UNIVERSITA’ IUAV DI VENEZIA Dipartimento di Progettazione e pianificazione in ambienti complessi Corso di Laurea Magistrale: Architettura e Innovazione Tesi di laurea A support tool for the early design phase of BiPV towards nearly zero energy districts Relatore Prof. Massimiliano Scarpa Correlatore Dott. David Moser Laureanda Jennifer Adami Mat. n° 280301 Anno Accademico 2014/2015
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UNIVERSITA’ IUAV DI VENEZIA
Dipartimento di Progettazione e pianificazione in ambienti complessi
Corso di Laurea Magistrale: Architettura e Innovazione
Tesi di laurea
A support tool for the early design phase of BiPV towards nearly zero energy districts
Relatore
Prof. Massimiliano Scarpa
Correlatore
Dott. David Moser
Laureanda
Jennifer Adami
Mat. n° 280301
Anno Accademico 2014/2015
A support tool for the early design phase of BiPV towards nearly zero energy districts
towards nearly zero energy districts Adami Jennifer
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1.2.2 BiPV as multifunctional technology
Building integrated Photovoltaics (BiPV) refer to photovoltaic cells and modules which can be
integrated into the building envelope as part of the building structure, and therefore can replace
conventional building materials [12]. It represents a “prerequisite for the integrity of the building’s
functionality” [13] and, if dismounted, it would have to be replaced by an appropriate building
component.
According to SUPSI [7], “BiPV definition excludes therefore building independent or overlapped
installations such as PV modules simply placed or mounted on pre-existing roofs or other PV systems
merely attached to parts of the building that do not assume other function than the solar power
generation”. The difference between an “integrated” or “applied” PV system is not always clear. An
accurate analysis of the system functions could be required. A BiPV module, which is used in
substitution of traditional elements of the building envelope, has to ensure all the functions of the
replaced element (e.g. weather protection, thermal insulation, shading, noise protection, security). It
has to comply not only electro‐technical requirements as stated in the low voltage directive
2006/95/IEC or CENELEC standards, related to the module itself, but also a function as defined in the
European Construction Product Directive CPD 89/106/EEC. BiPV must be generally designed and built
in such a way that it does not present risks of accidents or damages in service or in operation for
persons involved throughout the entire lifecycle of the building. It has to satisfy the basic requirements
for building component such as mechanical
resistance and stability, safety in case of fire,
hygiene and health of people, safety and
accessibility in use, protection against noise as
well as energy economy and sustainable use of
natural resources.
Besides these standard building
construction constraints, the integration of PV
implies other issues. It refers also to an
architectural, conceptual perspective, which looks at the building as a formal whole. In this case, BiPV
can be described with regard to the role that it plays in the concept of the envelope composition.
Several architectural criteria have been defined in the framework of the International Energy Agency
project IEA-PVPS Task 7 “Photovoltaic power systems in the built environment” [14] and they are
hereunder summarized.
Figure 1.11 - Glass ceiling with transparent BiPV modules [14]
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Naturally integrated: the PV system is a natural part of the building, it completes the building.
Without PV, the building would be lacking something.
Architecturally pleasing: based on a good design, the PV system adds eye-catching features to the
architecture.
Good composition: the colour and texture of the PV system is in harmony with the other materials.
Grid, harmony and composition: the sizing of the PV system matches the sizing and grid of the
building.
Contextuality: the total image of a building is in harmony with the PV system (e.g. for historic
buildings).
Well-engineered: the elegance of design details is taken into account. All details are well
conceived, the amount of materials is minimized.
Innovative new design: the PV system adds a value to building. The PV is an innovative technology
in the field of architecture, asking for innovative, creative, thinking of architects.
Figure 1.12 - Casa Solara, Laax, Switzerland [15]
Constructive, functional and formal quality must be simultaneously preserved in order to have
a suitable integration of a multifunctional technology such BiPV.
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1.2.3 BiPV “architectural systems”
Photovoltaic modules, available as flat or flexible surfaces, realized with cells or laminates, can
be used together with materials that are common in architecture, such as glass or metal, in opaque as
well as in semitransparent surfaces. They can be integrated into every part of the building envelope,
creating specific “architectural systems”. An architectural system, into the context of this work, refers
to a conceptual model defined by specific structures and components, and associated to specific
physical phenomena. A wide variety of BiPV architectural systems is available today. Several databases
collecting information on existing BiPV products on the market are provided on-line, e.g. in [16] and
[17].
Most of BiPV available can be grouped into three main categories: roofing systems, facade
systems and external devices. These categories will include different technological ways of using PV in
the envelope, which lead to different choices of the PV component.
Roofing system
As a roof element, the PV system is part of the building skin and requires attention to
weatherproofing, structural, and snow accumulation issues. It can displace traditional construction
materials and, depending on the substituted layers, they have to meet different requirements that
influence the choice of the most suitable PV component.
Both crystalline silicon and thin-film technologies are available for BiPV roofing solutions. The former
comfortably dominates the market, whereas the latter is used when chimneys, trees, or neighbouring
buildings cast a shadow or where crystalline modules are excluded because of their rigidity or weight.
Typical bluish or black c-Si solar cell patterns are the most widespread among roof-installed PV
modules. Semi-transparent, wafer-based solutions used as skylights are also an option, especially for
bigger roofs on public, commercial or industrial buildings.
Two first categorizations depending on the tilt of the roof (1) or on its portion dedicated to
BiPV (2) are shown as follows.
1. A pitched roof is made up of angled and sloped parts. It is known as a “discontinuous” roof
due to the presence of small elements (tiles, slates, etc.). Panel installation can be easier than
many other BiPV solutions, but an impermeable roof is required. The most common solutions
is with the use of photovoltaic roof tiles with mono- or polycrystalline solar cells integrated
with the classical roof tiles.
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A flat or curved roof, also known as “continuous roof”, is characterized by an uninterrupted
layer with the main function to be water resistant. Flat roofs have the advantage of good
accessibility and easy installation. On the contrary curved roofs need complex supports and
specific flexible panels, involving added costs. In both cases, care must be taken during the
fixing of the array to preserve the roof integrity. The added weight of the PV array on the
roof must be considered.
2. An in-roof system integrates photovoltaics in a limited portion of the roof, simply replacing
the tiles. In order to perform a good integration, the PV panels are required to be placed with
regard to the surrounding roof tiles. They can be framed into the roof structure or overlap it.
Water tightness has to be guaranteed, for instance, by means of an impermeable interlayer
underneath. Architects consider in-roof systems as a not so attractive option when
photovoltaics are additional and visible materials.
The full roof solution refers to a full solar roof concept, where the roof surface is exclusively
and specifically conceived as a solar collector for energy production. Besides in-roof
installations, a full-roof system is regarded as a more economic and more appealing
alternative choice [10]. All the traditional roof components are substituted and a maximum
surface area is dedicated to energy production.
Figure 1.13 - Full-roof BiPV installation on a 19th century Swiss farmhouse, Uettligen [10]
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Several categories within roofing system application area include:
- Solar tiles/shingles/slates
This system is usually designed to resemble the conventional ‘”roof tile” with a solar PV tiling.
The panel height is modular to the roof tile’s rows and it can be glazed or foil -based. There are several
varieties, semi-rigid and systems using various thin-film solar cell technologies. Commonly only a part
or the whole roof is used for PV, using the same sub-structure as the mounting system. There are also
smaller panel systems, with module size <0,5 m2,
which are adapted to the conventional roof tile
that becomes itself the PV element. Solar tiles and
solar shingles offer an alternative constructive
and aesthetic approach on account of their
likeness to ordinary roof tiles, but their use could
involve some disadvantages, like for example the
susceptibility to the ingress of water and
humidity, or the lack of compatibility with the multitude of existing different tile types and the
geometric variations. Moreover, in case of PV integration into existing buildings with this solution there
is a need for an additional mounting system and in most cases the reinforcement of the roof structure
due to the additional loads.
- Metal panels
This system consists in mounting flexible laminates PV on building materials such as metal
roofing. BiPV metal roofing can replace an architectural standing seam. The thin film amorphous silicon
PV material is laminated directly onto long metal roofing panels. A
protective waterproof membrane is attached covering the
photovoltaics. A traditional roofer followed by an electrician can
install these metal panels. In an interlocking tongue-and-groove
assembly, the panels are weighed down by pavers that surround
the system to provide access for maintenance and repairs. A cap is
placed over the standing edges to form a seam. This technology can
be used also to redo the roof of an existing building.
Figure 1.15 - PV applied on zinc
standing seam panels [9]
Figure 1.14 - Middle European solar tiles [9]
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- PV membranes
This technology uses waterproof membrane as a support of flexible laminates PV. The
mounting procedure of the panels on roofs as well as other building structures is different (easier) than
the conventional, rigid ones. The high performance membrane with integrated flexible and lightweight
PV modules can easily be adhesively bonded to roofing materials. This integration system brings along
many advantages like light weight and avoidance of heavy wind loads (because they do not allow wind
beneath them). Moreover, it allows easier planning permission on new build and retrofit projects.
- Solar glazing
Glazed PV laminates can be used as roof parts, often made by crystalline silicon cells with
adjusted spacing or by laser grooved thin-film which provides filtered vision (skylights).
This integration system is usually one of the most interesting. It combines the advantage of light
diffusion in the building while providing an
unobstructed surface for the installation of PV modules.
PV elements provide both electricity and light to the
building. The amount of light desired to go through the
designed structures can be customized by dimensioning
and adjusting the number and spacing of cells in the
case of crystalline silicon technology or by modifying the
manufacturing process in the case of thin-film. In both
cases the more transparent is the module, the lower is
the energy efficiency. They are also commonly used to
provide sun/wind protection to building surfaces and
interiors. Transparent modules can be used for open
and indoor atria. In both cases the glazing should meet
the standards for mechanical resistance, while in the latter also the thermal requirements of the
envelope. Skylights are used in flat roofs, pitched roofs, and sometimes in the top area of the façade.
They are employed usually for commercial and prestigious buildings, designed to provide a great deal
of natural lightning by using large areas of semitransparent window-like areas.
Figure 1.16 - Solar Glazing system [19]
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Façade system
The second field of BiPV application is that of façades where solar panels of all technologies
can be integrated as a conventional cladding system for curtain walls and single layer façades . Current
development is aimed at developing more advanced applications like adaptive modular PV façades
and intelligent ways of balancing daylighting and shading [18].
In many cases, standards modules (frame or frameless) are used for such application although the use
of tailored made modules is sometimes requested in order to match the façade specifications. For
example glass PV laminates, replacing conventional cladding material, are basically the same as tinted
glass. They provide long-lasting weather protection and can be tailor-made to any size, shape, pattern
and colour.
Façade applications typically include warm façades, cold façades and solar glazing.
- Warm façade
A warm façade is typically a continuous building envelope system in which the outer walls are
non-structural. A warm façade fulfils all building envelope requirements such as load bearing, thermal
insulation, weatherproofing and noise insulation. Since it is the building skin system, the parameters
related to solar gain control such as thermal and visual
comfort have to be controlled when using highly-glazed
curtain walls. In this case, a warm façade matches a solar
glazing. In general, it can also be represented by an opaque
curtain wall or by an insulating cladding panel (PV + thermal
insulation without an air gap) where there is no ventilation.
A vertical curtain-wall represents a fairly economical and
standard construction strategy, but a sloped one could be
more productive. However, both systems could have
problems with panels’ junctions and sealing. Figure 1.17 - Warm facade systems
with thin film photovoltaic [20]
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- Cold façade
This facade system typically consists of a load-bearing sub-frame, an air gap and a cladding
panel. The PV panel is used as a cladding element, which has no thermal connection with the wall
warm. It offers weather protection and cooling for the wall. Thanks to the naturally ventilated cavity
heat from the sun is dissipated through bottom and top openings. It provides a good ventilation behind
the modules, which enhances the PV system efficiency.
Figure 1.18 – Cold façade system, 28th Street Apartments, Los Angeles [19]
Some systems make use of vents to optimize the air ventilation. An example is shown in Figure
1.19. Poly-crystalline PV modules are installed in the façade in an integral form such that an air cavity
is created between the PV backside and the building envelope. The system thus constitutes an outer
layer (PV panels) and an inner layer (building envelope). The air gap formed between these two layers
interacts with the indoor environment by means of two vents (dampers), one located in the upper part
and the other located in the lower part of the brick wall. The vents can be controlled manually by the
occupants according to their individual comfort needs and weather conditions [8].
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Figure 1.19 - Cold facade system with vents [20]
Another possible option for PV integration is the double‐skin façade, a glazed surface that is a
non-load-bearing exterior wall suspended in front of the structural frame and wall elements [Figure
1.20].
Figure 1.20 - GDF Suez, Dijon, France [21]
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- Solar glazing
Solar glazing systems are often used as
windows or as a curtain wall semi-transparent
system, designed with extruded aluminium frames
(but also steel, woods, etc.) in-filled with glass. Since
it is part of the building envelope, parameters
related to solar gain control, such as thermal and
visual comfort, have to be controlled when using
highly-glazed curtain walls. The transparent
functional layer (glass) is replaced with PV glazed
panes, whilst the load-bearing part is equipped for
the electric wirings passages. The cell pattern and
assembly can provide the proper solar and
daylighting control replacing the traditional external
louvers and defining a particular architectural
appearance of the façade. The most common glass
types used are laminated glass, patterned or fritted
glass and spandrel panels. Some companies sell custom-made BiPV glazing products, available in any
size or dimension and consisting of any PV technology. The architect can indicate the spacing between
solar cells, which will determine the power supply and permit the design of passive solar features by
regulating the amount of daylighting allowed to enter into the building. Standard and custom products
are available in many sizes and in a range of thicknesses [18].
External devices
Photovoltaics can be also integrated into the design as external devices on the building skin.
These accessories may include balconies, shading systems, and several other smaller systems. Shading
systems are the most commonly used accessory [12]. The control of the indoor microclimate, especially
in glazed façade systems, usually requires the use of shading devices aimed to select the solar radiation
for ensuring the thermo-hygrometric and visual wellbeing through a proper use of the natural lighting.
Several types of shading devices are available: applied on roof or façade; external, interposed (for
Figure 1.21 - Solar glazing facade BiPV system [22]
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example in double skins) or internal; fix or tracking (manually or electrically); vertical, horizontal or
oriented; lamellar, micro-lamellar, sail, grid; curtain or blind; mobile screen [12].
Quite common these are semi‐transparent glass or glass components integrated as canopies
or louvers, but there are also movable shutters with semi‐transparent crystalline or thin film.
Opaque sun protections are also widely used, in most cases with an upper part without cells, to avoid
shading the PV cells when the overhangs overlap.
Figure 1.22 - Hofberg 6/7, Switzerland [22]
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- Spandrels, balconies parapets
Spandrels and parapet areas are also suitable for photovoltaic integration, mostly using glass
or glass semi-transparent modules made of security glass. Balcony fronts can either be two‐paned
solutions to protect the PV‐cells or single glasses to which the PV‐cell is laminated. In glazed verandas,
the heat generated at the back of the PV can be used to create thermal comfort in spring and autumn,
while the space can be opened for natural ventilation in summer time.
- Shading systems
PV modules of different shapes can be used as shading elements above windows or as part of
an overhead glazing structure. Since many buildings already provide some sort of structure to shade
windows, the use of PV overhangs should not involve any additional load for the building structure.
The exploitation of synergy effects reduces the total costs of such installations and creates added value
to the PV as well as to the building and its shading system. PV shading systems may also use one -way
trackers to tilt the PV array for maximum power while providing a variable degree of shading.
Figure 1.23 - Shading BiPV system [23]
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The main features of the different BiPV architectural systems described in the current
paragraph are highlighted through the comparative analysis outlined in Table 1.1.
Easy
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Hig
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ROOFING SYSTEMS
Tiles/shingles/slates
Metal panels
PV membranes
Solar glazing
FAÇADE SYSTEMS
Warm façade
Cold façade
Solar glazing
EXTERNAL DEVICES
Shading system
Table 1.1 - Main features of several BiPV solutions commonly used
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1.2.4 BiPV flexibility
From the building integration point of view,
the market is divided into two main categories: the
standard modules and bespoken products. Both
categories give some formal flexibility in their offer,
but obviously not to the same extent. As PV has to
compete with traditional technologies to generate
electricity at a reasonable price, most producers
have turned to very large factories to mass‐produce
PV modules, achieving economies of scale, but
limiting their offer to a few standard products. Their
fixed sizes and crude frames can make their
integration difficult, as they often do not match the
raster of the project. However, if the PV option is
considered at a very early design stage, an innovative and successful integration can be achieved.
Custom‐made products, developed for special projects for maximum flexibility, can be ordered with
specified formal characteristics (e.g. shape, size, colour, texture, etc.). This freedom usually comes with
an extra cost due to small quantities produced, and attention should be paid to the issue of spare parts,
which should be produced with the main order, to ensure replacement in case of incident.
- Shape and size
The size of standard crystalline photovoltaic modules ranges from 0.2 to 2 m². The small size
of the cells used in crystalline Silicon based modules (10x10 cm to 20x20 cm) gives the dimensional
possible “steps”, as the module will have as dimensions multiples of these cells size. Opaque and
translucent modules, with or without frame, can then be obtained in a large variety of shapes and
sizes, either from existing products or custom made.
About thin-film technology, producers generally offer standard glass modules with specific
size. A set of standardized building dedicated products is also available on the market. Due to the roll‐
to‐roll manufacturing process, there is only flexibility (a set of dimensions to choose from) in the length
of the laminates, that come with a fixed width. Several roofing companies are cooperating with the PV
producers to offer their products (metal roofs, corrugated sheets, insulated panels) with integrated
PV.
Figure 1.24 - SCHOTT Ibèrica SA, Barcelona, Spain [19]
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Figure 1.25 – Left: GreenPix media wall, Beijing, China [24]; Right: Opera House, Oslo, Norway [8]
- Colour, patterns, textures
The appearance of crystalline modules depends on the appearance and composition of the
cells, and marginally on the colour of the back coating (Tedlar). There is certain flexibility in choosing
the colour, but black or blue cells and black or white Tedlar are largely dominant. Only a few
manufacturers propose coloured cells, so these are quite rare and come with added cost and reduced
efficiency. The standard choice for crystalline modules front glass is an extra‐white, low iron, 3mm
glass, used by most producers. However, there are options for textured or etched extra‐white glass, or
very thick glass for increased static resistance or for combination with a back gl ass.
Figure 1.26 - Poli cristalline silicon wafer with different coloured anti-reflective
Thin-film manufacturers provide basic products in one single colour, brown, blue or black. New
developments offer now also some reddish brown, chocolate‐brown, hepatic and sage green colours.
Laminates are mainly available in dark blue and pinkish shades. Thin-film technology allows for the
possibility to produce semi‐transparent modules with a variety of patterns (point, stroke, stripes), by
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laser cutting. Some thin-film laminates, like in solar shingles that imitate traditional asphalt roof
shingles, present the general image of a dark blue or magenta cells ribbon, with two empty lateral
areas without cells, where the substrate is visible.
Figure 1.27 - Thin film modules
- Jointing/Framing
The crystalline modules jointing can be made through the aluminium framing, by integration
into curtain‐wall systems with mullion/transom, or modules can be integrated frameless in glazing
systems, with negative jointing. For roof applications, overlapping is often chosen for the horizontal
joint. Custom‐made products can be developed with their own specific fram ing as well, where the
frame becomes part of the module design.
For glass modules, the framing possibilities are the same as for crystalline products. The
jointing and framing of the laminates are defined by their substrate structure. Some laminate
producers are partnering with building component manufacturers to integrate their modules into
existing building products.
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1.2.5 Design stage
In order to have an effective integration of PV as a real building component, it is hence
necessary to include BiPV from the first design phases. Due to its multifunctional feature [1.2.2], BiPV
integration entails several considerations from different point of views (e.g. weather protection,
thermal insulation, shading, noise protection, security). This might require that the building team,
including architects, building designers, engineers, building owners and utility companies, work
together from the first phase of the project onwards. Some of the factors that must be taken into
account during the first design stages to optimized the performance of a BiPV system, according to
[19] , are reported in the following sections.
Minimizing electric loads
The first consideration in BiPV applications is to maximize efficiency in the building energy
demand or load. Designers should minimize the electricity load by utilizing integrated energy design
strategies such as building envelope improvements, daylighting techniques, natural ventilation
applications, and additionally installing energy-efficient lighting and cooling equipment. The goal is to
minimize the building energy needs and then supplement the remaining loads with the generated
electricity.
Matching electric loads
A BIPV system may produce the same amount of electricity as consumed in the building on a
yearly base, however the simultaneity of production and consumption needs to be evaluated. An
effective strategy to best optimize the PV energy use can be implemented by matching the offer with
the demand. It means that photovoltaics should produce energy when energy is needed with the final
aim to increase the direct self-consumption. The energy demand is defined by the building use.
Different electric load values are measured for example in residential and commercial buildings [25].
Increasing the correlation between the electricity production and the load profile can have a positive
impact on the PV economic case, optimizing the energy use. Designing a BiPV system, therefore,
cannot disregard considerations about load matching.
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Optimizing system configuration and electricity generation
Decisions regarding where and how to best integrate BiPVs into building designs are greatly
influenced by the potential amount of electricity generated from a specific application and its cost
effectiveness. For example, horizontal applications like roof BiPVs and vertical applications like curtain
walls have different material/installation costs and electrical output curves due to the different
position relative to the sun. The electricity generation widely depend on the solar access, the incidence
of solar radiation that reaches a PV surface at any given time. It is important to note that the availability
of solar radiation changes throughout the day and throughout the year. For maximum energy output,
it is important to determine the orientation, tilt angle, size and location of the BiPV system in relation
to the building site and design. As flexibility exists in the placement of BiPV, this gives a limited degree
of freedom to match the time of day, month, and season when peak solar generation is in close match
with the peak electrical needs of the building [9].
- Tilt. Maximum solar intensity occurs on a flat surface perpendicular to the sun’s rays. Inclining the
panels toward the sun increases the amount of sunlight striking the surface and wil l increase the
output. The sun path sweeps a daily arc that changes seasonally throughout the year. In this way, the
sun follows a prescribed solar position described by an altitude angle (vertical) and azimuth angle
(horizontal). By orienting the BiPV panels to be perpendicular to the sun at certain times of day and
year, it is possible to optimize solar exposure to match loads. As a general rule of thumb in the Northern
Hemisphere, BiPV installations produce the most energy over the course of a year when oriented true
south and tilted at an angle equivalent to the site latitude. However, instantaneous output varies
depending on cloud cover and the sun position. As a panel gets farther from a tilt equivalent to the
site latitude, the total annual output decreases. A vertical surface orientation may produce
approximately 30 percent less electricity, while a horizontal surface orientation may produce
approximately 10 percent less electricity than an optimally inclined installation at our latitudes [9].
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- Orientation. The total amount of energy that strikes
a surface is a function of both tilt and orientation. In
Figure 1.28 is shown the amount of the annual
irradiation evaluated on different orientation in
Bolzano. Vertical surfaces with east/west orientation
have a relevant decrease (around the 60%) of annual
irradiation compared to the optimally inclined
southern orientation. For these east/west
orientations, low sun angles at the beginning and end
of the day account for the majority of the power
generated.
- Sizing. For the selection of a BiPV type and for the sizing of a system, three main factors have to be
considered: energy loads, architectural or aesthetic considerations, and economic factors. To
determine the desired power rating of a BiPV system for a building, the electrical requirements of the
building should be evaluated at first. The optimum power rating of the system can be calculated and
sized, based on the share of the building electricity that will be supplied by the BiPV system.
Architecturally, the size of the BiPV system is physically limited to the dimensions of the building
available surface area. The balance between the amount of power required and the amount of surface
area available can determine the type of PV technology that will be used. Each technology has an
associated range of output in watts per square meter. For example, in order to have a certain energy
output, systems made of amorphous silicon can require a larger surface area than equivalent systems
composed of single crystal solar cells, placed with the same surface tilt and orientation.
- Location. BiPVs should be placed where they have secured long-term solar access. It is critical not to
locate BiPV panels where neighboring landscapes or structures that may cast shadows on the system
are present or planned in the future. Full or partial shading of the panels partially inhibits the
production of electricity. The system performs best if there is homogeneous solar access because the
solar cell with the lowest illumination level determines the operating current for all of the cells wired
in series.
100%
70%
84%
72%
62%
91%
94%
66% 45%
Figure 1.28 - Annual irradiation for different orientation evaluated in Bolzano (PVGIS)
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Figure 1.29 - Shading due to environmental factors [26]
In the design phase, it is also important to take into account the surface features that surround
the PV system. Their reflection capacity influences the amount of solar irradiati on that hits the PV
system.
Meeting aesthetic criteria
A BiPV solution may be an appropriate option for designers that wish to create aesthetically
appealing buildings. However, from the beginning of the design process, several aesthetic criteria
should be taken into account when designing distinctive “architectural features” [0]. All the formal
characteristic of the project must be architecturally
integrated into the context. This represents a
crucial issue especially when regarding to old
buildings, historical sites, and “protected”
landscapes [27]. The formal acceptance of a PV
integration is a matter of discussion, both in the
private dimension (i.e. willingness to adopt a
specific PV system) and in the public dimension (i.e.
acceptability of specific BiPV applications in the
urban context where the individual lives), as
reported in [28]. Surely, improving the BiPV design
could increase the acceptability of a PV technology. As shown in [1.2.4], the BiPV products already
available on the market today can make visual statements by adding patterns, textures, colours, and
visual notoriety to the roof or façade of a building. The wide flexibility of BiPV appearance can
contribute to achieve a good aesthetic integration level.
Figure 1.30 - Roof integration of PV on historic building, St Silas Church, Pentonville [24]
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Figure 1.31 – Left: Hotel Renovation, Paris [7]; right: Kollektihuset, Copenhagen, Denmark [29]
The economics of BiPV
While the prices of standards PV modules are very well known in the PV sector [30], when it
comes to BiPV, the subject gets much more complex since a combination of many factors may
contribute to affect the final price of BiPV system. It is important to carry out a comprehensive
economic analysis that evaluate all factors specific to the project. The best way to assess the economic
attractiveness of a building strategy like BiPV is to evaluate the total cost of the system over time [9].
A life-cycle cost (LCC) analysis gives the total cost accounting for all the expenses incurred and the cost
savings gained over the life of the system. It allows the designer to compare the economics of many
different power options as well as determine the most cost effective BiPV system design. Most LCC
analysis includes capital costs, installation costs, maintenance costs, energy costs, replacement costs,
energy cost savings, and salvage value. When using LCC to compare different systems, it is important
that each system configuration performs the same work with the same reliability.
Usually BiPV products are subject to comparison with other building materials since their aim
is to replace those conventional materials and their functionalities [1.2.2]. A review of a recent study
published by SUPSI and SEAC [12] displays the results of the price survey conducted to compare
conventional façade materials with BiPV façade solutions. The price is defined as the end-user price
and measured in €/m², which is the end-user PV system cost calculated over the area that the PV
systems covers on the roof or façade. Figure 1.32 shows a benchmark of the conducted price survey.
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Figure 1.32 - A benchmark of the conducted price survey, comparing conventional facade materials with BiPV
facade solutions [12]
Conventional façade technologies include fibrocement, brick-ceramic, metal, stone, wood,
window and curtain walls. Prices range all the way from 30-50 €/m2 for a low cost fibre-cement façade
(similar to a traditional plaster) to 1100 €/m2 for a special curtain wall (e.g. self-lighted, interactive
façade, etc.). The price of BiPV solution systems varied from 100-150 €/m2 for a thin-film PV cold façade
(with really simple sub-structures and a low efficiency solar technology) to 750 €/m2 for a high end PV
solar shading system [11].
The survey’s results show that, if considering the materials costs, BiPV systems seem to be
comparable in price with conventional façade materials. However, there is a wide value range, unable
to provide a clear benchmark. Furthermore, into a global costs context, this cost is not so indicative.
When designers have to choose between traditional building materials or photovoltaics, they have to
perform several evaluations. Firstly, they should compare materials with similar conditions, for
instance installation costs, maintainance and disposal costs, architectural integration, impact on the
building thermal balance or on the daylighting, etc. Since the comparison includes a technology that
produce electricity, other crucial issues to evaluate are all the factors connected with the current
electric energy cost and its growth.
A simplified economic analysis can be carried out by calculating what the max imum added cost
should be if the target is a return of investment (ROI) within a 10 year timeframe and represents a
correlation between additional costs, self-consumption and ROI.
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Assumption: BiPV makes sense if you can directly self-consume in the building (to go towards
nZEB) or in the district (to go towards nZED) (self-consumption can be increased by using early design
tool to place the BiPV modules accordingly using all façade direction/roof or with batteries).
Figure 1.33 - Economic benefits from generated electricity of BiPV modules on facades over a period of 10 years. Assumptions: cost of electricity in 2015, 0.2 Euros/kWh. Self-consumption: 100% in commercial and industrial buildings, 30% in residential buildings. The calculation was carried out for inflation rate of 1% and 3% and for
BiPV system efficiency of 5% and 10% (source: elaboration EURAC)
From the Figure 1.33 can extrapolate the maximal additional cost for two BiPV system
efficiencies of 5% and 10%, for the residential and for the industrial/commercial sector, to have a
return of investment within ten years (only considering self-consumption, energy to the grid is not
valorized). In the building sector price is given per m2. Clearly, the efficiency of PV here has a high
impact in the final figures if the production is calculated over m2 and not as a specific yield in kWh/kWp.
Within the framework of the EU Photovoltaic Technology Platform, EURAC has provided support to
the European Commission in setting future targets for BiPV Figure 1.34.
Figure 1.34 - Targets set by the EU PVTP for BiPV towards the European Commission
Installation year Inflation rate 5% 10% 5% 10%
1% 63 125 84 168
3% 69 137 92 184
1% 66 132 89 177
3% 79 159 107 214
Installation year Inflation rate 5% 10% 5% 10%
1% 19 38 25 50
3% 21 41 28 55
1% 20 40 27 53
3% 24 48 32 64
London
Economic benefits
[Euros/m2/10 years]
Rome
Economic benefits
[Euros/m2/10 years]
Economic benefits
[Euros/m2/10 years]
London Rome
2015
Economic benefits
[Euros/m2/10 years]
2015
2020
2020
Commercial and industrial
buildings
Residential buildings
Efficiency
semi-transparent opaque
Now (end 2015) 150 – 350€ 130 – 250€
2020 50% reduction on today 50% reduction on today
2030 75% reduction on today 75% reduction on today
Facade-integration
Additional cost
(€/m2)
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2 New methodology developed to support BiPV design
The growing demand for better energy performance in buildings is leading to an ongoing
development of strategies and technologies, involving an increasing complexity of the buildings design.
During the design phase, fundamental decisions are made that have an enormous impact on the whole
building lifecycle, on the durability and performance of any project. Especially the Early Design Phase
(EDP) plays a crucial role. It refers to the “stage of work where initial design ideas are being
conceptualized in tandem with the formulation of the building project requirements” [31]. In order to
achieve an optimal result, designers should be aware of the consequences of these design decisions.
Making informed design decisions requires the management of a large amount of information. An
overview of possible design options and their performance should be created to let the designer
choose the best solution. This can represent a critical task to implement. There is a distinct risk of
missing better design opportunities or obtaining undesirable effects if the design process is not
properly performed [32]. In order to better manage the complexities of a project, therefore, designers
should be supported by computer-based building simulation tools able to make quick and reliable
predictions, which let them evaluate the impact of early design choices on the building efficiency.
Several software tools are currently available. However, as will be explained through the following
sections, their capabilities seem not to satisfy the main designers’ needs. The present study aims to
provide a strong contribution in this sense, attempting to suggest an effective design methodology as
support to the design stage of a complex project.
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2.1 Barriers related to the use of software tools
There is a broad variety of digital tools that architects are using today. An international survey
of architects carried out within the framework of the IEA SHC Task 41 identified the most used [33].
The software tools reviewed are organized into the following two section.
- Graphical/physical tools section includes solar
charts/sun-path diagrams and physical models tool. They
allow the architect to perform a number of tasks quickly
and accurately such as determining shadows’ cast,
determining spatial relationships between buildings and
sun access to public space or to the internal spaces of
buildings, etc.
- Digital tools section includes CAAD tools (i.e. AutoCAD, ArchiCAD, GoogleSketchup, Revit,
VectorWorks) and Building Performance Simulation (BPS) tools (i.e. Ecotect, Project Vasari, RETScreen,
Radiance, IES VE, SolarBILANZ, bsol, DAYSIM, DPV, Lesosai, Polysun, PVsyst, PV sol, T sol). Digital tools
provide various outputs (e.g. 3D models, pre-sizing, simulation results, etc.) that can be used in
facilitating and improving communication between the actors of the design and construction process,
i.e. architects, clients, engineers, consultants, etc.
Figure 2.2 - Modelling and simulation outputs [33]
Figure 2.1 - Sun-path diagram [31]
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According to the survey, at the early design phase only some of the tools previously mentioned
are usually used [33]. Graphical/physical tools and CAAD, tools that mostly provide qualitative output
in solar modelling, can be helpful during the first design stage. Other tools, predominantly BPS tools or
specialised tools for sizing active solar components, provide quantitative output, but also require more
detailed and time consuming input. However, some BPS tools are considered acceptable also for the
early design stage. The results reported in [34], about simulation software used per design phase,
indicate Ecotect domination, due to its compatibility with CAD tools, its visual 3D output, its accuracy
at least for the scientific purposes, and capabilities that allows a comparison between various design
proposals. Second place in the acceptance for solar design tools in the early design stage is RETScreen,
which is relatively simple to use, despite of its completely numerical input and output. The following
in order are Radiance, PVSol, Polysun, PVsyst, eQUEST, IES VE, Design Builder, Lesosai, followed by
other software that are considered less compatible with early design phase. A wide review of the
software tools mentioned is available in [31].
Figure 2.3 - Output scenes provided by Radiance (left) [35] and IES VE (right) [36]
As stated by the IEA [34], architects and designers are aspiring to create sustainable built
environment and are taking serious consideration of the use of building performance simulation tools
that improves design reliability of energy efficiency. However, there are current obstacles preventing
architects from using existing methods and tools for solar building design. Although a great number of
digital tools for solar design exist today, they are not necessarily quite adequate for architects and the
EDP. The IEA international survey [34] identifies some barriers related to the use of available tools for
architectural integration of solar design [Figure 2.4].
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Figure 2.4 - Barriers related to the use of the tools for the architectural integration of solar design [34]
The results indicate that the tools are first of all too complex. Users have more difficulty in
using several tools (e.g. since capacities and software features are not well‐known) and especially in
interpreting the results. Some software tool are not able to provide architects with results that are
presented in a useful form for the EDP: most provide numerical results, as tables or graphs. Moreover,
most software are mainly suited for detailed design phase where extensive information is available
and important design decisions already have been taken. Another crucial barrier identified by the
survey is the lack of interoperability between software. Most of the used tools do not allow data
exchange and importing and exporting features (like 3D models), increasing errors probabilities and
waste of time during design phases. In addition, the lack of a graphical flexible representation to
interact visually instead of entering command lines represent a serious barrier. Users need a dynamic
interface that can respond in real time to their actions, allowing an iterative design process. A graphical
representation can also provide information more easily accessible than results presented in the form
of reports and/or spreadsheets.
Since EDP software tools have to support adequately architects and designers taking in account
all the factors related to a project, there is the need to employ “integrated” software tools, which can
be able to address all issues related to a project (e.g. energy balance, life‐cycle costs, economic issues,
architectural appearance, etc.). The use of multiple platforms for design and simulation not only slows
down the process, but also introduces interoperability issues which include the use of multiple models
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and interfaces. This represents a crucial problem when referring to the design of a multifunctional
technology like BiPV, which, as explained in [1.2.2], has an impact on several factors related to the
building.
2.2 Developed methodology and simulation tools for BiPV design
In the following paragraphs [2.2.1 and 2.2.2] the new method developed in this work and the
software used for the early design phase of BiPV is described. The methodology proposed aim to
provide a comprehensive evaluation of several issues related to the building integration of a
multifunctional technology as BiPV. It integrates design and analysis within a single parametric
environment, facilitating a smoother, more integrative and efficient workflow.
2.2.1 BiPV parametric design process
The procedure developed in this work evaluates several issues related to the integration of PV
on facades and the architectural system selected for the optimisation is BiPV as overhang. It compares
the output of energy production, shade of thermal gains and shade for daylighting purposes for a set
of different system of BiPV overhangs. Thanks to the aid of parametric design and genetic algorithm,
the three main aspects of the building integration (electricity production, daylighting performance and
thermal performance) are optimized. In the following sections, the main features of the methodology
proposed in this work are presented.
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Figure 2.5 – Developed BiPV parametric design process
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Parametric design
The process of parametric design consists in the characterization of a system or object through
a set of dimensions. These can be modified within a certain range of values. In explaining parametric
design, there are a few BiPV examples that deserve to be mentioned.
A glazing system with photovoltaic cells embedded can be described with a technical drawing down to
the smallest detail. Leaving the possibility to change the distance between neighbouring cells, without
changing the design of the element, could open the possibility of variating the overall transparency of
the glazed module. The distance between the cells, in this case, is a parameter. The distance not only
affects the transparency of the glass, but also the peak power per unit area. Analysing a BiPV
ventilated façade, as another example, the distance between the array and the façade could be the
parameter to be optimized. In BiPV overhangs, the type of system used in the following example,
illustrated with the Figure 2.6, the main parameter is the tilt angles. An array of overhangs could be
described with tilt angle, depth and distance between the rows.
Figure 2.6 - In yellow the length and angles characterizing the main parameters in an example facade with overhangs
When the behaviour and the physical performance of a building can be simulated, the main
advantage of the parametric design is its possibility of variating certain aspect of a system in order to
obtain maximum performance. The optimum is found by scanning a vast pool of solutions based on
the same basic design but with varying parameters. This gives an obvious advantage for systems such
as BiPV where simulation is essential to achieve design goals, and where performances and efficiency
are considered as key aspects. For the sake of BiPV architectural systems (defined in [1.2.3]), the
integration of photovoltaic has been grouped in few main categories, characterized by specific physical
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models and specific parameters. Several main solutions of PV integrated in roofs and façades,
accessible and not from within the building, are shown in Figure 2.7.
Figure 2.7 - Schematic subdivision in categories of main BiPV architectural system, each one could be parametrically described [13]
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Ray Tracing
The irradiation over the designated surfaces is calculated using a method called backward Ray-
Tracing. The procedure allows for the measurement of the irradiance (or illuminance) over specific
points on a surface. In the backward Ray-Tracing method, the light rays are traced in the opposite
direction to that which they naturally follow. As explained in [31], the process starts from the eye (the
viewpoint) and then traces the rays up to the light sources taking into account all physical interactions
(reflection, refraction) with the surfaces of the objects composing the scene. The idea behind
(backward) ray tracing is to simulate individual light rays in space to calculate the luminance
distribution in a room from a given viewpoint. Therefore, rays are emitted from the point of interest
and traced backwards until they hit either a light source or another object. In the former case, the
luminance distribution function of the light source determines the luminance contribution at the
viewpoint. If a ray hits an object other than a light source, the luminance of the object needs to be
calculated by secondary rays that are emitted from the object. Figure 2.8 shows a simplified model of
the Ray Tracing process.
Figure 2.8 - Ray Tracing method [37]
The Ray Tracing process requires as input at least one point associated to at least one vector.
The point will be the precise position where the irradiation is measured, the vector represents the
normal to the surface that should be measured.
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Weather data and environment modelling
To perform a building´s energy simulation the initial step is to create a model that emulates
the real site environmental condition. This means identifying all critical environmental factors that
influence the building. The main features in the model are the weather conditions and the geometries
and optical properties of the surroundings. In order to build a good model, a weather file should be
used. A weather file contains “typical” data derived from hourly observations at a specific location. It
represents local climatic conditions relative to the building models, such as hourly temperature,
humidity, wind speed and direction, atmospheric pressure and solar radiation or cloud cover
conditions.
Main part of the simulation model is based on the Ray Tracing procedure, using a sky function
as light source. The sky function is represented as a semispherical cap, composed of a system of points,
the luminance of which is determined by their coordinates. The value associated with each specific
point into the sky function can represent the radiation evaluated in a specific moment, when the sun
has a defined position (point in time evaluation) or it can be a total amount of annual radiation
(cumulative evaluation). The sky function, derived from the weather file adopted into the simulation
model, uses the value of direct and diffuse radiation combined with the sun position to generate a sky
vault. The sky function contains shadings determined by far objects. Far shadings usually cast an
instantaneous shadow on the whole simulated building. In order to evaluate the real surroundings
conditions, also close shadings have to be added to the model. The close obstructions, like surrounding
buildings and objects (trees, chimneys, etc.) could cast partial shadows on the simulated building (i.e.
leave some measuring point to the direct irradiation while shadowing others), so they have to be
considered (created or imported into 3D model).
As a last step, the definition of the real radiance properties of all the surrounding materials
(i.e. reflectance, roughness and specularity) is required to complete the environmental model, which
represents an essential input of every energy building simulation.
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Energy irradiation evaluation and early economic assessment
The developed method allows for an early assessment of the electricity production of the BiPV
system. The Ray-tracing procedure enables a measure of the irradiation over each measuring point of
a specific surface. For the early evaluation of the production and the economic performance of the
system an annual cumulative simulation is performed. For each measuring point, associated with a
single photovoltaic module, an annual cumulative irradiation is retrieved. The irradiation is then
multiplied by the efficiency of the module and by its ownarea, and by the performance ratio of the
system in order to get the system annual energy output. During the simulation every possible PV panel
irradiation is measured, and as results some modules are not worth installing, as their annual
production is low while their price is the same as the best performing ones. The method sorts all the
modules from the most to the least irradiated and calculate the annual electricity production removing
all the modules below a certain threshold with the simplified Equation 1
𝐸𝑡ℎ𝑟 = A ∙ 𝜂 ∙ 𝑃𝑅 ∙ ∑ 𝐻𝑛,
𝑛𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑
𝑛=1
Equation 1 - Annual electricity production for a configuration of modules chosen above a certain threshold of irradiation
where Ethr is the energy output [kWh] for one year, nthreshold is the number installed modules, A is the
area of a PV module [m2], η is the module efficiency, PR is the performance ratio of the system, and Hn
is the cumulative insolation of the nth module [kWh/m2 yr]. The performance ratio is a parameter that
evaluates the performance of a PV system over a certain period of time and it is defined as the ratio
between the final yield (energy produced by 1 kWp of PV system) and the reference yield (incident
energy from the sun divided by 1000 W/m2, irradiance defined as Standard Test Conditions). Typical
values of PR varies from 0.7 (roof mounted) to 0.9 (ground mounted).
Once the power production of the system is estimated, its profitability is accessed through the Net
Present Value (NPV), which is defined as
𝑁𝑃𝑉 = ∑𝐸𝑡ℎ𝑟 ∙ 𝐶𝑝(𝑡) ∙ 𝑃𝐸(𝑡) − 𝑚𝑡ℎ𝑟
(1 + 𝑟)𝑡 − 𝐼0
𝑙𝑖𝑓𝑒
𝑡=1
,
Equation 2 - Net Present Value after a precise time span of the PV system
Ethr is the energy output from Equation 1, Cp(t) is a coefficient of performance defined between 0 (worst
performance) and 1 (best performance), to consider degradation, PE(t) is the price of electricity in the
given year, mthr is the maintenance associated to the number of modules (threshold), I0 is the initial
investment and r is the discount rate, a measure of the future decrease in value of present money.
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With these two simple formulae, it is possible to evaluate a BiPV system’s annual electricity
production and maximize its NPV within a selected time frame by choosing an adequate threshold for
the minimum irradiation.
Thermal impact evaluation and thermal model
As a multifunctional technology included into the building envelope [1.2.2], BiPV necessarily
influences the building thermal balance. Its resulting impact can be evaluated through an energy
simulation, that, considering all boundary conditions and loads, models the building energy
consumption. Therefore, an optimization of the PV integration can also contribute to optimise the
building energy needs.
Firstly, a building model must be created for the simulation, identifying one or more thermal
zones. A thermal zone is a space within a building that has its own thermostat, served by the same
system and has the same building use. External and internal loads are assigned to the thermal zone.
- External loads come from the weather file. They are influenced by the building envelope: every
thermal zone has specific construction characteristics that condition the building energy
performances. The construction materials employed in the building envelope have their own thermal
properties and characteristics (i.e. thickness, conductivity, density, specific heat, roughness, thermal
absorptance). The external load of the thermal zones are deeply influenced by their boundary
conditions: the determination of the boundary conditions is thus required for the envelope surfaces
as adiabatic, outdoor, or ground exposed.
When the photovoltaics are integrated in the building envelope, they can have a thermal impact on
the building by modifying the external loads. BiPV are geometrically applied to the building model and,
as PV panels integrated in glazing surfaces or as shading elements, they influence solar gain, by varying
their position, size, tilt, transparency level, etc.
- Internal loads depend on the building use: user occupation, devices and type of activity conducted
condition the building energy balance determining the thermal loads. The internal loads are defined
hourly within schedules and assigned to the simulation model.
External and internal loads are employed as input in the Energy Plus simulation that provides hourly,
daily, monthly or annually energy need values (kWh or kWh/m2) for heating, cooling, ventilation,
lighting. These outputs have to be optimized, in favour of a nearly zero building energy balance.
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Study of daylight impact of BiPV
Daylighting concerns the illumination of the building interiors with sunlight or sky light and is
known to affect visual performance, lighting quality, health, human performance, and energy
efficiency. When the photovoltaic modules are integrated into the building glazed surfaces or as
shading elements, they have an impact on the internal illumination, controlling the solar radiation
reaching the interior of the building.
The daylighting is strictly connected with the human visual comfort as it is meaningful in
relation with the human eye. For this reason, it concerns the portion of the electromagnetic spectrum
included into the visible range of wavelengths (from about 390 to 700 nm). This requires the use of lux
(lumen/m2) as unit of the illuminance intensity in spite of raw irradiance (W/m2). The visual comfort is
related to quantity and distribution of light, it is a condition where people have enough light for their
activities and from the occupant’s viewpoint there is no blinding effect or discomfort glare. It is a
matter of subjective reaction, however there is an agreement about the necessity of glare control, and
about a few strategies for glare assessment in the design phase. One of these is to set a comfort range
of illuminance values on the measuring surface.
A daylighting analysis can be conducted through the Ray Tracing methodology previously
introduced, using a sky function as light source and a system of points and vectors. In this case , we
have to use a point in time evaluation to have hourly illuminance values. This is because the visual
conditions are evaluated calculating the percentage of hours in which the illuminance value is
maintained into the comfort range, so it is connected to the solar illumination level (weather file) and
to the occupancy of the building users (schedules), two factors that change every hour.
To perform a daylighting evaluation, we referred to two validate indices: Daylight Autonomy
(DA) and Simplified Daylight Glare Probability (sDGP), both based on the use of a system of points and
vectors. The first one, DA, is represented by the percentage of annual daytime hours that a given point
in a room is above a specified illumination level. The evaluation
is based on a horizontal point grid set on a specific level, which
could be, for example, the desk height [Figure 2.9]. A vector
indicates the normal to the plane on which the illumination is
measured, a vertical vector is associated to each of the points to
calculate horizontal illuminance.
Table height
Figure 2.9 - Points and vectors
system used in illumination analysis
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The second one, sDGP, is used for glare probability analysis,
evaluating the percentage of the annual daytime hours when people
are disturbed due to the high level of vertical eye illuminance. This
index also is based on a point evaluation, but in this case the points
are set at eye-level in particular positions where users are supposed
to stay for a long time during the occupied daytime [Figure 2.10]. Four
horizontal vectors with specific directions from each of the positions
have to be set to evaluate vertical illuminance. In cases of a
preferential viewpoint, the directions can be less than four, for
example if the position of the viewer is known there might be a single view direction. The sDGP index
does not sufficiently represent contrast based glare: to this extend it is required to generate luminance
renderings on a time-step basis. This solution is computationally expensive, and anyway cannot
represent the real daylight conditions of a building, as there is no way to foresee optical properties of
equipment and furniture chosen by the occupants. However, using the sDGP index together with
Daylight Autonomy index, it guarantees more details to the daylighting evaluation, revealing
characteristics of the simulated visual environment that cannot be directly inferred from predictions
of the horizontal work plane illuminance. A daylight simulation, with input the weather file, the points
set and the vectors attached to them, calculates these two percentage values, which can be crucial in
determining different design decisions. To avoid arbitrary choices at the end of the process the two
aspects of a minimum illuminance and glare performance were condensed in a DAI ( Daylight
Aggregated Index), described by following equation:
𝐷𝐴𝐼 = 1 − [ 0.5 ∙ (∑ 𝜏𝐸(𝑎)
𝑛𝑝𝑜𝑖𝑛𝑡𝑠
𝑎=1
𝑛𝑝𝑜𝑖𝑛𝑡𝑠+
∑ 𝜏𝐺(𝑏)𝑛𝑣𝑖𝑒𝑤𝑠𝑏=1
𝑛𝑣𝑖𝑒𝑤𝑠
)]
Equation 3 - DAI (Daylight Aggregated Index)
where τE(a) is the percentage of the time, over the total time of use of the building, when minimum
illuminance (300 lux) is not met on the specific ath point, npoints is the total number of measuring points
in the sample, τG(b) is the percentage of time when maximum irradiance (3473 lux) is exceeded on the
specific bth view while nviews is the total number of views in the sample.The DAI value resulting is
included in a range from 0 to 1.
The output of the daylight simulation provides a list of many different configurations with their
associated DAI. A solution might be excellent in avoiding glare while blocking too much daylight or
Eye height
Figure 2.10 - Points and vectors system used in glare
analysis
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vice-versa without affecting the index. Strongly unbalanced solutions (favouring light penetration or
glare protection) are characterized by lower DAI, while the best solutions are usually balanced.
Optimization algorithms
The core of the process does not lie in the evaluation itself but in the optimization. The method
has to evaluate many different samples with varying parameters and come out with a solution. To find
good performing combination of parameters, genetic algorithms are used. In the single target genetic
optimization, a performance metric (e.g. cumulative annual irradiation over the whole system) is
evaluated for every set of parameters. Each set of parameters can be considered as an individual, and
every single parameter can be considered as one of its genes. After the simulation of a certain number
of individuals (e.g. 100) they are sorted based on their fitness to the performance metric set before.
The low performing ones are excluded while the best performing ones are combined to form new
individuals, in a way passing on their genes to the offspring. In this way, it is possible to find high
performing set of parameters faster than a simple evaluation of every set in the solution space. The
genetic algorithm has as an output a list of individuals ranked from the best performing to the least
ones. In the use of BiPV, considering one simple fitness is not possible because there are, as we saw in
the previous paragraph, at least three criterion for the evaluation (electricity production, daylight
performance, thermal performance). In this case, a multi-target genetic algorithm should be used. The
output of this algorithm is a set of optimized solutions (a curve in case of 2 targets, a surface in case of
3 targets and a hypersurface for higher number of targets). Obviously, this type of output does not
provide the best solution, but a set of dominant solutions among which a choice must be made based
on considerations other than the three fitness metrics.
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2.2.2 Software tools for BiPV parametric design
The methodology previously explained in [2.2.1] developed in this work to support the design
stage of a BiPV system, is based on the use of validated software, able to provide a wide range of
building and environmental analysis.
Figure 2.11 shows a comprehensive overview of the software employed to develop the BiPV design
process. The main features of the tools proposed are briefly presented through the following sections.
Figure 2.11 - Software tools employed for the developed BiPV parametric design process
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Grasshopper
Grasshopper is a graphical algorithm editor that uses a visual programming language. It is
integrated with Rhino 3-D modelling software. The Grasshopper plug-in functions by associating
certain parts of a simple geometry with a graphical algorithmic editor. A geometry, created or imported
into Rhino platform, represents a static model. It can be transformed in a dynamic model within
Grasshopper [38]. A parametric transformation of
the geometry is implemented thanks to the “slider”,
a Grasshopper component customized to slide
along a range of numerical values. A slider can be
attached to the geometry, making its properties
parametric values. Every changing of the
parameters gets an instantaneous visual feedback
of the geometric effect in Rhino viewport, allowing
a better direct control of the system. A model based
on a parametric construction is used to perform dynamic evaluations of its performance, providing
different results for every variation of the parameters. In order to run building performance
simulations, Grasshopper is connected with specific validated simulation engines such as Radiance or
EnergyPlus. The model is firstly characterized with its specific properties in Grasshopper. It is then
submitted to the parametric simulations, which provide a dynamic visualization of the effect of the
design.
The BiPV design process I have developed in this work is based on the parametric modelling function of
Grasshopper, creating a complex analysis workflow based on a parametric definition and evaluation of
the BiPV system properties [2.2.1].
Grasshopper was developed by David Rutten at Robert McNeel & Associates [39].
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Internal loads and schedules are assigned to each of the buildings’ thermal zones. To define
the thermal zones the three building simulated are simplified and split into stories. One thermal zone
corresponds to one floor of one building. To limit the computation time, the floors designated to be
evaluated are three for each building (i.e. the two exposed stories and the middle one). Since the
buildings envelope has a crucial rule in controlling internal and external loads that must be considered,
the thermal zones are characterized by specific constructive properties. The construction materials
and components employed have their own thermal properties and characteristics (i.e. thickness,
thermal transmittance (U value), density, specific heat, roughness, thermal absorptance). They are
described in Table 3.3.
EXTERNAL ROOF U = 0.31 W/m2*K EXTERNAL WALL U = 0.26 W/m2*K
- green roof substrate 0.15 m - plaster 0.015 m
- EPS insulation 0.1 m - hollow brick 0.05 m
- reinforced concrete roof slab 0.25 m - reinforced concrete 0.25 m
- plaster 0.015 m - mineral wool insulation 0.13 m
EXTERNAL FLOOR U = 0.26 W/m2*K INTERNAL FLOOR adiabatic surface
- parquet flooring 0.01 m - acoustic ti le 0.02 m
- concrete screed 0.07 m - ceil ing air space resistance
- XPS insulation 0.05 m - l ightweight concrete 0.1 m
- screed 0.08 m WINDOW U = 1.5 W/m2*K
- XPS insulation 0.05 m - glass 3*10-3 m
- concrete floor slab 0.24 m - air 0.01*10-3 m
- plaster 0.015 m - glass 3*10-3 m
Table 3.3 - Druso Est construction materials. Definition of thermal transmittance and thickness
The thermal behaviour of the envelope components is
also characterized by specific boundary conditions. External
walls, floors and roofs are outdoor exposed, while internal floors
are adiabatic surfaces. The model is simplified through the
assumption that the thermal zones cannot exchange heat
among each other.
Once the thermal zone model is completed, it is
connected with the other inputs (i.e. buildings and BiPV
geometries, weather file) to the Grasshopper component able Figure 3.20 - EnergyPlus simulation
honeybee component
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to run the EnergyPlus simulation [Figure 3.20]. At the end of the evaluation process, several energy
demand values measured in kWh/m2/ y are provided. In this case, the outputs required are monthly
values.
The simulation results are immediately graphically visible within the Rhino viewport. The
thermal zones are coloured based on total energy (either cooling or heating) output data as shown in
Figure 3.21. The red zones represent those with the most energy demand, the blue ones require lower
energy supply.
Figure 3.21 - Thermal energy demand simulation output
The EnergyPlus simulation outputs are also provided as data in text format. They are available
in a “.csv” file that the software creates once completed the simulation. The file contains all the energy
demand data of each of the thermal zones for every month. Data are provided in a CSL format, which
can be edited in Excel.
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Figure 3.22 - Thermal energy demand simulation in the BiPV parametric design process
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3.2.6 Optimization algorithm
The energy simulations provide two performance indicators of the BiPV system: the annual
photovoltaics energy production and the impact of the overhangs integration on the annual buildings
energy demand for heating and cooling. These outputs are
connected with a specific BiPV configuration. Each configuration is
characterized by a combination of the three set of tilt angles (i.e. the
three model parameters): west, south and east [Figure 3.23]. A multi-
target genetic optimization algorithms, described in [2.2.1], is
performed to find the best set of angles to obtain specific
performance indicators. The first step is to connect, also into Grasshopper canvas, the performance
indicators and the three parameter to the optimizer component. Once started, the genetic
optimization process evaluates the current configuration and then the other ones, generated by
varying the parameters and each time restarting the simulation process. A genetic selection of the best
performance configurations, located as points on a Pareto curve, provides a set of optimized values of
electricity production and thermal energy demand. Figure 3.24 shows the distribution of the
configurations found by the two objectives optimization.
Figure 3.24 – Output of the genetic algorithm optimization. Each point corresponds to a BiPV configuration
40000
45000
50000
55000
60000
65000
70000
25.3 25.4 25.5 25.6 25.7 25.8 25.9 26
Pro
du
zio
ne
BiP
V (k
Wh)
Fabbisogno energetico (kWh/m2)
Figure 3.23 - Tilt angle Grasshopper sliders
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The two axis represent the indicators: the horizontal is the energy demand, the vertical is the
photovoltaic energy production. Each point corresponds to a BiPV configuration that have specific
performance values. The best configurations, i.e. those with optimized values of energy production
and/or of energy consumption, are the ones in red closer to the axes, not “dominated” by other
solutions [62]. In this case, the resulted optimized solutions are not so different in performance. The
one optimized for electricity production (on the right) differs by around 1000 kWh/year from the other
configurations. The one optimized for energy consumption (on the left) differs by less than 0.1 kWh/m2
year from the other configurations. The two target, therefore, seem not to be so competitive. The
reason could be found in the initial intelligent integration of the PV overhangs that, increasing the tilt
angle, distance itself from the windows, limiting the shade [3.2.2]. The resulted performance
differences, also, can be included into an estimated error range of 1.9 % (for the PV production) and
0.6% (for energy consumption), attributable to the software. Therefore, the best configurations can
be considered as a single solution. If otherwise (for e.g. with several optimized solutions with very
different performance), it would be left to designers to choose which solution to develop in the
following stage of the design. The choice would depend from the will of giving more weight to
electricity production or energy savings.
The output of the optimization process performed is shown in the following paragraph. A
resulted solution is analysed in terms of energy performances, evaluating the impact of the BiPV on
the total energy balance.
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Figure 3.25 - Genetic algorithm optimization phase in the BiPV parametric design process
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3.3 Optimization results
The optimization process provides a set of best solutions. One of them is analysed below. It is
characterized by the following specific combinations of the overhangs tilt angles: 48° on west façades,
36° on south, 56° on east. Figure 3.26 displays the results.
Figure 3.26 - One of the optimized BiPV configurations
The values reported in the following analysis are the output of the monthly EnergyPlus
simulation performed to evaluate an average energy demand of the three high-rise buildings
simulated. The aim is to evaluate the impact of the resulting shading system integration on the
buildings energy balance.
Figure 3.27 and Figure 3.28 show a comparison of energy demand values calculated in kWh/m2,
evaluated in two different buildings configurations. The first (in grey) represents a condition without
any shading system, the other one (in red and blue) is the result of the optimized overhang integration
on buildings façades.
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0
1
2
3
4
5
6
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
kWh
/m2
Ideal Heating Demand
Without BiPV
Optimized BiPV
According to the simulations results the effect of the overhangs appears to be beneficial for
the cooling load but has a negative impact on the heating load. This is an expected consequence since
the main function of a shading system is to block part of the incoming sunlight, and so the solar thermal
gain. However for an average increase of heating demand of 0.1 kWh/m2/month (i.e. 1140 kWh per
year for the three whole buildings), the results show an average decrease of cooling demand of 0.2
kWh/ m2/month (i.e. 2280 kWh per year for the three whole buildings). Therefore, the presence of
overhangs is more effective in decreasing the cooling load than increasing the heating load. It means
that BiPV overhangs can have an overall positive impact on the thermal energy balance, although not
much relevant.
Annual values
-without BiPV 16 kWh/m2
-with BiPV 17 kWh/m2
Annual values
-without BiPV 16 kWh/m2
-with BiPV 14 kWh/m2 0
1
2
3
4
5
6
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
kWh
/m2
Ideal Cooling Demand
Without BiPV
Optimized BiPV
Figure 3.28 - Ideal monthly Cooling Demand
Figure 3.27 - Ideal monthly Heating Demand
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Despite the comparison of the two different building configurations (without and with shading
BiPVs) in terms of ideal energy demand does not show a relevant improvement, it is not the same if
the electric energy balance is considered assuming that heat pumps are used.Figure 3.29 shows an
annual balance where the energy demand values, measured in kWh, were converted into electric
energy. Once a conversion factors is applied (2.5 for cooling and 3 for heating, supposing an heat pump
generation system and a low temperature emission system), the total electricity consumption was
calculated. For the first buildings configuration (in grey) it represents the amount of cooling and
heating demand values. For the other one (in blue) it was obtained by summing cooling and heating
demand values and subtracting the photovoltaic total energy production (in yellow).
Figure 3.29 - Electric Energy Balance of the district
Considering the total electricity consumption only, the two building configurations do not
show any relevant annual difference. Integrating the shading photovoltaics, the energy demand is
slightlyreduced, from 136500 to 134800 kWh per year. However, if the whole energy balance is
evaluated, the buildings consumption with overhangs integrated is considerably decreased, due to the
photovoltaic production contribution.
The overhangs benefit is clearly visible also at monthly level in the chart of Figure 3.30. In green
are represented values related to the consumptions savings resulting from the shading system
integration. The savings are calculated as the difference between the thermal energy demand (for
cooling and heating) converted in electric energy of the two different buildings configurations (without
and with shading BiPVs). It is relating with the BiPV energy production (in yellow) to provide a measure
of the monthly energy balance (in blue).
-100000
-50000
0
50000
100000
150000
kWh
Electric Energy Balance
Electric Energy Consumptionwith BiPV minus BiPV AnnualProduction
BiPV Annual Production
Electric Energy Consumptionwhitout BiPV
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Figure 3.30 - BiPV Energy Savings and Electric Energy Balance
As expected, during the heating season the savings values are negative, because the overhnags
are blocking part of the solar gain. On the other hand during cooling period the savings values are
positive, so the shading system has a favourable impact on the energy demand. This output means, as
already highlighted in Figure 3.27 and Figure 3.28, that this specific BiPV solution increases the heating
demand, but less then it decreases the cooling one. Moreover, if the photovoltaic added value of the
energy production is considered, the loss caused during heating season has a low influence on the total
electricity balance.
-2000
0
2000
4000
6000
8000
10000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
kWh
BiPV Energy Savings and Electric Energy Balance
Energy savings
BiPV Production
Electric EnergyBalance
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Figure 3.31 - Optimized models provided by the BiPV parametric design process
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4 Conclusions
This thesis work shows a methodology able to provide a broad support to design complex
projects integrating BiPV systems from the first design stage. At the beginning, a project is firstly
conceived as a concept, as an idea. Details and strategies are not defined. What is determined is the
final goal. The design process should represent the phase in which the concept evolves in a strategy
and effective solutions. The developed method is an instrument able to support the designers in
realizing their aims, justifying a potential project feasibility. This is enabled by the parametric approach,
which is the main feature of the methodology.
For the Druso Est case study, the developed method was applied to optimize the integration
of a building integrated photovoltaic system (i.e. BiPV overhangs), towards a low energy balance at
district level. The BiPV architectural system is a complex technology whose parameters need a fine
tuning to be optimized. The parameters are involved into a model that is submitted to a comprehensive
evaluation, taking into account all the main concurrent effects such as energy yield, cost analysis,
impact on the building energy demand and on the internal comfort. The simoultaneous consideration
of all these factors is necessary due to the multifunctional feature of the BiPV system. As a
consequence, the optimization does not provide a best solution according to a specific target but a set
of equally good ones. The designer can choose among the solutions according to several additional
considerations. In this case, a designer could choose to focus on the photovoltaic production towards
an optimal energy balance, or he could pursue the goal of minimizing the thermal demand for example
to reduce CO2 emissions, or he could again aim at achieving economic earnings after a given period.
Having a set of defined optimized solutions to select from, can be reflected in an active working
relationship with the customer. The designer can gather the client assessments and considerations,
once showed all the possible options, with peculiarities, limits, pros and cons of each solution. The
customer can see realistic data, previsions justified through a simulation process that, using validated
software tools, creates a model close to the truth. The customer can take advantage of graphic outputs
that make the results obtained more comprehensible and so have a clear representation of what it will
be the result of the project, with an appearance that will be more or less appreciated from an aesthetic
point of view.
One of the main positive aspects of the implemented method is that, after performing a series
of evaluations to determine energy efficient and economically feasible solutions, also aesthetically
good solutions can be considered. These will be solutions where the good appearance will not be
simply an added value but it will keep a “multifunctional” justification inside.
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The methodology developed and presented through this thesis is based on a specific linear workflow
workflow characterized by some main steps: create a model, test it, analyse the results, approve them or not. If or not. If not, go back to the beginning and, once changed the previous configuration, repeat the process to find
process to find the best solutions. Although it seems to be a simple process, when many inputs or targets to
targets to evaluate are involved into the optimization procedure, some issues connected to the processing processing timeframe could appear. Hence several questions can arise: How to reduce the time needed? How to
needed? How to simplify the process? Maybe limiting the accuracy? Maybe lowering the detail level? This is a This is a complex topic. These questions are of crucial importance for large the project scale. From the simple
simple inclusion of some buildings, it could comprehend a whole district and a city, etc. Implementing a method Implementing a method such as the one applied for the Druso Est district, able to include in a single
single evaluating environment many targets and to provide realistic previsions, could represent a suitable suitable support into a urban, regional, territorial development and energy planning. A suitable key of
development for the methodology could be the nZEB concept, that takes into account several different factors factors (and buildings, if talking about nZED design) into a global energy balance evaluation.
Figure 4.1 shows an example of SWOT analysis reporting strengths, weaknesses, opportunities
and threats, previously explained in the present paragraph, of the developed design methodology.
Figure 4.1 – Developed BiPV parametric design SWOT analysis
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4.1 Outlook
The suggested methodology and tools have been developed to facilitate the design of a
complex element such a BiPV system. However, it can represent a starting point for the development
of an overall method for building design. The previous paragraph highlighted strengths and
weaknesses, opportunities and threats of the developed design process. Each of them could represent
a development key of the methodology, in order to improve its potentiality. The main issues are
discussed below.
Multi-target evaluation
One of the main aspects characterizing the developed methodology is the capability to involve
in a single workflow, and in a single software platform, several different targets. The aim was to equate
them in a comprehensive evaluation to obtain a series of solutions for which economic feasibility,
photovoltaic production and energy demand are simultaneously considered. However, it was proven
to represent a complex purpose.
At the beginning of the optimization process, once the solar irradiation of the PV overhangs is
calculated [3.2.3], the employed procedure proceeds with the economic selection [3.2.4]. This
represents a critical phase. With the aim of guaranteing economically feasible results, the selection
excludes all the panels that, due to their low solar irradiation, do not allow a payback within a certain
timeframe (e.g. twenty years). Some of the discarded overhangs, however, could lead to an energy
saving (of heating or cooling) higher than the amount of the photovoltaics production. Otherwise,
some of the chosen ones could increase the thermal energy demand exceeding the energy generated.
In this case, the economic selection represent a simplification that could exclude some potentially good
solutions.
Several attempts were made to solve the issue. One of them consisted in basing the first
selection on an evaluation of the energy overhangs benefit. This would mean that, once both the
production and consumption simulations were performed, the panels would be selected according to
their impact on the energy demand. The overhangs that, with a specific outdoor temperature, lead to
the use of cooling or heating are preferably removed. This solution, also, is characterized by
simplifications. Considering as an input the heating and cooling consumption hourly values provided
by the thermal energy demand simulation [3.2.5], the dynamic internal gain are instead not taken into
account.
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Finally, the main issue connected with the complex purpose of simultaneously evaluate all the
targets seems to be the problem of simplification. When trying to consider at the same time several
evaluations, it seems complex to keep for each of them a good level of detail. Maybe, starting from a
simplified model to acquire then even more detail throughout the optimization process could be a
solution. Several trials have still to be carried out.
Figure 4.2 - Ideal multi-target BiPV parametric design process
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Daylighting simulation
When shading systems are integrated into the buildings, their impact on the daylighting can
be crucial, as previously seen in [3.2.1]. Optimizing an overhang can be an effective way to guarantee
visual comfort by controlling the glare and allowing enough natural lighting. Therefore, the daylighting
can represent an additional performance indicator to be included into the optimization process,
together with the photovoltaic electricity production and thermal energy building consumption. As
explained in [2.2.1], a daylight analysis uses the same method of the Ray Tracing as the simulation of
PV solar irradiation. However, in this case it is not cumulative but point in time, so the evaluation has
hourly time steps. Being a point grid simulation, it requires a series of points and vectors.
For Druso Est case study a preliminary daylight evaluation was performed. The living rooms of
the three high-rise building are selected to be submitted to the
analysis. Into each of these sites several points were disposed
on an horizontal grid set at desk height to evaluate the
percentage of annual daytime hours that they were above a
specified illumination level. Other three points, for the glare
analysis, were set at eye-level in particular positions where
users are supposed to stay for a long time during the occupied
daytime [Figure 4.3]. The vectors were assigned to each point
according principles already specified in [2.2.1]. Points and
vectors were connected to the Grasshopper component able to
run the Radiance simulation, together with the other input (i.e. 3D model, Druso Est weather file, BiPV
architectural system, users occupation schedule). Once performed the simulation, it provided a set of
values related to the annual percentage of hours in which the illuminance value is maintained into the
comfort range, from 300 [63] to 3473 [64] lux.
Also in this case the output are graphically visible in
Rhino viewport, connecting them to a multiple coloured
gradient component of Grasshopper. An example of a simulated
site, a living room on tenth floor, is shown in Figure 4.4. The
coloured circles are located at the desk level, the visible arrows
represent the vectors set for the glare analysis. The green
elements are indicative of visual comfort, a situation with
enough illumination intensity and an effective glare control.
Figure 4.3 - Points grid and vectors system visualized in Rhino viewport
Figure 4.4 - Daylighting simulation
output
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In the example shown above, the simulation was a long lasting process. The reason could be
found in the high density of the grid points set, excessively thick to be submitted to a point in time
evaluation. Some grid points could be plainly removed, or some sites can be excluded. Simplifying the
model could be a mean to excessively reduce the evaluation accuracy. Therefore, careful
considerations are needed. However, a potential development of the study could allow to achieve a
correct simplification level in order to include the daylighting simulation into the optimization process.
Figure 4.5 - Daylighting simulation in the BiPV parametric design process
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Load matching
One of the main goals usually pursued when installing a photovoltaic system is to maximize
the peak production from the installation over a 24-hour period, orienting the PV panel towards south
[65]. However, this could not result in a suitable approach. Often, the time of peak electric gains from
a PV array of southern azimuth does not coincide with peak electricity demand. As exemplified in [65],
if a load needs to be supplied during the second part of the daylight, from 1pm to 7pm, such a choice
does not ensure a correct energy management in the time interval where it is mainly needed. All the
energy extracted in the morning needs to be temporarily stored. It would be more efficient to point
the photovoltaic panels towards West, in order to maximize the energy converted from solar radiation
in the time interval within the load requirement is centred.
Analysing the interaction between the load required by the utility and the output of the PV
system seems to represent a decisive step during the design process. The load matching can widely
increase a PV system performance. It can be an effective strategy to best optimize the PV energy use,
for example reducing the energy cost associated with the storage system (in case of stand-alone
applications) [65] or to minimize the power injected into a grid, avoiding technical and potential
problems with it connected [66].
PV sizing and orientation are two crucial
aspects in conditioning the quality of the load
matching. Figure 4.6 shows the results arose
comparing the power output of two different PV
installation [67]: a single axis tracking systems
rotating from East to West and a fixed axis
system with south azimuth. The first one tracks
the sun throughout the course of each day,
maximizing the power gains during the morning and evening. The second one concentrate the time of
peak solar gain at midday. A clear difference of peak time and total production characterizes the two
applications. It can represent a useful criteria for evaluating which solution can better adapt to specific
buildings, according to the design goals.
A load matching analysis could be therefore included in an optimization process to evaluate a
PV system installation performance. It could result especially suitable in BiPV systems, due to their
several flexible solutions [1.2.3].
Figure 4.6 - Illustrative comparison of tracking PV (horizontal mount, E-W azimuths) to standard fixed
axis PV with South azimuth [50]
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With regard to Druso Est case study, an assessment of the electric energy consumption was
performed. In Figure 4.7, an hourly definition of the electricity demand profile (in blue) is compared
with the BiPV system production profile (in yellow).
Figure 4.7 - Hourly values of Energy Production and Consumption
The electric energy loads values come from the monitoring of a residential complex composed
of 40 units placed in north Italy [60]. The curve, relating to a typical summer day, shows a considerable
increase during the morning and the evening. Otherwise, the energy production curve is characterized
by a great growth around midday. There is a clear condition of mismatching. This BiPV system
configuration does not provide energy exactly when the buildings need the main electricity supply. The
reason could be that, with the current system solution, the most panels are south facing [Figure 4.8],
so they are especially irradiated around midday. Changing the photovoltaics configuration, for example
adding panels on east and west façades, could represent
the first step to increase the correlation between the
electricity production and the load profile. However, this
improvement of the load matching would have an impact
on other factors (e.g. thermal balance, daylighting, etc.).
Therefore, including the load matching could represent an
interesting development, an additional target to involve
into the design process to improve the optimization
results.
0
10
20
30
40
00:00 04:00 08:00 12:00 16:00 20:00 00:00
kWh
Electric Energy Production and Consumption
BiPV EnergyProduction
Electric EnergyLoads
Figure 4.8 - Resulted BiPV system configuration
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Figure 4.9 - Load matching integration phase in the BiPV parametric design process
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