Optics+Photonics 2015: Intense focus on photovoltaics

12 Aug 2015

Commercial and technical aspects of how to maximize returns from PV were under the microscope in San Diego.

by Matthew Peach in San Diego

The growing significance of renewable energy sources to the US energy sector was reflected by the high profile given to this subject at Tuesday’s packed Optics+Photonics Sustainable Energy Plenary session at the San Diego Convention Center.

A quartet of specialist speakers addressed various aspects of the subject ranging from technical presentations on cadmium telluride as an alternative to silicon as a solar cell medium; converting solar energy to hydrogen; upconversion of low- to high-energy photons; and the Sunshot project’s aim to make subsidy-free solar electricity cost competitive with conventional energy sources by 2020.

Cadmium telluride challenging silicon

Dr Wyatt Metzger, of the US National Renewable Energy Lab told the conference that the latest research into cadmium telluride solar cells is targeting 24% efficiency, “which is driving the cost down to less than $0.40/W, and could displace the dominant silicon market share, and reach grid parity,” he said.

“By maximizing photocurrent, CdTe cell efficiency has recently reached 21.5% and surpassed the performance of multicrystalline silicon. There is still headroom to increase performance further by improving hole density, lifetime, and thereby photovoltage.”

Metzger presented a review of the commonest types of PV materials in use and analyzed their pros and cons: “silicon is probably the best-known of these media, and most of it comes from China today; II-V materials such as gallium arsenide offer relatively ultra high efficiency – but also high cost; and II-VI films such as CdTe are now challenging the incumbent silicon.”

He emphasized this point, saying, “We have made cadmium telluride competitive with GaAs. Cadmium telluride can certainly capture the multicrystalline silicon piece of the solar power pie.” However, Metzger reported that global PV shipments by technology in 2014, according to analyst GreenTech Media, were: multicrystalline silicon (66%); monocrystalline silicon (23%); CdTe (5%); CIGS (3%); and amorphous silicon (3%).

Professor Timothy Schmidt, of the University of New South Wales, Australia, gave a technical presentation on the processes and energy benefits of the photochemical upconversion of light for renewable energy. He commented, “Certain molecular compositions are capable of photochemical upconversion, in which lower energy photons are converted to higher energies, sometimes with quantum efficiencies approaching 50%. PUC has been applied to solar cells, increasing the EQE of the devices in the region below the bandgap of the device.”

Sunshot initiative

Dr. Becca Jones-Albertus, Program Manager for Photovoltaics Research & Development in the US Department of Energy’s Solar Energy Technologies Office, oversees $200 million in funding, which is intended to reduce photovoltaic material and process costs, increase module efficiency and improve module reliability, towards and beyond the goals of the SunShot initiative. The Department of Energy’s SunShot Initiative was launched in 2011 to make subsidy-free solar electricity cost competitive with conventional energy sources by 2020.

She told the conference, “Research in reliability plays a major role in realizing these energy efficiency goals. Improving photovoltaic module lifetime and reducing degradation rates increase the lifetime energy output. Increasing confidence in photovoltaic system performance prediction can lower perceived investment risk and thus the cost of capital.

”In 2015, SunShot expects to award over $40 million to impactful reliability research through the SunShot National Laboratory Multiyear Partnership and Physics of Reliability: Evaluating Design Insights for Component Technologies in Solar 2 programs. There are three photovoltaics subprograms that are designed to work together to achieve our PV objectives, which are: improving system reliability and durability; increasing overall efficiency; and minimizing raw material costs.”

Solar hydrogen

The fourth presentation was entitled “Solar Hydrogen: Harvesting Light and Heat from Sun”, given by Dr Dengwei Jing (on behalf of Liejin Guo), International Research Ctr. for Renewable Energy, State Key Lab. of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, China.

Dr Jing’s research group has focused on renewable energy, especially solar hydrogen, for about 20 years. He presented the latest progress in hydrogen production using light and heat. Firstly, “cheap” photoelectrochemical and photocatalytic water splitting, including both nanostructured materials and pilot-scale demonstration in our group for light-driven solar hydrogen (so-called artificial photosynthesis) was introduced.

Then he described in more detail his group’s achievements on “thermal-driven solar hydrogen”, meaning the use of concentrated solar light and its application to biomass/coal gasification in supercritical water for large-scale and low-cost hydrogen production.

About the Author

Matthew Peach is a contributing editor to optics.org.