All life on Earth depends on energy from the sun. Solar energy is the source of energy for photosynthesis. It provides the warmth necessary for plants and animals to survive. The heat from the sun causes water on the Earth's surface to evaporate and form clouds that eventually provide fresh rainwater.
Solar energy is the result of thermonuclear fusion reactions deep within the sun. These reactions produce so much energy that they keep the surface temperature of the sun at about 10,300B0F (5,700B0C).
Even though solar energy is the largest source of energy received by the Earth, its intensity at the Earth's surface is actually very low due to the large distance betwee n the Earth and the sun and the fact that the Earth's atmosphere absorbs and scatters some of the radiation. Even on a clear day with the sun directly overhead, the energy that reaches the Earth's surface is reduced about 30 percent by the atmosphere.
When the sun is near the horizon and the sky is overcast, the solar energy at ground level can be negligible. It also varies from one point to another on the Earth's surface.
Nevertheless, in the 20th century, the sun's energy has become an increasingly attractive source for small amounts of direct power to meet human needs. A number of devices for collecting solar energy and converting it into electricity have been developed, and solar energy is used in a variety of ways. Solar energy is used to heat houses, and in many countries specially designed solar ovens are used for cooking.
The sun also supplies energy to electric generators that provide power for weather and communications satellites and for radio and television equipment.
Because the intensity of the sun's radiation at the surface of the Earth is so low, collectors designed to capture solar energy must be large. In the sunniest parts of the continental United States, for example, in orde r for a collector to gather enough energy to serve one person for one day, the area of the collector's surface must be about 430 square feet (40 square meters). The actual energy that can be used depends on the efficiency of the collector and of the device that converts the radiation into usable energy.
Flat-plate collectors. The most common flat-plate collectors consist of a dark metal plate, covered with one or two sheets of glass, that absorbs heat. The heat is transferred to air or water, called carrier fluids, that flows past the back of the plate. This heat may be used directly or it may b e transferred to another medium. Flat-plate collectors are used for home and hot-water heating . Flat-plate collectors typically heat carrier fluids to temperatures ranging from 150B0 to 200B0F (66B0 to 93B0C). The efficienc y of such collectors varies from 20 to 80 percent.
Concentrating collectors. When higher temperatures are required, a concentrating collector is used. These collectors reflect and concentrate sunlight from a wide area. One such device, called a solar furnace, was installed in the Pyrenees in France and has several acres of mirrors focused on a single target. The energy concentrated at the target is 3,000 times tha t received by any single mirror, and the unit produces temperatures of up to 3,630B0F (2,000B0C). Another structure, the so-called "power tower" plant near Barstow, Calif., generates 10,000 kilowatts of electricity. Here, the furnac e acts as a boiler and generates steam for a steam turbine-electric generator power plant.
In sophisticated concentrating collectors such as the California tower, each mirror is rotated by a heliostat that directs the sun's rays fro m the mirror to the target. Positioning motors, drives, and controllers make such systems expensive. Less costly collectors can produce temperatures lowe r than those of more advanced concentrating collectors but higher than those o f flat-plate collectors. For example, parabolic reflectors that concentrate sunlight on black pipes can produce fluid temperatures of about 400B0 to 55 0B0F (200B0 to 290B0C) and can concentrate the solar energy up to 50 times its original strength.
The small stand-alone system is an excellent replacement for propane or kerosene lights in a remote cabin, a recreational vehicle or a boat. The size of the photovoltaic (PV) array and battery will depend upon individual requirements. The actual sizing methods are discussed elsewhere. The PV arra y charges the battery during daylight hours and the battery supplies power to the loads as needed. The charge regulator terminates the charging when the battery reaches full charge. The load center may contain meters to monitor system operation and fuses to protect wiring in the event of malfunction or short circuit in the house.
The PV – Generator Combination system may be an economical alternative to a large stand-alone PV system, because the PV array does not have to be sized large enough for worst case weather conditions. A gasoline, propane or diesel generator combined with a battery charger can supply power when the PV array falls short. If the PV array is sized for average conditions, then during extended overcast situations or periods of increased load, the generator can be started. When batteries are low, the generator will power the AC loads in the house as well as a battery charger to help recharge the batteries. If the PV array is sized much smaller than needed fo r normal use, the generator can power peak loads such as doing laundry or pumping water and simultaneously run the battery charger to charge the battery bank. In addition to allowing for a smaller PV array, a back-up charging system may also allow use of a smaller battery bank. Generator and battery bank size must be chosen carefully for reliable system operation. Se e the system sizing section for more details on equipment choice.
The utility intertie system is also used in a grid connected house. Instead of storing power in batteries, it is sold to the utility company. Th e Utility Intertie System employs a special type of inverter, which inverts DC power from the PV array into low distortion AC, acceptable for purchase by the local utility power company. Batteries are not required for storage. The power is delivered through a kilowatt-hour (kWh) meter to the utility grid a s it is produced by the PV modules. A second kWh meter is used to measure the power consumed by the loads in the house. The user of this system will notic e no difference from any utility system, except lower utility bills or possibl y payments from the power company for excess electricity that is generated.
At last ordinary home owners can begin to reduce their dependence on utility power for their electricity. This type of utility sellback system is comprised of PV modules with small inverters mounted on them. This allows th e output of the inverter-module combinations to be connected directly to the A C line. The utility may require a second meter and disconnect. The installatio n cost of this type of intertie system is much lower than that of a large inverter system. A small system can be installed, and as finances allow, additional AC PV modules can easily be added to the system.
Conservation plays an important role in keeping the cost of a photovoltaic system down. The use of energy efficient appliances and lightin g as well as non-electric alternatives wherever possible can make solar electricity a cost competitive alternative to gasoline generators and in som e cases, utility power.
Conventional electric cooking, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges use 1500 watts or more per burner, so bottled propane or natural gas is a popular alternative to electricity for cooking. A microwave oven has about the same power draw, but since food cooks more quickly, the amount of kilowatt hours used may not be large. Propane and wood are better alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. Evaporative cooling is a more reasonable load, and in locations with low humidity, the results are almost as good. One plus for cooling – th e largest amount of solar energy is usually available when the temperature is the highest.
Lighting requires the most study since so many options exist in type, size, voltage and placement. The type of lighting that is best for one syste m may not be right for another.
The first decision is whether your lights will be run on low voltage direct current (DC) or conventional 110 volt alternating current (AC). In a small home, an RV, or a boat, low voltage DC lighting is usually the best. D C wiring runs can be kept short allowing the use of fairly small gauge wire. Since an inverter is not required, the system cost is lower. If an inverter is part of the system, the house will not be dark if the inverter fails if the lights are powered directly by the battery.
In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of DC fluorescent lights, which have 3 to 4 times the light output per watt of power used compared with incandescen t types. Halogen bulbs are 30% more efficient and actually seem almost twice a s bright as similar wattage incandescent because of the spectrum of light they produce. Twelve and 24 volt replacement ballasts are available to convert AC fluorescent lights to DC.
In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting is cost effective. In a large stand alone system with AC lighting, the user should have a back up inverter or a few low voltage DC lights in case the primary inverter fails. It is a good idea to have a DC powered light in the room whrere the inverter and batteries are in case there is a problem. AC light dimmers will not function on AC power from inverters unless they have pure sine wave output. Small fluorescent lights may not turn on with some "load demand start" type inverters.
Gas powered absorption refrigerators are a good choice in small systems if bottled gas is available. Modern absorption refrigerators consume 5 to 10 gallons of LP gas per month. If an electric refrigerator will be use d in a stand-alone system, it should be a high efficiency type. SunFrost refrigerators use 300 to 400 watt hours of electricity per day while conventional AC refrigerators use 3000 to 4000 watt hours of electricity per day at a 70 degree average air temperature. The higher cost of good quality DC refrigerators is made up many times over by savings in the number of sola r modules and batteries required.
Standard AC electric motors in washing machines, larger shop machinery and tools, "swamp coolers", pumps etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often, a 2000 watt or larger inverter will be required. These electric motors are sometimes hard to start on inverter power, they consume relatively large amounts of electricity, and they are very wasteful compared to high-efficiency motors, which use 50% to 75% less electricity. A standard washing machine uses between 300 and 500 watt-hours per load. If the appliance is used more than a few hours per week , it is often cheaper to pay more for a high-efficiency appliance (if one exists), rather than make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill press, etc.) , their standard electric motor can often be easily replaced with a high-efficiency type. These motors are available in either AC or DC, and com e as separate units or as motor-replacement kits.
Vacuum cleaners usually consume 600 to 1000 watts, depending on how powerful they are, about twice what a washer uses, but most vacuum cleaners will operate on inverters larger than 1000 watts because they have low surge motors.
Many small appliances such as irons, toasters and hair dryers consume a very large amount of power when they are used but by their nature require very short or infrequent use periods, so if the system inverter and batteries are large enough, they may be usable. Electronic equipment, like stereos, televisions, VCR's and computers have a fairly small power draw. Many of these are available in low voltage DC as well as conventional AC versions, and in general, DC models use less power than their AC counterparts. A portable stereo "boom box" that runs on 8 or 10 "D-cell" batteries will usually work on 12 volts DC. Some have a DC input, or you can connect wires from the battery contacts to the 12 volt system. This should be done by someone experienced in electronics repair.
In the 1950s scientists tinkering with semiconductors found that by introducing small, minutely controlled amounts of certain impurities called dopants to the semiconductor matrix, the density of free electrons could be shepherded and controlled. The dopants, similar enough in structure and valence to fit into the matrix, have one electron more or less than the semiconductor; for example, doping with phosphorus, which has five valence electrons, produces a (negative) n-type semiconductor, with an extra electro n which can be dislodged easily. Aluminum, boron, indium, and gallium have only three valence electrons, and so a semiconductor doped with them is (positive ) p-type, and has holes" where the missing electrons ought to be. These holes behave just like electrons, except that they have an opposite, positive charge. (Holes are theoretical, but so are electrons, and either or both may or may not exist, but we know for sure that if one exists, they both do, because we can't create something out of nothing in the physical world.) It is important to understand that, although loosely bonded or extra carriers exist in a substance, it is still neutral electrically, because each atom's electrons are matched one for one by protons in the nucleus.
The fun begins when the two semiconductor types are intimately joined in a pn-junction, and the carriers are free to wander. Being of opposite20 charge, they move toward each other, and may cross the junction, depleting the region they came from, and transferring their charge to their new region. This produces an electric field, called gradient, which quickly reaches equilibrium with the force of attraction of excess carriers. This field becomes a permanent part of the device, a kind of slope that makes carriers tend to slide across the junction when they get close.
When light strikes a Photovoltaic cell, atoms are bombarded with photons, and give up electrons. When an electron gets lopped off an atom, it leaves behind a hole, which has an equal and opposite charge. Both the electron, with its negative charge, and the hole, with its positive charge, begin a random walk generally down the gradient. If either carrier wanders across the junction, the field and the nature of the semiconductor material discourage it from recrossing. A proportion of carriers which cross this junction can be harvested by completing a circuit from a grid on the cell's surface to a collector on the backplane. In the cell, the light pumps" electrons out one side of the cell, through the circuit, and back to the other side, energizing any electrical devices found along the way.