It’s easy to tell if the steering wheel of your car, the surface of a parking lot, or a bicycle seat has been in the sun for a long time. Known (sensibly enough) “sensible heat materials,” substances like stone, cast iron, and aluminum get noticeably hotter as they absorb heat. With a quick touch, our senses tell us so. On the other hand, says Adam Paxson, a PhD candidate in MIT’s Mechanical Engineering department, there are phase change materials (PCMs), latent heat storage materials that absorb and release heat without rising in temperature themselves.
Every phase change material, he says, has its unique saturation temperature, the point at which it begins changing phase from liquid to vapor or solid to liquid. “When a PCM is changing from one phase to another, adding heat won’t change its temperature until all of the first phase is gone,” he explains. Placing a pan of ice cubes over a gas burner illustrates his point nicely. “The ice itself will stay at the same temperature until all of it is melted,” he says. “Only after all the water has changed phase from solid to liquid will it feel warmer to the touch.”
Thousands of materials are classified as PCMs, says Paxson, from organic substances like paraffins and fatty acids to inorganic salt hydrates and hydrocarbons. They’re especially of interest to engineers specializing in heat transfer and thermal energy storage, since they begin releasing their stored, latent heat only when the temperature is lowered below their freezing point. “Various waxes can be tailored to start melting and freezing at around room temperature, and are an energy-efficient form of insulation used in wall panels,” he says. On a hot August day, the wax inside the walls absorbs the sun’s heat, but its temperature remains constant at the wax’s melting temperature, keeping the home’s interior comfortably cool.
Latent heat technology can keep plants happy, too. PCMs such as calcium chloride and sodium sulfate decahydrate have been successfully used inside greenhouses to store solar energy. During the day, PCM-filled units inside the greenhouse collect warm air, and at night the direction of air flow is reversed, resulting in energy efficiency, cost savings, and tomatoes and tuberoses that thrive in the constant, optimal temperatures.
And though they may not realize it, brown baggers and picnickers rely on PCMs every time they slip a freezer gel pack into their lunchbox. Mixed with the water in the packs are propylene glycol and a dash of sodium carboxymethyl cellulose that lower its freezing point. The PCMs enable the pack to absorb the ambient heat of the school locker or the trunk of the car, yet never themselves become hot enough to melt, ensuring fresh sandwiches and chilled soft drinks when the lunch bell rings. —Sarah Jensen
Thanks to Jihan M. of Miami for this question.