studies along with container material and properties. In the cases where information was missing in this regard, it could be approximated from additional reported geometrical parameters and conservative assumptions. The summarized KPIs are portrayed in Figure 3. Experiments performed in the framework of the same study with different geometries are indicated with the same letter but different numbering. [PERSON] et al. [48] experimented with an AHEX unit embedded in PCM (B.4), and compared it to other common LHES approaches and it achieved an impressive NHTPC, several times higher than the next best performance within the same study, the graphite matrix enhanced tube-and-shell (B.3) unit previously discussed. This effect might be explained mainly by the sheer mass of heat exchanger material in the unit rather than the heat exchanger design as it presents a relatively low performance in both heat transfer and compactness when compared to other AHEX-based units. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & \(\mathbf{T_{PC}[^{\circ}C]}\) & \(\mathbf{V_{TOT}}\) & \(\hat{\mathbf{m}_{HTF}}\) & \(\mathbf{c_{p,HTF}}\) & **Geometry** & **Ref.** \\  & & \(\left[\mathrm{m^{3}}\right]\) & \(\left[\mathrm{kg\,h^{-1}}\right]\) & \(\left[\mathrm{k}\right]/\left(\mathrm{kg\cdot K}\right)\) & \\ \hline AHEX (B.4) & 35 & \(4.4\times 10^{-3}\) & \(4.00\times 10^{2}\) & 4.18 & [48] \\ \hline AHEX (S) & 4 & \(4.1\times 10^{-3}\) & \(5.34\times 10^{2}\) & 1.01 & [50] \\ \hline AHEX(T) & 69.3 & \(3.3\times 10^{-3}\) & \(7.13\times 10^{2}\) & 3.95 & [51] \\ \hline AHEX(D.2) & 93.8 & \(1.9\times 10^{-3}\) & \(2.21\times 10^{3}\) & 4.06 & [14] \\ \hline CTB (U) & 30 & 1.0 & \(3.60\times 10^{3}\) & 4.18 & [52] \\ \hline CTB (V) & 29 & \(8.5\times 10^{-1}\) & \(1.38\times 10^{3}\) & 4.18 & [53] \\ \hline CTB (W) & 28.5 & 1.5 & \(3.15\times 10^{3}\) & 4.18 & [54] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: References pertaining to AHEX and CTB based units. A similar situation was discussed in the study performed by [PERSON] et al. [14] their initial experiments were performed using a stock automotive HEX immersed in PCM (D.2). Based on their results, an FTB design with a higher capacity was produced and is discussed in Section 3.2 (D.1). Interestingly, their goal was achieved as their FTB custom design (33 [kW/m\({}^{3}\cdot\)K]) effectively matched the thermal performance of the AHEX-based unit (36 [kW/m\({}^{3}\cdot\)K]). Their design variations, however, required an additional sacrifice of 10% PCM volume fraction. A second iteration of an analog system for a diesel based system, explored by [PERSON] et al.