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- ton Christensen distinguishes between sustaining and disrup- tive technologies, and hybrid solar lighting has all the mark- ings of a disruptive technology. Christiansen asserts disrup- tive technologies are those that bring to market different value propositions than had been available previously. They are often simpler, more convenient, and cheaper. Compared to PVs, HSL is inherently simpler because no energy conver- sions are required and based on two economic analyses briefly described later, it will be considerably less expensive. Like- wise, HSL is a radically different approach to daylighting and provides architects, building owners, lighting designers, and Figure 5 occupants the convenience of flexible, easy to specify and September / October 2003 ELECTRICAL LINE 37 install, “plug-and-play” daylighting for the first time. For architects and building owners, HSL systems provide many new value propositions compared to conventional daylighting approaches. First, archi- tects are not limited by site orientation constraints as they are with many con- ventional daylighting approaches. Sec- ond, roof penetrations are few and small, drastically reducing the potential for leaks. Third, the infrared energy in sunlight is separated from the visible light rather than transmitted into the building where it adds to air condition- ing loads. Fourth, in retrofit markets, HSL can be more readily incorporated into existing building designs and the flexible optical fibres can be rerouted to different locations during space renova- tions. Fifth, HSL is better adapted to buildings with relatively low ceiling heights and interior walls that make tra- ditional daylighting difficult. For example, a single HSL system can dis- tribute sunlight to several rooms in a typical office building. Finally, HSL systems are more easily integrated with daylight harvesting control systems because the natural and electric light sources are located in the same place and emit light in the same direction and distribution pattern throughout the day. HSL offers lighting designers and building occupants other value proposi- tions. First, it provides better access to desirable, full-spectrum sunlight. As Frank Munger, reporting for the Knoxville News Sentinel, put it, after seeing distributed sunlight, “Perhaps the best part of the ORNL system is the qual- ity of the light. Natural light is much whiter than light from fluorescent bulbs, and the difference is obvious when seen side by side in the test facility.” Second, users can control where and how sunlight is used in buildings. The light emerging from the optical fibres about spatial variability, glare and over- subtle variations in lighting intensity can be tightly focused on a work surface illumination. and colour. HSL provides a subtle link for task lighting, directed to walls and A more subtle value proposition for to the variability that is a natural part of ceilings for indirect lighting, or used the building occupant is a sense of con- lighting in the outdoor environment. At with fluorescent lamps in traditional nectedness to the outside environment. dawn and dusk, the colour of distrib- downlighting applications. By moving In offices today, people in enclosed uted sunlight changes slowly for about the collector slightly off-sun, distributed rooms without windows often lose an hour to match the red and orange sunlight can even be dimmed or shut off track of time and have no idea whether hues of sunrise and sunset. At mid-day completely like electric lights. And in it’s sunny or cloudy outside. Lisa Hes- on sunny days, distributed sunlight is HSL systems, sunlight is routed to mul- hong, a nationally recognized daylight- abundant and dwarfs the output from tiple locations and better controlled to ing architect, argues that humans are co-located electric lamps in sunlit eliminate common occupant complaints naturally adapted to and actually prefer rooms. And on partly cloudy days, 38 ELECTRICAL LINE September / October 2003

there is a very subtle colour shift in ambient lighting condi- tions as clouds block sunlight and electric lights automati- cally compensate.

Economic & Market Potential Of Hybrid Solar Lighting Christensen noted in his book that it is impossible to pre- dict with any degree of certainty the market penetration of disruptive technologies like HSL. Nonetheless, an economic and market assessment of HSL was first developed by Antares Group Inc. engineers and economists for the Depart- ment of Energy in 1999. Their analysis was revised in June 2002 to reflect cost and performance estimates associated with initial prototype designs. In 2002, the University of Wisconsin at Madison also began modeling HSL systems. In both cases, the analyses were based strictly on the energy- savings potential of HSL in commercial buildings where lighting consumes a lot of energy and side-lighting through windows is either nonexistent or minimally effective. Elec- tricity displacement costs of from $1 to $2 per watt (peak) were deemed necessary to achieve payback periods of 4 years in the sunbelt. Both organizations suggest this is attainable in out-years as HSL units begin to be manufactured in large quantities, time-of-day electricity pricing proliferates, and renewable energy subsidies consistent with existing green power programs continue. The Antares Group predicts up to 1 million HSL systems could be operational by 2020 saving Jeff Muhs with the large-core optical fibre of the HSL system. ratepayers billions of dollars annually. John Richardson of the Tennessee Valley Authority’s Public Power Institute has suggested a distribution concept that could greatly facilitate the proliferation of HSL technology. Richard- son envisions the marketing of sunlight itself whereby an energy service provider would own and install HSL systems on its customers’buildings. The sunlight would be metered and customers billed on a cents per kilolumen-hour basis for the light delivered, just as electricity is today. Building owners would incur no up-front capital cost and assume no mainte- nance responsibilities. In the event a customer vacates a build- ing, collectors and optical fibres would be removed by the energy-service provider, leaving a fully functional electric lighting system in place. As with green power programs where customer demand outpaces renewable supply, some customers may even be willing to pay a small premium for distributed sunlight because it is derived from a renewable source and pro- vides other, lighting-quality related benefits.

Path To Commercialization There are three primary technical challenges that must be addressed before HSL technology will reach the maturity required for large-scale commercialization. First, the sys- tems must be designed so that they are easy to assemble, align, and calibrate. Second the materials and manufacturing processes for forming and finishing the collector mirrors must be made more affordable and robust. And third, the clarity and robustness of large optical fibre materials must continue to improve while costs continue to fall. The above challenges form the nucleus of a national R&D Duncan Earl, standing on ladder, installs optical fibre into a agenda that is being systematically addressed by members of hybrid luminaire while Jeff Muhs and Regina Parks look on. the Hybrid Lighting Partnership and other organizations. 40 ELECTRICAL LINE September / October 2003

Additional areas of ongoing research include the development of a wide variety of high-efficiency hybrid luminaires, dim- Is sunlight a renewable supply ming control systems that more seamlessly respond to or an energy-efficiency technology? dynamic lighting changes, and two-axis tracking controllers capable of periodic self-calibration. Since lighting is a huge consumer of electricity A Hybrid Solar Lighting Summit is scheduled for October in commercial buildings, common sense suggests 7 – 8, 2003 in Knoxville, TN where R&D needs and com- it may well be time to refocus solar research mercialization strategies will be the primary topics of dis- priorities and devote proportionate resources on cussion. In the coming year, alpha demonstrations are efforts to accelerate the use of sunlight in its planned in Mississippi, Alabama, and California. Other natural form. Roughly 10% of the electricity demonstration opportunities are welcomed as are additional used in the Unites States is used to light R&D and commercialization partners. commercial buildings. Today, almost all solar- Will HSL prove to be a disruptive technology that vastly based green power incentive programs are improves the payoff in solar energy investments while offer- limited to electricity generation options (mainly ing a noticeable improvement in interior lighting? No one PVs). Although it’s hard to imagine a more knows for sure but there is no doubt that HSL confronts con- renewable resource than natural sunlight, HSL ventional wisdom about how best to use sunlight in commer- wouldn’t qualify in traditional green power cial buildings where electric lighting is so prevalent. Com- programs simply because it displaces rather pelling evidence suggests that it may become the preferred than generates electricity. Since utilities are in use of solar energy in these buildings in the not-to-distant the business of selling electrons, it’s easy to future. Considering its unique value propositions for archi- understand why they might be reluctant to tects, lighting designers, building owners, and occupants, the classify a renewable electricity displacement future of this emerging technology looks even brighter. Ω technology like HSL as green power. In the future, green energy advocates must begin Jeff Muhs is Director of Solar Energy R&D at Oak Ridge viewing solar energy from a holistic perspective National Laboratory in Oak Ridge, Tennessee. so restrictions like this are lifted as technologies like HSL become commercially-available. Circle 35 on Reader Service Card