I’ve always been intrigued by “small-scale” energy harvest where the mechanism is relatively simple while the useful output is modest. These designs, which may be low-cost but may also use sophisticated materials and implementations, often make creative use of what’s available, generating power on the order of about 50 milliwatts.
These harvesting schemes often have the first-level story of getting “a little something for almost nothing” until you look more deeply in the detail. Among the harvestable sources are incidental wind, heat, vibration, incremental motion, and even sparks.
The most recent such harvesting arrangement I saw is another scheme to exploit the thermal differential between the cold night sky and Earth’s warmer surface. The principle is not new at all (see References)—it has been known since the mid-18th century—but it returns in new appearances.
This approach, from the University of California at Davis, uses a Stirling engine as the transducer between thermal energy and mechanical/electrical energy, Figure 1. It was mounted on a flat metal plane embedded into the Earth’s surface for good thermal contact while pointing at the sky.
Figure 1 Nighttime radiative cooling engine operation. (A) Schematic of engine operation at night. Top plate radiatively couples to the night sky and cools below ambient air temperature. Bottom plate is thermally coupled to the ground and remains warmer, as radiative access to the night sky is blocked by the aluminum top plate. This radiative imbalance creates the temperature differential that drives the engine. (B) Downwelling infrared radiation from the sky and solar irradiance are plotted throughout the evening and into the night on 14 August 2023. These power fluxes control the temperature of the emissive top plate. The fluctuations in the downwelling infrared are caused by passing clouds, which emit strongly in the infrared due to high water content. (C) Temperatures of the engine plates compared to ambient air throughout the run. The fluctuations in the top plate and air temperature match the fluctuations in the downwelling infrared. The average temperature decreases as downwelling power decreases. (D) Engine frequency and temperature differential remain approximately constant. Temporary increases in downwelling infrared, which decrease the engine temperature differential, are physically manifested in a slowing of the engine.
Unlike other thermodynamic cycles (such as Rankine, Brayton, Otto, or Diesel), which require phase changes, combustion, or pressurized systems, the Stirling engine can operate passively and continuously with modest temperature differences. This makes them especially suitable for demonstrating mechanical power generation using passive thermal heat from the surroundings and radiative cooling without the need for fuels or active control systems.
Most engines which use thermal differences first generate heat from some source to be used against the cooler ambient side. However, there’s nothing that says the warmer side can’t be at the ambient temperature while the other side is colder relative to the ambient one.
Their concept and execution are simple, which is always attractive. The Stirling engine (essentially a piston driving a flywheel), is put on a 30 × 30 centimeter flat-metal panel that acts as a heat-radiating antenna. The entire assembly sits on the ground outdoors at night; the ground acts as the warm side of the engine as the antenna channels the cold of space.
Under best-case operation, the system delivered about 400 milliwatts of electrical power per square meter, and was used to drive a small motor. That is about 0.4% efficiency compared to theoretical maximum. Depending on your requirements, that areal energy density is somewhere between not useful and useful enough for small tasks such as charging a phone or powering a small fan to ventilate greenhouses, Figure 2.
Figure 2 Power conversion analysis and applications of radiative cooling engine. (A) Mechanical power plotted against temperature differential for various cold plate temperatures (TC). (Error bars show standard deviation.). Solid lines represent potential power corresponding to different quality engines denoted by F, the West number. (B) Voltage sweep across the attached DC motor shows maximum power point for extraction of mechanical to electrical power conversion at various engine temperature differentials (note: typical passive sign convention for electrical circuits is used). Solid red lines are quadratic fits of the measured data points (colored circles). Inset shows the dc motor mounted to the engine. (C) Bar graph denotes the remaining available mechanical power and the electrical power extracted (plus motor losses) when the DC motor is attached. (D) Axial fan blade attachment shown along with the hot-wire anemometer used to measure air speed. (E) Air speed in front of the fan is mapped for engine hot and cold plate temperatures of 29°C and 7°C, respectively. White circles indicate the measurement points. (F) Maximum air speed (black dots) and frequency (blue dots) as a function of engine temperature differential. Shaded gray regions show the range of air speeds necessary to circulate CO2 to promote plant growth inside greenhouses and the ASHRAE-recommended air speed for thermal comfort inside buildings.
Of course, there are other considerations such as harvesting only at night (hmmm…maybe as a complement to solar cells?) are needing a clear sky with dry air for maximum performance. Also, the assembly is, by definition, fully exposed to rain, sun, and wind, which will likely shorten its operation life.
The instrumentation they used was also interesting, as was their thermal-physics analysis they did as part of the graduate-level project. The flywheel of the engine was not only an attention-getter, its inherent “chopping” action also made it easy to count motor revolutions using a basic light-source and photosensor arrangement. The analysis based on the thermal cycle of the Stirling engine concluded that its Carnot-cycle efficiency was about 13%.
This is all interesting, but where does it stand on the scale of viability and utility? On one side, it is a genuine source of mechanical and follow-up electrical energy at very low cost. But that is only under very limited conditions with real-world limitations.
I think this form of harvesting gets attention because, as I noted upfront, it offers some usable energy at little apparent cost. Further, it’s very understandable, requires exotic materials or components, and comes with dramatic visual of the Stirling engine and its flywheel. It tells a good story that gets coverage and likely those follow-on grants. They have also filed a provisional patent related to the work; I’d like to see the claims they make.
But when you look at its numbers closely and reality becomes clearer, some of that glamour fades. Perhaps it could be used for a one-time storyline in a “McGyver-like” TV show script where the hero improvises such a unit, uses it to charge a dead phone, and is able to call for help. Screenwriters out there, are you paying attention?
Until then, you can read their full, readable technical paper “Mechanical power generation using Earth’s ambient radiation” published in the prestigious journal Science Advances from the American Association for the Advancement of Science; it was even featured on their cover, Figure 3, proving The “free” aspects of this harvesting and its photo-friendly design really do get attention!
Figure 3 The harvesting innovation was considered sufficiently noteworthy to be featured as the cover and lead story of Science Advances.
What’s your view on the utility and viability of this approach? Do you see any strong, ongoing applications?
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References
- Applied Physics Letters, “Nighttime electric power generation at a density of 50 mW/m2 via radiative cooling of a photovoltaic cell”
- Nature Photonics, “Direct observation of the violation of Kirchhoff’s law of thermal radiation”
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