Ask Pablo: Do Solar Panels Actually Contribute to Climate Change?

Dear Pablo: Is it true that the heat absorbed by dark solar panels contributes to climate change?

The Source Of The Myth

This myth recently surfaced in the sequel to Freakanomics, call Superfreakanomics. Some people are very disappointed with the authors, who created quite a stir with their first book. The source of the myth is a quote by Nathan Myhrvold, the former Chief Technology Officer of Microsoft (commenting outside of his expertise):

"The problem with solar cells is that they're black, because they are designed to absorb light from the sun. But only about 12 percent gets turned into electricity, and the rest is reradiated as heat--which contributed to global warming."

With the new Catlin Arctic Survey report showing that the Arctic Sea is likely to be ice-free in summer months as soon as 10 years from now there is renewed urgency to address anthropogenic climate change ahead of the COP15 meeting in Copenhagen later this year. The prospect that solar panels, the main symbol of renewable energy, could be contributing more to the problem than they lessen it would certainly be a shocking revelation.

Reflection and Absorption

In addition to anthropogenic greenhouse gas emissions, which disrupt the earth's energy balance by acting like a blanket around the planet, another contributor to atmospheric warming (and therefore climate change) is the change in albedo of the earth's surface. Albedo is just a fancy word for reflectivity, and the problem of changing reflectivity is most important in the Arctic. The Arctic sea ice acts like a giant mirror, reflecting sunlight back into space. But as the sea ice disappears it exposes the Arctic Ocean, which is much darker, and therefore has a much lower albedo. So, not only is the Arctic sea ice melting caused by climate change, but it is also contributing to it.

What Does All This Have To Do With Solar Panels Contributing To Climate Change?

Photovoltaic panels range from blue to black but they are smooth and have an albedo around 0.3. But it is not the albedo itself that matters, it is the relative change in albedo from the status quo. Since most solar panels are roof-mounted, and most roofs are covered in dark tar paper shingles, covering the roof with solar panels may actually represent a positive change in reflectivity.But what if the panels are mounted on a hypothetical perfectly reflective surface and the solar panels absorb 30% of the solar energy that hits them? The average insolation, or the amount of the sun's energy hitting the earth, is approximately 6 (kWh/m²)/day. This means that, on the average day in the average location, the solar panels would absorb 1.8 kWh per square meter per day. The same solar panel, assuming a 15% efficiency would generate 0.9 kWh of electricity per square meter per day.

So Solar Panels Do Contribute To Climate Change?

Well no, not exactly. Even if solar panels absorb twice as much heat energy as they generate (and keep in mind that we are using very liberal estimates and the actual amount of heat created is much less) this is not the end of the story. Electric generating plants are only about 31% efficient, meaning that 2.9 kWh worth of fuel (almost 10,000 BTU) need to be combusted to generate 0.9 kWh of electricity. So the power plant directly adds at least 1.6 times more heat to the atmosphere than the solar panels. And keep in mind that the numbers for the solar panels are overestimates, while the numbers for the power plant are much more realistic.As if that didn't totally dispel the myth, we haven't even addressed greenhouse gas emissions yet. Naturally solar panels don't generate any greenhouse gas emissions, but coal-fired power plants emit about 2 pounds of carbon dioxide for every kWh. This CO₂ builds up in the atmosphere and continues to have a warming effect for a long time. So, not only do solar panels add less heat to the atmosphere, but they also don't emit any greenhouse gasses.