The potential for solar generation in Pennsylvania is relatively consistent across the Commonwealth although it is higher in the Philadelphia region than the Pittsburgh region. This is partly because Philadelphia has many reflective surfaces and receives about 9% more solar radiation on an annual basis than Pittsburgh as is graphically illustrated in Figure 3.2.1
When Philadelphia became designated a Solar America City in 2008, it set a goal to install 2.3 MW of solar generation capacity by 2011, which will be enough to provide electricity to more than 350 households. In 2009, the entire state had 4 MW of installed solar capacity2. To meet this commendable and aggressive goal, local solar contractors and developers must have a clear understanding of Philadelphia’s Permitting requirements and unique attributes of the City that might impact project development such as:
Before considering a solar photovoltaic system for a building (particularly a residential building), energy-efficient measures should be incorporated first. Energy-efficiency isnot only a cost-effective means of reducing carbon emissions (reducing energy usage thereby reducing the need for electric generation) but a home may also not need as large a PV system to meet its loads. Weatherization and conservation are low hanging fruit to achieve both energy and dollar savings. Figure 3-3 shows the profitability ofenergy efficiency upgrades and highlights the top 5 efficiency measures with the greatest return on investment:
Additional measures include replacing older refrigerators and inefficient room air conditioning units which can also be very cost effective. Implementing energy efficient measures are much more cost effective and less capital intensive than installing a solar PV system, and therefore should be done first or in concert with installing solar PV.
These measures are contrasted with other familiar investments such as 30-year bonds, money market accounts and dividends on common stocks to give perspective of the positive return on investment these actions can provide.
There are also many opportunities for efficiency within commercial buildings. Advancement in lighting, air conditioning, and control technologies has resulted in large increases in efficiencies. A PV system will have a synergistic effect when installed in conjunction with energy-efficiency measures for any building type.
Contact the following groups for more information about energy efficiency recommendations:
There are many variables that affect the output of a PV system. Some of these factors are discussed in Section 3.5 Estimating PV System Performance of this document. The key factors that have the greatest impact on how a PV system performs are described here.
Sunlight Intensity (Irradiance)
The performance/output of a PV system is relatively proportional to sunlight intensity. Therefore these systems can generate electricity even on cloudy days. Greater amounts and duration of sunlight increase system performance. Sunlight intensity is called irradiance, which is measured in watts per square meter (W/m2). In summer, when the sun is nearly directly overhead, its irradiance at the surface of the Earth, at sea level, is approximately 1000 W/m2. This irradiance is defined as ―full‖ or ―peak sun,‖ and it is the standard irradiance for testing and rating PV modules. At peak sun conditions, roughly 70% of the sun that is incident at the top of the atmosphere penetrates to the surface of the Earth. If PV modules are mounted perpendicular to the sun’s rays, it is possible to receive close to peak sun nearly every sunny day at noon for much of the continental United States.5
Over a one year period, PV modules in Philadelphia will receive the most perpendicular sunlight when the PV array is facing due South and is tilted at an angle that is slightly less than the latitude of the region (for Philadelphia, 5 degrees less than the local latitude – 39 degrees – is recommended). However, tilting the PV array at or below the angle of latitude is not always the best solution for a project.
Shading Shading portions of a PV array will have the most adverse effect on the system’s performance. It is important to determine during the site assessment if a potential location for the PV array will be shaded, especially between the hours of 9 a.m. and 3 p.m. This is important, as the output of PV modules may be significantly impaired by even a small amount of shading on the array.
A careful assessment using an hourly computer simulation program is necessary to determine the benefits of westerly orientations. A minimum of six hours of unshaded operation is important for best system performance.
Figure 3-4 shows the location of the sun in the Philadelphia sky relative to a particular point (as noted by the Solar Azimuth on the x-axis). For example, at 12 noon on December 21st, the sun will be due south and at an angle (relative to the horizon) of approximately 25o. On this same day at 3pm, however, the sun will be at an approximate angle of 12o and will be located slightly southwest in the sky. These angles should be used to determine how far apart rows of PV modules should be located to ensure adequate energy generation during the winter months and minimize shading (when the sun is the lowest in the sky).
However configuring a PV array to avoid shading 100% of the time is not always possible given space constraints both on building rooftops and on open land. For PV arrays that consist of multiple rows of PV modules and array structures, the losses that occur when one row shades an adjacent row should be accounted for when estimating the performance of the PV system.6 (Accounting for these losses is discussed further within this guidebook in Section 3.5 Estimating System Performance.)
Next to shading, orientation of the PV array is one of the more important aspects of the site assessment. Facing the PV array due South is ideal; however, slight deviations can also be feasible. For example, an unshaded PV array with a tilt of 35 degrees and facing +/- 45 degrees away from due South (SE or SW) will still receive 92% of the annual solar radiation as compared to the PV array facing due south.
Fully understanding what the orientation will be at construction must be understood very early in the project. Often the roof tilt is used as the orientation of a residential rooftop system due to the improved aesthetics of a parallel standoff roof mounted array rather than an array that is tilted at an angle greater than the roof tilt. Most roof orientations are not the most ideal for the array orientation so the impact of a less than optimal orientation must be understood prior to solidifying the system orientation.
There are several other factors that affect the performance of a PV system that should be communicated to potential system owners so they have realistic expectations of how their system will perform and the resulting economic benefits they can expect over time. These include:
Note that in most all cases, the actual DC power generated will be less than the nameplate rating of the PV modules; however, when the ambient temperature is very cold and the sky is very clear, which sometimes occurs in the months of October or February, it is possible for the PV modules to generate power above their nameplate rating.
There are also other factors used in determining how much the DC power of a PV system will be derated as it is converted into AC power (also known as the derate factor). The derate factor and its impact on system performance is examined in Section 3.5 Estimating PV System Performance in this document and includes a range of typical losses associated with each factor.
There are other variables that affect the design, cost and constructability of a PV system. In addition to optimizing the output of a PV system, other considerations that designers and installers should take into account include:
There are several computer modeling tools that will estimate the performance of a PV system in a particular region of the country and the world. Some of the more commonly used tools in the industry for grid-connected PV systems are listed below:
It is important to thoroughly understand how these tools operate and the limitations of their results based on the users inputs.
(Note: For a more comprehensive list of computer software tools, visit NREL’s Solar Technology Analysis Models and Tools http://www.nrel.gov/analysis/analysis_tools_tech_sol.html)
The performance of a PV system depends on many variables as was discussed earlier. When estimating the energy savings that a PV system will achieve for a customer, it is better to not ―over-sell‖ the performance. Many of the performance modeling tools that are available allow the user to input the amount that the DC output of a PV array will be reduced (or derated due to losses in the system) by the time the power is delivered to loads. This derate factor should not be used as a hard-fast value to estimate the amount of power or energy that a PV system will generate. Instead it is recommended that contractors provide an output range that the actual expected performance will fall within.
PV Watts provides a default derate factor based on losses associated with the system parameters it models. Although some designers consider it a conservative estimate, it is considered by others to be a reasonable estimate. Based on local experience, when comparing solar site assessment results and the actual annual performance of hundreds of PV installations in Southeastern Pennsylvania, an overall derating factor of 0.80 has been found to be a conservative value (not including shading impacts). This means that in this area, one can expect to get 80% of the output of a system. Table 3-1 summarizes the system parameter losses that determine the default derate factor.
Some tools (such as Solar Design Studio, SAM and PVSYST) will model the output of the PV system based on the electrical characteristics of specific equipment such as PV modules and inverters. This is helpful when evaluating PV modules of different electrical characteristics (such as temperature coefficients for voltage, current and power). Ambient temperature has a greater impact on the output and performance of some modules than others, which can be helpful (and often necessary) when completing the electrical design.
The commissioning process is well-accepted in the building industry and is typically applied to entire buildings. The process begins at project inception (during the pre-design phase) and continues through the life of the facility.
The main objective of a commissioning plan is to assure the safe and orderly handover of the unit from the constructor to the owner, guaranteeing its operability in terms of performance, reliability, safety and information traceability. According to the Building Commissioning Association, ―The commissioning process includes specific tasks to be conducted during each phase in order to verify that design, construction and training meet the owner’s project requirements.‖ 10 Additionally, when executed in a planned and effective way, commissioning normally represents an essential factor for the fulfillment of schedule, costs, safety and quality requirements of the project.
Translating this to the PV industry allows a means to formalize quality control of installed PV systems. It ensures that systems have been installed in a safe manner and will be high performing. Commissioning encourages integrators to be responsible for their installations and facilitates project closeout.
Although the initial start-up of a PV system is often referred to as ―commissioning the PV system,‖ the final commissioning event should occur after all inspections have occurred (utility and per local code) and permits are signed off. However, the commissioning plan provides details throughout the development of the system about the events, requirements and timeline.
Verifying the Actual Performance
compared to the Expected Performance Determining whether a PV system is performing as expected is one of the most important aspects of commissioning a PV system. There are several ways of accomplishing this, including web-based tools and calculators such as PVWATTS. Other available tools include performance calculators that account for electrical rate schedules and provide financial analyses. There are also calculations that can be performed in the field based on simple measurements to adjust the output based on actual conditions at the site (irradiance, temperature, assumed derate factors, etc.)
Part of the commissioning process should include some basic tests in the field. Before starting up a PV system for the first time, the following tests should be conducted to establish baseline data about the PV system:
There are also calculations that can be performed in the field based on simple measurements to adjust the theoretical power output based on actual conditions at the site (irradiance, temperature, assumed derate factors, etc.).12 The theoretical or calculated power can then be compared to the actual power output indicated by the inverter. Also, the DC current and DC voltage values can be measured to verify the inverter displayed power output is correct.
Thoroughly commissioning the PV system and documenting the results will also provide reassurance to the owner that their system is performing to their expectations. The commissioning results can be a useful resource for troubleshooting potential problems in the future.
Photovoltaic systems produce direct current (DC) power from sunlight. Some grid-connected PV arrays use hundreds of modules connected in series and parallel to produce large amounts of power. Operating voltages may exceed 600 volts dc and currents at the subfield level may be hundreds of amperes.13 Working safely with PV systems requires a fundamental understanding of electrical systems coupled with common sense. There should always be a licensed electrician employed on any solar PV installation project to perform some if not all of the electrical work. Some of the interlocking plug-in wiring can be safely carried out, but it is essential that proper lockout/tagout procedures are followed when making the final wiring connections for a grid-tied system. There are the usual subtle hazards, as well. These include nicks, cuts, and burns from sharp or hot components. As such, gloves should be used when handling anything that might be sharp, hot, rough, or that might splinter. More importantly, special insulated gloves and eye protection should be used when testing or working with the electrical parts of the PV system.
There is always the possibility of dropping tools or materials on either oneself, someone else, or on sensitive equipment or materials. Dropping tools across battery terminals is an especially dangerous hazard
Other safety measures that should be observed include:
Once a site has been deemed suitable for a PV installation, the following coordination must occur:
Section 4 of this guidebook details the steps that solar contractors must follow to ensure compliance with all local codes, regulations and requirements.