As a form of passive cooling, radiative cooling can reduce the temperature of objects below the ambient temperature without additional energy consumption, and has become a research hotspot in thermal management in recent years. Daytime radiative cooling has been achieved by radiating out heat through the atmospheric transparent window (ATW, 8-13 μm) and simultaneously reflecting incident sunlight to avoid heating, which has been employed in green buildings, solar cells, and other fields. Generally, the physical quantities that measure the cooling material performance are temperature difference and radiative cooling power. Compared to the variability value of temperature differences measured under different circumstances (e.g., locations, atmospheric temperatures), the radiative cooling power is stable and can reflect the cooling performance more objectively. However, there are no specialized instruments measuring the radiative cooling power, especially those compatible with various materials with different cooling capabilities, which has become an obstacle to standardized radiative cooling evaluation. Therefore, we design a kind of measurement device for measuring radiative cooling power, and hope to provide a helpful tool for the material development and large-scale applications of radiative cooling technology.

According to the thermal balance principle, we design a cooling power measurement system consisting of a single-chip microcomputer as the control unit, a DS18B20 digital temperature sensor and a platinum resistance of PT100 as the temperature sensor, and a positive temperature coefficient heater as the executive source. The temperature sensors measure the ambient temperature and surface temperature of the sample to be tested and transfer the data to the controller in the form of SPI communication and 1-wire bus communication. Thus, the temperature differences and their changing rates can be obtained. In data processing, we adopt the idea of fuzzy control. First, we finish the value fuzzification about temperature differences and their changing rates, and then determine the membership functions. After the fuzzy inference process, we leverage gravity method to defuzzify and obtain accurate control quantity. On this basis, the fuzzy proportional-integral-derivative (PID) control algorithm is utilized to output pulse width modulation waves with different duty cycles, and then adjust the effective voltage value and working power on the PTC. Then, we control the temperature of radiative cooling materials to be consistent with ambient temperature and finally calculate the radiative cooling power based on the electric power consumption. Additionally, we design a CAD model and fabricate the mechanical structure by empoloying the 3D printing process.

Radiative cooling is a surface cooling technology with zero energy consumption. During the measurement, cooling materials initially achieve a ambient temperature drop due to thermal emissions, and then can be heated and maintain consistency with the ambient temperature under the action of the PTC heater. Meanwhile, we adopt the effective voltage changes of the PTC as input and the temperature change of the sample material as output. After Laplace transformation, we can obtain the system's transfer function. On this basis, we develop a simulation model for a fuzzy PID controller (Fig. 9), which realizes higher-accuracy temperature control and enhances the system's anti-interference capability (Fig. 10). Traditional PID control cannot meet the accuracy requirements of temperature control when dealing with cooling materials with different cooling performance, which is because the steady-state errors will occur under the unchanged parameters. The employed fuzzy PID control algorithm can adaptively change the PID parameters according to the values and changing rates of the temperature differences returned from different materials. Additionally, it can adjust the controller which outputs multiple PWM waves without interfering with each other to meet the testing requirements of various radiative cooling materials. The outdoor test results show that the homemade PVDF films of different thicknesses achieve cooling temperatures of 5.7 ℃ and 7.0 ℃ respectively in the evenfall (Fig. 13). After adopting the fuzzy PID controller, the system achieves the ideal results with no overshoot, and the steady-state error remains within 0.1 ℃ (Fig. 14). We also obtain the actual cooling power. The calculation results show that the cooling power values are 53.33 W/m² and 67.45 W/m² respectively. The measured cooling power maintains the same trend as the theoretical prediction.

We design a radiative cooling power measurement system based on the fuzzy PID control algorithm by employing the STM32F103 chip as the main controller. From the component point of view, since the core components of the system are only the main controller, temperature sensor, and PTC heater, the system is simplified from the number of hardware and device complexity. Meanwhile, the system meets the power measurement requirements of different radiative cooling materials based on a fuzzy PID control algorithm. In the simulation model, the system shows excellent stability and anti-interference capability. The actual outdoor test results indicate that an ideal temperature control effect can be yielded for different materials, and the control accuracy after stabilization can reach ±0.1 ℃. On this basis, the system initially realizes the miniaturization and practical applications of the measuring device, with promising application prospects.