Real-Time Monitoring of Soiling Effects on PV Installations

Image Credits: Iakov Filimonov/shutterstock.com

The maximum efficiency displayed by solar cells has been incrementally enhanced by over 150 years of scientific effort. However, while photovoltaic technology keeps on improving, the amount of power that can be produced from solar cells is ultimately governed by environmental factors.

Chiefly, these are soiling, temperature, and solar irradiance. While the effects of temperature and solar irradiance are well-documented, the detrimental effects of soiling on solar panel performance frequently go unnoticed. However, the effects of panel soiling on photovoltaic performance can be measured easier than ever before with novel technologies.

Advances in Increasing Solar Cell Efficiency

When Edmond Becquerel was only 19, he created the world’s first photovoltaic device while performing experiments in his father’s laboratory.¹ Edmond, the son of eminent electro-/bio-chemist Antoine Becquerel, discovered that a platinum electric current coated with a thin layer of silver chloride could be manipulated into displaying a phenomenon previously unknown to science: when placed in an electrolytic solution with another electrode and illuminated, a current began to flow.²

His experiments were scrupulous. After eliminating thermoelectric effects and even managing to acquire a rough spectral response curve by shadowing the electrode with a series of colored filters, he rightly inferred the nature of the chemical reaction that took place at the photosensitive electrode. Becquerel had designed a liquid-phase photoelectric cell, the precursor to the popular (and much more convenient) silicon cell which would go on to become the fastest-growing renewable energy source in the beginning of the 21^st century.³

Photovoltaic cells have greatly progressed since Becquerel’s experiment in 1839. In 1876, scientist William Grylls Adams demonstrated that photovoltaic current could be generated without moving parts or heat transfer, paving the way for modern photovoltaic cells.⁴

Over the next 50 or so years, the photovoltaic effect (or “Becquerel effect”) was demonstrated in several different materials, but efficiency did not budge from around 1% until the 1950s when the power of silicon was found.

In 1954, Bell Labs made headlines with the first “practical” silicon solar cell, which had an efficiency of 6%.⁵ The next day, the front page of the New York Times proudly announced that the “Vast Power of the Sun is Tapped” – these cells indicated the advent of commercially viable solar cells that were compact and efficient.⁶

Since then, the continued endeavors of generations of scientists and engineers have increased the efficiency of solar cells by several folds. On December 2014, the world record for photovoltaic efficiency – the ratio of incident solar radiative energy to usable electrical output – was a remarkable 46%.

Environmental Factors are the Most Important

However, while the theoretical efficiency of photovoltaic cells is meticulously boosted by scientific advances, their true performance in situ depends greatly on environmental factors.

Clearly, variations in solar irradiance are a major concern. Not as evident is the effect of temperature on photovoltaic efficiency: charge-carrier recombination rates increase with increasing temperature, effectively putting electrons out of action in the cell, which lowers the generated voltage⁷. While both of these factors cannot be easily controlled, soling is one major threat to photovoltaic efficiency which can be easily prevented.

Measuring Photovoltaic Soiling in Real-Time

With causes such as plant products, salt, dust, soot, and even bird droppings, soiling can pose a serious threat to the power generation of photovoltaic cells. Soiling caused by dust is especially rampant in the dry areas which receive high sunshine levels – areas which are otherwise most suited for photovoltaic installations – sufficient to make investors reconsider setting up a solar cell array in desert climates.

The effects of soiling are not to be brushed aside. Soiling can result in losses in energy production of more than 10% per week. Accumulated grime and dust negates a considerable amount of each solar cell’s hard-earned capacity.⁸ For photovoltaic facility owners, stabilizing the cost of cleaning cells with losses due to soiling is vital. However, in contrast to relatively well-understood factors such as temperature and solar irradiance, it is difficult to predict the effects of soiling. This gives rise to a question: How can the effect of soiling on a given photovoltaic system be accurately measured?

Instrument manufacturer Kipp & Zonen has answered this question by developing DustIQ: a novel method to optically measure photovoltaic soiling. Other soiling-measurement systems work by comparing the signals from a clean cell and one that is permitted to accumulate dirt (requiring both sunlight and perpetual cleaning to work), whereas DustIQ measures soiling directly without requiring fortuitous weather conditions or maintenance. ^{9 10}

The DustIQ from Kipp & Zonen

The system is compact and designed to be mounted to the side of an existing photovoltaic panel. By means of an internal LED and photodetector, DustIQ analyzes the light reflected by the inside of a transparent window. Greater soiling leads to a higher proportion of reflected light. With the help of this information and considering the effects of different dust types, DustIQ calculates the exact soiling ratio, which can be converted into power losses in real time. DustIQ communicates digitally through Modbus^®, allowing multiple units to integrate into a single system measuring local variations in soiling across larger photovoltaic plants.¹¹

Download the Brochure for More Information

Equipped with up-to-date information on soiling even in the dark, photovoltaic plant operators can schedule cleaning of panels in direct response to actual soiling rates. This enables them to efficiently prevent substantial energy loss, including wastage of unnecessary expenditure on cleaning resources. DustIQ is accurate, user-friendly, and economical, providing the required information to keep photovoltaic installations working at their best.

References and Further Reading

1. Mémoire sur les effets électriques produits sous l’influence des rayons solaires. E. Becquerel. Comptes Rendus 561–567 (1839).
2. Becquerel photovoltaic effect in binary compounds. Williams, R. J. Chem. Phys. 32, 1505–1514 (1960).
3. Renewables 2017. Available at: https://www.iea.org/. (Accessed: 28^th February 2018)
4. Renewable and alternative energy resources : a reference handbook. Smith, Z. A. (Zachary A. & Taylor, K. D. (ABC-CLIO, 2008).
5. APS Physics -This Month in Physics History. (2009). Available at: http://www.aps.org/publications/apsnews/200904/physicshistory.cfm. (Accessed: 28^th February 2018)
6. Vast power of the sun is tapped by battery using sand ingredient. Murray Hill, N. J. New York Times 1 (1954).
7. Temperature Dependent Photovoltaic (PV) Efficiency and Its Effect on PV Production in the World: A Review. Swapnil Dubey & Jatin Narotam Sarvaiya, B. S. Energy Procedia 311–321 (2013).
8. Energy yield loss caused by dust deposition on photovoltaic panels. Sayyah, A., Horenstein, M. N. & Mazumder, M. K. (2014). doi:10.1016/j.solener.2014.05.030
9. Kipp & Zonen - DustIQ. Available at: https://cdn.pes.eu.com/v/20180916/wp-content/uploads/2018/06/PES-S-1-18-Kipp-Zonen-Think-Tank-1.pdf(Accessed: 1^st March 2018)
10. Kipp & Zonen - DustIQ for PV soiling monitoring. Available at: http://www.kippzonen.com/Product/419/DustIQ-Soiling-Monitoring-System. (Accessed: 1^st March 2018)
11. DustIQ the novel soiling monitoring solution for solar panels.

This information has been sourced, reviewed and adapted from materials provided by Kipp & Zonen.

For more information on this source, please visit Kipp & Zonen.