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Transpired Collectors as Solar Air Preheaters Course No: R01-002
Credit: 1 PDH
Harlan H. Bengtson, PhD, P.E.
Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980 P: (877) 322-5800 F: (877) 322-4774 [email protected]
The U.S. Department of Energyrequests that no alterations bemade without permission in anyreproduction of this document.
FederalTechnology
Alert
A publication seriesdesigned to speed the
adoption of energy-efficient and renewable
technologies in theFederal sector
The President's Million Solar RoofsInitiative aims to install 1 million solarenergy systems on residential, commercial,and public-sector buildings by 2010. Twentythousand of those systems will be installedon Federal buildings. In support of theInitiative, and as part of a continual effort to ensure U.S. buildings are energy efficientand environmentally sustainable, the U.S.Department of Energy's Federal EnergyManagement Program (FEMP) will helpinstall those solar systems targeted for theFederal sector.
FEMP is focusing on solar systems thatinclude photovoltaics (PV), solar hot waterfor buildings and swimming pools, and solarspace heating. Transpired solar collectors are a solar space-heating technol-ogy that is well proven, is readily available,and has considerable potential for applica-tion at Federal facilities. Transpired collec-tors use solar energy to preheat ventilation(outdoor) air as it is drawn into a building.The technology is ideally suited to buildingapplications in which large volumes ofspace are heated or where high ventilation
rates are required. By preheating ventilationair with solar energy, the technologyremoves a substantial load from a building’sconventional heating system, saving energyand money. This Federal Technology Alert(FTA) is designed to give Federal facilitymanagers the information they need todecide whether transpired collector tech-nology is suitable for their facility.
Energy-Saving MechanismA transpired collector reduces the load
on a building’s heating system by heatingintake air with solar energy. It preheats theambient air by up to 40°F, reducing all or aportion of the load on a heating system dur-ing daylight hours. Although the transpiredcollector may not be able to achieve therequired indoor air temperature on cloudydays or when the outside temperature plum-mets, it provides useful energy and reducesutility bills.
In addition to meeting a portion of abuilding’s heating load with clean, free solar energy, the transpired collector helps
Transpired Collectors (Solar Preheaters for Outdoor Ventilation Air)Simple, reliable technology can substantially reduce heating bills.
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save energy and money in other ways. Itrecaptures heat loss through a building’ssouth-facing wall; heat that escapes throughthe south wall is captured in the air spacebetween the structural wall and the tran-spired collector and returned to the interior.Also, by introducing make-up air throughceiling-mounted ducts, the system elimi-nates the wasteful air stratification that oftenplagues high-ceiling buildings.
Technology SelectionThe FTA series targets technologies that
appear to have significant untapped Federal-sector potential and for which some Federalinstallation experience exists. The new technologies presented in the series wereidentified through trade journals andthrough direct correspondence. Numerousresponses were obtained from manufactur-ers, utilities, trade associations, researchinstitutes, Federal sites, and other interestedparties. Based on these responses, the tech-nologies were evaluated in terms of poten-tial Federal-sector energy savings andprocurement, installation, and maintenancecosts. They were also categorized as eitherjust coming to market ("unproven" technolo-gies) or as technologies for which field dataand experience exist.
Transpired collectors are one of manyenergy-saving technologies to emerge in the last 20 years. They were judged to belife-cycle cost effective (at one or moreFederal sites) in terms of installation cost,net present value, and energy savings.Several other proven technologies have been slated for further study through theFederal Technology Alert series.
Application Any heated building in a cool, sunny
climate that has at least moderate ventilationrequirements and southern exposure couldbenefit from a transpired collector. Build-ings that require large volumes of heatedmake-up air such as machine shops, vehiclemaintenance buildings, and chemical storagefacilities, are good candidates for a tran-spired collector. A long-term storage ware-house that does not require ventilation,would not be a suitable candidate. Also,buildings that have 100% recirculation/filtration systems would be unsuitable.
Heat recovery systems, which are com-mon in many modern office buildings (butless common in industrial buildings), alsopreheat ventilation air. As such—because
of redundancy of function—buildings with existing heat recovery systems may not be suitable for transpired collector applications.
Field ExperienceAs of 1997, approximately 40 transpired
collector systems have been installed in theprivate sector and on two Federal sites—two systems at Fort Carson, Colorado, andone at the National Renewable EnergyLaboratory (NREL) in Golden, Colorado.Three of the 40 installations—the GeneralMotors (GM) of Canada Oshawa BatteryPlant, the Ford Plant in Oakville, Canada,and NREL—have been monitored extensively.
The installation at NREL is an idealapplication of transpired collector technol-ogy. The facility is a 1300-square-foot (ft2)chemical waste storage building thatrequires a ventilation rate of 3000 cfm tomaintain safe indoor conditions. Also,because of the danger of combustion, openflames are prohibited in the building, so outside air is heated with electricity insteadof gas. The 300-ft2 transpired collector saves about 14,310 kWh annually (25.7% of the energy required to heat the facility’sventilation air). With a local electric rate of$0.025/kWh, the annual savings equates toabout $360. The installation has a simplepayback of 4.7 years.
Case StudyThe GM Battery Plant in Oshawa,
Canada, is a 100,000-ft2 facility in whichautomotive batteries are manufactured. Theplant was built in the 1970s and consists of an open shop floor and a 28-foot-highceiling. GM operates two full-time produc-tion shifts within the plant and conductsmaintenance activities at night and on week-ends, so the building is continuously occu-pied. In 1991, plant management opted toinstall a transpired collector to correct theventilation problems.
The monitoring program showed that thetranspired collector saved GM 208,000 Btuper year for every square foot of installedsolar collector. The majority of this sav-ings—or 150,000 Btu/ft2/year—resultedfrom the thermal energy gained directly by the outside air as it passed through the collector. The balance of the savings—or 58,000 Btu /ft2/ year—came from recap-tured heat loss through the wall clad withthe transpired collector. The value of these
savings depends on the heating sourceassumed to be displaced.
At the GM plant, steam was the existingprimary heating source, but the system wasincapable of providing the necessary quanti-ties of heated outdoor air for ventilation. Toredress the airflow problems with a steamoption would have required the installationof a packaged rooftop steam-operated system with roof curb, steam piping, andoutlet ducting. These systems are installedfor about $2.16/cfm. To supply the samevolume of air that the transpired collectorsupplies, the steam system would have todeliver 25,200 cfm and would cost $55,000.Also, the fan on the steam system wouldrequire an additional 3.6 kW to operate(compared to the transpired collector sys-tem), which would increase electricity costsby $1,430 annually.
Implementation Barriers
The biggest hurdle transpired collectorsmust overcome is user acceptance. Manysolar technologies have been stigmatized bythe rapid expansion of solar markets in the1970s, when many poorly designed andpoorly performing systems were deployed.Many potential users are reluctant to com-mit to a solar technology if a proven con-ventional option is available. Transpiredcollector technology has been proven to be a valid, reliable technology for reducingenergy use and saving money, and the bodyof scientific data proving its effectivenesscontinues to grow.
AbstractTranspired collectors are a renewable
energy technology that is well proven, isreadily available, and has considerablepotential for application at Federal facilities.Transpired collectors use solar energy topreheat ventilation (outdoor) air as it isdrawn into a building. The technology isideally suited for buildings with at leastmoderate ventilation requirements in sunnylocations with long heating seasons.
Transpired collector technology isremarkably simple. A dark, perforated metalwall is installed on the south-facing side ofa building, creating approximately a 6-inch(15-cm) gap between it and the building’sstructural wall. The dark-colored wall actsas a large solar collector that converts solarradiation to heat. Fans mounted at the top of the wall pull outside air through the tran-spired collector’s perforations, and the thermal energy collected by the wall istransferred to the air passing through theholes. The fans then distribute the heated air into the building through ducts mountednear the ceiling. By preheating ventilation
air with solar energy, the technologyremoves a substantial load from a building’sconventional heating system, saving energyand money.
This Federal Technology Alert (FTA) of the New Technology DemonstrationProgram is designed to give Federal facilitymanagers the information they need todecide whether they should pursue tran-spired collector technology for their facility.
This FTA describes the transpired col-lector, its energy-saving mechanisms, andthe factors that influence its performance.Worksheets are included that let the readerperform the preliminary calculations todetermine if a given facility is suitable for atranspired collector system and to determinethe amount of energy such a system wouldsave annually. The FTA contains a casestudy documenting the performance of thetranspired collector installed at GeneralMotors’ battery plant in Oshawa, Canada.The document concludes with contacts foradditional information and a list of articles,conference papers, and academic theses pertaining to the technology.
Transpired Collectors (Solar Preheaterfor Outdoor Ventilation Air)Simple, reliable technology can substantially reduce heating bills.
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Figure 1. A transpired collector heating system being installed on a Federal Express facility in Littleton, Colorado.
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ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
About the Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Application Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Energy-Saving Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Other Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Federal-Sector Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Technology-Screening Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Estimated Savings and Market Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Laboratory Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Application Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Application Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Cost-Effectiveness Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Where to Apply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
What to Avoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Equipment Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Equipment Warranties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Codes and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Utility Incentives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Technology Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Field Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Energy Savings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Awards and Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Sizing and Energy-Savings Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Case Study—The General Motors Battery Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Facility Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Existing Technology Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
New Technology Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Energy Savings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Life-Cycle Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Performance Shakeout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
The Technology in Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
The Technology’s Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Technology Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Manufacturers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
For Further Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Appendixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Appendix A: Worksheets From Figures 8 and 9 Completed for a Hypothetical
Building in Chicago, Illinois . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Appendix B: Federal Life-Cycle Costing Procedures and the BLCC Software . . . . . . . . .19
Appendix C: General Motors Solar Ventilation Heat Case Study NIST BLCC
Comparative Ecomonic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
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About theTechnology
Transpired collectors use solar energy to preheat ventilation (outdoor) air as it isdrawn into a building. The technology isideally suited for buildings with at leastmoderate ventilation requirements in sunnylocations with long heating seasons.
Transpired collector technology isremarkably simple. A dark, perforated metalwall is installed on the south-facing side of a building, creating approximately a 6-inch(15-cm) gap between it and the building’sstructural wall (see Figure 2). The dark-colored wall acts as a large solar collectorthat converts solar radiation to heat. Fansassociated with the building’s ventilationsystem mounted at the top of the wall drawoutside air through the transpired collector’sperforations, and the thermal energy col-lected by the wall is transferred to the air passing through the holes. The fans thendistribute the heated air into the buildingthrough ducts mounted from the ceiling. Bypreheating outdoor air with solar energy, thetechnology removes a substantial load from
a building’s conventional heating system,saving energy and money.
A transpired collector is installed on allor part of a building’s south-facing wall,where it will receive the maximum exposureto direct sunlight during the fall, winter, andspring. The size of the wall varies dependingon heating and airflow requirements and climate, but in many applications, the tran-spired collector will cover the maximumsouth-facing area available. The amount ofenergy and money saved by a transpired collector depends on the type of conven-tional fuel being displaced, occupant usepatterns, building design, length of heatingseason, and the availability of sunlight dur-ing the heating season. In general, eachsquare foot of transpired collector will raisethe temperature of 4 cubic feet per minute(cfm) by as much as 40°F, delivering asmuch as 240,000 Btu annually per squarefoot of installed collector.
In addition to the metal sheeting that cap-tures solar energy, the transpired collectorheating system includes air-handling andcontrol components that supply the solar-heated air (see Figure 2). The ventilation system, which operates independently of a
building’s existing heating system, includesa constant-speed fan to draw air through thetranspired collector and into the distributionduct. Engineers typically use a 3-horse-power, 32-inch blade fan with about 10,000-cfm capacity.
As shown in Figures 3 and 4, the tran-spired collector system also contains abypass damper located directly in front ofthe fan inlet duct. During the summermonths when ventilation air does not needto be heated, this damper opens, circumvent-ing the air-heating system. The bypassdamper automatically opens when the airoutside reaches a predetermined tempera-ture, usually about 64°F.
Conserval Systems, Inc., of Toronto,Ontario, and Buffalo, New York, is cur-rently the only supplier of the technology.Conserval has been manufacturing andinstalling a range of heat recovery and solarproducts since 1977. They received the firstpatent for transpired collector technology in1989.
Application DomainAs of 1997, about 40 transpired collector
air-heating systems had been installed inlocations around the world on apartmentbuildings, warehouses, airplane hangars, fac-tories, and in many other applications. Threetranspired collector systems are in
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Figure 2. Transpired collector components.
A Wall by any Other Name...The solar ventilation air-heating
technology described in this FederalTechnology Alert has been referred to by a variety of different names since it wasfirst marketed in 1989. Conserval EngineeringInc., the company that holds the patent rights, refers to it as a“Solarwall®” heater. The research commu-nity, which has studied the technology indepth, refers to it as an “unglazed transpiredsolar collector” or “solar air-heating systemsusing perforated absorbers.” In other literatureit has been called “solar ventilation preheatsystem” or simply “transpired collectors.”
In this document, we use the term “transpired collector” to refer to the technology described. The reader seeking additional information (see the“Literature” section on page 15) should notbe confused by the different names by whichthe technology is presented in the scientificand popular literature.
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use in the Federal sector—two at FortCarson, Colorado, and another at the WasteHandling Facility at the National RenewableEnergy Laboratory (NREL) in Golden,Colorado. High-profile industrial usersinclude Ford Motor Company, GeneralMotors Corporation (GM), McDonnellDouglas, and Federal Express.
The Federal-sector potential for this tech-nology is considerable. Any heated buildingin a cool, sunny climate that has at leastmoderate ventilation requirements couldbenefit from a transpired collector. Build-ings that require large volumes of heatedmake-up air, such as machine shops, vehiclemaintenance buildings, and chemical storagefacilities are good candidates. There arethousands of such facilities in the Federalsector.
Energy-Saving MechanismA transpired collector reduces the load
on a building’s heating system by heatingintake air with solar energy. It preheats theambient air by up to 40°F, reducing all or aportion of the load on a heating system dur-ing daylight hours. Although the transpiredcollector itself may not be able to achievethe required indoor air temperature oncloudy days or when the outside tempera-ture plummets, it still provides useful energyand reduces utility bills.
The dark-colored transpired collector is a large solar collector, absorbing the solarenergy striking it. The wall capturesbetween 60% and 75% of the available solarenergy, making it one of the most efficientsolar collectors designed to date. In additionto capturing direct solar radiation, the tran-spired collector collects the indirect, scat-tered, and reflected sunlight known asdiffuse solar radiation. Typically, diffusesolar radiation, which includes a portion ofthe radiation on clear days and all the radia-tion on overcast days, makes up about 25%of the total annual radiation at the Earth’s surface.
The dark, corrugated metal sheets thatmake up the wall are 0.8-millimeter (mm)-thick and are typically manufactured fromaluminum or galvinized steel. The perfora-tions through which the air flows are1.6 mm in diameter and are placed at regu-lar intervals. The total percentage of the collector made up of these holes is referredto as the collector porosity.
In addition to meeting a portion of abuilding’s heating load with clean, free solarenergy, the transpired collector helps saveenergy and money in other ways. The col-lector recaptures heat loss through a build-ing’s south-facing wall; heat that escapesthrough the south wall is captured in the air space between the structural wall and the transpired collector and returned to theinterior. Also, by introducing make-up airthrough ceiling-mounted ducts, the systemeliminates the wasteful air stratification thatoften plagues high-ceiling buildings.
Other BenefitsThe solar energy collected by a tran-
spired collector displaces fossil fuel that
would otherwise be burned to produceheated ventilation air. Greenhouse gas (CO2)and acid rain emissions (SOx and NOx) arereduced proportionally. Electricity is con-sumed, though, by the distribution fans anddampers.
The transpired collector can also improvea building’s appearance, giving the south-facing side a neat, clean, uniform look.Although initially many black transpiredcollectors were installed, other colors workwell too, which gives the user some aes-thetic options. The product is now availablein a variety of shades, including brown,blue, gray, and red.
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Fan unit
Bypassdamper
Distribution ducting
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Figure 4. Transpired collector operation during the summer months.
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Fan unit
Heat loss through wallcaptured by incoming air
Outside air is heatedpassing through
perforated absorber
Distribution ducting
Figure 3. Transpired collector operation during the winter months.
InstallationThe transpired collector attaches to a
building’s existing structure. The buildingframe is usually sufficient to support thecollector sheeting, so racking requirementsand construction costs are minimized.Although many transpired collectors havebeen installed as retrofits, the system eco-nomics improve if installation takes placeduring initial construction or building reno-vation. The system payback period can bereduced by up to half if the transpired col-lector installation can be incorporated withother construction work.
The transpired collector is attached to thestructural wall with a support grid of verticaland horizontal Z-channels. These channelsare perforated to accommodate airflowbetween the collector and the structuralwall. The vertical channels are attached tothe existing wall, the horizontal channels areattached to the vertical channels, and theperforated metal sheets are affixed to thehorizontal channels.
The fan unit, which includes the fan,dampers, and thermostat controls, ismounted directly to the interior side of thesouth-facing wall. For each fan unitinstalled, a hole must be cut through thestructural wall to allow air to flow into thebuilding. A minimal amount of sheet-metalducting is required to form a proper sealbetween the fan and the wall.
Installation time varies depending on abuilding’s structural design and the total col-lector area being installed, but retrofit instal-lations typically require 10 to 14 days.
Building codes and regulations pertain-ing to issues such as the location of outdoorair openings, weather protection for ducts onthe building exterior, minimum duct thick-ness, and criteria for multistory applicationsshould be addressed on an installation-by-installation basis. The applicability of codesmay vary based on building height and area,construction type, and use group.
Federal-SectorPotential
The potential savings to be achieved byuse of this new technology were estimatedas part of the technology-screening processof the New Technology DemonstrationProgram.
Technology-Screening ProcessThe new technologies presented in the
Federal Technology Alert series were identi-fied through trade journals and throughdirect correspondence. Numerous responseswere obtained from manufacturers, utilities,trade associations, research institutes,Federal sites, and other interested parties.Based on these responses, the technologieswere evaluated in terms of potential Federal-sector energy savings and procurement,installation, and maintenance costs. Theywere also categorized as either just comingto market ("unproven" technologies) or astechnologies for which field data and experi-ence exist. Transpired collectors werejudged to be life-cycle cost effective (at oneor more Federal sites) in terms of installa-tion cost, net present value, and energy sav-ings. Several other proven technologieshave been slated for further study throughFederal Technology Alerts.
Estimated Savings and Market Potential
Figure 5 shows the average payback peri-ods for transpired collector systems in threedifferent geographic locations based onthree different prices for natural gas. Thedata were derived from analyses conductedby Science Applications InternationalCorporation (SAIC), which, in conjunctionwith the International Energy Agency, hasdeveloped a spreadsheet model to estimateeconomics of transpired collectors in differ-ent climates. The model outputs simple pay-back periods, taking into account the cost ofauxiliary heating fuel, the cost of electricityto operate the fan, the geographic location,and the building type.
SAIC has used this model to comparepaybacks at three locations—with greatlydiffering climates—in the United States:Denver, Colorado; Washington, D.C.; andSyracuse, New York. The model producedpayback comparisons for a range of energycosts, from $3/MBtu for natural gas to$24/MBtu for electricity ($0.08/kilowatt-hour [kWh]). These paybacks assume thecollector was installed during building con-struction and assume a net installed cost of$5/ft2 (see discussion of costs on page 7).Payback periods for retrofit installations—which would have a net cost of approxi-mately $10/ft2 —would be twice those fornew installations.
Laboratory PerspectiveResearchers at NREL in Golden,
Colorado, have been studying transpiredcollector technology since 1989. In fact,researchers at NREL and engineers atConserval simultaneously—and indepen-dently—developed the concept of usingunglazed, perforated metal sheets as solarcollectors for heating air. Since that time,researchers have improved the efficiency of the technology and gained a fundamentalunderstanding of its heat transfer and air-flow characteristics. The technology hasbeen the subject of a number of dissertationsconducted at universities in the UnitedStates and Canada. These investigations andother relevant publications are cited in theLiterature section on page 15.
The research community has monitoredand continues to monitor a number of thetranspired collectors in use in both theFederal and private sectors. These monitor-ing programs have demonstrated that tran-spired collector technology is a reliable andeffective way to save energy. They have alsodemonstrated that the current methods usedto estimate the performance of the technol-ogy are valid.
ApplicationThis section addresses technical aspects
of applying transpired collector technology.The range of applications and climates inwhich transpired collector technology canbest be applied are addressed. Design andintegration considerations are discussed,including equipment and installation costs,installation details, maintenance, and codesand standards.
Application ConsiderationsThe following subsections briefly discuss
the prerequisites for a successful transpiredcollector application and the factors thatinfluence project cost-effectiveness.
Application Prerequisites• Suitable South-Facing Wall—A suffi-
cient area of suitable south-facing exte-rior wall is required for installing thetranspired collector’s metal cladding. Awall with a high percentage of windowor door area will likely be unsuitable,as will a wall that is heavily shadedthroughout the day. A facade does nothave to face true south for a transpiredcollector to operate effectively. Any wall
5
within 45 degrees of true south willwork, but the best performance is real-ized when the wall is within 20 degreesof true south.
• Ventilation Load—A candidate buildingfor a transpired collector must have aminimum ventilation requirement. Along-term storage warehouse, for exam-ple, would not be a suitable application,because such a structure would not haveto be ventilated. Also, buildings that have100% recirculation/filtration systemswould be unsuitable. The Project Scalesection, below, discusses the influence of ventilation loads on the cost-effectiveness of an application.
• The Absence of a Heat RecoverySystem—Heat recovery systems, whichare common in many modern officebuildings (but less common in industrialbuildings), also preheat ventilation air. Because of redundancy of function,buildings with existing heat recovery sys-tems may not be suitable for transpiredcollector applications.
Cost-Effectiveness FactorsThe following paragraphs present the
major factors influencing the cost-effectiveness of transpired collector appli-cations. The worksheets presented in theSizing and Energy-Savings Calculations section (pages 9–11) provide quantitativeestimates of energy savings.
• The Cost of Conventional Energy—The cost of the conventional energy usedfor space heating has a dramatic impacton the overall cost-effectiveness of atranspired collector application. Whatkind of energy is being displaced by thethermal energy supplied by the transpiredcollector and how much does that con-ventional energy cost? The price of con-ventional energy (that is, electricity,fossil fuels, or steam) can vary greatlyfrom season to season and from region to region. The higher the price of theconventional energy used for space heat-ing, the more cost effective the transpired collector application becomes.
• Climate—Transpired collectors are mostcost effective in sunny climates with longheating seasons.
• Project Scale—Although transpired col-lectors have been installed on many largeindustrial buildings, they can also be costeffectively applied to smaller structures.A key to a successful application is thebuilding’s ventilation rate. For example, a
small building with a large outdoor-air ventilation rate, such as a machineshop, might be as good an application as a large manufacturing facility that has only a moderate ventilation rate. (Consult American Society of Heating,Refrigerating, and Air-ConditioningEngineers Standard 62 for recommendedventilation requirements.) Keep in mindthat small transpired collector installationswill have proportionally higher construc-tion costs, and that below a certain col-lector size—depending on the factorsinfluencing project cost-effectiveness—construction costs will become prohibitive.
• New Construction versus Retrofit—Installing a transpired collector duringbuilding construction can reduce systemcosts by as much as 50% compared to aretrofit application.
Where to ApplyThe following are the most common
transpired collector applications:
• Manufacturing plants
• Vehicle maintenance facilities
• Hazardous waste storage buildings
• Gymnasiums
• Airplane hangars
• Schools
• Warehouses requiring ventilation
What to AvoidThe following is a list of general applica-
tions and conditions that preclude the cost-effective use of transpired collectortechnology:
• Outdoor air not required
• Shaded or insufficient south-facing wall area
• Buildings with existing heat recovery systems
• Locations with short heating seasons
• Multiple-story buildings (because of pos-sible problems with fire codes).
Equipment IntegrationThe transpired collector system operates
independently of a building’s existing heating system (but may utilize existingventilation fans and ducts). The only inte-gration issues that arise are associated withinstalling the collector and the air-handlingcomponents onto the building’s existingframe (see page 5, “Installation”).
MaintenanceTranspired collectors are very reliable.
They have no moving parts (other than theventilation fans and dampers, which wouldbe part of any ventilation system). There areno problems from leaking, freezing, or over-heating, and to date, no problems have beenreported from users in the field regardingholes becoming plugged or degradation ofthe absorber surface. Even if the collectorbecomes dented, performance is notaffected.
Equipment WarrantiesThe fan is covered by a 1-year manufac-
turer’s warranty, and the color coatings usedon the collector sheeting are warranteed for20 years.
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16
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12
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6
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Syracuse Washington, D.C. DenverP
ayba
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B30
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Natural gas cost ($/MBtu)
6
Figure 5. Payback periods for transpired collectors for three different locations and natural gas costs.
Codes and StandardsWith regard to fire codes, transpired col-
lectors have been deemed safe for single-story applications, and the polyethylene airdistribution duct meets the fire code forflame-retardant materials.
Building codes and regulations pertain-ing to issues such as the location of outdoorair openings, weather protection for ductson the building exterior, minimum ductthickness, and criteria for multistory appli-cations should be addressed on an installa-tion-by-installation basis. The applicabilityof codes may vary based on building heightand area, construction type, and use group.
CostsAs a retrofit to an existing building, the
typical cost of a system is approximately$10 to $12/ft2 of installed wall. About 60%of this figure comprises material costs and40% comprises labor, which includes theinstallation of the collector and the air-handling components. Table 1 shows abreakdown of costs for a transpired collector system in a retrofit application.
In new buildings, builders can use lower-cost facades underneath collectorwalls. Doing so will likely realize savings of $3 to $5/ft2 by displacing more expensivefacade material. Taking into account thismaterial displacement credit, the installedcost of a collector on a new building is inthe range of $5 to $7 per square foot.
Utility IncentivesUtilities offer no incentives for using this
technology.
TechnologyPerformance
Transpired collector owners are generallypleased with their systems. Based on thesavings and improved ventilation created bythe transpired collector installed on its plantin Oakville, Canada, Ford Motor Companyinstalled five more systems on plants in theUnited States and Canada.
Field ExperienceAs of 1997, approximately 40 transpired
collector systems had been installed in theprivate sector, and on two Federal sites—two systems at Fort Carson, Colorado, and
one at NREL in Golden, Colorado (seeFigures 6 and 7). Three of these installa-tions—the General Motors of CanadaOshawa Battery Plant, the Ford Plant inOakville, Canada, and NREL—have beenmonitored extensively. Table 2 lists the transpired collector heating systems in usearound the world.
The installation at NREL (shown inFigure 7) is an ideal application of tran-spired collector technology. The facility is a 1300-ft2 chemical waste storage buildingthat requires a ventilation rate of 3000 cfmto maintain safe indoor conditions. Also,because of the danger of combustion, openflames are prohibited in the building, so outside air is heated with electricity insteadof gas. The 300-ft2 transpired collector savesabout 14,310 kWh annually (25.7% of the energy required to heat the facility’sventilation air). With a local electric rate of$0.025/kWh, the annual savings equates toabout $360. The installation has a simplepayback of 4.7 years.
Energy SavingsThe transpired collector at the Ford
assembly plant in Oakville, Canada, wasinstalled in 1990. The 20,000-ft2 transpiredcollector system supplies heated air to a28,000-ft2 area with a ceiling as high as45 feet in places. The system produces5811 MBtu of energy savings, worth about$30,000 U.S. annually. This savings repre-sents about 17% of the plant’s annual air-heating costs. The transpired collectorsystem paid for itself in a little more than5 years.
The transpired collector on the GM plantwas installed in 1991 with a total collectorarea of 4520 ft2. The GM system heats anarea of 39,000 ft2, and supplies 940 MBtuof energy savings annually. This systemsaves GM approximately $10,200 per year.The system is discussed in more detail inthe Case Study on page 13.
Maintenance Reports from system owners in the field
demonstrate that transpired collectors aremostly maintenance free. Except for servic-ing of air distribution units (fan belts, lubri-cation), the systems require minimalmaintenance.
Awards and RecognitionA transpired collector was an important
part of a comprehensive energy managementprogram at Fort Carson, Colorado. TheDepartment of the Army installed a collectoron an aviation hangar. The collector, com-bined with lighting and occupancy sensors,
7
Table 1. Installation Costs of aRetrofit Transpired Collector
Figure 6. The transpired collector on the Avum Hangar at Fort Carson, Colorado.
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Absorber $ 3.50/ft2
Supports, flashing, etc. $ 2.50/ft2
Installation $ 4.00/ft2
Other costs $ 1.00/ft2
Total $11.00/ft2
8
PLANT NAME LOCATION SIZE YEAR
CANMET Ontario, Canada 7,500 ft2 1997
Ft. Carson Battery Storage Building Colorado Springs, Colorado, U.S. 300 ft2 1997
Ontario Hydro and Maintenance Building Ontario, Canada 800 ft2 1997
Ontario Ministry of Housing Ontario, Canada 1,350 ft2 1997
Spices Board of India, spice drying test Cochin, India 100 ft2 1997
Steeltech Building Products Inc., metal fabrication plant Bloomfield, Connecticut, U.S. 6,200 ft2 1997
Canadair Division of Bombardier Airplane Manufacturer Quebec, Canada 108,000 ft2 1996
Federal Express Littleton, Colorado, U.S. 5,000 ft2 1996
Ontario Hydro and Maintenance Building Ontario, Canada 800 ft2 1996
Avum Hanger Colorado Springs, Colorado, U.S. 7,800 ft2 1995
Bombardier Quebec, Canada 5,000 ft2 1995
Bombardier Quebec, Canada 5,000 ft2 1995
Bombardier Inc., Rail Cars Manufacturer Quebec, Canada 4,420 ft2 1995
Canadian Tool & Die Winnipeg, Canada 5,400 ft2 1995
Domco Manufacturing Plant Quebec, Canada 1,880 ft2 1995
Ford New Holland Tractor Factory Manitoba, Canada 22,000 ft2 1995
Joint Reasearch Centre Engineering Research Facility Italy 5,630 ft2 1995
Office Building Switzerland 5,000 ft2 1995
Ontario Hydro North West Hydroelectric Headquarters Ontario, Canada 700 ft2 1995
Ontario Ministry of Housing Ontario, Canada 4,500 ft2 1995
Stampsli Factory Switzerland 1,000 ft2 1995
Versatile Farm Equipment (Ford New Holland) Manitoba, Canada 21,700 ft2 1995Tractor Manufacturer
ASEAN-Canade Project on Solar-Energy-Drying Malaysia 3,990 ft2 1994Processes, Tea-Processing Plant
AVEDA Corporation Blaine, Minnesota, U.S. 1,270 ft2 1994
Centre D’Enseignement Professional School St. Hyacinthe, Pennsylvania, U.S. 6,800 ft2 1994
Ontario Ministry of Housing Ontario, Canada 3,580 ft2 1994
Bombardier Inc., Manufacturing Plant Quebec, Canada 12,900 ft2 1993
C.I.M.I.C. Secondary Public School Quebec, Canada 1,330 ft2 1993
Eder Volkermarket, Austria 2,260 ft2 1993
General Motors of Canada Wastewater Plant Ontario, Canada 2,700 ft2 1993
Ontario Ministry of Housing Canada 1,200 ft2 1993
Ontario Ministry of Housing Ontario, Canada 2,820 ft2 1993
Stadtwerke Gottingen Cogeneration Plant Gottingen, Germany 3,980 ft2 1993
Acier CMP Steel Service Centre Quebec, Canada 4,225 ft2 1992
General Motors of Canada Oshawa Battery Plant Ontario, Canada 4,680 ft2 1992
National Renewable Energy Laboratory Golden, Colorado 351 ft2 1991
Ensite Engine Plant Ontario, Canada 10,500 ft2 1990
Ford Motor Company of Canada Oakville Assembly Plant Ontario, Canada 20,000 ft2 1990
Table 2. Transpired Collector Installations
digital controls, and boiler and motorreplacements, saved 31.3 Btu and loweredenergy costs by $144,800. This savings was recognized with a 1997 Federal Energyand Water Management Award from theU.S. Department of Energy (DOE).
R&D magazine awarded Conserval, Inc.,and researchers at NREL its prestigiousR&D 100 Award, recognizing transpiredcollector technology as one of the 100 mostimportant advances of 1994 (the year thenomination was submitted).
Popular Science also recognized thetechnology’s potential when it awardedConserval and NREL its 1994 “Best ofWhat’s New” Award.
In 1992, DOE’s Energy-Related Inven-tions Program rated the transpired collectorin the top 2% of energy inventions.
Conserval Systems, Inc., has received anumber of awards from Canadian organiza-tions, including the Toronto ConstructionAssociation’s Best New Building Product ofthe Year Award in 1994 and a Certificate ofRecognition for development and commer-cialization of the technology from NaturalResources Canada.
Sizing andEnergy-SavingsCalculations
Figure 8 through Figure 11 present work-sheets and supporting data that help calcu-late the optimum size of a transpiredcollector for a given facility and estimate theamount of energy that will be saved annu-ally with the system.
Figure 8 is a worksheet for determiningthe size of the collector based on a facility’savailable south-facing wall and its outdoorrequirement.
Figure 9 is a worksheet for estimating theamount of energy a transpired collector willsave annually based on the size of the collector, the facility’s operating and perfor-mance characteristics, and geographic location.
The map in Figure 10 presents the esti-mated annual Btu output per square foot of collector for geographic regions in theUnited States. This data is used in theannual energy-savings worksheet in
Figure 9. The energy output data presentedon the map are a function of solar radiationand length (and severity) of the heating season.
The map in Figure 11 shows the averageannual heating degree-days for geographicregions in the United States; it too is used in the energy-savings calculation.
Appendix A provides examples of completed sizing and energy-savings worksheets.
9
Figure 7. The transpired collector on the Waste Handling Facility at the NationalRenewable Energy Laboratory in Golden, Colorado.
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Figure 8. Collector sizing worksheet.
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Collector SizingCollector sizing depends on the magnitude of the building ventilation and the wall area available for mounting the transpired solar collector.
Vbldg = building outdoor airflow rate cfm
Aavai = available wall area for collector ft2
vmin = minimum collector flow rate cfm/ft2
(typically about 8 cfm/ft2)
vmax = maximum collector flow rate cfm/ft2
(typically about 8 cfm/ft2)
Amin = minimum collector area (ft2)
Amax = maximum collector area (ft2)
Acoll = design collector area (ft2)
Vcoll = total flow rate through the collector (cfm)
vcoll = flow rate per unit collector area (cfm/ft2)
Amin = ÷ = ft2
Vbldg Vmax
Amax = ÷ = ft2
Vbldg Vmin
1) if Aavail > Amax , then Acoll = Amax = ft2
Vcoll = Vbldg = cfm
vcoll = vmin = cfm/ft2
2) if Amin < Aavail < Amax , then Acoll = Aavail = ft2
Vcoll = Vbldg = cfm
vcoll = Vbldg ÷ Aavail = cfm/ft2
3) if Aavail < Amin , then Acoll = Aavail = ft2
Vcoll = Aavail x vmax = cfm
vcoll = vmax = cfm/ft2
11Figure 9. Energy-savings worksheet.
Annual Energy SavingsAcoll = collector area ft2
thours = time that there is airflow through the collector hours/day(length of collector operating day)
tdays = time that there is airflow through the collector days/week(length of collector operating week)
tweeks = time that there is airflow through the collector weeks/year(length of collector operating season)
qsolar = useful energy from the collector (from Map 1) kBtu/ft2-year
qfan = fan energy for airflow through the collector W/ft2
(typically about 1 W/ft2)
Uwall = heat loss coefficient for the building wall Btu/°F-ft2-hour
HDD = annual heating degree-days (from Map 2) °F-days/year
Ehtg = efficiency of the conventional heating system fraction
Qsolar = solar energy collected (MBtu/year)
Qwall = wall heat recapture (MBtu/year)(only significant for very poorly insulated walls)
Qsaved = thermal energy savings (MBtu/year)
Qfan = fan energy use (kWh/year)
Thermal Energy Savings:
Qsolar = x x ( ÷7) ÷ 103 = MBtu/yearAcoll qsolar tdays
Qwall = x x x ( ÷7) x ÷ 106 = MBtu/yearAcoll Uwall thours tdays HDD
Qsaved = ( + ) ÷ = MBtu/yearQsolar Qwall Ehtg
Electrical Energy Parasitics:
Qfan = x x x x ÷ 103 = kWh/yearAcoll qfan thours tdays tweeks
Figure 10. Useful energy delivered by the collector.
Figure 11. Annual heating degree-days.
12
13
Case Study—General MotorsBattery PlantFacility Description
The General Motors Battery Plant inOshawa, Canada, is a 100,000-ft2 facility in which automotive batteries are manufac-tured. The plant was built in the 1970s and consists of an open shop floor and a 28-foot-high ceiling. GM operates two full-time production shifts within the plant andconducts maintenance activities at night and on weekends, so the building is continuously occupied.
Existing Technology DescriptionUntil the early 1990s, GM relied solely
on a steam-operated fan coil system forspace heating, but the system was incapableof providing the necessary quantities ofheated outdoor air. As a result, the plant was not being adequately ventilated.
New Technology DescriptionIn 1991, plant management installed a
transpired collector (see Figure 12) to cor-rect the ventilation problems. During thefollowing 2 years the transpired collectorsystem was modified slightly to improve air-flow; the original fans and motors were
replaced with vane axial fans and high-efficiency motors, and the original ductingwas replaced with the upgraded fabric ducting.
The GM plant collector comprises4520 ft2 of absorber sheeting. The lower21 feet of the transpired collector is black,perforated, aluminum wall cladding with1.6-mm holes totaling 2% porosity. Theaverage depth of the plenum between thetranspired collector and the plant’s structuralwall is 6 inches. The canopy at the top ofthe wall acts as both a manifold for the air-flow and a solar heat collection device. Thecanopy face is made of perforated plate with1% porosity. The transpired collector coversabout 50% of the total area of the plant’ssouth-facing wall; the remainder of thesouth facade has shipping doors and otherobstructions that make it unsuitable formounting collector cladding.
The GM transpired collector has twofan/distribution systems, each consisting ofa constant-speed fan, a recirculation dampersystem, and a fabric distribution duct. Thetotal airflow delivered by the system’s fansis 40,000 cfm. Both recirculated air and airdrawn through the solar collectors make upthis flow; the percentages of each depend onthe temperature of the air coming from thecollector.
Energy SavingsThe GM Battery Plant’s transpired col-
lector has been monitored extensively sinceit was installed. The data in this case study
reflect the performance of the system duringthe 1993/94 heating season after the previ-ously mentioned modifications had beencompleted. An in-depth report on the moni-toring program is available (EnermodalEngineering Ltd. 1995).
The data shows that the annual energysavings for the 4520-ft2 collector was940 million Btu/year (see Table 3):678 MBtu resulted from the thermal energygained directly from the outside air as itpassed through the absorber (Qsolar); and262 MBtu resulted from heat loss recapturedby the wall (Qwall) from inside the building.Other possible energy-saving mechanisms—such as destratification and heat recapture—likely contributed to improved systemperformance; however, these effects arehighly structure-specific and have not beenincorporated into the savings reported here.
Life-Cycle CostThe cost of the transpired collector
system at the GM plant was $66,530, or$14.72/ft2 of installed collector. The cost per square foot is higher than typical instal-lations for two reasons: 1) this system wasinstalled soon after the technology wasintroduced, before design and installationprocedures had been streamlined, and 2) thecost includes the fan and ducting modifica-tions that were implemented during the first2 years of operation.
Figure 12. The transpired collector on the GM Battery Plant in Oshawa, Canada.
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Based on a National Institute of StandardTechnology (NIST) Building Life-CycleCosting (BLCC) comparative economicanalysis (see Appendix C), the present valueof life-cycle savings is $64,574, for a sav-ings-to-investment ratio of 1.97. Therefore,the project is cost effective according toFederal criteria (W CFR 43G). The simplepayback period is 6 years. (See the FederalLife-Cycle Costing Procedures in AppendixB and the NIST BLCC ComparativeEconomic Analysis in Appendix C.)
Performance ShakeoutThe GM transpired collector had a num-
ber of operational problems. After the sys-tem was initially installed, employeescomplained about fan noise and cold drafts,and they occasionally disabled the system.The fan and duct upgrades described previ-ously eliminated the problems on one of thefan systems; the other fan continues to gen-erate noise, and employees still disable itwhen working in the immediate vicinity.Conserval has addressed these complaintsby specifying smaller, but more numerous,fans in subsequent installations.
Also, both bypass dampers and a recircu-lation damper required additional mainte-nance. The recirculation damper becamestuck in full recirculation mode, and a newmodulating motor was installed to fix theproblem. The bypass dampers occasionallybecame bound, which led to unacceptablyhigh leakage rates. These dampers were keptclosed manually throughout the 1993–1994heating season.
The Technologyin Perspective
Transpired collectors have potential forwidespread use in the Federal sector, withthousands of warehouses, hangars, storagefacilities, vehicle maintenance shops,hazardous material storage, and other ventilation-intensive structures.
Users in the Federal sector can realize anumber of advantages from transpired col-lector technology. First, by reducing theamount of conventional energy used to heatventilation air, system users save money.Second, by replacing heat derived from fos-sil fuels with heat derived from clean solarenergy, users are reducing the detrimentalenvironmental side effects, such as the pro-duction of greenhouse gases (plus NOx andSOx) associated with the burning of fossilfuels. Third, the technology is remarkablysimple and maintenance free; the systemmakes few additional demands on the facil-ity’s engineering or maintenance staff.
The Technology’s DevelopmentPrior to 1989, solar air-heating systems
resembled flat-plate water-heating systems.These early systems contained a dark metalabsorber, but they also had glazing—a trans-parent cover—that prevented heat loss to theatmosphere. Rather than pull air through theabsorber, these systems heated the air flow-ing parallel to the absorber. Compared totranspired collector technology, these sys-tems were less efficient because solar radia-tion was reflected off the glazing and theyhad comparatively poor heat transfer fromthe absorber to the air. They were also moreexpensive because of the added materialcost.
The transpired collector design, devel-oped simultaneously at Conserval, Inc., andNREL in 1989, represented a major break-through in solar air-heating technology. By eliminating the glazing and pulling airthrough the absorber, researchers were ableto greatly improve efficiencies and reducesystem costs. Since the initial breakthrough,researchers and engineers have continued toimprove system performance by optimizingthe size and spacing of the perforations anddeveloping methods to ensure uniform air-flow through the absorber.
Technology OutlookThe future will likely see increased use
of transpired collectors in the Federal andprivate sectors. The biggest hurdle the tech-nology must overcome is user acceptance.Many solar technologies have been stigma-tized by the rapid expansion of solar mar-kets in the 1970s, when many poorlydesigned and poorly performing systemswere deployed. Many potential users arereluctant to commit to a solar technology ifa proven conventional option is available.Transpired collector technology has beenproven to be valid and reliable for reducingenergy use and saving money, and the bodyof scientific data proving its effectivenesscontinues to grow.
Further dramatic improvements in effi-ciency are unlikely; transpired collectors arealready among the most efficient solar col-lectors available, converting between 65%and 75% of the available solar energy.
ManufacturersConserval Engineering, Inc.200 Wildcat RoadDownsview, Toronto, ONTARIOM3J 2N5(416) 661-7057Fax: (416) 661-7146
Conserval Systems, Inc.4242 Ridge Lea Road, Suite 1Buffalo, NY 14226(716) 835-4903Fax: (716) 835-4904
For FurtherInformationCraig Christensen, Mail Stop 2722National Renewable Energy Laboratory1617 Cole BoulevardGolden, Colorado 80401 Phone: (303) 384-7510 Fax: (303) 384-7540e-mail: [email protected]
14
Energy-Saving Mechanism MBtu/year
Active Solar Heating 678
Recaptured Wall Heat Loss 262
Total Energy Savings 940
*The data presented in this table were derived from EnermodalEngineering Limited, 1995, Performance of the Perforated-Plate/Canopy Solarwall at GM Canada, Oshawa, DSS ContractNo. 007 SQ 23440-1-9552, Energy Technology Branch, CAN-MET, Department of Natural Resources Canada, Ottawa, Ontario.
Table 3. Savings from the GMTranspired Collector*
15
LiteratureAmerican Society of Heating, Refrigerating,and Air-Conditioning Engineers, Inc.ASHRAE Standard: Ventilation forAcceptable Indoor Air Quality,ANSI/ASHRAE 62-1989.
Brunger, A.; Hollands, K. (June 1996).“Back-of-Plate Heat Transfer in UnglazedPerforated Collectors Operated under Non-Uniform Air Flow Conditions.” Presented at the 22nd Annual Conference of the SolarEnergy Society of Canada, Inc., Orillia,Ontario, Canada.
Christensen, C.; Hancock, E.; Barker, G.;Kutscher, C. (March 1990). “Cost andPerformance Predictions for AdvancedActive Solar Concepts.” Proceedings of the American Solar Energy Society AnnualMeeting; Austin, Texas.
Dymond, C.; Kutscher, C. (March 1995). “A Computer Design Model for TranspiredSolar Collector Systems.” Presented at theASME International Solar EnergyConference. Also accepted for publicationin the journal Solar Energy.
Enermodal Engineering Limited. (1995).Performance of the Perforated-Plate/CanopySolarwall at GM Canada, Oshawa. DSSContract No. 007 SQ 23440-1-9552.Ottawa, ON: Energy Technology Branch,CANMET, Department of NaturalResources.
Gawlik, K. (May 1995). A Numerical andExperimental Investigation of Heat TransferIssues in the Practical Utilization ofUnglazed, Transpired Solar Air Heaters.Ph.D. thesis, Department of Civil,Environmental, and ArchitecturalEngineering, University of Colorado,Boulder, Colorado.
Golneshan, A. (April 1991). ForcedConvection Heat Transfer from aTranspired-Plate Having SlottedPerforations, with Applications to Solar Air Heaters. Ph.D. thesis, Department ofMechanical Engineering, University ofWaterloo, Waterloo, Canada.
Gunnewick, L. (April 1994). AnInvestigation of the Flow DistributionThrough Unglazed Transpired-Plate SolarAir Heaters. Master’s thesis, Department of Mechanical Engineering, University ofWaterloo, Waterloo, Canada.
International Energy Agency. (December1995). Low-Cost, High Performance SolarAir Heating Systems Using PerforatedAbsorbers (Draft). Final copies will beavailable from Doug McClenahan,CANMET, 580 Booth Street, 7th Floor,Ottawa, Canada K1A OE4.
Kutscher, C. (December 1992). AnInvestigation of Heat Transfer for Air Flowthrough Low Porosity Perforated Plates.Ph.D. thesis, Department of MechanicalEngineering, University of Colorado,Boulder, Colorado.
Kutscher, C.; Christensen, C.; Barker, G.(August 1991). “Unglazed Transpired SolarCollectors: An Analytical Model and TestResults.” Proceedings of the BiennialCongress of the International Solar EnergySociety; page 1245.
Kutscher, C.; Christensen, C.; Barker, G.(August 1993). “Unglazed Transpired SolarCollectors: Heat Loss Theory.” ASMEJournal of Solar Energy Engineering.
Peter, R.; Hollick, J. U.S. Patent No.4,899,728. (13 February 1990). Method andApparatus for Preheating Ventilation Air fora Building.
16
AppendixesAppendix A: Worksheets From Figures 8 and 9 Completed for a Hypothetical Building in Chicago,
Illinois.
Appendix B: Federal Life-Cycle Costing Procedures and BLCC Software
Appendix C: General Motors Solar Ventilation Heat Case Study NIST BLCC Comparative Economic Analysis
17
Appendix A: Collector SizingCollector sizing depends on the magnitude of the building ventilation and the wall area available for mounting the transpired solar collector.
Vbldg = building outdoor airflow rate cfm
Aavai = available wall area for collector ft2
vmin = minimum collector flow rate cfm/ft2
(typically about 8 cfm/ft2)
vmax = maximum collector flow rate cfm/ft2
(typically about 8 cfm/ft2)
Amin = minimum collector area (ft2)
Amax = maximum collector area (ft2)
Acoll = design collector area (ft2)
Vcoll = total flow rate through the collector (cfm)
vcoll = flow rate per unit collector area (cfm/ft2)
Amin = ÷ = ft2
Vbldg Vmax
Amax = ÷ = ft2
Vbldg Vmin
1) if Aavail > Amax , then Acoll = Amax = ft2
Vcoll = Vbldg = cfm
vcoll = vmin = cfm/ft2
2) if Amin < Aavail < Amax , then Acoll = Aavail = ft2
Vcoll = Vbldg = cfm
vcoll = Vbldg ÷ Aavail = cfm/ft2
3) if Aavail < Amin , then Acoll = Aavail = ft2
Vcoll = Aavail x vmax = cfm
vcoll = vmax = cfm/ft2
12,000
2,000
4
8
1,500
3,000
12,000 8
12,000 4
2,000
12,000
6
Annual Energy SavingsAcoll = collector area ft2
thours = time that there is airflow through the collector hours/day(length of collector operating day)
tdays = time that there is airflow through the collector days/week(length of collector operating week)
tweeks = time that there is airflow through the collector weeks/year(length of collector operating season)
qsolar = useful energy from the collector (from Map 1) kBtu/ft2-year
qfan = fan energy for airflow through the collector W/ft2
(typically about 1 W/ft2)
Uwall = heat loss coefficient for the building wall Btu/°F-ft2-hour
HDD = annual heating degree-days (from Map 2) °F-days/year
Ehtg = efficiency of the conventional heating system fraction
Qsolar = solar energy collected (MBtu/year)
Qwall = wall heat recapture (MBtu/year)(only significant for very poorly insulated walls)
Qsaved = thermal energy savings (MBtu/year)
Qfan = fan energy use (kWh/year)
Thermal Energy Savings:
Qsolar = x x ( ÷7) ÷ 103 = MBtu/yearAcoll qsolar tdays
Qwall = x x x ( ÷7)x ÷ 106 = MBtu/yearAcoll Uwall thours tdays HDD
Qsaved = ( + ) ÷ = MBtu/yearQsolar Qwall Ehtg
Electrical Energy Parasitics:
Qfan = x x x x ÷ 103 = kWh/yearAcoll qfan thours tdays tweeks
18
2,000
16
5
35
120
1
0.1
5,000
0.7
171
11
260
5,600
2,000 120 5
2,000 0.1 16 5 5,000
171 11 0.7
2,000 1 16 5 35
Appendix B: Federal Life-Cycle Costing Proceduresand the BLCC Software
Federal agencies are required to evaluate energy-related investments on the basis of minimum life-cycle costs (LCC) (10 CFR Part 436).A life-cycle cost evaluation computes the total long-run costs of a number of potential actions, and selects the action that minimizes thelong-run costs. When considering retrofits, sticking with the existing equipment is one potential action, often called the baseline condition.The LCC of a potential investment is the present value of all of the costs associated with the investment over time.
The first step in calculating the LCC is to identify the costs. Installed cost includes cost of materials purchased and the labor required to install them (for example, the price of an energy-efficient lighting fixture, plus cost of labor to install it). Energy cost includes annualexpenditures on energy to operate equipment. (For example, a lighting fixture that draws 100 watts and operates 2,000 hours annuallyrequires 200,000 watt-hours [200 kWh] annually. At an electricity price of $0.10/kWh, this fixture has an annual energy cost of $20.) Non-fuel operation and maintenance (O&M) includes annual expenditures on parts and activities required to operate equipment (for exam-ple, replacing burned-out lightbulbs). Replacement costs include expenditures to replace equipment upon failure (for example, replacing anoil furnace when it is no longer usable).
Because LCC includes the cost of money, periodic and a-periodic O&M and equipment replacement costs, energy escalation rates, andsalvage value, it is usually expressed as a present value, which is evaluated by
LCC = PV (IC) + PV(EC) + PV (OM) + PV (REP)
wherePV (x) denotes "present value of cost stream x",
IC is the installed cost,
EC is the annual energy cost,
OM is the annual non-energy cost, and
REP is the future replacement cost.
Net present value (NPV) is the difference between the LCCs of two investment alternatives, e.g., the LCC of an energy-saving or energy-cost-reducing alternative and the LCC of the baseline equipment. If the alternative's LCC is less than the baseline's LCC, the alternative issaid to have NPV, i.e., it is cost effective. NPV is thus given by
NPV = PV(EC0) - PV(EC1) + PV(OM0) - PV(OM1) + PV(REP0) - PV(REP1) - PV (IC)
or
NPV = PV(ECS) + PV (OMS) + PV(REPS) - PV (IC)
wheresubscript 0 denotes the baseline condition,
subscript 1 denotes the energy cost-saving measure,
IC is the installation cost of the alternative (the IC of the baseline is assumed to be zero),
ECS is the annual energy cost saving,
OMS is the annual non-energy O&M saving, and
REPS is the future replacement saving.
Levelized energy cost (LEC) is the break-even energy price (blended) at which a conservation, efficiency, renewable, or fuel-switchingmeasure becomes cost effective (NPV > = 0). Thus, a project's LEC is given by
PV(LEC*EUS) = PV(OMS) + PV(REPS) - PV(IC)
where EUS is the annual energy use savings (energy units/yr). Savings-to-investment ratio (SIR) is the total (PV) saving of a measuredivided by its installation cost:
SIR = (PV(ECS) + PV(OMS) + PV(REPS))/PV(IC)
Some of the tedious effort of LCC calculations can be avoided by using the BLCC software, developed by NIST. For copies of BLCC,call the FEMP Help Desk at (800) 363-3732.
19
Appendix C: General Motors Solar Ventilation HeatCase Study NIST BLCC Comparative Economic Analysis
******************************************************************************* N I S T B L C C: COMPARATIVE ECONOMIC ANALYSIS (ver. 4.4-97 ) *
******************************************************************************
Project: General Motors Solar Ventilation HeatBasecase: BasecaseAlternative: Solar Case
Principal Study Parameters
Analysis Type: Federal Analysis—Energy Conservation ProjectsStudy Period: 25.00 Years (Jan 1997 through Dec 2021)Discount Rate: 4.1% Real (exclusive of general inflation)Basecase LCC File: GMBASCAS.LCCAlternative LCC File: GMSOLAR.LCC
Comparison of Present-Value Costs
SavingsBasecase Solar Case from Alt.
Initial Investment item(s): —————— —————— ——————Cash Requirements as of Service Date $0 $66,530 -$66,530
—————- ————- —————-Subtotal $0 $66,530 -$66,530
Future Cost Items:
Annual and Other Recurring Costs $0 $3,092 -$3,092Energy-related Costs $136,822 $2,627 $134,195
—————- ————- —————-Subtotal $136,822 $5,718 $131,104
—————- ————- —————-Total Present Value of Life-Cycle Cost $136,822 $72,248 $64,574
Net Savings from Alternative Solar Case Compared to Alternative BaseCase
Net Savings = P.V. of Noninvestment Savings $131,104- Increased Total Investment $66,530
——————-Net Savings: $64,574
Note: the Savings-to-Investment Ratio (SIR) and AIRR computations include differential initial costs, capital replacement costs, and residual value (if any) as investment costs, per NIST Handbook 135 (Federal and MILCON analyses only).
SIR for Alternative Solar Case Compared to Alternative Basecase
P.V. of non-investment savingsSIR = ——————————————— = 1.97
Increased total investment
Adjusted Internal Rate of Return (AIRR) for Alternative Solar Case Compared to Alternative Basecase(Reinvestment Rate = 4.10%; Study Period = 25 years)
AIRR = 6.96%
Estimated Years to Payback: Simple Payback occurs in year 10; Discounted Payback occurs in year 12
ENERGY SAVINGS SUMMARY
Energy Units — Average Annual Consumption — Life-CycleType Basecase Alternative Savings Savings
—————— ———— —————- —————- ————— ———————Electricity kWh 0 2,362 -2,362 -59,050Central Steam MBtu 940 0 940 23,500
20
The Energy Policy Act of 1992, andsubsequent Executive Orders, mandatethat energy consumption in the Federalsector be reduced by 30% from 1985levels by the year 2005. To achieve this goal, the U.S. Department ofEnergy’s Federal Energy ManagementProgram (FEMP) is sponsoring a series of programs to reduce energyconsumption at Federal installationsnationwide. One of these programs,the New Technology DemonstrationProgram (NTDP), is tasked to acceler-ate the introduction of energy-efficientand renewable technologies into theFederal sector and to improve the rate oftechnology transfer.
As part of this effort FEMP is sponsoring a series of Federal Tech-nology Alerts (FTAs) that provide summary information on candidateenergy-saving technologies developedand manufactured in the United States.The technologies featured in theTechnology Alerts have already entered the market and have some experience but are not in general use in the Federal sector. Based on theirpotential for energy, cost, and environ-mental benefits to the Federal sector,the technologies are considered to be
leading candidates for immediateFederal application.
The goal of the Technology Alerts is to improve the rate of technologytransfer of new energy-saving tech-nolo-gies within the Federal sector and toprovide the right people in the field withaccurate, up-to-date information on the new technologies so that they can make educated judgments onwhether the technologies are suitable for their Federal sites.
Because the Technology Alerts arecost-effective and timely to produce(compared with awaiting the results of field demonstrations), they meet the short-term need of disseminatinginformation to a target audience in a timeframe that allows the rapiddeployment of the technologies—andultimately the saving of energy in theFederal sector.
The information in the TechnologyAlerts typically includes a description of the candidate technology; the results of its screening tests; a descrip-tion of its performance, applications and field experience to date; a list ofpotential suppliers; and important contact infor-mation. Attached
appendixes provide supplemental information and example worksheets on the technology.
FEMP sponsors publication of theFederal Technology Alerts to facilitateinformation-sharing between manufac-turers and government staff. While the technology featured promises sig-nificant Federal-sector savings, theTechnology Alerts do not constituteFEMP’s endorsement of a particularproduct, as FEMP has not indepen-dently verified performance data pro-vided by manufacturers. Nor do the Federal Technology Alerts attemptto chart market activity vis-a-vis thetechnology featured. Readers shouldnote the publication date on the backcover, and consider the Alert as an accurate picture of the technology andits performance at the time of publica-tion. Product innovations and theentrance of new manufacturers or suppliers should be anticipated since the date of publication. FEMP encourages interested Federal energyand facility managers to contact themanufacturers and other Federal sitesdirectly, and to use the worksheets inthe Technology Alerts to aid in theirpurchasing decisions.
Federal Energy Management ProgramThe Federal Government is the largest energy consumer in the nation. Annually, in its 500,000 buildings and 8,000 locations worldwide,it uses nearly two quadrillion Btu (quads) of energy, costing over $11 billion. This represents 2.5% of all primary energy consumption inthe United States. The Federal Energy Management Program was established in 1974 to provide direction, guidance, and assistance toFederal agencies in planning and implementing energy management programs that will improve the energy efficiency and fuel flexibility of the Federal infrastructure.
Over the years several Federal laws and Executive Orders have shaped FEMP's mission. These include the Energy Policy and Conserva-tion Act of 1975; the National Energy Conservation and Policy Act of 1978; the Federal Energy Management Improvement Act of 1988;and, most recently, Executive Order 12759 in 1991, the National Energy Policy Act of 1992 (EPACT), and Executive Order 12902 in 1994.
FEMP is currently involved in a wide range of energy-assessment activities, including conducting New Technology Demonstrations, to has-ten the penetration of energy-efficient technologies into the Federal marketplace.
This report was sponsored by the United States Government. Neither the United States nor any agency or contractor thereof, nor any oftheir employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness,or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specific commercial product, process, or service by trade name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency or contractor thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United StatesGovernment or any agency or contractor thereof.
About the Federal Technology Alerts
For More Information
FEMP Help Desk:(800) 363-3732International callers please use:(703) 287-8391Web site: http://www.eren.doe.gov/femp/
General Contacts
Bob McLarenNTDP Program ManagerFederal Energy Management ProgramU.S. Department of Energy1000 Independence Avenue SW, EE-92Washington, D.C. 20585(202) 586-0572
Steven A. ParkerPacific Northwest National LaboratoryP.O. Box 999, MS K5-08Richland, Washington 99352(509) [email protected]
Technical Contacts
Craig Christensen, Mail Stop 2722National Renewable Energy Laboratory1617 Cole BoulevardGolden, CO 80401Phone: (303) 384-7510Fax: (303) 384-7540e-mail: [email protected]
Produced for the U.S. Department of Energy (DOE) by the NationalRenewable Energy Laboratory,a DOE national laboratory
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