Solar Spectral Beam Splitting Simulation of Aluminum-Based Nanofluid Compatible with Photovoltaic Cells

Abstract

Solar photovoltaic/thermal (PV/T) systems can simultaneously solve PV overheating and obtain high-quality thermal energy through nanofluid spectral splitting technology. However, the existing nanofluid splitting devices have insufficient short-wavelength extinction and stability defects. To achieve the precise matching of the nanofluid splitting performance with the optimal spectral window of the PV/T system, this paper carries out a relevant study on the optical properties of Al nanoparticles and proposes an Al@Ag nanoparticle. The optical behaviors of nanoparticles and nanofluids are numerically analyzed using the finite-difference time-domain (FDTD) method and the Beer–Lambert law. The results demonstrate that adjusting particle size enables modulation of nanoparticle extinction performance, including extinction intensity and resonance peak range. The Al@Ag core–shell structure effectively mitigates the oxidation susceptibility of pure Al nanoparticles. Furthermore, coating Al nanoparticles with an Ag shell significantly enhances their extinction efficiency in the short-wavelength range (350–640 nm). After dispersing Al nanoparticles into water to form a nanofluid, the transmittance in the short-wavelength range is significantly reduced compared to pure water. Compared to 50 nm pure Al particles, the Al@Ag nanofluid further reduces the transmittance by up to 13% in the wavelength range of 350–650 nm, while having almost no impact on the transmittance in the photovoltaic window (640–1080 nm).

1. Introduction

In modern society, energy shortage and environmental pollution have prompted people to develop various renewable energy sources. Among them, solar energy, as a green energy source with wide distribution and huge reserves, is expected to replace fossil fuels to meet the energy demand of human beings and provide reliable and sufficient energy supplies [1,2]. However, due to the special spectral response characteristics of photovoltaic (PV) cells, only a portion of the solar radiation can be converted into electrical energy, while the rest is dissipated in the form of heat [3], and the unwanted heat leads to the overheating of PV cells, which further reduces their power generation efficiency [4]. In order to solve the above problems, it has been proposed to introduce the spectral beam splitting (SBS) technique into the PV/T system [5,6,7,8], which divides the solar radiation into two parts, one of which splits into the wavelength bands needed by the PV cell, while the remaining wavelength bands are converted into heat energy by the collector. The PV cell and collector are spatially separated from each other, avoiding the thermal coupling of the system, and the solar thermal and photovoltaic conversion can be carried out at different temperatures, which enables the PV cell to keep working efficiently while obtaining high-temperature thermal output, thus realizing the efficient utilization of the full spectrum of solar energy.

According to the principle of SBS, it can be divided into solid film interference beam splitter, liquid absorptive beam splitter, fluorescence beam splitter, and holographic beam splitter, etc. [4,5,9]. Among these, solid film interference beam splitters and liquid beam splitters are commonly used in CPV/T systems. The solid film interference beam splitter consists of periodically arranged materials with different refractive indices. When sunlight irradiates the splitter, it exhibits high reflectivity and transmittance for specific wavelength bands. While offering advantages such as high splitting efficiency and long service life, this approach suffers from complex fabrication processes, high costs, and difficulties in precisely controlling the light incidence angle. Thereby, this approach is not significant for PV/T systems in terms of either economy or overall efficiency improvement [10,11]. In contrast, liquid beam splitters allow for the high transmittance of the spectral band required by photovoltaic cells while directly absorbing and converting the remaining spectrum into heat, thereby avoiding thermal losses associated with multiple heat exchange processes [12]. However, conventional splitting liquids can typically only absorb near-infrared solar radiation, failing to achieve precise spectral splitting for the “ideal splitting window” required by photovoltaic cells, which hinders system performance improvement [13,14]. Looser et al. [15] suggested that for silicon PV cells in PV/T systems, the ideal spectral window should be 640–1080 nm, while other bands (<640 nm and >1080 nm) should serve as the photothermal window. Therefore, selecting appropriate splitting liquids and effectively regulating their spectral characteristics to match the spectral response of photovoltaic cells has become a key research focus. Nanoparticles possess unique light absorption properties, and the incorporation of nanoparticles into conventional liquids to form nanofluids offers a potential solution for controllable spectral splitting. This constitutes the fundamental significance of the present study.

In recent years, the unique localized surface plasmon resonance (LSPR) effect of metal nanoparticles has been generally emphasized in the field of near-field optics [16,17,18] and the field of enhanced fluid thermal conductivity [19,20]. Zhang et al. [21] synthesized Ag cubic-structured nanoparticles with a size range of 36–172 nm using the seed growth method and found an approximately linear relationship between the position of the LSPR absorption peak and the particle size. Han et al. [22] explored the performance of Ag/water and Ag/CoSO₄ nanofluids as spectral splitters for PV/T systems, and the results showed that Ag/CoSO₄ nanofluids exhibited superior spectral splitting performance, as well as an increase in the economic value of 35.9% compared to the PV cell alone. Wang et al. [23] tested the Ag/water nanofluid performance in a concentrating photovoltaic system, and the results showed high transmittance of the nanofluid at 500–1100 nm, with thermal and electrical efficiencies of 56% and 8.5%, respectively. Saroha et al. [24] tested the spectral splitting performance of water-based Au and Ag nanofluids. Although the Ag nanofluid demonstrated higher solar energy conversion efficiency, its spectral response range was relatively narrow and could not be effectively tuned according to practical application requirements. Abdelrazika et al. [25,26] prepared nanofluids by dispersing Ag nanoparticles and reduced graphene oxide-coated Ag nanoparticles (RGO-Ag) in water. Their results showed that the stability of the nanofluids gradually decreased over time, with higher nanoparticle concentrations leading to more pronounced stability degradation.

However, the absorption bandwidth of single-component LSPR nanoparticles tends to be limited to only a very narrow wavelength range, with low applicability and popularity to match the spectral frequency splitting requirements of specific applications [27,28]. Therefore, it is necessary to obtain nanoparticles with special structures through rational structural design to achieve continuous tunability of the optical properties of the particles. Most of the current studies are based on noble metal materials such as Au and Ag, because Au and Ag have excellent extinction properties in the long wavelength band; however, the ideal photovoltaic band of photovoltaic cells in a SBS-CPV/T system does not include the UV and near-UV bands, and the equipartition excitation resonance properties of the Au and Ag nanoparticles will not occur in the near-UV bands, so noble metal materials such as Au and Ag are not perfect nanomaterials.

Therefore, to enhance the absorption capability of the spectral splitting device in the short-wavelength range (200–640 nm), and to achieve a perfect match between the spectral splitting characteristics of the nanofluid and the ideal spectral window of the PV/T system, this paper selected Al, which has excellent LSPR characteristics in the near-ultraviolet wavelength band, as its research object, and water, which can absorb incident energy at longer wavelengths, as the base fluid, using the time-domain finite-difference method and the established optical transmittance model to simulate and analyze the effects of different particle sizes, core–shell structures, and optical paths on the optical properties of Al nanoparticles and fluids, providing theoretical guidance and optimization directions for finding nanofluids compatible with photovoltaic cells.

2. Numerical Simulation

2.1. Calculation Method and Physical Model

When the frequency of incident light matches the oscillation frequency of electrons, it induces a collective resonance of conduction electrons in the metal. This collective vibrational effect occurring inside metallic nanoparticles is termed localized surface plasmon resonance. The interaction between incident light and nanoparticles manifests as the propagation of light waves in nanoparticles in the form of electromagnetic waves. This phenomenon primarily arises from the reflection, transmission, and absorption of electromagnetic waves by the nanoparticles, which can be mathematically described by Maxwell’s equations [29]:

$$\nabla \times H=\epsilon {\displaystyle \frac{\mathit{\partial }E}{\mathit{\partial }t}}+\sigma E$$

(1)

$$\nabla \times E=-\mu {\displaystyle \frac{\mathit{\partial }H}{\mathit{\partial }t}}$$

(2)

where H and E are the magnetic field strength (A/m) and electric field strength (V/m), respectively; ε, μ, and σ are the particle permittivity (F/m), magnetic permeability (Ω/m), and electric conductivity (S/m), respectively; and t is the time (s).

The finite-difference time-domain (FDTD) is a differential representation of Maxwell’s system of equations that employs interleaved sampling in the space and time of the electric and magnetic field nodes and is used to solve problems related to the propagation of electromagnetic waves in a medium [30,31]. This method is now widely used in the nanophotonics community to effectively model the interaction of light with various materials and optical devices and can be used to study the optical properties of various materials, such as reflectivity, transmittance, absorption, and interference. In this paper, the optical properties of Al nanoparticles have been studied using FDTD Solutions software (version 2020R2).

In the calculations, two physical structure models of Al nanoparticles and Al@Ag core–shell nanoparticles are established, as shown in Figure 1. The perfectly matched layer (PML), originally proposed by Berenger in 1994, is an absorbing boundary condition capable of theoretically absorbing incident electromagnetic waves at arbitrary angles and wavelengths without reflection, thereby ensuring computational accuracy [32]. By implementing an absorbing layer outside the computational domain that perfectly matches the wave impedance of the medium, PML achieves reflectionless absorption of electromagnetic waves. Therefore, PML boundary conditions were used in the x, y, and z directions. The simulation domain should fully encompass the entire modeled region while avoiding excessive size that would increase computational memory and processing requirements. Based on these considerations, the simulation domain size in this study was set to 400 nm × 400 nm × 400 nm. The total-field/scattered-field (TFSF) source applies to both isotropic and anisotropic absorbing materials. When implementing the TFSF source, the following size hierarchy must be satisfied: simulation domain > scattered-field region ≥ mesh size > light source > total-field region > structure. Importantly, the scatterer (e.g., nanoparticles) must be entirely contained within the total-field region of the TFSF source. In this work, we employ the TFSF source to analyze the absorption and scattering characteristics of nanoparticles in both the ultraviolet and photovoltaic window spectral ranges. The wavelength range was set to 200–1000 nm because Al nanoparticles do not have extinction ability for long wavelength bands. The polarization direction of the light source is perpendicular to the YZ cross-section, and the incident direction is perpendicular to the XY cross-section pointing in the positive direction of the z-axis. The absorption cross-sectional area, scattering cross-sectional area and electric field distribution of the nanoparticles were obtained by inserting a monitor.

The absorption cross-sectional area $C_{\mathrm{abs}}$ (m²) and scattering cross-sectional area $C_{\mathrm{sca}}$ (m²) of the nanoparticles are obtained by FDTD simulation, and the sum of the two is defined as the extinction cross-sectional area $C_{\mathrm{np}}$ (m²) of the nanoparticles.

$$C_{\mathrm{np}}=C_{\mathrm{abs}}+C_{\mathrm{sca}}$$

(3)

In order to characterize the extinction capacity per unit area of metal nanoparticles, the parameter of extinction cross-sectional area of nanoparticles needs to be dimensionless, and here, the ratio of extinction cross-sectional area to the cross-sectional area of the particle perpendicular to the direction of incidence of the light is defined as the extinction efficiency, which is expressed by Equation (4):

$$Q_{\mathrm{np}}={\displaystyle \frac{C_{\mathrm{np}}(\lambda )}{S_{\mathrm{np}}}}$$

(4)

where $S_{\mathrm{np}}$ is the nanoparticle cross-sectional area in the direction of incidence of perpendicular light (m²).

In order to ensure that enough solar radiation can reach the surface of the PV cell through the beam splitter, the volume fraction of nanoparticles in the nanofluid is generally not more than 0.6%, and the scattering between particles is negligible at such a low volume fraction [33,34], and the extinction coefficients of the nanoparticle population are as follows:

$${\sigma }_{\mathrm{np}}={\displaystyle \frac{3f_v{Q}_{\mathrm{np}}}{2D}}$$

(5)

where $f_v$ is the volume fraction of nanoparticles; $Q_{\mathrm{np}}$ is the extinction efficiency of nanoparticles; and $D$ is the total particle size of nanoparticles (m).

When calculating the extinction coefficient of the base solution, the scattering effect is very weak, and only the absorption of light needs to be considered; hence, the extinction coefficient of the base solution can be expressed as follows:

$${\sigma }_{\mathrm{bf}}={\displaystyle \frac{4\pi \kappa }{\lambda }}$$

(6)

where $\kappa$ represents the absorption index of the base fluid (m⁻¹); and $\lambda$ represents the wavelength of the light (m), where the absorption index of water in the 200–2500 nm band is available in the optics handbook [35].

The optical properties of the nanofluid are jointly determined by the optical properties of the nanoparticles as well as the base fluid; therefore, the extinction coefficient of the nanofluid can be obtained by adding the extinction coefficient of the base fluid and the extinction coefficient of the nanoparticles.

$${\sigma }_{\mathrm{total}}={\sigma }_{\mathrm{bf}}+{\sigma }_{\mathrm{np}}$$

(7)

Finally, the formula for the spectral transmittance of the nanofluid is derived from the Lambert–Beer law [36]:

$${\tau }_{\mathrm{fluid}}={\displaystyle \frac{I}{I_0}}={\mathrm{e}}^{-L{\sigma }_{\mathrm{total}}}$$

(8)

where $I$ and $I_0$ are the solar radiation transmitted and absorbed by the nanofluid, respectively (W/m²); and L is the optical path (mm).

2.2. Model Validation

Although FDTD Solutions software has been widely used in the simulation of nanoscale optical components, the absorption and scattering characteristics of 60 nm Al metal nanoparticles in the 200–1000 nm band are further simulated and analyzed using FDTD Solutions in this study in order to ensure the accuracy and reliability of the calculation. The extinction efficiency of Al nanoparticles was obtained by monitoring the absorption cross-sectional area and scattering cross-sectional area in each band under a vacuum background; then, the simulation results were processed to obtain the extinction efficiency of Al nanoparticles. And the above calculations are validated using the classical Mie theory. It is worth noting that Mie theory is only capable of calculating the absorption and scattering properties of conventional spherical nanoparticles, whereas FDTD is capable of calculating a wide range of complex shaped structures, which is a major advantage of the FDTD method. As shown in Figure 2a, the maximum deviation of the calculation results of the two methods is about 2.8%, so it is considered to be reasonable and reliable to calculate the absorption and scattering characteristics of nanoparticles using the FDTD method. As shown in Figure 2b, Rycenga et al. [18] utilized Mie theory to analyze the extinction cross-section of Ag nanoparticles with a diameter of 40 nm. Compared with our FDTD calculations, the peak position deviation is approximately 4.6%, and the extinction area deviation is about 12.1%. These errors are acceptable, further validating the reliability of our computational model.

3. Results and Discussion

3.1. Effect of Particle Size on Extinction Characteristics of Al Nanoparticles

The absorption cross-sectional area and scattering cross-sectional area of nanoparticles of six particle sizes (30 nm, 40 nm, 50 nm, 60 nm, 80 nm, and 100 nm) were calculated using FDTD Solutions software, and the nanoparticle extinction efficiencies were obtained using Equations (3) and (4), as shown in Figure 3. It can be seen that the extinction efficiency curves of nanoparticles are not the same when the particle size of spherical Al nanoparticles is increased from 30 nm to 100 nm. In general, in small particles, there are only dipole excitations, whereas in large particles, there may be dipole and multipole excitations. The wavelength of the location of the resonance peak of Al nanoparticles increases with increasing particle size, indicating that the extinction properties of Al nanoparticles are affected by the nanoparticle size, which is due to the fact that the wavelength of the incident light is close to the particle size when the nanoparticles are large in size, which triggers the advanced vibrational modes of the nanoparticles. At the same time, the bandwidth of the resonance peaks increases with the particle size due to the fact that different polar vibrations have different energy peaks in the nanoparticles. Under the excitation of incident light, nanoparticles can achieve strong plasmon resonance, which is reflected in the spectrum as a pronounced absorption peak at a specific wavelength for metal nanoparticles. When the size of the nanoparticles is significantly smaller than the wavelength of the incident light, a broad and intense absorption band appears in the visible spectrum, resulting in the “hysteresis effect”. In addition, it can be found that as the particle size increases to 60 nm, two resonance peaks begin to appear in the figure, located at the wavelengths of 295 nm and 374 nm, respectively. This is because when the particle size of nanoparticles is much smaller than the wavelength of incident light, the interaction of nanoparticles with light has only dipole vibration modes, which means that there is only one dipole resonance peak on the extinction efficiency curve. However, as the particle size continues to increase, more multilevel vibrational modes at different levels are exciting, such as quadrupole resonance, octupole resonance, etc.

Figure 4 represents more intuitively the relationship between the resonance peak position and resonance peak bandwidth of Al nanoparticles and particle size, and it can be seen that the wavelength corresponding to the resonance peak of the nanoparticles shows an approximately linear relationship with the particle size, and the wavelength of the resonance peak increases from 226 nm to 576 nm when the particle size is increased from 30 to 100 nm. The linear slope of the resonance peak wavelength with the increase in the particle size is about 5. The resonance peak bandwidth increases from 84 nm to 320 nm, which is also nearly linear. Therefore, Al nanoparticles can produce an LSPR effect in the near-ultraviolet band; the particle size of the nanoparticles affects the extinction properties of the particles; the position of the resonance peak will be red-shifted with the increase in the particle size; and the extinction intensity will be reduced. Thus, changing the particle size can play a certain role in regulating the extinction properties of the nanoparticles.

Figure 5 shows the electric field distributions on the surface and in the vicinity of the spherical Al nanoparticles with different particle sizes. It can clearly be seen that a great near-field enhancement of the local equipartitioned exciton resonance effect is produced on the particle surface. This indicates that the Al nanoparticles are able to interact strongly with the incident light in the short wavelength band, leading to an enhancement of the local electric field, which, in turn, improves the absorption and scattering ability of the nanoparticles, as can also be seen from the extinction efficiency of the particles. From Figure 5, it can be seen that the electric field strength on the particle surface decreases gradually with the increase in particle size, from 3.24 at 30 nm particle size to 1.87 at 60 nm particle size, which corresponds to the decrease in the maximum value of the extinction efficiency of the nanoparticles with the increase in the particle size in Figure 3. Therefore, when modulating the position of the resonance peaks of the nanoparticles by changing their size, it is also important to consider that the increase in particle size decreases the extinction efficiency of the particles.

3.2. Effect of Core–Shell Structure on Extinction Characteristics of Nanoparticles

Considering that Al nanoparticles are easy to be oxidized, the dense oxide layer will affect their optical properties, and the core–shell structure nanoparticles can help to solve this problem. Meanwhile, when the core–shell of the nanoparticles are both metals, the resonance situation can cause the electric field enhancement inside the particles, and the optical nonlinear enhancement of the composite system of bimetallic core–shell-structured nanoparticles can be maximized by appropriately adjusting the core–shell ratio. For this reason, the optical properties of Al@Ag nanoparticles were investigated, and the extinction efficiency of the nanoparticles was analyzed for different shell thicknesses (where the particle size of the Al core was set as 50 nm and the Ag shell thicknesses were set as 2 nm, 4 nm, 6 nm, 8 nm, and 10 nm, respectively). Figure 6 gives comparisons of extinction efficiency of core–shell and single Al nanoparticles. It is shown that the extinction efficiency of nanoparticles in the 200–350 nm band gradually decreases with the increase in the thickness of the Ag shell layer. In contrast, the extinction efficiency of core–shell nanoparticles in the 350–640 nm band showed an opposite trend, where the extinction efficiency of the particles gradually increased with the increase in the thickness of the shell layer, and the bandwidth of the resonance peaks also gradually increased. The extinction efficiency of the resonance peak increases from 4.31 to 5.18 when the thickness of the shell layer is increased from 0 nm to 10 nm, but the trend of the increase gradually decreases, which can be visualized in Figure 7.

In Figure 6, it can be seen that the extinction efficiency of Al particles coated with Ag core–shells is significantly improved in the 350–640 nm band compared with that of single Al nanoparticles with a particle size of 50 nm. It can also be seen that the extinction efficiency of core–shell nanoparticles with a shell thickness of 10 nm (total particle size of 60 nm) is significantly higher than that of single Al nanoparticles with the same particle size in the 350–640 nm wavelength band and remains almost unchanged in the wavelength band larger than 640 nm. This is the effect that we would like to see, where the extinction efficiency is improved in the bands where the energy distribution is high, without affecting the high efficiency bands of photovoltaic power generation. The reason for the improved extinction efficiency of the core–shell structure can be effectively explained based on the electric field distribution in Figure 8. From the figure, it can be seen that the electric field strength of the core–shell nanoparticles is about twice as much as that of the single Al nanoparticles, which indicates that the nanoparticles after wrapping the Ag layer achieve a near-field enhancement of the localized surface plasmon resonances, which is able to enhance interactions with the light between the nanoparticles, thus leading to the increase in the extinction efficiency of the particles.

3.3. The Spectral Properties of Nanofluids

The optical properties of nanofluids are jointly determined by the optical properties of the base fluid and the nanoparticle population. By analyzing the optical properties of nanofluids at different particle sizes and different types, it is found that the extinction properties of nanoparticles can be reflected in the optical properties of nanofluids, and the changes of particle sizes and types can cause the nanofluid transmittance to change, as shown in Figure 9. The variation of transmittance with the wavelength of the nanofluids formed by dispersing Al@Ag with a shell thickness of 10 nm and single Al nanoparticles with particle sizes of 50 nm and 60 nm in water at a 50 mm optical path and 0.6% volume fraction was analyzed. Upon comparison, it can be seen that the spectral transmittance of the core–shell nanofluid increases near the 200–350 nm band and decreases near the 350–640 nm band, with a maximum decrease of about 13% compared to the nanofluid with a 50 nm particle size, and up to about 10% compared to the nanofluid with a 60 nm particle size, while there is no significant change in the transmittance in the bands larger than 640 nm. This indicates that wrapping Ag around the outer layer of Al enhances the extinction ability of the particles near the 350–640 nm band, which happens to be the thermally utilized band in the ideal splitting window of the PV/T system (640–1080 nm [15]), and at the same time, it has almost no effect on the transmittance of the photovoltaic window (640–1080 nm); thus, the efficiency of the system can be effectively improved.

In addition to the effect of nanoparticles, optical path, as a factor in the propagation of light through the fluid, also affects the transmission of radiation. Generally, the greater the optical path, the more the attenuation, so the optical path is also one of the important parameters affecting the optical properties of the nanofluid. As shown in Figure 10, the spectral transmittance of nanofluids at different optical paths (10 mm, 30 mm and 50 mm, respectively) was analyzed at a constant nanoparticle size of 60 nm and a nanofluid volume fraction of 0.6%. It can be seen that the spectral transmittance of the nanofluid decreases in the whole band with the increase in the optical path, which is consistent with the greater attenuation of electromagnetic wave radiation with the larger optical path. Meanwhile, in Figure 10, it can be seen that pure water has almost no extinction ability for light in the 300–700 nm band, so the decrease in the transmittance of nanofluid in this band is determined by the extinction performance of nanoparticles. And the extinction efficiency of Al nanoparticles in the rest of the band is very low, and the trend of the curve coincides with the trend of the transmittance of pure water. Therefore, the addition of Al nanoparticles in water reduces the spectral transmittance of the photothermal window (200–640 nm) in a wide range, which can effectively reduce the radiation reaching the PV panels, reduce the heat reaching the unavailable wavelengths of the PV cells, and reduce the surface temperature of the PV modules.

The volume fraction of nanoparticles also has an effect on the optical properties of the nanofluid. Figure 11 shows the spectral transmittance of nanofluids with different volume fractions by varying the volume fraction of nanoparticles in the nanofluid by 0.6%, 0.5%, and 0.4% while keeping the nanoparticle particle size (60 nm) and optical path (30 mm) constant, respectively. It is found that the spectral transmittance of the nanofluid in the 200–700 nm band decreases gradually with the increase in the volume fraction of nanoparticles in the nanofluid, and the transmittance in the photovoltaic window band decreases very little. This is determined by the extinction properties of Al nanoparticles, and increasing the volume fraction of the particles means increasing the concentration of the nanofluid and enhancing the extinction properties of the particles. The spectral transmittance in the long wavelength band (700–1000 nm), on the other hand, hardly changes, which is due to the weak extinction ability of Al nanoparticles in this band, coupled with the constant optical path, therefore reflecting the optical properties of the base liquid (water). Overall, increasing the volume fraction of nanoparticles decreases the spectral transmittance of the nanofluid at short wavelengths, while ensuring higher transmittance in the photovoltaic window band. Therefore, a higher volume fraction should be ensured as much as possible when using Al nanofluids as a beam splitter.

4. Conclusions

To achieve the precise matching of the nanofluid splitting performance with the optimal spectral window of the PV/T system, this paper carries out a relevant study on the optical properties of Al nanoparticles and proposes an Al@Ag nanoparticle. The optical behaviors of both individual nanoparticles and corresponding nanofluids were numerically analyzed using the finite-difference time-domain (FDTD) method combined with the Beer–Lambert law. The key findings are summarized as follows:

(1)

Changing the particle size enables the modulation of the extinction properties of nanoparticles, including extinction intensity and resonance peak range. Al nanoparticles with particle sizes of 50 nm and 60 nm have improved extinction in the photothermal window (200–640 nm).

(2)

The Al@Ag core–shell nanoparticle design effectively addresses the issue of rapid oxidation of pure Al nanoparticles in ambient conditions. Moreover, coating Al with a thin Ag shell significantly enhances extinction efficiency in the short-wavelength range (350–640 nm).

(3)

After dispersing Al nanoparticles into water to form a nanofluid, the transmittance in the short-wavelength range is significantly reduced compared to pure water. Compared to 50 nm pure Al particles, the Al@Ag nanofluid further reduces the transmittance by up to 13% in the wavelength range of 350–650 nm, while having almost no impact on the transmittance in the photovoltaic window (640–1080 nm).

From an application perspective, the use of Al@Ag nanofluids offers multiple advantages: it compensates for the insufficient extinction of traditional spectral splitting devices in the short-wavelength region, reduces the reliance on expensive noble metals, and improves the chemical stability and long-term durability of the working fluid. These features contribute to PV/T system efficiency and operational reliability.