Results and discussion
5.3 Experimental study on tube only vertical oscillating (M1) and sheet-tube vertical oscillating (M2) PV/T collectors
5.3.1 Performance evaluation
Figure 5.3 show the variation of electrical efficiency on a clear sunny summer day in the Indian Institute of Technology Guwahati (26.18° N, 91.69° E). The electrical conversion efficiency for M1 on a typical summer day (on 8th Sept 2018) is found to vary from 11.65% to a maximum of 12.30% at 12:00 hr at a corresponding solar irradiance of 940 W/m2. Whereas for M2 electrical efficiency found to vary from 12.38% to 13.6% at the same solar irradiance value. Under the same ambient condition, the PV conversion efficiency is observed to be in the range of 9.6% to 11.7%. The higher value of electrical conversion efficiency for M2 is due to the emission of radiation by the copper absorber sheet and the radiation incident on the PV surface. The average electrical efficiency for PV, M1 and M2 collectors are 10.5%, 11.6% and 12.26% respectively on 8th Sept 2018. The results show that there is an improvement in the PV electrical efficiency when cooling tubes are installed. It has been observed from the experimental results that cooling improves the electrical conversion efficiency of M1 and M2 by 10.5% and 16.7%, respectively, as compared to a PV module without cooling on a typical summer day. The electrical efficiency improvement in M1 and M2 are found to be 10.2% and
13.8% as compared to PV module without cooling on a typical winter day. Influence of heat transfer fluid circulation for cooling on electrical efficiency is not so significant in winter. The electrical efficiency is primarily dependent on the intensity of solar insolation, as compared to the other climatic parameters when cooling is employed. The performance of the collectors was also investigated during a winter day (on 25th Dec 2018). The electrical efficiency of M1 and M2 collectors was found to be varying from 10.8 - 13.14% and 11.5 - 13.6% respectively whereas, for PV module without cooling it ranges from 10.6% to 12.0%. Figure 5.5 presents the variation of electrical efficiency of M1 and M2 collectors from 9:00 to 16:00 hrs. The average electrical conversion efficiency is found to be higher in winter months as compared to summer months. This is due to lower ambient and lower PV operating temperatures during the winter months.
Figure 5.3. Electrical efficiency on a typical
summer day Figure 5.4. Thermal efficiency on a typical summer day
Figure 5.5. Electrical efficiency on a typical winter day
Figure 5.6. Thermal efficiency on a typical winter day
The thermal efficiency, on the other hand, is observed to be higher for M1 as compared to M2 collector. The thermal performance is influenced by the difference between inlet and outlet temperature as well as the solar irradiance pattern. On a clear sky, as the solar irradiance increases from the dawn reached a maximum at the noon, the temperature of the PV layer and absorber keep on increasing and the heat transfer to the fluid flowing through the tube increases, subsequently the outlet temperature of the fluid increases. The thermal efficiency for M1 gradually increases from 46.24% at 9:00 hr to a maximum value of 66.7% at 12:00 hr and then it starts decreasing and reaches a value of 33% at 16:00 hr, this can be seen from Figure 5.4. For M2, thermal efficiency is found to be in the range of 22-56%. The average value of thermal efficiency is observed to be 52% and 41.7% for M1 and M2, respectively. The higher thermal efficiency of the M1 is due to the higher heat transfer rate (less thermal resistance) from the top surface of the PV/T to the fluid flowing through the tubes as compared to M2 [12]. Thermal efficiency during a sunny winter day is lower as compared to that of a typical clear summer day. The average thermal efficiency on 25th Dec 2018 for M1 and M2 is 48% and 36%, respectively (Figure 5.6).
Figure 5.7. Thermal efficiency curves for M1 and M2 collectors
The collector efficiency curves over a range of T*values for M1 and M2 are shown in Figure 5.7. From the results, it is evident that the absence of absorber sheet (in M1) results in an improvement in the thermal efficiency. From the trend lines in Figure 5.7, the intercepts are 0.72 and 0.61, indicating the thermal efficiency at zero reduced temperature for M1 and M2 respectively. Also, the slopes are 16.72 Wm-2/°C and 13.98 Wm-2/°C for M1 and M2
respectively. It can be observed that the thermal efficiency of M2 experiences more deterioration at increased fluid temperatures as compared to M1.
The overall energetic efficiency of M1 varies from a value of 57.9% at 9:00 hr to a maximum of 79% at noon and then reduces to 43.99% on a summer day (8th Sept 2018) as shown in Figure 5.8. The overall energetic efficiency for M2 on the other side varies from 46.05% to 67.14% and then decreases towards the afternoon. It has been observed from the experimental results that overall energetic efficiency during a winter day fairly remains similar to that of a summer day for M1, but it reduces in the case of M2 during the winter as compared to a summer day. During winter, the temperature of the absorber sheet in M2 remains low as compared to summer. The overall energetic for M1 varies from 52.8% in the morning to 74.93% in the noon and then decreases to 37.8% at 16:00 hr (Figure 5.10). But for M2, overall energetic efficiency increases from 45.64% reaches a maximum of 61.2% and then falls to 24.07% in the latter half of the day.
Overall exergetic efficiency follows the same trend as that of electrical efficiency. As overall exergetic efficiency is a strong function of electrical efficiency. Thus, the effect of low- temperature thermal energy in overall exergy output of the PV/T is not significant. Overall exergetic efficiency increases from the morning as the solar irradiance value increases to a maximum at noon and then decreases. The overall exergetic efficiency varies from 11.14 - 13.6% and 12.7-14.49% for M1 and M2 respectively, on a summer day (8th Sept 2018). The average overall exergetic efficiency over the day for M1 and M2 is 12.26% and 13.6%, respectively. It can be observed from the Figures 5.9 and 5.11, that the average overall exergetic efficiency for the PV/T collectors (12.59% for M1 and 13.6% for M2) are higher as compared to summer (12.25% for M1 and 13.87% for M2). The improvement in the average overall exergetic efficiency is due to the low operating temperature of the PV.
Figure 5.8. Variation of overall energetic efficiency on a typical summer day
Figure 5.9. Variation of overall exergetic efficiency on a typical summer day
Figure 5.10. Variation of overall energetic efficiency on a typical winter day
Figure 5.11. Variation of overall exergetic efficiency on a typical winter day 5.3.2 Collector temperature rise
The time necessary to achieve the desired hot water temperature from the inlet water condition in the collector is an important parameter to define the applicability of a newly designed solar hot water generation system. Generally, water is allowed to get heated inside the collector under a steady-state for some duration of time; then, the heated water is taken out for end users. Therefore, it is necessary to know the temperature rise of the fluid at various time intervals so that the user can have more flexibility to decide on the end-use of generated hot water. The rate of temperature rise of fluid under the stagnant condition for PV/T collectors was performed. The weather parameters on 31st October 2018 is shown in Figure
5.12. The outlet temperature for the PV/T collectors and inlet temperature at each interval were measured. The variation of temperature for the collectors is shown in Figure 5.13. Fluid outlet temperature increases with an increase in the retention time of the fluid in the collector.
Temperature rise in M1 is higher than M2, the temperature rises of 9.39 °C and 7.59 °C are reported for M1 and M2 respectively at a heating time of 1 min. At 60 min heating time, the temperature rise of 34.5°C and 30.5°C for M1 and M2 have been observed. As seen from Figure 5.13, the heating rate for M1 decreases from 9.39 °C min-1 to 0.57 °C min-1 when heating time is increased from 1 min to 60 min. For M2, the heating rate decreases from 7.59 °C min-1 (1 min) to 0.50°C min-1 (60 min). The water reaches a saturation limit for thermal energy absorption after a particular time, so the temperature rise reduces.
Figure 5.12. Weather data on October 31, 2018
Figure 5.13. Collector temperature rise in PV/Ts