A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Evaluating the Performance and 

Economics of Transpired Solar 

Collectors for Commercial 

Applications 

Preprint 

E. Kozubal, M. Deru, S. Slayzak, and P. Norton 

National Renewable Energy Laboratory 

G. Barker 

Mountain Energy Partnerships 

J. McClendon 

Wal-Mart 

To be presented at the 2008 ACEEE Summer Study on 

Energy Efficiency in Buildings 

Pacific Grove, California 

August 17–22, 2008 

Conference Paper 

NREL/CP-550-43305 

July 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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Evaluating the Performance and Economics of Transpired Solar Collectors for 

Commercial Applications1

Eric Kozubal, Michael Deru, Steven Slayzak, and Paul Norton 

National Renewable Energy Laboratory 

Greg Barker 

Mountain Energy Partnership 

James McClendon 

Wal-Mart 

ABSTRACT 

Using transpired solar collectors to preheat ventilation air has recently become recognized as an 

economic alternative for integrating renewable energy into commercial buildings in heating 

climates. The collectors have relatively low installed costs and operate on simple principles. 

Theory and performance testing have shown that solar collection efficiency can exceed 70% of 

incident solar. However, implementation and current absorber designs have adversely affected 

the efficiency and associated economics from this initial analysis. The National Renewable 

Energy Laboratory (NREL) has actively studied this technology and monitored performance at 

several installations. A calibrated model that uses typical meteorological weather data to 

determine absorber plate efficiency resulted from this work. With this model, an economic 

analysis across heating climates was done to show the effects of collector size, tilt, azimuth, and 

absorptivity. The analysis relates the internal rate of return of a system based on the cost of the 

installed absorber area. In general, colder and higher latitude climates return a higher rate of 

return because the heating season extends into months with good solar resource. 

Wal-Mart has installed approximately 8,000 ft2

 of absorber at its experimental store in Aurora, 

Colorado. The delivered energy efficiency was measured at 8-11% during January and February 

2007. The low collection efficiency is largely due to the oversized absorber and to the 

multizone control strategy that limits the amount of air pulled through the collector. Analysis 

shows that more than 50% of the incident solar energy could be delivered with proper control 

strategy changes. 

Background 

Transpired Solar Collectors 

Transpired solar collectors are an emerging solar technology. The collector consists of a porous 

absorber plate with associated plenum and ducting. The absorber is mounted to collect sunlight 

and outdoor air is actively drawn through the porous plate. The air is heated as it transpires 

 

1 This work has been authored by employees of the Midwest Research Institute under Contract No. DE-AC36-

99GO10337 with the U.S. Department of Energy. The United States Government retains and the publisher, by 

accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, 

for United States Government purposes. 

1 
(perpendicularly) through the plate. The air is then collected in a plenum on the back side of the 

plate and delivered to the building. A transpired collector traditionally serves two purposes: 

1. Collect solar heat and transfer it to a ventilation air flow by virtue of air transpiration 

through the absorber. 

2. Perform all the normal functions of a building’s façade on which it is installed. 

The economics of such collectors depend on installation, solar resource, and sizing (BTUs 

delivered per square foot of collector). Figure 1 shows original designs which achieved 70%+ 

efficiency at 6 FPM and higher according to Kutscher et al. (1993). However, current designs 

are closer to 60%+ efficiency at the same flow rate. This results in an air temperature increase of 

about 36oF on a typical sunny day with a transpired collector facing due south. Increasing wall 

area (and thus lowering the approach velocity) can increase the total energy harvested, but 

because collector efficiency can drop sharply, it has diminishing economic returns. Furthermore, 

wind losses become much more prevalent at low approach velocities. A selective low-e coating 

(with high solar absorptivity and low long wave radiation emissivity) can enable high efficiency 

at lower approach velocity; however, the practical aspects of cost, visual appearance, and 

maintenance are outside this paper’s scope. 

Figure 1 Efficiency and temperature rise of transpired solar collector 

Figure taken from Kutscher et al. (1993), 0.01 m/s ≅ 2 FPM, 700 W/m2

2 
0

20

40

60

80

0 2 4 6 8 10 12

Face Velocity (FPM)

Pressure Drop (Pa)

0.00

0.08

0.16

0.24

0.32

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Face Velocity (m/sec)

Pressure Drop (inches water)

Figure 2 Pressure drop measurements of current south wall transpired collector products 

The induced pressure drop across the absorber plate should be sufficient to impede the effect of 

wind impingement, which could lead to air flow in and out of the absorber. To prevent this, the 

pressure drop should be greater than the wind’s dynamic pressure. For practical purposes, high 

winds and gusting do not coincide with large portion of collector operational hours, therefore, the 

highest expected mean wind speed is used. Eleven mph (5 m/s) is a typical high wind speed in 

the United States, which has a dynamic pressure of approximately 15 Pa. Typical pressure drop 

performance of current south wall-mounted products is shown in Figure 2. The pressure drop at 6 

FPM face velocity is marginal; however, it meets our 15 Pa criteria. 

United States Ventilation Energy Use 

Ventilation accounts for about 15% of the net commercial buildings heating load in the United 

States (DOE 2006). This heating energy is normally met with fossil fuel energy sources. 

Ventilation heating can be economically augmented from renewable energy sources because low 

grade heat (such as that from a transpired collector) can be utilized. Low-grade solar heat is 

attractive because high efficiencies and low absorber costs are possible. However, air-to-air heat 

recovery should be examined as an option when the exhaust air stream from a building is readily 

available. The total year-round energy savings and economics of heat recovery will likely 

exceed those of a transpired solar collector according to prior NREL analysis. The following list 

illustrates the advantages of heat recovery over a transpired collector. 

1. Transpired collectors are most useful during sunny days during heating months. This 

would typically be about 6-10 hrs/day for 6-10 months or about 12-35% utilization over 

the year. The highest utilization is realized only in special cases such as when used to 

ventilate the refrigeration section of a grocery store. 

2. Heat recovery can be used for nearly the entire year (cooling and heating), except when 

economizer cooling is more efficient. This would yield higher utilization than a 

transpired collector. Furthermore, a building’s total ventilation load may be the greatest 

during nighttime winter hours and humid summer days. 

3. Energy recovery can provide heating and cooling that can lead to downsized heating and 

cooling equipment. A transpired collector cannot downsize the heating (or cooling) 

equipment because of the non-coincident nature of solar resource and heating peak load. 

3 
Since not all exhaust air is recoverable (e.g., kitchen exhaust and exfiltration), the preceding 

arguments suggest that a good application for a transpired solar collector is ventilation make-up 

air to supplement exhaust air heat recovery. 

Analysis 

Transpired Collector Model 

A new transpired collector model was developed at NREL in order to accurately model the 

performance of current collector products. The model has been calibrated to several installations 

that have been monitored by NREL. It uses hourly TMY2 data sets to accurately calculate the 

absorber’s performance and capture some dynamic details such as the effects of thermal mass. 

The NREL model predicts about 14% less delivered energy than the RETscreen (NRCan 2007) 

transpired collector model, which is based on product offerings that are no longer available. The 

following is a list of attributes of the model. 

1. The model uses hourly TMY2 weather data to incorporate solar insolation, ambient 

temperature, sky temperature, and wind speed into the wall’s overall heat balance and 

efficiency calculations. 

2. The model uses the above data along with the specifics of the installation including: 

o Air transpiration face velocity (FPM) 

o Surface absorptivity and emissivity (both sides) 

o Wall insulation 

o Temperature of back wall (ambient or inside temperature, e.g., 70o

F) 

o R-value of insulation used 

o Specific heat and mass of the absorber plate and wall. 

3. The model takes all the inputs from 1 and 2, uses them to create a time-dependent energy 

balance, and calculates the air temperature increase from ambient to the delivered air. 

4. The model was calibrated using the methods by Kutscher et al. (1993) and data taken 

from an installation in New York. The calibration adjusts the coefficients used to 

approximate the absorber plate heat exchange effectiveness. 

5. As an approximation, building load is matched by modulating air flow from 100%, when 

the ambient temperature is below 55o

F, down to 0% at 65oF ambient temperature. 

Economic Analysis of South Wall and Modular Collectors 

The transpired solar collector may be thought of as a new construction technology that is 

attached onto the south wall of a building; however, retrofit installations are possible. In the case 

of big box retail, the south wall of a building may be located a distance prohibitively far away 

from the end use, or may be unsuitable for solar energy harvesting. In this case, the concept of a 

modular transpired solar collector may make sense. A modular collector would be a roof 

mounted collector with integral ducting and be placed in proximity to the HVAC equipment. 

Figure 3 shows a prototype of such a unit. Commercial models are becoming available. 

4 
Figure 3 A prototype modular transpired solar collector under test 

According to NREL analysis, one advantage of a modular design is that the collector can be 

tilted from vertical to about latitude +10o to 20o to maximize energy harvesting. For Denver, 

Colorado, and most U.S. cities where a transpired collector is suitable, 60o

 is close to an optimal 

tilt. A notable exception is Seattle, Washington, where winter solar is poor, making a 50o tilt 

optimal. Northern climates such as Fairbanks, Alaska (latitude ≅ 65o

) also have an optimal tilt 

near 60o because of the lack of winter solar resource. However, spring, summer, and fall months 

still require significant heating, making a transpired collector viable. Figure 4 shows the 

ASHRAE (2004) climate zones in the United States. Select cities in each zone are used for 

benchmarking building technologies. 

Figure 4 Map of DOE climate zones 

5 
Table 1 Climate zone locations of the cities selected for modeling 

Locations Climate Zone 

Baltimore, MD 4A 

Albuquerque, NM 4B 

Seattle, WA 4C 

Chicago, IL 5A 

Boulder, CO 5B 

Minneapolis, MN 6A 

Helena, MT 6B 

Duluth, MN 7 

Fairbanks, AK 8 

The typical ventilation flow for a 200,000 ft2 retail store is approximately 35,700 CFM. Given 

this flow rate, a wall size of 5,950 ft2

 with a face velocity of 6 FPM was determined to be a near 

optimal size based on efficiency and ability to deliver a significant amount of thermal energy, 

which will create a favorable economy of scale. This size also provides adequate leeway to shut 

off up to 50% of the ventilation air and still deliver up to 80% of the maximum heat capacity of 

the collector. Figure 7 through Figure 9 show the potential delivered energy and associated 

economics for select cities in the United States (Climate zones 4A-8) where a thermal preheating 

system is appropriate. The data are grouped into installations that take 100% of the ventilation 

air from either a south wall or a modular installation. The modular design can absorb more 

energy than the south wall installation because the collector is tilted at 60o

 and has higher 

absorptivity (0.94 versus 0.79, corresponding to black versus gray). Figure 6 shows the relative 

importance of azimuth for a south wall installation and a black absorber compared to a modular 

absorber in Boulder, Colorado. This analysis shows that a modular absorber could be as much as 

28-37% higher in installed cost over a south wall installation and have equal internal rate of 

return (IRR). A selective coating (with high solar absorbtivity and low long wave emissivity of 

less than about 0.20) added to a modular collector can further increase the delivered energy; 

however, special care to preserve the coating’s performance (which was not reflected in this 

analysis) complicate the analysis and should be studied further before determining its potential as 

an alternative design. 

6 
0

100

200

300

400

500

Energy Delivered (MWhr)

Modular Collector

Modular Collector + 0.2 Emissivity

Southwall Collector

Southwall Collector - 45deg Azimuth

Southwall Collector + 0.94 Absorbtivity

Figure 5 Comparison of annual delivered energy and economics of different collector design 

strategies for Boulder, Colorado 

0%

10%

20%

30%

40%

50%

$- $10 $20 $30 $40

Installed System Cost ($/sqft - Absorber)

IRR

Modular +0.20 Emmisivity

Modular

South Wall +0.94 Absorptivity

Southwall

South Wall -45deg Azimuth

Power (Modular +0.20 Emmisivity)

Power (Modular)

Power (South Wall -45deg Azimuth)

 

Figure 6 Economic comparison of south wall and modular collectors in Boulder, Colorado 

0

50

100

150

200

250

300

350

400

450

Baltimore, MN

Albuquerque, NM

Seattle, WA

Chicago, IL

Boulder, CO

Minneapolis, MN

Duluth, MN

Helena, MT

Fairbanks, AK

Energy Delivered (MWhr)

Modular Collector

Southwall Collector

 

Figure 7 Annual energy delivery analysis for climate zones 4A–8 

7 
0%

10%

20%

30%

40%

$- $10 $20 $30 $40

Installed System Cost ($/sqft - Absorber)

IRR

Baltimore, MN Albuquerque, NM

Seattle, WA Chicago, IL

Boulder, CO Minneapolis, MN

Duluth, MN Helena, MT

Fairbanks, AK Power (Seattle, WA)

Power (Duluth, MN)

 

Figure 8 Economic analysis for a southwall installation based on collector area (climate zones 4A–8) 

0%

10%

20%

30%

40%

$- $10 $20 $30 $40

Installed System Cost ($/sqft - Absorber)

IRR

Baltimore, MN Albuquerque, NM

Seattle, WA Chicago, IL

Boulder, CO Minneapolis, MN

Duluth, MN Helena, MT

Fairbanks, AK Power (Seattle, WA)

Power (Duluth, MN)

 

Figure 9 Economic analysis for a modular installation based on collector area (climate zones 4A–8) 

Notes and assumptions for Figure 5 through Figure 9: 

1 TRNSYS model developed by NREL and calibrated with current transpired collector installations. The model uses TMY2 data. 

2 Based on displacing natural gas burned in a 75% efficient boiler at $1.12/therm. Value taken from IEA website for the period 

of June 2006 to May 2007. No energy price escalation was included. 

3 Collector flow rate at 100% when the outside temperature is below 55o

F and modulating down to 0% at temperature above 

65o

F. 

4 6 FPM approach velocity. (35700 CFM, 5950 ft2

 collector area). 

5 Modular collector tilted at 60o

 except Seattle, Washington, which is at 50o because of poor winter solar resource. South wall 

collector is vertical. Both are due south except where noted. 

6 Life cycle = 30 years. 

7 Solar absorptivity = 0.79 (gray) for south wall installations and 0.94 (black) for modular. Emissivity is 0.96 for both except 

where noted. 

An hourly plot of delivered energy is shown in Figure 10 for a vertical south-facing transpired 

solar collector in Boulder, Colorado for a typical February based on the TMY2 data set. The 

8 
average efficiency is about 55%, and almost all energy collected is usable (extreme wind, max 

delivered air temperature of 90oF, and warm weather lower the efficiency at times). 

Modeled Energy Delivered vs. Insident Solar Energy

(Typical Meteorological Month of February)

-100

0

100

200

300

400

0 100 200 300 400 500 600 700 800

Incident Solar Energy (kWh)

Delivered Energy (kWh)

 

Figure 10 Modeled delivered energy plot for Boulder, Colorado (for typical meteorological month of 

February), south wall installation, and 0.79 absorptivity 

Measured Performance at a Wal-Mart Supercenter in Aurora, Colorado 

Installation Description 

The installed collector at the Aurora store is mounted on the south wall, faces due south, and has 

approximately 8,000 ft2

 of collector area. The wall stands nearly vertical with only a slight tilt 

angle (Figure 11). The minimum ventilation for the units attached to the collector was measured 

to be 12,275 CFM or 1.52 FPM through the collector. Maximum collector flow rate was 

measured to be 45,286 CFM (5.61 FPM). Each rooftop unit (RTU) has a dedicated duct leading 

from the top of the wall along the roof to each unit. 

9 
Figure 11 Transpired collector installed on south wall of the Wal-Mart Supercenter in Aurora, 

Colorado 

The store draws air through the collector via 11 smaller RTUs that range from 4,000 to 20,000 

CFM and two large air handling units (AHUs) that range from 8,000 to 25,000 CFM. 

Control Strategy and Design Intent 

The strategy for the store was to utilize the large collector to provide both ventilation preheating 

and space heating when the preheated air temperature was high enough. The collector is vastly 

oversized for ventilation air preheating. Thus, when space heat is required and the air 

temperature is hot enough, the flow through any given RTU or AHU quickly modulates to its 

maximum to provide additional space heating. During the data collection period presented in 

this paper, the two large AHUs had internal water coils that complicated the strategy and forced 

the system to reduce the preheated air flow if the ambient air was below freezing. The inclusion 

of water based coils has been identified as a design problem. The maximum air temperature 

supplied to the space is mixed with return air down to 90o

F if necessary. 

Measured Energy Performance 

The preheat system (collector + RTUs) was monitored during January and February 2007 which 

provided a wide range of climatic conditions to characterize its performance. Results of total 

energy delivered are presented in Table 2. January had significantly less delivered energy 

despite having higher solar insolation on the wall surface. This was primarily due to the large 

AHUs shutting off the preheated air during cold periods when the solar resource was high. The 

delivered energy to the space is less than that of the collected energy because of the space 

heating strategy. When the air flow is modulated higher than the required minimum outdoor 

airflow, this additional air stream must be heated above the return air stream (from the building) 

to offset gas heat. Thus the energy to raise the temperature of the additional outdoor air (that 

which is above the ventilation need) up to the return air temperature is not counted toward 

delivered energy. The collected energy utilization shows what percentage of the collected solar 

energy was delivered to the space. 

10 
Table 2 Measured energy delivered by the preheat system for January 4 to February 28, 2007 (MWh) 

January February 

Incident Solar on Collector 123 112 

Collected Energy 16.2 20.5 

Delivered Energy 9.6 12.8 

Delivered Energy Efficiency 8% 11% 

Collected Energy Utilization 59% 64% 

Figure 12 and Figure 13 show the delivered energy by each AHU and RTU for each month. The 

small AHU supplies 2,000 to 7,000 CFM of preheated outdoor air to the general merchandise 

zone in the rear of the store. This zone requires minimal heating because of lighting loads and 

minimal outdoor wall exposure. The large AHU supplies air to the grocery zone of the 

supercenter and supplies 6,000 to 18,000 CFM of preheated outdoor air. This zone requires 

heating for most of the year because of the amount of refrigeration in the zone. The entire sales 

floor ventilation requirement is 22,000 CFM, which is supplied by 9 AHUs. The RTUs supply 

air to the back rooms. RTUs 9, 10 and 11 supply the receiving dock area and adjacent zone, 

which have larger heating requirements than the rest of the store’s back rooms. 

0.0

2.5

5.0

7.5

10.0

Small AHU

Large AHU

RTU #1

RTU #2

RTU #3

RTU #4

RTU #5

RTU #6

RTU #7

RTU #8

RTU #9

RTU #10

RTU #11

MWh

Collected energy

Space heating

Ventilation preheating

 

Figure 12 Delivered energy for January 2007 (excluding January 1-3) 

0.0

2.5

5.0

7.5

10.0

Small AHU

Large AHU

RTU #1

RTU #2

RTU #3

RTU #4

RTU #5

RTU #6

RTU #7

RTU #8

RTU #9

RTU #10

RTU #11

MWh

Collected energy

Space heating

Ventilation preheating

Figure 13 Delivered energy for February 2007 

11 
Figure 14 and Figure 15 show the energy delivered (in 15-minute intervals) versus incident solar 

energy on the entire wall. The maximum collected energy efficiency is 56%, which occurs when 

the flow through the collector is near its maximum of 5.61 FPM. This compares well with 

modeled results that were done at 6 FPM. However, for a significant length of time, little or no 

air was drawn through the collector during high solar resource, resulting in an average delivered 

energy efficiency of about 8% for January and 11% for February. 

Figure 14 Energy delivery versus incident solar energy for January 2007 (excluding January 1-3) 

Figure 15 Energy delivery versus incident solar energy for February 2007 

Performance Discussion 

The collector performance is in line with expected results; however, the system under utilizes the 

solar resource for several reasons: 

• The AHUs are not drawing through the collector when the ambient temperature is below 

32o

F to prevent coil freezing. This is despite the fact that the preheated air may be above 

32o

F. 

12 
• The ventilation flow rate through all the units connected to the collector induces a face 

velocity of only 1.5 FPM through the collector. This causes the system to operate at low 

efficiency. 

• Control system programming bugs prevent the system from drawing ventilation air 

during low to medium solar resource through the AHUs. This shows up in Figure 14 and 

Figure 15 which show that the number of hours (points) with non-zero energy delivery is 

biased above 500 kWh of incident solar. 

• Because the collector system is a 100% outdoor air system, energy collected when the air 

flow rate is above the ventilation rate can offset gas use only when it is delivered above 

return air temperature. This negates a significant amount of heat collected by the 

absorber (see Figure 16). It is important to perform a thorough evaluation of this strategy 

to avoid a detrimental economic outcome. 

20

30

40

50

60

70

80

Temperature ( oF)

Return 

Air Space Heating Energy

Collected energy that 

does not offset gas use

Ventilation 

Preheating 

Energy

Ventilation Max Air 

Flow

Ambient

Preheat 

Air

Air Flow

Figure 16 Example of delivered energy portion (red and blue) of total collected energy for a given 

preheat temperature. The rectangle areas are proportional to energy. 

Solving these problems would significantly increase the value of the system installed at the store. 

Maximizing the preheated air flow through the two large AHUs during hours with favorable 

solar resource would raise the collector face velocity up to 5.6 FPM. This flow rate is sufficient 

to ventilate the entire sales floor, and would allow the ventilation in adjacent zones to be reduced 

or shut off. The intent would be to adjust the control strategy to better utilize the store’s existing 

infrastructure and the result would operate the system most economically as a ventilation preheat 

system. Consideration should be given to the ventilation air distribution, and to allow the 

immediate zone temperatures to float higher during the day and preheat the building for the 

nighttime. Furthermore, the system would still have to reduce the preheat air flow when it is 

below freezing because of the water coils in the AHUs. With these improvements, the system 

would deliver energy at an efficiency of 50%+ of incident solar. Table 3 below shows the solar 

resource available when the preheat temperature was above 35o

F and the subsequent 

improvement in delivered energy. 

13 
14 

Table 3 Estimated delivered energy (MWh) with recommended changes for January and February 2007 

 January February 

Incident Solar on Collector 113 106 

Delivered Energy 56 53 

Delivered Energy Efficiency 50% 50% 

% Improvement from Measured 486% 316% 

Conclusions 

Transpired solar collectors are a relatively new concept to deliver inexpensive solar heat to any 

building by preheating ventilation air. However, the economics and energy savings of heat 

recovery should be compared to that of the transpired collector to determine the best ventilation 

strategy. A good application for a transpired solar collector is ventilation air make-up to 

supplement exhaust air heat recovery. The choice between a south wall and modular collector 

largely depends on what the expected installed cost (given by $/ft2 of absorber). A modular 

design is attractive because it lends itself to retrofit and can deliver more heating because of the 

optimal tilting and the likelihood that a south wall installation would not be painted black to 

maximize absorptivity. The collector approach velocity should be designed to be at least 6 FPM 

(with a minimum 15 Pa pressure drop) when operated at 100% of ventilation flow. When 

operated correctly, a south wall collector can easily achieve a 10% or greater IRR across climates 

zones 4A-8 with a cost target of $13/ft2

 of absorber for all climates. Similarly for a modular 

design, the cost target is $18/ft2. Higher IRR rates are generally achieved in colder climates. 

Implementation and control strategy issues have limited the effectiveness of the transpired 

collector installed at the Wal-Mart Supercenter in Aurora, Colorado. The total utilized energy 

from the collector was measured to be between 8-11% compared to 50%+ that could be 

achieved. This is due to the combination of underutilizing the absorber area during moderate sun 

and a system configuration that attempts to supply space heating. The space heat strategy 

utilized only about 60% of the collected energy during good solar resource in January and 

February 2007. With simple changes to the controls and a reduction in adjacent zone ventilation 

during good solar resource hours, the collector can operate at an appropriate air flow rate and 

deliver 3 to 5 times more energy to the store. 

References 

ASHRAE. (2004). ANSI/ASHRAE/IESNA Standard 90.1-2004: Energy Standard for Buildings 

Except Low-Rise Residential Buildings. Atlanta, GA: American Society of Heating, 

Refrigerating, and Air-Conditioning Engineers. 

DOE (2006). Buildings Energy Data Book, 2006. U.S. Department of Energy. Washington, D.C. 

Energy Information Administration Website, http://www.eia.doe.gov/ 

Kutscher, C. F.; Christensen, C. B.; Barker, G. M. (1993). Unglazed transpired solar collectors: 

heat loss theory. Journal of Solar Energy Engineering. vol. 115, Aug. 1993, pp. 182-188. 

Kutscher, C. F. (1994). Heat Exchange Effectiveness and Pressure Drop for Air Flow Through 

Perforated Plates With and Without Crosswind. Journal of Heat Transfer. Vol. 116, May 

1994; pp. 391-399. Acc No. 15544. 

NRCan (2007). RETScreen International Version 3.1. Natural Resources Canada. Ottawa, 

Canada (www.retscreen.net). 
F1147-E(09/2007) 

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National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Using transpired solar collectors to preheat ventilation air has recently become recognized as an economic 

alternative for integrating renewable energy into commercial buildings in heating climates. The collectors have 

relatively low installed costs and operate on simple principles. Theory and performance testing have shown that 

solar collection efficiency can exceed 70% of incident solar. However, implementation and current absorber designs 

have adversely affected the efficiency and associated economics from this initial analysis. The National Renewable 

Energy Laboratory has actively studied this technology and monitored performance at several installations. A 

calibrated model that uses typical meteorological weather data to determine absorber plate efficiency resulted from 

this work. With this model, an economic analysis across heating climates was done to show the effects of collector 

size, tilt, azimuth, and absorptivity. The analysis relates the internal rate of return of a system based on the cost of 

the installed absorber area. In general, colder and higher latitude climates return a higher rate of return because the 

heating season extends into months with good solar resource. 

15. SUBJECT TERMS

transpired solar collector; renewable energy; solar collection; ventilation preheat

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OF ABSTRACT

UL 

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Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18