introduction
The Portuguese
Building Code “Regulamento Geral das Edificações Urbanas” (RGEU), does not
clearly adress passive solar. Revealing a special hygienical care that
prevailed in post-war Europe, the 1951 bylaw in force today without major
changes, joins “the lasting exposure to the solar radiation effects” and
hygienic ventilation (art. 58) and establishes (art. 59) a rough 45 degree
solar access rule, for the utilisation of daylight and the germicidal effect of
solar radiation. The 45 degree rule is applied to the rear court of
multi-family or collective buildings in art. 62, but is reduced to a “2/1
rule” (about 63 degree) in art. 73, respecting the relationship between windows
and walls facing them. The RGEU revision of 1960 proposed some minor improvements
concerning insolation and planning requirements and the revision of 1990 linked
RGEU as a “backbone code”, to other technical regulations, namely the thermal regulations RCCTE,
“Regulamento das Características do Comportamento Térmico dos Edifícios” [1]. But these
revisions were not implemented.
Anyhow,
concerning solar access, the Portuguese law, is not an exception in the UE, as
we can read in the “European Energy” Internet document: “At present, the
European figure of solar law, related to discipline adequately the solar access
rights is very poor. On the contrary, different indications come from the
United States.
The
experience in the U.S. is to be distinguished for the plurality of the
approaches adopted. But the tendency that emerges most often is to activate
contemporaneously several mechanisms for the protection of solar access. In
fact, frequently, to the decision of recommending (or imposing) local
governments the adoption of “solar zones”, in order to prefigure an area-wide
protection, is joined the predisposition of “lot by lot” approaches” [2].
Legislation
like the California Solar Rights Act of 1978, gives guidelines for the
establishment of solar easements that can be negotiated between property owners.
One
owner would receive assurances from the other that the sunlight which travels
over neighbour´s propriety would always be available.
If
such solar easements are properly recorded, the neighbour and subsequent
owners, will be restricted in building or planting non exempt vegetation that
could obstruct the sunlight.
Zoning
Ordinances like the Solar Access Provisions for New Development of the
Multnomah County, establish detailed guidelines and restrictions ensuring that
land is divided so that structures or non exempt vegetation can be oriented to
maximise solar access and to minimise shade on adjoining proprieties from
structures and trees. In this case very detailed rules are given, especially
for low density residential developments.
New
Pattonsburg, Missouri; Soldiers Grove, Wisconsin; Port Arthur, Texas; Boulder,
Colorado; S. José, California; Richmond, B.C.
among other cities, developed
legislation securing protected solar access, be it on a case-by-case
basis or with the broad protection scope of an ordinance.
ATTEMPTING TO INCLUDE PASSIVE SOLAR IN A NEW SOLAR ACCESS RULE
The
case-by case procedure as the ordinance one require a technical basis to define
and detail solar access requirements.
Solar
access has to be disentangled in three orders of different measures because
daylighting, the biochemical effect of solar radiation and passive (and active)
solar have different requirements concerning solar access:
1.Functional
daylighting is more tied to the sky and
exterior reflected components, so it is less influenced by orientation.
2.The
biochemical effect of solar radiation, namely the germicidal effect and the
synthesis of the “sunlight” vitamin D
which cannot be synthesised in absence of sunlight (ultraviolet B), are
associated with specific wavelengths (UV and IR) and enough dosing of solar radiation does
not necessarily occur in the cold Season.
3.Passive solar for
thermal comfort and energy efficiency has specific requirements concerning solar
access. To meet these requirements we will consider the following conditions and
possibilities:
à Passive
(as active) solar require about six hours of direct radiation in a southward
exposure for maximum efficiency.
à Passive
solar depends on local climatic conditions referred to thermal comfort
requirements and on available sunshine.
à Passive
solar can be a promote or a restrict
gain strategy. Emphasis on one of them depends on the building use (activity,
clothing and internal gains) and type (form factor).
à Finally, passive
solar can and must be coupled to
other bioclimatic strategies, according to local climatic liabilities
and assets, so it can be a less imperative strategy, especially in mild climates
that prevail in Portugal.
Solar
access and exposure
For
the purpose of this paper the solar access angle is the atg defined by the high and distance of a shading frontal building or tree, referred to the base line
of the lowest solar aperture in a wall. The solar aperture of direct or
indirect gain systems, has a southward exposure and complementary thermal
masses.
Considering only solar altitudes greater than 10
degree, for Lisbon (latitude 38.7 degree N), the solar aperture exposed to the
South, more or less 15 degree, should have an unimpeded solar access between
8:30 AM and 3:30 PM true local solar time in the winter solstice, to maximise
the utilisation of solar energy. Considering the winter solstice, this
unimpeded solar access requires a maximum vertical shadow angle of 15.0 degree,
for Lisbon; between 17.5 and 12.5
degree as we shift between 37.0 and 42.0 degree N, the extreme latitudes of
Portugal excluding the Islands of Madeira and Porto Santo [ see Figure 1].
Figure 1. Shadow angle at the
winter solstice as a function of true
local solar time and latitude
If
the exposure of the solar aperture is extended to the SE and SW (i.e., South, plus or minus 45.0 degree), as allowed by the
Portuguese building thermal regulations, "RCCTE", the vertical shadow angle attains an extremly low value; 10.8
degree at 8:30 AM (or 3:30 PM) for the latitude of Lisbon.
This
could be the solar access angle for passive solar, but such low value hinders a
suitable density of urban precincts by
social and economical criteria. Such low angles strongly restrict the urban
density, as clearly shown by a comparison between the floor space index (the Portuguese “índice de construção
líquido”) for the “45.0 degree” rule
and for a 15.0 degree solar access
angle, in courtyard or parallel flat block layouts [see Figure 2].
Things are a little better if we adopt this
low solar access angle only for the south facades in a courtyard layout (the
Portuguese “quarteirão”), as it will be.
Research
and projects after the Heiligenthall-Gropius model, consider the same solar access angle for any orientation, even
in a courtyard flat block layout [3].
It
would be more efficient to specify lower solar access angles for passive solar to
south facades and higher solar access angles to other orientations, based on criteria for
daylighting and the health effects of solar radiation.
Figure 2. Floor space index as a function of the floor number and solar access angle for residential areas
Anyway,
taking into account the desirable urban density such low solar access angles can only be
achieved and even exceeded, in low rise group housing, with diagonal housing following a natural slope.
A free solar access, a floor space index greater than 1.5, more than 500 persons per hectare, privacy
staying outdoors in secluded terraces and large views, can be attained with a
landscaped diagonal housing, following slopes up to 40.0 degree. One of
the rare (and fine) modern examples
of such diagonal
housing in Portugal, is the
first phase of Hotel do Mar, Sesimbra, designed by the Architect Conceição
Silva for a 35.0 degree southward slope.
It
is worthy to note that, in Portugal, for instance in the case of Lisbon, it happens
that higher southward slopes near 40.0 degree are warmer in winter and colder
in summer than lower ones. But the current interpretation of the Decreto-Lei nº
93/90, 19th March, prevents the residential use of slopes greater then 30%,
i.e., about 17.0 degree [4].
A
healthy social life in a living town
can be also strongly
favoured by comfortable urban exterior spaces, but solar access of buildings
and urban space may conflict with each other. For instance, in dense built up
areas commanded by the street pattern, the E-W axis can be better for solar gains of buildings, but
streets along the N-S axis have a better equilibrium between both sides,
concerning solar access in winter and summer. So, solar access and shading in cold and hot seasons of urban exterior
social space, would be properly considered using especial design tools such as those
proposed by Malato [5] and others, available for computer aided design.
Solar
access, local climatic conditions and building use
Generally, as
concerns thermal comfort, Portugal has
a mixed but mild, well-tempered climate. According to the “RCCTE”, the heating
degree-days (15°C)
have a typical value of 1600 DD for the
coldest zone (I3), exceeded only at altitudes over 1000 m.
The
hot and dry season has average temperatures in the comfort zone not exceeding
25°C, with
somewhat large swings. There is available sunshine in winter and clear skies
in summer. Shading is a must in summer and intermediate seasons. In winter,
solar gains do not face the high thermal loads due to low exterior
temperatures. In most cases, with
simple means, we can have the balance
point near the average exterior temperature.
Service
and commercial buildings, usually with somewhat high internal loads and even
housing blocks with a low form factor,
are less skin commanded, shifting the
priorities to the summer bioclimatic strategies. In medium and high
density urban areas, with a desirable mixed use, floors near the ground can be
commercial.
So,
for SE to SW exposures, we could adopt a solar access angle not so stringent
as what was defined above, in more skin
commanded low density residential areas. We
could accept a more loose application of a less stringent rule to more
dense urban areas. This less stringent rule,
could be very simple, following the solar altitude at noon of the winter solstice.
Starting from the general equation for the solar
altitude at noon hn:
|
hn =
90.0 - f + d |
(1) |
we can have the solar
altitude at noon in the winter solstice hnw
, taking it as the solar access angle:
|
hnw = 66.5
- f |
(2) |
where
f = latitude and d = declination.
With this rule, the average loss of exposed area between 9:00 AM and 15:00 PM
at the winter solstice is, in the worst case of latitude 42°N, less than
0.13 for a wall oriented due South.
Solar
access amongst the bioclimatic strategies
Solar access is
indispensable to passive (and active) solar, but passive solar is only one of
the bioclimatic strategies that we can use, according to local climatic
conditions, in order to help ourselves to attain a desirable thermal comfort
with simple means [6,7]...And, of course thermal comfort is a need among
others.
We
can balance lower solar gains with more insulation, a lower form factor, weather-stripping and wind protection, in winter. In summer we can
use thermal mass, earth cooling, ventilation, radiant and evaporative cooling,
according to local climatic conditions. But with strong hot seasons and clear
skies the sunshading is a first choice. It can be done with shading devices on
the south facade, but is better done with near buildings and trees for the West
and East walls.
A
comprehensive use of the bioclimatic strategies favours diversity and gives
more freedom in the urban layout, including a more free, but conscious,
adoption of a simple rule for the solar access angle, especially in urban
central areas with mixed uses.
CONCLUSION
Passive (and
active) solar requires the revision of the Portuguese building regulation
“RGEU”, to respect solar access. The following preliminary conclusions are a
contribution to this revision:
·
Solar access for passive (and active) solar, for
daylighting and concerning the health effects of sun exposure, should be
disentangled, because they require different measures;
·
Solar access for passive (and active) solar must
be applied only to the southward exposure,
more or less 15.0 degree (or extended to more or less 45.0 degree). Requirements for other
orientations should be established on the basis of daylighting and the health effects of sun
exposure, requiring other studies to define specific rules;
·
Solar access for the southward exposure, in
compliance with the eqn [2], could follow
the rule:
ACE = 66.5 -
f
where
ACE = solar access angle and f =
latitude;
·
Solar access for passive solar should be
mandatory in low density residential areas, and less mandatory, through a
balanced use of the bioclimatic strategies and functional “stratification”, in
medium and high density urban areas with mixed uses. But in any case, solar access angle for daylighting or other
health requirements should be exceeded.
This balanced use requires the detailed layout of the “Detail Plan”, as
it is defined in Portuguese planning regulations;
·
Solar access and shading, as wind protection and
natural ventilation of urban exterior spaces, should be properly demanded by
law, to encourage the staying outdoors
in a liveable city. The rule and design tool proposed by Malato [5] for convenient
solar access and shading of exterior spaces and others available for computer
aided design, should be considered for the phase of the detail plan (“Plano de Pormenor”);
·
Diagonal housing on stable slopes, more than
17.0 degree and up to 40.0 degree, should be allowed through a more flexible
interpretation of the Decreto-Lei 93/90;
·
Design tools for solar access and shading should
be adopted and included in the curricula of the architectural schools.
References
1. Ministério das Obras Públicas Transportes e Comunicações, Regulamento das Características de
Comportamento Térmico dos Edifícios, Decreto-Lei nº 40/90, 6 de Fevereiro,
Diário da
República -I série nº 31,. 490-504.
2. Committee
on Energy, Research and Technology of
the European Parliament, Relation of
Congressman V. Bettini
(Italy) on the use of
bioclimatic technologies for
residential buildings and
services approved the 9th February
1994. Internet URL: http://nmrc.ucc.ie/rems/ps/psleg.htm.
3. L. Martin, L.March, M. Echenique, La Estructura del Espacio Urbano. Editorial Gustavo Gili, S.A.
(Barcelona, 1975).
4. A. Pereira, J. Bento, M. Gomes and N. Cerejeira, Metodologias de
Avaliação da Reserva Ecológica Nacional (MAREN), Ambiente, Ordenamento, Gestão do Território e Sistemas de Informação
Geográfica, Faculdade de Letras da Universidade de Lisboa-PROGEO (Lisboa 1995) 33-41.
5. J. Malato, Insolação dos
Edifícios e dos Espaços Exteriores Urbanos. Comissão de Coordenação da
Região de Lisboa e Vale do Tejo (not published).
6. D.
Watson and K. Labs, Climatic Design. McGraw-Hill
Book Company (New York 1983).
7. F. Simões, Desenhar com o Clima - Clima Conforto Habitação. Energia
Solar e Qualidade de Vida. ISES,
SPES - RN (Porto 1997)
303-308.