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lights schedule and dimming interaction

asked 2015-04-16 08:55:45 -0600

Alex Vlachokostas gravatar image

updated 2015-11-09 13:22:37 -0600

I have difficulty understanding from the E+ manual how the lights dimming and lights schedule interact.

For example, let’s assume that I have:

  • illumination target of 500lux
  • lighting level of 200 W
  • ights schedule is "from 12am to 7am, 0.2; from 7am to 5pm, 0.9; and from 5pm to 12am, 0.5;”.

Case 1

At 11AM I have 600lux from daylight then the Artificial lights (AL) are off.

Hence, they would require 0(i.e., lights off) x 0.9 (schedule) x 200W (lighting level) = 0.

Case 2

At 11AM I have 400lux from daylight then the Artificial lights (AL) are on.

Hence, they would require X(i.e., lights on) x 0.9 (schedule) x 200W (lighting level) = 18X.

Case 3

At 11AM I have 50lux from daylight then the Artificial lights (AL) are on.

Hence, they would require Y(i.e., lights on) x 0.9 (schedule) x 200W (lighting level) = 18Y > 18X. Where X, Y are selected by the dimming algorithm

E+ just says:

For Lighting Control Type = 1 (continuous), the lowest power the lighting system can dim down to, expressed as a fraction of maximum input power (see figure, below). Fig. 54.

I want to confirm that when daylight is below the illumination target, artificial lights kick in at a complimentary level to achieve the 500 lux target. ASHRAE sets the schedule at 0.9 x 200W. Will I still get my 500lux or not?

I am asking because E+ says:

This is the lighting level that would be produced at this reference point at night if the overhead electric lighting were operating at full input power.

Full input power is 200W and not 0.9x200W as the ASHRAE schedule recommends putting in.

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Please always tag your question with the software you are using. Also, I understand markdown - the way to format a post here and on other similar sites - has a learning curve, yet formatting is important if you want people to read your question. You can see how I achieved this formatting by clicking on "edit" post and looking at the code.

Julien Marrec gravatar image Julien Marrec  ( 2015-04-16 09:28:22 -0600 )edit

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answered 2015-07-24 22:45:56 -0600

updated 2015-07-26 18:36:36 -0600

Your description leads to the documentation for Daylighting:DELight:Controls.

From the Engineering Reference, section DELight Daylighting Calculations, this description confirms that the electric + daylighting is the (total) illuminace used to meet the target:

For each point in time, DElight calculates the interior daylighting illuminance at user
specified reference points and then determines how much the electric lighting can be
reduced while still achieving a combined daylighting and electric lighting illuminance

Regarding your final question, from the Input Output Reference document:

Field: Illuminance Setpoint at Reference Point

The desired lighting level (in lux) at this Reference Point. This is assumed to be the
lighting level that would be produced at this reference point at night if the overhead
electric lighting were operating at full power. Recommended values depend on the
type of activity and may be found, for example, in the Lighting Handbook of the
Illuminating Engineering Society of North America. A typical value for general office
work (excluding computer terminals) is 500 lux.

This simply means that the target could be produced ONLY by electric lighting. One might ask "how do I know if my lighting system can attain that illumination level"? That's a good question, since you could define electric lighting levels, for example, at either 3 W/sqft or 1 W/sqft. How could both of these electric lighting levels produce the same lighting level (lux)? The answer depends on the type of lighting used, which in the case of EnergyPlus, is not an input. So the end result is that the user must pay close attention to the target value to ensure daylighting is modeled appropriately (and that the reference location is representative of the lighting level target potential).

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Asked: 2015-04-16 08:55:45 -0600

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Last updated: Jul 26 '15