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An Energy-Monitoring System
for Stanford University's Leslie Shao-ming Sun Field Station |
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Photovoltaic Array
 The
eight remaining sensors displayed on the dashboard monitor the parameters
associated with the PV array. The photovoltaic array (right) consists
of 275, 80-watt thin-film modules made by BP Solar that are mounted
on the south-facing portion of the building's inverted V-style roof.
Modules mounted on the west wing of the building are tilted at 15
degrees due south (roof angle). But the east wing of the building
is rotated so that the modules on its roof are turned to the west.
This means that roughly half of the modules will experience more sunlight
in the morning, while the other half receive enhanced evening sun.
This leads to some interesting performance issues, as San Francisco
weather tends to be foggy in the morning. Two pyranometers and two
thermistors were mounted to characterize the incident solar radiation
and module temperature respectively for the two module orientations.
The last four sensors record the array's DC voltage and the DC currents
produced by the three subarrays. These sensor values are combined
to calculate the DC power produced by the array before the inverter
(last display on the dashboard). The array's layout is shown below.
The array is unusual in that it is one of the first thin-film cadmium-telluride
(CdTe) arrays to be installed on a building. Crystalline silicon modules,
like those installed on the Lewis Center, are a mature technology
and have been widely used for many years. Competing thin-film technologies,
such as a-Si, CdTe, and CIS, offer the promise of low manufacturing
cost and building integration, but of these, only a-Si has been used
significantly in the marketplace. CIS modules and CdTe modules have
only recently become commercially available. Considerable interest
exists in the thin-film PV industry to learn how the CdTe array performs.
One of the immediate benefits of an energy-monitoring system is that
it can reveal performance problems that otherwise might go unnoticed.
Such was the case with the PV array. Operating since the building
opened in April 2002, the array clearly reduced electric bills because
there were times during the day when the PG&E electric meter actually
ran backward. But there was no information about how much energy the
PV array was producing until our monitoring system was installed in
April 2003. Within weeks, it was clear that the PV array was generating
between 10 percent and 30 percent less energy than expected.
The problem is illustrated in the May 12 data graphed below. The bell-curve-shaped
blue lines reflect the measured incident sunlight for the east and west pyranometers.
At solar noon the intensity of the incident sunlight peaks near 1000 W/m2.
The power output of the PV array is graphed in red; it peaks at about 13
kilowatts,
well below the 20-kilowatt inverter capacity. In early morning, the PV array
output increases with the increased sunlight. But after 10 a.m., although
the incident sunlight increases further, the PV array output flattens, failing
to
increase with the incident sunlight. For this reason, the PV power curve
has a flattened top, rather than the usual bell-curve shape.
 Further
investigation revealed the origin of the problem. As the array temperature
increased, its optimum operating voltage dropped below that achievable
by the inverter. The inverter (a device that converts direct current
to alternating current) could not track the maximum power point and,
instead, operated the array at its minimum allowable voltage of 330V,
well above the array's optimum voltage. The problem is being
addressed by rewiring the PV array to yield a higher voltage.
What Comes Next?
Because the building's energy consumption and the photovoltaic energy
production vary with the seasons, it is necessary to collect 12 months
of energy data before any significant conclusion regarding performance
can be drawn. When these data have been collected, they will be compared
with theoretical calculations to see how well actual performance aligned
with expectations. The building designers are hopeful that the PV
array will generate more energy than the building consumes. But the
sub-optimal performance of the PV array thus far is not encouraging.
As it is now configured, the energy-monitoring system does not collect information
about the performance of the solar hot-water-heating system. Its overall
performance can be inferred, however, by tracking the amount of propane that
is consumed.
In the next year, we expect to install a second group of sensors to monitor
various parameters associated with the HVAC system. These will provide much
better information
for characterizing this aspect of building performance.
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