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. production. 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 Application 3rd Precinct Police Station pre-heated supply air. Some application examples are: Minneapolis 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. (SolarWall©) ATAS International Inc. Additional Information (InSpire™) http://www1.eere.energy.gov/femp/technologies/renewable_svp.html Matrix Energy http://www.nrel.gov/docs/fy01osti/26748.pdf (MatrixAir™) http://www1.eere.energy.gov/femp/pdfs/FTA_trans_coll.pdf http://www.retscreen.net/ang/home.php Additional information can also be found in the full report which is available at: http://mavweb.mnsu.edu/tebbep/solarwall 6/14/2011 Performance Solar walls conserve energy by capturing solar radiation, capturing heat loss from the building, Perforated 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 Solar Radiation 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%. Ambient 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. Wall Cost Loss to Environment 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 0.7 0.6 The approach velocity (flowrate cfm/area ft2) Efficiency 0.5 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 0.1 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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