Radiative cooling is an emerging passive technique that exploits the coldness of outer space by radiating heat from a surface to the sky, especially through the atmospheric transparency window (8–13 µm) at night, enabling subambient cooling and potential energy harvesting. Compared to other nighttime energy harvesting methods, radiative cooling shows unique advantages as well as some limitations in performance:
Cooling Performance of Radiative Cooling
- Radiative cooling coatings can achieve substantial temperature reductions below ambient temperatures by reflecting solar radiation and efficiently emitting thermal radiation to the sky during day and night. Nighttime radiative cooling can lower surface temperatures by 10–15°C below ambient under clear sky conditions, with some advanced selective emitters reaching up to 40°C below ambient when combined with vacuum enclosures.
- The cooling power of particle-based radiative cooling (PBRC) films varies depending on particle size distributions and material properties, with typical cooling powers ranging from about 25 to 60 W/m², achievable in favorable conditions.
- Outdoor tests of TiO2-based radiative cooling coatings exhibited internal temperature reductions of up to around 7.9°C in humid environments and higher in drier conditions, showing practical cooling benefits despite some performance degradation due to humidity.
- Radiative cooling coatings on buildings can reduce indoor temperatures by several degrees Celsius, decreasing cooling energy consumption substantially (e.g., reductions in building cooling load by 18.5–40.5 kWh/m² per month depending on building type), making it an energy-efficient cooling technology.
Radiative-Cooling-Based Nighttime Electricity Generation
- Radiative cooling can be combined with thermoelectric generators (TEGs) to harvest electricity at night, by exploiting the temperature difference between a cooled emitter and the ambient air.
- Recent experimental setups have demonstrated nighttime power densities exceeding 100 mW/m², which represents a significant two-fold improvement over previous results (~50 mW/m²) and about four times higher than earlier demonstrations at 25 mW/m².
- This performance strongly depends on optimizing thermal design to minimize parasitic heat transfer and maximize the thermal gradient. Clear, dry nights are ideal for maximizing radiative cooling power and thus electricity generation.
Comparison to Other Nighttime Energy Harvesting Techniques
- Traditional nighttime energy harvesting methods often rely on stored energy (batteries charged during the day), or direct harvesting from vibration, ambient RF signals, or thermoelectric generators exploiting temperature gradients unrelated to radiative cooling.
- Radiative cooling offers a passive, renewable, and noise-free means to generate cooling or low-grade electricity at night without requiring external power inputs or moving parts.
- While the electrical power densities from radiative cooling-based TEGs (~100 mW/m²) are modest compared to solar photovoltaic power densities during the day (typically several orders of magnitude higher), radiative cooling uniquely offers a continuous energy harvesting opportunity at night, when solar is inactive.
- The main limitations of radiative cooling for energy harvesting include sensitivity to atmospheric humidity, cloud cover, and parasitic heat transfer losses, which can reduce cooling and power generation performance.
Summary Table: Radiative Cooling vs Other Nighttime Energy Harvesting
| Feature | Radiative Cooling Energy Harvesting | Other Nighttime Harvesting Methods |
|---|---|---|
| Power Density | ~100 mW/m² demonstrated experimentally | Varies; can be lower or comparable depending on source (e.g., ambient RF harvesting is usually <10 mW/m²) |
| Dependence on Environmental Factors | Strongly dependent on clear sky, low humidity | Varies; some methods less dependent on weather |
| Continuous Operation | Works continuously at night as long as sky conditions are favorable | Depends on energy source availability |
| Complexity and Cost | Passive system, low maintenance, no moving parts | Some methods require batteries, circuitry, or moving parts |
| Application Areas | Cooling, power generation for remote areas, building energy saving | Sensors, low power electronics, OFF-grid devices |
In conclusion, radiative cooling offers a promising passive technology for nighttime thermal management and energy harvesting with demonstrated cooling performance and growing electricity generation capabilities. While the power output is moderate compared to solar PV, its ability to operate at night and reduce cooling energy loads is a distinctive advantage. Optimization of materials, coatings, and system thermal design will be crucial to improving performance relative to other nighttime energy harvesting technologies.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/how-does-the-performance-of-radiative-cooling-compare-to-other-nighttime-energy-harvesting-techniques/
