The periods for comparison are based on the experimental hours between 07:30 hours and 17:30 hours of the 2nd August 2014. The most important performance characteristic parameters compared were:
• The thermal energy output, including the hot water and PV cell temperatures and useful heat energy generated.
• The efficiencies including thermal, electrical, overall energy, overall exergy and primary saving efficiencies.
5.2.1 Thermal energy outputs
The thermal outputs for the VIPV/T were compared in order to establish the performance in terms of useful heat output, out let (hot) water temperatures and the average PV cell temperatures which together had an effect of the performance, these quantities are analysed in the is section.
5.2.1.1 Useful heat
The comparison between the simulated and experimental useful heat were as shown in figure 59. The simulated results were represented by dashed lines. As can be seen, there was an average variation between simulated and experimental values of about 1.22 % and 1.53 % for VIPV/T and PV/T respectively. This deviation was within the acceptable limits of below 5 % as stipulated in the ISO 9806-1:1994 and compares well with the findings of Da Silva et al.,[31]. The maximum values attained for experimental and simulation studies were 208.87J/kg and 212.41J/kg for VIPV/T and 183.2J/kg and 187.17J/kg for PV/T respectively. However, the daily total values were almost the same for both systems at 5,376J/kg and 5,385.4J/kg (VIPV/T); 652.8J/kg and 4262.2J/kg respectively.
Figure 59: The variation of simulated and experimental useful heat energy transferred to hot water
5.2.1.2 Outlet water temperature
As an indicator of thermal energy output, the outlet water temperatures were measured from each module both in the experiment and in the simulation model. The variation of the outlet water temperatures for simulation (dashed lines) and experimental results were plotted as shown in figure 60. The maximum temperatures attained by VIPV/T during the experiment and simulation were 62.31oC and 57.97oC respectively while the PV/T attained 51.83oC and 52.02oC respectively. As can be seen, the maximum simulated values occurred at around 11:00 hours while experimental values peaked at around 12:00 hours.
The shift was caused by the estimations of the TMY weather files used in the simulation as opposed to the actual measured data for the experiments. In general, the variations
0 50 100 150 200 250
Time (hours)
Heat Energy (J/Kg)
Qu (VIPV/T sim) Qu (VIPV/T exp) Qu (PV/T sim) Qu (PV/T exp)
07:30 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30
between the simulated and experimental water temperatures were 0.4 % and 0.7 % for VIPV/T and PV/T systems respectively. The variation was less than 5 % and was found to satisfy the requirements of ISO 9806-1:1994, upon which Zondag et al., [24] also based his comparison.
Figure 60: The variation of experimental and simulated outlet water temperatures
5.2.1.3 The average PV cell temperatures
The average PV cell temperature is a very important parameter in electrical performance analysis. It is an established fact that electrical efficiency reduces with increase in PV cell temperatures, therefore, a well-designed hybrid PV/T system must be able to ensure these temperatures are kept as low as possible to keep the efficiencies at sufficiently near maximum levels.
Figure 61 shows the variation of simulated (dashed lines) and experimental average PV cell temperatures for the hybrid VIPV/T as well as the PV/T collectors. The highest values attained by VIPV/T were 179.88oC and 181.75oC for simulation and experiment respectively, both occurring at 12:00 hours both for simulation and experiment. On the other hand, the PV/T achieved simulated and experimental values of 173.65 oC and 172.39 oC respectively occurring at 11:30 hours. Overall, the simulation tallied with the experimental results to the degree of 0.8 % and 1.05 % for the VIPV/T and PV/T respectively. This variation was due to the assumptions made and ideal scenarios used in the simulation which was not the case in the experiment. However, the provisions of ISO 9806-1:1994 standard set at not more than 5 % variation was satisfied.
10 20 30 40 50 60 70
Time (hours)
Temperature (Deg. C)
To (VIPVT sim) To (PVT sim) To (VIPVT exp) To (PVT exp)
07:30 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30
Figure 61: The variation of simulated and experimental average cell temperatures
5.2.2 Efficiencies
The efficiencies for hybrid VIPV/T module obtained by simulation and experiment were compared and plotted as shown in figure 62. The variation between the simulated (dashed lines) and the experimental thermal, electrical and overall efficiencies were 1.05 %, 1.29 % and 3.57 % respectively.
Figure 62: The variation of simulated and experimental thermal, electrical and overall efficiencies for VIPV/T module
The individual simulated average values were 15.7 %, 9.7 % and 25.5 % while experimental values were 15.9 %, 10.5 % and 26.4 % respectively. Again the variations between simulated and experimental efficiencies were within the allowable tallying limits.
0 20 40 60 80 100 120 140 160 180 200
PV cell tenperature (Deg. C)
VIPV/T exp TCell VIPV/T sim
TCell PV/T exp TCell PV/T sim
07:30 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30 Time (hours)
0.05 0.1 0.15 0.2 0.25 0.3 0.35
Time (hours)
Efficiency (%)
hTh (VIPV/T) Exp hTh (VIPV/T) sim he (VIPV/T) Exp he (VIPV/T) Sim ho (VIPV/T) Exp ho (VIPV/T) Sim
07:30 08:30 09:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30
Figure 63 presents the variation of exergy and energy saving efficiencies for the hybrid VIPV/T collector. As can be seen there was a narrow gap of 3.6 % and 2.8 % between the simulated (dashed lines) and experimental values for exergy and energy saving efficiencies respectively. The average values as obtained for exergy efficiency were 10.9 % and 11.7 %, while, energy saving efficiencies were 19.48 % and 20.48 % by simulation and experiment respectively.
Figure 63: The variation of simulated and experimental exergy and primary energy saving efficiencies for VIPV/T module