might work here, see the photos on the websites

STOCK IMAGE, Outdoor photovoltaic facility, solar modules on a ...
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That would be my choice; an 'open-air' PVA could after all be mounted on the roof/roofs of a building, or group of buildings - houses,commercial buildings etc, couldn't it?!

is my preference.

'open-air, ground-based PVA' wenn man es überverdeutlichen will

Yet another term for a large outdoor photovoltaic plant- would seem to fit the source text as well.

as opposed to mounted facilities.

„open space“ is still a good choice (got 212.00 ghits with „photovoltaics“).
“open area” is another possibility (381.000 ghits with “photovoltaics”)

• System type | All
• On-roof
• In-roof
• Exteriors
• Open area
• Stand-alone system
http://www.tritec-energy.com/en/reference-cases/

Method and device for creating a configuration of a solar cell table arrangement with a number of solar cell tables for an open area photovoltaic power plant
http://www.google.com/patents/EP2570984A1?cl=en

Verfahren und Vorrichtung zum Erstellen einer Konfiguration einer Solarzellentischanordnung mit einer Vielzahl von Solarzellentischen für ein Photovoltaik-Freiflächenkraftwerk
Antragsteller: Siemens Aktiengesellschaft
http://www.google.com/patents/EP2570984A1?cl=de&hl=de

Field instalations
The panels can also be installed on an open area. The panels are attached to the steel structure anchored directly into the ground.
http://www.s-power.co.uk/technology

The HEINTZMANN SOLAR® sub-construction was especially designed for the use of photovoltaic power plants in open area, on roof tops, carports and greenhouses
http://www.heintzmann.eu/fileadmin/Dateien_Unternehmen/Solar...

Project developer for photovoltaic open area systems
SOLARINVEST seeks investors who wish to invest in solar power plants. Investors can be guaranteed a long-term, stable cash flow associated with the awareness of having made an intelligent and sustainable investment.
We are also seeking land owners and project developers to whom we can give some support in implementing projects financially and with our know-how.
http://www.advantageaustria.org/international/company/solari...


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Note added at 4 Tage (2013-07-04 13:29:07 GMT) Post-grading
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Some definition of Freiflächenanlage":

Freifläche f
1. open area, wide-open space (großräumlich bzw. nicht bebaubar); open field, open site, free space (noch unbebaut);
2. open-air space (z.B. auf Ausstellungsgelände);
3. (AE) concourse (z.B. an Staßenkreuzungspunkten, in Parkanlagen;
4. flank (an Gebäuden, Wänden usw.)
(Gelbrich, Fachwörterbuch Architektur und Bauwesen)

ground-level {adj}
ebenerdig
ground-mounted {adj}
bodenmontiert tech.
http://www.dict.cc/englisch-deutsch/ground.html

open space Freifläche {f}
http://www.dict.cc/?s=Freifläche

Freiflächenanlage f. open land system
http://www.dictindustry.de/deutsch/englisch.php/frei


Unter einer Photovoltaik-Freiflächenanlage versteht man eine Photovoltaikanlage, die nicht auf einem Gebäude oder an einer Fassade, sondern ebenerdig auf einer freien Fläche aufgestellt ist. Eine Freiflächenanlage ist ein fest montiertes System, bei dem mittels einer Unterkonstruktion die Photovoltaikmodule in einem bestimmten Winkel zur Sonne (Azimut) ausgerichtet werden. Neben diesen fest montierten Freiflächenanlagen gibt es auch nachgeführte Anlagen, sogenannte Tracker-Systeme, die dem Stand der Sonne folgen
http://de.wikipedia.org/wiki/Photovoltaik-Freiflächenanlage

and the corresponding English article:

Technology
Most solar parks use ground mounted (sometimes called free-field or stand-alone)[38] arrays. They can either be fixed tilt or tracking, either single axis or dual axis.[39] Tracking increases the output,[40] but also the installation and maintenance cost.

Solar array arrangements
The solar arrays are the subsystems which convert incoming light into electrical energy.[45] They comprise a multitude of solar modules, mounted on support structures and interconnected to deliver a power output to electronic power conditioning subsystems.[46]

A minority of utility-scale solar parks are configured on buildings[47] and so use building-mounted solar arrays. The majority are 'free field' systems using ground-mounted structures,[38] usually of one of the following types:
Fixed arrays
:
Dual axis trackers
:
Single axis trackers
:
http://en.wikipedia.org/wiki/Photovoltaic_power_station

Conclusion: "ground-mounted" is the translation of "bodenmontiert (ebenerdig aufgestellt)", not of "Freifläche". Freifläche means "open space", "open area", open land" or "free field".