Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)
Understanding the Components
Once you have completed your site survey, you know all the facts: how much power you need to generate, the suitability of your site and approximately how much it is going to cost you.
Now you need to look at the different technologies and products that are available, to see what best suits you and your application.
Your choice of components and the design of your system will depend on whether you are designing a stand-alone system (which also includes grid fallback and grid failover systems), or a grid-tie system that exports energy to the grid.
Because there are differences in the design of stand-alone and grid-tie systems, I have split this section into four chapters. This chapter looks at components that are common to both grid-tie systems and stand-alone systems. The next chapter looks specifically at what you require for grid-tie systems. The third chapter looks specifically at stand-alone systems and systems that incorporate their own battery store, whilst the fourth chapter looks at the component certification and regulations that you need to take into account when selecting solar energy equipment.
How to use these chapters
These next three chapters will go into much more detail about the different options available to you. There is a bewildering choice of solar panels, batteries, controllers, inverters and cables.
These chapters explain the technology in a lot more detail, so you can go and talk sensibly to suppliers and understand what they are saying.
Common components for all systems
Core to all solar energy systems are the solar panels themselves. Most solar panels can be used for either grid-tie or stand-alone use, and although recently some manufacturers have released higher-voltage solar panels designed specifically for grid-tie applications, the criteria for choosing one solar panel over another remain the same.
There are three different technologies used for producing solar panels. Each has its own set of benefits and disadvantages.
For the purpose of this handbook, I am ignoring the expensive solar cells used on satellites and in research laboratories and focusing on the photovoltaic panels that are available commercially at reasonable cost today.
Amorphous solar panels
The cheapest solar technology is amorphous solar panels, also known as thin-film solar panels.
These panels have had a bad reputation in the past, with poor product reliability and questionable lifespan. This has often been down to the chemistries used in older designs of panel breaking down under extremes of temperature over a period of a few years, or the poor quality of materials used in the production of cheap panels.
Thankfully, this technology has matured significantly over the past five years and amorphous solar is now regarded as being highly reliable, with some significant benefits over traditional solar panels. Big name manufacturers such as Mitsubishi, Sanyo and Sharp now manufacture high-quality amorphous solar panels, along with some exceptionally good specialist manufacturers such as Solar Frontier and Uni-Solar. Some manufacturers now even offer a ten- or twenty-year warranty on their amorphous panels.
On paper, amorphous solar panels are the least-efficient panels available, typically converting around 6–8% of available sunlight to electricity. This means that you need twice as much space available for installing amorphous solar panels compared to crystalline panels.
However, amorphous panels are good at generating power even on overcast and extremely dull days. In general, they are also far better in extreme temperature conditions, with significantly less power loss at higher temperatures than other solar panel technologies.
Unlike other solar panel technologies, amorphous solar panels provide excellent performance even when partially shaded. Whilst a best-case scenario is to eliminate shading whenever and wherever possible, amorphous panels continue to operate at a high level of efficiency even if part of the array is in shade.
Amorphous panels can also be manufactured into a shape or mounted on a curved surface. They can be made to be hard-wearing enough to be fitted onto surfaces that can be walked on. A few solar manufacturers have started manufacturing amorphous solar roof tiles (or shingles), so that new-build houses can incorporate solar into the structure of the roof.
This combination makes amorphous panels suitable for integration into consumer products such as mobile phones and laptop computers, and for mobile products such as the roof of an RV or caravan, where the manufacturer has no control over where the products are placed or how they are used.
Amorphous panels are the cheapest panels to manufacture and a number of manufacturers are now screen-printing low-cost amorphous solar films. Over the past three years, amorphous solar panel costs have dropped by around 30% each year. They are expected to drop to around half their current (2012) cost by 2015.
Because of their lower efficiency, an amorphous solar panel has to be much larger than the equivalent polycrystalline solar panel. As a result, amorphous solar panels can only be used either where there is no size restriction on the solar array or where the overall power requirement is very low.
In terms of environmental impact, amorphous panels tend to have a much lower carbon footprint at point of production, compared to other solar panels. A typical carbon payback for an amorphous solar panel would be in the region of 12–30 months.
Most amorphous solar panels have comparatively low power outputs. These panels can work well for smaller installations of up to around 300-watt outputs, but not so well for larger installations: larger numbers of panels will be required and the additional expense in mounting and wiring these additional panels starts to outweigh their cost advantage.
Consequently, amorphous solar panels are often more suited to OEM applications, as an energy source built into a manufactured product, or for large-scale commercial installations where the panels are incorporated into the structure of a roof-space on a new build.
Some of the most exciting advances in solar technologies over the past three years have come from amorphous technology. Products as diverse as mobile phones, laptop computers, clothing and roofing materials have all had amorphous solar panels built into them. An exciting technology, amorphous solar is going to get better and better over the coming years.
Polycrystalline solar panels
Polycrystalline solar panels are made from multiple solar cells, each made from wafers of silicon crystals. They are far more efficient than amorphous solar panels in direct sunlight, with efficiency levels of 13–18%.
Consequently, polycrystalline solar panels are often around one third of the physical size of an equivalent amorphous panel, which can make them easier to fit in many installations.
Polycrystalline solar panels typically have a life expectancy of about 25 years. This can often be exceeded: commercial solar panels only became available in the late 1970s and early 1980s and many of these panels are still perfectly functional and in use to this day.
The manufacturing process for polycrystalline solar panels is complicated. As a result, polycrystalline solar panels are expensive to purchase, often costing 20–30% more than amorphous solar panels. The environmental impact of production is also higher than amorphous panels, with a typical carbon payback of 3–5 years.
Prices for polycrystalline solar panels are dropping, thanks to both the increase in manufacturing capacity over the past few years and the increasing popularity for larger screen televisions, which use the same specification glass. For the past five years, prices have been dropping by around 25% per year. They will undoubtedly continue to reduce in price as the cost of amorphous solar technology continues to drop.
Monocrystalline solar panels
Monocrystalline solar panels are made from multiple smaller solar cells, each made from a single wafer of silicon crystal. These are the most efficient solar panels available today, with efficiency levels of 15–24%.
Monocrystalline solar panels have the same characteristics as polycrystalline solar panels. Because of their efficiencies, they are the smallest solar panels (per watt) available.
Monocrystalline solar panels are the most expensive solar panels to manufacture and therefore to buy. They typically cost 35–50% more than the equivalent polycrystalline solar panels.
Which solar panel technology is best?
For most applications, polycrystalline panels offer the best solution, with reasonable value for money and a compact size.
Amorphous panels can be a good choice for smaller installations where space is not an issue. They are usually not practical for generating more than a few hundred watts of power because of their overall size, unless you have an extremely large area that you can cover with solar panels.
What to look for when choosing a solar panel
Not all solar panels are created equal, and it is worth buying a quality branded product over an unbranded one. Cheaper, unbranded solar panels may not live up to your expectations, especially when collecting energy on cloudy days.
If you are spending a lot of money buying a solar energy system that needs to last many years, it is advisable to purchase from a known brand such as Kyocera, Panasonic, Clear Skies, Hyundai, Sanyo, Mitsubishi, Solar Frontier or Sharp. My personal recommendation is Kyocera polycrystalline solar panels, or Mitsubishi or Solar Frontier amorphous panels. I have found these to be particularly good.
Buying cheap solar panels
Not all solar energy systems have to last ten or twenty years. If you are looking for a small, cheap system to provide power to an RV or caravan, or your requirements are modest, such as installing a light in a shed, buying a cheap solar panel may well be the right option for you.
The quality of the cheaper solar panels has improved significantly over the past few years. Six or seven years ago, buying a cheap, unbranded Chinese-made product was a recipe for disaster. Many of the panels were poorly assembled, allowing water to seep through the frames and damaging the solar cells. A lot of them used plate glass, often a thin, low-grade glass that becomes clouded over time and is easily chipped or broken. The cells used by these manufacturers were often sub-standard reject cells and often degraded very quickly.
Thankfully, most of these problems are now resolved and if you buy a cheap solar panel on eBay, you are likely to have a good product that will reliably generate power for five to ten years, and in all probability a lot longer. If you are buying a solar panel from a manufacturer you have never heard of, here is a checklist of things to look for:
Buy bigger than you think you’ll need
If you are buying a very cheap solar panel, you are likely to be saving as much as 50% of the price when compared to buying a branded product. However, expect it to degrade slightly more quickly than a branded unit, and do not expect it to be quite so efficient.
To counter this, buy a solar panel with a higher watt rating (often shown as a watt peak, or Wp, rating) than you actually need, or buy additional solar panels if you are purchasing an entire array. Aim for 15% more power than you would otherwise have bought. You will still save a lot of money, but you will have an extra bit of assurance that the system will be up to the task.
With cheap solar panels, you’re not going to get a five-, ten- or twenty-year warranty, but you should still expect a one- or two-year warranty with any solar panel you buy. Check to see exactly what the warranty offers.
You are looking for a warranty that guarantees a minimum output under controlled conditions. The standard across the industry is to guarantee 80% of the quoted output under controlled conditions.
If you have a warranty claim, also check to see how you can claim on that warranty. Shipping a broken solar panel half way around the world and paying for return carriage is likely to cost as much as buying a new solar panel.
If you are buying a solar panel that is going to be fitted to a moving vehicle, or if you are buying a physically large solar panel, make sure that the solar panel uses tempered glass.
Tempered, or toughened, glass is around eight times stronger than plate glass. This makes it far more robust. If your glass is chipped on your solar panel, you will immediately see a significant drop in power output. If water gets into the solar panel itself, it can create a short circuit and becomes a fire hazard. Water and electricity do not mix.
It is worth noting that some amorphous solar panels cannot use tempered glass because of the way the thin film is applied to the glass. Some manufacturers reduce this problem by using thicker plate glass.
Second-hand solar PV panels
From time to time, second-hand solar panels appear for sale. They appear on eBay or are sold by solar equipment suppliers or building salvage yards.
So long as they come from a reputable brand, second-hand solar panels can be extremely good value for money and even old panels that are 25–30 years old may still give many more years of useful service. Although good quality solar panels should provide at least 25 years service, nobody knows how much longer they will last. The early commercially available solar panels (which are now over 30 years old) are still working extremely well, typically working at around 80–90% of their original capacity.
There are, however, a few points to look out for if you are considering buying second-hand solar PV panels:
· Never buy second-hand solar PV panels unseen. Take a multi-meter with you and test them outside to make sure you are getting a reasonable voltage and wattage reading
· Check the panels and reject any with chipped or broken glass. Also reject any panels where the solar cells themselves are peeling away from the glass or have condensation between the glass and the solar cell
· The efficiency of older solar PV panels is significantly lower than new panels. 30 years ago, the most efficient solar panels were only around 5–6% efficient, compared to 13–24% efficiency levels today. 10–15 years ago, most solar panels were around 10–12% efficient
· Consequently, a solar PV panel from the early 1980s is likely to be three times the size and weight of an equivalent modern crystalline panel
· These second-hand panels will not have any of the safety certification ratings that you get with new solar panels. This may cause issues with building regulations or building insurance, if you are installing these onto a building as part of a new solar installation
Fresnel lenses and mirrors
A very brief word here about Fresnel lenses and mirrors. The Fresnel lens was invented for lighthouses, as a way of projecting a light over a long distance. It does this by refracting the light to make it a concentrated beam. Scientists have been experimenting with Fresnel lenses in conjunction with solar panels for concentrating the power of the sunlight and focusing it on a solar panel.
In effect, by concentrating the sunlight into a smaller area and increasing the solar irradiance, significantly more energy can be captured by the solar panel, thereby improving its efficiency quite impressively.
However, there are problems with this technology. Most specifically, the heat build-up is quite considerable and, in testing, many solar panels have been destroyed by the excessive heat generated by the Fresnel lens. This is especially true of Fresnel lenses built by enthusiastic amateurs.
There are one or two companies now promoting Fresnel solar panels. These panels tend to be quite large and bulky. Due to the heat build-up, they also need to be very carefully mounted, with adequate ventilation around the panel. There are also questions about the long-term reliability of Fresnel solar panels. My advice would be to avoid these until other people have tried them for a number of years and found out how reliable they really are.
As an alternative, mirrors or polished metal can be a useful way of reflecting additional sunlight back onto solar panels and therefore increasing the solar irradiance. However, you must take care to ensure that the reflected light does not dazzle anyone. The practicalities of mounting, safety and ensuring that people are not dazzled by the reflected sunlight normally dissuade people from using mirrors in this way.
Solar panel mountings
You can either fabricate your own mounting for your solar panels, or purchase a ready-made modular system.
The design of the system must take into account wind loading, so that it is not damaged or destroyed in high winds. If you are installing solar in a hot climate, your mounting must also ensure there is adequate ventilation behind the panel to avoid excessive heat build-up.
Your support structure needs to be able to set the angle of the solar array for optimal positioning towards the sun.
If you have not installed solar electric systems before, it is usually a good idea to buy a modular support structure from the same supplier as your solar panels. Once you have more experience, you can then choose to fabricate your own, if you prefer.
For ground- or pole-mounted solar arrays, you can buy solar trackers that track the path of the sun across the sky and move the solar panels so they are facing the sun at all times.
The benefits of solar trackers are that they increase the amount of sunlight the solar panels can capture. They increase energy capture by up to 55% during the summer months and by around 15–20% during the winter months.
Unfortunately, the cost of these solar trackers means that they are rarely cost-effective. It is usually far cheaper to buy a larger solar array than it is to buy a solar tracker. Only if space is at a premium are solar trackers currently viable.
Solar array cables
Solar array cables connect your solar panels together and connect your solar array to the solar controller.
These cables are often referred to as ‘array interconnects’. You can purchase them already made up to specified lengths or make them up yourself. The cables are extremely heavy duty and resistant to high temperatures and ultra-violet light. They also have a tough, extra-thick insulation to make them less prone to animal damage.
If you are planning to wire your solar array in parallel rather than in series, you need to ensure that your solar array cables can cope with the current that you are going to be generating through your solar array. If you are designing a parallel design system, I explain how you can calculate the size of cable required in the chapter on stand-alone system components.
Fuses and isolation switches
The ability to isolate parts of the system is important, especially while installing the system and carrying out maintenance. Even comparatively low voltages can be dangerous to work on.
Even small systems should incorporate a fuse between the batteries and the controller and/or inverter. If something goes wrong with the system, far better to blow a cheap fuse than fry a battery or a solar controller.
For all but the smallest systems, you will also need to incorporate isolation switches into your solar design. This will allow a battery bank to be disconnected for maintenance purposes. For any installation with more than one solar panel, and for all grid-connected systems, an isolation switch to disconnect the solar array should also be installed: I would recommend installing an isolation switch on the solar array for all solar arrays capable of generating over 100 watts of power.
If your solar panels are mounted some way from your inverter or controller, it can also be a good idea to have an isolation switch fitted next to the solar panels, as well as one fitted next to the inverter or controller. You can then easily disconnect the solar panels from the rest of the system for maintenance or in case of an emergency.
Ensure that the isolation switch you choose is capable of handling high-current DC circuits, with contacts that will not arc. Suitable isolation switches are available from any solar supplier.
If you are planning a grid-connected system, you will also need AC isolation switches to allow you to disconnect the inverter from the grid supply. You will require an isolation switch next to the inverter, and a second one next to the distribution panel.
Ground fault protection
Ground fault protection ensures that if there is a short within the solar array, the current flow is cut off immediately. This averts the risk of damage to either the controller or the solar array, and significantly reduces the risk of electrocution.
Ground fault protection works by measuring the current entering and exiting a circuit. If everything is working correctly, the current in should equal the current out. However, if there is a ‘leak’ or a partial short circuit, the system will see a difference in current and immediately shut down. A partial short circuit could occur if a solar panel was broken or if somebody touched an exposed cable.
Most solar inverters and solar controllers incorporate ground fault protection, using a Residual Current Device (RCD) built into the unit (note: RCDs are known as Ground Fault Interrupters – GFIs – in the United States and Canada). Many experts say that it is prudent to install a separate ground fault protector, even if the controller or inverter has ground fault protection built in. As the cost of an RCD or GFI is low and the benefits they provide are high, this is good advice.
You will require separate ground fault protection for your DC and AC circuits:
· For anything larger than 100-watt solar panel systems, and for all systems mounted to a building, you should install ground fault protection between your solar panels and your controller or inverter
· If you are installing a DC power supply into a building for running appliances, you must install ground fault protection between your controller and this power supply
· If you are using an inverter, you should install ground fault protection between your inverter and any load
There are specific RCD units for DC circuits and these are stocked by solar panel suppliers.