Assessing climate to improve solar design

8 Pages·2001·0.676 MB·English

by  Energy Efficiency and Renewable Energy Clearinghouse (U.S.), National Renewable Energy Laboratory (U.S.)

DOE/GO-120001-1171
FS122
August 2001
Assessing Climate
to Improve Solar Design
Sunlight, weather patterns, and microcli- Designing for Climate
mates (the climate of a small area) affect
Taking into account the characteristics of
the performance of solar energy systems.
sunlight at a given location can greatly
The more direct sunlight a system receives,
enhance solar design. The amount of sun-
the more electricity or heat it produces. In
light, or solar radiation, reaching a specific
contrast, clouds, fog, frost, rain, and falling
site depends on latitude, time of day, and
snow reduce the amount of energy that
time of year. Generally speaking, locations
can be harnessed from the sun. Water and
nearer the equator receive more sunlight
vegetation can make a solar energy system
throughout the year than places at higher
work better or worse, depending on where
latitudes. In the United States, for exam-
they are located. Solar designers assess cli-
ple, the southern states receive more solar
mate to figure out how to work with spe-
radiation than the northern states.
cific environmental conditions to enhance
the performance of solar energy systems.
Two cities at the same latitude may vary
in the amount of sunlight they
receive during the year. Acity
that is often cloudy or rainy
will receive less sunlight than
one where the sun shines
nearly every day. Clouds scat-
ter and absorb solar radiation,
creating diffuse sunlight. Dust,
smoke, pollen, and suspended
water droplets in the atmos-
phere also scatter and absorb
the sun’s rays. Diffuse sunlight
contains less energy than direct
sunlight, which travels unim-
peded through the atmos-
phere. On cloudy days, most
or all solar radiation is diffuse.
On a clear day, about 85 per-
cent of the sunlight striking
the Earth is high-energy,
1 direct radiation. Locations
6
3
02 that enjoy clear, sunny days
X
L/PI throughout the year have the
E
NR best solar resources. In the
etz, United States, the Southwest
Gr
n is one of the world’s best areas
e
arr
W
Clouds are the predominant atmospheric condition that
determines the amount of solar radiation reaching the earth.
This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. The
document was produced by the Information and Outreach Program at NRELfor the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency
and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL/ DOE. The statements contained herein are based on
information known to EREC and NRELat the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC.
Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste
for sunlight. This region receives nearly Three factors are responsible for variations
twice as much sunlight in a year as other in the amount and quality of sunlight
areas of the country. Other parts of the striking the Earth: (1) the Earth is round,
world with lots of sunlight include desert (2) the Earth revolves around the sun in
and high plains regions of Asia, Africa, an elliptical orbit, and (3) the Earth rotates
and Latin America. on a tilted axis. Because the Earth is
round, the sun strikes the surface at differ-
You can get a general idea about the qual- ent angles ranging from 0°(just above the
The southwestern ity of sunlight in your community from horizon) to 90°(directly overhead). When
solar radiation maps. Bear in mind, how- the sun’s rays are vertical, the Earth’s sur-
United States receives ever, that the amount of sunlight available face gets all the energy possible. The more
for a solar energy system also depends on slanted the sun’s rays are, the longer they
nearly twice as much other factors, such as the angle of the solar travel through the atmosphere, becoming
system collector with respect to incoming more scattered and diffuse. Because the
sunlight in a year sunlight. Remember, too, that sunlight is Earth is round, the frigid polar regions
affected by the weather, microclimates never get a high sun. Because of the tilted
as other areas such as a foggy river valley, and nearby axis of rotation, these areas get no sun at
tall buildings, trees or shrubs, hills, and all during part of the year.
of the country. fences that block incoming sunlight. Even
building structures such as overhangs can Because the Earth revolves around the sun
shade a solar collector (or aperture). in an elliptical orbit, it is nearer the sun
Good design considers all these factors. during part of the year. When the sun is
nearer the Earth, the Earth’s surface
Solar Radiation Basics receives a little more solar energy. The
Earth is nearer the sun when it is summer
The evaluation of the solar resource for a
in the southern hemisphere and winter in
specific project requires understanding the
the northern hemisphere. However, the
basic principles of solar radiation. Every
presence of vast oceans moderates the
location on Earth receives sunlight, at least
hotter summers and colder winters one
part of the year. However, the amount of
would expect to see in the southern hemi-
sunlight reaching the Earth varies accord-
sphere as a result of this difference.
ing to geographical location, time of day,
and season.
The United States receives far mor solar radiation in summer, when the sun is nearly overhead,
than in winter when the sun’s rays are more slanted.
2
1
9
4
5
0
Assessing climate can PIX
L/
E
R
N
help you better define ns,
o
ars
P
system requirements d
avi
D
for solar heating and Active and passive solar features incorporated into the design of this off-the-grid Colorado
home provide all of its energy.
solar electric systems.
The 23.5°tilt in Earth’s axis of rotation is a Solar Resource Assessment
more significant factor in determining the
Calculating the amount of sunlight avail-
amount of sunlight striking the Earth at a
able for a solar energy system is complex
particular location. Tilting results in
and challenging. Atmospheric conditions,
longer days in the northern hemisphere
the changing position of the sun, and local
from the spring (vernal) equinox to the
terrain interact to moderate the solar
fall (autumnal) equinox and longer days
resources at a given time and place. Solar
in the southern hemisphere during the
engineers rely on historical data about
other 6 months. Days and nights are both
weather, climate, and air pollution to
exactly 12 hours long on the equinoxes,
design a solar energy system for a specific
which occur each year on or around
site. They must consider the sun’s position
March 23 and September 22.
throughout the year. They must take care
to use published information about solar
Countries like the United States, which lie
radiation that applies to tilted, south-fac-
in the middle latitudes, receive more solar
ing collectors. And they must understand
energy in the summer not only because
not only how seasonal weather patterns
days are longer, but also because the sun
affect sunlight, but also how they affect
is nearly overhead. The sun’s rays are far
solar energy system requirements.
more slanted during the shorter days of the
winter months. The result is a big differ-
Whether sunlight is direct or diffuse can
ence in the amount of direct sunlight avail-
significantly affect solar energy system
able for a solar energy system. Cities like
performance. Solar electric systems
Denver (near 40°N latitude) receive nearly
require direct sunlight. Collectors for solar
three times more solar energy in June than
heating systems, including passive solar
they do in December. That's why winter is
buildings, capture both direct and diffuse
so much colder than summer.
solar radiation. In some areas, solar
energy systems will capture significant
The rotation of the Earth is responsible for
amounts of sunlight reflected from nearby
hourly variations in sunlight. In the early
white surfaces such as snow, light-colored
morning and late afternoon, the sun is low
sand, or salt flats. Solar designers need to
in the sky. Its rays travel further through
account for such surface reflection effects
the atmosphere than at noon when the sun
where they exist.
is at its highest point. On a clear day, the
greatest amount of solar energy reaches a
solar collector around solar noon.
3
Designing according
to your climate can
also help you keep
your home from
over heating in the
summer.
Obtaining specific data on the solar resources in your area will help you design the most
effective PV system for your home.
Scientists measure the amount of sunlight resource maps of the United States.
falling on specific locations at different The maps are available for both horizontal
times of the year, and then estimate the and tilted surfaces for each month of the
amount of sunlight falling on regions at year. (See "Resources" below to obtain
the same latitude with similar climates. basic information and maps.)
Measurements of solar energy are typi-
cally expressed as total radiation on a System Requirements
horizontal surface. Radiation data for pho- and Climate
tovoltaic (PV) systems, which produce
Assessing climate can help you better
electricity, are often represented as kilo-
define system requirements for solar heat-
watt-hours per square meter (kWh/m2).
ing and solar electric systems. Seasonal
Direct estimates of solar energy may also
weather patterns affect heating and cool-
be expressed as watts per square meter
ing requirements as well as the solar
(W/m2). Radiation data for active and
resource. In winter, heat loss is affected by
passive solar energy systems, which
the outdoor, or ambient, temperature. The
produce heat, are usually represented
colder it is outside, the faster a solar collec-
in British thermal units per square foot
tor will lose heat, becoming less efficient.
(Btu/ft2). The following
Low temperatures also increase heat loss
equation allows you to con-
from buildings. The lower the daily aver-
vert from one system of
age outdoor temperatures, the greater the
measurement to the other:
heating requirements will be.
317 Btu/ft2 = 1 kWh/m2.
Winter heating requirements usually
You can obtain basic infor-
increase when there is less solar energy
mation about the nation’s
available. For this reason, engineers often
solar resource from the
1
004 National Renewable Energy design solar heating systems to capture the
0
PIX Laboratory in Golden, maximum amount of energy from the win-
EL/ ter sun. Then they can apply the principles
R Colorado, or the National
N
etz, Climatic Data Center in of climatic design to keep the system from
n Gr Asheville, North Carolina. producing too much heat in the summer.
arre The National Renewable For example, a roof overhang can be
W
designed to shade south-facing windows
Energy Laboratory also
Aresearcher measures solar radiation at the in the summer.
provides solar radiation
NationalRenewable Energy Lab.
4
People use more electricity in the winter certain areas, increasing heating require-
when days are shorter. Solar electric sys- ments and reducing the amount of solar
tems produce more electricity in the sum- energy available in the winter months.
mer when days are longer and there is The solar designer who factors in such
more direct sunlight. Detailed information considerations will reap dividends in
about your climate can help you figure out terms of improved system performance.
whether a solar electric system makes
sense for you. If you live in the southwest- Winds can affect how a solar system per-
ern United States, for example, the answer forms. It is important to analyze whether
may well be "yes." This region enjoys a nearby hills, trees, and buildings funnel
significant amount of direct sunlight year- winds into or away from a particular site.
round, and its wintertime heating require- In winter, cold winds can accelerate heat
ments are moderate. Your answer may loss from a building, increasing solar heat-
also be "yes" if you want electricity for a ing requirements. In summer, cool breezes
The microclimate summer cabin in Montana that is miles can help disperse heat from a well-
from the nearest power line. By carefully designed building, making it more com-
of a building can positioning your array of solar (PV) cells, fortable and reducing cooling
you should be able to capture enough requirements.
make or break a solar direct sunlight to run your lights and a
small refrigerator. Using climate assessments in design
project. involves more than creating a building
Adetailed discussion of solar resource, or solar system within the context of a
heating-, and electric-system requirements specific environment. Solar engineers can
is beyond the scope of this fact sheet. use landscape design and building struc-
These requirements vary throughout the tures themselves to enhance solar system
United States and are specific to a particu- performance or to keep buildings warmer
lar project. Excellent software for deter- in winter and cooler in summer. For
mining solar energy system capacity instance, overhangs and eaves can shade
requirements is available on the Internet windows when solar heating is not
from Web site resources listed at the end desired. Landscape designers can strategi-
of this publication. cally place deciduous trees and shrubs
outside to help keep buildings cooler and
Microclimates more comfortable during the warmer
months. Courtyards filled with trees, flow-
The microclimate of a building site can
ering vines, shrubs and a large fountain
make or break a solar project. Shadows
can create a cool oasis even in the hottest
cast by nearby buildings, trees, or hills are
desert climate.
important considerations in orienting a
solar collector or design-
ing a passive solar build-
ing. So, too, are local
conditions that predis-
pose a site to fog or an
unusual amount of
snow. For instance, low-
lying areas near rivers
and lakes may have an
early morning fog that
burns off before noon. In
such a location, a solar
9
5
designer might want to 00
1
orient a collector to take PIX
L/
advantage of the clear RE
N
afternoon sky. Similarly, ni,
local weather patterns celli
may result in frequent ul Tor
a
and larger snowfalls in P
Overhangs shade windows in hot climates or in summer.
5
Software tools give
homeowners access to
high-technology mea-
surements of solar
radiance in various 02
1
4
0
X
PI
regions. L/
E
R
N
etz,
Gr
n
e
arr
W
This instrument in Billings, Oklahoma measures total hemispheric irradiance.
Design Assistance patterns. All you need to do is fill in your
exact location (latitude and longitude).
Climates can play an important role at
Other resources for climate and design on
four stages in the solar energy system
the Internet include:
design process: 1) the evaluation of the
solar resource, 2) the definition of system
• Learning tools for newcomers to
requirements, including the impact of
green design
climate-related factors on system design,
• Green design checklists
3) the analysis of existing site-specific
• Strategies for finding the best locations
microclimates, and 4) the design of sys-
for new solar energy systems
tems, buildings, and landscapes to maxi-
• Strategies for reducing the time and
mize performance. Until recently, solar
cost of pre-feasibility studies
designers had to perform a series of com-
• An easy-to-use method for estimating
plex calculations to determine the size of
the performance of PV systems
the solar resource, figure out heating and
connected to the U.S. power grid
cooling requirements in different parts of
the country at different times of the year, • Aprogram that calculates the location
and decide the optimal size and orienta- of the sun in the sky—any place, any
tion of a solar heating or electric system. day, any time.
Once this information was obtained,
designers then integrated it into plans Ongoing efforts in countries around the
for a new system or building and the world to develop and refine new design
landscape surrounding it. assistance resources will continue to sup-
port the use of climate assessment in
Today, the design process is made easier design and other improvements to solar
because of the number of resources avail- energy system design.
able on the Internet or on CD-ROM. Inter-
net resources offer something for
everyone, from the amateur home builder
to the experienced solar designer. Several
programs can provide you with a detailed
analysis of your local solar resource along
with information on long-term weather
6
Resources Fax: [phone]
E-mail: [email protected]
The following are sources of additional information on
Web site: http://www.ncdc.noaa.gov/
the role of sunlight and microclimates in solar energy
system design. The list is not exhaustive, nor does the The world's largest active archive of weather data. Produces
mention of any resource constitute a recommendation or numerous climate publications and responds to data requests
endorsement. from all over the world.
Ask an Energy Expert National Renewable Energy Laboratory (NREL)
DOE Energy Efficiency and Renewable Energy 1617 Cole Blvd.
Clearinghouse (EREC) Golden, CO 80401
Web site: http://www.nrel.gov
P.O. Box 3048
The U.S. Department of Energy’s premier laboratory for
Merrifield, VA22116
renewable energy research & development, and a lead
Phone: 1-800-DOE-EREC (1-800-363-3732)
lab for energy efficiency.
TDD: 1-800-273-2957
Fax: [phone] Renewable Resource Data Center
E-mail: [email protected] E-mail: [email protected]
Web site: http://rredc.nrel.gov
Online submittal form:
http://www.eren.doe.gov/menus/ Provides maps and information on solar radiation,
energyex.html biomass, geothermal, and wind energy resources.
Consumer Energy Information Web site: The Center for Buildings and Thermal Systems
http://www.eren.doe.gov/consumerinfo/ Web site: www.nrel.gov/buildings_thermal
Energy experts at EREC provide free general and technical Provides information on energy-efficient buildings.
information to the public on many topics and technologies
Sustainable Buildings Industry Council (SBIC)
pertaining to energy efficiency and renewable energy.
1331 H Street, NW, Suite 1000
DOE Energy Efficiency and Renewable Energy Washington, DC 20005-4706
Network (EREN) Phone: [phone]
Fax: [phone]
Web site: http://www.eren.doe.gov/
E-mail: [email protected]
Acomprehensive online resource for DOE's energy Web site: http://www.sbicouncil.org
efficiency and renewable energy information.
Promotes the use of energy-efficient and passive solar
building design and construction.
Organizations
American Solar Energy Society, Inc. (ASES) Web Sites
2400 Central Avenue, G-1
Building Energy Software Tools
Boulder, CO 80301
DOE Office of Building Technology, State and
Phone: [phone]
Community Programs
Fax: [phone]
Web site:
E-mail: [email protected]
http://www.eren.doe.gov/buildings/tools_directory/
Web site: http://www.ases.org
Describes many energy-related software tools for buildings,
Anational advocacy organization dedicated to the use
with an emphasis on renewable energy and energy efficiency.
of solar energy in buildings.
National Climatic Data Center
Federal Building
151 Patton Avenue
Asheville, NC 28801-5001
Phone: [phone] (Continued on page 8)
7
(Continued from page 7) Reading List
The Climatic Dwelling: An introduction to climate-responsive
Dynamic Maps of Monthly Direct Normal residential architecture,E. Cofaign, J. Olley, and J. Lewis.
Solar Radiation London: James & James Ltd., 1996. Phone: [phone]
8558; http://www.jxj.com.
NRELCenter for Renewable Energy Resources
Web site: http://maps.nrel.gov/
CRC Handbook of Energy Efficiency,F. Kreith and R. West,
Online atlas of monthly solar resource maps includes a editors. Boca Raton, Florida: CRC Press, Inc., 1997.
user demonstration. Phone: [phone]; http://www.crcpress.com/us/.
Energy Design Tools Energy and Climate in the Urban Built Environment,
Department of Architecture and Design M. Santamouris. London: James & James, Ltd., 2000.
University of California, Los Angeles Phone: [phone]; http://www.jxj.com.
Web site: http://www.aud.ucla.edu/
The Green Vitruvius – Principles and Practice of Sustainable
energy-design-tools/
Architectural Design,E. Cofaign, E. Fitzgerald, R.
Features downloadable software to help design McNicholl, and J. Lewis. London:
energy-efficient homes, including those concerning James & James Ltd., 1999. Phone: [phone];
solar and climatic design. http://www.jxj.com.
PV Watts: APerformance Calculator for The Role of the Sun in Climate Change,
Grid-Connected PV Systems D. Hoyt and K. Shatten. New York: Oxford University
NRELRenewable Resource Data Center Press, 1997. Phone: [phone];
Web site: http://www.oup-usa.org.
http://rredc.nrel.gov/solar/codes_algs/PVWATTS
Solar Radiation and Daylight Models for the Energy Efficient
Provides non-experts with performance estimates for grid- Design of Buildings (with Software Compact Disk), H.
connected PV systems. Kambezidis, T. Muneer, and P. Tregenza. Oxford, UK:
Butterworth-Heinemann, 1997.
Solar Position and Intensity
Phone: [phone]; http://www.bh.com.
NRELRenewable Resource Data Center
Web site:
http://rredc.nrel.gov/solar/codes_algs/solpos/
Provides a code that computes the sun’s position and
intensity in the sky. Includes links to other solar and climate
analysis tools.
Solar Radiation Data Manual for Buildings
NRELRenewable Resources Data Center
Web site: http://rredc.nrel.gov/solar/pubs/bluebook
Provides methods for calculating solar radiation and
deriving climatic data.
Solar Radiation Data Manual for Flat-Plate
and Concentrating Collectors
NRELRenewable Resources Data Center
Web site: http://rredc.nrel.gov/solar/pubs/redbook
Provides methods for calculating solar radiation for
flat-plate and concentrating collectors.
8