ASHRAE-Standard-55 - Page 031
Extracted Content
To obtain ERF solar with Equation C-6 and the fixed default values given above, the required inputs are fsvv, Idir, fbes, Tsol, SW, , posture, and the sun’s horizontal angle relative to person (SHARP). These are described further in Section C2.
ERF solar is converted to short-wave mean radiant temperature trsw using Equation C-1.
C2. INPUTS TO CALCULATION PROCEDURE
The calculation requires eight input values as listed in Table C-2 and explained below.
a. Short-wave absorptivity SW : The short-wave absorptivity of the occupant will range widely, depending
on the color of the occupant’s skin as well as the color and amount of clothing covering the body. A value of 0.7 shall be used unless more specific information about the clothing or skin color of the occupants is available. ( Informative Note: Short-wave absorptivity typically ranges from 0.57 to 0.84, depending on skin and clothing color. More information is available in Blum [ 4] . b. Sky-vault view fraction fsvv : The sky-vault view fraction ranges between 0 and 1 as shown in Table C-3. It
is calculated with Equation C-7 for windows to one side. This value depends on the dimensions of the window (width w, height h ) and the distance d between the occupant and the window.
fsvv
tan– [1] ---- h - tan– [1] ---- w - 2 d 2 d
= ------------------------------------------------90 180
(C-7)
where the arctan function returns values in degrees. When calculating fsvv for multiple windows, the fsvv for each may be calculated and summed to obtain a total fsvv . Exterior objects obstructing the sky vault shall not be considered because they have a similar diffuse reflectivity as the sky vault. c. Total solar transmittance Tsol : The total solar transmittance of window systems, including glazing unit,
blinds, and other façade treatments, shall be determined using one of the following methods:
- Glazing unit Tsol shall be provided by manufacturer or from the National Fenestration Rating Council approved Lawrence Berkeley National Lab International Glazing Database.
- Glazing unit plus interior fabric shade shall be calculated as the product of glazing unit Tsol (in item C2[a]) multiplied by the shade openness factor.
- Glazing unit plus venetian blinds or other complex or unique shades shall be calculated using National Fenestration Rating Council approved software or Lawrence Berkeley National Lab Complex Glazing Database.
When direct solar radiation that falls on a representative occupant is transmitted through more than one window system with differing solar transmittances, the solar transmittance Tsol impinging on the occupant shall be calculated as the area-weighted average of the solar transmittance of each window system. d. Direct-beam solar radiation Idir : Direct-beam solar radiation data for a standard cloudless atmosphere are
presented in Table C-4. ( Informative Note: Idir is based on elevation above sea level up to 900 m [3000 ft]. Above 900 m [3000 ft], increase these values 12%; above 1200 m (4000 ft) increase values 15%; above 1500 m [5000 ft], increase values 18%; and above 1800 m [6000 ft], increase values 21%. e. Fraction of the body exposed to solar beam radiation fbes : The fraction of the body’s projected area factor fp
that is not shaded by the window frame, interior or exterior shading, or interior furniture. See Figure C-2. f. Solar altitude : Solar altitude ranges from 0 degrees (horizon) to 90 degrees (zenith). Also called “solar elevation.” See Figure C-3. g. Solar horizontal angle relative to the front of the person (SHARP): Solar horizontal angle relative to the
front of the person ranges from 0 to 180 degrees and is symmetrical on either side. Zero (0) degrees represents direct-beam radiation from the front, 90 degrees represents direct-beam radiation from the side, and 180 degrees represent direct-beam radiation from the back. SHARP is the angle between the sun and the person only. Orientation relative to compass or to room is not included in SHARP. See Figure C-3. h. Posture: Inputs are “seated,” “standing,” or “horizontal.”
ANSI/ASHRAE Standard 55-2023 29