In recent years, solar photovoltaic technology has developed rapidly, with reliable technical solutions and mature commercial products. Most photovoltaic modules are based on crystalline silicon, yet their electrical conversion efficiency remains below 20%. Detailed investigations have shown that, among the solar radiation absorbed by a photovoltaic panel, less than 20% is converted into electricity, while the remainder dissipates as heat. Consequently, the panel temperature rises considerably. Further studies indicate that the internal resistance of a solar panel increases with its operating temperature, which in turn lowers the electrical efficiency. Typically, for every 10 °C increase in panel temperature, the electrical efficiency drops by approximately 0.5%. To mitigate this effect, many researchers have adopted active cooling strategies, such as forced air circulation, water cooling, or even direct refrigerant evaporation on the back of the panel. However, these active methods require additional electrical energy to drive pumps or fans, which partially offsets the gain in photovoltaic output.
In this context, my research focuses on the thermal and electrical performance of solar panels under passive cooling conditions. Passive cooling is advantageous because it consumes no additional power and can be integrated with building structures or solar collectors. The main objectives of this study are twofold. First, I investigate the effects of natural ventilation on the electrical performance of solar panels with and without attached fins. The parameters considered include inclination angle, solar radiation intensity, ambient temperature, wind velocity, fin height, and fin spacing. Second, I develop and test a heat pipe–based solar photovoltaic hot water system, in which heat pipes are attached to the back of the solar panel and a header box is installed on the top. This system not only cools the panel but also produces useful hot water. I examine the effects of solar radiation intensity, inlet water temperature, and water circulation flow rate on both the electrical and thermal efficiencies.
To achieve these objectives, I combined numerical simulation with experimental testing. I established one-dimensional steady-state mathematical models for both configurations and solved them using Matlab. I also built experimental platforms to measure the relevant parameters under real outdoor conditions. The experimental data were used to validate the numerical results. This study provides useful insights for improving the performance of solar panels through passive thermal management, and it contributes to the practical application of solar photovoltaic/thermal (PV/T) technologies.
1. Background and Significance
Energy is the foundation of human civilization, and the world is facing increasing energy demand. According to the International Energy Agency, world energy demand will double by 2025, while conventional fossil fuel reserves are being depleted rapidly. Solar energy, as an abundant and renewable resource, offers a promising alternative. The solar constant outside the Earth’s atmosphere is about 1376 W/m², and after atmospheric attenuation, the irradiance at ground level is typically around 1000 W/m² under clear skies. Solar energy reaching the Earth every second is equivalent to approximately 500 million barrels of oil. This huge potential has motivated extensive research on solar energy conversion technologies.
Solar energy can be converted into both electricity and heat. Photovoltaic (PV) cells directly convert sunlight into electricity, but their efficiency is limited. Solar thermal collectors convert sunlight into heat, which is a mature technology. Combining both functions into a single system, known as photovoltaic/thermal (PV/T) technology, can substantially improve the utilization of solar energy. However, there exists a trade-off between electrical and thermal outputs. As the panel temperature rises, the electrical efficiency decreases, while the thermal efficiency may increase if the heat is effectively removed. Thus, the design of a PV/T system must balance these two aspects.
The motivation for my research is to explore passive cooling techniques that can lower the operating temperature of solar panels without consuming extra energy. In particular, I focus on natural convection fins and heat pipe–based water heating systems. These approaches not only improve the electrical output of solar panels but also produce usable heat, thereby enhancing the overall solar energy utilization efficiency.
2. Fundamentals of Solar PV and PV/T Systems
Photovoltaic generation is based on the photovoltaic effect of semiconductor materials. When photons with energy greater than the band gap of the semiconductor are absorbed, electron–hole pairs are generated. The built‑in electric field of a p–n junction separates these carriers, producing a current if an external circuit is connected. The electrical power output is proportional to the incident radiation intensity, provided that the panel temperature remains constant. In practice, however, the panel temperature tends to increase under intense sunlight, and this negatively affects the output voltage and efficiency.
PV modules are usually made of silicon (monocrystalline or multicrystalline). The conversion efficiency of commercial silicon modules is typically between 14% and 20%. The remaining incident energy is converted into heat, which raises the module temperature. For silicon cells, the temperature coefficient is typically about 0.4–0.5% per °C, meaning that for every 10 °C rise, the output power decreases by 4–5%. Therefore, thermal management is crucial for PV systems.
PV/T systems combine a solar collector with a PV panel. The working fluid (air, water, or refrigerant) removes heat from the panel and delivers it to a storage tank or to a heat pump evaporator. The thermal energy can be used for domestic hot water, space heating, or industrial processes. The electrical output is improved because the panel temperature is lowered. Numerous studies have shown that PV/T systems can achieve high combined efficiencies, often exceeding 60%, while the electrical efficiency may be improved by 10–20% relative to conventional PV panels.
In my study, I focus on passive cooling methods. The first method uses fins attached to the back of the PV panel to enhance natural convection heat transfer. The second method uses heat pipes attached to the back of the panel, with a water header box at the top. The heat pipes effectively transport heat from the panel to the water, cooling the panel and generating hot water simultaneously.
3. Natural Cooling Performance of Solar Panels
3.1 Mathematical Model for a PV Panel without Fins
For a PV panel without fins, the energy balance can be written by considering the absorbed solar radiation, the electrical output, the convective heat loss to the surroundings, and the radiative heat loss to the sky. The steady-state energy equation is:
$$0 = Q(\tau\beta)_p A_p – E – h_{pa} A_{pa}(T_p – T_a) – h_{ep} A_{ep}(T_p – T_e) \tag{3.1}$$
where:
- \(Q\) = solar radiation intensity (W/m²)
- \(A_p\) = area of the PV panel (m²)
- \((\tau\beta)_p\) = effective absorptivity of the PV cell
- \(E\) = electrical output power (W)
- \(h_{pa}\) = convective heat transfer coefficient between the panel and ambient air (W/(m²·K))
- \(T_p\) = panel temperature (K)
- \(T_a\) = ambient temperature (K)
- \(h_{ep}\) = radiative heat transfer coefficient between the panel and sky (W/(m²·K))
- \(T_e\) = effective sky temperature (K)
The electrical output is computed as:
$$E = Q(\tau\beta)_p \eta_{ref} [1 – \kappa (T_p – 298.15)] \tag{3.2}$$
Here, \(\eta_{ref}\) is the reference efficiency under standard test conditions (1000 W/m², 25 °C), and \(\kappa\) is the temperature coefficient (K⁻¹).
3.2 Model for a PV Panel with Fins
When fins are attached to the rear side of the panel, additional heat transfer pathways are introduced. The heat flow diagram is shown conceptually in the figure below. The energy balance equations for the panel, the metal base plate, the fins, and the air between fins must be solved together.
For the PV panel with fins, the energy conservation equation becomes:
$$0 = Q(\tau\beta)_p A_p – E – h_{pb} A_{pb} (T_p – T_b) – h_{pa} A_{pa} (T_p – T_a) – h_{ep} A_{ep} (T_p – T_e) \tag{3.3}$$
where \(T_b\) is the temperature of the back metal plate and \(h_{pb}\) is the contact conductance between the PV cell and the back plate. In this work, I use a contact resistance of \(R_{pv-b} = 0.04\) m²·K/W, yielding \(h_{pb} = 1/R_{pv-b} = 25\) W/(m²·K).
The back plate exchanges heat with the fins and with the air between fins. The energy equation for the back plate is:
$$0 = h_{pb} A_{pb}(T_p – T_b) – h_{bf} A_{bf}(T_b – T_f) – h_{bm} A_{bm}(T_b – T_m) \tag{3.4}$$
where \(T_f\) is the fin temperature, \(T_m\) is the air temperature between fins, and \(h_{bf}\), \(h_{bm}\) are the corresponding heat transfer coefficients. The fin equation is:
$$0 = h_{bf} A_{bf}(T_b – T_f) – h_{mf} A_{mf}(T_f – T_m) \tag{3.5}$$
Finally, the air gap energy balance is:
$$0 = h_{bm} A_{bm}(T_b – T_m) + h_{mf} A_{mf}(T_f – T_m) + \dot{m} c (T_a – T_m) \tag{3.6}$$
with the air mass flow rate due to natural convection \(\dot{m} = \rho u_f (H – \delta_f) y_f\).
3.3 Numerical Simulation Results
I performed simulations for Beijing conditions. The geographical location, solar angles, and daily radiation data were obtained from standard meteorological databases. For a typical day (October 15, 2012, at noon), the direct normal irradiance was set to 800 W/m², and various inclination angles were tested. The following tables summarize the simulated electrical efficiency and power output with and without fins.
| Inclination (°) | Without fins – Efficiency (%) | With fins – Efficiency (%) | Without fins – Power (W) | With fins – Power (W) |
|---|---|---|---|---|
| 20 | 14.45 | 14.70 | 108.7 | 110.6 |
| 30 | 14.38 | 14.65 | 114.8 | 117.0 |
| 45 | 14.31 | 14.60 | 118.6 | 121.0 |
| 60 | 14.35 | 14.63 | 115.6 | 117.8 |
The results show that the electrical efficiency reaches its lowest point at an inclination of 45°, while the power output reaches its maximum at the same angle. This is because the total solar radiation absorbed by the panel depends on the inclination. The optimum angle for maximum power output in Beijing was found to be around 45°, which aligns with practical recommendations for fixed solar panels.
I also investigated the effect of wind velocity. The simulations were performed at a constant radiation of 800 W/m², ambient temperature of 16 °C, fin height of 0.1 m, and fin spacing of 0.08 m. The results are provided in the following table.
| Wind velocity (m/s) | Without fins efficiency (%) | With fins efficiency (%) |
|---|---|---|
| 3 | 14.36 | 14.64 |
| 4 | 14.51 | 14.71 |
| 5 | 14.65 | 14.79 |
| 6 | 14.78 | 14.87 |
Wind velocity enhances convective heat transfer, thus lowering the panel temperature and increasing the efficiency. The effect is more pronounced for the panel without fins because the fins already improve convective heat transfer. On average, the finned panel performed about 0.17% better in efficiency than the unfinned one within the studied wind range.
Ambient temperature has a linear effect on the efficiency. I simulated a temperature range from −4 °C to 26 °C. The results are shown below.
| Ambient temperature (°C) | Without fins efficiency (%) | With fins efficiency (%) |
|---|---|---|
| −4 | 15.72 | 16.00 |
| 6 | 15.04 | 15.32 |
| 16 | 14.36 | 14.64 |
| 26 | 13.67 | 13.95 |
The average temperature coefficient for both cases was approximately 0.68% per 10 °C, which is slightly higher than the commonly cited 0.5% per 10 °C. This difference arises from the specific operating conditions and the assumption of a constant absorptivity.
I also analyzed the daily variation of efficiency for a clear day. A typical result is shown in the following table.
| Time of day | Without fins efficiency (%) | With fins efficiency (%) |
|---|---|---|
| 8:00 | 15.63 | 15.67 |
| 10:00 | 15.01 | 15.20 |
| 12:00 | 14.58 | 14.85 |
| 14:00 | 15.10 | 15.31 |
| 16:00 | 15.65 | 15.71 |
As expected, the efficiency is lowest around noon when the solar radiation is strongest and the panel temperature reaches its maximum. In the early morning and late afternoon, the panel is cooler, leading to higher efficiency.
Fin height and spacing were also optimized. The simulation predicted an optimal fin height of approximately 86 mm for the specified conditions. Increasing the fin height beyond this point increased the effective heat transfer area but also restricted the natural convection airflow between fins. Similarly, larger fin spacing reduced the number of fins and hence the total heat transfer area, causing a decrease in efficiency. The following data illustrate the effect of fin spacing.
| Fin spacing (mm) | Efficiency (%) |
|---|---|
| 10 | 14.74 |
| 20 | 14.68 |
| 40 | 14.60 |
| 60 | 14.55 |
| 80 | 14.52 |
| 100 | 14.50 |
The effect is relatively small but consistent. For engineering purposes, a fin spacing of 10–20 mm appears to be a good choice to balance performance and material usage.
3.4 Experimental Testing of Finned and Unfinned Solar Panels
To validate the numerical simulations, I built an experimental setup. The test system consisted of two similar PV panels (one with fins and one without), a support frame with adjustable inclination, a set of resistive loads, thermocouples, a pyranometer, a data logger, and a fan to control wind speed. The PV panel used was a 110 W multicrystalline module with dimensions 1172 mm × 660 mm × 35 mm. The fins were made of aluminum with a thickness of 0.8 mm and were attached to the back side using thermal adhesive. The air gap between the fins was left open to allow natural convection.
I measured the open-circuit voltage and output power for various load resistances to determine the optimal load. The following table shows the measured output voltage and power for different resistance values at a solar irradiance of about 662 W/m².
| Resistance (Ω) | Output voltage (V) | Output power (W) |
|---|---|---|
| 50 | 21.6 | 9.3 |
| 30 | 21.5 | 15.4 |
| 20 | 21.2 | 22.5 |
| 10 | 20.3 | 41.2 |
| 5 | 15.3 | 46.8 |
| 3.33 | 12.0 | 43.2 |
| 2.5 | 8.3 | 27.6 |
| 1.67 | 6.3 | 23.1 |
The maximum power occurred at 5 Ω. To avoid overheating the resistor, I used two 10 Ω resistors in parallel, providing an equivalent resistance of 5 Ω while distributing the power dissipation.
Using this load, I performed experiments under various conditions. The trend of the experimental results matched the simulations, although the absolute values of efficiency were lower because of the higher ambient temperatures and lower irradiance during the test period. For example, when the panel inclination was varied from 30° to 60°, the measured efficiency of the finned panel ranged from a minimum of 10.53% at 45° to about 11.09% at 60°. The unfinned panel showed a similar trend with lower values, with an average efficiency of 10.17% compared with 10.97% for the finned panel. The improvement due to fins was about 0.8 percentage points in these experiments.
The experimental investigation also confirmed that the output power increased with increasing solar radiation. Over a radiation range of 284 to 685 W/m², the finned panel’s output power rose from 9.3 W to 49.7 W, while the unfinned panel rose from 9.4 W to 37.4 W. The average increase per 50 W/m² was 5.1 W for the finned panel and 3.5 W for the unfinned panel. These differences reinforce the benefit of passive cooling fins.
Wind speed experiments were conducted in a low-radiation environment (300±50 W/m²) because high radiation levels caused the panel temperature to dominate. Increasing the wind speed from 1 m/s to 3 m/s raised the efficiency from 4.85% to 6.32% for the finned panel and from 4.25% to 6.47% for the unfinned panel. Interestingly, the unfinned panel showed a larger relative improvement with wind speed, because it had a higher thermal resistance and therefore was more sensitive to convective cooling. Nevertheless, the finned panel still gave a higher average efficiency.
3.5 Comparison between Experimental and Numerical Results
To compare, I used the measured meteorological conditions as inputs for the simulation. For the effect of inclination, the simulation predicted an average efficiency of 10.02% while the experiment gave 10.97% for the finned panel. The difference was about 0.95 percentage points, which can be attributed to the simplified one-dimensional model and uncertainties in the measured contact resistance.
For the wind speed test, the simulation predicted an efficiency of about 11.4%, whereas the experiment measured only 5.45% on average. The large discrepancy was mainly because the experimental radiation level was only 300 W/m², while the simulation assumed 800 W/m². When I adjusted the simulation input to the actual radiation level, the mismatch was reduced, but there remained a systematic offset due to the simplified heat transfer coefficient correlations. This highlights the necessity of comprehensive experimental validation.
The daily efficiency comparison showed a reasonable agreement. The experimental average efficiency over the day was 11.33% for the finned panel, while the simulation predicted 11.64%—a difference of only 0.31 percentage points. The output power difference was even smaller, with experiment giving 54.6 W and simulation giving 54.2 W. These results indicate that the numerical model is sufficiently accurate for engineering design when the input parameters are carefully determined.
4. Heat Pipe–Based Solar Photovoltaic Hot Water System
4.1 Concept and Structure
In this section, I describe a novel heat pipe–based solar photovoltaic hot water system. The system integrates ten heat pipes onto the backside of a 200 W polycrystalline PV panel. The evaporator section of each heat pipe is attached to the panel using thermally conductive silicone adhesive. To improve the heat collection effect, a thin iron sheet was also bonded onto the backside of the PV panel over the heat pipes. The heat pipe condenser sections are inserted into a water header box (manifold) located at the top of the panel. The header box is connected to a water tank via pipes, and a small pump drives the water circulation. A flowmeter measures the flow rate, and thermocouples measure the inlet/outlet water temperatures and the panel temperature.
The working principle is that heat generated by the PV panel vaporizes the working fluid (water) inside the heat pipe. The vapor rises to the condenser, releases its latent heat to the water in the header box, and condenses back to liquid. The condensate returns to the evaporator section by gravity or capillary action. This cycle passively transports heat from the PV panel to the water, thereby lowering the panel temperature and increasing its electrical efficiency while producing hot water.
The advantages of this system include:
- Passive operation without additional electrical energy for cooling.
- No moving parts in the heat pipes, ensuring high reliability.
- Utilization of low-grade heat for domestic hot water or other applications.
- Improved electrical performance of solar panels due to effective heat removal.
4.2 Mathematical Model
I developed a one-dimensional steady-state model for the heat pipe PV/T system. The energy balance for the PV panel is written as:
$$0 = Q(\tau\beta)_p A_p – E – h_{pa} A_{pa}(T_p – T_a) – h_{pe} A_{pe}(T_p – T_e) – h_{pt} A_{pt}(T_p – T_t) \tag{4.1}$$
where \(T_t\) is the temperature of the heat pipe outer wall, and \(h_{pt}\) is the equivalent contact conductance between the PV panel and the heat pipe outer wall. The model includes conduction through the PV layer, the adhesive layer, and the heat pipe wall.
For the back metal plate, the equation is:
$$0 = h_{pb} A_{pb}(T_p – T_b) – h_{bt} A_{bt}(T_b – T_t) – h_{bi} A_{bi}(T_b – T_i) \tag{4.2}$$
For the insulation layer and the heat pipe outer wall, I derived the following balance equations:
$$0 = h_{bi} A_{bi}(T_b – T_i) – h_{it} A_{it}(T_i – T_t) – h_{ia} A_{ia}(T_i – T_a) \tag{4.3}$$
$$0 = h_{bt} A_{bt}(T_b – T_t) + h_{it} A_{it}(T_i – T_t) + h_{pt} A_{pt}(T_p – T_t) – h_{te} A_{te}(T_t – T_{evap}) \tag{4.4}$$
The heat pipe evaporator temperature is governed by:
$$0 = h_{te} A_{te}(T_t – T_{evap}) – h_{HP} A_{HP}(T_{evap} – T_{cond}) \tag{4.5}$$
Finally, the energy balance for the condenser section and the circulating water is:
$$0 = h_{HP} A_{HP}(T_{evap} – T_{cond}) – h_w A_w (T_{cond} – T_w) \tag{4.6}$$
$$0 = h_w A_w (T_{cond} – T_w) – \dot{m}_w c_w (T_{out} – T_{in}) \tag{4.7}$$
In these equations, \(T_{evap}\) and \(T_{cond}\) are the temperatures of the evaporator and condenser sections of the heat pipe, \(T_w\) is the average water temperature in the header box, and \(h_w\) is the water-side heat transfer coefficient.
4.3 Simulation Results for the Heat Pipe System
I used the model to simulate the performance under typical conditions. The base parameters were a solar radiation intensity of 650 W/m², ambient temperature of 20 °C, and water flow rate of 6 L/min. The header box inlet water temperature was varied from 20 °C to 44 °C. The thermal efficiency was calculated using the expression:
$$\eta_{th} = \frac{\dot{m}_w c_w (T_{out} – T_{in})}{Q S \alpha} \times 100\% \tag{4.8}$$
where \(S\) is the total area of the PV panel and \(\alpha\) is the absorptance (taken as 0.9).
The simulated heat efficiency decreased linearly with increasing inlet water temperature. When the inlet temperature increased from 20 °C to 44 °C, the heat efficiency fell from 14.72% to 5.60%—a decrease of 0.38% per °C. This is because the temperature difference between the heat pipe condenser and the water decreases, which reduces the heat transfer rate. The electrical efficiency also decreased slightly, from 12.92% to 12.73%, because a higher water temperature results in less cooling of the PV panel.
I also varied the solar radiation intensity between about 500 and 816 W/m² while maintaining the inlet water temperature at 20 °C and flow rate at 6 L/min. The simulated heat efficiency rose linearly, while the electrical efficiency declined with radiation intensity. The following table shows the simulated results.
| Solar radiation (W/m²) | Heat efficiency (%) | Electrical efficiency (%) |
|---|---|---|
| 507 | 18.12 | 13.13 |
| 600 | 19.15 | 12.88 |
| 700 | 20.32 | 12.58 |
| 816 | 21.83 | 12.42 |
These simulated data suggest that, for every 100 W/m² increase in solar radiation, the heat efficiency increases by about 1.18 percentage points, while the electrical efficiency decreases by about 0.23 percentage points. The decrease in electrical efficiency is a consequence of the higher operating temperature, despite the cooling effect of the heat pipes.
4.4 Experimental Results for the Heat Pipe System
An experimental platform was constructed as described in Section 4.1. The PV panel used in this experiment was a 200 W multicrystalline module with an area of 1.24 m². Ten copper heat pipes, with an outer diameter of 8 mm and evaporator length of 1000 mm, were attached to the back of the panel. The condenser section of each heat pipe (60 mm long) was inserted into the header box. The water tank had a volume of 30 L and was insulated with 30 mm thick foam. A small centrifugal pump circulated water through the header box, and a rotameter measured the flow rate.
I performed a series of tests on clear days. In one representative experiment, the system was operated from 9:00 in the morning to 13:30 in the afternoon. The measured heat efficiency decreased over time as the water temperature in the tank increased. The experimental heat efficiency at 9:00 was 17.32%, and by 13:30 it had dropped to 2.56%, giving an average of 7.91%. The simulation predicted an average heat efficiency of 5.67% for the same period. The higher experimental values may be due to a higher actual solar radiation and a lower ambient temperature than the assumed values, as well as to the unsteady nature of the real system. The average electrical efficiency measured during the test was 11.90%, while the simulation predicted 12.88%.
I studied the effect of inlet water temperature by adjusting the water tank temperature before each run. The water flow rate was fixed at 6 L/min, and the solar radiation was maintained around 650 W/m². The experimental results are summarized in the following table.
| Inlet water temperature (°C) | Heat efficiency (experiment, %) | Electrical efficiency (experiment, %) |
|---|---|---|
| 20 | 15.90 | 14.11 |
| 26 | 12.88 | 13.45 |
| 32 | 9.73 | 12.90 |
| 38 | 7.05 | 12.18 |
| 44 | 4.31 | 11.62 |
Both heat and electrical efficiencies decreased with increasing inlet water temperature. The slopes were approximately −0.48% per °C for heat efficiency and −0.05% per °C for electrical efficiency. These measured temperature coefficients are larger than the simulated ones, likely due to additional heat losses in the experimental setup.
The influence of solar radiation intensity on the experimental system was also analyzed. With a constant inlet temperature of 20±1 °C and flow rate of 6 L/min, the solar radiation was varied between 500 and 816 W/m². Over this range, the heat efficiency increased from 13.48% to 17.45%, and the electrical efficiency decreased from 13.83% to 10.72%. The experimental slopes were 1.26% per 100 W/m² for heat efficiency and −0.99% per 100 W/m² for electrical efficiency. The negative slope for the electrical efficiency was more pronounced than in the simulation, which is expected because the real panel temperature rose more significantly at high irradiation.
Finally, I investigated the effect of water flow rate on the average thermal and electrical efficiencies. In this series, the average solar radiation was about 700 W/m², ambient temperature about 13 °C, and inlet water temperature about 14 °C. The flow rate was varied from 5 L/min to 9 L/min using the bypass valve. The results are shown in the table below.
| Flow rate (L/min) | Average thermal efficiency (%) | Electrical efficiency (%) |
|---|---|---|
| 5 | 18.91 | 12.44 |
| 6 | 18.12 | 12.15 |
| 7 | 17.42 | 11.90 |
| 8 | 16.75 | 11.58 |
| 9 | 16.07 | 11.28 |
Unexpectedly, both efficiencies decreased as the flow rate increased. This may seem counterintuitive because higher flow rates normally improve heat transfer. However, in this experimental configuration, the water residence time in the header box was shorter, and the circulating water did not have enough time to extract heat efficiently from the heat pipe condensers. Moreover, the increased flow rate increased the heat transfer coefficient, but this effect was outweighed by the reduced temperature rise, resulting in a lower thermal output. The panel temperature also remained higher, which reduced the electrical efficiency.
5. Discussion
My research confirms that passive cooling can significantly benefit the performance of solar panels. For the finned panel, the improvement in electrical efficiency compared with the unfinned panel was observed under various conditions. In the numerical study, the average enhancement ranged from 0.17 to 0.27 percentage points, while in the experiments, the improvement was up to 0.8 percentage points. The difference is due to the more realistic measured conditions, which included higher panel temperatures and a moderate wind speed. The optimal fin height and spacing identified in this work can be used as design guidelines for practical passive cooling systems.
The heat pipe PV/T system demonstrated even more promising results. By integrating heat pipes with a water header, the system achieves dual functions: electrical power generation and hot water production. The electrical efficiency of the PV panel in the heat pipe system was generally higher (around 11–14%) than that of a standalone panel because the heat pipes continuously remove heat. At the same time, the thermal efficiency was moderate, around 10–18% depending on the operating conditions. The combined efficiency (electrical + thermal) of the heat pipe system could reach 25–30% or even higher, which is a significant improvement over conventional solar panels.
The experimental data also exposed some limitations. For instance, the thermal efficiency decreased dramatically with time because the water in the storage tank became hot and the temperature difference between the condenser and water diminished. This is a common issue in batch‑type solar water heaters. To maintain a high thermal efficiency throughout the day, a more sophisticated control strategy, such as variable flow rate or a heat exchanger with storage, would be necessary. However, for a simple passive system, the declining efficiency is acceptable because the stored hot water is the desired product.
Another important observation is the trade‑off between heat and electricity. Higher water temperatures improve the usability of heat but reduce the electrical output. In regions where hot water is valuable, the system can be optimized by operating at a higher water temperature, whereas for maximizing electricity, the water temperature should be kept as low as possible. This trade‑off can be quantitatively described by an exergy analysis, which evaluates both energy streams on a common basis. My results provide the necessary data for such an analysis.
6. Conclusions
Based on the numerical and experimental investigations presented in this work, the following conclusions can be drawn:
- Attaching fins to the rear side of solar panels is an effective passive cooling method. The finned panel consistently outperformed the unfinned panel in terms of both electrical efficiency and output power.
- The inclination angle of a solar panel significantly affects its performance. For Beijing, an inclination of about 45° yielded the maximum output power, despite the fact that the efficiency was lowest at that angle. The trade‑off between total radiation and temperature effect must be considered when choosing the optimal tilt.
- Wind velocity improves the electrical performance of solar panels by enhancing convective heat transfer. The effect is stronger for an unfinned panel because the fins already reduce thermal resistance.
- Ambient temperature negatively affects both the efficiency and output power. The temperature coefficient was measured to be about 0.7% per 10 °C for both finned and unfinned panels, which is slightly higher than the commonly assumed 0.5%.
- For the fin design, an optimal fin height exists (around 80–90 mm in my simulations). Fin spacing should be as small as practical to maximize heat transfer area, but small spacing may lead to manufacturing difficulties and airflow restriction.
- The heat pipe–based PV/T system effectively combines power generation and hot water production. The heat pipe cooling reduces the PV panel temperature and thus increases the electrical efficiency, while the recovered heat provides useful thermal energy.
- In the heat pipe system, the heat efficiency decreases linearly with increasing inlet water temperature, and increases with increasing solar radiation. The electrical efficiency behaves oppositely, decreasing with higher radiation and higher water temperature.
- The water flow rate has a nuanced effect. In the present system, higher flow rates decreased both thermal and electrical efficiencies due to reduced residence time. This indicates that the optimum flow rate should be determined based on the specific system design and the desired output.
In summary, passive cooling of solar panels is a promising and cost‑effective approach to improve their performance. The use of fins or heat pipes not only lowers the PV panel temperature but also provides a means to utilize the waste heat. This dual functionality is especially beneficial for building‑integrated solar systems, where both electricity and hot water are needed. Future work could extend the current model to transient three‑dimensional analyses, incorporate phase‑change materials for thermal storage, and investigate the integration of a heat pump with the heat pipe PV/T system to further improve the overall coefficient of performance.