How’s your Solar in Winter? (Author: The Economist)
Winter sneaked up on our solar electricity system during July. We hardly
took notice of the change in the length of a day before installing solar and
now it matters. We lose up to 6 hours in daylight from the longest day in
summer to the shortest day in winter. That is a lot. The days shrink by almost
2 minutes per day, that’s about 20 minutes every 10 days! The result is that
the supply of solar electricity shrinks as the days get shorter and the demand
for reservoir electricity increases as the nights get longer.
The daylight and night time effects combine with a third
negative, a drop in temperatures for a triple punch to the solar electricity system.
The drop in temperature means that all heat based electricity needs increase
and here I exclude home heating. The heat pump takes 3 hours to get the geyser
temperature to 60 degrees when in summer it would take between 1.5 and 2 hours
to do the same job. The kettle takes a few minutes more every time it is used.
The dishwasher has to contend with much lower temperature water inflows to be
heated up and the tumble dryer also has to work harder to dry the clothes. It
all adds up to increased electricity demand when days get shorter and reservoir
needs increase.
Winter is when you find out how robust (or not) your solar
electricity system is. So, let’s look at supply of electricity from the solar
panels.
There is one additional observation with regards to supply
in winter which matters for us here in Gauteng South Africa. Winter sees
atmospheric interference which causes hazy mornings and afternoons. So the
solar electricity production is muted even with good sunshine at around 08h30.
Once the hazy conditions clear up around 09h30 we experience a steep rise in
electricity production. Sadly the loss of electricity generation due to
haziness has a significant effect on total daily electricity production in
winter and haziness is the norm not the exception, so we show hazy conditions on
the graph as the “best” case. The electricity generation in winter is about 55%
of summer generation with hazy conditions, but on clear winter days we can get
around 65%-70% of summer electricity production.
The change is season requires a resource management response
from us. We have to use the electricity with more care with the most important
effect having to spread the use of electricity. An example is not to do the
washing only once a week, which requires the tumble dryer to run 4 times during
the day but to spread the washing over two non-consecutive days. Thus, the
tumble dryer runs only twice on those days. Demand management will be the
subject of our next post.
Our solar electricity system can supply up to 56.5kWh
electricity per day in high summer. We usually only need around 20kWh per day
in summer on high use days, which means that we have surplus supply of more
than double our needs. Here the “use it or lose it” principle applies so one
can store it in ever larger battery banks or use it for unusual needs such as
cooling the house (aircon) or processing of excess fruit and vegetables into
jams or for drying (which uses the oven), etc.
In winter the picture changes fairly dramatically. We
produce only around 31kWh electricity per day while a high use winter day can
easily get to 28kWh. Our winter surplus now falls to only about 3kWh per day
slightly more than a 10% surplus margin. Our daylight supply in deep winter is
tight and our overnight reservoir is also under pressure. I’ve discussed the
battery banks here, Part 2: Living with Solar in Gauteng, South Africa. Our solar electricity supply is adequate
albeit tight as the few high winter weeks are manageable and soon after we are
back to ever increasing surpluses as the length of days increase by 20 minutes
every 10 days.
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| Sun position in the sky at 18h00 from January to December 2015 (read from- top left to bottom right). Notice differece in available light; this has an effect on solar production throughout the year. Picture generated with screenprints from Stellarium 0.11.2 software. |
The solar experience so far is a positive adventure with no
load shedding and with significant economic benefits!
Program reference: Stellarium - A free planetarium software, allows you to look up and track stars, planets and constellations in the night sky.
Our solar system was designed and installed with the assistance of Jurie Venter, cellphone 083 557 6031 and email jurie@sunor.co.za . For details on the whole system, see the post How to go off grid permanently.
Related Posts:
Part 1: Solar Power in South Africa - How to go off grid permanently (The System Set-up)
Part 2: Living with Solar in Gauteng, South Africa (Batteries)
Part 1: Solar Power in South Africa - How to go off grid permanently (The System Set-up)
Part 2: Living with Solar in Gauteng, South Africa (Batteries)
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