In 2019, the Netherlands tested a 1,312-foot highway noise barrier with two-sided solar panels; it generated 325.5 MWh over 18 months

You may drive past them every day and never even seem to notice as such: the highway sound walls. They are tall, grey, and built to block traffic noise from reaching nearby homes, infrastructure that has quietly lined densely populated highway corridors for over 20 years, lowering noise levels and, in some cases, contributing to better air quality. In the town of Uden in the Netherlands, some engineers wondered about that 1,300-pound mass of dead weight and pondered a pretty good question: Why is this thing just standing there?

Their response was a noise wall of double-sided solar glass that generated 325.5 megawatt-hours in an 18-month testing period from January 2019 to June 2020, enough to power approximately 60-70 homes for a year, while remaining part of the working roadside mounted directly on the barrier structure alongside the A50 motorway's traffic, according to the European Commission's official results page for the Solar Highways project. No farmland levelled and no open field taken away. The wall halfway there, doing a partial job. Now it does two, which is a mildly infuriating thing to learn, if you'd been told there's no room left for clean energy.

A note on chronology: the 2019–2020 monitoring period described here was the demonstration phase of a project that ran from 2014 to mid-2020. The studies and projections discussed later in this piece: from 2017, 2018, and 2024, are separate research efforts that build on or analyze this same system; they aren't sequential updates to the Uden numbers themselves.


A wall that pulls double duty

It's not a showy barrier like solar technologies typically are sold as. It is 400m (1,312ft) long, 5m-high and constructed from 136 integrated panels covering approximately 1,600sq m by construction company Heijmans, which was switched on for the Dutch infrastructure ministry, Rijkswaterstaat, in December 2018. The twist is that both sides of the panels are generating power.

The wall faces approximately east-west, so one side receives the morning sun and the other the evening sun, thus creating an unusual double-humped output curve, as opposed to the typical peak that you would get on a rooftop array. This was not the first solar-noise wall in the world: In 1989, one was installed on one of the back panels in Switzerland. Uden was different because of its scale; at this length, it was the first time the double-sided panels were fully incorporated into a barrier's structure, not bolted on top.

The facts behind the numbers

The wall's net yearly energy yield for 2019 came in at 202.8 megawatt-hours, enough to power around 68 typical Dutch households for that year. Over the full 18-month monitoring period, the total was 325.5 megawatt-hours, averaging to roughly 220 megawatt-hours per year, with an overall system performance ratio of 74.5%, and an estimated 121 tons of avoided carbon emissions per year over an expected barrier lifespan of about 30 years.

It's no coincidence that that performance was achieved in good weather. The orientation, tilt, and placement of the cells can make a difference that is "meaningful" when it comes to extra electricity supplied by the second side of the bifacial noise barrier. The researchers from Delft University of Technology built and monitored their own full-scale bifacial noise barrier, as documented in a 2018 peer-reviewed study titled “Maximizing annual yield of bifacial photovoltaic noise barriers,” published in the journal Solar Energy. According to a separate 2024 study titled “Spectro-angular analysis of roadside-integrated bifacial solar power systems with reflecting sound barriers,” in the Journal of Physics: Photonics, which carried out the simulations directly on this Dutch system, a simple diffuse reflector behind the barrier could increase the annual output by some 70%, and by almost double in the winter months when solar power is most often challenged.

Meanwhile, American walls just sit there

America has a lot of this idle real estate, and almost none of it is doing anything. The Michigan Technological University researchers' 2017 study titled “Power and energy potential of mass-scale photovoltaic noise barrier deployment: A case study for the U.S.,” in Renewable and Sustainable Energy Reviews, estimates that, with no new land being cleared, covering all current highway noise barriers with solar panels could release 7-9 gigawatts of solar power. The authors highlight the capacity that can be achieved in just one state, similar to some of the largest solar arrays that already exist in the country.

But there is no other large-scale US project like Uden even 8+ years after going online. It's partly jurisdictional, though: highways fall under the control of state transportation departments, solar incentives are handled by utilities and energy agencies, with no coordination between them. Inertia is a part of it, because it is no one‘s job description to say, "notice the wall.”

Why should this matter

This may be a frustrating reality for millennials and Gen Z renters as they see electricity bills rise during summer, as well as hear that the grid must become cleaner: some of that electricity is already blocking the sound while you're on your way to work. The Dutch didn't discover a miracle technology in Uden; it's just that they saw a wall. But the real question is, why haven't more locations paid attention to their own retrofit? After all, there are federal incentives for clean energy already in place for the same kind of infrastructure retrofit.