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To obtain the maximum input, solar modules generally need to face into the sun. But before going too deeply into building complex mechanisms or buying costly trackers it is worth quantifying the probable gain.
In our part of the world, the further one moves south (i.e., toward higher latitudes) the lower in the sky the sun tracks from east to west. A horizontally mounted solar module will be effectively exposed to less sunlight than one facing into that low sun. At high latitudes it is thus well worthwhile tilting the module accordingly. The higher the latitude, and the closer to winter, the lower is the sun and the greater the increase gained.
But where, as across a major part of Australia, the sun tracks close to, or actually overhead, for a fair part of the year, a horizontally mounted solar module is already facing into the sun. There will be some minor gain (typically <10%) during winter, and particularly down south, but by and large even a perfectly tracking solar mechanism will not be as remotely effective in much of Australia as in places like northern Europe.
Precise solar module orientation is not necessary. The maximum irradiation is generally obtained when the module is facing directly into the sun, but the sun’s effect is far from a ‘shaft of light’. It is often quite diffused and even substantial non-alignment makes little difference. Here are two examples:
The Australian Solar Radiation Data Book shows that, for Adelaide (latitude 35 degrees south) during January, the difference in solar input between horizontal mounting and the more accurate 10º tilt is a mere 0.16%. Even 20º error makes only 4% difference in solar input.
The same sources show that, on a yearly basis, the irradiation for Adelaide with modules at the optimum fixed tilt (of 30º) results in an average gain of about 8% compared with horizontal mounting. There, variations of plus/minus 20 degrees, both in azimuth (north facing) or tilt cause less than 5% daily difference.
Tracking the sun in an East/West plane makes surprisingly little difference at low latitudes during summer, but there is an appreciable gain in winter. In Brisbane (latitude 27.25 S) for example, the difference in January is less than 6%. The average maximum gain during the three winter months is about 40%.
Worldwide, there is less irradiation as the sun rises and sets and tracking in the North/South plane will capture more of that available. But unlike East/West tracking, the main gains are in summer. Compared with horizontal mounting, and again using Brisbane as an example, the gain (over and above tracking in the East/West plane) is about 17.5% in January, and about 12.5% in June. The obtainable gains become much greater at higher latitudes. Within Australia (excluding the Antarctic!) the greatest percentage gain you are likely to obtain from tracking in both planes will be during mid-winter at Hobart’s latitude of 43 degrees. There, the June input will more than double (but bear in mind the starting point is very low). This progressively drops off as summer approaches. Caution is needed however when assessing performance claims. Those given only as a percentage may impress, but have little meaning unless the change refers to the input over a whole day (or preferably longer). For example increases of 100% or more are common for the hour or so each day following sunrise and before sunset – but the actual gain may be only a watt/hour or two for a 130-watt module.
One (European) vendor of sophisticated tracking systems stresses the effect of latitude and seasonal effects on the gain from solar tracking. He told The Wanderer that: “comparing a flat mounted panel and an optimised panel during noon in summer and equatorial areas will show only a small benefit [for] the optimised panel.” But doing the same thing in winter and high latitude areas “will show a great benefit for the optimised panel.”
It thus seems prudent to establish where test results have been obtained – and/or obtain assurance that the claimed gains can be realised in the latitudes and seasons where the modules are to be used.
A surprisingly large amount of the available gain is obtained by simply having modules more or less facing north, at somewhere around the latitude angle. Doing this alone will add about 33% more input during the winter months and up to 10% or so in the summer down south. This is now the preferred method of most professional installers.
Where tilting is less feasible, as with fixed solar modules on RVs, the increasing lower cost (in real money terms) of solar modules is resulting in it being simpler and cheaper to gain more solar input by adding extra module capacity. Whether or not tracking is worthwhile depends very much on where you live, what you use solar for and, very much when you use it. As this article has shown, real gains are massively dependent on latitude and season.
As a rough general guide the Solar Energy Industries Association of Australia’s ‘Remote Area Power Supply Systems’ (1995) commenting on fully tracking solar arrays, states that: ‘It is generally accepted that a 30% increase in energy production should be expected averaged out over the whole year.’ The increase will however be lower at low latitudes –(CR).
For clarity I need to note that the solar output shown in Peak Sun Hour maps (as in my books) give the average irradiation incident on a horizontal surface. It is that obtained using a horizontally mounted solar module. Tilting or tracking will increase the output but only if that module is producing less than maximum. The increase may also be sometimes less than expected. It may even be negative: where for example a module already producing full power becomes hotter with full sun exposure - and loses output through heat loss.
This is a totally different technology that, like tracking, is intended to increase a solar module’s output. A typical solar module produces maximum power (volts times amps) around 17.2 volts, but as a battery requires anywhere from 14.4-14.7 or so volts to fully charge, that part between the solar output and the battery’s need is normally ‘lost’. The MPPT unit ‘juggles’ the volts and amps to optimise watts thus recovering some part of that lost area. The technology is particularly effective where (a) the battery is low in charge, and (b) during the early and late hours of the day. These devices are typically claimed to ‘save’ 25-30%. One (French-designed and Chinese-made) claims to save 40% but, as this is more than is lost, there is clearly a major misunderstanding about the limitations imposed by basic physics. General and my own experience with a large MPPT unit (handling 50-amps at 72-volts) is that the gain varies from 12.5-17.5%. This is certainly enough to justify its use – but short of some vendors’ claims.
There is a great deal more about solar and allied topics in the new and totally revised edition of Solar That Really Works (now one single edition). It is available from CMCA Headquarters and is also sold at major CMCA Rallies.
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