Novel thermal model and its validation
4.3 Validation of the novel thermal model .1 Temperature variations
to wind. The copper absorber plate in the PV/T collector was attached to the tubes which carried the water, thus extracting heat from the absorber plate leading to lowering the temperature of the absorber plate surface. The difference between the outlet and inlet fluid temperatures is in the range 1-6.3 K. The highest temperature rise in the fluid was observed at noon and thereafter the fluid temperature decreased with time.
Figure 4.3. Variations of irradiance, ambient temperature, relative humidity and wind speed
Figure 4.4. Variations of different temperatures observed in experiment
4.3.2 Performance evaluation
It is always important to assess the performance of a solar collector to justify its applicability and acceptability. In the present study energy and exergy analysis was carried out for obtaining values of various performance parameters, such as electrical, thermal, overall energetic and exergetic efficiencies of the developed PV/T system. Figure 4.5 shows variations in these four efficiencies of the present PV/T system with the time of the day. It can be observed that the electrical efficiency varies from 11.78 - 13.02% and the thermal efficiency varies from 29.97 – 76.37%. The highest value of both the electrical and thermal efficiencies was observed at 12:00 p.m., at solar insolation of 940 W/m2. The overall energetic efficiency of the system, which combines electrical and thermal efficiencies was found to be in the range of 33.9 – 80.7 %. The efficiency of a PV/T collector is directly influenced by the solar insolation and therefore the efficiency variation is dictated by variation in the solar irradiance. The overall exergetic efficiency is however having lower values as compared to the energy efficiency, it varies in the range 12.7 - 14.97%. The lower value of overall exergetic efficiency is due to the low-temperature fluid output from the system.
Figure 4.5. Experiemental values of electrical, thermal and overall efficiencies 4.3.3 Comparison of simulation and experimental results
For assessing the model developed in the present study, its results were compared with the ones obtained using the set of equations proposed by Huide et al. [192]. The experimental data for solar irradiance and ambient temperature were used as the inputs to
both the models. The values of different temperatures obtained were compared with the present experimental data. Variations in the temperature of various components of the PV/T collector obtained from the present simulations are presented in Figure 4.6. All the temperatures increase from the morning and attain the maximum values at approximately 12:15 noon and thereafter these start decreasing . The maximum temperature of the glass, PV cell, water outlet and absorber are found to be 329.3 K, 330.8 K, 320.5 K and 316 K, respectively.
The observed trends in temperature variations are due to variation in solar irradiance during the day, which increases from the morning, reaches the maximum at noon and then starts decreasing towards evening.
Figure 4.6. Variations of temperature of different layers obtained from the present model Figures 4.7 - 4.9 provide a comparison of glass, cell and water outlet temperatures obtained from the models and experiment. It can be observed that the simulation results of temperature obtained from the present model are in better agreement with the experimental data than those by the reference model [192]. A close agreement in the glass temperature observed might be due to the fact that the present model accounts for the radiative heat loss to the sky as well as to the ground along with the convective heat loss due to wind. The root mean square error for the glass temperature for the developed model with respect to the experimental data is calculated to be 1.36 K whereas for the reference model it is 2.27 K.
The PV cell temperature is an important parameter for performance evaluation of PV systems since its variation has a direct impact on the electrical efficiency of PV systems. With a rise in the temperature of a PV cell its efficiency decreases [281,282]. Therefore, it is always important to have a precise prediction of a PV cell temperature. The experimental results for cell temperature were found to vary from 315 K to 331.6 K between 8:00 a.m. to 16:00 p.m. PV
cell temperature was found to be 310.56 - 330.86 K and 308.96 – 327.12 K for the present and reference models, respectively. The RMSE value of the PV cell temperature for the developed model was calculated to be 2.71 K whereas for the reference model it was 11.02 K. A lower RMSE value of the cell temperature in the case of the developed model is due to the consideration of the Ohmic heat dissipation due to the internal resistance of the cells while formulating the present set of equations.
Figure 4.7. Comparison of experimental and simulated values of glass temperature
Figure 4.8. Comparison of experimental and simulated values of PV cell temperature
Figure 4.9. Comparison of experimental and simulated values of water outlet
temperature
The experimental value of the water outlet temperature of the PV/T collector was observed to be less than the values predicted by the two models. However, the experimental
values were found to be closer to the simulation results for the water outlet temperature obtained from the present model compared to that using the reference model (Figure 4.9). The RMSE value of water temperature for the developed model and the reference model were 3.75 K and 11.89 K, respectively. The difference between the results obtained from the two models is significant. The water temperature obtained from the developed model increased from 306 K at 8:00 a.m. to the maximum value of 316 K at 12:20 p.m. Thereafter it started to decrease and reached a value of 307.8 K at 4:00 p.m. The variation in the water outlet temperature obtained using the reference model was 309 K to 327.12 K. The water outlet temperature obtained from the reference model was higher than the results obtained from the present model as well as the experimental data. A higher deviation of the water outlet temperature obtained from the reference model [25] and a closeness of the results using the present model is due to the fact that the developed model accounts for the contact resistances that the thermal energy faces in its flow direction from PV cells to absorber plate and finally makes its way to the incoming fluid through the tube walls. The combined thermal resistance represented by R1and R2 (shown in Figure 4.2 (b)) are calculated to be 3.263 10 3m K W2 and 1.10m K W2 respectively. The thermal contact resistance between the Tedlar back sheet and the absorber plate (Rc ted, 1.098 m K W2 ) is found to be significant among all the thermal contact resistances that exist in a PV/T collector. The thermal model developed by Hulide et al. did not consider the thermal contact resistance parameter in their formulation and thus deviation in the results of the reference model is large compared to that by the present model.