PV
Overview
Solar PV technology converts energy from solar
radiation directly into electricity. Solar PV cells are the
electricity-generating component of a solar energy system. When sunlight
(photons) strikes a PV cell, an electric current is produced by stimulating
electrons (negative charges) in a layer in the cell designed to give up
electrons easily. The existing electric field in the solar cell pulls these
electrons to another layer. By connecting the cell to an external load, this
current (movement of charges) can then be used to power the load (e.g., light
bulb).
PV cells are assembled into a PV panel or
module. PV modules are then connected to create an array. The modules are
connected in series and then in parallel as needed to reach the specific
voltage and current requirements for the array. The direct current (DC)
electricity generated by the array is then converted by an inverter to useable
alternating current (AC) that can be consumed by adjoining buildings and
facilities or exported to the electricity grid. PV system size varies from
small residential (2–10 kW), to commercial (100–500 kW), to large utility scale
(10+ MW). Central distribution plants are also currently being built in the
100+ MW scale. Electricity from utility-scale systems is commonly sold back to
the electricity grid.
3.2 Major System Components
A typical PV system is made up of several key components,
including:
• PV modules
• Inverter
• Balance-of-system (BOS) components.
These, along with other PV system components, are
discussed in turn below.
3.2.1 PV
Module
Module technologies are differentiated by the type of PV material
used, resulting in a range of conversion efficiencies from light energy to
electrical energy. The module efficiency is a measure of the percentage of
solar energy converted into electricity.
Two common PV technologies that have been widely used for
commercial- and utility-scale projects are crystalline silicon and thin film.
3.2.1.1 Crystalline Silicon
Traditional solar cells are made from silicon. Silicon is quite
abundant and nontoxic. It builds on a strong industry on both the supply
(silicon industry) and product side. This technology has been demonstrated for
a consistent and high efficiency for more than 30 years in the field. The
performance degradation, a reduction in power generation due to long-term
exposure, is under 1% per year. Silicon modules have a lifespan in a range of
25–30 years but can keep producing energy beyond this range.
Typical overall efficiency
of silicon solar panels is between 12% and 18%. However, some manufacturers of
mono-crystalline panels claim an overall efficiency nearing 20%. This range of
efficiencies represents significant variation among the crystalline silicon
technologies available. The technology is generally divided into mono- and
multi-crystalline technologies, which indicates the presence of grain
boundaries (i.e., multiple crystals) in the cell materials and is controlled by
raw material selection and manufacturing technique. Crystalline silicon panels
are widely used based on deployments worldwide.
Two examples of crystalline solar panels: mono- and
multi-silicon installed on tracking mounting systems.
3.2.1.2
Thin Film
Thin-film PV cells are made from
amorphous silicon (a-Si) or non-silicon materials, such as cadmium telluride
(CdTe). Thin-film cells use layers of semiconductor materials only a few
micrometers thick. Due to the unique nature of thin films, some thin-film cells
are constructed into flexible modules, enabling such applications as solar
energy covers for landfills, such as a geomembrane system. Other thin-film
modules are assembled into rigid constructions that can be used in fixed-tilt
or, in some cases, tracking system configurations.
The
efficiency of thin-film solar cells is generally lower than for crystalline
cells. Current overall efficiency of a thin-film panel is between 6% and 8% for
a-Si and 11% and 12% for CdTe. Figure 4 shows thin-film solar panels.
Figure 4. Thin-film
solar panels installed on a (left) solar energy cover and (middle/right)
fixed-tilt mounting system. Photos by (left) Republic Services, NREL 23817,
(middle) Beck Energy, NREL 14726, and (right) U.S. Coast Guard Petaluma Site,
NREL 17395
Industry standard
warranties of both crystalline and thin-film PV panels typically guarantee
system performance of 80% of the rated power output for 25 years. After 25
years, they will continue producing electricity at a lower performance level.
3.2.2
Inverter
Inverters convert DC electricity from
the PV array into AC and can connect seamlessly to the electricity grid.
Inverter efficiencies can be as high as 98.5%.
Inverters also sense the utility power
frequency and synchronize the PV-produced power to that frequency. When utility
power is not present, the inverter will stop producing AC power to prevent
“islanding” or putting power into the grid while utility workers are trying to
fix what they assume is a de-energized distribution system. This safety feature
is built into all grid-connected inverters in the market. Electricity produced
from the system may be fed to a step-up transformer to increase the voltage to
match the grid.
There are two primary types of inverters
for grid-connected systems: string and micro-inverters. Each type has strengths
and weaknesses and would be recommended for different types of installations.
String inverters are most common and typically range in
size from 1.5 kW to 1,000 kW. These inverters tend to be cheaper on a capacity
basis, as well as have high efficiency and lower operation and maintenance
(O&M) costs. String inverters offer various sizes and capacities to handle
a large range of voltage output. For larger systems, string inverters are
combined in parallel to produce a single point of interconnection with the
grid. Warranties typically run between 5 and 10 years with 10 years being the
current industry standard. On larger units, extended warranties up to 20 years
are possible. Given that the expected life of the PV panels is 25–30 years, an
operator can expect to replace a string inverter at least one time during the
life of the PV system.
Micro-inverters are dedicated to the conversion of a single PV
module’s power output. The AC output from each module is connected in parallel
to create the array. This technology is relatively new to the market and in
limited use in larger systems due to the potential increase in O&M
associated with significantly increasing the number of inverters in a given
array. Current micro-inverters range in size between 175 W and 380 W. These
inverters can be the most expensive option per watt of capacity. Warranties
range from 10 to 20 years. Small projects with irregular modules and shading
issues typically benefit from micro-inverters.
With string inverters, small amounts of
shading on a solar panel will significantly affect the entire array production.
Instead, it impacts only that shaded panel if micro-inverters are used.
3.2.3.1
Mounting Systems
The array has to be secured and oriented
optimally to maximize system output. The structure holding the modules is
referred to as the mounting system.
3.2.3.1.1 Ground-Mounted Systems
For ground-mounted systems, the mounting
system can be either directly anchored into the ground (via driven piers or
concrete footers) or ballasted on the surface without ground penetration.
Mounting systems must withstand local wind loads, which range from 90–120 mph
for most areas or 130 mph or more for areas with hurricane potential. Depending
on the region, snow and ice loads must also be a design consideration for the
mounting system.
Typical ground-mounted systems can be
categorized as fixed tilt or tracking. Fixed-tilt mounting structures consist
of panels installed at a set angle, typically based on site latitude and wind
conditions, to increase exposure to solar radiation throughout the year.
Fixed-tilt systems have lower maintenance costs but generate less energy (kWh)
per unit power (kW) of capacity than tracking systems.
Tracking systems rotate the PV modules so they are
following the sun as it moves across the sky. This increases energy output but
also increases maintenance and equipment costs slightly. Single-axis tracking,
in which PV is rotated on a single axis, can increase energy output up to 25%
or more. With dual-axis tracking, PV is able to directly face the sun all day,
potentially increasing output up to 35% or more. Depending on underlying
soiling conditions, single- and dual-axis trackers may not be suitable due to
potential settlement effects, which can interfere with the alignment
requirements of such systems.
System Type
|
Fixed-Tilt Energy Density (DC-Watts/ft2)
|
Single-Axis Tracking Energy Density
(DC-Watts/ft2)
|
Crystalline Silicon
|
4.0
|
3.3
|
Thin Film
|
3.3
|
2.7
|
Hybrid High Efficiency
|
4.8
|
3.9
|
The selection of mounting type is dependent on many factors,
including installation size, electricity rates, government incentives, land
constraints, latitude, and local weather. Contaminated land applications may
raise additional design considerations due to site conditions, including
differential settlement.
Selection of the mounting system is also heavily dependent on
anchoring or foundation selection. The mounting system design will also need to
meet applicable local building code requirements with respect to snow, wind,
and seismic zones. Selection of mounting types should also consider frost
protection needs, especially in cold regions, such as New England. The TechCity
East Campus has no areas for large ground-mounted PV systems due to the high
concentration of buildings on the site.
3.2.3.1.2 Roof-Mounted Systems
Installing PV on rooftops has many of the same considerations as
installing ground-mounted PV systems. Factors, such as available area for an
array, solar resource, shading, distance to transmission lines, and distance to
major roads at the site, are just as important in roof-mounted systems as in
ground-mounted systems. Rooftop systems can be ballasted or fixed to the roof,
and it is recommended that the roof be relatively new (less than 5 years old)
to avoid having to move the PV system in order to repair or replace the roof.
System Type
|
Fixed-Tilt Energy Density
(DC-Watts/ft2)
|
Crystalline Silicon
|
10.0
|
Thin Film
|
4.3
|
3.2.3.2
Wiring for Electrical Connections
Electrical connections, including
wiring, disconnect switches, fuses, and breakers, are required to meet
electrical code (e.g., NEC Article 690) for both safety and equipment
protection.
In most traditional applications, wiring
from (1) the arrays to inverters and (2) inverters to point of interconnection
is generally run as direct burial through trenches. In RCRA applications, the
wiring might be required to run through above-ground conduit due to
restrictions with cap penetration or other concerns. Therefore, developers
should consider noting any such restrictions, if applicable, in requests for
proposals in order to improve overall bid accuracy. Similarly, it is
recommended that PV system vendors reflect these costs in the quote when
costing out the overall system.
3.2.3.3
PV System Monitoring
Monitoring PV systems can be essential
for reliable functioning and maximum yield of a system. It can be as simple as
reading values such as produced AC power, daily kilowatt-hours, and cumulative
kilowatt-hours locally on an LCD display on the inverter. For more
sophisticated monitoring and control purposes, environmental data, such as
module temperature, ambient temperature, solar radiation, and wind speed, can
be collected. Remote control and monitoring can be performed by various remote
connections. Systems can send alerts and status messages to the control center
or user. Data can be stored in the inverter’s memory or in external data
loggers for further system analysis. Collection of this basic information is
standard for solar systems and not unique to landfill applications.
Weather
stations are typically installed in large-scale systems. Weather data, such as
solar radiation and temperature, can be used to predict energy production,
enabling comparison of the target and actual system output and performance and
identification of under-performing arrays. Operators can also use this data to
identify required maintenance, shade on panels, and accumulating dirt on
panels, for example. Monitoring system data can also be used for outreach and
education. This can be achieved with publicly available, online displays;
wall-mounted systems; or even smart phone applications.
3.2.4
Operation and Maintenance
PV panels typically have a 25-year performance warranty. Inverters,
which come standard with a 5-year or 10-year warranty (extended warranties
available), would be expected to last 10–15 years. System performance should be
verified on a vendor-provided website. Wire and rack connections should be
checked annually. This economic analysis uses an annual O&M cost of
$20/kW/year, which is based on the historical O&M costs of installed
fixed-axis grid-tied PV systems. In addition, the system should expect a
replacement of system inverters in year 15 at a cost of $0.25/W.
3.3 Siting Considerations
PV modules are very sensitive to
shading. When shaded (either partially or fully), the panel is unable to
optimally collect the high-energy beam radiation from the sun. As explained
above, PV modules are made up of many individual cells that all produce a small
amount of current and voltage. These individual cells are connected in series
to produce a larger current. If an individual cell is shaded, it acts as
resistance to the whole series circuit, impeding current flow and dissipating
power rather than producing it.
The NREL solar assessment team uses a
Solmetric SunEye solar path calculator to assess shading at particular
locations by analyzing the sky view where solar panels will be located. By
finding the solar access, the NREL team can determine if the area is
appropriate for solar panels.
Following
the successful collection of solar resource data using the Solmetric SunEye
tool and determination that the site is adequate for a solar installation, an
analysis to determine the ideal system size must be conducted. System size
depends highly on the average energy use of the facilities on the site, PPAs,
available incentives, and utility policy.
3.4 Useable Acreage for PV System Installation
Typically,
a minimum of 2 useable acres is recommended to site PV systems. Useable acreage
is typically characterized as "flat to gently sloping" southern
exposures that are free from obstructions and get full sun for at least a
6-hour period each day. For example, eligible space for PV includes
under-utilized or unoccupied land, vacant lots, and/or unused paved areas
(e.g., a parking lot or industrial site space), as well as existing building
rooftops.
3.5 Solar Investor versus Developer Owned
The
choice between going with a solar investor or developer ownership will depend
on the desire for involvement and the risk appetite of the developer. While
ownership of the system will bring a higher payback for the developer, it will
also require hiring the contractors to permit, install, and maintain the
system. A solar investor inherits that risk and profit, and the solar farm in
turn will receive power from the PV system at a rate determined by the
investor. The recommendation of the feasibility team is to not pursue a PPA
because the PPA is much higher than the current utility price. If a PPA was
pursued, the tilted PV panels are recommended because that system is modeled to
be slightly cheaper than including the flat panels. The predicted price is
$0.03/kWh.