Generating Power Using Earth’s Ambient Thermal Radiation

Tapping Temperature Gradients

Most of our power generation methods rely on a temperature differential. This is often created by heating one part through the combustion of fossil fuels (coal, oil, gas), nuclear fission, drilling deep underground (geothermal), or concentrating sunlight (concentrated solar).

This thermal difference is then used to heat water or another liquid (such as molten salt) to activate a turbine that generates electricity.

So while directly capturing sunlight (photovoltaics) or natural movements (wind power, hydroelectricity, tidal) is also a possibility, thermal gradients are the most common form of power generation, from the days of the steam engine to today.

Another thermal gradient that could be theoretically exploited is the difference in temperature between Earth and outer space.

Earth’s average surface temperature is roughly 15°C (59°F), while outer space sits at −270°C (−454°F). This enormous theoretical thermal differential has long intrigued researchers, but tapping it is far from trivial.

Emitting Heat Into Space

For thermal radiation at wavelengths between 8 and 13 μm, the atmosphere is fully transparent and allows Earth’s heat to escape into space. This is the principal mechanism that allows our planet to cool down after receiving energy from the Sun.

In theory, an engine able to emit at this wavelength, or a close enough frequency emitting energy into the cooler sky (compared to the ground), could generate electricity from the ambient temperature.

In fact, this method has already been demonstrated, using either low-gap semiconductor devices or thermoelectric generators. But these methods are not practical for economical power generation due to their low power output and the need for rare-earth elements.

But scientists Tristan J. Deppe and Jeremy N. Munday, working at the University of California, might have found an alternative using Stirling engines. They published their work in the prestigious scientific review Science¹, under the title “Mechanical power generation using Earth’s ambient radiation”.

Stirling Engines Explained

While most temperature differentials are used to generate power with turbines moved by steam, an alternative is the Stirling engine.

These engines create a mechanical motion when one side of the engine is hotter or cooler than the other. In contrast to internal combustion engines or turbines, it requires no burning of material.

The mechanical motion can then be converted into electricity with a simple alternator.

Stirling engines are remarkably durable, although relatively heavy, limiting their applications for transportation.

Their yield is also slightly lower than that of turbines, which explains why they are not commonly used in thermal or nuclear power plants. However, they can function with even a small temperature gradient, while turbines require hundreds of degrees of difference between hot and cold.

How Stirling Engines Capture Ambient Thermal Energy

The basic concept of the ambient thermal power generation used here has 2 components:

• The engine’s bottom plate is in direct thermal contact with Earth’s surface.
• The top plate is optically coupled to the sky.

To manage the emission of heat into the air for the top part of the engine, an infrared-emissive paint is used.

Source: Science Advances

This method leverages the small temperature difference between the ground and the air, especially at night, which only a Stirling engine is able to capture into motion/energy.

Our proof-of-concept demonstration radiatively couples the engine to the sky and delivers >400 mW/m2 of continuous power on Earth throughout the night.

Testing In Real-Life Conditions

The method was tested in Davis, California, with temperature differences of 10°C (18°F) and a 1-Hz rotation of the engine’s flywheel. Testing was performed throughout the year, with most of the period working, although winter with rain and clouds was less efficient. More than the absolute temperature, it is the air moisture content that affects the efficiency of this system the most.

Source: Science Advances

In high-humidity conditions, the difference between day and night temperatures is reduced due to high concentrations of water in the atmosphere, which reduces radiative cooling power, impacting the overall energy potential.

Mapping Ambient Energy Potential

Using their experimental results, the scientists went on to model the areas with the best potential for their invention.

They drew a few conclusions:

• The power density is highest in arid regions and mountain ranges, where downward radiation is lowest.
• Higher-humidity areas are lower in energy potential.
• Power generation is close to zero in heavily forested regions, where increased humidity prevents the cooler from effectively exhausting heat to the sky.

Using these data, they created a map showing the areas of Earth with the best potential to deploy ambient radiative Stirling engines.

Source: Science Advances

The regions with the best potential are:

• Saharan Africa.
• The Eurasian Steppe.
• Antarctica during the summer.
• Inland regions of the US West Coast.
• The Andes mountains
• The Tibetan plateau.

Future Improvements

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This work was very much a proof-of-concept, so several design elements could be improved.

The first element would be to improve the radiative cooling power. This could be achieved by using a tailored radiative cooling material instead of commercial paint.

The second element would be to increase the conductive coupling to Earth, for example, by using a larger contacting surface area and higher thermal conductivity materials like copper.

A larger engine could also increase the total power output and efficiency. Using helium or hydrogen instead of air in the Stirling engine piston could also reduce friction and increase yield.

Lastly, our industrial civilization generates significant waste heat from greenhouses, factories, HVAC systems, and heated residential buildings in winter, among other sources. This could increase the temperature difference between ground and sky greatly, boosting the energy production.

In practice, a temperature differential of 35-40°C (72°F) can generate almost 4x the power compared to a 15°C differential.

Source: Science Advances

Toward “Reverse Solar Panels”

Because this design works best at night (although it could also run during the day with design changes), it seems to make a good complement to photovoltaic solar panels.

It could also be a great way to maximize the usage of waste heat, be it from other forms of power generation, industrial processes, warm buildings (offices, apartments, houses), or greenhouses.

Lastly, it could be designed as an extra cooling method to install on buildings, with the system absorbing heat and radiating it back into space.

If deployed at a large enough scale, it could even generate power while reducing the Earth’s overall captured heat, which is rather unique compared to all other power generation methods.

Stirling Engine Companies

Aerojet Rocketdyne and L3 Harris: Leading Stirling Engine Innovators

Stirling engines are a niche application in energy generation, but are still a $1.17B market in 2025, expected to grow by 8.5% CAGR until 2029, reaching $1.62B. However, few companies active in the sector are publicly listed.

Aerojet Rocketdyne, a branch of the aerospace & defense contractor L3 Harris, is collaborating with partners like NASA and SunPower Inc. to develop Stirling engines for space applications.

Aerojet Rocketdyne was acquired by L3 Harris in July 2023 for $4.7B, adding a 4^th department to the company.

Sunpower Inc (to not mismatch for Sunpower, the solar panel company (SPWR )) is the inventor of an advanced design of Stirling engine: Free-Piston Stirling Engine (FPSE). FPSE can be used to both produce power from heat and to cool using power.

This technology is especially applicable to Radioisotope Power Systems (RPS), which uses the natural decay of radioactive material to generate heat, which the Stirling engine converts into usable electrical power. One major project for such an engine would be to power equipment on the Moon, or even a small lunar base.

Source: NASA

NASA has been interested in Stirling engines for a long time now, thanks to their reliability, maintenance-free operations, and long lifetime, especially with the Advanced Stirling Radioisotope Generator (ASRG).

Besides lunar Stirling engines, L3 Harris is a major military and aerospace company. It generated 60% of its revenues from the US Department of Defense (DoD), 20% from international defense orders, and 20% from the civilian industries.

Notably, Harris controls 45% of the global tactical radios market, several times larger than the next competitor.

Regarding unmanned systems, L3Harris has a vertical take-off drone, the FVR-90, the marine autonomous boat Shadowfox (13m long), the family of underwater drones Iver, and is the prime contractor for the U.S. Navy’s first major contract award for the Medium Unmanned Surface Vehicle (MUSV).

Aerojet is also a developer of hypersonic missiles and other missile systems.

Overall, L3 Harris is a leading technology company when it comes to autonomous systems, rocketry, and aerospace energy systems, with a solid technical expertise for both civilian and military contracts.

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Study Referenced

^{1. Tristan J. Deppe and Jeremy N. Munday. Mechanical power generation using Earth’s ambient radiation. Science Advances. 12 Nov 2025. Vol 11, Issue 46. https://www.science.org/doi/10.1126/sciadv.adw6833}