The “nighttime solar” technology developed by the Photovoltaic and Renewable Energy Engineering team at the University of New South Wales (UNSW) has broken the conventional understanding that traditional solar energy relies on sunlight. Leveraging the phenomenon of the Earth releasing infrared thermal radiation into space at night, the team has created a device called a “thermal radiation diode,” which absorbs the Earth’s nighttime heat and converts it into electricity, enabling power generation without sunlight. Its unique principle and application potential have attracted widespread attention, making it a highlight in the latest engineering news of cutting-edge technological innovations.
Traditional solar cells generate electricity by absorbing photons from the sun to excite electron movement, while the operating mode of “nighttime solar” is almost the opposite. After absorbing solar energy during the day, the Earth continuously dissipates heat into the cold universe in the form of infrared radiation at night. This “Earth glowing” phenomenon, observable with infrared cameras, serves as the core basis for the technology’s development. The thermal radiation diode captures this infrared thermal radiation and converts the contained thermal energy into electrical energy. However, the current power generation efficiency of the device is relatively low, with an output only one-hundred-thousandth that of traditional solar panels. The electricity generated by human body heat can barely power an electronic watch, mainly due to gases such as water vapor and carbon dioxide in the atmosphere absorbing heat, which reduces the temperature difference between the Earth’s surface and the night sky.
The research team points out that the core potential of this technology lies in space. In space, unobstructed by the atmosphere and with extremely low ambient temperatures, the efficiency of thermal radiation is greatly enhanced, perfectly matching the operational requirements of the thermal radiation diode. Satellites have become its priority application scenario. Near-Earth orbit satellites orbit the Earth approximately every 90 minutes, spending about half the time in sunlight and the other half in the Earth’s shadow. Traditional satellites rely on solar panels and storage batteries for power; while solar panels only work when there is sunlight, satellites have to depend on pre-charged batteries once entering darkness. In contrast, the thermal radiation diode can continuously generate electricity at night using the heat absorbed by the satellite during the day. Additionally, it is lightweight and can be deployed on the idle surfaces of satellites, aligning with the development trend of smaller satellites with the same functionality as large ones.
Although NASA has shown interest in this technology, it believes that batteries are more cost-effective in near-Earth orbit, and the thermal radiation diode will only be considered for use if its cost becomes sufficiently low. Its greater value lies in deep space exploration. Missions such as probes heading to the outer edges of the solar system or rovers operating in the permanent shadow regions of the moon can barely rely on solar energy. Currently, such missions mainly use special thermoelectric generators that generate electricity through heat produced by the decay of radioactive isotopes (e.g., plutonium). However, these generators are not only bulky—weighing around 45 kilograms and having a volume of approximately 200 liters—but also use radioactive materials that are extremely scarce, making their manufacturing costly and difficult. As a result, the generators are expensive and only suitable for large-scale flagship missions. The thermal radiation diode, with its simple structure and light weight, is expected to be a more efficient energy solution for deep space tasks. Although plutonium is still needed to provide heat for the thermal radiation diode, its characteristics allow for smaller and more efficient heating panels, thereby conserving plutonium resources.
The current technology still faces challenges. The semiconductor materials used in the thermal radiation diode need to withstand the high temperatures generated by the decay of radioactive isotopes. Currently, power supply systems in space that use these isotopes as heat sources operate at temperatures of around 540°C or 1000°C. Researchers note that no one has previously attempted to operate such semiconductors at high temperatures, so their service life remains poorly understood. However, space missions require these semiconductors to last for 10 years, 20 years, or even longer. At present, the UNSW team has received research funding from the U.S. Air Force and plans to conduct high-altitude balloon tests to verify the technology in an environment close to space for the first time. Meanwhile, researchers are exploring material systems similar to those used in the traditional photovoltaic industry to enable large-scale manufacturing in the future. If the research and development progress goes smoothly, the thermal radiation diode is expected to enter practical application within the next five years, providing a more efficient and economical energy solution for space exploration and promoting the extension of renewable energy technology to broader scenarios.
