Solar Energy for Buildings

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Abstract:

Solar Energy for Buildingspresents basic information on solarbuilding design, which includes passive solar heating, ventilationair heating, solar domestic water heating and shading. The articlesuggests ways to incorporate solar design into multi-unit residentialbuildings, and provides calculations and examples to show howearly design decisions can increase the useable solar energy.

This Introduction to Solar Design Issues, presents basic notions ofsolar design and describes different passive, active and hybridsystems and the solar aspects of design elements, which include

window design, cooling and control, and water heat ing.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 residentialbuildings (MURB).

THE PRINCIPLES OFSOLAR DESIGN

Benefits of solar energy

For both new and retrofit projects,solar energy can substantiallyenhance building design.

Solar energy offers these advantagesover conventional energy:

s Free after recovering upfront

capital costs. Payback time canbe relatively short.

s Available everywhere and

inexhaustible.

s Clean, reducing demand for fossilfuels and hydroelectricity, and

their environmental drawbacks.

s Can be building-integrated,

which can reduce energydistribution needs.

Solar Energy forBuildings

Introduction: Solar Design Issues

By Keith Robertson and Andreas Athienitis


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Solar Energy for Bui ldings

2 Canada Mortgage and Housing Corporation

The amount of energy that reaches earths

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 fromabout 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.

Energy from the sun reaches earth as direct,

reflectedand diffuseradiation.

Direct radiationis highest on a surface

perpendicular to the suns rays (angle of

incidence equal to 0 degrees) and provides

the most usable heat.

Diffuse radiationis 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 suns energy reaching

the surface of the earth also depends on

cloud cover, air pollution, location and thetime 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

suns 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

materialsradiation, 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 systemsare commonly used for:

s Water heating;

s Space conditioning;

s Producing electricity;

s Process heat; and

s 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 grid-

supplied or diesel-generated electricity.

Hybrid systems are the most common, except

for the direct gain system, which is passive.

S o l a r E n e r g y o n a V e r t i c a l P la n e

0 . 0 0

1 . 0 0

2 . 0 0

3 . 0 0

4 . 0 0

5 . 0 0

6 . 0 0

7 . 0 0

J a n F e b M ar A p r M a y J u n J u l A u g S e p O c t N o v D e c

H a l i f a x

T o r o n t o

E d m o n t o n

Yel l ow k ni fe

V a n c o u v e r

Source: RETScreenFigure 1 kWh/m2/day on a vertical surface, for selected Canadian cities

1 RETScreenis 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

kWh/m2/day


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3

Glossary

AbsorptanceThe ratio of absorbed to

incident radiation.

Active solarA solar heating or coolingsystem that operates by mechanical means

such as motors, pumps or valves to sort

and distribute the sun's heat to a buidling.

Energy rating (ER)A rating system that

compares window products for their

heating season efficiency under average

winter conditions.

Evacuated tube collectorsSolar

collectors that use individual, sealed

vacuum tubes surrounding a metalabsorber plate.

Flat-plate collectorsThe most common

type of solar collector. Can be glazed or

unglazed.

Hybrid power systemsCombines active

and passive solar power systems or involves

more than one fuel type for the same device.

Latent HeatAlso called heat of

transformation. Heat energy absorbed orreleased by a material that is changing state,

such as ice to water or water to steam, at

constant temperature and pressure.

Low-emissivity (low-e)Coatings applied

to window glass to reduce inside heat loss

without reducing outside solar gain.

Passive solarA solar heating or cooling

system that operates by using gravity, heat

flow or evaporation to collect and transfer

solar energy.

Photovoltaic (PV) systemSystem that

onverts sunlight into electricity. Can be

autonomous or used with another energy

source. (Can be connected to the main

power grid, for example).

R-value (imperial), RSI-value (metric)

A measure of resistance to heat flow

through a material or assemblya

numerical inverse of the U-value.

Solar balconyAn enclosed balcony that

acts as a solar collector.

Solar constant1,350 W/m2 The

average amount of solar energy reachingthe earths upper atmosphere.

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.

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.

Solar south180 degrees from true or

grid (not magnetic) north.

SolarwallA proprietary system that

uses perforated metal panels to pre-heat

ventilation air.

Switchable glazingGlazing 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-valueA measure of heat flow through

a material or assembly. Measured in

Watts/m2/C.

Warm-edge spacersSeparate a window's

glazing layers with thermal break or a low-

conductivity material.


Canada Mortgage and Housing Corporation

S o la 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

0 . 0 0

1 . 0 0

2 . 0 0

3 . 0 0

4 . 0 0

5 . 0 0

6 . 0 0

7 . 0 0

J a n F e b Ma r A p r M a y Ju n Ju l A u g S e p O c t N o v D e c

Halifax

Montral

Toronto

Winnipeg

Edmonton

Yellowknife

Vancouver

Figure 2 kWh/m2/day on a south-facing horizontal surface, for five Canadian cities

Source: RETScreen

kWh/m2/day


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Building design issues

Careful solar design can:

s Maximize possible solar transmission and

absorption in winter to minimize or reduceto zero the heating energy consumption,

while preventing overheating.

s Use received solar gains for

instantaneous heating load and store

the remainder in embodied thermal

mass or specially built storage devices.

s Reduce heat losses using insulation and

windows with high solar heat gain factors.

s

Employ shading control devices orstrategically planted deciduous trees to

exclude summer solar gains that create

additional cooling load.

s 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.

s 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.

s Use solar radiation for daylighting,2

which requires effective distribution

into rooms or onto work planes, while

avoiding glare.

s Integrate passive solar systems with

active heatingcooling/air-conditioning

systems in both design and operation.

What is design

integration?

The most important factor for a successful

solar building is integration. Thisconcept includes not only the integration

of design professionals at the projects 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.

The architect may design the buildingenvelope 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

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 traditionalworking relationships between architects,

engineers, property managers and other

professionals do not foster an integrated

design approach.3

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.

Passive solar heating systems (thermal) areseparated into two broad categories, direct

gain and indirect gain (seeFigure 3). An

indirect passive system insulated from the

heated space is an isolated system.

Solar Facade

DirectGain

Collector-storageWall



2 See Daylighting Guide for Buildingsat: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_001.cfm

3 See Integrated Design Process Guideat: http://www.cmhc.ca/en/inpr/bude/himu/coedar/coedar_002.cfm

Figure 3 Two major options for thermal mass placement in passive solardesign: direct gain and Trombe wall, or collector-storage wall


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5

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 afloor-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:

s Location and orientation of a building;

s Fenestration area, orientation and type;

s Thermal massing and envelope

caracteristics;

s Amount of insulation;

s Shading devicestype, location

and area;

s Effective thermal storage insulated from

the exterior environment, as well as

amount and type;

s sensiblesuch as concrete in the

building envelope with exterior

insulation, or

s 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

gainsthe 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 heatingcooling system.



Figure 4 Sixteen of the 42 units in this apartment 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 buildingsdomestic hot water energy.

Source: CMHC, at http://www.cmhc-schl.gc.ca/en/imquaf/himu/buin_018.cfm


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Design procedure

The initial design steps in solar design are to:

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 anddetailed simulations in evaluating

options at the early design stages and

later to assess alternatives.

Building orientation

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 northmagnetic

declinationvaries between east and west

coasts. In Nova Scotia, the compass

points west of true north; in B.C.,

east of true north.

The maximum difference (as a percentage)

between south-facing and 30E (or W)

orientations occurs when the sun is lowest

and the days shortest (Dec. 21). When solar

facades or roofs generate photovoltaicelectricity that is sold to the grid at time-

of-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

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 meanshalf 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 Montral angles the

windows to the south creating a sawtooth

plan, to avoid east-and west-facing

windows.



Generally, deviations up to 30from due south reduce solar gainsby up to about 12% and are thusacceptable in solar building design,providing significant freedom in

choice of form.


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7

Building Conditions

NRCs EE4 software4was used to modelthe 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:

s Four-storey, double-loaded corridor,

wood-frame.

s Window-to-wall ratio: 19 per cent on

primary facades.

s Double-glazed, low-e vinyl windows.

s High insulation levels.

Simulation Results

s Using a higher Solar Heat Gain

Coefficient (SHGC) glazing reduced

the total annual heating cost by three

to four per cent.

s Orienting the building along the long

eastwest axis instead of northsouth

axis reduced annual heating cost by

about one per cent.

s Increasing the window area on thesouth-facing suites reduced annual

heating cost by less than one per cent.

s Increasing the interior mass reduced

annual heating cost by about two per

cent.

Although differences in assumptions and

input data make comparisons difficult, a

study of a Toronto building produced

different results. RETScreens passive solar

energy module was used for the Torontobuilding. 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.):

s Increasing the glass Solar Heat Gain

Coefficient (SHGC) from .45 to .65

saved 1,100 to 1,200 kWh annually.

s 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 high-

mass (concrete-frame) building.

s Increasing the glass R-value and

maintaining a high SHGC saved about

900 kWh annually.

s 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.



Figure 5 Foyer hongrois in Montral. South angled windows on a building with along north-south axis. Sunshades shadow these windows in the summertime.

4 EE4 is the software developed for NRCans Commercial Building Incentive Program to check for compliance to its program requirements.


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More details on the design of windows and

glazing selection are presented in Selection

and Commissioning of Window Installations5

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, Matlabor a

spreadsheet-based program such as

RETScreen.

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

approximately equal to the amplitude of the

cyclic heat flow into the mass divided by its

surface temperature amplitude or swing.)

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.

Another approach is control of solar gains

with motorized blinds, which are widely

used in airports, atriums and somecommercial 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..


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9

Direct-gain passive solartechniques

Pure passive solar design uses the suns

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.

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. Thermalmass 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 sufficientmass, 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 tobuild the heavy structure outweighed its

operational advantage in a lifecycle analysis

of costs.

Mass is known to be able to reduce peakcooling 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.

WindowsWindow 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.

It can be calculated as a percentage of thetotal area of the south-facing exterior

wallof limited use because it is not

affected by what goes on beyond the

wallor as a percentage of heated floor

areawhich accounts for the volume of

the building.

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

Practiceswebsite, 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.

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 possiblyapproach 50 per cent when a large atrium

is included with adequate thermal storage



Figure 6 Effect of internal mass on internal temperature swings

7 Stahl, Fredrik, The effect of thermal mass on the energy during the life cycle of a building, presented at Building Physics 26th Nordic Symposium

Outdoor temperature

Light timber-framed building

Heavy building withexternal insulation

Heavy building set into andpartially covered with earth

Time of day

Airtemperature


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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:

s 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, low-emissivity (low-e) coatings to be

applied onto a protected glass surface.

Low-e coatings still allow solar gain

(short wavelength radiation) and they

help retain heat by reducing longwave

(infrared) radiation losses. This is very

helpful from a passive solar heating

point of view.

s 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.

s 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 usedin a glazing unit.

High-performance windows may make it

possible to move heating outlets further from

windows to eliminate ducting or piping.

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 beable 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.



Figure 7 Double-glazed, low-e window

8 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


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11

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 low-

conductivity material, such as wood, vinylor 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.

Warm-edge spacers use low-conductivity

materials, rather than aluminum, and areimportant 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 buildings latitude

and wall orientation. The earth rotates15 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 thesummer; a south-facing surface receives its

annual maximum in the late fall or winter.

Figure 8 shows a sun path chart for latitude

44N. The suns path varies by a projects

latitude. The X-axis gives the direction of

the sun; the Y-axis the suns 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.



Table 1 Visible Light Transmissionsolar heat gain coefficient (per cent)

Glazing system (6 mm glass) Clear Blue-green Grey Reflective

Single 8981 7562 4356 2029

Double 7870 6750 4044 1821

Double, hard low-e, argon 7365 6245 3739 1720

Double, soft low-e, argon 7037 5929 3524 1615

Triple, hard low-e, argon 6456 5538 3236 1517

Triple, soft low-e, argon 5531 5229 3026 1413

Source: ASHRAE Fundamentals 1997,Table 11, page 29

9 Website:Advanced Buildings: Technologies and Practiceshttp://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm

10 Website:Advanced Buildings: Technologies and Practiceshttp://www.advancedbuildings.org/_frames/fr_t_building_low_conduct_window.htm

11 Website:Advanced Buildings: Technologies and Practiceshttp://www.advancedbuildings.org/_frames/fr_t_building_warm_edge_windows.htm


12/34



Superimposing Figure 9Solar 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 indirectsunlight 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 Canadas

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

Figure 8 Sun path chart

Figure 9 Solar energy intensity

Figure 10 Energy striking a south window for latitude 44N

Figure 11 Energy striking a west window for latitude 44N

Adapted from Edward Magria The Passive Solar Energy Book


13/34

13



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 northfacade) 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:

s Determine the window size, height

and glazing treatments separately for

each facade.

s Maximize southern exposure.

s Optimize northern exposure.

s Minimize western exposure when the

sun is lowest and most likely to causeglare and overheating. Windows

themselves can be oriented differently

from the plane of the wall in a

sawtooth arrangement.

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.

Glare from windows can occur when theincoming 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.

Shading

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.

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 partnerof daylight, to enter the building. Light-

coloured interior shading will reflect some

of this energy back through the window.

However, a minimum of about 2030 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 possiblyto have airflow within the cavitya

double-facade.

Figure 12 Common types of exterior shading

Overhang Louvred Overhang Lightshelf Vertical Fins


14/34



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 west-

facing windows are best shaded withvertical 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 RatingER

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.

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 EnergyRating 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 CSA-

A440.2 Energy Performance of Windows

and Other Fenestration Systemsstandard.

Figure 13 Double facade in a residential building,

Klosterenga, Oslo, Norway

Figure 14 Glazed solar pacade from the outside of

Klosterenga


15/34

15



Solar cooling

Traditionally,passive solar coolingis

associated with much warmer climates than

Canadas. 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 rooflevel. 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 corefloors to store coolness. During the day,

room air is recirculated through the cool

floor to provide free cooling.

Absorption cooling involves high-

temperature solar collectors connected to

an absorption chiller operating at around

100C (212F). The device uses a solar

collector to evaporate a pressurized

refrigerant from an absorbentrefrigerant

mixture. Absorption coolers require littleelectricity to pump the refrigerant

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.

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

The Rankine-cycle cooling process is avapour 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

OverheatingOverheating 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 loadin some climates.

Solar balconies

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.

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 lesseffective 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.

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.

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


16/34



Courtyards, atriums andcommon 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.

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 re-heat 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.

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 thermosiphonsystems, common in southern Europe, that

eliminate the need for pumps.

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

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 1,600m3 to 6,300 m3 (56,503 cu. ft. to 222,482

cu. ft.).16

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

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.

Figure 15 Glazed flat-plate collector

15 http://www.cmhc-schl.gc.ca/en/imquaf/himu/upload/Fire-Safety-in-High-Rise-Apartment-Buildings.pdf16 D. Lindenberger et al., Optimization of solar district heating systems: seasonal storage, heat pumps and cogeneration, May 199917 B. Mahler et al. Central solar heat plants with seasonal storage in Hamburg and Friedrichschafen

Outlet

Inlet

Header

Tube

Box

Glazing

Inlet

The design shown isan example of a typicalliquid-cooled collector.

Air-cooled collectordesign will vary

accordingly

Bottom Plate

Absorber Plate

Note: for further information oncollector design and performance,see manufacturers specifications

Insulation


17/34

17



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

30C (86F) for outdoor and indoorswimming 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 six-

year-year payback.18 In Canada, their use is

limited to non-heating seasons.

Simple RETScreencalculations 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

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 energyand 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 verycold climates. Evacuated tube collectors are

able to provide higher water temperatures,

but are also more expensive, with longer

payback periods. RETScreencalculations

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.

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


18/34

Active solar space heating

SDHWSolar domestic hotwater 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.

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. Thesesystems 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.



Figure 17c Rooftop evacuated tube collector

Source: Architectural Graphic Standard

Figure 17a Evacuated tube collectors

Source: Natural Resources Canada

Figure 17b Evacuated tube collectors

Source: Natural Resources Canada

Evacuated tube

Glazing

Outlet

Inlet

Cross section ofevacuated tube Outer Glass Tube

Inner Glass Tube

Fluid Tube

Copper Sheet

Evacuated Space


19/34

19



Table 2 Cost and benefits of solar collectors

Collector Typical uses Advantages DisadvantagesCapital

cost $/m2Energy deliveredannually kWh/m2

Unglazed Swimming poolsEconomical, efficient at lowtemperature differentials.

Not for freezingtemperatures.

150350210250

(summer only)

Glazed DHW pre-heat Economical.Needs glycol protection

from freezing.450750 500600

Evacuated tube DHW pre-heat Provides hotter water.

Expensive; needs glycol

protection fromfreezing.

1,1001,500 800840

Figure 18 Solar domestic hot water system

Source: www.AdvanceBuilding.org

SolarCollectors

Gas orElectricWaterHeater

HotWater

to House

SolarHeatedWater

HeatExchanger

Solar StorageTank

Pump

ColdWater in

PumpController

Anti-Freeze Solution


20/34



Solar air heating

The following summary is based on Solar

Air Systems: A Design Handbook, edited by

S. Robert Hastings and Ove Mrck. The

authors looked at European and North

American applications. Cost analyses are in

Canadian dollars, unless otherwise noted.

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.

A total system can consist of any combination

of the four different components.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.

Design procedure

The Solar Air Systemsdesign handbookrecommends the following design steps.

More technical details can be found in

the guide.

s Define necessary basic data about

building and climate.

s Determine if it is possible to obtain

enough collector area.

s Determine ventilation rate through the

solar air collector.

s Determine if there are restrictions on

inlet temperature from ventilation

system.s Investigate if it is appropriate to

include storage in system.

s Define the required control strategy.

s Choose a solar collector.

s Investigate if the system may serve

other purposes.

s Determine the collector area.

s Size the ducting.

s Choose a fan.

s

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.

Table 3 Common elements in solar air-heating systems

Collector systems Storage systems Control systems Distribution

s Flat-plate collector

s Window air

collector

s Perforated

unglazed collector

(Solarwall)

s Double facades

and double-shell

collector

s Spatial collector

(atriums,

sunrooms,

greenhouses)

s Hypocaust

(ceiling or

floor slab)

s Murocaust

(wall)

s Rock beds

s Water

s Phase-change

material

s Continuous

performance

s Temperature

control

s Solar cell

control

s Timer control

s Usually

through

ducting.


21/34

21



System 1: Solar heating ofventilation air, such as Solarwall

This system provides the simplest, and

usually least costly way to bring solar-

heated fresh ventilation air into a building.

It uses mainly off-the-shelf componentsin its design. Its major disadvantage is

that it will reduce cost-effectiveness of the

buildings ventilation heat recovery unit.

An example of this type of system developed

in Canada is Solarwall, in which a south-

facing wall is clad with dark metal panels,

typically steel or aluminum, perforated with

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

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, a24-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.

Table 4 Solar heating of ventilation air

Benefits Limitations

Less cost to heat ventilation air

Recaptures heat loss through wall

May replace conventional cladding(new construction)

Conceals old cladding (retrofit)

Requires large, south-facing wall area

Reduces opportunity for south facing

glazing

Reduces the cost-effectiveness of

ventilation heat recovery (becauseowner pays less to heat incoming air)

Doesn't replace normal heating system

ConservalFigure 19 Ouellette Manor, Windsor,uses Solarwall to pre-heat corr idor

ventilation air

SolarCollector

System 1

PreheatedVentilation Air

Figure 20 System 1, solar air pre-heat system concept


22/34



System 2: Open collection loopwith 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.

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 alayer of solar-heated air. This creates a

buffer space that reduces the buildings

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 arefor glass-enclosed systems. Questions of

cleaning and maintenance for this type of

system must be addressed.

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.

Figure 21 System 2, without storage

Figure 22 System 3a, double-envelope system with storage

19 Meyer Boake, Terry et al. Canadian Architect, August 2003, p. 38

SolarCollector

System 2

Open LoopAir Circulation

SolarCollector

System 3a

Solar AirSurroundsEnvelope

CavityWall

RadiantHeat


23/34

23



There are numerous concepts for double

facade. The following example

demonstrates the heating effect of an air-

heating solar collector with a motorized

blind as the surface absorbing the solarradiation. 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 Montral.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.

Figure 23 System 3b, double-facade design option

20 Seville Theatre Redevelopment Project Integrated Design Process, CMHC Technical Series (63175) Research Highlight03-102

Open LoopAir

Circulation

System 3b

SolarFacade


24/34



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: 0.10.2 m/secw=width of the space=3.6 m,

Outdoor air temperature of -5C

L= the distance between the two skins;

Outer glazing, clear double; innerglazing single

low-emissivity coating on inner side ofouter 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.

1. For L=20 cm: for v=0.1 m/sec, the

collector air temperature will rise to

about 15C (rise of 20C) when the

blind is closed with incident solar

radiation of 600 watts/m2.

2. For L=30 cm: for v=0.2 m/sec (L=30

cm), the collector air temperature will

rise to about 5C (rise of 10C) when

the blind is closed with incident solar

radiation of 600 watts/m2.

System 4: Closed-collection loopwith radiant discharge storage

In this system, an air collector is connected

to the buildings integrated heat storage. The

air is circulated in a closed loop, normallywith 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.

Figure 24 Fresh-air pre-heating in double facade (Klosterenga, Oslo, Norway)

Figure 25 System 4, with storage

SolarCollector

System 4

Radiant

Heat

SolarCollector

ClosedLoop

ChargeMass


25/34

System 5: Closed-collectionloop 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

systems 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.

System 6: Closed-collection loopwith 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. Itrequires that the air temperature in the

system be hotter than ventilation pre-heat

systems. It is usually bulkier than liquid

systems.

System design

For more technical details, see pp 103-104

ofSolar Air Systems: A Design Handbook

s Step 1-Profile the loads

s Step 2-Select collector type

s Step 3-Decide on air mass flows

s Step 4-Specify the heat exchanger

s Step 5-Size the storage and determine

heat loss

25



Figure 26 System 5, with storage

Figure 27 System 6, with hot-water storage

SolarCollector

System 5

RadiantHeat

SolarCollector

System 6

Air to WaterHeat Exchanger

Solar PreheatedWater

SolarCollector

Mass

Open LoopDischarge


26/34


Table5Compariso

nofsixsolar-airheatingsystems

Savedenergy

110550kWh/m

2

(sunnyco

ld)

903

00kWh/m

2

(cloudytemperate

)

802

00kWh/m

2

(sunnyco

ld)

407

5kWh/m

2

(cloudytemperate

)

150400kWh/m

2

(sunnyco

ld)

100225kWh/m

2

(cloudytemperate

)

100425kWh/m

2

(sunnycol

d)

502

00kWh/m

2

(cloudytemperate

)

301

50kWh/m

2

(sunnyco

ld)

101

00kWh/m

2

(cloudytemperate

)

300400kWh/m

2

(sunnyco

ld)

120130kWh/m

2

(cloudytemperate

)

System

performance

600800kWh/m

2

Paybac

k25years

Paybac

ktime

depends

on

integrat

ion

802

40kWh/m

2

(Heatingseason)

175375kWh/m

2

Cost

Solarwal

l

$194

/m2

$200

/m2

$90

$475/m2

$250

$650/m

2

Disadvantages

Decreasedperformanceof

heatrecord

ing

unit

Co

llectormater

ialsmust

benon-toxic

Mayrequ

ireseparatevent

ilation

Relat

ivelymoreexpens

ive

Window

collectorsmayover

heatad

jacent

rooms;rockedstorage

bulky

Furn

iture

can't

beplacedagainstwal

ls

Increase

dinstal

lationcosts

becauseof

faca

desh

ell

Bul

kiert

han

liqui

dsystems

Riskoffreez

ing

inheatexchanger

Overalle

fficiencyre

duce

ddueto

temperature

dropover

heatexchanger

Advantages

Simple,

inexpensive

Lowtemperatureairusab

le

Allco

llectorsusa

ble

Systemusesstandar

dcomponents

Simple,can

beu

sedw

ithorw

ithoutstorage

Highdegreeof

integrat

ionposs

ible

,even

in

retro

fits

Usingco

llector

asparto

fbuildingenve

lope

lowersextracosts

fortheso

larsystem

Existing

building

componentsused

Cancom

binew

ithheatan

dvent

ilation

system

Nopro

blemwith

freezing,b

oilingan

dleak

ing

inco

llector

Stan

dardventilationequipmentcan

beused

Can

beusedto

heat

hotwater

insummer

System

1 2 3 4 5 6

Solar

Energy

for

Bu

ildings


27/34

27



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

overheatingtemperatures in excess of

70C (158F) 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 systemsinclude 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 theanticipated load and solar access. A

weakness of the system is that the supply of

solar energy may be intermittent.

Hybrid PV systemshave at least one

additional electricity source, such as a fuel-

fired 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.

While autonomous systems can be

immediately cost-effective in remote

locations, they are not likely to be cost-beneficial for MURBs.

Grid-connected PV systemscancel 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.

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 break-

even point for PV products in Canada usingmarket 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.

Building-Integrated PV systems(BIPVs)

A more recent trend is the development

of BuildingIntegrated Photovoltaics

(BIPVs). The PV cells are incorporated

into a building element. Currently there is

much development in PV roofing, PV

shading elements (seefigure 28) and PV

cladding or semi-transparent curtain wall

components (seefigure 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.

Figure 28 PV as window shadingelements (overhangs) at QueensUniversity, Kingston

Source Kawneer


28/34



conventional roofing and is available in

shingles, tiles and metal standing-seam

roofing (seeFigure 30). PV shading can be

effective as a window shading element,

entrance canopy or walkway shading. PVpanels 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 cur-ta in wa ll elements at the Mata rLibrary, Matar, Catalonia, Spain.

(The facade is also used for freshair pre-heating).

Sol Source Engineering

Figure 30 BIPV metal-standing seam roof,Toronto


29/34

29



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, utilityinterconnection 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 heatingsystem (PV-thermal system)

A typical crystalline silicon PV panel has

an efficiency of 1015 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

50C (122F) 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 4070

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 togenerate both electricity and useful heat

substantially improves the overall efficiency

(electrical plus thermal). The payback for a

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.

Consider a simplified analysis of a PV-

thermal systema PV panel with airflowbehind it (seefigure 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 510 per cent of incident solar

radiation is reflected but the rest becomes

heat. By bringing in fresh air through aninlet 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

of thermal power. This gives the following

energy balance on the PV panels:

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 aboutthree times the revenue of a simple PV

system on the facade.

This simplified analysis shows why

PVthermal 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 smalloverhangs to large continuous facade areas.

Figure 30 shows a double facade with PV

overhangs, in Freiburg, latitude 48o

N.

Simplified Analysis

Incident solar

radiation=reflected+ electricity+

heat transfer

to air+

heat lost

to exterior

1,000 W= 100+ 100+ 400+ 400

22 Ayoub, J., Dignard-Bailey, L. and Filion, A., IBID.


30/34



Figure 31 Double facade with PV overhangs in Freiburg,Germany

Table 6 Description of collector types

Collector type Advantages Disadvantages Capital cost $/kW Efficiency

Single crystal High efficiency High cost, fragile,

uniform look

5,00010,000 1115%

Polycrystalline High efficiency High cost, fragile,

non-uniform look

5,00010,000 1014%

Thin-film amorphous Flexible, can be applied to different

types of surfaces

Low efficiency,

degrades

58%

Spheral solar (crystalline

family)

Low cost, flexible, can be applied to

different types of surfaces.

Low efficiency 4,500 910%

Table 7 BIPV manufacturers

BIPV product Manufacturercountry

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., JapanUnited 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.


31/34

31



Summary

Passive solar is best for buildings that have

low internal heat gains and in which direct

solar gain is directed to absorbent thermal

ma

Documents

Solar Energy for Buildings