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Letting the Sun Shine in by Jeff Muhs

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Letting the Sun Shine in by Jeff Muhs The secondary mirror and fibre receiver of the hybrid solar lighting collector system. At left is ORNL’s Alex Fischer, director of technology transfer & economic development, and Jeff Muhs (plus his reflection in mirror at right), director of solar energy R&D. Letting The Sun Shine In By Jeff Muhs n emerging technology called hybrid solar lighting is turning them up as clouds move in or the sun sets. As a causing experts to rethink how best to use solar result, HSL is close to an order of magnitude more efficient Aenergy in commercial buildings where lights than the most affordable solar cells today and has many consume a third of the electricity. advantages over conventional daylighting approaches. Imagine a day when newswires report a low-cost solar technology achieving efficiencies an order of magnitude bet- Solar Options For Illuminating Commercial Buildings ter than the most cost-effective solar cells available today. Until just over a hundred years ago, the sun provided light Although it may sound like a distant fantasy, a recent for illuminating the inside of buildings during the day. Even- research effort led by the Department of Energy’s Oak Ridge tually, the cost, convenience, and performance of electric National Laboratory (ORNL) is quickly proving otherwise. lights improved until sunlight was no longer needed. Electric Rather than converting sunlight into electricity, paying the lights revolutionized the way we designed buildings, making price of photovoltaic (PV) inefficiency, Hybrid Solar Light- them minimally dependent on natural daylight. Couple this ing (HSL) uses sunlight directly. Roof-mounted collectors with an ever-growing number of people working indoors, and concentrate sunlight into optical fibres that carry it inside it’s easy to understand why electric lighting now represents buildings to “hybrid” light fixtures that also contain electric the single largest consumer of electricity in commercial build- lamps (see Figure 1). As the two light sources work in tan- ings (around 30% of the electricity in a typical school, store, dem, control systems keep rooms at a constant lighting level or office building) costing businesses billions annually. by dimming the electric lights when the sunlight is bright and Today, there are two commercially-available solar options September / October 2003 ELECTRICAL LINE 35 for lighting the inside of commercial buildings, i.e. conven- tional topside daylighting systems such as skylights that reduce electric lighting use during the day and photovoltaic (PV) cells used to power electric lights. Conventional topside daylighting approaches have enjoyed a resurgence in the past decade, and for good reason. In addition to energy savings, several studies show that shoppers, students, and workers prefer daylight to artificial light. However, several limitations make daylighting inconvenient in a majority of commercial buildings. Daylit buildings are comparatively more costly to design, more constraining in terms of space util- ity, more difficult to reconfigure during space renovations, more difficult to cool during the summer months, more difficult to illuminate evenly, and more likely to develop maintenance problems caused by large roof penetrations. Because HSL uses fewer and smaller roof penetrations and flexible light distribu- tion systems, it eliminates these problems. Figure 1 The attractiveness of converting sunlight into electricity so that it can be used for lighting and end-use appliances has Why Now? been well documented. Unfortunately, PV technology In the early 1980s, researchers in Japan developed a pre- remains costly and complex to implement and suffers from cursor to HSL technology. At the time, tracking the sun accu- gross inefficiencies in the photoelectric energy conversion rately was difficult, expensive and often unreliable. Light dis- process. The conversion efficiency of PV cells is relatively tribution losses in polymer optical fibres were quite high and low in the dominant visible portion of the solar spectrum and different portions of sunlight were attenuated more than oth- is more responsive in the infrared region. Thus, most of the ers, making emerging light look different from natural sun- visible light is converted into heat and wasted. light to observers. And on cloudy days and at night, no meth- When PVs are used to power electric lamps approximately ods of automatically adjusting electric lights were available. 10% of the sunlight is converted into electricity; then less than Over the past two decades, advances in microprocessors half of the resulting electricity is converted back into visible and control algorithms have made tracking the sun a rela- light by electric lamps. The remainder of the electricity is con- tively easy, reliable and inexpensive task. Light losses in low- verted into heat that adds to the cooling load of buildings. The cost polymer optical fibres have dropped by a factor of three, amount of visible light generated in this conversion/reconver- and dimmable electronic ballasts capable of automatically sion process compared to the amount of sunlight incident on adjusting fluorescent lamps are now commonplace. the PV cell is between 1% and 5% depending on the type of electric lamp used. In contrast, preliminary HSL prototypes Initial HSL Feasibility Demo (described later) deliver about 50% of the available sunlight Has Shown Exciting Results into the rooms below providing close to an order of magnitude The development of HSL began in earnest in 1998, when the improvement in end-to-end efficiency (see Figure 2). Hybrid Lighting Partnership was formed. Hardware develop- ment began in 2001 when DOE’s Office of Solar Energy Tech- nologies began supporting the R&D program. In less than 18 months close to 30 organizations including private industry, universities, utilities, and national labs worked together to turn HSL conceptual designs into experimental reality. In September 2002, ORNL installed the first HSL system in a commercial building in Knoxville, TN. The sunlight col- lector (pictured in Figure 3) consists of a parabolic primary mirror with a total collection area of 1 m2 that tracks the sun throughout the day. A segmented secondary mirror reflects the visible portion of the converging sunlight into eight large core (12.6 mm) optical fibres while allowing the ultraviolet and infrared energy to pass harmlessly out of the system. The collector is mounted on a 4-inch pipe through which the eight optical fibres are routed into the building. Future designs will include thermo-PVs that generate electricity using the otherwise wasted infrared (IR) energy. The amount of light transmitted through each fiber is in the range of 5000 - 6000 lumens on a sunny day, which is equivalent to two state-of-the-art 32-W T8 fluorescent Figure 2 lamps. Light is routed to eight separate luminaires that are 36 ELECTRICAL LINE September / October 2003 traditional 2’x 4’light fixtures containing four lamps each. The fixtures were modified to accommodate two 3M side- emitting acrylic diffusers located between fluorescent lamps, as shown in Figure 4. The acrylic rods spatially distribute the sunlight similarly to the collocated fluorescent lamps. Figure 5 shows the inside of ORNL's Hybrid Lighting Lab- oratory with distributed sunlight illuminating the left side of the lab and fluorescent lamps illuminating the right side with the same amount of light. Lab measurements indicate that distributed sunlight is virtually indistinguishable from direct sunlight in terms of colour temperature, colour rendering index, and spectral power distribution. Based on early experiments, HSL already appears to be a viable and practical option for illuminating the top two floors of commercial buildings. Even this early capability - sure to improve rapidly in the coming decade - is applicable to roughly two-thirds of the commercial floor space in the United States. The total electrical power displaced by the proof-of-con- cept prototype is between 500 and 2400 watts per square metre of incident sunlight depending on the type of electric lights being displaced by the distributed sunlight. By adding the energy-savings in cooling load associated with using electric lamps less often and the performance improvements anticipated in a system redesign, the electrical power dis- placed in a commercial prototype is expected to improve con- siderably. Estimates are that between ~7 to 31 m2 of PV pan- Figure 3 els would be required to generate the same amount of elec- tricity as the next generation 1 m2 HSL system will displace. The existing prototype is only about half the size of antici- pated commercial units that will collect ~2 m2 of sunlight and illuminate ~1000 ft2 of floor space in a typical office building. Future designs are being developed in an open-architecture “plug-and-play” format to ensure compatibility with a multi- tude of new fibre optic lamp/luminaire combinations being developed for other remote source lighting applications. Curious Novelty Or Disruptive Technology? Many building energy experts measure the viability of a new technology like HSL solely from the standpoint of its Figure 4 simple payback comparing it with traditional lighting retro- fits and relamp programs having simple paybacks of under 2 years. If energy-savings were the only value proposition con- sidered, HSL could very easily be categorized as just another curious novelty because its projected simple payback is 4 years or more. But in his book, The Innovators Dilemma, Harvard’s Clay-
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