Residential Solar Site Measurements: Page 6 of 7
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Shade measurements are always more accurate when made on-site using a tool such as the Solar Pathfinder or the Solmetric SunEye. These tools take into account everything within the array’s field of view that can cause a shadow, from distant mountains to nearby trees to utility wires. They see what the array sees and correctly capture the current size of trees and other obstructions. They also enable the user to make measurements at the locations where the modules will be installed, such as 6 inches off the roof for a flush–mount system. (For a comprehensive review of on-site shade measurements, see “Solar Site Evaluation,” December/January 2009, SolarPro magazine.)
Leasing and PPA contracts typically guarantee a minimum level of performance. The contract may stipulate that the building owner is responsible for controlling shading. If a system begins to underperform after several years of operation, shading may be suspected. New on-site measurements may be required for comparison with the original measurements made when the system was installed. Accurate and repeatable solar access measurements can identify tree growth that may cause performance reductions.
System Performance Measurements
Currently, solar lease and PPA financial vehicles are driving the residential market in many states. For example, according to Clean Power Finance, 55% of the residential systems installed in California in 2011 were financed. This number rose to a staggering 80% for the month of December 2011. System performance guarantees are a standard component in financed systems, and increased attention is being paid to system commissioning and ongoing performance measurements as a result.
The performance verification process can be separated into two phases. Phase one includes commissioning, when performance should be verified and documented to establish an initial benchmark for the system. Phase two covers the ongoing monitoring of the system over its lifetime, which is typically performed remotely. When systems are leased or financed, this phase is important to ensure that customer expectations and performance guarantees are met. “We can verify systems on-site through voltage, irradiance and temperature measurements,” states Sungevity’s Williford. “Through remote monitoring, we have diagnostic tools that allow us to determine if a system is performing as expected.”
At the time of PV system installation, all stakeholders benefit from a comprehensive and well-documented system commissioning and performance verification procedure. (See “PV System Commissioning,” October/ November 2009, SolarPro magazine.) While commissioning residential systems is a straightforward process compared to commissioning commercial or utility-scale projects, its importance should not be undervalued. Proper commissioning is an essential aspect of limiting risk over the life of systems of any scale.
Jerry Shafer, CEO of Affinity Energy, confirms the importance of performance verification at the time of system commissioning. “We develop an as-built data sheet for the system to use as a starting point for the module and/or inverter output performance,” he says. “It is a type of insurance policy for us and the investor to see the actual data. In the event of an output question, whether it is the result of dirt, shade, inverter operation or anything else that can affect performance, we know what we started with.”
Industry best practices are evolving rapidly in the area of system commissioning and performance verification. Standard commissioning includes verifying system workmanship, operation, performance and acceptance documentation. Electrical testing including string open-circuit voltage, operating current and insulation resistance should be performed. Once the system is on line, system performance should be verified. A typical procedure for residential performance verification is to measure the module backsheet temperature and plane of array (POA) irradiance and simultaneously record the inverter power reading. Then a model is used to predict instantaneous power based on the irradiance, temperature, number of modules and other variables of the system. This number is compared to the inverter power that was recorded at the time of measurement. The ratio of actual power to expected power is often called the power performance index.
For systems using string or central inverters, more complete performance verification is possible through measurement of string I-V curves and comparisons with modeled performance (see “Field Applications for I-V Curve Tracers,” August/September 2011, SolarPro magazine.) This approach is common for commercial and larger residential applications. An I-V curve tracer measures and quantifies how a string is performing compared with how it should be performing under current irradiance and temperature conditions. Confidence that a new system is performing optimally on day one is important to system owners, whether they are homeowners or finance providers.