Solar Panels: Which Ones are the Best?

A study by US-based MIT researchers has revealed that choosing the best solar cellfor a given location or use is not as simple as it could be.

Their conclusions in assessing the best options for a given location and type of installation were:

For household-scale rooftop systems in relatively dry locations, the more efficient but more costly panels would be better.

For grid-scale installations or for those in wetter climates, the established, less efficient but cheaper panels are better.

While solar cell costs have dropped significantly, installation costs have remained static, so the benefits and drawbacks have become more convoluted.

However, the study points a clear way to estimating the best technology for a project.

The findings, reported in the journal Nature Energy, were outlined in a paper by MIT graduate student Sarah Sofia, associate professor of mechanical engineering Tonio Buonassisi, research scientist I. Marius Peters, and three others at MIT and at First Solar and Siva Power, solar companies in California.

The study compared two solar cells, a single type photovoltaic cell and advanced designs that combine tandem cells to capture more of the energy in sunlight.

For the tandem cells, the researchers compared different varieties: those in which each of the two cells are connected together in series, called two-junction tandem cells, and those where each cell is separately wired, called four-junction tandem cells.

Instead of just looking at the amount of power each kind can deliver, the team analyzed all the associated installation and operational costs over time, to produce a measurement called the levelized cost of electricity (LCOE), a measure that incorporates all the costs and revenues over the lifetime of the system.

“Standard single-junction cells have a maximum efficiency limit of about 30 percent,” Sofia explains, whereas “tandem cells, using two materials, can have much higher efficiency, above 40 percent.”

But while higher efficiency is obviously an advantage in principle, “when you make a tandem, you basically have two solar cells instead of one, so it’s more expensive to manufacture. So, we wanted to see if it’s worth it,” she says.

The team looked at three types of environment — arid (Arizona), temperate (South Dakota), and humid (Florida) — because the amount of water vapor in the air can affect how much sunlight reaches the solar cell.

In each of these locations, they compared the standard two kinds of single-junction solar cells (cadmium telluride, or CdTe, and copper-indium-gallium-selenide, or CIGS) with two different types of tandem cells, two-junction or four-junction.

Thus, a total of four different technologies were studied in each environment. In addition, they studied how the overall LCOE of the installations would be affected depending on whether overall energy prices remain constant or decline over time, as many analysts expect.

“For residential systems, we showed that the four-terminal tandem system [the most efficient solar cell available] was the best option, regardless of location,” Sofia says.

But for utility-scale installations, the cell with the lowest production costs is the best deal, the researchers found.

The new findings could be significant for those planning new solar installations, Sofia says. “For me, showing that a four-terminal tandem cell had a clear opportunity to succeed was not obvious. It really shows the importance of having a high energy yield in a residential system.”

The study could help to guide research priorities in solar technology, Sofia says. “There’s been a lot of work in this field, without asking this first [whether the economics would actually make sense]. We should be asking the question before we do all the work. I hope this can serve as a guide to where research efforts should be focused,” she says.

“The paper breaks new ground because it precisely quantifies the cost of solar energyfor different solar-panel technologies, in different climate zones, and for different application scales,” says Raffi Garabedian, the chief technology officer at First Solar, who was not involved in this research.