Innovative Way for Choosing Best Solar Cell for Specific Location and Installation Type

A team of researchers at MIT and elsewhere has come up with a way to figure out the best type of solar panel for a given location and type of installation. (Credit: MIT)

Which one sounds better— a groundbreaking kind that delivers more power for a specific area but is expensive, or a conventional, off-the-shelf kind of solar panel?

Although this question is complicated, a group of researchers from MIT and other institutions has devised a technique to find out the best choice for a specific type and location of installation. The essential point is that for household-scale rooftop systems in comparatively dry locations, highly efficient but more expensive panels would suit better; however, for those in wetter climates or for grid-scale installations, the less efficient, conventional but inexpensive panels will be better.

Although the costs of solar cells have continued to fall, the costs the associated equipment and for installation have remained fairly constant. Therefore, finding out the conventions in planning a new installation has become more complex. However, according to the authors, the new research offers a clear path to evaluate the most suitable technology for a specific project.

The outcomes of the study have been reported in the Nature Energy journal on April 30, 2018, in a paper by MIT graduate student Sarah Sofia, associate professor of mechanical engineering Tonio Buonassisi, research scientist I. Marius Peters, and three others from MIT and from First Solar and Siva Power, solar companies in California.

The research involved the comparison of two fundamental types of solar cells— advanced designs in which two different types (known as tandem cells) are combined to tap more energy from sunlight, and standard designs in which a single type of photovoltaic material is used. For the tandem cells, the scientists also compared distinctive types—one type in which each cell is separately wired, known as four-junction tandem cells; and the other type in which the two cells are connected together in series, known as two-junction tandem cells.

Rather than merely evaluating the amount of power each type can deliver, the researchers investigated all the associated operational and installation costs over time, to generate a measurement known as the levelized cost of electricity (LCOE), a measure that takes into account all the revenues and costs over the service life of the system.

Standard single-junction cells have a maximum efficiency limit of about 30 percent .Tandem cells, using two materials, can have much higher efficiency, above 40 percent. 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.

Sarah Sofia, Graduate Student, MIT

For their investigation, the researchers considered three kinds of environment—humid (Florida), temperate (South Dakota), and arid (Arizona)—since the amount of sunlight reaching the solar cell can be influenced by the amount of water vapor in the air. In each of the above locations, the team compared the two standard types of single-junction solar cells (copper-indium-gallium-selenide, or CIGS; and cadmium telluride, or CdTe) with two different kinds of tandem cells, two-junction or four-junction. Hence, in total, four distinctive technologies were analyzed in every environment. Moreover, they analyzed the way in which the comprehensive LCOE of the installations would be influenced based on whether overall energy costs stay constant or fall over time, as anticipated by various analysts.

The outcomes were quite astonishing. “For residential systems, we showed that the four-terminal tandem system [the most efficient solar cell available] was the best option, regardless of location,” stated Sofia. However, the team found that for utility-scale installations, the cell with the least production costs would be ideal.

According to Sofia, the new outcomes could be crucial for those planning new solar installations. “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.”

However, since utility-scale systems can distribute the costs of the control systems and the installation over several panels, and since space is likely to be less restricted in such installations, “we never saw an opportunity” for the more efficient, expensive cells in these environments. In large arrays, “because the installation costs are so cheap, they just want the cheapest cells [per watt of power],” she stated.

According to Sofia, the research could assist in guiding research priorities in solar technology.

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.

Sarah Sofia, Graduate Student, MIT

The technology developed by the researchers for making the comparisons can be applied to several other comparisons of solar technologies, not only the particular kinds selected for this research, stated Sofia. “For thin-film technologies, this is generalizable,” she stated.

Since the materials analyzed by the researchers for the four-terminal case have already been commercialized, Sofia stated, “if there was a company that had an interest,” affordable, practical four-junction tandem systems for residential applications could prospectively be commercialized very quickly.

This paper breaks new ground because it precisely quantifies the cost of solar energy for different solar-panel technologies, in different climate zones, and for different application scales. As the authors point out, high-efficiency tandem cells, once fully developed, should have the edge in high-installation cost environments such as residential rooftops.

Raffi Garabedian, Chief Technology Officer, First Solar

Jonathan Mailoa at MIT, Dirk Weiss at First Solar Inc., and Billy Stanbery at Siva Power, both companies in Santa Clara, California, were also part of the research team. The study was supported by the National Research Foundation Singapore through the Singapore-MIT Alliance for Research and Technology (SMART), the Bay Area Photovoltaic Consortium, the U.S. Department of Energy, and the National Science Foundation.

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