By Daniel HayesConventional silicon solar panels fall idle the moment the sun dips below the horizon, forcing power grids to rely on expensive battery banks. Now, photovoltaic researchers at the University of New South Wales have demonstrated a solid-state thermoradiative diode made from mercury cadmium telluride that generates electricity at night by harvesting infrared heat radiating from Earth into space.
Turning Earth’s Nighttime Heat Flow Into Electricity
Every day, Earth’s crust and atmosphere absorb vast amounts of solar energy and warm up. Once night falls, that accumulated thermal energy radiates back outward into the freezing expanse of deep space as invisible mid-infrared light.
Any thermodynamic system where heat flows from a warm source toward a cold sink can theoretically perform useful work. Just as a standard photovoltaic cell generates electricity by absorbing photons arriving from the hot sun, a thermoradiative cell generates power by emitting infrared photons toward a colder environment.
Led by photovoltaic physicist Ned Ekins-Daukes, the research team proved this principle experimentally in the laboratory. Their solid-state semiconductor junction captures a fraction of outgoing thermal radiation and converts it directly into continuous electrical current.
Inside The Mercury Cadmium Telluride Infrared Diode
Capturing low-energy thermal photons requires a narrow-bandgap semiconductor normally reserved for military night-vision cameras and cooled infrared astronomy sensors. The researchers selected mercury cadmium telluride, or HgCdTe, which can be precisely tuned to emit and respond to mid-infrared wavelengths between four and twelve micrometers.
When heated to just 20 degrees Celsius—typical of solar-warmed ground or building rooftops after sunset—the photodiode radiates infrared photons toward a cooler ambient target. As electrons drop across the semiconductor energy gap to emit those photons, an imbalance of charge carriers drives a measurable voltage across external contacts.
Initial laboratory measurements recorded a power density of roughly 2.26 milliwatts per square meter with an efficiency near 1.8 percent. While modest compared to daytime silicon panels, this milestone in clean energy technology mirrors the earliest silicon solar cell demonstrations of the 1950s.
Suppressing non-radiative Auger recombination inside the crystal lattice remains the primary engineering focus for boosting output voltage. Optical resonant cavities and anti-reflection surface coatings are now being integrated to channel more mid-infrared photons through the atmospheric transparency window.
Pathways To 24-Hour Hybrid Panels And Space Power
Thermodynamic modeling indicates that optimized thermoradiative diodes could eventually produce up to one-tenth of the power output of a conventional daytime solar module. Engineers are also investigating lower-cost III-V compound semiconductors to replace expensive mercury-based alloys for commercial manufacturing.
In the near term, miniature thermoradiative harvesters can trickle-charge remote environmental sensors, wearables powered by human body heat, and industrial waste-heat recovery monitors. Eliminating replaceable coin-cell batteries in low-power microelectronics would dramatically reduce maintenance across distributed sensor networks.
Industrial foundries, data centers, and geothermal pipelines also reject vast streams of low-grade heat below 100 degrees Celsius that conventional steam turbines cannot recover. Lining hot exhaust ducts with solid-state thermoradiative arrays offers a silent, maintenance-free method to convert industrial thermal losses into auxiliary grid electricity.
Looking further ahead, pairing thermoradiative layers with rooftop photovoltaic modules could enable hybrid panels that generate electricity around the clock. Satellites orbiting in Earth’s shadow could similarly use radiative diodes to extract steady auxiliary power as their sun-warmed hulls cool in space.
