1、 Patrick Tebbe is an associate professor in the Department of Mechanical and Civil Engineering, Minnesota State University, Mankato, MN. Saeed Moaveni is Dean of the David Crawford School of Engineering, Norwich University, Northfield, VT. Louis Schwartzkopf is an emeritus professor in the Departmen
2、t of Physics and Astronomy, Minnesota State University, Mankato, MN. All students are former or current students in mechanical engineering, Minnesota State University, Mankato, MN. Study of Unglazed Transpired Solar Collector Installations in the Twin Cities Minnesota Climate Patrick A. Tebbe, PE Sa
3、eed Moaveni, PE Louis Schwartzkopf Joseph Dobmeier ASHRAE member and SBA ASHRAE member Joseph Gehrke Matthew Simones ASHRAE Student members ABSTRACT An unglazed transpired solar collector, sometimes called a solar wall or UTC, can be used to preheat ventilation air with incident solar energy. In thi
4、s system a dark colored collector made of sheet metal absorbs solar energy, transfers it to outdoor air passing through its perforated surface, and supplies it to a building air handling system. This system offers energy savings with a simple, efficient, and reliable design. Minnesota State Universi
5、ty, Mankato has undertaken a study of UTC installations in the Minneapolis-St. Paul region for the Minnesota Office of Energy Security. This study includes several different UTC installations; including a corporate building, a K-12 institution, and a public services building. In each case the UTCs a
6、nd/or buildings have different characteristics from each other. This paper will begin with a brief background on UTCs and will then discuss typical climate conditions, determined through weather logging, and the resulting solar wall performance, determined from temperature and energy management syst
7、em logging. The paper will conclude with a discussion of the suitability of unglazed transpired solar collectors for buildings in the Minnesota climate. INTRODUCTION An unglazed transpired solar collector can be used to preheat ventilation air with incident solar energy. This system offers energy sa
8、vings with a simple, efficient, and reliable design. Over the last two decades a number of sites have installed such systems. In 2008 Minnesota State University, Mankato received a grant from the Minnesota Department of Commerce to study installations in the Minneapolis-St. Paul region. The purpose
9、of the grant is to develop data-driven evidence documenting the effectiveness of this technology for energy savings in the Minnesota climate. Historically unglazed transpired solar collectors have been referred to by a number of additional names, such as solar transpired walls, unglazed perforated-a
10、bsorber collector, or just perforated collectors. For this paper the term unglazed transpired collectors (UTC) will be used. Marketed for many years by Conserval Engineering, Inc. and developed with research from the National Renewable Energy Laboratory (NREL) UTCs have found their way to a number o
11、f new and LV-11-C071 2011 ASHRAE 5792011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital f
12、orm is not permitted without ASHRAES prior written permission.retrofit building projects. With a push for the use of more renewable energy and the availability of possible LEED credits for their installation, UTC systems are receiving even more interest of late. While the underlying technology is st
13、raightforward, the interacting physical phenomena can be quite complex to fully understand and model. Simple table (DOE 1998) or spreadsheet (RETScreen 2005) calculations can be used to predict possible energy savings. However, to fully examine the energy exchanges taking place within the system a c
14、omputational fluid dynamics (CFD) package or a package such as TRNSYS must be used. The measurement of thermodynamic properties on existing installations not only demonstrates possible energy savings, but offers an attractive verification technique for existing tools and climate assumptions. This pa
15、per will briefly review the science and operation behind UTCs. Next, several buildings under study will be described along with the experimental measurements that are taking place. Similarities and differences in installation and operation will be examined for each of the described buildings. Typica
16、l climate conditions, determined through weather logging, and the resulting energy savings, determined from UTC exiting temperature and energy management system logging, will be referenced for the main test building. The paper will conclude with a discussion of the suitability of UTCs for different
17、buildings in the Minnesota climate. BACKGROUND ON UNGLAZED TRANSPIRED COLLECTORS (UTC) UTCs are constructed from corrugated aluminum or steel panels. They are generally dark colored to improve solar thermal radiation absorption, although the color can be adjusted to aesthetically match the rest of t
18、he building. The sheeting is perforated with small pin holes or slits; typically thousands per square meter. This metal sheeting can be mounted with a simple support structure to an existing structural wall, ideally a south facing wall. While options do exist for many types of buildings, for angled
19、roof-top units, and for combination with photovoltaic cells; this paper will focus solely on the vertical wall mounted unit for commercial buildings (Figure 1a). Solar radiation heats the metal surface where part of the energy is transferred to a thin boundary layer of air. The space between the she
20、eting and the building wall forms a type of air plenum. With the use of a fan, or other suitable air distribution system, the heated outside boundary layer air can be pulled, or transpired, through the perforations and then channeled through this plenum space to an exit ductwork connection. This air
21、 can then be supplied directly to the building space as conditioned ventilation air or to a heating unit as pre-heated ventilation air. Consequently, the system is most effective for buildings that require large volumes of ventilation air. When ventilation air is required without additional heating
22、a bypass damper can be installed allowing fresh air to be provided without passing through the UTC. The temperature rise across the UTC will be a function of several parameters. In general, as solar radiation increases or the airflow rate decreases the exiting temperature increases. The airflow rate
23、 through the UTC will be, at least partially, determined by the required ventilation airflow rate and the area available for the UTC. If enough area is not available the flow rate per area will need to be increased to satisfy the buildings total demand. This will reduce the exiting temperature. Alte
24、rnatively, a minimum UTC flow rate is necessary to produce the desired flow fields on the surface and interior of the UTC, thus reducing convective losses and ensuring optimum efficiency. The suction pressure inside the UTC also needs to be high enough to overcome the influence of wind pressure, whi
25、ch could cause air to flow in reverse out of the UTC. A typical airflow rate is 4.0 CFM/ft2(0.02 m3/s/m2) (Summers 1995). As described the major energy savings from a UTC comes from providing fresh air for ventilation that is at a temperature above ambient. It is not uncommon to see a temperature ri
26、se of 40 F (22 C) through UTCs (Hollick 1994). There is also an added energy cost to using the UTC in the form of extra energy needed by the fan to overcome increased flow resistance due to the UTC (i.e. parasitic fan power). While the majority of energy used to raise the airs temperature comes from
27、 solar radiation, there can be a substantial amount that comes from recaptured wall losses. As with all surfaces of 580 ASHRAE TransactionsExiting AirSolarRadiationAmbientAirBuildingWallPerforated Collector PlateRecapturedWall LossWallLoss to Environment(a) (b) Figure 1 (a) Schematic of an unglazed
28、transpired collector. (b) View of the Breck School UTC installation on the field house southeast facing wall. the building envelope during the heating season, energy is lost by heat transfer through the building wall and into the environment. For the portion of the building wall covered by the UTC t
29、his energy passes into the UTC plenum space, thereby helping to heat the ventilation air. Since the buildings structure also stores energy during the day a portion of this can then be recaptured at night. The stored thermal energy can be significant if the exterior wall to which the UTC is mounted i
30、s massive. This allows the UTC to continue providing heated air even after solar radiation starts to taper off for the day. In addition, the elevated temperature in the plenum space and the additional insulation of the UTC itself reduces energy loss due to conduction through the building wall, even
31、when the flow rate is zero. EXPERIMENTAL SITE DESCRIPTIONS For this study several possible sites were researched in, or near, the Minneapolis-St. Paul region. The sites selected for this study span a range of building types (Table 1). To date, the majority of data has been gathered at the Breck Scho
32、ol. The Breck School is an Episcopal school that teaches grades K 12. In 2002, the field house was opened and included a UTC on the southeast side of the building. Its purpose was to preheat the air entering the building thereby reducing the amount of energy needed to heat it to a comfortable temper
33、ature. The Breck UTC is dark brown in color and split into 8 individual sections (Figure 1b). Each section has a vertical panel with horizontal corrugation mounted and installed in its own steel tube frame. Four fans are connected to the UTC system; one for every two sections. In total, the UTC syst
34、em can provide 5,800 cfm (2.74 m3/s) of preheated air used for ventilation. All fresh air for the field house is provided by the UTC before being mixed with return air, filtered, and heated by four independent two stage natural gas fired heaters. The air is then distributed to the field house throug
35、h ceiling mounted ductwork and diffusers. Since this system was installed without a bypass damper all ventilation air for the building must be pulled through the UTC. Therefore, the system is only operated during the heating season. Only one of the four units was monitored, thus, the associated area
36、 is of that reported in Table 1. The 3rd Precinct is one of five police precincts in Minneapolis, MN. From 2003 to 2005, the original precinct building (built in the early 1980s) was renovated and expanded with a new addition. During this time, new mechanical and electrical systems were installed in
37、 the building, including a UTC on the buildings south side. The UTC supplies ventilation air to a rooftop heating unit and is equipped with a bypass damper and a relief damper that allows hot air to naturally rise and exit the 2011 ASHRAE 581Table 1. Basic characteristics of physical sites studied.
38、Sites Total Area ft2(m2) Flow Rate CFM (m3/s) Year Installed Approach Velocity* ft/min (m/s) R value of Building Wall ft2 F h /Btu ( m2K/W) Building Wall Construction Breck 3300 (306.6) 5800 (2.74) 2002 1.76 (0.009) 15 (2.64) Precast Concrete with insulated core 3rdPrecinct 765 (71.1) 3000 (1.42) 20
39、05 3.92 (0.020) 43 (7.57) Wood frame with EIFS exterior Aveda 1270 (118) 8000 (3.78) 1994 6.30 (0.032) 11 (1.94) Steel framing with insulated exterior * The approach velocity is defined as the total flow rate divided by the total area. wall during summer. The buildings energy management system deter
40、mines whether to open the damper for the UTC or the bypass damper for the unconditioned outdoor air depending on whether it is in heating or cooling mode. After drawing in outside air from either source it is mixed with return air and filtered. Finally, the air is conditioned to the required supply
41、dry-bulb temperature and relative humidity before being distributed throughout the building. The third installation studied is at the Aveda Corporation headquarters in Blaine, MN. In 1992 during an energy audit it was suggested that the company install a UTC to preheat incoming ventilation air. This
42、 resulted in a UTC being installed on the upper (roof level) south wall to help reduce energy costs. Since the system uses 100% ventilation air, there is no mixing with return air as with the Breck and 3rd Precinct sites. The ventilation air is drawn from either the UTC or from the outdoor bypass ai
43、r intake louvers. The air is then filtered and conditioned before being supplied to the manufacturing floor. The conditioning is done with a pre-heat coil, an electric heat pump, and a steam humidifier. At each site a weather station was installed with the following components: outdoor temperature a
44、nd relative humidity sensor with shield, solar radiation sensor, wind speed and direction sensor, data logging system, wireless data transceiver, and tripod mounting system. These stations logged data at 8 minute intervals. At Breck and 3rd Precinct two additional temperature sensors were added to t
45、he system and mounted in the exiting duct from the UTC. Aveda already had an exiting temperature sensor installed as part of the UTC system. Other data such as heat stage status, interior temperature, and fan status were recorded via each buildings energy management system. PERFORMANCE AND ENERGY SA
46、VINGS RESULTS Climate Verification In measuring the impact of the UTC on energy savings it is important to consider whether the climatic conditions during the period of study represent a typical year. ASHRAE Standard 90.1 specifies that the Minneapolis-St. Paul region is in climate zone 6 with 7981
47、annual heating degree days (ASHRAE 2007). Outdoor air temperature was measured locally at each site and used to calculate heating degree days for each month, based on a 65 F (18 C) base point (shown in Figure 2 for the Breck location). For this same site the normal high and low temperature found for
48、 each month were compared in Table 2 to the 30 year averages from the National Climatic Data Center (NCDC 2010). The majority of heating days occurred between October and April with a total for the year of study of 7375 (7.5% lower than the Standard 90.1 listed value). While temperatures for the pri
49、mary UTC heating months of December, January, and February were at or below the NCDC averages, November was approximately 10 F (5.5 C) above average. This is also seen in Figure 2 which shows November equal to October in terms of heating degree days. Therefore, it is reasonable to conclude that energy savings estimates will be depressed for the year of study, with the major difference coming in November. 582 ASHRAE TransactionsFigure 2 Heating degree days (HDD) measured at the Breck School. Values are computed based on a 65 F base point. Calculation Pr
