News release: "MIT opens new 'window' on solar energy"
A Q&A by the MIT research team led by Marc A. Baldo, the Esther and Harold E. Edgerton Career Development Associate Professor of Electrical Engineering, on solar concentrators.
What did we do? We demonstrated a large improvement in the performance of low-cost solar concentrators. Our new devices increase the power obtained from solar cells by a factor of over 40 without needing to track the sun. Our results are at least a factor of four better than previous results.1
Why is this important? The sun is an inexhaustible source of clean power. The major impediment to widely deployed solar-power systems has been cost. Unsubsidized solar electricity is over three times as expensive as the average grid prices for electricity derived from conventional energy sources, according to the U.S. Department of Energy. Dramatic cost reductions are needed. Clean, renewable electricity at affordable prices would be an attractive alternative to conventional electricity and the related fossil-fuel dependence, greenhouse-gas emissions and peak-time grid constraints.
What is a solar cell? Solar cells transform sunlight into electricity by using a semiconductor device, typically made of silicon. Solar cells are packaged into solar panels, which can be installed on rooftops or large fields. The solar cells are typically some of the most expensive parts of an installed solar panel.
What is a solar concentrator? Solar concentrators collect light over large areas and focus it onto smaller areas of solar cells. This increases the electrical power obtained from each solar cell. Solar concentrators can reduce the cost of solar power since more electricity is obtained per solar cell, and fewer solar cells are needed. Â
What is wrong with existing solar concentrators? Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain. Solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighboring concentrators.
What is our technology? Our devices are based on a concept from the 1970's that was largely abandoned: the luminescent solar concentrator (LSC). Our version of this device consists of a piece of transparent glass or plastic plate with a thin film of dye molecules deposited on the face and inorganic solar cells attached to the edges. Light is absorbed by the dye coating and reemitted into the glass or plastic for collection by the solar cells.
Why did LSCs fail in the 1970's? Two reasons: the collected light was absorbed before it reached the edges of the glass or plastic plates, and the dyes were unstable.
What precisely did you do to reduce loss of the collected light? We borrowed some ideas from lasers, introducing what is known in lasers as a four-level system. In practice, we added a small concentration of an extra dye that collected all the absorbed light from its surrounding dye molecules. We also introduced a new class of dye molecules, known as molecular phosphors, that are extremely transparent to their own light emission.
What about stability? We tested one of our devices and found that it was stable (to 92 percent of initial performance) for three months. This isn't good enough yet for products but we are confident that the technology developed for organic light emitting devices (OLEDs) in televisions will be portable to this application.
When will these concentrators make it into production? The technology is being further developed for commercialization by Covalent Solar, a company being spun out of MIT by three of its inventors: Michael Currie, Jon Mapel, and Shalom Goffri. The team believes that it could be implemented within three years.
References
1. Currie, M. J., Mapel, J. K., Heidel, T. D., Goffri, S. & Baldo, M. A. High-efficiency Organic Solar Concentrators for Photovoltaics. Science. In Press.
