It's a cold December day in England and the
temperature outside is maybe 4°C (39°F). Even so, I'm sitting by the
window in the Sun feeling really quite warm (and I have
outside in the sunlight that I expect will be 50–75 percent done when
I bring them indoors later on). What I'm making use of here is
passive solar energy: my body is capturing the hidden heat in
sunlight, storing it in its mass, and allowing it to distribute
itself to other parts of my body that are not directly in the heat of
the Sun. Architects design buildings the same way to capture, store,
and distribute solar energy so they have lower heating and
air-conditioning costs and reduced environmental impact. How do
passive solar buildings work? Let's take a closer look!
Photo: This eco-friendly home in Idaho Springs, Colorado is designed around passive solar
technology, including large areas of sun-facing glass and a Trombe wall (explained below) to trap sunlight. Overhangs provide shade and prevent overheating in summer. Photo by Warren Gretz courtesy of US DOE/NREL (NREL photo id#26343).
The easy answer is "energy from the Sun." But sun
light is actually a mixture of light and heat,
and the light itself is a mixture of different frequencies of
including invisible ultraviolet (the sunlight that gives you sunburn)
and infrared (the invisible light you feel as radiated heat if you
stand near something like a camp fire or barbecue). The great thing about solar energy
is that it's plentiful and free. In theory, every horizontal square meter of
Earth receives about 1kW of power from the Sun
(assuming overhead Sun at midday and a cloudless sky): that's 1000 joules of energy
every single second, which adds up to a huge amount over the course
of a (sunny) day. Colder, cloudier countries receive much less energy
than this, but even correcting for latitude, seasons, and cloud, dull
countries like Britain can still usefully access at least 100 watts
per square meter (according to the calculations of physicist David MacKay).
Artwork: At noon on a cloudless day, 1000 watts (1 kilowatt) of solar
power hits one square meter (roughly a square yard) of Earth. An even more impressive way to frame this (according to
physicist Richard Muller's Physics for Future Presidents, p.78) is that there's potentially a gigawatt of solar power in a square kilometer of land—which the same as you'd get from a typical nuclear plant.
What can we do with solar energy? Broadly, we can
capture it with two different approaches known as active and passive
solar. Active solar means things like photovoltaic solar cells (which
turn sunlight into electricity) and roof-mounted solar hot-water
systems (which capture the sun's heat in water and use a
heat exchanger system to store it in a tank for baths and showers).
Passive solar generally means capturing and trapping the Sun's heat
inside a building—and that's what the rest of this article will look
at in more detail.
Photo: If you have a conservatory like this, you'll be well aware of passive solar energy. Unless you fit it with blinds or low-e windows.
What is passive solar?
You can buy homes off the shelf—prefabricated
boxes that you put together like self-assembly IKEA furniture.
Prefabricated buildings are, by definition, identical and they look and work the same,
whether you build them in Finland, Germany, Mali, or Japan. But a
passive solar home is a completely different concept: key to the
design is the idea that the house is geared to a very specific local
climate (and a particular location within it). At its very simplest,
that would mean lots of large, sun-facing windows (south-facing in the northern
hemipshere, in somewhere like the UK or the United States, or north-facing in
a southern-hemisphere country such as Australia). "Passive" solar
means what it says: unlike solar panels and solar-thermal water
heating, it uses no electrical or mechanical devices to move heat or
light through the building. Instead, the building is designed to soak
up, store, and distribute energy naturally.
Passive solar buildings are meant to be
environmentally friendly. There would be no point in designing a
building that saved 75 percent of its winter heating costs if that same design led to a 300 percent
increase in air conditioning expenses in summer. So an
essential aspect of passive solar design is achieving year-round
effectiveness. Typically, that means being able to screen intense, overhead sunlight
(or otherwise reduce its effects) in the heat of summer.
Basic elements of passive solar building design
It's generally agreed that there are several
distinct aspects to passive solar design. Broadly speaking, they boil
down to capturing heat from the Sun, storing that heat, transmitting
or releasing the heat gradually (especially at night or after the Sun
has gone down), and finally (another separate aspect) preventing the
building from overheating on really hots days (particularly in summertime). Let's consider these in
1. Capturing heat
There are three basic ways of catching the Sun's
heat, known as direct, indirect, and isolated solar gain.
Direct gain essentially means that the
Sun's heat streams directly, through one or more large sun-facing
windows (sometimes referred to as the aperture), into the
parts of the building where the heat is required (the main daytime
living areas rather than the bedrooms or kitchen, which you most
likely want to be cooler). The greater the area of glass, the
higher the gain—that's why passive solar homes are generally
characterized by huge windows. The windows typically need to be
double-glazed to ensure that the building not only traps heat but
retains it when the Sun has gone down.
Indirect gain means that the Sun's energy
is captured by a wall or window that doesn't directly lead into the
living area. Inside, the energy is trapped and feeds slowly into the
living areas (and the rest of the home) by conduction, convection,
and radiation (the three modes of heat transfer through
solids, liquids, and gases). One of the best known examples of indirect gain
is the Trombe wall and consists of
a window that admits light onto a really thick, dark-colored wall. The wall heats up very gradually
and stores the solar energy that it releases slowly into the house
for some hours afterward (generally in the evening and at night). In
another design, you might have a wall made of pipes in which water
sits, soaking up the solar energy and slowly releasing it to the
house; a rooftop pond would work in a similar way.
(Water has a very high specific heat capacity, which means each liter of water is
capable of storing a very large amount of heat energy. That's why
it's used as the heat-transfer fluid in central-heating and
Photo: Indirect gain: A Trombe wall is like a cross between a window and a wall: it's
a wall with a glazed surface that heats up and releases its energy into the building behind. Photo by Paul Torcellini courtesy of US DOE/NREL. (NREL photo id#26485)
Isolated gain means the building has a
sun-trap of some kind built onto the side—maybe a conservatory,
solarium, greenhouse, or other Sun space. It's built almost entirely
of glass, heats up quickly, and stores its energy in its floor or
walls, slowly releasing it to the rest of the building.
Photo: Isolated gain: The solarium on the side of this office building captures heat and slowly feeds it to the building it's attached to. Photo by Warren Gretz courtesy of US DOE/NREL.
(NREL photo id#56223)
2. Storing heat
The Sun might seem like a spotlight in the sky,
but it doesn't shine consistently, from the same inclination or
direction, all day and all night; passive solar homes need to be able
to store up daytime solar energy and slowly release it in the cooler
evenings, nights, and early mornings. How? They need large walls or
floors with a high thermal mass, made from dense, solid
materials such as brick, stone,
concrete, blocks of compressed earth,
adobe, straw bales, or lighter materials filled with something
like water that can store large amounts of heat per unit of volume.
Often (but not always), thermal mass is painted black or colored darkly so that it
absorbs the maximum (and reflects the minimum) amount of solar heat
energy falling upon it. It's not just walls and floors that provide a
building's thermal mass: something like a heavy stone or brick
fireplace, stone pillars inside a room, or even very heavy wooden
doors could also do the job. Ideally, thermal mass will release the
heat captured by a passive solar building for 6–10 hours afterward,
until the Sun comes around again. The time between a thermal mass
absorbing heat and releasing it is known as its thermal lag.
Chart: Cooling effect: An effective thermal mass delays the peak temperature and reduces both the maximum and minimum temperatures inside a building. The blue line shows the rising and falling outside temperature (assuming it's exactly the same over several days). The orange line shows the effect of a small thermal mass. The red line shows how more thermal mass has a bigger effect.
Since thermal mass is designed to release heat
over a long period, it's important that it's insulated against heat
losses so it doesn't lose energy too quickly, especially in cold
climates. That could mean insulating the mass itself (maybe lining the
underside of a stone floor), but it also means insulating the
building as a whole with, for example, double or triple glazing, air
locks, roof insulation, or more sophisticated systems such as
(which lets fresh air into a building without allowing too much heat to escape)
or low-e windows
(also referred to as low-emissivity or heat-reflecting glass).
Photo: Thermal mass provides temporary heat storage in a passive solar building,
but it doesn't have to be as lumpen and unattractive as it sounds. Here's an attractive example of
walls and flooring with high thermal mass from the Visitor Center at Zion National Park.
You'd never know that one of its primary functions is to store solar heat. Photo by Robb Williamson courtesy of US DOE/NREL. (NREL photo id#26697)
3. Moving heat
In a centrally heated home, water from a furnace or boiler
(usually powered by natural gas, electricity, oil, or some other
fuel) is pumped around a continuous circuit of pipe, through
radiators, to keep the entire building warm. Ideally, passive solar
buildings don't use things like boilers and pumps to move heat, or
even air ducts and blowers; instead, the heat captured by glazing and
stored by thermal mass has to move around the building by the natural
processes of conduction (heat flow between solid materials
that are touching one another), convection (heat flow through
the movement of air), and radiation (where hot objects give
off heat by emitting infrared radiation).
4. Winter-summer balance
Keeping a passive solar building cool in summer is
just as important as keeping it warm in winter. That's why passive
solar buildings feature such things as eaves/overhangs
(carefully designed to prevent too much hot, high-level, summer Sun
entering a building without cutting off the all-important low-level
winter Sun), as well as temporary, adjustable devices such as blinds,
awnings, or shutters. Adjustable ventilation obviously also plays an
important part, though devices like air vents, exhaust fans, and
opening windows can drastically compromise the effectiveness of the
building in winter if they increase "air leaks" that allow heat
to escape accidentally.
Photo: Overhangs above these windows allow low winter sunlight to penetrate and heat the building, but shade out hotter summer sun. Note how the south side of the building (which receives intense daytime sun) has overhangs, but the north side (which receives little or no direct sun) omits them. Photo by Warren Gretz courtesy of US DOE/NREL. (NREL photo id#16849)
Planning for passive solar
Apart from catching, storing, and moving heat, the
general design of a passive solar building is also hugely important.
The architect would need to decide which rooms require most heating
in winter and arrange those close together so they soak up most
daytime sunlight and pass it on to one another, in turn, by
conduction (direct contact) and convection (air movement). A living
room would be a prime candidate for direct solar heating, while a
kitchen or dining room would more likely want indirect heat from
thermal mass in the evenings. Bedrooms and dining rooms need less
When people talk about passive solar,
super-modern, eco-friendly, architect-designed buildings spring to
mind. But it's important to remember that passive solar principles
can be applied to existing buildings as well. You can add something
like a conservatory to your home, add more glazing to capture more
solar energy (or use double, triple, or low-e glazing), increase the
solar mass or the insulation in the main living areas, or simply use
the rooms in your home in a different way (maybe living in the
upstairs rooms instead of the downstairs ones, or the front of the
house instead of the back, to take advantage of greater winter
sunlight). These things are simple examples of maximizing passive solar energy.
Advantages and disadvantages of passive solar buildings
A passive solar building is environmentally
friendly and economical and should prove cheap to run all year round.
It doesn't have to be hugely expensive; the basic principles are
simple and in an ideal passive solar building, there are no noisy,
expensive, and even potentially dangerous furnaces or pumps
requiring maintenance or refueling. Having large areas of glazing
makes a home light and bright, while open-plan interiors (designed to
improve air and heat movement) give a feeling of spaciousness.
Passive solar works anywhere on Earth (even at the poles, where
there is sunlight for at least half the year!) and can be applied to new or existing buildings.
It's hard to think of disadvantages, except where
passive solar principles are applied with absolute, idealistic,
environmental zeal. Even then, the worst that's likely to happen is
that, left to the mercy of the climate, the building could prove too
hot in summer or too cold in winter. That's why knowledge of the local
climate is particularly important when you design a passive solar building from scratch. There's nothing to stop you
having a partly passive solar building with other, more
conventional forms of heating and air-conditioning to fall back on if
you want to.
Photo: By definition, passive solar homes are light, bright, and sunny, which
makes them very attractive living spaces. This is a home designed by Auburn University
for a competition called the Solar Decathlon house, in which university and college studies try to build the most attractive and economic eco-friendly homes. Photo by Warren Gretz courtesy of US DOE/NREL.
(NREL photo id#105567)
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