Solar power
We've
always tried to be efficient, to use resources and energy sparingly, and to generate minimum waste. In October 2010, we
took the next logical step by putting our south facing roof to good use with
PV (photo-voltaic) panels that generate solar electricity. The panels have a peak rated output of 3.3kW, but the actual
output is generally less than that, and varies with time of day, time of year and the weather. Using published monthly averages
for sunshine in this part of the country, I estimated that the panels would generate around 2.7MWh per year, which is about
10% more than our annual consumption. Of course that doesn't make us 'self sufficient', because we still need to import
energy when we aren't generating, eg to power our lights at night, but we should export more than we import.
Installation was straight forward – we used Ardenham Energy .
The panels are mounted on brackets that go under the tiles, and are fixed to the rafters. They feed DC power to an inverter
mounted in the loft (see pictures below) which sends AC power down to the total generation
meter in the meter cupboard (see pictures below). When the panels are generating, the light
on the meter flashes at a rate proportional to the power. We wanted to be able to see the power without going to the cupboard
and timing the flashes, so we use an Owl electricity monitor that sits on the kitchen
worktop (see pictures below) and communicates with a sensor clipped round the power cable.
We have another Owl meter to show the power that we are using, see below.
Results so far
The
first graph shows energy generated since installation (October 2010 – December 2011). For tidiness the graph starts
with January (week 1). The blue line is for 2011 and the red line is for 2010. The grey line is what I expected as an average.
I derived it from published monthly averages for sunshine in Berkshire, normalised so that the total energy is the same
as derived from the manufacturer's figure, Generation fluctuates from week to week with the weather, but during the winter
months it more or less followed the model. In the spring it was so far above my prediction, that I began to think the model
was wrong, but it must just have been the abnormally sunny weather. For most of the summer generation was below prediction.
It rose again briefly during the very sunny period in September and early October.
The
second graph shows cumulative energy generation. Like the first graph, it starts in January, with 2011 shown in blue and
the tail end of 2010 added in red where it would be if the previous part of the year had been as it was in 2011. Overall,
the energy generated in the first year of operation (October to October) was about 10% more than my predicted figure (3MWh
v 2.7MWh) and the energy generated in 2011 was about 15% higher (3.17MWh).
Daily power variation
The
power varies during the day (even with a clear sky), and the daily pattern varies round the year. In summer, the days are
long and the sun rises high, while in the winter the days are short and the sun is low in the sky, with spring and autumn
in between, and you might expect the power output to do the same. The curve in winter is indeed lower and narrower, but
there is relatively little variation between spring, summer and autumn. Partly this is because the sun is behind the panels
at either end of the long summer days.
These graphs represent the spring (end April), mid summer, autumn (mid September) and winter (mid December).
The power fluctuates quite a lot with passing clouds, so I took readings at or near each hour over several days, and used
the highest figure for each hour. NB – All times are shown in BST, so midday is 13.00.
Import and export
Before
we bought the panels, I predicted that we would export about 85% of what we generate. I had produced a model for generation
at different times of day round the year using various published data, and a similar model for consumption based on knowing
our approximate pattern of using various devices, and I had then compared the two models.
I can't measure exported energy directly, because I opted not to pay extra for a certified export meter.
(We could have got paid a bit more than the nominally assumed 50% if you don't have a meter, but the extra payment of a
few pence per kWh did not justify the cost of the meter.) I still wanted to see how my model matched reality, so I use an
Owl electricity monitor that shows how much power we are using, and also has a cumulative
energy readout. Knowing consumption (from the Owl), import (from the normal meter) and generation (from the generation meter),
I can work out export by subtraction, but it is not accurate because. The Owl measures current and calculates power assuming
a fixed voltage and unity power factor, which is perfectly adequate for the Owl's intended educational use. Quite a few
of our devices, notably fluorescent lights, have a power factor less than 1, so the display reads a bit high. For the graphs
I have included an approximate correction factor.
In the graph, energy generated is shown above the axis. The green part is exported, and the pale blue part
is consumed by us. The dark blue part below the axis is imported energy. So taken together, the two blue arther represent
the total energy that we consumed. The graph includes 9 months of 2011, and the start of 2012. In the period April to December
2011 (the right hand part of the graph) we exported 85% of the energy we generated, and we imported 75% of the energy that
we consumed. The import figure for the six summer months was slightly lower at 66%, and inclusion of the early part of the
year will obviously push up the average over the whole year a little. I will update these figures when I have a full year's
record.
Ecotricity
We sell the power that we generate through Ecotricity's Microtricity
scheme, which is backed by the Government's Feed-In Tariff. We also buy our gas and electricity from Ecotricity
because they invest all of their profit in building new green energy sources, while charging the same prices as our regional
supplier. Ecotricity invests far more per customer in new green production facilities than the large supplies, including
those that offer 'green tariffs'.
Pictures
Click to enlarge.
