The case for solar-powered LED lighting

June 24, 2005
Rapid developments in solar cells, LED lighting and energy storage are creating great opportunities for solar-powered solid-state lighting, says Moneer Azzam of SolarOne Solutions.
The outdoor lighting industry, as with so many other application-oriented industries, assumes that the power source is infinite and always available. This is a tribute to the reliability of the electric grid - at least in developed-world nations. It has certainly made life simpler for the lamp/lighting designers who, for the most part, are able to divorce the lamp characteristics from the energy source. The developed world is starting to learn that the power source is far from infinite.*
Fig. 1.

Some companies are not waiting to see the bottom of the barrel, and are exploring alternatives that range from replacement power sources to energy demand reduction. A number of companies are developing technologies that achieve both: SolarOne Solutions' work in solar-powered solid-state lighting under the SOLED™ brand name is an example of this.

[* Estimates indicate at current levels of energy usage, the earth has about 150 years of energy stored in the form of oil, coal and uranium. However, if the entire population of the earth consumed as much as Americans' do on a per capita basis, that amount drops to 15 or 20 years.]

Solar-powered lighting

Solar electric lighting systems do in fact connect to a truly "infinite" power source - the sun. However, as we all know, this source is intermittent. In the case of solar outdoor lighting, the power source is inversely related to the load (the lights turn on when the sun goes down). This relationship leads to an important conclusion; the system must rely on energy storage (e.g. batteries), unless it remains connected to grid.

Fig. 2.

Now in order for a solar lighting system to perform reliably, the solar panel and battery must be sized for the period of longest nights, shortest days and cloudiest weather, all of which occur at the same time each year (see figure 2). Historically, the solar industry has addressed this worst-case scenario by seeking out the most efficient lumen per watt DC lamps and over-sizing the system for the rest of the year. That translated to DC fluorescent bulbs, bigger solar panels, more batteries, higher costs and less-than-appealing appearance.

This approach confined the market for solar lighting to areas closer to the equator with highest average levels of solar irradiation and temperatures that did not affect the performance and lifetime of the DC fluorescent bulbs. It also left the markets in the higher latitudes, typically with higher per capita wealth levels and greater lighting needs rather under-served.

Fig. 3.

SolarOne perceived the shortfalls in the status quo products and the under-served market as an opportunity to integrate white LED lighting into its product offering and an area on which to focus its development efforts. With a deep understanding of solar cell characteristics and system operation along with resources in circuit design and micro-processor programming and strong relationships to major manufacturers of key components, the company saw their role in integrating the components into useful products and developing the brains that most cost-effectively manage their operation. At this year's LightFair International, SolarOne announced the first embodiment of such a system in a lamp, driver, and system manager combination named SOLED™ mc2 Technology. With the proverbial umbilical cord to the electric utility severed, the mc2 Technology seeks to create a more symbiotic relationship between the lighting load, solar cells, battery storage and the user. While the use of high efficiency components and circuit designs are a vital aspect of the technology, attaining the highest level of resource utilization is paramount to the mc2 philosophy. This is where the LEDs make an enormous contribution.

1. More effective illumination

Fluorescents, while providing a cool color temperature at a very efficient lumen per watt (lm/W basis still have fundamental deficiencies. In addition to their bulk and inherent fragility, fluorescents project light poorly, in all directions around the lamp and in almost 180 degrees in the plane of the lamp. Thus their utilization, (the amount of usable light projected) is lower than the technical calculations of efficiency suggest. A reflector can redirect the light around the lamp downward, but there are efficiency costs. Further, as the fluorescent lamp is controlled to direct light downward for ambient lighting, approximately 30% or more of the light is lost. For more demanding task lighting, when even greater control is required, the loss from the fluorescent is approximately 50%.

In contrast, LEDs produce directional beams of light, up to 90% of which is usable for ambient lighting and 70% of which is useable for task lighting (assuming that the lighting design incorporates multiple LED installations to compensate for shadows in a space measuring 4 feet square). The relatively low lumen output ratings of LED lamps are often times compensated for when looking at the foot-candle levels at the illuminated surfaces. In other words, for outdoor lighting applications, SolarOne estimates that an LED lamp rated at 45 lumens per watt will perform equivalently to a fluorescent bulb rated at 75 lumens per watt. This facet of LEDs offer an enormous side benefit to areas with dark-sky mandates.

2. Optimized system efficiency

Solar cells and LEDs share many characteristics - even in the assembly process. For example, both solar cells and LEDs require sorting and balancing to optimize performance. The SOLED mc2 LED Lamp and Lamp Driver is configured to effectively eliminate the need for balancing resistors and their associated losses. Perhaps more significantly, through its range of testing and field experience, SolarOne has identified "sweet spots" in LED operation that optimize current flows and light levels with solar panel and battery costs. The trade-offs are quite different than grid-connected or even automotive applications. This translates into almost a 10% improvement in overall system efficiency.

3. Fine tuned to user needs

Fig. 4.

Unlike conventional lighting, LED's lend themselves to programmability and fine-tuning. Delivering light not only where, but at the level and time it's most needed is another cornerstone of the mc2 philosophy. This aspect alone can reduce solar panel size and battery capacity by as much as 50%. The mc2 System Manager offers the user with 16 set lighting profiles they can select for their application. The company also offers factory installed custom profiles, depending on the size and nature of the project.

4. Battery runtime extension

Fig. 5.

SolarOne's extensive field experience with solar systems highlighted the need to give options to users in selecting the preferred intervention methods for those cases where battery drops to lower states of charge. By giving the user the ability to define their "hierarchy of needs" and apply it to system operation, mc2 adds a new tool to address solar panel size, system cost and siting issues to the satisfaction of the customer.

5. Cold weather

Another shared characteristic between Solar cells and LED lights is improved efficiency, performance and lifetime at colder temperatures. This brings advantages over other types of lamps whose performance and lifetime drops in colder climates - such as DC fluorescents. An LED lamp is expected to last at least 5 to 10 times as long as a DC florescent in these environments. It is another arrow for solar/LEDs' quiver in serving markets at latitudes further from the equator.


SolarOne and the other companies in this business space are just scratching the surface of opportunities for outdoor lighting. Quantum leap developments are underway in the fields of LED lighting, solar cells and energy storage alike. With each advancement, the benefits multiply throughout the system. As an example, the latest generation of high-brightness LEDs are playing a key role in realizing the potential of the SOLED mc2 concept.

Cutting the tether from the outdoor light fixture to the electric grid is akin to what happened in the telecommunications market with the advent of the cellular phone. LED bulbs are conceptually solar cells in reverse - solid-state devices converting electricity into light. By combining them we create wireless, solid-state lighting. Who knows what markets will emerge? One thing seems certain, great opportunity exists in the compatibility between solar cells and LED lighting, it is just a matter of seizing it.