Tuesday, 5 November 2013

What will PV panels do to your school's energy bill?

In a previous post I showed you some analysis of gas use data from some schools in Hertfordshire (Potential energy savings in Schools). I also have electricity data from some of the schools from the same source. I am working as a volunteer on a project to raise financing for PV panels for community buildings rooftops, including schools, so I did some analysis to see what impact the panels would have for the sample schools - how much of the power from the panels would be used by the school and how much would be exported to the grid. The schools varied considerably in how much electricity they used at weekends and in the holidays. Also some had a relatively high base load and others presumably are more careful to turn things off at night. Apart from the holidays, their use pattern is probably similar to many other public buildings and offices. First, here are some charts.

Out of the five schools I have selected two with different patterns of use. School 1 has a lot of out of hours activity and a high base load - around 5 kW overnight - and school 2 much less with only about 2 kW overnight.

This analysis uses half hour logged data for each school for a year and 'typical' PV generation data from PVWATTS for London/Gatwick. The generated power is sampled hourly from selected months from the last few decades: January from 1991, February from 1994 and so on.

These charts show a particular school day in May; both demand and generation vary from day to day with activities and weather. The lines above the X-axis represent electricty used - either from the panels (green) or the grid (red). The blue lines below the line represent electricity exported.

On this day both schools use most of the electricity generated. This is not so at weekends or in the holidays. Also the charts are for a day only a month away from the summer solstice. When the days are shorter the PV panels will contribute over less of the day.

School 1 also uses a lot of power in the evening when there is nothing coming from the panels and drops down to base load only at about 11pm on this particular day. School 2 uses very little power after 6pm but even so it has to draw from the grid in the later afternoon.

My analysis is slightly optimistic as the power use is averaged over quite a long period - 30 minutes for the schools. This smooths out brief, high power loads such as kettles or printers. A laser printer can easily use 700 or 800 W while it is working. A kettle could be 2kW or 3kW. If this makes the load more than the PV power available there will be more power drawn from the grid. However, other loads such as lights, screens and computers are fairly steady so assuming the printers and kettles don't contribute too much to the overall bill the analysis is fair. Also, the PV generated power is averaged over an hour and in practice will fluctuate up and down somewhat.

The next two charts show a year, with each vertical line representing a day. In school 2 you can clearly see the weekends and holidays because there is much less electricity used on those days - so more is exported

Finally, here are some charts showing the effect of changing the size of the PV array. Increasing the size decreases the energy bills slightly but with dimininishing returns.
This table shows the savings for the two schools with a 50 kWp array in tabular form (all units are MWh/year). School 2 would reduce its bills by more than a third, school 1 rather less because more of its energy use is outside the solar day.

School 1School 2
Total used 7148
PV exported2223
PV used1918
PV provides fraction27%37%

If there is one thing to be learned from this it is that it is hard to predict the impact of PV panels on your electricity bills without quite detailed information about your current use. This makes a difference to your savings estimates because electricity you export earns you the export tariff - currently 4.64p/kWp whereas the electricity you use saves you what you would have paid for it - probably 9p/kWh or more. Also the export tariff rises with RPI whereas your electricity price could rise much faster than that.

The other lesson to take away from this, though it should have been obvious anyway, is that even if you have PV panels your electricity is not free, especially the fraction that you use overnight. So even if you have PV panels, if you shut down the office overnight make sure it really is shut down as far as you can, and not just wastefully idling.

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