Performance Factors of a PV installation
Actual performances of PV installations have shown great variations from expections. Many systems are largely ignored until someone notices great discrepancies in incidental billing statements.
Like any electric system exposed to weather, a photovoltaic system, with robust solar panels, and inverter may develop malfunctions, or degradation, or deterioration in electrical connections. Performance of such systems will hence be affected, and in most instances are not detected until being notified, or indirectly through discrepancy in billing, or when periodic financial statement is being examined. Many factors affect performance of a photovoltaic installation, such as shading, dust layer, fallen objects on solar panels, corroded connections, component failure in inverter or solar modules, etc. The occurrence of these factors will result in under-performance of a PV system, hence affect the financial and environmental returns on investment. Few installations come with web monitoring service, as a result, most under-performing systems go undetected. In order to assess the performance of an installation, one needs to quantify the factors that influence a system's output.
A mathematical model is used for determining PV output (with crystalline cells)
National Renewable Energy Laboratory[1] and Sandia National Laboratory [2] had developed mathematical model and software to determine output of solar arrays. For practical purpose, a greatly simplified equation may be adopted to help assess theoretical output of an installation. This idealized output embodies the assumption of Maximum Power Point Tracker. This equation may take the form:
where is power in watts, is Sun's intensity in W/m*m, is the incidence angle the Sun's ray impinging on an array, is total area of cells in square meters. It can be calculated by multiplying cell area listed in manufacturer's datasheet, of the module, by the number of modules in the system. is system efficiency nominally around 77%, is temperature coefficient of power at -0.5% per degree C, and is cell temperature above 25C. This equation, is useful for tracking relative performance of a PV installation, since the coefficients vary in value from brands and models of solar panels. Specific skills and equipment are required to accurately assess these parameters directly. However, if a set of parameters related to the above equation is available, an 'educated' estimate of output for an installation is possible; without physically touching the electrical components in the array. Sun's irradiance for some well-known photovoltaic sites are available on some publicly accessible PV monitoring websites, including hundreds served by Sunnyboy Portal, and FatSpaniel Technologies. A laboratory-grade well instrumented web monitoring site, exemplifies possibly the best practice in PV performance monitoring --Jasper Ridge Biological PreserveJRPV. For small PV system owners, UV Index and ambient temperature, sky condition, visibility, etc. are readily available through data querying from www.weather.com, or www.wunderground.com. Without sensor data for Sun's irradiance, the UV Index may serve as a surrogate indicator for this purpose. Under clear sky, the Sun's irradiance is expected to be proportional to the UV Index, or via a piece-wise interpolation regression line. Cell temperature, above 25C is also directly related to the ambient temperature, if the solar panels are well-vented underneath. This equation has been demonstrated to be close to 1% accuracy, in some actual system verified with instruments. Although actual accuracy may vary greatly, a good relative accuracy –for tracking to a PV site, has been demonstrated to be sufficient for non-commercial purposes.
