Photovoltaic
Electricity Systems
Photovoltaic
(PV) collectors can be integrated into the building envelope
as “building-integrated photovoltaics” (BIPV),
or mounted on separate dedicated supports (“stand-off
arrays”).
BIPV
collectors integrate weather protection and solar electricity
generation for greater economy, since they integrate collector
support structures, and displace conventional building materials.
As well, they are typically more visually appealing, avoiding
the clutter of support structures.
BIPV
systems are available for roofing, curtain walls, skylights,
roof monitors, awnings, light shelves and semi-transparent
glazing.
Stand-off
PV arrays are suited for both new and retrofit applications,
but do not save the cost of displaced finish materials.
For
the greatest annual energy generation in Santa Monica, orient
collectors southwest with a slope equal to the latitude
(~35°) from horizontal). Vertical wall-mounted collectors
facing southwest outperform collectors facing south on an
annual energy basis. Morning fogs and clouds in Santa Monica
reduce sunlight availability on southeast and east orientations.
Horizontally mounted collectors yield an average of ~124.2
kWh/sq.ft./year. (All figures include reductions in solar
radiation by typical cloud cover.)
Photovoltaic
arrays generate direct current (DC) electricity. An inverter
is required for alternating current (AC) devices, or to
connect to the utility grid. Stand-alone PV systems with
battery storage are much more expensive than grid-connected
systems and, since charging batteries generate hydrogen
gas, can be hazardous. In Santa Monica these are recommended
only for emergency power applications.
Grid-connected
BIPV systems are cost-effective on a life-cycle basis in
Los Angeles today. For example, in Santa Monica PV atrium
glazing can have a payback period of less than 10 years
when the savings of displaced exterior finish materials
are considered.
Other
electrical components, such as inverters and wiring, should
be specified to minimize energy losses. AC systems allow
for smaller wire sizes, a wider range of UL rated safety
equipment, and lower costs.
Inverters
come in two general categories: stand-alone, for battery
storage systems; or utility-interactive, for connection
to the utility grid. Modern solid-state utility-interactive
inverters are available complete with safety disconnects,
automatic transfer switches, maximum power point tracking
and other features for code-approved installation.
Key
considerations when selecting inverters are:
- sizing
for peak loads and high surge currents of induction loads;
- power
quality (output should be a true sine wave);
- high
conversion efficiency (>90%);
- low
standby power consumption; and
- staged
or dedicated inverters for higher operating efficiency.
Ventilation
to keep PV collector temperatures low is important for maximum
power output. Custom designs can integrate with building
HVAC systems to collect heat from PV collectors to pre-heat
outdoor air.
Low-interest
loans are available from the Federal Million Solar Roofs
program; both loans and grants are available from the California
Energy Technologies Advancement Program (ETAP). Tax credits
and accelerated depreciation are available to improve economic
viability.
Cautions
- Equipment
and systems should be certified by the Solar Rating Certification
Corporation (SRCC) and other nationally-recognized electrical
certification agencies.
- Codes
and regulations governing building-integrated photovoltaics
are under development.
- System
design and installation should be performed by licensed
specialists, and properly commissioned.
- Multiple
trades may be involved for system maintenance/replacement,
requiring extra coordination.
-
System sizing should account for slight output degradation
over time, however many systems guarantee 90% of their
initial effectiveness for 20 years.
- Photovoltaic
modules must be clean for best performance; easy access
must be considered.
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Average
Annual Solar Radiation in Santa Monica, kWh/(sq.ft.-year)
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Orientation
|
Array
Slope= 20 degrees
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Array
Slope= 35 degrees
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Array
Slope= Vertical
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South
|
135.2 |
135.9
|
130.1
|
87.3
|
|
SW
|
140.7
|
140.6
|
134.8
|
96.5
|
|
SE
|
121.7
|
121.5
|
116.5
|
83.4
|
|
W
|
130.4
|
125.8
|
118.4
|
87.3
|
|
E
|
112.8
|
108.7
|
102.4
|
75.5
|
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NOTE: To estimate annual energy from
a PV array, multiply value by 0.14 for crystalline
silicon PV, and 0.05 for amorphous silicon PV
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