The topic of Photovoltaics (PV), also commonly referred to as “solar power” or “solar electric,” has been a growing part of our personal lives for the past five years. Homesteading in the rural mountains of far west Texas has been a crash-course lesson in living with solar power. For simplicity, we will refer to solar power as PV in the rest of this article.
Solar Module Basics
A solar panel or module is compromised of PV cells arranged in different series and/or parallel configurations to provide different voltage and current outputs. These cells form modules, commonly referred to as panels. The modules are typically rigid, but thin-film modules can be incorporated into flexible designs. The front of the module uses glass or another composite to protect the electrical connections from rust and corrosion caused by the elements. The back of the module can be plastic, wood, metal, glass or composite. The frame can be composed of metal, plastic, composite, wood and there are even some frameless designs. PV modules can be arranged together to form PV arrays. These arrays are usually mounted to rooftops, ground mount systems, the tops of poles, orbiting satellites, or anywhere else they can receive precious photons.
Mono-Crystalline is the granddaddy of solar cell technology, and has been making usable electricity since the 1950s with ever increasing efficiency and production. In 1958 the NASA Vanguard satellite was launched into orbit with a monocrystalline backup PV array. The use of photovoltaics on satellites would push crystalline and all other PV technology forward. To this day, monocrystalline solar panels still boast the highest efficiency in the mainstream PV market. Their downsides are a bigger price tag, and arrays that don't function well in partial shade or the low light of morning and evening sun.
Polycrystalline solar modules were an important advance in PV science. Instead of relying on larger, harder to manufacture slices of solid silicon crystals, polycrystalline uses small flakes of PV silicon. The resulting cells are cheaper to manufacture, though slightly less efficient. But polycrystalline arrays have better power output in partial shade than monocrystalline, and start producing usable power earlier in the day, and later into the evening. Polycrystalline modules are easy to identify by their multitude of randomized, shimmering flakes.
Thin Film and Amorphous cell technology came along in the late 1970s and had many people in the solar field quite excited. It promised and delivered much lower production costs, lower energy input to produce, the possibility of much larger cells, and an ability to work in more varied light conditions such as low light. But there is an important catch; the thin film panels are much less efficient than crystalline, meaning that the panels need to be larger to achieve the same Watt output. Therefore if space is an issue, thin film panels are not ideal. The technology typically utilizes amorphous silicon as the photovoltaic layer, although other types do exist. Flexible thin film is one of the newest PV marvels, and these flexible modules can be installed in a variety of places, built into roofs, or even laminated onto cars or planes. The flexible thin film is available commercially in many forms, but it is still an infant technology, prices are high and efficiencies are ever improving. Look for thin film technology to gain much more market share in the next 5-10 years.
Sizing a PV System
The first step to sizing a PV system is to determine the Watt hours (Wh) of electricity you use per day. You can determine the wattage of your appliances by looking at the device for a label that gives you watts consumed. If it does not tell you the watts, it will tell you the ampere draw and input voltage, and simply multiply these together to get the Watts it consumes.
Next, you need to determine how long each device runs every day. For some devices you can just use a timer. Another good way of determining Wh for AC devices is a nifty little product (one popular version is known as the Kill-a-Watt) that plugs inline with the device to tell you how many hours a day it runs and how many watts it draws.
It is also good to keep in mind that every device that draws through an AC power inverter will have losses; typically 10-20% of the energy is lost through the inverter inefficiencies. So for those of you living off the grid, you should try to use as many DC devices as possible. To get a total on how much energy you use, make a chart like in fig A and fig B which separates AC and DC loads (if you plan on using both).
Now that you have determined the Wh you use, you need to do some calculations.
1. (Note: if you are doing a grid-tied solar system without a battery bank, skip ahead to step 3.) Since you will be you be using an inverter, you need to account for the extra energy it uses, so divide your total AC load by your inverter efficiency, and then add your DC watt hours (if you have any) to determine the total.
Inverter 87% efficient
AC loads 525 Wh
525/0.87=603.44 or 604Wh
604 AC Wh + 478.5 DC Wh=1082.5 Wh or 1.1 kWh
2. Now with a battery bank there are charging and discharging losses to take into account. This lowers the efficiency to typically 80%, so divide your result from step 1 by 0.80.
3. With your total Wh determined, you can now calculate how many Watts of PV you will need. Find out how many full sun hours you get in your area in the worst case scenario such as winter (this information is available online), and then divide the Wh by these sun hours.
1353.12Wh/5.5=246.02 watts, rounded up to the nearest tens place 250 Watts
4. So now you need to decide how many PV panels you will need to fulfill the electrical needs of your home. In this case, I have chosen 60 watt amorphous thin film panels. These modules are produce 18.5 volts open circuit voltage and I will be using them on a 12 volt system. Since the panel voltage is higher than the system voltage, adequately charging the batteries will be a little easier. PV modules are available in many different voltages and can be arranged in different series/parallel configurations for to achieve the desired current and voltage. For our own system we need 250 Watts, so we divide that number by 60 (our module Watts) and round up to determine that we will need 5 modules, resulting in a 300 Watt PV array.
The above info is intended as a guide or estimate. Electricity is inherently dangerous, and even deadly. If you undertake any experiments described here, it is at your own risk. Please use common sense and take all reasonable precautions. Please consult books and/or your local professional, especially for grid-tied solar systems.
Antonio Luque (Editor), Steven Hegedus. June 2003.
Handbook of Photovoltaic Science and Engineering