PHOTOVOLTAIC SYSTEMS

 Photovoltaic systems convert the energy in sunlight to electrical
energy. This conversion is accomplished by using "solar cells" which
are semiconductors constructed of cadmium or silicon. While a single
solar cell can produce only a small amount of electricity, these cells
may be interconnected to form large photovoltaic arrays capable of
producing a considerable amount of electrical energy [Cheremisnoff and
Regino, 1978]. Figure A-1 presents a basic schematic diagram of a
photovoltaic system which is being used to power an irrigation pump
motor. When sunlight (solar insolation) is received on the photovoltaic
array (solar cells) a direct current (DC) of electricity is produced.
This DC electricity is fed into an inveter which transforms the electri-
city from DC to alternating current (AC). The inverter also upgrades
the waveform of electricity produced by the array to be compatible with
the utility grid current and regulates current flows. During periods
when the electricity demand of the pump motor is greater than array
output, the inverter feeds supplemental power from the utility to the
pump motor. During periods when array generation exceeds the pump motor
demand, surplus electricity is routed into the utility grid.
 Becuase photovoltaic systems rely on solar insolation to produce
electricity, the output of these systems will vary with the movement of
the sun both hourly and seasonally. Figure A-2 presents the average
hourly insolation for a solar day. The insolation profile shown in this
figure was estimated using the Orlando/Herndon Solmet Data [National
Climatic Center, 1980]. On average, a positive level of solar insolation
(KWH/m2) occurs between solar hours 6 and 19. This however varies
seasonally. As would be expected, the greatest insolation levels occur
at solar noon.
 The shape of the insolation curve shown in Figure A-2 demonstrates
why an array with no electricity storage capabilities must use supple-
mental electricity purchased from the utility at certain times. Assume
that operation of the pump motor requires an array output of electricity
equivalent to .35 KWH/m2 of array. It can be seen that array output
 per m is below this level during several hours of the day. During
per m is below this level during several hours of the day. During