The U.S. Department of Energy

requests that no alterations be

made without permission in any

reproduction of this document.

Federal

Technology

Alert

A publication series

designed to speed the

adoption of energyefficient and renewable

technologies in the

Federal sector

The President's Million Solar Roofs

Initiative aims to install 1 million solar

energy systems on residential, commercial,

and public-sector buildings by 2010. Twenty

thousand of those systems will be installed

on Federal buildings. In support of the

Initiative, and as part of a continual effort 

to ensure U.S. buildings are energy efficient

and environmentally sustainable, the U.S.

Department of Energy's Federal Energy

Management Program (FEMP) will help

install those solar systems targeted for the

Federal sector.

FEMP is focusing on solar systems that

include photovoltaics (PV), solar hot water

for buildings and swimming pools, and solar

space heating. Transpired solar 

collectors are a solar space-heating technology that is well proven, is readily available,

and has considerable potential for application at Federal facilities. Transpired collectors use solar energy to preheat ventilation

(outdoor) air as it is drawn into a building.

The technology is ideally suited to building

applications in which large volumes of

space are heated or where high ventilation

rates are required. By preheating ventilation

air with solar energy, the technology

removes a substantial load from a building’s

conventional heating system, saving energy

and money. This Federal Technology Alert

(FTA) is designed to give Federal facility

managers the information they need to

decide whether transpired collector technology is suitable for their facility.

Energy-Saving Mechanism

A transpired collector reduces the load

on a building’s heating system by heating

intake air with solar energy. It preheats the

ambient air by up to 40°F, reducing all or a

portion of the load on a heating system during daylight hours. Although the transpired

collector may not be able to achieve the

required indoor air temperature on cloudy

days or when the outside temperature plummets, it provides useful energy and reduces

utility bills.

In addition to meeting a portion of a

building’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.

Keith Gawlik, NRE/PIX04118
save energy and money in other ways. It

recaptures heat loss through a building’s

south-facing wall; heat that escapes through

the south wall is captured in the air space

between the structural wall and the transpired collector and returned to the interior.

Also, by introducing make-up air through

ceiling-mounted ducts, the system eliminates the wasteful air stratification that often

plagues high-ceiling buildings.

Technology Selection

The FTA series targets technologies that

appear to have significant untapped Federalsector potential and for which some Federal

installation experience exists. The new 

technologies presented in the series were

identified through trade journals and

through direct correspondence. Numerous

responses were obtained from manufacturers, utilities, trade associations, research

institutes, Federal sites, and other interested

parties. Based on these responses, the technologies were evaluated in terms of potential Federal-sector energy savings and

procurement, installation, and maintenance

costs. They were also categorized as either

just coming to market ("unproven" technologies) or as technologies for which field data

and experience exist.

Transpired collectors are one of many

energy-saving technologies to emerge in 

the last 20 years. They were judged to be

life-cycle cost effective (at one or more

Federal sites) in terms of installation cost,

net present value, and energy savings.

Several other proven technologies have 

been slated for further study through the

Federal Technology Alert series.

Application 

Any heated building in a cool, sunny 

climate that has at least moderate ventilation

requirements and southern exposure could

benefit from a transpired collector. Buildings that require large volumes of heated

make-up air such as machine shops, vehicle

maintenance buildings, and chemical storage

facilities, are good candidates for a transpired collector. A long-term storage warehouse 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 common in many modern office buildings (but

less common in industrial buildings), also

preheat ventilation air. As such—because 

of redundancy of function—buildings 

with existing heat recovery systems may 

not be suitable for transpired collector 

applications.

Field Experience

As of 1997, approximately 40 transpired

collector systems have been installed in the

private sector and on two Federal sites—

two systems at Fort Carson, Colorado, and

one at the National Renewable Energy

Laboratory (NREL) in Golden, Colorado.

Three of the 40 installations—the General

Motors (GM) of Canada Oshawa Battery

Plant, the Ford Plant in Oakville, Canada,

and NREL—have been monitored 

extensively.

The installation at NREL is an ideal

application of transpired collector technology. The facility is a 1300-square-foot (ft2)

chemical waste storage building that

requires a ventilation rate of 3000 cfm to

maintain safe indoor conditions. Also,

because of the danger of combustion, open

flames are prohibited in the building, so 

outside air is heated with electricity instead

of gas. The 300-ft2 transpired collector 

saves about 14,310 kWh annually (25.7% 

of the energy required to heat the facility’s

ventilation air). With a local electric rate of

$0.025/kWh, the annual savings equates to

about $360. The installation has a simple

payback of 4.7 years.

Case Study

The GM Battery Plant in Oshawa,

Canada, is a 100,000-ft2 facility in which

automotive batteries are manufactured. 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 and conducts

maintenance activities at night and on weekends, so the building is continuously occupied. In 1991, plant management opted to

install a transpired collector to correct the

ventilation problems.

The monitoring program showed that the

transpired collector saved GM 208,000 Btu

per year for every square foot of installed

solar collector. The majority of this savings—or 150,000 Btu/ft2/year—resulted

from 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 recaptured heat loss through the wall clad with

the transpired collector. The value of these

savings depends on the heating source

assumed to be displaced.

At the GM plant, steam was the existing

primary heating source, but the system was

incapable of providing the necessary quantities of heated outdoor air for ventilation. To

redress the airflow problems with a steam

option would have required the installation

of a packaged rooftop steam-operated 

system with roof curb, steam piping, and

outlet ducting. These systems are installed

for about $2.16/cfm. To supply the same

volume of air that the transpired collector

supplies, the steam system would have to

deliver 25,200 cfm and would cost $55,000.

Also, the fan on the steam system would

require an additional 3.6 kW to operate

(compared to the transpired collector system), which would increase electricity costs

by $1,430 annually.

Implementation Barriers

The biggest hurdle transpired collectors

must overcome is user acceptance. Many

solar technologies have been stigmatized by

the rapid expansion of solar markets in the

1970s, when many poorly designed and

poorly performing systems were deployed.

Many potential users are reluctant to commit to a solar technology if a proven conventional option is available. Transpired

collector technology has been proven to 

be a valid, reliable technology for reducing

energy use and saving money, and the body

of scientific data proving its effectiveness

continues to grow.
Abstract

Transpired collectors are a renewable

energy technology that is well proven, is

readily available, and has considerable

potential for application at Federal facilities.

Transpired collectors use solar energy to

preheat ventilation (outdoor) air as it is

drawn into a building. The technology is

ideally suited for buildings with at least

moderate ventilation requirements in sunny

locations with long heating seasons.

Transpired collector technology is

remarkably simple. A dark, perforated metal

wall is installed on the south-facing side of

a building, creating approximately a 6-inch

(15-cm) gap between it and the building’s

structural wall. The dark-colored wall acts

as a large solar collector that converts solar

radiation to heat. Fans mounted at the top 

of the wall pull outside air through the transpired collector’s perforations, and the 

thermal energy collected by the wall is

transferred to the air passing through the

holes. The fans then distribute the heated 

air into the building through ducts mounted

near the ceiling. By preheating ventilation

air with solar energy, the technology

removes a substantial load from a building’s

conventional heating system, saving energy

and money.

This Federal Technology Alert (FTA) 

of the New Technology Demonstration

Program is designed to give Federal facility

managers the information they need to

decide whether they should pursue transpired collector technology for their facility.

This FTA describes the transpired collector, its energy-saving mechanisms, and

the factors that influence its performance.

Worksheets are included that let the reader

perform the preliminary calculations to

determine if a given facility is suitable for a

transpired collector system and to determine

the amount of energy such a system would

save annually. The FTA contains a case

study documenting the performance of the

transpired collector installed at General

Motors’ battery plant in Oshawa, Canada.

The document concludes with contacts for

additional information and a list of articles,

conference papers, and academic theses 

pertaining to the technology.

Transpired Collectors (Solar Preheater

for Outdoor Ventilation Air)

Simple, reliable technology can substantially reduce heating bills.

1

Figure 1. A transpired collector heating system being installed on a Federal Express 

facility in Littleton, Colorado.

Federal

Technology 

Alert

Warren Gretz, NREL/PIX04257
Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .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

2
About the

Technology

Transpired collectors use solar energy 

to preheat ventilation (outdoor) air as it is

drawn into a building. The technology is

ideally suited for buildings with at least

moderate ventilation requirements in sunny

locations with long heating seasons.

Transpired collector technology is

remarkably simple. A dark, perforated metal

wall is installed on the south-facing side of 

a building, creating approximately a 6-inch

(15-cm) gap between it and the building’s

structural wall (see Figure 2). The darkcolored wall acts as a large solar collector

that converts solar radiation to heat. Fans

associated with the building’s ventilation

system mounted at the top of the wall draw

outside air through the transpired collector’s

perforations, and the thermal energy collected by the wall is transferred to the 

air passing through the holes. The fans then

distribute the heated air into the building

through ducts mounted from the ceiling. By

preheating outdoor air with solar energy, the

technology removes a substantial load from

a building’s conventional heating system,

saving energy and money.

A transpired collector is installed on all

or part of a building’s south-facing wall,

where it will receive the maximum exposure

to direct sunlight during the fall, winter, and

spring. The size of the wall varies depending

on heating and airflow requirements and 

climate, but in many applications, the transpired collector will cover the maximum

south-facing area available. The amount of

energy and money saved by a transpired 

collector depends on the type of conventional fuel being displaced, occupant use

patterns, building design, length of heating

season, and the availability of sunlight during the heating season. In general, each

square foot of transpired collector will raise

the temperature of 4 cubic feet per minute

(cfm) by as much as 40°F, delivering as

much as 240,000 Btu annually per square

foot of installed collector.

In addition to the metal sheeting that captures solar energy, the transpired collector

heating system includes air-handling and

control components that supply the solarheated air (see Figure 2). The ventilation 

system, which operates independently of a

building’s existing heating system, includes

a constant-speed fan to draw air through the

transpired collector and into the distribution

duct. Engineers typically use a 3-horsepower, 32-inch blade fan with about 

10,000-cfm capacity.

As shown in Figures 3 and 4, the transpired collector system also contains a

bypass damper located directly in front of

the fan inlet duct. During the summer

months when ventilation air does not need

to be heated, this damper opens, circumventing the air-heating system. The bypass

damper automatically opens when the air

outside reaches a predetermined temperature, usually about 64°F.

Conserval Systems, Inc., of Toronto,

Ontario, and Buffalo, New York, is currently the only supplier of the technology.

Conserval has been manufacturing and

installing a range of heat recovery and solar

products since 1977. They received the first

patent for transpired collector technology in

1989.

Application Domain

As of 1997, about 40 transpired collector

air-heating systems had been installed in

locations around the world on apartment

buildings, warehouses, airplane hangars, factories, and in many other applications. Three

transpired collector systems are in 

3

Figure 2. Transpired collector components.

A Wall by any Other Name...

The solar ventilation air-heating 

technology described in this Federal

Technology Alert has been referred to 

by a variety of different names since it was

first marketed in 1989. Conserval Engineering

Inc., the company that 

holds the patent rights, refers to it as a

“Solarwall®” heater. The research community, which has studied the technology in

depth, refers to it as an “unglazed transpired

solar collector” or “solar air-heating systems

using perforated absorbers.” In other literature

it has been called “solar ventilation preheat

system” 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 not

be confused by the different names by which

the technology is presented in the scientific

and popular literature.

M75-B308401
use in the Federal sector—two at Fort

Carson, Colorado, and another at the Waste

Handling Facility at the National Renewable

Energy Laboratory (NREL) in Golden,

Colorado. High-profile industrial users

include Ford Motor Company, General

Motors Corporation (GM), McDonnell

Douglas, and Federal Express.

The Federal-sector potential for this technology is considerable. Any heated building

in a cool, sunny climate that has at least

moderate ventilation requirements could

benefit from a transpired collector. Buildings that require large volumes of heated

make-up air, such as machine shops, vehicle

maintenance buildings, and chemical storage

facilities are good candidates. There are

thousands of such facilities in the Federal

sector.

Energy-Saving Mechanism

A transpired collector reduces the load

on a building’s heating system by heating

intake air with solar energy. It preheats the

ambient air by up to 40°F, reducing all or a

portion of the load on a heating system during daylight hours. Although the transpired

collector itself may not be able to achieve

the required indoor air temperature on

cloudy days or when the outside temperature plummets, it still provides useful energy

and reduces utility bills.

The dark-colored transpired collector is 

a large solar collector, absorbing the solar

energy striking it. The wall captures

between 60% and 75% of the available solar

energy, making it one of the most efficient

solar collectors designed to date. In addition

to capturing direct solar radiation, the transpired collector collects the indirect, scattered, and reflected sunlight known as

diffuse solar radiation. Typically, diffuse

solar radiation, which includes a portion of

the radiation on clear days and all the radiation on overcast days, makes up about 25%

of the total annual radiation at the Earth’s 

surface.

The dark, corrugated metal sheets that

make up the wall are 0.8-millimeter (mm)-

thick and are typically manufactured from

aluminum or galvinized steel. The perforations through which the air flows are

1.6 mm in diameter and are placed at regular intervals. The total percentage of the 

collector made up of these holes is referred

to as the collector porosity.

In addition to meeting a portion of a

building’s heating load with clean, free solar

energy, the transpired collector helps save

energy and money in other ways. The collector recaptures heat loss through a building’s south-facing wall; heat that escapes

through the south wall is captured in the 

air space between the structural wall and 

the transpired collector and returned to the

interior. Also, by introducing make-up air

through ceiling-mounted ducts, the system

eliminates the wasteful air stratification that

often plagues high-ceiling buildings.

Other Benefits

The solar energy collected by a transpired collector displaces fossil fuel that

would otherwise be burned to produce

heated ventilation air. Greenhouse gas (CO2)

and acid rain emissions (SOx and NOx) are

reduced proportionally. Electricity is consumed, though, by the distribution fans and

dampers.

The transpired collector can also improve

a building’s appearance, giving the southfacing side a neat, clean, uniform look.

Although initially many black transpired

collectors were installed, other colors work

well too, which gives the user some aesthetic options. The product is now available

in a variety of shades, including brown,

blue, gray, and red.

M75-B308403

Fan unit

Bypass

damper

Distribution ducting

4

Figure 4. Transpired collector operation during the summer months.

M75-B308402

Fan unit

Heat loss through wall

captured by incoming air

Outside air is heated

passing through 

perforated absorber

Distribution ducting

Figure 3. Transpired collector operation during the winter months.
Installation

The transpired collector attaches to a

building’s existing structure. The building

frame is usually sufficient to support the

collector sheeting, so racking requirements

and construction costs are minimized.

Although many transpired collectors have

been installed as retrofits, the system economics improve if installation takes place

during initial construction or building renovation. The system payback period can be

reduced by up to half if the transpired collector installation can be incorporated with

other construction work.

The transpired collector is attached to the

structural wall with a support grid of vertical

and horizontal Z-channels. These channels

are perforated to accommodate airflow

between the collector and the structural

wall. The vertical channels are attached to

the existing wall, the horizontal channels are

attached to the vertical channels, and the

perforated metal sheets are affixed to the

horizontal channels.

The fan unit, which includes the fan,

dampers, and thermostat controls, is

mounted directly to the interior side of the

south-facing wall. For each fan unit

installed, a hole must be cut through the

structural wall to allow air to flow into the

building. A minimal amount of sheet-metal

ducting is required to form a proper seal

between the fan and the wall. 

Installation time varies depending on a

building’s structural design and the total collector area being installed, but retrofit installations typically require 10 to 14 days. 

Building codes and regulations pertaining to issues such as the location of outdoor

air openings, weather protection for ducts on

the building exterior, minimum duct thickness, and criteria for multistory applications

should be addressed on an installation-byinstallation basis. The applicability of codes

may vary based on building height and area,

construction type, and use group.

Federal-Sector

Potential

The potential savings to be achieved by

use of this new technology were estimated

as part of the technology-screening process

of the New Technology Demonstration

Program.

Technology-Screening Process

The new technologies presented in the

Federal Technology Alert series were identified through trade journals and through

direct correspondence. Numerous responses

were obtained from manufacturers, utilities,

trade associations, research institutes,

Federal sites, and other interested parties.

Based on these responses, the technologies

were evaluated in terms of potential Federalsector energy savings and procurement,

installation, and maintenance costs. They

were also categorized as either just coming

to market ("unproven" technologies) or as

technologies for which field data and experience exist. Transpired collectors were

judged to be life-cycle cost effective (at one

or more Federal sites) in terms of installation cost, net present value, and energy savings. Several other proven technologies

have been slated for further study through

Federal Technology Alerts.

Estimated Savings and 

Market Potential

Figure 5 shows the average payback periods for transpired collector systems in three

different geographic locations based on

three different prices for natural gas. The

data were derived from analyses conducted

by Science Applications International

Corporation (SAIC), which, in conjunction

with the International Energy Agency, has

developed a spreadsheet model to estimate

economics of transpired collectors in different climates. The model outputs simple payback periods, taking into account the cost of

auxiliary heating fuel, the cost of electricity

to operate the fan, the geographic location,

and the building type. 

SAIC has used this model to compare

paybacks at three locations—with greatly

differing climates—in the United States:

Denver, Colorado; Washington, D.C.; and

Syracuse, New York. The model produced

payback comparisons for a range of energy

costs, from $3/MBtu for natural gas to

$24/MBtu for electricity ($0.08/kilowatthour [kWh]). These paybacks assume the

collector was installed during building construction 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 approximately $10/ft2 —would be twice those for

new installations.

Laboratory Perspective

Researchers at NREL in Golden,

Colorado, have been studying transpired

collector technology since 1989. In fact,

researchers at NREL and engineers at

Conserval simultaneously—and independently—developed the concept of using

unglazed, perforated metal sheets as solar

collectors for heating air. Since that time,

researchers have improved the efficiency 

of the technology and gained a fundamental

understanding of its heat transfer and airflow characteristics. The technology has

been the subject of a number of dissertations

conducted at universities in the United

States and Canada. These investigations and

other relevant publications are cited in the

Literature section on page 15.

The research community has monitored

and continues to monitor a number of the

transpired collectors in use in both the

Federal and private sectors. These monitoring programs have demonstrated that transpired collector technology is a reliable and

effective way to save energy. They have also

demonstrated that the current methods used

to estimate the performance of the technology are valid.

Application

This section addresses technical aspects

of applying transpired collector technology.

The range of applications and climates in

which transpired collector technology can

best be applied are addressed. Design and

integration considerations are discussed,

including equipment and installation costs,

installation details, maintenance, and codes

and standards.

Application Considerations

The following subsections briefly discuss

the prerequisites for a successful transpired

collector application and the factors that

influence project cost-effectiveness. 

Application Prerequisites

• Suitable South-Facing Wall—A sufficient area of suitable south-facing exterior wall is required for installing the

transpired collector’s metal cladding. A

wall with a high percentage of window

or door area will likely be unsuitable,

as will a wall that is heavily shaded

throughout the day. A facade does not

have to face true south for a transpired

collector to operate effectively. Any wall

5
within 45 degrees of true south will

work, but the best performance is realized when the wall is within 20 degrees

of true south.

• Ventilation Load—A candidate building

for a transpired collector must have a

minimum ventilation requirement. A

long-term storage warehouse, for example, would not be a suitable application,

because such a structure would not have

to be ventilated. Also, buildings that have

100% recirculation/filtration systems

would be unsuitable. The Project Scale

section, below, discusses the influence 

of ventilation loads on the costeffectiveness of an application.

• The Absence of a Heat Recovery

System—Heat recovery systems, which

are common in many modern office

buildings (but less common in industrial

buildings), also preheat ventilation air. 

Because of redundancy of function,

buildings with existing heat recovery systems may not be suitable for transpired

collector applications.

Cost-Effectiveness Factors

The following paragraphs present the

major factors influencing the costeffectiveness of transpired collector applications. The worksheets presented in the

Sizing and Energy-Savings Calculations 

section (pages 9–11) provide quantitative

estimates of energy savings.

• The Cost of Conventional Energy—

The cost of the conventional energy used

for space heating has a dramatic impact

on the overall cost-effectiveness of a

transpired collector application. What

kind of energy is being displaced by the

thermal energy supplied by the transpired

collector and how much does that conventional energy cost? The price of conventional energy (that is, electricity,

fossil fuels, or steam) can vary greatly

from season to season and from region 

to region. The higher the price of the

conventional energy used for space heating, the more cost effective the transpired 

collector application becomes.

• Climate—Transpired collectors are most

cost effective in sunny climates with long

heating seasons.

• Project Scale—Although transpired collectors have been installed on many large

industrial buildings, they can also be cost

effectively applied to smaller structures.

A key to a successful application is the

building’s ventilation rate. For example, a

small building with a large outdoorair ventilation rate, such as a machine

shop, 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-Conditioning

Engineers Standard 62 for recommended

ventilation requirements.) Keep in mind

that small transpired collector installations

will have proportionally higher construction costs, and that below a certain collector size—depending on the factors

influencing project cost-effectiveness—

construction costs will become prohibitive.

• New Construction versus Retrofit—

Installing a transpired collector during

building construction can reduce system

costs by as much as 50% compared to a

retrofit application. 

Where to Apply

The 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 Avoid

The following is a list of general applications and conditions that preclude the costeffective use of transpired collector

technology:

• 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 possible problems with fire codes).

Equipment Integration

The transpired collector system operates

independently of a building’s existing 

heating system (but may utilize existing

ventilation fans and ducts). The only integration issues that arise are associated with

installing the collector and the air-handling

components onto the building’s existing

frame (see page 5, “Installation”).

Maintenance

Transpired collectors are very reliable.

They have no moving parts (other than the

ventilation fans and dampers, which would

be part of any ventilation system). There are

no problems from leaking, freezing, or overheating, and to date, no problems have been

reported from users in the field regarding

holes becoming plugged or degradation of

the absorber surface. Even if the collector

becomes dented, performance is not

affected.

Equipment Warranties

The fan is covered by a 1-year manufacturer’s warranty, and the color coatings used

on the collector sheeting are warranteed for

20 years.

18

16

14

12

10

8

6

4

2

0

3

6

24

Syracuse Washington, D.C. Denver

Payback (yr)

B308404

Natural gas cost ($/MBtu)

6

Figure 5. Payback periods for transpired collectors for three different locations 

and natural gas costs.
Codes and Standards

With regard to fire codes, transpired collectors have been deemed safe for singlestory applications, and the polyethylene air

distribution duct meets the fire code for

flame-retardant materials.

Building codes and regulations pertaining to issues such as the location of outdoor

air openings, weather protection for ducts

on the building exterior, minimum duct

thickness, and criteria for multistory applications should be addressed on an installation-by-installation basis. The applicability

of codes may vary based on building height

and area, construction type, and use group.

Costs

As 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 and

40% comprises labor, which includes the

installation of the collector and the airhandling components. Table 1 shows a

breakdown of costs for a transpired 

collector system in a retrofit application. 

In new buildings, builders can use 

lower-cost facades underneath collector

walls. Doing so will likely realize savings 

of $3 to $5/ft2 by displacing more expensive

facade material. Taking into account this

material displacement credit, the installed

cost of a collector on a new building is in

the range of $5 to $7 per square foot.

Utility Incentives

Utilities offer no incentives for using this

technology.

Technology

Performance

Transpired collector owners are generally

pleased with their systems. Based on the

savings and improved ventilation created by

the transpired collector installed on its plant

in Oakville, Canada, Ford Motor Company

installed five more systems on plants in the

United States and Canada.

Field Experience

As of 1997, approximately 40 transpired

collector systems had been installed in the

private sector, and on two Federal sites—

two systems at Fort Carson, Colorado, and

one at NREL in Golden, Colorado (see

Figures 6 and 7). Three of these installations—the General Motors of Canada

Oshawa Battery Plant, the Ford Plant in

Oakville, Canada, and NREL—have been

monitored extensively. Table 2 lists the 

transpired collector heating systems in use

around the world.

The installation at NREL (shown in

Figure 7) is an ideal application of transpired collector technology. The facility is 

a 1300-ft2 chemical waste storage building

that requires a ventilation rate of 3000 cfm

to maintain safe indoor conditions. Also,

because of the danger of combustion, open

flames are prohibited in the building, so 

outside air is heated with electricity instead

of gas. The 300-ft2 transpired collector saves

about 14,310 kWh annually (25.7% 

of the energy required to heat the facility’s

ventilation air). With a local electric rate of

$0.025/kWh, the annual savings equates to

about $360. The installation has a simple

payback of 4.7 years.

Energy Savings

The transpired collector at the Ford

assembly plant in Oakville, Canada, was

installed in 1990. The 20,000-ft2 transpired

collector system supplies heated air to a

28,000-ft2 area with a ceiling as high as

45 feet in places. The system produces

5811 MBtu of energy savings, worth about

$30,000 U.S. annually. This savings represents about 17% of the plant’s annual airheating costs. The transpired collector

system paid for itself in a little more than

5 years.

The transpired collector on the GM plant

was installed in 1991 with a total collector

area of 4520 ft2. The GM system heats an

area of 39,000 ft2, and supplies 940 MBtu

of energy savings annually. This system

saves GM approximately $10,200 per year.

The system is discussed in more detail in

the Case Study on page 13.

Maintenance 

Reports from system owners in the field

demonstrate that transpired collectors are

mostly maintenance free. Except for servicing of air distribution units (fan belts, lubrication), the systems require minimal

maintenance.

Awards and Recognition

A transpired collector was an important

part of a comprehensive energy management

program at Fort Carson, Colorado. The

Department of the Army installed a collector

on an aviation hangar. The collector, combined with lighting and occupancy sensors,

7

Table 1. Installation Costs of a

Retrofit Transpired Collector

Figure 6. The transpired collector on the Avum Hangar at Fort Carson, Colorado.

Warren Gretz, NREL/PIX00599

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 1995

Tractor Manufacturer

ASEAN-Canade Project on Solar-Energy-Drying Malaysia 3,990 ft2 1994

Processes, 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 motor

replacements, saved 31.3 Btu and lowered

energy costs by $144,800. This savings 

was recognized with a 1997 Federal Energy

and Water Management Award from the

U.S. Department of Energy (DOE).

R&D magazine awarded Conserval, Inc.,

and researchers at NREL its prestigious

R&D 100 Award, recognizing transpired

collector technology as one of the 100 most

important advances of 1994 (the year the

nomination was submitted).

Popular Science also recognized the

technology’s potential when it awarded

Conserval and NREL its 1994 “Best of

What’s New” Award.

In 1992, DOE’s Energy-Related Inventions Program rated the transpired collector

in the top 2% of energy inventions.

Conserval Systems, Inc., has received a

number of awards from Canadian organizations, including the Toronto Construction

Association’s Best New Building Product of

the Year Award in 1994 and a Certificate of

Recognition for development and commercialization of the technology from Natural

Resources Canada.

Sizing and

Energy-Savings

Calculations

Figure 8 through Figure 11 present worksheets and supporting data that help calculate the optimum size of a transpired

collector for a given facility and estimate the

amount of energy that will be saved annually with the system.

Figure 8 is a worksheet for determining

the size of the collector based on a facility’s

available south-facing wall and its outdoor

requirement.

Figure 9 is a worksheet for estimating the

amount of energy a transpired collector will

save annually based on the size of the 

collector, the facility’s operating and performance characteristics, and geographic 

location.

The map in Figure 10 presents the estimated annual Btu output per square foot 

of collector for geographic regions in the

United States. This data is used in the

annual energy-savings worksheet in

Figure 9. The energy output data presented

on the map are a function of solar radiation

and length (and severity) of the heating 

season.

The map in Figure 11 shows the average

annual heating degree-days for geographic

regions 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 National

Renewable Energy Laboratory in Golden, Colorado.

Warren Gretz, NREL/PIX00147
Figure 8. Collector sizing worksheet.

10

Collector Sizing

Collector 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
11

Figure 9. Energy-savings worksheet.

Annual Energy Savings

Acoll = 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/year

Acoll qsolar tdays

Qwall = x x x ( ÷7) x ÷ 106 = MBtu/year

Acoll Uwall thours tdays HDD

Qsaved = ( + ) ÷ = MBtu/year

Qsolar Qwall Ehtg

Electrical Energy Parasitics:

Qfan = x x x x ÷ 103 = kWh/year

Acoll qfan thours tdays tweeks
Figure 10. Useful energy delivered by the collector.

Figure 11. Annual heating degree-days.

12
13

Case Study—

General Motors

Battery Plant

Facility Description

The General Motors Battery Plant in

Oshawa, Canada, is a 100,000-ft2 facility 

in which automotive batteries are manufactured. The plant was built in the 1970s 

and consists of an open shop floor and a 

28-foot-high ceiling. GM operates two fulltime production shifts within the plant and

conducts maintenance activities at night 

and on weekends, so the building is 

continuously occupied.

Existing Technology Description

Until the early 1990s, GM relied solely

on a steam-operated fan coil system for

space heating, but the system was incapable

of providing the necessary quantities of

heated outdoor air. As a result, the plant 

was not being adequately ventilated.

New Technology Description

In 1991, plant management installed a

transpired collector (see Figure 12) to correct the ventilation problems. During the

following 2 years the transpired collector

system was modified slightly to improve airflow; the original fans and motors were

replaced with vane axial fans and highefficiency motors, and the original ducting

was replaced with the upgraded fabric ducting.

The GM plant collector comprises

4520 ft2 of absorber sheeting. The lower

21 feet of the transpired collector is black,

perforated, aluminum wall cladding with

1.6-mm holes totaling 2% porosity. The

average depth of the plenum between the

transpired collector and the plant’s structural

wall is 6 inches. The canopy at the top of

the wall acts as both a manifold for the airflow and a solar heat collection device. The

canopy face is made of perforated plate with

1% porosity. The transpired collector covers

about 50% of the total area of the plant’s

south-facing wall; the remainder of the

south facade has shipping doors and other

obstructions that make it unsuitable for

mounting collector cladding.

The GM transpired collector has two

fan/distribution systems, each consisting of

a constant-speed fan, a recirculation damper

system, and a fabric distribution duct. The

total airflow delivered by the system’s fans

is 40,000 cfm. Both recirculated air and air

drawn through the solar collectors make up

this flow; the percentages of each depend on

the temperature of the air coming from the

collector.

Energy Savings

The GM Battery Plant’s transpired collector has been monitored extensively since

it was installed. The data in this case study

reflect the performance of the system during

the 1993/94 heating season after the previously mentioned modifications had been

completed. An in-depth report on the monitoring program is available (Enermodal

Engineering Ltd. 1995).

The data shows that the annual energy

savings for the 4520-ft2 collector was

940 million Btu/year (see Table 3):

678 MBtu resulted from the thermal energy

gained directly from the outside air as it

passed through the absorber (Qsolar); and

262 MBtu resulted from heat loss recaptured

by the wall (Qwall) from inside the building.

Other possible energy-saving mechanisms—

such as destratification and heat recapture—

likely contributed to improved system

performance; however, these effects are

highly structure-specific and have not been

incorporated into the savings reported here.

Life-Cycle Cost

The 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 installations for two reasons: 1) this system was

installed soon after the technology was

introduced, before design and installation

procedures had been streamlined, and 2) the

cost includes the fan and ducting modifications that were implemented during the first

2 years of operation.

Figure 12. The transpired collector on the GM Battery Plant in Oshawa, Canada.

Conserval Systems, Inc./PIX04030
Based on a National Institute of Standard

Technology (NIST) Building Life-Cycle

Costing (BLCC) comparative economic

analysis (see Appendix C), the present value

of life-cycle savings is $64,574, for a savings-to-investment ratio of 1.97. Therefore,

the project is cost effective according to

Federal criteria (W CFR 43G). The simple

payback period is 6 years. (See the Federal

Life-Cycle Costing Procedures in Appendix

B and the NIST BLCC Comparative

Economic Analysis in Appendix C.)

Performance Shakeout

The GM transpired collector had a number of operational problems. After the system was initially installed, employees

complained about fan noise and cold drafts,

and they occasionally disabled the system.

The fan and duct upgrades described previously eliminated the problems on one of the

fan systems; the other fan continues to generate noise, and employees still disable it

when working in the immediate vicinity.

Conserval has addressed these complaints

by specifying smaller, but more numerous,

fans in subsequent installations.

Also, both bypass dampers and a recirculation damper required additional maintenance. The recirculation damper became

stuck in full recirculation mode, and a new

modulating motor was installed to fix the

problem. The bypass dampers occasionally

became bound, which led to unacceptably

high leakage rates. These dampers were kept

closed manually throughout the 1993–1994

heating season.

The Technology

in Perspective

Transpired collectors have potential for

widespread use in the Federal sector, with

thousands of warehouses, hangars, storage

facilities, vehicle maintenance shops,

hazardous material storage, and other 

ventilation-intensive structures.

Users in the Federal sector can realize a

number of advantages from transpired collector technology. First, by reducing the

amount of conventional energy used to heat

ventilation air, system users save money.

Second, by replacing heat derived from fossil fuels with heat derived from clean solar

energy, users are reducing the detrimental

environmental side effects, such as the production of greenhouse gases (plus NOx and

SOx) associated with the burning of fossil

fuels. Third, the technology is remarkably

simple and maintenance free; the system

makes few additional demands on the facility’s engineering or maintenance staff.

The Technology’s Development

Prior to 1989, solar air-heating systems

resembled flat-plate water-heating systems.

These early systems contained a dark metal

absorber, but they also had glazing—a transparent cover—that prevented heat loss to the

atmosphere. Rather than pull air through the

absorber, these systems heated the air flowing parallel to the absorber. Compared to

transpired collector technology, these systems were less efficient because solar radiation was reflected off the glazing and they

had comparatively poor heat transfer from

the absorber to the air. They were also more

expensive because of the added material

cost.

The transpired collector design, developed simultaneously at Conserval, Inc., and

NREL in 1989, represented a major breakthrough in solar air-heating technology. 

By eliminating the glazing and pulling air

through the absorber, researchers were able

to greatly improve efficiencies and reduce

system costs. Since the initial breakthrough,

researchers and engineers have continued to

improve system performance by optimizing

the size and spacing of the perforations and

developing methods to ensure uniform airflow through the absorber.

Technology Outlook

The future will likely see increased use

of transpired collectors in the Federal and

private sectors. The biggest hurdle the technology must overcome is user acceptance.

Many solar technologies have been stigmatized by the rapid expansion of solar markets in the 1970s, when many poorly

designed and poorly performing systems

were deployed. Many potential users are

reluctant to commit to a solar technology if

a proven conventional option is available.

Transpired collector technology has been

proven to be valid and reliable for reducing

energy use and saving money, and the body

of scientific data proving its effectiveness

continues to grow.

Further dramatic improvements in efficiency are unlikely; transpired collectors are

already among the most efficient solar collectors available, converting between 65%

and 75% of the available solar energy.

Manufacturers

Conserval Engineering, Inc.

200 Wildcat Road

Downsview, Toronto, ONTARIO

M3J 2N5

(416) 661-7057

Fax: (416) 661-7146

Conserval Systems, Inc.

4242 Ridge Lea Road, Suite 1

Buffalo, NY 14226

(716) 835-4903

Fax: (716) 835-4904

For Further

Information

Craig Christensen, Mail Stop 2722

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401 

Phone: (303) 384-7510 

Fax: (303) 384-7540

e-mail: craig_christensen@nrel.gov

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 Enermodal

Engineering Limited, 1995, Performance of the PerforatedPlate/Canopy Solarwall at GM Canada, Oshawa, DSS Contract

No. 007 SQ 23440-1-9552, Energy Technology Branch, CANMET, Department of Natural Resources Canada, Ottawa, Ontario. 

Table 3. Savings from the GM

Transpired Collector*
15

Literature

American Society of Heating, Refrigerating,

and Air-Conditioning Engineers, Inc.

ASHRAE Standard: Ventilation for

Acceptable Indoor Air Quality,

ANSI/ASHRAE 62-1989.

Brunger, A.; Hollands, K. (June 1996). “Back-of-Plate Heat Transfer in Unglazed

Perforated Collectors Operated under NonUniform Air Flow Conditions.” Presented 

at the 22nd Annual Conference of the Solar

Energy Society of Canada, Inc., Orillia,

Ontario, Canada.

Christensen, C.; Hancock, E.; Barker, G.;

Kutscher, C. (March 1990). “Cost and

Performance Predictions for Advanced

Active Solar Concepts.” Proceedings of 

the American Solar Energy Society Annual

Meeting; Austin, Texas.

Dymond, C.; Kutscher, C. (March 1995). “A Computer Design Model for Transpired

Solar Collector Systems.” Presented at the

ASME International Solar Energy

Conference. Also accepted for publication

in the journal Solar Energy.

Enermodal Engineering Limited. (1995).

Performance of the Perforated-Plate/Canopy

Solarwall at GM Canada, Oshawa. DSS

Contract No. 007 SQ 23440-1-9552.

Ottawa, ON: Energy Technology Branch,

CANMET, Department of Natural

Resources.

Gawlik, K. (May 1995). A Numerical and

Experimental Investigation of Heat Transfer

Issues in the Practical Utilization of

Unglazed, Transpired Solar Air Heaters.

Ph.D. thesis, Department of Civil,

Environmental, and Architectural

Engineering, University of Colorado,

Boulder, Colorado.

Golneshan, A. (April 1991). Forced

Convection Heat Transfer from a

Transpired-Plate Having Slotted

Perforations, with Applications to Solar 

Air Heaters. Ph.D. thesis, Department of

Mechanical Engineering, University of

Waterloo, Waterloo, Canada.

Gunnewick, L. (April 1994). An

Investigation of the Flow Distribution

Through Unglazed Transpired-Plate Solar

Air Heaters. Master’s thesis, Department 

of Mechanical Engineering, University of

Waterloo, Waterloo, Canada.

International Energy Agency. (December

1995). Low-Cost, High Performance Solar

Air Heating Systems Using Perforated

Absorbers (Draft). Final copies will be

available from Doug McClenahan,

CANMET, 580 Booth Street, 7th Floor,

Ottawa, Canada K1A OE4.

Kutscher, C. (December 1992). An

Investigation of Heat Transfer for Air Flow

through Low Porosity Perforated Plates.

Ph.D. thesis, Department of Mechanical

Engineering, University of Colorado,

Boulder, Colorado.

Kutscher, C.; Christensen, C.; Barker, G.

(August 1991). “Unglazed Transpired Solar

Collectors: An Analytical Model and Test

Results.” Proceedings of the Biennial

Congress of the International Solar Energy

Society; page 1245.

Kutscher, C.; Christensen, C.; Barker, G.

(August 1993). “Unglazed Transpired Solar

Collectors: Heat Loss Theory.” ASME

Journal of Solar Energy Engineering.

Peter, R.; Hollick, J. U.S. Patent No.

4,899,728. (13 February 1990). Method and

Apparatus for Preheating Ventilation Air for

a Building.
16

Appendixes

Appendix 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 Sizing

Collector 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 Savings

Acoll = 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/year

Acoll qsolar tdays

Qwall = x x x ( ÷7)x ÷ 106 = MBtu/year

Acoll Uwall thours tdays HDD

Qsaved = ( + ) ÷ = MBtu/year

Qsolar Qwall Ehtg

Electrical Energy Parasitics:

Qfan = x x x x ÷ 103 = kWh/year

Acoll 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 Procedures

and 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 the

long-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 annual

expenditures on energy to operate equipment. (For example, a lighting fixture that draws 100 watts and operates 2,000 hours annually

requires 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 example, replacing burned-out lightbulbs). Replacement costs include expenditures to replace equipment upon failure (for example, replacing an

oil 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, and

salvage 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 energycost-reducing alternative and the LCC of the baseline equipment. If the alternative's LCC is less than the baseline's LCC, the alternative is

said 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-switching

measure 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 measure

divided 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 Heat

Case 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 Heat

Basecase: Basecase

Alternative: Solar Case

Principal Study Parameters 

Analysis Type: Federal Analysis—Energy Conservation Projects

Study Period: 25.00 Years (Jan 1997 through Dec 2021)

Discount Rate: 4.1% Real (exclusive of general inflation)

Basecase LCC File: GMBASCAS.LCC

Alternative LCC File: GMSOLAR.LCC

Comparison of Present-Value Costs

Savings

Basecase 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 -$3,092

Energy-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 savings SIR = ——————————————— = 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-Cycle

Type Basecase Alternative Savings Savings 

—————— ———— —————- —————- ————— ———————

Electricity kWh 0 2,362 -2,362 -59,050

Central Steam MBtu 940 0 940 23,500

20
The Energy Policy Act of 1992, and

subsequent Executive Orders, mandate

that energy consumption in the Federal

sector be reduced by 30% from 1985

levels by the year 2005. To achieve 

this goal, the U.S. Department of

Energy’s Federal Energy Management

Program (FEMP) is sponsoring a 

series of programs to reduce energy

consumption at Federal installations

nationwide. One of these programs,

the New Technology Demonstration

Program (NTDP), is tasked to accelerate the introduction of energy-efficient

and renewable technologies into the

Federal sector and to improve the rate of

technology transfer.

As part of this effort FEMP is 

sponsoring a series of Federal Technology Alerts (FTAs) that provide 

summary information on candidate

energy-saving technologies developed

and manufactured in the United States.

The technologies featured in the

Technology Alerts have already 

entered the market and have some 

experience but are not in general use 

in the Federal sector. Based on their

potential for energy, cost, and environmental benefits to the Federal sector,

the technologies are considered to be

leading candidates for immediate

Federal application.

The goal of the Technology Alerts 

is to improve the rate of technology

transfer of new energy-saving tech-nologies within the Federal sector and to

provide the right people in the field with

accurate, up-to-date information 

on the new technologies so that they 

can make educated judgments on

whether the technologies are suitable 

for their Federal sites.

Because the Technology Alerts are

cost-effective and timely to produce

(compared with awaiting the results 

of field demonstrations), they meet 

the short-term need of disseminating

information to a target audience in 

a timeframe that allows the rapid

deployment of the technologies—and

ultimately the saving of energy in the

Federal sector.

The information in the Technology

Alerts typically includes a description 

of the candidate technology; the 

results of its screening tests; a description of its performance, applications 

and field experience to date; a list of

potential suppliers; and important 

contact infor-mation. Attached 

appendixes provide supplemental 

information and example worksheets 

on the technology.

FEMP sponsors publication of the

Federal Technology Alerts to facilitate

information-sharing between manufacturers and government staff. While 

the technology featured promises significant Federal-sector savings, the

Technology Alerts do not constitute

FEMP’s endorsement of a particular

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the Federal Technology Alerts attempt

to chart market activity vis-a-vis the

technology featured. Readers should

note the publication date on the back

cover, and consider the Alert as an 

accurate picture of the technology and

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entrance of new manufacturers or 

suppliers should be anticipated since 

the date of publication. FEMP 

encourages interested Federal energy

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Federal Energy Management Program

The 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 in

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Over the years several Federal laws and Executive Orders have shaped FEMP's mission. These include the Energy Policy and Conservation 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 hasten 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 of

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About the Federal Technology Alerts
For More Information

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Web site: http://www.eren.doe.gov/femp/

General Contacts

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Technical Contacts

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