ASHRAE-Standard-55 - Page 030
Extracted Content
(This is a normative appendix and is part of this standard.)
NORMATIVE APPENDIX C PROCEDURE FOR CALCULATING COMFORT IMPACT OF SOLAR GAIN ON OCCUPANTS
C1. CALCULATION PROCEDURE
Solar gain to the human body is calculated using the effective radiant field (ERF), a measure of the net radiant energy flux to or from the human body ( ASHRAE Handbook—Fundamentals [ 1], Chapter 9). ERF is expressed in W/m [2] (Btuh/ft [2] ), where “area” refers to body surface area. The surrounding surface temperatures of a space are expressed as mean radiant temperature tr, which equals long-wave mean radiant temperature trlw when no solar radiation is present. The ERF on the human body from long-wave exchange with surfaces is related to t by
ace are expressed as mean radiant temperature tr
trlw when no solar r
trlw
ERF = feff hr trlw - ta
(C-1)
where feff is the fraction of the body surface exposed to radiation from the environment (= 0.696 for a seated person and 0.725 for a standing or horizontal person), hr is the radiation heat transfer coefficient (W/m [2] ·K
[Btuh/ft [2] ·°F]), and ta is the air temperature (°C [°F]).
The energy flux actually absorbed by the body is ERF times the long-wave absorptivity LW of skin and clothing (0.95 is the default value for skin and clothing).
Solar radiation absorbed on the body’s surface can be equated to an additional amount of long-wave flux, ERF solar :
LW ERF solar = SWEsolar
(C-2)
where Esolar is the short-wave solar radiant flux on the body surface (W/m [2] [Btuh/ft [2] ]) and SW is shortwave absorptivity.
Esolar is the sum of three fluxes that have been filtered by fenestration properties and geometry and are distributed on the occupant body surface: diffuse solar energy coming from the sky vault ( Ediff ), solar energy reflected upward from the floor ( Erefl ), and direct-beam solar energy coming directly from the sun ( Edir ). These fluxes are defined below.
Ediff = 0.5 feff fsvvTsol Idiff
(C-3)
where fsvv is the fraction of sky vault in the occupant’s view (see Figure C-1); Idiff is diffuse sky irradiance received on an upward-facing horizontal surface (W/m [2] [Btuh/ft [2] ]); and Tsol is the total solar transmittance, the ratio of incident short-wave radiation to the total short-wave radiation passing through the glazing unit and shades of a window system.
The reflected radiation from natural and built surfaces protruding above the horizon is assumed to equal the Idiff they have blocked.
The total outdoor solar radiation on the horizontal is filtered by both Tsol and fsvv and multiplied by the reflectance of the floor and lower furnishings Rfloor .
Erefl = 0.5 feff fsvvTsol ITH Rfloor
(C-4)
where ITH is the total horizontal direct and diffuse irradiance outdoors (W/m [2 ] [Btuh/ft [2] ]), and the floor reflectance Rfloor is 0.6.
Direct radiation is incident only on the projected fraction of the body fp, which depends on solar altitude , the sun’s horizontal angle relative to the front of the person (SHARP), and posture (seated, standing, horizontal). The fp values are tabulated in the computer program in Section C4.
The direct radiation is also reduced by any shading of the body provided by the indoor surroundings, quantified by the body exposure fraction fbes (see Figure C-2).
Edir = fp feff fbes Tsol Idir
(C-5)
Idir is the direct-beam (normal) solar radiation (W/m [2 ] [Btuh/ft [2] ]). The meteorological radiation parameters are related as follows:
ITH = Idir sin + Idiff Idiff is approximated as (0.2 Idir).
ERF solar is therefore calculated from the following equation:
ERF solar = 0.5 fsvv Idiff + 0.6 ITH + fp fbes Idir feff Tsol SW LW
(C-6)
28 ANSI/ASHRAE Standard 55-2023