# [TRNSYS-users] Leap Year

Michaël Kummert kummert at engr.wisc.edu
Tue Jun 29 16:01:50 PDT 2004

```Juan,

> I’m doing some experimental validation of the Type 45 code with a solar
> collector manufactured in Brazil. The year 2004 is a leap year, with 366
> days. The TRNSYS solar radiation processor calculates the solar
> declination angle based on a year with 365 days. My question is, should
> I use the simulation start and stop time according to a “normal year” or
> a “leap year”? For example, June 29^th is the day number 180 of a normal
> year and the day 181 of a leap year. What I want to assure is that the
> declination angle calculated by Trnsys corresponds to the position of
> the sun during the experiment. The solar declination angle is
> calculated, I guess, once per day. If that is true, a difference from
> one day to the next would be in the order of 0.01 degrees. It probably
> won’t make a difference at all. Please tell me if otherwise!
>

TRNSYS uses a simple equation to calculate the declination angle, and
that equation does not take the year into account. So you will have
errors in the declination angle that can reach 1 degree, depending on
the year you are simulating (the actual declination angle has a 4-year
cycle + a slight "long-term drift" when you look at values over a
100-year period).

Those errors have a very small impact on the energy performance of solar
systems or buildings and are consistent with other hypotheses used in
the simulation (e.g. using a hourly data file and calculating the sun's
position at the middle of the hour).

In your case, the difference in declination angle if you use 180 or 181
as the day number is about 0.4 degrees (note that the maximum change in
the declination angle in 24 h, which occurs at the equinoxes, is 0.5°).

If you really want to take the time to do this, you can minimize the
error introduced by comparing the sunrise/sunset in the extraterrestrial
radiation outputted by Type 16 and your data and selecting the shift
angle and the starting day that give the best results. I would think
that selecting the correct shift angle is much more important than
choosing 180 or 181 as the starting day - For that reason, it is always
a good idea to plot the extraterrestrial radiation and the measured data
when using non-standard weather files.

I hope this helps,

Michaël Kummert

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Michaël Kummert

Solar Energy Laboratory - University of Wisconsin-Madison
1303 Engr Res Bldg, 1500 Engineering Drive