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A cloudy

PV?future for

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Could Europe’s solar dream be derailed by

something as elemental as... the weather?

Weather Lead.indd 20 15/08/2011 15:55

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Some believe, following some of the harshest winter weather on record, that the Earth is entering a sustained period of low temperatures. If true, what effect will the gathering clouds have for PV? PES investigates…

In 2010, northern Europe suffered its coldest winter in over 300 years. Now, some researchers and scientists are predicting an increasingly cold climate for the next 20 to 30 years. The theory predicting up to three decades of colder weather is based on the idea of ‘global cooling’; a theory as controversial with those who believe the planet is warming through man’s intervention as global warming is with climate change sceptics.

Much of the thinking involved with global cooling is centred on research that has found low solar activity – marked by a decrease in the sun’s magnetic field – influences the weather conditions across northern Europe. The link between weaker solar activity and cold winters was made after experts found similarities between early weather records and data collected in 2010.

“Last year, winter in the UK, for example, was the fourteenth coldest in the last 160 years and yet the global average temperature for the same period has been the fifth highest,” said Professor Lockwood, a space physicist at the University of Reading’s department of meteorology. “We have discovered that this kind of anomaly is significantly more common when solar activity is low,” he added. “We can expect an increased number of cold winters.”

A paper published online last year in the Journal of Geophysical Research, Atmospheres lends credence to the cosmoclimatology theory that increases in Galactic Cosmic Ray counts (GCRs) have a statistically significant correlation to increased cloud formation.

“Scientists are discovering that the level of solar activity determines how many cosmic rays strike our atmosphere, which then determines the extent of cloud coverage”

What this means is during solar minimum (as we are experiencing) there are a lot more cosmic rays which means a lot more clouds which means less heat hitting the earth and the earth cooling.

Of course, if Europe, and indeed the world were to fall into a sustained period of colder weather, a marked increase in cloud cover could have significant impact on the PV industry in Europe.

Despite the global financial crisis, the PV industry is currently still in growth and operating in a relatively strong market. Though the cost of implementing PV power plants is still high, it is expected to drop as the technology develops further. However, as utilities get more serious about incorporating large amounts of photovoltaics (PVs) into the grid, they have some reservations about how cloud cover and weather will affect PVs.

Both large-scale and small-scale PV arrays are affected by the variability of solar insolation, which is the amount of solar radiation experienced by a surface over a given period of time.

Solar insolation and solar irradiance are often used interchangeably to describe the amount of solar radiation power received by a photovoltaic array. While smaller PV generation’s power tends to follow the fluctuations in solar insolation more closely, larger PV arrays tend to have more of an inherent time delay and a smooth, more

gradual power ramp for a given change in solar irradiance.

Regardless of where you stand on the issue of global cooling or warming, the issue of cloud cover affecting PV efficiency is still a concern for the utilities companies and their investors.

Recently, studies to determine the effects of shifting cloud over PV arrays has been conducted and is ongoing, with the hope that clear, definitive research will help better prepare the industry for the potential effects of variable weather – information that could be invaluable in Europe.

At the end of September 2010, Sandia National Laboratories in the US announced that it would be working with SunPower Corporation to study the impact of shifting cloud cover on large-scale solar photovoltaic plants, using the 1.5MW La Ola Solar Farm on the island of Lana’i, Hawaii as a site on which to base its tests.

Sandia says that it has developed new systems to monitor weather movements so as to prepare for fluctuations in PV output. Sandia and SunPower will jointly publish the findings from the study over the next two years. “Currently, a utility company that wants to build a large solar PV power plant might have a lot of questions about the plant’s output and variability at a proposed site.” Work being done at the La Ola plant is leading to new methods that eventually can be used to answer these questions,” said Sandia researcher Josh Stein. “These techniques will allow a developer to place a sensor network at a proposed site, make measurements for a period of time and use that to predict plant output variability.”

Sandia notes that this information is particularly important as more large PV plants come online, to allow for utility companies to prepare to turn on and off backup generation.

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Given the potential for increased cloud cover and lower temperatures there is also a concern, as we’ve seen in the last couple of years that winter months will bring with them increased snowfall – another potential implication for PV arrays.

From small scale residential to large multi-MW ventures, developers are interested in the effect of snow on the performance of different types of panels and on the optimization of racking angles. There have been very few comprehensive studies performed which attempt to quantify the effects of snowfall, and none which provide universally applicable estimations of snow related losses.

Globally, this kind of research could be incredibly useful, given that in 2010 74 per cent of all installed PV resources were in countries which experience some amount of snowfall, with the top four countries for PV installations being Germany, Italy, USA, and Japan.

“There have been very few comprehensive studies performed which attempt to quantify

the effects of snowfall, and none which provide universally applicable estimations of snow-related losses,” explained Adegboyega Babasola, lead researcher at St. Lawrence College’s Sustainable Energy Applied Research Centre.

“The first goal of our study is to attempt to gain a better understanding of the effects, and to provide some recommendations for reducing these losses” said Babasola.

Snow or ice cover “shades” cells in the array and thus limits their ability to generate electricity. The electrical performance of a uniformly covered array will be linearly related to the level of solar radiation penetrating the snow cover. However, the electrical performance of a non-uniformly covered array will be nonlinearly related to the average level of solar radiation penetrating the snow cover.

In a series string of cells, the current passing through the cells is limited by the most severely shaded cell. The less-shaded cells force this cell to operate at a current above

its short circuit current, and thus the cell becomes reverse-biased, acts like a resistive component, and dissipates electrical energy as heat.

Though research is ongoing, another preliminary study of how snow affects panel performance has revealed some positive data. “Snow effect increases albedo, which had a significant (positive) effect on the performance of panels,”

Albedo is the reflective power of a surface, and as snow reflects light, the environment in which the PV array operates can be conducive to the performance of a panel. “While snow on the panels reduced their power output, snow on the ground increased albedo, and thus increased solar irradiation onto the panels,” said Babasola, adding that measurements varied by panel angle and style.

Whether harsh winters experienced in the last few years are an anomaly or a signal marking the beginning of a sustained period of worsening weather, there is positive news for the PV industry that suggests that gloom does not necessarily mean doom.

Researchers at the Lawrence Berkeley National Laboratory recently issued “Implications of Wide-Area Geographic Diversity for Short-Term Variability of Solar Power,” a study published in early September 2010 refuting findings of earlier studies, which said that PV plants could be greatly affected by short-term events—like cloud cover, potentially limiting their practicality as a large-scale power generating source.

The new study, which measured PV array output in Oklahoma and Kansas concludes that the geographic diversity of large and/or distributed solar generating sites are “are not substantially different from the costs for managing the short-term variability of similarly sited wind in this region.” in other words, it shows that the effect of cloud cover and other temporary events didn’t have as great an effect on PV arrays as formerly thought.

Research is still being done to better understand the variability and uncertainty of PV power generation. System operators and planners need to know what sort of variability and uncertainty are associated with PV plants. Such knowledge would allow them to better manage a PV connected power grid. Moreover, the effects of such PV variations and uncertainties on power system performance must also be studied so as to develop solutions for remediating unfavourable situations. Understanding PV power fluctuations will allow for the creation of enforceable reliability standards. Conversely, while standards provide recommended limits for reliable operating conditions, they do not, however, dictate how to meet such requirements.

“System operators and planners need to know what sort of variability and uncertainty are associated with PV plants”

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PES insight: global dimming decodedWe are all seeing rather less of the sun, according to scientists who have been looking at five decades of sunlight measurements. They have reached the disturbing conclusion that the amount of solar energy reaching the Earth’s surface has been gradually falling.

The effect was first spotted by Gerry Stanhill, an English scientist working in Israel.

Comparing Israeli sunlight records from the 1950s with current ones, Dr Stanhill was astonished to find a large fall in solar radiation.

“There was a staggering 22 per cent drop in the sunlight, and that really amazed me.” Intrigued, he searched records from all around the world, and found the same story almost everywhere he looked. Sunlight was falling by 10 per cent over the USA, nearly 30 per cent in parts of the former Soviet Union, and even by 16 per cent in parts of the British Isles.

Although the effect varied greatly from place to place, overall the decline amounted to one to two per cent globally every decade between the 1950s and the 1990s.

Dr Stanhill called it “global dimming”, but his research, published in 2001, met a sceptical response from other scientists.

It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.

Global dimming is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide - the principal greenhouse gas responsible for global warming - but also tiny

airborne particles of soot, ash, sulphur compounds and other pollutants.

This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds.

Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the Sun’s rays back into space.

There is a belief among some researchers that the cooling effects of global dimming have had a positive effect, acting as a counter balance to global warming “Global cooling may explain why the world has not already warmed more strongly - the cooling effect of particle pollution has been offsetting the warming from carbon dioxide” says David Sington, who produced a BBC film covering the subject in 2006.

Clearly the issue of global dimming could have implications for the PV industry; less sunlight reaching the Earth could lower the energy production capabilities of any installation, whether industrial or domestic.

As the industry strives for ever-greater levels of efficiency from their products, the concern that photovoltaics could be operating in declining conditions and negating - by whatever extent - developments in technology could become an issue the industry needs to address.

Of course, there is a strong argument that the quicker we develop and implement PV and other sources of renewable energy production, the lesser the effect of global dimming as fewer pollutants reach the atmosphere. It is unknown however - should the theory that global dimming is counteracting warming be proved correct - what the implications are for the temperature of the planet in the future.

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