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BUILDING OFF THE GRID: Solar is the Future By Edwin Lapschies
S
olar electrical systems have come a very long way since the 60’s. Without getting into equipment details, here are a few actual system design considerations. Have you got a place to install the solar panels which are somewhat fragile and very valuable ? And although you might be out in the middle of nowhere, can you get a full day’s sun on them ? Here on the Baja we are blessed with at least 5.5 hours of full useful sunlight- a good consideration in your solar design. Moreover, relative to many parts of North America, it is very consistent. So let’s look at a few details beyond last month’s discussions. RACKING THEM UP ! I vote for the flat roof and quality racking system over yard pole mounted ones. However, if you don’t have one, find a safe place where the children do not think about using them as a daytime play apparatus. In addition, bolt them down on a rack built of aluminum or stainless steel. Regular galvanized steel lasts less than one year here, even if it is powder coated. Make sure they are fastened well, because come hurricane season, they can act like a sail. The panels are rated by the manufactures to withstand the typical 150 m.p.h. wind, so your racking and roof fittings need also to be strong. Use at least a 3/8” anchor bolt when fixing the rack to a concrete roof. The alternate is to make your racking capable of folding flat onto your roof. Now there is much talk about the correct angle at which to tilt the solar panels to the sun. This is important because the solar electric panels have a glass finish, and any sunlight that reflects from the glass is not converted to electric energy. (morning sun and late afternoon sun reduces collected energy by at least 60%, which is why we like to use the figure of 5.5 hours of full sun as a design number.) If you are here all year, and need that extra power for cooling in the summer, 23 degrees is
about right. If you are a seasonal home occupier, then a tilt angle of 35 degrees may be better suited to your needs. If you are just installing a small system, you could also easily adjust the rack twice a year for the best seasonal sun angle. Sound like a bit of a chore? But hey, a bit of sunshine and a reason to drink a cold beer is never easy. I like to practice preventive measures, so if the roof surface is surrounded by a 24” parapet wall, that is my maximum mounting height. POWER IN THE DARK Batteries store energy for use during non sunny periods. This is important for a few reasons. Your lifeline usually starts with refrigeration, so you need to make sure there is enough energy to get through at least 1 day of low sunlight levels. Sizing a battery bank is quite complex, but the first rule of thumb is become and educated shopper. The second rule of thumb is that you may never have enough useful storage capacity. All batteries have an expected life span. The more they are used (cycled from full power to less than half power) the shorter their life becomes. The specific qualities and features of a battery can be found on the manufacturer’s web site, and the figures are usually based on years of official testing and consumer experience. Thus if you need 100 units of electricity a day and your batteries are designed deliver 100 units of energy a day, with and expected life span of 7 years, then that is your design criteria. This is much different from the rating on the battery label. For example; A typical brand model L16 battery is rated at a storage capacity of 300 amp hours, but the detailed specifications state that the average life of the battery is only 3 years if you cycle the battery down to 66% capacity, or 100 amp hours. Battery model SU-200 is a 200 amp hour battery. It is rated at 7 years life for a 50% cycle. Both can readily deliver the energy you need, but with different life expectancies, at different costs. In either case, you need to
consider the battery cycle rating and the useful energy available to you when you design for your storage needs. In this example, the useful energy form either battery is only 100 amp hours, not the rating as stated on the battery itself. Have you got a place to store the batteries near the electronics but separate from the electronics? Can you get the collectors, batteries and electronics close together ? Batteries use very heavy special cable to transfer energy between the batteries and the electronics, so closer is better. Count on a typical battery bank being 2 feet wide by 4 feet long, by 4 feet high. A well ventilated area is very important as battery acid vapors will exist at almost all times to some extent. And as they become fully charged, batteries also heat up considerably. The electronics that make up your charge controllers and your inverters are not conducive to the acidic conditions in and around batteries, therefore, they are best placed indoors. I have seen some real misnomers that have caused fires, explosions and rather quick electronic failures. Please don’t think you can ever store your propane cylinders in with the batteries or the electronics, or gasoline. Most architects and builders just build a separate outdoor closet space attached to the house, outside of the electrical room. The batteries will weigh between 1200 and 2400 pounds. Make sure the concrete is reinforced correctly, and put a lock and key on it. Remember, batteries have to be serviced at least once a month. So if you are stacking them on a racking system, you need to leave at least 24” between the top of one battery and the bottom of another. Yes, you will need to look inside them, and add just enough distilled “ battery grade” water. Adding too much water will dilute the power capacity of the battery. And draining out the water /acid blend is not an option Next issue of Destino HOME magazine we will cover the electronics of solar electric systems