Solar Panel Cleaning

The Newdoll Panel-Cleaning solution

Solar's dirty little secret

The solar industry has a “dirty little secret”. While they are a cheap and sustainable source of electricity, solar panels need frequent cleaning, up to four times a year, depending on their location and environment. Dirt and dust build-up on the panels prevents sunlight from reaching the silicon, reducing electrical output by up to 25%. A test in July of 2006, after a six-month period with no cleaning showed a 25% increase in electrical output after washing on a true side-by-side comparison. This is what photovoltaic solar system sellers aren't telling you. However, there is a solution which can reduce the cost for cleaning, saving you time and money, while ensuring you get the most out of your solar system.

Ronald Newdoll, an early adopter of photovoltaic solar power generating systems, completed installation of a 400 KW system on the roofs of his Edison Technology Industrial Complex in January 2005. The $3.6 million that he invested was not only a sound fiscal investment, but also provided a test bed for advances in solar equipment technology. He installed three different brands of solar panels, inverters from three different manufacturers and various panel installation arrangements. Ronald's installation became a veritable proving ground for the latest thinking about solar. Dissatisfaction with the performance of inverters led to an investment in cutting edge engineering to develop new approaches. Meanwhile, side-by-side testing showed that a sophisticated, individual panel monitoring system would greatly increase the value of the system by immediately identifying problem areas. This was the genesis of Accurate Solar Power - a cutting edge power electronics and monitoring company spun out to commercialize this product line.

Having realized the importance of clean solar panels, Ron set out to determine the best method for cleaning them. A highpressure wash system proved very ineffective and left much of the panel dirty as well as requiring lots of water. A lowpressure water system and soft bristle brushes, requiring thorough manual scrubbing, was very effective but labor intensive. Dissatisfied with both, Ron set off on a worldwide search for methods offering a more economical and effective cleaning solution. This led to the discovery of a patented nano technology material which forms a molecular bond between the material and the glass covering the solar panel. This material fills in the microscopic voids and fissures in the glass, making it smoother, and creates a surface that rejects dust and dirt and does not allow contaminants to adhere to the glass. The panel can be cleaned by lightly washing it with water and does not require the heavy scrubbing with harsh soaps and other cleaning materials.

Accurate Solar Power has developed a panel-monitoring device that measures the voltage, current, and temperature of the panel and wirelessly transmits it to a web-based monitoring system. If any panel drops below a certain performance level, the software sends an alarm indicating a problem. Monitoring of our 400 KW system here at Edison Technology Park could have saved many thousands of dollars in electricity bills as incidences of panel failure, which were only discovered by manually inspecting the panels. These outages were caused by, in one case, a panel being hit by a rock, in a second case by a bullet and in two cases, panels failed due to hot spots burning through the copper traces. We have no way of knowing how long these panels were out of commission, but they could have been down for six to eight months before detection. Not only did we lose the performance of the afflicted panel, but the weak-link effect brought down the performance of several of the connected strings, exacerbating the problem and loss of electricity.

In addition, Accurate's SMART-monitoring device ensures that fire and maintenance crews remain safe when operating around solar systems by allowing panels to be isolated using a remote system with a fail-safe.

The Newdoll comprehensive solar solution

For our current and future solar power customers Newdoll is developing a complete solar optimization solution:

  1. Recommendation of the proper racking structure for the panels to accommodate at least a 10-degree mounting angle to the south, to facilitate self-cleaning during rainfall and robotic cleaning. This helps avoid the maintenance and cleaning problems associated with flat mounted panels.
  2. Application of a patented nano-technology solution to the solar panel glass, providing a non-stick coating to reduce cleaning and an anti-glare layer, which allows more of the light to penetrate the glass and strike the silicon instead of being reflected away.
  3. A robotic watering system installed along the top of the racking system, provides a slow cascade of water to run down the face of the panels at optimal intervals to keep the system operating at maximum efficiency. Water caught at the base of the racking is delivered to a station, filtered and pumped back into the storage tank for the next cleaning cycle. In areas where the water contains calcium and other harsh chemicals that may be harmful to the panel, the water treatment would also include de-ionization. Additional boost in electrical output would be gained by cooling the panels during the heat of the day, as the panels decrease output when exposed to higher temperatures.
  4. Each panel has a system installed that monitors the voltage, current, and temperature (as well as allowing them to be individually shut down for emergencies). This allows you to tell at a glance the output of each panel and when the efficiency drops by as much as 5% the watering cycle is activated to clean the panels. The system is fully robotic, no longer requiring workers on the roof for cleaning, saving labor, water and insurance costs, while ensuring full production out of each solar panel year round.
  5. Ron's goal is to boost the total power output from his 400 KW system by at least 5% and early data is proving that this is feasible.

We have estimated that the additional power realized by proper monitoring and cleaning, and by detecting failed panels, would cover the cost of installation of the monitoring and cleaning systems, with an R.O.I. of two years and increase the total electrical harvest by 5-25% for the life of the system.

If you are experiencing problems similar to those mentioned above, we would be pleased to inspect your system to configure a cleaning/monitoring system that would meet your needs. For further information, please call us.

What is Dirt?

Soiling of solar and solar mirrors - effects and remedies

When a window becomes soiled, the amount of light passing through the glass is reduced. We are generally unaware of this, but are cognizant of streaks, haze, and other optical anomalies due to dirt on the surface. When we clean the window, we are generally surprised by how much clearer the view is and by the sudden brightness of the room.

In the case of a solar panel or a solar mirror, this dirt build up has an economic effect. The amount of energy generated by the solar device is reduced by the dirty build-up on the surface.

This paper will address the phenomena of dirty surfaces (emphasis on glass), quantify the energy reduction due to dirt build up, and show how the new self-cleaning coatings work to minimize this soiling, and simplify the cleaning.

So how does glass (or any solid surface for that matter) get dirty?

First some definitions:
  • Hydrophilic- the tendency of a solid surface to allow wetting or sheering of liquids.
  • Hydrophobic- the tendency of a solid surface to resist wetting, thereby forcing liquids such as water to bead up.
  • Surface tension- the property of a liquid that resists tearing or shearing at the liquid I air interface. This phenomenon causes water to form a convex surface and climb the wall of a glass or tube. It also causes a bead or droplet to form.
  • Surface energy- the property of a solid that attracts liquid to wet. If the surface energy of a solid is greater than the surface tension of a liquid, the liquid will wet or spread out on the surface.
  • Contact angle- the angle formed by water that incompletely wets a surface.
  • Dielectric- broadly speaking, any electrical insulating material. Ordinary glass and all transparent polymers are dielectric materials (they do not conduct electricity, but they can build up electrical charges on their surfaces).
  • Electrostatic (attraction)- the build up of charge on the surface of objects due to contact with other surfaces. Principally concerned with dielectric objects (or surfaces) as opposed to electrical conductors.
  • Photoactive (photo-catalytic)- materials that generate free electrons when exposed to UV light.
  • TCO Coating- A transparent conductive oxide coating; it conducts electricity, is transparent, reflects heat, and is generally very durable.

So, a solid with high surface energy will tend to attract a liquid and cause it to wet. This results in a hydrophile surface with a low contact angle. (Figure A)

A solid with a low surface energy will not be able to overcome the surface tension of the liquid, thereby not allowing it to wet, resulting in a bead of liquid with a high contact angle. This is a hydrophobic surface. (Figure B)

Most transparencies such as glass or plastic are dielectric, which means they will build up static charge on their surface in dry climates. (There are transparent electrical conductive coatings that can be applied to these surfaces, and these will be discussed later).

Back to the soiling:

Soiling on an exposed outdoor surface will be a mix of organic and inorganic solids, some windbome, and some deposited from evaporated liquid (rain water, dew, fog).

In a very dry environment, the bulk of the soiling on glass or polymer materials may be inorganic windbome solids that are electrostatically attracted to the glass or plastic dielectric material. Dry winds can create significant electrical charge, and the dust and dirt particles will be electrostatically attracted to the charge built up on the glass or plastic dielectric surface.

In desert environments, the bulk of this dust will be silica and other inorganic minerals, some of which may be quite abrasive, causing plastic surfaces to scratch and abradc.

Organic deposits can include windbome dirt, bird and other animal droppings, pollution (soot from burning coal or diesel), as well as decomposing organic plant matter from leaves, pollen, etc. When these materials become wet, depending on the pH, there may be greater attractive forces to the surface, and potential corrosion and leaching of ions.

In coastal areas, salts can be deposited from water spray.

The presence of moisture (rain, dew, fog, or spray) can help spread these deposits, and the polar (OH) molecule can contribute to the adhesion forces bonding the deposits to the surface.

So soiling is a complex phenomenon that will vary by climate and location. A surface in Arizona will be subject to electrostatically attracted inorganic materials; a surface in coastal Florida will be subject to dried salts and rain driven dirt; and a surface in Ohio will be subject to organic windblown dirt, deposits from evaporated rain, and atmospheric pollutants from fossil fuels.

Effect of Dirt Build Up on Solar Performance

Photovoltaic solar systems have been well studied over the last 25 years, and the sizing and energy prediction tools have been validated with field studies. One aspect of array performance that was unanticipated in the early models was the effect of dirt, or soiling, on performance. It was found that the output of a photovoltaic solar array could decrease by over 30% over a period of months due to dirt build up on the coverplate. Particularly, in dry climates (favored because of the high percentage of sunshine), where it may not rain for 6 months or more, dirt build up is a significant issue with respect to efficiency and power output.

One widely available design tool from NREL (National Renewable Energy Laboratory), a la601atory of the U.S. Department of Energy (DOE) is "PVWATTS v.1, available free of charge on line at .

This program calculates the output of a solar array anywhere in the USA, using historic climate data and inputted values for the array size, tilt, etc.

The final step when using this program is to calculate the derating when going from the DC output rating of the solar array to the final AC electricity delivered to the load. There is a table of derate factors that have defaults, but the user can change these if actual data is available. The fascinating fact is that the default derating results in only 77 AC watts for every 100 DC watts nameplate rating. There are 11 loss categories, and in rank order soiling is the #I or #2 loss factor, depending on severity of the build up. The default derating for soiling is 0.95 (5% loss), but the range of acceptable values is 0.30 (7W loss) to 0.995 (0.5% loss). No other derating factor even approaches the low end of this range. The DC to AC inverter derate factor is 0.92 (8% loss), whereas all others are greater than 0.95 (5% loss).