By Paul Grana, co-founder, Folsom Labs
East-west arrays are an increasingly popular configuration in the commercial solar market. With low module prices, it makes sense to trade a slight decrease in module productivity for greater overall system power production. By placing modules back-to-back, designers can avoid clearances for module shading, and can fit many more modules per unit area. Typical east-west arrays will have energy yield 5 to 10% worse than equivalent south-facing arrays, but 20 to 25% better power density.
Typical east-west analysis assumes that the commercial rooftop is facing north-south. Unfortunately, in the real world, many rooftops are not so orderly in their design. In the cases where there is no true “south” (and therefore, no true “east” or “west” either), how do these new racking products pencil out?
Why does the building orientation matter for the solar array’s orientation?
First, it is worth clarifying why the building’s orientation has any bearing on the module orientation. Solar arrays are typically designed to go with the direction of the building. There are a number of reasons for this:
- Cost-effectiveness. Positioning the array with the direction of the building makes it easier to install, secure to the roof as necessary, and maintain. In short, having the array not oriented with the rooftop incurs many additional costs.
- Packing density. Most commercial arrays are space-constrained rather than demand-constrained. Designers can pack more modules into the roof when they are going “with the grain” of the roof versus against it.
- Aesthetics. Solar arrays look better when they are aligned with the building. Symmetry is always beautiful, and aesthetics are often quite important to building owners.
In short, the building’s direction often dictates the orientation of the array –system engineers cannot simply require that their modules perfectly align based on the equator.
While this may be reasonable to consider for traditional south-facing racking, the concept of being off azimuth for an east-west array seems unacceptable for many engineers. To these designers, having modules pointed to the north is a cardinal sin that good engineers simply don’t do. By extension, having an array that is a combination of, for example, southwest- and northeast-facing modules is seen as a fundamentally un-sound PV system. But is it?
The penalty for being off-azimuth
It is no secret that modules do best when they are facing toward the equator. If we look at an example system in Charlotte, NC (a ballasted commercial array at a 10° tilt), the array 30° off of due-south produces 1% less energy than the south-facing array, while the array 60° off produces 4% less energy.
However, we get truly interesting results if we vary the azimuth of the east-west array. The east-west array has no penalty at all for being off of due south – and in fact, produces slightly more energy (0.3% improvement for an array that is 60° off of due south). In absolute terms, note that the east-west array actually “catches up” with the south-facing array when the building is 60° off of due south. In that case, the array is benefiting from all of the packing density benefits of dual-tilt arrays, with none of the yield problems.
Why east-west isn’t as bad as it seems
How can it be that the energy yield of east-west arrays is so unchanged based on the azimuth angle? Much of it comes down to the offsetting impacts of the two sets of modules. As an east-west array rotates, one half of the modules gets pointed away from the equator, while the other half is rotated toward the equator. The net impact is that there is essentially no penalty for being off-center for east-west racking systems. Interestingly, since there is a drop in yield for a single-tilt array (1-4%, as seen above), the comparative economics for east-west arrays actually looks better for non-south-facing rooftops.
“Dual-Tilt” versus “East-West”
East-west racking systems are becoming popular due to their ability to greatly improve system size in exchange for a modest downgrade in energy yield. Even for buildings that are not facing south, east-west racking (while not actually east-west) can provide all of the density benefits, with even smaller yield downgrades. In light of this, we should perhaps start calling the products “Dual-Tilt” rather than “East-West.”
Read more Solar Boot-up articles from Folsom Labs here.