Self-cleaning solar panels – ditching the dirt

Materials World magazine
,
1 Feb 2014

What are self-cleaning materials, and why are they so important to solar panel efficiency? Eoin Redahan reports.

When you hear a material is self-cleaning, it conjures cartoon-like images of materials scrubbing themselves. The reality, unsurprisingly, is more prosaic. When a solar panel is called self-cleaning, it tends to mean that the surface is hydrophobic. Water droplets cannot stick to these surfaces. The spurned droplets then roll off, taking pesky dust particles with them.

In essence, these surfaces aren’t exactly self-cleaning. Rather, they minimise the buildup of dirt and mineral residue that can block the sun from the panel. The importance of this should not be underplayed. In a dusty environment, a single solar cell could lose more than 20% of its conversion efficiency if part of the panel is shaded. Your panel may boast the most awe-inspiring conversion efficiency in the world, but if it doesn’t contain the right coating material and isn’t cleaned regularly, it won’t perform well.

On the flip side, Dr Ranga Pitchumani, Chief Scientist and Director of solar programmes with the US Department of Energy’s SunShot Initiative, based in Washington DC, USA, says that while hydrophobic coatings and integral electrodynamic elements can help keep panels clean, ‘It is important to ensure that the self-cleaning advantages do not come at the expense of the optical properties of high transmission (in PV modules) or high reflectance (in mirrors used to concentrate solar power systems) of the surfaces.’

So what self-cleaning materials protect against the buildup of dirt without undermining solar cell performance?

Ready-salted coating
From his base in dusty Houston, USA, C-Voltaic’s Shay Curran has developed a bespoke nanolayer coating (the materials used are confidential) that combines these properties. According to the researcher, the transparent hydrophobic coating doesn’t interfere with the PV’s absorption spectrum. ‘You spread a bit of water on it and all the dust particles come off. The hydrophobic coating makes it easier to clean, and you can clean with a lot less fluid.’

Curran adds that the coating’s most exciting attribute may not be felt in the UK, or even Houston, but in the Middle East. ‘You can clean it with saltwater,’ he says. ‘If you’re using saltwater on a regular solar panel, it will build up quickly, and there is the risk that you will short out the aluminium from the corrosion. The saltwater can get inside and damage the electronics. With the coating, the aluminium and the inside of the solar panel are protected, and you don’t have salt buildup.’

The ability to clean solar panels with saltwater makes the coating particularly useful in parched, desert climates. He explains, ‘In places such as Kuwait, Qatar and the UAE, there is a huge amount of solar being erected. They don’t have an abundance of fresh water, but they do have an abundance of saltwater. They can use the coating to protect the solar panel with saltwater and save the most valuable commodity, which is fresh water.’

Adopting the lotus position
Another material worth its salt is the lotus leaf. Several self-cleaning pioneers have used its hydrophobic structure as something of an inspiration. Researchers at Oak Ridge National Laboratory (ORNL) in Tennessee, USA, are among those mimicking the leaf’s properties.

Pitchumani, of the aforementioned SunShot Initiative, explains that ORNL’s self-cleaning, optically transparent coatings can be applied to the surfaces of heliostats and curved mirrors in concentrating solar power systems. ‘The coatings are made by modifying high surface-area, using nanostructured silica particles with self-assembled monolayers of low surface energy. They will be transparent over the entire UV visible near-infrared range and will be applicable to surfaces other than glass. The reflector coatings are expected to provide as much as a 90% reduction in mirror cleaning and maintenance costs and about a 20% improvement in the average amount of reflected solar energy.’

This project is one of many involving the SunShot Initiative. Another technology, developed at Boston University, USA, in partnership with Abengoa Solar, headquartered in Seville, Spain, and Sandia National Laboratories in New Mexico, USA, is adopting a different approach to dust prevention with its transparent electrodynamic screen (EDS) for solar concentrators. To make this screen, EDS electrodes based on oxides or silver nanowires are printed onto glass substrates.

Pitchumani says, ‘The EDS uses travelling-wave electric fields to lift and move dust particles across the collector surfaces, including flat and curved mirrors, ultimately removing dust entirely. Laboratory-scale prototype EDS systems have been demonstrated to remove more than 90% of deposited dust in minutes, using a very small fraction of energy produced by the solar collectors. This cleaning is achieved without water, moving parts or manual labour, and the process is scalable.’

Undoubtedly, the breadth of self-cleaning material development is impressive. However, it will be a long time before self-cleaning does exactly what it says, especially when it comes to the solar panels used on domestic roofs. It could be decades before we see average householders sitting idly by as their solar panels wash the dirt from their hydrophobic faces.

Curran notes, ‘People forget that it’s not just efficiency that is important. I’ve heard arguments from people saying solar panels are no good because their efficiency goes down after a couple of years. Then you have to ask them if they’ve ever cleaned their panels. And the answer is generally no.’

 

Cleaning up for the sun
Just how important is solar panel cleaning? Steve Williams, Managing Director of Clean Solar Solutions, in Shropshire, UK, offers his views.

How much efficiency can you gain by cleaning your solar panels?
It’s circumstantial. We regularly see increases of 8–10% after 12 months on residential installations. On farms we often see gains of 20%, and the highest we’ve seen is 48.2%.

Why will rainwater not do the job?
Rainwater won’t do it because the dirt adheres to the panel too much, especially if you’ve got a dry spell and the sun has baked the dirt onto the panels. There’s no way rainwater can shift that kind of dirt. It’s the same principle as a car windscreen. If you leave a car parked for months on end, that windscreen will get really dirty and block out some of the light.

What cleaning fluid do you use?
We tend to use ultrapure water, which is mains water that goes through several stages of purification until it is hospital grade. The water out of our taps comes out at 330 parts per million impure. We take that water all the way down to zero.

Could the development of selfcleaning solar panels place your business at risk?
Not at all. Self-cleaning glass has been around for a long time, but the reality is that it doesn’t self-clean. It has the potential to be self-cleaning provided a panel is mounted at the correct angle, but you need exact weather conditions for that to happen. Solar panels are not mounted at the angle required for that chemical process to happen.

How busy do you anticipate business being in 2014?

Last year we cleaned 53,350 panels. This year, we’ve already booked and confirmed 52,000 panels. I anticipate 75,000, but the number could be as high as 250,000 panels.