Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power .Solar power is an intermittent energy source, meaning that solar power is not available at all times, and is normally supplemented by storage or another energy source, for example with wind power and hydropower.
Basic Information about How a Solar Electric System Works
A Grid-Tied Solar System With No Battery Back-Up
The Solar Module
The workhorse of any solar electric (PV) system is the module, which is usually mounted upon the roof.
There are four kinds of photovoltaic solar 'panels' commercially available today:
- Single crystal modules are the most efficient (10 to 17%) and the most expensive. This kind of technology has been around longer than any other and is most frequently used for outer space (satellites) and remote or off grid tracking applications. It has demonstrated long term (30 year) stability in its ability to produce power in hot desert to marine environments. They are usually recognizable as the modules with polka dots or octagons
- Poly or Multicrystalline units are less expensive but demonstrate lower efficiencies (9 to 14%). Polycrystalline modules are pure blue in appearance and since there are no gaps or openings in the face of the collector, its size is about the same as its more efficient polka-dotted single cell cousin.
- Amorphous (Thin Film) cells are manufactured by vaporizing and depositing silicon on either glass, ceramic or steel. The process to manufacture this module is simple and cheap, but efficiency (5 to 7%) is so low that a very large area is required to produce the same kind of power made by the single or polycrystalline modules. This technology is most often seen in toys and calculators as well as in Building Integrated PV (BIPV) where the solar module is actually built into the roof or structure.
- String Ribbon manufacturing produces efficiencies of 7 to 8% but are relatively inexpensive. Long term performance is similar to other polycrystalline technologies, but low efficiency requires relatively larger systems to produce the same energy as single or polycrystalline modules.
Ratings
Manufacturers rate their individual products in peak watts (STC) according to their performance under ideal laboratory conditions. Thus, a module that is pegged at 120 peak watts can only be compared to another 120 watt brand when they are both in the same lab at the same temperature and all other conditions are the same. A more realistic figure used by the California Energy Commission is the PTC rating. A module's peak rating of 120 watts may only be 105 watts under 'real world' conditions. Also, PV electrical output decreases as the temperature increases, while the opposite also holds true. A system may be producing in excess of its peak (STC) rating on a very cold clear winter day (especially if snow is reflecting the sun) and less than its rated PTC output on a hot clear day.
Modules produce electricity only when the sun is shining. Their power output changes during the day as the sun moves through the sky and the temperature changes. We can expect a stationary module to get 5.5 hours of "usable" sun per day on an average annual basis in most of Southern California. It may produce more accumulated energy in summer because the days are longer while the opposite is true in the winter. However, cells will be more efficient on a clear winter day because their lower temperature will enhance performance. A 120 watt module de-rated to its PTC output of 105 watts will produce 105 watts x 5.5hrs/day x 365 days = 210,788 DC watt hours (210.8 kWh DC) in a year if it is positioned at the optimum orientation and angle.
Collector angle or tilt and orientation are also important but not critical for grid-tied system's annual output. When a panel is used to generate electricity its power is sent into the grid and the meter will go backwards when production exceeds the usage of the household. If the panel is tilted high for good winter performance when days are short, the summer output will suffer when the days are long. If you are trying to attain the best annual outcome in the Los Angeles area the system should be placed at about 30o to 32o facing due South. But this angle is much higher than most residential roofs which are commonly 19o from the horizontal (or commonly referred to as a '4 in 12 pitch'.) The increase in annual performance from propping the system up at 31o versus 19o is only three percent! It would seem preferable to keep the system in profile with the architectural lines of the roof rather than to maximize output and create a solar eyesore.
Orientation of modules is important to performance, but going east or west (instead of south) at our latitude will only decrease performance by 9% on a typical (4/12) Southern California roof. So, if you place the system flush to a west or east facing roof, overall annual performance will decline by only twelve percent ( 9% + 3%) from ideal 30o true south. Laying the modules flat will decrease from peak by 11 percent. Facing the system north is not advisable.
Note: These comments concerning collector angle and orientation do not apply to stand alone or off-grid or battery back-up power applications where daily energy generation (rather than overall annual performance) is critical.
PV modules are ganged up and mounted in series or in parallel. A modest 1.2 kW (DC) system may employ sixteen 75 watt modules and a larger 6 kilowatt installation will need 80! The larger a system becomes the more important it is to use bigger individual units. The same 5 kilowatt system will need only forty 150 watt modules, which means less wiring and less labor. However, racking costs may not decrease by much, because roughly the same amount of material would be necessary to mount the system. When choosing a method to attach the modules to a roof it is important to use a product that has been engineered to handle the load plus resist wind shear and to distribute the weight of the system throughout the structure of the roof. It is absolutely essential to mount the substructure of the PV mounts to rafters, as this will allow attachments to be weather-tight for the life of the roof and will provide a firm secure mount for the entire load. Eighty 75 watt modules with rack can weigh as much as 2000 pounds, and you should not rely upon bolts in just plywood sheathing to hold this kind of load securely! Over time, even much lighter pool heating solar panels (average system weight = 400 pounds with water) can pull lag bolts out of plywood sheathing. The module support framework should raise the entire system off of the roof by at least 3.0 to 6.0 inches. This will allow the air to circulate around the array keeping the system cool and the roof dry. In hotter dessert climes six inches would be recommended.
The Solar Inverter
While the workhorse of any PV system is the module, its power usually cannot be useful unless it is converted into a form that is compatible with our appliances. This transformation is accomplished by a device know as an inverter. Its role is vital in applications where the user is also relying upon a utility to provide electricity.
The inverter's main job is to convert the DC (direct current) output of PV modules into AC (alternating current) electricity, which is the kind of energy our homes need to operate. When Thomas Edison first started experimenting with electricity, he envisioned a future using DC electricity, the type of power provided by batteries. When power plants were erected in New York at the turn of the century, generators had to be constructed every half mile or so, because DC electricity dissipated so quickly and traveled poorly over long distances at safe voltages. Almost every home uses AC power which is generated at plants far away by turbines driven by oil, coal, uranium, natural gas, wind or water. The turbines create electricity by rotating coils through magnetic fields creating 'waveform' electricity which oscillates at 60 times per second (in the U.S.) If your eyes were quick enough you would actually be able to see a light bulb flicker as the electric current to that bulb oscillates 60 times a second.
When an inverter is operating it takes the electricity from the solar array and causes it to oscillate until it matches the frequency of the grid (60cps or Hz). An inverter with ground fault protection will also be constantly checking the DC wiring for shorts or bad connections. If one of the wires is cut or frayed or if a live wire touches a grounded path, the inverter will shut down. In some instances a GFCI fuse will blow. This is one of the many safety features that will help protect the home if something is wrong with the wiring. Any inverter that is attached to the grid must also possess an "anti-islanding" feature. This means that the inverter will NOT continue to operate if there is a power outage. If there is no electricity supplied to the inverter from the grid, the solar system will shut down to prevent electricity from backfeeding the wires in your neighborhood. When the utility company arrives to make repairs in your local area they should not have to worry about whether or not there is electricity in the wiring coming from your house.
Some of the electricity is lost when an inverter turns DC into AC. According to the California Energy Commission's list of approved inverters, efficiencies range from 87 to 94% depending upon the make & model. The operating efficiency for any single inverter also varies depending upon load and temperature. If the inverter is placed in the sun, the hot summer months will compromise its ability to operate efficiently. So it's a good idea to place the unit in a cool location. Some inverters use fans to keep cool while others rely upon a heat sink, which is a large mass of metal with fins that will radiate heat to the outside of the inverter.
Wiring and Disconnects
When an inverter and solar panels are added to a home or business, the city, county or utilities involved have separate and sometime overlapping requirements. Although the inverter may have its own fuse, sometimes separate fusing is required. Although an inverter may also have its own on/off switch, a separate DC switch may also be required. Even though an inverter will be attached to its own dedicated circuit breaker(s), your utility may also require a separate switch that will shut off the AC power between the inverter and the grid. The utilities that require this item obviously do not trust the ability of the inverter to 'drop-out' during power outage nor do they want their linesmen hunting for the appropriate circuit breaker switch during a black-out. Usually they want the switch or disconnect within nine feet of the meter so the utility person can easily spot it and make sure that the inverter is off-line. These additional switches & fuses add to the cost and complexity of a system, but nobody wants to short circuit safety. As solar systems become more commonplace some redundant features may be dropped.
It is important to keep line losses as low as possible so you can take advantage of every kilowatt the solar system will produce. Wiring should be of sufficient gauge (size) and run (length) in order to keep transmission losses to less than 3%. While twelve gauge stranded may be sufficient for most runs between array and inverter, ten gauge would be better for 100 feet or more. While this may apply for higher voltage (>100 volt) inverters, wiring for low voltage units would be thicker. Low voltage inverters are typically more useful in battery back up or off-grid applications.
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