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Once again, as in the CIGS case, module efficiency needs to be increased to reduce the area-related costs.
Emerging Technologies
The PV field is moving so quickly that by the time information appears in print, it is generally outdated.
Reliability of cells, modules, and system components continues to improve. Efficiencies of cells and
modules continue to increase, and new materials and cell fabrication techniques continue to evolve.
One might think that Si cells will soon become historical artifacts. This may not be the case. Efforts
are underway to produce Si cells that have good charge carrier transport properties while improving
photon absorption and reducing the energy for cell production. Ceramic and graphite substrates have
been used with thinner layers of Si. Processing steps have been doubled up. Metal insulator semiconductor
inversion layer (MIS-IL) cells have been produced in which the diffused junction is replaced with a
Schottky junction. By use of clever geometry of the back electrode to reduce the rear surface recombi-
nation velocity along with front surface passivation, an efficiency of 18.5% has been achieved for a
laboratory MIS-IL cell. Research continues on ribbon growth in an effort to eliminate wafering, and
combining crystalline and amorphous Si in a tandem cell to take advantage of the two different bandgaps
for increasing photon collection efficiency has been investigated.
© 2001 CRC Press LLC
At least eight different CIS-based materials have been proposed for cells. The materials have direct
bandgaps ranging from 1.05 to 2.56 eV. A number of III-V materials have also emerged that have favorable
photon absorption properties. In addition, quantum well cells have been proposed that have theoretical
efficiencies in excess of 40% under concentrating conditions.
The PV market seems to have taken a strong foothold, with the likelihood that annual PV module
shipments will exceed 200 MW before the end of the century and continue to increase by approximately
15% annually as new markets open as cost continues to decline and reliability continues to improve.
PV Applications
PV cells were first used to power satellites. Through the middle of the 1990s the most common terrestrial
PV applications were stand-alone systems located where connection to the utility grid was impractical.
By the end of the 1990s, PV electrical generation was cost-competitive with the marginal cost of central
station power when it replaced gas turbine peaking in areas with high afternoon irradiance levels.
Encouraged by consumer approval, a number of utilities have introduced utility-interactive PV systems
to supply a portion of their total customer demand. Some of these systems have been residential and
commercial rooftop systems and other systems have been larger ground-mounted systems. PV systems
are generally classified as utility interactive (grid connected) or stand-alone.
Orientation of the PV modules for optimal energy collection is an important design consideration,
whether for a utility interactive system or for a stand-alone system. Best overall energy collection on an
annual basis is generally obtained with a south-facing collector having a tilt at an angle with the horizontal
approximately 90% of the latitude of the site. For optimal winter performance, a tilt of latitude +15° is
best and for optimal summer performance a tilt of latitude 15° is best. In some cases, when it is desired
to have the PV output track utility peaking requirements, a west-facing array may be preferred, since its
maximum output will occur during summer afternoon utility peaking hours. Monthly peak sun tables
for many geographical locations are available from the National Renewable Energy Laboratory (Sandia
National Laboratories, 1996; Florida Solar Energy Center).
Utility-Interactive PV systems
Utility-interactive PV systems are classified by IEEE Standard 929 as small, medium, or large (ANSI/IEEE,
1999). Small systems are less than 10 kW, medium systems range from 10 to 500 kW, and large systems
are larger than 500 kW. Each size range requires different consideration for the utility interconnect. In
addition to being able to offset utility peak power, the distributed nature of PV systems also results in
the reduction of load on transmission and distribution lines. Normally, utility-interactive systems do not
incorporate any form of energy storage they simply supply power to the grid when they are operating.
In some instances, however, where grid power may not be as reliable as the user may desire, battery back-
up is incorporated to ensure uninterrupted power.
Since the output of PV modules is DC, it is necessary to convert the module output to AC before
connecting it to the grid. This is done with an inverter, also known as a power conditioning unit (PCU).
Modern PCUs must meet the standards set by IEEE 929. If the PCU is connected on the customer side
of the revenue meter, the PV system must meet the requirements of the National Electrical Code® (NEC®)
(National Fire Protection Association, 1998). For a system to meet NEC requirements, it must consist of
UL listed components. In particular, the PCU must be tested under UL 1741 (Underwriters Laboratories,
1997). But UL 1741 has been set up to test for compliance with IEEE 929, so any PCU that passes the
UL 1741 test is automatically qualified under the requirements of the NEC.
Utility-interactive PCUs are generally pulse code modulated (PCM) units with nearly all NEC-required
components, such as fusing of PV output circuits, DC and AC disconnects, and automatic utility dis-
connect in the event of loss of utility voltage. They also often contain surge protectors on input and
output, ground fault protection circuitry, and maximum power tracking circuitry to ensure that the PV
array is loaded at its maximum power point. The PCUs act as current sources, synchronized by the utility
© 2001 CRC Press LLC
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