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Rate capability and Ragone plots for phase change thermal energy storage


Phase change materials can improve the efficiency of energy systems by time shifting or reducing peak thermal loads. The value of a phase change material is defined by its energy and power density—the total available storage capacity and the speed at which it can be accessed. These are influenced by material properties but cannot be defined with these properties alone. Here we show the close link between energy and power density by developing thermal rate capability and Ragone plots, a framework widely used to describe the trade-off between energy and power in electrochemical storage systems (that is, batteries). Our results elucidate how material properties, geometry and operating conditions influence the performance of phase change thermal storage. This research sets a clear framework for comparing thermal storage materials and devices and can be used by researchers and designers to increase clean energy use with storage.

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Fig. 1: Schematics of electrochemical and thermal energy storage devices, showing analogous inputs and outputs.
Fig. 2: Rate capability and Ragone plots for electrochemical and thermal energy storage.
Fig. 3: Thermal resistances and their relationship to temperature profiles in the PCM.
Fig. 4: Effect of thermal conductivity on device performance for PCM thickness of 3.5 cm.
Fig. 5: Effect of PCM thickness on Ragone curves for two thermal conductivities.
Fig. 6: Effect of material properties on gravimetric and volumetric Ragone plots for building cooling energy storage.

Data availability

All data generated or analysed during this study are included in the published article, its Supplementary Information and Source Data files

Code availability

The numerical model developed for this work was generated in MATLAB R2019b. Unrestricted access to the source code is available at


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This work was authored by the National Renewable Energy Laboratory (NREL), operated by Alliance for Sustainable Energy, LLC, for the US DOE under contract no. DE-AC36-08GO28308. Funding provided by US DOE Building Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the US Government. We thank S. Mumme from the DOE Building Technologies Office for his support and insight on this paper. We also thank J. Vidal and M. Bianchi from NREL for their feedback on our work.

Author information




J.W. developed the concept of thermal rate capability and Ragone plots and oversaw the project, A.M. developed the numerical model, performed experiments on the thermal storage device, analysed the data and created the figures, A.G. and E.K. helped design the thermal storage device experiments and contributed to developing the concept, A.O. measured material properties and contributed to developing the concept, R.J. helped develop the idea and provided guidance during the project. J.W. and A.M. wrote the paper.

Corresponding author

Correspondence to Jason Woods.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Energy thanks Patrick Shamberger and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–8, Notes 1–6, Tables 1 and 2 and references.

Supplementary Video 1

The PCM phase and the PCM and fluid temperatures when the baseline device is discharged at 1 C.

Supplementary Video 2

The PCM phase and the surface heat flux at y = 0 when the baseline device is discharged at 1 C.

Supplementary Table 1

The raw experimental data used to generate Supplementary Figs. 3 and 4.

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Woods, J., Mahvi, A., Goyal, A. et al. Rate capability and Ragone plots for phase change thermal energy storage. Nat Energy 6, 295–302 (2021).

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