Me and My Panel Therese Peffer c.1992 Therese Peffer I did it. I decided to make my own power. I live in a trailer about an 100 foot extension cord length from Home Power Office & Power. The batteries are filled with electricity from photovoltaic panels and a wind generatorÄI can't complain about the source. But, well, sometimes the extension cord gets "borrowed", and there are "black outs" when Richard changes inverters (he counters that I haven't been paying my bill). I decided to create my own system, to learn, and to be able to set up a system for my folks. The Plan I decided to use a photovoltaic module to make electricity from the sun, and a battery to store it in, but what kind of PV module or what size battery? First I made a list of the appliances I use and whether they use Direct Current (DC) or alternating current (ac). I also have a clock, but it is a wind-up model. Since my only ac loads right now are lights, I may buy a DC light instead of buying an inverter to change DC to ac current. I've seen DC compact fluorescents and halogen lights from 11 to 50 Watts. Then I looked at how much power or watts each appliance draws. The figure is usually stamped on the back or bottom of the appliance and often is not exact, but gives a fair estimate. Next, I listed how long I use each during the week. I also thought about expanding in the future. Our area is really dusty, so a small car vacuum would be nice. I've been thinking about getting a computer someday, too. I multiplied wattage drawn by each appliance by the hours used per day to get an idea of how much power I need. Now I have an idea of how much electricity I needÄabout 113 Watt-hours per day. In the future, I may need about 191 Watt-hours. Consumption Chart How much Storage? The capacity of a battery (how much it can store) is rated in Ampere- hours. To figure out how big a battery bank I need, I converted Watt-hours to Ampere-hours. Since Power (watts) equals Volts times Amperes, I just divided the number of Watt-hours by the Volts. I decided to use a 12 Volt battery, so I divide 113 Watt-hours by 12 Volts to get 9.4 Amp-hours. But what if the sun doesn't shine? We have stretches of cloudy days, say about three in a row on average. Since I want to be able to turn on my lights during this period, I want to have a battery capacity of at least three days: 9.4 Amp-hours per day x 3 days = 28.2 Amp-hours. Another concern is the usable capacity of the battery. I'll start off with a lead-acid car battery. You can't use the full capacity of lead-acid batteries. In other words, a 40 Amp-hour lead-acid battery cannot deliver 40 Amp-hours. If the battery is a deep-cycle battery (designed for deeper or fuller discharges), one should only use about 80% of the capacity. For car batteries, only 70% of the capacity should be used to prolong the life of the battery. So I divided 28.2 Amp-hours by 70% to get 40.3 Amp- hours. I need a battery rated at least 40.3 Amp-hours. Figuring Energy Storage Chart Different batteries are rated in Ampere-hours at certain charge/discharge rates. For example, a battery may be rated 40 Amp-hours at a C/10 rate. A C/10 rate is the rate of charge or discharge. The rate of charge (in Amperes) is equal to the rated capacity of the battery (in Ampere-hours) divided by the cycle time (time to totally charge or discharge the battery in hours). In this case, C/10 equals 40 Amp-hours divided by 10 hours, or 4 Amps. If you discharge a battery at a higher amperage than its rating, you won't get the full capacity of the battery. If I plug in a load that needs more than 4 Amps, I would deplete the battery much faster. If the load is only on for a few minutes, than this is not a problem. Another consideration I have for choosing a battery is the rate of charge. I have to look at my appliances and see how much current they draw for how long. Currently, the maximum amps drawn is three Amps. One future appliance uses 8 AmpsÄa C/5 rateÄbut only for a few minutes. A C/10 rate will work fine for me now and for my future loads; a car battery should work just fine. Choosing a panel Next I wanted to buy a photovoltaic module. But which one? There are so many brands and sizes! I decided to buy a new panelÄI want this to be a portable system, so greater power per size is a factor. Another factor is voltage. I may decide to use Nickel-Cadmium or better yet, Nickel-Iron batteries someday. Generally, these batteries get up past 16 Volts under charge; lead acid batteries generally do not reach over 15 Volts. Considering the voltage loss through wiring and a regulator and due to heat, I need a panel that can deliver a respectable current at 17 Volts to fill nicad batteries. We can get five months of 90ø F weather here sometimes, and heat degrades the voltage output of most modules (about 15-25% for every 25øC above 25øC (77øF)). Modules can reach 50øC on a sunny day. Crystalline photovoltaic modules are made up of many cells wired in series; each cell has about 0.5 Volt drop across it. So I need a module with at least 36 cells (36 times 0.5 equals 18 Volts). Modules with 33 cells are called self-regulating for a reasonÄthe 13 Volts or so that they produce is not enough to overcharge lead acid batteries and not enough to fully charge nicads. Heat does not seem to affect amorphous silicone cells, but presently these are more expensive per watt. And yes, another factor is the cost. Although I was willing to pay for a new module, I wanted to get the most watt per dollar! I looked at the specifications of a few modules. Time for a lesson in alphabet soup! This is another area that has always confused me. I flipped through Home Power #24 to the article where Richard and Bob-O tested different photovoltaic modules. Let's see, Isc is the short circuit current. If I directly connected the positive terminal of the module to the negative terminal, I create a short circuit pathway for the electrons set into motion when full sun is on the panel. There is no load and no voltage. Next is Voc, the open circuit voltage. With full sun on my panel, this is the voltage difference from the positive to the negative terminal. There is no current flowing. The panel's maximum power is labled Pmax. The voltage (Vpmax) and current (Ipmax) at maximum power are also listed. The part of the "soup" important to me was Pmax (maximum power), Vpmax, and Ipmax. The final decision was fairly arbitrary. I called up my local dealer, Bob-O, and was told he dealt primarily in Solarex modules. Since the Solarex modules have 36 cells, 17.1 Volts at peak power, and were a fair price per watt, I opted for the Solarex MSX60 photovoltaic module. I decided to buy the 60 Watt panel instead of the 50 Watt, because I wanted plenty of power for future expansion. Maybe I'll run my toaster oven in my trailer.... The specifications for my particular module are Isc = 3.86 Amps, Voc = 21.4 Volts, Pmax = 61 Watts, Vpmax= 17.2 Volts, and Ipmax = 3.55 Amps, rated at 1000 Watts-meter2 (called solar insolation) at 25øC. Solarex provides real figures at 49øC and 800 Watts-meter2 to account for the loos in power due to heat. For my panel, at 49ø C, maximum power (Pmax) drops to 44.4 Watts and the current at max power (Ipmax) is 2.91 AmpsÄabout 15.3 Volts! The Frame I had my panel! The next step was to find a place for it and mount it. I found a fairly clear place about 20 feet from the trailer. Using the Solar Pathfinder, a device that shows the sun's path over a particular spot over the course of the year, I found that my site will get 5 hours of sun in the winter and about 8 hours in the summer. The hours in a day are not equal in the eyes of PV's; photovoltaics produce more power when directly facing the sun. My panel faces south on a stationary mount and will not track the east to west movement of the sun; when looking for a site for the panel, the hours just before and after solar noon are the most important. The 60 Watt panel will deliver the energy I need. In the winter, my panel should produce about 3.2 Amps times 5 hours of sun per day equals 16 Amp-hours, or 192 Watt-hours; in the summertime, my panel will only produce 2.9 Amps but for 8 hoursÄ23.2 Amp-hours or 278 Watts. Great! At first I looked at some iron angle rack to mount the panel, but found out that we had some Echo Lite PV racks. I understand this company is out of business, but the racks work great! I had to modify the rack to fit the Solarex module (the module is about 20 inches by 44 inches long); I drilled two extra holes in the two-module rack holder. I screwed the bottom of the rack into two 3 foot long 2 by 4s, and that was it. Shall I compare Thee to a Nose... Photovoltaic modules perform best when perpendicular to the sun in the north-south axis, just like your nose gets burnt from the sun before your arms or legs. Your nose is at an angle to catch more of the sun's rays. The angle of photovoltaic modules have to be adjusted throughout the year to follow the sun's angle in the skyÄlow in the winter and high in the summer. I have the panel set at 45ø from horizontal for fall sun, but the rack is adjustable to three more angles: 60ø for winter sun, 30ø for summer sun, and 0ø folds up for easier carrying. Next Time Whew! I feel I've done and learned a lot, but I'm not finished yet! The next step is wiring, and building a homebrew regulator, which I'll write about next time. Access Therese Peffer, c/o Home Power, POB 520, Ashland, OR 97520 ù 916- 475-3179