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14. Use of windbreaks for home energy conservation
Agriculture, Ecosystems and Environment, 22/23 (1988) 243-260 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
243
14. U s e of W i n d b r e a k s for H o m e E n e r g y Conservation DAVID R. DEWALLE 1and GORDON M. HEISLER 2 1Institute for Research on Land and Water Resources and School of Forest Resources, The Pennsylvania State University, University Park, PA 16802 (U.S.A.). 2Northeastern Forest Experiment Station, U.S.D.A. Forest Service, University Park, PA 16802 (U.S.A.)
ABSTRACT DeWalle, D.R. and Heisler, G.M., 1988. Use of windbreaks for home energy conservation. Agric. Ecosystems Environ., 22/23: 243-260. Windbreaks are effective in reducing energy needs for home heating by reducing air-exchange rates in homes. Air exchange in homes is caused by pressure differences caused either by temperature differences between inside and outside air or wind forces on the exterior surfaces of the home. Windbreaks are only effective in reducing air exchange caused by wind forces. Thus, windbreaks are more effective in saving energy in windy climates. Available data indicate that annual energy savings for home heating using windbreaks can be up to 10-15% in the northeastern U.S.A. and 15-25% in the northcentral U.S.A. Economic analyses indicate that windbreaks are cost effective with the value of heating energy savings exceeding costs for windbreak establishment and maintenance over a 20-year windbreak life.
INTRODUCTION T h e p u r p o s e of t h i s c h a p t e r is to discuss t h e use of w i n d b r e a k s to c o n s e r v e e n e r g y u s e d for s p a c e h e a t i n g in h o m e s , w i t h special r e f e r e n c e to u r b a n a n d s u b u r b a n settings. T h e effects of w i n d b r e a k s on m i c r o c l i m a t e a n d w i n d b r e a k a e r o d y n a m i c s as well as use of w i n d b r e a k s a r o u n d f a r m s t e a d s h a v e b e e n discussed in p r e v i o u s c h a p t e r s ( H e i s l e r a n d DeWalle, 1988; M c N a u g h t o n , 1988; W i g h t , 1988). T h i s c h a p t e r deals specifically w i t h h e a t - e x c h a n g e p r o c e s s e s in h o m e s a n d h o w to utilize w i n d b r e a k s to reduce e n e r g y u s e d for h e a t i n g of t h e i n t e r i o r air s p a c e to t h e b e s t a d v a n t a g e . S p a c e c o n d i t i o n i n g ( h e a t i n g a n d cooling) of h o m e s is a m a j o r use of e n e r g y ( M c P h e r s o n , 1984 ). E n e r g y use in t h e r e s i d e n t i a l s e c t o r r e p r e s e n t s a b o u t 17% of t h e t o t a l e n e r g y use in t h e U.S.A. N e a r l y t w o - t h i r d s of t o t a l r e s i d e n t i a l energy use is for h e a t i n g ( 5 3 % ) a n d cooling ( 1 2 % ) . H e n c e , r e s i d e n t i a l s p a c e c o n d i t i o n i n g r e p r e s e n t s a b o u t 11% of t o t a l U.S.A. e n e r g y use.
0167-8809/88/$03.50
© 1988 Elsevier Science Publishers B.V.
244
To reduce energy used for heating or cooling, windbreaks must influence air exchange, heat conduction or solar radiation transmission. Air exchange is the simultaneous passage of air into and out of a home through cracks around windows and doors, porous materials, open doors and windows, and other openings to the outside. Heat conduction is the passage of energy from molecule to molecule through the solid portions of the structure from hot to cold sections. Solar radiation transmission is the entry of solar energy through glazed surfaces such as windows. Air exchange is the heat-transfer process most affected by windbreaks. The authors will draw upon their windbreak research experience from several experiments conducted using mobile homes (DeWalle and Heisler, 1983; Walk et al., 1985; Heisler, 1986) and supplement that with information from other studies. AIR EXCHANGE IN HOMES
Air exchange in homes, typically represents one-third of the heat loss from a home in winter and can represent over half of heat loss on selected days. Thus, if windbreaks can be used to reduce air exchange, considerable energy savings could result. Air exchange occurs when air is forced or drawn through openings in the house by pressure differences. From a fluid mechanics viewpoint the volume flow rate of gas through an orifice can generally be computed as (Albertson et al., 1960):
Q=CcCvAo~p
(1)
where Q is the volume flow rate of air, Cc is a contraction coefficient, Cv is a velocity coefficient, Ao is the cross-sectional area of the orifice, Ap is the difference in pressure in the fluid across the orifice and p is the fluid density. The coefficients C~ and Cv vary with the geometry of the orifice. Orifices or openings in the exterior shell of a home vary widely in geometry and location, with no two homes exactly alike. Pressure differences also vary in magnitude and from positive to negative values over the exterior shell of the home. Pressure differences are caused by forces of the wind over the exterior surfaces of the home and by temperature differences between inside and outside air. Surfaces of the home facing the wind will experience increased pressure as windspeed increases and air will enter the home through openings in these surfaces (see Fig. 1 ). Passage of air into the home will force an equal mass of interior air out of the home. The location of air entry and exit is dependent upon wind direction and home characteristics. Windbreaks reduce air exchange by reducing wind-pressure forces on upwind home surfaces. Air infiltration is sometimes used to refer to air exchange, but air infiltration ignores the fact that there is also concurrent air exfiltration occurring in the
245
Hot air out
Air in
aI
"1
wind effect
I
•
I
"
Air out
C o l d ' C o l d air
.n.,
"-'-- If
,X. j o
air
~ - I "-"
Temperature difference effect
Fig. 1. Air exchange in homes occurs because of pressure differences between inside and outside air created by wind forces and temperature differences in winter (DeWalleand Heisler, 1980b). Windbreaks only reduce air exchange caused by wind forces. home. Air exchange will be used generally in this chapter, unless we are referring to a specific author's use of air infiltration. To understand wind-induced air exchange better, it is useful to consider the vertical and horizontal distribution of air pressure over the surface of a home. Typical wind-tunnel results for pressure distributions over a single model home with a gable roof and with the long dimension perpendicular to the wind are shown in Fig. 2. Pressures are expressed as a pressure coefficient (Cp) which is computed as the ratio of the pressure difference (Ap) between inside and outside air at a point on the house surface to the dynamic pressure of the wind measured upwind from the model home. Dynamic pressure may be computed as 1/2 pu 2 where p is the fluid density and u is the upstream speed. Infiltration of outside air will occur through any openings in the structure where a positive Cp is found, while exfiltration of interior air occurs through openings in regions with negative Cp. The mass flow rate of air into the structure over areas with positive Cp is equal to the concurrent mass flow rate out of the structure over negative Cp areas. In Fig. 2, positive Cp values occur over all of the front wall and part of the front roof surface. Negative Cp values are found on the side walls, the lee wall, the lee roof surface and part of the front roof surface. Such pressure distributions are dictated by the shape and orientation of the structure to the wind. The distribution and type of openings in the structure will also play an important role in determining the actual air-exchange rate. If it is assumed that openings are uniformly distributed over the structure, a dimensionless air-infiltration parameter (AI) can be computed for model homes in a wind tunnel as (Harrje et al., 1982): M +
AI= ~ CpkAk/Achar
(2)
k=l
where the product of each measured pressure coefficient (Cpk) and the area represented by each pressure measurement (Ak) is summed over the number
246 VERTICAL DISTRIBUTION
½ .~101
( ~ WIND DIRECTION
Cp
-1 0
-1 0 1 Cp
-1
0
1
Cp
HORIZONTAL DISTRIBUTION -1
Cp -1 0 1
LEEWALL LEEROOF
0 Cp 1 Cp 1 0-1 ~
.'ii
FRONTROOF WIND DIRECTION I FRONTWALL
-0 Cp
Fig. 2. Vertical and horizontal distribution of pressure coefficients (Cp) over the surface of a model home in a wind tunnel. Positive Cp indicates potential areas for infiltration of exterior air into the structure, while negative C, indicates potential areas for exfiltration of interior air (adapted from Buckley et al., 1978).
(M ÷ ) of measurement points with positive Cp and divided by a characteristic area (Achar). The characteristic area chosen by Harrje et al. (1982) was the area of the front wall of the structure. The AI parameter is an index of windinduced air exchange in model homes and is not a true air-exchange rate. However, it is very useful in translating wind-tunnel test data into practical results and will be referred to later in this paper. Because warm air rises, temperature differences between inside and outside air will also create a natural circulation of air in the home (Fig. 1). Warm interior air in winter will rise and flow out of the home through openings near the top of the home, while cold outside air is drawn into the home through lower openings. This process of air exchange is known as the "chimney" or "stack" effect. Windbreaks are not effective in reducing this type of air exchange. In summer, cooler, denser interior air can sink to create a reverse type of flow. However, temperature differences in winter are generally greater than in summer. For example in the U.S.A., temperature differences between interior and exterior air can reach 50°C (90°F) in winter, but seldom exceed 30°C
247 12
AIR = - 0 . 0 7 8 8 - 0 D 9 9 4 ( ~ T ) - 0 . 3 0 6 8 ( U ) + 0.1080(U z) r 2 = 0.61
A T = -35°C AT = -25°C
I0
L~T = - 1 5 ° C
zs ¢J
~6
4
2
0
i
i
4
i
t
6
i
t
8
i
i
I0
WIND VELOCITY IN OPEN, rn/s
Fig. 3. Relationship between air-exchange rates in a mobile home and climatic variables, illustrating that temperature differences largely control air exchange below windspeeds of 3-4 m s - ] (DeWalle et al., 1983 ).
(54 °F) in summer. Thus, temperature-induced air exchange in winter should be greater than in summer, unless openings are modified through use of storm windows, plastic sheeting or some similar device. Air exchanges caused by temperature differences and by wind often occur simultaneously and generally are subadditive (Sinden, 1978). That is, windinduced air exchange may actually reduce exchange caused by temperature differences. However, the degree of interference will be unique to each dwelling because of variations in the type and distribution of openings and building shape. Consequently, models relating air exchange to windspeed and insideoutside temperature differences cannot be generalized. One such relationship for a small mobile home (Fig. 3) indicated that reductions in windspeed at low speeds where temperature-difference effects predominated would be quite ineffective in reducing air-exchange rates. At higher windspeeds, e.g. greater than 4 m s -1, windspeed reductions produced with windbreaks would be much more effective. Rapid increases in air exchange with increasing windspeed, at speeds greater than 4 m s- 1 are probably caused by wind-pressure forces which vary with the square of windspeed. Because each home is unique, this example only serves to illustrate what can happen. Generalizations on windspeeds at which windbreaks can be effective for other homes cannot be made. The influence of turbulence in the air on air-exchange rates is not well understood. Generally, air-exchange rates in houses have only been related to mean horizontal windspeeds rather than some measure of turbulence level. Because turbulence levels may be altered in the wake zone downwind from windbreaks (see Heisler and DeWalle, 1988), an understanding of turbulence
248 effects on air exchange may eventually improve the ability to describe windbreak effectiveness. Wind direction is also important in controlling air exchange in homes. In homes where the distribution of openings in the exterior shell is not equal on all sides or surfaces, e.g. in an interior row townhouse, air exchange rates for a given windspeed and temperature will be sensitive to wind direction (Malik, 1988). This is an important consideration in placement of windbreaks such that surfaces of homes with openings through which air exchange will occur, are protected. Air-exchange rates in homes currently cannot be estimated with confidence. For design purposes air exchanges are generally assumed to be 0.5 change per hour for tightly sealed homes, 1 change per hour for a typical home, and 2-3 changes per hour for poorly-sealed homes without storm windows or weather stripping. Where more precise information is needed, exchange rates are generally measured using a tracer gas-dilution methodology (ASHRAE, 1985; Hunt et al., 1980). In homes without significant sources or sinks for water vapor, a simple method utilizing commonly-available humidity-control equipment can be employed (DeWalle and Heisler, 1980a). In summary, air exchange in homes is a result of pressure differences caused by wind forces and temperature differences. Windbreaks can only be used to influence air exchange caused by wind forces. Thus, the windbreak effect on air exchange is controlled by the balance between wind and temperature-difference air exchange for a specific home and climatic condition. WINDBREAKEFFECTS ON AIR EXCHANGE Several wind-tunnel and field studies have been conducted to determine windbreak effects on air exchange in homes. An early wind-tunnel study of pressure distributions over the surface of a 50-mm high model home with and without a wire-screen windbreak was conducted by Blenk and Trienes (1956). They showed variations in pressure distributions with distance between the house and windbreak and with wind direction. Minimum mean pressures, and presumably minimum wind-induced air-exchange rates, were found with the windbreak at 4 h upwind (h = windbreak height) from the house for a 2 h long windbreak and normal winds. For winds departing from normal, mean pressures on the house increased rapidly and at an angle of 40-60 ° from normal, no windbreak effect was measured. Mattingly and Peters (1977) conducted wind-tunnel studies with small model houses to determine windbreak influence on wind-induced air exchange computed from pressure coefficients as previously indicated in eqn. (2). They found the most effective windbreak tested was a row of simulated conifer trees, slightly taller than the model home, located about two house heights upwind. Windinduced air exchange in the model home was reduced by about 42%.
249 In a later study with a scale model of a two-story house with a gable roof, model conifer trees were used to test effectiveness of various arrangements of trees and windbreaks in reducing air exchange with wind perpendicular to the tree row (Harrje et al., 1982). They found maximum wind-induced air-exchange reductions in the model house occurred when windbreaks were located 2-4 tree heights upwind. Other results observed were that air exchange decreased linearly with increased model tree height, air exchange decreased in a nearly-linear fashion with increasing density in a single-row windbreak, only marginal additional air-exchange reductions were obtained with multiple-row windbreaks compared to single-row windbreaks, and model trees with pruned trunks were much less effective than those with foliage to ground level. Several field tests of shelter effects on air exchange have also been conducted. Mattingly et al. (1979) conducted a field test to confirm previous windtunnel results by erecting a 30-m long, single-row windbreak of 8-m tall eastern white pine (Pinus strobus L. ) trees upwind from a townhouse bounded on either side by adjacent townhouses. They found a 42% reduction in air exchange when winds blew perpendicular to the v~indbreak at a speed of 5.6 m s- 1 and for an inside-outside temperature difference of 18 ° C. The results were strongly related to wind direction, since the interior-row townhouse studied was protected on each side from the wind by adjacent townhouses. DeWalle and Heisler (1983) conducted a field test to determine the distance between windbreak and home which produced the lowest air-exchange rates, windspeeds and energy needs for home heating. In a series of tests a small camper-type mobile home was located 1, 2, 4 and 8 h downwind from a 61-m long, 3.05-m high, single-row windbreak of white pine trees. Windspeeds in the open during the tests ranged from 1.7 m s -1 to 12.0 m s -1. Relative to open conditions, air-exchange rates in the mobile home were reduced a maximum of 54% at the 1 h downwind position, while maximum reductions in wind velocity of 54% occurred 2 h downwind (Fig. 4 ). Reductions in air-exchange rates were 41, 31 and 30% relative to open conditions at 2, 4 and 8 h, respectively. Based upon both wind-tunnel and field tests with a variety of model and real homes, it is clear that there is potential to reduce air-exchange rates significantly using windbreaks. The overall influence of such reductions in air exchange in homes on energy used for home heating depends on the fraction of total heat loss occurring by air exchange and the average windspeeds during the heating season. WINDBREAKEFFECTS ON OTHER HOME HEAT EXCHANGEPROCESSES Although it is generally conceded that the major action of a windbreak is to reduce home air exchange, windbreak effects on solar radiation transmission through windows and heat conduction through exterior walls may not be negligible. Windbreaks located near a home, where the windbreak shades the walls
250 A
8O
6O
40
20
Ih
2h
4h
8h
Ih
2h
4h
8h
Ih
2h
4h
8h
80
60
40
20
C
3O
.~
20
-I0 ._c "6 -20
Downwind Position
Fig. 4. Reductions in wind velocity (A), air infiltration rates (B) and heating energy needs (C) in a small, camper-type mobile home at varying distances downwind from a single-row, eastern white pine windbreak (DeWalle and Heisler, 1983). Maximum winter energy savings occurred close to the windbreak.
or windows in winter, can produce reductions in surface temperature, increased heat loss by conduction, and reduced transmission of solar radiation through windows. Obviously, such a situation is counter-productive in winter. Reduced wind velocities can also reduce heat loss by conduction/convection from the exterior building surfaces. DeWalle and Heisler (1983) showed that of the heating energy savings realized from the use of a windbreak upwind from a mobile home about one-fourth of the savings were attributed to reduced conduction/convection.
251 WINDBREAK EFFECTS ON ENERGY NEEDS FOR HOME HEATING
The classic study of windbreak effects on energy for home heating was conducted by Bates (1945). To collect basic data in outdoor tests Bates employed small 1.32-m cubical test homes with three windows. Wooden snow fences simulated windbreaks. For several Great Plains locations Bates estimated from field tests that windbreaks which reduced wind velocity by 40% would reduce energy needs for home heating for an entire heating season by 23-25%. Bates clearly understood that such results would pertain only to windy climates such as the Great Plains and to "leaky" homes where air exchange dominated heat loss; however, these results have been widely quoted as being generally achievable. Another early major study of windbreak effects on home heating was conducted by Woodruff (1954) using 12.7-cm cubical model homes with windows in a wind tunnel. He developed curves showing reductions in heating energy possible in the electrically-heated homes for various distances downwind from a 10-row windbreak composed of tree branches and for various windspeeds. For high windspeeds (15.6 m s -1 ) and a home location of 2 h downwind, maximum savings of 41% were predicted; however, at a low velocity (2.23 m s -1) and an 18 h downwind position only a 3% energy saving was found. These results clearly showed the effects of climate as well as proximity of the windbreak. Woodruff estimated that energy savings for home heating in Topeka, Kansas would equal 15% for a heating season for the windbreak and house tested. In more recent tests, Mattingly et al. (1979) estimated annual heating energy savings for a New Jersey interior-row townhouse which was protected by a single-row of eastern white pine trees. Savings for an entire season were only 3%, based upon air-exchange measurements in the house and adjustment for the frequency of wind directions which put the townhouse downwind from the trees. Energy savings in this interior-row townhouse from use of a windbreak were modest because the house was already protected from the wind on two sides by the adjacent houses. Harrje et al. (1982) used wind-tunnel air-exchange measurements in model two-story homes to estimate seasonal energy savings. Homes were 1/48th scale models of simple two-story houses with a gable roof. Their computations were also adjusted for the frequency distribution of windspeeds and directions for an hypothesized home site in central New Jersey with mean windspeeds of 3.15.8 m s-1. They showed annual heating energy savings of 9% for a house on a 0.30-ha (0.75 acre) lot with windbreak protection from the west and north. DeWalle and Heisler ( 1983 ) found seasonal heating energy savings of about 12% for a small 4.3-m long, camper-type mobile home downwind from an eastern white pine windbreak in central Pennsylvania. During these experiments 37% of total heat loss from the mobile home was from air exchange, which is
252 similar to the 33% rule often assumed for conventional homes. For wind directions when the home was downwind from the windbreak, heating energy savings of 17-18% were found at both I and 2 h (Fig. 4). In a follow-up study with mobile homes (Walk et al., 1985), a conifer windbreak did not significantly reduce heating energy use in any of the units in an entire 66-unit mobile home park in central Pennsylvania. A low density, 6-m tall, 100-m long, single-row, windbreak of white spruce (Piceaglauca (Moench) Voss) trees, erected along only part of the upwind edge of the park, reduced windspeed within the park by a maximum of only 9%. Windspeeds in the park were already reduced by up to 1/3 relative to upwind velocities by pre-existing vegetation, topography and the homes themselves. Thus, air exchange and heating energy exchange in the homes was not significantly affected by the windbreak. If windspeed had been reduced by 50%, energy-use equations for the homes indicate heating energy needs would have declined by about 11%, a value which agrees well with earlier studies. However, these results indicate that the opportunity to use a single windbreak to influence heating energy use in multiple dwellings may be limited. Research is also being conducted in Indian Head, Saskatchewan, Canada by the Prairie Farm Rehabilitation Administration on the benefits of windbreaks for home heating. This work involves monitoring energy used for heating construction trailers with and without windbreak protection on all sides. Preliminary unpublished results indicate energy savings of about 26% for an entire winter heating season at Regina, Saskatchewan. Results of quantitative studies of windbreak effects on air exchange and heating energy use are summarized in Table I. It is these studies which form the basis for guidelines on the use of windbreaks to reduce energy needs for home heating. WINDBREAKS AND HOME COOLINGENERGY NEEDS Limited data are available on windbreak effects on cooling energy needs, but home cooling energy needs could be reduced by reducing air exchange in windy, hot environments. Air-conditioning energy needs could be reduced if the infiltration of hot, humid, exterior air was reduced. Landscape architects generally prescribe vegetation which shades the exterior surface of homes in hot environments to reduce cooling energy needs. As long as cool breezes into the home environment are not blocked at other times, windbreaks to reduce strong hot winds are beneficial. GUIDELINES FOR USING WINDBREAKSTO CONSERVEHOME HEATINGENERGY The effectiveness of a windbreak in conserving energy for home heating will vary considerably, depending upon characteristics of the home, homesite, occupants, climate and degree of protection.
Snow fence windbreaks and 1.22m cube-shaped homes with windows in field
Wind-tunnel study with 10-row model tree windbreak and 12.7-cm cube-shaped model homes
Wind-tunnel study with model fence and/or tree windbreaks and a 1/48 scale model interior-row townhouse
Field study of single-row white pine windbreak upwind from interior row townhouse
Wind-tunnel experiment with model trees and a 2-story, gable roof, 16-cm high model house
Single-row white pine windbreak upwind from small mobile home
Single-row, white spruce windbreak upwind from 66-unit mobile home park
Field study with multiple-row shelterbelt protection on all sides of construction-type trailers
Bates (1945, Table 6)
Woodruff ( 1954, pp. 21- 23 )
Mattingly and Peters (1977)
Mattingly et al. ( 1979 )
Harrje et al. (1982)
DeWalle and Heisler ( 1983 )
Walk et al. (1985)
Preliminary results, Prairie Farm Rehabilitation Administration, Indian Head, Saskatchewan, Canada
'Corrected for frequency of wind directions at the site.
Type of study
Source
35 ~
10 ~
81
60
40
--
--
Air exchange reduction (%)
Effect of windbreaks on air exchange rates and space heating energy needs in homes
TABLE I
Seasonal estimate for Regina, Saskatchewan, Canada
Seasonal estimate for central Pennsylvania, 66-unit mean for assumed 50% wind velocity reduction
11
26'
Seasonal estimate for central Pennsylvania, wind velocity was only reduced 9% by windbreak
0'
Seasonal estimate for central Pennsylvania, 1-h downwind position
Seasonal estimate for central New Jersey, protection on west and north sides
91
12'
Seasonal estimate for central New Jersey, 3 trees upwind
Seasonal estimate for central New Jersey
Single row of trees and solid fence, wind normal to windbreaks
Single row of trees, wind normal to windbreak
Seasonal estimate for Topeka, Kansas
Seasonal estimate for several windy, cold Great Plains sites
Comments
31
3'
--
15
23-25
Heating energy savings (%)
t~ ¢9n
254
Home Home characteristics of importance are those which influence the fraction of total heat loss occurring as wind-induced air exchange. The ratio of airexchange heat loss to total heat loss (F) is generally assumed to be about 0.33. Homes with relatively few openings for air exchange ( F < 0.33) will generally be less responsive to windbreak effects than homes which are "leaky" (F> 0.33 ). Tightness of construction, use of vapor/air barriers and extent of weather stripping and caulking are all factors influencing air exchange. Orientation of the house on the lot and the distribution of openings for air exchange, relative to the direction of prevailing cold, strong winter winds must also be considered. If the side of the home with numerous or large openings faces into the wind, heat loss by wind-induced air exchange and benefits from use of a windbreak are likely to be higher than if most openings exist on the downwind side. Taller homes may induce more air exchange because of the "chimney effect", thereby reducing the importance of wind-induced air exchange. Taller homes are also exposed to higher windspeeds found with increased height above ground and are more difficult to protect fully with a windbreak.
Homesite The local characteristics of a homesite, of importance in controlling windbreak effectiveness, are related to the wind exposure of the site. Sites without upwind obstacles, such as other buildings, vegetation, or terrain to protect the home, and sites which occur in topographic positions where winds are accelerated, such as upper slope, ridgetop or mountain pass sites, are likely to produce greater benefits from use of a windbreak (see Wight, 1988).
Occupants Occupant habits also influence wind-induced air exchange heat loss. High winter thermostat settings and frequently opened doors and windows enhance windbreak effectiveness.
Climate Climate controls wind directions, windspeeds and air temperatures to which a home is subjected in winter. High-speed, cold air, from directions in which the home is particularly vulnerable, produces the greatest wind-induced air exchange heat loss. For example, windbreak benefits reported in Table I are generally greater in the windy Great Plains than in the eastern U.S.A. Climate is a function of such macro-scale climate differences as well as the local cli-
255 mate, e.g. upwind homes channel winds towards a particular house. In urban areas, windspeeds at ground level can be increased 4-fold by channelization around tall buildings (Oke, 1978). Windbreak Windbreak characteristics which must be considered to reduce energy needs for home heating in approximate order of importance are: ( 1 ) orientation relative to the house; (2) windbreak height and distance from the house; (3) length of windbreak; (4) windbreak elements: fence vs plants; (5) number of windbreak rows: (6) spacing of windbreak plants; (7) windbreak plant species. Heisler (1984) has considered many of these factors in a recent review. A summary of basic principles follows. Climatological analysis of wind directions is needed to determine which directions require the greatest winter protection. Ideally, the windbreak would be perpendicular to prevailing winds; however, winter wind directions of importance are usually variable and property lines often dictate directions for orientation of the windbreak. Thus, windbreaks are commonly located along one or more property lines in urban settings. Protection on western and northern property lines is typical. Protection can also be provided from all directions. However, blockage of cool breezes in summer should be minimized. Desirable views from windows in the home, locations of underground or overhead utility lines; positions of downwind snowdrifts which may be created on driveways, sidewalks, etc. and winter shading of the home by the windbreak are all factors to consider. The distance a windbreak should be placed upwind from a home is a compromise among achieving maximum reductions in heating energy use, prevention of shading of the home by the windbreak, prevention of formation of snowdrifts around the home and available space. From several studies (Woodruff, 1954; Blenk and Trienes, 1956; Harrje et al., 1982; DeWalle and Heisler, 1983) optimum windbreak distance upwind from a home to achieve winter energy conservation is about 1-4 h. Wind-tunnel experiments under laminarflow conditions by Harrje et al. (1982) for two model windbreak densities show the minimum air infiltration parameter computed from pressure coefficients (see eqn. (2)) at about 4 h (Fig. 5). Minimum air infiltration occurred at 1 or 2 h in the field study by DeWalle and Heisler (1983) shown in Fig. 4. Because h changes as windbreak trees grow, a better frame of reference is probably the height of the home (h'). In Fig. 6, variation in the air-infiltration parameter with windbreak tree height from wind-tunnel tests by Harrje et al.
256 H 1.2 <
e-
O P E N R O W 4 . 9 m spacing
~
C L O S E D R O W 2.4 m spacing
IlJ I-UJ
:E
.4 z,
z
~c <
0
2'h
4'h
6h
8h
10h
HOUSE TO WINDBREAK DISTANCE (h = WINDBREAK HEIGHT)
Fig. 5. Variation in the air-infiltration parameter for a model home in the wind tunnel with distance between the windbreak and home for two windbreak tree spacings. Model tree height was equal to house height and windbreak length was approximately two house heights (adapted from Harrje et al., 1982). 1.2 Z
_o~ I--
ee-
.
.8
.4
o 0.0
i 0 5
i 1.0
i 1.5
TREE HEIGHT/HOUSE HEIGHT
Fig. 6. Variation in the air-infiltration parameter for a model home in the wind tunnel with windbreak tree height. Windbreak was at 1.2 house heights upwind from the house and windbreak length was approximately two house heights (adapted from Harrje et al., 1982).
(1982) shows that air infiltration decreases linearly with increasing tree height for trees ranging from 0.6 to 1.6 h'. Beyond 1.6 h' the effects of increasing tree height are unknown, but the benefits of rapid tree growth or planting large trees are obvious. To prevent shading when the trees reach the height of the home it is desirable that the windbreak be located at about 2 h', thus for a 6-m high two-story home windbreak trees should be planted about 12 m upwind (see Fig. 7A). For east- and west-facing walls the 2 h' is probably acceptable; however, for southfacing surfaces which receive considerable solar energy in winter a 3 or 4 h' rule is needed, to prevent shading at latitudes typical in the U.S.A. (Fig. 7B ). Windbreaks along north-facing walls could be closer than 2 h'. All this assumes adequate space is available on the lot. If it is not available, a homeowner probably should not use a windbreak to the south of the house.
257 A.I
NORTH/WEST
SOUTH/EAST WIND
windbreok
~
_ _ -
~ 2 h ' - -
B.
1 I'
~q
SOUTH
NORTH WIND
3o~"--~__2o'~ 3h ~
C.
-
"I
WEST
EAST WIND snow tence
It
I"
windbreak
- - 2 0 - 30h - - - - - - - - ~ ' ~
2 h] ~
Fig. 7. Recommendedplacement of a windbreakupwind from a home when (A) cold, high-speed winter winds originate from a westerlyor northerly direction, (B) winter winds originate from the south and home shading must be prevented and (C) westerlywinter winds are accompanied by blowingsnow for which a snowfence is also needed. Windbreaks protecting north-, east-, or west-facing surfaces can be planted in the space available with recognition t h a t they may grow too tall and need trimming. Shade on solar collecters will be particularly detrimental. Shade from windbreaks in summer on east or west walls can be beneficial in keeping the home cool, because these walls receive more solar energy t h a n a south wall in summer. Large snow drifts may form downwind from a windbreak where a b u n d a n t snow, high winds and few other obstacles exist to trap snow upwind. In these areas, drifts could form around homes hampering normal activities. Under such circumstances, and where space permits, it is best to locate the windbreak as suggested and use a snow fence (s) to trap snow before it reaches the windbreak (Fig. 7C ). Equilibrium drifts can reach lengths of up to 20-30 times snow fence
258 heights downwind from porous snow fence (see Tabler, 1980); however, smaller drifts are usually confined to lesser downwind distances. In reality, most homes in urban and suburban areas are protected from large drift formation by upwind obstacles. The length of a windbreak should be at least as great as the length of the home surfaces to be protected. Any extra length can be beneficial for protection from a wider range of wind directions. Protection on two or three sides of a property, e.g. to the west, north or east of a home may be a wise investment. Care must be exercised to minimize blockage of cool summer breezes with the windbreak. Windbreaks constructed from various materials such as wood can be used instead of trees and shrubs and have the advantage of providing instant benefits. Although windbreaks often consist of up to 10 rows of trees and shrubs, windbreaks for home energy conservation must often be used in urban/suburban areas with severe space limitations. Thus one, possibly two, rows of trees are commonly employed. Wind-tunnel studies (Harrje et al., 1982 ) have shown the marginal effectiveness of adding windbreak rows in reducing air exchange in homes. Spacing within rows of 2-3 m and 3-4 m between rows is often used (DeWalle and Farrand, 1978) with the lesser spacings producing the quickest results, but with some thinning possibly needed later. Trees should be planted in a staggered arrangement between rows, rather than directly downwind from one another (Wight, 1988). Species used for single-row windbreaks are generally conifers. Conifers with stiff branches which will not streamline in the wind and which reach to ground level are most desirable. A second row of rapid-growing, closely-spaced deciduous trees, such as hybrid poplar (Populus spp. ), can also be used to provide some quick protection while the slower-growing conifers reach effective height. Other characteristics of importance are climatic and edaphic requirements, insect and disease resistance, wildlife benefits and aesthetic qualities. ECONOMICS OF WINDBREAKUSE TO CONSERVEENERGY Economic analyses of windbreaks used for home energy conservation have been conducted (DeWalle, 1980; Heisler, 1984). These studies show that windbreaks can be cost-effective in terms of energy savings. The net benefit is strongly related to the type of planting material chosen. Planting seedlings keeps initial costs to a minimum, but considerable time is needed for the trees to grow before significant benefits are received. Planting large growing stock requires greater initial expense and professional help in planting, but paybacks may occur more quickly. Heisler (1984) found that with fuel escalation rate (FER) at twice the inflation rate (i) the net present value of a 20-year windbreak investment was greatest when medium-sized ( 1.5-m high ) planting stock was used, as opposed
259
to either seedling or larger ( 3.0-m high ) stock. When he assumed, either F E R = i or F E R = 0 and i = 4%, he found lesser net windbreak present values than for F E R = 2i. In these cases the use of seedlings was preferable. Without considering fuel price escalation, DeWalle (1980) concluded that use of seedling planting stock was economical, but that use of 2-m high balled planting stock was not cost effective. However, if any of the other windbreak benefits can be quantified and added to that for energy savings, the use of large trees could be economical. In many instances, proper care and maintenance will make the planting of smaller stock more economical and help reduce the waiting period. ACKNOWLEDGEMENT
Research by the senior author was largely funded by the United States Department of Agriculture Forest Service, Northeastern Forest Experiment Station through the Consortium for Environmental Forestry Studies.
REFERENCES Albertson, M.L., Barton, J.R. and Simons, D.B., 1960. Fluid Mechanics for Engineers. PrenticeHall, New Jersey, 561 pp. ASHRAE, 1985. ASHRAE Handbook: 1985 Fundamentals. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA, pp. 22.8-22.12. Bates, C.G., 1945. The value of shelterbelts in house heating. J. For., 43: 176-196. Blenk, C.G. and Trienes, H., 1956. Model testing of wind pressures on a house with and without a windbreak. Abh. Braunschw. Wiss. Ges., S., pp. 74-84 (in German). Buckley, C.E., Harrje, D.T., Knowlton, M.P. and Heisler, G.M., 1978. The optimum use of coniferous trees in reducing home energy consumption. Princeton University, Cent. Environ. Stud., Rep. PU/CES 71, 76 pp. DeWalle, D.R., 1980. Manipulating urban vegetation for residential energy conservation. Proceedings, National Urban Forestry Conference, 1979. College Environ. Sci. For., SUNY, Syracuse, ESF Publ. 80-003, pp. 267-283. DeWalle, D.R. and Farrand, E.P., 1978. Windbreaks and shade trees - their use in home energy conservation. The Pennsylvania State University, College of Agriculture, Agric. Extn. Serv., Spec. Circ., 245, 8 pp. DeWalle, D.R. and Heisler, G.M., 1980a. Water vapor mass balance method for determining air infiltration rates in houses. U.S.D.A. For. Serv. Northeast. For. Exp. Stn., Res. Note NE-301, 7 pp. DeWalle, D.R. and Heisler, G.M., 1980b. Landscaping to reduce year-round energy bills. In: Cutting Energy Costs, The 1980 Yearbook of Agriculture, U.S.D.A., pp. 227-237. DeWalle, D.R. and Heisler, G.M., 1983. Windbreak effects on air infiltration and space heating in a mobile home. Energy Build., 5: 279-288. DeWalle, D.R., Heisler, G.M. and Jacobs, R.E., 1983. Forest home sites influence heating and cooling energy. J. For., 81 (2): 84-87. Harrje, E.T., Buckley, C.E. and Heisler, G.M., 1982. Building energy reductions: Optimum use of windbreaks. Proceedings of the American Society of Civil Engineers. J. Energy Div., 108: 143154.
260 Heisler, G.M., 1984. Planting design for wind control. In: E.G. McPherson (Editor). Energy Conserving Site Design, Am. Soc. Landscape Architects, Washington, DC, Chapter 9, pp. 165-184. Heisler, G.M., 1986. Energy savings with trees. J. Arboricult., 12 (5): 113-125. Heisler, G.M. and DeWalle, D.R., 1988. Effects of windbreak structure on wind flow. Agric. Ecosystems Environ., 22/23: 41-69. Hunt, C.M., King, J.C. and Trechsel, H.R. (Editors), 1980. Building air change rate and infiltration measurements. Am. Soc. Test. Mater., Spec. Tech. Publ., 719, 185 pp. Malik, N., 1978. Field studies of dependence of air infiltration on outside temperatures and wind. Energy Build., 1 (3): 281-292. Mattingly, G.M. and Peters, E.F., 1977. Wind and trees: Air infiltration effects on energy in housing. J. Ind. Aerodynamics, 2: 1-19. Mattingly, G.M., Harrje, D.T. and Heisler, G.M., 1979. The effectiveness of an evergreen windbreak for reducing residential energy consumption. A.S.H.R.A.E. Trans., 85, (Part 2), pp. 428444. McNaughton, K.G., 1988. Effects of windbreaks on turbulent transport and microclimate. Agric. Ecosystems Environ., 22/23: 17-39. McPherson, E.G., 1984. Benefits and costs of energy-conserving site design. In: E.G. McPherson (Editor), Energy Conserving Site Design. Am. Soc. Landscape Architects, Washington, D.C., Chapter 2, pp. 17-32. Oke, T.R., 1978. Boundary Layer Climates. Methuen and Co., Ltd., London. 372 pp. Sinden, F.W., 1978. Wind, temperature and natural ventilation: theoretical considerations. Energy Build., 1 (3): 275-280. Tabler, R.D., 1980. Geometry and density of drifts formed by snow fences. J. Glaciol., 26(94): 405-419. Walk, M.-F., DeWalle, D.R. and Heisler, G.M., 1985. Can windbreaks reduce energy use in a mobile home park? J. Arboricult., 11 (6): 190-195. Wight, B., 1988. Farmstead windbreaks. Agric. Ecosystems Environ., 22/23: 261-280. Woodruff, N.P., 1954. Shelterbelt and surface barrier effects on wind velocities, evaporation, househeating, and snowdrifting. Kans. Agric. Exp. Stn., Tech. Bull., 77: 21-23.
243
14. U s e of W i n d b r e a k s for H o m e E n e r g y Conservation DAVID R. DEWALLE 1and GORDON M. HEISLER 2 1Institute for Research on Land and Water Resources and School of Forest Resources, The Pennsylvania State University, University Park, PA 16802 (U.S.A.). 2Northeastern Forest Experiment Station, U.S.D.A. Forest Service, University Park, PA 16802 (U.S.A.)
ABSTRACT DeWalle, D.R. and Heisler, G.M., 1988. Use of windbreaks for home energy conservation. Agric. Ecosystems Environ., 22/23: 243-260. Windbreaks are effective in reducing energy needs for home heating by reducing air-exchange rates in homes. Air exchange in homes is caused by pressure differences caused either by temperature differences between inside and outside air or wind forces on the exterior surfaces of the home. Windbreaks are only effective in reducing air exchange caused by wind forces. Thus, windbreaks are more effective in saving energy in windy climates. Available data indicate that annual energy savings for home heating using windbreaks can be up to 10-15% in the northeastern U.S.A. and 15-25% in the northcentral U.S.A. Economic analyses indicate that windbreaks are cost effective with the value of heating energy savings exceeding costs for windbreak establishment and maintenance over a 20-year windbreak life.
INTRODUCTION T h e p u r p o s e of t h i s c h a p t e r is to discuss t h e use of w i n d b r e a k s to c o n s e r v e e n e r g y u s e d for s p a c e h e a t i n g in h o m e s , w i t h special r e f e r e n c e to u r b a n a n d s u b u r b a n settings. T h e effects of w i n d b r e a k s on m i c r o c l i m a t e a n d w i n d b r e a k a e r o d y n a m i c s as well as use of w i n d b r e a k s a r o u n d f a r m s t e a d s h a v e b e e n discussed in p r e v i o u s c h a p t e r s ( H e i s l e r a n d DeWalle, 1988; M c N a u g h t o n , 1988; W i g h t , 1988). T h i s c h a p t e r deals specifically w i t h h e a t - e x c h a n g e p r o c e s s e s in h o m e s a n d h o w to utilize w i n d b r e a k s to reduce e n e r g y u s e d for h e a t i n g of t h e i n t e r i o r air s p a c e to t h e b e s t a d v a n t a g e . S p a c e c o n d i t i o n i n g ( h e a t i n g a n d cooling) of h o m e s is a m a j o r use of e n e r g y ( M c P h e r s o n , 1984 ). E n e r g y use in t h e r e s i d e n t i a l s e c t o r r e p r e s e n t s a b o u t 17% of t h e t o t a l e n e r g y use in t h e U.S.A. N e a r l y t w o - t h i r d s of t o t a l r e s i d e n t i a l energy use is for h e a t i n g ( 5 3 % ) a n d cooling ( 1 2 % ) . H e n c e , r e s i d e n t i a l s p a c e c o n d i t i o n i n g r e p r e s e n t s a b o u t 11% of t o t a l U.S.A. e n e r g y use.
0167-8809/88/$03.50
© 1988 Elsevier Science Publishers B.V.
244
To reduce energy used for heating or cooling, windbreaks must influence air exchange, heat conduction or solar radiation transmission. Air exchange is the simultaneous passage of air into and out of a home through cracks around windows and doors, porous materials, open doors and windows, and other openings to the outside. Heat conduction is the passage of energy from molecule to molecule through the solid portions of the structure from hot to cold sections. Solar radiation transmission is the entry of solar energy through glazed surfaces such as windows. Air exchange is the heat-transfer process most affected by windbreaks. The authors will draw upon their windbreak research experience from several experiments conducted using mobile homes (DeWalle and Heisler, 1983; Walk et al., 1985; Heisler, 1986) and supplement that with information from other studies. AIR EXCHANGE IN HOMES
Air exchange in homes, typically represents one-third of the heat loss from a home in winter and can represent over half of heat loss on selected days. Thus, if windbreaks can be used to reduce air exchange, considerable energy savings could result. Air exchange occurs when air is forced or drawn through openings in the house by pressure differences. From a fluid mechanics viewpoint the volume flow rate of gas through an orifice can generally be computed as (Albertson et al., 1960):
Q=CcCvAo~p
(1)
where Q is the volume flow rate of air, Cc is a contraction coefficient, Cv is a velocity coefficient, Ao is the cross-sectional area of the orifice, Ap is the difference in pressure in the fluid across the orifice and p is the fluid density. The coefficients C~ and Cv vary with the geometry of the orifice. Orifices or openings in the exterior shell of a home vary widely in geometry and location, with no two homes exactly alike. Pressure differences also vary in magnitude and from positive to negative values over the exterior shell of the home. Pressure differences are caused by forces of the wind over the exterior surfaces of the home and by temperature differences between inside and outside air. Surfaces of the home facing the wind will experience increased pressure as windspeed increases and air will enter the home through openings in these surfaces (see Fig. 1 ). Passage of air into the home will force an equal mass of interior air out of the home. The location of air entry and exit is dependent upon wind direction and home characteristics. Windbreaks reduce air exchange by reducing wind-pressure forces on upwind home surfaces. Air infiltration is sometimes used to refer to air exchange, but air infiltration ignores the fact that there is also concurrent air exfiltration occurring in the
245
Hot air out
Air in
aI
"1
wind effect
I
•
I
"
Air out
C o l d ' C o l d air
.n.,
"-'-- If
,X. j o
air
~ - I "-"
Temperature difference effect
Fig. 1. Air exchange in homes occurs because of pressure differences between inside and outside air created by wind forces and temperature differences in winter (DeWalleand Heisler, 1980b). Windbreaks only reduce air exchange caused by wind forces. home. Air exchange will be used generally in this chapter, unless we are referring to a specific author's use of air infiltration. To understand wind-induced air exchange better, it is useful to consider the vertical and horizontal distribution of air pressure over the surface of a home. Typical wind-tunnel results for pressure distributions over a single model home with a gable roof and with the long dimension perpendicular to the wind are shown in Fig. 2. Pressures are expressed as a pressure coefficient (Cp) which is computed as the ratio of the pressure difference (Ap) between inside and outside air at a point on the house surface to the dynamic pressure of the wind measured upwind from the model home. Dynamic pressure may be computed as 1/2 pu 2 where p is the fluid density and u is the upstream speed. Infiltration of outside air will occur through any openings in the structure where a positive Cp is found, while exfiltration of interior air occurs through openings in regions with negative Cp. The mass flow rate of air into the structure over areas with positive Cp is equal to the concurrent mass flow rate out of the structure over negative Cp areas. In Fig. 2, positive Cp values occur over all of the front wall and part of the front roof surface. Negative Cp values are found on the side walls, the lee wall, the lee roof surface and part of the front roof surface. Such pressure distributions are dictated by the shape and orientation of the structure to the wind. The distribution and type of openings in the structure will also play an important role in determining the actual air-exchange rate. If it is assumed that openings are uniformly distributed over the structure, a dimensionless air-infiltration parameter (AI) can be computed for model homes in a wind tunnel as (Harrje et al., 1982): M +
AI= ~ CpkAk/Achar
(2)
k=l
where the product of each measured pressure coefficient (Cpk) and the area represented by each pressure measurement (Ak) is summed over the number
246 VERTICAL DISTRIBUTION
½ .~101
( ~ WIND DIRECTION
Cp
-1 0
-1 0 1 Cp
-1
0
1
Cp
HORIZONTAL DISTRIBUTION -1
Cp -1 0 1
LEEWALL LEEROOF
0 Cp 1 Cp 1 0-1 ~
.'ii
FRONTROOF WIND DIRECTION I FRONTWALL
-0 Cp
Fig. 2. Vertical and horizontal distribution of pressure coefficients (Cp) over the surface of a model home in a wind tunnel. Positive Cp indicates potential areas for infiltration of exterior air into the structure, while negative C, indicates potential areas for exfiltration of interior air (adapted from Buckley et al., 1978).
(M ÷ ) of measurement points with positive Cp and divided by a characteristic area (Achar). The characteristic area chosen by Harrje et al. (1982) was the area of the front wall of the structure. The AI parameter is an index of windinduced air exchange in model homes and is not a true air-exchange rate. However, it is very useful in translating wind-tunnel test data into practical results and will be referred to later in this paper. Because warm air rises, temperature differences between inside and outside air will also create a natural circulation of air in the home (Fig. 1). Warm interior air in winter will rise and flow out of the home through openings near the top of the home, while cold outside air is drawn into the home through lower openings. This process of air exchange is known as the "chimney" or "stack" effect. Windbreaks are not effective in reducing this type of air exchange. In summer, cooler, denser interior air can sink to create a reverse type of flow. However, temperature differences in winter are generally greater than in summer. For example in the U.S.A., temperature differences between interior and exterior air can reach 50°C (90°F) in winter, but seldom exceed 30°C
247 12
AIR = - 0 . 0 7 8 8 - 0 D 9 9 4 ( ~ T ) - 0 . 3 0 6 8 ( U ) + 0.1080(U z) r 2 = 0.61
A T = -35°C AT = -25°C
I0
L~T = - 1 5 ° C
zs ¢J
~6
4
2
0
i
i
4
i
t
6
i
t
8
i
i
I0
WIND VELOCITY IN OPEN, rn/s
Fig. 3. Relationship between air-exchange rates in a mobile home and climatic variables, illustrating that temperature differences largely control air exchange below windspeeds of 3-4 m s - ] (DeWalle et al., 1983 ).
(54 °F) in summer. Thus, temperature-induced air exchange in winter should be greater than in summer, unless openings are modified through use of storm windows, plastic sheeting or some similar device. Air exchanges caused by temperature differences and by wind often occur simultaneously and generally are subadditive (Sinden, 1978). That is, windinduced air exchange may actually reduce exchange caused by temperature differences. However, the degree of interference will be unique to each dwelling because of variations in the type and distribution of openings and building shape. Consequently, models relating air exchange to windspeed and insideoutside temperature differences cannot be generalized. One such relationship for a small mobile home (Fig. 3) indicated that reductions in windspeed at low speeds where temperature-difference effects predominated would be quite ineffective in reducing air-exchange rates. At higher windspeeds, e.g. greater than 4 m s -1, windspeed reductions produced with windbreaks would be much more effective. Rapid increases in air exchange with increasing windspeed, at speeds greater than 4 m s- 1 are probably caused by wind-pressure forces which vary with the square of windspeed. Because each home is unique, this example only serves to illustrate what can happen. Generalizations on windspeeds at which windbreaks can be effective for other homes cannot be made. The influence of turbulence in the air on air-exchange rates is not well understood. Generally, air-exchange rates in houses have only been related to mean horizontal windspeeds rather than some measure of turbulence level. Because turbulence levels may be altered in the wake zone downwind from windbreaks (see Heisler and DeWalle, 1988), an understanding of turbulence
248 effects on air exchange may eventually improve the ability to describe windbreak effectiveness. Wind direction is also important in controlling air exchange in homes. In homes where the distribution of openings in the exterior shell is not equal on all sides or surfaces, e.g. in an interior row townhouse, air exchange rates for a given windspeed and temperature will be sensitive to wind direction (Malik, 1988). This is an important consideration in placement of windbreaks such that surfaces of homes with openings through which air exchange will occur, are protected. Air-exchange rates in homes currently cannot be estimated with confidence. For design purposes air exchanges are generally assumed to be 0.5 change per hour for tightly sealed homes, 1 change per hour for a typical home, and 2-3 changes per hour for poorly-sealed homes without storm windows or weather stripping. Where more precise information is needed, exchange rates are generally measured using a tracer gas-dilution methodology (ASHRAE, 1985; Hunt et al., 1980). In homes without significant sources or sinks for water vapor, a simple method utilizing commonly-available humidity-control equipment can be employed (DeWalle and Heisler, 1980a). In summary, air exchange in homes is a result of pressure differences caused by wind forces and temperature differences. Windbreaks can only be used to influence air exchange caused by wind forces. Thus, the windbreak effect on air exchange is controlled by the balance between wind and temperature-difference air exchange for a specific home and climatic condition. WINDBREAKEFFECTS ON AIR EXCHANGE Several wind-tunnel and field studies have been conducted to determine windbreak effects on air exchange in homes. An early wind-tunnel study of pressure distributions over the surface of a 50-mm high model home with and without a wire-screen windbreak was conducted by Blenk and Trienes (1956). They showed variations in pressure distributions with distance between the house and windbreak and with wind direction. Minimum mean pressures, and presumably minimum wind-induced air-exchange rates, were found with the windbreak at 4 h upwind (h = windbreak height) from the house for a 2 h long windbreak and normal winds. For winds departing from normal, mean pressures on the house increased rapidly and at an angle of 40-60 ° from normal, no windbreak effect was measured. Mattingly and Peters (1977) conducted wind-tunnel studies with small model houses to determine windbreak influence on wind-induced air exchange computed from pressure coefficients as previously indicated in eqn. (2). They found the most effective windbreak tested was a row of simulated conifer trees, slightly taller than the model home, located about two house heights upwind. Windinduced air exchange in the model home was reduced by about 42%.
249 In a later study with a scale model of a two-story house with a gable roof, model conifer trees were used to test effectiveness of various arrangements of trees and windbreaks in reducing air exchange with wind perpendicular to the tree row (Harrje et al., 1982). They found maximum wind-induced air-exchange reductions in the model house occurred when windbreaks were located 2-4 tree heights upwind. Other results observed were that air exchange decreased linearly with increased model tree height, air exchange decreased in a nearly-linear fashion with increasing density in a single-row windbreak, only marginal additional air-exchange reductions were obtained with multiple-row windbreaks compared to single-row windbreaks, and model trees with pruned trunks were much less effective than those with foliage to ground level. Several field tests of shelter effects on air exchange have also been conducted. Mattingly et al. (1979) conducted a field test to confirm previous windtunnel results by erecting a 30-m long, single-row windbreak of 8-m tall eastern white pine (Pinus strobus L. ) trees upwind from a townhouse bounded on either side by adjacent townhouses. They found a 42% reduction in air exchange when winds blew perpendicular to the v~indbreak at a speed of 5.6 m s- 1 and for an inside-outside temperature difference of 18 ° C. The results were strongly related to wind direction, since the interior-row townhouse studied was protected on each side from the wind by adjacent townhouses. DeWalle and Heisler (1983) conducted a field test to determine the distance between windbreak and home which produced the lowest air-exchange rates, windspeeds and energy needs for home heating. In a series of tests a small camper-type mobile home was located 1, 2, 4 and 8 h downwind from a 61-m long, 3.05-m high, single-row windbreak of white pine trees. Windspeeds in the open during the tests ranged from 1.7 m s -1 to 12.0 m s -1. Relative to open conditions, air-exchange rates in the mobile home were reduced a maximum of 54% at the 1 h downwind position, while maximum reductions in wind velocity of 54% occurred 2 h downwind (Fig. 4 ). Reductions in air-exchange rates were 41, 31 and 30% relative to open conditions at 2, 4 and 8 h, respectively. Based upon both wind-tunnel and field tests with a variety of model and real homes, it is clear that there is potential to reduce air-exchange rates significantly using windbreaks. The overall influence of such reductions in air exchange in homes on energy used for home heating depends on the fraction of total heat loss occurring by air exchange and the average windspeeds during the heating season. WINDBREAKEFFECTS ON OTHER HOME HEAT EXCHANGEPROCESSES Although it is generally conceded that the major action of a windbreak is to reduce home air exchange, windbreak effects on solar radiation transmission through windows and heat conduction through exterior walls may not be negligible. Windbreaks located near a home, where the windbreak shades the walls
250 A
8O
6O
40
20
Ih
2h
4h
8h
Ih
2h
4h
8h
Ih
2h
4h
8h
80
60
40
20
C
3O
.~
20
-I0 ._c "6 -20
Downwind Position
Fig. 4. Reductions in wind velocity (A), air infiltration rates (B) and heating energy needs (C) in a small, camper-type mobile home at varying distances downwind from a single-row, eastern white pine windbreak (DeWalle and Heisler, 1983). Maximum winter energy savings occurred close to the windbreak.
or windows in winter, can produce reductions in surface temperature, increased heat loss by conduction, and reduced transmission of solar radiation through windows. Obviously, such a situation is counter-productive in winter. Reduced wind velocities can also reduce heat loss by conduction/convection from the exterior building surfaces. DeWalle and Heisler (1983) showed that of the heating energy savings realized from the use of a windbreak upwind from a mobile home about one-fourth of the savings were attributed to reduced conduction/convection.
251 WINDBREAK EFFECTS ON ENERGY NEEDS FOR HOME HEATING
The classic study of windbreak effects on energy for home heating was conducted by Bates (1945). To collect basic data in outdoor tests Bates employed small 1.32-m cubical test homes with three windows. Wooden snow fences simulated windbreaks. For several Great Plains locations Bates estimated from field tests that windbreaks which reduced wind velocity by 40% would reduce energy needs for home heating for an entire heating season by 23-25%. Bates clearly understood that such results would pertain only to windy climates such as the Great Plains and to "leaky" homes where air exchange dominated heat loss; however, these results have been widely quoted as being generally achievable. Another early major study of windbreak effects on home heating was conducted by Woodruff (1954) using 12.7-cm cubical model homes with windows in a wind tunnel. He developed curves showing reductions in heating energy possible in the electrically-heated homes for various distances downwind from a 10-row windbreak composed of tree branches and for various windspeeds. For high windspeeds (15.6 m s -1 ) and a home location of 2 h downwind, maximum savings of 41% were predicted; however, at a low velocity (2.23 m s -1) and an 18 h downwind position only a 3% energy saving was found. These results clearly showed the effects of climate as well as proximity of the windbreak. Woodruff estimated that energy savings for home heating in Topeka, Kansas would equal 15% for a heating season for the windbreak and house tested. In more recent tests, Mattingly et al. (1979) estimated annual heating energy savings for a New Jersey interior-row townhouse which was protected by a single-row of eastern white pine trees. Savings for an entire season were only 3%, based upon air-exchange measurements in the house and adjustment for the frequency of wind directions which put the townhouse downwind from the trees. Energy savings in this interior-row townhouse from use of a windbreak were modest because the house was already protected from the wind on two sides by the adjacent houses. Harrje et al. (1982) used wind-tunnel air-exchange measurements in model two-story homes to estimate seasonal energy savings. Homes were 1/48th scale models of simple two-story houses with a gable roof. Their computations were also adjusted for the frequency distribution of windspeeds and directions for an hypothesized home site in central New Jersey with mean windspeeds of 3.15.8 m s-1. They showed annual heating energy savings of 9% for a house on a 0.30-ha (0.75 acre) lot with windbreak protection from the west and north. DeWalle and Heisler ( 1983 ) found seasonal heating energy savings of about 12% for a small 4.3-m long, camper-type mobile home downwind from an eastern white pine windbreak in central Pennsylvania. During these experiments 37% of total heat loss from the mobile home was from air exchange, which is
252 similar to the 33% rule often assumed for conventional homes. For wind directions when the home was downwind from the windbreak, heating energy savings of 17-18% were found at both I and 2 h (Fig. 4). In a follow-up study with mobile homes (Walk et al., 1985), a conifer windbreak did not significantly reduce heating energy use in any of the units in an entire 66-unit mobile home park in central Pennsylvania. A low density, 6-m tall, 100-m long, single-row, windbreak of white spruce (Piceaglauca (Moench) Voss) trees, erected along only part of the upwind edge of the park, reduced windspeed within the park by a maximum of only 9%. Windspeeds in the park were already reduced by up to 1/3 relative to upwind velocities by pre-existing vegetation, topography and the homes themselves. Thus, air exchange and heating energy exchange in the homes was not significantly affected by the windbreak. If windspeed had been reduced by 50%, energy-use equations for the homes indicate heating energy needs would have declined by about 11%, a value which agrees well with earlier studies. However, these results indicate that the opportunity to use a single windbreak to influence heating energy use in multiple dwellings may be limited. Research is also being conducted in Indian Head, Saskatchewan, Canada by the Prairie Farm Rehabilitation Administration on the benefits of windbreaks for home heating. This work involves monitoring energy used for heating construction trailers with and without windbreak protection on all sides. Preliminary unpublished results indicate energy savings of about 26% for an entire winter heating season at Regina, Saskatchewan. Results of quantitative studies of windbreak effects on air exchange and heating energy use are summarized in Table I. It is these studies which form the basis for guidelines on the use of windbreaks to reduce energy needs for home heating. WINDBREAKS AND HOME COOLINGENERGY NEEDS Limited data are available on windbreak effects on cooling energy needs, but home cooling energy needs could be reduced by reducing air exchange in windy, hot environments. Air-conditioning energy needs could be reduced if the infiltration of hot, humid, exterior air was reduced. Landscape architects generally prescribe vegetation which shades the exterior surface of homes in hot environments to reduce cooling energy needs. As long as cool breezes into the home environment are not blocked at other times, windbreaks to reduce strong hot winds are beneficial. GUIDELINES FOR USING WINDBREAKSTO CONSERVEHOME HEATINGENERGY The effectiveness of a windbreak in conserving energy for home heating will vary considerably, depending upon characteristics of the home, homesite, occupants, climate and degree of protection.
Snow fence windbreaks and 1.22m cube-shaped homes with windows in field
Wind-tunnel study with 10-row model tree windbreak and 12.7-cm cube-shaped model homes
Wind-tunnel study with model fence and/or tree windbreaks and a 1/48 scale model interior-row townhouse
Field study of single-row white pine windbreak upwind from interior row townhouse
Wind-tunnel experiment with model trees and a 2-story, gable roof, 16-cm high model house
Single-row white pine windbreak upwind from small mobile home
Single-row, white spruce windbreak upwind from 66-unit mobile home park
Field study with multiple-row shelterbelt protection on all sides of construction-type trailers
Bates (1945, Table 6)
Woodruff ( 1954, pp. 21- 23 )
Mattingly and Peters (1977)
Mattingly et al. ( 1979 )
Harrje et al. (1982)
DeWalle and Heisler ( 1983 )
Walk et al. (1985)
Preliminary results, Prairie Farm Rehabilitation Administration, Indian Head, Saskatchewan, Canada
'Corrected for frequency of wind directions at the site.
Type of study
Source
35 ~
10 ~
81
60
40
--
--
Air exchange reduction (%)
Effect of windbreaks on air exchange rates and space heating energy needs in homes
TABLE I
Seasonal estimate for Regina, Saskatchewan, Canada
Seasonal estimate for central Pennsylvania, 66-unit mean for assumed 50% wind velocity reduction
11
26'
Seasonal estimate for central Pennsylvania, wind velocity was only reduced 9% by windbreak
0'
Seasonal estimate for central Pennsylvania, 1-h downwind position
Seasonal estimate for central New Jersey, protection on west and north sides
91
12'
Seasonal estimate for central New Jersey, 3 trees upwind
Seasonal estimate for central New Jersey
Single row of trees and solid fence, wind normal to windbreaks
Single row of trees, wind normal to windbreak
Seasonal estimate for Topeka, Kansas
Seasonal estimate for several windy, cold Great Plains sites
Comments
31
3'
--
15
23-25
Heating energy savings (%)
t~ ¢9n
254
Home Home characteristics of importance are those which influence the fraction of total heat loss occurring as wind-induced air exchange. The ratio of airexchange heat loss to total heat loss (F) is generally assumed to be about 0.33. Homes with relatively few openings for air exchange ( F < 0.33) will generally be less responsive to windbreak effects than homes which are "leaky" (F> 0.33 ). Tightness of construction, use of vapor/air barriers and extent of weather stripping and caulking are all factors influencing air exchange. Orientation of the house on the lot and the distribution of openings for air exchange, relative to the direction of prevailing cold, strong winter winds must also be considered. If the side of the home with numerous or large openings faces into the wind, heat loss by wind-induced air exchange and benefits from use of a windbreak are likely to be higher than if most openings exist on the downwind side. Taller homes may induce more air exchange because of the "chimney effect", thereby reducing the importance of wind-induced air exchange. Taller homes are also exposed to higher windspeeds found with increased height above ground and are more difficult to protect fully with a windbreak.
Homesite The local characteristics of a homesite, of importance in controlling windbreak effectiveness, are related to the wind exposure of the site. Sites without upwind obstacles, such as other buildings, vegetation, or terrain to protect the home, and sites which occur in topographic positions where winds are accelerated, such as upper slope, ridgetop or mountain pass sites, are likely to produce greater benefits from use of a windbreak (see Wight, 1988).
Occupants Occupant habits also influence wind-induced air exchange heat loss. High winter thermostat settings and frequently opened doors and windows enhance windbreak effectiveness.
Climate Climate controls wind directions, windspeeds and air temperatures to which a home is subjected in winter. High-speed, cold air, from directions in which the home is particularly vulnerable, produces the greatest wind-induced air exchange heat loss. For example, windbreak benefits reported in Table I are generally greater in the windy Great Plains than in the eastern U.S.A. Climate is a function of such macro-scale climate differences as well as the local cli-
255 mate, e.g. upwind homes channel winds towards a particular house. In urban areas, windspeeds at ground level can be increased 4-fold by channelization around tall buildings (Oke, 1978). Windbreak Windbreak characteristics which must be considered to reduce energy needs for home heating in approximate order of importance are: ( 1 ) orientation relative to the house; (2) windbreak height and distance from the house; (3) length of windbreak; (4) windbreak elements: fence vs plants; (5) number of windbreak rows: (6) spacing of windbreak plants; (7) windbreak plant species. Heisler (1984) has considered many of these factors in a recent review. A summary of basic principles follows. Climatological analysis of wind directions is needed to determine which directions require the greatest winter protection. Ideally, the windbreak would be perpendicular to prevailing winds; however, winter wind directions of importance are usually variable and property lines often dictate directions for orientation of the windbreak. Thus, windbreaks are commonly located along one or more property lines in urban settings. Protection on western and northern property lines is typical. Protection can also be provided from all directions. However, blockage of cool breezes in summer should be minimized. Desirable views from windows in the home, locations of underground or overhead utility lines; positions of downwind snowdrifts which may be created on driveways, sidewalks, etc. and winter shading of the home by the windbreak are all factors to consider. The distance a windbreak should be placed upwind from a home is a compromise among achieving maximum reductions in heating energy use, prevention of shading of the home by the windbreak, prevention of formation of snowdrifts around the home and available space. From several studies (Woodruff, 1954; Blenk and Trienes, 1956; Harrje et al., 1982; DeWalle and Heisler, 1983) optimum windbreak distance upwind from a home to achieve winter energy conservation is about 1-4 h. Wind-tunnel experiments under laminarflow conditions by Harrje et al. (1982) for two model windbreak densities show the minimum air infiltration parameter computed from pressure coefficients (see eqn. (2)) at about 4 h (Fig. 5). Minimum air infiltration occurred at 1 or 2 h in the field study by DeWalle and Heisler (1983) shown in Fig. 4. Because h changes as windbreak trees grow, a better frame of reference is probably the height of the home (h'). In Fig. 6, variation in the air-infiltration parameter with windbreak tree height from wind-tunnel tests by Harrje et al.
256 H 1.2 <
e-
O P E N R O W 4 . 9 m spacing
~
C L O S E D R O W 2.4 m spacing
IlJ I-UJ
:E
.4 z,
z
~c <
0
2'h
4'h
6h
8h
10h
HOUSE TO WINDBREAK DISTANCE (h = WINDBREAK HEIGHT)
Fig. 5. Variation in the air-infiltration parameter for a model home in the wind tunnel with distance between the windbreak and home for two windbreak tree spacings. Model tree height was equal to house height and windbreak length was approximately two house heights (adapted from Harrje et al., 1982). 1.2 Z
_o~ I--
ee-
.
.8
.4
o 0.0
i 0 5
i 1.0
i 1.5
TREE HEIGHT/HOUSE HEIGHT
Fig. 6. Variation in the air-infiltration parameter for a model home in the wind tunnel with windbreak tree height. Windbreak was at 1.2 house heights upwind from the house and windbreak length was approximately two house heights (adapted from Harrje et al., 1982).
(1982) shows that air infiltration decreases linearly with increasing tree height for trees ranging from 0.6 to 1.6 h'. Beyond 1.6 h' the effects of increasing tree height are unknown, but the benefits of rapid tree growth or planting large trees are obvious. To prevent shading when the trees reach the height of the home it is desirable that the windbreak be located at about 2 h', thus for a 6-m high two-story home windbreak trees should be planted about 12 m upwind (see Fig. 7A). For east- and west-facing walls the 2 h' is probably acceptable; however, for southfacing surfaces which receive considerable solar energy in winter a 3 or 4 h' rule is needed, to prevent shading at latitudes typical in the U.S.A. (Fig. 7B ). Windbreaks along north-facing walls could be closer than 2 h'. All this assumes adequate space is available on the lot. If it is not available, a homeowner probably should not use a windbreak to the south of the house.
257 A.I
NORTH/WEST
SOUTH/EAST WIND
windbreok
~
_ _ -
~ 2 h ' - -
B.
1 I'
~q
SOUTH
NORTH WIND
3o~"--~__2o'~ 3h ~
C.
-
"I
WEST
EAST WIND snow tence
It
I"
windbreak
- - 2 0 - 30h - - - - - - - - ~ ' ~
2 h] ~
Fig. 7. Recommendedplacement of a windbreakupwind from a home when (A) cold, high-speed winter winds originate from a westerlyor northerly direction, (B) winter winds originate from the south and home shading must be prevented and (C) westerlywinter winds are accompanied by blowingsnow for which a snowfence is also needed. Windbreaks protecting north-, east-, or west-facing surfaces can be planted in the space available with recognition t h a t they may grow too tall and need trimming. Shade on solar collecters will be particularly detrimental. Shade from windbreaks in summer on east or west walls can be beneficial in keeping the home cool, because these walls receive more solar energy t h a n a south wall in summer. Large snow drifts may form downwind from a windbreak where a b u n d a n t snow, high winds and few other obstacles exist to trap snow upwind. In these areas, drifts could form around homes hampering normal activities. Under such circumstances, and where space permits, it is best to locate the windbreak as suggested and use a snow fence (s) to trap snow before it reaches the windbreak (Fig. 7C ). Equilibrium drifts can reach lengths of up to 20-30 times snow fence
258 heights downwind from porous snow fence (see Tabler, 1980); however, smaller drifts are usually confined to lesser downwind distances. In reality, most homes in urban and suburban areas are protected from large drift formation by upwind obstacles. The length of a windbreak should be at least as great as the length of the home surfaces to be protected. Any extra length can be beneficial for protection from a wider range of wind directions. Protection on two or three sides of a property, e.g. to the west, north or east of a home may be a wise investment. Care must be exercised to minimize blockage of cool summer breezes with the windbreak. Windbreaks constructed from various materials such as wood can be used instead of trees and shrubs and have the advantage of providing instant benefits. Although windbreaks often consist of up to 10 rows of trees and shrubs, windbreaks for home energy conservation must often be used in urban/suburban areas with severe space limitations. Thus one, possibly two, rows of trees are commonly employed. Wind-tunnel studies (Harrje et al., 1982 ) have shown the marginal effectiveness of adding windbreak rows in reducing air exchange in homes. Spacing within rows of 2-3 m and 3-4 m between rows is often used (DeWalle and Farrand, 1978) with the lesser spacings producing the quickest results, but with some thinning possibly needed later. Trees should be planted in a staggered arrangement between rows, rather than directly downwind from one another (Wight, 1988). Species used for single-row windbreaks are generally conifers. Conifers with stiff branches which will not streamline in the wind and which reach to ground level are most desirable. A second row of rapid-growing, closely-spaced deciduous trees, such as hybrid poplar (Populus spp. ), can also be used to provide some quick protection while the slower-growing conifers reach effective height. Other characteristics of importance are climatic and edaphic requirements, insect and disease resistance, wildlife benefits and aesthetic qualities. ECONOMICS OF WINDBREAKUSE TO CONSERVEENERGY Economic analyses of windbreaks used for home energy conservation have been conducted (DeWalle, 1980; Heisler, 1984). These studies show that windbreaks can be cost-effective in terms of energy savings. The net benefit is strongly related to the type of planting material chosen. Planting seedlings keeps initial costs to a minimum, but considerable time is needed for the trees to grow before significant benefits are received. Planting large growing stock requires greater initial expense and professional help in planting, but paybacks may occur more quickly. Heisler (1984) found that with fuel escalation rate (FER) at twice the inflation rate (i) the net present value of a 20-year windbreak investment was greatest when medium-sized ( 1.5-m high ) planting stock was used, as opposed
259
to either seedling or larger ( 3.0-m high ) stock. When he assumed, either F E R = i or F E R = 0 and i = 4%, he found lesser net windbreak present values than for F E R = 2i. In these cases the use of seedlings was preferable. Without considering fuel price escalation, DeWalle (1980) concluded that use of seedling planting stock was economical, but that use of 2-m high balled planting stock was not cost effective. However, if any of the other windbreak benefits can be quantified and added to that for energy savings, the use of large trees could be economical. In many instances, proper care and maintenance will make the planting of smaller stock more economical and help reduce the waiting period. ACKNOWLEDGEMENT
Research by the senior author was largely funded by the United States Department of Agriculture Forest Service, Northeastern Forest Experiment Station through the Consortium for Environmental Forestry Studies.
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260 Heisler, G.M., 1984. Planting design for wind control. In: E.G. McPherson (Editor). Energy Conserving Site Design, Am. Soc. Landscape Architects, Washington, DC, Chapter 9, pp. 165-184. Heisler, G.M., 1986. Energy savings with trees. J. Arboricult., 12 (5): 113-125. Heisler, G.M. and DeWalle, D.R., 1988. Effects of windbreak structure on wind flow. Agric. Ecosystems Environ., 22/23: 41-69. Hunt, C.M., King, J.C. and Trechsel, H.R. (Editors), 1980. Building air change rate and infiltration measurements. Am. Soc. Test. Mater., Spec. Tech. Publ., 719, 185 pp. Malik, N., 1978. Field studies of dependence of air infiltration on outside temperatures and wind. Energy Build., 1 (3): 281-292. Mattingly, G.M. and Peters, E.F., 1977. Wind and trees: Air infiltration effects on energy in housing. J. Ind. Aerodynamics, 2: 1-19. Mattingly, G.M., Harrje, D.T. and Heisler, G.M., 1979. The effectiveness of an evergreen windbreak for reducing residential energy consumption. A.S.H.R.A.E. Trans., 85, (Part 2), pp. 428444. McNaughton, K.G., 1988. Effects of windbreaks on turbulent transport and microclimate. Agric. Ecosystems Environ., 22/23: 17-39. McPherson, E.G., 1984. Benefits and costs of energy-conserving site design. In: E.G. McPherson (Editor), Energy Conserving Site Design. Am. Soc. Landscape Architects, Washington, D.C., Chapter 2, pp. 17-32. Oke, T.R., 1978. Boundary Layer Climates. Methuen and Co., Ltd., London. 372 pp. Sinden, F.W., 1978. Wind, temperature and natural ventilation: theoretical considerations. Energy Build., 1 (3): 275-280. Tabler, R.D., 1980. Geometry and density of drifts formed by snow fences. J. Glaciol., 26(94): 405-419. Walk, M.-F., DeWalle, D.R. and Heisler, G.M., 1985. Can windbreaks reduce energy use in a mobile home park? J. Arboricult., 11 (6): 190-195. Wight, B., 1988. Farmstead windbreaks. Agric. Ecosystems Environ., 22/23: 261-280. Woodruff, N.P., 1954. Shelterbelt and surface barrier effects on wind velocities, evaporation, househeating, and snowdrifting. Kans. Agric. Exp. Stn., Tech. Bull., 77: 21-23.