the photovoltaic system on the roof of the Physics Department of Bologna University index: solar energy and photovoltaic roofs the future

Solar radiation and electric power generated

Fig. 1 shows, at different times of day, the incident solar radiation measured by a radiometer (with a precision of 5%) and the electric power generated in alternate current, on 2 July 2003 - both expressed in W/m2.

On that day there were no clouds and only slight unsettled weather for short periods. The curves for insolation and electric energy generated are typical "bell curves", with maximums at 12.00 hours solar time. We can see that the electric power generated follows with a regular pattern the power of the incident radiation, but reduced by roughly a factor of 10.

Fig. 2 shows the incident solar radiation measured by the radiometer and the electric power obtained on a winter day. We can see the shorter duration of the day and the presence of variations due to clouds.

Fig. 1: Incident solar radiation (in W/m2) and electric power generated (in W/m2) on 2 July 2003. Data recorded every 10 minutes.


Fig. 2: Incident solar radiation and electric power generated on 23 February 2003.

Fig. 3 shows the monthly insolation (in kW/m2) from January 2001 to December 2005, while Fig. 4 shows the a.c. electric energy generated every month (in KWh) in the same period. We can see again that the production of electric energy follows insolation with a regular pattern.

Fig. 3: Monthly incident insolation.
Fig. 4: Electric energy generated monthly.

If we consider five years, from 1 January 2001 to 31 December 2005, there was a total insolation of 6990 kWh/m2. The total electric energy in alternate current generated in the same 5-year period was 10403 kWh, which corresponds to 150 kWh/m2 per year and to 1112 kWh/year/kWpeak installed. We can note that the overall efficiency E of the system is

R=[electric energy generated/m2]/[insolation/m2] = 150/1400 = approx. 10.7%

The table below summarizes the values obtained in the period in question:

  2001 2002 2003 2004 2005 Annual average
Annual insolation (kWh/m2) 1440 1350 1443 1389 1368 1398
Energy generated (kWh/m2) 157 142 158 148 146 150
Energy generated (kWh) 2180 1965 2185 2055 2018 2080
Efficiency (%) 10.9 10.5 10.9 10.7 10.6 10.7

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