Ragone plots: Understanding the tradeoff between power and energy for thermal energy storage
A new Ragone framework for thermal energy storage provides guidance for researchers on how to optimize new thermal storage materials or devices for both energy and power density. This framework will accelerate the development of novel thermal storage technologies.
At the National Renewable Energy Laboratory (NREL), we are researching cutting-edge technologies to address climate change and to build a clean and sustainable energy economy. Achieving this requires new energy storage technologies, including thermal energy storage. Thermal storage can be used to shift building heating and cooling loads, extend the availability of electricity from solar-thermal power plants, enable pumped-heat electric grid storage (the so-called Carnot battery), or cool photovoltaic cells or electrochemical batteries.
There is considerable research worldwide on thermal energy storage, often looking at phase change materials with their large enthalpy of fusion. When a material changes from a solid to a liquid, it absorbs a lot of energy, which can be used for cooling. Likewise, liquid-to-solid phase change releases energy, which can be used for heating. Thousands of publications exist on new phase change materials for these applications, with many of them focused on increasing the power capability of these materials due to their low thermal conductivity. However, there is no clear guidance on what level of enhancement is needed or how power is related to the accessible storage capacity in thermal storage devices.
While participating in a multi-disciplinary energy storage consortium between building technology and electric vehicle researchers (www.nrel.gov/research/behind-the-meter-storage-consortium.html), we were able to better understand the tradeoff between power and energy in electrochemical batteries. For many decades, this power-energy tradeoff has been characterized by Ragone plots, based on foundational work by David Ragone in 1968 (https://doi.org/10.4271/680453). These Ragone plots are now used extensively to select the right battery chemistry and architecture for the application. For example, a designer can easily choose one battery type to power a clock (low power) and a different one for an electric vehicle (high power) with guidance from a Ragone plot.
Our team wanted to create these Ragone plots for thermal energy storage, in the hopes that it could elucidate the tradeoff between power and energy for materials and thermal-science researchers. It would also help guide material property selection—how high does the thermal conductivity need to be for these phase change materials?
As described in our recent article titled Rate capability and Ragone plots for phase change thermal energy storage (https://doi.org/10.1038/s41560-021-00778-w), we devised a method and framework for creating Ragone plots for thermal storage. We used a finite-difference numerical model to estimate how material properties, geometry, and operating conditions change Ragone plots. We also built a prototype phase change storage device, illustrating this power-energy tradeoff in practice.
Our paper is the first to create thermal Ragone plots, which set a clear objective for researchers: push the Ragone curve up (higher power) and to the right (higher energy). This framework allows for easy comparisons between different thermal storage materials or devices, whether it is used as an experimental characterization technique or to develop potential improvements through modeling and design. The framework also provides a fundamental basis for defining material property targets and can be a critical design and optimization tool for researchers and practitioners developing thermal storage materials and devices.
We leveraged the knowledge of fellow battery researchers in our behind-the-meter storage consortium, who understand the ins-and-outs of developing Ragone plots to ensure we used appropriate analogies between phase-change and electrochemical storage, and that we had an appropriate framework for these thermal Ragone plots.
Using this framework, we found some non-intuitive results. Below is a figure from the paper, showing the tradeoff between power and energy for different materials in a thermal storage device. You’ll notice that pure water/ice phase change outperforms the high-conductivity material (tetradecane/graphite composite; 10 W/m-K thermal conductivity) for low power as well as very high powers. But there is an intermediate power where the graphite/tetradecane is the best material on a ‘per volume’ perspective.
This research was funded by the U.S. Department of Energy Building Technologies Office, in collaboration with researchers from the Department of Energy Vehicle Technologies Office, to increase energy efficiency in buildings and strengthen a clean energy economy.