By Canute Haroldson, account manager and Paul Grana, co-founder, Folsom Labs
Installing solar on the side of a building is rarely the first choice for solar developers, but sometimes the customer prefers a wall-mounted array. In one instance that caught our eyes, New York installer Quixotic Systems built a 37-kW array on the side of Urban Health Plan’s Simpson Pavilion. The hospital’s limited roof space made a traditional rooftop array impractical, but at four stories high, there was ample room on the south side of the building. This unique array made us wonder when vertical installations can make sense.
High-quality roofs for installation are becoming difficult to come by. A wall-mounted array may not be the first choice, but when a roof is almost completely obstructed, it may be a decent option. So Folsom Labs decided to run a few tests to see how walls compare to roofs for solar panel installation.
The first comparison we wanted to make was the output of the wall-mounted array vs. the output of a fixed-tilt array on this specific roof. The complexity of the rooftop provides very little space to fit modules. Taking the space that was available, there was only enough room for an 11.8-kWdc installation, with significant shading losses. Even given the orientation of a wall-mounted array, this rooftop would only make around 12.1 MWh per year, vs. an estimated 33.9 MWh for the wall-mounted array. It was quickly apparent that the wall installation was the correct choice for reducing the hospital’s energy needs.
It is also worth asking how a solar wall would perform across the board. While it may make sense to install wall-mounted arrays when space is constrained, what are the cases where a solar wall will be more useful than a fixed-tilt array? What’s the relative performance between two equally sized systems?
To study this, we created a low-tilt roof-mounted design and a wall-mounted design in New York, Alaska and Florida. These designs all use the same components and system size.
Comparing the two approaches in New York, we can see some interesting interactions. The wall-mounted array performs equal to or better than the roof-mounted design for most of the fall and winter. In the spring, production falls off moderately for the wall-mounted array and underperforms compared to the winter months.
Ultimately, a wall-mounted array in New York produces 30% less than the equivalent roof-mounted array. Not ideal, but arguably quite respectable.
Not surprisingly, that difference can change dramatically based on geography. A 30% production gap in New York grows to 56% in Florida but is just 9.8% in Alaska. The production graphs for Florida and Alaska show a summer production dip for the wall-mounted array in Florida, and peak in Alaska due to the extreme northern latitude. Despite this reduction in available light, wintertime gains in production contribute significantly to the difference in production only being 9.8%.
Most of the difference in production that we see here is driven by the angle of the sun relative to the array. Florida’s generation comes when the sun is at a high angle in the sky, with an average elevation of 47°, while Alaska and New York get sunlight from lower angles (because of their higher latitudes). In fact, there is a 15° difference in the average sun angle that hits Alaska versus Florida.
Incorporating snow losses
The difference grows even bigger if we incorporate losses from snow. A critical advantage of wall-mounted arrays is that they will be effectively immune to snow losses. Areas that see significant snowfall can often see monthly production losses of 40-100% for a roof-mounted array. To make sure that this was considered in our comparison, we made a conservative estimate of snow losses for both our New York and Anchorage roof-mounted arrays, which can be seen below.
Comparing all the sites side by side and including our snow losses, solar walls prove their efficiency in the winter. New York and Alaska both see ~1,000 kWh of energy in favor of a wall-mounted array for four months of the year. Traditional solar arrays typically fall off in productivity during the winter, but a wall-mounted array will perform the best during this time. The Alaskan wall-mounted array even outperforms the roof-mounted array in overall annual production when snow is considered.
Ultimately, wall-mounted solar is not a crazy idea in New York and may be optimal in a place like Alaska. We found some solar companies in Alaska already designing systems with these considerations in mind. For example, Lime Solar showcases a number of wall-mounted or steep-tilt arrays at high-tilt or on the side of buildings as seen in the photo on the right from Lime Solar. With this analysis, it’s easy to tell just how appropriate that choice is.
It’s important to note that there are very likely additional costs for a wall-mounted array that wouldn’t exist for traditional arrays. Installation and labor costs will likely involve additional scaffolding and safety equipment, and so will almost certainly be more expensive. Developers will either need to create their own processes, or the customers will have to pursue developers that already have this expertise, which will likely come at a premium price. Racking will also factor into this assessment. A wall-mounted array will include unique structural requirements and mounting equipment. These costs will vary widely from region to region, and will likely be a deciding factor in installation in areas with a lower cost of electricity.
Finally, we ran our calculations without changing the albedo for each location. Albedo is an adjustment for the reflected gain of sunlight off the ground—and will be particularly helpful for wall-mounted arrays! When including the albedo in calculations, we would expect the output from the wall-mounted array in Alaska and New York to go up even more.
Have you had any experiences with wall-mounted solar installations? Are there things we forgot to consider in our analysis? Let us know in the comments below!