Active daytime radiative cooling for air conditioning and refrigeration systems
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Solution Overview
Problem
Existing cooling technologies face challenges in achieving efficient daytime radiative cooling, especially when exposed to direct solar irradiation, as they struggle to balance reflectivity at short wavelengths and emissivity at long wavelengths, leading to suboptimal heat rejection and increased energy consumption.
Innovation Solution
A heat exchanger system incorporating a spectrally selective surface material with high reflectivity at short wavelengths and high emissivity at long wavelengths, combined with an active cooling system, which can utilize photovoltaic panels or thermoelectric modules to enhance heat rejection through both radiation and convection, even when temperatures exceed ambient levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If traditional cooling surfaces are used, then they can reject heat through convection, but they absorb significant solar irradiation at short wavelengths, reducing cooling efficiency
Solution Approach 1:
The cooling surface is designed with spectrally selective properties that vary by wavelength: high reflectivity in the solar spectrum (short wavelengths) and high emissivity in the thermal infrared range (long wavelengths). This local quality differentiation allows the surface to simultaneously reject solar heating and emit thermal radiation effectively, resolving the contradiction between minimizing solar absorption and maximizing heat rejection.
Solution Approach 2:
The patent employs composite or engineered materials with tailored optical properties that combine high solar reflectivity and high thermal emissivity. These composite surfaces integrate multiple functional characteristics to achieve spectral selectivity, enabling the surface to differentiate between short-wavelength solar radiation and long-wavelength thermal radiation, thus improving heat rejection while minimizing solar absorption.
2Object-affected harmful factors
If surfaces with high solar reflectivity are used, then solar absorption is reduced, but emissivity at long wavelengths may be compromised, limiting radiative cooling
Solution Approach 1:
The surface is engineered with wavelength-dependent optical properties: high reflectivity in the solar spectrum region and high emissivity in the thermal infrared region. This spectral differentiation allows the surface to simultaneously achieve both high solar reflection and high thermal emission, resolving the contradiction between minimizing solar absorption and maximizing radiative cooling.
Solution Approach 2:
The patent utilizes materials or structures whose optical parameters (reflectivity and emissivity) are specifically tuned or changed across different wavelength ranges. By adjusting these spectral parameters, the surface achieves optimal performance in both reflecting solar radiation and emitting thermal radiation, thereby resolving the contradiction between solar reflectivity and thermal emissivity.
3Temperature
If active cooling systems operate at temperatures above ambient, then cooling capacity is maintained, but conventional methods require large condenser units and high energy consumption
Solution Approach 1:
The patent replaces the conventional mechanical convection-based cooling system with a radiative cooling mechanism. Instead of relying on large condenser units and high-velocity air flow to reject heat, the system uses spectrally selective surfaces to emit thermal radiation directly to the cold sky, significantly reducing the size and complexity of cooling equipment while maintaining effective heat rejection at elevated temperatures.
Solution Approach 2:
The patent converts the typically harmful effect of high operating temperatures (which increase heat rejection demands) into a benefit by utilizing the temperature difference between the hot heat exchanger and the cold sky as a driving force for enhanced radiative heat transfer. The high temperature operation, which would normally require larger cooling equipment, actually enhances the radiative cooling effect when combined with spectrally selective surfaces.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables effective daytime radiative cooling, reducing temperatures by up to 40°C relative to outdoor ambient, increasing heat transfer to deep space, and potentially reducing global temperatures, while offering advantages over traditional air conditioning methods by eliminating the need for condenser units and reducing energy consumption.
Implementation Method 1
Surfaces that exhibit high reflectivity at short (e.g., solar) wavelengths and high emissivity at long (e.g., terrestrial) wavelengths can experience net radiation cooling
Implementation Method 2
high reflectivity at short (e.g., solar) wavelengths
Implementation Method 3
high emissivity at long (e.g., terrestrial) wavelengths
Implementation Method 4
The system operates at a temperature that exceeds that of outdoor ambient for active radiative and convective cooling
Implementation Method 5
The active cooling system can include one or more photovoltaic panels to provide power to the active cooling system
Implementation Method 6
Hot refrigerant from the active cooling system can be cooled or condensed within the heat exchanger
Data Source
AI summary
Examples of heat exchanger systems for active radiative cooling are described. In one example, the system includes a heat exchanger and a spectrally selective surface material on at least one surface of the heat exchanger. The spectrally selective surface material exhibits high reflectivity at shorter wavelengths and high emissivity at longer wavelengths. The system can also include an active cooling system in some cases to actively transfer heat to the heat exchanger. The use of spectrally selective surfaces that operate at temperatures exceeding that of the outdoor ambient for which convective losses augment radiation losses have advantages over passive cooling, such as but not limited to: providing a better match to cooling loads, reducing the heat rejection surface area required to achieve a desired cooling rate, and increasing the heat transferred to deep space through the atmospheric window so as to simultaneously cool infrastructure, devices, buildings, and Earth.