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Solar (photovoltaic) Energy
|Solar Cell Basics||Building a Photovotaic Panel||Mounting a Photovoltaic Panel|
Solar (PV) Cell Basics
I believe that producing electricity from the sun is the best way for the average person to get involved in alternative energy. Photovoltaic panels are relatively small and can be placed most anywhere the sun will shine on them for a good part of the day. With a micro-scale project, they can be left mobile and brought in at night, thus circumventing the possible need for zoning approval. Even city apartment dwellers might find that several small panels can be set in or hung out of south-facing windows to generate power during the day. As with wind power, solar suffers from the "feast or famine" syndrome, in that little power is generated on a cloudy day and more than is needed is produced when the sun shines brightly. That is why a larger system that is tied into the grid makes so much sense - excess energy can be sent to others who need it rather than being wasted once the storage batteries of a single user have been charged to capacity. Since we're dealing with tiny, 12 Volt systems on this website, I won't get into how one sets up a system that ties into the electrical grid, which is really a subject for and involving the professionals.
A very brief explanation of how photovoltaic cells operate is in order. In general, the solar cells you will encounter are made from a very thin slice of silicon, called a wafer. These may be formed from a single crystal (monocrystaline) or multiple crystals (polycrystaline). The former type tends to be more efficient, thus smaller in size for a given output as well as being more expensive to produce. Regardless of the crystal structure, the pure silicon has had a miniscule percentage of impurity added to each side (doping) that will enable electron flow between the two to occur when it is exposed to light. The side onto which the sun shines is the negative side and is given an anti-reflective coating, which usually makes it appear blue. A conductive gridwork has been burned into this side of the wafer, to which one or more flat ribbon-like conductors can be soldered for making the electrical connection. The back side of the solar cell, which is the positive, is also made electrically conductive and contains one or more points to which the tinned flat conductors may be soldered. Each cell will produce approximately one-half Volt in bright sunlight at a current that is determined by its physical size. The bigger the surface area, the higher the amperage. We can therefore say that in order to produce 12 volts of electricity we would need 24 cells connected together in series to form a solar panel. However, the typical solar panel for charging a 12 Volt storage battery will actually contain about 36 individual solar cells, the reason being that extra voltage is necessary to overcome the loss various resistances in the circuit will cause and to ensure an adequate amount of current will flow into the battery from a higher potential. To put this in simple terms, a state of equilibrium would exist if both solar panel and battery voltage were the same and no charging of the latter would take place. To prevent current from flowing from the battery back into the solar panel when the latter's output drops, a blocking diode is generally connected in series with one lead of the panel. A diode is an inexpensive semiconductor device that acts like an electrical valve, allowing current flow in one direction only. Diodes are rated for PIV (peak inverse, i.e. reverse voltage) as well as for maximum current (Imax) and must be properly sized for the output of the panel. Although it is common practice to install a series diode on homebuilt panels, they are not generally included in large, commercially-produced panels. Truthfully, the amount of reverse current flow I measured at night from my panel is in the order of 10mA (1/100th Amp), which is quite insignificant. Therefore, I didn't bother adding diodes to the two commercial panels I purchased. That said, a commercial panel will often contain a pair of diodes in its junction box that are used for a different purpose. This is done to bypass a poorly-producing (shaded) section in a multi-panel, series-connected, high-voltage array, which most of these panels are intended to be used in.
Building a Solar Panel
Although the temptation to build one's own solar panel is great, I'd like to advise against this. Truthfully, a commercially-built panel is likely to last many times longer than something built at home. The reason for this is that most of us do not have the experience, nor the correct tools, nor the proper materials for building a panel that will hold up to the ravages of nature. Yes, the Internet is full of do-it-yourself plans and videos showing how others have constructed their own solar panels, often of wood and Plexiglas or Lexan. What you generally don't see or read about is what these panels look like after a year or two out in the weather. One fellow actually did make a video that showed what Mother Nature had done to his panel and the results weren't pretty--cells dangling loose, black mildew covering large areas inside and so on. This was the preface, the real subject of the clip being to show off the two commercially-made panels he was installing in its place. Professionally-made panels consist of low-iron tempered glass, encapsulated cells and a moisture-proof backing mounted in a weatherproof aluminum frame. The construction is weather-tight, and built to withstand rough weather (including hail). Yes, you could theoretically purchase all the right materials, but the cost is going to be high. Plus, a commercial panel will generally come with a long warranty on the power output, although that's not to say the company will still be around in twenty or so years to honor it.
That said, there are certain advantages to making your own solar panel. There's a feeling of satisfaction and empowerment that comes from creating something yourself, together with the knowledge gained from hands-on experience. If the panel is built in a manner that allows it, you can take it apart to replace a bad cell or redo a poor solder connection, which is definately not the case with the encapsulated variety. In addition, building your own homemade solar panel generally will cost less than buying a commercial product. This is especially true if you can scrounge some of the materials, as I did. However, prices on commercial solar panels have been dropping and as of 2016 one could sometimes pick up a 60 Watt panel for around a hundred dollars (US).
Most amateurs choose to build their solar panels mostly out of wood. They start with a sheet of plywood and add some wood strips as a frame around it. Following the lead of others, they'll often glue the cells in rows to a sheet of pegboard, which is then screwed to the inside of the thin box made earlier. Finally, the entire assembly gets covered with a sheet of acrylic plastic or maybe, if one splurges, glass that may or may not be thick and may or may not be tempered. It looks great, until the condensation starts forming within or the plastic warps or the glass cracks and so on. Realistically, wood absorbs moisture. It also cracks as it shrinks and expands, and pretty soon the paint job that looked perfect is showing little fissures that are letting humidity in. The clear sheet covering the panel will experience its own stresses as the temperature inside the panel becomes quite hot. Assuming the well-meaning do-it-yourselfer caulked and screwed it tightly to the wooden frame, the sheet will twist and warp significantly as it expands with no place to stretch. It may bow inward to the point that it actually presses against some of the fragile cells. Or, it may start pulling away from the wood in several places. I call your attention to these things because they actually happened to me.
Now that I've explained why you should purchase a panel rather than build your own, I will give you some pointers in the event you still want to pursue your project. After all, I successfully built my own rudimentary panel. A solar panel may be constructed in a number of different sizes and configurations, but I wanted a 60 Watt panel that was suitable for charging a 12-Vot system. In this case, I needed 36 individual solar cells, each measuring about 6 inches by a bit under 3.25 inches. I also wanted to use stock sizes of wood and plexiglass for their easy availability, so I chose overall panel dimensions of two feet by four feet. I began with a 2' x 4' piece of plywood, a bit thicker that 3/8 inch. Around this I placed strips of inch lumber (3/4 in. actual finished thickness) that were 2.5 inches wide. The wood strips were mounted flat on top of the plywood and flush with its edges. Having a miter box, I cut 45-degree beveled, mating ends such as you'd find on a picture frame, but there's no reason butted ends wouldn't work. As I had many screw clamps available, I simply glued the pieces to the plywood and did not use any screws. I also worked several coats of the water-resistant glue into the raw edges of the plywood to help seal them up. My construction technique allowed an interior space for the cells measuring 19 inches by 43 inches, more than ample. The entire wooden assembly was given a coat of exterior primer and later, a topcoat of white exterior latex paint. It was allowed to dry for almost a week before any furthur work was done. The addition of moisture from the water-based wood glue resulted in warping of the panel, but this did not end up causing any significant problems. Still, you might do better to employ only solvent-based adhesives and paints in your own project.
Although some use pegboard as a backing for a separate cell assembly, I did not see any good reason why the PV cells could not be mounted directly on the inside of the plywood back. This choice seems to have worked out well. After making a sketch on paper and determining where I wanted my lines, I used a yardstick and a ruler to mark out a grid-like guide on the painted plywood for mounting the solar cells. Knowing that I'd need room to run some quarter-inch flat bus wire along the bottom and one side, as well as space for a pair of terminals on one end, I carefully offset my layout accordingly. A quarter inch of space was left between solar cells, which were laid out in three rows of twelve cells each, the longer edge of the cells being in line with the narrower dimension of the panel.
The home-brewers seem to prefer a single dab of silicon caulk/glue in the center of each solar cell for mounting them to the panel. The thought is that this allows the fragile cell to have some play which might avoid cracks from stress due to expansion and contraction. I endorse this policy, and I therefore made a mark in the center of each of the rectangles I'd drawn so I'd know precisely where to place the adhesive. This job was made easier by cutting a rectangular template the same size as a cell from a piece of cardboard and punching a hole in the center of it for the mark.
I'm going to add a step in here that I didn't perform until after the panel had been in use, but that really should be done at this time. I had erroneously intended that my solar panel should be a tightly-sealed unit and determined that the best way to accomplish that would be to use clear silicon caulk to adhere the plexiglas to the wood frame. I did not see a need for screws, feeling that the holes would allow another place for moisture to potentially leak in. As I soon discovered, however, the acrylic sheet expanded with heat and before long a good part of it had pulled away from the frame. I then removed it, carefully sanded along the edges for better adhesion and also drilled a screw hole every 5 inches around the perimeter (about a half inch in from the edge). This drilling was done in four steps. The positions were marked on the plexi and I used an awl with the tip heated up by a soldering iron to make positioning dimples at the correct spots. Next, I drilled a small pilot hole right through the correctly-positioned plastic and into the wood and then enlarged the holes in just the plastic sheet to accommodate the screws. Finally, I reapplied the silicone, screwed down the sheet and let everything cure thoroughly, figuring the problem was licked. It did hold together for a time, but the plexiglass bowed inward considerably as it warmed up and I had visions of cracked cells in the center of the panel where it appeared to actually be touching them. Within a week, it also began to pull away again in two spots between screws. I mentioned this problem to a friend who had constructed his own panel the previous year and he gave me a solution. Because the dimensions of the sheet change with temperature, it cannot be firmly secured to the panel. Instead, he advised me to oversize the screw holes in the plexiglas and put a washer under the head of each screw, which would not be tightened fully. In this way, the sheet would be able to move around as it expanded rather than warping. I tried this method and it worked, even more so after I had used a small, round file to elongate some of the holes furthur. Of course, this meant that my panel was no longer weatherproof. That was not a concern the other fellow had as his panel was intended for occasional use only on sunny days. Mine being in daily use, I needed to find a workaround. What I came up with was to make a "raincoat" for the panel out of a sheet of clear, heavy-duty vinyl of the type used to make storm windows and weatherize screened-in porches. This I cut so the dimensions were about 4 inches longer on all sides than the panel itself. I folded the edges around behind the panel and taped them at each corner. In retrospect, I should have used a vinyl cement on three of them as the tape loosens in the heat and leaves a gooey residue. The fourth corner could have Velcro, tape or another method of temporarily fastening the vinyl to itself if you'll want to be able to pull the cover off for maintenance. I suppose one might use a screw and washer at each corner to both secure the vinyl to itself and to the underside of the panel. I might just add here that although my friend had used stainless steel screws on his panel, I went with those made of regular plated steel. However, I gave the head of each a coat of clear nail polish to help retard oxidation. As cold weather approached that first winter I decided that the panel really needed to be made more weatherproof against blowing snow. Therefore, I cut a second piece of vinyl to fill in the open area on the back and ran tape along all its edges to secure them to the front piece. Because the majority of the tape was shielded from the sun, it held together as long as the sunlight-exposed front vinyl did, which was about a year. The clear vinyl sheeting I used was 2.7 mils in thickness, but thicker material is available that might last longer. In addition to being sold in home-improvement stores, this product may be carried in bulk rolls at your local craft/fabric stores.
I have not yet explained how one goes about wiring the individual solar cells together in series. It is general practice for the self-builder to purchase a bundle of 36 or so "untabbed" cells, that is, plain cells without the flat tabbing wire pre-attached. They are cheaper to buy this way. The hobbyist then either purchases the tabbing wire, liquid soldering flux and tinned "bus" wire seperately, or sometimes these items come in a kit with the cells. Either way, assuming 3 x 6 cells are being used, six inch long pieces of the tabbing wire are cut from the hank provided and a pair are soldered to either the back or the front of each of the solar cells. To which side of the cell this is attached is up to the preference of the individual, but doing it to the back makes more sense to me and will be assumed to be the procedure followed in this discussion. It is not necessary to extend the wire across the entire cell on the back side, for you want to make sure that you have the right amount of excess length to bridge the 1/8 or 1/4 inch gap between cells and go most of the way up the front of the next cell in the row. In other words, each piece of tabbing wire connects the positive (back) of one cell with the negative (front) of the next and two pieces are used for each cell since there are two sets of connection points. Six of the cells will actually need a shorter piece of tabbing wire of roughly half the length. These are the cells at the beginning and end of each row. Those cells will be attached to the next row, or to your terminals, by the tinned bus wire. I should note here that a soldering iron with a fairly high tip temperature is necessary in order to properly solder photovoltaic cells since the wafer tends to conduct the heat away. I used a temperature-controlled iron with an 800-degree, 1/8-inch screwdriver (flat) tip. This worked well for me. Flux is applied to the silvered connection area of the cell before soldering. It is important that the bus wire be attached so it lines up properly with the thin connection lines on the front of the cell. That is, when soldering to the back, try to keep the wire centered in the squares or rectangles provided. Some side-to-side adjustment will be possible when mounting the cells, of course, but there is not a lot of leeway if you want a smart-looking job. Another caution I should pass along is that the silver, solderable coating on the cells likes to vaporize when heated. I found that a quick slide off the tabbing wire with a tip that has a drop of melted solder on it made a better connection than leaving the tip in contact with the joint for very long. I would also advise soldering the very end of the tabbing wire first, once it has been roughly positioned. You can then hold it taut against the cell and properly centered and hit the solder points at the center and then the opposite end with your iron.
Once the cells have been tabbed on the back, they are fastened to the panel with a small dab of silicone caulk as explained earlier and this is allowed to cure for a day or more before the next step is begun. Great care must be taken when soldering and handling the solar cells, for they are very thin and fragile. My friend broke several in building his panel, probably when he went to solder the tabbing to the front of the cells in this next step. Flux is applied along the two thin, solderable lines on the front of the cell and the tabs to be connected are laid over these lines. You may find it handy to have a small piece of wood or cardboard available to hold down the far end of the tabbing wire. Have a good-sized drop of melted solder on the tip of your iron and run it slowly up the tab, starting from the end that connects to the previous cell, or, the end you are holding in place if it's going to be connected to bus wire. If you feel the iron start to drag, remove it and add another drop of solder before continuing along the tab from where you left off. Although the tabbing is pre-tinned, a bit of molten solder is absolutely essential between tip and tab to aid in heat transfer. A "dry" tip won't solder well at all.
Now that I have gone over how most panel builders solder the solar cells together, I'll tell you how I went about it. You see, I felt there was too much danger of cell breakage trying to solder to them once they were mounted in the panel. Therefore, I elected to put the tabbing wire on each one, front and back, before it was placed in the panel. Since my panel design allowed for a bit more than a quarter of an inch between cells, there was plenty of room for inserting the tip of a soldering iron between them in order to make a connection. I therefore ended up cutting my tabbing wire into lengths of roughly 3.25 inches, the actual snipping of each being made after it had been soldered on rather than before. I also devised a method of holding down the free end, which was to thumbtack it in position on the piece of board I was using as a work surface. I could then line up the end still attached to the roll and hold it taut with one hand, while soldering with the other. I did waste some of the tabbing because I trimmed off the tack hole, which was slightly beyond the 3/16-inch I needed to protrude beyond the cell's edge. I used this procedure for attaching the tabbing to both front and back, so I ended up with two tabs coming out one end and two on the other end of each of the solar cells. Once the cells were fastened into the panel, I simply soldered the tabs to the next/previous cell in the row or to the bus wire, as the case might be. Of course, I had to be careful when mounting each cell that it was facing the right way so that the tabs coming from the back slid under those on the front of the adjoining cell. In this way, positive connected to negative and all 36 cells were connected in series electrically. This system of construction worked well for me and eliminated breakage. In retrospect, I probably would simply purchase the slightly more expensive pre-tabbed (short-tabbed) cells if I built another panel. I'd advise others, especially the less-experienced, to do the same as it would eliminate much of the hassle and risk of breakage or having poor solder joints.
Mounting a Solar Panel
In order to successfully generate electricity, a solar panel must be located in such a way that the sun's rays strike the surface of the cells in as perpendicular a fashion as possible. That is, the front of the panel should be pointed right at the sun. Generally speaking, the tilt of the panel will be roughly the same as the latitude (in degrees) of your particular location on the globe. Finding the correct amount of tilt at any time of the year is easily accomplished by simply monitoring the output current from the panel while adjusting it up and down until the reading is the highest. You will find that this is not particularly critical. For those not on or near the equator, the ideal setting will vary somewhat during the year since the sun's zenith changes with the seasons. Rotating the panel side-to-side to follow the sun's daily movement across the horizon, called "tracking" will yield a greater overall output, but is often not feasible in a small installation.
One thing I've discovered that doesn't seem to get as much attention as it should is that maximum output on overcast days will be achieved with the panel in a close-to-horizontal position. This puts its surface more nearly perpendicular to the diffuse light rays filtering through the cloud layer. A gain of around 17 percent is possible over the traditional tilt directly towards the sun. Therefore, you may wish to make some arrangement that allows for easy repositioning on days when the sun is likely to be obscured. My panel is in a wooden "cradle" which may be tilted to a new position and held there by selecting the appropriate link in each of of a pair of restraining chains.
You obviously will want to locate your panel where it will receive direct sunlight for as much of the day as possible. You may find that a position that works well during the summer may not be suitable in months when the sun sits closer to the horizon. This was exactly the situation I found myself in, which necessitated the raising of the panel to avoid lengthening shadows from the peak of the house as the season progressed. The opposite can also occur to some extent when a previously shaded area improves with the annual falling of the leaves from deciduous trees. If you live in a cold climate, keep in mind that you'll need to access the panel in order to remove snow accumulation during the winter months. Climbing a ladder may not be particularly handy in the winter. There's often no reason that a panel cannot be moved with the seasons if it is desireable to do so.
Finally, I want to report back on how my home-built panel with its clear vinyl protection has fared over these five or six years. I discovered that by the end of each summer there would be brown patches and spots on the vinyl, and these spots would generally have adhered themselves to the plexiglass underneath. They were very difficult or impossible to scrape off and I suspect occured as a result of dirt on the film, including bird and insect droppings, which caused the temperature to increase in that spot due to increased solar absorption. I finally decided to only use the panel during the colder months of the year. Another thing that occured over time was the development of tiny fissures in the plexiglass, and an odd whiteness along the inside of its lower edge, which was permanent and could not be scrubbed away. This appeared in the area that condensation was occurring. Despite my best efforts, some moisture continued to get inside, causing significant rusting of the screws I'd used. None of this has created a problem with the panel's electrical output, however.
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