By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 2 2024A recent article published in the Journal of Materials Chemistry proposed an in situ switchable photoselective polymer (PSP) comprising alternating light-reflecting and light-absorbing layers. This asymmetric assembly prepared by electrospinning can aid in year-round energy saving in buildings.
Study: Dynamic Photoselective Polymer for Building Energy Management. Image Credit: Audio und werbung/Shutterstock.com
Background
Thermal management in buildings consumes 51% of the world’s energy use. Intelligent thermal management systems can reduce this energy consumption. Such systems combine solar thermal technologies with radiative cooling for buildings towards zero-energy consumption and low carbon emissions.
However, simultaneous utilization of solar thermal and radiative cooling under static conditions is challenging due to opposite operational principles. Additionally, materials with fixed properties cannot flexibly respond to daily and seasonal temperature changes, resulting in overheating or overcooling.
Dynamic thermal management strategies employing smart temperature- or mechanical-responsive materials with in-situ switching capabilities offer promising solutions. However, most existing materials exhibit long switching times and fixed switching temperatures.
Thus, this study proposed a novel composite PSP film using electrospun nanofibers of polymers with contrasting properties for rapid in-situ switching between highly reflective (radiative cooling) and highly absorbing (solar heating) regimes.
Methods
Polypyrrole-polyimide (PPy-PI) and polyacrylonitrile (PAN) were used to prepare the PSP due to their contrasting optical properties. Firstly, PPy-PI film was prepared by polymerizing pyrrole monomer on electrospun nanofibers on PI. Subsequently, a PAN fiber film layer was spun over the PPy-PI film.
The fabricated PSP films were dried at 60 °C for three hours to remove residual solvent. In addition, an index-matching liquid (ethanol) was introduced into the top light-reflecting PAN nanofibers layer to realize the dynamic switching of the photothermal properties of the PSP film.
The outdoor thermal performance of the in-situ PSP film was evaluated through a thermal test conducted on the roof of a college in Harbin on June 26, 2021. The morphologies of all specimen surfaces and sections were assessed by scanning electron microscopy (SEM).
The solar spectral reflectance of the PSP specimens was analyzed using an ultraviolet-visible-near infrared spectrophotometer while their infrared spectral reflectance was determined using a Fourier-transform infrared spectrometer. Additionally, thermal images of the samples were captured using an infrared thermal imager.
Furthermore, energy-saving simulations were performed on EnergyPlus 9.2.033 software. From these computations, the equivalent CO2 emission reductions and corresponding power savings achievable by applying the proposed PSP films on the roof of a building (2224.52 m2 in size with a window-to-wall ratio of 20%) were estimated.
Results and Discussion
Sequential assembly of electrospun nanofibers of PAN and PPy-PI resulted in a two-layer PSP film. The comparable refractive indices of PAN and PPy-PI (above 1.5) resulted in substantial optical contrast with air, enabling intense light scattering.
PAN was chosen as the top layer of the PSP film as it has almost zero extinction coefficient (low absorbance) at solar wavelengths. Alternatively, PPy-PI with a much larger extinction coefficient was employed as a highly absorbing layer at the bottom. Cross-sectional SEM images of the PSP film revealed different morphologies of the two layers due to distinct raw materials and pre-treatment processes.
The PSP film exhibited a highly porous structure resulting from multiple micrometer-sized air gaps between the adjacent nanofibers. Such microstructure facilitated fluid transport (almost unaffected by diffusion) through the film to perform a full cycle (cooling/heating/cooling) switching within five minutes. Specifically, switching from the radiative cooling to the solar heating state upon ethanol injection took only 15.5 seconds.
In the outdoor thermal performance tests, the temperature of the PAN layer (device performance at the cooling state) decreased by 2.4 °C from the average ambient temperature. Alternatively, the temperature of the PPy-PI material increased by 2.3 °C (device performance at the heating state) compared to the ambient. Overall, the PSP film exhibited enhanced solar light reflectance (97.7%), broadband emissivity (94.9%), and radiative cooling power (111.1 W/m2).
The numerical calculations further highlighted the fabricated PSP film's high potential in the thermal management of buildings located at high latitudes. It could help realize energy savings of up to 89.74 GJ/m2 and reduce CO2 emissions to 21.69 kg/m2 in typical Chinese cities at high latitudes.
Conclusion
Overall, the researchers successfully demonstrated the composite PSP film's rapid in-situ switching capabilities. Electrospinning was used for simple and scalable production of the two-layer PSP film using light-reflecting PAN and light-absorbing PPy-PI.
The dynamic switching of the PSP film between radiative cooling and solar heating was realized by injecting ethanol as an index-matching liquid. Outdoor tests revealed PSP’s heating (cooling) capability by 5.1°C (9.4°C) from the ambient temperature while switching to heating (cooling) states.
Notably, the proposed temperature management method for material production and photoselective switching consumed very little energy. The researchers claim that the PSP films can be adapted to regional environments, paving the way for sustainable thermal regulation in carbon-neutral buildings.
Journal Reference
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