Solar Energy Systems - Research - Concentrating Sunlight

Research: Concentrating Sunlight

Although an immense amount of energy is constantly coming from the sun to the earth, all that energy is spread out over a very large area. New possibilities are opened up if we can collect sunlight over a large area and concentrate it down to a much smaller area. For example, the sunlight can be used to produce heat – similar to the way that you can burn a piece of paper by focusing sunlight down to a small spot with a magnifying glass, but on a much grander scale. Alternatively, the concentrated sunlight can be absorbed by a photovoltaic cell, directly generating electricity but requiring a much smaller solar cell than would be needed without concentration.

Concentrated solar thermal systems

Concentrated solar power system

Concentrated solar thermal systems produce heat from sunlight. These systems use large lenses or mirrors to focus direct sunlight hitting a large area onto a small area where it is absorbed. Tracking systems allow the concentrators to follow the sun as it moves through the sky during the day. The absorbed light is most commonly used to boil water and run steam turbines, as in conventional fossil-fuel or nuclear power plants. Ongoing research is looking into using the heat to power chemical reactions, perhaps making it possible to directly generate liquid fuels from sunlight.

A broad range of concentrating technologies exist that vary in the way they track the Sun and focus light. Among the most common approaches is a parabolic trough, which consists of a long, curved reflector that concentrates light onto a tube positioned along the center of the reflector. Light is focused onto the tube, which is filled with a heat-absorbing working fluid. As the reflector tracks the sun, the working fluid flows through the tube, is heated to several hundred degrees Celsius, and is then used as a heat source for a power generation system.

Luminescent solar concentrators

One approach to concentrating sunlight involves a stacked structure where different parts of the solar spectrum are captured over a large area and directed to matched high-efficiency solar cells.

Lenses and mirrors can also be used to focus sunlight onto photovoltaics, but these systems are expensive and only work in direct sunlight. They work well in large-scale solar plants in sunny areas, but a different solution is needed for concentrating sunlight in regions, such as the Chicago area, that see a large number of cloudy and hazy days. Luminescent solar concentrators (LSCs) offer an alternative that may eventually be inexpensive enough for small-scale, distributed generation, such as roof-top systems on single-family homes. These concentrators are typically large slabs filled with a material that absorbs sunlight and then re-emits light with a particular color. The light is absorbed over the large face of the slab, but the emission is directed towards a photovoltaic cell on the small face of the slab, resulting in a concentration effect. By choosing a color of emitted light that can best be absorbed by the photovoltaic material, the total system can efficiently convert sunlight into electricity, while requiring much less photovoltaic material than would otherwise be required. A stack of properly designed LSCs, coupled to appropriate photovoltaic cells, would allow the entire spectrum of sunlight to be absorbed, re-emitted, and then converted into electricity.

March 2013