The global pursuit of sustainable energy solutions has intensified the focus on harnessing solar power, one of the most abundant and promising renewable resources of the 21st century. Among various technologies, solar thermoelectric power generation (STEG) offers a direct method for converting thermal energy into electricity via the Seebeck effect, bypassing the need for mechanical intermediaries. This technology boasts significant advantages, including no moving parts, silent operation, environmental friendliness, and long operational life. However, its widespread commercial application has been hindered by relatively low conversion efficiency. A primary pathway to enhancing STEG performance lies in maximizing the temperature differential across the thermoelectric generator (TEG) modules. This work investigates a novel approach by integrating a spectrally selective absorption nanofilm on the hot side and a biomimetic heat dissipation structure on the cold side of a TEG system. The core innovation centers on replacing conventional commercial solar paints with a custom-engineered selective absorption nanofilm, which demonstrates superior photothermal conversion and infrared suppression capabilities. This study details the design, experimental characterization, and performance analysis of this enhanced STEG device, comparing it directly against a baseline system using commercial coatings under varying solar irradiance and incidence angles.
The fundamental principle of thermoelectric power generation is rooted in the Seebeck effect. When a temperature gradient is established across a junction of two dissimilar conductive or semiconducting materials, an electromotive force (EMF) is generated. The performance of a thermoelectric material is commonly evaluated by its dimensionless figure of merit, \(ZT\):
$$ZT = \frac{S^2 \sigma}{k} T$$
where \(S\) is the Seebeck coefficient (V/K), \(\sigma\) is the electrical conductivity (S/m), \(k\) is the thermal conductivity (W/(m·K)), and \(T\) is the absolute temperature (K). A higher \(ZT\) value indicates better material performance. For practical TEG modules composed of multiple p-n junctions, the open-circuit voltage \(U_{oc}\) is directly proportional to the applied temperature difference \(\Delta T\):
$$U_{oc} = \alpha \Delta T$$
where \(\alpha\) is the effective Seebeck coefficient of the module. The maximum output power \(P_{max}\) delivered to a matched load occurs when the load resistance equals the internal resistance \(R_{in}\) of the TEG:
$$P_{max} = \frac{U_{oc}^2}{4 R_{in}} = \frac{\alpha^2 \Delta T^2}{4 R_{in}}$$
Therefore, for a given TEG module, maximizing \(\Delta T\) is paramount for increasing power output. This research focuses on augmenting \(\Delta T\) through advanced thermal management at both interfaces of the TEG.
The experimental system was designed to simulate and measure STEG performance under controlled conditions. The hot-side assembly consisted of a copper plate serving as the solar absorber. For the enhanced device (hereafter referred to as C-TEG), this plate was coated with a sol-gel derived selective absorption nanofilm. This film was engineered to exhibit high absorptance (\(\gt 90\%\)) in the solar spectrum (300–2500 nm) and low emittance (\(\approx 20\%\)) in the mid-infrared range (2.5–25 μm), thereby efficiently capturing solar radiation while minimizing re-radiative losses. The baseline device (S-TEG) used an identical copper plate coated with a standard commercial solar paint. The absorber plate was attached to the hot side of a Bismuth Telluride (Bi₂Te₃) TEG module (TEG1-241) using a thermal interface material (TIM) to reduce contact resistance.
The cold side of the TEG was attached to a copper heat sink featuring a biomimetic surface pattern inspired by squid fin morphology, known to enhance convective heat transfer. This heat sink was housed within a water-cooled jacket. A low-temperature thermostatic bath maintained the cooling water at a constant inlet temperature of 0°C, which was circulated by a peristaltic pump. A xenon lamp solar simulator (CEL-S500/AM1.5) provided adjustable and uniform irradiation to the absorber surface. The system’s key performance parameters—temperatures at strategic points on the hot and cold sides, open-circuit voltage, and short-circuit current—were recorded using a data acquisition system (Keysight 34970A). The properties of the TEG module were first characterized to establish baseline parameters, yielding an average internal resistance \(R_{in} = 7.48 \, \Omega\) and an effective Seebeck coefficient \(\alpha = 0.06521 \, \text{V/°C}\).
The thermal and electrical performance of both C-TEG and S-TEG systems was evaluated under varying solar irradiance levels (500 to 800 mW/cm²) with normal incidence (0° angle). The temporal evolution of the hot-side temperature and open-circuit voltage revealed distinct behaviors. Both systems showed a rapid temperature rise upon illumination, reaching a quasi-steady state. Crucially, the C-TEG system consistently achieved a higher equilibrium hot-side temperature, approximately 2°C greater than the S-TEG system under identical conditions, directly attributable to the superior optical properties of the selective nanofilm.
The corresponding open-circuit voltage and temperature difference (\(\Delta T\)) across the TEG are summarized in Table 1. The data clearly shows that the selective nanofilm contributes to a consistently higher \(\Delta T\), which directly translates to a higher \(U_{oc}\). The maximum power output for each condition, calculated using the formula for \(P_{max}\), is also presented.
| Irradiance (mW/cm²) | System | Avg. \(\Delta T\) (°C) | Avg. \(U_{oc}\) (mV) | \(P_{max}\) (mW) | Power Increase (C-TEG vs. S-TEG) |
|---|---|---|---|---|---|
| 500 | C-TEG | 17.8 | 894 | 26.7 | 29.8% |
| S-TEG | 15.3 | 757 | 20.6 | ||
| 600 | C-TEG | 19.9 | 1038 | 36.0 | 23.5% |
| S-TEG | 18.2 | 952 | 29.2 | ||
| 700 | C-TEG | 22.5 | 1216 | 49.4 | 13.2% |
| S-TEG | 21.2 | 1153 | 43.6 | ||
| 800 | C-TEG | 24.6 | 1345 | 60.5 | 9.7% |
| S-TEG | 23.0 | 1287 | 55.1 |
The results demonstrate a significant performance enhancement from the nanofilm, particularly at lower irradiance levels. At 500 mW/cm², the C-TEG system produced 29.8% more power than the S-TEG system. This relative advantage decreases as irradiance increases but remains substantial. The diminishing relative improvement is likely due to the increasing dominance of other heat loss mechanisms (e.g., convection) at higher temperatures, which affects both systems similarly. Nevertheless, the absolute \(\Delta T\) and power output remain higher for C-TEG across all tested irradiances. This underscores the critical role of advanced optical coatings, a principle central to the development of high-efficiency thin film solar panels and absorbers, in low-concentration or non-concentrating STEG applications.
In real-world scenarios, fixed (non-tracking) STEG systems experience varying solar incidence angles throughout the day. To evaluate this effect, experiments were conducted at a constant high irradiance of 800 mW/cm² but with varying incidence angles (0°, 30°, 45°, and 60°). The effective irradiance on the absorber plate decreases with the cosine of the incidence angle. As expected, both the hot-side temperature and the resulting \(\Delta T\) decreased significantly with increasing angle for both systems.
The performance data under angular variation is compiled in Table 2. The selective absorption nanofilm again proves advantageous. While both systems suffer a performance drop, the C-TEG system maintains a higher \(\Delta T\) relative to the S-TEG at every angle. The power enhancement offered by the nanofilm becomes even more pronounced at steeper angles. At a 60° incidence, where received solar flux is halved, the C-TEG system outperformed the S-TEG by a remarkable 42.6%. This highlights a key strength of engineered selective surfaces: their ability to more effectively capture and retain low-intensity or oblique radiation, thereby stabilizing the energy output of systems like thin film solar panels and STEG devices under non-ideal lighting conditions.
| Incidence Angle (°) | System | Avg. \(\Delta T\) (°C) | Avg. \(U_{oc}\) (mV) | \(P_{max}\) (mW) | Power Increase (C-TEG vs. S-TEG) |
|---|---|---|---|---|---|
| 0 | C-TEG | 24.6 | 1345 | 60.5 | 9.7% |
| S-TEG | 23.0 | 1287 | 55.1 | ||
| 30 | C-TEG | 19.1 | 1121 | 42.0 | 18.3% |
| S-TEG | 17.5 | 1025 | 35.1 | ||
| 45 | C-TEG | 14.8 | 926 | 28.7 | 33.5% |
| S-TEG | 12.9 | 808 | 21.8 | ||
| 60 | C-TEG | 10.2 | 721 | 17.4 | 42.6% |
| S-TEG | 8.6 | 596 | 11.9 |
The underlying mechanism for the superior performance of the selective absorption nanofilm can be analyzed through a simplified energy balance on the absorber plate at steady state. The net heat flux into the TEG’s hot side, \(Q_{in}\), can be expressed as:
$$Q_{in} = A \left[ \alpha_s G \cos(\theta) – \epsilon \sigma_{SB} (T_h^4 – T_{amb}^4) – h_c (T_h – T_{amb}) \right]$$
where \(A\) is the absorber area, \(\alpha_s\) is the solar absorptance, \(G\) is the solar irradiance, \(\theta\) is the incidence angle, \(\epsilon\) is the thermal emittance, \(\sigma_{SB}\) is the Stefan-Boltzmann constant, \(T_h\) is the hot-side temperature, \(T_{amb}\) is the ambient temperature, and \(h_c\) is the convective heat loss coefficient. The selective nanofilm is designed to maximize \(\alpha_s\) while minimizing \(\epsilon\). A high \(\alpha_s\) ensures more solar energy is captured, and a low \(\epsilon\) reduces the radiative heat loss term \(\epsilon \sigma_{SB} (T_h^4 – T_{amb}^4)\), which becomes increasingly significant at higher operating temperatures. Commercial solar paints typically have a higher emittance, leading to greater radiative losses. This fundamental principle of spectral selectivity is a cornerstone in the design of efficient solar thermal collectors and is increasingly being applied to the emerging field of thermally-integrated thin film solar panels for hybrid energy harvesting.
Furthermore, the system’s overall thermal resistance network influences the final \(\Delta T\). The temperature difference across the TEG itself is related to the heat flow through it and its thermal resistance \(R_{th,TEG}\):
$$\Delta T = Q_{in} \cdot R_{th,TEG} – \frac{1}{2} I^2 R_{in} – S I T_{avg}$$
where the second and third terms represent Joule heating and Thomson effect (often neglected) within the TEG, respectively. By increasing \(Q_{in}\) through superior optical properties, the selective nanofilm directly boosts the primary driving potential \(\Delta T\). The integration of the biomimetic cooler on the cold side ensures a low and stable \(T_c\), further contributing to a large \(\Delta T = T_h – T_c\). This synergistic approach—enhancing heat input and strengthening heat removal—is essential for unlocking the full potential of thermoelectric materials.
The implications of this research extend beyond laboratory-scale demonstrations. The development of low-cost, durable, and high-performance selective absorption coatings is directly relevant to the commercial viability of STEG systems. These coatings can be applied not only to dedicated STEG absorbers but also potentially integrated with photovoltaic (PV) modules in hybrid PV-TE systems to utilize the waste heat from thin film solar panels, thereby increasing the total solar conversion efficiency. The demonstrated resilience to off-angle irradiation is particularly valuable for building-integrated applications or systems in regions with high diffuse solar radiation.
In conclusion, this study successfully designed and characterized an enhanced solar thermoelectric generator utilizing a sol-gel derived selective absorption nanofilm. Through comparative experimentation with a system employing a commercial solar paint, the nanofilm-based device demonstrated consistently superior performance. It achieved an increase in the TEG’s cold-hot side temperature difference of 1–2°C, leading to a maximum power output enhancement of 29.8% under low irradiance (500 mW/cm²) and a striking 42.6% under a high incidence angle (60°). These results unequivocally validate the effectiveness of spectral selectivity as a powerful strategy for improving the thermal input management in STEG systems. The principles explored here, mirroring advancements in optical engineering for thin film solar panels, highlight a critical pathway toward higher efficiency and more practical solar energy harvesting through thermoelectric conversion. Future work will focus on optimizing the film’s durability, scaling up the fabrication process, and integrating the technology into full-scale prototype systems for long-term field testing.