SOLAR ENERGY. 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.
Small Stand-Alone DC System
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.
PV - Generator Combination
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.
Utility Intertie
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.
AC Photovoltaic Module Intertie
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
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.
Cooking, Heating ; Cooling
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
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.
Refrigeration
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.
Major Appliances
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.
Small Appliances
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 batterie
s
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 onl
y
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.
