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Collecting sunlight using a rooftop parabolic mirror.

Hybrid solar lighting

by Chris Woodford. Last updated: December 12, 2013.

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: HSL systems collect sunlight with a rooftop mirror dish like this, 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. Photo by Chris Gunn Photography courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What is hybrid solar lighting?

Vertical strip light fed by daylight from a rooftop collector.

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. 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.

Photo: Luminaire: This might look like an electric, fluorescent strip light, but it's fed by daylight piped in from a fiber-optic cable. Photo by Chris Gunn Photography courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

Who invented hybrid solar lighting?

Who came up with this brilliant idea? It was originally 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, Lonnie 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 (only a fraction of the electricity they take in emerges as light), but because they 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?

Simple artwork showing how hybrid solar lighting collects sunlight from the roof and pipes it into your home.

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.

Detailed design of a fiber-optic, hybrid solar lighting collector using two mirrors.

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: Right: 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).

Left: 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.

Advantages and disadvantages

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.

Warehouse lit by a combination of natural daylight and fluorescent artificial light.

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).

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 fluorescent lamps, or LEDs instead.

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Text copyright © Chris Woodford 2010, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2010) Hybrid Solar Lighting. Retrieved from http://www.explainthatstuff.com/hybrid-solar-lighting.html. [Accessed (Insert date here)]

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