This article was originally published in the March/April 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1995
Many utilities are serious about the potential of shade trees as a demand-side management (DSM) resource. Tree planting and care partnerships with local community groups can also provide benefits beyond cost-effective energy savings. Shade tree programs offer opportunities for utilities to take civic leadership roles with respect to environmental issues, conservation education, neighborhood revitalization, and job training (see Utilities Grow Energy Savings, p.14).
Shade trees reduce solar heat gain by transferring the active heat-absorbing surface from an inert building envelope to living foliage. Because the heat capacity of leaves is low, most of this energy is transferred to the surrounding air. If ample soil moisture is present and environmental conditions are suitable, water in the leaves evaporates in a process known as evapotranspiration and the air is cooled.
The evapotranspirational cooling produced by a single tree is difficult to measure because the cool air is rapidly diffused into the larger volume of air moving through the tree crown. However, large parks or residential neighborhoods with extensive vegetation can produce air temperature reductions as great as 10deg.F compared to nearby areas with little vegetation. At this neighborhood scale, large trees increase the aerodynamic roughness of the urban canopy layer, thereby reducing wind speeds by as much as 50%. Individual trees or massings near buildings can further modify air flow near buildings. Windspeed and air temperature modifications due to the aggregate effect of trees at the scale of a neighborhood are called indirect effects. Shading of a building by trees, on the other hand, is a direct effect.
Trees can increase or decrease energy used for space conditioning. Shade is beneficial when it reduces solar heat gain during the cooling season, but detrimental in the heating season. Trees that overhang roofs can reduce radiative longwave infrared heat loss, especially to the cool summer sky at night, although the benefits of daytime shade outweigh this penalty. Wind shielding that reduces air infiltration rates is beneficial during winter and to a lesser extent in summer, but trees that block summer breezes can reduce cooling from natural ventilation. Little is known regarding the influence of trees on atmospheric humidity and latent loads inside extensively landscaped buildings.
Measuring Tree Power
A 1992 review of the research found that measured air conditioning savings from landscaping range from 25%-80%, depending on experimental design, building type, landscape design, and climate.1 Since then, two other studies have measured tree shade effects on residential energy use (see Figure 1).
In 1993, Kim Clark and David Berry placed one to six trees of 14-ft average height principally on west, but also on south and east sides of 22 homes in Phoenix, Arizona (See Targeting Residential Conservation Measures, HE Sept/Oct '94, p.14). They accounted for differences due to structural characteristics and occupant behavior to estimate electrical savings. For a dark-roofed, 1,800-ft2 house with 15% of all walls, 10% of west-facing wall, and 15% of south-facing wall in windows, they predicted average 3-5 pm summer weekday demand savings of 0.48 kW (12%) and energy savings of 4.7 kWh/day (7%). Assuming air conditioning represents 70% of peak demand and 50% of total electrical energy use, the researchers estimated air conditioning demand and energy savings of 17% and 13%, respectively.
In a 1993 study conducted in Sacramento, California, researchers from Lawrence Berkeley Laboratory placed 20 boxed trees, evenly divided between 10- and 20-ft stock, on south, southwest, and southeast sides of one of two residences, while the other was unshaded. Air conditioning energy use of both residences was monitored for 28 days immediately following a baseline period of 55 days when both were monitored without shading. Trees were then moved to the second residence and monitoring continued for another 41 days, resulting in a combined before and after parallel experimental design. Measured capacity and energy savings were 0.61 kW (25%) and 3.6 kWh/day (26%) for the home shaded for 28 days in August; 0.79 kW (50%) and 4.8 kWh/day (47%) for the home shaded for 41 days in September and October. Simulation results found minimal impact on savings estimates due to changes in season or changing weather patterns (see Urban Heat Islands, HE, May/June '94 p.16).
In general, the largest energy savings appear to be associated with more dense and extensive shade and milder climates where solar radiation is often the predominant mode of heat gain. Differences in the amount of shading were probably a major reason for the differences in savings found between Sacramento and Phoenix. In Sacramento, 20 boxed trees were used and photographs indicate shading of walls was virtually complete. In Phoenix, fewer trees (1 to 6) were used, and these were not as optimally located for shading as in Sacramento.
Difficulties associated with applying data from monitoring studies to the real world include isolating the effect of individual tress when they exist with other trees in lawns and accurately apportioning savings between shade, air temperature, and wind shielding effects. Computer simulations can overcome some of these limitations.
Computer analyses use shading simulators to account for irradiance reductions due to trees and use preprocessed weather data to account for modifications to air temperature and wind speed. Models estimate shading effects more accurately than they project air temperature reductions and wind shielding, because the former involves straightforward geometric calculations while the latter involves complex meteorological processes. We used the Shadow Pattern Simulator and Micropas 4.0 to obtain estimates of energy savings. Computer simulations for cities across the United States indicate that shade from a single well-placed, mature tree (about 25-ft crown diameter) reduces annual air conditioning use by 2%-8% (40-300 kWh) and peak cooling demand by 2%- 10% (0.15-0.5 kW). Simulations indicate that air temperature reductions associated with evapotranspirational cooling reduce annual cooling energy by 2%-8%, and reduce peak cooling by 1%-10% per tree. These findings suggest evapotranspirational cooling can produce air conditioning savings that rival those attributed to tree shade. Wind shielding from a single tree is projected to reduce annual use of natural gas for space heating by 1%-5%.
Shady Characters in California
In a study of the five climate zones found in the service territory of Pacific Gas and Electric Company (PG&E) in central and northern California (Santa Rosa, Sunnyvale, Red Bluff, Sacramento, and Fresno areas), we simulated the effects of tree shade on peak and annual cooling and heating loads with computer models. Typical 5-, 10-, and 15-year-old trees were located singly and in groups on east, south and west sides of a sin gle-story residence. We calculated peak and annual space conditioning energy use for the various shading and climate scenarios. In addition, we used three levels of wall and ceiling insulation: no insulation, R-19 in ceiling only, and R-38 in ceiling and R-19 in walls. This conservative analysis did not incorporate the effects of lower air temperatures and wind speeds often associated with increased urban tree cover.
We modeled a typical single-story frame house with characteristics consistent with California Energy Efficiency Standards (Title 24) for residential buildings as a test structure. Floor area was 1,500 ft2 (slab-on-grade) and windows were evenly distributed (16% of floor area). Gas furnace efficiency was 78%, and the air conditioner SEER (Seasonal Energy Efficiency Rating) was 10. We assumed that cooling by natural ventilation occurred when outside temperatures dropped below the thermostat set point of 78deg.F.
We used a single deciduous tree species, the Chinese Lantern Tree (Koelreuteria bipinnata), to represent all trees in these simulations, and assumed the trees blocked 85% of incoming solar radiation when in leaf from April through November, and 30% during the December to March leaf-off period. At planting (15-gallon stock) and years five, ten and 15, tree height and crown diameter were 6-, 13-, 19-, and 24-ft, respectively. The rate of growth decreased with age from 1.5 to 1 ft per year, an extremely conservative growth rate for this tree in California. We investigated the impact of shade from individual trees on building energy use for trees growing opposite east, south, and west walls. We excluded the north wall because of the negligible shading that occurs there. We also simulated the effect of two trees on the west, and two on the west combined with one on the east.
Trees shading a west exposure had the largest impact on cooling savings (see Figure 2). In Sacramento, annual savings for a single 15-year-old tree on the west were up to 15% compared to the no shade case, or 450 kWh ($50). The addition of a second tree on the west was 80% of savings from the first tree on the west; savings from east and west trees were approximately additive. Savings for younger trees decreased in proportion to tree age, since younger trees shaded less wall area than the older trees with larger crowns.
Annual cooling savings were partially offset by small negative impacts of shade that reduced winter solar access and increased heating requirements. This energy penalty was most pronounced for trees on the south, and in near-coastal climates, where increases in heating load were larger than annual cooling savings, sometimes resulting in an increase in net space-conditioning costs. Obstruction of irradiance during the heating season by trees to the south and east can be minimized by selecting solar friendly species such as redbud, green ash, and honey locust that have open crowns during the leaf-off period, drop their leaves relatively early during the fall, and leaf out in late spring.
In-leaf crown density and tree form influence the amount of building surface area shaded and air-conditioning savings. When selecting trees to maximize shade, tree form may be more important than crown density. For example, crown diameters of mature tree species can range from 10 to 50 ft, but the range of summer crown densities is relatively less, 60%-90% attenuation. A tall, narrow tree with a dense crown could produce less shade than a broad spreading, open-crowned tree in the same location. Trees with broad crowns and dense foliage provide the greatest shade.
Effects of Climate Zone
Relations between cooling savings, climate, and building insulation level were consistent. As cooling degree-days and building insulation levels increased, annual percentage savings increased and absolute savings decreased. Three 24-ft tall shade trees reduced annual air-conditioning energy use 20%-50% (300-600 kWh, or $35-$70) for residences with R38/R19 insulation, 20%-40% (600-1,000 kWh, or $70- $130) for residences with R19/R0 insulation, and 10%-20% (800-1,100 kWh, or $90-$140) for those with no insulation. Annual air conditioning energy savings (kWh) for heavily insulated buildings were about 45% of the savings for uninsulated buildings, while percentage savings were two to three times greater for insulated compared to uninsulated buildings as a result of increased relative importance of solar gain through glazing on insulated structures.
Savings (kWh) in near coastal climates were two-thirds of those in valley climates because of the shorter cooling season and relatively cooler air temperatures in near-coastal climates. Percentage savings were about 50% greater in near-coastal climates since conductive gain is smaller due to lower air temperatures, so that solar gain becomes a relatively larger portion of total cooling load.
Trees Break Wind
For residents of mid- and northern-latitude cities who pay more for space heating than cooling, wind protection from trees may be more valuable than summer shade. To evaluate potential heating and cooling savings, we ran computer simulations for five prototypical buildings: one-, two-, and three-story brick buildings similar to residences in Chicago, and one- and two-story wood-frame buildings representing suburban construction. (We validated energy performance of several prototypes by calculating building performance indices of occupied buildings using whole-house metered data and comparing results with indices of the simulated prototypes.) We simulated space heating and air conditioning savings due to tree shade, as well as evapotranspirational cooling, and wind shielding associated with a 10% increase in tree cover (corresponding to about three trees per building).
We projected annual space-heating savings of about $50 per tree (10 million Btu, 1.5%) for the three-story (six unit) buildings--$15-$20 more than air conditioning savings. Heat transfer in these large, old buildings (1930s construction) is dominated by infiltration and conduction. On the peak heating day, a single deciduous tree (36-ft tall and 24-ft wide) was projected to reduce the average building air exchange rate and air infiltration heat loss by about 8%. This suggests that trees in cities like Chicago can not only mitigate summer heat islands, but also provide annual savings in heating energy, especially for older buildings.
The relative magnitude of projected indirect and direct shade effects on cooling savings varied with building type (see Figure 4). Annual air conditioning attributed to evapotranspirational cooling exceeded savings from shade for the three-story buildings, probably because a relatively large amount of wall area was unshaded by the single tree (36-ft tall and 24-ft wide) and lower wall and ceiling insulation levels magnified the importance of a reduced exterior-interior thermal gradient. Also, lower wind speeds associated with increased tree cover reduced infiltration of hot outside air, thereby reducing air conditioning loads. Net annual cooling savings from wind shielding ($2 per tree, 2.5 kWh) confirm that given the building and modeling assumptions we used, the benefits of reduced infiltration in summer can offset reduced natural ventilation from lower wind speeds.
The two-story wood frame building, with its tight construction, large amount of window area, and high level of insulation, provides a contrast to the three-story building. We projected cooling savings from shade to be over three times greater than from evapotranspirational cooling, with most of the savings due to reduced solar heat gain through windows.
Although further research is needed to validate our models, simulation results from Chicago and 12 other U.S. cities indicate that effects of trees on air temperature and windspeed can produce significant residential heating and cooling energy savings. Trees not located to directly shade a building can still provide benefits due to their aggregate effect on urban climate. This is important in areas where dense, multi-story residential development often limits tree placement to streetsides and backyards. We found that street trees alone accounted for about one-third of total tree cover in the city of Chicago. New construction of large homes on small lots can yield a similar situation. In both cases, savings from shade are sensitive to spatial relations between street direction, building orientation, and window placement, while the aggregate effect of trees on air temperature and wind are more pervasive.
Trees Can Pique
Trees that shade west walls can also reduce peak demand for air conditioning and shift the hour of building peak to reduce the total utility system peak. As an example, Commonwealth Edison is a summer peaking utility, with electricity demand usually greatest in July or August. In 1992, peak demand occurred on July 22. Electricity demand by residential customers peaked from 6 pm to 7 pm, while total system peak occurred at 4 pm. Midday peaking by commercial and industrial users shifted the system peak from early evening to late afternoon.
In one computer simulation, the peak demand for air conditioning for a two-story brick building in Edison's service territory was 10-11 kW between 3 and 5 p.m. Shading and indirect effects associated with a single large shade tree on the west reduced the peak demand by 2 kW (19%) at 5 pm. The effect of this tree was to shave the peak between 4 pm and 6 pm and shift the building peak from 5 pm to 3 pm, or one hour before the system peak. A single 25-ft tall tree reduced the peak of the two-story wood-frame base case by 1 kW (20%) at 5 pm, but the time of building peak remained 5 pm. The brick building's responsiveness to tree shade and drybulb temperature between 4 pm and 6 pm was in part due to its relatively large amount of west-facing window area (25% of total wall area) and low amount of insulation compared to the wood-frame building.
I Think that I Shall Never See, a Heat Pump as Lovely as a Tree
If Joyce Kilmer wrote his tree poem today, he might ask if a shade tree program that provides substantial environmental, social, aesthetic, and public relations benefits has to be cost-effective to warrant utility support. Our research suggests that the economic value of trees' non-DSM benefits (removal of air pollutants, heating energy savings, reduced stormwater runoff, increased property values, scenic beauty, and biological diversity) can be two to three times greater than costs for tree planting and care. Many of these benefits extend beyond the site where a tree grows, to influence quality of life in the local neighborhood, community, and region. Although the act of planting a tree is simple, it has a multitude of consequences that we are just beginning to discover.
Shade trees that are carefully selected, located, and maintained can be cost-effective energy conservation measures. However, the DSM benefits are highly site-specific, with greatest savings in areas with relatively long cooling seasons, large numbers of air-conditioned buildings, and ample space for new tree planting.
1.Cooling our communities: A Guidebook on Tree Planting and Light-Colored Surfacing, Akbari, H.; Davis, S.; Dorsano, J.; Huang, J.; Winnett, S., Eds. 1992. U.S. Environmental Protection Agency, Office of Policy Analysis, Climate Change Division. 401 M Street, SW (PM-221), Washington, D.C. 20460. Tel: (202)260-8825; Fax: (202)260-6405.
2. The Impact of Trees and White Surfaces on Residential Heating and Cooling Energy Use in Four Canadian Cities, Akbari, H; Taha, H. 1992. Energy. 17(2): 141-149. 25 Van Zant Street, Norwalk, CT 06855. Tel: (203)853-4266; Fax: (203)853-0348.
3. The wind-shielding and shading effects of trees on residential heating and cooling requirements, Huang, J.; Akbari, H.; Taha, H. 1990. ASHRAE Transactions. 96:1:1403-1411. ASHRAE, 1791 Tullie Circle, NE, Atlanta, GA 30329, Tel: (404)636-8400; Fax: (404)321-5478.
4. Evaluating the Cost Effectiveness of Shade Trees for Demand-Side Management, by Greg McPherson, Electricity Journal, November 1993. 1932 First Ave, Suite 809, Seattle, WA 98101-1040. Tel: (206)448-4078; Fax: (206)382-0098.
5. Energy Saving Potential of Trees in Chicago, McPherson, E. G. 1994. In McPherson, E.G.; Nowak, D.J.; and Rowntree, R. (Eds.) Chicago's Urban Forest Ecosystem: Final Report of the Chicago Urban Forest Climate Project. GTR-NE-186. USDA Forest Service, Northeastern Forest Experiment Station, Radnor, PA. p. 95-115. USFS Publications Group, 359 Main Rodad, Delaware, Ohio, 43015-8640. Tel: (614)368-0127; Fax: (614)368-0152.
5. Strategic Landscaping and Air-Conditioning Savings: A Literature Review. Meier, A. 1990/91. Energy and Buildings 15-16: 479-486. Elsevier Science Inc, 655 Avenue of the Americas, New York NY, 10010. Tel: (212)633-3764; Fax: (212)633-3764.
6. Planting for Energy Conservation in Minnesota Communities, Summary report for 1991-1993 LCMR research project. Sand, M. A.; Huelman, P. H. 1993. St. Paul, MN: Department of Natural Resources, Forestry. 46 p. Contact: Pat Huelman, University of Minnesota, 203 Kaufert Lab, 2004 . Folwell Ave , St Paul, MN 55108. Tel: (612)624-1293; Fax: (612)625-6286.
7. Energy-Efficient and Environmental Landscaping, by Anne Simon Moffat, Marc Schiler, and the staff of Green Living. Appropriate Solutions Press, Dover Road Box 39, South Newfane, VT 05351. Tel: (802)348-7441.
Gregory McPherson is a project leader and James R. Simpson is a forest meteorologist, both with the USDA Forest Service's Western Center for Urban Forest Research at the University of California in Davis, California.
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