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Building and Wiring a Micro-scale System
|Using Your Energy||12-Volt Circuit||120VAC Inverter Circuit|
Using Your Home-generated Electricity
How does one utilize the energy generated by alternative means? Basically, in just the same way as you use the electricity purchased from the power utility. For most of us who construct our own tiny system there will be limitations, of course. One or two small solar panels or a micro wind turbine simply aren't going to generate enough electricity to power most household appliances, at least for any appreciable length of time. In my household, the 12 Volts is used for two main applications - lighting in the evening and recharging of the batteries in and for the various portable electronic devices I own throughout the day. These include a PDA, digital cameras, a laptop computer and the like.
It's important to be realistic about what your system can deliver. Truthfully, a single home-built solar panel in the 60 Watt range is only going to provide enough electrical energy to run two or three 60-Watt-equivalent compact fluorescent lamps (CFLs) or LED lamps or for 3 or 4 hours each evening if you don't want to discharge your battery beyond a safe level during periods of cloudy weather. This figure is dependent on where in the country you live, the time of year and so on, so I'm just giving you a general idea here. Adding more panels will increase this capacity, but will also probably require the purchase of more batteries in which to store the electricity. Also bear in mind that maximum power generation only occurs under absolutely optimal conditions, and is by no means continuous. Plus, some energy is lost to resistance in the wiring of the charging circuit and in the inefficiency of the storage battery (or batteries). Therefore, don't assume that a 60 Watt solar panel will really be supplying your system with that amount of power.
12 Volt DC Circuit
One of the great things about wiring your home with some 12 Volt DC lines is that many items come with adapters to plug into a car's cigarette lighter outlet. That is, they convert, where necessary, the 12VDC source into whatever voltage is needed by the particular device. That means they can be run directly off the current provided by an automobile or marine-type battery, which is just what your system will be using to store its power. There is, unfortunately, a catch to running low-voltage wiring, and that has to do with voltage loss. Due to the internal resistance of the wire, a certain voltage drop will occur per foot of length for a given current flowing through it. The amount of loss is inversely proportional to the diameter of the wire, thick cables having less of a loss than thin ones. Since we don't want to spend a small fortune on heavy-duty wire, nor wish to deal with its unwieldiness when making connections, we are going to be limited in just how much current can realistically be drawn by what we plug in. Safety issues aside, at some point the voltage at the end of the wire is just going to be too low for the device to operate properly. Again, this point is going to be determined by the gauge of the wire and how far it is from the battery to the outlet. For example, if you were to install fifty feet of 16ga. wire, you could perhaps operate something that draws up to a couple of Amps. However, if you were to use the same length of 12ga. wire, the allowable current could be more than doubled. If you're not familiar with the American Wire Gauge (AWG), the preceeding may confuse you. Suffice it to say that the smaller the wire gauge, the thicker the wire. 18 or 16ga. is what is found in the typical electrical extension cord, 14 or 12ga. cable is used in house wiring. Just what type of connection one should use for low-voltage applications is a matter of preference and what is available. Standard automotive cigarette lighter sockets mounted in a project box would certainly work well and should be safe enough, at least with respect to proper polarity. I happened to have a large number of banana plugs and jacks on hand and used those in my system. However, it's very important they be color coded for polarity and that those using them recognize the importance of connecting red to red and black to black. Obviously, I had to make up some adapters that included sockets.
Depending on how reliable the power from your local electrical grid is, you may wish to consider installing some low voltage "emergency lighting" as part of your system while you're at it. This can be controlled by an SPDT relay with a 115VAC coil that closes a set of contacts when power from an outlet is lost. If you use small automotive bulbs or LEDs, the 12-volt wiring for this can be 18 gauge. I installed such a system in our home back in the late 1970s when most storms resulted in a loss of lights. Although the reliability of the grid is much improved, this backup system, now upgraded to LED technology, remains an integral part of my alternative energy system. This can also become a house-wide nightlight system if you'd like to have a small amount of illumination for navigating through rooms in the evening without having to switch lights on and off.
120 Volt AC Inverter Circuit
I spoke above about voltage loss in the wiring and how it limits what can be done with a 12 Volt system. Fortunately, there is a great workaround to this problem, used by both hobbyists and the power utility. It involves stepping up the voltage so that the number of Volts lost in the wiring becomes a much smaller percentage of the total. If you've ever wondered why AC power is transmitted on high-tension lines and then reduced to household voltage by transformers before entering the building, this is precisely the reason. In the case of alternative energy, a loss of 1.2 Volts through the wiring is a problem when you're only dealing with a 12 Volt supply. However, if we were to increase the supply voltage by a factor of ten, that small voltage loss now becomes quite insignificant. Plus, for a given wattage current draw will be less for a higher voltage than a lower one, which puts us ahead even furthur. This is one of the reasons most micro and small-scale alternative energy projects will include a DC-to-AC inverter. This handy device electronically converts the 12 Volts from the storage battery and its charging system into 120 Volts AC that is able to operate many standard household items, including CFL and LED lamps. These inverters have come down in price and are more efficient, to say nothing of being more widely available than ever before, so there's no reason not to include one in any system. A couple notes of caution are in order. The high voltage from the output of an inverter requires that the external wiring be suitable for use at 120VAC. Only use extension cords or electrical cable approved for household wiring. Don't wire it up with speaker cable or so-called "primary" wire which is intended for low-voltage use. The other thing you'll need to know is that the inverter must draw its power from a storage battery. You cannot connect its input directly to a solar panel or wind turbine, despite what some eager salespeople may tell you. This is easily explained by the example of a cloud passing in front of the sun, temporarily reducing the voltage from a solar panel. What do you think happens to the inverter connected to that panel? It shuts off. Yet, if we have a storage battery connected to the circuit, it will continue to furnish power to the inverter despite the diminished output of the panel.
An inverter must be connected to the battery with appropriate cables, preferably red and black in color. Primary wire is fine, as is household wire. The size of cable you'll need for battery connections depends on the wattage you'll be demanding from the inverter. Heavy loads will require extremely thick wire, probably 6ga or even heavier. This is something most people won't have on hand, but may be available by the foot in home improvement stores. In any case, you'll want to position the inverter near the battery in order to avoid excessive voltage drop in the wiring. Most inverters will stop functioning if the input gets below about 11V, so you can see how critical this becomes if you're drawing a lot of current. However, don't place the inverter so close that it can get damaged by any acidic emissions from the battery's cell vents. Most inverters require that you operate a switch on their front panel, which can be inconvenient. Since mine is located in the basement, I rigged up a relay with contacts connected in parallel with those of the switch so it could be turned on remotely from the living room, above. I wouldn't advise attempting this modification, however, unless you have some electronics background and don't mind voiding any warranty on the device.
Two different types of power inverters are available. Those that produce something called a "modified sine wave" are the most common and least expensive. Their output is essentially a stepped square wave, which seems to work fine powering most things with a few exceptions. I have personally verified that shaded-pole (induction) motors such as those used in many electric fans will run much slower on this type of waveform. I have read that microwave ovens, and perhaps refrigerator compressors will also not run as well as they should. Because of the high-harmonic content of the switched output waveform, this type of inverter will cause an audible buzzing in coils/transformers as well as potentially produce annoying noise in audio systems and interfere with non-FM radio reception. A second, more expensive type of inverter puts out a pure sine wave, essentially identical to what is supplied by your local electric utility. This smoothly-undulating waveform will run all electronic devices properly and contains very little distortion that would produce interference. However, my research indicates that in addition to being scarcer and much more expensive, this latter type of inverter is somewhat less energy-efficient and, at least with the smaller-wattage models, has lower consumer satisfaction due to breakdown.
Getting the output from an inverter to lights, small appliances and devices throughout a house can be a challenge, short of running extension cords between rooms. You absoutely must not connect it to any existing house circuits, except through an approved transfer switch such as are used for generators. Don't believe ads that show inverters being plugged right into an electrical outlet to power the whole house. I chose to mount plastic outlet boxes connected to 14ga household electrical wire on my walls, despite the relative unsightliness of this arrangement. I'm fortunate in that I have pipes for steam heat running up two walls and the oversized holes surrounding them allowed space for running both DC and AC wiring through. Obviously, only one dedicated outlet per room still requires the use of some short extension cords in places. You can easily see another advantage the large, professional systems that use a grid-tie inverter have over what we can put together on a micro scale. However, the do-it-yourselfer simply does what they can to build a workable system on a budget. One advantage to wiring the house for inverter power is that this same wiring can easily be plugged into a gasoline-powered generator, should you happen to have one for emergencies.
My particular system uses an inexpensive 400 Watt modified sine wave inverter, which is more than adequate for the small amount of lighting we use. It has performed well, the only thing being that it tends to generate RFI (radio-frequency interference) on AM radios. I added an external filter between its output and the dedicated household wiring I installed, which helped somewhat.
Before leaving the topic of converting voltages, I should mention that electronic devices are readily available to change the 12V DC from the storage battery (batteries) to a lower or higher DC voltage, and to do so quite efficiently. These are so-called "switching converters" and are available on the large Internet auction site from Asian vendors for under $10., shipped. The terms "buck" and "boost" are commonly applied to their respective function. Some of these devices are good for 100 Watts output and perhaps more if a fan is added to cool them off. I have purchased unenclosed circuit boards of both types and use one of the voltage-lowering variety as part of the charge control system for my bank of storage batteries.
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