Ar-Rehman

Newly invented device by UC Davis Engineers can generate electricity from the cold night sky

Engineers at the University of California, Davis (UC Davis) have demonstrated a novel device capable of generating electricity by harvesting the natural cooling of the cold night sky. On clear nights, Earth quietly leaks heat into space. This natural cooling can create a steady energy flow. Their small outdoor engine used the night sky as a cold reservoir, maintained a strong temperature difference overnight, and delivered enough power to run a small fan. The study demonstrates a new way to harvest night time energy using passive physics.

This technology utilizes the principle of radiative cooling, where the Earth continuously leaks heat into space as infrared light on clear nights. The work was led by Dr. Jeremy Munday at the University of California, Davis (UCDavis). His research centres on clean energy photonics and radiative thermal devices. On clear nights, the sky acts as a heat sink that draws energy away through radiative cooling – heat lost to space as infrared light. A sky-facing surface which emits strongly can fall well below the air temperature without pumps or compressors. At night, the radiator cools through a clear band of wavelengths in the atmosphere which leaks heat to space. This band is often called the atmospheric window, a set of infrared colours where the air is most transparent.

The small, outdoor engine uses the night sky as a cold reservoir. The setup effectively maintains a strong temperature difference between its warm ground plate and its cool sky-facing radiator. This temperature difference is enough to power a Stirling engine, an external heat engine which thrives on small temperature gaps. The bottom plate follows soil warmth, which changes through the night compared to the air. This steadiness helps the engine keep turning while the top plate sheds heat to space. In Davis, the setup kept a steady temperature difference of about 18 °F (-7.8 °C) between its warm and cool plates for long stretches. This was enough to run the engine near one turn/second and deliver usable shaft power. “These engines are very efficient when only small temperature differences exist. If you just set it on the table, it’s not going to produce any power on its own,” said Munday. The team reports potential for several watts per square meter using better components. In one trial the device directly turned a fan and, with a small motor attached, also generated a modest electric current.

In experiments conducted in Davis, the device successfully delivered usable shaft power, running near one turn/second and capable of running a small fan. Dry air and clear skies help the radiator shed heat. Humid nights cut the effect because water vapour glows in the same infrared bands the radiator needs to use. Global maps built from NASA’s CERES radiation data show where the down welling infrared from the sky is lowest, which favours larger temperature gaps. Land surface temperatures from MODIS, an Earth-observing instrument, provide the warm side of the picture. NASA’s MODIS land surface temperature product supplies monthly global maps. The strongest potential appears in arid zones and at high, dry elevations where the air is thin and moisture is scarce.

The team reported potential for outputting several watts per square meter with improved components. The group showed air movement near one foot/second in a greenhouse-like temperature setup, enough to circulate CO2 around leaves. The speed aligns with comfort ranges near 0.5 to 0.7 foot/second, according to ASHRAE, the US standards body for building comfort. They also noted airflow rates that approach about 5 cubic feet/minute/person, which appears in ventilation guidance for many public spaces. The figure appears in an official interpretation of ASHRAE 62.1 which shows how designers combine/person and per area outdoor air. Performance can rise if the radiator couples more strongly to the sky and the warm plate couples better to the ground. Tailored coatings and thin film stacks can boost emissivity in the atmospheric window and can reflect sunlight during the day. A vacuum enclosure around the radiator would curb convective heat leaks. Careful seals and lightweight supports make that upgrade realistic. A Stirling engine, an external heat engine which turns a temperature difference into motion using a sealed gas, thrives on small temperature gaps. It uses two pistons and a regenerator to shuttle gas between warm and cool zones so gas expands, then contracts, keeping a flywheel turning. Real machines are limited by Carnot efficiency, the absolute ceiling for any heat engine which depends only on the hot and cold temperatures. The UC Davis setup runs well at low differences because Stirling engines waste little when designed for gentle pressure swings.

This breakthrough demonstrates a new way to harvest night time energy passively, with the strongest potential appearing in arid zones and at high, dry elevations where air moisture is scarce. Future versions could run in daytime by reflecting sunlight while still radiating heat in the right infrared bands. Stronger thermal contact with soil or water could also amplify the temperature gap without enlarging the radiator. Reducing friction and matching the motor to the torque and speed would lift electrical output. The engine could also use waste heat from farms or factories on the warm side to raise the overall temperature gap without new fuel. Running a fan without a grid connection sounds modest, yet it fills an overlooked need. Greenhouses need steady air movement at night when plants take up CO2 and humidity creeps up. Buildings need gentle flow for comfort, even when heating and cooling systems are idle. A rooftop unit which moves air with no electricity could support health goals while trimming loads after dark. The study’s global maps relied on down welling infrared, the heat the sky sends toward the surface from air and clouds. This value, combined with local ground temperature, sets the ceiling on the temperature gap a radiator can achieve at a given time and place. As with any passive system, output rises and falls with local weather and siting. Shade, wind and surface materials can change the effective temperatures which the engine actually sees.