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. Here's
how it works!
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.
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.
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.
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.
How does hybrid solar lighting work?
On your roof, you have one or more mirror dishes (a bit
bigger than home satellite dishes, roughly 1m or 4ft wide) that collect
sunlight and funnel it through thick, clear plastic fiber-optic rods and then natural light fittings coupled to them. Unlike the hair-thin fiber-optic cables made from glass used in telecommunications systems, the fiber-optic rods and cables used in HSL are more like the fatter optical fibers used in a medical endoscope, and are made with relatively thick acrylic (plastic) cores to transmit sunlight more efficiently.
Artwork: How it works: 1) A parabolic dish ("solar concentrator") on the roof collects sunlight and feeds it into a thick plastic rod and fiber-optic cable. 2) The light bounces down the fiber-optic cable, reflecting off the walls inside. 3) A light fitting ("luminaire") inside your home allows the light to escape and illuminate your room. 4) A photoelectric light sensor monitors the light level. 5) If it gets too dark, the sensor switches on an ordinary electric light (and switches off again automatically when daylight levels increase again).
Just like sunflowers, the dishes swivel to follow the sun
through the sky over the course of the day so they always collect the
maximum light for as long as possible. Each dish can feed light to about ten hybrid light fittings, which are called luminaires and blend the incoming light from the roof with
artificial light. The luminaires are designed to diffuse the light
softly in all directions, providing very even lighting for about 100 square meters
(~1000 square feet) of space.
Artwork: How the collectors work: 1) A large primary mirror (yellow), roughly 1m (3.3ft) wide collects the sunlight and shines it back upward onto a smaller secondary mirror (orange). 2) The small mirror, roughly 20cm (8in) wide, concentrates the sunlight, focusing it onto the fiber-optic rod (blue). 3) The rod, which is about 2.5cm (1 inch) wide and 1m (3.3 ft) long, carries the light down into the building. 4) The entire thing is mounted on moving supports so it can track the sun. This artwork is from the inventors' original patent design, courtesy of US Patent and Trademark Office.
Here's a slightly different illustration of the setup:
Photo: (1) is the mirror, roughly 1.6m in diameter.
(2) is a 12-part secondary reflector, which bounces and concentrates light from the mirror onto
a fiber mount (3) containing 12 or more fiber-optic cables, one of which is highlighted in yellow (4).
The tracking mechanism is shown by (5) and (6). Artwork 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.
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).
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
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
fluorescent lamps and doesn't increase the need for
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.
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),
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.
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