– Water Consumption in PV Panel Cleaning –
Studies show water use in PV panel cleaning can be reduced significantly by using Solar Panel Wash™ (SPW) from American Polywater Corporation. SPW lowers water consumption by creating a thin, continuous sheet of water on the PV panel’s protective glass. Less water is needed to wet the panel for cleaning than would be required if water alone were used. Details from PV cleaning field studies that show significant reduction of water use in India and Central America are reviewed in this paper.
When PV panels first became available commercially, panel manufacturers suggested that rainfall alone was sufficient to maintain panel generation efficiency. This worked for installations with enough annual rainfall. In those areas where it was inadequate or where rainfall did not remove persistent contaminants, power generation quickly degraded and return on investment was compromised. To overcome the inefficiencies of the rainwater-only regime, many PV plant operators now use local water supplies and/or treated water to supplement the PV cleaning process. Supplemental water use increases operating costs. They must be managed. There are a number of ways to lower water costs of PV cleaning:
- Reduce or eliminate water treatment
- Recycle wash and rinse water
- Use less water in general for cleaning operations.
Water treatment includes distillation, deionization (DI) or reverse osmosis (RO) processes. Water treatment removes impurities from the water to minimize streaking and spotting of the panel’s protective glass. Recycling can be used to both re-treat panel wash water and to reduce the total amount of water used. Water use reduction through cleaning process changes and the use of compounds that require less water are easy changes to make. The control of incremental costs from these process changes is important to maintaining the economic viability and environmental stewardship of many PV operations. We discuss below how the use of Polywater’s Solar Panel Wash makes this possible.
Water Consumption Tests—American Polywater has quantified water use in a number of PV installations around the world. In all comparisons, American Polywater’s Solar Panel Wash™ (SPW) reduced water use significantly. There are three basic steps in cleaning PV panels: Soaking/cleaning, scrubbing and rinsing. Water is always consumed in the soaking and rinsing steps. When special cleaning equipment is employed, water can also be consumed in the scrubbing step. In the tests described below, water is either cast, e.g., thrown from large plastic cups/pails and/or hosed, or sprayed onto the surface of the protective glass in the soaking and rinsing steps. These comparison tests measured the amount of water used in the soaking and rinsing steps using only water vs those same steps using an H2O/SPW mix in a 25:1 ratio.
India Test——For example, in an H2O consumption test done in India, it was determined that over 10 L of water were used to clean and rinse a panel using only untreated local water. In both soaking/cleaning and rinsing steps, the H2O was cast onto the panels. In comparison, when both soaking and rinse water were sprayed on to the panel, only 230 ml of water/SPW mixture was used. This translates to more than 40 times more water consumed when panels are cleaned with water alone. (See graph below.)
Central American Test—In another comparison test done in Central America, it was estimated that 3.5 L of water were needed for soaking and rinsing each panel when water only was used. (See graph below.) Using the H2O/SPW mix, only 30 mL were needed in the soaking stage. In the rinse stage, where H2O only was used instead of the SPW mixture, another 1.5 L of water were used. This translates into a total of 1.53 L of total H2O used per panel, or a reduction of 56.3% from using water alone for both wash and rinse. It is believed that the reduction could have been greater had the H2O/SPW mixture been used in the rinse stage.
Analysis—As can be seen in the table below, there are significant differences in the amount of water used in these two scenarios. But, in both cases, SPW reduced total water consumption. How and where these reductions were realized are discussed below:
|Location||H2O - Clean|
|H2O - Rinse|
|India - H2O Only||5.0||5.0||10.0|
|India - SPW Mix||0.03||0.2||0.23||97.7%|
|C.A. - H2O Only||1.5||2.0||3.5|
|C.A. - SPW Mix||0.03||1.5 (H2O)||1.53||56.3%|
• Differences in cleaning procedures—Water consumption is most affected when only water is used in both the soaking/cleaning and rinse stages. The surface-tension-reducing properties of SPW help to form a thin, continuous H2O /SPW sheet across the entire panel. This property is called wetting. Because of its high surface tension, water alone will tend to form droplets on the surface of the glass and will not wet completely. More water is required to completely wet the panel when the H2O/SPW mix is not used.
A cleaning procedure change that results in significantly lower water consumption is the use of the H2O/SPW mix in both the cleaning and rinsing steps. More water was used when only H2O was used to rinse in Central America than when the H2O/SPW mixture was used in India. This shows that the cleaning procedure employed can dramatically impact water use.
• H2O Spraying vs. Casting—In both H2O Only cleaning scenarios described above, water was cast onto the panel’s surface. Spraying was used in the case of the application of the H2O/SPW mix in both instances. Spraying resulted in lower water use as a spray pattern covers more of the panel’s surface area with less water. This is evident in the table above. The H2O Rinse column shows that the water used in the Indian SPW Mix-Rinse scenario was 87% lower when sprayed onto the panel than when rinse water was cast onto the panel. Compare the bold data in the table above to see the differences in the Indian and Central American rinsing scenarios.
Water application methods result in different levels of water consumption during PV panel cleaning. Sprayed water in both cleaning and rinsing stages uses significantly less water than when water is cast onto the panel.
• Time of Day—Panel cleaning in the early morning or evening will result in lower water use than when cleaning is done during the heat of day. When the sun is high in the sky and the panels are at their peak power generation capacity, the heat generated by the panels causes wash and rinse water to evaporate quickly. More water is required as a result.
In addition to higher water use, cleaning panels during periods of peak solar irradiance can result in micro-cracks or fissures in the panel’s protective glass. These are caused by the large temperature gradient created between the hot glass surface and the lower temperature water. These micro-cracks not only compromise the protection of the underlying solar cells but also interfere with incoming sunlight, which can reduce overall power generation.
Conclusion—Water consumption in PV panel cleaning operations can be a major operating cost over the lifetime of a solar panel installation. Control of water use is a key element to the economic viability and environmental stewardship of many PV installations. There are a number of strategies that can be used to control water consumption costs. This paper reviewed one of these strategies: water use reduction. Water consumption can be reduced significantly with the use of Solar Panel Wash™ (SPW) from American Polywater Corporation. SPW helps to lower water consumption as it allows for the formation of a thin, continuous sheet of water on the PV panel’s protective glass. Less water is used to wet the panel completely than would be required if water were used alone. SPW is easily sprayed on the glass in both cleaning and rinsing steps of the cleaning process to further reduce water consumption. When panels are cleaned during early morning hours or after dark, further reductions in consumption are possible due to slower water evaporation.