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By Jonathan McHugh,Patrick J. Burns, Ph.D.,Associate Member ASHRAEandDouglas C. Hittle, Ph.D.Fellow ASHRAE

lectric lighting and its associated cooling require-ment consume 30% to 50% of the energy used in atypical commercial building.1,2 The Energy Informa-tion Agency3 estimates that

lighting U.S. commercial (office, retail,hospital, etc.) buildings consumes be-tween 205 and 321 billion kWh/yr., cost-ing Americans between $15 and $23 bil-lion annually. Electric lighting replacementby daylighting using advanced glazings(windows) offers the possibility of reduc-ing the cooling load.

In addition to energy savings, someevidence indicates that exposure to day-light reduces stress.4 A post-occupancyevaluation5 of daylit buildings found thatover 90% of those with offices near win-dows had the right amount of sunlight asopposed to 61% of people with more in-terior offices. While construction costsof commercial buildings are typicallyaround $150/ft2 and total utility costs areabout $1/ft2·yr, the salaries and overheadfor office workers are estimated at $150/ft2·yr. Thus, it is important that anydaylighting strategy either enhances, or at the very least, doesnot reduce worker productivity.

The Zero Energy BuildingThe building investigated here is a small 17,400 ft2 or (1,620

m2) two-story, commercial building (Figure 1). The building isdubbed the “Zero Energy Building,” because it was designedto be eventually self-sufficient in energy, i.e. to take zero en-ergy from the electric and natural gas grids. Details of the HVACsystem are available elsewhere.6 This article describes the re-sults of Phase I of the design effort, using advanced lightingand HVAC strategies. The energy conserving features describedhere add only 3% to the construction costs of the building.7

The Zero Energy Building was designed using load avoid-ance rather than using alternative sources of energy. The walls,roofs and floors would be insulated to a thermal resistance (R-)values of 19°F · ft2 · h/Btu (3.35°C · m2/W), 32°F · ft2 · h/Btu(5.64°C · m2/W); and 14°F · ft2 · h/Btu (2.47°C · m2/W), respec-tively. The windows would have a center of glass R-value of

8.1°F · ft2· h/Btu (1.43°C · m2/W) and an overall resistance of5.5°F · ft2 · h/Btu (0.97°C · m2/W). This is because the frame ismore conducting than the glass assembly of two low-emissiv-ity (low-e) Mylar sheets sandwiched between two sheets ofglass. This proposed daylit building would have a window towall area ratio of 17.4%.

For maximum cooling capacity, an indirect-direct evapora-tive cooling system was specified for the Zero Energy Building.For only a few hours per year, this enhanced evaporative cool-ing system would not meet the required cooling load. Duringhot and humid periods, the water in the direct evaporative cooler

can be mechanically chilled using a ther-mal energy storage system that sensiblycools the air leaving the indirect evapora-tive cooler. However, the thermal storagesystem is not modeled here so that theinteractions between the HVAC systemand daylighting would be simplified.Phase II of the effort would further exam-ine a thermal storage system.

Though single story spaces can easilybe daylit using skylights and roof moni-tors, these are only readily usable onsingle story buildings. Since this designwas to be applicable to multistory com-mercial buildings, skylights and roof moni-tors were ruled out. Light pipes and fiberoptic daylighting systems also were notconsidered, as it was desired to investi-gate a simpler, less expensive alternative.An interior-exterior specular light shelfwas chosen for several reasons: utilizingdirect beam light reduces the area of low-

R glazing, the exterior light shelf reduces the seasonal variationof light entering the clerestory window and provides control ofsunlight entering the view window (more in the winter when theheat is needed and less in the summer when it is not), whicheliminates much of the overheating typical in spaces with sig-nificant daylighting.

The Energy Impact of Daylighting

E

About the AuthorsJonathan McHugh works on daylighting and energy man-agement at the energy consultancy Energy Research, inDunedin, Colo.

Patrick J. Burns, Ph.D., works in alternative energy at Colo-rado State University, Fort Collins, Colo. He is the foundingfaculty advisor of the ASHRAE student section at the univer-sity.

Douglas C. Hittle, Ph.D., is professor and director of the SolarEnergy Applications Laboratory at Colorado State University.

A post-occupancyevaluation of daylitbuildings found that

over 90% of those withoffices near windowshad the right amountof sunlight as opposedto 61% of people withmore interior offices.

A SHRAE JOURNALThe following article was published in ASHRAE Journal, May 1998. © Copyright 1998 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

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The south-facing room geometry modeled for this buildingis shown in Figure 2. This room measures 20 ft by 20 ft (6 m by6 m) and has an 8 ft (2.4 m) dropped ceiling. A 3-ft (0.91 m) tallview window that is 4 ft (1.2 m) above the floor extends theentire 20 ft (6 m) width of the perimeter wall with mullions andstructural supports that occupy 20% of the nominal glazingarea. The 12-ft (3.6 m) long light shelf is a dropped ceiling thathas a specularly reflective Mylar film on its top surface. ThisMylar film is applied also to the ceiling and walls above thelight shelf and is illuminated by a 2-ft (0.61 m) tall by 20-ft (6.1 m)wide clerestory window. The rear of the specular light shelfopens up to the ceiling of the rear half of the room on the northside of the room.

The specular light passage above the dropped ceiling main-tains the interior direction of the light until the last bounce offof a diffuse surface to spread the light downward. A daylightingsimulation without the view window indicates that the specularlight shelf lights nominally therear two-thirds of the room,while the view window lightsthe front one-third. Horizontalmini-blinds are used on theview windows during the win-ter to control glare and to directthe sunlight up onto the dif-fusely reflecting ceiling for amore even distribution of lightthrough the room. Perforatedmini-blinds are recommendedso occupants can see outsideeven when the blinds areclosed.

The primary means ofdaylighting the north rooms isto provide a view to a large partof the sky. The first floor roomsfacing north have a 3 ft (0.91 m)high view window, a 2 ft (0.61m) high clerestory window anda 3 ft (0.91 m) high skylight that span the entire width of theroom. The second floor offices have two 3 ft (0.91 m) highskylights. A cross-section of one of the north rooms can beseen in Figure 1.

The lighting system is designed with a luminous efficacy of80 lm/W (T-8 fluorescent lamps with dimmable electronic bal-lasts) and fixture coefficients of utilization that range from 0.62to 0.28 depending upon fixture type and room geometry. Thetotal connected lighting power necessary to provide an aver-age 75 fc (750 lux) in the offices and 20 fc (200 lux) in the hall-ways is 29.2 kW or 1.68 W/ft2. This lighting power density isbelow the more stringent (whole building) method of comply-ing with the ASHRAE/IESNA Standard 90.1-1989, Energy Ef-ficient Design of New Buildings Except Low-Rise ResidentialBuildings energy code. Given the credits for daylighting con-trols, the adjusted connected power is about half of the allowedlighting power budget. Dimming ballasts are used to optimizethe amount of energy savings possible from “daylight harvest-

ing,” i.e. providing only as much electric light as is necessary tomeet desired interior light levels. Though it costs more to pro-vide dimming controls for fluorescent lights, fluorescent light-ing provides four times as much light per input watt as doesincandescent lighting.

Predicting Daylight Inside BuildingsTo predict the amount of daylight inside of buildings, two

questions must be answered: 1) how much direct beam anddiffuse light are available outside of the building throughoutthe day and over the course of the year, and 2) what is thespatial distribution and intensity of light inside the buildingenclosure?

The weather data used herein for the whole building energyanalysis are the Typical Meteorological Year (TMY) data com-piled by the National Climatic Center.8 Unfortunately, there is adearth of measured illuminance data available, so a model for

illuminance is used. To achieveconsistency between illumi-nance and weather data, welinked the thermal simulation tothe daylighting simulation byusing hourly TMY solar datain the daylighting simulation.The irradiance data are con-verted to illuminance data us-ing the luminous efficacy (lu-mens/Watt) model of Perez.9

The illuminance data areused in a ray tracing analysis todistribute the incident light. Raytracing follows the path of a“particle” of light from its ori-gin in the “sky” (defined as aplane just outside of the win-dow) across the interior spaceas it bounces from reflectivesurface to reflective surfaceuntil it is absorbed. Ray tracing

models are robust: radiative properties (reflection, transmissionand absorption) of any material can be modeled, only if prob-abilities of reflection, transmission and absorption and their an-gular variations can be described mathematically.

In particular, specular reflectors are modeled accurately us-ing ray-tracing methods. A further discussion of this methodand a procedure for determining the accuracy of this method isgiven in Maltby & Burns.10 McHugh describes how this methodis applied to calculate illuminances in a space.6 This method iscomputationally intensive—the geometry modeled here withapproximately 100 surfaces requires the tracing of 1.2 millionphotons for each position of the sun modeled. Separate runsare performed for direct beam sunlight and diffuse skylight.Since the diffuse light is independent of the position of the sun(isotropic diffuse model), the number of diffuse runs was de-pendent only on the number of positions modeled for the mini-blinds. Each simulation requires about 10 minutes of computa-tion time on a fast processor. Over 1,000 runs were performed to

Figure 1: Cross-section of the Zero Energy Building.

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optimize the performance of thedaylighting design shown in Figure 2.

Any daylight simulation predicts theillumination factor, defined as the ratio oflight transmitted through the “work plane”(an imaginary surface located at a heightabove the floor where work is performed),to that incident outside. In an office, thework surface is typically at the height ofthe desktop. Here, the work plane is di-vided into a regular 10 by 10 grid of sub-surfaces to yield the spatial distributionof illuminance. For this study, the subsur-face size is 4 ft2 (0.37 m2). Where the illu-minance level is low, it can be augmentedwith electric lighting according to a con-trol scheme. Millions of photons are emit-ted from the “solar” surface, which repre-sents the position of the sun at the par-ticular date and time for the run.

A photon may pass through the win-dow, then through the work surface (tal-lied for the subsurface through which itpasses), and be reflected (prob-abilistically) from the floor and the ceil-ing before it again passes through a dif-ferent subsurface (again tallied). It is even-tually absorbed and tracing proceedswith a new emission. Note that a photonmay be tallied more than once as it passesthrough the work surface. Given the am-bient diffuse and direct beam illuminationfor every hour of a “typical” meteorologi-cal year (TMY), this is multiplied by theappropriate illumination factor whichspecifies how much of the ambient lightpasses through a given subsurface of thework plane. Thus, for every hour of ev-ery day, the footcandles of daylight oneach subsurface are available on all workplane subsurfaces.

Predicting Whole BuildingEnergy Usage

The novel aspects of this approach arethe detailed daylighting calculations. Forthis, a representative room is divided upinto nine lighting zones and the averagelight level for each zone is compared tothe lighting set point, in this case 75 fc(750 lux). Where a zone is underlit, elec-tric lighting is used to achieve 75 fc (750lux). A variety of lighting control strate-gies could be used such as: on-off,stepped lighting, continuous dimming;and functions could be generated to de-scribe the relationship between light out-

put and electrical input into the lightingsystem. However, for simplicity’s sake, adirectly proportional relationship is used.This yields the hourly fraction of electriclighting that is displaced by daylightingfor the entire nine zones (room), which iswritten to a file for each of the 8,760 hoursof the year for each class of room.

The Building Loads and System Ther-modynamics (BLAST) program is thenused to calculate the energy used by theentire building: lights, plug loads, andHVAC system and plant. Using theBLAST program, a direct-indirect evapo-rative system and a conventional electricchiller system are simulated. The BLASTprogram accepts inputs that describebuilding orientation and construction,HVAC system type, and schedules of setpoints, fan operation, light and plug loads.Besides providing a thermal model of howthe building responds to these inputs andweather data, BLAST accumulates theseloads to result in annual energy and peakdemand for each run.

The BLAST program was altered sothat the daylight fraction file is read, andelectric lighting loads are reduced accord-ingly. Because both the daylight fractionsand the weather data used to calculatesolar thermal loads derive from the samesource, daylighting and electrical usageare predicted coincident with HVAC sys-tem performance. This enables the deter-mination of the monthly peak demand andthe energy usage, thereby allowing ac-

curate estimates of energy (usage anddemand) cost savings to be made.

ResultsCombining the results of a daylighting

simulation into an annual building energysimulation captures the interaction ofdaylighting with other building energymechanisms. A building energy simula-tion also predicts the effect on electricaldemand. Electrical demand (in units ofkW) typically costs the building owneras much as the cost of electrical energy(in units of kWh).

The six-building/HVAC system con-figurations considered are:

A. The base case. No clerestory win-dows or skylights, but with an overhangabove the south-facing view windows,electric lights are not dimmable and theair-conditioning system uses a vapor-compression chiller with an evaporativecondenser.

B. The same as Case A except that cool-ing is provided by a direct-indirect evapo-rative cooling system with a chillerbackup.

C. Daylit building with north skylights,south clerestory windows with lightshelves, dimming (daylight harvesting)controls on electric lighting, and a chillerto provide cooling.

D. The same as Case C except that cool-ing is provided by a direct-indirect evapo-rative cooling system with a chillerbackup.

Figure 2: Daylighting geometry—south rooms.

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E. The same as Case C except withno daylight controls on lighting (i.e.electric lighting power usage is thesame as Case A).

F. The same as Case C except with noclerestory windows and skylights butusing the same reduction of lightingpower as in Cases C and D. Case F ishypothetical since the elimination of clere-story windows and skylights would af-fect the amount of daylight available todisplace electric lighting.

As seen in Figure 3, daylight displaces70% of the annual electric lighting usage.As expected, most of the lighting sav-ings occur in the middle of the day and inthe summer. Indeed, many hours existwhere no electric lighting is required.Daylighting reduces chiller usage, butonly by 5%; the plug loads are slightlyless than the peak lighting loads and chiller energy consump-tion is not that large due to the economizer operation. Thus, theelectric lighting savings dwarf the chiller energy savings. Asexpected, evaporative cooling all but eliminates chiller energyusage. However, since the evaporative cooling system cannotprovide cooling on the peak cooling day, the chiller size andpeak energy usage are approximately the same as without anevaporative cooling system.

The heating energy results are initially surprising. One mightexpect that boiler fuel consumption would increase under thedaylit case; the glazings are more thermally conductive thanthe opaque wall assemblies and electric lighting internal gainshave been reduced. However, an additional 588 ft2 (55 m2) ofsouth-facing glazing in the daylit case admits more solar heatgain which is coincident with the reduction in internal heatgains from electric lighting.

In Case F with no clerestory windows and skylights, thebase case building has the same electric lighting load profile asa daylit building, and the boiler usage, increases by 14% overthe base case, thus substantiating the solar gain hypothesis.Conversely, if the daylighting geometry is used without light-ing controls, the results given in Figure 3 (Case E) show thatboiler usage decreases by 11% over the base case; again illus-trating that the daylighting geometry also provides significantpassive solar energy.

Boiler fuel use is also reduced for the evaporatively cooledsystem. The higher airflow rates of an evaporative system al-low higher supply air temperatures to provide the same amountof cooling than for the supply air temperatures of a mechanicalcooling (chiller) system with lower flow rates. During the swingseason, the core zones in an evaporative system are cooled witha greater flow rate of warmer air than would be with a chillersystem. Because the supply air temperature is higher for theevaporatively cooled system, this supply air does not have to bereheated as much in the perimeter zones. (A terminal reheat sys-tem is used as an easy and convenient way to provide heat toperimeter zones.) Thus the evaporatively cooled building uses

less boiler fuel because there is less reheating of cooled air.The evaporative cooling system uses more fan energy (1.94

kBtu/ft2 [22 038 kJ/m2]) than its mechanical cooling counterpart(1.14 kBtu/ft2 [12 950 kJ/m2]) since a greater volume of air isnecessary to remove the same cooling load due to the highersupply air temperatures. Daylighting reduces the amount ofcooling loads due to electric lights, thus in Case C less fanenergy is used than in Case A, and in Case D less fan energy isused than in Case B. Notice that in Case E, greater fan energy isused than in any of the other chiller cooling cases—Cases Aand C, because without daylight controls on the electric light-ing, this building must reject the heat from the sunlight enteringthe clerestories and skylights and the heat of the electric lights.

Figure 4 shows the monthly peak demand for all cases. Thedaylighting in the Zero Energy Building reduces electric de-mand by eliminating the need for electric lighting for most ofthe day. The effect is not as pronounced due to the small cool-ing loads, but daylighting decouples the coincidence of cool-ing and illumination needs. Due to this decoupling, the annualpeak (which drives power plant construction) is not only re-duced significantly but shifted from the summer to the winter.

Without daylighting, the evaporatively cooled building hasa large demand spike in August, because for a few hours in thesummer, wet bulb temperatures are so high that the evaporativecooler is unable to provide the required cooling. As a result, thebackup DX chiller is engaged, and the monthly demand is setby these few hours of chiller operation. When evaporative cool-ing is used in conjunction with daylighting, on the hot sunnydays when the chiller is needed, the demand by the chiller isoffset by the reduced electrical and cooling loads of the electriclighting. Thus, daylighting enhances the value of evaporativecooling to electric utilities interested in reducing their summerpeak.

Relatively low energy costs, for Fort Collins, Colo., wereused to predict annual energy costs. Annual energy usage costsare, for the various cases, in $/ft2-yr: a) 0.714, b) 0.660, c) 0.538,d) 0.500, and e) 0.726. The operating costs are lowest for the

Figure 3: Annual energy end uses from BLAST results.

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building that is evaporatively cooled anddaylit, Case D. Daylighting the buildinghas a much greater impact on operatingcosts than the addition of evaporativecooling. This corroborates the results ofelimination para-metrics as discussed inTernoey et al.11—reducing internal loadsin commercial buildings will have the larg-est impact on reducing energy costs.Case E, the building designed fordaylighting with clerestory windows andskylights but without the lighting con-trols, costs the most to operate. Thispoints to the necessity of following adaylighting project through tothe end. Scrimping on lightingcontrols, as often happens as abuilding project is nearingcompletion, can produce abuilding that is more costly tooperate than one which is notcarefully designed.

ConclusionsA novel daylighting design

has been presented that utilizesbeam radiation, thereby avoid-ing the classic problem of over-heating, while simultaneouslydisplacing over 70% of the an-nual electric lighting load. Thenovel aspects of the design are: 1) geom-etry, including a specular light shelf, anexterior reflector and an overhang over aview window, 2) advanced glazings (low-e and high R-value, yet passing more than50% of the visible light) and 3) evapora-tive cooling. The simple payback of theenergy conservation measures listed hereis about 13 years. Though a building con-structed now should last until the 22ndcentury, energy efficiency design featureswith more than a 10-year payback are of-ten eliminated in the design process. How-ever, the cost-effectiveness of such de-signs can be increased as follows:

• Over time, the cost of dimming bal-lasts should drop as the technology ma-tures.

• Judicious use of dimming controlsshould also be considered. The luminairesclosest to the windows are not needed mostof the time during the work day; a simpleon-off control may capture the most sav-ings whereas a dimming control may bemore appropriate for a luminaire at an in-termediate distance from the windows.

Since the productivity benefits fromdaylighting and evaporative cooling arehard to quantify, the building communityin general has not assigned a monetaryvalue to these benefits. A multi-disciplin-ary approach that elevates the importanceof occupant satisfaction with temperature,humidity, air quality, lighting quality,daylighting quality and view of the out-doors may allow these aspects to bequantified.

AcknowledgmentsWe would like to thank William Seaton

from the national headquarters ofASHRAE and Joseph Wojek of the RockyMountain Chapter of ASHRAE for theirsupport of this work. The visionary sup-port of the “Zero Energy Building” con-cept by the Public Service Company ofColorado resulted in this work and gaveus the opportunity to work with JackBrinkley, the lead architect who was ableto take our engineering concepts andmold them into an aesthetic building.Thanks also are due to William Emslie andJohn Bleem of Platte River Power Author-ity, sponsors of a DEED scholarship fromthe American Public Power Association.We are also indebted to Toni Smith atColorado State University and theBLAST support office at the Universityof Illinois for the assistance rendered onunderstanding the intricacies of BLAST.

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2. Robins, C. 1986. Daylighting Design andAnalysis, Van Nostrand.

3. Energy Information Agency. 1992. “Light-ing in Commercial Buildings,” US Departmentof Energy publication DOE/EIA-0555(92)/1,also see http://www.eia.doe.gov/bookshelf/consumer.html.

4. Hollich, F. 1979. The Influence of OcularLight Perception on Metabolism in Man &Animal, New York: Springer-Verlag.

5. Heerwagon, J., et al. 1991. “Energy EdgePost-Occupancy EvaluationProject: Final Report,” U.S. De-partment of Energy Contract #DE-AC06-89RLL11659. Seattle,Wash.

6. McHugh, J., P. Burns, D. Hittle,and B. Miller, 1992. “DaylightingDesign Via Monte Carlo,” Devel-opments in Heat Transfer, HTD-Vol. 203, ASME, pp. 129–136.

7. Miller, B., et al. 1992. “InitialEnergy Conserving Design of aLow Energy Office Building,” Pro-ceedings, ASME Solar EnergyConference, Maui, April 5–9,1992.

8. National Climatic Center. 1981. TypicalMeteorological Year User’s Manual TD-9734:Hourly Solar Radiation - Meteorological Ob-servations. Asheville, N.C.

9. Perez, R., et al. 1990. “Modeling DaylightAvailability and Irradiance Components fromDirect and Global Irradiance,” Solar Energy,Vol. 44, No. 5, pp. 271–289.

10. Maltby, J. and P. Burns, 1991. “Perfor-mance, Accuracy, and Convergence in a Three-Dimensional Monte Carlo Radiative HeatTransfer Simulation,” Numerical Heat Trans-fer, Part B, Vol. 19, pp. 191–209.

11. Ternoey, S., et al. 1985. The Design ofEnergy Responsive Buildings. New York: JohnWiley & Sons.

Figure 4: Electrical demand impacts of daylighting.

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