through the use of photovoltaics (PV), sunlight
can be converted directly into electricity. No fluids or
moving parts are involved.
Solar photovoltaics are made of semiconductor
materials that produce voltage and current when exposed to
sunlight. These semiconductor materials are made into PV
cells. The electricity generated by PV cells is direct current
(DC) like that produced by batteries. As more light falls on
a cell, more electricity is generated; therefore, a PV system
must not be shaded (i.e. by shadows, snow, or wet leaves)
because such shading can substantially reduce
performance.
A typical PV cell made of crystalline silicon is 12
centimeters in diameter and 0.25 millimeters thick. In full
sunlight, it generates 4 amperes of direct current at 0.5 volts
or 2 watts of electrical power. PV modules consist of PV
cells connected in series (to increase the voltage), and in
parallel (to increase the current), so that the output of a PV
system can match the requirements of the load to be
powered. If more power is required, modules can be
appropriately connected in series or parallel to form what is
called a PV array (see Figure 1). Thin film PV cells are
newer forms of PV with different semiconductor materials,
geometries, and dimensions but the series and parallel
connection of thin film cells and modules is similar to
crystalline PV.
The energy output of a PV system can be increased by
having the PV modules track the sun as it moves across the
sky on a daily basis instead of being fixed in one
orientation. Tracking arrays work well with certain
applications such as direct PV water pumping. Most
tracking systems require climates with a high fraction of
direct beam versus diffuse sunlight to work well. Climates
with high humidity have lower fractions of direct beam
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sunlight. The added complexity, maintenance and costs of
tracking arrays needs to be compared with the increased
energy benefit of tracking arrays in order to make an
informed decision of which type of array, fixed or tracking,
is the best option for each PV application.
TYPES OF PV SYSTEMS
Stand-Alone PV Systems
PV installations not connected to a utility power line
are referred to as stand-alone systems. The two basic
types of stand-alone systems are: (1) direct systems, which
utilize the PV electricity as it is produced, and (2) battery
storage systems, which have the capability of storing PV
generated electrical energy for use when the sun is not
shining. Some stand-alone systems with battery storage
include a DC to AC inverter to allow alternating current
(AC) appliances to be used. AC appliances are generally
cheaper and more readily available. Figure 2 shows the
basic layout of a stand-alone system with battery storage
and both AC and DC circuits. An example of a direct system is a water pumping
facility which pumps water during the (sunny) day to a
storage tank for later use. A battery storage system, on the
other hand, stores PV electricity in a battery to power an
electrical device (e.g., a light , a pump) even when the sun
is not shining.
The range of applications for stand-alone systems is
tremendous. Whenever the economics of providing
electricity from the utility grid are in question, photovolta-
ics should be considered. Examples of situations where the
economics of grid power are in question are (1) when the
utility monthly facilities charge (meter charge) is a majority
of a monthly electric bill, (2) when electricity is needed far
away from a utility distribution line or (3) where hard
surfaces such as parking lots or sidewalks are between a
proposed electrical load and a utility connection.
For remote or portable large power needs (greater than
500 watts), it has been common to use gasoline, propane, or
diesel powered generators. Associated with this option is
the high cost of maintenance, as well as the purchase and
transportation of fuel. With low maintenance costs and no
fuel requirements, PV is an ideal source of power for
remote or portable applications.
It is also possible to couple a PV system with a fossil-
fuel generator. In this type of system, the generator is used
to recharge the PV battery system during long periods of
cloudy weather. This hybrid system requires much less
fuel and maintenance for the generator, while extending
battery life.
Utility-Interactive Systems
Unlike stand-alone systems, utility-interactive systems
are connected to the electrical utility grid. These systems
are typically located on residential or commercial build-
ings. The house pictured in Figure 1 has a utility-interac-
tive PV system. Figure 3 shows the basic layout of a utility-
interactive system. These systems have a PV array that
supplies power to the building through a high quality
inverter. This inverter converts PV-generated DC electric-
ity to AC electricity compatible with the utility grid. This
AC electricity is supplied to the main electrical service of
the building, offsetting the purchase of power from the
utility grid. When the PV system is not generating as much
power as the building is using, the utility grid provides the
additional needed power. When the PV system is generating
more power than the building is using, excess power is sold
into the utility grid.
Selling power back to an electric utility may not be as
attractive as it sounds. Some utilities meter electrical energy
purchased and sold separately. Customers pay a retail rate
for electricity purchased from the utility but are paid a lower
avoided cost (wholesale) rate for electricity sold to a utility.
There is usually a significant difference between these two
rates. Under current conditions, it is much more cost
effective to use a PV system to displace the need for utility
power than to generate revenue with it. Until photovoltaic
power becomes cheaper relative to utility power, utility-
interactive systems should be sized so that very little power
has to be sold back to the utility.
Figure 2. Stand-Alone PV System A major benefit of utility-interactive PV is that it can be
widely distributed. This asset will lead to the installation of
many small scale distributed generation power plants
located on buildings. Over time, this could reduce the need
for additional large centralized power plant construction
and costly utility grid distribution network expansion. The
nature of distributed generation can increase national
energy security by increasing the number of power plants
that would have to be targeted in order to shut down the
national utility grid. Other examples of distributed genera-
tion technologies are wind turbines, fuel cells and micro
turbines.
Another benefit of utility-interactive PV systems is that
they produce electrical power when the sun is shining
strongest and the electrical utility grid needs it the most.
They produce the most electrical power when the demand
on the electrical utility grid is highest due to building air
conditioning. This benefit is currently not well rewarded
but may be in the future through the use of solar load
control devices, demand management systems and real time
pricing scenarios.
PHOTOVOLTAIC SYSTEM SIZING AND
APPLICATIONS
PV Module Electrical Power
PV modules power is rated by peak approximate DC
power output at standard testing conditions (STC). STC
are laboratory test conditions and are very different from
the conditions that the modules will see when operating in
the sun. These conditions are solar intensity of 1,000 watts
per square meter (317 BTU/hr-ft
2
) and a module tempera-
ture of 25ºC (77ºF). Actual operating conditions on a
sunny day may be solar intensity of 800 Watts per square
meter (254 BTU/hr-ft
2
) and module temperature of 50ºC
(122ºF). PV module power is proportional to solar inten-
sity so it will be less than rated at lower solar intensity
levels. Also, PV module power is reduced as the operating
temperature increases. Typically, PV module power is
reduced by about 5% for every 10ºC (18ºF) increase in
operating temperature above STC temperature. Failure to
consider this reduction could cause major design errors.
Solar Resource
During July in North Carolina, there are 14 hours
between sunrise and sunset. Much of that time the sun does
not shine at peak intensity (1,000 Watts/m
2
) because clouds,
haze and the atmosphere reduces solar intensity. The solar
intensity seen by a PV array is reduced when the suns rays
are not perpendicular to the surface. To estimate the output
Figure 3. Utility-Interactive System of a PV system, we use peak sun hours which is the
number of hours the sun would had to have shone at peak
intensity on a PV array to equal the amount of radiation
that was actually received by the array during the day. This
value is often reported as kilowatt-hours per square meter
(kWh/m
2
). Thus, a PV array receiving solar radiation of 7
kWh/m
2
per day has received the equivalent of 7 hours of
sunshine at peak conditions (i.e., 7 peak sun hours).
Tables 1 and 2 show the average daily peak sun hours
on a monthly basis for fixed (Table 1) and tracking (Table
2) arrays for the Raleigh-Durham area.
PV Module Electrical Energy Generation
To estimate the energy that a PV module will produce,
we have to multiply module power and time. The power is
not the STC power rating but instead is the ex