New System Can Help Predict Photovoltaic Solar Output Variability


The variability of cloud patterns is a problem for solar powercapturing, especially for larger solar farm operations. If gridoperators could forecast or predict these changes before they occur,they would be in a better position to manage load on their grids.

Researchers at Sandia National Laboratories looked into the problem andcame up with a new system to monitor how clouds affect large-scale solar photovoltaic (PV) power plants.

The sensors observe cloud shape, size and movement in order toprovide a way for utility companies to predict and prepare forfluctuations in power output due to changes in weather. The resultingmodels will give utility companies data to assess potential power plantlocations, ramp rates and power output.

“Clouds can cause a PV plant to change from full power to about 20%of full power in a relatively short period of time, which depends on the size and layout of the plant and the type and velocity of the clouds”,researcher Josh Stein told Energy Refuge.

“Currently, with solar plants accounting for a very small fraction of generation on most electrical grids these changes do not cause anyproblems for grid operators. However, if solar penetration levelsincrease in the future, as they likely will, such variability will poseproblems for grid operators. A fast change in generation from cloudtransients will need to be balanced by other sources of generation tobalance load”, he added.

The researchers’ work is currently focused at the 1.2-megawatt La Ola Solar Farm on the Hawaiian island of Lana’i. La Ola is the state’slargest solar power system, and can produce enough power to supply up to 30 per cent of the island’s peak electric demand, which is one of thehighest rates of solar PV power penetration in the world. The highpenetration of PV power on Lana’i, combined with the sun and cloud mixat the 10-acre La Ola plant, provided the researchers with the idealenvironment for prediction and modeling research.

Research could not interfere with the ongoing operations of theplant, which currently sells power to Maui Electric Company ( MECO ), so Sandia engineers connected 24 small, nonintrusive sensors to theplant’s PV panels and used a radio frequency network to transmit data.The sensors took readings at one-second intervals to provide researchers with unprecedented detail about cloud direction and coverage activity. A radio frequency transmission system has the added benefit of beingportable.

“The sensor network in Lanai is providing information about how large PV plants respond to spatially variable irradiance due to cloud shadows in Hawaii. Sandia is working with its industry partner, SunPowerCorp., to deploy a larger sensor network at another location early in2011. If this deployment proves successful, perhaps commercialapplications will be identified”, said Josh.

Image: Josh Stein at work. Source: Sandia National Laboratories



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