SOLAR ELECTRIC SYSTEMS
Energy produced by the sun has been used to improve our lives since ancient times. Early housing was constructed facing it for heat and more daylight through openings or windows. The sun was regarded a deity by all cultures. Known life cannot exist without its radiation and gravitational pull. Huge pyramids built thousands of years ago have openings correlating to the sun’s position during summer and winter solstices or other times of year considered important. It was thought the sun moved across the sky.
Modern understanding of our solar system and outer space is considerably different with exception of life’s entire dependence on this star. Thought to exist in the current state for four billion years the sun will continue doing so the same amount of time. Although slightly over one-hundred times earth’s size, compressed density makes its mass nearly ninety-nine percent of all orbiting planets.
Radiation and gravity go the speed of light traveling ninety-three million miles to earth in over eight minutes. Some radiation bounces off the atmosphere although one-thousand watts per square meter deposit on this planet’s surface on a clear day, as much electrical energy people use in a year. Heat and vegetation produced by it are abundant.
PHOTOSYNTHESIS
Plants make their own food by photosynthesis. Vegetation converts light into chemical energy by absorbing carbon dioxide from air, making sugars and oxygen. In comparison mammals, reptiles, fish and insects breath oxygen, eat sugars and exhale or deposit carbon. The cycle of life totally depends on light radiated from the sun.
Green plants contain chlorophyll proteins which ironically absorb light from either side of the visible spectrum. Being in the middle, green and yellow are left to reflect making these plants appear that color. When red and blue lights are absorbed, chlorophyll separates water into hydrogen and oxygen. Hydrogen bonds with carbon dioxide forming glucose sugars and oxygen releases into the air. Our atmosphere consists of seventy-eight percent nitrogen. Temperature is a variable for all life forms.
When plants and animals decay carbon remains and when concentrated by fossilization under heat and pressure for a long time become coal, natural gas and oil which burn hot producing heat or mechanical and electrical energy. Under extreme conditions carbon transforms into diamonds, the hardest substance on earth. It is estimated there are hundreds of millions of diamonds in earth’s crust buried too deep to extract.
LIGHT WAVES AND PHOTONS
The sun is basically a massive nuclear reactor fusing hydrogen atoms into helium, converting four million tons of matter into energy per second. When this energy eventually escapes sun’s gravity it radiates at various wave lengths. Longer ones are infrared with shorter ultraviolet and X-rays. Visible light to us is only a short span somewhere in the middle. All waves have a velocity of 186,282 miles per second in a vacuum in outer space. First it was thought radiation is strictly waves, but then realized light bends toward objects with large gravitational pull. Einstein concluded light is made of both waves and particles in his theory of relativity. These particles are called photons with no measurable mass or electric charge. Photons are attributed for exchanging electrons between molecules in photosynthesis and passing electrons from one substance to another in electric solar systems.
PHOTOVOLTAICS
Tangible matter is made of tiny atoms. Atoms contain protons and neutrons forming a nucleus with electrons orbiting around at different distances. Several electrons can be the same distance by orbiting at different angles imagined as clouds or shells. The outer most is the valance orbit. Protons have a positive, neutrons none, and electrons a negative charge. Depending on composition different elements form. They combine into molecules, carbon dioxide and water for example. Electromagnetic force is extremely stronger than gravity at small atomic levels.
Atoms with more protons than electrons have a positive charge and those with more electrons a negative charge. The difference often occur in the outer valance shell. Substances consisting of equal proton and electron charge are semi-conductors. These are made to charge one way or other by adding small amounts of atoms from other material.
Crystalline silicon might not be the best semi-conductor but is abundant and easy to manufacture. Silicon’s ability to stop or allow electricity is essential for computer chips. Originally shining light on some metals produced small amounts electricity. Transparent silicon is more efficient.
SOLAR PANELS
Solar panels are made of several modules subdivided into solar cells. Each cell by itself doesn’t produce much energy. It’s common for sixty cells in a module connected in series, plus to minus, resulting in substantial voltage. Twelve volts is one standard for direct-current. Twelve volt batteries are used in cars. Modules can be connected in parallel by tying negative and positive connections together increasing amperage. The panel is covered with a transparent sheet and solid backing. Several panels connected parallel at a fused combiner boxes make arrays providing more available electricity.
Solar cells made of semi-conductors convert sunlight into electricity. One semi-conductor layer is mixed, called doping, with material having less electrons than protons. Positively charged the P side is bonded by a layer not doped to the N side mixed with extra electrons. The layer in the middle is neutral forming a P-N junction. Photons excite extra electrons displacing them from silicon on the N side. When connected by a wire they want to fill vacant electrons called holes on the P side creating a potential difference across the P-N junction. Voltage forces electrons back to the N side repeating the cycle. If a wire feeds resistive and inductive loads, voltage potential is measured across those devices as well. Solar panels produce direct-current in one direction.
Some solar panels are more efficient than others producing more electricity with the same amount of irradiation. Conversion efficiency is calculated by short circuit current, open circuit voltage and fill factors. Depending on type of silicon and how it’s doped determines electron movement by photons from different light frequencies. Expensive panels have cells with several layers of different semi-conductors increasing electrical production. Panels with transparent coverings resistive to glare but reflecting infrared rays are better. Infrared rays create heat without displacing electrons by photons. Solar panels work best at lower temperatures.
PHOTOVOLTAIC SYSTEMS
Photovoltaic or PV systems are proper names for solar electric systems.
The simplest system connects twelve volt devises such as heaters, lights and motors to twelve volt panels. They only operate when sun shines. Adding battery backup charged by some electricity produced allows operation for a while during night or on cloudy days. An advantage for lights at night and locations without power lines, satellites being an extreme example. Controllers prevent batteries from over or under charging. Trade off exists between size of panel or array and number of batteries. No point installing more batteries than panels can charge. Automotive batteries work but lead-acid, deep-cycle ones are made for this purpose charging up to eighty percent storage capacity. Larger solar panels can output much more than twelve volts.
Inverters added to photovoltaic systems decrease or increase voltage and changes direct into alternating current. 120 and 240 volts at 60 cycles per second are standard. Inverters aren’t complicated like variable drives because only one voltage and frequency are created. Inverters should be capable of suppling circuit breakers protecting smaller wires to ac loads. Unless other production sources are connected to PV wiring, it is called a stand-alone photovoltaic system.
Without many solar panels, arrays produce only enough electricity for some devises or supplement energy delivered by generators, windmills and utility companies. Tying sources together requires computer controlled power conditioning equipment insuring electricity is all the same voltage with frequencies in phase. Companies generally supply most power for property and houses, but electricity from other sources reduce that amount lowering utility fees. Renewable energy provided by sun and wind is considered free.
PHOTOVOLTAICS AND THE NEC
The National Electric Code doesn’t determine how solar power is made but that it’s installed efficient and safe. The NEC (NFPA 70) is one volume in an extensive library published by the National Fire Protection Association dedicated to preventing, restricting damage and stopping fires. NFPA also concerns safety for fire fighters and other emergency responders.
Photovoltaic systems are addressed by Article 690 in Chapter 6 titled Special Equipment in the NEC. General Requirements in Part I necessitate installation of PV equipment and all associated wiring and interconnections be performed only by qualified persons with skills, knowledge and safety training to recognize and avoid hazards involved. However, electricians and homeowners become involved with PV systems during troubleshooting and maintenance.
Solar panels exposed to sunlight continue to produce voltage. Voltages often exist on both sides of disconnect switches when other production sources and inverters interconnect. Parts VII & VIII of Chapter 6 make clear Energy Storage Systems and Interconnected Electric Power Productions Sources be installed according to Special Conditions in Chapter 7. PV stand-alone systems are addressed by Articles 690.10 and 710.15.
Throughout the electrical code requirements for circuits, disconnection devices, wiring methods, grounding and bonding, and marking depend on the type of installation. Requirements become complex with PV systems comprised of different voltages and currents. Equipment and components are listed or field labeled appropriate for photovoltaics. Disconnect switches subject to electrical back feeds are suitable and labeled for that purpose. Polarity for direct-current branch circuits more than 60 volts are identified according to Article 210.5(C)(2) with red positive (+) wires and black negative (-) ones or accurately labeled.
Assuring safety for fire fighters, first responders and maintenance personnel, many PV circuits need ground fault or arc fault protection. Inverters and power conditioning equipment contain capacitors storing energy necessitating means of rapid shut-down limiting power output to less than 30 volts and 240 watts within 10 seconds. Disconnects are put at solar panels and between its equipment. The main PV disconnect switch initiates rapid shut-down of the entire system in or on a building, labeled to code and located next to the utility main panel easily found by firefighters. PV service panels also operate remote-controlled disconnects to completely protect buildings from photovoltaic systems. PV system dc circuits cannot be greater than 600 volts on or in one and two family dwellings. Exception for rapid shut-down is made for buildings only containing PV equipment.
CONSIDERATIONS
Solar powered systems are efficient and productive in some geographic locations more than others. Lower latitudes closer the equator receive more daylight and higher altitudes with thinner atmosphere allow more sunshine. Angles solar panels are mounted vertically depend on geographic latitude, although sun is higher during summer than winter months. Solar panels do not work when covered by snow.
Sun’s radiation naturally heat buildings and blocking it with panels mounted on roof tops might be a good idea in the south but create a dilemma farther north. Air conditioning often require more electrical consumption than heating given many furnaces burn fuels. Solar panels mounted on structures away from buildings have an advantage for programmable tracking systems using electric motors to position panels toward the sun as its location changes during the day and throughout seasons.
When a building produced more electricity than used meters ran backwards lowering if not reversing utility costs. Electronic meters are smarter detecting this occurrence. Some electric companies do not refund the difference. Others might but don’t pay the same as they charge to provide it. Some locations where consumption is growing faster than electricity is produced give significant financial incentives to homeowners installing photovoltaic systems. Cost of solar installations compared to utility savings over a period of time at different locations determine efficiency. Solar panels are expected to last about thirty years.
Storage batteries emit toxic vapors possibly creating explosive mixtures, particularly when charging. The room they’re in must have sufficient ventilation eliminating these hazards and have an entrance door designed for quick exit. Not a bad idea housing lead-acid batteries outdoors. Solar battery back-up is certainly an advantage during power outages to keep lights and furnaces working.
Global warming by burning carbon fuels is a prevalent concern and using sun’s energy a popular idea. People want to do their part. Good news is, as solar and wind generated power becomes more efficient with less installation costs industrial companies are finding it economical installing solar and wind farms instead of massive power plants requiring substantial maintenance and large quantities of expensive, transported fuel.
Eberling@www.thndrsns.com
