SOLAR ENERGY. All life on Earth depends on energy from the sun. Solarenergy is the source of energy for photosynthesis. It provides the warmthnecessary for plants and animals to survive. The heat from the sun causeswater on the Earth’s surface to evaporate and form clouds that eventuallyprovide fresh rainwater.
Solar energy is the result of thermonuclear fusion reactions deepwithin the sun. These reactions produce so much energy that they keep thesurface temperature of the sun at about 10,300B0F (5,700B0C). Even though solarenergy is the largest source of energy received by the Earth, its intensityat the Earth’s surface is actually very low due to the large distance betweenthe Earth and the sun and the fact that the Earth’s atmosphere absorbs andscatters some of the radiation. Even on a clear day with the sun directlyoverhead, the energy that reaches the Earth’s surface is reduced about 30percent by the atmosphere. When the sun is near the horizon and the sky isovercast, the solar energy at ground level can be negligible. It also variesfrom one point to another on the Earth’s surface.
Nevertheless, in the 20th century, the sun’s energy has become anincreasingly attractive source for small amounts of direct power to meethuman needs. A number of devices for collecting solar energy and convertingit into electricity have been developed, and solar energy is used in avariety of ways. Solar energy is used to heat houses, and in many countriesspecially designed solar ovens are used for cooking. The sun also suppliesenergy to electric generators that provide power for weather andcommunications satellites and for radio and television equipment.
Because the intensity of the sun’s radiation at the surface of theEarth is so low, collectors designed to capture solar energy must be large.
In the sunniest parts of the continental United States, for example, in orderfor a collector to gather enough energy to serve one person for one day, thearea of the collector’s surface must be about 430 square feet (40 squaremeters). The actual energy that can be used depends on the efficiency of thecollector and of the device that converts the radiation into usable energy.
Flat-plate collectors. The most common flat-plate collectors consistof a dark metal plate, covered with one or two sheets of glass, that absorbsheat. The heat is transferred to air or water, called carrier fluids, thatflows past the back of the plate. This heat may be used directly or it may betransferred to another medium. Flat-plate collectors are used for home andhot-water heating . Flat-plate collectors typically heat carrier fluids totemperatures ranging from 150B0 to 200B0F (66B0 to 93B0C). The efficiency of suchcollectors varies from 20 to 80 percent.
Concentrating collectors. When higher temperatures are required, aconcentrating collector is used. These collectors reflect and concentratesunlight from a wide area. One such device, called a solar furnace, wasinstalled in the Pyrenees in France and has several acres of mirrors focusedon a single target. The energy concentrated at the target is 3,000 times thatreceived by any single mirror, and the unit produces temperatures of up to3,630B0F (2,000B0C). Another structure, the so-called “power tower” plant nearBarstow, Calif., generates 10,000 kilowatts of electricity. Here, the furnaceacts as a boiler and generates steam for a steam turbine-electric generatorpower plant.
In sophisticated concentrating collectors such as the Californiatower, each mirror is rotated by a heliostat that directs the sun’s rays fromthe mirror to the target. Positioning motors, drives, and controllers makesuch systems expensive. Less costly collectors can produce temperatures lowerthan those of more advanced concentrating collectors but higher than those offlat-plate collectors. For example, parabolic reflectors that concentratesunlight on black pipes can produce fluid temperatures of about 400B0 to 550B0F(200B0 to 290B0C) and can concentrate the solar energy up to 50 times itsoriginal strength.
Small Stand-Alone DC SystemThe small stand-alone system is an excellent replacement for propaneor kerosene lights in a remote cabin, a recreational vehicle or a boat. Thesize of the photovoltaic (PV) array and battery will depend upon individualrequirements. The actual sizing methods are discussed elsewhere. The PV arraycharges the battery during daylight hours and the battery supplies power tothe loads as needed. The charge regulator terminates the charging when thebattery reaches full charge. The load center may contain meters to monitorsystem operation and fuses to protect wiring in the event of malfunction orshort circuit in the house.
PV – Generator CombinationThe PV – Generator Combination system may be an economicalalternative to a large stand-alone PV system, because the PV array does nothave to be sized large enough for worst case weather conditions. A gasoline,propane or diesel generator combined with a battery charger can supply powerwhen the PV array falls short. If the PV array is sized for averageconditions, then during extended overcast situations or periods of increasedload, the generator can be started. When batteries are low, the generatorwill power the AC loads in the house as well as a battery charger to helprecharge the batteries. If the PV array is sized much smaller than needed fornormal use, the generator can power peak loads such as doing laundry orpumping water and simultaneously run the battery charger to charge thebattery bank. In addition to allowing for a smaller PV array, a back-upcharging system may also allow use of a smaller battery bank. Generator andbattery bank size must be chosen carefully for reliable system operation. Seethe system sizing section for more details on equipment choice.
Utility IntertieThe utility intertie system is also used in a grid connected house.
Instead of storing power in batteries, it is sold to the utility company. TheUtility Intertie System employs a special type of inverter, which inverts DCpower from the PV array into low distortion AC, acceptable for purchase bythe local utility power company. Batteries are not required for storage. Thepower is delivered through a kilowatt-hour (kWh) meter to the utility grid asit is produced by the PV modules. A second kWh meter is used to measure thepower consumed by the loads in the house. The user of this system will noticeno difference from any utility system, except lower utility bills or possiblypayments from the power company for excess electricity that is generated.
AC Photovoltaic Module IntertieAt last ordinary home owners can begin to reduce their dependence onutility power for their electricity. This type of utility sellback system iscomprised of PV modules with small inverters mounted on them. This allows theoutput of the inverter-module combinations to be connected directly to the ACline. The utility may require a second meter and disconnect. The installationcost of this type of intertie system is much lower than that of a largeinverter system. A small system can be installed, and as finances allow,additional AC PV modules can easily be added to the system.
ConservationConservation plays an important role in keeping the cost of aphotovoltaic system down. The use of energy efficient appliances and lightingas well as non-electric alternatives wherever possible can make solarelectricity a cost competitive alternative to gasoline generators and in somecases, utility power.
Cooking, Heating ; CoolingConventional electric cooking, space heating and water heatingequipment use a prohibitive amount of electricity. Electric ranges use 1500watts or more per burner, so bottled propane or natural gas is a popularalternative to electricity for cooking. A microwave oven has about the samepower draw, but since food cooks more quickly, the amount of kilowatt hoursused may not be large. Propane and wood are better alternatives for spaceheating. Good passive solar design and proper insulation can reduce the needfor heat. Evaporative cooling is a more reasonable load, and in locationswith low humidity, the results are almost as good. One plus for cooling – thelargest amount of solar energy is usually available when the temperature isthe highest.
LightingLighting requires the most study since so many options exist in type,size, voltage and placement. The type of lighting that is best for one systemmay not be right for another.
The first decision is whether your lights will be run on low voltagedirect current (DC) or conventional 110 volt alternating current (AC). In asmall home, an RV, or a boat, low voltage DC lighting is usually the best. DCwiring 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 inverteris part of the system, the house will not be dark if the inverter fails ifthe lights are powered directly by the battery.
In addition to conventional size medium base low voltage bulbs, theuser can choose from a large selection of DC fluorescent lights, which have 3to 4 times the light output per watt of power used compared with incandescenttypes. Halogen bulbs are 30% more efficient and actually seem almost twice asbright as similar wattage incandescent because of the spectrum of light theyproduce. Twelve and 24 volt replacement ballasts are available to convert ACfluorescent lights to DC.
In a very large installation or one with many lights, the use of aninverter to supply AC power for conventional lighting is cost effective. In alarge stand alone system with AC lighting, the user should have a back upinverter 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 inverterand batteries are in case there is a problem. AC light dimmers will notfunction on AC power from inverters unless they have pure sine wave output.
Small fluorescent lights may not turn on with some “load demand start” typeinverters.
RefrigerationGas powered absorption refrigerators are a good choice in smallsystems if bottled gas is available. Modern absorption refrigerators consume5 to 10 gallons of LP gas per month. If an electric refrigerator will be usedin a stand-alone system, it should be a high efficiency type. SunFrostrefrigerators use 300 to 400 watt hours of electricity per day whileconventional AC refrigerators use 3000 to 4000 watt hours of electricity perday at a 70 degree average air temperature. The higher cost of good qualityDC refrigerators is made up many times over by savings in the number of solarmodules and batteries required.
Major AppliancesStandard AC electric motors in washing machines, larger shopmachinery and tools, “swamp coolers”, pumps etc. (usually 1/4 to 3/4horsepower) require a large inverter. Often, a 2000 watt or larger inverterwill be required. These electric motors are sometimes hard to start oninverter power, they consume relatively large amounts of electricity, andthey are very wasteful compared to high-efficiency motors, which use 50% to75% less electricity. A standard washing machine uses between 300 and 500watt-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 oneexists), rather than make your electrical system larger to support alow-efficiency load. For many belt-driven loads (washers, drill press, etc.),their standard electric motor can often be easily replaced with ahigh-efficiency type. These motors are available in either AC or DC, and comeas separate units or as motor-replacement kits.
Vacuum cleaners usually consume 600 to 1000 watts, depending on howpowerful they are, about twice what a washer uses, but most vacuum cleanerswill operate on inverters larger than 1000 watts because they have low surgemotors.
Small AppliancesMany small appliances such as irons, toasters and hair dryers consumea very large amount of power when they are used but by their nature requirevery short or infrequent use periods, so if the system inverter and batteriesare large enough, they may be usable. Electronic equipment, like stereos,televisions, VCR’s and computers have a fairly small power draw. Many ofthese are available in low voltage DC as well as conventional AC versions,and in general, DC models use less power than their AC counterparts. Aportable stereo “boom box” that runs on 8 or 10 “D-cell” batteries willusually work on 12 volts DC. Some have a DC input, or you can connect wiresfrom the battery contacts to the 12 volt system. This should be done bysomeone experienced in electronics repair.
In the 1950s scientists tinkering with semiconductors found that byintroducing small, minutely controlled amounts of certain impurities calleddopants to the semiconductor matrix, the density of free electrons could beshepherded and controlled. The dopants, similar enough in structure andvalence to fit into the matrix, have one electron more or less than thesemiconductor; for example, doping with phosphorus, which has five valenceelectrons, produces a (negative) n-type semiconductor, with an extra electronwhich can be dislodged easily. Aluminum, boron, indium, and gallium have onlythree valence electrons, and so a semiconductor doped with them is (positive)p-type, and has holes” where the missing electrons ought to be. These holesbehave just like electrons, except that they have an opposite, positivecharge. (Holes are theoretical, but so are electrons, and either or both mayor 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.) Itis important to understand that, although loosely bonded or extra carriersexist in a substance, it is still neutral electrically, because each atom’selectrons are matched one for one by protons in the nucleus.
The fun begins when the two semiconductor types are intimately joinedin a pn-junction, and the carriers are free to wander. Being of opposite20charge, they move toward each other, and may cross the junction, depletingthe region they came from, and transferring their charge to their new region.
This produces an electric field, called gradient, which quickly reachesequilibrium with the force of attraction of excess carriers. This fieldbecomes a permanent part of the device, a kind of slope that makes carrierstend to slide across the junction when they get close.
When light strikes a Photovoltaic cell, atoms are bombarded withphotons, and give up electrons. When an electron gets lopped off an atom, itleaves behind a hole, which has an equal and opposite charge. Both theelectron, with its negative charge, and the hole, with its positive charge,begin a random walk generally down the gradient. If either carrier wandersacross the junction, the field and the nature of the semiconductor materialdiscourage it from recrossing. A proportion of carriers which cross thisjunction can be harvested by completing a circuit from a grid on the cell’ssurface 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 theother side, energizing any electrical devices found along the way.