To store heat for days, weeks, or months, you need to trap the energy in the bonds of a molecule that can later release heat on demand. The approach to this particular chemistry problem is called molecular solar thermal (MOST) energy storage. While it has been the next big thing for decades, it never really took off. //

Molecular batteries, in principle, are extremely good at storing energy. Heating oil, arguably the most popular molecular battery we use for heating, is essentially ancient solar energy stored in chemical bonds. Its energy density stands at around 40 Megajoules per kilo. To put that in perspective, Li-ion batteries usually pack less than one MJ/kg. One of the problems with heating oil, though, is that it is single-use only—it gets burnt when you use it. What Nguyen and her colleagues aimed to achieve with their DNA-inspired substance is essentially a reusable fuel. //

The researchers achieved an energy storage density of 1.65 MJ/kg—nearly double the capacity of Li-ion batteries and substantially higher than any previous MOST material. //

One of the biggest fears with chemical storage is thermal reversion—the fuel spontaneously discharges because it got a little too warm in the storage tank. But the Dewar isomers of the pyrimidones are incredibly stable. The researchers calculated a half-life of up to 481 days at room temperature for some derivatives. This means the fuel could be charged in the heat of July, and it would remain fully charged when you need to heat your home in January. The degradation figures also look decent for a MOST energy storage. The team ran the system through 20 charge-discharge cycles with negligible decay. //

Still, we’re rather far away using MOST systems for heating actual homes. To get there, we’re going to need molecules that absorb far more of the light spectrum and convert to the activated state with a higher efficiency. We’re just not there yet.