Solar Panel Cleaning -Window Cleaning Add-On

Panel Cleaning: by Bryan Banke, Solar Power Partners: Panel cleaning is where we can see immediate and dramatic results. It is also the least structured area of preventative maintenance with schedules being elusive and dependent upon the environment. However, because our aim is more about optimizing energy production and less about preventing component failure, we have a goal with measureable parameters (energy production) guiding us to a flexible cleaning schedule.

To statistically judge when a system should be cleaned, we take actual energy production as measured at the meter, adjust it using the ratio of actual solar insolation to expected solar insolation for the solar electric facility (SEF) and compare the result to projected production adjusted for degradation over time. The result is our system efficiency.

At SPP, we define a soiling boundary for each SEF below which any further loss in efficiency will cost us more than the cleaning cost of the SEF. Before we hit this boundary, a trigger tells us that we should start to consider cleaning the system.

Calculating system efficiency and knowing when an SEF should be cleaned is simple. Placing that information into the context of a budget is a different story.

For PPA (power purchase agreement) providers and large commercial and utility scale system owners, expenses associated with a system are modeled before a SEF is built. Doing this enables financial backers to calculate the risk and rewards of their investment. We’ll speak more on this subject later, but here the budget directly affects when we clean an array and has little connection to a capricious environment.

Ideally, we would like to clean an array the moment its efficiency drops to the calculated soiling boundary. In the real world we must work within the constraints of a budget that often provides for a set number of cleanings per year. We may cross the soiling boundary four or five times in any given year, but we are constrained to one or two cleanings. The question then is how to make the most efficient use of those cleanings. The answer depends upon countless variables that we whittle down to a few generalizations:
When is peak production? We want to optimize production in peak season. Late spring, summer and early fall are peak production times in California so we want to optimize output during these seasons.
What seasonal events create exceptional soiling? Pollen is a huge soiling problem in agricultural areas. Windblown dust collects in dry seasons. Birds tend to congregate in specific seasons (and pigeons all the time).
Is it too hot to clean panels? Midsummer panel temperatures can reach precarious heights. Cleaning hot panels with water can result in thermal fractures.
Is rain expected? Rain events can delay the need for cleaning.
When is site access possible? Some businesses restrict access to their facilities. Examples include schools in session, airports requiring access pre-approval, and agricultural facilities in peak harvest.

You can see why scheduling panel cleaning is such a nuanced science requiring the insight and intuition of a seasoned asset manager with complete knowledge of each SEF and its operating environment. It is a balancing act between three disparate subjects: budget constraints, environment and production.

To add yet another layer of complexity, different installations require different cleaning methods. Ground-mount and flat-roof installations are relatively cheap and easy to clean provided a ready source of water exists. Tilted roof mounted arrays have personnel safety considerations and roof arrays built edge to edge require special equipment to reach long spans of panels without walking on them. Cleaning elevated arrays such as car shades often involves the use of a mechanical lift. Some arrays are designed in such a way that access with cleaning equipment is impossible and a water jet must be employed. These various methods all carry specific maintenance costs that must be budgeted before construction; otherwise, the systems will experience perennial budget shortfalls or improper cleaning.

Panel cleaning services exist in the marketplace today. Many are inexperienced entrepreneurs with inadequate PV knowledge, insufficient insurance coverage and an unrealistic pricing structure. At a minimum, each individual in a cleaning crew should know how to identify broken panels, deteriorated wiring and other dangerous SEF conditions; each worker should be covered under a general liability insurance policy; and pricing should reflect the nature of the job. The simplest of ground mounted and flat rooftop mounted systems should not cost any more than $.0025/watt to clean in 2009-10 dollars. More complex jobs will require a larger budget but should not exceed $.01/watt. As we tell every window cleaner who approaches us with a $.05/watt bid: “These aren’t windows.”

At SPP we clean panels with a weak vinegar solution (2 Tbsp/Gal. water). We do not use de-ionized water or special filters and we do not squeegee the panels dry. The vinegar breaks the water’s surface tension causing it to sheen from the glass without spotting. It is also safe for roofing surfaces and the environment and it is inexpensive. We rinse the heaviest material away then use wide, soft-bristle brushes to mechanically remove soil before following with a rinse. For larger unbroken spans of panels, we use extendable window washing poles and brushes. For those spans too large for such devices, we use a water pump that forces a large volume of water great distances. It is not a pressure washer and will not damage the panel seals. The key to all of these methods is to save money and time. We typically clean a 250kW flat-roof array mounted at 20 degrees in four hours with three laborers.

Also see - Solar Panel Cleaning: Add-On For Window Cleaners.