DESIGNING WITH THE SUN
The first step in creating comfort and thermal delight in buildings is to understand
the relationship between the climate and our need for shelter. There is
an enormous variation in climates that buildings experience. These can be at
the scale of global climates, from the Arctic to the Sahara. They can be
regional climates in the centre of a continent or on the seashore. They can be
local climates on the sunny or the shady side of a hill or street. All will influence
the way in which a building should be designed in relation to the sun.
The sun can be a friend or an enemy in buildings. Poor climatic design of
buildings, all too often seen in ‘modern’ architecture, causes many buildings to
overheat, even in temperate or cold climates where such problems traditionally
never existed. The power of the sun should be understood and respected
by good designers of well-designed, passive solar buildings in which the free
energy of the sun is used to power the building but not allowed to interfere
with the comfort and economy of the building’s occupants.
The five things a designer needs to know for a good passive solar design are:
1 how strong the sun at the site is at different times of the year
2 where the sun will be at different times of the year in relation to the site
3 how much of the sun’s heat a building will need, or not need, at different
times of the year to enable the building occupants to be comfortable
4 how much storage capacity the building should have in relation to the
available solar gain at the site to meet those needs
5 what the additional requirements are for controlling the heat gain from
direct solar radiation, convection or conduction in a design and how they
can be met by envelope performance, building form and ventilation.
There are a number of factors that influence the incidence, or strength, of
solar radiation at the site including:
• the latitude of the site
• the altitude and azimuth of the site
• how much shade will be given by any obstacles that exist between the
building and the site
• the weather above the site.
AZIMUTH AND ALTITUDE OF THE SUN AT A SITE
The angle with which the sun strikes at a location is represented by the
terms altitude and azimuth. Altitude is the vertical angle in the sky (sometimes
referred to as height); azimuth is the horizontal direction from which it
comes (also referred to as bearing). Altitude angles range from 0° (horizontal)
to 90° (vertical: directly overhead). Azimuth is generally measured clockwise
from north so that due east is 90°, south 180° and west 270° (or 90°).
Because the Earth revolves around the sun once a year, we have four seasons.
The Earth’s axis remains in a constant alignment in its rotation so
twice a year the incoming solar radiation is perpendicular to the latitude of
the equator and only once a year is it perpendicular to the tropics of Cancer
and Capricorn,
The changing values of azimuth and altitude angles are predominantly a
reflection of the changes in the relative positions of Earth and sun. These are
governed by:
• the rotation of the Earth around the sun
• the rotation of the Earth about its axis.
One of the simplest tools we can use for the derivation of altitude and
azimuth angles is a graph using Cartesian coordinates
incorporates two types of line. Firstly, those representing the variation in altitude
and azimuth over the period of a day (given for the 21st or 22nd day of
each month). Secondly, those joining the points on the altitude–azimuth lines
for a specific hour. Thus the solar angles for 11 a.m. on 21 March may be read
off on the horizontal and vertical axes where these two lines meet (altitude
36°, azimuth 19°). Values for other days may be read by interpolating between
these lines.
It may appear that these are the only determinants of angular position,
however we are actually concerned with the direction of the sun’s radiation
rather than the Earth–sun position. Also, radiation does not travel in an
entirely straight line but is bent slightly by the Earth’s atmosphere.
The distance between the Earth and the sun is approximately 150 million
km, varying slightly through the year with the variation of the azimuth and
altitude angles with time.
All passive solar features involve the transmission of solar radiation through
a protective glazing layer(s) on the sun side of a building, into a building space
where it is absorbed and stored by thermal mass (for example thick masonry
walls and floors or water-filled containers). The typical processes involved are:
• collection – to collect solar energy, double-glazed windows are used on
the south-facing side of the house
• storage – after the sun’s energy has been collected, some heat is immediately
used in the living spaces and some is stored for later use. The storage,
called thermal mass, is usually built into the floors and/or interior walls.
Mass is characterised by its ability to absorb heat, store it and release it
slowly as the temperature inside the house falls. Concrete, stone, brick and
water can be used as mass
• distribution – heat stored in floors and walls is slowly released by radiation,
convection and conduction. In a hybrid system, fans, vents and
blowers may be used to distribute the heat.
There are several types of passive solar system that can be used in homes. The
most common are direct gain, indirect gain and isolated gain.
SYSTEM COMPONENTS
There are three key components to all passive solar systems for heating:
• collector
• mass
• heated space.
DIRECT GAIN SYSTEMS
Direct gain systems are most commonly used in passive solar architecture.
The roof, walls and floor are insulated to a high level. Solar radiation enters
through the windows and is absorbed by the heavy material of the building.
The whole building structure gradually collects and stores solar energy during
the day. Heavy building materials provide thermal storage. The collected
solar energy is gradually released at night when there is no solar gain.
Direct gain systems commonly utilise windows or skylights to allow solar
radiation to directly enter zones to be heated. If the building is constructed
of lightweight materials, mass may need to be added to the building interior
to increase its heat storage capacity. The proportion of a building’s heating
needs that can be met by solar energy increases as the area of sun-facing
glazing increases. Additional mass must therefore be used to reduce interior
temperature swings and delay the release of solar energy into occupied
spaces. While the mass that is directly illuminated by the incident energy,
sunshine, is the most effective for energy storage, long-wave radiation
exchanges and convective air currents in the solar heated rooms allow nonilluminated
mass to also provide effective energy storage.
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