# Comparison of Heat Transfer Enhancement Techniques in Latent Heat Storage
## 5 Conclusions  
A pair of performance indicators to evaluate the heat transfer performance and compactness for latent heat energy storage (LHES) units were presented. These key performance indicators (KPIs) were calculated for several LHES units reported in literature allowing a leveled performance comparison with regard to operating conditions at different scales, while remaining intrinsic to the geometry, heat exchanger structure materials, and PCM. The robustness of the proposed KPIs was confirmed with varying HTF mass flow rate and temperature levels, for units at different size scales, with a coefficient of variance below 6% in every case, and were compared to other reported KPIs for LHES.  
Finned tube bundle (FTB) and tube bundle (TB) units showed the widest range of performance, and a great potential mainly dependent on the quality of the HEX design. Composites in general require further experimental work but show very promising potential.  
Automotive heat exchanger (AHEX)-based units showed promise especially on their heat transfer performance, and are interesting for further study as they are already mass produced in a very wide range of variations. Capillary tube bundles (CTBs) show great potential especially in terms of compactness, but due to the added practical challenges require some optimization work for ideal operation. For both AHEX and CTBs, material limitations regarding compatibility and operation range are some of the main concerns, and should be thoroughly considered in further studies.  
The macro encapsulated (ME)-based systems considered showed in general low performance and compactness, but a very large potential for improvement and flexibility, especially in terms of capsule shape and size optimization to customize their performance, for instance.  
It is necessary to highlight that the publications taken into account had clear application-oriented objectives. This implies that achieving the highest possible heat transfer rate was not the focus during their conception, but only the required performance for a specific application under given conditions. It is possible to infer that optimized versions of the mentioned technologies would deliver considerably higher performance indicators. Therefore, the conclusions drawn from this analysis cannot be considered as final in any way regarding the technological approaches but more so as a look at the general potential of each approach.  
Additionally, a key aspect of the thermal response is relatively overlooked by the analysis, as the methodology proposed only accounts for the stability of the outlet temperature indirectly, within the approximation of the average outlet temperature. Further dimensionless analysis is required to account for this effect.  
Conceptualization, S.M. and R.W.; Formal analysis, W.D.-D.; Funding acquisition, A.S.; Investigation, W.D.-D.; Methodology, W.D.-D.; Project administration, W.D.-D. and A.S.; Resources, A.S.; Supervision, A.S. and J.W.; Writing-original draft, W.D.-D.; Writing-review and editing, A.S., S.M., R.W. and J.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Swiss Competence Center for Energy Research Heat and Electricity Storage (SCCER HaE).