View images, links and information on Sullivan County's "Solar Renaissance" at sullivanrenaissance.org.

Good Solar Design Practices and LED Lighting

Courtesy of SolarOne® Solutions

In today’s highly energy environment, people rarely question the promise and benefits of solar power. It’s no longer a question of if, but when and in what forms. As it turns out, “Solar Power” comes in many forms. All too often those forms are confused by the general public. Most laypeople still think of solar power for heating their pool or domestic hot water. However there is another form of solar power that generates electricity (photovoltaics). Under this category, there are several sub-categories that range from huge power plants that may employ concentrators generating mega-watts (MW) of power into the electrical grid, all the way to tiny “grid-independent systems” that use solar cells to charge a battery with micro-watts (µW) or milli-watts (mW) of power that in turn provides power to a dedicated load – such as a calculator or flash light.

When a solar power plant is connected to an electrical grid, it joins many other power plants, powered by different energy sources (e.g. coal, natural gas, hydro), each one offering different power characteristics (e.g. base load, peak) – all working in tandem to provide reliable power for all the homes, businesses and institutions connected to that grid. If one power plant goes down, then the other plants should be able the carry the extra load. Solar power plants for these types of applications are not considered “mission critical” – and therefore can be design with very thin margin. Designers of these systems tend to look at optimizing annual energy production and don’t worry about how the system performs during long periods of inclement weather.

This is not the case for grid-independent solar power systems, such as communication towers and of course, lighting. For these systems, designers must consider the worst case situations. For solar powered general illumination, that situation is well understood. It’s the longest duration of low/no sun weather during the period of the year that has the longest nights and shortest days. In the Northern Hemisphere, that’s mid-November through mid-January. It’s the time that the lighting load is on the longest, placing the biggest demand on the batteries, while at the same time the sun is at its weakest ability to recharge the batteries. An example of how the “worst case” must be treated is depicted in the chart below.

The challenge is to properly design for that condition, which, as with the lighting aspect of the job, starts with good specifications. It’s should be noted that systems with the largest solar arrays and battery banks don’t necessarily offer the best reliability, but almost always come at the highest cost.

Solar Panel Capacity

The first step in specifying a grid-independent solar power system is sizing the solar array to produce sufficient energy to serve the electrical load under for all the conditions and seasons that the load must operate. For year round solar lighting in the Northern Hemisphere, the worst case is unequivocally December where the hours of daylight for charging the battery are the fewest and the hours of darkness requiring lighting are the most abundant.

While the objective is simple, (akin to a bank account, putting in more money than is taken out), there are a great number of factors that go into more accurately assessing the required solar panel size. Some of them include:

Solar Panel Temperature De-rating: The power and voltage of Solar Panels are rated under factory test conditions. The voltage and power characteristics can change dramatically the more operating temperatures vary from room temperature. Although solar panels produce more power under colder conditions, the power comes in the form of higher voltages which is typically not useful to the battery unless it’s conditioned through a Maximum Power Tracker (MPT) controller.

• Solar Panel Orientation: For maximum solar power production on the winter solstice (shortest day, longest night), the optimal solar panel orientation is true south, tilted at an angle equal to about the latitude of the site + 15◦, off of the horizontal.
• Solar Panel Shading: Solar panel production can be adversely affected by shading from trees, buildings or any object that casts a shade on a solar panel during the day. For year round solar lighting, as a rule of thumb in the U.S. this would be objects that shade the panels between about 9:30 am and 2:30 pm in the winter.

Energy Storage Capacity

The second key step for specifying a grid-independent system is establishing the amount of energy storage – or days of no/low sun. While the calculation is simple, the big challenge in assessing days of storage relates to defining the load and how the load and storage behaves under different conditions. For example, the efficiency of certain lamps, such as fluorescent lamps, drops precipitously in colder temperatures and should be accounted for by using a factor that increases the current draw. Battery capacities also drop in colder temperature.

Structural Loading

With key issues involving lighting characteristics and energy balance addressed, the last distinctive issue in specifying solar lighting is structural loading of the elements. That is not to say structural loading assessment is not essential for non-solar lighting. It is that the solar panels present a particular challenge. Depending on their surface area and orientation, solar panels can impose significant wind loading on light poles. The poles must be rated to withstand both the weight and the Effective Projected Area (EPA) of the solar panels and fixtures mounted to the pole for a given wind regime. The solar panels alone can double or triple the EPA, often times requiring a pole of different material, wider diameter or thicker wall. The more efficient a lighting system, the smaller the solar panels, the less of a load the poles have to withstand. Specifications need only to point out that all elements must be designed to withstand the rated wind speed for the area.

Other requirements, common to all lighting systems such as protection from corrosion and proper grounding, must also apply. It’s important to point out that most solar lighting systems stay under 50 VDC and therefore not subject to as many safety guidelines as applicable to a high voltage (>50 volts) system.

The Benefits of Solar LED

Past commercial-scale, solar lighting projects relied on fluorescent or low pressure sodium (LPS) light sources – although some solar lighting companies have started to offer systems with metal halide sources. While each of these light sources have distinct color, efficiency or lifetime advantages, they are also accompanied by significant shortfalls. For example low pressure sodium lamps have remarkable lumen/watt efficiency, but offer very poor color rendering and so do not perform well for mesopic (partially dark-adapted eye) vision. LPS lamps also have under 18,000 hour lifetimes. Fluorescent lighting, on the other hand, provides clean white light, with great color rendition and 20,000 hour lifetimes, but its efficiency and lifetime diminish dramatically with colder temperatures. Metal Halide does not suffer the temperature degradation fluorescent lighting does while offering clean white lighting, but has reputation for very short lifetimes with consequently very high maintenance costs. All of these light sources are single point bulbs that shine light in all directions, requiring significant manipulation via reflectors and lenses, to provide effective uniform lighting. None of these light sources truly lend themselves to low voltage DC and precision control. These shortfalls present basic incompatibilities with the low voltage DC and intermittent nature of solar power. In effect, these are fundamental inefficiencies in the system, only over come with additional solar panel and battery.

LEDs are fundamentally compatible with solar energy. On a very conceptual basis an LED is the inverse of a solar cell. A solar cell is a semi-conductor device that converts light to electricity, while an LED is a semi-conductor device that converts electricity to light. LED’s “control-ability” enables them, through intelligent controls, to adapt to the ebb and flow of the solar energy through changing weather patterns and seasons. Like solar cells, LEDs offer “solid state reliability – lasting at least a decade, if not longer. LED’s efficiency and lifetimes improve under colder condition – when the system needs it the most. And then of course, well designed LED lighting systems can reduce the number of lighting systems on a project by 20% or more and still achieve exceptional lighting results

Gaining Wider Acceptance

Clearly the rapid pace in the development of LEDs is helping to make solar powered area lighting viable for mainstream deployment. There are other critical factors playing out that further diminish barriers and catalyze acceptance. Obviously one key factor is the skyrocketing costs of energy and copper associated with conventional lighting. This, overlaid with the declining lumen-hr per day costs of solar powered lighting through efficiency and volume increases, makes the economics more compelling. Another key factor is improving awareness and education with regard to good lighting practices. Dark sky mandates, glare standards and studies in spectrally enhanced lighting all translate into better lighting using less energy. As communities and institutions learn about these trends and see them at work, they tend to adopt them, because they represent cost savings and fewer complaints. Lighting layouts requiring less energy make the solar power option that much more attractive. Then of course there is the public’s growing acceptance of a “new look” to energy, even to the point of “demanding it in certain cases. More uniform lighting facilitates a reduction in light level standards without any compromise in security or visual acuity. From town councils to retail stores, facility owners are have to respond to public requests to dim lighting and reduce lighting. In the same vein, many architects, who in the past have sought to hide solar panels from public view, now want to show them off. Lighting manufacturers are starting to recognize the changing landscape and developing new products that are melding the technology with needs, standards and trends – making adoption of solar lighting even more desirable.

Widespread acceptance starts with proper specifications; ones that define the objectives without over-constraining the problem. Simply specifying a unit quantity of a particular type of solar lighting system with only mature light sources, such as LPS or fluorescent along with fixed solar panel wattage and battery size will keep the technology from advancing. Solar lighting projects, like any lighting project, are best served by defining the foot-candles, color temperature and color rendering on a surface rather than calling out the lumens from a light sources or worse yet, mandating a specific light bulb. Lighting uniformity should always take a high priority amongst the lighting metrics. On the solar power side, establishing the baseline “Array-to Load” ratio and “Days Storage” for the average worst-case period are the two most important requirements. In order for these metrics to be effective, proper selection of the “derating” factors that account for losses related to things like temperature and voltage mismatch must be integral to any calculation that supports meeting the requirements. At this level of definition, lighting designers and manufacturers are more liberated to apply “best practices” and “state-of-the-art” technology to achieve the desired result for the best value.