Are finally we on the cusp of economically viable thermal batteries — devices to store and transmit heat? A perspective article published in this week's Science seems to think so, and it could mean a major change for the way we think about heating and cooling.
Anyone who's lived in a climate that suffers from both extremes of heat and cool (Midwest, I'm looking at you) has pondered the concept of the thermal battery — after all, why can't we find a way of bottling the summer heat, and releasing it in winter? That's what a thermal battery does, primarily through two methods: thermochemical, and thermophysical. The latter literally stores heat in an object, such as molten salt, to use later, but requires substantial insulation to prevent heat loss. Thermochemical reactions are reversible chemical reaction that can store energy, but tend to have low energy density.
The authors believe that advances in materials science could dramatically alter the way we heat (and cool) our lives. They point out better thermal storage could help solar power break even in poor conditions, could replace air conditioning in buildings, and a separate thermal battery could cut 35%-40% of the electrical draw of an electric car.
So what technologies are going to take us to this thermal powered future?:
Recent advances in flexible design and synthesis of new materials offer an exciting set of possibilities. For example, the versatility of metal-organic frameworks and ionic liquids is now being exploited to modulate binding energies and adsorptivity of adsorbents. 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. 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 nano-tube templates. Lastly, binary mixtures of fluids have shown anomalous enhancements in heat capacity near the critical point which, in combination with recent advances in high-strength materials, may enable the use of supercritical fluids as a storage medium.
We're still a ways off from getting anything near as efficient as modern electrical batteries, but thermal batteries will also have the advantage of needing no moving parts, making them potentially less expensive.
As global warming becomes more of a problem, and urban sprawl makes waste heat a bigger factor, the concept of using higher temperatures to warm us in winter and take strain off the grid sounds like a remarkably good one.
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