PV Applications and Components
The first decision when surveying a project site is to determine the type of PV
application to use. There are two categories of PV functions:
Standalone systems, which usually have a battery backup.
Grid-tied systems, which depend on energy from the utility grid to support
electrical loads on dark or cloudy days.
The second decision is to verify the PV system size and components neces-
sary for the chosen PV system. The collective name for all PV system components,
except the PV modules, is the balance of system (BOS). These include the charge
controller, battery, inverter, fuses, and circuit breakers.
Sun Hours
The amount of energy that PV modules can produce depends on the available
solar energy. The more hours of sunlight at the PV system site, the more energy
the modules can generate.
A factor that PV installers must consider when designing PV systems is the
measurement of peak sun. Peak sun hours or sun hours are the equivalent
number of hours per day as if the irradiance (density of radiation on a surface)
averages one kW/m². It is based upon a predetermined maximum solar irradiance
available on a clear day of 1,000 W/m
2
. One hour of peak sun is a peak sun hour
or simply a sun hour. There are maps of the earth and localized charts that indi-
cate the average daily insolation in sun hours measured as kWh/m²/day. This
information may be found for monthly or annual averages, and it may be at lati-
tude or some other angle for fixed or tracking systems.
For example, in Phoenix, Ariz., on an annual basis, the daily average of sun
hours fixed at latitude is about 6.85. This is based on 30 years of data, so in other
words, the average daily insolation is an average of 6.85 kWh per square meter.
This means the energy received during total daylight hours approximates the
energy that would have been received as if the irradiance had been constant for
6.85 hours at a constant one kW/m
2
.
Peak sun hours are a tool for estimating solar irradiation for a site. Solar irra-
diation for a PV site also is called its solar insolation or simply insolation. The PV
industry uses insolation to compare solar resources in various locations in the
world. Insolation information also is used to determine the amount of energy
available in a location to size a PV system to more specific requirements.
CHAPTER 4 Site Review, Layout, Mounting Systems, and Building Integration 67
Assessing PV Sites
A detailed site assessment should be conducted for every PV system. Usually, the
PV designer does them. To save time and money, many customers do them as
well. An initial site assessment determines if a site is suitable for a PV system. The
Energy Efficiency and Renewable Energy Clearinghouse (EREC) has material on
organizing site feasibility evaluation.
There are several considerations for every prospective site:
There must be southern exposure between 8 a.m. to 4 p.m.
The site must be shade-free.
For ground-mounted PV systems, there must be flat landscapes with a
southern exposure.
There must be enough space to install the PV system.
A true south facing installation surface is always ideal. Unfortunately, cir-
cumstances rarely provide surface areas that face true south. To address orienta-
tions off true south, designers and installers must adjust for the solar input loss
for this variation. There is a penalty in reduced solar input to the array the further
you move from true south. This is more serious after you move 30 degrees off
true south. Twenty percent of prime sunshine may be lost for orientations
30 degrees off true south. Adjust the array size to counteract the off-true-south
orientations away from true south.
How Does Weather Affect PV Module Output?
Understanding and developing an awareness of the site climate and its impact on
technology and performance is critical to PV system design and performance.
A comprehensive understanding of local temperature, rain, clouds, freezing, and
other microclimate issues is essential to good design and system performance.
String sizing and system modeling must be carefully adjusted for real conditions.
Do not rely on local information alone.
Understand how panels and inverters work at a variety of temperatures and
climatic conditions.
Know climate extremes and their impact on performance and
equipment longevity.
Learn to design for real conditions, not standard test conditions (STC) or
performance test conditions (PTC).
Study climatic data and local climatological, agricultural, or other
information available to understand PV site irregularities.
Measure both summer and winter panel and cell temperatures.
Take into consideration anything that preheats the air or the array for
calculating the cell temperature.
Look for boost of input from anything that may reflect light from surfaces,
such as green grass, gray or white roofing, or snow.
68 ADVANCED PHOTOVOLTAIC INSTALLATIONS
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.