Design daylight dependant regulation (DDR)

The purpose of this document is to explain how to design projects using Daylight Dependant Regulation (DDR) and achieve the best possible energy savings, while maintaining the desired light levels.

Introduction

Daylight Dependant Regulation (DDR) refers to the capability of a sensor to measure light levels and maintain a desired calibrated lux level (set point) by dimming up or down a single or a set of luminaires. In other words, when natural light starts entering the building, the luminaires will dim down to save energy, and when it gets dark, the lights will automatically dim to maintain the desired light level.

Terms and definitions

  • Set point: Refers to the value in Lux levels, which the sensor is calibrated to achieve, this set point is then exposed on the system as a percentage. The calibrated level is 100%, and it can be dynamically adjusted by using the circadian programs feature.

  • Daylight calibration: Refers to the process to measure the target lux level to be used as a set point.

  • Closed loop DDR: Refers to the case where the sensor measures both the luminaire brightness and the natural light, this means if the sensor is installed on the ceiling it will be facing down to measure all reflections from both artificial and natural light. Any increase or decrease on the light levels provided by the luminaire will also impact the sensor measurement, meaning the control loop is closed (any change done by the control system is also received as feedback).

  • Open loop DDR(currently not supported by the system): Refers to the case where the sensor measures only natural light, this means if the sensor is installed on the ceiling it will be facing up or directly to a source of natural light, then based on this measurements the system can set different light levels, any increase or decrease on the light levels provided by the luminaire will not have any impact on the sensor measurement, meaning the control loop is open (no direct feedback on changes).

  • Switch on level: When combining DDR with an occupancy sensor, the "task level" as configured on the template parameters of a lighting area will behave as a switch on level, meaning as soon as occupancy is detected, luminaires will go to task level and then start regulating to the setpoint.

Why and when to use DDR

DDR is an energy saving feature, it is always desired for any building which can leverage natural light to save energy, however the technology comes with a few limitations to be considered before designing a project to use this feature. DDR might not be the best solution when the following scenarios are present: * Natural light is directly reflected to the sensors on the ceiling, for example, when there are windows close to the ceiling only and or surfaces that can reflect sun light directly to the sensors, in these cases the DDR behavior will decrease the dim level of the luminaires to resulting in lower light levels than required. * Sensors cannot be positioned according to the design guidelines, avoiding close proximity to direct sun light reflections. * Non-dimmable luminaires.

Common applications for DDR:

  • Office, schools or any buildings with a lot of natural light.

  • Industry or warehouse buildings.

  • Retail stores.

  • Any indoor space with natural light or higher lux levels than required for the space to to design.

How does DDR works?

DDR is enabled by using a template with the DDR letters at the end, for example "AUTO ON AUTO OFF with DDR". Depending on the settings, DDR can be enabled by an occupancy sensor, a local switch, BACnet, the UI or a personal control app.

When DDR is enabled, the sensor will measure lux levels, which will come from the reflection on the surfaces below the sensor of the light provided by the luminaires together with the light provided by natural sources. These measurements will then be compared to the set point (calibrated value). If the set point is lower, then the sensor will slightly increase the dim level of the luminaires. If it is higher, then the sensor will decrease it, this cycle happens multiple times until the measured level falls between +-5% of the target setpoint. The 5% is used to stabilize the behavior and to prevent continuous system corrections, which will keep on happening forever and is also known as "system hysteresis". This 5% means that if the lights are calibrated to reach 500 Luxes, then the measurement range can be between 475-525 Luxes.

How to calibrate a sensor to use DDR

Calibrating a sensor means setting a reference lux level on the sensor to use it as the target level when using DDR.

The calibration process should always be done at night, to ensure there is no natural light present during the calibration, this will ensure the light levels are properly configured and will achieve the best outcome.

During a calibration process, the luminaires on the complete floor will continuously switch On and Off while each sensor takes measurements, this process will last for about 7 minutes. The sensor will be calibrated with 100% of the luminaires output as a setpoint. In case a lower setpoint is required, the high-end trim parameter can be used to limit the maximum level that the areas will use during the calibration process.

After changing the high-end trim levels for a luminaire or an area, a re-calibration of the sensors must be done to update the set point.
DDR performance

DDR is not an accurate way to measure Lux levels, while it performs well to save energy, due to the nature of the measurement it is not 100% reliable. The reason is that it uses an indirect measurement of light and assumes a proportionality to the level on the floor or desk.

CLO (Constant light output)

Constant light output refers to the capability of a luminaire to maintain the designed light output over time. LEDs will degrade overtime, resulting in less light output, and will need to be replaced when the output cannot reach the desired lux level. A maintenance factor is used on lighting designs, meaning the luminaires will provide more output than required. This will guarantee when the LEDs reach the end of their lifetime, the lux level will not be below the minimum required for the specific application. Note that by dimming down an LED luminaire, the decrease of its light output over a period of time is less than that of when used on full brightness. All documentation on expected lifetime in hours (usually close to 50.000 hours), relates to the maximum degradation when used at full brightness.

CLO can be achieved by two commonly used strategies:

  1. Using DDR + High end trim to guarantee the desired lux level, then increasing high end trim gradually to 100%. This will allow the luminaires to increase their output over a period of time to compensate for their normal degradation.

  2. Using the driver CLO feature, this option will ensure the maximum output of the luminaire that follows a degradation curve based on use. It means the luminaire will start increasing its maximum output over time, without relying on the system. This option needs to be configured directly into the driver, (Multione tool for SR drivers with this feature) and the system does not have any means to enable/disable this functionality.

DDR maintenance

To keep DDR levels and savings, it is important to re-do the calibration of the sensors over the lifetime of the project at a minimum 1-2 times per year. A calibration should be done in any of the following scenarios: * Building is recently painted. * Mobiliary change (desks, chairs, etc…​) * Floor change or deep cleaning * Cleaning of the luminaires * Replacing an old/broken luminaire with a new one. * Any changes which will result in to different light level reflections into the ceiling.