Performance of PV modules in arid areas – effect of soiling and module technology on yearly yield
Test station in South Africa
A large test station consisting of about 30 solar cell panels, some on tracking-systems, and a well-equipped weather station is being build within the fences of Scatec’s 75 MW Kalkbult plant in South Africa. Data and samples from this test stations is analyzed along with production data from Scatec’s Kalkbult plant. In addition to the experimental work, IFE has developed an optical model of a solar module, which is now further adapted to study soiling effects. By the term soiling, we mean dust and dirt that accumulates on top of the solar module glass.
Soiling of solar cells
A solar plant consists of individual solar modules which operates outdoor for at least 25 years, perhaps much longer. The accumulated dust will reduce the amount of light transmitted through the solar module glass and lead to reduced production from the solar cells. Therefore it has been assumed that soiling of the solar modules will give significant efficiency losses if the dust is not removed, particularly in dry areas with little rainfall.
Methods for keeping clean
There is a range of possible solutions to keep the glass clean: manual washing, self-cleaning glass, washing robot, and electrostatic curtain to name a few. Which solution is best suited depends on both the amount and type of dust, the solar cell technology applied in the park, weather and access to water for washing. And cost of course. It is a need for more quantitative information on the subject of soiling and washing, and also great potential for new innovative products designed to match the different environment of solar parks around the globe.
Solar modules at the test station in South Africa
How much dust must accumulate before it becomes a problem? Solar modules can stand unwashed for years without drastic efficiency losses. Losses corresponding to a few percent of the incoming solar irradiation seem to be relatively common. That is why washing has not been the top priority. However, let us put things in perspective. The last 10 years, the cost of silicon solar cells has been cut by incredible 90%. In the same period, the efficiency of the best silicon solar cells that you can by has increased from 20.5% to 22.5%. Significant research resources lie behind every tenth of a percent. When the solar cells are connected into modules and used in the real world, there are several factors that determine how much current the PV system actually produce: The number of sun-hours, their inclination, how the solar panels are connected, and how clean they are.
The picture to the right shows visible dust on the solar cells after long time without rain.
Electricity from the sun to supply 100 000 households
South Africa has great solar resources, large areas of dry climate and a huge need for more and renewable power. Today 70% of the electricity consumption in South Africa is based on coal, and 30% of the population is still without electricity.
To reduce emissions and increase access to electricity, South Africa launched the REIPPPP program in 2011. Scatec Solar has build three solar power plants within this program, and now has another three projects in the pipeline. The three power plants Scatec has already build generates enough electricity to supply approximately 100 000 households.
Analysis and recommendations
The project will give us better analysis of the amount and type of dust that accumulates on the module glass under various weather conditions. It will also give information about how the dust affects the transparency and efficiency of the modules. The amount of dust is extremely dependent on the location of the site. Therefore our goal is not only to study the losses and find solutions for Kalkbult, but also to be able to predict the soiling conditions at other sites, and recommend optimal, place-specific cleaning solutions.
This kind of predictions will again improve the accuracy of PV cost analysis. Even a few percentage differences in net irradiation may be of importance to the economy in large solar power projects. In 2014 alone, almost 50 GW new solar power was installed worldwide.
A large test station consisting of about 30 solar cell panels, some on tracking-systems, and a well-equipped weather station is being build in South Africa
2015-06-02 Josefine Krogh Selj