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Energy Savings
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definition
In U.S. cities with populations over 100,000, peak utility loads increase 1.5-2.0% for every 1°F (0.6°C) increase in summertime temperature. Steadily increasing downtown temperatures over the last several decades mean that 3-8% of community-wide demand for electricity is used to compensate for the heat island effect.
Installing cool roofs and cool pavements, and planting shade trees and vegetation are sensible steps that communities can take to decrease urban air temperatures and summertime energy use.
How Do Heat Island Reduction Measures Reduce Energy Demand?
Heat island reduction measures can directly and indirectly reduce the need for summertime air conditioning. For example, installing cool roofs and strategically planting shade vegetation around homes and buildings directly reduces cooling loads. Cool roofs reflect the sun's energy, while trees and other vegetation prevent it from striking windows and walls. Both strategies decrease heat transfer into homes and buildings. This lowers the need for indoor air conditioning, which saves consumers money on summertime energy costs.
Cool roofs, cool pavements, and trees and vegetation
also result in
indirect
energy savings
. Cool roofs and cool pavements, which have higher
solar reflectance than traditional options, minimize
heat transfer to the surrounding air. Trees and
vegetation keep surfaces cooler by providing shade, and
can lower air temperatures through evapotranspiration
(the evaporation of water from leaves). These effects
indirectly contribute to reduced demand for summertime
air conditioning.
For more information on opportunities for building-level energy savings, see What Can Be Done .
What Are the Energy Savings Where You Live?
The magnitude of potential energy savings from heat island reduction measures varies from city to city across the country. However, almost all U.S. communities can realize some level of savings by taking action to reduce summertime urban temperatures. There are a number of factors that influence the extent to which heat island reduction measures will result in energy savings where you live:
- In general, the larger the area using heat island reduction measures, the more substantial the overall energy reductions;
- Cities where the summer season is long, sunny, and hot will also typically achieve substantial energy savings, as well as emission reductions and peak electric power avoidance;
- The concept of
cooling
degree days
is another useful way to assess annual cooling
energy demand in a particular city or region. A
higher number of summertime cooling degree days makes
it likely that heat island reduction strategies can
save both energy and money for building owners and
residents.
While these factors provide an indication of the potential for energy savings in a city, other variables also affect the demand for air conditioning. Weather conditions such as humidity and cloud cover are important, and in individual buildings, installing energy efficiency measures can increase savings.
How Much Energy Can U.S. Cities Save?
As shown in the graph, annual energy savings in the three metropolitan areas ranged from $4-15 million (1997 dollars). Results for all three cities suggest that implementing reduction measures would also result in significant peak power avoidance and a reduction in annual CO 2 emissions.