Achieving high power and energy density thermal storage in phase change materials

In this study, we propose an approach that achieves spatial control of the melt-front location of pure phase change materials using pressure-enhanced close contact melting, enhancing thermal management and storage to support a rapidly-electrifying energy infrastructure.

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The United States Energy Information Administration (EIA) AE2020 Reference Case projects that, by 2050, 79% of all energy generated will be derived from either wind or solar photovoltaics (PV)1. That is great news for the planet, but while renewable energy technologies are extremely effective in generating clean energy, they are intermittent. Wind and solar resources are often not always available at the exact moment we need electricity. This creates a classic challenge to renewable energy—addressing the mismatch between the supply and demand curves, often termed as the “duck curve” problem. High power and energy density energy storage can play a critical role in ensuring that renewable energy can address this challenge and maintain its energy production penetration projections. Understanding that the long-term viability of renewable energy is inextricably linked to advancements in energy storage, we became passionate about investigating existing energy storage technologies, understanding their shortcomings, and investigating how to address these shortcomings. One such technology, phase change material thermal energy storage, in particular, stood out to us.

Phase Change Materials (PCMs) have been proposed as a means of thermal energy storage for well over a century. In these systems, thermal energy can be stored or released via the latent heat absorbed or released in the PCM. PCMs offer a wide variety of opportunities for thermal energy storage, whether in transportation, energy generation, or thermal management. While PCMs represent a significant opportunity for cost-effective energy storage, a challenge to their implementation is their relatively poor transport properties (i.e., low thermal conductivity) for typical PCMs such as paraffin wax and ice2. A second challenge to the application of PCMs is their rapid melt front propagation away from the energy source, which increases the source-to-interface thermal resistance over time. This increasing conductive thermal resistance places a limit on the maximum power density that phase-change thermal storage can achieve3. Our study introduces a novel method to address both of these challenges.

When approaching this challenge, the natural question that presents itself is, “How can this thermal resistance be reduced while also ensuring minimal energy loss and added cost?” We could consider means by which thinner layers of phase change material could be applied over a larger surface area. However, this sort of solution has a much more limited set of end-uses due to the sheer area required. Furthermore, this solution would present a practical limit on the energy density achievable by the thermal energy storage system. Instead, we looked to turn the challenge posed by melt front propagation into an advantage by considering the increased mobility of the molten PCM when compared to solid PCM. We began considering ways in which this melted liquid PCM could be drained from the system while simultaneously pushing the solid PCM closer to the heat source. A mechanism that accomplished both of these goals is dynamic close-contact melting via our pressurized PCM (dynPCM) concept.

In our system, an external force is exerted on the solidified PCM to compress it against a heat source, melting the solidified PCM to a predetermined melt thickness and draining the melted PCM out of the system. Liquid PCM is then pumped back into the void area, optimizing the solidification front thickness as well, and allowing for cyclical operation. This approach not only allows us to achieve higher power and energy density thermal energy storage when compared with conventional methods, it enables a robust temperature control mechanism, which makes our technology suitable for thermal management applications. Our preliminary modeling results showed that the concept was promising and able to achieve high heat fluxes that could be held constant over time (see Fig. 1).

Figure 1: Comparison of conventional and dynamic phase change materials (PCM) a, Schematic showing the transient response for conventional PCM cooling. A solid bulk PCM (blue) constrained by container side walls absorbs heat from the heat source (yellow). Liquid PCM (red) accumulates on top of the heat source and the melt thickness (δ) increases over time and move away from the heat source. b, Schematic depicting the transient response during dynPCM cooling. A pressure applied on the solid PCM column (black arrows on the top) helps to squeeze out melted PCM (red horizontal arrows). A thin and constant-thickness liquid layer (red) forms between the solid PCM (blue) and the heat source (yellow). c, Conduction thermal resistance (Rth) of different PCMs as a function of melt-front distance from the heat source (δ ~ √t). The yellow shaded region represents dynPCM performance with a paraffin wax, resulting in a time-independent Rth < 10 (cm2·K)/W. d, Experimental comparison of heated surface temperature (Ts) for conventional and dynPCM cooling at a heat flux of q” = 1.15 W/cm2. e, Simulation heat flux results for conventional PCM cooling (bulk PCM, blue) compared with dynPCM cooling (red). The simulation boundary condition was the constant base temperature of the heat source at 60 ⁰C

To experimentally validate the promising findings observed in our modeling study, we built an experimental setup using paraffin wax, a cost effective and widely available PCM. We imposed an external force on the PCM and provided a constant heat flux to the system to measure the steady-state temperature required to maintain that applied heat flux. The results confirmed our analytical findings, showing that, even at higher applied heat fluxes (> 3W/cm2), we were able to maintain a stable base temperature in contrast to what is achievable using conventional PCMs. We also tested thermal energy storage cycling using our dynPCM, which showed that the system was consistently able to lower a heat source temperature of 110°C to an equilibrium temperature of 61°C once the PCM was applied to the heat source.

Thermal energy storage traditionally exhibits a trade-off between the allowable power and energy densities. Here, the discharge rate (power density) causes the equilibrium temperature of the system to reach the maximum allowable (or cutoff) temperature for the heat source before the entire PCM is melted. Conversely, to achieve high energy density (wherein the PCM is near-completely or completely melted), the power density is restricted to much lower discharge rates. The dynPCM approach, however, circumvents this trade-off and allows the entire PCM column to be melted prior to reaching the cutoff temperature. Power densities near 1 W/cm3 and energy densities of 300 J/cm3 were achieved using our dynPCM, which was an order of magnitude larger than conventional methods. Our approach is also promising in its ability to handle high heat fluxes approaching ~1 kW/cm2 when using appropriate PCM materials.

DynPCMs show significant potential for thermal management and thermal energy storage applications, using cost-effective and mass manufactured commercial PCMs. Compared with state-of-the-art PCM devices, dynPCMs provide stable heat source temperatures without requiring the addition of highly thermally conductive, but also energy density reducing, additives. Our project presents fundamental advances in thermal energy storage technologies to enable the growth of renewable energy generation. In heating, ventilation, and air conditioning applications, dynamic thermal storage will allow for balancing of air conditioning demand with energy supply, reducing the need for fossil fuel based peaker plants and shifting peak demand. Electrification also demands advances in thermal management techniques, as electric vehicles and many applied semiconductor technologies require robust thermal management. The dynPCM technology achieves stable temperature control under extreme heat fluxes at the expense of a relatively available and inexpensive resource, mechanical force or pressure. We are excited to continue this work and are beginning to implement this technology in a variety of end uses in thermal management and energy storage, maintaining the lifetime of electrified technologies, reducing fossil fuel usage, and facilitating the transition toward a clean energy future.

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[1] Energy Information Administration, "Renewable Energy Generation, Including End Use".

[2] Mohamed, S.A. et al, Renewable and Sustainable Energy Rev., vol. 70, pp. 1072-1089, 2017.

[3] Woods, J. et al. Rate capability and Ragone plots for phase change thermal energy storage. Nat. Energy 1–8 (2021).

Vivek Garimella

Graduate Research Assistant, University of Illinois at Urbana-Champaign