A research team led by Minjoo Larry Lee, associate professor of electrical...

Yale University’s latest contribution to solar power is a hot topic, literally.

A research team led by Minjoo Larry Lee, associate professor of electrical engineering, has been awarded USD$2.5m to develop dual-junction solar cells that can operate efficiently at temperatures of more than around 400ºC.

As well as converting a portion of the sunlight directly into electricity, the intention is to use the CPV technology to transfer the heat to high-temperature fluids used to power a steam turbine and generate electricity.

The project, sponsored by the US Department of Energy’s Advanced Research Projects Agency for Energy (ARPA-E), overcomes two of the big challenges with traditional PV panels: temperature-related performance degradation and intermittency.

Standard PV panels become significantly less efficient as they get hotter by absorbing sunlight, but with this technology the heat will also be captured and used, improving performance.

Specifically, Lee’s panels will be built from materials that can operate efficiently at temperatures far higher than the typical panel, and will integrate with a solar thermal collector that absorbs the unused portion of the light spectrum and converts it into heat.

This can help the technology overcome intermittency.

The thermal fluids can be easily stored so that the heat energy can be dispatched when the sun is not shining or whenever electrical demand rises; this method of storing solar energy is more cost-effective than storing energy in batteries, notes Lee.

To find out more, PV Insider spoke to Lee and also to Dr Paul Sharps, chief scientist for research and development at Emcore Photovoltaics, the main partner in the project alongside the US National Renewable Energy Laboratory (NREL).

Q. Can you provide further details of this initiative? What are its objectives and scope?

Lee: Our project is funded under ARPA-E's programme called Full-spectrum Optimized Conversion and Utilization of Sunlight. One of the main goals is to reduce the cost of photovoltaic energy by adding low-cost storage in the form of heat.

Essentially, we want to hybridise the high efficiency and rapidly declining cost of PV and CPV with the energy storage offered by concentrating solar power systems.

Low-cost storage is very important, since it allows solar energy to be deployed when there is less sunlight or whenever demand is high. In fact, without low-cost storage, ARPA-E predicts that the remarkable growth of PV we've been seeing will eventually halt.

The initiative with NREL and Emcore aims to demonstrate a III-V dual-junction solar cell that can operate efficiently and reliably at the high temperatures characteristic of CSP absorbers.

A number of interesting materials science and device physics challenges arise in solar cells at elevated temperatures, but the potential payoffs are very exciting.

Q. What role is Emcore playing in the project?

Lee: Emcore is playing a wide range of important roles in the project. In addition to their incredible knowledge of CPV cells, we are also lucky to have their expertise with cell reliability.

Furthermore, technology-to-market is extremely important to ARPA-E, and Emcore is an ideal partner as they have a great track record of commercialising innovative compound semiconductor products.

Q. What current problems with CPV do you think this might be able to address?

Lee: The main advantage that the FOCUS programme will offer to CPV is the addition of energy storage.

Q. How does this project fit into your current plans for the CPV sector?

Sharps:While Emcore is no longer directly in the CPV systems business, we still are a large supplier of CPV cells to other companies that make CPV systems.

Q. In general, what are your current CPV target markets and goals for the next 12 months?

Sharps: This may not be the next 12 months, but Emcore would see the same kind of relationship for the cell being developed on the Yale/ARPA-E programme. We would be a cell supplier to companies interested in the hybrid CPV/CSP system that is under consideration.

Q. How do you view the current market opportunity for CPV? What challenges and opportunities are there?

Sharps: Emcore is very interested in commercialising the cell that is developed on the programme, provided that it makes business sense. Commercialisation potential of what is developed in the programme is a big part of what we will be evaluating.