Solar Energy Handbook

Guidance of city of Vienna on Combining Solar Technology with Green Roofs & Vertical Greening Systems

Solar Energy Handbook's cover

Using solar energy and building surfaces in the city - Now and in the Future

1. Using solar energy and building surfaces in the city – Now and in the Future

The last few decades have seen significant changes in the demands placed on urban built environments. At the same time, climate-related problems in new and existing buildings are being aggravated by more and more surface sealing, the urban heat island effect and less water evaporation from green spaces, including parks, roofs and building facades. The number of very hot days in Vienna is expected to double over the next 100 years, causing the amount of energy needed for cooling to increase threefold over the next 50 years. Alongside the creation of green spaces, the surface areas of existing and newly built structures can be used to generate energy. Photovoltaic systems and solar thermal systems placed on walls and roofs can generate electricity and heat. As space and surfaces are limited resources, it is important to integrate different needs in designs for multi-purpose use.

Figure 1: Potential effects of building greening on the building, its immediate surroundings and the wider urban space
Potential effects of building greening on the building, its immediate surroundings and the wider urban space

Meeting future climate and energy challenges in urban environments requires solutions that combine and draw upon synergies from solar and photovoltaic technologies, “Green City” solutions and nature-based systems.
Under the headline theme “Using Solar Energy and Building Surfaces in the City – Now and in the Future”, this handbook presents the technologies that are available to harness solar power as we look to the future. By describing in detail how electricity and heat generation technologies can be applied in combination with vegetation on buildings and which synergies can be expected from this design approach, the authors hope to provide a tool that will help building designers to optimise the use of solar energy. The combination of technology and nature enables us to use solar energy most effectively and to improve the urban climate, thus raising the quality of life in the city.

The roofs of existing buildings offer great potential for the installation of solar technology, and we should make use of it. Nearly two-thirds of the total roof surface area of the City of Vienna are at least theoretically suitable for retrofitting with solar technology. The surface area suitable for greening has been calculated at about 5,800 hectares of rooftops and 12,000 hectares of wall surfaces (net figures).

And the relevant technologies are evolving all the time. In photovoltaics, for example, solar cells are now available in different colours. They no longer have the typical look of a PV array and can be integrated as design features in building facades. Thanks to these advances, solar technology has become very versatile and can be integrated in urban settings with hardly any constraints on design. Likewise, coloured glass for solar heating systems has been on the market for several years now, enhancing the appearance of solar thermal collectors on building walls.

Vegetation on roofs and walls is a valuable asset in densely built-up areas, providing benefits in terms of microclimate, energy, economy and ecology as well as prolonging the useful life of buildings. Moreover, greening of buildings creates amenity value; providing a more pleasant environment for people, greenery contributes to human well-being and quality of life.

The combination of building greening and solar technology gives rise to synergy effects which help to increase the efficiency and performance of existing surface systems. Greening/cooling the rear of a PV facade has a positive effect on the performance of the PV modules. Combining PV and solar heating with a green roof creates shaded areas on the roof that function as new habitats for diverse plant and animal species.

Technologies that can be used on building surfaces – photovoltaics, greening with vegetation and solar thermal systems

2. Technologies

The following section presents the three technologies that can be used on building surfaces – photovoltaics, greening with vegetation and solar thermal systems, and describes their design and how they function. Additionally, there are sections on community installations and the Renewable Energy Development Act.

2.1 Photovoltaics

2.1.1 How does a photovoltaic system work?

Photovoltaic (PV) technology uses solar cells to transform the energy from sunlight intoelectrical energy. PV installations can be mounted directly on a building (on the roofarea) or integrated into the building facade. There are also free-standing PV systemsmounted on the ground.The components of a PV system are: the PV modules, wiring, safety devices and acurrent inverter. A power storage unit (battery pack) can be added to store energy andthus improve the PV system’s effectiveness.The sunlight that is intercepted by the PV modules is transformed into electricity, withboth direct and indirect solar radiation being used. The PV modules produce directcurrent, which has to be converted to alternating current to be usable in the building. This is done by means of a current inverter. The solar-generated electricity is now readyfor use, to run electrical appliances in the building, power vehicles and generate heat.PV systems are designed to use most of the electricity generated on site, reducing the need to buy electricity from the grid. When the PV system generates more power than is needed, the surplus may be either stored in a battery or fed into the grid, i.e. sold to a utility company.

Figure 2 - Design of a grid-connected PV system
Design of a grid-connected PV system

2.1.2 Structure of a PV module

a) Solar cells:
The smallest unit in a PV module is the solar cell, which converts sunlight to electrical energy. The module is made up of a number of interconnected solar cells. The most frequently used PV cells are made of crystalline silicon, either in monocrystalline or polycrystalline form.

b) Polycrystalline silicon solar cells:
This is the solar cell type which is most frequently installed. The characteristic crystal structure of the cells resembles frost patterns. Each flake in such a pattern is a small silicon crystal. The crystals grow naturally during the production process. This is why polycrystalline cells are easier to make than monocrystalline ones, which require additional processing. As a result, polycrystalline cells come at lower production costs than monocrystalline ones, but they are also slightly lower in performance. Because of the lower cost, polycrystalline solar cells are often used in PV systems on large surface areas.

c) Monocrystalline silicon solar cells:
As opposed to polycrystalline solar cells, monocrystalline cells are made from a single silicon crystal, which improves performance. They are regarded as highly efficient in harvesting direct solar irradiation, requiring less surface area than polycrystalline solar cells to generate the same amount of power. Monocrystalline-cell modules are the technology of choice for high outputs on a small (rooftop) area. The surface of monocrystalline cells has a homogenous appearance, with surface colours ranging from dark blue to black.

d) Thin-film solar cells
Thin-film solar cells exist in different material compositions and structures. What they have in common is the manufacturing process and the thickness of the various layers (in the μm range), which are produced by vapour deposition of the material. Different materials are combined so that a broad range of radiation can be taken up and used for energy generation. The greatest advantages of thin-film modules are flexibility and low weight. But as a rule, their efficiency is lower than that of other solar cells, which means they require a larger surface area to produce the same amount of power. Surface colours typically range from orange-brown to black. Thin-film cells allow much greater freedom of design than crystalline cells.

e) Special solar cells:
In addition to standard cells, special designs are available, such as coloured cells, cells with printed surfaces or designs with holes (to allow some sunlight to shine through the cell). Any modification of a cell affects performance, however. When coloured glass is used, energy output is reduced by 10–20%.

2.1.3 Types of PV modules

The power output of a PV system is measured in kilowatt-peak (kWp). “Watt peak” is defined as the power output produced by a module under standardised test conditions.
A standard PV module today is made up of 60 solar cells, has a power output of up to 370 watt, a size of 1.7 m2 and weighs about 18 kg. PV modules with 72 cells are also on the market, with top performers in the range of slightly over 400 watt. Their size is 2 m², and they weigh about 25 kg.
Half-cell modules are a new trend in the production of solar modules. The solar cells are cut in half for this application. The smaller cell size reduces the module’s internal resistance, thus enhancing its efficiency. Special designs are now available, in which colours are printed on the glass pane of a module which may then be used for installation on a building facade, for example. Such installations are often no longer recognisable as PV systems.

a) Standard photovoltaic modules
At the core of a PV module are the solar cells, which are connected to each other. To protect the solar cells against mechanical impact, weather and humidity, modules are made up of several layers. In standard modules, the outermost layer consists of glass. The solar cells are connected by solder tabbing wire. The rear side of a standard module consists of a synthetic back sheet and an aluminium frame.

b) Glass-glass modules
In glass-glass PV modules, both the front and rear layers on the outside of the module are made of glass. With the stronger protective effect of the glass layers on either side, glass-glass modules are more robust and have a longer service life. Moreover, they are designed to let sunlight shine through the module, illuminating the area below. The drawbacks of glass-glass modules are slightly lower efficiency and slightly higher prices, and they are less easy to mount than standard modules. Some manufacturers offer customised module assemblies that allow shorter or longer distances between PV cells. With this option, the buyer can choose how much of the area below the module will be shaded, for example when installing the PV system as part of a conservatory roof.

c) Flexible photovoltaic modules
The special structure of flexible PV modules allows them to be bent, opening up a wide range of installation options, including mobile applications; examples are arched roofs (on caravans or boats) and roll-up shading elements. Combining very low weight and high performance, these modules are especially suitable for roofs that
can only bear little additional weight load.

d) Bi-facial photovoltaic modules
Bi-facial modules generate electricity from direct radiation on the module front and from (indirect) light on the back of the module. The rear side of bi-facial modules is transparent – as opposed to standard modules, which have an opaque back. The solar cells are designed to process light from both sides. Light can thus be harvested from the front and rear side of the module, raising the energy output over that of standard (uni-facial) modules. Power output can thus be increased by up to 30%, depending on the reflective properties of the surface behind the module.

Structure of a bi-facial
PV module
Structure of a bi-facial PV module

Bi-facial modules are best suited for reflective sub-surfaces, carports, noise protection barriers and for vertical installation, for example on fences, which makes them suitable for private as well as public use.
An advantage of vertically mounted bi-facial modules is that solar power generation is strong both in the morning and the afternoon. This helps to even out electricity generation and use over the day.

e) Plug-in modules
Plug-in modules are easy to install and may be used by any household without first obtaining approval for grid integration from a power utility. Their maximum rated output is 800 watt (TOR Erzeuger – Technical and organisational rules for power generators,
Type A, version 1.1). For further information (in German), see

f) Current inverter
A current inverter is required to transform the direct current generated by the PV modules into alternating current, which is used in households. Systems with a rated power of 3.68 kVA and more have to have three-phase feed-in to avoid substantial unbalanced loads. These and other technical requirements are specified in the TOR Erzeuger (Technische und organisatorische Regeln für Erzeuger, Technical and organisational rules for power generators). In most systems, the inverter is selected to be capable of dealing with slightly more than the rated power load. This is done because a system runs at highest efficiency in the upper output ranges. The inverter serves as an interface between the PV system and the public grid and ensures smooth operation. Important parameters of inverters are efficiency, service life, functionalities and ease of installation.

g) Power optimiser
Even with optimum planning, shading of some PV modules at certain times cannot always be completely avoided. Shading or soiling cause the power output of the affected module to drop, affecting the performance of other modules as well. Power optimisers, which are attached directly to individual modules, monitor and optimise performance of each single module. This prevents shaded modules from affecting the performance of other modules. Many manufacturers are already integrating power optimisers into
their modules at the production stage. An online tool is used to monitor the individual PV modules and detect drops in power output in good time.

h) Power storage unit
By using a battery pack for temporary storage of PV-generated electricity, even more solar power can be utilised at the place of production, raising the on-site harvesting rate significantly. As a result, the owner of the battery pack is less dependent on the
local energy utility, and potential price increases will have less of an effect.
Lithium-ion batteries are the state-of-the-art technology today. They are maintenancefree and highly efficient and offer large depths of discharge for up to 8,000 charge/discharge cycles. Moreover, they are small and light-weight. In addition to lithium batteries, there are also saltwater-based battery systems. These are a very safe and
eco-friendly option. A power storage system should always be chosen based on the specific needs of a given project.
The average lifespan of a battery pack is 20 years. As batteries are charged only when surplus energy is produced, the system can be expected to go through 100 to 200 charge/discharge cycles per year. This translates to about 4,000 cycles for the full lifespan of a solar power battery used in a PV system.
Given the ongoing developments in the field of e-mobility, battery prices are expected to go down further in the future.

2.1.4 Options for installing a photovoltaic system
There are three basic options for the installation of PV modules:

  1. Mounting the PV modules directly on the building roof
  2. Integration of the PV modules into the building design, where they may replace parts of the roof and/or walls, serve as shading elements or fall barriers, etc.
  3. PV systems on the ground may have a double function, for example as roofing for a carport or car park, or in the management of agricultural land

Rooftop photovoltaic systems
Depending on the pitch of the roof, PV modules are either installed parallel to the roof surface or mounted on racks at an angle to the surface. Parallel installation is usually the preferred option for roofs with a pitch of between 20 and 50 degrees. The solar power yield of PV systems on roofs that are flat or with very little pitch can be
increased by mounting the PV modules at an angle of 15 to 35 degrees. In addition, rain will keep the modules clean, and snow will be able to slide down the module surface. PV systems on pitched roofs are mounted directly on the rooftop, anchored into the roof structure. Rooftop- and wall-mounted PV systems require sufficient rear ventilation to keep them cool. The distance between the module body and the roof surface should be 10 to 15 cm. A PV system with an east-west orientation on a flat roof produces power mainly in the morning and the afternoon.

The following pictures show various options for mounting PV systems on building roofs:

Rooftop PV system on a multi-unit apartment building
Rooftop PV system on a multi-unit apartment building
Rooftop PV plus solar thermal system on a single-family home
Rooftop PV plus solar thermal system on a single-family home
Rooftop PV system on an apartment building, used as a community PV plant, Lavaterstrasse, Vienna
Rooftop PV system on an apartment building, used as a community PV plant, Lavaterstrasse, Vienna
East-west-orientated PV system on the roof of a multi-storey car par
East-west-orientated PV system on the roof of a multi-storey car par
PV system on the roof of Technology Centre 2, Seestadt Aspern
PV system on the roof of Technology Centre 2, Seestadt Aspern
Figure 14: Flexible, lightweight PV modules supply power for the Ottakring station on underground line U3
Flexible, lightweight PV modules supply power for the Ottakring station on underground line U3

An array of bi-facial PV modules provides power for Vienna’s aquarium (“Haus des Meeres”) and shade for the rooftop café. Special steel brackets and custom-made semi-transparent solar panels have been deployed to preserve the impressive view of the city from the building roof:

PV array atop Vienna’s aquarium (Haus des Meeres)
PV array atop Vienna’s aquarium (Haus des Meeres)

Building-integrated photovoltaic systems
Given the significant potential of building surface areas, integrating PV modules into a building’s walls is a viable solar design option. In addition to generating electrical power, building-integrated photovoltaic (BIPV) systems also have a functional role in the building design. Made of customised glass-glass modules, BIPVs may be used instead of conventional facade or roofing elements or for shading. In this design approach, the BIPV system replaces and fulfils the functions of the roof or facade. Integrated designs may be more aesthetically pleasing, they do not require racking, and the conventional roof/facade surfaces will be smaller and thus cost less. On the other hand, BIPV modules which have to fulfil the functions of a roof (moisture and noise protection, thermal insulation) often come at a higher cost compared to conventional modules, and their installation is more complex.
The power output of facade-integrated modules that are mounted at right angles to the ground is 30% lower than that of arrays with an optimum tilt angle.
BIPV systems can be used freely in very diverse designs, as the following pictures show.

BIPV system takes on the functions of the roof
BIPV system takes on the functions of the roof

The BIPV system on the administrative building of TU Vienna is one of Austria’s largest building-integrated PV systems. The modules are mounted on the building’s facade, roof, staircase and terrace and also function as shading elements. The power generated by the system is used directly in the building, with surpluses being transmitted to neighbouring buildings on the University campus.

BIPV glass-glass modules are integrated into the facade of TU Vienna
BIPV glass-glass modules are integrated into the facade of TU Vienna

The Solaris residential building in Zurich sets a standard for sensitive development in an urban setting. Here, state-of-the-art solar panels have been integrated into the building envelope, giving the apartment building a homogenous appearance. Creative solar panel solutions combine structured cast glass on the front and ceramic digital printing on the back; they are not immediately recognisable as PV modules and add to the building’s appeal.

Solaris residential building in Zurich; facade made of PV modules that generate electricity
Solaris residential building in Zurich; facade made of PV modules that generate electricity

More examples from Switzerland and Germany of different BIPV system designs and ways of integrating them into buildings are shown below:

Printed PV modules on the facade of the "Energiehaus" in Lucerne generate electricity
Printed PV modules on the facade of the “Energiehaus” in Lucerne generate electricity
Apartment building with proactive energy design in Zurich. A project by the Stiftung Umwelt Arena Schweiz foundation in collaboration with Rene Schmid Architects
Apartment building with proactive energy design in Zurich. A project by the Stiftung Umwelt Arena Schweiz foundation in collaboration with Rene Schmid Architects

The envelope of the Pierre Arnaud Foundation building in the francophone region of Switzerland is an example of a near-perfect solution that combines several distinct functions. The solar cell facade generates electricity and protects the works of art within against harmful UV radiation, and it is also capable of producing LED projections that are reflected in the adjacent lake surface at night.

Headquarters of the Pierre Arnaud Foundation at Lac Louche Lens, Switzerland by night
Headquarters of the Pierre Arnaud Foundation at Lac Louche Lens, Switzerland by night

The single-pitch flying roof at the Fraunberg municipal offices is a good example of PV roofing in a public space. The overhead structure between the L-shaped building tracts provides a canopy that allows the space to be used as an assembly area.

PV canopy in front of the Fraunberg (GER) municipal office
PV canopy in front of the Fraunberg (GER) municipal office

One of Austria’s first commercially used energy-plus building complexes can be found at the Seestadt Aspern development in Vienna. Electricity for on-site use is generated by a PV system which has been integrated into the building facade and the roof structure, including eaves with projecting solar panels. The site comprises two building complexes, Technology Centres 1 and 2.

PV modules integrated into the facade of Technology Centre 2 at Seestadt Aspern
PV modules integrated into the facade of Technology Centre 2 at Seestadt Aspern
PV modules on the facade of Technology Centre 1 at Seestadt Aspern
PV modules on the facade of Technology Centre 1 at Seestadt Aspern

Free-standing photovoltaic systems

PV systems need not always be installed on buildings; they can also be mounted on the ground, for example in car parks, on public land, in waiting areas, and many more sites besides. There are no limits to the designer’s imagination. With no pre-defined building orientation to take into account, ground-mounted PV arrays can be more freely placed and aligned. They can also perform multiple functions, such as shading or cooling in combination with power generation.
PV systems can be used as canopies for public circulation and parking areas, such as traffic lanes, carports and bicycle parks. Carports with integrated PV modules provide shade and generate electricity at the same time.

Figure 29 Bicycle park with glass-glass PV module roof
Bicycle park with glass-glass PV module roof

Installing PV systems on agricultural land may create a variety of synergy effects. Many different options for placement of PV arrays have become available, and land use can continue as before when panels are mounted vertically or at a height of several metres. The PV modules can be either fixed in place or mounted on trackers that follow the sun.
When the panels are placed in a single continuous line, agricultural use of the surrounding land can continue as before. The benefits are manifold: the solar panels shade the nearby ground, protecting the topsoil and crops against direct solar irradiation (harmful UV radiation). Less water is needed, and the soil is protected against drying out.
PV systems may also be used as fencing, both on agricultural land (enclosures for farm animals) and in private gardens.

Double-function PV system
on agricultural land
Double-function PV system on agricultural land

2.1.5 Community PV systems

When several parties join forces to run a community PV system, they can move from being exclusively buyers of electricity to being so-called “prosumers”, using the selfgenerated electricity to achieve a degree of self-sufficiency. Building owners and tenants in residential buildings as well as operators of office blocks and shopping centres stand to benefit from a communally run PV system, making full use of the roof area available in each case.

2.1.6 Participating in a community PV system

Every resident who wants to draw on the solar power generated by a community PV system has to buy a share in the system. At least two or more parties have to participate. Before building the system, the parties have to make a solar power distribution agreement (with fixed or dynamic allocation).

The community PV system is installed alongside the power supply from the public grid.
As before, the parties can freely choose the utility from which they want to buy gridsupplied electricity. All parties in the building are free to share in the community solar power. The parties have to appoint a system manager who acts as liaison with the grid operator and power utilities.
There are several operating models for community PV systems. The list below presents the main options, which may be modified as required in practice.

Model 1: PV system as infrastructure and “free-of-charge power”
The property owner invests in a PV system and supplies the power to the residents (similar to a communal bicycle storage or laundry room).
Model 2: Installation and utilisation by a residents’ association
Residents/property co-owners invest in a PV system and agree on its operation and the allocation of the power output (e.g. through a residents’ association established for this purpose).
Model 3: An external company installs the PV system and leases usage rights to the residents
An external company makes the initial investment and operates the PV system; the residents lease the right to a share of the power output.
Model 4: An energy utility builds and operates a PV system as single power supplier
The PV system is built and run by an energy utility that supplies on- and offgrid power to the residents.

Models 3 and 4 are a type of facility contracting.
Further information on community PV systems and standard contract forms are available (in German) on the community PV systems platform .

2.1.7 Outlook: Energy communities

Taking community PV systems (multi-party power generation systems) a step further, the future will see the establishment of energy communities, specifically renewable energy communities and citizens’ energy communities. As the relevant EU directives (Renewable Energy Directive and Internal Market in Electricity Directive) are translated into member states’ national legislation, they will promote the establishment of energy communities that can act jointly across property boundaries and regionally, over a certain distance, to
– generate
– use
– store
– consume and
– trade electricity,
and such energy communities may also own and operate grids of their own.
Energy communities will facilitate the local consumption of regionally generated power from renewable resources. Optimising each PV system for self-consumption will be less of a concern as soon as the electricity from a local system can be used in a community that includes more than one building. This will ensure that existing potentials are most efficiently harvested. With less power needed from the grid, the cost of grid tariffs will be reduced. The renewable power systems can thus be run economically while making the most efficient use of available surfaces.

Schematic diagram of an energy community
Schematic diagram of an energy community

2.1.8 Costs of PV systems

The prices of photovoltaic systems have dropped significantly over the last few years. The cost of installing a new system has come down by 50% since 2011. Specific costs are lower for bigger systems than for smaller ones. The payback period of a PV system is shorter if more of the solar power generated is used on site, as this means that the PV system owner has to buy less grid power from a utility and will therefore pay less in tariffs and taxes. The cost of a ready-to-operate PV system is currently €1,880 /kilowatt-peak, or €1,520/kilowatt-peak for larger systems.
Building-integrated PV systems, which are custom-designed for each project, entail higher costs. The costs of PV systems also vary depending on the products built into the system and the performance they offer.

Prices of 5kWpeak grid-connected PV systems 2011-2020
Prices of 5kW peak grid-connected PV systems 2011-2020

2.1.9 Potential of photovoltaics

The electricity produced by PV systems can be used for a variety of purposes. It can power electrical fixtures and appliances in the building, but may also be used for all types of e-mobility, as well as for hot water and heating systems. Any surpluses not needed at the moment can be stored in a battery pack.

Self-produced electricity from a rooftop PV system can be used for a variety of purposes
Self-produced electricity from a rooftop PV system can be used for a variety of purposes

If a photovoltaic system is connected to the public grid, any surplus electricity can be fed into the grid. The PV system operator receives payments for this power in the form of feed-in tariffs. As feed-in tariffs are usually lower than the price of electricity from the grid, it makes economic sense to use as much of your own solar power as possible.
To be able to feed electricity into the grid, the PV system operator has to conclude a contract with an energy utility that will buy the solar power.
Stand-alone PV systems are not connected to the public grid and produce solar power exclusively for on-site use. They are not usually found in cities, but are deployed in remote locations, such as mountain shelters.

2.1.10 Output of PV systems

PV systems in Austria generate 900 to 1,100 kilowatt hours (kWh) per kilowatt-peak per year. A surface area of about 7 m² is required per kilowatt-peak.

The annual power consumption of an average four-person household is about 4,000 kWh. So a photovoltaic system with four kilowatt-peak output produces roughly the amount of solar power in a year which the household needs over the same period. The size of this PV system would be 28 m². About 30% of the solar power could be used directly in the building. The rest could be stored in a battery pack for later use or fed into the grid. As electricity is increasingly used for heating (heat pumps, heating rods, electric water heating), cooling and transport, overall power consumption is bound to rise despite more energy-efficient appliances. New buildings in particular are designed to require very little energy for heating. This can be covered by a heat pump and a heating rod for water heating. The power to operate the equipment can be generated by a PV system.

The influence of orientation and tilt angle on the annual solar power output
The influence of orientation and tilt angle on the annual solar power output

2.1.11 Orientation of a PV system
A PV system will produce its best performance when it is orientated towards the south, with a tilt angle of about 30–40 degrees. Deliberately choosing east or west instead of a southerly orientation may make sense in cases where the need for electricity is highest in the morning and afternoon. East- or west-orientated systems may still reach about 80–85% of the maximum possible output. To ensure optimum performance it is much more important to prevent any shading of the modules by other buildings or trees, or soiling of the surface of the array.

2.1.12 Energy payback period
The energy payback period is the time for which a PV system has to operate until its cumulative power output over time equals the energy expended in making and setting up the system. Typical energy payback periods in Vienna are 2 to 4 years for systems that use crystalline PV modules and 1 to 2 years for systems based on thin film modules.

2.1.13 Service life
Photovoltaic systems have a service life of over 25 years. The only exception are inverters, which can be expected to function for about 10 to 15 years. The systems have to be properly designed, correctly mounted and serviced at regular intervals to achieve the best possible operating results.

2.2 Solar thermal systems

Solar thermal systems transform solar radiation into heat, which is then used to supply hot water or support room heating systems. The basic elements of a solar thermal system are the solar collectors, pump, heat exchanger, and buffer storage tank or boiler.

2.2.1 How do solar thermal systems work?
The energy contained in the sun’s rays is collected and transformed into heat by the solar collectors. The central component of a solar collector is the solar absorber, which contains a heat transfer liquid. This liquid, usually a mix of water and an antifreeze fluid, absorbs the heat and transports it to the heat exchanger. In the heat exchanger, the heat energy is transferred to the buffer storage tank. The cooled-down liquid is then pumped back into the collector, where the heating cycle starts again.

2.2.2 Components of a solar thermal system
The components that are built into a solar collector vary according to collector type (see the section on “Types of solar collectors” below). The following paragraphs describe the components of the most widely used glazed flat-plate collector (97% of collectors installed in Austria were glazed flat-plate collectors in 2018), the heat storage unit and the heat exchanger.

2.2.3 Structure of a solar collector
a) absorber
The central component of a solar collector is the absorber. This is a metal sheet which catches the solar energy. It is usually made of copper or aluminium, with a dark coating. The absorber transfers the heat to a transfer liquid in copper or aluminium pipes underneath the absorber, which are connected to the storage unit.
b) thermal insulation
To minimise solar energy losses, the collector is insulated with special solar glass on the front; the rear insulation typically consists of mineral wool. Collectors are usually contained in aluminium frameworks.
c) heat storage unit and heat exchanger
A so-called buffer storage tank is used in most cases to store the water heated by the sun for days with no sunshine. Packed into at least 10 cm of insulating material, the tank can conserve the collected heat for a few days. Storing heat through the winter months requires a much larger seasonal storage tank with about 50 cm of insulation. The heat exchanger, which transfers heat from the sun-warmed water cycle to the building’s hot water system, is built into or, more rarely, onto the heat storage unit.

2.2.4 Types of solar collectors
a) unglazed flat-olate collectors
Unglazed flat-plate collectors are the most cost-efficient option in terms of production and installation. They generate about 300 kWh/m² energy per year and are suitable for use in low-temperature systems. Ideal applications include water heating for swimming pools and pre-heating of water in building supply systems. They consist of a bundle of plastic pipes or an absorber plate with a hydraulic system plus insulation underneath.

b) glazed flat-plate collectors
The price of glazed flat-plate collectors is about double that of unglazed flat-plate collectors. The annual energy yield is also higher, at about 400–600 kWh/m². Glazed flat-plate collectors are the most widely used type in the European Union. They are deployed mainly for water heating and as an auxiliary heating technology. Glazed flat-plate collectors consist of an absorber, a hydraulic system and insulation. The glass cover provides additional insulation between the absorber and the ambient air.

c) evacuated tube collectors
The price per square metre of evacuated tube collectors is about triple that of unglazed flat-plate collectors. The annual energy output is 450–650 kWh/m². Evacuated tube collectors are suitable for very high working temperatures, but they can also be used to heat water and as auxiliary room heating systems. The absorber and the hydraulic system are contained in evacuated glass tubes. The vacuum has a very strong insulating effect.
Evacuated tube collectors may also be integrated into a building’s architectural design. The benefit of such solutions is their additional functionality, for example as parapets or shading elements.

d) air collectors
Air collectors differ from the collector types described above in that the fluid that is heated as it passes through the system is not water, but air. As air does not heat up as readily as water, air collectors are not as well suited to running room heating or hot water supply systems. Their main application is drying of agricultural products. The warm air can be used to dry fruits, herbs, hay, wood chips and many other goods.

2.2.5 Options for installing a solar thermal system
The collectors for a solar thermal system are usually mounted on the building roof. If the roof is pitched, the collectors may be either mounted on top of it, with a gap of 5−15 cm, or they may be integrated into the roof, replacing part of the actual roofing.
The latter, more complex option entails higher costs, but is more aesthetically pleasing in most cases. If the roof is flat, the collectors have to be mounted on racks. This allows optimised pitch and orientation of the collectors, though care must be taken to avoid mutual shading. The same applies to collectors mounted on the ground and used for district heating or industrial purposes.
When the system is mounted on a building, the collectors may also be integrated vertically into the facade, with or without rear ventilation.

2.2.6 Costs and benefits of a solar thermal system
Installing a solar thermal system for water and/or space heating requires expert analysis and planning. The energy yield of the system varies over the winter and summer seasons. The system has to be designed for optimised utilisation of solar energy on both winter and summer days. A good compromise has to be found between the capital investment needed to install the solar thermal system and the savings to be had in terms of conventional energy costs.
On average, the energy needed for water heating in a three-person household is about 3,400 kWh per year. An average solar thermal system with a size of 6 m² and 400 kWh/m² annual energy output produces 2,400 kWh per year. This will be enough to supply about 70% of the average amount of hot water needed per year by a family of three.
The total cost of a typical solar thermal system for water heating (6 m² collector surface, 300-litre storage tank) comprises about €3,800 for the system, €500 for additional materials needed and €1,500 for the installation. If the system is to be used for water and space heating (15 m² collector surface, 1,000-litre storage tank), the cost is about €8,100 for the system, €800 for additional material and €2,700 for the installation. The exact numbers vary from system to system.

2.2.7 Other applications
Process heat
Large solar thermal systems are capable of producing a significant part of the heat needed in industrial processes. They are more or less universally deployable in the low and medium-temperature range, and many companies are already using them to save money and reduce CO2 emissions.
District heating
Large solar thermal systems can make an important contribution to the heat supply of cities and smaller communities. With production costs of 4 to 6 eurocents per kWh, solar district heating is generally a more cost-effective option than oil or gas.

2.2.8 Output of solar thermal systems
The combined surface area of all solar thermal collectors operating in Austria at year-end 2019 was 5 million m², corresponding to a rated output of about 3.5 GWth. The useful heat output of the systems amounted to 2,081 GWh. This translates into a reduction of CO2 emissions by 353,713 tonnes per year.
To harvest as much solar energy as possible, collectors are precisely orientated towards the south. However, deviations of up to 45 degrees towards south-east or south-west are possible without major heat losses. Orientation is also a determining factor for a system’s output and the required collector size. The farther a collector faces away from due south, the bigger it has to be to yield an equal amount of heat. Most solar thermal systems are mounted at an angle of 30 to 45 degrees. However, ideal pitch depends on the intended use and the place where the system is installed.
Collectors that are used to heat swimming pools are mounted at 0 to 30 degrees, those for water heating at 25 to 55 degrees, for auxiliary room heating at 50 to 70 degrees (suitable for catching the winter sun), and facade-mounted collectors are mounted at an angle of 90 degrees to the ground. Design and installation by an experienced contractor is recommended to ensure maximum yields, with the goal of harvesting energy for direct use in periods of strong irradiation and storing surpluses for periods of less solar heat.

2.2.9 Energy payback period
Solar thermal systems have a very short energy payback period. It takes them about a year to produce an amount of energy equal to the energy input that went into the production of the system.

2.2.10 Service life
According to the Solar Heating and Cooling Programme (SHC) of the International Energy Agency (IEA), solar thermal systems have a statistical service life of 25 years. In practice, however, the systems tend to work for much longer.

Roof-integrated collectors
Roof-integrated collectors
Rack-mounted rooftop
Rack-mounted rooftop

2.3 Hybrid collectors

Hybrid collectors, or photovoltaic-thermal (PVT) collectors, combine solar thermal and photovoltaic technology to cogenerate both electricity and heat. A PVT collector is basically a PV module with piping fixed to its rear side, in which transfer fluid carries heat to a buffer storage tank. In purely PV systems, only a part of the solar radiation intercepted by the PV modules is transformed into electrical power. A large part becomes heat, which remains unused.

Hybrid (PVT) collectors harvest this heat and carry it to a buffer storage tank. As a result, the PV cells do not heat up so much, and output increases. PV modules work most efficiently at about 25 degrees Celsius; when temperatures rise above this level, efficiency drops at a rate of about 0.5% for each one-degree increment. In contrast, PVT collectors create a synergy effect that enhances efficiency per size unit by cogenerating electricity and heat from the same surface area. The diagram below shows the systems side by side for comparison. Given the similarity in design, the life expectancy of PVT collectors is about the same as for PV modules and solar collectors.

Annual energy output of PV, solar thermal and hybrid collectors
Annual energy output of PV, solar thermal and hybrid collectors

2.4 Greening of buildings, a contribution to green

The deteriorating microclimate of our cities is something we are all experiencing firsthand. More and more frequent heatwaves in the last few years are causing increased mortality among vulnerable groups, especially elderly people with restricted mobility and children. Austria has reached a point where deaths caused by excessive heat are believed to outnumber traffic-related deaths. These developments have been caused, among other things, by the spread of sealed, heat-conserving surfaces in combination with insufficient air movement. The difference in temperature levels between urban areas with their growing proportion of sealed surfaces and the surrounding region has been described in the City of Vienna’s Urban Heat Island Strategy. Strategic greening with vegetation has been recognised as an effective means of climate change adaptation.
In densely built-up urban areas, vegetation on the exterior surfaces of buildings is an important contribution to the city’s green infrastructure. It adds to the visual appeal of the cityscape and is an effective way of using hitherto unused surfaces. The following section discusses green roofs and vertical greening systems.

2.4.1 What is the impact of greening on the immediate environment?

The deployment of green infrastructure in the city has an important role to play in many aspects of proactive urban development policies, both with respect to new urban developments and the existing built environment.

Green outdoor air-conditioning
In contrast to sealed surfaces – asphalt, concrete, glass and mineral surfaces, among others – vegetation-covered surfaces provide an active cooling infrastructure. While the temperature on sealed surfaces may climb to 70 Co and even higher on summer days, the surfaces of plant leaves hardly heat up to more than ambient
air temperature. Vegetation also produces shaded areas. Water evaporates from the plant leaves and planting medium, cooling the surrounding air. The cooler air sinks to the ground, where it heats up again. As the heated air rises, the cycle starts over again.
The “latent” heat in this cycle is not perceptible, but is transformed to perceptible heat in the condensation process. The cooling effect of evaporation from plants continues through the night as well. This has the effect of keeping the ambient air around a building cooler.

Rainwater retention
Deployed for rainwater management purposes, even sparsely vegetated green roofs are capable of holding back up to 90% (annual mean) of the total amount of precipitation. With special designs and intensive roof greening, even 100% rainwater retention is possible, including the efficient retention of stormwater after heavy rainfalls. The rainwater is taken up by the plants, and either stored or given off through evaporation. Because green roofs prevent immediate stormwater run-off and release the water over time, they are much more efficient than conventional roofs in reducing stormwater flow to the sewer system. Moreover, greywater may become usable thanks to innovative green roof and facade systems. Green roofs could, for example, function as water filters, and the filtered water could be used to flush toilets inside the building.

Well-being and biodiversity
Greening buildings improves thermal comfort, helps to purify ambient air and mitigates noise, thus promoting human well-being and contributing to a healthier environment and enhancing quality of life. This leads to greater productivity and contentment, increases the recreational value of the urban environment and even reduces time spent on sick leave.

Another major role of vegetation on the exterior surfaces of buildings is that it helps to preserve and improve biodiversity. Green roofs and facades are valuable stepping-stone biotopes at a safe distance from the pesticides and herbicides that are used in agriculture. When implemented as part of an urban ecological strategy, they can provide diverse habitats for endangered species such as wild bees, butterflies and birds.

Figure 40 Benefits of greening bildings at the level of the city
Benefits of greening bildings at the level of the city
Figure 41 Benefits of greening at the level of the building
Benefits of greening at the level of the building
Benefits of greening at the level of the building

Benefits at the level of the building
The greening of buildings creates benefits not only for the city and society as a whole, but also at the level of the individual building: vegetation has a positive effect on temperatures on the building envelope and in the nearby environment. Synergies can be used in terms of natural ventilation, energy production, efficient water use, shading and the durability of materials. More specifically, a greened outer shell prolongs the life of the building because the vegetation acts as a temperature buffer, protecting materials physically and mitigating the effects of extreme weather conditions.

The plants protect the building against solar irradiation, hail and rain. At the same time, the vegetation enhances the value of the property. Plants store CO2, produce oxygen, filter fine particle dust from the air and reduce the building’s energy requirement, improving lifecycle assessments and carbon footprint.

Adiabatic cooling (shading and evaporation chill prevent the building from heating up) and high-quality insulation help to reduce the operating cost of heating and cooling systems. As regards heat insulation in winter, measurements on an ivy-covered wall
have shown a temperature difference of 3°C between the outer leaves and the wall surface. Using plants effectively as a natural shading element will reduce the amount of primary energy needed by a building. Well-planned and properly tended vegetation also incurs lower maintenance costs than technical solutions for shading on the outside of a building. Deciduous plants shed their leaves in winter so that sunlight
can pass through. Because the air around a greened building is preconditioned by the improved ambient microclimate, natural ventilation becomes possible again, minimising the need for air-conditioning.

Considering the growing amounts of energy required to operate buildings and the challenges ahead as a result of climate change, which will further drive up energy consumption, a number of measures will have to be taken, ranked by priority as follows. First of all, educational efforts are needed to bring about changes in user behaviour. At the same time, passive strategies – including thermal insulation and shading of buildings – have to be pursued to reduce energy consumption, and building environments have to be actively cooled by vegetation to allow natural ventilation.
Additionally, active strategies have to be implemented as needed. These include the decentralised generation of energy from renewable resources, helped by greenery on and around buildings. Suitable combinations of photovoltaics and solar thermal
energy will increase the overall efficiency of solar technology and contribute meaningfully to preserving and enhancing biodiversity through the creation of habitats.

2.4.2 Green roofs

Which types of green roof systems are there?
The main difference in green roof types is between extensive and intensive (deepsoil) greening. Both types are described in detail in the City of Vienna’s green roof guidance Leitfaden für Dachbegrünung (2020) and the relevant Austrian Standard ÖNORM L 1131 (2010) on green roofs. The documents specify regulatory requirements with respect to green roofs, including certification of the different layers and overall structure. Green roofs differ in thickness, weight, functionality and plant communities.
They can be installed on flat or pitched roofs and may have a single- or multi-layer structure. Depending on the roof’s intended functions, a green roof system may be installed on the whole or part of the roof surface and may also be designed as a hybrid system, i.e. a combination of different types in terms of thickness and habitat structure. A connection to the water supply system is required for an intensive green roof and recommended for an extensive system. In addition to primary horticultural work – planting and initial care until plants are established – a green roof also requires regular care and maintenance.

The minimum thickness of a lightweight, extensive green roof is 8 cm. Low-growing plant species, including succulents, mosses, herbs and grasses, dominate the picture.
Extensive green roofs require little care. As this type of green roof is not usually used by humans, it provides a valuable habitat for plants and animals. Its weight is 80 to 150 kg/m². An extensive green roof can easily be combined with a solar installation (see Chapter 3).

Intensive green roofs are at least 20 cm thick and heavier in weight. Depending on the thickness of the substrate, a wide variety of plants can be grown, including trees, which need a planting substrate of at least 80 cm and have to be protected against strong winds. An intensive green roof may fulfil all the functions of a garden, or even
a park, provided it receives sufficient care and is supplied with water as needed. It can serve as a recreational space or a communal gardening area; people may use it for sports or vegetable growing. The weight load of an intensive green roof is 300 to 1,000 kg/m². It may be combined with a solar energy system mounted on a pergola
(see Chapter 3).

Figure 44: Extensive green roof on Dresdner Strasse
Figure 45: Intensive green roof on Favoritenstrasse

Components of a green roof
Different plant communities, from ground-covering succulents to shrubs and trees, may exist on a green roof, depending on substrate depth, available root space and care of the green roof. Initial greening may be done with seeds, cuttings, potted or baled plants, or even pre-cultivated sod mats and turf.
The planting medium provides space for the plant roots and stores water and nutrients. Planting mediums that meet the relevant standards consist of mineral openpore aggregates, such as hard-fired clay, expanded clay or shale, or recycled brick chippings,
which are mixed with organic materials (for example, quality-certified compost) in varying proportions. The share of organic material is low in extensive green roofs and high in intensive systems. Austrian Standard ÖNORM L 1131 contains a detailed description of the planting medium requirements (storage stability, pore air capacity,
water retention capacity, drainage effect, proportion of fines, etc.).

Figure 46: Typical green roof structure Vegetation
Planting medium
Filter layer
Drainage and retention layer
Protective membrane/retention mat
Waterproofing (roof-sealing) membrane
The filter layer separates the planting layer from the drainage and retention layer. This prevents the drainage layer from becoming clogged with fine particles which could otherwise be washed down from the planting layer. With the filter layer in place, the fine particles, which are also important for the plant roots, are retained in the planting medium. The filter layer is made of a geotextile that is water-permeable and can be penetrated by plant roots. It has to keep its permeability for decades.
The drainage and retention layer is designed to control stormwater runoff after heavy rainfalls and acts as a reservoir for rainwater. It consists either of mineral (recycled) materials, prefabricated synthetic filling materials or material mixes with or without water storage characteristics.
The protective membrane is a geotextile as specified in Austrian Standard ÖNORM L 1131. It protects the waterproofing membrane from damage during construction and after completion of the project.

An additional root barrier is necessary if the waterproofing membrane itself does not protect against root penetration. Standard root barriers consist of mechanically arranged membrane layers.

The waterproofing (roof-sealing) membrane has to comply with the standard requirements and protect against root penetration. It is made of a bituminous membrane plus a root barrier or consists of several layers of sealing membranes. The City of Vienna’s rules for subsidies granted for green roof construction specify that the waterproofing membrane has to be asbestos- and PVC-free. Waterproofing membranes containing substances that inhibit root growth or are biocides as defined in the EU Biocidal Products Regulation (No. 528/2012) are not allowed in green roof systems and materials.
The root-barrier functionality of products has to be assessed pursuant to the relevant guideline of the Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FFL), a German industry and research association in the field. A list of roof-barrier membranes
is published and annually updated by the Austrian industry association Verband für Bauwerksbegrünung.

2.4.3 Roof designs that are suitable for greening
The following roof types are suitable for greening (regardless of roof pitch):
Cold deck roof: ventilated double-skin design with load-bearing upper layer, single-skin design without thermal insulation
Warm deck roof: single-skin design with thermal insulation below the waterproofing layer
Inverted roof: single-skin design with insulation above waterproofing*
Plus roof: a warm deck roof with additional inverted insulation on top of the waterproofing membrane (to be treated in the same way as an inverted roof)

Please note: Water storage and vapour permeability have to be
taken into account in the build-up of an inverted roof, which needs to be adjusted accordingly. Inverted roofs are always constructed as multi-layer systems.
In principle, roofs are capable of being greened regardless of their pitch. Standardcompliant greening is possible on roofs with pitches of between 1.8% (1-degree pitch) and 58% (30-degree pitch). Roofs with a pitch of 9% or more require slippage or thrust protection to prevent the root barrier and waterproofing layer from slipping, those
with 26% or more have to be protected against slippage of the entire construction (see Chapter 5.5, Planning aids and tools from the various specialist fields). Green roofs outside this pitch range are special engineering projects with highly stringent demands in
terms of professional planning and construction (Austrian Standard ÖNORM L 1131, “Garden design and landscaping – green building roofs – design, installation and maintenance requirements”

Figure 47: Biodiversity roof, Sargfabrik residential development

Green roof options:

a) Flat roofs on new buildings
Multi-storey residential buildings and major industrial complexes offer highly scalable options for green roofs. In principle, any flat roof surface can be used to establish a green roof, with a wide variety of potential uses and benefits. The land use and development
plans of the City of Vienna can mandate greening of flat roofs. See relevant content of the development plans (Bebauungspläne) § 5 (4) k (

b) Flat roofs on existing buildings
When a flat roof is in need of renovation, the possibility of installing a green roof should always be considered. Existing gravel roofs in particular harbour great potential, as they are usually designed with sufficient load-bearing capacity for an extensive
green roof. In some specific areas of Vienna, this is prescribed by regulatory mandate.

c) Retention roofs – a special type of roof
Retention roofs are capable of holding large quantities of stormwater runoff from heavy rainfall events, effectively reducing the workload of sewer systems. State-of-the-art retention roof technologies include regulator systems for controlled water
accumulation. We expect the number of completely flat (zero-degree) roofs to grow in future as a result of the improvements made in stormwater management and because of the favourable microclimatic effects to be had.

d) Pitched roofs
Extensive greening is possible on pitched roofs (standard-compliant solutions up to 30 degrees), with slippage/thrust protection required for slopes of 9 or more degrees.

e) Biodiversity roofs
Extensive or semi-intensive green roofs may be designed specifically to create nearnatural spaces. Dedicated to biodiversity in terms of habitats and species, these roof system variants contain a broad variety of structures (planting media, deadwood, rocks, nest boxes, temporary water surfaces, etc.). Existing green roofs can be modified
to create a biodiversity roof.

2.4.4 Greening of facades

Facades that are suitable for greening

The requirements which facades have to meet depend on the desired outcome. Facade type is a constraint on vertical greening projects and determines the choice of greening system in both new-build and existing buildings.

Among facades suitable for greening, these are the three types most often found in Austria:

  1. Composite thermal wall insulation systems
  2. Solid brick or concrete walls
  3. Rear-ventilated cladding

In existing structures, suitability for greening has to be determined by an expert on a case-by-case basis. In addition to load-bearing aspects, the surface characteristics have to be considered, as some surfaces (e.g. sandy/crumbling or highly reflective surfaces) are unsuitable or present major difficulties for greening.

2.4.5 Which types of vertical greening systems are there?
The different technical approaches to facade greening are regulated by guidance published by the City of Vienna, the Leitfaden der Stadt Wien für Fassadenbegrünung (2020), and Austrian Standard ÖNORM L 1136 (2021), including certification of vertical greening systems. Green facades differ in terms of system build-up, weight, functionality and plant communities. The main types are ground-bound systems with climbing plants, mixed systems based on containers, and wall-bound vertical greening systems.
The vegetation consists either of climbing plants or, in the case of living walls, grasses, herbs and herbaceous perennials. In addition to primary horticultural work – planting and initial care until plants are established – a green facade also requires regular care and maintenance work.

Components of vertical greening systems
Different system components are needed, depending on the vertical greening system that has been selected based on suitability for a specific facade and greening objectives. They comprise plants, growing media, plant support systems, supply technologies, etc.

Ground-bound vegetation
If the ground next to the wall offers enough space for plant roots, ground-bound vegetation is usually the most cost-effective facade greening solution. The ground should thus be examined and soil conditioning carried out where needed. The design should
ensure that plants receive enough water, either by supplying rainwater to them or via
an irrigation system. The root space size requirements are determined by the greening
objective, the plant species and the envisaged height of the vegetation cover. A minimum
depth of 60 cm and a minimum volume of 1 m³ per plant are advisable.
Climbers can either be self-clinging, such as Virginia creeper, or require a support structure.
Different support structures are in use, depending on the growth patterns of the plants and whether or not they have adhesive organs. There are more than 20 different species of climbers in Austria, some of which may grow to heights of up to 30 m.

Figure 49: Ground-bound facade greening, Kandlgasse

A range of different support structures are in use, such as cables, mesh or trellises.
Important aspects to consider are how the system is fixed to the wall (minimising penetration), load-bearing characteristics, and that it has to match the growth pattern of the plants (twining, attaching by tendrils, etc.). This will ensure a pleasant appearance of the greened facade.

Figure 50: Different climbing strategies of climbing plants

Green vegetation curtains may also serve as external shading elements. The load-bearing characteristics have to be specified taking into account the plant type (weight, climbing patterns), maintenance intervals and additional loads that have to be expected (e.g. wind, snow).

Plants in containers
If ground root space is not available or if the selected plant species cannot be expected
to grow to the desired height from the ground, root space has to be created in
front of the facade. The containers used for this purpose must be made of materials
suited to long-term use (e.g. fibre cement, mineral wool composite, high-strength
concrete, metal, etc.) and capable of being recycled. Requirements in terms of material
properties have to be taken into account. The technical specifications include
frost, UV and fire resistance, stability against external mechanical forces and vandalism.
Additionally, ecological criteria (such as regional production, short transport
routes) should be applied.
When containers are used as planter boxes, the planting substrate has to be as durable
and structurally stable as in a green roof. This is why it is typically built up in a
layered structure. If water can accumulate in the container, this helps to supply water
to the plants as needed and save on overall water consumption.
Figure 51: Facade greening with climbing
plants in containers, Zedlitzhalle
59 Solar Energy Handbook – Guidance on Combining Solar Technology with Green Roofs & Vertical Greening Systems 60 Technologies
Figure 52: Wall-bound vertical
greening system, Municipal
Department 48
Wall-bound vertical greening system
Structurally speaking, any wall-bound greening system, also called a green wall, living
wall, or vertical garden, is a rear-ventilated mounted facade. Water-bearing levels are
separated from the building. The plants receive water and nutrients as needed from
automated irrigation and nutrient-supply systems. Suitable plantings range from
intensive vertical gardens with flowering ornamental shrubs to flower meadows with
a more extensive look. In most cases, this type of vertical greening also functions as insulation and protective cladding for the building. Its significant microclimatic effect is based on evaporation chill from the plants and the growth medium.
In systems of a certain size, the installation of sensors and self-learning control systems is recommended to ensure optimum water and nutrient supply as needed.
Control systems must be housed in a frost-free utility room and accessible for maintenance

2.4.? Where to use vertical greening
Large-scale vertical greening is potentially possible on nearly all facade types, both on new and existing buildings. An examination of relevant site conditions will reveal potential benefits, such as the replacement of technical external shading solutions or optimisation of energy efficiency, as well as constraints, e.g. fire safety requirements, in a timely manner.

If self-clinging plants are to be used on an existing facade, this first has to be examined to ensure that it is structurally intact. Separator bars can be used to prevent self-clinging plants from growing onto neighbouring buildings, roof structures or windows. Some producers of composite thermal wall insulation systems are already designing and offering systems specifically for vertical greening. Special considerations apply regarding climbers with shoots that turn away from the light (e.g. ivy). They may grow into the space behind ventilated walls or underneath windowsills, cracking open the structures. The same is true of twining plants with similar characteristics that are trained on structural supports. Particular care is necessary when existing facades have external technical shading installations which could be damaged by plants pushing underneath and eventually bursting the structure.

The orientation of the facade also plays a role in greening. Climate conditions on eastand west-facing facades are much more moderate than on north- and south-facing walls. On north-facing walls, plants have to make do with less natural sunlight. On the south side, they are exposed to hot and dry conditions and need much more water as a consequence. Suitable plant communities can be selected for all orientations and greening objectives (see Chapter 3). This has to be done in the context of professional project design. Likewise, planting and ongoing care also require professional expertise.
The substructure and system components of a vertical greening project have to be adjusted to the type of greening to be implemented.

2.4.? Costs of roof and facade greening
The factors determining implementation and maintenance costs are: project size, materials used, presence/absence of connections to electricity and water supply, as well as accessibility, which in turn determines the use of elevating work platforms and other equipment, industrial climbers, and integrated maintenance systems. Project planning costs are typically 5−15% of the total project realisation cost.

Prices of Green roof systems (ÖNORM L 1131 ASI 2010)

Indicative price ranges for the realisation of building greening projects by professional contractors, compliant with Austrian ÖNORM standards, net of VAT (as of 2019):
Cost of extensive roof greening (thickness 8 cm or more): 25 to 50 Euro/m2
Cost of intensive roof greening (12–30 cm or more): 50 to 100 Euro/m2
Cost of solar green roof/PV green roof system (green roof installation plus solar PV system racking, without modules): from 65 Euro/m2
Professional care and maintenance of extensive and intensive green roofs: Depending on working conditions – 55 to 100 Euro/hour

The price ranges are indicative, based on 1,000 m2 projects. Pitched green roofs require slippage protection, in line with the angle of pitch.
Care and maintenance work on extensive green roofs that is carried out according to the relevant standards is comparable to that on gravel roofs in terms of intervals and the extent of work needed. The standard differentiates between primary horticultural work – planting and initial care until plants are established – and ongoing care and maintenance. Design quality has an important influence on the extent of care required for intensive green roofs.

Prices of vertical greening systems (ÖNORM L 1131)
Indicative price ranges for the realisation of building greening projects by professional contractors, net of VAT (as of 2019)

Relevant cost factors in greening projects using climbing plants are: whether or not scaffolding is needed for the installation of the system; which type of system (with or without plant support structures) is used; and whether plumbing work is required/included in the contract. Design quality has an important influence on the extent of care required for vertical greenery.

Cost of ground-bound facade greening (climbers with or without support structures): 50 to 500 Euro/m2
Cost of facade greening with climbing plants in containers on the ground* (climbers with or without support structures): 250 to 800 Euro/m2
Cost of wall-bound vertical greening systems (green or living walls – herbs, grasses, herbaceous shrubs): 500 to 1,500 Euro/m2
Care and maintenance (at 2- to 5-year intervals) of groundand wall-bound facade greening systems (depending on the selected plants, ease of access, technical equipment, e.g. sensors and control system): 10 to 50 Euro/m2 per year

To be continued!

2.5 Buildings and neighbourhoods for a sustainable future

Combined options of solar and green roofs/vertical greening systems and usage synergies

3. Combined options and usage synergies

3.1 Solar technology and green roofs/vertical greening systems

3.2 Combined solutions and model projects

Model projects of solar and combined solar & green roof/vertical greening systems

4. Model projects

4.1 Solar photovoltaics

4.2 Solar thermal systems

Planning guidance to ensure the long-term efficient operation of solar technology and combined solar & green roof/vertical greening systems

5. Planning guidance

5.1 Guidance on combining solar technology with
green roofs/vertical greening systems

5.2 Building type, usage and ownership – factors to consider in connection with greening of buildings

5.3 Example of a combined solar green roof system

5.4 Fire safety

5.5 Planning aids and tools from the various specialist fields

5.6 Guidance on care and maintenance of solar technology and green roofs/vertical greening systems

5.7 Step-by-step implementation guides for solar technology and green roof/vertical greening systems

Government funding to help fund the installation of solar
technology and green roofs/vertical greening systems.

6. Government funding

6.1 Funding from the Province of Vienna

6.2 Federal grants & subsidies