Small Particulate Dust Cleaning



Potential customers in urban Chinese markets have given resistance to purchasing solar panels due to the perceived negative effect of excessive PM 2.5 and PM 10.0 dust particles in the air and accumulation of these particles on the solar panel. It is the purpose of this paper to discuss the negative effects of small dust particles, such as PM 2.5, on solar panels. Specifically, a cost benefit analysis has been performed to show exactly how frequently solar panels should be cleaned. 

 Solar irradiance is a measure of the rate of energy transmitted from the sun to a given surface. Likewise, solar insolation is a measure of how much energy is transmitted from the sun to a given surface over a certain amount of time. This is typically measured in kilowatt-hours per square meter per day. Special thermal sensing camera systems called pyranometers have been developed to measure the solar irradiance, and pyranometers have been deployed all over the globe. Pyranometers measure solar irradiance every second for a general location and are then averaged into monthly data sets. This measurement is very important in the solar panel industry because it is a theoretical measure of how much energy can be produced by a perfectly efficient solar panel. The energy coming from the sun can be blocked by dust, snow or leaves casting a shadow on the surface. When there is an accumulation of these materials on a solar panel, it is best to remove the obstructions to maximize energy production.

In urban Chinese markets, dust can affect the power production of a solar panel. Dust is characterized by the size of the diameter of the dust grain. If a grain of dust has a nominal diameter of 2.5 micrometers, then it is referred to as PM 2.5. Unlike larger sized dust, PM 2.5 and PM 10.0 tend to stay suspended in the air which reduces visibility and solar insolation. Reduced solar insolation leads to reduced energy production.However, this reduction of the light transmittance from the sun to the solar panel is taken into account during System Design’s site analysis. This is because the solar insolation is measured from the ground as close to the actual site as possible. Insolation and irradiance measurements taken by local meteorological stations already include the reduction of energy due to the dust particles. Thus, UGE’s technical analysis on power production is accurate and inherently accounts for airborne PM 2.5 and PM 10.0.

However, in order to accurately determine the expected annual energy production from a solar array, both the losses from airborne dust and the dust that settles on the solar panel itself must be accounted for. The impact of dust settling on the solar panel, also called soiling, is a function of the time between cleanings of the solar array. The following analysis identifies the optimal time between cleanings. 

It is has been measured that the accumulation rate of PM 10.0 translates to a loss of about 0.089% transmittance per day in an urban environment (Appels, R., et. Al. (2013)). Measurements of loss of transmittance over a multi month period confirm that the rate of loss of transmittance due to dust settling on the panel face is reasonably linear. 

In order to determine the optimal time between cleanings the transmittance information above was analyzed in relation to the following financial data. The cost of energy in China can be approximated to be about $0.10 per kWh, based on actual samplings of Chinese residential, industrial and commercial energy costs. Given the annual average solar insulation for the area in and around Beijing, a solar panel with a 250W nameplate rating produces approximately 1.06kWh per day on average. From all of this, we can come up with an equation (See Figure 1) to determine the cumulative cost of lost energy per day as a result of the setting particles. 

Comparing this loss to the fixed cost of cleaning a solar panel, it can be determined how long it will take before the rising cost of lost energy will overtake the cost of labor to clean the solar panel. This comparison can be seen in the following graph:

Graphically, we can see that after about 1 month, the cumulative lost energy cost is greater than the cost to clean the solar panel. Thus, cleaning the solar panels once a month is the best case scenario. As particles of PM 2.5 are smaller, the voids between them are also smaller resulting in a slightly higher rate of transmittance loss than shown in the analysis above, however we expect most customers will want to maintain a monthly cleaning schedule for practical purposes. 

It’s important to note that the y-axis of this graph is denoting a cost due to either total lost energy or labor. This study was performed using a single solar panel. If there were 1,000 solar panels, the total lost energy and labor costs to clean this solar panel farm would increase, but the break even time would be static. 

Finally, it’s important to note that PM 2.5 and PM 10.0 does not wash away due to rainwater due to the impurities in the rainwater. One should clean the panels using soft tap water or distilled water and a lint-free microfiber cloth to increase dust removal from the solar panel glass and reduce scratching. 

References

Appels, R., et. al. (2013). Effect of Soiling on Photovoltaic Modules. Solar Energy, 283-291.

- Clifford M. Long, Engineer & Scott Van Pelt, Engineering Director