McPherson : Tree Shade and Energy Use POTENTIAL OF TREE SHADE FOR REDUCING RESIDENTIAL ENERGY USE IN CALIFORNIA by

Electric utilities in California currently sponsor planting of approximately 75,000 yard trees annually as an energy conservation measure. In this study we evaluated the potential effects of tree shade on residential air conditioning and heating energy use for a range of tree orientations, building insulation levels and climate zones in California using computer simulation. Trees shading a home's west exposure produced the largest savings, both annual (kWh) and peak (kW), for all climate zones and insulation levels considered. Next largest savings were for southwest (annual and peak) and east (annual only) locations. Three trees (two on the west, one on the east side) reduced annual energy use for cooling 10 to 50 percent (200 to 600 kWh, $30 to $110) and peak electrical use up to 23 percent (0.7 kW). Except in climates with little air-conditioning demand, cooling load reductions were always greater than increased heating loads associated with shade from south side trees in winter. Air-conditioning savings, both peak and annual, were larger in warmer climates and uninsulated buildings; percentage savings were larger in cooler climates and for more energy efficient buildings. Recommendations are made regarding locating yard trees to maximize energy savings. Strategically placed shade trees affect energy consumption for residential space conditioning by reducing solar gain. In summer, decreased solar thermal gain resulting from direct tree shade can reduce energy used for air conditioning. In winter, reduced solar access, even from leafless trees, can increase heating requirements. Nationwide, it is estimated that $10 billion is expended annually for residential cooling [1]; approximately $500 million is spent in California alone [4]. Therefore, even small percentage changes in cooling loads due to trees can have potentially large dollar impacts. The effect of tree shade on cooling and heating loads depends on factors such as climate; tree size, shape and shading coefficient; direction and distance of trees relative to buildings; type, size and vintage of building construction, and occupant behavior [13]. (Shading coefficient refers to the fraction of solar radiation blocked, ranging from 0.0 when all solar radiation is blocked to 1.0 when all is transmitted). Measurements of shade effects on space conditioning have thus far been limited primarily to single buildings or scale models, due largely to the time and expense involved with monitoring large populations [19]. A review of four measurement studies with a range of experimental designs, building types, landscaping, and climates found measured air-conditioning savings ranged from 25 to 80 percent [19]. Larger savings were associated with more dense and extensive shading, and milder climates where solar radiation is often the predominant mode of heat gain. More recently, Akbari et al. [2] measured peak demand and energy savings of 0.61 to 0.79 kW (25 50%) and 3.6 4.8 kWh/day (26 47%) for a heavily shaded home in Sacramento. Clark and Berry [5] found weekday demand savings during the summer of 0.17 kW (7%) for houses with air conditioners, and 0.35 kW (14%) for houses with air conditioners and evaporative coolers, in Phoenix, Arizona. The latter study measured whole house electrical use for 175 homes; a subsample of 24 had an average of 3 trees (24 inch box) planted nearby after data were collected without trees. Average peak (3 to 5 p.m.) whole house electrical demand for the entire sample was 3.55 kW; percentage savings were based on the assumption that air conditioning represents 70 percent of peak demand (K. Clark, Arizona Energy Commission, 1994, personal communication). Factors that influence building energy use can be investigated by computer simulations. These show that shade from a single well-placed, mature tree (about 25 ft crown diameter) would reduce annual air-conditioning use by 2 to 8 percent (40 300 kWh) and peak cooling demand by 2 to 10 percent (0.15 0.5 kW) for cities across the U.S. [1,9,10,11,15,18,21,22]. Journal of Arboriculture 22(1): January 1996 11 Reduced solar access from winter shading can be appreciable even for deciduous trees, potentially increasing energy used for space heating [7]. Computer simulations [26] found that shade from street trees positioned 15to 35 feet from a building's south side often increased annual (kWh) building energy use for combined space conditioning and hot water heating compared to the same building without trees. The effect was most pronounced for energy efficient homes with heavy window shading and solar-assisted hot water heaters than for more conventional homes, and for the cooler of the five California climate zones considered. This paper extends Thayer and Maeda's analysis to 11 California climate zones, considers yard trees in a variety of orientations around a building, includes effects of building energy efficiency, and considers peak (kW) and annual (kWh) energy use. Shade on buildings from various tree configurations is calculated; results are used in an energy use simulation model to predict the effect of the tree shade on space conditioning. Electric utilities in California currently sponsor planting of approximately 75,000 yard trees annually as an energy conservation measure, stimulating interest in quantifying their energy impacts. Information from studies of this type should suggest strategies useful for reducing peak generating requirements of electric utilities, developing energy-efficient landscape incentives for new construction, planning landscape retrofits for existing residential and small commercial buildings, and producing educational materials that deal with the design of energy efficient landscapes. Arborists, urban foresters, landscape professionals, utilities and utility sponsored nonprofits can employ the results as a basis for practical siting and selection guidelines for energy conserving trees. Methods Computer simulations. The relative effects of various landscape tree configurations on space conditioning use were determined from computer simulation. Shading of buildings by trees was determined using the Shadow Pattern Simulator (SPS) program [17]. SPS calculates hourly tree shade for each wall and roof surface based on building and tree sizes and their relative orientations and distance from buildings. MICROPAS ver. 4.01 [6] uses building thermal characteristics, weather data and information related to occupant behavior (described subsequently), combined with SPS results, to provide hourly estimates of building energy use. Energy savings are determined by comparing predictions for identical unshaded (base case) and shaded buildings. Since simulations may overestimate measured energy use [3], base case results were checked by comparison with residential energy use data available for Sacramento [2,25]. Dollar savings were calculated using local utility rates of $0.12/kWh for zones 2, 4, 11,12 and 13 (except Sacramento County), $0.08/ kWh for Sacramento County, $0.11/kWh for zone 7and$0.13/kWhforzones8, 9, 10, 14 and 15. MICROPAS determines the peak day (defined as the day with the maximum hourly energy use value) based on calculated daily building energy use and California Energy Commission weather data. Peak demand or capacity (kW) is important to electric utilities in terms of providing enough generating capacity to meet demand, which is due to air conditioner operation for summer peaking utilities. Peak savings were calculated on the peak day for the hour with the greatest energy use; this occurred between 1300 and 1600 hours here (i.e., between 1:00 and 4:00 p.m.). Weather data. Hourly shading impacts were simulated for residential structures in representative cities (Table 1) in 11 of California's 16 climate zones (Figure 1). These climate zones were developed by the California Energy Commission as part of its building energy efficiency standards, and are based largely on average dry bulb temperature [12]. Representative weather information for these zones is included with MICROPAS; average monthly and annual dry bulbtemperatures are given in Table 2. Zones selected here had utility sponsored tree planting programs and significant cooling seasons (Table 1). The range of climates considered represent many cooling dominated climates in the United States, except for the hot/humid southeast. Hence, energy and demand savings presented as percentage differences from corresponding unshaded base cases here are expected to be representative of a wide 12 Simpson & McPherson: Tree Shade and Energy Use Table 1. Climate zone, base case annual (kWh) and peak (kW) electrical energy use for cooling, and peak and annual cooling savings summary for two trees on the west and one on the east for the energy efficient home. Climate Major zone city