June 8, 2008
By now you’ve read a lot in recent news about solar panels, or “modules” as they are called within the industry. You want to slap one down on your roof, ,and tap into some free power, right? Getting a solar module to produce power is the easy part (more on that in an upcoming article). Successfully utilizing that power is something else.
This is the first of a series, which will cover the considerations involved, in getting that solar cell to produce usable power. You might start first, by thinking of WHAT you want to power; that has a lot to do with the considerations herein. Keep in mind that some things will require power 24 hours a day, or throughout a 24 hour day (i.e. refrigerator, which cycles on & off all day), and some things may need to only be powered while the sun is out, or only when the sun is down, like outdoor lighting.
Much of the content here will be based on an ongoing observation of a work-in-progress solar installation. That is to say, I am basing this article largely on real-world findings, not just by-the-book calculations. Things usually do not work out as they do on paper, thus we are going to focus on what is real-world application, more so than the book calculations, wherever possible!
I have started with a 150W Kyocera Module. This particular model is a PL-KYOC-FL158. It is cleverly designed to become an integrated part of the roofing system, as it provides additional insulation, and requires none of the typical mounting (rails). So with 150 watts on tap, where do we begin . . .
It is not quite that simple, as 150 watts is the output only at optimal operating conditions, such as full sun, and the proper alignment angle to the sun. Since the sun arcs across the sky throughout a given day, for example, the panel will not be aligned all of the time. That is, unless you also install a ‘Solar Trackers’, a device which tilts the panel towards the sun automatically, throughout the day. There are such devices, but they add complexity and cost. For now, that goes way beyond the scope of what we’ll cover in this article. There is also a cost to benefit gained factor to consider with such a device.
I decided to go conservative on the device(s) that I will run off of this panel, and I wanted to try circuitry that would run via a charged battery, thus I need a device known as a Charge Controller. This amazing all-in-one device not only charges up the battery during the day, but it also controls how much energy is drawn out of that battery later, when the sun is down. In other words, it protects the charge state of the battery, ensuring that it is not drawn down too much. Additionally, it charges the battery within a voltage range which is acceptable to most batteries, with a maximum limit of somewhere around 14 volts. This is much like the alternator regulator in a car.
I will be drawing approximately 20 watts of power from the battery at any given time, as I will be powering a very efficient set of outdoor landscaping lights. There are approximately 80 lights in all, many of which were home-built into fixtures which originally housed incandescent light bulbs. In all, the entire string of lights, and even a motorized color wheel on a light in a tree, use 1.6 amps total. Before converting the incandescent bulb fixtures to LED, one such bulbs used 1 amp by itself. There are 12 fixtures like that. They now use 60 ma each (.72 watts), and that is with two Ultra-Bright LEDs in them! The saving in electricity usage is quite impressive.
With a power requirement of < 2 amps , which is only needed at night, I simply multiply the current requirement by the number of hours used in a given night, to obtain the amp-hours used. 1.6 amp x 12 hours, yields 19.6 amp/hrs. Rounding it off to 20 amp/hrs, we’ll go from there, to size the battery and access the charging cycle.
Now remember, each night this circuit must charge the battery up, so that it can be ready for the next discharge cycle, the following night. If I use a 20 amp/hr battery for this application, it will be fully exhausted by the morning, with no reserve. Then, just in case the following day is a cloudy/rainy day, the charging will be shortchanged, and there will not be enough reserve to power the light the next night!
In light of this, I selected a 40 amp/hr battery, about twice the nightly draw. My 150 watt solar module has an output of approximately 15 volts, at 10 amps. If I can successfully draw 10 amps out of it, I can charge that battery in 4 hours. I live in Phoenix, where there are 6 hours of solar sunlight, which is according to a solar charging authority.
In summary, I have a battery that will last for two days without a charge, and it can be fully charged, by around noon of a given day, thus I really only need ½ of a day’s sun, to fully restore the battery back to where I need it to be! This design gives me 4 times the reserve, being that it can charge in ½ day, and the battery last 2 x days.
To simplify the lighting circuitry, since it is only needed at night, I wanted the lights to cut out during the day, as to not compete with the charging of the battery. A simple method by which to accomplish this was to simply add a relay on the output of the solar module. Using the NC (normally-closed) contacts of the relay, the power produced by the panel holds the contacts open! That is to say, until there is no more power being produced (i.e. night time), and the relay closes, sending no power to the lights. This allows the charging circuit to not have to compete with the lighting circuit, getting all the benefit of the sun, to the batteries ASAP. In essence, you have an automatic night lighting system! The relay, with is powered directly off the solar module, draws only 20 ma of power; insignificant at this level of available current. No matter what time of year it is, when the sun goes down to a certain point, i.e. dusk, the lights come on. Perfect!
I keep on adding more lights to the entire circuit too, as I see fit, and the system can accommodate the extra load easily, since there is such a large reserve. That is how it should be designed.
You may decide that you want to power some light during the day instead, as I too will do, to add some light inside the house during the day. This is an excellent way to add brightness inside, without the risk of adding heat (i.e. solar light ‘tubes’). This method is provides some of the surplus power during the day, as sort of an extra benefit. That can be easily accomplished if you have enough reserve on the output of your solar module!
I will run two 2.5 watt 12v bulbs, installed in light ‘cans’, flush-mounted in the ceiling. These bulbs will provide light that is close in the light spectrum to natural sunlight, in my living room. If this works out as I envision, will likely install more such fixtures in my bathrooms and kitchen, as long as the overall charging system can keep up with the additional daytime load. This way, I will have solar-powered light during the day, while charging the batteries, then at night, the outdoor lights run solely off the batteries, to then be charged up the next day, repeating the cycle, day after day.
In an article to come, I will provide more specifics on the wiring of the controller, including schematic diagrams. The controller basically keeps the system voltage from exceeding a maximum voltage of approximately 14 volts, monitors the charge level of the battery, and has a cut-out on the load side of the circuit, if the battery voltage gets too low, thus protecting the battery from having too deep a discharge, which would drastically shorten battery life.
Modern day charge controllers greatly simplify the wiring of the overall circuitry needed, and do a terrific job at policing the system voltage. With three connections, it is a pretty simple hookup: battery, solar module, load. That is it! My particular circuit employs a few extras, for a night-time circuit (when there is no solar module output), a daytime circuit (when the solar module is outputting voltage), and a few meters to monitor system parameters.
Look for the next article in this series, for the wiring specific, and more ideas on what you can feasibly run form a solar module, as well as how to hook it all up. Also coming in forthcoming articles is a article on making your own LED fixtures, as well as converting incandescent fixtures to led.
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