Me and "My  solar panel " ...




 I did it. I decided to make my own power. I live in a trailer about a 100 foot extension cord length from Home Power Office & Power. The batteries are filled with electricity from photovoltaic panels and a wind generator — I 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 do a system for my folks.   



1. The Plan

PV-Power-Pack (5W, 10W, 25W, 50W, 75W, 100W )


Solar Panels USP




I decided to use a photovoltaic (PV) module to make electricity from the sun, and a battery to store it in, but what kind of PV module and 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 ranging 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 the 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 need about 100 Watt-hours per day.  In the future, I may need over 212 Watt-hours.



 2. How much Storage?  


The capacity of a battery how much it can store is rated in Ampere-hours. To figure out how big of a battery bank I need, I converted Watt-hours to Ampere-hours. Since power (Watts)  equals voltage  (Volts) times current (Amperes),  I divided the number of Watt-hours by the Volts. I will use a 12 Volt battery, so I divide 100 Watt-hours by 12 Volts to get 8.3 Amp-hours.

Another concern  is  the  usable  capacity  of  the  battery. Lead-acid batteries should not be fully discharged — you can’t regularly  use  the  full  capacity.  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 never use more than 80% of the capacity. For car batteries, only 20% of the capacity should be used any more will decrease the life of the battery. I’ll use a car battery a friend gave me for now, and maybe get a deep cycle or alkaline battery in the future. So I divided 8.3 Amp-hours by 30% to get 27.7 Amp-hours. I need a battery rated at least 27.7 Amp-hours; the car battery is rated 40 Amp-hours.

But  what  if  the  sun  doesn’t  shine?  We  have  stretches  of cloudy days, 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: 27.7 Amp-hours per day x 3 days = 83.3 Amp-hours.  I don’t have this capacity right now, but will make do with what I have. I will just watch my use on cloudy days until I get a different battery.



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 draws more than 4 Amps, I would deplete the battery faster. If the load is only on for a few minutes, no big deal.

I looked at my consumption chart to see how much current each appliance draws for how long. (Watts divided by Volts equals Amperes.) Currently, the maximum amps drawn are three Amps. The vacuum uses 8 Amps a C/5 rate but only for a few minutes. A C/10 rate would work for my current and future loads. The car battery can take a high discharge rate for a short time, so no problem there!



3. 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 panel; I want this to be a portable system, so greater power per  size  is  a  factor.  Another  factor  is  voltage.  I  may  use Nickel-Cadmium  or  Nickel-Iron  batteries  someday;  these alkaline batteries may be fully discharged. Generally, these batteries  get  up  past  16  Volts  under  charge;  lead  acid batteries generally do not get past 15 Volts. Some voltage will be lost through wiring and a regulator (to prevent the PV from overcharging the battery), and also due to heat. We can get five months of 90° F weather here, and heat degrades the voltage output of most PVs (about 15–25% for every 25°C above 25°C (77°F)). Modules heat up to 50°C on a sunny day. I need a panel that has a high enough rated  voltage to deliver a respectable current to fill nicad batteries. With a panel rated at 17 Volts, 15 Volts may reach the battery. Crystalline PV modules are made up of many cells wired in series; each cell produces about 0.5 Volts. So I need a module with at least 36 cells (36 x 0.5 V = 18 V). Modules with 33 cells are called self-regulating for a reason the 13 Volts or so that reaches the battery cannot overcharge lead acid batteries and is not enough to fully charge NiCads. Heat does not seem to affect amorphous   silicone   PVs, but   currently   these   are   more expensive per watt. Yes, another factor is price. I was willing to pay for a new module, but wanted the most watts 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 connect the positive terminal of the module to the negative terminal,  I  create  a  short  circuit  pathway  for  the electrons set into motion when sun hits the panel. There is no load and very little 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. No current is flowing. The panel’s maximum power is labeled 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, produce 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 25°C. 

Solarex  provides  real  figures  at  49°C  and  800 Watts-meter2 to account for the loss in power due to heat. For my panel, at 49°C, maximum power (Pmax) drops to 44.4 Watts and the current at max power

(Ipmax) is 2.91 Amps — voltage drops to 15.3 Volts!



4. 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 across a particular spot over the course of the year, I found that my site will get 4.5 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; photovoltaic produce the most power when perpendicular to the sun. My panel faces south on a stationary mount and will not track the east to west path of the sun. When looking for a site for the panel, the two 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 will produce about 3.5 Amps times 4.5 hours of sun per day, which equals 16 Amp-hours, or 189 Watt-hours. In the summertime, my panel will only produce 2.9 Amps due to heat but for 8 hours 23.2 Amp-hours or 278 Watts. Great! I’ll have plenty of power.



At first I looked at some perforated angle iron 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.  



 5. Shall I compare Thee to a Nose...  



Photovoltaic modules perform best when perpendicular to the sun, 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. Unlike your nose, you can adjust your panels throughout the year to follow the sun’s elevation in the sky: low in the winter and high in the summer. I have the panel set at 45° from horizontal for fall sun, but the rack is adjustable to three more angles: 60° for winter sun, 30° for summer sun, and 0° folds up for easier carrying.  



 6. Next Time  



Whew! In planning a system, I can see why using efficient appliances and shutting that light off when not in use is so important. 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.


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