Incorporating photovoltaics with sound barriers is an application that has been steadily gaining interest since the installation of the first 100 kW grid-connected plant alongside the Swiss N13 motorway seven year ago. Since then utilities in several European countries have been 'sounding-out' the prospects for PV noise barriers.

The initial attraction of mounting photovoltaic modules on sound barriers is similar to that of mounting PV on buildings. A proportion of land and support structure costs can be avoided. Operating experience gained from a number of noise-barrier systems demonstrates the technical feasibility and electrical generation potential of such installations.

The Swiss plant mentioned above, for instance, has fed on average 110 000 kWh into the grid each year since it was installed. A 40 kWp system installed by the Austrian utility, Oberösterreichische Kraftwerke AG, shows similar performance promise: the solar generator, which extends for 264 m along the A1 near Gmunden, has produced some 31 500 kWh/year and has a mean performance ratio of 0,67 since it became operational in February 1992. The entire installation process from design to completion took around 9 months at a cost of ATS 6,4 million (approximately USD 600 000), with by far the greatest single cost component being the modular purchase, which constituted 43% of the total.

Developments are now concentrating on true integration of PV to allow the complete substitution of a simple noise protection element with one that offers both noise protection and electricity generation capabilities.

In the Netherlands, a 52 kWp noise barrier integrated PV-system has been in operation since April 1995. Over the period from July 1995 to July 1996, a performance ratio of 0,73 was recorded from the PV generator.

The German utility Stadtwerke Saarbrücken AG, has installed two such integrated systems near the intersection of the A6 and A620 motorways on the German-French border. One is a 40 kWp design incorporated into a 232 m stretch of transparent sound reflecting acrylic plastic, while the other - a 20 kWp array - extends 217 m along a highly sound-absorbing, non-transparent section of barrier. The systems were installed in the second half of 1995, so no performance data is available as yet. However, in respect of installed costs, 15% of the total system costs can be offset through saving on conventional noise protection elements, resulting in installed costs of 8 000 ECU/kWp (approximately 10 000 USD/kWp).

Theoretically, there is a sizeable potential for electricity generation from PV sound barriers. TNC Consulting engineers have evaluated the short-term potential for Germany (based on sound barriers planned for erection in the next few years) to be 115 MW. Similar studies confirm that Switzerland also has a considerable resource.

As part of an ongoing development programme funded by the Swiss and German governments, TNC recently organized a design competition to stimulate ideas, which could take advantage of this potential. Nineteen teams composed of noise reduction and PV companies submitted a total of 31 competition entries. Six winning entries were selected, based on noise damping properties, PV power output, integration innovation, architectural design and cost. 10 kWp Prototoypes of these designs will be built in the near-term to assess the practical performance issues ant to provide feedback to enable large-scale installations (100 kWp) to be realized.

Conventional PV-barrier arrangements need to be sun-facing and inclined at a suitable angle to maximize electrical yields. This restricts their application largely to routes running in an East-West direction. One innovation, which could extend the range to include many North-Soouth routes as well is the bi-facial or 'dual sided active' module. In theory, a vertically mounted bi-facial module oriented to face East and West (representing placement alongside a North-South route) could give electrical yields up to 6% higher than optimally positioned singe-faced modules. A design based on bi-facial modules was among the six entries selected for further development. See also article

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