Refrigeration is one of those things we completely take for granted, until we don’t have it. It keeps food fresh, medicines safe, and entire supply chains moving. But traditional fridges are far from perfect. They use electricity, often from fossil fuels, and rely on refrigerants that contribute massively to global warming (looking at you, HFCs). And in off-grid or rural communities, where electricity is either expensive or unavailable, keeping things cool is still a major challenge.
In a recent paper I co-authored, we explored a low-tech but high-potential solution: Solar Thermal Adsorption Refrigeration (STAR). Instead of using electricity, STAR systems use heat from the sun to drive the cooling cycle. No compressor, no grid power, just sunlight, clever materials, and some physics.
At the heart of it is the adsorption cycle, which relies on materials that can trap refrigerant vapors on their surface. Here’s how it works: during the night, a refrigerant like ethanol evaporates inside the system, absorbing heat and creating a cooling effect. The vapor gets adsorbed onto a solid material, like activated carbon. Then, during the day, solar heat is used to drive that vapor off the adsorbent, a process called desorption. The vapor condenses, turns back into liquid, and cycles back into the evaporator to repeat the cooling process.
We looked at all the recent progress in STAR tech, from better working pairs (like ethanol and activated carbon) to materials like metal-organic frameworks (MOFs) that improve adsorption capacity. Performance-wise, the key metric is the Coefficient of Performance (COP), the ratio of useful cooling to heat input. That number’s been climbing thanks to smarter designs, like multi-bed systems and improved heat transfer techniques. Researchers have also started applying machine learning and simulation tools to fine-tune these systems even before building them.
Our team went hands-on and developed two small STAR prototypes, one horizontal, one vertical, and tested them in the lab. Then we ran a life cycle assessment (LCA) to see how they stack up environmentally compared to conventional fridges. The results were eye-opening. Since STAR systems don’t use electricity during operation, their “use phase” emissions are basically zero. Most of their impact comes from the materials, aluminum, copper, glass, etc., used in construction.
Between the two prototypes, the vertical design performed better but also had a larger material footprint (mostly due to its thick plywood frame). Still, both prototypes showed drastically lower life cycle environmental impacts than traditional fridges, even the modern energy-efficient ones. The bottom line? STAR has huge potential, especially in regions where the grid is unreliable or non-existent.
The Numbers Behind the Cooling
While the system itself feels refreshingly low-tech, we still needed a way to quantify performance, and that’s where the math comes in. First, we calculate the ideal cooling capacity, the maximum possible cooling effect based on how much refrigerant is evaporated and its latent heat:
$$ Y_i = \mu \times \lambda $$
Here, $( \mu )$ is the mass of refrigerant evaporated, and $( \lambda )$ is the latent heat of vaporization.
Then we measured the actual cooling achieved in our lab setup, say, how much heat was removed from a bucket of water, using:
$$ Y_e = \mu_w \times \phi_w \times \Delta T $$
Where $( \mu_w )$ is the mass of the water, $( \phi_w )$ is its specific heat, and $( \Delta T )$ is the observed temperature change.
Any difference between the ideal and experimental values is counted as losses:
$$ Y_i = Y_e + \xi $$
This gives us a sense of system inefficiency, where heat was lost or cooling wasn’t fully utilized. To get a sense of how close we are to real-world applications, we used a scaling factor:
$$ Q = \frac{Y_v}{Y_i} $$
This compares the prototype’s cooling capacity $( Y_i )$ to the cooling requirement $( Y_v )$ of something like a vaccine fridge. From that, we can estimate how much adsorbent material we’d need to scale up to a usable device.
What does this all mean in plain terms? STAR systems don’t just work, they’re measurable, comparable, and scaleable. If you can get a basic prototype to deliver a few hundred watt-hours of cooling per cycle, you’re already in the zone for small-scale medical refrigeration or food preservation in rural settings.
Looking at the big picture, STAR could be a game changer. It drastically reduces operational emissions, eliminates dependence on electricity, and still delivers functional, reliable cooling. The main challenge lies in design, reducing material usage, improving heat transfer, and lowering costs for advanced materials like MOFs or composite adsorbents. But with ongoing research and smarter engineering, this is an area with serious potential.
If you’re interested in all the gritty details, performance graphs, and LCA breakdowns, check out the full paper here: https://doi.org/10.1016/j.resenv.2025.100195