A thermoelectric generator and water-cooling assisted high conversion efficiency polycrystalline silicon photovoltaic system

Abstract

Solar energy has been increasing its share in the global energy structure. However, the thermal radiation brought by sunlight will attenuate the efficiency of solar cells. To reduce the temperature of the photovoltaic (PV) cell and improve the utilization efficiency of solar energy, a hybrid system composed of the PV cell, a thermoelectric generator (TEG), and a water-cooled plate (WCP) was manufactured. The WCP cannot only cool the PV cell, but also effectively generate additional electric energy with the TEG using the waste heat of the PV cell. The changes in the efficiency and power density of the hybrid system were obtained by real time monitoring. The thermal and electrical tests were performed at different irradiations and the same experiment temperature of 22°C. At a light intensity of 1000 W/m², the steady-state temperature of the PV cell decreases from 86.8°C to 54.1°C, and the overall efficiency increases from 15.6% to 21.1%. At a light intensity of 800 W/m², the steady-state temperature of the PV cell decreases from 70°C to 45.8°C, and the overall efficiency increases from 9.28% to 12.59%. At a light intensity of 400 W/m², the steady-state temperature of the PV cell decreases from 38.5°C to 31.5°C, and the overall efficiency is approximately 3.8%, basically remain unchanged.

Graphical abstract

Keywords

photovoltaic (PV) / thermoelectric generator / conversion efficiency / hybrid energy systems / water-cooled plate (WCP)

1 Introduction

With the reduction of fossil fuel supply and the increase in world energy demand, the provision of a sustainable energy supply will become the main social problem [1]. As a safe and renewable clean energy source, solar energy is usually applied in both light and heat, and the cumulative capacity of the global solar photovoltaic market in 2018 increased by approximately 25% to 505 GW [2–4]. Solar energy is usually utilized by PV cells. When the visible and some near-infrared portion of the solar radiation are received on the PV cell, the light energy can be converted into electrical energy [5,6]. However, limited by the conversion efficiency of the PV technology, only a certain percentage of the incident energy absorbed is converted into electrical energy, and the rest is lost on the surface of the module in the form of heat, resulting in an increase in temperature, and as a result, the efficiency of photovoltaic (PV) cells decrease significantly [7–9]. Especially for crystalline silicon solar cells, when the operating temperature of the polycrystalline silicon solar cell rises, the conversion efficiency of the module decreases at a constant rate of about –0.45%/K [10]. Therefore, it is necessary to optimize and improve the PV cell or its use environment.

Thermoelectric materials are functional materials that use solid internal carrier motion to convert thermal energy into electrical energy. By combining with the thermoelectric device, it cannot only reduce the surface temperature of the PV cell, but also introduce additional thermoelectric power through the thermoelectric device to improve the utilization efficiency of energy. Many studies were conducted on modeling and improving the performance of PV/TEG hybrid systems. Ju et al. designed a spectrum separation hybrid system with a photovoltaic receiver and a TEG, the thermal electronic system provided an output power of about 10% [11]. However, the system had a complicated structure, occupied a lot of space, and had a weak applicability to distributed power stations and photovoltaic buildings. From the perspective of material, Deng et al. proposed a new photoelectrode configuration for light and heat integrated utilization. The CdTe/Bi₂Te₃/FTO photoelectrode showed a significantly enhanced power conversion efficiency of ~10 times higher than the CdTe/FTO [12]. Li et al. controlled the temperature of PV cells by attaching phase change material (PCMs) to the back of PV cells, and the PCMs further transferred heat to TEG to realize thermoelectric composite utilization. The conversion efficiency of the PV-TEG hybrid system using PCMs is 0.56% higher than that of the PV cell system alone [13]. For the concentrated photovoltaic thermoelectric (CPV-TE) system, Kil et al. demonstrated a CPV-TE hybrid system using a single-junction, GaAs-based solar cell and a conventional thermoelectric module as a model system, which increases the conversion efficiency by approximately 3% higher than the single CPV cell at a solar concentration of 50 suns [14]. Wu et al. [15] explored the influence of various parameters on system performance by establishing a theoretical model. In addition, many researchers have modeled and solved photovoltaic-thermoelectric hybrid systems through numerical simulation, and analyzed the results of performance, load matching, and transient response of hybrid systems [16–19]. Their results above demonstrated that the PV-TEG composite system can realize the rational use of energy and improve the overall energy conversion efficiency. TEG could reduce the operating temperature of solar energy systems and improve the conversion efficiency of PV cells. At present, the combination of TEG and PV cells at the device level is the most obvious efficiency improvement and the most widely applicable utilization method. However, more research is to be conducted to further decrease the operating temperature of PV cells and increase the temperature difference between the two sides of the TEG, and explore the efficiency changes under various conditions, so as to improve the utilization of solar energy.

In this work, a commercially available polycrystalline PV cell and a Bi-Te TEG hybrid system was designed and manufactured, and the WCP was introduced into the PV/TEG system as an effective cooling device, forming a multi-layer composite structure. As a result, by changing the light intensity (1000, 800, 400 W/m²) and the utilization area of TEG (40 mm × 40 mm, 30 mm × 30 mm), the efficiency and power density of composite systems was monitored in real time. At a temperature of 22°C and a light intensity of 1000 W/m², the steady-state temperature of the PV in the PV/TEG (40 × 40)/WCP composite system decreased from 86.8°C to 54.1°C, the steady-state power density of the TEG is 5.3 W/m², and the overall efficiency increased from 15.6% to 21.1%. This proves the feasibility of the PV/TEG/WCP composite system for efficiency improvement at different light intensities, and lays a foundation for future study of the relationship between TEG coverage area and overall efficiency and cost.

2 Experimental

2.1 Design and working-progress of hybrid systems

In photovoltaic-thermoelectric hybrid systems, the PV cell and thermoelectric device are thermally connected and electrically isolated. Figure 1 is the structural schematic diagram of the PV-TEG composite device and the position of temperature sensing point. According to Fig. 1, the simulated sunlight (E_in) emitted from the Xenon lamp was incident on the photovoltaic device. After being absorbed by the PV cell, a proportion of the radiation was converted into electrical energy (P_PV) by the PV module, and the rest was converted into heat energy. A proportion of the thermal energy was transmitted to the environment through convection (E_con), reflection energy (E_ref), and radiation (E_rad) on the surface of the PV module, while the other proportion of the thermal energy (Q_h) moved into the thermoelectric device from the back of the PV cell. The generated waste heat was used as the heat source, and the WCP was used as the cold source of the thermoelectric component. The TEG converts a proportion of the thermal energy absorbed by the photovoltaic device into electric energy (P_TEG) by the Seebeck effect, while most of the rest of the heat energy (Q_c) was lost to the liquid and the environment. The temperature measurement point was selected as the center of the PV cell, and the temperature change of the PV cell was monitored. By measuring the output electrical signal of the PV cell, the power density and PV conversion efficiency can be calculated.

2.2 Theoretical models and measurement methods

PV cells are semiconductor devices that convert solar or light energy into electricity. The current-voltage relationship of ideal PN junction in steady-state illumination is the Shockley equation [20], expressed as

(1)J=-{{\displaystyle J}}_{\mathrm{ph}}+{{\displaystyle J}}_{\mathrm{0}}\left[\exp \left({\displaystyle \frac{qV}{nkT}}\right)-\mathrm{1}\right],

where J_ph is the photogenerated current density, V is the terminal voltage, k is Boltzmann constant, and n is the ideality factor.

When solar cells are irradiated, only photons with an excitation energy higher than the band gap energy (E_g) of the semiconductor can be absorbed and produce electron-hole pairs. Therefore, the cut-off wavelength of the photon is determined by E_g. According to the Varshni relation, the relationship between semiconductor bandgap and temperature can be obtained, as expressed in Eq. (2).

(2){{\displaystyle E}}_{\mathrm{g}}\left(T\right)={{\displaystyle E}}_{\mathrm{g}}\left(\mathrm{0}\right)-{\displaystyle \frac{\alpha T^{\mathrm{2}}}{T+\beta }},

where E_g(T) is the band gap of semiconductor at temperature T, E_g(0) is the band gap of semiconductor at the temperature of 0 K, and α and β are constants.

The short circuit current density, J_sc depends on the given solar spectral irradiance, as shown in Eq. (3).

(3){{\displaystyle J}}_{\mathrm{s}\mathrm{c}}=q{\displaystyle {\int }_{h\upsilon ={{\displaystyle E}}_{\mathrm{g}}}^{\infty }\left({\displaystyle \frac{\mathrm{d}{{\displaystyle N}}_{\mathrm{p}\mathrm{h}}}{\mathrm{d}h\upsilon }}\right)}\mathrm{d}\left(h\upsilon \right),

where N_ph is the density of photogenerated carriers, h is Planck’s constant, and υ is the frequency of incident light.

The open-circuit voltage (V_oc) is the maximum voltage available from a PV cell. Substitute J= 0 into Eq. (1) to obtain the expression of V_oc , as described in Eq. (4).

(4){{\displaystyle V}}_{\mathrm{o}\mathrm{c}}=\left({\displaystyle \frac{kT}{q}}\right)\ln \left({\displaystyle \frac{{{\displaystyle J}}_{\mathrm{s}\mathrm{c}}}{{{\displaystyle J}}_{\mathrm{0}}}}+\mathrm{1}\right).

The maximum power output of a TEG is defined as the power output produced when the module resistance matches the load resistance. The maximum power output of the TEG is calculated as [21]

(5){{\displaystyle P}}_{\max }={\displaystyle \frac{{{\displaystyle V}}_{\mathrm{1}}^{\mathrm{2}}}{\mathrm{4}{{\displaystyle R}}_{\mathrm{L}}\left({{\displaystyle V}}_{\mathrm{1}}/{{\displaystyle V}}_{\mathrm{2}}-\mathrm{1}\right)}},

in which R_L is a load resistor including the wire and the contact resistance in a circuit, and V₁ and V₂ are voltages at terminals a and b, respectively, when the switch is open and closed (Fig. S1, Electronic Supplementary Material).

The resistance R_m of the thermoelectric module is calculated as [22]

(6){{\displaystyle R}}_{\mathrm{m}}={{\displaystyle R}}_{\mathrm{L}}\left({\displaystyle \frac{{{\displaystyle V}}_{\mathrm{1}}}{{{\displaystyle V}}_{\mathrm{2}}}}-\mathrm{1}\right).

Conversion efficiency {{\displaystyle \eta }}_s can be calculated by

(7){{\displaystyle \eta }}_s\left(\mathrm{\%}\right)=\left({\displaystyle \frac{{{\displaystyle P}}_{\max }}{{{\displaystyle P}}_{\mathrm{i}\mathrm{n}}}}\right)\times \mathrm{100}\mathrm{\%},

(8){{\displaystyle P}}_{\mathrm{i}\mathrm{n}}={{\displaystyle E}}_{\mathrm{i}\mathrm{n}}\times S,

where P_in is the optical power incident on the surface of the solar cell, E_in is the power density of sunlight, and S is the area of the PV cell.

According to the previous reports and the experimental data [23–25], for PV/TEG(40 × 40)/WCP, at a room temperature of 22°C, the temperature of the PV cell can reach 55°C. The reflection energy is calculated as 123.2 W/m², the thermal radiation as 136.1 W/m², and the thermal conduction as 165 W/m². The energy that can be transferred to the hot end of TEG is 385.7 W/m². If the temperature difference between the two sides is 30 K and the figure of merit (Z) of the Bi₂Te₃ module is close to infinity, the theoretical power density of TEG is 35.1 W/m², and the theoretical efficiency of the system is 25.1% (More details can be found in Eq. (S11) in Electronic Supplementary Material).

2.3 Material components and experimental procedures

The PV cell used in this work was a polycrystalline solar cell, and the thermoelectric device used was bismuth telluride-based TEG with two different sizes. Due to the position limitation of the power contacts on the back of the PV cell, the TEG device with the largest area that can be selected is 40 × 40. The two different types of TEG used in this work are of the same thermoelectric material, thermoelectric particle size, and internal PN junction arrangement structure. Therefore, the change in area will cause the change in the corresponding number of PN junction. The water-cooled plate was made of aluminum with built-in pipes. The parameters of the component and equipment of the hybrid system are listed in Table 1. The solar cell output performance test was realized by the TRM-JX1 solar PV cell experimental system test bench. With a 1000 W xenon lamp as an analog light source, the light intensity can be changed by adjusting the voltage and the current. Since the International Organization for Standardization has set the irradiance of AM1.5 to 1000 W/m², this work mainly uses 1000 W/m² optical irradiance for measurement, and selects 800 W/m² and 400 W/m² for multi-scenario experiments. Due to the limitation of the test environment and instrument conditions, the temperature for all tests is controlled to 22°C and the cooling water temperature is set to 25°C. The irradiance parameter was obtained from the National Renewable Energy Laboratory (NREL) Solar Radiation Research Laboratory (BMS).

The brief operation steps of the experiment are as follows: ① Clean the back of the solar cell and the surface of the thermoelectric film with alcohol cotton, evenly smear thermal conductive silicon grease on the surface of thermoelectric film, and stack the solar cell, thermoelectric film and water cooling plate from top to bottom, as shown in Fig. 1(a), apply appropriate pressure and fix them; ② observe the radiation value of the fixed test point on the software surface of the ‘temperature control system’, and make it reach the experimental value (1000, 800, 400 W/m²); ③ place the sample on the test rack, so that the light can evenly illuminate the experimental module. Open the ‘solar cell module test system’ software to test the volt-ampere characteristics. Meanwhile, use the infrared thermometer to measure the temperature of the module, and use the multimeter to record the output voltage of the thermoelectric sheet. Test every 30 s to one minute until the temperature is stable. During the experiment, it should be noted that the xenon fan should be turned on first and then the xenon lamp. If the PV cell needs replacing, the light source should be adjusted to the darkest, and then adjusted back to the brightest after the replacement. The output signal of the solar cell, the temperature and the output voltage of the thermoelectric device were measured every 30 s.

3 Results and discussion

To explore the feasibility of the PV-TEG composite system and find out the effect of TEG with different size and PN junction, four different photovoltaic cells (single PV, PV/WCP, PV/TEG(40 × 40)/WCP, PV/TEG (30 × 30)/WCP) were compared.

3.1 Properties of PV and TEG

During the whole process from the start of the PV cell receiving light to the stable state, the test results were recorded every 30 s, and the I-V curves of the PV cell at temperatures of 40°C, 65°C and 85°C were selected for comparation, as demonstrated in Fig. 2. As the temperature increased, short-circuit current (I_sc) increased and open-circuit voltage (V_oc) decreased.

The PV was tested at a light intensity of 1000 W/m² and room temperature of 22°C. By comparing the surface temperature of photovoltaic cells with the efficiency at the same time, it can be found that the temperature rises approximately linearly in the first 5 min, reaches a stable level in about 10 min, and finally reaches a stable level of about 86.8°C. At this time, the efficiency is only 15.6%, a decrease of 3.4%, and the efficiency is seriously attenuated, as shown in Fig. 3(a). The variation of the power density of the two TEGs used in this paper with the temperature difference is illustrated in Fig. 3(b).

3.2 Comparison of temperature changes

The comparison of the temperature-time changes of the four composite systems is exhibited in Fig. 4 and Table 2. The steady-state operating temperature was reduced from 86.8°C to 52.1°C after the direct addition of WCP, and stayed at 59.5°C and 54.1°C after the addition of TEG (30 × 30) and TEG (40 × 40) between the PV cell and the WCP, respectively. It can be found that the steady-state temperature of the PV/WCP composite device is the lowest, which is also consistent with popular perception. The steady-state operating temperature of the PV/TEG (40 × 40)/WCP composite device is similar to the former, only slightly higher than 2°C, while the PV/TEG (30 × 30)/WCP composite device can also achieve a cooling temperature of 27.3°C. Therefore, the WCP could have a good cooling effect, and the additional thermal resistance introduced by the TEG will not significantly reduce the heat dissipation effect of PV/TEG/WCP composite device. According to Figs. 4 and 5, the optimal temperature range for the energy conversion efficiency of the polycrystalline silicon photovoltaic cells used in the tests in this paper is about 40°C–50°C.

3.3 Comparation of power density and efficiency

The comparing of the overall output efficiency suggests that the single PV cell has a severe attenuation efficiency at an irradiation of 1000 W/m², as displayed in Fig. 5. The PV power density of the PV/WCP increased from 156.8 W/m² to 220.7 W/m², and the overall heat dissipation effect was significantly improved. The addition of the TEG (40 × 40) and the WCP not only increased the PV power density but also introduced a thermoelectric power density of 5.3 W/m², which improved the total energy utilization rate. The total efficiency of the composite component reached 21.12%. When the TEG (30 × 30) was used, the coverage area of the thermoelectric sheet was reduced by 43.8% compared to the former, and the number of PN junction pairs was reduced from 127 to 71, resulting in a slight decrease in the cooling effect. However, the temperature difference between the hot and cold sides of the TEG can rise to 35.5°C, which makes its power density reach 18.5 W/m². The total efficiency of composite components increased from 15.68% to 20.54%. Although the efficiency was 0.58% lower than that of the former, the reduced use of bismuth telluride thermoelectric materials decreases the cost of thermoelectric components, which is more realistic to some extent.

For the perspective of heat dissipation, when directly combined with the WCP, a good heat dissipation effect can be achieved, and the efficiency of the PV cell is increased by 6.39%, but the WCP absorbed a lot of heat, resulting in a waste of energy. From Fig. 5(a), it can be seen that the power density of the photovoltaic modules in the PV/WCP and PV/TEG (40 × 40)/WCP did not attenuate, and the PV/TEG (30 × 30)/WCP attenuated only by a small range. Therefore, it can be concluded that the PV/TEG/WCP have a good heat dissipation effect and a significant improvement in efficiency, which has reached the goal proposed in the preamble. The PV/TEG (40 × 40)/WCP has a higher efficiency improvement and temperature control performance while the PV/TEG (30 × 30)/WCP has a relatively lower cost.

3.4 Other light intensity conditions

To explore the performance of the hybrid system in practical scenarios, light intensities of 800 and 400 W/m² were also selected to simulate at different times of a day. According to the data of BMS on September 22, 2019, Global Normal LI-200R [22], it can be found that the light intensity of 800 W/m² and 400 W/m² are around 6:50 am and 6:10 am on that day, respectively. The thermoelectric device used in this section was the TEC1-12715 (40 × 40). The whole process was tested at 22°C, and the I-V curves of the PV cell at temperatures of 40°C, 55°C, 70°C (800 W/m²) and 30°C, 40°C (400 W/m²) were selected for comparison, respectively, as presented in Fig. 6(a). As the operating temperature increased, I_sc increased wile V_oc decreased. A comparison of Fig. 2 and Fig. 6(a) can show that the power generation capacity of the PV cell decreased significantly with the decrease in light intensity. At a low light intensity such as 400 W/m², the V_oc of the PV cell was about one tenth of 1000 W/m².

As shown in Fig. 6(b), with the extension of the working time, the steady-state operating temperature of a single PV at 800 W/m² is 70.0°C, and the temperature of a PV/TEG/WCP composite system is 45.8°C. The steady-state operating temperature of a single PV is 38.5°C, and the temperature of the PV/TEG/WCP composite system is 31.5°C at 400 W/m². More specific parameter values are listed in Table 3. It is found that both the PV/WCP and the PV/TEG/WCP have a good heat dissipation performance at the light intensities of 800 W/m² and 400 W/m², and their steady-state temperature is almost the same. When the light intensity is 800 W/m², the temperature can be maintained in the optimal temperature range. When the light intensity is 400 W/m², due to light restriction, both the steady-state temperature of PV and the PV/TEG/WCP cannot reach the optimal temperature range.

As shown in Fig. 7(a), a comparison of the results at the two light intensities indicate that at a light intensity of 800 W/m², the power density of a single PV cell attenuated from 89.3 W/m² to 74.3 W/m², and the efficiency decreased from the highest value of 11.2% to 9.3%. After using the composite structure, the power density can be maintained at about 94.8 W/m² without any significant decrease, with an overall efficiency of 12.1%, while at the light intensity of 400 W/m², the power density of both the PV and the PV/TEG/WCP is remained around 15 W/m² with no significant decrease. Therefore, the use of the composite system at this time cannot improve the comprehensive utilization efficiency of energy.

By corresponding with the three light intensities to the time of the day, the efficiency change of in a day can be simulated, as shown in Fig. 7(b). When the light intensity is 400 W/m², there is almost no difference in efficiency. On a sunny day, the PV/TEG/WC composite system can achieve an overall efficiency improvement over a single PV most of the time. As light intensity continues to increase, the efficiency of the composite components increases. Temporarily, 1000 W/m² is taken as the maximum light intensity in a day. It is not excluded that the light intensity exceeds 1000 W/m², the overall efficiency is further improved.

The test results at the three light intensities are also compared with previously reports, as tabulated in Table 4. The researchers in Table 4 adopted the lamination structure to compound PV and TEG, and obtained a certain efficiency improvement. The difference between them lies in the PV cell material, the heat dissipation way, and the test method. For example, Motiei et al. [27] presented a two-dimensional PV-TEG numerical model. The model takes into account solar irradiation, wind speed, and ambient temperature in addition to convective and radiative heat losses from the front and rear surfaces of the system. Their results showed that adding the TE module at the back of the PV can improve the PV efficiency by 0.59%. It can be found that under the same test conditions, the temperature of the PV/TEG/WCP system in this work changes greatly and the efficiency increases most. The reasons for this is that, the multi-layer structure can effectively make use of the solar energy, and the WCP can provide good cold end conditions for TEG. Besides, the WCP has a good cooling effect, and the extra thermal resistance introduced by TEG has little influence on the whole. Furthermore, for different photovoltaic cells, the more the photovoltaic devices are affected by the temperature effect, the better the lifting effect of the composite devices will be.

4 Summary

In this work, a novel PV/TEG/WCP system were designed and manufactured. The residual heat of the PV cell was utilized to a certain extent by the TEG, and the purpose of cooling the PV cell was realized by cooperating with the WCP. When the light intensity were 1000 and 800 W/m², the steady-state temperature of the PV/TEG (40 × 40)/WCP system decreased to 32.7°C and 24.2°C, and the overall efficiency improved by 34.7% and 35.7%, respectively. It is found that if the coverage and distribution of thermoelectric devices are further optimized, the utilization rate of residual heat can be increased. Of course, there is still a lot of work to be done in the future.

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