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					       Solar Ventilation Preheating
       With Unglazed Transpired Solar Collectors
       Results of a Study of Sites in the Twin Cities, MN Region

                                An unglazed transpired solar collector can be used to preheat ventilation air using incident solar
                                energy. This system offers energy savings with a simple, efficient, and reliable design. Over the
Benefits                        last several decades a number of sites have installed such systems. Historically unglazed
 Saves energy and reduces      transpired solar collectors have been referred to by a number of additional names, such as solar
  greenhouse gas                transpired walls, unglazed perforated-absorber collector, perforated collectors, or just solar walls.
                                Technology Description
 Essentially maintenance       Solar walls are constructed from corrugated
  free.                         aluminum or steel panels. They are generally dark
                                colored to improve solar thermal radiation
 Aesthetically appealing       absorption, although the color can be adjusted to
  and structurally simple.      aesthetically match the rest of the building. The
                                sheeting is perforated with small pin holes or slits;
 Has been shown to have a      typically thousands per square meter. This sheeting
  short payback period.         can be mounted with a simple support structure to
                                an existing or new structural wall, ideally a south
 Potential for satisfying      facing wall.
  LEED credits.
                                Solar radiation heats the metal surface where a thin
                                boundary layer of air is heated. With the use of a
                                fan, or other air distribution system, the heated layer
Sampling of Regional            air can be pulled through the perforations and then               Source: U.S. Department of Energy
Sites                           channeled to an exit ductwork connection. This air
                                can then be supplied directly to the building space
 Breck School                  as conditioned ventilation air or to a heating unit as
 3rd Precinct Police Station   pre-heated supply air.
                                                                                          Some application examples are:
 AVEDA Corporation                                                                          Manufacturing plants
  Headquarters                                                                               Industrial sites
 FAIR School Downtown                                                                       Commercial buildings
 St. Anthony Village High                                                                   Schools and public buildings
  School                                                                                     Garages and workshops
 Hibbing Courthouse Annex                                                                   Multi-residence buildings
                                                                                             Barns
                                                                                             Preheating combustion air for furnaces

                                                                                          There is a minimum ventilation requirement
                                                                                          for the solar wall to be effective. If the
                                                                                          space does not require additional fresh air or
Sampling of Suppliers                                                                     uses 100% recirculation it is not well suited
 Conserval Engineering Inc.                                                              for an installation.
 ATAS International Inc.         Additional Information
 Matrix Energy
  (MatrixAir™)                    

                                  Additional information can also be found in the full report which is available at:
Solar walls conserve energy by capturing solar radiation, capturing heat loss from the building,
and increasing the effective building insulation (R value). During summer months solar walls                                                    Collector
have the potential to reduce cooling loads by helping “shed” solar heat gain by the building.                                                    Plate

                                                                                                                                                                               Exiting Air
Manufacturers predict intake air temperatures can be increased by 30-70° F with use of a solar
wall. Studies for installations in Minnesota found that increases of 40° F were not uncommon
during winter months. Due to recaptured heat loss smaller temperature differences were seen
even at night. Average solar efficiencies ranged from 30 to 55%.
                                                                                                                                       Air                             Recaptured
Total energy savings is dependent on design/installation factors as well as the amount of time the                                                                      Wall Loss

solar wall is used. Values of energy saved per area for this study ranged from 73 to 238 kBtu/ft2,
with cost savings of approximately $2000 per year (depending on total area and hours used).
This was equivalent to a 10-20% reduction in energy use for the associated system.

Cost                                                                                                                                                                 Loss to
Retrofit installations tend to cost more than initial construction. Typical cost estimates are                                                              Building

$10/ft2installed, however; some manufacturers quote values as high as $17/ft 2. There is an                                                                  Wall

associated electricity cost due to the additional fan power required. This is generally estimated at
1 W/ft2.

Solar walls are essentially maintenance free. While some manufacturers only warranty the
surface coating for 20 years, the expected lifetime of the solar wall can easily be 30-40 years.

Reported payback periods vary from 1 to 8 years. The payback is better for buildings with long
heating seasons, south facing walls, high priced conventional energy sources, and high
ventilation requirements. Part of the cost is often offset be the fact that structural and/or HVAC
components of the system are pre-existing or would be required regardless of the solar wall.

Design Considerations                                                        Efficiency vs. Approach Velocity
 A sufficient unobstructed south facing wall
  area with solar access is needed.                                0.8
 The approach velocity (flowrate cfm/area ft2)

  is ideally set near 4.0 cfm/ft2. Lower values                    0.4
  result in larger temperature increases but at the                0.3
  expense of lower efficiency. Higher values                       0.2
  result in smaller temperature increases.                           0
                                                                         0         2            4           6        8
 A bypass damper is recommended; especially                                                                    2
                                                                                       Approach Velocity (cfm/ft )
  if the system is to be used during the summer.

Savings Prediction Tools
There are several tools available to predict potential energy and cost savings. Their accuracy is
highly dependent on the inputs used, particularly amount of time the wall is in service.

 Department of Energy worksheets provide simple calculations for sizing and savings, but tend
  to overestimate savings by assuming 68% efficiency.

 RETScreen is a free Excel based tool that uses monthly average weather data. RETScreen
  version 4 has the potential to closely predict savings if the default solar irradiance and wind
  speeds can be adjusted to more closely match the site under study.

 To monitor the performance of a solar wall installation the volumetric air flowrate and
  temperature difference across the solar wall is needed. This will often require the addition of a
  sensor to measure the temperature of the air exiting the collector.

 This research was conducted by Minnesota State University, Mankato with support
 from the State of Minnesota Office of Energy Security.

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