POTENTIAL ENERGY SAVINGS WITH EXTERIOR SHADES IN LARGE OFFICE BUILDINGS AND THE IMPACT OF DISCOMFORT GLARE Sabine Hoffmann, Eleanor Lee
Objectives of the simulation study 1. Determining the best geometry for fixed exterior slat shades (lowest energy use intensity - EUI) 2. Evaluating the interaction of exterior shades and daylighting controls 3. Evaluating the occurrence of discomfort glare with exterior shades and the influence of glare control on energy use intensity
Simulation model: DOE prototype Large office building 12 story high rise Variation of WWR (window-to-wall):, 1, 3, 4, 6 % Layout: Perimeter zone Core zone Open plan office
Simulation parameters Heating Cooling: Variable Air Volume (VAV) Lighting power density: 7. W/m 2 at % Target illuminance: lux Base glazing: U-value (COG) = 1.4 W/(m 2 K) (.238 Btu/(h-sf-F)) SHGC =.3 Tvis =.6 Shades: fixed exterior shades automated interior shades for glare control: deployed when discomfort glare occurred for the reminder of the day
Simulation parameters Radiance evalglare 9 viewpoints Discomfort Glare Probability DGP, Index DGI Glare control: if DGP.38 or DGI 24 then interior shade gets deployed
Solving the trade-off Transparent areas: solar heat gain daylight Desirable in a Chicago winter climate Not desirable in summer for internal load dominated office buildings in most US climates Transparent areas: solar heat gain daylight With daylighting control?! More daylight: less electric light more solar heat gain risk of discomfort glare Less daylight: more electric light heat gain from electric light less discomfort glare
Slat shading shd 1 shd 2 Variation of slat angle and cut-off angle 4, 3, 1, shd 3 shd 4 shd
kwh/(sf-yr) kwh/(sf-yr) best geometry for fixed exterior slat shades without daylighting control Blue bars: Not window related energy use intensity simulation model without windows Grey bars: Window related energy use intensity 28 % reduction for both climates, Chicago and Houston WWR 6, NDC, all orientations WWR 6, NDC, all orientations Chicago WWR Chicago WWR 6 Houston WWR Houston WWR 6 3 3 2 2 1 Source energy shd Site energy 3 3 2 2 1 Source energy shd Site energy
kwh/(sf-yr) kwh/(sf-yr) exterior slat shades with daylighting control 26 % reduction for Chicago Light blue bars: overall energy use intensity White contours: savings compared to fully opaque wall 24 % reduction for Houston WWR 6, DC, all orientations WWR 6, DC, all orientations Chicago WWR Chicago WWR WWR 6 Houston WWR Houston WWR 6 3 3 2 2 1 Source energy shd 2 Site energy 3 3 2 2 1 shd 2 Source energy Site energy
More shades shd 6 shd 8 shd shd 6 shd 8 shd
Occurrence of discomfort glare Discomfort glare: DGP.38 or DGI 24 Cut-off angle 4 Cut-off angle 3 Rvis =. Rvis =. Cut-off angle 3 Cut-off angle 3 Rvis =.87 Rvis =.4
South West Shd 2 Shd 7a Shd 7c
kwh/(sf-yr) kwh/(sf-yr) impact of glare control on energy use intensity Without glare control, all 12 exterior shades behave similar With glare control, there can be a huge spread With daylighting control and glare control, benefits in Chicago are moderate (WWR > 4 %) For small WWR exterior shades are not recommended Chicago, DC, west, source, 2 no glare control 2 1 % 7% 17% 11%,1,3,4,6 WWR Chicago, DC, west, source, 2 glare control code % % code 8% shd 1 2 6% shd 2 4% shd 3 1 6% 2% shd 4 % 12% shd % shd 6-2% shd 7a -4% shd 7b -6% shd 7c -8% shd 8 -% shd 9,1,3,4,6 shd WWR
kwh/(sf-yr) kwh/(sf-yr) impact of glare control on energy use intensity With daylighting control and glare control, benefits in Houston are still significant for WWR > 4 % for most exterior shades Glare control really impacts the EUI significantly Houston, DC, west, source, 3 no glare control 2 2 1 % 11% 23% 22%,1,3,4,6 WWR Houston, DC, west, source, glare control 3 code % % code shd 1 2 shd 2 % shd 3 2 % % shd 4 22% 2% 1 % shd shd 6 shd 7a -% shd 7b shd 7c shd 8 -% shd 9,1,3,4,6 shd WWR
Conclusions Without daylighting control (NDC), the most blocking slat shades perform best for cooling load dominated buildings Obvious! With daylighting control (DC), there is an optimum cut-off angle. Check it for your location! With glare control, overall savings that can be achieved through exterior shades are less, and the spread amongst the different shades is bigger than without glare control. It matters which shade you use!