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Daylighting and Thermal Assessment of Combined Dynamic Shading Systems on Energy Consumption in Educational Buildings.

INTRODUCTION

The last two decades have seen a remarkable surge in the number of superior and innovative building products that better assist in solar energy utilization in buildings. These include high performance glazing products, integrated shading and lighting controls, building-integrated photovoltaic-thermal systems and personalized comfort delivery systems. Many of these systems are complicated and often their optical, thermal and/or other system properties are not completely defined or understood. Complex shading devices fall under this category. The authors in their previous work have studied the thermal performance of a combined dynamic shading system consisting of an overhang, a tiltable internal fabric light shelf and automated bottom-up roller shades. It was compared to a widely used contemporary shading system--automated roller shades (Rao and Tzempelikos, 2012). However, it did not include a comprehensive daylighting study of the system.

Daylighting analyses of reflective overhangs and light shelves have confirmed that they reflect a significant amount of natural light through the auxiliary aperture and redirect it deeper into the room resulting in uniform interior illuminance distributions, and an enhanced perception of openness associated with it (Claros and Soler, 2001, Ochoa and Capeluto, 2006). The coupling of such daylight harvesting systems with electric lighting controls provides a comfortable and visually interesting environment for the occupants of a space while reducing energy consumption (Galasiu et al., 2003, Bourgeois et al., 2006, Ihm et al. 2009). A joint dynamic control strategy may also be designed to take advantage of thermal mass resulting in shifted peak load. However, inappropriate use of shading devices can result in problems such as increased overall energy use, excessive glare and thermal discomfort, as well as limited view to the outside. Hence, it is critical that for the successful implementation of complex shading systems, the effects on the following parameters be studied in detail - i) the physical characteristics of the device (geometry, position, reflectance), ii) the ability to reduce glare, iii) the potential to improve illuminance distribution in the room and iv) the potential to reduce energy use for both electric lighting and air-conditioning.

Therefore, for this analysis, two fundamental study objectives can be identified - a) the development of an integrated energy simulation engine to assess the daylighting and thermal performance of building shading devices, and b) to propose a universal metric to evaluate their impact on overall building energy use to facilitate the design process.

MODEL DETAILS AND METHODOLOGY

This study was focused on a classroom located in Chicago, IL measuring 10 m (33 ft) x 10 m (33 ft) in area with a 3 m (10 ft) ceiling height (Figure 1 a). It was side lit, with windows oriented due south. The facade consisted of a curtain wall with clear glazing and a lower spandrel section. The window to wall ratio was 72%. Given the apparent need for shading, two systems were analyzed - a) automated top-down roller shades (baseline system) and b) a combined dynamic shading system consisting of an overhang, an internal tiltable fabric light shelf and automated bottom-up roller shades (advanced system). In order to reduce the possibility of visual and thermal discomfort, both systems were actively controlled to block all direct sunlight to avoid over-heating and excessive daylight.

In case of the advanced system, the overhang and the light shelf divided the window into two independent sections - i) the view aperture (bottom window), and ii) the auxiliary aperture (top window). The overhang provides effective external shading to the bottom window only, while reflecting daylight into the classroom through the auxiliary aperture. For all sun positions, the overhang and the internal bottom-up roller shades together provide complete solar protection for the bottom window. The roller shades remain open from sunset to sunrise. The tilt of the internal light shelf (controlled in steps of 30[degrees]) determines the solar radiation entering the classroom through the top window. The light shelf remains closed at night. During the day, depending on the position of the sun, it tilts to keep out all direct light coming in through the top window. Previous work by the authors may be referenced to obtain more details on system setup and control strategies (Rao and Tzempelikos, 2012).

The simulation engine required the user to specify inputs such as building location, facade orientation, classroom dimensions, fenestration characteristics, shading device selection and properties, building construction, etc. Using solar angle calculations, an all-weather anisotropic sky model (Perez et al., 1990) was implemented to calculate the irradiance on the different portions of the facade resulting from the varying shading patterns. Solar angle dependent window optical properties were obtained from LBNL's Window 7 (Mitchell R et al., 2008) tool for representative glazing materials. Thus, natural light entering the classroom through the window, and the solar thermal radiation incident on the facade and entering the space through the fenestration were computed. At each time step, the daylighting module calculated the luminous exitance of the shaded and non-shaded window areas depending on the position of the shading device(s). Then, with the luminous exitance of window known, a hybrid ray-tracing and multiple bounce radiosity calculation was implemented to determine the luminous exitance of all internal surface after infinite inter reflections using the following radiosity formulation.

[mathematical expression not reproducible] (1)

where, [M.sub.i] was the final luminous exitance of a Lambertian surface 'i' [lm/[m.sup.2] (lm/[ft.sup.2])], [M.sub.oi] was the initial luminous exitance of surface 'i' [lm/[m.sup.2] (lm/[ft.sup.2])], [[rho].sub.i] was the diffuse light reflectance of surface 'i', [M.sub.j] represented the final exitance of all other surfaces in the room [lm/[m.sup.2] (lm/[ft.sup.2])] and [F.sub.ij] was the form factor between surface 'i' and all other enclosing surfaces in the room. Dynamic view factors were calculated in this time varying nine surface enclosure.

Once the final exitance of all room surfaces was established, configuration factors were computed from nine equidistant grid points on the work plane to all the surfaces that contribute to light at these discreet points. For example, configuration factor from all the points on the work plane to the floor was zero. Horizontal illuminance on the desks due to light transmittance through the light shelf was also accounted for. Then, illuminance at a point was calculated using the following formula.

[mathematical expression not reproducible] (2)

where, [E.sub.p] was the total horizontal illuminance at point 'p' [1x (fc)], [C.sub.pj] is the configuration factor from point 'p' to a surface room 'j', [M.sub.j] is the luminous exitance of surface 'j' after infinite inter-reflections [lm/[m.sup.2] (lm/[ft.sup.2])], [C.sub.pls] is the configuration factor from a point to the light shelf and [M.sub.ls] is the light flux transmitted through the shelf per unit area [lm/[m.sup.2] (lm/[ft.sup.2])].

Then, based on these work plane illuminance levels, daylighting metrics such as average daylight autonomy and useful daylight illuminances are calculated. When daylight-linked electric lighting control was implemented, depending on the control strategy, electric lighting loads were decreased to incorporate the effect of natural light. Since the zone can either fall into the IESNA illuminance category D or E, a work plane horizontal illuminance setpoint of 500 lx (46 fc) was used.

Concurrently, at each time step, the thermal module adjusted the internal heat gains from electric lighting to account for reduced lighting needs. Based on the solar loads, scheduled occupant and equipment loads and the resulting electric lighting loads, the thermal analysis module then utilized the explicit finite difference method to solve a thermal network model for each case and compute cooling/heating loads at each time step. The lighting and thermal loads were used to calculate an Annual Load Based Energy Consumption (ALBEC) value. Details on the algorithm implemented for the thermal analysis may be found in Rao (2011).

Annual Load Based Energy Consumption (ALBEC) Value

Annual Load Based Energy Consumption (ALBEC) value is defined as the integral of instantaneous loads contributing to energy consumption in a zone over time for the entire year. Three main ALBEC values are identified - a) ALBECc (Annual Load Based Energy Consumption - Cooling), ALBE[C.sub.H] (Annual Load Based Energy Consumption - Heating) and ALBE[C.sub.L] (Annual Load Based Energy Consumption - Lighting). Mathematically, each of them is defined as follows.

[mathematical expression not reproducible] (3)

where, [mathematical expression not reproducible] is the average hourly cooling load in the zone, [mathematical expression not reproducible] is the average hourly heating load in the space and [mathematical expression not reproducible] is hourly net electric lighting load in the space.

ALBEC is a direct indicator of the effectiveness of an envelope element in tapering off the effect of external factors affecting annual energy consumption in building perimeter zones. It is a universal overall energy performance metric; a single value that factors in all the key elements affecting the energy performance of building envelope elements, in this case, specially applied to complex shading devices in an educational setting. It accounts for three key parameters affecting the energy performance of all building envelopes - i) thermal performance (assembly U-value and thermal mass of opaque construction, glazing properties, internal and external heat transfer coefficients, etc.), ii) daylighting performance (visible light transmittance) and iii) infiltration. However, it must be noted the ALBEC depends on a number of zone parameter such as internal surface reflectance, internal heat gain contributors, building location and orientation. These affect the overall ALBEC value.

RESULTS

A number of parametric studies were carried out to contrast the daylighting, thermal and overall energy performance of the two shading systems using a variety of shade fabrics, control strategies, glazing types, shading device placement, etc. Representative results are presented below.

Enhanced Spatial Illuminance Distribution

One of the key benefits of using the overhang and light shelf combination is that the overhang provides effective external shading to the bottom window, and that it works in tandem with the light shelf to redirect light further into the classroom. Consequently, high illuminance levels close to the window are eliminated while facilitating deeper penetration of natural light. Since the bottom window is not completely closed, it also provides view to the outside (that would not have been possible with standard automated roller shades). This is clearly demonstrated in Figure 2, where horizontal illuminance along the center of the classroom is plotted as a function of distance from the window, for three shade fabrics with a diffuse light transmittance of 3%, 7% and 12%, for the baseline (Base) and advanced (Adv) shading systems on March 3rd at 10 am standard time.

As expected, for the baseline it was observed that desk illuminance is greatly dependent on visible light transmittance of the shade fabric. This is particularly true for grid points close to the window. For a light weave fabric with high visible light transmittance, the illuminance in the occupied zone (1 m (3.3 ft) from the window to 9 m (29.5 ft) from the window) was found to be between 963 lx (89 fc) and 245 lx (23 fc). For a medium weave fabric, the range was 562 lx (52 fc) to 131 lx (12 fc).

For the combined system, the internal light shelf is tilted at 30[degrees] and the bottom window is 25% open. Therefore, although illuminance close to the window increases with increasing shade transmittance, the effect is not as profound as in case of the baseline. The shade transmittance had a smaller effect on horizontal illuminance beyond 4 m (13 ft) from the window; illuminance in this portion of the classroom was more affected by the top window size and overhang and light shelf properties.

Comparison of the two systems shows that for all shade fabrics, the advanced system provides higher work plane illuminance. Besides, for a low transmittance shade, the ratio of maximum work plane illuminance at any point in the classroom, to the minimum work plane illuminance was 2.3 for the advanced system. The ratio increased to 4.6 for the baseline system, indicating that combined shading provides better spatial illuminance distribution.

Effective Shading System Characteristics & Lighting Controls

Annual electric lighting energy savings resulting from the combined use of the advanced shading system and modern lighting control strategies was compared to the baseline system with identical controls. Six different shading system configurations and three prevailing electric lighting control strategies were tested; the results are presented in Figure 3. System setup, shade fabric properties, and shading device arrangement are outlined in Table 1.

Three types of lighting control strategies - i) on/off, ii) three stepped on/off and iii) continuous dimming were tested for an illuminance setpoint of 500 lx (46 fc). For on/off control, the photosensor was assumed to be placed at 7.5 m (25 ft) from the window to ensure adequate light was available on majority of the desks when all the lights were turned off. In case of stepped control, the lighting fixtures were divided into three batches of luminaires along the breadth of the room. Each batch was switched on or off based on the illuminance recorded by the photosensors places directly under it (2.5 m (8 ft), 5 m (16 ft) and 7.5 m (25 ft) from the window). For continuous dimming, each luminaire was independently controlled to supplement natural lighting to ensure that only the minimum threshold for horizontal workplane illuminance was met.

Unsurprisingly, a baseline system with higher shade transmittance resulted in greater annual lighting energy savings. With on/off lighting controls, a 10% transmittance shade cut down annual lighting energy use by 10%, while a fabric with 5% transmittance resulted in 4% electric lighting energy savings.

For the same shade fabric, irrespective of the lighting control strategy utilized, the advanced system provided greater electric lighting energy savings compared to the baseline. Annual energy savings for the Adv-1 system with continuous dimming was found to be 50%, much higher than that for the baseline (28%). Since the light shelf redirects light deeper into the room, the effect is more prominent for on/off lighting control.

For the different configurations of the advanced system that were tested, it was observed that a large overhang (0.6 m (2 ft) ) and large auxiliary aperture combination (Adv-2) resulted in minimum lighting energy consumption for all three control strategies. When comparing cases Adv-3 and Adv-4, the increased effect of a larger auxiliary aperture on electric lighting energy savings is not as prominent since the smaller overhang has higher losses associated with it and is less effective in reflecting light to the top window. With on/off lighting controls, the use of Adv-4 configuration resulted in 23% decrease in annual lighting energy use compared to a 20% decrease with Adv-3 configuration. The independent and proportional dimming of each luminaire resulted in much higher savings compared to the two on/off control strategies.

Peak Cooling and Heating Loads

One of the key benefits of using external shading is the reduction in peak cooling loads. Automated roller shades with three shade fabrics (absorptance = 18%, 23% and 27% and constant reflectance = 70%) were tested and compared to two different arrangements of the advanced system for a single shade fabric with an absorptance, [alpha] = 23%. The results are presented in Figure 4.

In case of the baseline, the automated roller shades occupy a large area and experience high solar irradiance. Depending on the fabric, this could either be reflected back through the window or absorbed by the shades. With higher shade absorptance, its peak temperature increases, resulting in greater peak cooling loads. Peak cooling load went up 9% (from 3.20 kW to 3.53 kW) when the shade absorptance was increased from 18% to 27% for a constant reflectance of 70%.

In case of the combined system, with the overhang placed 1.2 m (4 ft) above the windowsill, two overhang widths were tested, 0.3 m (1 ft) and 0.6 m (2 ft). The larger overhang shades the bottom window more effectively, and consequently the area of the bottom window covered by the shades was reduced. Comparing the combined system with a large overhang to the baseline, peak cooling load reduced was by 14%, from 3.40 kW to 2.91 kW.

In the winter, because of the low sun angles and the tilt of the light shelf, the advanced system allows more heat gains through the auxiliary aperture. Therefore, compared to the automated roller shades, the combined system decreased peak heating load by 8%.

Room Comfort Setpoints, Lighting Controls and the Positioning of Shading Device

A parametric study was carried out to analyze zone setpoints and lighting control strategies to quantify their potential for overall energy saving in the classroom (Figure 5 a). Both shading systems used the same shade fabric with 5% transmittance. The combined system was assumed to have 0.6 m (2 ft) overhang with the bottom window being 1.2 m (4 ft) high. It was observed that independent of the type of shading system used or the lighting control strategy implemented, lowering the illuminance setpoint from 500 lx (46 fc) to 450 lx (42 fc) resulted in overall energy savings. When using on/off control with the advanced system for a 500 lx (46 fc) setpoint, the ALBEC value was 7,302 kWh, it reduced to 7,133 kWh for a 450 lx (42 fc) setpoint, a potential energy saving of up to 2.5%. This was mainly attributed to the reduction in ALBECc that went down from 3,236 kWh to 3,187 kWh and ALBECL that reduced from 2,023 kWh to 1,903 kWh. ALBECH largely remained unchanged. A classroom equipped with the combined shading system using continuous dimming with an illuminance setpoint of 450 lx (42 fc) demonstrated a 20% potential for energy saving over the automated roller shades with identical control and setpoints. ALBEC decreased from 7,851 kWh for the baseline to 6,256 kWh for combined shading.

ALBEC can be used to easily and clearly compare various system characteristics such as the placement of the overhang and its width. Figure 5 b) shows ALBEC values for an analysis conducted to compare the baseline system with 5% transmittance shades to the advanced system with the same fabric. On/off lighting controls were implemented with an illuminance setpoint of 500 lx (46 fc). The baseline ALBEC was found to be 8,717 kWh. For the combined system, with a 0.6 m (2 ft) overhang placed 1 m from the sill, the ALBEC was 7,185 kWh, an 18% reduction over the baseline. In addition, if a smaller overhang (0.3 m (1 ft)) is used, because of its reduced efficacy in shading the building envelope, ALBECc increased 5% from 3,174 kWh to 3,339 kWh while ALBECH decreased 2% from 2,110 kWh to 2,071 kWh.

CONCLUSION

Integrated energy simulation models were developed to determine established thermal and daylighting performance metrics for a contemporary shading device and a novel combined dynamic shading system. A new energy performance metric called the ALBEC was proposed to evaluate building envelope elements, specifically complex shading devices. Together they provide a solid platform to compare the energy benefits of one system over another.

The analysis showed that daylight-linked electric lighting control is an important energy saving strategy, and it should be implemented irrespective of the shading system being used. Selecting a system based on ALBEC values, the combined shading system with daylight-linked continuous dimming lighting controls has the greatest potential to save energy, and thus it is highly recommended for perimeter classrooms. The auxiliary window and the light shelf help in redirecting natural light deeper into the space while reducing over-illumination close to the window, consequently providing good quality natural light in this learning environment. Combined shading also helped to reduce peak cooling and heating loads, thus reducing equipment first cost. However, if automated roller shades are used, given the large shade area, the choice of fabric properties is a key design decision. A light weave fabric with appropriate visible light transmittance and low absorptance is recommended to allow more sunlight into the classroom, while reducing cooling loads. It must be noted that elevated shade temperatures can also be a source of thermal discomfort for occupants.

REFERENCES

Bourgeois D., Reinhart C. 2006. "Adding advanced behavioral models in whole building energy simulation: A study on the total energy impact of manual and automated lighting control." Energy and Buildings Vol. 38, No. 7, p. 814- 823.

Claros T., Soler A. 2001. "Indoor Daylight Climate-Comparison between Light Shelves and Overhang Performances in Madrid for Hours with Unit Sunshine Fraction and Realistic Values of Model Reflectance." Solar Energy, Vol.71, No.4, p. 233-239.

Galasiu A., Atif M., MacDonald R. 2003. "Impact of window blinds on daylight-linked dimming and automatic on/off controls." Solar Energy, Vol.76 No. 5, p. 523-544.

Ihm P., Nemri A. and Krarti M. 2009. "Estimation of lighting energy savings from daylighting." Building and Environment, Vol. 44, No. 36, p. 509-514.

Mitchell R., Kohler C, Klems J. 2008. "Window 6.2/Therm 6.2 Research Version User manual." Lawerence Berkeley National Laboratory Ochoa C.E., Capeluto I.G. 2006. "Evaluating visual comfort and performance of three natural lighting systems for deep office buildings in highly luminous climates." Building and Environment, Vol. 41, No. 8, p. 1128-1135.

Perez R., Ineichen P., Seals R., Michalsky J., Stewart R. 1990. "Modeling Daylight Availability and Irradiance Components From Direct and Global Irradiance." Solar Energy, Vol. 44, No. 5, p. 27 -289.

Rao and Tzempelikos. 2012. "The Impact of a Combined Dynamic Shading System on the Thermal Performance of Building Perimeter Zones". ASHRAE Winter Conference, Chicago, USA.

Rao S., 2011. "Thermal and daylighting analysis of building perimeter zones equipped with combined dynamic shading systems". M.S.E. Thesis, Purdue University.

Sagar Rao

Associate Member ASHRAE

Athanasios Tzempelikos, PhD

Associate Member ASHRAE

Sagar Rao is a Sustainable Systems Analyst with Affiliated Engineers, Inc., Madison, Wl. Athanasios Tzempelikos is an Assistant Professor at the School of Civil Engineering at Purdue University, West Lafayette, IN.
Table 1. Cases Analyzed & System Characteristics

Test                              Fabric      Bottom Window  Overhang
Configuration  Shading System  Transmittance  Height [m]     Width [m]
                                    [%]

Base-1         Baseline              5        N/A            N/A
Base-2         Baseline             10        N/A            N/A
Adv-1          Advanced              5        1.2 (4 ft)     0.6 (2 ft)
Adv-2          Advanced              5        1.0 (3.3 ft)   0.6 (2 ft)
Adv-3          Advanced              5        1.2 (4 ft)     0.3 (1 ft)
Adv-4          Advanced              5        1.0 (3.3 ft)   0.3 (1 ft)
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Author:Rao, Sagar; Tzempelikos, Athanasios
Publication:ASHRAE Conference Papers
Date:Dec 22, 2014
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