The Dover Sun House relied on several vats of Glauber salts, or sodium sulfate, placed throughout the living quarters. (Our illustrator depicted them as a fanciful "heat bin" in the attic.) The salts melt at 90 degrees Fahrenheit and store energy more efficiently than water. The system, devised by chemist Mária Telkes and architect Eleanor Raymond, channeled the sun's warmth to the sodium-sulfate tanks, which meted out heat on cold or overcast days. It worked better in principle than in practice: Tenants were chilly in winter, and the tanks tended to leak. They moved out after a few years. Today, the Tesla Powerwall stores solar energy more effectively with lithium-ion batteries.