Solar Photovoltaics Energy.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
Photovoltaics (PV) is the term
derived from Greek word for light - photos- and the name for unit of
electromotive force - volt. Photovoltaics means direct generation of electricity
from light. Recently this process is utilised by means of solar cells. The
“solar cells”, made from semiconductor materials such as silicon, produce
electric currents when exposed to sunlight. By manufacturing modules which
contain dozens of such solar cells and connecting the modules large power
stations can be built. The largest photovoltaic power station that has yet been
constructed is the 5 MW system at Carrisa Plain, California. The efficiency of
photovoltaic power stations is presently about 10% but individual solar cells
have been fabricated with efficiencies exceeding 20%.
HISTORY OF PHOTOVOLTAICS
The history of
photovoltaics dates back to 1839 and major developments evolved as follows:
 In 1839
Edmund Becquerel, a French physicist observed the photovoltaic effect.
In 1883
Selenium PV cells were built by Charles Edgar Fritts, a New York electrician.
Cells converted light in the visible spectrum into electricity and were 1% to 2%
efficient. (light sensors for cameras are still made from selenium today).
In the
early 1950’s the Czochralski meter was developed for producing highly
purecrystalline silicon.
In 1954 Bell Telephone Laboratories
produced a silicon PV cell with a 4% efficiency and later achieved 11%
efficiency.
In 1958 the US Vanguard space satellite used a small (less than one
watt) array to power its radio. The space program has played an important role
in the development of PV’s ever since.
During the 1973-74 oil price shock
several countries launched photovoltaic utilization programmes, resulting in the
installation and testing of over 3,100 PV systems in USA alone, many of which
are in operation today.
PV MARKET
 |
| The present PV market is characterised by a fairly high and stable
increase of over 20 % per year, however on a still fairly low level of
production volume. The world-wide module production for 1998 amounted to
about 125 MW while prices have dropped from $50/W in 1976 to $5/W in 1999.
Nevertheless kWh prices of electricity produced by PV systems are still
too expensive by a factor 3 to 10 (depending on the site and system
design) as compared to conventional electric energy. The PV market is thus
a small niche market, however with steadily increasing market segments
where PV is already cost competitive as e.g. in many stand alone system
applications. |
Progress is visible in many parts of the world. The Japanese government is
investing $250 million a year to increase manufacturing capacity from 40 MW
(1997) to 190 MW (2000) and national programs are being launched in Europe,
driven by energy independence and environment. These programs,
combined with environmental pressures such as climate change, can
accelerate growth of the PV industry. Shell Solar has built the world’s
largest PV manufacturing facility in Germany, with current annual production of
10 MW and future growth to 25 MW. The cost was 50 million Mark.
PV UTILISATION
 For a
range of applications solar cells are technically feasible and economically
viable alternative to fossil fuels. A solar cell can directly convert the sun’s
irradiation to electricity and this process requires no moving parts. This
results in a relatively long service life of solar generators. PV systems have
been the best choice for many jobs since the first commercial PV cells were
developed. For example, PV cells have been the exclusive power source for
satellites orbiting the earth since the 1960s. PV systems have been used for
remote stand-alone systems throughout the world since the 1970s. In the 1980s,
commercial and consumer product manufacturers began incorporating PV into
everything from watches and calculators to music boxes. And in the 1990s, many
utilities are finding PV to be the best choice for thousands of small power
needs.
PV systems are now generating electricity
to pump water, light up the night, activate switches, charge batteries, supply
the electric utility grid, and more. PV systems produce power in all types of
weather. On partly cloudy days they can produce up to 80% of their
potential energy delivery; on hazy/humid days, about 50%; and on extremely
overcast days, they still produce up to 30%.
PV cells are no longer
just available in panels. Different companies are incorporating PV into
light-weight, flexible and durable roofing shingles, as well as inverted curtain
walls for building facades. These new products make the economics of
photovoltaics more attractive by incorporating the PV cells into building
materials. In remote areas or locations, PV is the most cost-effective, reliable
and durable energy solution available. For grid-connected systems PV can
provide, in some regions, a cost competitive energy solution. In all regions,
both remote and grid connected, PV provides clean energy without the polluting
effects of conventional power sources.
Solar powered water pumping
systems are effective and economical for virtually any water pumping need.
Electric utilities in the USA found that it is more economical to use PV powered
water pumps than to maintain distribution lines to remote pumps. Several
utilities are offering photovoltaic water pumping systems as customer service
options.
Other agriculture solutions include electric fence charging and lighting.
In greenhouse or hydroponics operations, solar can provide the power for water
circulation, fans, lights and climate control equipment.
 |
PV modules supplied electricity also to the Breitling Orbiter 3
balloon during its non-stop trip around the world. For three weeks
in March 1999, the balloon’s on-board equipment was powered by 20 modules
suspended under the nacelle. Each module was tilted to ensure even
power output during rotation, and recharged five lead batteries for
navigation instruments, satellite communications systems, lighting and
water heating. The modules functioned perfectly throughout epic
voyage. |
PV is successfully utilised also in village electrification. Today two
billion people in the world are without electricity. A large portion live in the
developing world, where 75% of the population lives without electricity. There
is rarely a utility grid in these remote, rural or suburban villages. Experience
shows that PV delivers cost-effective electricity for basic services, such
as:
 light
water pumping
communications
health
facilities
businesses
People not
served by a power grid often rely on fossil fuels like kerosene and diesel.
There are a number of problems associated with the use of fossil fuels.
Imported fossil
fuels drain foreign currency.
Transporting is difficult because of infrastructure.
Maintenance of fossil fuel
generators is difficult because of lack of spare parts.
Generators pollute the environment by
loud noises and exhaust.
Electric lights powered by PV are more effective than kerosene lights in
developing countries, and installing a PV system is usually less expensive than
extending the power lines. Moreover, many developing countries are located in
areas with high insolation levels, providing them with a free abundant source of
energy year round. Using photovoltaics to generate electricity from sunlight is
simple and has proven reliable in tens of thousands of applications
world-wide.
During the next decades, a large part of the world’s
population will be introduced to electricity produced by PV systems. These PV
systems will make the traditional requirements of building large, expensive
power plants and distribution systems unnecessary. As the costs of PV continue
to decline and as PV technology continues to improve, several potentially huge
markets for PV will open up. For example, building materials that incorporate PV
cells will be designed right into homes, helping to ventilate and light the
buildings. Consumer products ranging from battery-powered hand tools to
automobiles will take advantage of electricity - producing components containing
PV materials. Meanwhile, electric utilities will find more and more ways to use
PV to supply the needs of their customers.
The EU wants to double the share
of renewables by 2010, and key actions include one million PV systems (500,000
roof and the export of 500,000 village systems) with total installed capacity of
1 GW. BP Amoco (one of the world’s leading marketers of petroleum products) will
incorporate solar energy into 200 of its new service stations in Britain,
Australia, Germany, Austria, Switzerland, the Netherlands, Japan, Portugal and
Spain, France and the US. The $50 million program will involve 400 panels,
generating 3.5 MW and saving 3,500 tonnes of CO2 emissions every year. The
project will make BP Amoco one of the world’s largest users of solar power, as
well as one of the largest manufacturers of cells and modules. The solar panels
will generate more power than consumed for lighting and pump power, and will be
grid-connected to allow excess electricity to be exported during the day and the
shortfall imported at night. The world market for photovoltaics will reach 1,000
MW by 2010 and 5 million MW by 2050, according to the president of BP Solar.
TECHNOLOGY
Solar electric
systems are simple to operate and have no moving parts; however, PV cells employ
sophisticated semiconductor devices, many of which are similar to those
developed in the integrated circuit industry. PV cells operate on the physical
principle that electric current will flow between two semiconductors with
different electrical properties when they are put in contact with each other and
exposed to light. A collection of these PV cells constitutes a PV panel, or
module. PV modules, because of their electrical properties, produce direct
rather than alternating current (AC). Direct current (DC) is electric current
that flows in a single direction. Many simple devices, such as those that run on
batteries, use direct current. Alternating current, in contrast, is electric
current that reverses its direction at regular intervals. This is the type of
electricity provided by utilities and required to run most modern appliances and
electronic devices. In the simplest systems, DC produced by PV modules is used
directly. In applications where AC is necessary, an inverter can be added to the
system to convert the DC to AC.
PV CELLS
 Today’s solar cell production is
almost exclusively based on silicon. About 80% of all modules are fabricated
using crystalline silicon cells (multicrystalline and single crystalline) and
about 20% are based on amorphous silicon thin film cells. The crystalline
cells are the more common, generally blue-coloured frosty looking ones.
Amorphous means noncrystalline, and these look smooth and change color depending
on the way you hold them. Monocrystalline silicon has the best efficiency
- about 14% of the sunlight can be utilized - but it is more
expensive than multicrystalline silicon, which typically has 11%
efficiency. Amorphous silicon is widely used in small appliances
such as watches and calculators, but its efficiency and long-term
stability are significantly lower; consequently, it is rarely used in
power applications.
On a laboratory and/or a pilot production
scale there are, however, several alternative thin film solar cells under
development which may penetrate the market in the future. The most advanced of
the presently investigated thin film systems are:
amorphous silicon (a-Si: H) cells,
cadmium
telluride/cadmium sulfide (CTS) cells,
copper indium diselenide or copper
indium/gallium diselenide (CIS or CIGS) cells, crystalline silicon thin film
(c-Si film) cells and
nanocrystalline dye sensitised electrochemical (nc-dye) cells.
 PV
cells are “sandwiches” of silicon, the second most abundant material in the
world. Ninety-nine percent of today’s solar cells are made of silicon (Si), and
other solar cells are governed by basically the same physics as Si solar cells.
One layer of silicon is treated with a substance to create an excess of
electrons. This becomes the negative or “N” layer. The other layer is treated to
create a deficiency of electrons, and becomes the positive or “P” layer.
Assembled together with conductors, the arrangement becomes a light-sensitive NP
junction semiconductor. It’s called a semiconductor, because, unlike a wire, the
unit conducts in only one direction; from negative to positive. When exposed to
sunlight (or other intense light source), the voltage is about 0,5 Volts DC, and
the potential current flow (amps) is proportional to the light energy (photons).
In any PV, the voltage is nearly constant, and the current is proportional to
the size of the PV and the intensity of the light.
Photovoltaic cells
are made from hyper pure silicon that is precisely doped with other materials.
The hyper pure silicon substrates used to make PV cells are very expensive.
After all, the same amount of hyper pure silicon used in a single 50 Watt PV
module could have been made into enough integrated circuits for about two
thousand computers. The remainder of the materials used by PV cells are
aluminum, glass, and plastic - all inexpensive and easily recyclable
materials.
PV production facility.
SOLAR MODULES
Solar modules are an array of solar
cells which are interconnected and encapsulated behind a glass
cover. The stronger the light falling down on the cells and the larger the cell
surface, the more electricity is generated and the higher the
current. Modules are rated in peak watts (Wp). A watt is the unit used to
express the power of a generator or the demand of a consumer. One peak
watt is a specification which indicates the amount of power
generated under rated conditions, i.e. when solar irradiance of 1
kW/m2 is incident on the cell at a temperature of 25 deg. C. This level of
intensity is achieved when weather conditions are good and the sun is at
its zenith. No more than a cell of 10 x 10 cm is necessary to generate a
peak watt. Larger modules, 1 m x 40 cm in size, have an output of
about 40-50 Wp. Most of the time, however, the irradiation is below 1
kW/m2. Furthermore, in sunlight the module will warm up beyond the rated
temperature. Both effects will reduce the module’s performance. For typical
conditions an average output of about 6 Wh per day and 2000 Wh per
year per peak watt can be expected. To have the idea of how much
that is, 5 Wh is the energy consumed by a 50 W lamp in 6 minutes (50W x 0,1h =
5Wh) or by a small radio in one hour (5W x 1h = 5Wh).
 Although some differences still exist in product quality,
most international companies produce fairly reliable units which can be
expected to work for 20 years. Meanwhile, suppliers guarantee the
specified power output for a period of up to 10 years. The most decisive
criterion for the comparison of different modules is the price per peak
watt. In other words, it is possible to get more power for the money with
a 120 Wp module which costs USD 569 (4,74 USD/Wp) than with a
“cheap” 90 Wp module that costs USD 489 (5,43 USD/Wp). The rated efficiency of a
system is a less important consideration.
PV ADVANTAGES
High Reliability
PV cells were originally developed for use in space, where repair is
extremely expensive, if not impossible. PV still powers nearly every
satellite circling the earth because it operates reliably for long periods of
time with virtually no maintenance.
Low Operating Costs
PV cells use the energy from sunlight to produce electricity - the fuel
is free. With no moving parts, the cells require low-maintenance.
Cost-effective PV systems are ideal for supplying power to communication
stations on mountain tops, navigational buoys at sea, or homes far from utility
power lines.
Non-polluting
Because they burn no fuel and have no moving parts, PV systems are clean
and silent. This is especially important where the main alternatives for
obtaining power and light are from diesel generators and kerosene lanterns.
Modular
A PV
system can be constructed to any size. Furthermore, the owner of a PV
system can enlarge or move it if his or her energy needs change. For
instance, homeowners can add modules every few years as their energy usage and
financial resources grow. Ranchers can use mobile trailer-mounted pumping
systems to water cattle as they are rotated between fields.
Low Construction Costs
PV systems are usually placed close to where the electricity is used,
meaning much shorter wire runs than if power is brought in from the utility
grid. In addition, using PV eliminates the need for a step-down transformer from
the utility line. Fewer wires mean lower costs and shorter construction
time.
HOW MUCH DOES PV-GENERATED ELECTRICITY COST?
There is no simple
answer. Many small PV systems designed to power a few fluorescent lights
and a small TV in remote hoses are much cheaper than the next best alternatives
running a new power line, replacing and disposing of primary batteries (those
batteries that are used once and then disposed of, such as flashlight
batteries), or using an engine generator. The cost of electricity from larger
systems, those able, for example, to power a modern home, is evaluated according
to the cost per kilowatt hour (kWh). The cost depends on the initial cost,
interest on the loan (for paying the initial cost), the cost of system
maintenance, the expected lifetime of the system, and how much electricity it
produces. Using typical borrowing costs and equipment life, the cost of
PV-generated energy in USA in 1998 ranged from $0,20 to $0,50/kWh.
HOW MUCH SPACE DOES PV TAKE?
 |
The most common modules (using cells made from crystalline silicon)
generate 100-120 watts per square meter (W/m2). Thus, one square
meter module generates enough electricity to power a 100 W light
bulb. At the upper end of the range, a PV power plant laid out on a
square piece of land measuring approximately 160km on a side could supply
all the electricity consumed annually be the entire United States. Better
alternative than to use open land area is to place PV modules on the roofs
of buildings or integrate them into facades of the walls. This option is
usually cheaper because it can replace traditional building materials
which have to be used anyway. |
Simple PV Systems
 The
sunlight that creates the need for water pumping and ventilation can be
harnessed using the most basic PV systems to meet those same needs. Photovoltaic
modules produce the most electricity on clear, sunny days. Simple PV systems use
the DC electricity as soon as it is generated to run water pumps or fans. These
basic PV systems have several advantages for the special jobs they do. The
energy is produced where and when it is needed, so complex wiring, storage, and
control systems are unnecessary. Small systems, under 500 W, weigh less than 70
kilograms making them easy to transport and install. Most installations take
only a few hours. And, although pumps and fans require regular maintenance, the
PV modules require only an occasional inspection and cleaning.
Solar Water Pumping
 |
Photovoltaic pumping systems provide a welcome alternative to fuel
burning generators or hand pumps. They provide the most water
precisely when it is needed the most - when the sun shines the brightest!
Solar pumps are simple to install and maintain. The smallest systems can
be installed by one person in a couple hours, with no experience or
special equipment
required. | Advantages of using
PV-powered pumps include:
low maintenance
ease of installation
reliability
scalability
Solar power differs fundamentally from conventional electric or
engine-powered systems, so solar pumps often depart from the conventional. PV
arrays produce DC power, rather than the AC from conventional sources. And, the
power available varies with the sun’s intensity. Since it costs less to store
water (in tanks) than energy (in batteries) solar pumps tend to be low in power,
pumping slowly through the duration of the solar day.
Simple,
efficient systems are the key to economical solar pumping. Special, low-power DC
pumps are used without batteries or AC conversion. Modern DC motors work well at
varying voltage and speed. The better DC motors require maintenance (brush
replacement) only after periods of 5 years or more. Most solar pumps used for
small scale application (homes, small irrigation, livestock) are “positive
displacement” pumps which seal water in cavities and force it upward. This
differs from faster, conventional centrifugal type pumps (including jet and
submersible pumps) which spin and “blow” the water up.
Positive displacement pumps include piston, diaphragm, rotary vane, and
pump jacks. They work best for low volumes, particularly where variable running
speeds occur. Centrifugal, jet and turbine pumps are used for higher volume
systems. Electronic matching devices known as Power Trackers and Linear Current
Boosters allow solar pumps to start and run under low-light conditions. This
permits direct use of the sun’s power without bothersome storage batteries.
Solar trackers may be used to aim the panels at the sun from morning to sunset,
extending the useable period of sunlight. Storage tanks usually hold a 3-10 day
supply of water, to meet demands during cloudy periods. Solar pumps use
surprisingly little power. They utilize high efficiency design and the long
duration of the solar day, rather than power and speed, to lift the volume of
water required.
In areas where photovoltaic pumps have entered into
competition with diesel-driven pumps, their comparatively high initial cost is
offset by the achieved savings on fuel and reduced maintenance expenditures.
Studies concerning the economic efficiency of photovoltaic pumping systems
confirm that they are often able to yield cost advantages over diesel-driven
pumps, depending on the country-specific situation.
PV SYSTEMS WITH BATTERIES
The most simple
solutions have certain drawbacks - the most obvious one being that in case
of PV powered pump or fan could only be used during the daytime, when the
sun is shining. To compensate for these limitations, a battery is added to
the system. The battery is charged by the solar generator, stores the energy and
makes it available at the times and in the amounts needed. In the most
remote and hostile environments, PV-generated electrical energy stored in
batteries can power a wide variety of equipment. Storing electrical energy makes
PV systems a reliable source of electric power day and night, rain or shine. PV
systems with battery storage are being used all over the world to power lights,
sensors, recording equipment, switches, appliances, telephones, televisions, and
even power tools.
A solar module generates a direct current (DC), generally at a voltage of
12 V. Many appliances, such as lights, TV’s, refrigerators, fans,
tools etc., are now available for 12V DC operation. Nevertheless the majority of
common electrical household appliances are designed to operate on 110 V or
220 V alternating current (AC). PV systems with batteries can be designed to
power DC or AC equipment. People who want to run conventional AC equipment add a
power conditioning device called an inverter between the batteries and the load.
Although a small amount of energy is lost in converting DC to AC, an inverter
makes PV-generated electricity behave like utility power to operate everyday AC
appliances, lights, or computers.
PV systems with batteries operate
by connecting the PV modules to a battery, and the battery, in turn, to the
load. During daylight hours, the PV modules charge the battery. The battery
supplies power to the load whenever needed. A simple electrical device called a
charge controller keeps the batteries charged properly and helps prolong their
life by protecting them from overcharging or from being completely drained.
Batteries make PV systems useful in more situations, but also require some
maintenance. The batteries used in PV systems are often similar to car
batteries, but are built somewhat differently to allow more of their stored
energy to be used each day. They are said to be deep cycling. Batteries designed
for PV projects pose the same risks and demand the same caution in handling and
storage as automotive batteries. The fluid in unsealed batteries should be
checked periodically, and batteries should be protected from extremely cold
weather.
A solar generating system with batteries supplies
electricity when it is needed. How much electricity can be used after sunset or
on cloudy days is determined by the output of the PV modules and the nature of
the battery bank. Including more modules and batteries increases system cost, so
energy usage must be carefully studied to determine optimum system size. A
well-designed system balances cost and convenience to meet the user’s needs, and
can be expanded if those needs change.
DESIGNING PV HOME SYSTEM WITH BATTERIES
 |
A solar-powered system with batteries can run quite a lot of
consumer devices, but only, of course, if the energy demand does not
exceed the generator output. The right sizing of the system is thus
necessary. The first step towards having such a system that will provide
energy needs is specification of the system. |
CALCULATION OF ENERGY DEMAND
 In case of designing PV powered home
system the first step to make is to create a list of all electrical appliances
in the household. Check the power input required for the operation of these
appliances and put this on the list.
As an example average data on
power consumption for some devices are in the table below, but it is important
to bear in mind that these are only rough estimations. To calculate power
consumption (E) of the system with inverter (using AC devices) it is needed to
make correction (multiply average consumption by C to calculate the total power
demand Ptot).
|
DEVICE |
Pave |
C |
P
tot |
| Fluorescent lamps |
18 W |
1,5 |
27 W |
| Radio/Cas.tape,6V |
2W/8W |
2,0 |
4W/16W |
| Radio/Cas.tape,12V |
8W/12W |
1,0 |
8W/12W |
| Small b/w TV |
18 W |
1,0 |
18 W | To operate other
electrical appliances such as refrigerators, irons, big fans, cooking plates,
etc., you would need a bigger and more expensive system. Since such a system is
not standardized but will be tailored specifically to your needs, calculation
have to be done by an expert.
Second step is to estimate the amount of time per day that the specific
appliances are in operation. This maybe as much as 10 hours for a lamp in the
living room, but perhaps only 10 minutes for one in the store. Add these data to
your list in a second column in table bellow. Finally, you should make a third
column where you list the daily energy requirement. Calculate this figure by
multiplying the power by the operating period, e.g. 27 W x 4 h = 108 Wh. When
you have added up all the figures in this column, you will have your overall
energy demand (E).
|
DEVICE |
Pave |
No.of
h/d |
E |
|
Fluor.Lamp |
27 W |
4 |
108 Wh |
|
Fluor.Lamp |
27 W |
1 |
27 Wh |
|
Fluor.Lamp |
27 W |
0,5 |
13,5 Wh |
|
Radio 6 V |
4 W |
10 |
40 Wh |
|
TV |
15 W |
2 |
30 Wh |
|
Fan |
12 W |
3 |
36 Wh |
|
TOTAL |
|
|
254 Wh | The next step
consists of estimation of the amount of solar insolation which can be expected
at home site. In most cases, these figures can be obtained from local PV
suppliers or at a local weather station. Important figure is the annual average
solar insolation as well as the average in the month with the worst climatic
conditions (some general data can be found in chapter on Solar radiation).
Using the first figure, PV system can be adjusted to the average
insolation per year, which means there are some months with more energy than
required or calculated and some months with less. If you use the second (low
case) figure, you will always have at least enough energy to meet your
requirements, except in unusually bad weather periods. However, the PV module
will have to be larger and it will also be more costly.
Now you can
calculate the rated power of the PV module. Use your energy demand figure (in
Wh/d), multiply it by 1,7 to allow for energy losses in the system and divide it
by the solar insolation figure (in Wh/d), e.g. 280 (Wh/d) x 1,7/ 5 (kWh/d) =
96,2 W. Unfortunately, PV modules are only available with a few power
ratings. Using a 50 W module, for example, you can build generators of 50 W, 100
W, 150 W, etc.. With a power demand of 95 W, a two-module system would be the
best match. Choose the number of modules whose total power rating corresponds
approximately to the value you have calculated. If the two figures differ
significantly, you have to undersize or oversize the generator. In the first
case, the PV system will not be able to meet overall energy demand. Decide
whether this partial supply option would be acceptable to you. In the second
case, you will have surplus energy.
Designing the battery size
depends on energy demand and the number of PV modules. For above mentioned
example battery capacity of 60 Ah per module as a minimum should be used and 100
Ah as an optimum. Such a battery can store 1200 Wh at 12 V. This capacity can
cover 4 days of energy needs for above mentioned example with daily energy
consumption of 280 Wh.
SYSTEM DC VOLTAGE
In the
past, almost all systems used 12 V DC as their base voltage. This was because
the systems were small and extensively employed 12 V DC appliances powered
directly from the battery. Now, with the arrival of efficient and reliable
inverters, 12 Volt use has declined and 24 V DC is becoming the favored battery
voltage. At this moment, the system’s DC voltage should be determined by how
much power the system cycles daily. Systems producing and consuming less than
2,000 Watt-hours daily are best served by 12 Volts. Systems cycling over 2,000
and less than 6,000 Watt-hours daily should use 24 V DC as a base voltage.
Systems cycling over 6,000 Watt-hours daily should use 48 Volts.
System voltage is a very important factor effecting the choice of
inverter, controls, battery chargers, and system wiring. Once these components
are bought, they usually cannot be changed. While some hardware, like PV
modules, can be reconnected from 12 to higher voltages, other hardware like
inverters, controls, and wiring is specified for a particular voltage and must
operate there.
BATTERY
A battery stores the energy delivered
by the solar generator and provides power for various appliances. As
a component of an SHS a battery has to fulfil three tasks:
It covers peak loads which
the PV modules cannot meet on its own (buffer).
It provides energy during the
night (short-term storage).
It compensates for periods of bad
weather or of unusually high energy demand (medium-term
storage).
 Automotive batteries, which are available all over the world at
reasonable prices, are the most commonly employed type of battery.
However, they are designed to deliver high currents over short periods.
They cannot withstand the continuos cycles of charging and
discharging that are typical for solar systems. The industry has developed
batteries, sometimes called solar batteries, which meet these
conditions. Their main feature is low sensitivity to cyclic operation.
Unfortunately, there are only a few developing countries in
which such batteries are produced, and imported batteries may be very
expensive owing to transport costs and customs duties. In such cases, a
heavy-duty truck battery may be an appropriate, easily accessible
alternative, even if it has to be replaced more often.
In the
case of large PV systems, the capacity of one battery may not be
sufficient. If so, more than one battery, can be switched in
parallel, i.e. all poles marked + and all marked - are connected to each
other. Thick copper wires, preferably less than 30 cm long, should
be used for the connection. During charging, batteries produce gases which
are potentially explosive. Thus, you should avoid using an open fire
nearby. However, gassing is relatively low, especially if a charge
regulator is used; the risk is thus no greater than that normally involved
in the use of automotive batteries in cars. Nevertheless, the
batteries need to be well ventilated. Therefore you should not cover them
up or put them in boxes.
The capacity of a battery is indicated in ampere-hours (Ah). A 100
Ah, 12 V battery, for instance, can store 1,200 Wh (12 V x 100 Ah).
However, the capacity will vary, depending on the duration of the charging
or discharging process. In other words, a battery will deliver more
energy during a 100 h discharging period than during a 10 h period.
The charging period is indicated by an index to the capacity c, e.g. C100
for 100 hours. Note that suppliers may use different reference
periods.
When storing energy in batteries, a certain amount of
energy is lost in the process. Automotive batteries have efficiencies of
about 75%, while solar batteries may perform slightly better. Some
of the battery capacity is lost in each charging-discharging cycle and
eventually drops to a level at which the battery has to be replaced.
Solar batteries have a longer lifetime than heavy-duty automotive
batteries, which last about 2 or 3 years.
SIZING THE PV SYSTEM’S BATTERY
It is important to size the PV systems battery
with a minimum of four days of storage. Consider the system that consumes 2,480
watt-hours daily. If we divide this figure by system voltage of 12 V DC, we
arrive at a daily consumption of 206 Ampere-hours from the battery. So four days
of storage would be 4 days X 206 Ampere-hours per day or 826 Ampere-hours. If
the battery is a lead-acid type, then we should add 20% to this amount to ensure
that the battery is never fully discharged. This brings our ideal lead-acid
battery up to a capacity of 991 Ampere-hours. If the battery is nickel-cadmium
or nickel-iron, then this extra 20% capacity is not required because alkaline
batteries don’t mind being fully discharged on a regular basis.
THE CHARGE REGULATOR
 A battery can only be expected to
last several years if a good charge regulator is employed. It
protects the battery against overcharging and deep-discharging, both of which
are harmful to the battery. If a battery is fully charged, the
regulator reduces the current delivered by the solar generator to a level
which equalizes the natural losses. On the other hand, the regulator
interrupts the amount of energy supplied to the load appliances when the
battery has discharged to a critical level. Thus, in most cases a
sudden interruption in supply is not a system failure, but rather an effect of
this safeguard mechanism.
 Charge regulators are electronic components and, as such, may be
affected by malfunctions and improper handling of the systems.
Improved designs are equipped with safeguards to prevent damages to the
regulator and other components. These include safeguards against
short circuit and battery reverse polarity (mixing up of the
batteries’ +/- poles) as well as a blocking diode to prevent overnight
battery discharge. Many models indicate certain states of operation and
malfunctions by means of LEDs (light emitting diodes = small lamps). A few
even indicate the state of charge. Nevertheless the state of charge is
difficult to determine and can only be roughly estimated.
THE INVERTER
 The
inverter converts low voltage DC power (stored in the battery and produced by
the PVs) into standard alternating current, house power (120 or 240 V AC, 50 or
60 Hz). Inverters come in sizes from 250 watts (about 300 USD) to over 8,000
watts (about 6,000 USD). The electric power produced by modern sine wave
inverters is far purer than the power delivered to your wall sockets by your
local electric utility. There are also “modified sine wave” inverters that are
less expensive yet still up to most household tasks. This type of inverter may
create a buzz in some electronic equipment and telephones which can be a minor
problem. The better sine wave inverters have made great improvements in
performance and price in recent years. Inverters can also provide a “utility
buffer” between your system and the utility grid, allowing you to sell your
excess power generated back to the utility for distribution by their grid.
CABLES
A simple means of avoiding unnecessary losses is to use appropriate
cables and to attach them properly to the devices. Cables should always be
as short as possible. The ones connecting the different appliances should
have a cross-sectional area of at least 1.6 mm2. To ensure that the
voltage loss does not exceed 3%, the cable between the PV generator and the
battery should have a cross-section of 0.35 mm2 (12 V- system) or 0.17 mm2
(24 V-system) per metre and module. Thus, a 10 m cable for 2 modules would
require at least 10 x 2 x0,35 mm2 = 7 mm2. Since cables with a
cross-section exceeding 10 mm2 are difficult to handle and even difficult
to get, higher losses have to be accepted in some cases. If a part of this
cable is exposed to the open air, it should be designed so that will withstand
all weather conditions. Tolerance to ultraviolet rays may be an important
feature.
TRACKERS
PV
modules work best when their cells are perpendicular to the Sun’s incoming rays.
Adjustment of static mounted PV modules can result in from 10% (in winter) to
40% (in summer) more power output yearly. Tracking means mounting the array on a
movable platform which follows the sun’s daily motion. A tracker is a special PV
mounting rack that follows the path of the sun. In general the extra energy
captured by following the sun must be weighed against the costs of installing
and maintaining the tracking system.
Trackers cost money just like PV
modules. In many countries it is not cost effective to track less than eight
modules (e.g. in the USA). Under eight modules, we will get more power output
for money if we spend the money on more panels rather than a tracker. At eight
panels in the system, the tracker starts to pay off. There are exceptions to
this rule, for example array direct water pumps. If PVs are directly driving a
water pump, without a battery in the system, then it is cost effective to track
two or more PV modules. This has to do with technical details like the peak
voltage required to drive the pumps electric motor.
THE LAMPS
Due
to their excellent efficiency and long lifetime, energy saving lamps
should always be used in PV operated systems. Fluorescent tubes or the new
compact fluorescent lamps (CFL) are suitable in many cases, 18 W CFL lamp is
able to substitute traditional 100 W incadescent light bulb. CFL lamps require
electronic ballasts to be operated with a DC system. The quality of such
ballasts varies considerably and sometimes proves to be very poor.
Low-quality ballasts will result in high costs for continuous replacement of
worn-out tubes. It is important for ballasts to have a good efficiency, a high
number starting cycles, reliable ignition at low temperatures and low voltages
(10.5 V), and protection against short-circuit, open circuit, reverse polarity
and radio interference. Despite the fact that most CFL lamps on the market are
working only with AC current there are few companies offering also DC powered
lamps.
LIFETIME AND PRICING OF COMPONENTS
A very important consideration in the economic analysis is the lifetime
of a PV system. Lifetimes of the various components of a PV power supply have
been estimated, based on experiences gained over the past few years.
The lifetime of PV
panels is estimated at 20 years. Proper encapsulation and the use of low-iron
tempered glass ensure a lifetime which may go well beyond.
Galvanized iron frames and
anchors are part of most PV systems. Properly galvanized material should last as
long as the panels although some
maintenance may be required.
Batteries. Depending
on the character of the charge/discharge cycles, the average lifetime of the
so-called “Solar Batteries”, has been 4 years.
Battery chargers are assumed to last at
least 10 years.
Inverters are assumed to last for 10 years.
Rough guidelines for pricing of the several components:
Inverters - USD 0.50/W
Frames (galvanized) -
USD 0.30/Wp
Control Devices - USD 0.50/Wp
Cables - USD 0.70/m
Local stationary batteries -
USD 100/kWh capacity
PV modules - USD 5 /Wp.
PV WITH GENERATORS
Working together, PV and other electric generators can meet more varied
demands for electricity, conveniently and for a lower cost than either can meet
alone. When power must always be available or when larger amounts of electricity
than a PV system alone can supply are occasionally needed, an electric generator
can work effectively with a PV system to supply the load. During the daytime,
the PV modules quietly supply daytime energy needs and charge batteries. If the
batteries run low, the engine-generator runs at full power its most constant
fuel-efficient mode of operation until they are charged. And in some systems the
generator makes up the difference when electrical demand exceeds the combined
output of the PV modules and the batteries. Systems using several types of
electrical generation combine the advantages of each. Engine-generators can
produce electricity any time. Thus, they provide an excellent backup for the PV
modules (which produce power only during daylight hours) when power is needed at
night or on cloudy days. On the other hand, PV operates quietly and
inexpensively, and does not pollute. Using PV and generators together can also
reduce the initial cost of the system. If no other form of generation is
available, the PV array and the battery storage must be large enough to supply
night time electrical needs.
However, having an engine-generator as
backup means fewer PV modules and batteries are necessary to supply power
whenever it is needed. Including generators makes designing PV systems more
complex, but they are still easy to operate. In fact, modern electronic
controllers allow such systems to operate automatically. Controllers can be set
to automatically switch generators or to supply AC or DC loads or some of each.
In addition to engine generators, electricity from wind generators, small hydro
plants, and any other source of electrical energy can be added to make a larger
hybrid power system.
GRID-CONNECTED PV
 |
Where utility power is available, a grid-connected home PV system
can supply some of the energy needed and use the utility in place of
batteries. Several thousands of homeowners around the world are using PV
systems connected to the utility grid. They are doing so because they like
that the system reduces the amount of electricity they purchase from the
utility each month. They also like the fact that PV consumes no fuel and
generates no pollution. The owner of a grid-connected PV system buys and
sells electricity each month. Electricity generated by the PV system is
either used on site or fed through a meter into the utility grid. When a
home or business requires more electricity than the PV array is
generating, for example, in the evening, the need is automatically met by
power from the utility grid. When the home or business requires less
electricity than the PV array is generating, the excess is fed (or sold )
back to the utility. Used this way, the utility backs up the PV like
batteries do in stand-alone systems. At the end of the month a credit for
electricity sold gets deducted from charges for electricity purchased. In
some countries utilities are required to buy power from owners of PV
systems (and other independent producers of
electricity). |
An
approved, utility-grade inverter converts the DC power from PV modules into AC
power that exactly matches the voltage and frequency of the electricity flowing
in the utility line, and also meets the utility safety and power quality
requirements. Safety switches in the inverter automatically disconnect the PV
system from the line if utility power fails. This safety disconnect protects
utility repair personnel from being shocked by electricity flowing from the PV
array into what they would expect to be a dead utility line. In some
countries utilities are establishing rate structures that may make PV
grid-connected systems more economical. (At today‘s prices, when the cost of
installing a utility-connected PV system is divided by the amount of electricity
it will produce over 30 years, PV- generated electricity is almost everywhere
more expensive than power supplied by the utility.) For example, some utilities
charge higher prices at certain times of the day. In some parts of the USA, the
highest charges for electricity under this time-of-day pricing structure are now
nearly equal to the cost of energy from PV. The better the match between the
electrical output of the PV modules and the time of highest prices, the more
effective the system will be in reducing utility bills.
Grid
connected systems are growing especially in USA and Europe. One such a project
was commissioned in California. Twelve homes in a major housing development in
Compton (southern California) are using integrated solar roof tiles to provide
household electricity from sunlight. Central Park Estates, an affordable
single-family housing development, uses solar roof tiles as an integral and
aesthetically pleasing part of the homes. The solar roofs are connected to the
local power grid, and meters will ‘spin backwards’ when the PV cells produce
excess power.
UTILITY-SCALE PV
 Electric, gas, and water utilities have been using small PV systems
economically for several years. Most of these systems are less than 1 kW and use
batteries for energy storage. These systems are performing many jobs for
utilities, from powering aircraft warning beacons on transmission towers to
monitoring air quality of fluid flows. They have demonstrated the reliability
and durability of PV for utility applications and are paving the way for larger
systems to be added in the future.
Utilities are exploring PV to
expand generation capacity and meet increasing environmental and safety
concerns. Large-scale photovoltaic power plants, consisting of many PV arrays
installed together, can prove useful to utilities. Utilities can build PV plants
much more quickly than they can build conventional power plants because the
arrays themselves are easy to install and connect together electrically.
Utilities can locate PV plants where they are most needed in the grid because
siting PV arrays is much easier than siting a conventional power plant. And
unlike conventional power plants, PV plants can be expanded incrementally as
demand increases. Finally, PV power plants consume no fuel and produce no air or
water pollution while they silently generate electricity. Unfortunately, PV
generation plants have several characteristics that have slowed their use by
utilities. Under current utility accounting, PV-generated electricity still
costs considerably more than electricity generated by conventional plants.
Furthermore, photovoltaic systems produce power only during daylight hours and
their output varies with the weather.
Utility planners must therefore
treat a PV power plant differently than a conventional plant in order to
integrate PV generation into the rest of their power generation, transmission,
and distribution systems. On the other hand, utilities are becoming more
involved with PV. For example in USA utilities are exploring connecting PV
systems to the utility grid in locations where they have a higher value. For
example, adding PV generation near where the electricity is used avoids the
energy losses resulting from sending current long distances through the power
lines. Thus, the PV system is worth more to the utility when it is located near
the customer. PV systems could also be installed at locations in the utility
distribution system that are servicing areas whose populations are growing
rapidly. Placed in these locations, the PV systems could eliminate the need for
the utility to increase the size of the power lines and servicing area.
Installing PV systems near other utility distribution equipment such as
substations can also relieve overloading of the equipment in the substation.
Photovoltaics are unlike any other energy source that has ever been
available to utilities. PV generation requires a large initial expense, but the
fuel costs are zero. Coal- or gas- fired plants cost less to build initially
(relative to their output) but require continued fuel expense. Fuel expenses
fluctuate and are difficult to predict due to the uncertainty of future
environmental regulations. Fossil fuel prices will rise over time, while the
overall cost of PVs (and all renewable energy resources) is expected to continue
to drop, especially as their environmental advantages are valued.
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