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Shading and Solar Radiation

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Shading and Solar Radiation Framing the Issue: Shading Measuring and Calculating Solar Radiation Prudence Ferreira, CPHC, CEA, LEED AP January 20, 2012 The Importance of Solar Radiation The availability of solar energy is affected by location (including latitude and elevation), season, and time of day. Additionally cloud cover and other meteorological conditions which vary with location and time, greatly affect the available solar energy for any site. As illustrated in the figure below, average solar radiation exposure differs dramatically between Germany and the US. The map below shows kWh/kW/yr production differences between the two locations. Source: Solar Maps: National Renewable Energy Laboratory, European Commission The PHPP Manual states that” Utilization of special tools to more accurately calculate specific shading situations is not typically justified, and usually has low impact on energy balance results” however, due to the marked difference between Germany and the US, and the broad range of US insolation levels as shown above, the case can be made that modeling the shading conditions for projects in the US requires greater accuracy and attention than may be warranted for projects in Germany. Overestimating the effect of shading can lead to discrepancies in the modeled and implemented design, where a completed building exhibits higher frequency of overheating, increased cooling loads and increased occupant discomfort relative to modeled conditions. Conversely, underestimating the impact of shading in Passive House energy balance calculations can lead to an actual annual heat demand and associated increases in peak heat loads that are higher than modeled conditions. The consequences of inaccuracy are far greater in the US than they are in Germany. In considering solar radiation and the role it plays in performing energy balance calculations for Passive House projects, there are several types of radiation that must be considered: 1 1. Direct Beam Radiation: The solar radiation that travels through the atmosphere in a straight line to the surface of the earth and objects on the earth is known as direct beam radiation. Because the rays of direct beam radiation are all travelling in the same direction, an object can block them all at once. Shadows are created only when direct radiation is blocked. Beam radiation may be up to 80Wm² 2. Diffuse Solar Radiation: As solar radiation passes from the sun through the earth’s atmosphere, a portion of the energy is scattered by molecules, clouds, other aerosols and particles of dirt. The portion of the solar radiation that is scattered in the atmosphere but has still made it down to the surface of the earth is known as diffuse solar radiation or sky radiation. Diffuse radiation varies according to sky conditions and location but may be around 300Wm². 3. Some solar radiation strikes the earth or is reflected by surrounding surfaces. This is called reflected radiation. Light colored surfaces reflect more than dark ones. The reflectivity of a surface is expressed as albedo, measured as percent of light reflected. Asphalt reflects about 4% of the light that strikes it and a lawn about 25%. An exception is in very snowy conditions, which can sometimes raise the percentage of reflected radiation quite high. Fresh snow reflects 80 to 90% of the radiation striking it. In Fairbanks, Alaska, USA (64.5° North) there is still snow on the ground in April and May and the reflected radiation portion of the total radiation can be 25%. Reflected radiation is also included in the diffuse solar radiation category. 4. Global Solar Radiation aka Global Insolation: Together, the sum of the direct beam and diffuse and reflected solar radiation make up the global solar radiation (total solar radiation) falling on a horizontal surface. Further distinction of solar radiation can be made to account for Angle of Incidence. The angle at which solar radiation strikes glass has a significant impact on the amount of heat transmitted. An angle of incidence of 0 occurs when the sun’s position is perpendicular to the glass. The PHPP describes the angle of incidence with a Θ-value for a window placed horizontally, for example a skylight, the Θ=0°. For a vertically placed window, the angle of incidence is 90° For standard clear single pane glass with a SHGC of 0.86 and a 0° angle of incidence, 86% of solar heat is transmitted. As the angle increases, more solar radiation is reflected, less is transmitted. Solar heat transmission through the glazing falls sharply once the angle of incidence exceeds 55°. Also, as the angle of incidence increases, the effective area of exposure to solar radiation reduces. So, the same window can have hugely different solar gain, depending on the angle of incidence. The angle of incidence is influenced by the position of the sun according to location, season and time of day and the orientation of the glazing. 2 The SHGC declared by glazing manufacturers is always calculated as having a 0° angle of incidence i.e. the maximum solar heat gain. The PHPP takes account for all non- perpendicular incidence of radiation with the rincidence angle reduction factor due to inclined radiation (Standard value: 0.85). Since the same value is used for all cases and all cardinal directions, Logic dictates that this is an average of the reduction in heat gain due to non-perpendicular incidence for all seasons at all locations. Further research is needed to ascertain the exact source of the rincidence angle reduction factor value. Solar Gains in Passive House Energy Balance In calculating, annual solar gains for Passive House energy balance, the following formula is used: QS = r * g * Aw * G Where QS = Annual solar gains (kBTU/yr) r = (%) reduction factor for the frame portion, shading, dirt and non- perpendicular incidence of radiation (Diffuse solar radiation) g = (%) total energy transmittance via perpendicular (Direct Solar Beam) radiation, aka SHGC or g-value 2 2 Aw = (ft or m ) window area G = (kBTU/ft2yr or kWh/m2a) global radiation (the sum of the Direct Solar Beam and Diffuse solar radiation) averaged over during the heating season The total reduction factor (r), used in calculating the solar gains above, is further broken down as such: rT = rshade* rdirt * rincidence angle * rframe% Where rT = (%) total solar gain reduction factor rshade= (%) solar gain reduction due to shade (Standard value: 0.75) rdirt = (%) solar gain reduction due to dirt on panes (Standard value: 0.95) rincidence angle =(%) solar gain reduction factor accounting for the reduced energy transmittance due to non-perpendicular radiation (Standard value: 0.85) rframe% = (%) solar gain reduction due to frame % of overall window 3 rshade solar gain reduction due to shade (Standard value: 0.75), can be further classified as: rS = rH * rH * rO * rother * z Where rH = Shading by a neighboring row of houses or surrounding landscape elements rR= Shading by window reveal rO = Shading by overhangs rother = Additional shading by other obstruction factors z = Activation factor for temporary sun protection devices The inputs needed to calculate the impacts shading by window reveals and overhangs are easy to obtains since these dimensions are part of the projects plans or directly measureable on the project building itself. rH requires the consultant to obtain or calculate the height (hhori) and distance (dhori) of horizontal shading objects. However in many situations this can be challenging. Physical measurements are often constrained by property lines and though laser distance meters can assist in overcoming some challenges, they may not always be suitable. Calculating heights of objects that are not within the projects property lines can be accomplished with the shadow method or angle of elevation method, but this can be very time consuming, especially for projects with a large number of windows. Measuring Solar Radiation and Shading There are several devices that may assist in measuring solar radiation and shading as we look for solutions to help us model conditions with increased accuracy: Global solar radiation is best measured through the use of a Precision Spectral Pyranometer, and is measured in terms of Watts per square meter (W/m2). Direct beam solar radiation is best measured through the use of a Normal Incidence Pyrheliometer and is measured in Watts per square meter (W/m2) as well. In addition, there are shade analysis devices, which measure shading based on the sun’s path and use algorithms to estimate the direct beam radiation by month. http://www.tbcl.com.tw/Product/EPPLEB/EPPleb.htm Members of the PHIUS Technical Committee have been experimenting for the last two years with the use of the shade analysis devices to more accurately model solar gain reduction factors due to landscaping and neighboring buildings (rH - Horizontal Obstruction Shading Factor). However, while use of these devices can save considerable time versus the shadow or angle of elevation method, the results of onsite measurements with these devices yield only shading reduction information as relates to direct beam radiation; these devices are not able to account for shading from diffuse radiation. 4 Diffuse solar radiation can be measured by shielding a pyranomter from the direct beam through the use of a shade. The shade however must be adjusted frequently to account for changes in the path of the sun. The shade as well presents a potential problem when accounting for the diffuse solar radiation. In order to shade the pyranometer, the shade itself must be larger than the pyranometer. This then can cast a shadow on the ground, which in turn reduces the amount of diffuse solar radiation. This approach is problematic and prone to inaccuracy. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/18605/Steven_and_Unsworth_Q_J_R_ Met_Soc_1980_Shade-ring.pdf?sequence=1 Measurements of diffuse solar radiation are crucial for confirming radiative transfer values used in climate models.
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