Hybrid solar lighting is a new type of lighting that transmits sunlight directly into buildings and supplements this with conventional electric lighting when solar light levels are low. Rather than using solar modules, which transform solar energy into electricity, store that electricity, and use it to power lights throughout a building, hybrid solar lighting (HSL) pipes the solar energy directly into the structure using optical fibers. By bypassing this intermediary step, hybrid solar lighting has a much higher efficiency.

 

Traditional solar modules are able to utilize about 15% of the sunlight they receive, while standard light bulbs lose the bulk of their energy in the form of heat. The end result is a total efficiency of only about 2% of the original sunlight. By contrast, HSL systems are able to utilize as much as 50% of the original sunlight.

 

Hybrid solar lighting systems generate much less heat than traditional bulbs (the optical fibers are cold enough to be touched with no danger of burning), saving on cooling costs. They also yield indirect sunlight, meaning the light you receive is full-spectrum rather than the narrow band found in most light bulbs. The infrared radiation in sunlight is directed to a concentrating thermo-photovoltaic cell that efficiently converts it into electricity. The resulting electric power can be used for other purposes in the building.

 

How it works

Hybrid solar lighting technology is being developed and tested, among others, by Oak Ridge National Laboratory (ORNL) in collaboration with the Department of Energy and several industry partners. The ORNL system uses rooftop collectors consisting of four-foot-wide mirrored dishes that track the sun with the help of GPS chips. The collectors focus the sunlight onto 127 optical fibers, bundled into a single chord as wide as a quarter. The fibers are connected to hybrid light fixtures equipped with special diffusing rods to spread out the light in all directions. One collector powers about eight hybrid light fixtures, which are sufficient to illuminate about 10,000 square feet (930 square meters).

 

Hybrid solar lighting systems are hybrids because they also utilize artificial light: on a normal day 20% of the light generated is created artificially. Photo sensors keep the room at a steady lighting level by adjusting the electric lights based on the sunlight available. Because the optical fibers lose light the longer they are, hybrid solar lighting is limited at present to use in rooms with direct roof access.

 

Applications

Hybrid solar lighting will be used initially in industrial and commercial buildings, where lighting can account for a major chunk of the electric bill. Residential uses may be farther down the road, since the cost advantages are not as great. However, many environmentally-conscious homeowners are already beginning to look at HSL as a viable source of lighting.

 

Hybrid solar lighting system in commercial building

 

Researchers are also investigating the use of HSL as a key component in new hybrid solar photobioreactors that sequester carbon via enhanced photosynthetic-based bio-processing at power plants.

 

Other advantages

Research has shown that natural light is important for setting our body's internal rhythms and some doctors also attribute Seasonal Acquired Depression to a lack of full-spectrum natural light. Hence, hybrid solar lighting can be expected to lead to improvements in health and work efficiency over conventional artificial lighting.

 

Surprisingly, too, it has been found that people tend to buy more in a sunlit store. In 1999, the California Board for Energy Efficiency had a study done on the relationship between retail sales and the presence of skylights. The results showed a 40% increase in sales due to natural lighting. The sales boost may be attributable to a feeling, expressed by some shoppers, that full spectrum light makes the store seem cleaner and more spacious. Knowing that this 40% is a strong incentive, a spin-off company, called Sunlight Direct, has been set up by a group of ORNL researchers to market HSL to retailers. Sunlight Direct plans a project launch in 2007.

 

Cost

Currently a full HSL setup costs about $40,000 for 10,000 sq. ft.. However, advancing technology and switching to plastic optical fibers should make it possible for prices to drop more than an order of magnitude to $3,000 for 10,000 sq. ft. within a few years. At this level, a building owner in a sun-rich location such as Hawaii could pay for implementing the new technology in 2-3 years, with the savings on electricity bills. In other areas that receive less average detail sunlight, the payback would take longer.