The Soiling Issue

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Most people know that if you leave a car in the driveway for long enough it will naturally accumulate dirt. In the photovoltaic (PV) industry this causes a significant and avoidable loss of power known as "soiling". Studies have found that the soiling effects on various PV systems have exhibited over 25% energy loss during dry periods.

Soiling is the greatest avoidable performance loss in PV systems. Soiling comes from many anthropogenic and natural sources which makes it difficult to develop a standard model for accumulation. The issue of soiling is compounded when PV system owners clean arrays at set-time intervals. This method is potentially cash flow detrimental to the lifetime performance of the PV system. Due to these factors, a system should consider local measurement analysis with an automated device such as ARES.

The cost of washing a photovoltaic system can be a large portion of operating and maintenance expenses. The normalized cost for cleaning an array can be anywhere from $2-$4 per kW. This means that a 1,000kW system can cost $2000 – $4000 to clean. We must also factor in the cleaning equipment required, location, and access to the panels. Washing too often, or not enough can drastically reduce both the levelized cost of energy (LCOE) and return on investment (ROI) over the lifetime of the system. Therefore, determining the best day to wash is critical to increase the operational efficiency of the array. A non-cost safety risk to washing PV systems is also important to consider. Cleaning a PV array only when necessary avoids risks to personnel by avoiding work-at-height and arc flash dangers. These account for the vast majority of serious injuries when dealing with PV array maintenance.

At present, system owners are forced to undergo field experimentation in a very manual fashion if they wish to determine the soiling rate of their arrays. These experiments fail for a number of reasons. Three of which are the most glaring:

• The experiments are normally done for a short time and assume extrapolation over the entire lifetime of the system
• They are usually performed using non-calibrated sensors (generally utilizing the modules in the system) - which experience other losses that are not accounted for in these studies
• The cost of all labor and site visits. These experiments cost more (and deliver far less) than the installed cost of an ARES device

As financing institutions are more frequently becoming the owner of systems across the country, the direct effect of soiling to system ROI has become an important factor when determining whether or not to finance a project.

• Accumulation of dirt on solar panels is the greatest avoidable performance loss in photo-voltaic systems
• Cleaning is often incorporated without true knowledge of the current effect of soiling loss at the time of the scheduled cleaning
• Washing without soiling knowledge will impact ROI and LCOE negatively
• Scheduled cleanings can be a costly endeavor and should be based on an economic decision
• The soiling problem can be addressed appropriately by using Fracsun’s ARES device.