Photovoltaic Tutorial:

Anatomy of a PV Panel

Although the term "panel" is still widely used, it's more accurate to say "module" when referring to a rectangular unit of solar cells encased in an aluminum frame. When connected in series or in parallel, Installers in the field will refer to the several modules as a solar panel. And any number of modules wired in series or parallel make up a solar array.

Most people are unaware that Albert Einstein won his Nobel Prize, not for relativity, but for a paper he wrote in 1905 on the photoelectric effect. It described the physics of photons striking atoms in metal and putting electrons into motion. Following on Max Planck's earlier work, Einstein believed that light delivers energy in bundles of particles, or quanta (which later became known as photons). Significantly, this force can be measured, using Planck's constant times the frequency of the light. A few decades earlier, French physicist Edmund Becquerel first discovered what came to be known as the photovoltaic effect. In 1954, engineers at Bell Laboratories created the first semi-conductor PV solar cells and packaged them into a portable unit. And the solar module was born.

Created in 1954 at Bell Labs, the first solar modules were used to power telephone service in rural areas. The U.S. Department of Energy has prepared a nice timeline outlining the history of this field.

So how does a module produce electricity that can be used in the home? Each unit is packed with 60 or 72 individual solar cells that generate power when exposed to sunlight. The next step is to collect all the electrons agitated by the light and transport them out of the solar cells. To do this, two layers of silicon, oppositely charged, are sandwiched together during the manufacturing process. Like a typical battery, it's this molecular disparity that produces voltage when a module is exposed to light. Now it can start producing kilowatts for your home.

The center divider between the oppositely charged silicon layers is known as the P-N junction. At the junction, the electrons flow through the traces towards one collection point, or contact, inside the module's junction box. Each module has two external cables attached at the other side of this junction box. This allows PV installers to hook the modules up (either in series of parallel) and then send the electricity from the solar array downstream towards the main service panel in a home.

You may notice in the illustration above that solar cells produce direct current (DC), which is electricity that always moves in one direction. However, most electrical loads in the home require alternating current (AC). Consequently, another component called an inverter is added to the PV system circuit to convert DC into AC. The inverter is considered the brains of a PV system because it regulates the array's voltage and turns the electrical current into the same type used by the utility grid (i.e. 240/120 AC volts at 60 cycles per second). To complete the electrical circuit of the PV system, building and electrical codes may require isolation switches (called disconnects) and a few other components for circuit safety and equipment protection.

So what makes a solar module a renewable resource? Well, when their trip around the circuit is done, the valence electrons in the silicon layers recombine with the holes created when the photons first knocked them into motion. At this point the affected atoms return to a state of equilibrium. As far as the module goes, nothing is lost in its structure. That's why PV arrays can produce power indefinitely.

It's estimated that a module's efficiency decreases at a rate between one-half and one per cent each year. That's not because the solar cells get tired. More often, it's the plastic laminates surrounding the semi-conductive material that starts degrading. The laminates are there to prevent moisture from seeping in and corroding the metal traces. Either the seals will start to break, or a laminate will discolor from UV exposure, blocking the absorption of photons.

In short, it takes a long, long time for a PV module to stop producing energy. Regardless, it's a good idea to buy these components from a manufacturer with a proven track record and a facility that uses the best technology and materials available. Each module should generally come with a 25-year free replacement warranty.

How designers measure sunlight and kilowatt hours

PV modules capture three types of proton-generating activity, otherwise known as solar irradiance.

Irradiance can be measured by instruments called pyranometers and pyroheliometers, and its value is expressed as "watts per square meter", or w/m2 for short. Each day, the most useful irradiance occurs three hours before and after solar noon, when the sun reaches its zenith in the sky. Thus, experts have dubbed the hours between 9 a.m. and 3 p.m. daily as the golden hours for PV, or the solar window. This is the time when you don't want anything shading your PV array.

Of course, not every day of the year is filled with sunshine. Also, in the winter the sun may be too low in the sky, even between 9 a.m. and 3 p.m., to generate much irradiance. So in order to quantify the power generating potential in any given locale, the National Renewable Energy Laboratory (NREL) publishes charts for different U.S. latitudes and cities. Most places in the United States receive on average 4-5 hours of usable irradiance daily. The NREL estimates are referred to alternatively as peak sun hours, insolation or kWh/m2/day. This information is key to estimating how much power a PV system will generate annually.

The unit of measure "kWh" stands for kilowatt hour, the rate of energy that's used for any calculation related to electrical generation and consumption. One kWh represents 1,000 watts produced in the course of 60 minutes. If you check your utility bill, you'll find that you're being charged per kilowatt hour of electricity you consume. So, to find out how many actual kilowatts of energy a PV array must to produce each year to cover this consumption, just divide your annual kilowatt hours charged by the peak sun hours for your local area, then multiply that result by 365 days a year.

For instance, if your kilowatt hour consumption for the last 12 monthly bills totals 7,000 and your city averages 5 peak sun hours daily, then 7000 / 5*365 = 3.84 kilowatts. That's just a ballpark estimate, of course. A solar designer will then have to multiply this figure by a variety of derate factors, which account for wire losses, DC to AC conversion losses, occasional soiling of the modules (causing shading losses), age of the modules, etc. But once the numbers get crunched, he or she will know how much juice is required to cover a portion (or all) of your electricity needs each year.

In system sizing, each module contributes an equal part of the array's wattage total. For example, an array of 20 modules that are rated to provide 235 watts each would produce a total of 4,700 watts. In the parlance of a solar salesperson, this is referred to as a 4.7K PV system. (Incidentally, if you like math, you'll find a lot more of these calculations in our Steps to Going Solar guide.)

One last thing to consider: Not only is this new energy from the sun clean and devoid of emissions, but the whole process is automatic. You don't have to shovel coal into a furnace, hose down steaming nuclear power rods, or any of that labor- intensive stuff. There are no hinges to oil or batteries to replace (unless you purchase the battery backup option). From time to time, you'll check the array for debris or snow, wash the glass on the modules, prune nearby trees that may cause new shading, and maybe once a year have someone inspect the wiring for any loose connections. No other repair or maintenance is ever needed. A clean, unshaded, obstructed array oriented in any direction but northward is pretty much all it takes to operate your personal power generation plant for several decades.

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Next: Module specs and features

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