Effect of Shading
Devices on Residential
Energy Use in
by
Randy
K. Pletzer, Jerold W. Jones, and Bruce D. Nunn, PhD
Center
for Energy Studies
The
June 1988
EXECUTIVE SUMMARY
This
report presents the results of an analytical study of the effect of shading
devices on the annual heating, cooling, and total energy use, on summer
peak electric demand, and on energy cost savings in single-family residences
in
Results
are presented in terms of annual heating and cooling energy use and energy
costs, with each shading device in place, as compared to baseline reference cases for three
prototypical residences. The devices are ranked in terms of energy cost savings. While
the five bests performing devices have annual cooling energy savings
ranging up to 32%, the annual energy cost savings (at 1985 Austin, TX utility
ratings)range from $5 to 14%. Another
significant result is the multiple-regression correlation of normalized annual
heating and cooling energy
savings with Shading Coefficient (SC) and U-value. By means of this correlation the results of the study can be
applied to any single-family residence in
Performance Analysis of Shading
Device
Shading devices of three basic types
were analyzed in this study:
·
reflective and tinted glazing (including films)
·
interior devices (louvered blinds, draperies and curtains, planar
roller or hanging shades, and shutters)
·
exterior devices (solar screens, awnings, overhangs, and the effects of recessed windows and vegetation)
A data base of the
thermal/optical characteristics (principally Shading Coefficient and U-value)
of these devices, which was developed from manufacturer's and technical
literature, is included in Appendix A of this report.
Direct
and diffuse solar gains were analyzed using the standard heat gain methodology
for fenestrations developed by ASHRAE, The American Society of
Heating, Refrigerating, and Air-Conditioning Engineers. This methodology was implemented in the DOE-2 building energy analysis
computer program that was used for the simulations. Each
interior shading device was characterized by a Shading
Coefficient and a U-value; where applicable, these values were scheduled
to represent managed (operable) devices. The exterior devices were characterized by Shading Factors (time-averaged
Shading Coefficients) calculated external to the simulation program; however,
at each simulated hour the program calculated the shading pattern on the
glazing and calculated the transmitted solar gain.
Parametric Analysis of Prototypical
(Baseline) Residences
The set of three prototypical
residences was representative of small, medium, and
large houses, as well as a range of building age, thermal integrity, and occupant energy use
patterns. A nominal baseline (gas heating, single-pane glazing, and nominal shading from eaves and neighboring buildings) was
established to represent the most likely configuration for each of the three houses.
However, to test the sensitivity of the shading device effects to key features of the baseline models, baseline variants
were developed for an all- electric
house, a house with double glazing, and one with no eaves or neighbors
(bare case). these baseline variants were run only for selected shading
strategies.
Residence 1,
representative of pre-1964 vintage, was a single-story, two-bedroom house
of 1,008 ft representing old frame
construction. It has an R-19 ceiling, R-11 floor, and R-2 (un-insulated) walls,
and room air conditioners (EER =6.55). Residence 2 was a single-story,
four-bedroom, 1,543-ft house of 196473 vintage.
It was moderately insulated with R-19 ceiling and
R-11 walls, and had a 2.5-ton central air conditioner of EER = 7.78. Resident3
was a 2,782-ft, two story, four bedroom
house representative
of new, large home construction. Thermal
integrity was high with-R-19 ceiling, R-11 walls, and tight construction. Two central
air conditioners (1.5-ton and 3-ton units) had EERs of 7.92. The Residence 2
and 3 prototypes were calibrated and validated by comparing DOE-2 simulated monthly and annual electric and gas use with
metered data.
Sensitivity cases were run for
the nominal
baselines to establish the maximum effect
of the use of shading devices and to determine the sensitivity of energy use to
building orientation and to the distribution of shading on each facade.
The results showed that if all solar gains were eliminated (zero Shading
Coefficient) the annual energy cost decreases 7-9%. Thus, the insulation value of a shading device can be
significant, even in the cooling- dominated
Results
of the baseline variant sensitivity runs showed that the double-pane
baseline has slightly lower energy use (5-7% lower) and
energy cost (3-5% lower) than the nominal baseline case. In contrast,
the annual energy costs for the bare
baseline slightly exceed (by about 3%) those for the nominal baseline.
This difference indicates the magnitude of the effect of nominal eaves and neighbor shading.
Results of Shading Strategy Analysis
Performance
Relative to Nominal Baseline Residences
Comparisons
of performance relative to the nominal baseline residences show that
for all three
residences the interior strategies (including solar screens)
consistently outperform the exterior strategies in terms of the energy cost savings. The likely reason for this
finding is that the exterior devices, assumed to be in place year round, provide no U-value improvement while they
reduce beneficial wintertime solar gains. Thus awnings should be removed in the wintertime to be effective.
However, despite the very modest annual
energy savings of the exterior devices, they do significantly reduce
summer peak loads more than 7% in the largest house.
The annual energy
cost savings for the top strategy (solar screens with the best properties) range from 10 to 14% ($140-$240/Year) (calculated at 1985 energy prices your savings will be higher depending on the
increase in electric costs since 19851; the comparable
savings
for the top exterior device (3-foot awning with side fins)
are only 2%. The relative rankings are generally independent of residence size and thermal integrity, although
the greatest relative savings occur for Residence 3.
In terms of annual energy
cost savings, the best five strategies are:
·
Best Available Solar Screen (SC x 0.14)
·
Reflective Film (SC = 0.23)
·
Best Drapery/Curtain (SC = 0.15)
·
Worst Available Solar Screen (SC = 0.44)
·
Operable Blinds with Closed Position at 45 (SC = 0.51
Although annual cooling energy savings for these top
strategies range from 22 to 32%, and
summer peak reductions range from 4 to 22% for the 2,782 square foot house, the annual energy cost savings range only from 10
to 14% for the three residences. Annual
cost savings include the net
effect of cooling and heating load tradeoffs and the differential effects of gas
and electricity utilization efficiencies and prices.
Among the interior
strategies, the operable drape/curtain with the worst available properties and
tinted windows rank the lowest, saving only 1 to 3% in annual energy costs. The
1-foot overhang and the 6-inch recessed windows rank lowest among the exterior
strategies, saving energy costs of at most
0.3%.
Performance
Relative to Baseline Variants
When
selected shading strategies were simulated for Residence 2, but with electric space heating (either resistance or heat
pump) and electric resistance water
heating instead of gas space and water heating, the performance ranking and
relative savings did not change significantly, with one exception: the Worst Solar
Screen was replaced by the Planar Shade in the top five performers.
Because interior strategies provide an extra measure of window insulation, more of them rank above the 50th
percentile in performance for the all-electric baseline as compared with the
mixed-fuel (nominal) baseline.
Similarly, with the bare
baseline variant (no eaves or neighbor shading) the annual energy cost rankings of the strategies analyzed are unchanged
compared to those for the nominal baseline. However, the presence of the
nominal shading diminishes the effect of the
shading devices on energy cost, so that houses with completely un-shaded
fenestration experience 2 to 5% greater energy cost savings with shading devices
than houses with nominal shading to begin with.
The
annual energy cost of strategies applied to double-pane windows is less than
that for single-pane windows; the difference is most pronounced for Residence 3. However,
because the relative reduction in heating and cooling energy differs for the singe- and double-pane baselines, some shading
devices perform better when applied to double-pane windows than when applied to
single-pane_ windows._
Selected
shading strategies were compared with a 30% reduction in infiltration and the installation of
clear storm windows. The results showed that these alternative strategies are more effective in reducing annual energy costs than is
the 3-foot awning with
sides, but they are only half as effective as the Best Solar Screen. The
solar screens are three times more effective as the alternative strategies in
reducing summer peak demand.
Finally, the effect of shading device management is
illustrated by comparing results for the operable mode versus those for the closed mode (device
in place during all hours). The results
indicate that this extreme approach (closed mode) increases energy cost savings by a factor of two over the managed mode savings.
Any shading device to any
single-family residence in
Conclusions
The most significant conclusions of this study are that:
1.
The top strategy in terms
of energy performance and energy cost savings is the last Solar Screen (IC at
0.14). Although the annual cooling energy savings were 32% for this device, the annual energy cost
savings were 14%, the summer peak load reduction was 22%.
2.
As a group the interior strategies (which include solar screens) perform
better, with very few
exceptions, than the exterior strategies. The reason is that the interior
strategies not only provide effective solar gain control, but also improve
the U-value of the window. Thus even
for the cooling-dominated climate of
3.
Annual heating and
cooling energy savings, normalized by glazing area, correlate well with the weighted Shading Coefficient and U-value of a
shading device, allowing a generalized method for predicting the annual
energy savings for a residence of any size and thermal integrity in