Introduction
Global warming increases the demand for air conditioning, and air conditioning contributes to global warming: a pattern called "positive feedback". As global warming increases, air conditioning increases, which in turn increases global warming. We need to think, therefore, how to minimize use of air
conditioning during hot weather.
The
purpose of this text is to help you reduce global warming in small ways
through personal behavior and increased understanding of heat transfer.
You may be interested from a purely practical standpoint. Or you may
find that the vocabulary and ideas inspire you to want to learn more.
If so, you are fortunate, because there are several relevant fields of
study: physics, biology, architecture, and engineering.
References below to cooling methods used in "very hot"
places are based on touring un-airconditioned buildings in India. You
may think the references are irrelevant for milder climates with
widespread air conditioning. But global warming may increase the
frequency of extreme weather events throughout the globe. So
observation of un-airconditioned spaces in very hot climates today may
actually lead us toward practices that will be beneficial in many parts
of the world as a means to reduce air conditioning and increase comfort
during hot weather.
Conduction
Conduction is heat transfer between materials that are in direct
contact. When you are feeling overheated, try lying down on a bare
section of the basement floor, or
drinking a glass of cold water from the refrigerator. The bigger the temperature difference
between your body and the floor or the water, the greater the rate of
heat transfer. Conduction isn't mysterious.
To cool off in hot weather using conduction, try eating cold foods such as gazpacho soup, salads, and
sandwiches.
Sometimes we want to slow down conduction. To reduce the rate of heat transfer by conduction, add insulation. Wall
insulation is helpful for keeping a house warm in the winter and cool in the summer.
Other times, we want speedy conduction. Now
that you have lain on a bare basement floor when you were too hot, try
lying on a part of the basement floor that has a rug on it. Where do
you cool down
faster? You will discover immediately that the rug is a form of
insulation.
Solar Radiation
Radiation is a type of heat transfer that can travel through a void.
The amount of radiation given off by an object increases dramatically
(to the fourth power) as the temperature of the object rises. Due to the
sun's high temperature, it gives off a lot of radiation and heats the
earth even though the sun and the earth are far apart. When direct
sunlight travels first through empty space, then through the earth's
atmosphere, and finally onto and through your window, it is likely to
heat up the room significantly.
If you can block the sunlight before it comes through the glass, that is the most powerful way to keep rooms cool that have large windows facing the sun.
Consider planting deciduous trees outside
windows facing south, west or east. The trees will grow faster than
you think, soon offering sunlight through bare branches in the winter
and shade from a full set of leaves in the summer. Ideal! If you like,
you can choose a tree that will attract insects and birds to blossoms
in the spring. Small tree seedlings are relatively inexpensive, easy to
plant, fun to care for, and fun to watch through the window.
In very hot places, there are usually shutters or louvers outside the
windows. You can experiment with an inexpensive roll-down blind that
can be hung outside a window, to see what difference it makes. When I
am desperate, during extreme heat, I have sometimes temporarily taped
newspaper sheets to the outside of my windows and found it makes a huge
difference. Through these experiments, one learns the effects of solar radiation in heating up a space. That heat is very useful during cold
weather, but not during the summer.
Shading inside the window
isn't as effective, but it's easier to install and maintain. For
interior shades, combine shade and insulation, to obtain benefits for
both summer and winter weather. Today you can easily purchase
attractive cellular shades that look like accordions, and create a
layer of still air between two pieces of cloth, reducing heat transfer
by radiation, convection and conduction in both hot and cold weather.
Some
are translucent, others opaque. Some pull up from the bottom, others
pull down from the top. They are easy to install yourself, and once
installed should last for years.
If you pay attention, you can learn a lot about radiation simply by
walking around on a hot day. A black surface exposed to bright sunlight
for a few hours becomes hot. The darker the surface, the faster it
warms up in the sun. When you walk along, notice whether a black patch
of concrete is warmer than a white patch of concrete. If you or your
neighbor has a thermal solar hot water heater, what color is the
collector on the roof? Black! Yes... Why? Have your tried wearing
black pants on a hot sunny day? White clothes will tend to keep you cooler on a sunny day. Some people choose to have white
roofs. Why might they do that?
Radiation can be stopped by a thin opaque surface, so if you want
to "get out of the sun", try moving into the shade. Does it make a
difference? It does! (On the other hand, if the surface blocking the
sun heats up to a high temperature as it absorbs sunlight, you may feel
the hot opaque object radiating energy to you.) If you want to keep
your face cool even when you walk out into the direct sunlight, how
about a hat with a wide brim?
Radiation can be reflected as
well as blocked, so some people put mirror-like films on their windows
to cool down the house by reflecting the sunlight, but it may then hit
someone else's house!
Convection
Heat can also be transferred into the house by convection, which means it comes in with the air through natural air circulation.
Attempt to keep the windows closed when the air is hotter outside than
inside. (The only exception to this--often considered by people who live
in places that get very hot--is to let hot air move on a breeze through
the house that has high thermal mass, where the thermal mass is likely
to cool down the air, and the people will cool themselves through a more
rapid evaporation of sweat in the presence of air movement. See "latent
energy", below.)
On the other hand, at night, when the air
outside has cooled down, you will find that opening up the windows
allows you to drop the temperature in the house before the outside air
heats up again the following day. The typical daily temperature swings
are called "diurnal cycles". The outside air temperature drops
gradually at night, and is generally lowest just before the sun comes
up. Even if you just get up early and throw open the windows for an
hour, you can cool the house somewhat before starting the day.
While
considering the fact that that the house can be cooled down by bringing
in cold outside air, you may wonder what makes the air move when you
open a door or window.
Remember that one stream of air can
enter a room only if another stream of air leaves. Opening windows
across from each other rather than all on the same side of the house
helps increase the flow of air. This is called "cross ventilation".
Equally important: warm air is lighter than cold air,
so hot air rises. Outside, this tendency creates winds. Inside, it create s
gentler air movements. When a window is open, the rate of air flow
through the window is affected by the general wind outside, by the
temperature differences inside the house, and finally by the difference
between inside and outside temperatures.
Opening windows
toward the top and bottom of the house tends to create air movement as
the hot, lighter, air rises out through the high openings while the
colder, heavier, air is drawn into the house through the bottom
openings. Give it a try! The taller the building, and the larger the
temperature difference between the inside air temperature and the
outside air temperature, the stronger the "thermal buoyancy" that drives
this type of air flow. If an attic happens to have exhaust fans, hot air can be drawn up through the house at night, drawing cool outside air in through the windows below if the attic door is open to allow vertical airflow.
Another benefit of the fact that hot air is lighter than cold air is
that sleeping on the floor on a hot day allows you to distance yourself
from the hottest air in the room, which will be near the ceiling.
Even
the slightest breeze outside will help the air move through your house
once windows are opened, particularly if they are opened on two sides.
You
will find that controlling for radiation and convection can be tricky.
Shades brought down to keep out the sun interfere with air flow through
the window. Fortunately the shades are most effective during the day,
while opening the windows is most effective at night. Louvered shades
may serve both purposes if the air movement isn't too brisk.
In very hot places decorative security bars may be installed so that the
windows can be opened for convective cooling without risking entry by a
person or animal who hasn't been invited.
These
are some of the principles of "natural convection", meaning air flow
without use of a fan, and "passive cooling", meaning cooling without an
air conditioner. When a fan is used to move air, it can offer many of
the same benefits, but requires energy to operate and adds a small
amount of heat to the air. Natural convection and air movement using a
fan to bring fresh outside air into a house are both forms of
"ventilation". The first is called "natural ventilation", the second,
"mechanical ventilation".
Thermal Mass
Some buildings always seem to be cool on the interior, even without air
conditioning, due to "thermal mass". The higher the thermal mass, the
more heat is required to increase the temperature by one degree.
Concrete, stone, and bricks have high thermal mass, as does soil,
particularly if it's wet. Wood, plastics, and panes of glass have lower
thermal mass. During hot weather, a basement, or a concrete building
with few windows, is likely to stay cool longer during hot weather, than
a wooden building with lots of windows. In very hot places, people who
can afford to may choose to build a home with high thermal mass, such
as thick-walled adobe, and may even place decorative concrete grills
across the windows to cool hot air as it enters the building.
Latent Heat
The most mysterious part of heat transfer for most people is "latent
heat". When water evaporates from your skin, that evaporation process
requires energy, because the molecules move faster in a gas than in a
liquid. The energy used to evaporate the water is subtracted from your
skin, producing a cooling effect. If you don't believe this, try
spraying some water onto your skin on a hot day, or putting on a wet
shirt. The cooling effect of evaporation is why many creatures,
including humans, sweat when they are hot. Amazing! If you are suffering
terribly from the heat, wet your shirt under a faucet or in a stream, then put it back on. Soon the shirt will be dry again and you will be
cool. If you prefer to simply sweat, that is fine, but do not under any
circumstances forget to drink a lot of water in hot weather,
because whether you like to sweat or not, you are likely to be
sweating. You will need to replace the water lost to sweat.
If
the air is particularly dry, as in a desert, you may be sweating a lot
without realizing it, because the sweat will evaporate easily, cooling
you effectively. If the air is humid, sweat evaporates more slowly from
your skin, so you will feel hotter on a humid day than on a dry day at
the same air temperature. In hot weather, wear clothes that breathe easily
so that your sweating can do its work. White cotton is handy in hot
weather because it handles both latent energy (breaths well) and radiation well for
staying cool. White linen is also effective.
When you drink a lot of water, you will want to be sure you get enough electrolytes.
Ask an athlete about which might be the most effective electrolytes
for you. It may be enough to eat an apple, raisins, banana, or celery,
but it's an important enough topic that it's worth consulting an expert
directly. At the very least, eat some salty food in very hot weather.
If you lick your arm (but not sun screen!) after exercising on a hot
day, you will find it tastes salty. Sweat naturally includes salt.
When the water in the sweat evaporates, it leaves behind the salt, which
can form a little invisible crust on your skin and/or your clothes.
Eat some salty foods during hot weather so that the salt lost to sweat
can be replenished.
You may wonder why having a fan on your desk makes you more comfortable
when the room around you is hot. The fan doesn't have an air
conditioner. It's just moving the air around in circles. So what's it
doing exactly? Well, for one thing, it's consuming electricity and
adding that energy ultimately as heat to the air, so turn off the fan
when you're not near enough to feel the air movement. The benefit of
a table fan is that it increases the rate at which sweat evaporates
from your skin (and your clothes if they are wet). So even though the
air temperature hasn't dropped, and the amount of radiation hasn't
changed, the latent cooling has increased, as you will discover for
yourself by standing in front of a fan when you are sweaty or wearing a
wet shirt. Heat transfer between your body and the air increases with
the velocity of the air against your skin even if your skin is dry,
without the latent cooling effect, but the latent cooling effect is the
stronger of the two.
Opening windows during cool periods of the night and early morning not
only cools the house by bringing in cold air using natural convection.
It also increases the rate of "latent" cooling by increasing the air movement, or velocity, and thereby increases the rate at
which sweat evaporates from your skin, particularly if one of the open
windows is near your bed.
When you bicycle on a hot sunny
afternoon along a road without trees or tall buildings, you may find you
are hotter than when you bicycle through a wooded area. In fact, I've
found the difference quite startling. The trees also the bike
path, reducing the radiation reaching you from overheated concrete. The
amount of radiation given off, from the sun or from a surface on the
earth, is strongly dependent on temperature. This we've already covered
under "radiation". However, there is another reason you will be cooler
bicycling through a wooded area than in the open: plants transpire.
Some of the water taken up through the roots exits through pores in the
leaves and evaporates into the air, producing a latent cooling effect.
The soil around the trees may be somewhat moist and evaporating, further contributing to latent cooling in wooded areas. Walk around a
bit on a hot day, paying attention to the surfaces and trees and bushes
and grass; see whether you agree with the above observations or discover
a different perspective.
Biology
Some
people like hot weather while others do not. How mysterious! There
could be many reasons, including whether or not their homes and offices
are easily kept cool in hot weather. But it also seems that over time
people exposed to hot weather slowly become adapted so that the
heat bothers them less, in the same way that people exposed to cold
weather over time become less bothered by cold weather, and less able to
handle heat. So it's not just through sweat that the body is clever in
handling heat; there are also ways the human body creates longer term
adaptations.
All Together
Thermal
comfort is influenced by many factors including conduction, radiation,
thermal mass, convection, latent energy, and biology. It is difficult
to talk about each of these factors in isolation. The amount of
temperature increase from solar radiation is determined largely by the
thermal mass of the building. Convection increases cooling of the skin
by the latent energy transfer from the skin. The amount of insulation
in the walls affects the rate of conduction through the wall, which
moderates the temperature of the interior wall surfaces, which in turn
determine the amount of radiation emitted from the wall into the house.
One doesn't think about this much, but hot interior walls can make a
person feel warmer. In short: it's not just the temperature of the air
in the room, as measured by the thermostat, that determines the degree
of thermal comfort. Understanding the physics, engineering and
architecture of design for passive cooling can be both interesting and
of practical use in everyday life as we seek to slow down global warming
while at the same time we must cope with its effects.
Written in Seattle, Washington, USA, June, 2021,
during a hot spell that produced outside air temperatures up to 108F.
from Thought Tools Timpani
pw 7.2.21
