Modeling of photovoltaic systems with bidirectional use of thermoelectric module

During photovoltaic (PV) conversion, most of the solar energy captured by the PV cells is dissipated as heat, resulting in a large amount of energy waste. While PV cells contribute only a certain percentage of incoming solar radiation to electricity generation, the remaining energy is dissipated as heat, causing the temperature of the PV cells to increase. Many studies have been conducted to eliminate or use the residual heat that causes PV cells to heat up. Some of the studies conducted within the scope of increasing the efficiency of PV systems by eliminating residual heat are the use of PV systems together with thermoelectric (TE) systems. TE systems are systems that can convert heat energy into electrical energy or, in the opposite way, operate as a heat pump (cooler-heater) using electrical energy. Studies on PV-TE hybrid systems aim to convert the residual heat generated in PV systems into electrical energy with thermoelectric generator (TEG) modules and to increase the total efficiency. However, there are also studies aiming to increase efficiency by cooling PV modules with thermoelectric cooler (TEC) modules or to protect PV cells from excessive heat. This thesis presents a bidirectional model for thermoelectric modules (TEMs) to be used in circuit simulations in MATLAB/Simulink software. The aim of this study is to develop a single block like other circuit elements in the Simulink library by combining both thermoelectric generator (TEG) and thermoelectric cooler (TEC) module parameters and their operations. Since TEG and TEC are mostly thermoelectric devices with the same main material such as Bi2Te3 for similar temperature ranges, a TEG can be operated as a TEC and vice versa. In some systems such as PV-TE, fuel cell-TE and battery temperature management of electric vehicles, the modules can be used in both modes. In addition, in some of these systems, sometimes the TEM may need to be operated as a generator and sometimes as a cooler. The bidirectional operation of TEM is a cost-saving approach since it is not necessary to use two types of TEM together. When the simulation models in the literature are examined, it is seen that the existing models are not fully compatible with the bidirectional use of TEMs and a new model suitable for bidirectional operation is proposed in this thesis. This model deals with the polarities of both thermal and electrical ports of TEM to decide the operating mode, i.e. TEG or TEC. Maximum parameters and performance parameters for both TEG and TEC operations are included in the module block in order to observe and control the device for the desired operating conditions. It is seen that the simulations made with temperature-dependent parameters give results very close to the graphs in the data sheet. However, the simulations where Thomson heat is not ignored have more deviations compared to the data sheet. Moreover, the simulations with constant material properties give better results than those with Thomson heat at higher ∆T levels. When ∆T is 10 °K, COPmax without Thomson heat is found to be 3.399, which is the same as in the data sheet. Meanwhile, COPmax with Thomson heat is found as 3.7147, and COPmax with fixed properties is found as 3.8057. However; when ∆T is 50 °K, COPmax without Thomson heat is 0.247, which is the same as in the data sheet, but COPmax with Thomson heat is 0.341 and COPmax with fixed material properties is 0.268. Furthermore, it is shown that bidirectional operation of TEM is successfully achieved under the variable polarities of thermal and electrical ports, and TEM is operated as both TEG and TEC in the same simulation. In the thesis, a heat transfer model was established in the MATLAB/Simulink environment for the TEM model proposed to be used in two ways in PV-TE hybrid systems. In the simulations, the same TEM model was used for electricity generation in the PV-TEG system and for cooling PV cells in the PV-TEC system. In addition, a sample PV-TE hybrid experimental system was implemented in order to compare the operation of the simulation model and the real system. Specially cut PV cells with dimensions of 39x39 mm2 were directly connected to TEMs with dimensions of 40x40 mm2, with heat transfer material in between. Heat sinks (aluminum radiators) were attached to the other surfaces of the TEMs. An Arduino UNO microcontroller-based measurement and data recording system was created to experimentally obtain the electrical characteristics of the PV and TE systems used in the set. A buck type DC-DC converter controlled by Arduino was designed, and the voltage, current and power values of the system under variable load were obtained by changing the PWM duty cycle. An Arduino-based pyranometer was designed for solar irradiation measurement and its accuracy was calibrated with a pyranometer. In order to record the measurement values and compare them with the simulations, an Arduino UNO compatible data logger add-on was used. With this add-on, which includes a real-time clock integration, real-time date, hour, minute and second information of the recorded data was obtained. The necessary programming was done to record the values measured with the sensors in the CSV file format. A solar radiation measurement set was designed instead of a laboratory type Pyranometer and data logger. With this set, the size was reduced and the measurements were made more practical. Another advantage of the set is that it was installed under the Arduino platform. In this way, the measured values can be recorded. Arduino nano, 128x32 OLED screen and TSL2561 luxmeter sensor were used in the measurement set. The diffuser model prepared with Thinkercad software was printed with a 3D printer. In order to regulate and filter the light distribution coming to the sensor, the sensor was placed inside the diffuser and PTFE tape was wrapped on the diffuser. Since the maximum detection threshold of the TSL2561 luxmeter sensor was exceeded at approximately 400 W/m2 irradiance levels, it was not suitable to be used directly for pyranometer purposes. Therefore, in order to increase the maximum irradiance level that can be measured by reducing the light coming to the sensor, filtering was done with PTFE tape and approximately 25% irradiance was provided to the sensor. For the calibration of the new measurement system, measurements were made simultaneously with a laboratory type pyranometer at different day and light levels. The luxmeter sensor was calibrated by adjusting the measurement values to the reference laboratory pyranometer values with the necessary multiplier adjustments in the Arduino software. In the simulations, it was observed that the cooling capacities of the heat sinks connected to the TE modules were important in heating the PV cells. When the experimental conditions were simulated, when the h.A cooling capacity was taken as 0.8 W/K, the PV cell surface temperatures were found to be 303 °K (30 °C) and the ∆T TEG module surface temperature difference was 1.79 °K, while when h.A was taken as 0.03, the PV cell surface temperatures were found to be 321.9 °K (48.9 °C) and the ∆T TEG module surface temperature difference was 1.172 °K. Since the PV cell temperatures were measured around 49 °C, it is thought that using the h.A 0.03 W/K value in the simulations will yield more realistic outputs. In the experiment, the ambient temperature is 27 °C and the solar radiation is 950 W/m2; these values were used in the simulation of the experimental conditions. Since the temperature difference between the TEG surfaces is very small at these levels of solar radiation, the power produced by the TEG is low. If the CPV-TEG system in which the light is concentrated is considered and the radiation is 2000 W/m2 with the assumption of 2 times concentration, the PV cell temperature was found to be 349.3 °K (76.3 °C) and the ∆T TEG module surface temperature difference was found to be 2.62 °K. If the cooling capacity of the TEM heat exchanger is increased by 10 times in these conditions, the PV cell temperature was found to be 311 °K (38 °C) and the ∆T TEG module surface temperature difference was found to be 3.98 °K. If the CPV-TEC system simulation is performed, if the TECs are supplied with a voltage of 2% of their maximum voltage values, the PV cell temperature is found to be 348.5 °K (75.5 °C), that is, the PV cell temperature is reduced from 76.3 °C to 75.5 °C with TE cooling. Since the cooling capacity of the heat sinks used in the experimental set is limited, it is found that the PV cell temperature can be reduced up to 48.34 ℃ with TEC cooling in the simulation, and the PV cell temperature cannot be further reduced with the existing heat sink system, although more cooling power is consumed by increasing the current further. In the systems controlled by MPPT, the simulation and experimental results for the average PV system power values were very close to each other, with the simulation result being 1.375 W and the experimental result being 1.382 W. The difference between them is 0.29%. The average power output in the TEG system was 1.517 mW in the simulation and 1.590 mW in the experiment, with a difference of 4.59% between the results. The PV cell temperature was 49.14°C in the simulation and 49°C in the experiment, with a difference of 2.8% between the results. According to the results of the simulations comparing the systems with and without TEG, the total PV+TEG power produced in the system with TEG is lower than the power produced in the system with only PV cells, i.e. the efficiency is higher in the system where the TEM is removed and the heat sink is directly connected to the PV cells compared to the system where TEM and heat sink are used together with PV cells. For example, in the PV-TEG system, the PV cell temperature was 48.88 ℃, the PV system power was 1.49 W and the TEG system power was 1.536 mW, while the PV cell temperature was 48.17 ℃ and the PV system power was 1.497 W in the system with only PV cells (with heat sink, without TEG). The simulation and experimental results are generally similar, with differences arising from various factors. To include the effect of wind in the simulations, wind speed was set at 1 m/s. The experiments were conducted in a semi-open balcony environment under sunny and cloudless conditions, assuming that wind speed and radiation remained constant throughout the experiment. Steady-state equations were used for the heat transfer model in the simulations. The experiments, however, were conducted under variable environmental conditions, and the results were obtained after steady-state conditions were established. Additionally, the PV cells and TEMs used in the experiments exhibited behavior different from their characteristics listed in the data sheets. Even cells and modules of the same type showed different operating results. Therefore, the PV cell temperatures measured in the experiments varied, and evaluations were made using average values. Errors from temperature, radiation, current, and voltage measurement sensors also contributed to the differences between the experimental and simulation results. On the DC-DC converter side, the characteristic values of the electronic components such as material internal resistances are obtained from data sheets and included in the simulation model. However, it is assumed that the material characteristics in the electronic circuit are not affected by environmental and operating conditions. In the experiment, the PWM switching of the DC-DC converter was performed with an Arduino UNO microcontroller with an 8-bit resolution operating ratio. The MPPT was controlled with the P&O algorithm and the iteration steps of the run rate changes were set to correspond to 1.916% (5/255). After processing the data received from the measurement system, the step value was not lowered in order not to cause incorrect direction changes in the algorithm. In the MPPT algorithm in the simulations, if 1.916% change in D is applied, the fluctuations in the PV system power were higher than the fluctuations in the experiment. According to the simulation results, in order to obtain more power output from the TEG in the PV-TE system, the radiation level and the cooling capacity of the TEM's heatsink must be high. As a result, it is seen that the established simulation model will be a helpful tool in issues such as system design and system sizing before moving on to the physical application phase.