Commercial PV System Data Monitoring, Part One: Page 7 of 11
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Weather sensors. Weather sensors measure the environmental conditions in which the PV system is operating. Examples of weather sensors include pyranometers to measure sunlight intensity (irradiance) and sun hours (insolation); thermometers, thermistors, thermocouples or other devices to measure cell or ambient temperature; anemometers to measure wind speed and vanes to measure wind direction; barometric pressure sensors; precipitation meters to measure rainfall; and many others.
While revenue-grade plant metering is essential for billing and reporting purposes, its value is limited within the context of O&M. Plant operators need additional information in order to determine if a PV asset is performing as expected and to optimize scheduled and unscheduled maintenance activities. Weather sensors provide this additional information.
According to Blake Gleason, director of engineering at Sun Light & Power, an integrator based in Berkeley, California, “Plane-of-array irradiance and module cell temperature are all that you really need to check an array’s performance ratio.” A pyranometer or reference cell mounted in the same plane as the PV array typically provides irradiance data. A thermistor or thermocouple mounted on the back of one or more PV modules typically provides cell temperature data.
As characterized by Matt Taylor and David Williams in part one of their SolarPro article on PV performance guarantees (June/July, 2011), the performance ratio for an operational PV plant “separates out the uncertainty and variability of irradiance and is intended to normalize out weather factors to produce a consistent measure of system performance.” It is an index of PV plant performance, usually expressed as a percentage rather than in units, that represents the ratio of actual metered PV output power as compared with the ideal irradiance- and temperature-corrected output power.
In the context of unscheduled O&M activities, the value of monitoring a PV plant’s performance ratio is that this index can provide an early indication of installation or commissioning problems. Tracking this index can also be used to optimize scheduled maintenance activities, such as array cleaning. This is because the cumulative effects of dirt and dust buildup show as a steadily declining performance ratio. A stepwise or gradually progressing decline in performance ratio can indicate other issues, such as blown string or subarray combiner box fuses, stolen modules or inverter failures.
“While it might seem unnecessary,” notes Southern Energy Management’s Whitley, “I recommend getting a full weather station package, with sensors for ambient air temperature, wind speed and direction, global horizontal irradiance and precipitation. The worst kind of data is the data you wish you had when you need it.”
Power-One’s Tansy concurs, explaining: “Precipitation data is useful for predicting when to wash panels. Wind speed and direction are useful for discriminating the effects of ambient temperature on performance versus some other heat-induced system defect.” According to Tansy, skimping on environmental monitoring is one of the most common mistakes made when data monitoring solutions are implemented: “Installers routinely omit environmental monitoring from their plant installations and then scratch their heads when system owners ask ‘Why didn’t my plant produce last Tuesday morning?’” he says. “The answer is often ‘it was raining’ or ‘the ice hadn’t melted from the panels’ or something similar.”
When you are designing PV systems for large acreages or sites with variable terrains, consider incorporating multiple weather stations for more specific performance ratio calculations. In many cases, projects scaled for utility-power production require multiple weather stations for this very reason. Tansy recommends that designers “segment large plants into subsegments and use a cascading design for plant data collection devices.”
It is also important to follow the manufacturer’s instructions when installing weather monitoring hardware. For example, the correct placement of irradiance and temperature sensors is critical.
“Many weather sensors provide output signals on the order of microvolts or millivolts,” notes Draker Laboratories’ Reaugh. “As a result, the sensitivity of the datalogger or analog-to-digital converter is of prime interest when reading the output of these sensors,” he continues. “An analog-to-digital converter that cannot distinguish measurements smaller than 1 millivolt is useless when connected to a sensor that outputs in microvolts.”