Searching for a Better Thermal Battery

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Science  23 Mar 2012:
Vol. 335, Issue 6075, pp. 1454-1455
DOI: 10.1126/science.1218761

Energy storage has mainly focused on electrochemical systems (1). However, more than 90% of the world's primary energy generation is consumed or wasted thermally. Thermal energy storage has a broad and critical role to play in making energy use more sustainable for heating and cooling, solar energy harvesting, and other applications. Thermal storage technologies are still based on solutions developed decades ago, such as molten salt, ice, and paraffin phase-change systems, whose performance and cost do not merit widerscale adoption. Progress in materials science, chemistry, and engineering may lead to dramatic breakthroughs in thermal energy storage that could improve the efficiency with which we produce, distribute, and consume energy.

Energy density of various thermal batteries.

Theoretical volumetric and gravimetric energy densities for leading thermal storage materials are plotted, illustrating the distinct advantages of thermochemical and thermophysical approaches. The energy densities have been calculated for a wide range of temperatures. As a point of reference, the active materials in a state-of-the-art lithium ion battery have volumetric and gravimetric energy density of roughly 5000 MJ/m3 and 1.3 MJ/kg, respectively; no existing thermal energy storage material has comparable performance. New materials and system designs that achieve performance metrics in the gray region at the upper right could enable wider adoption of thermal batteries.

Thermal storage materials can be broadly grouped into two classes, thermophysical and thermochemical. Thermophysical approaches rely on changes in a system's physical state and use sensible heat (by increasing the temperature of the storage material), latent heat (absorbed at a constant temperature, as in a phase change), or both. For example, storing solar energy by heating molten salts is currently used to extend output and reduce production cost of solar thermal power plants (2). In thermophysical systems, thermal insulation is needed to minimize heat losses to the environment. In thermochemical approaches, chemical reactions reversibly store energy without a need for insulation. Despite the advantages of enabling high energy density and insulation-free long-term storage, thermochemical technologies have yet to be widely used.

One key performance metric for any energy storage technology is energy density. In the figure, we plot the theoretical volumetric and gravimetric energy densities for approximately 50 thermal storage materials. The data illustrate the limitations of each of the two classes of thermal storage. Thermophysical storage relies primarily on liquids and solids and offers high volumetric energy density but low gravimetric energy density. Thermochemical systems generally contain at least one gas-phase component, affording light weight but requiring large volume in the absence of mechanical compression. If mechanical compression is used, it can reduce the system-level gravimetric energy density and also the round-trip efficiency of storage. An example of a reaction that is often suggested for thermochemical storage is CaCO3 ↔ CaO + CO2.

The recently developed ARPA-E High Energy Advanced Thermal Storage (HEATS) program (3) illustrates that new thermal storage materials that achieve best-in-class gravimetric and volumetric performance simultaneously could enable several new energy applications. In buildings, modular on-demand heating and cooling could reduce or eliminate the use of inefficient centralized air conditioning. Advanced high-temperature solar thermal plants could integrate thermal storage directly into concentrating dishes. This approach offers a low-cost solution to solar intermittency while increasing the capacity factor of the power block and thus reducing the levelized (“break-even”) cost of electricity.

Electric vehicles, which today can draw as much as 35 to 40% of their electrical battery capacity for cabin heating and cooling, could dramatically increase their driving range by using a separate thermal battery, charged from the grid or waste-heat sources, for cabin conditioning (4). Even conventional combustion-engine vehicles could use thermal storage to avoid cold-engine starts, which can temporarily reduce fuel efficiency by 10% or more, depending on the ambient temperature (5). When connected to the electricity grid, thermal storage would offer the added benefit of load shifting for applications such as air conditioning or refrigeration (6), or it could be used directly to store electricity from the grid (7).

Irrespective of the storage mechanism, a breakthrough will depend on finding a reversible phenomenon with a high enthalpy change based on components with low molecular weight and high volumetric density. Recent advances in flexible design and synthesis of new materials offer an exciting set of possibilities. For example, the versatility of metalorganic frameworks and ionic liquids is now being exploited to modulate binding energies and adsorptivity of adsorbents (8, 9). Condensed-phase chemical reactions can store thermal energy in covalent bonds or through the entropy of mixing of condensed phases. One possible approach could be the exploration of entirely condensed phase reactions of relatively small organic molecules (10). High-density isomerization reactions may show renewed potential for thermal storage; a recent calculation on the photoisomerization of azobenzene suggests that stored volumetric energy density could be enhanced by nearly four orders of magnitude (approaching the energy density of lithium ion batteries) by anchoring the molecules on carbon nanotube templates (11). Lastly, binary mixtures of fluids have shown anomalous enhancements in heat capacity near the critical point (12), which, in combination with recent advances in high-strength materials, may enable the use of supercritical fluids as a storage medium.

Practical thermal storage solutions not only need high energy density but must operate in the appropriate temperature range and provide sufficient power, cycle life, and efficiency for a given application. Optimization of these factors requires that new materials be coupled with advanced engineering designs and system-level innovations, such as novel direct-contact heat exchangers or thermally conductive nanoscale binders for efficient thermal power delivery. Despite these challenges, a key metric for commercial viability—cost—is one area where thermal systems may have an inherent advantage over electrochemical or mechanical energy storage, given the potential for solutions based on simple active chemistries and purely thermal systems with no moving parts.

References and Notes

  1. Acknowledgments: We thank A. Gidwani for help with the figure.
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