Photovoltaic cells can generate electricity without adding greenhouse gases to the atmosphere, but solar power is significantly more expensive than the electricity produced by coal- and gas-fired plants. To boost the competitiveness of solar energy, researchers have striven to make solar cells convert sunlight into electricity more efficiently.

Inspiration may come from the most basic scientific research. Investigators are starting to delve into the intricacies of photosynthesis, which converts sunlight into chemical energy with almost 100 percent efficiency. A group led by Gregory S. Engel, formerly at the University of California, Berkeley, and now at the University of Chicago, cooled a green sulfur bacterium to 77 kelvins (–321 degrees Fahrenheit) and then zapped it with ultrashort pulses from a laser, enabling the tracking of the energy flow through the bacterium’s photosynthetic apparatus.

The researchers found that by using this spectroscopy technique, they could explain how plants efficiently transfer solar energy to molecular reaction centers for conversion into chemical energy. The previous view of photosynthesis postulated that light-harvesting molecules called chromophores absorbed energy from the sun and then transferred it from one such molecule to another along one of various possi­ble routes until reaching a reaction center.

The study found that in contrast to the prevailing notion, energy moves in a wavelike motion along all the pathways in the system at once, a quantum effect that ensures that the energy takes the most efficient route, arriving at its destination almost instantaneously. Eventually this new understanding may become the basis for an artificial photosynthesis process that can be incorporated into the design of more efficient photovoltaic cells.

Other scientists are devising better ways to use sunlight to heat and cool buildings. Steven Van Dessel of the Rensselaer Polytechnic Institute and his colleagues have developed a prototype system called the Active Building Envelope (ABE), which couples solar panels to thermoelectric heat pumps. Electricity produced by the solar cells goes to the heat pumps, which can either heat or cool the building’s interior, depending on the direction of the flow of the current. The research group is now investigating the possibility of creating a transparent ABE system using thin-film photovoltaic cells and thermoelectric materials instead of bulky components. The transparent films could be applied like a glaze to the windows of buildings and to the windshields and sunroofs of cars.
—Mark Alpert