Capture "stealth" solar energy at night

When the sun goes down and the sun goes away from the solar panels, what is the supply of energy throughout the night? Battery or old diesel generator? Recently, researchers at the Harvard University School of Engineering and Applied Science (SEAS) proposed a new idea: use the infrared heat radiation emitted by the Earth to generate electricity.

Due to the sun, the earth is warmer than outer space. Researchers say that using current technology, heat imbalances can already be converted to direct current, which is also possible between Earth and outer space, making the Earth's infrared heat radiation a huge, never-used source of energy. . Related papers were published in the "Proceedings of the National Academy of Sciences".

"The first is how to harness the Earth's infrared radiation to space to generate electricity. This is not obvious," said Federico Cappaso, professor of applied physics at Harvard University's Robert Wallace. "Using radiant power instead of absorbing light. It sounds bizarre. Although counterintuitive, it is plausible in physics. This is a new application of physics in the nano field."

Challenge traditional development of new energy

When heat is transferred from a hotter object to a cooler object, renewable energy can be generated. From the warm surface to the cold outer space, there is also this heat transfer, which is infrared radiation. The earth releases 10 billion megawatts of energy in the form of infrared radiation. Such a huge amount of energy has been ignored.

"Mid-infrared is largely ignored by people. Even in spectroscopy, until people have a quantum cascade laser, people still think that this band is very difficult to operate. But this is people's preconceptions about it." Capasso Say. Their latest research shows that it is possible to capture energy from infrared radiation emitted from Earth.

Capasso is an expert in semiconductor physics, photonics and solid-state electronics. He co-invented the infrared quantum cascade laser in 1994, pioneered the energy band engineering research field, and proved a kind of quantum electrics phenomenon known as “Casimir repulsion”.

"Sun is an energy source, so photovoltaic cells make sense. You only need to collect energy. But things are not that simple. It is still difficult to capture the energy of infrared light," said Stephen Burns, the paper's lead author and postdoctoral fellow of SEAS. "It is not obvious how much electricity can be generated in this way. Is it worthwhile to study economically? We must also sit down and carefully calculate it."

Power generation is small but actually available

Burns pointed out: "For example, combining this type of device with a solar cell can provide extra power at night without additional equipment costs."

In order to prove the feasibility of infrared radiation power generation, Capasso Group proposed two different types of radiant energy harvesters (EEH): one is a thermal EEH, similar to a solar thermal generator; and the other is an optoelectronic EEH, just like Photovoltaic panels.

The first type of equipment consists of "hot" and "cold" panels. The temperature of the "hot" panels is the same as the temperature of the Earth and the ambient air. The "cold" panels are mounted on the "hot" panels, facing upwards, from a high level. Made of radioactive materials, it can efficiently radiate heat to the sky. The researchers conducted an experimental measurement in Lamont, Oklahoma, and calculated that the difference in heat between the two plates could emit several watts per square meter per day. Although it is still more difficult to keep the "cold" panel temperature below ambient temperature, this equipment proves that thermoelectric power generation is indeed feasible.

"This method is more intuitive, and we are combining the familiar heat engine principle with the radiation cooling principle," said Burns.

The principle of the second device goes deeper into the level of electronic behavior and is less intuitive. It relies on the temperature difference between the nanoelectronics, the diode and the antenna, which is not the temperature that people can sense with their hands. "If you have two components with the same temperature, you obviously cannot do anything. If the two components are at different temperatures, you can do the work," said Capasso. Its working principle is similar to a photocell. Its core is a rectenna, which uses a temperature difference between different electronic components after absorbing external heat to generate current.

In the paper, the researchers designed a monolithic flat device that printed many of these microcircuits and turned it toward the sky to generate electricity. They also pointed out that at present, rectenna technology can only produce “negligible power,” but technological advances may increase power generation efficiency.

Technical challenges and future prospects

Researchers are more optimistic about the second option. Although the optoelectronic method is still very new, it is still feasible with the advancement of current technology, with advances in plasma science, microelectronics, new materials, and nanofabrication. The paper also pointed out the technical challenges and future prospects for future research.

“People have been investigating infrared diodes for at least 50 years, and there has been no major progress. Recent advances in nanomanufacturing have allowed people to create better, scalable, and renewable nanomaterials,” Burns said. However, there are still problems with using the best infrared diodes. "The more electricity that flows in a single circuit, the lower the voltage is when you collect energy from infrared radiation. This also means that it is very difficult to make efficient infrared diodes."

Engineers and physicists, including Burns, have been devising new types of diodes that can operate at low voltages, such as tunnel diodes and ballistic diodes. Another method is to increase the impedance of the circuit element to increase the voltage to a more feasible level.

Burns also pointed out that speed is another challenge. "In addition to dealing with voltage and impedance problems, we also need to meet speed requirements. At present, there is only one type of diode that can be selected and can switch 30 trillion times in one second, which is the frequency required by infrared signals."

The researchers pointed out in the paper: "Today's technology is not enough to create an effective and cost-effective photoelectric radiant energy harvester, but we have described some possible ways to achieve this goal in the future. We hope to open up this frontier area in Renewable energy plays a role as a radiant energy collector."