Page 164 - Alternative Energy Systems in Building Design

140 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

be noted that flatly laid solar PV arrays may incur about a 9–11 percent power loss,
but depending on the number of installed panels, it could exceed 30–40 percent on the
same mounting space.
An important design criterion when laying out solar arrays is grouping the proper
number of PV modules that would provide the series-connected voltages and current
required by inverter specifications. Most inverters allow certain margins for dc inputs
that are specific to the make and model of a manufactured unit. Inverter power capaci-
ties may vary from a few hundred to many thousands of watts. When designing a solar
power system, the designer should make decisions about the use of specific PV and
inverter makes and models in advance, thereby establishing the basis of the overall
configuration.
It is not uncommon to have different sizes of solar power arrays and matching
inverters on the same installation. In fact, in some instances, the designer may, for
unavoidable occurrences of shading, decide to minimize the size of the array as much
as possible, thus limiting the number of PV units in the array, which may require a
small-power-capacity inverter. The most essential factor that must be taken into
consideration is ensuring that all inverters used in the solar power system are com-
pletely compatible.
When laying out the PV arrays, care should be taken to allow sufficient access to
array clusters for maintenance and cleaning purposes. In order to avoid deterioration
of power output, solar arrays must be washed and rinsed periodically. Adequately
spaced hose bibs should be installed on rooftops to facilitate flushing of the PV units
in the evening only, when the power output is below the margin of shock hazard.
On completing the PV layout, the designer should count the total number of solar
power system components and, by using a rule of thumb, must arrive at a unit-cost
estimate, such as dollars per watt of power. This will make it possible to better approxi-
mate the total cost of the project. In general, net power output from PV arrays, when
converted to ac power, is subjected to a number of factors that can degrade the output
efficiency of the system.
The CEC rates each manufacturer-approved PV unit by a special power output per-
formance factor, referred to as the power test condition (PTC). This figure of merit is
derived for each manufacturer and PV unit model by extensive performance testing
under various climatic conditions. These tests are performed in a specially certified
laboratory environment. Design parameters that affect system efficiency are as follows:

■ Latitude and longitude
■ Climatic conditions (in cold weather, PV units work more efficiently)
■ Associated yearly average insolation
■ Temperature variations
■ Building orientation (north, south, etc.)
■ Roof or support structure tilt
■ Inverter efficiency
■ Isolation transformer efficiency
■ DC and ac wiring losses resulting from the density of the wires in conduits
■ Solar power exposure