Hybrid solar lighting

by Chris Woodford. Last updated: April 27, 2023.

In the dark depths of winter, there's nothing like a sunny day to set your spirits soaring. Trees and flowers reach for the sky, cats bask in the warm sunlight, and even humans take time to sit in gardens or stroll along the beach. The message is clear: everything living loves natural light! So wouldn't it be great if we could use more of it in our buildings instead of the sterile, artificial electric light we usually have to put up with? Windows and skylights are great for this, but they can't always get light deep into large commercial buildings such as stores and warehouses, and they may not give enough lighting on dull days. A relatively new idea called hybrid solar lighting (HSL) aims to solve these problems by harvesting natural light and piping it right to the heart of your home. Here's how it works!

Photo: Collecting sunlight with a rooftop heliostat (solar mirror). Photo by Warren Gretz courtesy of National Renewable Energy Laboratory (NREL) (photo id #16847).

What is hybrid solar lighting?

Photo: Luminaire: This might look like an electric, fluorescent strip light, but it's fed by daylight piped in from a fiber-optic cable.

HSL is an automated system that lights a room using a combination of artificial light (usually from energy-saving fluorescent lamps) and daylight piped in from the roof along fiber-optic cables. HSL systems collect sunlight with a rooftop mirror dish, which tracks the Sun as it moves across the sky. A fiber-optic cable in the center takes the light collected by the dish into the building. If it's very sunny outside, the majority of the light (maybe three quarters or more) comes from daylight; when the Sun is obscured, photocells in the room detect the lower light level and increase the artificial light to compensate (providing 90 percent or more of the room lighting). Overall, on sunny days, HSL might reduce your need for artificial lighting by 50 percent.

Who invented hybrid solar lighting?

People have been trying to figure out better ways of getting sunlight into buildings for hundreds of years, though systems like the one I've described here essentially date back to the arrival of fiber optics in the 1950s. In a 2019 review of the subject, Dr Mohammed Mayhoub of Al-Azhar University traces the idea of "building core sunlight systems" (BCSS)—ones that pipe light into buildings—back to the heliostat, a sun-collecting mirror developed in the 18th century by Willem Jacob 's Gravesande (1688–1742) and others. [1] Modern systems that use fiber optics to catch and "refine" daylight for use inside buildings were first mooted in the 1970s. One notable version, The Japanese company Himawari solar lighting system, was launched in 1978 and is still available today. With a distinctive eye-shaped dome made of acrylic plastic, it uses either 12 or 36 lenses to catch and funnel sunlight into 2 or 6 fiber-optic cables. [2] Other proposed systems have used mirrors to split sunlight into visible and infrared components, with the former piped into the building through fiber-optic cables and the latter turned into useful electricity by photovoltaic solar cells. [3]

Who came up with the idea of HSL? It was first proposed in the mid-1990s and more fully developed between about 2000 and 2007 at Oak Ridge National Laboratory (the US-government-sponsored energy lab) by a team that included Jeffrey Muhs, Dennis Earl, David Bashears, L. Curt Maxey, John Jordan, and Randall Lind. According to the patent they filed for this invention, they were trying to save energy wasted in daytime electric lighting, which "represents the single largest consumer of electricity in commercial buildings." Artificial lights are inefficient not just in the way they produce light from electricity (even energy-saving lamps, such as CFLs and LEDs, waste some energy), but because they often squander the light they make with shades, fittings, dimmers, diffusers, and so on. While recognizing the advantages of using natural light, the inventors also understood the drawbacks: the Sun's energy carries heat, as well as light, and it's important to separate the two if you want to make a building light without making it hot at the same time.

Advantages and disadvantages

Photo: Saving energy the easy way: this environmentally friendly ProLogis warehouse in Poland is lit by a combination of natural daylight and fluorescent artificial light. Photo courtesy of ProLogis and US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

Pros

On a bright sunny day, HSL can dramatically reduce the need for artificial lighting in a building, offering considerable energy savings. That's good news for the environment and one more positive step to reducing climate change. Another good feature is that HSL delivers the visible light energy from sunlight but blocks the ultraviolet and infrared (the heat, in other words), so it doesn't warm a building as much as incandescent or fluorescent lamps and doesn't increase the need for air-conditioning or refrigeration (in grocery stores, for example). Stores have discovered that customers much prefer daylight to artificial lighting and will spend more in naturally lit buildings, so an investment in HSL can bring considerable sales benefits as well as energy savings.

Cons

The main drawback of HSL is that fiber-optic cables absorb light as it travels down them, so they can carry light energy only for limited distances without amplification. In practice, this means HSL is best used in rooms close to roof level: on the top-floor of a building or in something like a one-story warehouse or grocery store. The technology can bring great savings in places like superstores, schools, public buildings, or offices that use huge amounts of artificial light during daylight hours. In homes, where people use far less artificial lighting in the daytime, energy savings are unlikely to recover the extra cost of the HSL equipment; if you're looking into using HSL at home, you'll probably find it makes more sense to invest in skylights, energy-saving lamps (CFLs or LEDs), instead.

Photo: Best of both worlds: This fitting combines a large natural light diffuser, fed by a fiber-optic cable, with some traditional fluorescent tube lights for night-time and dull days. From US patent #6,603,069: Adaptive, full-spectrum solar energy system by Jeffrey D. Muhs et al, courtesy of US Patent and Trademark Office.

Find out more

On this website

• Energy-saving fluorescent lamps
• Fresnel lens
• Heat insulation
• Incandescent lamps
• Light (general introduction to the science of light energy)

Books

• "Chapter 5: Flexible Sunlight—The History and Progress of Hybrid Solar Lighting" by L. Curt Maxey in Emerging Environmental Technologies: Volume 1 by Vishal Shah (ed), Springer, 2008, pp.83–104.

Articles

• Hybrid solar lighting earns national technology transfer award: ORNL News, June 1, 2007. Read how hybrid solar was developed by a team from Oak Ridge National Laboratory.

Videos

• Hybrid solar lighting: A quick three-minute tour of a real installation (an archived news report).

Patents

• US patent #7,231,128: Hybrid solar lighting systems and components by Jeffrey D. Muhs et al, granted August 5, 2003. The main patent covering HSL, this gives a very detailed description of how the system works and how it performs under various lighting conditions.
• US patent #6,603,069: Adaptive, full-spectrum solar energy system by Jeffrey D. Muhs et al. An earlier description of the same invention, granted August 5, 2003.

1. ↑   Fifty years of building core sunlighting systems—Eight lessons learned by M.S. Mayhoub, Solar Energy, Volume 184, 15 May 2019, Pages 440–453.
2. ↑   [PDF] Himowari Solar Lighting System, Himowari/La Foret Engineering Co. Ltd.
3. ↑   "Chapter 5: Flexible Sunlight—The History and Progress of Hybrid Solar Lighting" of Emerging Environmental Technologies: Volume 1 by Vishal Shah (ed), Springer, 2008, p87.