ReNew magazine gives their top tips for finding the right solar panel
By Lance Turner, COURTESY of ReNew Magazine
Incentives such as feed-in tariffs and rebates mean that many more households and businesses are considering solar photovoltaics for electricity. Best of all, this electricity source is clean and renewable.
Photovoltaic panels produce electricity directly from sunlight. They are used to power houses (on and off the mains grid), water pumps and remote communications systems as well as in large commercial solar power installations.
In its most common form, a solar panel consists of a number of photovoltaic cells connected together. These cells are usually coated in a plastic such as ethylene vinyl acetate (EVA) and sandwiched between layers of glass and/or plastic, or sometimes plastic and metal.
The collection of cells is usually surrounded by a metal or plastic frame for strength and to allow easy mounting of the panel. A junction box is often mounted on the back of the panel to allow easy electrical connection, though some panels have flying leads for connection.
Where glass is used as a covering for solar panels, it is usually low-iron glass to allow as much light transmission as possible, thus maximising power output. Many panels have glass on the front and a plastic, such as Tedlar, on the back to seal the panel. There are also panels designed to replace windows and other glass panels in architectural uses and they may have glass on both sides of the cells, depending on their intended use. This means the homeowner can offset some of the cost of the solar panels, as the panels themselves double as building materials.
PV Solar Energy roof tiles fall into this category. Most other solar panels are designed to be mounted on external frames, themselves mounted to a building’s roof or other frame, such as a solar tracker, but there are also flexible stick-on panels that can simply be stuck to suitable roofs or structures.
The Different Technologies
There are three common types of solar cells: monocrystalline, polycrystalline and thin film.
Both mono and polycrystalline cells are made from wafers cut from blocks of silicon, which are then modified by a process known as “doping”.
This involves heating the cells in the presence of boron and phosphorus, which changes the structure of the silicon in such a way as to make it a semiconductor. This is the same method that’s used to make integrated circuits.
Once the wafers have been doped, they then have a fine array of electrically conductive current-collecting wires applied to each side of them. Thin-film technology uses a different technique involving the deposition of layers of different materials directly onto metal or glass. The most common thin-film panels are the amorphous silicon type, which are found everywhere from watches and calculators right through to large grid-connected PV arrays.
In recent years, other types of thin-film materials have started to appear. These include CIGS (copper indium gallium [di] selenide) and CdTe (cadmiun telluride). They tend to have higher efficiencies than amorphous silicon, with CIGS cells rivalling crystalline cells for efficiency.
Flexible panels are a spin-off of thin-film technology. These are manufactured on a plastic or thin metal substrate and can be rolled up or attached to curved surfaces. They are commonly used for camping and boating, but are generally quite expensive on a dollarper- watt basis, although larger ones designed for mounting on buildings are competitive with conventional rigid panels.
Another recent development is the tubular solar panel, manufactured by Solyndra and available in Australia through Clear Solar. They look more like an evacuated tube solar hotwater collector than a PV panel. The design involves coating the inside of glass tubes with thin-film PV material. The resulting panel supposedly increases output and reduces wind resistance of the panels, meaning they can be mounted on a roof without penetrating the roofing material (according to the manufacturer).
As far as material use is concerned, crystalline panels use a great deal more semiconductor material than an equivalent output thin-film panel. This occurs for two reasons. The first is that a lot of material is lost in the process of cutting the silicon boule or billet into slices (wafers). The cutting is done with a diamond saw or wire, which may well be thicker than the resulting wafers, so more than half of the silicon may be lost in this process. Manufacturers have been working on reducing this wastage, but it is still a considerable proportion of the total material.
The other reason for greater material use with crystalline cells is that, because they are handled as individual cells, they must be robust enough to withstand mechanical handling. So a good proportion of the cell is actually there just to provide support to the active junction. This is also an issue that manufacturers are working to improve, with cells slowly becoming thinner in recent years (though not by a great deal).
Thin-film panels don’t have these problems, so may use less than one per cent of the semiconductor material as a crystalline panel. An example is the Kaneka thin-film modules. These have an active material thickness of just 0.3 micrometers. Compared to a typical crystalline cell thickness of 100– 200 micrometers, this is as little as 1/600th of the silicon and that doesn’t take into account the silicon wasted by the cutting process for crystalline cells.
There are two reasons why silicon use can be an issue. The first is the embodied energy of the silicon — it takes a lot of energy to make the highly purified silicon used in solar panels. The second is the fact that highgrade silicon suitable for this sort of use can be in short supply due to the demand for it in both solar cells and integrated circuits, although with recent economic events, there is now a glut of manufacturing capacity driving panel prices down.
Indeed, prices in the US have been less than US$3 per watt for many brands for a while now. While Australia hasn’t reached that level yet, there are signs that prices are indeed becoming more realistic, with panels approaching the $5 per watt mark and some direct imports being cheaper still.
PANEL RATINGS
There’s a number of different ratings on solar panels, so let’s have a look at what they are and what they mean.
Rated (peak) power: This is the maximum sustained power output of the panel, assuming a level of insulation (strength of light falling on the panel) of one kilowatt persquare metre. In general, the solar panel’s rating is the rated peak power.
Nominal voltage (Vn): The system voltage that the panel is designed to be used in. A 12-volt panel is designed for a 12-volt system but will produce voltages well above 12 volts. Some panels can be rewired to suit six- or 24-volt systems. Other panels are designed for grid-interactive systems and have nominal outputs of 48 volts or even higher.
Voltage at peak power (Vp): This is the voltage measured across the panel when the panel is producing peak power. Current at maximum power (Im): The maximum current available from the panel at peak power.
Open circuit voltage (Voc): The maximum voltage available from the panel with no load attached. This is usually around 21 volts for a 36-cell, 12-volt unit.
Short circuit current (Isc): The current obtained when the output of the panel is short-circuited with an insolation level of 1000 watts per square metre at a panel temperature of 25°C.
Temperature at rated power: This is the temperature the solar panel manufacturer rates its panels at. Most panels are rated to put out their maximum power at 25°C, a rather unrealistic figure given that the panel temperature under typical Australian conditions can be up to 70°C.
Temperature coefficient: The figure that tells you at what rate a panel’s output decreases with rising temperature. For instance, a panel with a temperature coefficient (of power) of -0.3per cent/°C means that for every degree of panel temperature above 25°C, the output decreases by 0.3 per cent. This doesn’t sound like much of a decrease until you realise that the panel might be running at 70°C. In this case, the decrease is 45 (the increase above 25°C) multiplied by 0.3, or 13.5 per cent, a significant amount. If you live in a hot climate, you should look for panels with as low a temperature coefficient as possible. Temperature coefficients can be specified as a change in output voltage, output current or maximum power. Sometimes, only the power figure is given, sometimes all are provided and sometimes none is!
Current-voltage (IV) curves: These are graphs of output voltage versus current for different levels of insulation and temperature. They can tell you a lot about a panel’s ability to cope with temperature increases as well as performance on overcast days. Obviously, the most important ratings when doing calculations for a power system are the voltage and current at maximum power. A system is rarely calculated using panel wattage ratings, as this is a function of both the voltage and current. Some panels are rated at slightly higher or lower voltages than others and this affects the amount of current available. The open-circuit voltage and shortcircuit current ratings are important from a safety point of view, especially the voltage rating. An array of six panels in series, while having a nominal 72-volt rating, can output over 120 volts DC — more than enough to be dangerous.
Heat and shading
These are two factors that can greatly affect solar panel performance. In general, solar panel performance decreases as temperature increases, and a panel rated at 25°C will not perform as well when operating at the hotter temperatures experienced in most parts of Australia. A typical operating temperature in summer can be up to 60°C or higher.
Some companies also supply ratings for temperatures higher than 25°C, so check to see whether these are available. Also bear in mind that, generally, thin-film panels perform better when hot than crystalline panels do. In many cases, a thin-film panel will perform as well as or better than a crystalline panel that’s rated at up to 10 per cent higher wattage. For example, a thin-film 65-watt panel will often perform as well as a 70-watt crystalline unit on an “overall energy produced per year” basis. Shading affects different panels in different ways.
The reduction in performance of the crystalline panel types, even when a single cell from a panel is shaded, is quite considerable.
Thin-film panels often perform somewhat better, especially panels that have bypass diodes built into each cell. Also, because thinfilm panels usually have cells that are long and thin, they are less likely to have individual cells fully shaded by birds and debris buildup. However, shade falling on the panels should be eliminated if at all possible — there’s not much point in investing large amounts of money in power-generating equipment if you don’t allow it to do its job!
Embodied energy
This is the amount of energy required to produce the panel in the first place and includes all energy used to make every part of the panel, including cells, frame, cable or junction box and assembly.
Some panels, especially the thin-film units, will repay their energy “debt” within a year or two, while others, especially monocrystalline panels, take longer. However, all panels on the market will produce more energy than they use over their lifetime if installed and used correctly. Manufacturers are starting to provide embodied energy information on request.
What to look for
It’s important to buy a panel that has the correct ratings in both voltage and current, with consideration given to their performance as determined by their IV curve.
You also need to look for a few other things when buying, such as construction quality, frame type and panel shape and weight. Some panels may be more suited to your roof shape than others, especially when used on small buildings such as sheds or outdoor toilets. Panel quality is very important. Many of the small amorphous panels manufactured in China are of variable quality — some last many years, while others die a quick death — so be wary of these. However, the overall quality of Chinese panels has improved considerably in the past few years thanks to intense competition, so don’t discount a panel just because it comes from China. Indeed, the biggest panel manufacturer in the world is based there.
Any solar panel worth buying will come with a long warranty. If the manufacturer doesn’t have enough faith in its product to offer a good warranty, why would you buy it?
Most panels come with a warranty of at least five years and some warranties are up to 25 years. We have not included any panel with less than a two-year warranty in this guide. Warranties come in different forms.
Some are just a power output warranty but don’t cover things like construction quality, while others are a bit more comprehensive. Ask questions before you hand over any money.
Installation and rebates
The federal government rebate for PV panels recently finished and has been replaced with the Solar Credits scheme, whereby eligible PV installations receive many more RECs (renewable energy certificates) than represent the actual energy generated. This equates to a cash-back similar to the previous rebates, but it also means that in order to get this benefit you must surrender your RECs (usually to the installer at the time of installation).
This scheme awards an artificially inflated number of RECs to each system, distorting the annual targets achieved under the RET (Renewable Energy Target), as the additional RECs produced will allow more carbon to be emitted from electricity generation, not less.
Fortunately, it’s becoming economically viable now to ignore the scheme and buy lower-cost solar panels and do much of the installation yourself. Indeed, it should be possible to do a self-installed system without RECs for a cost comparable to that of some of the more expensive commercially installed systems that requires the surrender of the RECs.
Of course, a self-installed system still requires any 240-volt components to be installed and wired by a suitably qualified person. For your system to be eligible for solar credits you must use panels that are certified for the scheme. A list of certified panels can be found at the Clean Energy Council website at www.cleanenergycouncil.org.au
