Today's solar street LED lights are able to provide reliable, quality lighting both in developing and developed countries, thereby reducing light poverty and the economic and environmental costs of electric outdoor lighting. Rapid technical innovation and dramatic price reduction in the LED, PV module, and battery components, which has occurred in the last 5 years, will accelerate the penetration of solar street LED lights across the world. Applications will not be limited to countries with significant insolation only but will extend to Northern regions as well. This study provides a critical overview of a technology that will play an important role en route to global sustainability. For further resources related to this article, please visit the WIREs website.
Solar street lighting based on photovoltaic(PV) electricity accumulated in digitally con-trolled, reliable batteries and used at night to powerhighly efﬁcient light-emitting diode (LED) light sources is an advanced renewable energy technology increasingly used across the world to eradicate light poverty in developing countries and to provide inex-pensive quality lighting in afﬂuent areas of the world.
1 Dramatic cost reduction coupled with technical hardware and software advances in all main compo-nents (PV module, LED lamp, battery, and micro-controller) has increased the adoption of these stand-alone systems, resolving historic reliability problemssuch as in the case of the 450 poles installed in south-ern Sicily in 2005 that went out of work a fewmonths after installation.
2 Today’s solar street lighting is able to bring low-cost and environmentally friendly lighting to populations with no access to electricity, such as inthe case of several hundred solar streets installed in 2010 in Conakry, Guinea, for years providing the only lighting in town.
3 Contrary to expectation (see below), the technology today can also be successfully used in Northern regions, providing ﬁnancial relief to municipalities. Furthermore, glare-free LED lighting presents signiﬁcant opportunities to mitigate sky glow and the ecological impacts of light pollution. 4 This study intends to show the relevance of this critically important technology in the context of global efforts aimed to achieve a more sustainable development. Following an overview on the main technology trends, we provide economic arguments based on recent results achieved in different regions of the world with different solutions that demon-strate how solar lighting systems have evolved to become affordable, versatile, and reliable, pointing to a forthcoming boom in demand and utilization.
OWARDS INTEGRATED, SMARTSOLAR LIGHTSThe working principle of solar LED street lighting is simple. Solar cells, either on top of the pole or verti-cally integrated into the mast, generate electricity during the day. Electricity is accumulated in a battery through a charge controller, and when the sun hasset, it is used to power highly efﬁcient LED sources of white light with a small (typically 350 mA) direct current. It is emphasized here that the current output that powers the LED lamp comes from the charge controller (Scheme 1) and not from the battery so asto avoid its unsafe and undesirable deep discharge.Certain manufacturers opt for burying the bat-tery underground, whereas others leave the battery on the top or at the bottom of the pole (mast).Others, ﬁnally, elegantly insert the battery within the pole, a technical solution suitable for light, energy dense Li-ion batteries.Solar street lights were initially used in remote locations and disaster-prone areas, with the poor lumi-nous efﬁciency of incandescent or ﬂuorescent lampsforcing the industry to over-size the solar lighting sys-tem, especially the solar module and the battery, to facethe longest nights and cloudiest weather of winter.
5 This lack of balance translated into wasting 50–60% of the energy generated throughout the year, high costs due to over-sized solar modules and battery, and poor appearance, conﬁning the market for solar lighting to areas closer to the equator with the highest average levels of solar irradiation and temperatures that did not affect the performance of the incandescent or ﬂuorescent lamps.All this has changed in the last decade with the widespread diffusion of highly efﬁcient solid-state LED lights. Today’s reliable solar LED luminaires offer a documented long lifetime of at least 15 years(60,000 working hours, namely at least three times longer than conventional lighting discharge technolo-gies) and are currently used to light a broad set of outdoor environments:
-Pedestrian and bike paths
-Gardens and parks
The advantages of this off-grid solar technology are clear:
(1) no monthly electric bill;
(2) no risk of elec-tric shock thanks to the inherently safe 12/24 Voltcircuit;
(3) long-lasting and consistent high performance due to digitally controlled battery and LED;
(4) high-quality, white, ﬂicker-free light emitted byLED chips of high luminous efﬁcacy (>130 lm/W)equipped with advanced optics for uniform light dis-tribution;
(5) no permitting, line voltage, trenching,wiring, transformer, and meter costs; and
(6) lightingremains on in case of power outage.
To understand the scope of the innovation in this ﬁeld, it is enough to compare a solar lighting sys-tem commercialized in the late 1980s with a typicalstate-of-the-art system installed today.In 1988, villagers in Indonesia used in candes-cent bulbs coupled to a lead-acid battery powered, in turn, by an 11.1% efﬁcient polycrystalline Si module(80 W).6After a decade, out of the 15 street lighting systems initially installed, nine were still working,with three having been converted to solar home sys-tems and three out of order. Yet, pointing to the rele-vance of street lighting, all the villagers considered the solar street lights to be ‘very useful and necessary in the village.’Today’s state-of-the-art solar light systems use PV modules of no less than 16% efﬁciency, equipped with one or more bypass diodes for optimal production on cloudy days, multichip power LEDs of noless than 100 lm/W luminous efﬁcacy, Li-ion batteryseven times more durable than lead-acid batteries(see below), and digital micro-controllers for char-ging the battery and driving the LED light, resultingin solar street lights fully capable to meet demandinginternational lighting standards.Furthermore, today’s systems often have a wire-less monitoring system to remotely monitor and man-age each light (operating schedule, self-regulatingdimming, power metering, performance monitoring).In further detail, the latest design uses an inte-grated systems design approach, extracting the bestperformance from each component. Today’s state-of-the-art charge controllers (or charge regulators) uti-lize a maximum peak power tracking (MPPT) chargerthat ensures maximum electricity production and
efﬁcient battery charging (compared to a basic charger, an MPPT charger transfers ~30% more charge from the module to the battery by increasing the charging current to the battery). A typical battery bank charge algorithm presents three different modes:7•If the PV module available current is lower than capacity (in Ah), the converter will track the maximum power point (MPPT) in order to provide the highest current possible for the battery bank.•If the PV module available current is higher than 0.1 capacity (in Ah), the converter limits the current to 0.1 CAh and disables the search for the MPPT.•If the batteries are already charged, the controlalgorithm will apply a constant voltage level in order to keep the batteries (undergoing self-dis-charge) fully charged.Newly developed digital micro-controllers, such as the SO-Bright introduced by Azzam in 2010, 8 feature a control algorithm that, besides maximizing the energy going into the battery, reacts to the storage capacity by comparing future energy needs with available storage, making incremental adjustments to the lighting proﬁle to balance them.As mentioned above, the poor performance of conventional lead-acid rechargeable batteries, with their high weight, short lifetime (3 years), and limited(50%) depth of discharge (DoD), coupled to faster than forecast cost decline of Li-ion batteries (about 14% a year between 2007 and 2014) 9 is leading to the widespread commercialization of solar streetlights directly equipped with Li-ion batteries.The initial higher upfront cost of these batteries is rapidly recovered due to a far longer lifespan and performance of the Li-ion technology, which is also much cleaner as well as safer for the environment.For example, a lithium titanate battery works well at temperatures between −20C and 55C, has anexpected lifetime of 20 years, with 15,000 chargecycles (1200 cycles for the obsolete lead-acid bat-tery), each complete in about 2 h (versus 6–7 h of acorresponding lead accumulator).10As a further comparison, the cost of a singlelithium titanate battery capable of storing and releas-ing 1140 Wh of electricity amounts to $987, whereasthe cost of two lead-acid/lead gel batteries of thesame capacity amounts to $811 (the energy densityof the Li-ion battery is triple that of its lead acidcounterpart). The lead-acid batteries must breplaced every 3.3 years, namely six times in 20 years,which translates into a four times higher cost for theobsolete lead-acid technology.The technology trend towards integrated, smartsolar lights is analogous to that taking place inbuilding-integrated photovoltaics (BIPV).11Accord-ing to this approach, rather than separately, the solarmodule, battery, controller, and the LED lightingsource are all designed in one lamp. For example, the40 W streetlight developed in China, shown inFigure 1, includes a 60-W, 18.2% efﬁcient, monocrystalline Si module; a lithium ferrophosphateenergy-dense battery; high-brightness white LEDs;and a micro-controller all integrated into a singlelamp, also featuring light and motion sensors.12An even more radical design approach involvesthe vertical integration of the solar cells along themast of the lighting pole, such as in the case of streetlights comprising solar cells integrated into the poleand protected by a tough polycarbonate shell, asshown in Figure 2.13The battery is vertically insertedin the concrete foundation of the pole, which has apassive ventilation system that cools the control andbatteries, improving performance and system life.The pole houses all batteries and control unitsfor increased energy storage capacity, allowing up to three days of autonomy even in northern climatessuch as Denmark, while the advanced optics
employed in the LED lamp allows an extremely accu-rate control of the output and direction of the light,minimizing light pollution and allowing the use oflamps requiring less power to meet demanding streetlighting standards.
INSTALLATION OF SOLAR LEDSTREET LIGHTS
The quality-driven process to properly install solarLED lights is guided by a preventive approach.Inspection of the site allows the insertion of the cor-rect street area features in the lighting software,which, in turn, simulates different options. A typicalstate-of-the-art application provides a three-dimensional project with illumination levels, uni-formity, luminance and shadows, spacing, and lumi-naire photometric curves.Installation is then followed by a proper main-tenance program:
1. Site inspection
2. Project proposal with simulations reports
3. Approval and commissioning
Maintenance, though limited and low-cost, is veryimportant, especially to address soiling of the PVmodule, which rapidly reduces electricity generation.For example, due to the proximity to the desert andhigh levels of volatile organic compounds in the air,losses of the output power due to soiling of ﬁxed solar modules powering PV street lights in Baghdadreached 26% in just one month.15Economic AspectsDue to a dramatic fall in price and increase in efﬁ-ciency of PV solar modules and LED lights thatoccurred in the last 5 years, most studies dealing withcost analysis of solar street lighting today are of his-toric interest.For example, a study comparing the installationcost (in 2006) of 10-km roadway lighting with LEDlights powered by grid and solar electricity includedan $835 cost for a 167-W solar module ($5/W) and$1000 for a 100-W LED lamp emitting 7200 lm($139/klm).16Eight years later, another economicanalysis of solar roadway lighting carried out inMalaysia,17indicated more than halved costs for thesolar module ($306, 140 W) and LED lamp($410, 112 W).Yet, by the end of 2015, the price of PV mod-ules had dropped to $0.58/W.18In turn, the price ofLED lights was less than $1 per kilolumen(Table 1),19decreasing at a rate of 28% per yearbetween 2011 and 2015, corresponding to an 18%price reduction for each doubling of cumulativeshipments.20We remind here that the fact that a given tech-nology follows a learning curve means that the costwill fall by a ﬁxed fraction for each doubling incumulative production as the curve characterizes thecost of manufacturing as a declining power law func-tion of cumulative manufacturing.Given such dramatic price reductions, it is per-haps not surprising that in a cost breakdown of thesingle units of solar LED streetlights completed in2012 (Figure 3), the metal pole was already the mostexpensive component, accounting for 46% of theoverall cost.21With the advancements in the LED, battery,and PV technologies, the total price of a solar streetsystem is decreasing at a rate proportional to that ofthe learning curve of the three technologies, com-pounded according to the relative weight of eachcomponent in the speciﬁc solar streetlight (21.5% forPV modules,2218% for LED, and between 6% and9% for Li-ion batteries9).
Furthermore, the dramatic increase in the lumi-nous efﬁcacy of LEDs employed in commercial street-lights, from 70 lm/W in 2006 to 140 lm/W in 2015,has made the use of PV lighting also possible in northern areas, such as Denmark or Russia, where the ﬁrst off-grid solar street systems for lightingstreets, playgrounds, courtyards, and parks were installed in 2013.23
SELECTED LARGE PROJECTS
Solar street lighting is currently expanding at a fast rate across the world. The concomitant fall in the cost ofPV modules, LED lights, and batteries has made thetransition to solar outdoor lighting in developing coun-tries more convenient than extending the electric grid.In India, for instance, about 300 million peoplelive in rural areas not served by the grid. The govern-ment thus recently took initiative to bring solar light-ing to said rural regions. Two LED solar streetlighting contracts were awarded in late 2015 to amajor solar streetlight manufacturer to provide morethan 76,000 solar LED street lights across 800 vil-lages in Uttar Pradesh that will be lit with LED lumi-naires emitting 1200 lumen (absorbing 12 W, withlead-acid battery charged by a pulse width modula-tion charger). Small towns and villages of Manipur,instead, will be lit with 1400 solar LED lamps emit-ting 3500 lumen (absorbing 43 W, with lead batteryequipped with MPPT charge controller mounted ona pole capable of withstanding strong winds).24In Guinea, in 2015, the government started anation-wide solar lighting campaign with a total of30,000 solar streetlights to be installed in more than300 towns and villages. Since late February 2015,every 3 days, a new neighborhood is given access to outdoor solar lighting.2 n Zimbabwe, the capitol Harare started the installation of 10,000 solar-powered streetlights in2015 (a $15 million project, translating into a $1500cost for each pole, including installation). Streetligthingwas also a measure used to halt a recent rise in streetcrimes and enhance overall road safety (Figure 4).In Brazil, in 2015, a civil engineering companycompleted the installation of 4310 solar street light-ing poles (supplied by a large PV module manufac-turer) that now provide excellent white light for73 km of the highway connecting the ﬁve main high-ways crossing Rio de Janeiro (Arco Metropolitanodo Rio de Janeiro, Brazil’s largest highway). Eachstreetlight includes a 150-W LED lamp, three 250-Wsolar modules, one MPPT controller, a photocell(light sensor), a pole with mounting structures, andfour 240-Ah lead-acid batteries.Another beneﬁt identiﬁed by the local governmentwhen opting to install 3.2-MW off-grid solar lights wasto avoid overburdening the local electric grid.