The ever-increasing global demand for electricity, coupled with the urgent need to transition towards sustainable energy sources, has placed solar energy at the forefront of renewable technology research. Among the various pathways for solar energy conversion, hybrid systems that combine different physical principles offer a promising route to overcome the limitations of single-technology approaches. One particularly intriguing concept is the integration of photovoltaic (PV) cells with thermoelectric generators (TEGs) to form a unified solar power harvesting unit. This hybrid solar system aims to utilize the full spectrum of sunlight: the PV cell converts the high-energy ultraviolet and visible photons into electricity, while the TEG scavenges the thermal energy from the low-energy infrared photons and the waste heat generated by the PV cell itself, converting temperature differences directly into electrical power. This synergistic approach has the potential to significantly boost the total power output and overall efficiency of a solar installation compared to standalone PV or TEG systems.
However, the conventional design for such a hybrid solar system, where the TEG is placed in series behind the PV cell (PV-on-TEG), faces fundamental thermodynamic challenges. In this configuration, the TEG is sandwiched between the hot PV cell and a cold-side heat sink. The typically low thermal conductivity of the TEG module creates a substantial thermal barrier, impeding the efficient flow of waste heat from the PV cell to the heat sink. This results in excessive heat accumulation at the PV cell, elevating its operating temperature. Since the efficiency of a silicon PV cell decreases markedly with increasing temperature—typically around 0.4-0.5% per °C—this heat buildup can severely degrade the PV performance, potentially negating the additional power contributed by the TEG. Furthermore, the temperature difference across the TEG itself is limited because its hot side is thermally choked, and its cold side is not optimally cooled. This mismatch in operating conditions and the high interfacial thermal resistances can render the conventional hybrid solar system suboptimal.
To address these critical shortcomings, this work investigates an alternative, bifacial structural configuration for the hybrid solar system. The proposed design re-architects the thermal and electrical pathways. Instead of a linear series, it adopts a “sandwich” structure: a central active cooling element (like a microchannel heat sink) is flanked on one side by the PV cell and on the other side by the TEG. In this arrangement, both the PV cell and the TEG have independent, low-thermal-resistance interfaces with the central cooling unit. The PV cell’s waste heat is conducted directly and efficiently into the coolant, maintaining the cell at a lower, more favorable temperature. Simultaneously, the coolant removes heat from the cold side of the TEG. The hot side of the TEG is independently heated, often by a secondary heat source or by concentrating the infrared portion of sunlight. This decoupling allows both the PV and TEG subsystems to operate closer to their individual optimal temperature ranges. Previous theoretical and simulation studies have suggested that such a bifacial hybrid solar system could outperform the traditional series-connected design, but comprehensive experimental validation under controlled conditions is still needed.
This article presents a detailed experimental study and performance analysis of this novel bifacial hybrid solar system. An indoor experimental setup was constructed to allow precise control and measurement of key parameters. The system’s performance was evaluated under steady-state conditions by varying two critical operational factors: the simulated solar irradiance and the flow rate of the cooling water. The primary objectives are to: 1) Quantify the temperature profiles of the PV and TEG components within the bifacial structure, 2) Measure the electrical power output and conversion efficiency of each subsystem and the overall integrated solar system, 3) Analyze the impact of irradiance and cooling intensity on system performance, and 4) Provide a comparative assessment against the performance characteristics expected from a traditional series-connected hybrid solar system. The findings aim to offer concrete empirical evidence and design insights for advancing high-performance, thermally managed hybrid solar energy systems.
Experimental Setup and Methodology
The core of the bifacial hybrid solar system, as schematically represented, consists of four primary components arranged in a layered structure: a custom-fabricated monocrystalline silicon photovoltaic cell, a copper microchannel heat sink (water-cooled), a commercial Bismuth Telluride thermoelectric generator module, and a silicone rubber heater. The photovoltaic cell, with dimensions of 0.05 m x 0.05 m, was engineered with a standard laminate structure: a glass top layer, an ethylene-vinyl acetate (EVA) encapsulant, the silicon wafer, another EVA layer, and a Tedlar/PET/Tedlar (TPT) back sheet. Under Standard Test Conditions (STC: 1000 W/m², 25°C), the cell provides a maximum power of 0.25 W at 2 V and 0.125 A, with an open-circuit voltage (Voc) of 2.4 V and a short-circuit current (Isc) of 0.135 A.
The thermoelectric generator (model SP1848-27145) has an active area of 0.0016 m² (0.04 m x 0.04 m) and contains 127 Bismuth Telluride thermocouples. At a nominal temperature difference of 40 K, its open-circuit voltage and short-circuit current are approximately 1.8 V and 0.368 A, respectively. The central cooling element is a copper microchannel heat sink with the same footprint as the TEG (0.0016 m²). Its high thermal conductivity (400 W/m·K) and internal microchannel structure are designed for efficient heat extraction. The silicone rubber heater, attached to the back of the TEG, simulates an independent thermal input, representing absorbed infrared radiation or other waste heat sources in a practical hybrid solar system. To minimize contact thermal resistance, high-thermal-conductivity silicone grease was applied at all interfaces (PV/heat sink and TEG/heat sink), and a clamping fixture ensured uniform and consistent pressure across the assembly.
The experimental apparatus was housed indoors to maintain strict control over ambient conditions. A xenon lamp solar simulator provided the adjustable irradiance source for the PV cell. The electrical characteristics of the PV cell were measured using a precision digital source meter, while the TEG’s current-voltage (I-V) curve was obtained by connecting it to an electronic load. K-type thermocouples were strategically attached to measure temperatures at critical locations: the front and back surfaces of the PV cell, the hot and cold sides of the TEG, and the inlet/outlet of the cooling water. All temperature and electrical data were recorded at 10-second intervals via a data acquisition system connected to a computer. Cooling water was circulated using a pump, and its flow rate was precisely controlled and monitored using a rotameter.
Performance Parameters and Data Analysis
The performance of the hybrid solar system is evaluated using several key metrics derived from the measured temperature and electrical data. The average temperature for a component (PV back surface, TEG hot side, etc.) is calculated from multiple thermocouple readings:
$$T_{av} = \frac{\sum_{i=1}^{n} T_i}{n}$$
where \( T_{av} \) is the average temperature (K), \( T_i \) is the temperature at the i-th measurement point, and \( n \) is the number of points.
The electrical power output of the photovoltaic cell is determined by:
$$P_{PV} = V_{PV} \times I_{PV}$$
where \( P_{PV} \) is the PV output power (W), and \( V_{PV} \) and \( I_{PV} \) are its operating voltage and current, respectively. The maximum power point is found by scanning the I-V curve. The PV conversion efficiency is:
$$\eta_{PV} = \frac{P_{PV}}{G \times A_{PV}} \times 100\%$$
where \( G \) is the incident solar irradiance (W/m²) and \( A_{PV} \) is the PV cell area (m²).
Similarly, the electrical power output of the thermoelectric generator is:
$$P_{TEG} = V_{TEG} \times I_{TEG}$$
The TEG conversion efficiency, considering the thermal power input to its hot side (\( Q_{in, TEG} \)), is:
$$\eta_{TEG} = \frac{P_{TEG}}{Q_{in, TEG}} \times 100\%$$
In this experiment, \( Q_{in, TEG} \) is the electrical power supplied to the silicone rubber heater (\( V_{heater} \times I_{heater} \)), simulating a thermal flux.
The total power output of the hybrid solar system is the sum of the contributions from both subsystems:
$$P_{total} = P_{PV} + P_{TEG}$$
The overall system efficiency can be defined relative to the total energy input (optical to the PV and electrical/thermal to the TEG heater):
$$\eta_{sys} = \frac{P_{total}}{(G \times A_{PV}) + (V_{heater} \times I_{heater})} \times 100\%$$
Results and Discussion: System Performance Under Controlled Conditions
Temperature Evolution and Steady-State Behavior
The transient temperature response of the bifacial hybrid solar system is crucial for understanding its thermal dynamics. Figure 3 illustrates the temperature profiles of the PV front/back and TEG hot/cold sides over time under a constant irradiance of 2000 W/m² and a cooling water flow rate of 16 mL/min. All temperatures exhibit a rapid initial increase within the first 15-20 minutes, followed by a gradual approach to steady-state conditions by the 60-minute mark. The PV front temperature rises to about 317 K, while the PV back temperature stabilizes near 307 K. The 10 K difference is attributed to the direct irradiation on the front and the efficient conductive cooling from the back via the heat sink. The TEG hot side reaches approximately 318 K, and its cold side is maintained at about 306 K by the coolant, establishing a steady-state temperature difference (\( \Delta T_{TEG} \)) of 12 K. This stable \( \Delta T_{TEG} \) is a direct result of the effective heat removal by the central cooling block, a key advantage of this bifacial architecture for the hybrid solar system.
Impact of Solar Irradiance
The influence of incident solar power on the hybrid solar system was investigated by varying the simulated irradiance from 1000 W/m² to 5000 W/m² while keeping the cooling water flow constant. Table 1 summarizes the key steady-state results.
| Irradiance (W/m²) | PV Back Temp. (K) | ΔT_TEG (K) | P_PV (W) | P_TEG (W) | η_PV (%) |
|---|---|---|---|---|---|
| 1000 | 304.8 | 8.2 | 0.242 | 0.008 | 14.25 |
| 2000 | 307.1 | 11.9 | 0.462 | 0.023 | 13.59 |
| 3000 | 310.5 | 16.7 | 0.645 | 0.038 | 12.67 |
| 4000 | 314.3 | 21.4 | 0.802 | 0.055 | 11.81 |
| 5000 | 318.0 | 25.1 | 0.934 | 0.071 | 11.19 |
As shown, increasing irradiance raises the operating temperature of both subsystems. However, the PV back temperature increases only from ~305 K to ~318 K (a ~13 K rise) across a 4000 W/m² increase in irradiance, demonstrating exceptional thermal management. This translates to a mere ~3.25 K rise per 1000 W/m², which is significantly lower than what is typically observed in standard PV modules or series-connected hybrid systems. Consequently, the PV power output (\( P_{PV} \)) rises substantially with irradiance, from 0.242 W to 0.934 W. The PV efficiency (\( \eta_{PV} \)), however, declines from 14.25% to 11.19% due to the negative temperature coefficient of the silicon cell, despite the effective cooling. Concurrently, the temperature difference across the TEG (\( \Delta T_{TEG} \)) grows from 8.2 K to 25.1 K, driving a nearly nine-fold increase in its power output (\( P_{TEG} \)) from 0.008 W to 0.071 W. The bifacial configuration successfully allows both components of the hybrid solar system to benefit from higher energy input: the PV produces more absolute power, and the TEG generates power from a meaningfully larger温差.
Impact of Cooling Water Flow Rate
The effect of active cooling intensity was studied by adjusting the water flow rate through the central heat sink. Experiments were conducted at low (1000 W/m²) and high (5000 W/m²) irradiance levels with flow rates of 16, 32, and 64 mL/min. The results are consolidated in Table 2.
| Test Condition (Irradiance, Flow) | PV Back Temp. (K) | ΔT_TEG (K) | P_PV (W) | P_TEG (W) | P_total (W) |
|---|---|---|---|---|---|
| 1000 W/m², 16 mL/min | 304.8 | 8.2 | 0.242 | 0.008 | 0.250 |
| 1000 W/m², 32 mL/min | 303.1 | 8.5 | 0.247 | 0.008 | 0.255 |
| 1000 W/m², 64 mL/min | 301.8 | 8.8 | 0.250 | 0.009 | 0.259 |
| 5000 W/m², 16 mL/min | 318.0 | 25.1 | 0.934 | 0.071 | 1.005 |
| 5000 W/m², 32 mL/min | 313.5 | 26.8 | 0.962 | 0.076 | 1.038 |
| 5000 W/m², 64 mL/min | 309.2 | 28.3 | 0.984 | 0.081 | 1.065 |
The data reveals that increasing the cooling flow rate consistently lowers the PV operating temperature and increases the TEG’s temperature difference. The benefit is more pronounced at higher irradiance. At 5000 W/m², increasing the flow from 16 to 64 mL/min reduced the PV temperature by nearly 9 K (from 318.0 K to 309.2 K) and increased \( \Delta T_{TEG} \) by 3.2 K. This thermal improvement directly boosted electrical performance: \( P_{PV} \) increased by 0.05 W (from 0.934 W to 0.984 W), and \( P_{TEG} \) increased by 0.01 W. The total system power gain was 0.06 W. At low irradiance, the absolute changes are smaller because the thermal loads are lower, but the trend remains. This demonstrates that active cooling control is a powerful tool for performance optimization in this type of hybrid solar system, especially under high-flux conditions.
Comparative Advantage of the Bifacial Architecture
The fundamental merit of the proposed bifacial hybrid solar system becomes clear when its performance is juxtaposed with the expected behavior of a traditional series-connected (PV-on-TEG) system. The key differentiator is thermal resistance management. In the series configuration, the total thermal resistance between the PV cell and the ultimate heat sink is the sum of the contact resistance and the TEG’s internal resistance. This leads to higher PV temperatures. For instance, literature indicates that under similar water-cooling and irradiance conditions (e.g., 5000 W/m²), a series-connected system might see average PV temperatures exceeding 343 K, with a TEG ΔT often below 10 K due to the thermal bottleneck.
In contrast, the bifacial system establishes two parallel, low-resistance thermal paths: one from the PV to the coolant, and another from the TEG cold side to the coolant. The PV temperature in our experiment remained below 320 K even at 5000 W/m², and the TEG ΔT reached 25 K. This superior thermal management directly translates to superior electrical output. The PV component operates at higher efficiency due to lower temperature, and the TEG component delivers more power due to a larger usable temperature difference. Therefore, for a given solar input and cooling resource, the bifacial architecture for a hybrid solar system consistently provides higher total energy yield by allowing both sub-systems to function in more favorable thermal regimes.
Conclusion and Future Perspectives
This experimental study successfully demonstrated the operational advantages of a bifacial, sandwich-structured hybrid solar system that integrates photovoltaic and thermoelectric generation. The central findings can be summarized as follows:
- Effective Thermal Decoupling: The proposed architecture successfully decouples the thermal management of the PV and TEG units. By providing a dedicated, low-thermal-resistance cooling path for the PV cell, it maintains the cell at significantly lower operating temperatures compared to traditional series-connected hybrid solar systems. Simultaneously, it establishes a stable and appreciable temperature difference across the TEG by efficiently cooling its cold side.
- Positive Response to Irradiance: Increasing the solar irradiance boosts the absolute power output of both the PV and TEG subsystems within this bifacial solar system. The PV output increases due to higher photon flux, and the TEG output increases due to a larger temperature difference driven by the increased thermal load. However, the PV conversion efficiency exhibits the characteristic decrease with temperature, underscoring the critical importance of the integrated cooling.
- Cooling Intensity as a Performance Lever: Enhancing the cooling water flow rate effectively reduces the operating temperature of the PV cell and increases the TEG’s temperature difference. This improves the electrical performance of both components, with the benefits being more substantial under conditions of high solar irradiance. This highlights the potential for adaptive thermal management to optimize the hybrid solar system’s output under varying environmental conditions.
- Demonstrated Superiority: The experimental data confirms that the bifacial configuration enables both the PV and TEG components to outperform their expected performance in a conventional series arrangement. The system achieves this by fundamentally resolving the thermal mismatch and high interfacial resistance problems inherent in the traditional design.
The present work provides a solid experimental foundation for the bifacial hybrid solar system concept under controlled, steady-state indoor conditions. To advance this technology towards practical application, several important directions for future research are recommended:
- Outdoor Field Testing: Validating the system’s performance under real-world, dynamic conditions—with natural solar spectrum, varying irradiance, ambient temperature swings, and wind effects—is an essential next step. An outdoor test rig would provide critical data on reliability, long-term performance, and practical energy yield.
- Advanced Heat Sink Integration: The central cooling block is a pivotal component. Future investigations should explore alternative and potentially more efficient thermal management solutions integrated into the sandwich structure, such as oscillating heat pipes, two-phase microchannel coolers, or even latent heat storage using phase change materials (PCMs). These could offer lower pumping power, more uniform cooling, or thermal buffering capabilities.
- Optical and Spectral System Design: For a fully integrated solar system, the independent heating of the TEG needs to be addressed. Research into spectral beam splitters or selective absorber coatings that divert infrared radiation directly to the TEG’s hot side while transmitting visible light to the PV cell would create a truly synergistic, full-spectrum hybrid solar system.
- System-Level Optimization and Economics: Comprehensive techno-economic analysis and lifecycle assessment are needed to evaluate the cost-effectiveness, net energy gain, and environmental payback period of the bifacial hybrid solar system compared to standalone PV arrays and other hybrid configurations.
In conclusion, the bifacial hybrid solar system presents a compelling redesign that addresses the core thermal limitations of previous PV-TEG integrations. By enabling superior thermal management, it unlocks higher simultaneous performance from both energy conversion technologies, paving the way for more efficient and powerful solar energy harvesting systems.