Photovoltaic (PV)

Photovoltaic (PV) 30/03/2011:

Corporate HQ North of England.

In 2006 I was asked by an associate to assist him with investigations, using energy monitoring, into unpredicted energy usage at the corporate headquarters of an environmentally conscious company in the north of England. Its design should have achieved <100kWh/m²/annum but, had only managed 180kWh/m²/annum, which was considered acceptable by comparison with similar office buildings, but did not match the owners expectations. The premises had been open for three years and included a substantial array of PV on a south east facing roof. At the time we had little information about photovoltaic installations; this has now changed.

We concluded that poor or non existent time control of the heat recovery ventilation system, gas boiler and electric water heating, also a lack of interlocks on primary hot water system (HWS) pumps contributed to annual dual fuel energy usage of 95000kWh/annum (180kWh/m²). Improvements may have reduced energy consumption to 60000kWh/annun (113kWh/m²). At the time it was also stated that “the photovoltaic contribution is low and not commercially viable”, which the designers had expected.

Now we have greater knowledge of PV systems and their potential output which can be calculated on the internet and this has uncovered some interesting facts about the system installed at this location. The system had 40 cells over two rows on a south east facing roof with approximately 35° inclination; it is estimated the peak system power should be 8kW. The table below is derived from meter readings taken during the site visits.

Date  Days   Time PV PV
        kWh kWh
04-Aug       8088  
18-Aug 14 12.5 12:05 8263 175
08-Sep 21 12.5 11:00 8525 262
26-Sep 18 11.5 10:05 8732 207
11-Oct 15 9.07 10:00 8868 136
    11     780

The meter readings highlight that the PV system had delivered, on average, less than 10kWh/day over 3 years. The table below is from a calculator which estimates what should have been generated by this system 

Fixed system: inclination=35°, orientation=-68° 
Month Ed Em Hd Hm
Jan 4.57 142 0.75 23.3
Feb 8.79 246 1.42 39.8
Mar 13.9 431 2.28 70.5
Apr 20.6 617 3.46 104
May 26.6 824 4.56 141
Jun 25.8 773 4.5 135
Jul 25.9 804 4.55 141
Aug 21.6 670 3.76 117
Sep 16.7 501 2.83 84.8
Oct 10.2 316 1.69 52.4
Nov 5.37 161 0.88 26.5
Dec 3.12 96.7 0.52 16.1
 
Yearly average  15.3 465 2.61 79.3
Total for year 5580 951

 By comparing the meter reading table with the estimated calculator it is evident that the system has under performed by almost 50%! However, if the estimates had been matched it would have reduced the annual energy consumption by a further 2600kWh.

 

Access to recent data from other projects confirms that, correctly installed, the yield from PV arrays can match or better pre installation calculations.

 

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