Research highlights:
1. Energy source innovation: This system uses organic photovoltaic modules to obtain energy directly from sunlight without the need for additional power sources. This allows thermally regulated clothing to function in a variety of harsh environments, providing convenience to users.
2. Two-way adjustment function: The electrothermal effect device has a two-way adjustment function, which can both heat and cool. This flexibility is important to respond to individual needs in different temperatures and environmental conditions, increasing the garment's practicality.
3. Flexible design: The organic photovoltaic and electrothermal effect devices in the system adopt flexible designs, making the clothing more adaptable and comfortable, and can flexibly fit the curves of the human body and be suitable for personal thermal adjustment needs.
4. All-weather use: Since the system uses solar energy as its energy source and has low energy consumption and high-efficiency electrothermal effect devices, the heat-adjusting clothing can operate around the clock, even in challenging environments such as extremely cold polar regions and space travel. It can keep the human body within a comfortable temperature range.
5. Self-powered wearable platform: This system is self-sufficient and does not require external power supply support, forming a wearable thermal regulation platform.
1. Development Goals of Wearable Thermal Adjustable Clothing
Clothing plays an important role in maintaining body thermal comfort, especially in the face of ambient temperature fluctuations. In order to cope with changing environmental temperatures, the human body needs to have the ability to adapt quickly to keep body temperature within a safe range. Wearable thermally regulated clothing that maintains human skin temperature within a comfortable range in harsh environments, such as extremely cold polar regions or space travel, has always been a challenging goal. There are currently two thermal regulation systems. Passive systems include radiative, phase change, and adsorption thermal regulation systems, while active systems allow rapid cooling or heating of the human body. However, most current active systems require complex mechanical compressors or have some limitations. It is of great significance to develop an all-day, self-sufficient, two-way thermally regulated clothing system. This system needs to be able to quickly respond to various complex or rapid changes in ambient temperature and achieve round-the-clock thermal regulation by collecting solar energy.
2. Introduction to results
For comfort and safety, the human body must be maintained within a certain temperature range. However, thermally regulated clothing faces several challenges in demanding applications such as all-weather cycling, cold polar regions, and space travel. To solve these problems, Chen Yongsheng's research team at Nankai University developed a flexible and sustainable personal thermal-regulating clothing system by integrating flexible organic photovoltaic (OPV) modules to harvest energy directly from sunlight, and a bidirectional electrothermal effect (EC) device. This flexible OPV-EC thermally regulated garment (OETC) enables rapid thermal regulation, extending the body's thermal comfort zone from 22°–28°C to 12.5°–37.6°C. The EC device has low energy consumption and high efficiency, allowing the system to achieve 24-hour controllable dual-mode thermal adjustment under 12 hours of sunlight energy input. This self-sufficient wearable thermal regulation platform has a simple structure, compact design, high efficiency, and exhibits strong adaptability when sunlight is the only energy source.
In this study, JV was measured using Enlitech products.
3. Results and Discussion
Point 1: OETC system enables autonomous thermal regulation between hot and cold environments
A large flexible OPV module with a thickness of only 180 microns was fabricated to serve as a solar energy collection unit in the OETC system. The effective area of the entire flexible OPV module is 25.2 square centimeters. Under standard air quality 1.5 global lighting conditions, it can provide a total voltage of 5.75 volts and a photoelectric conversion efficiency of 11.85%.
In the thermal conditioning unit of OETC, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF- TrFE-CFE) as material, fabricated a flexible EC thermal conditioning device and demonstrated the same thermal management performance as a rigid device.
Integrating these two flexible units together forms the OETC system. Under sunlight, the Organic Photovoltaic (OPV) module efficiently converts solar energy into electrical energy and directly drives the EC device to provide a cooling effect. Excess energy can be stored in additional energy storage systems due to the low energy consumption of EC devices. In the absence of sunlight, the OETC system can use the energy storage provided by the ESS to maintain body temperature, enabling all-day operation. Cooling and heating modes can be switched at any time to achieve individual thermal comfort.
Figure 1 OETC system achieves autonomous thermal adjustment between hot and cold environments
Point 2: How the OETC system works in cooling/heating mode
The authors show photos of a flexible OETC thermal conditioning system assembled from one OPV module and two EC units. This compact assembly provides effective cooling/heating of the body as needed. The OETC system works in cooling mode in the same way as when powered by the mains, but in this system it is driven directly by the electricity generated by the OPV module.
Cooling mode includes the following steps:
(i) Electrostatic actuation of the EC polymer layer towards the top flexible heat transfer layer (acting as a heat sink with large thermal capacity); (ii) The EC polymer layer is heated by applying an electric field on the EC polymer layer, whereby heat is transferred from the EC The polymer layer passes to the flexible heat transfer layer;
(iii) Electrostatic actuation of the EC polymer layer towards the underlying human skin (as a heat source);
(iv) The EC polymer layer is cooled by removing the electric field, so that heat is transferred from human skin to the EC polymer layer, achieving primary skin cooling.
For the heating mode, by changing the sequence of the above four steps (by simply adjusting the phase of the square wave voltage), the change in the direction of heat transfer is achieved. Correspondingly, the warming mode has similar steps to the cooling mode, but with the opposite heat transfer effect.
Electrostatic actuation is a simple and fast method to control the rate of heat transfer by adjusting the operating frequency of the EC device. The authors compared the temperature span of the OETC system at different frequencies through a solar simulator under standard AM 1.5G (100 mW/cm2). Although OETC systems can operate at higher frequencies, the frequency capable of providing a maximum temperature span of 2.9 K is 0.75 Hz (one complete cycle is approximately 1.33 seconds), in part due to the need to transfer heat from the EC stack to human skin and Time required for flexible heat transfer layer.
Figure 2 Performance of the flexible OETC system
Point 3: Temperature span of OETC system under different working scenarios
The temperature span of this OETC thermal regulation system can also be easily achieved by adjusting the light intensity. As the light intensity increases, the flexible OPV module can reach higher voltage (power), thereby increasing the input voltage of the EC device, thus improving the thermal regulation performance of the OETC system. The authors measured the temperature difference (ΔT, the difference between the real-time temperature and the initial temperature) of the OETC system under different light intensities (55, 70 and 100 mW/cm2) with a frequency of 0.75 Hz (Figure 2C). The OETC system performs well under different light intensities. When the light intensity is standard AM 1.5G sunlight (100 mW/cm2), the maximum temperature span can reach 2.9 K.
In addition, the authors also demonstrated the outdoor thermal conditioning performance of the OETC system through outdoor experiments conducted under clear-sky conditions in Tianjin, China (August 3, 2022). Although the outdoor sunlight intensity varies greatly with time, the OETC system still exhibits good and stable thermal control capabilities under different light intensities. When the outdoor sunlight intensity is the same as the simulated light intensity, the OETC system shows almost the same thermal control effect. The entire process requires no external energy supply, achieving self-sufficient thermal regulation with zero energy consumption.
Although an external power source was previously required to power the EC device in order to achieve effective thermal management reported in the literature, the authors demonstrated that the EC device can actually receive power directly in the field through an integrated flexible OPV module. The integrated device exhibited the same excellent performance, including the same temperature difference (ΔT) under the same electric field (Figure 2D).
Point 4: Excellent sustainable performance of OETC system and performance of OPV-EC array
The authors compared the thermal regulation performance of an OETC system powered by a flexible OPV module at an irradiation intensity of 100 mW/cm2 with a commercial rigid TE device of the same size as the EC device. Under the same irradiation intensity, the temperature span and heat flow of the OETC system are 2.9 K and 28.76 mW/cm2, respectively, while the OPV-TE system only has a temperature span of 1.2 K and a heat flow of 16.79 mW/cm2. In addition, the authors also compared the thermal regulation performance of OETC systems powered by OPV modules with perovskite photovoltaic modules of the same size as EC devices. Compared with the thermal management performance of the EC device powered by OPV modules in Figure 2C, the EC device powered by perovskite photovoltaic modules showed almost the same results. At the same time, the author calculated the energy consumption of the EC device under different irradiation intensities, and the results showed that at an irradiation intensity of 100 mW/cm2, the average energy consumption of the EC device was only 1.91 mW/cm2 due to its low energy consumption.
Considering that the photoelectric conversion efficiency of this OPV module is 11.85% under standard AM 1.5G (100 mW/cm2), while the energy consumption of the EC device is only 15.28 mW (1.91 mW/cm2 × 8 cm2 = 15.28 mW), a simple The estimate indicates that the total power generation is 298.58 mW (100 mW/cm2 × 11.85% × 25.2 cm2 = 298.58 mW). Therefore, the authors benefit from the low energy consumption of the EC device (15.28 mW), and the excess 283.30 mW (298.58 mW − 15.28 mW = 283.30 mW) energy can be stored in the ESS under ideal conditions. The excess energy stored in the ESS can be automatically switched at night to power the entire system, enabling an all-weather thermal regulation cycle. In addition, it is worth noting that energy recovery can also be achieved during the depolarization process of the EC effect, further improving the efficiency of the OETC system.
The EC device has good array cooperability, and a single OPV module with a size of 25.2 cm2 has enough power to drive two 16 cm2 EC device arrays simultaneously. For example, under standard AM 1.5G (100 mW/cm2), these two EC parallel devices can be fully synchronized and both can reach a temperature span of 2.9 K, demonstrating their bidirectional thermal regulation performance. To further extend its application in wearable thermal regulation, the authors also evaluated the performance of four parallel EC arrays driven by one OPV module under 100 mW/cm2 irradiation intensity. Four parallel EC arrays can simultaneously achieve bidirectional controllable thermal regulation, indicating that this OETC system has good scalability required in practical wearable thermal regulation.
Point 5: OETC’s thermal regulating properties on the human body
Figure 3 Wearable temperature regulation performance of OETC
In order to demonstrate the wearable performance of OETC in meeting the flexible needs of human body thermal regulation, the authors measured the stable performance of OETC cooling and heating modes in the bent state. During operation, the authors observed minimal changes in the thermal regulation properties of the OETC in the flat, bent, and released states, demonstrating excellent flexibility.
The authors further applied flexible OETC to human skin for thermal regulation. At a light intensity of 100 mW/cm2, the ambient temperature is 26°C. The flexible OETC cools human skin from 36.8° to 31.7°C at an average rate of 6.1°C/min, achieving rapid thermal regulation.
The human body must remain within a certain temperature range (skin temperature) to ensure comfort and safety, but this range varies between individuals. The authors set a comfort range based on observed human skin temperatures (32° to 36°C), requiring an ambient temperature range of 22° to 28°C. Thermal regulation properties were measured directly on human skin, with an initial temperature of human hands of 34.0°C and a corresponding ambient temperature of 25.0°C (the midpoint of the comfort zone). Under standard AM 1.5G (100 mW/cm2), the skin is moved to a low temperature environment (12.5°C), the skin temperature drops to 29.2°C, and the OETC heating mode starts to work, raising the skin temperature to 32.0°C. Correspondingly, when the skin was moved to a higher temperature environment (37.6°C), the skin temperature increased to 38.3°C. OETC cooling mode activates, reducing skin temperature to 36.0°C.
Therefore, this OETC maintains the human skin temperature within a comfortable range between 32.0°C and 36.0°C when the ambient temperature changes between 12.5° and 37.6°C. Compared with bare human skin (comfort temperature range of 6 K), this OETC extends the skin's comfort temperature range by 19.1 K at this module size and light intensity. In addition, within the first 5 seconds, the skin can heat up at a maximum rate of 15.6°C/min or cool down at a maximum rate of 14.0°C/min, achieving rapid thermal regulation. When the irradiation intensity is lower than 100 mW/cm2 (75 or 90 mW/cm2), the OETC system still has bidirectional thermal regulation performance. Under a light intensity of 90 mW/cm2, the OETC heating mode can increase the skin temperature from 29.4° to 31.3°C, while the OETC cooling mode can reduce the skin temperature from 38.3° to 36.5°C. These temperatures are only slightly above the comfort range. When the light intensity is 75 mW/cm2, the OETC system can still increase the skin temperature from 29.4° to 30.8°C in heating mode and reduce the skin temperature from 38.3° to 37.2°C in cooling mode. The comfort temperature range of bare artificial skin is 23° to 27°C (4.0 K) (46); the OETC extended the comfort temperature range of artificial skin to 16.6 K. Although OETC cannot restore the temperature of the artificial skin to the comfort zone in harsher environments, it still has good thermal regulation properties. The comfort zone can be improved by improving the performance or efficiency of the OPV or EC unit. Additionally, the relative proportions and sizes of OPV and EC units can be further optimized.
Point 6: OETC’s outdoor thermal conditioning performance and its application prospects in space
The authors measured and compared the temperature changes of bare artificial skin, skin covered with cotton clothing, and skin covered with OETC at 100 mW/cm2, 26.0°C ambient temperature, and 0°C ambient temperature. Under standard AM 1.5G (100 mW/cm2) irradiation, at an ambient temperature of 26.0°C, the temperature of bare skin and skin covered with cotton clothing can increase from 34.0°C to 50.9°C and 48.4°C, respectively. However, the temperature of the artificial skin covering the OETC is only 40.8°C. The refrigeration capacity of OETC reaches 10.1 K, showing excellent cooling effect. In addition, the OETC can also be driven to heat the skin at 0°C ambient temperature by using ESS. Compared with bare artificial skin, the heating performance of artificial skin covered with OETC is 3.2 K higher than that of covered cotton clothing and bare skin, demonstrating its superior heating ability.
Bidirectional thermal regulation using solar energy makes the OETC system potentially applicable to traditional space suits, helping to reduce total power consumption. In individual space travel, the theoretical area of the spacesuit is approximately 1.85 m2. With the continuous improvement of solar cell performance, including flexible OPV modules, assuming the use of solar cell equipment with a photoelectric conversion efficiency of 45%, the author estimates that the OPV module area required to provide all-weather human body thermal regulation is only 1.12 m2. The author believes that the OETC system can be optimized in terms of performance and practicality in the future to adapt to harsher environments. To improve the thermal regulation performance of the OETC system, the temperature span of the EC device can be increased. First, in terms of materials, the double-bond modified P(VDF-TrFE-CFE) material can provide a larger temperature change of 7.8 K at 118 MV/m. Secondly, by using a cascade device to optimize the device, the temperature span can be increased to 4.8 K (double layer) and 8.7 K (quadruple layer cascade). Finally, the EC performance can be further improved by adding nanofillers to increase the thermal conductivity of P(VDF-TrFE-CFE) or using an active EC regenerator to further increase the temperature span. Clearly, further research is needed to develop practical products based on the prototypes and concepts demonstrated in this study.
Figure 4 Thermoregulatory properties of OETC compared to cotton clothing and prospects for personal space travel
4. Summary
The authors developed an advanced self-powered wearable thermal regulation system that integrates a flexible OPV module and an EC thermal regulation unit to achieve efficient personalized thermal regulation. Its active control feature allows for rapid cooling/heating dual-mode thermal regulation based on human needs. In addition, OETC expands the thermal comfort zone from 6.0 to 25.1 K through rapid thermal adjustment, ensuring human body safety and comfort in various complex and unstable environments. Due to the advantage of low energy consumption of EC devices, OETC can achieve controllable all-weather dual-mode thermal regulation. In addition, OETC also has outstanding features such as simple and compact structure, high efficiency, and strong adaptability. With more optimization, the authors believe that OETC has potential applications in the field of high-end thermal regulation, and can even extend human survivability in extreme environments such as polar regions and individual space walks.
5. References
Ziyuan Wang et al., Self-sustaining personal all-day thermoregulatory clothing using only sunlight. Science 382,1291-1296(2023).
DOI:10.1126/science.adj3654
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