Solar Panel Cleaning Robot Developed Using Building Cleaning Knowhow (1)

Orders received for 25 units through demonstrations across Japan

2015/10/26 16:59
Shinichi Kato, Nikkei BP CleanTech Institute

The output of solar panels can be increased by removing dirt from the panel surfaces using a cleaning robot. Companies and research institutes in Japan and abroad are developing such robots. J.E.T, based in Miyazaki City, started the rental of cleaning robots and cleaning services (Fig. 1).

Fig. 1: The cleaning robot J.E.T rents out. Building maintenance knowhow is incorporated in the robot for solar panel cleaning. (source: Nikkei BP)

The company was established in May 2000 and is financed by Hirofumi Shibata, chairman of building maintenance company Daiichi Building Maintenance in Miyazaki City. The company started to focus on solar panel cleaning and began operations in 2014, taking advantage of the knowhow it obtained from the building cleaning business.

The technologies and knowhow for "cleaning every corner precisely," "efficient cleaning," "limiting water use to a minimum" and "securing water at any location" are incorporated in solar panel cleaning. The cleaning robot was developed jointly with a domestic robot company.

The cleaning robot developed by the company is set on a rail installed on the solar panels. The robot moves on the solar panels guided by the rail and cleans the panels with brushes while spraying water on them.

The rail is attached to two wheels that are in contact with the upper frame of the panels in the uppermost array (row of solar panels) (Fig. 2). The wheels move in a transverse (east/west) direction along the panel frames of the uppermost array.

Fig. 2: Wheels are installed by attaching them on the frame of the panels in the uppermost array. The wheels move in a transverse (east/west) direction along the panel frames. (source: Nikkei BP)

Building cleaning knowledge reflected in brushes

The rail also has 16 wheels that are in contact with the solar panels (Fig. 3). The wheels support the movement of the rail on the panels in a transverse direction. They also distribute the load of the cleaning robot on the panels by attaching four wheels per panel. The total weight of the rail and the robot is approximately 28kg.

Fig. 3: The load is distributed by four wheels per solar panel. The wheels are adjusted so they do not make contact with the upper/lower frames or metal brackets. Pillars are attached to frames of the uppermost panels for adjustment, if needed, as shown in the picture at the bottom. (source: Nikkei BP)

The wheels are adjusted so that they travel without running over the upper/lower frames of the solar panels and metal brackets of the mounting system. The adjustment is required for more accurate contact of the brushes on the panel surfaces and better cleaning performance. The adjustment is also needed because unnecessary pressure will be applied to the panels if the wheels run over the metal brackets.

The cleaning robot moves up and down on the rail in a longitudinal (south/north) direction. It can clean in a range of 3 to 7m in a longitudinal direction, which is equivalent to three to seven arrays of panels laid out horizontally.

The rail moves in a transverse direction without support if there is no gap that exceeds a certain width. For the travel in a transverse direction, the rail can pass over gaps up to 3cm in width with almost no support. If the gap is wider, the rail has to be lifted up to move it to the adjacent panels (Fig. 4).

Fig. 4: The rail can travel over the gap between arrays without support if it is less than about 3cm in width. If the gap is wider, the rail has to be lifted up to move it to the adjacent panels (source: Nikkei BP)

The movement of the cleaning robot and the rail on arrays are controlled by a radio controller. However, it is difficult to use this robot to clean arrays that are tilted by more than about 5° in a transverse direction.

Building cleaning knowhow is concentrated most in the brushes and the driving method of the brushes (Fig. 5). The brushes were designed so that the entire area of panels including the boundary between the frame and the glass can be cleaned properly.

Fig. 5: The knowhow is incorporated in the brushes and the driving method of the brushes. Water and dirt are washed away toward the bottom using a helical brush. The upper picture shows the robot at standby, and the lower picture shows the robot in cleaning operation. (source: Nikkei BP)

Water and dirt are washed away toward the bottom by a helical brush. According to the company, the material, shape, dimensions, turning method and speed of the brush were decided in pursuit of efficient cleaning without damaging the panel.

Does the increase in output outweigh the cost and labor of cleaning?

Solar panels are installed outdoors and are continuously exposed to the natural environment. Solar panel surfaces are dirtied by accumulation of various substances including bird droppings, soil dust, fallen leaves, pollen, yellow dust and volcanic ash, posing the risk of a drop in power generation efficiency.

It is said in Japan, where the rainfall is high, that substances on the panel surfaces are washed away by the rain even if the surfaces are stained. However, if the solar panel installation angle is reduced to increase the number of panels by minimizing the distance between the panels, soil dust are hardened and not washed away by rain and remain on the panels. And such substances are sometimes very difficult to remove. If they remain uncleaned, they could affect the output.

Panels at solar power plants where many panels are installed, such as mega (large-scale) solar power plants, are normally washed manually by using high-pressure washers or brushes (Fig. 6).

Fig. 6: Manual panel cleaning using high-pressure washers and brushes. High labor cost and many hours of hard work are required. (source: J.E.T)

The cleaning requires high labor cost and many hours of hard work. In addition, according to the company, there is a risk of generating (minor) cracks in photovoltaic cells (power generation elements) due to excessive pressure when high-pressure washers are used.

The company explains that it will take four persons four to five days if panels for 1MW output are cleaned manually. The operators handle high-pressure washers while standing or continue to scrub the surfaces with brushes and flowing water. Therefore, the cleaning quality tends to become unreliable when the persons are tired.

Furthermore, issues remain, namely that traces of water drops on panel surfaces have to be wiped off after cleaning and dirt tends to remain unremoved from the boundary between the frame around the panel and the cover glass.

The cleaning robot may solve these issues and improve the cleaning quality. If the increase in income due to the increase in output outweighs the cost and labor needed for manual cleaning, the advantage of using the robot will be clearer.

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