# Comparison of Heat Transfer Enhancement Techniques in Latent Heat Storage
[36] concerns the ratio of energy required to pump the HTF through the LHES unit to the stored energy. In addition, in a similar approach, [PERSON] et al. [37] also proposed the performance analysis of a wide range of LHES operational and material parameters, as both energetic and exergetic efficiencies. Even though the previously described KPIs provide very valuable information about an LHES unit, they address particular aspects and consider mostly capacity and efficiency perspectives, but provide no indication on the rates of heat transfer and the required material to achieve it. Directly addressing the thermal response, [PERSON] et al. [38] proposed various KPI for both short and long term scales: Average power, 5 min-peak-power, 5 min peak power-energy ratio (based on 5-min-peak-power over total capacity of the unit), and finally the discharge time. The set of KPIs was intended for the evaluation of four LHES units of very similar scales, operating at uniform conditions and equal PCM. Although useful while the experimental conditions and the geometry remain similar enough, they remain intrinsically connected to the current operation conditions and scale. This leads to non-representative results, especially when comparing across different studies and applications. Similarly, [PERSON] et al. [39] consider the specific charging rate (\(\gamma[1/\mathrm{h}]\)) and specific energy loss rate. The specific charging rate directly addresses the heat transfer performance of the unit, but it is ultimately an average power to capacity ratio, without any normalization with respect to the driving forces. Similar analyses include, for instance, the use of the average temperature effectiveness (\(\epsilon_{avg}\)) by [PERSON] et al. [40] and [PERSON] et al. [41] to represent the efficiency of the heat exchange. Additionally, [PERSON] et al. [40] present a NHTPC (\(h_{v}\)), directly addressing the heat transfer performance of the units. It considers the average heat transfer rate divided by the average temperature difference (which ultimately can be interpreted as the enthalpy flow of the HTF), with respect to the volume of PCM only in the unit. Although it directly addresses the heat transfer capability of a unit, it still remains dependent on the HTF conditions, and thus varies with different mass flow and temperature difference. The Effectiveness-NTU Method (\(\varepsilon\)-NTU) allows the calculation of the heat transfer rate and temperature profiles in a heat exchanger using the enthalpy flows, and defining a heat exchanger effectiveness based on the actual heat transfer rate over the maximum achievable by the system [42]. It has been previously used to analyze LHES systems as performed by [PERSON] et al. [43; 44] as design and sizing tool on specific designs. The methodology presented in this paper is inspired on the previous (\(\varepsilon\)-NTU) analyses and the view of LHES units as a heat exchanger core acting as boiler or condenser. It focuses on two KPIs, which represent the normalized heat transfer performance and degree of compactness of the LHES design. This new approach allows the comparison of LHES systems reported in literature in terms of their heat transfer capabilities and compactness regardless of their geometry, scale, and operation conditions. The developed KPIs are applied to several technologies reported in literature and the results of this comparison are presented and discussed. Based on the authors' knowledge, this is the first time such an extensive quantitative comparison across different LHES technologies with a focus on high power applications has been performed.