From: Essential Information <ei@igc.apc.org>
Newsgroups: sci.energy
Date: 31 Dec 92 12:42 PST
Subject: Re: Energy Ideas - Thermal Env.
Nf-ID: #R:cdp:1466300127:cdp:1466300129:000:11987
Nf-From: cdp.UUCP!ei    Dec 31 12:42:00 1992



WINDOWS - A NEW LOOK AT AN OLD SCENE

Known as "glazing," glass areas on a building shell are a
necessary part of a pleasant work environment.  Offices with
windows are, almost without exception, considered to be the most
desirable.  Being able to see out and to fill an office with
natural light makes office space feel more relaxed and natural. 
Windows also provide unique opportunities to use sunlight to
reduce lighting and heating loads.  Unfortunately, most buildings
today suffer from poor glazing design, resulting in increased
heating and cooling loads.  New design techniques and glazing
products have been developed to use sunlight to increase building
energy efficiency.

Providing Light with Daylight

Smart buildings can take advantage of sunlight to reduce their
electricity consumption for lighting.  According to the U.S.
Department of Energy (DOE), more than 20 times the light required
to light building interiors is available from natural sunlight. 
Conventional window placement does not take full advantage of the
available light.  Simply placing windows in outer, or perimeter,
offices only illuminates those offices.  However, direct sunlight
which enters those windows will also increase the room
temperature.  This is helpful in winter but it adds substantially
to cooling loads in summer.  Traditionally, interior offices have
not been lit by windows.  

New glazing techniques provide lighting to all offices while
avoiding overheating from direct sunlight.  These technologies
are very cost-effective in new construction.  However, the design
techniques require integration with the building's layout and
heating, ventilating and air conditioning systems.  Consequently,
these retrofits are extremely costly.  The notable exception to
this is the replacement of inefficient windows.

Atria

One new building design, the atrium, provides a bright and
pleasant central area for office buildings and also creates a
light source for interior offices.  An atrium is an area in the
center of a building which is open from the ground floor to the
roof and is illuminated by natural light.  Interior offices with
windows facing an atrium need lower levels of electric lighting. 


Light Shelves and Clerestories

Perimeter offices may be illuminated by new window arrangements
which admit diffused and reflected light while blocking direct
light (which causes unwanted heating and glare).  Light shelves
project outward from windows in perimeter offices.  Located about
a third of the distance from the top of the windowthey reflect
light through the upper portion of the window.  A light-colored
ceiling will scatter this light, illuminating the office without
heating it.  Clerestories are skylights where the glass has been
mounted perpendicular to the roof (see photo on page two). 
Mirrors on the roof reflect light into the window and the inner
surface of the clerestory diffuses the light and directs it
downward into the building.  The clerestory may also contain
photocells to activate artificial lights when natural light falls
below a certain level.

New Developments

Two new technologies can transport light into interior office
spaces.  The first, light "pipes," reflect sunlight along their
length, similar to a optical fiber, and allow some light to
escape with each reflection.  These pipes are excellent light
sources for hallways and cubicle offices.    The second,
holographic films, refract light to change its direction.  A
holographic film on a window can make light appear as if it is
shining directly into the room regardless of the angle at which
it strikes the window.  Thus, even incidental light can be
projected 30 feet into a hallway or office space.

Controlling Heat Transfer through Windows

All objects emit light at different frequencies; the hotter the
object, the more light emitted at higher frequencies.  Relatively
cool objects, such as ourselves, sun-warmed asphalt, warm rooms
and other objects near 80 degrees fahrenheit, primarily emit
infrared radiation (IR), which we experience as heat.  All
objects seek a state of equal radiation; the net result of this
radiation is that cooler objects will get warmer.  Since glass is
transparent to IR, warm rooms will lose heat through radiation in
winter and cool rooms will absorb IR heat in summer.

IR is not the only way that glass facilitates energy transfer. 
Energy may be transferred by visible light as well.  When
sunlight shines directly into a room, all of the energy
associated with that light is absorbed by dark surfaces in a room
(carpeting, desks, chairs, etc.).  These surfaces get warm and
radiate heat in the room.  This will reduce heating loads in
winter, but it will increase cooling loads in summer.  

Lastly, windows allow heat transfer by conduction.  Glass has a
very low thermal resistance.  While a fiberglass-insulated wall
typically has an R-value (a measure of thermal resistance) of 15,
called "R-15," single-paned glass windows have R-1 or, at best,
R-2.  Thus, these windows allow warm rooms to lose heat in winter
and cool rooms to gain heat in summer.

Low-Emissivity Windows Block IR

Low-emissivity (low-E) windows minimize the entrance of infrared
radiation into a room while permitting the passage of visible
light.  Emissivity is a material property.  An object with a low
emissivity has a highly reflective surface; it neither absorbs
nor emits radiation.  A low-E window will bounce radiant heat
back to its source; warmth in a room on a cold day will be
trapped inside while warmth outside on a warm day will be kept
outside.  According to Lawrence Berkeley Laboratory (LBL), these
windows cost about $2/square foot and will pay back their
investment in two to six years, depending upon climate and energy
costs.  Low-E films may also be placed directly on existing
windows as part of an efficiency upgrade.

A new type of window, an electrochromic window, has recently been
developed which could reduce a building~s energy costs by 30 to
50 percent.  According to a September 29, 1992, article in the
New York Times ("Researchers Develop ~Smart~ Window to Cut Energy
Consumption," p. C4), the electrochromic window uses an electric
current to react to heat and light changes.  The window consists
of seven very thin layers: the two outer layers coat the window;
the next two layers function as positive and negative terminals
for current flow through the window; the next two layers contain
a metal which changes color in response to an electric current;
and the central layer contains an ion conductor.  As the two
metals change color in response to a electric current, the amount
and type of radiation which passes through the window may be
controlled.  These electrochromic windows may be placed between
two panes of glass to increase further the insulating value of
the window.  

Casting Shadows Can Also Save Energy

Light-blocking structures can prevent the entrance of direct
sunlight through windows.  Exterior blinds prevent light from
striking a building during the day (though that eliminates the
view).  According to Energy & Economics: Strategies for Office
Building Design (see Resources, page seven), these blinds add
very little to the construction cost of a building (less than 1
percent) and can reduce energy consumption by 10 percent.  At a
modeled cost of $60,000, the estimated payback time for a
60,000-square-foot building is six years.  Other less expensive
devices, such as awnings,  may be more cost-effective, depending
upon their effectiveness at blocking direct sunlight.  Exterior
window plants provide shade in summer and allow sunlight in
winter.  Interior blinds are not as effective in preventing heat
gain, since the heat has already passed through the window before
it strikes the blind, though they can prevent heat loss at night
in cold climates.

Improving Window R-Values

The thermal resistance of windows may be improved by adding panes
of glass and by filling the space between the panes with a
non-conducting gas.  Multi-paned or storm windows filled with an
inert gas (such as xenon, krypton or argon gas have an R-value (a
measurement of thermal resistance) of R-6.
Windows may also have "aerogel" placed between the individual
panes.  Aerogel is a solid and transparent material made of
nearly pure silica which contains millions of microscopic air
cavities.  These cavities increase the resistance of the
material.  Just one-half inch of aerogel between two panes of
glass can increase the R-value of a window by 500 percent, making
a window more resistant to heat conduction than a wall.

CASE STUDIES:

Mt. Airy Public Library, North Carolina
In 1982, the town commissioners of Mt. Airy, North Carolina,
sought to construct a library which would consume 70 percent less
energy than a conventional building.  By using clerestories
across the top of the library, the building provides glare-free,
diffuse light to all corners of the library without allowing the
stacks to be directly illuminated, thereby preventing damage to
the books from sunlight.  The building design also incorporates
insulation and air lock areas (see pages 5-6) and a zoned system
of heat pumps.  Electricity used for lighting accounts for only
one-eighth of the energy consumption in the building.  The
library uses 90 percent less energy than the Mt. Airy City Hall,
a building of comparable size (about 13,000 square feet).  The
construction cost was $88 per square foot as compared to $79 per
square foot for a conventional building, but was within the range
of other innovative and uniquely-designed libraries.  After
construction, the library was found to use 53 percent less energy
that was estimated for a conventional design.  (Contact: Ralph
Cooke, 838 Cross Creek Drive, Mt. Airy, NC  27030).

Abrams Elementary School in Bessemer, Alabama
In designing a replacement for a condemned elementary school, the
Board of Education in Bessemer made quick energy-saving
adjustments to conventional designs.  Since heating and lighting
account for almost 70 percent of the energy used in a typical
elementary school, the school was designed with modified
clerestories.  These shine light directly onto water-filled
plastic pipes placed in the ceilings of the school~s 20 hexagonal
teaching areas.  These tubes intercept and diffuse light.  Other
energy-saving measures led to the construction of a building
which consumes 60 percent less energy than a similar, non-solar
building.  (Contact: Stuart Wells, Passive Solar  Industries
Council, 1511 K Street NW, Suite 600, Washington, DC  20005,
(202) 371-0537.)
Elementary Schools in Laveen, Arizona
As a part of a major energy retrofit, in 1991, Laveen replaced
inefficient, leaking aluminum windows in one school with a glass
block window wall system.  The project also added insulation to
the roofs and walls of the building.  The school superintendent
reported that the improvements in the quality of lighting, air
distribution, air temperature and noise level in buildings has
improved the learning environment.  The projects were repeated in
two additional buildings in the summer of 1992.  (Contact: Dennis
Umber and Associates, 5535 Osborn Road, Suite 203, Scottsdale, AZ 
85251 (602) 423-8474.)

Office Complex in Southfield, Michigan
In 1992, The Northland Towers Office Complex added a blue low-E
film to the outside of 39,000 square feet of window space on
their two towers.  The previous film on their glass, which
accounts for 90 percent of the building~s exterior, had begun to
peel.  The project cost $69,960.  While reflective film lowers
the cooling load in summer, it increases the heating load in the
winter because the reflective film prevents sunlight from
entering and warming rooms.  The net annual savings are 
estimated at $22,571, resulting in a payback of 2.8 years. 
(Contact: Chris Pedigo, Executive Vice-President, Northland
Towers Management, 15565 Northland Drive, Suite 510 West,
Southfield, MI  48075, (313) 569-3180.)