Steerable solar panels are more efficient than fixed units, but there are costs. Phil Kreveld looks at all the angles.

The typical domestic solar panel in Australia faces north, or as close to north as allowed by the orientation of the house, and at an angle determined by the roof pitch.

It’s to be hoped that most installations maximise the energy available during daylight hours. The emphasis here is on ‘hope’, because there can be a wide variation from the ideal. A steerable solar panel array would track the sun as it moved across the sky, but we are not aware of any commercially available domestic units in this country.

However, there are experimental set-ups. A prototype tracker has been described in a paper by Dr Jamal Rizk (University of Western Sydney) and Yelena Chaiko (Riga Technical University).

The device is a small solar panel with a 9W rating mounted on a table rotated by a stepper motor. The authors show that the total energy generated in 12 hours of daylight is about double compared with fixed-angle geometry based on maximum irradiance.

The device is actuated by a signal indicating the initial panel position, which is detected by a photocell mounted on each side of a V-shaped reflecting surface. With the solar panel positioned other than ‘normal’ to the sun, there will be more light to one photocell than to the other.

The difference in photocell output provides an electrical difference signal, as well as the direction in which the stepper motor must rotate to maintain the normal position.

Some tracking systems have been developed in colder climes, such as the Lorentz system from Germany, employing a linear motor for actuation of single and dual tracking (zenith and azimuth).

A steerable array would be based on the cosine law. For example, if sunlight is falling onto a panel at 60º (see Figure 1: the angle is defined between the panel surface and perpendicular to the panel), the intensity as measured in available watts per square metre would be half the level obtained with the sunlight normal to the panel surface. The latter state pertains only on the equator at midday.

Some arrays can be tilted to make the best use of the sun’s power (the technical term is ‘insolation’). However, they are not steerable in the strict sense of being remotely controllable or controllable via software using an algorithm appropriate to the co-ordinates of the site.

There are several suppliers of adjustable frames, some providing dualaxis angular adjustment. The two-axis variety permits adjustment from east to west (azimuth) and elevation (zenith).

Mechanical considerations – resistance to wind in particular – make these adjustable solar panel frames expensive. They are not suitable for roof mounting but would be appropriate for installation in a large yard, depending on the maximum power rating required.

How important is it to have an adjustable panel? In practical terms a great deal depends on the insolation patterns at a particular site, and on the energy-price advantage of tracking the sun.

First of all, the number of bright days must be considered. Bright days offer the best conditions for drawing benefit from the cosine law. On those days most of the sunlight is ‘specular’ – that is, up to 80% of the sun’s energy is directly beamed rather than scattered, as it is on hazy and cloudy days.

(Such scattering is responsible for a general light level that makes it possible to read a book in the shade of a backyard tree.)

The lower the incidence of bright days, the less attractive an adjustable array would be.

It is also important to take account of some basic physics concepts. When the sun is overhead, as at the Equator at noon, its rays traverse the shortest distance through the atmosphere (about 40km) so that energy absorption is minimal.

As dusk approaches, the distance to be traversed increases greatly, as does the energy absorbed.

Furthermore, there are shifts towards the red end of the spectrum, and solar cells are less efficient at longer wavelengths (quantum mechanics indicates that the shorter the wavelength, the higher the energy of the light quanta).

UV radiation (ie: beyond blue) is so powerful that it tends to travel through the panel without being absorbed, so there is no useful result. Beyond red (infrared) the quanta pack too little energy to interact usefully, that is, to produce charge carriers.

Steerable arrays are costly. They need to be strong enough to resist high winds and require an adequate foundation.

Although possessing high inertia, they do not as a rule require powerful motors to drive them, as the velocity requirements are low.

According to some proponents of tracking systems, there are gains to be made. Inclining panels in accordance with latitude will achieve an improvement in efficiency of about 30% (relative, not absolute), and tracking on the basis of a single axis (zenith) can account for a further 6% relative gain.

Depending on feed-in tariffs, there may be an economical basis for a tracking system. However, in view of the political nature of these tariffs, it would be difficult to make a case for the expenditure in typical urban solar installations.

In the case of isolated installations, tracking can be warranted. Yet Australia offers high insolation levels, and it makes more sense to install extra panels rather than putting up with the complications and possible service problems of a tracker.

Developments in panel technology won’t obviate the need for tracking but will make it relevant only for certain infrastructure installations involving, for example, heliostat technology that reflects sunlight onto panels.

For urban capture of solar energy, advances in panel technology are more likely to offer a practical return.

Major developments in thin-film technology allow solar panels capable of generating energy at very acceptable efficiency and in less than ideal weather.

Many of the panels sold in Australia are of the crystalline and poly-crystalline type. This is proven technology and efficiencies are generally in the 14-16% area.

Amorphous silicon and other thin-film technology panels are manufactured by depositing thin layers of noncrystalline silicon materials on inexpensive substrates, with typical efficiency of 7-10%.

Other thin-film technology modules – based on cadmium telluride, gallium arsenide or CIGS (cadmium indium gallium selenide) show moderate efficiency of about 8% and are durable. Some must be recycled at the end of their life, as they contain hazardous material.

One substantial advantage of thin-panel technology is its reduced dependence on angle of incidence. In practice it is possible to achieve an average of 10% more usable energy (kilowatt/hours) per 24 hours. There is also much less deterioration when partial shading occurs.

One interesting option is buildingintegrated photovoltaics (BIPVs). Conventional building materials are replaced by photovoltaic materials, which can be architecturally pleasing as cladding and roofing, and in light wells.

There is a high demand in world markets for CIGS technology, but so far it hasn’t made inroads in Australia.

The claimed advantages include reduced drop-off in peak power through temperature rise (about half, compared with polycrystalline silicon), and shading is less of a problem when compared with silicon. For example, a mid-panel shadow for CIGS causes a reduction in peak power of 5%, and for silicon it can be in the order of 40%.

Although perhaps not of primary interest, the carbon footprint for CIGS panels is considered to be much smaller than for poly and crystalline silicon panels.

Solar-cell generation is still one of the most expensive ways of generating green energy, so it is difficult to predict commercial success for thin-film panels. However, the worldwide market for the technology is growing – 60% in the past five years, according to some sources – and falling prices are putting pressure on thin-film panel manufacturing.

Still, this ‘angle’ on the future of solar technology should not be overlooked.