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Solar Energy for
Buildings
Introduction: Solar Design Issues
By Keith Robertson and Andreas Athienitis
Abstract:
Solar Energy for Buildings presents basic information on solar
building design, which includes passive solar heating, ventilation
air heating, solar domestic water heating and shading. The article
suggests ways to incorporate solar design into multi-unit residential
buildings, and provides calculations and examples to show how
early design decisions can increase the useable solar energy.
This Introduction to Solar Design Issues, presents basic notions of
solar design and describes different passive, active and hybrid
systems and the solar aspects of design elements, which include
window design, cooling and control, and water heating.
THE PRINCIPLES OF
SOLAR DESIGN
Benefits of solar energy
For both new and retrofit projects,
solar energy can substantially
enhance building design.
Solar energy offers these advantages
over conventional energy:
I
Free after recovering upfront
capital costs. Payback time can
be relatively short.
I
Available everywhere and
inexhaustible.
I
Clean, reducing demand for fossil
fuels and hydroelectricity, and
their environmental drawbacks.
I
Can be building-integrated,
which can reduce energy
distribution needs.
Upon reading this article, the reader will understand:
1. The benefits of solar energy in building design.
2. The difference between passive, active and hybrid solar technologies.
3. The design opportunities available for multi-unit residential
buildings (MURB).
Solar Energ y for Buildings
Energy from the sun reaches earth as direct,
reflected and diffuse radiation.
Direct radiation is highest on a surface
perpendicular to the sun’s rays (angle of
incidence equal to 0 degrees) and provides
the most usable heat.
Diffuse radiation is energy from the sun
that is scattered within the atmosphere by
clouds, dust or pollution and becomes
non-directional. On a cloudy day, 100 per
cent of the energy may be diffuse radiation;
on a sunny day, less than 20 per cent may
be diffuse.
The amount of the sun’s energy reaching
the surface of the earth also depends on
cloud cover, air pollution, location and the
time of year. Figure 1 shows the solar
energy available in five Canadian cities at
different times of the year.
The amount of solar energy reaching a
tilted collector significantly changes the
result. Figure 2 shows the amount of solar
energy received by a horizontal collector,
such as window, for a passive solar design.
Note that even Yellowknife receives a
significant amount during part of the
heating season.
Passive, active and hybrid solar
Solar buildings work on three principles:
collection, storage and distribution of the
sun’s energy.
A passive solar building makes the greatest
use possible of solar gains to reduce energy
use for heating and, possibly, cooling. By
using natural energy flows through air and
materials—radiation, conduction,
absorptance and natural convection.
A passive building emphasizes passive
energy flows in heating and cooling. It can
optimize solar heat gain in direct heat gain
systems, in which windows are the
collectors and interior materials are the
heat storage media.
The principle can also be applied to water
or air solar heaters that use natural
convection to thermosiphon for heat
storage without pumps or fans.
An active solar system uses mechanical
equipment to collect, store and distribute
the sun's heat. Active systems consist of
solar collectors, a storage medium and a
distribution system. Active solar systems
are commonly used for:
I
Water heating;
I
Space conditioning;
I
Producing electricity;
I
Process heat; and
I
Solar mechanical energy.
Hybrid power systems combine two or more
energy systems or fuels that, when integrated,
overcome limitations of the other, such as
photovoltaic panels to supplement gridsupplied or diesel-generated electricity.
Hybrid systems are the most common, except
for the direct gain system, which is passive.
S o la r E n e r g y o n a V e r t ic a l P l a n e
7 .0 0
6 .0 0
kWh/m2/day
The amount of energy that reaches earth’s
upper atmosphere is about 1,350 W/m2—
the solar constant. The atmosphere reflects,
scatters and absorbs some of the energy. In
Canada, peak solar intensity varies from
about 900 W/m2 to 1,050 W/m2,
depending on sky conditions. Peak solar
intensity is at solar noon, when the sun is
due south.
Ha lif a x
5 .0 0
To r o n to
4 .0 0
Ed m o n t o n
3 .0 0
Yell ow kn i f e
2 .0 0
V anc ouv er
1 .0 0
0 .0 0
Jan
Fe b
Ma r
A pr
Ma y
Ju n
Ju l
A ug
Sep
Oc t
No v
Dec
Source: RETScreen1
Figure 1 – kWh/m /day on a ver tical surface, for selected Canadian cities
1
2
RETScreen is free energy assessment software that assesses renewable energy options against a base building model. Software modules are available at
http://www.retscreen.net/ang/menu.php
2
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Glossary
S o l a r En e r g y o n a H o r iz o n t a l S u r f a c e
Absorptance—The ratio of absorbed to
incident radiation.
Energy rating (ER)—A rating system that
compares window products for their
heating season efficiency under average
winter conditions.
kWh/m 2/day
Active solar—A solar heating or cooling
system that operates by mechanical means
such as motors, pumps or valves to sort
and distribute the sun's heat to a buidling.
7 .0 0
6 .0 0
Halifax
5 .0 0
Montréal
Toronto
4 .0 0
Winnipeg
3 .0 0
Edmonton
2 .0 0
Yellowknife
1 .0 0
Vancouver
0 .0 0
Jan
Feb
Mar
A pr
May
Ju n
Ju l
A ug
Sep
Oct
No v
De c
Evacuated tube collectors—Solar
collectors that use individual, sealed
vacuum tubes surrounding a metal
absorber plate.
Figure 2 – kWh/m2/day on a south-facing horizontal surface, for five Canadian cities
Source: RETScreen
Flat-plate collectors—The most common
type of solar collector. Can be glazed or
unglazed.
R-value (imperial), RSI-value (metric)—
A measure of resistance to heat flow
through a material or assembly—a
numerical inverse of the U-value.
Hybrid power systems—Combines active
and passive solar power systems or involves
more than one fuel type for the same device.
Latent Heat—Also called heat of
transformation. Heat energy absorbed or
released by a material that is changing state,
such as ice to water or water to steam, at
constant temperature and pressure.
Solar balcony—An enclosed balcony that
acts as a solar collector.
Solar constant—1,350 W/m —The
average amount of solar energy reaching
the earth’s upper atmosphere.
2
Low-emissivity (low-e)—Coatings applied
to window glass to reduce inside heat loss
without reducing outside solar gain.
Solar Domestic Hot Water (SDHW)—A
supplement to traditional domestic hot
water heating. The most common system
uses glazed, flat-plate collectors in a closed
glycol loop.
Passive solar—A solar heating or cooling
system that operates by using gravity, heat
flow or evaporation to collect and transfer
solar energy.
Solar Heat Gain Coefficient (SHGC)—
Equal to the amount of solar gain through
a window, divided by the total amount of
solar energy incident to its outside surface.
Photovoltaic (PV) system—System that
onverts sunlight into electricity. Can be
autonomous or used with another energy
source. (Can be connected to the main
power grid, for example).
Solar south—180 degrees from true or
grid (not magnetic) north.
Solarwall®—A proprietary system that
uses perforated metal panels to pre-heat
ventilation air.
Switchable glazing—Glazing materials
that can vary their optical or solar
properties according to light (photochromic),
heat (thermochromic) or electric current
(electrochromic).
Thermosiphon solar collector —A system
in which the circulation of hot water in the
loop is based only on buoyancy.
U-value—A measure of heat flow through
a material or assembly. Measured in
Watts/m2/°C.
Warm-edge spacers—Separate a window's
glazing layers with thermal break or a lowconductivity material.
Canada Mortgage and Housing Corporation
3
Solar Energ y for Buildings
Building design issues
Careful solar design can:
I
Maximize possible solar transmission and
absorption in winter to minimize or reduce
to zero the heating energy consumption,
while preventing overheating.
I
Use received solar gains for
instantaneous heating load and store
the remainder in embodied thermal
mass or specially built storage devices.
I
Reduce heat losses using insulation and
windows with high solar heat gain factors.
I
Employ shading control devices or
strategically planted deciduous trees to
exclude summer solar gains that create
additional cooling load.
I
Employ natural ventilation to transfer
heat from hot zones to cool zones in
winter and for natural cooling in the
summer; use ground-source cooling
and heating to transfer heat to and
from the underground, which is more
or less at a constant temperature, and
utilize evaporative cooling.
Integrate building envelope devices
such as windows, which include
photovoltaic panels as shading devices,
or roofs with photovoltaic shingles;
their dual role in producing electricity
and excluding thermal gain increases
their cost-effectiveness.
I
Use solar radiation for daylighting,2
which requires effective distribution
into rooms or onto work planes, while
avoiding glare.
I
Integrate passive solar systems with
active heating–cooling/air-conditioning
systems in both design and operation.
I
What is design
integration?
not use the building enclosure as part of
an integrated energy system in which
the components fit together well.
Collaboration between architects and
engineers is increasing, but the traditional
working relationships between architects,
engineers, property managers and other
professionals do not foster an integrated
design approach.3
The most important factor for a successful
solar building is “integration.” This
concept includes not only the integration
of design professionals at the project’s start,
but also the integration of those who are
responsible for the systems operation. This
potential for synergy is usually overlooked
because architects and engineers
traditionally do not explore the concepts
together closely enough to truly integrate
systems, and they infrequently discuss new
concepts with property managers, except
when auditing a building failure.
A preferable approach is to consider the
building and its HVAC system as one
energy system and to design them together,
taking into account possible synergies such
as electricity generation, thermal storage
and control strategies.
Direct
Gain
Solar Facade
Collector-storage
Wall
Figure 3 – Two major options for thermal mass placement in passive solar
design: direct gain and Trombe wall, or collector-storage wall
The architect may design the building
envelope to passive solar design principles
while the engineer designs HVAC to
extreme design conditions, ignoring the
benefits of solar gains and natural cooling.
The result is an oversized system that does
2
See Daylighting Guide for Buildings at: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_001.cfm
3
See Integrated Design Process Guide at: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_002.cfm
4
Canada Mortgage and Housing Corporation
Passive solar heating systems (thermal) are
separated into two broad categories, direct
gain and indirect gain (see Figure 3). An
indirect passive system insulated from the
heated space is an isolated system.
Solar Energ y for Buildings
Depending on climate and building
function, certain heating/cooling systems
are more compatible with passive systems.
For example, the thermal mass in a floor
may store passive solar gains and act as a
floor-heating system. This is a control
challenge that must be carefully planned if
it is to achieve acceptable thermal comfort
for the occupants.
The key aspects of passive solar design are
interlinked, dependent design parameters:
I
Location and orientation of a building;
I
Fenestration area, orientation and type;
I
Thermal massing and envelope
caracteristics;
I
Amount of insulation;
I
Shading devices—type, location
and area;
I
Effective thermal storage insulated from
the exterior environment, as well as
amount and type;
I
sensible—such as concrete in the
building envelope with exterior
insulation, or
I
latent such as phase-change
materials.
The ultimate objective of design integration
is to minimize energy costs while
maintaining interior comfort. A larger
thermal mass within a building can delay
its response to heat sources such as solar
gains—the thermal lag effect. This thermal
lag can avoid comfort problems if taken
into account in selecting the thermal mass,
choosing appropriate control strategies
and sizing the heating–cooling system.
Source: CMHC, at http://www.cmhc-schl.gc.ca/en/imquaf/himu/buin_018.cfm
Figure 4 – Sixteen of the 42 units in this apar tment building in Amstelveen, the Netherlands, take advantage of solar
energy from the atrium as an air pre-heating system. Solar domestic hot water panels provide about half the building’s
domestic hot water energy.
Canada Mortgage and Housing Corporation
5
Solar Energ y for Buildings
Design procedure
Building orientation
The initial design steps in solar design are to:
Orientation is crucial since it can provide
free savings from the concept stage. There
is a difference between true north and
magnetic north. The deviation between
magnetic north and true north—magnetic
declination—varies between east and west
coasts. In Nova Scotia, the compass
points west of true north; in B.C.,
east of true north.
1. Set performance targets for energy
sources and uses.
2. Minimize heating and cooling loads
through orientation, massing, envelope
and landscape design.
3. Maximize solar and other renewable
energy to meet the building load, then
to design efficient HVAC systems
that are integrated with the building
envelope performance characteristics.
4. Use simple energy simulation tools and
detailed simulations in evaluating
options at the early design stages and
later to assess alternatives.
Generally, deviations up to ±30º
from due south reduce solar gains
by up to about 12% and are thus
acceptable in solar building design,
providing significant freedom in
choice of form.
6
The maximum difference (as a percentage)
between south-facing and 30ºE (or W)
orientations occurs when the sun is lowest
and the days shortest (Dec. 21). When solar
facades or roofs generate photovoltaic
electricity that is sold to the grid at timeof-day rates, these rates may change the
optimal orientation if their peak value is
not at noon.
Further information about magnetic
deviation and a calculation routine is
available at
http://www.geolab.nrcan.gc.ca/geomag/ma
gdec_e.shtml
Canada Mortgage and Housing Corporation
Generally, buildings with long axes running
east and west have greater solar-heating
potential if their window characteristics are
chosen accodingly. For MURBs with a
typical double-loaded corridor, this means
half the units face south and half face
north. A partial solution could be a
south-facing central atrium or solar heater
that pre-heats and delivers air for the
north-facing units.
Buildings with east-and west-facing
orientations have greater potential for
overheating in the non-heating season and
get little solar gain in winter. In figure 5 the
Foyer hongrois in Montréal angles the
windows to the south creating a sawtooth
plan, to avoid east-and west-facing
windows.
Solar Energ y for Buildings
Although differences in assumptions and
input data make comparisons difficult, a
study of a Toronto building produced
different results. RETScreen’s passive solar
energy module was used for the Toronto
building. The RetScreen model of a 110 m2
(1,184 sq. ft.), south-facing suite in
Toronto with 7.2 m2 (75 sq. ft.) of windows
(similar to the suites in Halifax) gave the
following results. (Increases in cooling load
were not calculated, as this was assumed to
be an unconditioned building.):
Figure 5 – Foyer hongrois in Montréal. South angled windows on a building with a
long nor th-south axis. Sunshades shadow these windows in the summer time.
Building Conditions
NRC’s EE4 software4 was used to model
the energy use of a Halifax MURB, and
showed modest energy reductions from
orientation, window performance and
window size. The advantage of energy
reductions due to orientation is that they
are free, and the savings continue for the
life-time of the building. Note also that
these energy simulation results are specific
to a particular location. The MURB had
the following characteristics:
I
Four-storey, double-loaded corridor,
wood-frame.
I
Window-to-wall ratio: 19 per cent on
primary facades.
I
Double-glazed, low-e vinyl windows.
4
I
High insulation levels.
Simulation Results
I
Using a higher Solar Heat Gain
Coefficient (SHGC) glazing reduced
the total annual heating cost by three
to four per cent.
I
Orienting the building along the long
east–west axis instead of north–south
axis reduced annual heating cost by
about one per cent.
I
Increasing the window area on the
south-facing suites reduced annual
heating cost by less than one per cent.
I
Increasing the interior mass reduced
annual heating cost by about two per
cent.
I
Increasing the glass Solar Heat Gain
Coefficient (SHGC) from .45 to .65
saved 1,100 to 1,200 kWh annually.
I
Doubling window area and increasing
the SHGC gave a slight annual energy
loss in a low-mass (wood-frame)
building and a slight saving in a highmass (concrete-frame) building.
I
Increasing the glass R-value and
maintaining a high SHGC saved about
900 kWh annually.
I
The best results came from increasing
the R-value, increasing the mass,
increasing the window area, and
maintaining a high SHGC.
These results are expected from basic solar
design principles. Increasing the resistance
of windows to thermal loss (low-e glazing)
while admitting high solar gains reduces
heating energy consumption if the building
is well insulated and there is enough thermal
mass to store the solar gains and prevent
overheating. Obviously, the thermal
performance of windows cannot be
separated from solar gains, which relate to
form, orientation and solar transmittance.
Optimizing requires rigorous energy
modelling and project-specific analysis.
EE4 is the software developed for NRCan’s Commercial Building Incentive Program to check for compliance to its program requirements.
Canada Mortgage and Housing Corporation
7
Solar Energ y for Buildings
More details on the design of windows and
glazing selection are presented in Selection
and Commissioning of Window Installations5
approximately equal to the amplitude of the
cyclic heat flow into the mass divided by its
surface temperature amplitude or swing.)
The analytical tool selected depends on
the detail required. For basic energy flows,
an analysis based on solar heat gain
coefficients and thermal conductance
provides an approximate estimate of the
net energy transfer across the building
envelope. The calculations can be
performed in MathCAD, Matlab or a
spreadsheet-based program such as
RETScreen.
A good design strategy for building
orientation is to “tune” windows to admit
or exclude solar energy based on their
orientation. Generally, south-facing
windows should admit winter solar gain
and east- and west-facing windows should
exclude low-angle solar gain. Window
design strategies are discussed in more
detail later.
To determine room-temperature swings
and associated thermal mass response,
more detailed simulation tools are needed.
However, even for the calculation of
temperature swings and the effectiveness
of thermal mass, simplified models exist
which are based on thermal admittance
calculations.6 “Thermal admittance” is
essentially a dynamic U-value and is
typically calculated for a daily cycle. (It is
Another approach is control of solar gains
with motorized blinds, which are widely
used in airports, atriums and some
commercial buildings in Europe. Along
with other control technologies, such as
electrochromic coatings, motorized blinds
may soon become cost-effective. If active
solar control is taken into account in sizing
cooling systems, there may be significant
savings from reduced energy consumption
and reduced equipment sizing.
Obstructions to sunlight
Obstructions can have a significant effect
on solar potential. For low- to mid-rise
buildings, obstructions are usually
buildings, terrain or trees. For larger
buildings, obstructions are usually other
large buildings.
Obstructions can be identified on the sun
path chart in figure 8. East and west
obstructions can reduce solar gain in the
summer and admit energy in the winter,
when the sun rises in the southeast and sets
in the southwest.
5
See http://www.cmhc.ca/en/inpr/bude/himu/coedav/upload/Article_Design_Aug31.pdf
6
Athienitis A.K. and Santamouris M., 2002. Thermal analysis and design of passive solar buildings, James and James, London U.K..
8
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Direct-gain passive solar
techniques
Internal thermal mass reduces temperature
swings within a space. In a properly
designed passive solar system, thermal
mass absorbs solar energy during the day,
preventing the building from overheating,
and releases the energy at night. Thermal
mass is most effective when it can gain
energy directly from the sun. An ideal
thermal mass for passive solar heating has
high heat capacity, moderate conductance,
moderate density and high emissivity.
Additional cost is negligible if the material
is also structural or decorative. Concrete
and masonry are good thermal mass
materials. (Plaster, drywall, and tile are also
useful in this respect, but calculations are
needed to determine if they have sufficient
mass, as was done in the Halifax study.)
Passive solar design in single-family
residences shows that operational energy
can be reduced by 30 to 50 per cent
through window sizing and thermal mass
storage. A recent study of MURBs in
Sweden reported that operational energy
use in a heavy structure is only slightly
lower than in a similar, lightweight
structure.7 The additional energy used to
build the heavy structure outweighed its
operational advantage in a lifecycle analysis
of costs.
7
Outdoor temperature
Air temperature
Pure passive solar design uses the sun’s
energy directly, without mechanical
intervention. In its simplest form, the sun
shining through a window directly heats
the space. Thermal mass within the
building can absorb some of the heat
and release it at night.
Light timber-framed building
Heavy building with
external insulation
Heavy building set into and
partially covered with earth
Time of day
Figure 6 – Effect of internal mass on internal temperature swings
It can be calculated as a percentage of the
total area of the south-facing exterior
wall—of limited use because it is not
affected by what goes on beyond the
wall—or as a percentage of heated floor
area—which accounts for the volume of
the building.
Mass is known to be able to reduce peak
cooling load when night temperatures are
cooler than day temperatures. Exterior and
interior masses cool down at night and
reduce peak cooling demand while also
delaying the time of the peak solar gain
during the day. However, the effectiveness
of thermal mass is proportional to the
allowable room temperature variation over
a day.
A typical passive solar-heated building may
have south-facing glazing equal to 10 to 15
per cent of the heated floor area. As the
area of south glass increases, the amount
of mass inside must also increase. The
Advanced Buildings Technologies and
Practices website, at
http://www.advancedbuildings.org,
recommends a window-to-exterior wall
area ratio (WWR) of 25 to 35 per cent,
similar to a typical MURB.
Windows
Window orientation, layout and
performance are important in passive solar
design. The goal is to provide an
appropriate amount of window area in the
right place. Where there is no fenestration,
a conventional insulated wall is a solar
barrier, transmitting little energy to the
inside.
Window sizing
There are two ways to quantify a building's
south-facing glass.
WWR may increase with proper control of
solar gains (for example, with motorized
shading) and transfer of excess energy to
north-facing zones. This could possibly
approach 50 per cent when a large atrium
is included with adequate thermal storage
Stahl, Fredrik, The effect of thermal mass on the energy during the life cycle of a building, presented at Building Physics 2—6th Nordic Symposium
Canada Mortgage and Housing Corporation
9
Solar Energ y for Buildings
capacity. Utilization of double facades with
blinds in the cavity, or exterior controlled
shading reduces cooling loads during summer.
(Figure 4 – Urban Villa, Amstelveen)8
Glazing
This section describes some the most
important parameters of window and
glazing design.
Solar heat gain coefficient (SHGC)
The Solar Heat Gain Coefficient (SHGC) is
a useful measure of a window's ability to
admit solar energy. SHGC is the amount
of solar gain a window allows, divided by
the amount of solar energy available at its
outside surface, a number between zero
(solid wall) and one (open window).
SHGC can be measured for the window
unit, including the frame, or the glazed
area. The higher the SHGC, the better the
window will perform as a solar collector. If
overheating is a concern, low-SHGC
windows exclude solar energy to reduce
cooling loads.
A single pane of clear glass facing the sun
will admit most of the visible solar radiation,
some of the infrared and very little ultraviolet
and have the highest heat loss from inside
to outside. Ways to modify windows to
enhance their performance include:
I
Adding a second or third layer of glass,
which can dramatically lower the
U-value (increase the R-value), while
maintaining a large SHGC. Additional
layers of glass also permit thin, lowemissivity (low-e) coatings to be
applied onto a protected glass surface.
Low-e coatings still allow solar gain
(short wavelength radiation) and they
Figure 7 – Double-glazed, low-e window
help retain heat by reducing longwave
(infrared) radiation losses. This is very
helpful from a passive solar heating
point of view.
I
There are reflective coatings that block
unwanted solar gain (reduce the SHGC)
to reduce the cooling load. There are many
types of spectrally selective glazings that
block out selective wavelengths that can
change the SHGC and levels of visible
light transmittance.
I
Evacuating the space between the
panes, using an inert gas such as argon
or krypton, or transparent insulation,
can reduce heat loss by conduction and
convection. Because gas-fills perform
well and are low cost, they should be
used whenever a low-e coating is used
in a glazing unit.
High-performance windows may make it
possible to move heating outlets further from
windows to eliminate ducting or piping.
8
A recent glazing development is switchable
glazing. These can vary their optical or solar
properties according to light (photochromic),
heat (thermochromic) or electric current
(electrochromic). Initial computer simulations
show that electro chromic glazing has the
most promise for improving comfort. These
are prototype systems. They will likely be
able to reduce cooling loads and glare and
improve visual comfort if high solar
transmittance is not needed. Switchable
glazings may have poorer optical properties
and not be suitable in residential buildings.
Visible light transmittance
Visible light transmittance (VT) measures
the visible spectrum admitted by a window.
Typical daylight strategies require windows
with a high VT. A low SGHC is also desirable
where heat gain is a concern. Reflective
glass is not recommended for daylighting.
Table 1 shows typical values for light
transmittance and SHGC of common
glazing systems.
See Innovative Buildings Case Studies — “Atrium, Solar shading and ventilation for residents’ confort”, Amstelveen :
http://www.cmhc.ca/en/inpr/bude/himu/inbu_001.cfm#CP_JUMP_58686
10
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Table 1 – Visible Light Transmission–solar heat gain coefficient (per cent)
Glazing system (6 mm glass)
Clear
Blue-green
Grey
Reflective
Single
89–81
75–62
43–56
20–29
Double
78–70
67–50
40–44
18–21
Double, hard low-e, argon
73–65
62–45
37–39
17–20
Double, soft low-e, argon
70–37
59–29
35–24
16–15
Triple, hard low-e, argon
64–56
55–38
32–36
15–17
Triple, soft low-e, argon
55–31
52–29
30–26
14–13
Source: ASHRAE Fundamentals 1997,Table 11, page 29
Frames
Frames are often the weakest thermal part
of a window. Although frames (sash and
mullion assemblies) are only 10 to 25
per cent of window area in commercial
buildings, they can account for up to half
the window heat loss and be the prime site
for condensation.9
Thermal performance of frames is improved
either by using a low-conductivity thermal
break in metal frames or a frame of a lowconductivity material, such as wood, vinyl
or fibreglass. Low-conductivity window
frames reduce energy consumption in all
types of buildings. For MURBs the
designer should note that Canadian fire
codes state that the area of windows with
combustible framing materials must be less
than 40 per cent of the building wall area
and that non-combustible materials must
separate windows.10
Spacers
Spacers separate panes of glass in a sealed
window to prevent the transfer of air and
moisture in and out of the glass cavity.
9
Warm-edge spacers use low-conductivity
materials, rather than aluminum, and are
important in reducing heat loss through
the window. By reducing the likelihood of
condensation on the glass surfaces, they
can also influence daylighting performance.
The low cost and good performance of
warm-edge spacers make them suitable for
all window systems and should be
considered mandatory whenever low-e
coatings and inert gas fills are used.11
Window orientation
The greatest amount of solar energy is
generated at noon on any given day in the
year. The greatest amount of energy
received through a window is when the sun
is perpendicular to the window and 30 to
35 degrees above the horizon. South, east
and west windows receive about the same
annual maximum of solar radiation. The
time and date that the maximum energy is
received depends on the building’s latitude
and wall orientation. The earth rotates
15 degrees an hour; when a window is
oriented 30 degrees east of south, the
maximum heat gain will be about two
hours before solar noon. East and west
facades receive maximum solar gain in the
summer; a south-facing surface receives its
annual maximum in the late fall or winter.
Figure 8 shows a sun path chart for latitude
44ºN. The sun’s path varies by a project’s
latitude. The X-axis gives the direction of
the sun; the Y-axis the sun’s angle above the
horizon. The curved lines show the arc of
the sun across the sky on the 21st day of
each month. The dashed lines show the
time of day. An accurate location of the
sun can be determined by plotting the time
of day and month.
Obstructions are also plotted to show when
a building will be shaded. Sun charts for
any latitude can be generated through a
University of Oregon online program at
http://solardat.uoregon.edu/SunChartProgr
am.html
Figure 9 shows the intensity of solar energy
striking a vertical surface facing the sun. The
maximum energy entering a window occurs
when the sun is 30 to 35 degrees above the
horizon and directly in front of the window.
Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm
10
Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm
11
Website: Advanced Buildings: Technologies and Practices http://www.advancedbuildings.org/_frames/fr_t_building_warm_edge_windows.htm
Canada Mortgage and Housing Corporation
11
Solar Energ y for Buildings
Superimposing Figure 9—Solar energy
intensity – over the sun path chart shows the
effect of window orientation on solar gain
Figure 10 aligns the solar intensity chart to
south on the sun path chart. This shows
that the maximum solar gain occurs at
noon in October and February.
To indicate the solar gain on a west
window, align the solar intensity chart with
west on the Sun path chart, as shown in
Figure 11. This clearly shows how window
orientation affects the time of day and the
time of year of maximum solar gain.
North-facing windows provide consistent
indirect light with minimal heat gains, but
can also create heat loss and comfort problems
during the heating season. South-facing
windows provide strong direct and indirect
sunlight that varies during the day. Controlling
heat gain can be a problem during the
cooling season. Shading is easily done with
horizontal shading devices in these windows.
East- and west-facing windows can create
more problems with glare and heat gain
and are more difficult to shade because the
sun is closer to the horizon. In Canada’s
North, the sun is at a low angle in the sky
during winter, when sunlight is most
needed to contribute to heating. This is
when south-facing clerestory windows have
an advantage over horizontal roof glazing.
However, the sun also creates glare.
Overhangs over south windows may need
to be large to prevent this. Also, when the
sun is low, buildings and trees can create
shade, which is desirable in some seasons.
Note that south-facing surfaces receive
more energy in the winter and less in the
summer than east- and west-facing
surfaces. A strategy to control overheating
Adapted from Edward Magria “The Passive Solar Energy Book”
Figure 8 – Sun path char t
Figure 10 – Energy striking a south window for latitude 44ºN
Figure 9 – Solar energy intensity
Figure 11 – Energy striking a west window for latitude 44ºN
12
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
is to maximize window area on the south
and use less on the east and west. For
mainly cloudy regions, where overheating
is less of a problem, interior spaces benefit
from larger windows (including the north
facade) that allow more light into a
building. There can be a trade-off between
allowing more daylight and increasing heat
loss. In mainly clear regions, glare and heat
gain are more problematic. In direct
sunlight, smaller windows can provide
adequate daylight. Direct sunlight can also
be reflected or diffused, or both, with
window shading.
Window performance and tuning
Window orientation, size, layout and
performance are important in passive solar
design. Proper glazing and frame selection
can enhance daylighting and energy
performance. General rules for tuning
window orientation include:
I
Determine the window size, height
and glazing treatments separately for
each facade.
Overhang
I
Maximize southern exposure.
Shading
I
Optimize northern exposure.
I
Minimize western exposure when the
sun is lowest and most likely to cause
glare and overheating. Windows
themselves can be oriented differently
from the plane of the wall in a
“sawtooth” arrangement.
Shading may be exterior, interior, fixed,
motorized or between an exterior glazing
and an interior facade in double-facade
systems. Figure 12 shows some examples
of shading systems. A good shading system
permits lower levels of artificial illumination,
because the eye can accommodate itself
without strain to function within a wide
illumination range.
Larger window areas increase the risk of
glare, overheating in summer and heat loss
in winter. For areas with direct sun,
shading needs to reduce transmittance to
10 per cent or less to prevent glare.
Exterior shading devices are the most
effective at controlling solar gain. Interior
window shading allows much of the solar
energy into the building and allows more
heat, sometimes an unwanted partner
of daylight, to enter the building. Lightcoloured interior shading will reflect some
of this energy back through the window.
However, a minimum of about 20–30 per
cent of incident solar radiation will come
indoors as transmitted or be absorbed and
re-emitted as heat when interior blinds
are used. Exterior blinds collect dust and
may be difficult to maintain and clean.
One solution is to place reflective blinds
between the two glazings and possibly
to have airflow within the cavity—a
double-facade.
Glare from windows can occur when the
incoming light is too bright compared
with the general brightness of the interior.
Punched windows can create strong
contrasts from the interior between
windows and walls. Horizontal strip
windows provide more even daylight
distribution and, often, better views. This
article discusses other interior design
guidelines later.
Louvred Overhang
Lightshelf
Vertical Fins
Figure 12 – Common types of exterior shading
Canada Mortgage and Housing Corporation
13
Solar Energ y for Buildings
South-facing windows are the easiest to
shade. Horizontal shading devices, which
block summer sun and admit winter sun,
are the most effective. East- and westfacing windows are best shaded with
vertical devices, but these are usually harder
to incorporate into a building and not
limit views from the window. On lower
buildings, well-placed deciduous trees on
the east and west reduce summer
overheating and allow desirable winter
solar gains. Some practioners are testing
vines hung on metal lattices to reduce
overheating. Interior shading is most
effective at controlling glare and can be
controlled to suit the occupants.
Energy Rating—ER
Energy Rating (ER) is a rating system
developed by the Canadian Standards
Association and the window industry. It
compares window products for their
heating season efficiency under average
winter conditions. The ER is the value of
energy gained or lost in watts per square
metre (W/m2). RSI value is a misleading
measure of energy efficiency because it
often only accounts for the heat loss
through the centre of the glass. The ER
considers all the energy flows through the
window, the total glass R-value, the frame
R-value, air infiltration and average solar
gain. The solar gain is an average of the
four orientations.
Figure 13 – Double facade in a residential building,
Klosterenga, Oslo, Norway
14
Canada Mortgage and Housing Corporation
Because the ER relies on an average solar
gain, it cannot be used to compare actual
performance for a specific location
orientation and window size. Further
calculations can determine the Energy
Rating Specific (ERS). This determines a
specific ER value for a window based on
the climate of a particular location, the
window-to-floor area ratio and the window
orientation on the building.
Both the ER and ERS are part of CSAA440.2 Energy Performance of Windows
and Other Fenestration Systems standard.
Figure 14 – Glazed solar pacade from the outside of
Klosterenga
Solar Energ y for Buildings
Solar cooling
Traditionally, passive solar cooling is
associated with much warmer climates than
Canada’s. In Canada, the most effective
method is to exclude solar gain through
fenestration design, window glazing
selection and shading devices. Another
common strategy is to use the mass of the
building, which cools down at night to
mitigate overheating by absorbing solar
energy during the day.
Harnessing the stack effect, that is the
upward movement of warmer, more buoyant
air, is possible if a building is designed to
capture solar heat and exhaust it at roof
level. This warm air can be released to the
outside, drawing cooler ground-level air
into and up through the building. An
atrium can act as a solar chimney with
motorized windows to harness the stack
effect and help the natural ventilation
process. Using thermal mass in an atrium
helps prolong the chimney effect well into
the night to draw cool air into the
building. In Europe, cool night air is
passed (using fans) through hollow core
floors to store coolness. During the day,
room air is recirculated through the cool
floor to provide free cooling.
Absorption cooling involves hightemperature solar collectors connected to
an absorption chiller operating at around
100°C (212°F). The device uses a solar
collector to evaporate a pressurized
refrigerant from an absorbent–refrigerant
mixture. Absorption coolers require little
electricity to pump the refrigerant
Solar balconies
compared to that of a compressor in a
conventional electric air conditioner or
refrigerator. This system is not yet efficient
enough for conventional buildings and
requires a large, upfront investment.
Glazed, stacked balconies can also work as
passive collectors. They passively re-radiate
heat or actively ventilate to the rest of the
unit or to the outside.
Desiccant cooling uses a desiccant, a
chemical drying agent, in contact with the
air to be cooled. The air becomes so dry
that moisture can be injected into it
without affecting comfort. The moisture
droplets evaporate and cool the air. The
drying agent is regenerated by hot air that
is heated through solar air collectors or a
coil connected to liquid-based collectors.12
An effective method is to inset the balcony
into the building envelope. This simplifies
the building envelope and eliminates the
need to separately support or cantilever the
balcony. It also reduces the amount of
thermal bridging across the envelope, but
may require additional shading devices if
the room is to be occupied regularly or if
temperature fluctuations are not desirable.
Of course, the balcony becomes less
effective as a solar collector as it is oriented
away from south. An enclosed balcony
partially or entirely projecting from the
exterior allows solar gains in units without
direct southern exposure.
The Rankine-cycle cooling process is a
vapour compression cycle similar to that of
a conventional air conditioner. Solar
collectors heat the working fluid, which
has a very low vaporization point, which
then drives a Rankine-cycle heat engine.
This technology is mainly experimental
and not used often because it needs a large
system size to do any meaningful amount
of cooling.13
In the CMHC study of renewable energy
at the building envelope, energy modelling
of a six-storey MURB in Halifax predicted
that solar balconies would reduce energy
consumption by about four per cent.
Overheating
Overheating tends to occur more from
unshaded west-facing windows and, to a
lesser extent, east windows. Late summer is
often the most crucial time of year. Design
strategies include minimizing the amount
of east- and west-facing glass, selecting
glazings with a low SHGC to exclude
heat and provide shading. Thermal mass
inside the building can also have the
effect of reducing the peak-cooling load
in some climates.
A Dutch study14 looked at solar balconies
in renovating post-war, multi-family
residential buildings with aged and failing
envelopes. The study showed that the new
solar elements were a cost-effective way to
upgrade while reducing energy
consumption by about 35kWh per square
metre. Optimizing thermal, glazing and
ventilation parameters and using simple
venting and solar shading enhanced
occupant comfort.
12
Natural Resources Canada website: http://www.canren.gc.ca/tech_appl/index.asp?CaID=5&PgID=164#desiccant
13
U.S. Department of Energy website: http://www.eren.doe.gov/consumerinfo/refbriefs/ac2.html
14
Advanced glazed balconies: Integration of solar energy in building renovation, W/E consultants, The Netherlands, EuroSun'96
Canada Mortgage and Housing Corporation
15
Solar Energ y for Buildings
Cour tyards, atriums and
common spaces
A south-facing atrium can collect pre-heating
air to be circulated throughout the building.
This requires airtight construction and a
high level of insulation. Overheating in the
atrium can be avoided with properly sized
and located motorized shades and a passive
ventilation system. Architects must
recognize the fire safety issues of atriums
and provide protection for their occupants.
This is addressed in a separate article on
the CMHC website Fire Safety in High-rise
Apartment Buildings.15 The difficulty in
dealing with smoke control and using an
atrium to pre-heat building air becomes
a challenge.
In high-rise and mid-rise apartments, it
may be easier to consider common spaces,
such as entry and elevator lobbies and
stairwells, as solar space. This makes
orientation of individual units more
flexible and may allow greater variations in
temperature swings.
An efficient flat-plate solar hot water heater
can collect approximately 2GJ of energy
per m2 (550 kWh/m2) of collector area per
year in most of southern Canada. Other
systems available include thermosiphon
systems, common in southern Europe, that
eliminate the need for pumps.
project consists of 100 well-insulated units;
each with 140 m2 (1,506 sq. ft.) heated
area, and assessed configurations of
collector area (900 m2 to 1,500 m2) and
insulated underground water storage 1,600
m3 to 6,300 m3 (56,503 cu. ft. to 222,482
cu. ft.).16
Several projects in Europe are working with
prototypes of seasonal storage, the “Holy
Grail” of the solar world. These projects
use large solar arrays to collect heat in the
summer and store it in large, well-insulated
underground water tanks. The heat is
extracted from the water during the heating
season. To illustrate the scope of such
systems, they use approximately 10 m2 to
20 m2 (107 sq. ft. to 215 sq. ft.) of
collector and 20 m3 to 40 m3 (706 cu. ft.
to 1,412 cu. ft.) of storage for every flat or
house. Performance projections indicate
that they would provide from 30 to 60 per
cent of a buildings’ energy. Planning for a
100-unit solar demonstration housing
project in Bavaria assessed systems capable
of providing 60 to 90 per cent of heating
using seasonal solar heat storage. The
In Hamburg, 24 single-family, detached
houses used 3,000 m2 of collector with
4,500 m3 (158,916 cu. ft.) insulated
underground water storage. A sister project
in Friedrichschafen used 5,600 m2 (60,277
sq. ft.) of collector with 12,000 m3
(423,776 cu. ft.) of storage for
570 flats in eight buildings. Both projects
anticipate solar energy will cover 50 per
cent of heating and hot
water needs.17
Solar water heating
Solar domestic hot water heating systems vary
in complexity, efficiency and cost. Modern
solar water heaters are relatively easy to
maintain and can pay for themselves in
energy savings well within their lifetimes.
In MURBs, they may pre-heat water for
the boiler in hot water heating systems.
This works best in large projects that have
significant system heat losses (when the return
water is cooled sufficiently that solar can reheat it). For boilers heating water for space
heating and hot water, solar panels may allow
the boiler to be shut down in the summer and
provide hot water from solar energy alone.
In much of Canada we have clear cold
winters and under these conditions a
substantial amount of solar energy is
available when needed, so short-term
(1-2 days) storage is more cost effective.
An equivalent climate in Canada for these
European examples would be the lower
mainland of British Columbia.
Glazing
Box
Tube
Outlet
Inlet
Inlet
Header
Absorber Plate
Note: for further information on
collector design and performance,
see manufacturers’ specifications
Insulation
Bottom Plate
Figure 15 – Glazed flat-plate collector
15
http://www.cmhc-schl.gc.ca/en/imquaf/himu/upload/Fire-Safety-in-High-Rise-Apartment-Buildings.pdf
16
D. Lindenberger et al., Optimization of solar district heating systems: seasonal storage, heat pumps and cogeneration, May 1999
17
B. Mahler et al. Central solar heat plants with seasonal storage in Hamburg and Friedrichschafen
16
The design shown is
an example of a typical
liquid-cooled collector.
Air-cooled collector
design will vary
accordingly
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Unglazed flat-plate collectors are the most
common North American collectors, as
measured by area installed per year. They
are used most for warming water up to
30°C (86°F) for outdoor and indoor
swimming pools.
They are inexpensive, simple systems that
can provide all the heating needs for
residential outdoor swimming pools,
eliminating both fossil fuel consumption
and the capital costs of conventional
heating equipment. They are simple to
install and generally have a three- to sixyear-year payback.18 In Canada, their use is
limited to non-heating seasons.
impractical on high-rise buildings. To avoid
heat loss during transit, a glycol collector
with a well-insulated circuit may be used
close to the pool. Southern or overhead
glazing can also provide direct solar energy
and cut conventional lighting costs. Solar
energy can supply between 30 and 100
per cent of the required heat, depending
on variables, including location, collector
angle and orientation, desired pool
temperature, size of pool and use of
a pool cover.
Evacuated tube collectors are individually
sealed vacuum tubes surrounding a metal
absorber plate. The vacuum minimizes
conductive heat loss, like a thermal jug.
These collectors are commonly used in very
cold climates. Evacuated tube collectors are
able to provide higher water temperatures,
but are also more expensive, with longer
payback periods. RETScreen calculations
show that an evacuated tube collector can
deliver about 1.2 kWh/m2/day in winter
and up to 2.9 kWh/m2/day in June.
Simple RETScreen calculations show that
unglazed collectors deliver about 2.0 to
2.4 kWh/m2/day during summer. Outdoor
pools are usually seasonal and in warmer
months a solar blanket can be used, or
solar collectors and pumps can heat the
pool directly. When indoor pools are at
or below grade, rooftop collectors are
Natural Resources Canada
Figure 16 – Unglazed flat-plate collector
18
Sheltair Group et al, Healthy High-Rise: A guide to innovation in the design and construction of high-rise residential buildings,
(Canada: CMHC, 1996) p. 49
Canada Mortgage and Housing Corporation
17
Solar Energ y for Buildings
Active solar space heating
Evacuated tube
SDHW—Solar domestic hot
water systems
Solar Domestic Hot Water (SDHW)
systems supplement traditional hot water
heating. The most common system uses
glazed, flat-plate collectors in a closed
glycol loop. A heat exchanger transfers the
energy from the glycol to one or more solar
storage tanks. These are usually connected
in series to the hot water system. The
traditional water heater comes on to keep
the water at the required temperature if the
solar heat is not enough.
Glazing
Inlet
Outlet
Figure 17a – Evacuated tube collectors
Source: Natural Resources Canada1
Cross section of
evacuated tube
Outer Glass Tube
Inner Glass Tube
Fluid Tube
Copper Sheet
Evacuated Space
There are seasonal variations in the energy
they collect, depending on location, collector
efficiency, collector angle and orientation,
ranging from about 0.6 to 1.0 kWh/m2/day
in winter up to about 2.4 kWh/m2/day in
summer. It is easy to get 50 per cent of hot
water energy from the sun. A reasonable
target for fossil fuel displacement is 30 to
40 per cent. This allows the panels to operate
at a more efficient temperature. These
systems are easily integrated into current
hot water systems and have a payback in
the range of 10 years. In Canada, this
varies tremendously, depending on funding
incentives and fuel cost.
18
Figure 17b – Evacuated tube collectors
Source: Natural Resources Canada
Figure 17c – Rooftop evacuated tube collector
Source: Architectural Graphic Standards
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Anti-Freeze Solution
Solar
Heated
Water
Hot
Water
to House
Solar
Collectors
Pump
Controller
Pump
Cold
Water in
Gas or
Electric
Water
Heater
Heat
Exchanger
Solar Storage
Tank
Figure 18 – Solar domestic hot water system
Source: www.AdvanceBuilding.org
Table 2 – Cost and benefits of solar collectors
Collector
Capital
cost $/m2
Energy delivered
annually kWh/m2
Not for freezing
temperatures.
150–350
210–250
(summer only)
Economical.
Needs glycol protection
from freezing.
450–750
500–600
Provides hotter water.
Expensive; needs glycol
protection from
freezing.
1,100–1,500
800–840
Typical uses
Advantages
Unglazed
Swimming pools
Economical, efficient at low
temperature differentials.
Glazed
DHW pre-heat
Evacuated tube
DHW pre-heat
Disadvantages
Canada Mortgage and Housing Corporation
19
Solar Energ y for Buildings
Solar air heating
Table 3 – Common elements in solar air-heating systems
The following summary is based on Solar
Air Systems: A Design Handbook, edited by
S. Robert Hastings and Ove Mørck. The
authors looked at European and North
American applications. Cost analyses are in
Canadian dollars, unless otherwise noted.
Collector systems
I
Flat-plate collector
I
Window air
collector
I
Six principal solar air-heating systems are
summarized below. All systems consist of the
following common elements in one form
or another: collector, distribution system
(ducting), storage unit and control system.
I
A total system can consist of any combination
of the four different components.
I
The applications analyzed in the study
were for industry, dwellings (apartments,
row and single-family houses), offices,
schools, sports halls and swimming pools.
The factors affecting system performance
are type and mass of building, insulation
level and climate.
I
I
Design procedure
The Solar Air Systems design handbook
recommends the following design steps.
More technical details can be found in
the guide.
I
I
Define necessary basic data about
building and climate.
Determine if it is possible to obtain
enough collector area.
20
Perforated
unglazed collector
(Solarwall®)
Double facades
and double-shell
collector
Storage systems
I
Hypocaust
(ceiling or
floor slab)
I
Murocaust
(wall)
I
Rock beds
I
Water
I
Phase-change
material
I
Continuous
performance
I
Temperature
control
I
Solar cell
control
I
Timer control
Distribution
I
Usually
through
ducting.
Spatial collector
(atriums,
sunrooms,
greenhouses)
Determine ventilation rate through the
solar air collector.
Determine if there are restrictions on
inlet temperature from ventilation
system.
I
Investigate if it is appropriate to
include storage in system.
I
Define the required control strategy.
I
Choose a solar collector.
I
Investigate if the system may serve
other purposes.
Canada Mortgage and Housing Corporation
Control systems
I
Determine the collector area.
I
Size the ducting.
I
Choose a fan.
I
Choose diffusers.
Using an integrated design approach will
enable the building design team to better
consider any possible alternative purposes
for the various systems, which could help
reduce the payback time or provide other
benefits to the occupants.
Solar Energ y for Buildings
Preheated
Ventilation Air
Solar
Collector
System 1
Conserval
Figure 19 – Ouellette Manor, Windsor,
uses Solarwall ® to pre-heat corridor
ventilation air
System 1: Solar heating of
ventilation air, such as Solarwall ®
This system provides the simplest, and
usually least costly way to bring solarheated fresh ventilation air into a building.
It uses mainly off-the-shelf components
in its design. Its major disadvantage is
that it will reduce cost-effectiveness of the
building’s ventilation heat recovery unit.
An example of this type of system developed
in Canada is Solarwall®, in which a southfacing wall is clad with dark metal panels,
typically steel or aluminum, perforated with
Figure 20 – System 1, solar air pre-heat system concept
small holes. A gap is left between the cladding
and the wall so that outside air passes
through the holes in the collector panel.
Air is aspirated into the airspace between
the collector and the wall, is heated, and
rises as a result of the stack effect and the
lower pressure zone above, which is created
by fans moving air through the system to
the interior. This pre-heated ventilation air
is then incorporated into the building's
normal distribution system. A recirculation
damper controls the mix of air from the
collectors and from inside the building to
maintain a constant air temperature for
distribution. Using the sun to pre-heat air
for ventilation in this way is a fairly new
Table 4 – Solar heating of ventilation air
Benefits
Limitations
Less cost to heat ventilation air
Requires large, south-facing wall area
Recaptures heat loss through wall
Reduces opportunity for south facing
glazing
May replace conventional cladding
(new construction)
Conceals old cladding (retrofit)
Reduces the cost-effectiveness of
ventilation heat recovery (because
owner pays less to heat incoming air)
technology. In the last 10 years, about
35,000 m2 (376,737 sq. ft.) of Solarwall®
collector systems have been installed in
buildings, including low-rise and high-rise
residential. Pre-heated ventilation air
systems can be integrated into new
construction or as a retrofit (see figure 19).
In the early 1990s, Ouellette Manor, a
24-storey, 400-apartment seniors residence
in Windsor, reclad part of its complex with
Solarwall®. The new Solarwall® had an
incremental cost of about $30,000 and the
energy savings provided a simple payback of
about six years. There is more information
about Ouellette on the CMHC website at
http://www.cmhc-schl.gc.ca/en/imquaf/
himu/buin_006.cfm
Solarwall® is ideally suited for applications
that require large quantities of ventilation air
during the day and has proven effective at
pre-heating ventilation air in MURBs. In
new and retrofit situations, it has the benefit
of offsetting the cost of traditional cladding
materials. As a result, it can have very quick,
and sometimes immediate, payback.
Doesn't replace normal heating system
Canada Mortgage and Housing Corporation
21
Solar Energ y for Buildings
System 2: Open collection loop
with radiant discharge storage
In this system, air circulates, either
naturally or mechanically, through the
collector, distribution system, room space
and back to the collector. It can be built
with or without storage, and may require a
separate ventilation system.
Open Loop
Air Circulation
System 3: Double envelope
(facade) systems
In a double-envelope or double-facade
solar air system, solar heated air is
circulated through cavities in the building
envelope, surrounding the building with a
layer of solar-heated air. This creates a
buffer space that reduces the building’s
heating and cooling load. Inner comfort is
improved because inner surfaces of the
external walls are warmer. The outer
envelope can be made of opaque materials
(traditional cladding materials with an air
space) or glass. The Klosterenga project in
Oslo, Norway uses the space between
double layers of south-facing windows to
preheat the air. The figures in Table 5 are
for glass-enclosed systems. Questions of
cleaning and maintenance for this type of
system must be addressed.
Solar
Collector
System 2
Figure 21 – System 2, without storage
Solar
Collector
Cavity
Wall
This system is versatile and integrates into
most existing heating systems, but is
usually much more expensive than other
systems. In North America, costs are
reported to be four to five times that of
traditional, low-cost cladding systems,19 but
the effective cost may drop if the double
facade reduces energy consumption.
Radiant
Heat
Solar Air
Surrounds
Envelope
System 3a
Figure 22 – System 3a, double-envelope system with storage
19
Meyer Boake, Terry et al. Canadian Architect, August 2003, p. 38
22
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
There are numerous concepts for double
facade. The following example
demonstrates the heating effect of an airheating solar collector with a motorized
blind as the surface absorbing the solar
radiation. Major design parameters are the
spacing between the two skins of the
facade, the air velocity and the properties
of the blind, which is controlled by the
building automation system, with manual
override and automatic refresh every hour
or so.
The blind, even when closed, must allow
enough daylight into the space. This
requires a 20 per cent transmittance
depending on window area. The glazing
must be clear. The airflow-window type of
double facade was considered for the
Seville adaptive reuse project in Montréal.20
Each floor may be separate (with box
window types) with individual inlets and
outlets or connected to form one large
“chimney.” Figure 23 shows double glazing
for the outer skin with low-emissivity
coating facing the skin cavity to reduce
heat losses in winter. However, this coating,
which increases the outlet temperature
by a couple of degrees, may possibly be
excluded as it can deteriorate in this case.
The inner glazing may be operable. The
inlets and outlets of the airflow window
need to be carefully designed.
20
Open Loop
Air
Circulation
Solar
Facade
System 3b
Figure 23 – System 3b, double-facade design option
“Seville Theatre Redevelopment Project Integrated Design Process,” CMHC Technical Series (63175) Research Highlight 03-102
Canada Mortgage and Housing Corporation
23
Solar Energ y for Buildings
The following results show an example of
the air temperature rise of the solar
collector due to varying the distance
between the two “facades” or skins.
v=air velocity: 0.1–0.2 m/sec
w=width of the space=3.6 m,
Outdoor air temperature of -5ºC
L= the distance between the two skins;
Outer glazing, clear double; inner
glazing single
low-emissivity coating on inner side of
outer glazing (double)
blind solar absorbance: 60 per cent,
transmittance 20 per cent.
Height of the space=4 m
Note that the larger the gap width between
the skins, the smaller the air velocity
needed to achieve the required fresh-air
flow rates.
Figure 24 – Fresh-air pre-heating in double facade (Klosterenga, Oslo, Norway)
1. For L=20 cm: for v=0.1 m/sec, the
collector air temperature will rise to
about 15ºC (rise of 20ºC) when the
blind is closed with incident solar
radiation of 600 watts/m2.
Solar
Collector
2. For L=30 cm: for v=0.2 m/sec (L=30
cm), the collector air temperature will
rise to about 5ºC (rise of 10ºC) when
the blind is closed with incident solar
radiation of 600 watts/m2.
System 4: Closed-collection loop
with radiant discharge storage
In this system, an air collector is connected
to the building’s integrated heat storage. The
air is circulated in a closed loop, normally
with the aid of fan-forced convection, through
the collector to the storage and back to the
collector. The room-facing surface of the
storage discharges heat by radiation and
convection to the room space. The collector
system can be used as part of the building
envelope, with lower extra costs.
24
Solar
Collector
Radiant
Heat
Closed
Loop
Charge
Mass
System 4
Figure 25 – System 4, with storage
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
System 5: Closed-collection
loop with open-discharge loop
This system provides comfort, even in
rooms with high internal and solar gains
and small losses, because it allows
controlled discharge of stored solar energy
to the heated room. This increases the solar
system’s efficiency and reduces the risk of
overheating. It can use existing building
components and can be combined easily
with existing HVAC systems. It is more
expensive than other systems.
Solar
Collector
Solar
Collector
Radiant
Heat
System 6: Closed-collection loop
with heat exchange to water
The closed-loop solar-air system has
advantages over liquid systems, as there is
no risk of leaking, boiling or freezing. It
might also be chosen for its economy or
for architectural reasons. Solar-air heated
water can provide space heating, domestic
hot water heating or be used for industrial
applications. Apart from the collector, the
system consists of standard HVAC
components. This system can be used for
heating hot water during the summer. It
requires that the air temperature in the
system be hotter than ventilation pre-heat
systems. It is usually bulkier than liquid
systems.
System design
Open Loop
Discharge
Mass
System 5
Figure 26 – System 5, with storage
Air to Water
Heat Exchanger
Solar Preheated
Water
Solar
Collector
For more technical details, see pp 103-104
of Solar Air Systems: A Design Handbook
I
Step 1-Profile the loads
I
Step 2-Select collector type
I
Step 3-Decide on air mass flows
I
Step 4-Specify the heat exchanger
I
Step 5-Size the storage and determine
heat loss
System 6
Figure 27 – System 6, with hot-water storage
Canada Mortgage and Housing Corporation
25
26
Canada Mortgage and Housing Corporation
6
5
4
3
2
1
System
Furniture can't be placed against walls
Increased installation costs because of
facade shell
Bulkier than liquid systems
Risk of freezing in heat exchanger
Overall efficiency reduced due to
temperature drop over heat exchanger
Existing building components used
Can combine with heat and ventilation
system
No problem with freezing, boiling and leaking
in collector
Standard ventilation equipment can be used
Can be used to heat hot water in summer
lowers extra costs for the solar system
Window collectors may overheat adjacent
rooms; rocked storage bulky
Using collector as part of building envelope
retrofits
Relatively more expensive
May require separate ventilation
Simple, can be used with or without storage
High degree of integration possible, even in
Decreased performance of heat recording
unit
Collector materials must be non-toxic
Disadvantages
Simple, inexpensive
Low temperature air usable
All collectors usable
System uses standard components
Advantages
Table 5 – Comparison of six solar-air heating systems
$250–$650/m2
$90–$475/m2
$200/m2
Solarwall®
$194/m2
Cost
175–375 kWh/m2
80–240 kWh/m2
(Heating season)
Payback time depends
on integration
Payback 25 years
600–800 kWh/m2
System
performance
300–400 kWh/m2
(sunny–cold)
120–130 kWh/m2
(cloudy–temperate)
30–150 kWh/m2
(sunny–cold)
10–100 kWh/m2
(cloudy–temperate)
100–425 kWh/m2
(sunny–cold)
50–200 kWh/m2
(cloudy–temperate)
150–400 kWh/m2
(sunny–cold)
100–225 kWh/m2
(cloudy–temperate)
80–200 kWh/m2
(sunny–cold)
40–75 kWh/m2
(cloudy–temperate)
110–550 kWh/m2
(sunny–cold)
90–300 kWh/m2
(cloudy–temperate)
Saved energy
Solar Energ y for Buildings
Solar Energ y for Buildings
Photovoltaic (PV) systems
The photovoltaic effect converts solar
energy directly into electricity. When
sunlight strikes a photovoltaic cell,
electrons in a semiconductor material are
freed from their atomic orbits and flow in
a single direction. This creates direct
current electricity, which can be used
immediately, converted to alternating
current or stored in a battery. Whenever
sunlight arrives at its surface, the cell
generates electricity. PV cells normally have
a lifespan of at least 20-25 years; however,
they usually last longer if frequent
overheating—temperatures in excess of
70ºC (158ºF) is prevented.
PV systems can be used as a building's sole
electricity supply or with other sources,
such as a generator or a grid connection.
Autonomous PV systems include an array of
PV cells and a power conditioner that
connects to the building's electrical loads.
To have electricity when there is no sun,
this system must have storage batteries.
Battery storage must be sized to the
anticipated load and solar access. A
weakness of the system is that the supply of
solar energy may be intermittent.
Hybrid PV systems have at least one
additional electricity source, such as a fuelfired generator or a wind turbine. These
systems can still be off the utility grid and
can minimize or eliminate the problem of
intermittent solar energy.
The cost of PV technology is now much
more expensive than traditional electricity
and has a very long payback period. In 2000,
Natural Resources Canada assessed the breakeven point for PV products in Canada using
market data from the past 25 years. Based
on annual growth rates of 20 per cent
(growth has been closer to 30 per cent for
the past six years), the break-even point for
competing with bulk electricity generation
was calculated to be between 2020 and
2030.21 This was based on lowest production
cost but does not consider technological
advancements or the advantage of reduced
greenhouse gas production.
Figure 28 – PV as window shading
elements (overhangs) at Queen’s
University, Kingston
Source Kawneer
While autonomous systems can be
immediately cost-effective in remote
locations, they are not likely to be costbeneficial for MURBs.
Grid-connected PV systems cancel out the
need for onsite generators and batteries and
eliminate the problem of intermittent solar
energy. In many jurisdictions it is possible
to supply excess solar-generated electricity
to the grid and receive credit from the
power company.
Building-Integrated PV systems
(BIPVs)
A more recent trend is the development
of Building—Integrated Photovoltaics
(BIPVs). The PV cells are incorporated
into a building element. Currently there is
much development in PV roofing, PV
shading elements (see figure 28) and PV
cladding or semi-transparent curtain wall
components (see figure 29). With PV
cladding it is best to have a vented cavity
behind the panels so as to operate at lower
temperatures. By following such a
construction approach one may also
develop an effective rainscreen system
which hinders rain penetration. PV
roofing is installed much the same way as
21
Ayoub, J., Dignard-Bailey, L. and Filion, A., Photovoltaics for Buildings: Opportunities for Canada: A Discussion Paper, Report # CEDRL-2000-72 (TR),
CANMET Energy Diversification Research Laboratory, Natural Resources Canada, Varennes, Que., November 2000.
Canada Mortgage and Housing Corporation
27
Solar Energ y for Buildings
conventional roofing and is available in
shingles, tiles and metal standing-seam
roofing (see Figure 30). PV shading can be
effective as a window shading element,
entrance canopy or walkway shading. PV
panels can be opaque, used where no light
transmission is needed, or semi-transparent
for areas where light is wanted, such as
atriums or skylights, but some shading is
needed to reduce cooling loads.
Figure 29 – PV integrated in curtain wall elements at the Mataró
Librar y, Mataró, Catalonia, Spain.
(The facade is also used for fresh
air pre-heating).
Sol Source Engineering
Figure 30 – BIPV metal-standing seam roof, Toronto
28
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Until the use of BIPVs becomes more
widespread, there are barriers to overcome.
Canadian utilities are often not familiar
with small, decentralized energy
production. Consequently, utility
interconnection for BIPVs is a major
barrier to their use. Another barrier is the
absence of technical standards and
installation codes. Non-technical barriers
include the lack of experience among
builders and electrical inspectors; lack of
financing for systems with large capital
costs; additional permit, insurance and
inspection fees for net-metering systems;
unawareness of potential and long-term
benefits to system integrators.22
Photovoltaic hybrid heating
system (PV-thermal system)
A typical crystalline silicon PV panel has
an efficiency of 10–15 per cent. PV solar
panels produce more than four times as
much heat as electricity. This heat is
normally lost to the environment. A PV
cell can have a stagnation temperature of
50°C (122ºF) above the ambient if the
heat is not removed. The cooler the PV cells,
the higher the efficiency. A solar air collector
has a typical thermal efficiency of 40–70
per cent. Drawing outside air in across the
back of panels pre-heats the HVAC supply
air and also increases the PV efficiency by
keeping them cooler.
Combining these two systems produces
both heat and electricity. (This is
equivalent to a co-generation power plant.)
Test results show that using PV panels to
generate both electricity and useful heat
substantially improves the overall efficiency
(electrical plus thermal). The payback for a
22
Simplified Analysis
Incident solar
radiation=
reflected+
electricity+
heat transfer
to air+
heat lost
to exterior
1,000 W=
100+
100+
400+
400
grid-connected PV electrical system was
decades. By 2004, the cost of panels had
fallen to $4.5 per kW-peak (one kW-peak
is electricity generated with 1,000 W/m2
incident solar radiation). Annual electrical
production is generally in the 70-200
kWh/m2 range, depending on climate.
of thermal power. This gives the following
energy balance on the PV panels:
Consider a simplified analysis of a PVthermal system—a PV panel with airflow
behind it (see figure 10, page 12).
Assume that 1,000 W/m2 of solar radiation
is incident on the solar panel, which
converts 10 per cent to electricity to
produce 100 watts of electricity for one
square metre of panel (a panel costs about
$450 at mid-2004 prices).
About 5–10 per cent of incident solar
radiation is reflected but the rest becomes
heat. By bringing in fresh air through an
inlet at the bottom and passing it behind
the panels the air is heated the same way as
in a Solarwall® system. The faster the
airflow, the more heat is transferred to the
flowing air and less is lost to the outside air.
Optimal cavity width and air velocity are
selected by taking into account fan energy,
required outlet temperature and fresh air
requirements. The PV can extend over
multiple stories, with multiple inlets. In any
case, if the inside heat transfer coefficient hi
is equal to the exterior film coefficient ho
(about 12 W/m2 for still air), then the
flowing air may capture about 400 watts/m2
This shows that four times more thermal
energy is generated than electricity. The
electrical efficiency is 10 per cent, while the
thermal efficiency is 40 per cent. This gives
an overall efficiency of 50 per cent. If
thermal energy is worth half as much as
electricity, then this system generates about
three times the revenue of a simple PV
system on the facade.
This simplified analysis shows why
PV–thermal applications are the key to
early, cost-effective use of PV.
Integration into MURBs
For MURBs, facades have the highest
potential for cost-effective BIPVs. In
facades, they can easily generate thermal
energy. Semi-transparent panels can also
provide daylighting. There are two main
options for using the hot air. Like the
Solarwall® system, the PV system can be
applied in vertical strips with a fan drawing
the air into the HVAC system. An
alternative is installation into box-type,
airflow windows. If they project from the
facade, they need a separate support
structure, adding to installation costs.
The applications may range from small
overhangs to large continuous facade areas.
Figure 30 shows a double facade with PV
o
overhangs, in Freiburg, latitude 48 N.
Ayoub, J., Dignard-Bailey, L. and Filion, A., IBID.
Canada Mortgage and Housing Corporation
29
Solar Energ y for Buildings
Table 6 – Description of collector types
Collector type
Advantages
Disadvantages
Capital cost $/kW
Efficiency
Single crystal
High efficiency
High cost, fragile,
uniform look
5,000–10,000
11–15%
Polycrystalline
High efficiency
High cost, fragile,
non-uniform look
5,000–10,000
10–14%
Thin-film amorphous
Flexible, can be applied to different
types of surfaces
Low efficiency,
degrades
Spheral solar (crystalline
family)
Low cost, flexible, can be applied to
different types of surfaces.
Low efficiency
5–8%
4,500
9–10%
Table 7 – BIPV manufacturers
BIPV product
Manufacturer–country
Sloped roof
Atlantis Solar Systeme AG, Switzerland
Ecofys,The Netherlands
BMC Solar Industrie GmbH, Germany
BP Solar, United Kingdom
Canon Inc., Japan
Lafarge Brass GmbH, Germany
MSK Corp., Japan
United Solar Corp, U.S.A.
Facades
Atlantis Solar Systeme AG, Switzerland
Pilkington Solar Inter., Germany
Isophoton Inc., Spain
Saint-Gobain Glass Solar, Germany
Sanyo Solar Engineering Ltd., Japan
Schuco Int. KG, United Kingdom
Shading
Ecofys, Netherlands
Colt Solar Technology AG, Switzerland
Kawneer, U.S.A.
Flat roof
Powerguard, U.S.A.
Figure 31 – Double facade with PV overhangs in Freiburg,
Germany
30
Canada Mortgage and Housing Corporation
Solar Energ y for Buildings
Summary
Passive solar is best for buildings that have
low internal heat gains and in which direct
solar gain is directed to absorbent thermal
mass. The housing market today may object
to hard floor surfaces out of concern for
comfort and impact noise, but increased
drywall thickness and concrete ceilings may
compensate for the lack of hard flooring.
Mass is most effective if it receives direct
solar gains, i.e. usually on the floor. However,
if this is not possible, a concrete ceiling will
absorb much of the energy from air heated
by the floor; this air will rise through
buoyancy. Generally, about 5-10 cm of
concrete—or equivalent—on the floor
provides adequate mass.
I
I
The cost of passive solar is minimal,
but must be planned during the initial
design stages.
Orientation in most MURBs provides
challenges. An effective strategy is to
tune the selection of glazing based on
the orientation of each facade.
I
Solar domestic hot water can have a
reasonable payback time and is
relatively easy to install in new
buildings and retrofits.
I
Solar water heating is less likely to be
effective for space heating, except in
very large heating systems.
I
Solar water heating for swimming pools
is very effective for seasonally used
pools, with short payback times.
I
Solar air-heating systems for preheating ventilation air can have very
short—even immediate—payback.
Their drawbacks are the need for prime
southern exposure and their industrial
aesthetic. Such considerations need to
be part of the architectural design of
MURB installations.
I
Presently, photovoltaics are an
expensive way to provide electricity and
are more cost-effective combined with
heat recovery as well. However, the cost
of building-integrated PV (BIPV)
systems is coming down as competition
and market share increase.
Tools and resources
Canada Mor tgage and Housing
Corporation
www.cmhc.ca
key word: Innovative Buildings
NRCan and CMHC’s Advanced
Buildings Technologies
program is being phased out in 2007.
However the EE4 software and CBIP goals
are part of the LEED prerequisites.
Renewable Energy Deployment
Initiative (REDI)
REDI provides an incentive of 25 per cent
of the installed cost of renewable energy
systems for space and water heating and
cooling. Eligible systems include:
I
active solar hot water systems
I
active solar air heating systems
I
highly efficient, low-emitting biomass
combustion systems.
www.advancedbuildings.org
PV systems are not eligible but receive
accelerated depreciation under Class 43 of
the Income Tax Act.
C2000 Commercial Building
Program
Natural Resources Canada
RETScreen
This is a Natural Resources Canada
demonstration program for design
assistance in energy-efficient commercial
buildings. The goal is 50 per cent less
energy consumption than a building
constructed to the Model National Energy
Code of Canada for Buildings (MNECB).
RETScreen is free energy-assessment
software that assesses renewable energy
options against a base model. RETScreen
has specific modules for passive solar, solarair heating and solar domestic hot water
heat. Output includes financial analysis,
payback periods and energy displaced.
Modules are available at
http://www.retscreen.net/ang/menu.php
Commercial Building Incentive
Program (CBIP)
CBIP is probably the largest federal
government initiative to reduce energy use
in commercial buildings. Building owners
are given a financial incentive of three
times the annual energy savings if the
predicted building energy use is 25 per
cent below that required by the MNECB.
Computer simulations are done with
NRCan's EE4 software. This program has
no provision for analyzing PV systems.
Energy from PV is a credit towards
meeting the energy target and contributes
to the eligibility of the incentive. The
Canada Mortgage and Housing Corporation
31
32
Canada Mortgage and Housing Corporation
Atrium pre-heats ventilation air, solar DHW,
heat recovery ventilation, SOcLAR shading of
some units
Thermally isolated balconies; meeting
room–greenhouse; external shading
Passive solar heating; large thermal mass; 21 m2
solar panels for DHW
336 m2 Solarwall® pre-heats ventilation air
228 m2 solar DHW pre-heat system
47.5 m2 flat-plate solar collectors heat DHW
with surplus going to radiant slab; 2 PV panels
for pumps; no fossil fuels consumed on site
Combined heat and power units (CHP)—wastewood combustion delivers electricity and space
heat via centrally located domestic hot water
cylinders; PV array to recharge cars
Greenhouse in common area; space heating
from rooftop solar collectors; solar balconies
218 m2 rooftop solar collectors for radiant slab
and DHW pre-heating; passive solar double
glazed south facade to pre-heat ventilation air;
summer shading
4-storey apartments, new
construction
10 units, 4-storey MURB,
1997 new construction
400 units, 24 storeys
232 units, 10 storeys
2-storey inn (9 suites plus
common areas)
Carbon-neutral residential
development
Social housing project,
10,000 residents; “green
retrofit” of 1970 s building
35-unit 6-storey green
apartment demonstration
Conservation Co-op,
Ottawa
Uster Condominiums,
Switzerland
Ouellette Manor,
Windsor
Quinpool Towers,
Halifax
Chantrelle Inn, North
River, N.S.
BedZED, U.K.
Gardsten, Gothenberg,
Sweden
Klosterenga, Norway
Technologies used, solar features
42-unit MURB new
construction
Building description
Amstelveen,
Amsterdam,The
Netherlands
Project–location
Case studies — visit CMHC website for other examples
11,589 euros/m2
$36,700 ($772/m2)
after incentive
$93,000 total
system ($408/m2)
before incentive
$90/m2 incremental
cost
200 USD/sq. ft.
construction cost
Incremental cost
$733 per balcony
System cost
$645/ m2
$152, 386
Cdn per unit
Project
cost
Part of IEA solar program
International Energy Agency
60% decrease in total unit energy
demand; 90% decrease in total heat
demand
46 GJ (12,800 kWh) produced each
year—50% of DHW and 25% of
space heat requirements; savings of
$2,660/year—10.5-year payback
(with incentive)
630 GJ/yr solar delivered; 6-year
payback (with incentives)
Collector delivers 584 kWh/ m2;
6-year payback; 2,000–4,000 Cdn
savings in energy/yr
Total energy consumption 112 kWh/
m2-35% of typical MURB
5,320 GJ/ m2 ;
energy savings small but other
benefits
Energy use: 13 kWh/m2 for space
heating, 15 kWh/ m2 for DHW,
Comments
Solar Energ y for Buildings
Solar Energ y for Buildings
References
Tap the Sun: Passive Solar Techniques and
Home Designs (Canada: CMHC, 1998)
Canada Mortgage and Housing
Corporation (CMHC)
Sheltair Group, Healthy High-Rise: A
Guide to Innovation in the Design and
Construction of High-Rise Residential
Buildings, (Canada: CMHC, 1996)
Ayoub, J., Dignard-Bailey, L. and Filion,
A., Photovoltaics for Buildings: Opportunities
for Canada: A Discussion Paper, Report
# CEDRL-2000-72 (TR), CANMET
Energy Diversification Research Laboratory,
Natural Resources Canada, Varennes, Que.,
November 2000, pp. 56 (plus appendices).
Solar Air System: A Design Handbook
Editors S. Robert Hastings, Ove Morck,
James & James 2000
Edward Mazria, The Passive Solar Energy
Book, Innovative Building Case Studies,
CMHC website.
Canada Mortgage and Housing Corporation
33
Solar Energ y for Buildings
Questions
1. Name three benefits of using
solar energy.
2. Why is passive solar a good
choice for new construction
MURBs?
3. What four window technologies
can improve a window’s
performance as a solar
collector?
34
4. What is the difference between
active and passive solar energy
systems?
5. What are the main elements of
a hybrid grid-connected
photovoltaic system ?
6. What are the main elements of
a typical solar domestic hot
water system?
Canada Mortgage and Housing Corporation
7. Name three types of active solar
collectors.
8. What are three pros and three
cons of a Solarwall® type of
ventilation air preheat system?
9. Describe two ways of integrating
PV in MURBs and improving
their cost-effectiveness