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Climate change

A hotter planet will make solar power less efficient

Photovoltaic cells are notoriously sensitive to temperature. Now a new study reveals how global warming will reduce output across the globe by 2100.

Solar power is a green, renewable energy source and an important way to reduce the planet’s reliance on fossil fuels. The great hope is that this and other renewable sources can help reverse the heating that threatens our climate.

But there is a problem: climate change is likely to have a significant impact on solar power generation, not least because of changes in the amount of incident sunlight in different parts of the world. Some areas will see more sunlight and others less because of changes in cloud cover, atmospheric water content, aerosols, and so on. Most places will see greater variability.  

The exact changes in specific locations are hard to calculate. Which is why scientists, policymakers, energy suppliers, and others are struggling to predict how their areas will be affected.

Today, Ian Peters and Tonio Buonassisi at MIT show why another factor needs to be taken into account: temperature. The output of photovoltaic cells drops as they get hotter. But how this will stack up as the planet warms is poorly understood, which is where Peters and Buonassisi step in.

Photovoltaic cells create current when sunlight transfers energy to electrons in the material. This process also creates positively charged “holes” in the structure that flow in the opposite direction to electrons.

Change in photovoltaic energy output between 2000 and 2100

An important factor in the efficiency of photovoltaic cells is the rate at which electrons recombine with the holes, a process that takes them out of the conduction band and so reduces output. This rate is highly sensitive to temperature: the hotter it is, the higher the rate of recombination.

That’s likely to play a significant role as the planet warms, say Peters and Buonassisi. They calculate that, on average, photovoltaic power output reduces by 0.45% for each degree increase in temperature.

They then study the effect of this change using the temperature change predicted by International Panel on Climate Change. The IPCC has made several predictions based on different estimates of future greenhouse-gas emissions. Peters and Buonassisi use the so-called Representative Concentration Pathway 4.5 scenario, in which emissions peak in 2040 and increase the average global temperature 1.8 degrees Kelvin by 2100.

“As temperatures raise nearly everywhere on the land mass of our planet, energy output is reduced everywhere,” they say. But some areas will be worse off than others. “Areas that are especially affected include the Southern United States, Southern Africa and Central Asia.”

They go on to produce a global map that shows the extent of the change in power output in any area.

Of course, the researchers point out a number of caveats. They say that other factors will also influence the power output from photovoltaic cells. In particular, the amount of incident sunlight will depend on cloud cover and atmospheric water content (i.e., humidity).

And they point out that the -0.45%/K figure they use to calculate the drop in power output is a just a representative number. Breakthroughs in materials science could significantly change it in the future. For example, materials with a higher band gap, such as cadmium telluride, have a significantly smaller drop in efficiency.

Nevertheless, the message is clear. Global warming will reduce the amount of power that photovoltaic cells can produce, in some places by a significant amount. And that will need accounting for when it comes to future energy planning.

Ref: arxiv.org/abs/1908.00622 : The Impact of Global Warming on Silicon PV Energy Yield in 2100

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