This chapter explains the development of the “PV Rooftop Garden” and demonstrates exemplary use cases. The design development was based on the results of the user survey, the literature review, and an analysis of the data from the testing facility, as outlined above. In the following section, the architectural and construction details, as well as the specific plant selection and garden design, are outlined.
4.2. Construction Design: Key Results
The literature review showed that the Viennese zoning and building code is very important for the design of the structure. In buildings that have already reached the maximum permissible building height, only temporary structures—and not space-forming structures—can be built on a roof. The maximum building height also stipulates that a certain distance from the edge of the building must be maintained in peripheral areas. A "glass cube" may only be exceeded on one-third of the length of the building. The "glass cube" is the area that can be used at an angle of 45° to the horizon for roof extensions or similar structures [
33].
At the outset of the design study, the necessary framework conditions were established. The “PV Rooftop Garden” was designed to fulfill the following requirements, which should be suitable for a wide range of flat roof applications:
A combination of an integrated system with planting, recreational areas, and PV elements within the same roof surface area;
Applications for new and existing buildings;
No fixed connection to the existing roof;
Consideration of the supporting structure of the building; and
Integrated rainwater management.
Figure 5 provides a detailed overview of the technical concept of the “PV Rooftop Garden”. It consists of (1) transparent PVs creating a weather protection layer and (2) a secondary support structure that transfers the load to the (3) main support structure. The main support structure is mounted on three columns (4), which are fitted on a frame of steel beams (5). The steel beams and the steel plates (6) are installed in the ground structure of the green roof. The soil acts as ballast and counteracts the wind suction force. In addition, plant troughs (7) are mounted on the main support structure as an additional load. They are also used as a rain gutter to collect water, store it, and transfer it to the columns for discharge. In the following section, the concept as well as the influence of the framework conditions on the design are explained in detail.
The main goal of the project, as discussed above, was to develop a
fully integrated system where a green roof can be combined in a new way with PV panels, simultaneously creating a recreational area for the residents of the building. Thus, each square meter must be used multiple times for these different functions. In order to fulfill these requirements, a new mounting option for the PVs had to be found. To create a usable space between the top edge of the roof and the bottom surface of the PVs, at least 3.5 m were needed so as to avoid the impression of a confined space. By increasing the height of the supporting structure, the design of the PV mounting had to be redeveloped and the strength significantly increased compared to a conventional assembly. In addition to the load transfer, power lines, irrigation hoses, and the drainage of the PV modules had to be included into the design. Subsequently, a three-column prototype made out of steel, as seen in
Figure 5, was found to be the ideal configuration, as it could meet all requirements and was advantageous from a constructional point of view, since due to the incline the necessary stiffening could be realized. In addition to a slim design that could be aesthetically integrated into existing roof landscapes and was optimized from a constructional point of view so that additional loads were as low as possible, it was important to have all installation cables inside the structure. This clean design is especially important for publicly accessible roofs, so that the risk of vandalism is reduced. One column is used for all cables to connect the PV elements to the inverter and to implement an additional lightning system, which can be mounted under the PVs to make the roof usable in the evening. In one column, the rainwater, which is gathered in the plant troughs and stored there until the storage capacity is reached, is collected and transferred into the soil structure, where a second storage layer is located. To minimize the amount of tap water needed for watering the plants, additional water storage is needed. This also reduces the peak drain into the sewer, as the storage acts as a buffer. The amount of water that evaporates directly onsite is subsequently significantly increased. Both water storage layers act as a rainwater management system. The third column is used for the irrigation system for the plant troughs. In times of drought, it may be necessary, despite storage, to water the plants and therefore increase the cooling effect.
In new buildings, considerations of additional loads and the coordination of construction with load-bearing elements in the building grid can be adapted well to the building design. The weight of any standard PV panels mounted directly on the roof is usually unproblematic, if the building structure is sufficiently strong. Mounting them at a height of 3–4 m above the roof, as suggested for the “PV Rooftop Garden”, adds complexity. In addition to the load imposed by the substructure of the PVs and the panels themselves, wind and snow loads must be taken into account. The main focus of the research project was nevertheless to bring the additional quality of the “PV Rooftop Garden” to existing buildings.
For existing buildings, the initial outset may be more challenging. The structure of the building and the ability to carry additional loads are crucial in this context. Roof penetrations in existing buildings are very difficult to carry out, and there is a considerable risk of water ingress through leaky joints. This can potentially lead to serious structural damage, which must be ruled out under all circumstances. In addition, a fixed connection to the existing roof carries the risk that vibrations caused by wind in the PV support structure are transmitted to the building. This can spread, especially in reinforced concrete buildings, throughout the whole building [
44] and can significantly affect the living or working quality of the users of the building.
Taking into account all of these requirements, it was crucial to find a solution where no fixed connection to the existing roof was necessary. Using the weight of the soil substrate from the green roof was an obvious option. Two conditions were decisive for the design. Due to the dry weight of the substrate, requirements with regard to wind loads had to be fulfilled. The plant troughs are an optional element of the “PV Rooftop Garden” and therefore could not be used as an additional weight. The load-bearing capacity of the building must be checked in a water-saturated state. Due to the use of different substrates, drainage layers, and degrees of saturation with water, the weight of green roofs can vary greatly even with the same construction height [
20]. This fact can be taken into account for different residual capacities of existing buildings.
The three columns are mounted on a base plate (1.5 m × 1.5 m), which is highlighted in red in
Figure 6. The base plate is weighted down by the applied green roof structure. In addition, the surcharge on the steel beams is used as an additional load.
Figure 7 shows the different layers of the roof structure. The ground plate is placed on a protection fleece so that the roof sealing is not damaged.
In order to be applicable to most existing building types, consideration must be given to the underlying supporting structures and the alignment of the columns. In the development of the system, different building types were studied.
For further development for real-life applications, the project focused on three typical building types, which are shown in
Figure 8. The single-staircase residential type (1) offers a zone between the individual apartments. The staircase opens up directly into several apartments without any corridors. There are apartments with both one-sided and multisided exposure. Depending on the number of apartments, the tract depth is typically between 4 and 8 m and 10 and 13 m. In the central-staircase residential type (2), several apartments are accessed via a centrally located corridor. Depending on the depth of the apartments and the design of the corridor tract depths, between 9 m and 13 m is possible. The open-plan office type (3) is accessed centrally via a staircase. The tract depths vary more in office buildings than in residential buildings, but since offices are generally larger, they have more structural elements to allow for wider spans. In the project, a typical tract depth of 27 m was assumed as an exemplary case study.
In addition to the requirements of the different building types, the choice of PV modules also has a significant influence on the spans. For the exemplary design, PV modules with a size of 1.00 m × 0.84 m were selected. These glass-glass modules are made of translucent PVs with a residual permeability of about 30%.
Due to the dimensions of the individual PV modules, the design allows for a gradual increase or decrease in the dimensions of the system by 1.6 m or 0.8 m, respectively (see
Figure 9).
During the design development, it became clear that the optimal maximum size of one “PV Rooftop Garden” element, called the “PV Pergola”, was 7.78 m × 6.80 m. The size was defined based on the size of the PV elements, the structural elements of the buildings, and the dimensions of the supporting structure. With smaller units, it is possible to react to different framework conditions and to adjust the size accordingly. The increments of 1.6 m or 0.8 m enable the positioning of the system to be on or in close proximity to supporting elements of the building structure.
To ensure adequate drainage, the minimum incline of the PV panels is approximately 2.1°. If the number of module rows is reduced, this also increases the incline. The relatively low angle of incline also allows for the modules to be aligned both north–south and east–west without causing any significant losses in the PV yield. Due to the modular design, several “PV Pergolas” of different dimensions can be combined, as in a modular system (see
Figure 10). This is of particular importance for existing roofs, where chimneys, staircases, or lift shafts have to be considered.
Figure 10 and
Figure 11 both show four “PV Pergola” elements with different numbers of rows of PV modules connected together to demonstrate how individual elements can be attached to each other. Combined with the plant troughs, the “PV Pergolas” create a weatherproof shell. The troughs act as a rain gutter and subsequently store and direct the water to the columns.
The plant troughs are designed to be used as an additional element in the “PV Rooftop Garden” system. They are, however, not mandatory, because there might be applications where they are not needed or cannot be implemented.
Figure 12 shows a variant of the “PV Rooftop Garden” where no troughs are used. It is also possible to use the plants troughs just at some points to form a green hem or provide green accents.
4.3. Plant Selection: Key Results
This chapter gives a summery of the performance of the vegetation under the shade of PV-Panels. The results of the pretesting provided important knowledge for the choice of the transluscence of the PV-Panels and the chosen vegetation in the in field prototype-testing.
4.3.1. Plant Selection: Pretesting
During a prestudy for the “PV Rooftop Garden”, practical tests were undertaken in order to gain more in-depth results for the use of ornamental (gardening) plants on intensive green roofs and to create representative planting beds. In the first step, the grade of translucence, which would allow for a broader range of plants to grow under the panels, had to be evaluated. Therefore, four types of PV modules ranging from 10%, 20%, and 30% to 40% translucency were used to monitor the plant health and growth rate of indicator plants with various light demands (
Koeleria glauca, Dianthus carthusianorum, Phlox paniculata Peppermint Twist,
Geranium cantabrigiense, Thymus vulgaris, Salvia officinalis, and
Fragaria vesca). The plants were tested in 1-m
2 fields for a full vegetation period. The plants under the modules with 20% and 30% translucency showed the highest growth combined with the best vitality [
45].
4.3.2. Plant Selection and Green Roof Development: In-Field Testing within the Prototype
For the plant selection and design of the “PV Rooftop Garden”, in general the same requirements used for any type of extensive or intensive green roof regarding load weight, waterproofing, drainage, growing media, and suitable plants should be fulfilled [
19,
20,
21,
46,
47]. As the “PV Pergola” is not connected to the building, the weight of the substrate will provide the needed weight to hold the construction down against wind suction. In the case of a retrofit, the possible and needed height of the substrate has to be calculated based on the actual structural requirements of the building and depending on the specific wind and snow loads and the additional weight of the construction. In the testing facility of the BOKU (on the third floor of a university building in Vienna) (see
Figure 4, the green roof layer had to be 20 cm (with a maximum substrate weight of 1400–1500 kg/m
3) to fulfill the minimum structural requirements. Additional irrigation is necessary if not enough rainwater for sufficient watering of the plants can be stored in the substrate. This also has to be taken into account in the dimensioning of the substrate.
In the testing facility at the BOKU, three different light zones that were to varying degrees influenced by the shading of the PV panels were identified with hemispheric photography:
Zone 1: mostly unshaded areas with little influence from the PVs (260–360 MJ/(m2year));
Zone 2: semishaded areas (160–260 MJ/(m2year)); and
Zone 3: mostly shaded areas with strong influence from the PVs (under 160 MJ/(m2year)).
Within these areas, different plant species, annuals, herbaceous perennials, grasses, and climbing plants were surveyed over a period of three years. On the basis of their vitality, growth, and flowering success after the three-year monitoring period, suitable plants for the different zones could be identified.
Lawn: This was a mixture of species in a dry lawn area consisting of 80% grass (Cynodon dactylon, different species of Festuca, Lolium perenne, and Poa compressa), 8% leguminoses (Anthyllis vulneraria, Lotus corniculatus, Medicago lupulina, and Trifolium repens), and 10% herbs (Achillea millefolium, Bellis perennis, Dianthus carthusianorum, Galium verum, Hieracium pilosella, Petrorhagia saxifraga, Plantago media, Potentilla tabernaemontani, Salvia nemorosa, and Thymus pulegioides) seeded in a semishaded to sunny zone. The grasses grew fast and successfully, while the herbs and leguminoses only grew in the mostly sunny border zones.
Edible plant species/plants for urban gardening:
Fragaria vesca and
Allium schoenophrasum grew in all zones, but there was less flowering in zones 2 and 3, which had more shade.
Capsicum annuum,
Foeniculum vulgare, and
Ocimum kiliman x
basilicum had a very good vitality and growth rate in all light zones. In addition,
Mentha x
piperita,
Eruca sativa, and
Origanum vulgare successfully grew in all light zones and spread so dominantly that they had to be reduced after the second year.
Solanum lycopersicum grew in both the mostly unshaded and semishaded zones.
Raphanus sativus var.
sativus,
Brassica oleracea convar.
capitate,
Beta vulgaris subsp.
Vulgaris, and
Lactuca sativa var.
capitata had no satisfying growth in zones 2 and 3.
Phaseolus vulgaris var.
vulgaris had no satisfying growth rate in any zone. A summary of the suitability of edible plant species for the different light zones is shown in
Table 2.
Annual and biannual plant species:
Antirrhinum majus self-spread wildly over all light zones,
Verbascum nigrum grew in the sunny and semishaded zones, and
Calendula officinalis disappeared after the first year.
Echium vulgaris and
Silene vulgaris self-developed during the third year in the semishaded and sunny zones. A summary of the suitability of the annual and biannual plant species for the different light zones is shown in
Table 3.
Grass species:
Deschampsia cespitosa,
Calamagrostis x
acutiflora,
Carex ornithopoda, and
Festuca gautieri grew successfully in all light zones. The vitality, flowering, and growth rate of
Bouteloua gracilis was better in the sunny zone.
Koeleria glauca and
Pennisetum alopecuroides did not survive at all. It was also clear that
Bouteloua gracilis and
Carex ornithopoda were very easily overgrown by dominant neighbors and therefore needed to have suitable adjoining plants. A summary of the suitability of the grass species for the different light zones is shown in
Table 4.
Table 5 gives an overview of the suitability of ornamental perennial plant species in the different light zones of the PV Rooftop Garden: Different species of
Hosta as well as
Hemerocallis lilioaspodelus, Aster divaricatus, and
Primula denticulata were suitable for all light zones.
Coreopsis lanceolata had good vitality in all zones and self-spread wildly across the entire planting bed.
Iris foetidissima (the only
Iris species suitable for semishading) successfully grew and flowered in all areas and the bright orange fruit added a great visual aspect in winter. Other species of
Iris are only recommended for sunny areas.
Campanula portenschlagiana showed good vitality in all light zones; however, their bloom and growth was reduced with less light.
Poligonatum humile was too small and not competitive and was overgrown, but
Poligonatum multiflorum grew and flowered successfully in the semishaded and shaded light zones. The same effects were monitored for
Aruncus aethusifolius, which did not survive in any zone, while the bigger form
Aruncus dioicus survived successfully in zone 3.
Salvia nemorosa as well as
Sedum x
telephium is recommended for the sunny zones.
Campanula lactiflora had a bad performance in all three zones.
Aurinia saxatile died in the shaded zones during the first year but also showed bad vitality in the sun as well.
The performance of Fern species is shown in
Table 6:
Phyllitis scolopendrium had a good performance in all light zones during summer, but some samples died in winter, most likely due to the lack of irrigation of this wintergreen plant. A similar downgrade of vitality in winter was shown for
Polystichum sp. in zone 3.
Asplenium trichomanes was healthy in all light zones, but had to have small neighboring plants due to its fragile form.
Polypodium vulgare grew successfully in the semishaded and shaded zones but was also not very competitive.
Table 7 shows the suitability of Climbing plant species:
Jasminum nudiflorm and
Rosa filipes performed very well in all light zones; however, especially for the rambler rose, there was a better bloom in the sunny areas.
Vitis vinifera had better growth and more fruit on the sunny edges with a higher light intensity, but also grew well more or less in the shadows. Different species of
Lonicera grew well and flowered in all zones.
Actinidia arguta had bad vitality in all zones.
4.4. Examples of Photovoltaic Rooftop Garden Designs
The following two case studies highlight the various layouts and configurations that can be implemented with the developed rooftop garden design. As outlined above, one of the key requirements is to provide maximum flexibility for a series of flat roof applications to cater to a large variety of building types. Even though the design offers multiple layout options in the overall rooftop configuration as well as in the actual garden design, the examples and their visualization show what actual implementation could look like and which plants could be selected. The case studies presented here are based on two out of the three typical building types described above (central-staircase residential type and open-plan office type) to provide options for residential as well as office use (see
Figure 7).
The first layout shows an example of the central-staircase residential type, as shown in
Figure 13. In this type, the roof is entered via two staircases with sufficiently large elevators to ensure barrier-free access to the roof terrace. The elevators are ideally also used to transport heavy loads or objects, which are needed for maintaining the rooftop garden. It is assumed that an elevator is already installed in the building or that it is part of the retrofitting measures.
The user profile for the rooftop garden of this residential building was chosen to be shared use with recreational and relaxation areas to create social meeting places. A covered, weather-protected common area is centrally located and sheltered from the wind: it has spacious seating (a fixed table bench and groups of chairs), a small tea kitchen, and a lockable storage room (e.g., for furniture and other appliances). The furniture is fixed on the roof so that it is protected against wind and vandalism and cannot become a hazard. Adjacent to the staircase there are covered areas for gardening, with planting tables, composters, and water connections provided. In the peripheral zones of the PV garden, there are rest and relaxation areas, which are equipped with plant beds and seating opportunities. These areas are only partially covered by the PV canopies to cater to mixed uses and to offer sunny areas during the shoulder seasons and in particular during the colder winter months. In
Figure 14, visualizations of the “PV Rooftop Garden” for the proposed case study are provided.
In this example, 179 m2 of PV area is accommodated on the roof surface, with “PV Pergolas” of different sizes. This corresponds to a usage of approximately 65% of the usable roof area, allowing for a 1-m distance to the building edge. If in different use cases, unshaded areas are not needed, practically the entire usable roof can be equipped with PVs. The remaining areas are largely covered with PV construction, and greening plant troughs are added in all possible locations. The more heavily trafficked areas are equipped with floor slabs and the remaining areas with lawn. In addition, enclosed plant beds are made available, some of which are covered by the “PV Pergolas”. In order to also use the roof terrace in the evening, lighting elements can be attached to the underside of the construction. The building service elements (e.g., the sewage ventilation pipes) are integrated into the design so that the shading of the PV modules by other applications can be minimized. For the case study, it was assumed that heating and hot water would be supplied by a district heating connection, meaning no chimneys to interfere with the roof structure. To increase the efficiency of the buildings, it is expected that a central ventilation system with heat recovery would be installed on the roof. Care was taken to ensure that there would be at least 1–1.5 m of space between the ventilation units and the PVs in order to not affect airflow.
The height of the drainage layer of the green roof for this example was chosen to be 5 cm, with the substrate layer being 20 cm. The plants and their applications in the mostly unshaded (zone 1), semishaded (zone 2), and mostly shaded areas (zone 3) and their respective blooming times are outlined in
Figure 14. The species on the lawn include
Festuca ovina, F. valeisiaca, F. nigrescens, Festuca rupicola, Poa compressa, Anthyllis vulneraria, Lotus corniculatus, Medicago lupulina,
Trifolium repens, Achillea millefolium, Dianthus carthusianorum, Galium verum, Hieracium pilosella, Petrorhagia saxifraga, Potentilla tabernaemontani, and
Thymus pulegioides. The lawn is not irrigated automatically and consequentially has to be watered by hand during longer drought periods. In the planting beds with integrated seating, the substrate layer is 45 cm tall, and irrigation is provided by dripping pipes and automatized irrigation. The plants used for the enclosed planting beds with full shading by the PVs are
Mahonia aquifolium and
Carex morrowii in combination with
Bergenia cordifolia and
Primula veris. The planting in the semishaded trays consists of
Calamagrostis x
acutiflora,
Geranium sanguineum, Coreopsis lanceolate, and
Primula denticulata. In the sunny areas,
Sedum floriferum,
Sedum telephium,
Teucrium camaedrys, Hemerocallis Summer Wine, and
Rudbeckia nitida are in addition to the species mentioned for the semishaded trays. In addition,
Antirrhinum majus is seeded. Inhabitants are invited to add plants themselves to the provided mix of species, and for this
Capsicum annuum and
Eruca sativa are recommended. The chosen plants will offer nice aspects over the entire vegetation period, starting with the early pink and yellow bloom of
Primula and the yellow flowers of
Sedum floriferum and
Coreopsis in summer. The grasses and the late bloom of
Sedum telephium and
Rudbeckia will be the eyecatchers in autumn. A detailed summary of the plant species for the residential building is shown in
Figure 15.
The second layout shows a typical example for an open-plan office building.
Figure 16 depicts the rooftop floor plan of this case study. The roof is accessed via a central core with sufficiently large-sized elevators to allow for barrier-free access and the possibility of bringing heavy loads and equipment to the roof. The user profile for this type of building was chosen to be joint use for rest, recreation, breaks, meetings, and light office work, as well as for representative purposes, celebrations, or events. In the southwest, adjacent to the central core, the “recreational garden” is located. This is supposed to be a largely covered, weather- and wind-protected common and retreat area for rest and relaxation that offers seating on fixed benches mounted between hills that are formed out of the green roof substrate (Substrate height of 20–60 cm). The major plants in the mostly shaded areas are various fern species such as
Polystichum sp.,
Phyllitis scolopendrium, Dryopteris erythrosora, Poligonatum multiflorum, Hosta sp., and
Carex grayi. The roof is irrigated via dripping pipes, and additionally there is the option to install water sprayers to enhance the recreational quality in summer.
To the southeast there is a covered, weather-protected break and work area with fixed tables and bench groups, as well as plant beds with additional seating. The application includes moisture protection sockets so that users can work in this area. In one specific part, which is separated by higher plants (Aster divaricatus, Anemone hupehensis, Deschampsia cespitosa, Aquilegia vulgaris, and Campanula persicifolia), there is an area for private and more confidential meetings.
In the northeast, an event area is foreseen, which is equipped with a kitchen and bar area with an attached storage room. This area should be directly accessible from the elevator, with sufficient space for bar tables and additional seating. An uncovered area with a water basin and integrated seating should emphasize the representative character of this space. The plants suitable for this mostly sunny area are Lilium candidum, Calamintha nepeta, Festuca glauca, Iris barbata nana, and Iris barbata elatior. Since in office buildings, there is more electricity demand compared to other energy demands, dense PV coverage on the roof was chosen for this exemplary office “PV Rooftop Garden” design. In office buildings, there is an almost constant demand for electricity for cooling, ventilation, lighting, and small power, and thus the aim is to cover as much of the surface area of the roof with PVs as possible to ensure a high percentage of production coverage. For the described example, a total of 284 m2 of PV area was planned for the roof surface with different-sized “PV Pergolas”. This corresponds to a usage of approximately 37% of the usable roof area, while keeping a 1-m distance to the building edge. Plant troughs are arranged only at the edges to allow for an open and free roof space. An ornamental mixture of flower species consisting of Campanula portenschlagiana, Aster divaricatus, Anemone hupehensis, Deschampsia cespitosa, Aquilegia vulgaris, Campanula persicifolia, Helleborus foetidus, Iris foetidissima, Festuca glauca, and F. gautierii was selected for this roof garden application.
The more frequented areas, such as those used for meetings, should be fitted with tile flooring, and the remaining areas should be fitted with a tread-resistant, permeable lawn area. The furniture is fixed to the roof to secure it against wind. Lights can be placed on the underside of the “PV Pergola” in order to use the rooftop in the evening and for events. See
Figure 17 for visualizations of the exemplary design for a prototypical office building.
The necessary building services are located in the northwestern part of the roof. They are partially covered, depending on the requirements of the technical equipment. The entire area is lockable and separated by a fence, which is greened with
Euonymus fortunei. As with the example of the residential type, the building is considered to be connected to the district heating grid and is thus provided with heating and hot water without any chimney interfering with the roof design. Office buildings also require cooling due to their high internal loads. Therefore, in addition to a controlled ventilation system with heat recovery, necessary cooling units are placed on the roof. To ensure undisturbed operation of the building equipment, certain areas are consequently not covered by a garden design. The infrastructure of the sanitary facilities on the roof of the building does not shade the PV elements, since they are arranged around the elevator core. Overall, the layout should provide ample room for all required facilities for recreational purposes and at the same time deliver a share of the building’s energy demand. A detailed summary of the plant species for the office application can be seen in
Figure 18.