When the band Bananarama penned the ditty, “A cruel summer” back in 1983, I’m sure they must have been singing about solar power. Well, upon reflection, they probably weren’t. Anyway, it is an appropriate metaphor for solar power generation in the Macedon Ranges in Victoria and indeed elsewhere across the country for this summer given the La Nina influence on our weather. In the article A Solar Powered Life – part II, I wrote about how many solar panels you would need in order to generate your average electricity requirements. These were all based on a best case scenario which is rarely achieved in the real world. Not to fear though, as solar power has plenty to offer people even in less than perfect conditions.
Solar panels are affected by all sorts of factors in the real world. Usually, anything that affects the performance of a solar panel will tend to reduce its electrical output, so it’s worth understanding these factors so that you can set-up your solar panels so that they provide the best possible output.
There are various things that affect the output of solar panels and they include:
- Heat. Generally, as the air temperature increases the electrical output of solar panels decreases. At an air temperature over 25 degrees Celsius, you’ll find that the electrical output begins to decrease. By around 30 degrees Celsius I’ve noticed electrical output can drop by around 15%, and by 35 degrees Celsius it can be reduced by as much as 33%. It’s worth noting that some types of solar panels are better adapted to warmer environments than others (for example the type of solar panels installed at Zaytuna Farm). People tend to confuse the optimal conditions for electricity generation with that of the production of solar hot water where you require the heat from the sun to warm the water. Solar panels simply require strong direct sunlight, without the heat.
- Shading. Any shading across a solar panel will reduce its electrical output. Sunlight is a diffuse form of energy and the trees are also competing with your solar panels for the same available sunlight. If you have large trees shading the area where you are considering installing a solar panel array, then it’s probably worth installing the solar panels in a less shaded area. There are panels that perform slightly better in shaded conditions, however no solar panel will produce maximum output without strong direct sunlight available across the entire solar panel. A solar panel in shade will probably only produce about 15% of its rated output.
- Available sunlight, summer versus winter. In the previous article I explained that where you are located on the planet will affect how much peak sunlight you receive over summer and winter. I’m at 37 ½ degrees latitude south (for the technically minded) so receive 6.5 hours of peak sunlight over summer and 3.13 hours of peak sunlight over winter. As you move closer to the equator, the difference between available summer and winter peak sunlight will be less, although it will also become much hotter.
- The optimal angle of solar panels differs between summer and winter. For solar panels to receive the greatest amount of sunlight possible at any time of the year, then they need to directly face the sun. As previously mentioned, over winter the sun is low in the sky. For the solar panels to directly face the sun, you can imagine that they’d need to be on quite a steep angle which can sometimes be close to vertical. Over summer, with the sun high overhead you need to have the solar panels angled more or less flat to receive the greatest amount of sunlight. In the real world, you can achieve an average (between vertical in winter and horizontal in summer) by angling the solar panels at the same angle as the latitude of your house. This is a general rule applied to most well installed fixed solar panel installations. I’m at 37 ½ degrees latitude south, so the panels themselves are mounted at about a 37 ½ degree angle from horizontal. Fortunately for me, the angle of the roof of my house roughly matches this. You never really get the maximum electrical output, but it’s a good compromise for both winter and summer.
- Trackers. With some installations, where the budget exceeds my meagre resources, people install their solar panels with a solar tracker, rather than just at a fixed angle. A solar tracker is usually a pole mounted in the ground with a frame at the top of the pole which can hold a number of solar panels. The wonderful thing is that the frame can change angles and directions so that it can always face the sun at the best possible angle. The adjustment can be either manual or for the really well resourced, automatic. These tracker systems are the single most effective thing you can do to increase the electrical output of your solar power system. The tracker system can increase output by approximately 20% to 30%.
- Panels facing towards the equator are preferable in fixed installations. Most fixed solar panels face roughly north in the southern hemisphere or south if you’re in the northern hemisphere. The reason for this is that the sun rises in the east and sets in the west, so a panel facing towards the equator will on average face the sun for the longest amount of time each day of the year. If a solar panel is facing slightly north west (or south west in the northern hemisphere) output won’t be noticeably reduced. If you live on an easterly facing sloping block, you’ll probably find that you have predominantly morning sun. If on the other hand, you live on a westerly facing block, you’ll probably find that you have predominantly afternoon sun. As a general rule west facing slopes tend to receive more sunlight than easterly facing slopes so they are more suitable for the generation of solar power.
- Maximum output is usually around the summer equinox. In the southern hemisphere the summer equinox is usually around the 21st December. The summer equinox is literally the day of the year with the longest available sunlight. What a lovely Christmas present for those in the southern hemisphere who have warm to hot Christmases, in that you also receive the maximum output from your solar power system! The simple reason for this is that the sun is high overhead and in the sky for a longer number of hours than at other times of the year, both of which increase the amount of electricity generated.
- The cloud edge effect can be quite substantial. The cloud edge effect is where the sky is predominantly sunny with some dispersed clouds. What happens is that the sunlight literally bounces off the edge of the clouds so that you can receive more than the normal quantity of sunlight on the face of your solar panel. I’ve seen around the summer equinox and on a cool, partly cloudy day, that electrical output can exceed the rated output of the solar power system. I’ve also had anecdotal evidence of this effect from other people with solar power systems.
- Rain has a greater effect than cloudy days. Rainy days tend to also be days where the cloud layer is quite dense and relative humidity is quite high. This heavy cloud has the effect of reducing the available sunlight. A rainy, cloudy or misty day can reduce output to about 15% of the rated capacity of the system. Generally, days with a higher layer of cloud can reduce the output to about 40% of the rated capacity of the system.
In A Solar Powered Life – part II I wrote about a 260Wh solar panel and how it related to the electricity requirements in a household. How do the above factors affect that 260Wh solar panel?
It means that you will only ever generate 260Wh on a cool partly cloudy day around the middle of summer, and only then if that solar panel is exactly facing the sun! For the remaining part of the year in different conditions, it will be less than this.
What type of panel should I buy?
There are different types of solar panels commercially available, made of differing materials. If you look at some sales blurbs for solar panels they may include specifications like: advanced mono-crystalline, polycrystalline, thin film, flexible, HIT etc. These different materials and technologies may mean that some panels may be more efficient or better constructed than other panels, or perform slightly better under certain conditions, but they all ultimately perform the same function. All solar panels take light from the sun and then output electrical energy which we can then use. It’s that simple.
I run two separate solar power systems at my house, one based on mono-crystalline and the other on polycrystalline solar panels and they both perform equally well. The differences aren’t as great as you may be lead to believe, so I am very dubious of claims of massively increased outputs from a particular panel relative to other types of solar panels. It is also worth noting that under laboratory conditions with exotic materials very high conversions of sunlight to electricity have been achieved, but these are not commercially available.
For my house, I have a system that is rated to generate 1.44kWh. It includes 8 x 180Wh mono-crystalline solar panels of dubious parentage (ie. no name brand). The solar panels are fixed at an appropriate angle for the latitude and they face approximately north-north-west which is quite acceptable. I’m also surrounded by quite tall trees but with no over shadowing on a westerly sloping block. I’m pretty happy with the system and can’t afford to increase or optimise it’s performance.
Getting back to Bananarama though (or was it La Nina?) the system has been challenged by excessive rainfall in recent times. Over January and February this year (2011) I received about 437.2mm (about 17 inches). In the previous calendar year (2010) I received in total a record breaking 1,420mm (about 56 inches) of rainfall spread throughout the year (rainfall has been recorded in this area from about 1870 onwards). All of this intense weather means that what buffer I had between my usage and my generation capacity has been tested at times.
So, you can get an idea of the current end of summer / early autumn sky conditions and how they affect the output of the solar power system, below you’ll see photo’s which are representative of each day this week and the respective power output per day of the system (remember that the maximum potential output.
Sunday 27th February – Output 0.6kWh
Monday 28th February – Output 0.84kWh
Tuesday 1st March – Output 3.6kWh
Wednesday 2nd March – Output 5.2kWh
Thursday 3rd March – Output 3.6kWh
Friday 4th March – Output 2.3kWh
Saturday 5th March – Output 6.1kWh
Total Output from the solar panels for the week is 22.24kWh (or an average of 3.18kWh). I use about 2.9kWh per day and the system itself uses about 0.4kWh per day so have had to draw on the batteries to be able to sustain my usage
In part 2, I wrote about the differences between a grid tied solar power system and an off grid solar power system, but it’s useful reiterating those differences again. Aside from the technology and cost differences, the main difference is where a household obtains their electricity from when the renewable energy sources are not generating power. With a grid tied system, the excess requirements comes the electricity grid and with an off grid system, it comes from a battery (or other source) connected to the system.
In the next article I’ll look at batteries and how they perform in the real world.
Continue on to read Part IV…