Towards an improved architectural quality of building integrated solar thermal systems (BIST)

Abstract

Architectural integration is a major issue in the development and spreading of solar thermal technologies. Yet the architectural quality of most existing building integrated solar thermal systems (BIST) is quite poor, which often discourages potential new users. In this paper, the results of a large web survey on architectural quality, addressed to more than 170 European architects and other building professionals are presented and commented. Integration criteria and design guidelines established and confirmed through the analysis of these results are proposed. Subsequently, a novel methodology to design future solar thermal collectors systems suited to building integration is described, showing a new range of design possibilities. The methodology focuses on the essential teamwork between architects and engineers to ensure both energy efficiency and architectural integrability, while playing with the formal characteristics of the collectors (size, shape, colour, etc.). Finally a practical example of such a design process conducted within the European project SOLABS is given; the resulting collector is described, and integration simulations are presented.

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Solar Energy
journal homepage: www.elsevier.com/locate/solener
Criteria and policies to master the visual impact of solar systems in urban
environments: The LESO-QSV method
Maria Cristina Munari Probst, Christian Roecker
⁎
Laboratoire d’Energie Solaire (LESO), Ecole Polytechnique Fédérale de Lausanne(EPFL), Switzerland
ARTICLE INFO
Keywords:
Urban acceptability
Solar policies
Architectural integration quality
Visual impact
Solar map
Criticity
ABSTRACT
Increased use of solar collectors in buildings is necessary but poses major challenges in existing built environ-
ments, especially where architectural quality is an issue. The large size of solar systems at the building scale
requires careful planning, as they may easily end up compromising the aesthetics of buildings, threatening the
identity of entire contexts. A new method named LESO-QSV(for Laboratoire d’Energie SOlaire – Qualité-
Sensibilité-Visibilité) has been developed to help authorities promote solar energy use while preserving the
quality of pre-existing urban areas. The vision underlining the approach is that solar integration is possible also
in delicate contexts, if appropriate design efforts and adequate cost investments are made. The issue is then no
longer to be in favour or against the use of solar systems in cities, but rather to define appropriate local levels of
integration quality, and to identify the factors needed to initiate smart solar energy policies able to preserve the
quality of pre-existing urban contexts while promoting solar energy use.
The LESO-QSV method helps tackle these issues with clear and objective proposals:
First it clarifies the notion of architectural integration quality and proposes a simple evaluation method, based
on a set of criteria derived from pre-existing literature.
Then it helps authorities set and implement local acceptability requirements, introducing the notion of ar-
chitectural “criticity” of city surfaces (LESO-QSV acceptability). The concept of “criticity”, at the basis of the
whole approach, is defined by the Sensitivity of the urban context where the solar system is planned, combined
with its Visibility (close and remote) from the public domain. The more sensitive the urban area and the more
visible the system (high “criticity”), the higher the need for integration quality. In practice, authorities will be in
charge to set the desired integration quality levels for each of the defined “criticity” situations, considering local
specificities (energy context, available energy sources, political and social considerations, city identity and to-
pography, among others). To help authorities set these quality expectations, the software LESO-QSV Grid has
been developed. It illustrates the acceptance impact of pre-defined sets of quality requirements, using a large
number of integration examples (150 emblematic cases). These detailed examples are provided to show au-
thorities how to objectively evaluate integration quality, but they also constitute a large set of learning examples,
good and bad, for architects, installers and building owners.
Finally the method proposes a way to tailor solar energy policies to local urban specificities by mapping the
architectural “criticity” of city buildings surfaces, and crossing this information with a city solar irradiation map
(LESO-QSV crossmapping), hence completing the characterization of the building surfaces with the potentially
required effort of integration.
1. Introduction
In the last decade global warming has become a major concern for
the community, leading political authorities to take increasingly drastic
actions in order to achieve energy savings and encourage the use of
renewable energy sources. In the built environment these new concerns
have led to the introduction of strict energy standards whose
requirements continue to rise.
In Switzerland, many standards and regulations have been estab-
lished, both voluntary (Minergie, Minergie P, Eco, A) and mandatory
(SIA 380/1), clearly indicating a trend towards a zero energy balance of
buildings.
In Europe, the framework is equally strict. A recent directive of the
European Parliament rules that from 2020 onwards, each and every
https://doi.org/10.1016/j.solener.2019.03.031
Received 21 September 2018; Received in revised form 26 February 2019; Accepted 10 March 2019
⁎
Corresponding author at: Laboratoire d’Energie Solaire EPFL, Station 18, 1015 Lausanne, Switzerland.
E-mail address: christian.roecker@epfl.ch (C. Roecker).
Solar Energy 184 (2019) 672–687
Available online 17 April 2019
0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
T

new building will have to meet the requirements of the NZEB standard
(Nearly Zero Energy Building). For public buildings these requirements
will have to be fulfilled starting January 2019. As stated by the
Commission, “Nearly zero-energy buildings need to have very high
energy performance”, and “The low amount of energy that these
buildings require has to come mostly from renewable sources”.
(Directive 2010/31/EU European Union Parliament).
In order to meet these new standards, it will no longer suffice to cut
heating needs through an effective insulation of the envelope – it will
also be necessary to plan and enforce new, long-lasting strategies for the
production of operational power.
Its abundant availability and flexible use makes solar energy natu-
rally one of the resources that we will turn to in priority (Fig. 1, left).
A combination of passive and active solar energy is in fact able to
meet the various energy requirements of buildings in an effective and
economically sustainable way (Fig. 1, right):
– appropriate daylighting strategies are able to decrease artificial
lighting needs to a minimum;
– passive collection of solar energy through windows can cover a very
large part of heating needs;
– solar thermal collectors are an excellent solution to produce hot
water, for domestic use and space heating.
– photovoltaic modules can provide the power for household appli-
ances and lighting, and can also operate a heat pump.
Clearly though, a consistent implementation of all these strategies
will not be possible without a global architectural reflection. On the one
hand, the position and size of openings play an essential part in lighting
and passive heating strategies; on the other hand, the size of active solar
systems is such that these systems have a major impact on building
appearance (Dessi, 2013). As much as new insulation requirements
have changed the way the materiality and language of the envelope are
conceived, the use of solar technology will have a radical influence on
the layout of buildings’ exposed surfaces. Without the skills needed to
integrate these new elements in a consistent design, the result will fail
to be satisfactory from an architectural point of view.
2. Energy versus architecture?
The new energy regulations and mandatory solar fractions for
electricity and domestic hot water are introducing new materialities
and geometries in buildings, which are leading to new forms of archi-
tectural expression that will slowly modify our city landscapes. NZEB
designers most often choose a compact geometry (optimizing the heated
volume/envelope losses ratio) and then need to artificially expand the
envelope surface to intercept a higher amount of solar radiation and
convert it through dedicated solar devices into electricity or heat
(Fig. 2).
This increased use of active solar collectors in buildings is clearly
necessary and welcome, but brings major challenges, especially in al-
ready existing built environments. The large size of solar systems at the
building scale asks for thoughtful planning, as these systems may end
up compromising the quality of the building, threatening the identity of
entire contexts (Fig. 3).
Accepting to sacrifice architectural quality to promote solar spread
can be very counterproductive, leading right to the opposite effect in
the long term. Animated discussions are already ongoing in most cities
between “solar pros” on one side, concerned by the urgency of max-
imizing renewable energy use and asking for total freedom to install,
Fig. 1. Left: Building energy evolution (Switzerland); Right: Available solar technologies in relation to the different energy needs.
Fig. 2. New Solar Buildings (left: 3M office building, Milan, M.Cucinella; right: Endesa pavilion, IAAC, Barcelona).
M.C. Munari Probst and C. Roecker Solar Energy 184 (2019) 672–687
673

and architects and building heritage institutions on the other side, ex-
pressing their worries about the urban impact of such systems and
asking for a restriction of their use to certain urban contexts only.
3. The LESO-QSV mediation solution
De facto, both concerns of maximizing solar energy spread and
protecting the architectural quality of the built environment are justi-
fied, and both should possibly be satisfied at the same time.
This is even more true when considering that good architectural in-
tegrations can often be possible even in very critical contexts, under the
condition that appropriate design and cost investments are made
(Fig. 4).
Starting from these considerations, and convinced that the devel-
opment of solar energy in cities is one of the prominent challenges of
the near future, we looked for an “inclusive“ solution adressing Energy
and Architecture.
As the issue is then no longer to be in favour or against the use of
solar energy systems in cities, we propose to define minimal local levels
of integration quality, and to identify the factors needed to establish
smart solar energy policies able to preserve the quality of pre-existing
urban contexts while allowing solar energy use.
The LESO-QSV(Quality-Sensitivity-Visibility) approach (Munari
Probst and Roecker, 2011,2015) gives clear and objective answers in
this debate:
(a) First it introduces the innovative notion of “Architectural criticity“
of city surfaces, in relation to their need for integration quality (see
Section 4).
(b) Then it clarifies the notion of “Architectural integration quality”,
proposing a simple and objective evaluation method (see Section
5);
(c) Based on a) and b) it helps authorities set and implement precise
“Local acceptability requirements” (“LESO-QSV acceptability”, see
Section 6);
(d) d) Finally it proposes a way to tailor “Solar energy promotion po-
licies” to local urban specificities by combining architectural “cri-
ticity“ and solar irradiation information (”LESO-QSV-crossmapping“,
Fig. 3. Solar renovation at Schloss Walbeck-Castel, Germany (XVIII century).
Fig. 4. Photovoltaic system integrated on the roof of Aula Pierluigi Nervi, Vatican.
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674

see Section 7).
4. Architectural “criticity” of city surfaces
Good integration quality is always desirable, but not always crucial,
or even necessary. To facilitate the spread of solar energy, expectations
toward integration quality may be reduced where the perceived impact
of the installed system on the urban quality is lower.
In order to structure this idea, the new concept of “criticity” of city
surfaces is introduced, which is at the basis of the whole LESO-QSV
approach. The “criticity” of a surface defines the impact that mod-
ifications on that surface can have on the perceived global quality of the
context.
Two main factors are defining the criticity of city surfaces: the ar-
chitectural sensitivity of the building/urban context of the surface lo-
cation, and the visibility of that surface from the public domain.
4.1. System visibility
The visibility of the surface from the public domain is one of the two
factors influencing criticity. The higher the visibility of the surface from
the public domain, the more important its impact on the perceived
context identity. As for the context sensitivity, visibility is articulated
into three levels High,Medium and Low. The visibility has two compo-
nents: visibility from close range (close visibility) and from far away
(remote visibility) (Figs. 5 and 6).
For the first one, the determining elements will be the geometry of
the buildings and of the public space, while the second will be influ-
enced by the topography of the city and its surroundings.
A detailed description of the practical ways to assess the visibility of
city surfaces and the impact various city configurations have on this
factor is given in Appendix A.An automatized geometric calculation
tool is also under development at Ecole Polytechnique Fédérale de
Lausanne (EPFL). The first promising results are presented in a PhD
thesis just completed at EPFL (Florio, 2018).
4.2 Context sensitivity
The pre-existing quality of an urban context/building clearly in-
fluences the level of quality we can expect an integration to fulfil. If the
pre-existing context has no clear identity, nor other specific
architectural qualities, it is not pertinent to ask for a perfectly designed
and integrated solar system. By contrast, it seems very important to
push for integration quality in valuable areas or buildings.
To practically structure the issue, the QSV method proposes to
classify the sensitivities of existing contexts into 3 categories:
–High sensitivity (heritage protected or meaningful contexs/build-
ings);
–Medium sensitivity (contexts/buildings with no specific archi-
tectural/urban qualities, but with a meaningful identity for the
community, like most post world war residential or tertiary urban
developments);
–Low sensitivity (contexts with poor urban/architectural qualities,
and no specific identity, like many recent industrial/commercial
urban developments) (Fig. 7a–c)
This categorization is meant to be coherent and coordinated (even if
simplified), with both the city zoning defined in the building regulation
codes, and the official categorization of protected areas and buildings
released by national and regional Heritage commissions (Fig. 7d).
In a situation of rehabilitation or new affectation of a neighborhood
the sensibility categorisation should clearly refer to the projected quality
level planned by the authorities, and not to the present status.
4.3. Derived “criticity” matrix
To structure the issue of architectural criticity of city surfaces and
related needs for integration quality, a matrix,called “criticity grid”, is
established by crossing the three identified levels of visibility (low-
medium-high, Figs. 5 and 6) with the three identified levels of sensi-
tivity (low-medium-high, Fig. 7), defining nine different criticity si-
tuations for which quality expectations will have to be set (cf. chap. 6).
As shown in Fig. 8, the matrix displays a “criticity gradient” of in-
creasing sensitivity, from the top left corner to the bottom right.
5. Assessing architectural integration quality
Requesting a certain level of integration quality implies being able
to assess quality. Often this is considered a matter of personal taste, but
recent studies have confirmed the existence of implicit criteria shared
by the architects community and actually leading architectural
Fig. 5. Visibility of city surfaces from the public domain.
Fig. 6. Remote visibility is influenced by city topography.
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integration quality perception (Krippner and Herzog, 2000; Munari
Probst, 2008; Munari Probst and Roecker, 2012).
To be perceived as integrated, the system has to be designed as an
integral part of the building architecture. This means that all the formal
(i.e. visual) characteristics of the solar energy system, have to be co-
herent with the global building design logic (Munari Probst, 2008):
–collectors field size and position
–visible materials
–surface textures
–colours
–modules shape/size
–jointing system
Identifying these key characteristics opens the way to an objective
evaluation procedure.
5.1. Evaluation procedure
Based on these findings the LESO-QSV approach proposes a sim-
plified qualitative assessment method, grouping the above described
integration characteristics into three global sets of characteristics,
leading to three integration criteria. This makes the procedure lighter
while keeping the evaluation as objective as possible (Fig. 9).
The quality evaluation consists then in assessing separately the level
of coherence of the three sets of characteristics with the global building
logic – coherence of the System Geometry, the System Materiality and the
Modular Pattern - using a three-level scale (fully coherent - partly co-
herent – not coherent).
This allows to concentrate sequentially on the specific character-
istics of the integration while evaluating their level of coherence:
– First the system geometry is evaluated, considering the size of the
collectors field, its shape and its position in relation to the building.
– Second, the coherence of the system materiality is evaluated, con-
sidering mainly the colour, texture and reflexivity of the modules.
– Finally, the pattern formed by the juxtaposition of modules and their
jointing system is considered, to rate the coherency of the modular
pattern.
(In the cases where the most critical visibility situation is the remote
visibility (§ 4.1, Fig. 6) this last criteria can often be validated as fully
coherent, as the modular pattern is no longer perceived from afar.)
This being a global qualitative evaluation, the partial results cannot
be expressed by numbers and cannot be synthesized in a single mean
value; hence the choice to represent each partial evaluation as a co-
loured arc of a circle (green, yellow or red, according to the level of
coherency) to be combined with the others to form a complete three
sectors circle expressing the global system quality (Fig. 9).
Fig. 8. Criticity matrix and gradient.
Fig. 7. (a, b, c) different degrees of sensitivity of existing urban context – (d) Swiss ISOS offcial mapping of heritage protected enclosure.
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5.2. Criteria evaluation examples
The following examples show the system evaluation principles ap-
plied to existing cases, demonstrating the idea of separate evaluations
for each of the three global criteria (see Figs. 10,11 and 12).
partly coherent not coherent
Fig. 10. Different levels of “System geometry” coherency.
Fig. 9. Integration quality evaluation method: criteria grouping – sectors evaluations– resulting quality circle.
partly coherent not coherent
Fig. 11. Different levels of “System materiality” coherency.
partly coherent not coherent
Fig. 12. Different levels of “Modular pattern” coherency.
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Fig. 13. Continous vs discrete scale evaluation: explaining apparent discrepancies.
Fig. 14. Discrete vs continous evaluation scale and examples of cross evaluations by MAS students in Architecture.
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5.3. Evaluation scale, discrete vs continuous
Proposing only three discrete values for qualifying the integration
has the major advantage of greatly simplifying the tool, but it somehow
reduces the possibilties of nuancing the appreciation.
To evaluate the impact of this simplification a continuous scale has
been proposed to take this aspect into consideration (Fig. 13). This al-
lows to evaluate the integration using the continous scale and to then
read the result considering the three coloured “zones”, giving the dis-
cretized evaluation score.
The continous scale helps evaluators set the discrete values, and can
also explain why two expert’s evaluations, once discretized, can slightly
differ on particular cases.
The closer the “linear” evaluations are to a limit, the higher the risk
of getting different discretized appreciations. For example, the closer a
green evaluation is to the yellow zone for an expert, the higher the
chances of getting different results (green or yellow) when the case is
analysed by a different person (Fig. 13 System materiality). The ap-
proach was effective, as shown in the validation section below.
5.4. Validation
To validate the process of quality evaluation, the approach has been
tested in several academic courses and seminars dedicated to solar in-
tegration and has been used as teaching material (Munari Probst and
Roecker, 2009;Munari Probst, 2015a,b).
Participants were asked to individually evaluate integration cases,
placing coloured stickers on the linear scales. As shown in the examples
in Fig. 14, while not totally uniform, the participant’s evaluations
showed a very good general coherency, well validating the approach. A
relatively low spread due to subjectivity and to the qualitative character
of the evaluation process still does exist, but it indicates rather a
variability in the intensity than a fundamental difference in perception.
This procedure can be very useful when operating within specialized
commissions having to find a consensus over the evaluation of delicate
situations/objects.
Altogether this shows that the method can be considered reliable,
and the discretization into a three-value scales a satisfactory practical
choice.
A complete “case description and evaluation” sheet has also been
developed, which uses the adapted version of the continuous-discrete
evaluation scale (Section 6.2,Fig. 17).
These sheets are used within the Grid Software (Chapt. 6.1) and for
education purposes (Chapt. 6.2)
6. “LESO-QSV Acceptability”
With a reliable method to evaluate integration quality finally
available, quality expectations can be set for each criticity situation (see
Chapter 4).
However, the quality level to be required for each criticity situation
of the matrix is neither absolute nor constant, but depends on many
local and temporal factors, such as city identity and image, energy
context, availability of other renewable energy sources, quality and
availability of market products for good integration solutions, political
orientation, economic structure, etc.
For this reason the method does not provide one set of absolute
quality requirements, but offers flexibility.
A specific tool, the LESO-QSV GRID software (§6.1) is provided to
support authorities in establishing their own grid of local quality ex-
pectations, which will be more or less severe depending on the local
reality. Three predifined grids, of different “severity levels” are pro-
posed (Fig. 15), and the application provides the possibility to elaborate
acustom grid, to adress specific situations.
Note that in the acceptability phase, as only the number of sectors of
each colour is relevant, no specific position in the circle is attributed to each
criterion.
Fig. 15. Different possible levels of severity for the acceptability grid (from left to right : standard, permissive, demanding).
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6.1. LESO-QSV GRID software
To help authorities set these requirements, a multi-purpose software
simulation tool has been developed, called LESO-QSV GRID (Fig. 16).
Quality expectations are represented by the same three circle-sector
symbols used for the evaluation of the integration quality described in
Section 5. Three “standard” sets of quality requirements with gradual
severity (demanding - standard – permissive, Figs. 15 and 16/1) are
available for authorities to choose from, together with the additional
option of setting a fully customized grid. As can be seen in the ex-
amples, most propsed grids will have a “severity gradient” matching
somehow the criticity gradient.
To help authorities choose the most appropriate “acceptability
grid”, a large selection of integration cases is displayed (Fig. 16/3) that
shows in real time which integration approaches would be accepted and
which ones would have to be rejected with the selected settings. The
examples database can be scrolled through, showing the effect of the
acceptability grid over a very extensive set (more then 150 real cases)
of integration approaches and criticity situations.
The same software is intended to be used, with minor adaptations,
also as an education tool for architects, authorities, installers and
building owners. The wide palette of examples provides inspiration
from good examples, shows errors to be avoided or gives ideas on how
to improve the quality of a project which would be rejected in its
present state.
It can also help municipalities explain in an interactive and visually
convincing way how the method works and justify to users possible
project rejections.
Selection buttons are available in the bottom part of the screen to
display a chosen subset of integration examples in selected situations
(system visibility/context sensibility/integration quality level…)
(Fig. 16/5)
1
2
4
5
6
3
Fig. 16. Main screen of the LESO-QSV GRID program : 1 – Acceptability grid of the specific city: i.e. required integration quality for each criticity level f (system
visibility; context sensitivity). These are the criteria to be met for the installation to be accepted; 2 – Acceptability grid setting bar (for Municipality use only):
integration requirements can be selected by using pre-established grids (more or less severe), or built to measure; 3 – Integration examples showcase: a database of
more than 150 cases is shown according to a selected filter setting (5). This showcase is meant to: help Municipalities set an appropriate acceptability grid by showing
the impact in acceptancy of pre-defined sets of quality requirements; present a model for authorities of how to objectively evaluate integration quality; inspire
architects, installers, building owners… ; 4 – Case details window: The window appears while clicking on a specific case. The detailed evaluation of quality becomes
visible, together with other more precise information and additional pictures of the case; 5 – Filter bar: The case studies can be filtered according to solar system type,
position, dimension, context sensitivity, system visibility, integration quality; 6 – Accepted/not accepted cases button filter.
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6.2. Educational use of the LESO-QSV GRID database
To better valorize the education potential of the database, more
then 150 cases can be accessed and downloaded as separate case sheets
classifying and describing in some detail each installation example and
using both the discrete and the continous evaluation scale (Fig. 17).
These cases were collected with the help of the the students of different
courses at EPFL and at IUAV (Venice) who were asked to collect existing
solar integration examples and use the sheets as learning material to classify
and analyse each case (more than 500 examples were collected in three
years of teaching in three different courses on the topic of Architecture and
Solar energy (Munari Probst and Roecker, 2009).
The proposed selection is meant to give a significant and compre-
hensive outlook of the different approches available today to integrate
active solar strategies in buildings. New, existing and historical build-
ings are equally represented with respectively 50, 62, and 44 cases
each. For each category, different integration approches, solar products,
integration qualities and criticity situations are proposed, all together
Fig. 17. Case study description and evaluation sheet example, with continuous evaluation scales and final discretization.
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informing extensively on the state of the art of the topic.
The cases can then be used as unique education material, providing
at the same time a collection of integration references and a set of
educated evaluations.
The integration examples, good and bad, inform students, architects
and building owners on present possibilities and limits in terms of
products and technologies, within various architectural contexts.
To ease its use, the complete collection can be sorted according to
various criteria of interest: building type; system size (small, medium,
large), system position (roof or façade), solar technology (PV, solar
thermal, hybrid), integration approach (basic to enhanced); etc.
6.3. Implementing the LESO-QSV method locally
As the main goal of the method is to help Municipalities manage the
impact of active solar systems locally, all the needed practical elements
to handle the acceptability have been prepared in the form of a “user
kit”. This kit includes a method implementation manual, a quality
evaluation manual, the described GRID software and the set of example
sheets. A customized “Application form” for approbation/rejection of
new installations is also produced, using the “severity grid” established
by the local authorities (Fig. 18).
Once an application form is filled by a citizen for a proposed
installation, the local QSV commission can accept or refuse it, using the
detailed quality evaluation to explain which aspect of the system needs
to be improved in case of refusal, possibly allowing to correct the
proposed system.
7. LESO-QSV “Crossmapping” tool
While the above described tools are reactive, and meant mainly for
context protection and users education, another tool derived from the
QSV criticity concept, called “LESO-QSV Crossmapping”, is conceived
as proactive and meant for energy policy planning.
Presently, the only information available to planners and authorities
to make decisions on solar policies (promotion, regulations, financial
incentives, among others) is the amount of solar energy received by the
various city surfaces, usually displayed on solar maps (GIS). These maps
vary in accuracy and detail levels but their only goal is to assess the
gross solar irradiation potential of city surfaces, without concern for
their urban specificities. As shown in Fig. 19, these integration related
specificities do have a major impact on solar application real potential
and should therefore also be made available to planners. To cover this
need, the “LESO QSV-Crossmapping” tool proposes to map the archi-
tectural criticity of city surfaces, as defined in Section 4, and to su-
perimpose this information over the available GIS solar irradiation
Fig. 18. Proposal of application form.
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map. This allows to weight the pure irradiation potential of each sur-
face with the expected architectural integration effort needed to collect
it.
Differentiated policies and educated decisions can then be based on
this more comprehensive information, keeping in mind that solar in-
tegrations are possible also in delicate situations (Fig. 4). In these cases
though, design efforts and cost investments will probably be higher. If
extra efforts cannot be afforded, it might be preferable to postpone the
operation, as poor integrations usually end up just discouraging new
users. By contrast, if well designed, such examples can be among the
strongest driving forces for the solar change, repaying by far their extra
cost.
7.1. Next steps
The criticity map mentioned above indicates for each city surface its
visibility from the public domain, and its sensitivity in relation to the
urban context. A process to automatically establish the visibility of the
surfaces in the 3D models of cities is currently being developed at the
EPFL LESO-PB Laboratory, as part of a PhD thesis (Florio, 2018). The
information related to surface visibility should not only consider the
purely physical visibility from the public domain, but should also take
into account the hierarchy of the different points of view in relation to
their importance for the perceived city identity (the view from a major
city square being usually more crucial than the one from a secondary
parking lot).
Possible crossed graphic representations of insolation and criticity
are currently under development in the Laboratory.
8. Conclusion
As more and more pressure is building up to increase the use of solar
energy as a replacement for fossil energies, there is an urgent need for
new responsible ways to implement the solar collecting elements in
urban contexts.
We have presented a new method able to concile the spread of ac-
tive solar solar systems in buildings with the protection of existing
urban environments. The method is based on the novel concept of city
surfaces criticity, as a function of visibility from the public domain and
architectural sensitivity of urban environments. Criticity is used to set
the quality expectations for solar integrations in the different situations.
The needed objective integration criteria have been defined and a
simple qualitative evaluation method proposed.
A software program has been produced to assist municipalities in
the practical application of the method. The program with its 150
commented integration cases will also serve as education tool for stu-
dents, architects and the general public.
Finally, a new element for urban solar planning is introduced by
combining criticity of city surfaces with their irradiation potential and
finally establish smart solar promotion strategies.
We strongly believe that the concepts of urban “criticity” and ar-
chitectural integration quality at the basis of the LESO-QSV method offer
practical means to implement such responsible policies. We do hope
that all together the inferred tools will contribute to the elaboration of
valuable solutions to the problematic “Solar Energy Promotion and
Urban Context Protection” equation.
The method has been used within the recent works of IEA SHC Task
51 “Solar Energy in Urban Planning” (IEA SHCP Task 51 2018;Munari
Fig. 19. LESO QSV Crossmapping tool (roof irradiation data from http://www.uvek-gis.admin.ch/BFE/Sonnendach?lang=fr; Sensitivity plan from ISOS: http://
data.geo.admin.ch/ch.bakbundesinventar-schuetzenswerte-ortsbilder/PDF/ISOS_4397.pdf.
M.C. Munari Probst and C. Roecker Solar Energy 184 (2019) 672–687
683

Probst and Roecker, 2016) as a basis to assess the quality and accept-
ability of the different solar integration approaches proposed by the set
of case studies collected within Subtask C, and as core resources in three
courses currently taught at EPFL (Ecole Polytechnique Fédérale de
Lausanne, Switzerland) and University IUAV in Venice (Italy).
In November 2016 the method was rewarded by the Innovator of
the Year Prize Årets Framtidsbyggare in Sweden (Prize Årets
Framtidsbyggare, 2016), and recently the city of Malmö in Sweden and
the village of Valangin in Switzerland have shown a strong interest to
implement it as a pilot project.
A professional supporting structure to help municipalities and
heritage preservation commissions implement the method is currently
under finalisation with the support of the innovation transfer programs
Enable and Innoseed of EPFL.
Acknowledgments
This work was financially supported by :
– The Swiss Federal Office for Energy(OFEN)
– The Swiss Federal Institute of Technology of Lausanne(EPFL).
– The Innoseed Incentive Program at ENAC/EPFL.
– The Enable Technology Transfer Program at EPFL.
The authors wish to thank herewith the following persons :
– Laurent Deschamps, Paul Becquelin, Sebastien Hausamann and
Julien Grandin for their support in developing the software LESO-QSV
GRID.
– Barbara Smith for revising and proofreading the english.
– Ariane Delahaye for her support in developing the graphic layout
of the software and the example sheets.
– Professor Maria Wall for supporting the method and promoting its
diffusion.
– André Catana from the Technology Transfer Office at EPFL for his
continuous support.
– All the Innoseed commission members at EPFL.
Appendix A. Assessing visibility levels in standard urban situations
As presented in the main text, the visibility has two components: visibility from close range (close visibility) and from far away (remote visibility)
(Section 4.1 Figs. 5 and 6).
While the second will be influenced by the topography of the city and its surroundings, for the first one, the determining elements are the
geometry of the buildings and of the public space. It is therefore possible to pre-estimate the visibility levels for the most paradigmatic city
configurations.
A first simplified estimation of the impact of city geometry has been done by calculating the variablity of the surface vision angle (β) according to
viewer distance, building height and roof tilting (Fig. A1).
The table below (Fig. A2) shows the values both of β (degrees) and of the actual visibilty reduction (% of viewed surface) for different relevant
city situations:
– Various roof types : flat roof - roof tilting at 25°- roof tilting at 40°- roof tilting at 60°
– Various building heights; from 1 to 6 floors (most buildings higher than 6 floors have flat roofs!)
– Various distances of view: across a street of 5 m. (medieval city street); 10 m. (medium city center carriage street); 20 m. (wide city center
carriage street/suburb street with buildings located far from plot boundaries); 30 m. (small city square); 100 m. (large city square).
A series of visual simulations (Figs. A3 and A4) have allowed to establish that below 10% (β < 6°) the surface can be considered not critical at all
(low visibility). Starting at around 40% (β = 24°) and up, the surface starts to become very visible (high visibility). Between these two limits, the
visibility can be considered medium (the system details are not clear but the presence of the system is well perceived).
Analysis
Plotting on the same graph (Fig. A5), the visibility of roofs as a function of building heights, tilt angles and observer distances allows to extract
very significant findings:
– in narrow to medium wide city streets (up to 10 m.) building roofs are invisible, unless the building is extremely low and the roof extremely tilted. In
Fig. A1. Vision angle (β).
M.C. Munari Probst and C. Roecker Solar Energy 184 (2019) 672–687
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these situations the only concern may be the remote visibility from the surroundings, mainly depending on the city topography.
– In medium to large city streets or in suburb areas where buildings are located far from the plot boundary (around 20 m. width), the visibility is still
generally quite moderate, but steep roof tilting (40° or more) becomes a major concern; depending on the building height it will in fact induce a
medium to high visibility.
– Finally , when the building is in front of a large space such as a city square (50 m. or more), the crucial factor for visibility becomes the roof tilting,
while the building height starts to have a much reduced impact. Medium tiltings (around 20°) always result in medium visibility, while high
tiltings (around 40°) always lead to a high visibility.
A more comprehensive calculation process, aiming at automating the calculation of visibility of city surfaces based on available city 3D re-
presentations (eg LIDAR) has been conducted at EPFL-LESO in the form of a PhD thesis (Florio, 2018). This work takes into consideration city
topography, visual obstructions, as well as all the different angles of vision) (Munari Probst and Roecker, 2015; Florio et al., 2016, 2017, 2018).
5 6 7 8
Fig. A4. Simulations of the visual impact of distance on a 3 floors building with a 25° tilted roof (upper numbers refer to situations in Fig. A5).
1 2 3 4
Fig. A3. Simulations of the visual impact of roof tilting on a 2 floor building seen from a 10 m distance (upper numbers refer to situations in Fig. A5).
Fig. A2. Values of β and of the actual visibilty reduction (% of viewed surface) for different vision distances, building height, roof tilting.
M.C. Munari Probst and C. Roecker Solar Energy 184 (2019) 672–687
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Academic courses
MC Munari Probst, Master course “Energie Solaire et Architecture”, Ecole Polytechnique
Fédérale de Lausanne (EPFL) since 2015.
MC Munari Probst, 2015b. Master course “SOLAR ENERGY & ARCHITECTURE”.
Università IUAV di Venezia, Italy, since.
Munari Probst, M.C., Roecker, C., 2009. Bachelor course“Intégration architecturale de
l’Energie solaire”. Ecole Polytechnique Fédérale de Lausanne (EPFL) since.
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