Source: Ars Technica

Keeping the Heat

Sunlight can cause a molecule to change structure, and then release heat later.

The system works a bit like existing solar water heaters, but with chemical heat storage.

Credit: Kypros

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Heating accounts for nearly half of the global energy demand, and two‑thirds of that is met by burning fossil fuels such as natural gas, oil, and coal. Solar energy is a promising alternative, but while we have become reasonably good at storing solar electricity in lithium‑ion batteries, we’re not nearly as good at storing heat.

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. This chemistry challenge is known as molecular solar thermal (MOST) energy storage. Although it has been touted as the next big thing for decades, it never really took off—until now.

In a recent Science paper, a team of researchers from the University of California, Santa Barbara, and UCLA demonstrate a breakthrough that might finally make MOST energy storage effective.

The DNA Connection

In the past, most energy‑storage solutions have been plagued by lackluster performance. The molecules either didn’t store enough energy, degraded too quickly, or required toxic solvents that made them impractical. To find a way around these issues, the team led by Han P. Nguyen, a chemist at the University of California, Santa Barbara, drew inspiration from the genetic damage caused by sunburn. The idea was to store energy using a reaction similar to the one that allows UV light to damage DNA.

How UV Light Damages DNA

• When you stay out on the beach too long, high‑energy ultraviolet (UV) light can cause adjacent bases in DNA—specifically thymine (the “T” in the genetic code)—to link together.
• This creates a structure known as a (6‑4) lesion.
• If that lesion is exposed to even more UV light, it twists into an even stranger shape called a “Dewar” isomer.

In biology, this is rather bad news: Dewar isomers introduce kinks into the DNA double‑helix, disrupting replication and potentially leading to mutations or cancer.

Nature’s Solution: Photolyase

Evolution shaped a specialized enzyme called photolyase to hunt (6‑4) lesions down and snap them back into their safe, stable forms.

Turning a Problem into a Solution

The researchers realized that the Dewar isomer is essentially a molecular battery. Its “snap‑back” reaction releases a substantial amount of heat, providing the exact energy‑release mechanism Nguyen’s team was seeking for a new class of energy‑storage materials.

Rechargeable Fuel

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 is ≈ 40 MJ kg⁻¹. For comparison, Li‑ion batteries usually pack < 1 MJ kg⁻¹.

The problem with heating oil is that it is single‑use: it is burned the moment you need heat. Nguyen and her colleagues set out to create a reusable fuel inspired by DNA.

How It Works

1. Molecule design – The team synthesized a derivative of 2‑pyrimidone, a chemical cousin of the thymine base in DNA.
2. Photo‑induced isomerisation – Under sunlight the molecule folds into a Dewar isomer (high‑energy form).
3. Controlled release – The stored energy is released on command when the molecule re‑opens to its original structure, returning to a “relaxed” state ready for the next charging cycle.

Context: Molecular Solar‑Thermal (MOST) Systems

Nguyen’s Breakthrough

• Energy‑storage density: 1.65 MJ kg⁻¹
• Significance:
  • Nearly double the capacity of Li‑ion batteries.
  • Substantially higher than any previously reported MOST material.

Thus, the pyrimidone‑based system demonstrates a rechargeable molecular fuel that bridges the gap between high‑density, single‑use fuels and low‑density, reusable storage technologies.

Double Rings

The jump in performance was attributed to what the team called compounded strain.

When the pyrimidone molecule absorbs light, it doesn’t simply fold; it twists into a fused, bicyclic structure containing two different four‑membered rings: 1,2‑dihydroazete and diazetidine. Four‑membered rings are under immense structural tension, and fusing them together creates a molecule that is eager to snap back to its relaxed state.

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From Paper to Real‑World Application

Achieving high energy density on paper is one thing; making it work in practice is another. A major limitation of previous Molecular Solar Thermal (MOST) systems is that they are solids that must be dissolved in solvents such as toluene or acetonitrile. Solvents dramatically reduce energy density—diluting the fuel to 10 % concentration cuts the energy density by 90 %.

Nguyen’s team addressed this by designing a version of the molecule that is liquid at room temperature, eliminating the need for a solvent. This simplification allows the liquid fuel to be pumped through a solar collector for charging and then stored in a tank.

• Water‑compatible: Unlike many organic molecules, Nguyen’s system tolerates aqueous environments. If a pipe leaks, no toxic solvents (e.g., toluene) are released.
• High‑energy release: The researchers demonstrated that the molecule can operate in water and release enough energy to boil it.

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Proposed MOST‑Based Heating System

1. Charging: The liquid fuel circulates through roof‑mounted panels that capture sunlight, storing the energy in a basement tank.
2. Discharge: Fuel from the tank is pumped to a reaction chamber containing an acid catalyst, triggering the energy‑release reaction.
3. Heat Transfer: A heat exchanger transfers the released heat to the water in a conventional central‑heating system.

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The Catch

… (continue with the limitation or challenge that follows).

Looking for the Leak

The first hurdle is the spectrum of light that puts energy into Nguyen’s fuel.

• Solar spectrum: The Sun delivers a broad range of wavelengths—from infrared (IR) through visible to ultraviolet (UV).
• Molecule absorption: The pyrimidone molecules used in the system only absorb light in the UV‑A and UV‑B range (≈300–310 nm).
• Coverage: This narrow band represents ≈5 % of the total solar spectrum.
• Consequence: The vast majority of solar energy (visible + IR) passes straight through the molecules without being captured.

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Quantum Yield

Quantum yield is a measure of how efficiently absorbed photons drive the desired chemical change.

• Definition: “For every 100 photons that hit the molecule, how many actually switch to the Dewar isomer state?”
• Current performance: The pyrimidones exhibit a single‑digit quantum yield (well below 10 %).
• Impact: Low quantum yield means the fluid must be exposed to sunlight for a longer period to achieve a full charge.

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The “Leak” Hypothesis

Researchers suspect a fast, non‑radiative decay pathway:

• What it is: After photon absorption, the excited molecule may dissipate the energy as heat rather than converting to the storage (Dewar) form.
• Why it matters: This “leak” reduces the amount of usable stored energy.
• Goal: Identify and block this decay route to improve overall efficiency.

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Acid Catalyst Issue

In the experimental setup, an acid catalyst was mixed directly into the storage material.

• Problem: In a closed‑loop device, the residual acidity must be neutralized after heat release.
• Potential drawback: If the reaction products cannot be removed or purified, the energy density of the system will drop.

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Outlook

Despite the efficiency challenges outlined above, the stability of Nguyen’s system appears promising, making it a worthwhile target for further optimization.

The MOST storage?

One of the biggest fears with chemical storage is thermal reversion—the fuel spontaneously discharges because it gets a little too warm in the storage tank. However, 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 remain fully charged when you need to heat your home in January.

The degradation figures also look decent for a MOST energy‑storage system. The team ran the system through 20 charge‑discharge cycles with negligible decay.

The problem of separating the acid from the fuel could be solved in a practical system by switching to a different catalyst. The scientists suggest that, in a hypothetical setup, the fuel would flow through an acid‑functionalized solid surface to release heat, thus eliminating the need for neutralization afterwards.

Still, we’re rather far from using MOST systems for heating actual homes. To get there, we’ll need molecules that:

• absorb a larger portion of the light spectrum, and
• convert to the activated state with higher efficiency.

We’re simply not there yet.

Science, 2026. DOI: 10.1126/science.aec6413

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Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.

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