Roots 'n' Shoots: Solar Power in South Africa Part 3: Solar & Seasonality, Living with Solar in Gauteng, South Africa

Saturday, 5 September 2015

Solar Power in South Africa Part 3: Solar & Seasonality, Living with Solar in Gauteng, South Africa


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.

Overcast days in winter are terrible for solar electricity production but fortunately Gauteng is a summer rainfall area with very few overcast winter days. We’ve discussed generator back-up in our first article here, Part 1: How to go off grid permanently (The System Set-up).


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.

Here is a table of our summer and winter electricity supply and demand data. Keep in mind that it applies to our system and our household needs, which for each household would differ.




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.

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)





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9 comments:

  1. Wow - 6 hours of daylight loss??? Where is this guy living? We lose max. 3 hours of sunlight between mid-summers day and mid-winters day i.e. the sun rises at 6.00a.m. in summer, and 7.45a.m. in winter. Sets at +/-7.45p.m. in summer, and 5.30p.m. in winter.

    What he doesn't mention is that the cooler winter temps on the panels actually increases electricity production - when the panels overheat in summer they lose efficiency - but in winter, even with a deeper angled sunlight and a cooler ambient temperature, they actually produce more.

    That's what we've found anyway...

    And as for using the oven to dry fruit and vegetables - hasn't he heard of a dehydrator? Would use less power than switching on the oven to the lowest setting, help to keep the house cooler (no oven heat escaping through the opened doorway) and would obviate the necessity of switching on air-con.

    ReplyDelete
    Replies
    1. Would be extremely surprised if your panels produced more in winter. While cooler temperatures under the same levels of solar radiation do produce more power, this is a relatively slight effect. The lower levels of radiation in winter (due to the shorter days and lower angles of the sun) mean you produce substantially less electricity. This is what all the research shows including South African research), and my personal experience with my panels shows this too. Sorry.

      Jason (commented on part 2 as well)

      Delete
  2. Daylight, daytime and day length do not all equate to a 3 hour difference between winter and summer solstice. The day length increases by 3 hours and 17 minutes (for 2015) between winter and summer solstice, whereas the amount of daylight will differ. For example, our inverters do not switch from solar to battery exactly at the point when the sun sets or rises; it is a few minutes before sunset and after sunrise. This transition zone shifts with the sun as well as increasing the amount of time before switching over as the season moves towards winter. Extra atmospheric noise in winter (such as mist and smoke - which contributes to haziness and increases during El Niño) compacts the problem - hence you get a 3 hour loss in day length + a 3 hour loss in daylight, which leads to a 6 hour loss in total daylight, especially during overcast days (please refer to the graph as well). The increase in solar panel production during the cooler temperatures is not significant enough to make up for the daylight loss due to decrease in solar irradiation.

    Dehydrators are generally expensive (when you calculate the amount of cost per dried fruit it is likely cheaper to buy store dried ones or it would take huge amounts of dried fruits to make back the cost of the dehydrator) and would not necessarily be suitable for all families who already own an oven. We have surplus electricity during summer that allows for essentially free drying of fruit in the oven, no extra costs involved. Also we do not have aircon, but many houses are equipped with units and using the surplus electricity allows for cooling of the house with essentially free electricity. This is very useful since peak surplus electricity time falls within peak heat of the day. It is an illustration of how to use up the surplus so that nothing goes to waste (as it is more economically viable to use the surplus electricity for 'special' uses than to buy additional battery bank capacity to store the surplus).

    ReplyDelete
  3. Thanks for this article.
    I didn't think about the haziness we experience as such a big factor.

    ReplyDelete
  4. You’re more than welcome! We were also quite surprised to see the impact of winter on the solar system - but with a bit of savvy energy management one can get through the tightest times quite reasonably. I think we had a day towards the end of heavy overcast conditions where the solar was depleted, but other than that we didn't experience any load shedding during the winter!

    ReplyDelete
  5. Summer is making a huge difference to our system performance. We are finding that our battery bank is too small as by midday our batteries are charged and we are therefore "losing" power that we could potentially be keeping. Winter (July) we struggled.

    Jason

    ReplyDelete
  6. Thanx for the input Jason - we experience a similar situation in summer, an unfortunate side-effect of the solar system - hopefully the design and technology of the future solar systems will improve on this - hence why it should be mentioned so that the solar can continue to develop and become viable for more households by scrubbing out some of the inconveniences. We are still very happy with our solar system, but also recognise that it might not be suited to everyone's needs and that some limitations still exist with the system overall.

    ReplyDelete
  7. After installing a heatpump and our solar system, our daily consumption has gone down from 53kwh to between 20kwh and 25kwh, which is great, though perhaps slightly short of what I hoped for. Once we connect our garden cottage to the solar system and change a couple of old appliances, and the pool pump, we should get closer to 15kwh.

    And when there's load shedding or electricity issues (the other day our suburb was out for 12 hours) it's just the best :)

    Jason

    ReplyDelete
  8. We are also very impressed with our solar system and as of Oct 2015 have not used any municipal electricity! I have written another update, which you can find at http://rsandss.blogspot.co.za/2016/02/solar-power-in-south-africa-part-4.html

    ReplyDelete

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