Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)
Appendix E. Other Solar Projects
Grid fallback system/ grid failover system
Grid fallback and grid failover are both often overlooked as a configuration for solar power. Both these systems provide AC power to a building alongside the normal electricity supply and provide the benefit of continued power availability in the case of a power cut.
For smaller systems, a solar electric emergency power system can be cost-competitive with installing an emergency power generator and uninterruptable power supplies. A solar electric emergency power system also has the benefit of providing power all of the time, thereby reducing ongoing electricity bills as well as providing power backup.
The difference between a grid fallback system and a grid failover system is in the configuration of the system. A grid fallback system provides solar power for as much of the time as possible, only switching back to the grid when the batteries are flat. A grid failover system cuts in when there is a power cut.
Most backup power systems provide limited power to help tide premises over a short-term power cut of 24 hours or less. Typically, a backup power system would provide lighting, enough electricity to run a heating system and enough electricity for a few essential devices.
As with all other solar projects, you must start with a project scope. An example scope for a backup power project in a small business could be to provide electricity for lighting and for four PCs and to run the gas central heating for a maximum of one day in the event of a power failure.
If your premises have a number of appliances that have a high-energy use, such as open fridges and freezer units, for example, it is probably not cost-effective to use solar power for a backup power source.
Installing any backup power system will require a certain amount of rewiring. Typically, you will install a secondary distribution panel (also known as a consumer unit) containing the essential circuits, and connect this after your main distribution panel. You then install an AC relay or a transfer switch between your main distribution panel and the secondary distribution panel, allowing you to switch between your main power source and your backup source:
In this above diagram, a second consumer box has been wired into the electrical system, with power feeds from both the main consumer box and an inverter connected to a solar system.
Switching between the two power feeds is an automatic transfer box. If you are configuring this system to be a grid fallback system, this transfer box is configured to take power from the solar system when it is available, but then switches back to grid-sourced electricity if the batteries on the solar system have run down.
This provides a backup for critical power when the normal electricity supply is not available, but also uses the power from the solar system to run your devices when this is available.
If you are configuring this system to be a grid failover system, the transfer box is configured to take power from the normal electricity supply when it is available, but then switches to power from the solar system if it is not.
One issue with this system is that when the transfer box switches between one power source and the other, there may be a very short loss of power of around / of a second. This will cause lights to flicker momentarily, and in some cases may reset electronic equipment such as computers, TVs and DVD players.
Many modern transfer boxes transfer power so quickly that this is not a problem. However, if you do experience this problem it can be resolved by installing a small uninterruptable power supply (UPS) on any equipment affected in this way.
You can buy fully built-up automatic transfer boxes, or you can build your own relatively easily and cheaply using a high-voltage AC Double-Pole/ Double-Throw (DPDT) Power Relay, wired so that when the inverter is providing power, the relay takes power from the solar system, and when the inverter switches off, the relay switches the power supply back to the normal electricity supply.
Portable solar power unit
A popular and simple project, a briefcase-sized portable power unit allows you to take electricity with you wherever you go. They are popular with people who go camping, or with repair people who need to take small power tools to locations where they cannot always get access to electrical power.
In essence, a portable power unit comprises four components: a small solar panel, a solar controller, a sealed lead acid gel battery and an inverter, all built into a briefcase.
Many people who build them add extra bits as well. A couple of light bulbs are a popular addition, as is a cigarette lighter adapter to run 12-volt car accessories.
For safety purposes, it is important to use a sealed lead acid gel battery for this application, so that you can place the unit on its side, if necessary, without it leaking.
For occasional use, a portable solar power unit can be a good alternative to a petrol generator: they are silent in operation and extremely easy to use. Their disadvantage is that, once the battery is flat, you cannot use it until it is fully charged up again and on solar power alone this can take several days.
For this reason, solar powered generators often include an external charger so they can be charged up quickly when necessary.
Boating on inland waterways has been undergoing a revival in recent years, especially with small craft powered with outboard motors.
Electric outboard motors are also becoming extremely popular: they are lighter, more compact, easier to use and cheaper to buy than the equivalent petrol outboard motor. Best of all, their silent running and lack of vibration make them ideally suited to exploring inland waterways without disturbing the wildlife.
For a small open boat, a 100-watt electric motor will power the boat effectively. Depending on what they are made from, small, lightweight boats can be exceptionally light – a 5m (15 foot) boat may weigh as little as 20kg (44 pounds), whilst a simple ‘cabin cruiser’ constructed from alloy may weigh as little as 80kg (175 pounds). Consequently, they do not require a lot of power to provide ample performance.
An 80 amp-hour leisure battery will provide around eight hours of constant motor use before running flat. This is more than enough for most leisure activity. Because most boats are typically only used at weekends during the summer, a solar panel can be a good alternative to lugging around a heavy battery (the battery can quite easily weigh more than the rest of the boat!).
Provided the boat is moored in an area where it will capture direct sunlight, a 50–60 watt panel is normally sufficient to charge up the batteries over a period of a week, without any external power source.
Solar shed light
There are several off-the-shelf packages available for installing solar shed lights and these often offer excellent value for money when bought as a kit rather than buying the individual components separately.
However, the manufacturers of these kits tend to state the best possible performance of their systems based on optimum conditions. Consequently, many people are disappointed when the ‘four hours daily usage’ turns out to be closer to twenty minutes in the middle of winter.
Of course, if you have done a proper site survey and design, you will have identified this problem before you bought the system. If you need longer usage, you can then buy a second solar panel when you buy the kit in order to provide enough solar energy.
Solar electric bikes
Electric bikes and motorbikes are gaining in popularity and are an excellent way of getting around on shorter journeys.
Electric bikes with pedals and a top power-assisted speed of 15mph are road-legal across Europe, Australia, Canada and the United States. You can ride an electric bike from the age of fourteen.
Legally, they are regarded as normal bicycles and do not require tax or insurance. They typically have 200-watt or 250-watt motors (up to 750-watt motors are legal in North America). Most electric bikes have removable battery packs so they can be charged up off the bike and usually have a total capacity of 330–400 watt-hours and a range of 12–24 miles (20–40km).
Thanks to their relatively small battery packs, a number of owners have built a solar array that fits onto a garage or shed roof to charge up their bike batteries. This is especially useful when you have two battery packs. One can be left on charge while the second is in use on the bike.
A number of people have also fitted solar panels onto electric trikes in order to power the trike while it is on the move. Depending on the size of trike and the space available, it is usually possible to fit up to around 100 watts of solar panels to a trike, whilst some of the load-carrying trikes and rickshaws have enough space for around 200 watts of solar panels. Such a system would provide enough power to drive 15–20 miles during the winter, and potentially an almost unlimited range during the summer, making them a very practical and environmentally friendly form of personal transport.
Whilst the solar-only range may not seem that great, there are many drivers who live in a sunny climate and only use their cars for short journeys a few times each week. For these people, it could mean that almost all their driving could be powered from the sun.
Even in colder climates such as the United Kingdom and Northern Canada, solar power has its uses in extending the range of these cars: by trickle-charging the batteries during the daytime, the batteries maintain their optimum temperature, thereby ensuring a good range even in cold conditions.
Meanwhile, a number of electric car owners have already made their cars solar powered by charging up their cars from a larger home-based solar array, providing truly green motoring for much greater distances. Several electric car clubs have built very small and lightweight solar powered electric cars and tricycles and at least one electric car owners’ club is planning to provide a solar roof to fit to existing electric cars in the coming year.