A team of chemists at UC Santa Barbara has developed a liquid compound that absorbs ultraviolet sunlight, locks that energy into its molecular bonds for months, and then releases it as heat on command. The molecule, a pyrimidone derivative that rearranges into a stable form called Dewar pyrimidone, stores more thermal energy per kilogram than lithium-ion batteries and needs no electricity, no insulation, and no solvent to work. If the technology scales beyond the lab bench, it could rewrite the rules for how buildings capture summer sun and spend it as winter warmth.
How Dewar Pyrimidone Traps Sunlight
The core chemistry relies on a process called molecular solar thermal energy storage, or MOST. When exposed to ultraviolet light around 300 nanometers, the pyrimidone molecule snaps into a higher-energy structural form, the Dewar isomer, which is metastable enough to hold that energy without bleeding it away as heat. In a peer-reviewed study, researchers from UC Santa Barbara, Brandeis University, and UCLA reported in Science that the liquid can achieve a gravimetric energy-storage density above 1.6 megajoules per kilogram. That figure is significant because it approaches twice the roughly 0.9 MJ/kg typically associated with lithium-ion batteries, according to a UC Santa Barbara news release that described the material as functioning like a “rechargeable solar battery” in liquid form.
What makes this compound stand out among MOST candidates is that it operates as a neat liquid without added solvent and still mixes readily with water, two traits highlighted in the researchers’ PubMed-listed abstract. Earlier MOST work, including studies of norbornadiene-based photoswitches, showed that careful molecular and oligomer design could stretch storage lifetimes and raise energy density; one Nature Communications paper demonstrated how tailoring conjugated systems improves performance but often at the cost of requiring organic solvents. Those solvents dilute energy density and complicate pumping, containment, and safety. By eliminating that requirement, the Dewar pyrimidone system avoids a major bottleneck that had kept many MOST fluids closer to laboratory curiosities than to practical energy-storage media.
On-Demand Heat From a Drop of Acid
Storing sunlight is only half the story; the other half is releasing it exactly when needed. In the Dewar pyrimidone system, a small amount of acid acts as a catalyst that triggers the reverse molecular rearrangement, converting the strained Dewar form back to its lower-energy structure and releasing the stored energy as heat. In laboratory demonstrations, adding the acid to a sample that had been charged under ultraviolet light produced enough heat in a short burst to boil roughly half a milliliter of water. That may sound modest, but it shows that the energy release is both rapid and concentrated, in contrast to materials that simply warm up slowly as they cool down to ambient conditions.
This controllable trigger differentiates MOST fluids from phase-change materials, which passively absorb and release heat when they melt or solidify at a set temperature. It also contrasts with conventional solar hot water systems, which, as the U.S. Department of Energy explains in its overview of active solar heating, rely on collectors, pumps, heat-transfer fluids, and insulated tanks. Those tanks inevitably lose heat over days or weeks, making them well-suited to daily cycles but poorly matched to seasonal storage. A chemical fluid that holds energy for months and discharges only when exposed to a catalyst could, in principle, replace some of that bulky infrastructure with compact storage vessels that sit idle until the moment heat is required.
Why Seasonal Storage Changes the Calculus
The mismatch between when the sun shines and when people need heat is a fundamental challenge for solar technology. As solar-fuels researcher Takashi Hisatomi has noted in a discussion of sunlight-driven hydrogen production, solar conversion systems simply cannot operate at night or during extended bad weather, which underscores the need for ways to capture energy when it is available and use it later. That temporal gap is even more pronounced on a seasonal scale: rooftops in many temperate regions soak up the most solar energy in late spring and summer, while heating demand peaks in late autumn and winter. Existing approaches to seasonal heat storage, such as large underground thermal reservoirs or tanks filled with phase-change materials, can smooth out some of that imbalance but still suffer from gradual heat loss over months.
A MOST fluid that locks energy into covalent bonds rather than relying on hot water or molten salts could, at least in theory, hold onto that energy with minimal losses until a catalyst is introduced. Industries that require process heat for drying, sterilization, or chemical reactions are already turning to solar thermal collectors to cut fossil-fuel use, but they must currently size their systems around daily or weekly storage horizons. A liquid solar fuel that remains charged for months could enable factories, district heating networks, or campus-scale systems to bank surplus summer energy and deploy it in midwinter, potentially reducing the need for backup boilers and easing stress on electric grids during cold snaps.
What Still Stands Between Lab and Rooftop
Translating a promising molecule into a practical energy product will require solving challenges that extend far beyond photochemistry. One hurdle is scalability: researchers must show that Dewar pyrimidone and its precursors can be synthesized in large quantities from affordable feedstocks without creating problematic byproducts. Another is durability, since a real-world MOST fluid would need to endure many charge–discharge cycles without decomposing or forming side products that foul catalysts or reduce capacity. The Science paper reports stable performance over laboratory timescales, but multi-year testing under outdoor conditions (with fluctuating temperatures and impurities) remains an open question. Engineers will also need to design collectors that can efficiently expose the liquid to ultraviolet-rich sunlight while preventing overheating and degradation.
System-level integration poses its own set of questions. Any rooftop or ground-based installation must safely store thousands of liters of an energy-dense fluid, manage the controlled addition and removal of acid catalysts, and route the resulting heat into existing hydronic or air-based heating systems. Safety standards, building codes, and insurance requirements will all influence how quickly such technology can move from demonstration to deployment. Public and private funding programs can accelerate that path: the U.S. Department of Energy has already highlighted solar-thermal fuels and long-duration thermal storage as priorities through its FY23 concentrated solar-thermal initiative, and broader clean-energy efforts such as the DOE’s Genesis portal and the Office of Scientific and Technical Information repository provide pathways for researchers and companies to connect, share data, and seek support. Whether Dewar pyrimidone itself or a next-generation derivative ultimately reaches rooftops, the concept it embodies, a storable, dispatchable solar heat fuel, has already begun to expand how scientists and engineers think about capturing the sun’s energy for the coldest days of the year.
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*This article was researched with the help of AI, with human editors creating the final content.
