By lining an affordable solar box cooker with crushed volcanic gravel, researchers have found a simple way to store thermal energy for eight hours, enabling families to boil water and cook multiple hot meals even after sunset.
What to know about Volcanic rocks give solar cookers the power to
Scientists have found an ingenious and remarkably low-cost way to solve one of solar energy’s biggest headaches: how to cook with the sun when the sun is no longer shining. By packing tiny granules of crushed volcanic rock beneath the cooking plate of a wooden solar box, an international research team has demonstrated that families can store solar heat for more than eight hours. Developed and tested over a full year in northern India, the system reaches temperatures high enough to boil water in all seasons, costs about $68 to build, and can easily prepare dinner long after dusk without a single stick of firewood or spark of electricity. The team included researchers from Dev Bhoomi Uttarakhand University, Dehradun, Universidad de Tarapacá, Chile, Graphic Era (Deemed-to-be-University), Dehradun, Shri Mata Vaishno Devi University, COER-University, Roorkee, National Institute of Technology Hamirpur, and Gachon University, South Korea
Tested under the changing skies of Moradabad, in Uttar Pradesh, the cooker proved remarkably robust across summer, rainy, and winter weather. In the intense heat of summer, the cooker’s absorber plate soared to a peak temperature of 136.2 degrees Celsius, achieving an overall thermal efficiency of 40.1% and bringing water to a boil in just over an hour. Because the volcanic rock bed acted as a miniature energy reservoir, it stored surplus heat during peak daylight hours and released it slowly over eight hours and four minutes.
In practical culinary tests, the researchers cooked a traditional meal of moong pulses under direct sunlight in 72 minutes, then swapped the pot for uncooked rice and cooked it in 73 minutes using only the stored heat from the rocks. They then immediately placed a third batch of sliced potatoes into the oven, which cooked through in 71 minutes entirely on residual heat, showing that a single household could prepare up to five meals a day in summer and two substantial meals during sunny winter afternoons.
Key context and latest developments
To evaluate how well the system worked, the researchers monitored the cooker during both empty trials and loaded tests containing standardised volumes of water, checking digital temperature sensors, sunlight intensity, and wind speed every twenty minutes. They observed that the cooker acts as a miniature greenhouse. Sunlight passes through a tilted double-glazed glass lid, which traps infrared thermal radiation inside an insulated wooden box lined with glass wool. Beneath a blackened aluminium tray, the team packed just under two and a half kilograms of basaltic volcanic gravel crushed to millimetre-sized granules. As the blackened tray absorbs visible light and converts it into heat, energy conducts directly downward into the rock bed.
Volcanic basalt is an ideal heat-storage material because it combines high density with a high capacity to soak up heat without degrading over thousands of heating and cooling cycles. The fine granules pack tightly together, leaving minimal air pockets. Because air is a poor conductor of heat, reducing large voids allows heat to travel smoothly across grain boundaries and spread evenly through the bed. During peak midday sunshine, the rock bed acts as a thermal flywheel, absorbing excess solar energy that would otherwise overheat the box and escape through the glass. Once the sun dips and the absorber plate begins to cool, a natural thermal inversion occurs: the rocks become hotter than the chamber above them, reversing the flow of energy and continuously pumping heat back upward into the cookware through conduction and radiant infrared warmth.
Although phase-change materials such as paraffin waxes or hydrated salts can store energy chemically, they are often expensive, toxic, prone to degrading, and vulnerable to leaking. Volcanic rocks are a safe, economical alternative, making the technology viable. Furthermore, by burying the volcanic granules beneath the metal plate rather than piling them on top, the researchers eliminated the mechanical wear and surface scratching that typically ruins the light-absorbing black coatings of older solar ovens. Compared directly with an identical pebble-based solar oven, the volcanic-rock system boiled water thirteen minutes earlier and retained usable heat for an extra hour and twenty-one minutes.
Across developing nations, millions of rural families still rely on expensive combustible gases, smoky or wood-burning stoves and open fires, which drain their pockets. They also accelerate regional deforestation and cause severe respiratory illnesses that disproportionately afflict women and young children. By using cheap, locally sourced volcanic gravel and everyday materials that local carpenters can assemble without specialised tooling, this cooker delivers an off-grid appliance that pays for itself in avoided fuel purchases in less than three years. Beyond preparing lunch and dinner, it can double as a fruit dehydrator and milk pasteuriser, freeing vulnerable communities from the daily grind of gathering firewood while providing a clean, flame-free path toward health and energy independence.
