In their efforts to deliver solar energy systems capable of maximum power production, designers consider numerous factors to ensure they install effective, reliable systems capable of optimum performance in any environment. In New England, for example, solar technologies have to operate in challenging climate conditions that range from harsh, freezing winters with frequent snowstorms to high-temperature summers. Other primary considerations In solar installations include site conditions, structural issues, panel-mounting options, and project lifecycle costs.
Given the need to generate as much energy as possible from their solar installations, residential and commercial customers are increasingly considering ground-mounted solar tracking systems as alternatives to fixed roof- or ground-mount systems where conditions allow. Solar trackers follow the sun during its entire daily trajectory to maximize the amount of energy they capture. Ground-mounted tracking systems comprise photovoltaic (PV) arrays built on pole mount units, which incorporate either a single- or dual-axis design. A solar tracker’s ability to keep PV panels at a perpendicular angle to the sun from dawn to dusk yields considerably higher energy than fixed PV systems of the same size.
Tracker movement relies on either passive or active drive mechanisms. Passive trackers follow the sun’s movement using thermal actuators or light sensors that identify the sun’s position so the unit can rotate to face it. Active solar trackers, on the other hand, use motors to propel units and incorporate GPS technology to track the sun based on the array’s geographical location. An advantage of active tracking systems is that rather than follow the sun as passive systems do, they use preprogrammed algorithms to automatically align themselves according to the sun’s daily coordinates. Data shows that this precise tracking of the sun as it traverses the sky results in solar energy production that’s 35%-45% higher than that provided by fixed roof- or ground-mounted systems and 10%-15% higher than passive tracking systems.
Optimizing Performance: Siting and Climate Considerations
The higher energy produced by solar trackers depends on them having a clear view of the horizon so they can leverage their rotational capacity to capture energy from sunrise to sunset. Ideal tracker sites have at least 90% annual solar exposure, requiring a relatively clear view of the horizon to the East, South, and West. Under these conditions, systems produce electricity early and late in the day, times during which fixed roof-mount and ground-mount systems don’t face the sun.
For installations in higher latitudes like the Northeast, the best option for solar trackers is a dual-axis design that swivels PV arrays on an x-y axis. In these latitudes, trackers have to adjust to the sun’s changing elevation above the horizon as well as track its East-West azimuth, making dual-axis designs a superior choice to single-axis trackers, which are best-suited to lower latitudes because they only track East and West. Dual-axis trackers are also suited to installations in higher latitudes because they’re able to adjust their pitch to withstand certain external conditions, such as snowstorms or high winds.
When oriented toward the sun, trackers should be designed to withstand winds of up to 90 mph, but as an additional safety precaution, they’re ideally equipped with anemometers to measure wind speed. If wind speeds exceed, for example, 30 mph, the trackers can orient themselves in a horizontal position, at which point they should be capable of withstanding extremely high winds— more than 120 mph for some trackers — while still producing electricity.
Wind conditions factor into other tracker design considerations as well. While some designs rely on bearings at the unit’s pivot point to overcome the friction that occurs as the system rotates from East to West, this lowered resistance also means the unit has to work harder to stay aligned — particularly so in high winds — which transmits stress to internal components that can result in premature component failure. A design that incorporates brake pads at the pivot point can reduce overall stress on components. When operating in high winds, the tracker maintains its position by exploiting friction created by the brake pads.
Snow is another element that New England-based solar trackers must be able to handle. Trackers with the ability to rotate 360 degrees can orient themselves to the North after the sun goes down and set themselves at a steep angle so any snow that might fall during the night doesn’t settle on the PV panels, and then reorient themselves at dawn based on programmed coordinates.
Configurations and Costs
For residences whose occupants have relatively modest electricity needs and the right site profile, a single tracker typically suffices. Large commercial installations requiring a considerable amount of electricity will require multiple tracking units. In this case, trackers should be placed roughly 50’ apart East to West, and 50’ North to South, to ensure there’s no shading from tracker to tracker. Individual trackers should incorporate an SMA inverter that delivers AC power whether the system is grid-tied or grid-independent, so that failure of one inverter does not affect any other tracker within an installation.
No one solar design solution meets every residential or commercial site variable, budget, or client preference. Flush roof-mount systems provide the best initial cost-to-production ratio because the structure mounting is already in place, while trackers, with their moving parts, can cost more to install and maintain than fixed systems. However, in prime conditions, the higher power produced by trackers versus similarly sized roof-mounted systems typically offsets installation cost differentials. With trackers among the technologies in the solar energy toolbox, homeowners and commercial entities have a number of viable installation options available for consideration as they move to lower their energy costs and carbon footprint.
Geoff Sparrow is the director of engineering at ReVision Energy and has been involved in the design and installation of hundreds of commercial and residential systems. He has a BS in Mechanical Engineering from the University of New Hampshire and is NABCEP-certified in both photovoltaics and solar thermal.