Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I don't want to get into the details of EnergyPlus or DView, but from a building physics and astronomical point of view, there are several reasons why solar noon is not simply 12 p.m. 1. The "Equation of Time" - because earth's orbit is not perfectly circular, its revolution around the sun varies depending on the time of year,which can cause up to 15 minutes difference between solar and clock time. 2. If the location is not at the Time Zone meridian, solar time will differ from clock time by 1 hour for every 15 degrees difference in longitude. Even if the Time Zone boundary is perfectly rational, i.e., +/- 7.5 degrees from the meridian, locations near the boundaries will be off by 30 minutes.
The other thing to take into consideration is that the sun position in EnergyPlus is calculated for the midpoint of the previous time step, since the solar irradiance is the total over the previous time step, NOT the average at the time stamp. For example, if Hour 1 is from 00:00 to 01:00, then the sun position is calculated for 00:30.
| | 2 | No.2 Revision |
I don't want to get into the details of EnergyPlus or DView, but from a building physics and astronomical point of view, there are several reasons why solar noon is not simply 12 p.m.
1. p.m.
The "Equation of Time" - because earth's orbit is not perfectly circular, its revolution around the sun varies depending on the time of year,which can cause up to 15 minutes difference between solar and clock time.
2. time.
If the location is not at the Time Zone meridian, solar time will differ from clock time by 1 hour for every 15 degrees difference in longitude. Even if the Time Zone boundary is perfectly rational, i.e., +/- 7.5 degrees from the meridian, locations near the boundaries will be off by 30 minutes.
The other thing to take into consideration is that the sun position in EnergyPlus is calculated for the midpoint of the previous time step, since the solar irradiance is the total over the previous time step, NOT the average at the time stamp. For example, if Hour 1 is from 00:00 to 01:00, then the sun position is calculated for 00:30.
