pvl_dirint

Determine DNI from GHI using the DIRINT modification of the DISC model

Contents

Syntax

pvl_dirint = pvl_dirint(GHI, Z, doy, pressure)
pvl_dirint = pvl_dirint(GHI, Z, doy, pressure, UseDelKtPrime)
pvl_dirint = pvl_dirint(GHI, Z, doy, pressure, UseDelKtPrime,
 DewPtTemp)

Description

Implements the modified DISC model known as "DIRINT" introduced in [1]. DIRINT predicts direct normal irradiance (DNI) from measured global horizontal irradiance (GHI). DIRINT improves upon the DISC model by using time-series GHI data and dew point temperature information. The accuracy of the DIRINT model improves with each additional input.

Inputs

Outputs

Example

This example shows the measured and DIRINT-model-estimated direct normal irradiance for August 6 (TMY3 file for Albuquerque, NM). Note that the model-estimated quantity is not a match. DNI is a difficult quantity to estimate and there are large uncertainties associated with using model estimates for DNI.

TMYData = pvl_readtmy3('723650TY.csv');
TimeMatlab = TMYData.DateNumber;
Time = pvl_maketimestruct(TimeMatlab, ones(size(TimeMatlab))*TMYData.SiteTimeZone);
dayofyear = pvl_date2doy(Time.year, Time.month, Time.day);
DNI = TMYData.DNI; % Read in for comparison with results
DHI = TMYData.DHI; % Read in for comparison with results
GHI = TMYData.GHI;
Location = pvl_makelocationstruct(TMYData.SiteLatitude,TMYData.SiteLongitude,...
TMYData.SiteElevation); %Altitude is optional
PresPa = TMYData.Pressure*100; %Convert pressure from mbar to Pa
%
% Models that rely on sun position that are run at specific time steps
% (e.g., hourly) run into numerical problems during timesteps when the sun
% straddles the horizion (i.e., spends part of the timestep above and part
% below the horizion).  Since in this example we use TMY day, which is
% hourly and reported at the end of the hour, we adjust the sun position in
% the following two ways.  (1) For hours when the sun is above the horizion
% we adjust sun position so it is for the midle of the hour.  For example,
% sun position reported at 4PM is the sun position at 3:30 PM.  (2) For
% hours where the sun straddles the horizion, we report the position half
% way between the horizion and the position of the sun at the end of the
% hour.  This is appropriate because the GHI data is essentially a sum of the GHI
% measurements made in the past hour.
%
%Run sun position twice:  Once for end of hour positions.
% A second time for mid hour positions.
% Adjust sun elevation and GHI for hours when the sun traverses the horizion
% Include these adjusted values in the mid hour positions
%
[~, ~, AppSunEl, ~] = pvl_ephemeris(Time,Location,PresPa,TMYData.DryBulb);
Time.hour = Time.hour-.5; % shift times back 1/2 hour for sun position calculation because of tmy
% timestamps
[~, ~, AppSunEla, ~] = pvl_ephemeris(Time,Location,PresPa,TMYData.DryBulb);

A=diff(sign(AppSunEl)); %identifies hour before sun straddles horizion (2,-2)
ind1 = find(A==2)+1; % AM hour where sun straddles horizion
ind2 = find(A==-2)+1; % PM hour where sun straddles horizion
%AM Adjustment
    AppSunEl(ind1) = AppSunEl(ind1)/2; %change sun elevation to mid way above the horizion
 %PM Adjustment
    AppSunEl(ind2) = AppSunEl(ind2)/2; %change sun elevation to mid way above the horizion
AppSunEla(ind1)=  AppSunEl(ind1);
AppSunEla(ind2)=  AppSunEl(ind2);
DNI_model = pvl_dirint(GHI,90-AppSunEla, dayofyear, PresPa);

figure
tfilter = and(Time.month == 8,Time.day == 6);
plot(Time.hour(tfilter),DNI_model(tfilter),'-s')
hold all
plot(Time.hour(tfilter),DNI(tfilter),'-o')
legend('DNI (DIRINT Model)','DNI (TMY3)','Location','NW')
xlabel('Hour of Day')
ylabel('Irradiance (W/m^2)')
title('Albuquerque Direct Normal Irradiance Comparison - Aug 6','FontSize',14)

References

[1]Perez, R., P. Ineichen, E. Maxwell, R. Seals and A. Zelenka, (1992). "Dynamic Global-to-Direct Irradiance Conversion Models". ASHRAE Transactions-Research Series, pp. 354-369

[2] Maxwell, E. L., "A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation", Technical Report No. SERI/TR-215-3087, Golden, CO: Solar Energy Research Institute, 1987.

See also

pvl_disc , pvl_erbs , pvl_louche , pvl_orgill_hollands , pvl_reindl_1 , pvl_reindl_2 , pvl_ephemeris , pvl_date2doy , pvl_alt2pres

Copyright 2014 Sandia National Laboratories