Understanding the influence of clouds on concentrated solar power

Posted on June 13, 2022

In a year, humanity consumes roughly 500 exajoules of energy at present. That is 500 followed by 18 zeros! To put this in perspective, boiling a kettle of water for a cup of coffee takes roughly a million joules. If you made a cup for every single person in the world (approximately 8 billion) you wouldn’t even make a dent in the total energy consumed. We use a LOT of energy! At the moment, the majority of that energy comes from burning fossil fuels to generate electricity. The energy need has grown in leaps and bounds, but the fossil fuel sources have not. Once they’re gone, they’re gone. It’s not all doom and gloom, however. We have a nigh unlimited source of energy right on our proverbial doorstep: the sun. In a year, the earth absorbs 3.85 YOTTAjoules of solar energy. That’s 3.85 followed by 24 zeros, dwarfing the amount consumed by humanity by quite a margin! If only we could tap into that!

This is where sustainable energy production methods play a key part. Solar photovoltaic panels are one of the most common ways of obtaining clean, sustainable energy from sunlight. For residential scale applications, this is feasible, but what about large, commercial, industrial and city-wide applications? That’s where photovoltaics’ lesser known, but more promising, sibling comes in: Concentrated Solar Power. This relies on the fact that one can concentrate sunlight to increase temperature (think burning your initials into wood with a magnifying glass). The replacement of traditional fossil-fuel powered boilers in power stations by solar concentration is very appealing, and great strides have been made in recent years to bring this to fruition.

As always, with new technology, there are many challenges to overcome. What happens if the sun is obscured by clouds? By how much should one oversize the solar collection field to account for this? Is a concentrated solar power plant even feasible given the natural climate of a geographical area, or should a different location be sought?

All these questions need urgent investigation, and was partly the topic of Dr Wilhelm Johann van den Bergh’s doctoral thesis at the University of Pretoria’s Department of Mechanical and Aeronautical Engineering. This formed part of a larger investigation into harnessing unsteady phase-change heat exchange in high-performance concentrated solar power systems. A collaborative effort between Imperial College of London, the University of Mauritius, University of Lagos and the University of Pretoria was launched to address this, and was funded by the United Kingdom’s Royal Society-FCDO Africa Capacity Building Initiative.

In his work, Dr van den Bergh designed and constructed a laboratory scale boiling facility, attempting to replicate parabolic trough concentrating solar power plants used in industry that boil water using the sun’s rays to produce power. Based on real weather data from the station on top of the Engineering 3 building on the University of Pretoria’s Hatfield Campus, the transient nature of solar radiation due to cloud cover on a typical day was quantified. This information was applied to the constructed experimental facility, and resulted in some interesting findings.

The research team visited an existing concentrating solar power parabolic trough plant to get valuable input into their study

Small, residential scale concentrated solar power systems benefit from a surprising increase in heat transfer at very low water flow rates. This may make these types of systems more attractive than traditional photovoltaics. Under cloudy conditions, the boiling did not follow the expected patterns, and questions were raised on the validity of traditional ways of calculating various factors critical for power stations. As with many doctorates, more questions than answers resulted, with the field primed for enthusiastic researchers to expand the envelope.

This is just one area of active research into solar energy at the University of Pretoria, placing it at the forefront of sustainability efforts.

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