DayltgSkyLuminance Function

private function DayltgSkyLuminance(ISky, THSKY, PHSKY)

Arguments

Type		Name
integer	::	ISky
real(kind=r64)	::	THSKY
real(kind=r64)	::	PHSKY
Return Value real(kind=r64)

Called By

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Source Code

DayltgSkyLuminance
Source Code

FUNCTION DayltgSkyLuminance(ISky, THSKY, PHSKY)

          ! SUBROUTINE INFORMATION:
          !       AUTHOR         Fred Winkelmann
          !       DATE WRITTEN   July 1997
          !       MODIFIED       na
          !       RE-ENGINEERED  na

          ! PURPOSE OF THIS SUBROUTINE:
          ! Called by CalcDayltgCoefficients, DayltgExtHorizIllum AND DayltgInterReflectedIllum.  gives
          ! luminance in cd/m2 for four different sky types, as described in R.Perez, P.Ineichen,
          ! R.Seals, J.Michalsky and R.Stewart, "Modeling daylight availability and irradiance
          ! components from direct and global irradiance," Solar Energy 44, 1990, 271-289.
          ! The luminance distributions in this routine are normalized such that
          ! the zenith luminance is 1.0, i.e., DayltgSkyLuminance =
          ! (sky luminance at THSKY, PHSKY)/(zenith luminance), which is dimensionless.
          ! The sky types are:
          ! 1. Standard CIE clear sky
          ! 2. Standard CIE high-turbidity clear sky
          ! 3. CIE intermediate sky
          ! 4. CIE overcast sky

          ! METHODOLOGY EMPLOYED:

          ! REFERENCES:
          ! Based on DOE-2.1E subroutine DSKYLU, which did only clear and overcast skies.

          ! OTHER NOTES:
          ! THSKY ranges from 0 to 2Pi starting with 0 directly East and rotating clockwise.
          ! PHSKY ranges from 0 to Pi starting with 0 at the horizon and Pi/2 at the zenith.

          ! USE STATEMENTS: na

  IMPLICIT NONE ! Enforce explicit typing of all variables in this routine

          ! FUNCTION ARGUMENT DEFINITIONS:
  INTEGER           :: ISky                      ! Sky type: 1=clear, 2=clear turbid, 3=intermediate, 4=overcast
  REAL(r64)         :: THSKY,PHSKY               ! Azimuth and altitude of sky element (radians)

          ! FUNCTION PARAMETER DEFINITIONS:
          ! na

          ! INTERFACE BLOCK SPECIFICATIONS
          ! na

          ! DERIVED TYPE DEFINITIONS
          ! na

          ! FUNCTION LOCAL VARIABLE DECLARATIONS:
  REAL(r64) :: SPHSKY                    ! Sine of PHSKY
  REAL(r64) :: G                         ! Angle between sun and element of sky (radians)
  REAL(r64) :: COSG                      ! Cosine of G
  REAL(r64) :: Z                         ! Solar zenith angle (radians)
  REAL(r64) :: Z1,Z2,Z3,Z4               ! Luminance factors (intermediate variables)
  REAL(r64) :: DayltgSkyLuminance        ! Luminance of sky element divided by zenith luminance

          ! FLOW:
  SPHSKY = MAX(SIN(PHSKY),0.01d0) ! Prevent floating point underflows
  Z = PIOVR2 - PHSUN
  IF (ISky >= 1 .AND. ISky <= 3) THEN ! Following not needed for overcast sky
    COSG = SPHSKY * SPHSUN + COS(PHSKY) * CPHSUN * COS(THSKY - THSUN)
    COSG = MAX(constant_minusone,MIN(COSG,1.0d0)) ! Prevent out of range due to roundoff
    G = ACOS(COSG)
  END IF

  SELECT CASE(ISky)
    CASE(1) ! Clear Sky
      Z1 = 0.910d0 + 10.d0 * EXP(-3.d0 * G) + 0.45d0 * COSG * COSG
      Z2 = 1.d0 - EXP(-0.32d0 / SPHSKY)
      Z3 = 0.27385d0 * (0.91d0 + 10.d0 * EXP(-3.d0 * Z) + 0.45d0 * SPHSUN * SPHSUN)
      DayltgSkyLuminance = Z1 * Z2 / Z3

    CASE(2) ! Clear turbid sky
      Z1 = 0.856d0 + 16.d0 * EXP(-3.d0 * G) + 0.3d0 * COSG * COSG
      Z2 = 1.d0 - EXP(-0.32d0 / SPHSKY)
      Z3 = 0.27385d0 * (0.856d0 + 16.d0 * EXP(-3.d0 * Z) + 0.3d0 * SPHSUN * SPHSUN)
      DayltgSkyLuminance = Z1 * Z2 / Z3

    CASE(3) ! Intermediate sky
      Z1 = (1.35d0 * (SIN(3.59d0 * PHSKY - 0.009d0) + 2.31d0) * SIN(2.6d0 * PHSUN + 0.316d0) + PHSKY + 4.799d0) / 2.326d0
      Z2 = EXP(-G * 0.563d0 * ((PHSUN - 0.008d0) * (PHSKY + 1.059d0) + 0.812d0))
      Z3 = 0.99224d0 * SIN(2.6d0 * PHSUN + 0.316d0) + 2.73852d0
      Z4 = EXP(-Z * 0.563d0 * ((PHSUN - 0.008d0) * 2.6298d0 + 0.812d0))
      DayltgSkyLuminance = Z1 * Z2 / (Z3 * Z4)

    CASE(4) ! Overcast sky
      DayltgSkyLuminance = (1.0d0 + 2.0d0 * SPHSKY) / 3.0d0

  END SELECT

  RETURN

END FUNCTION DayltgSkyLuminance
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