Energy nomographs as a design tool for daylighting

Energy and Buildings, 6 (1984) 197 - 205

197

Energy Nomographs as a Design Tool for Daylighting ALVIN M. SAIN, PETER G. ROCKWELLand JAMES E. DAVY Burt Hill Kosar Rittelmann Associates, 400 Morgan Center, Butler, PA 16001 (U.S.A.)

SUMMARY The purpose o f this paper is to inform commercial building designers about an energy analysis tool which can aid them in making appropriate decisions about daylighting. The energy nomographs are an energy design tool which calculate the annual energy consumption o f commercial buildings, including lighting, heating, cooling, domestic hot water, fans, pumps, and miscellaneous items. This paper specifically discusses the daylighting aspects o f the tool. The calculation procedure is presented with an example to explain how this design tool can be used to make good energy decisions early in the design process.

INTRODUCTION Lighting now exceeds heating or cooling as the major energy user in most commercial buildings because of current energy costs and codes requiring insulation and solar shading. Although new energy-efficient artificial lighting systems are available, designers have been skeptical of their efficiency claims and are concerned with color rendition. Daylighting has become a popular design strategy since it reduces lighting energy consumption and, at the same time, introduces new variables which the designer can manipulate to increase visual stimulation within the space. Daylighting also avoids the "closed-in b o x " approach to energy-conscious design and can enhance the habitability of interior spaces. As with any passive feature, decisions about daylighting must be made early in the design process since basic architectural elements of a building are involved. An effective design m e t h o d must be available to determine the effects of various daylighting options quickly and accurately, including the effect 0378-7788/84/$3.00

on heating and cooling energy consumption as well as lighting. The energy nomographs are a graphic design tool developed to aid this design method. They can be used quickly in the schematic phase to determine the total impact on energy consumption of different building designs, including buildings with daylighting features. The nomographs can also be used to predict the energy effects of other energy-conserving features, such as building configuration, orientation, and insulation levels. There are a total of 18 homographs in the package and an input data manual. The first three nomographs are used to determine Rvalues, U-values, and shading coefficients in conformance with the ASHRAE 90A-80 Standard. The next four are the lighting/ daylighting nomographs described in detail in this paper. The next eight are nomographs used to determine peak and annual heating/ cooling loads. The annual loads are calculated using a modified degree-day m e t h o d based on building balance point temperature and building solar characteristics. This m e t h o d allows an hourly weather tape (such as TMY or Solmet) to be summarized in a one-page table of degree-days for each of four building types (office, schools, retail stores, and multifamily residential). The final three nomographs determine the domestic hot water (DHW), miscellaneous, fan, and pump loads, and convert all the loads into annual consumption based on annual equipment efficiencies. The input data manual provides default values for all the non-architectural input data, such as people/m 2 (people/ft:), typical miscellaneous equipment loads, skylight and window data, etc. It also includes all of the daylighting, heating, and cooling weather data for m a n y cities throughout the United States. The total package will be publicly available in the early months of 1984. © Elsevier Sequoia/Printed in The Netherlands

198 USING THE ENERGY NOMOGRAPHS

Required 'building information Only a minimum of information about the building is required to begin using the energy nomograph procedure. All of this information is normally available at the schematic design phase, including basic dimensions of walls, floors, rooms, windows and the roof. Additional inputs are determined during the calculation procedure. Once the basic building information has been collected, the daylighting calculation can begin.

Daylighting Design Nomograph for Vertical Windows The energy nomographs used for daylighting allow the designer to see the effects of changes in architectural elements without bogging him down with the need to develop conversion factors, locating proper weather data, or performing repetitive calculations. Figure 1 illustrates the Daylighting Design Nomograph for Vertical Windows. This nomograph is used to determine the coefficient of utilization for a design strategy and to approximate the light level provided by daylighting. The example calculation shown is for the wall section in Fig. 2. The graphic procedure (Fig. 1) begins at scale 'A' -- distance from window 3 m (10 ft), then follows through block 'B' -- window height 1.8 m (6 ft), block 'C' -- glazing light transmittance (55%), block 'D' -- ceiling reflectance {80%), block 'E' -- back wall reflectance {no back wall), and ends at scale 'F' -- daylight coefficient of utilization (0.075). Each turning point represents a mathematical relationship graphically determined with a pen and straight edge; no calculations are necessary. Block 'G' - - l u x (footcandles) on the exterior {outside of the glazing} and scale 'H' -- average lux (fc) level (on the work surface) are optional turning points that determine the average light level for various exterior conditions provided in the input data manual.

Daylight Design Nomograph for Skylights The Daylighting Design Nomograph for Skylights, Fig. 3, is used for areas where an even pattern of small skylights is used to provide general daylighting throughout a space. (Roof monitors, clerestories, atriums, and

large area skylights are not covered by this nomograph.) The result of this nomograph is also the coefficient of utilization for a skylight system and the average daylight level provided. The graphic procedure for this nomograph is similar to the one described above: Fig. 3 shows a sample calculation with results.

Weather data The next step is selecting the appropriate weather data from the input data manual. The daylighting weather data (Fig. 4) is given as full-load hours of artificial lighting. This data has been determined using a computer program which calculates the hourly daylight levels on each orientation and determines the quantity of artificial light required to supplement the daylight for two control types. Therefore, the full-load hours must be selected for the following conditions: -- location; - - b u i l d i n g type {which determines the standard occupancy profile}; - - lighting level desired in the space (lux (fc)); --daylight coefficient of utilization (determined by the Daylight Design Nomographs); -- control device {dimming or switching); -- daylight orientation (H, N, S, E, or W). Full-load hours for the vertical windows can be found for each glazing orientation (N, S, E and W) or, as was done in this case, the full-load hours for the four orientations can be averaged. The number of full-load hours for an interior space without daylighting is 2280, {based on standard office business hours}, the average for the exterior areas with vertical windows is 875, and for the skylit area the average is 1040.

Lighting Consumption Nomograph After completing the Daylighting Design Nomographs and selecting the appropriate weather data, the final step is completing the Lighting Consumption Nomograph, Fig. 5. Locate the watts/ft 2 on scale ' A ' - - l i g h t i n g system. The value of 18.8 W/m 2 (1.75 W/ft 2) was determined using a Lighting Design Nomograph (not shown) for a lighting level of 540 lux (50 fc). Continue through block 'B' -full-load hours {from the weather data), then through block 'C' -- diversification factor (90%) to scale 'D' or 'E' -- zone lighting load

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in kWh/ft 2 per year or MBtu/ft 2 per year. Proceed through block 'F' -- area of zone per building. For this calculation the interior zone is assumed to be 55% of the total area and the daylit zone is 45%. Conclude at scale 'H' or 'G' -- building lighting load. The results indicate that the interior zone uses 19.6 MWh/ year per m 2 (6.2 MBtu/year per square ft) of building and the daylit zone uses 5.7 (1.8) for a total of 25.3 MWh/yr m 2 (8.0 MBtu/year ft 2) of building.

Other nomographs The energy nomographs are capable of calculating energy consumption for heating, cooling, DHW, fans and pumps as well as lighting. A reduction in lighting consumption due to daylighting will affect the heating and cooling consumption and possibly the fans and pumps; consequently, the magnitude of this effect must be determined in order to analyze the true value of a daylighting scheme. The heating and cooling consumption is calculated in a three-step process. First, the

thermal properties of all building components are combined to describe the building with two numbers: (1) the balance point temperature (BPT} which takes into account the shell losses and the internal gains, and (2) the solar factor (SOL" F), which addresses the quantity and type of glazing on all orientations, and the shell losses. Second, the BPT and the S O L . F are used to select the proper weather data for the location in question. (The weather data has been produced using a computer program and TMY or SOLMET weather tapes for a variety of locations in the United States.) Data is given in sol-aire degree-hours for conduction, sensible and latent degreehours for outside air, full-load hours for internal gains, and Btus/ft 2 for solar heat gain. The third and final step is to complete a calculation for each c o m p o n e n t using the weather data found in step two. The various component loads are summarized to give the annual heating and cooling loads. Annual heating and cooling system efficiencies are then determined based on the annual loads and a peak load analysis (also completed using

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the energy nomographs). The heating and cooling consumption is then calculated using the loads and annual efficiencies. The energy nomograph calculation of heating and cooling is affected by a daylighting strategy in the following ways. A reduction in internal gains due to daylighting will change the balance-point temperature of the building which changes the degree-day weather data for all components. Daylighting may also change the peak cooling load which, in turn, changes the annual efficiency and the fan kilowatts required. The interdependence of various building loads is complex. The energy nomographs provide a simple technique which addresses this interdependence and provides quick

answers which are accurate enough to eliminate poor design strategies and verify successful ones. Two strategies which have similar performances according to the energy nomographs may be analyzed by a more advanced calculation technique to assist in selection of the optimum strategy.

EXAMPLE CALCULATIONS FOR SCHEMATIC DESIGN

Base building The base building is a 5-story office building in Pittsburgh, Pennsylvania. The information which was available at schematic design is shown in Fig. 2. The base building was

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analyzed assuming that no automatic controls were used to accommodate daylighting and that the building consisted of a perimeter and an internal zone only.

Daylighting modifications to the base building Various daylighting modifications were analyzed to determine the most cost-effective option. In all cases, the heating and cooling zones and variables were held constant. The daylighting options analyzed were: A -- To add automatic lighting controls to dim the fluorescent lights in the 4.6 m (15 ft) perimeter zone by installing a sensor 3 m (10 ft) from the window. {This option does not require any additional glazing since the owner requested a 1.8 m (6 ft) window in the base case.) B -- To install the sensor 4.6 m (15 ft) from the window and thus control fixtures in a 6.1 m (20 ft) perimeter. This option will reduce the daylight coefficient of utilization but will increase the number of lights being controlled, and, C -- To install skylights in the interior zone of the top floor. Since the performance of option 'B' was predicted to be slightly better than option 'A', the 6.1 m (20 ft) perimeter zone was maintained.

Results o f calculations Figure 6 illustrates the results of the energy nomograph calculation procedure for heating, cooling and lighting costs only. The lighting portion required approximately 30 min of the 3 hours required by an experienced user to complete the entire nomograph procedure. From examination of Fig. 5, it is apparent that all daylighting options will realize a reduction in annual energy costs when compared to that of the base building. In compar-

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ing cases A, B and C, the change in annual energy costs is not appreciable and further analysis may need to be carried out to justify any one such alternative. The lighting designer may now choose a direction in which such further study may be pursued.

Flexibility in design process Rather than beginning with strategies and determining energy consumption, the designer may set an energy goal and then select strategies to meet that goal. Designing to meet a goal can be accomplished with one calculation using the energy homographs. (Since the subject of this paper is daylighting, this discussion will be limited to the lighting portion of the energy homographs, although it applies to the heating and cooling portions as well.) If the designer has set his goal at 32 MWh/yr m 2 (10 MBtu/ft 2 yr) for lighting, he can start with the Lighting Consumption Nomograph (Fig. 5) and determine the watts/ft 2 of artificial lighting required for the total occupied hours. Assuming that it is physically impossible to design such an efficient lighting system, he must look at daylighting to reduce the fullload hours. The next step is to look at the daylighting weather data (Fig. 4) to find the appropriate full-load hours for the light level required. Associated with this will be an orientation and a coefficient of utilization. If the orientation is horizontal, he should use the "Daylighting Design -- Skylights" homograph (Fig. 3) and work from both ends to determine the best skylight shape and the proper a m o u n t of skylight to use in the space. If the orientation is vertical, he can use the "Daylighting Design -- Vertical Windows" nomograph (Fig. 1) to determine the best window configuration and sensor location. (Sensor location should be checked to ensure that enough fixtures can be controlled to reduce the overall lighting load and meet the goal.) Although this procedure for using the energy nomographs is a bit more complicated, it allows a unique approach to design by starting with the desired answer and then determining the required building parameters.

HEATING BASE

A

B

C

Fig. 6. Estimated energy costs for a base building and three daylighting options determined by the energy nornographs.

Verification o f the energy nomographs In the past 2% years during their development, the energy nomograph procedure has been tested by Burt Hill Kosar Rittelmann

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Associates on office projects and in an independent design tool evaluation project conducted at SERI. Several in-house design projects showed their ease of use and applicability. Another project required similar results for the nomograph technique and the TRNSYS computer program. In comparing these two design tools on five different buildings in various climates, the error was always less than 10% and normally less than 5%. The design tool evaluation conducted at SERI compared five design tools (DOE-2, BLAST, Energy Graphics, One Node Wonder, and the Energy Nomographs) to determine whether these tools would give the same direction to a designer who would use them in early design. Although the results have never been published formally, the five tools did give the same direction in each test case. The actual numbers did vary by +20% but none of the tools were out of line significantly enough to give a designer the wrong design approach. It was agreed that the design tools tested were "accurate" for design purposes, while the "precision" for each could be argued. It should be pointed o u t that this crosstool testing was done with no daylighting since most of the other tools could not incorporate this design strategy. CONCLUSIONS

The energy nomographs are a quick, simple-to-use design tool for daylighting.

They are capable of analyzing not only whether daylighting is an effective solution, but also what t y p e of control to use, where to m o u n t the control and what effect the artificial light level has on daylighting. Also considered are other factors which affect daylighting decisions such as lighting W/m 2 (W/ft2), HVAC system efficiency, and various fuel costs. This total flexibility and accuracy which has been built into the energy nomographs make them ideal for energy-conscious design of daylighting systems. An important aspect of this analysis technique is its graphic procedural quality. As each design decision is made in the course of completing the nomograph procedure, the effects of each may be seen graphically and alternatives chosen without having to complete the entire process. This will save time, but,. even more important, will act as an educational tool for the designer. As more calculations are completed, the user realizes how results can be positively influenced by certain design strategies.

ACKNOWLEDGMENTS

The authors wish to thank the Tennessee Valley Authority and the U.S. Department of Energy for jointly sponsoring the development of the energy nomographs and the preparation of this paper.