Make sense of presence detection

Lighting can account for more than 50% of a building’s electrical consumption. It is an obvious area to target for making energy savings.

The most obvious control is the humble wall switch or dimmer. Unfortunately, while we are very good at turning the lights on, we are very poor at turning them off, either when we leave or when natural light levels are high enough.

You can help overcome this problem by installing intelligent sensors to control the luminaires.

For most projects sensors are used with a marshalling box. This allows a single sensor to control multiple luminaires. Typically, this could be anything from two to 12 units.

At the simplest level, sensors detect presence and switch the lights on or off accordingly. Over the years this occupancy detection has improved dramatically so that now they will detect even the smallest movement. The days of having to wave frantically at the sensor to turn the lights back on, thankfully, are over.

Modern sensors also have more functionality. Some include a photocell so that they can also manage the artificial lighting in response to natural daylight levels. Variable time-out allows switching or dimming after a set period.

A ‘when vacant’ facility takes this time-out functionality even further. In this case the lights dim down to 25% after a set period and then switch off after a further time-out period of three hours. Another function worth considering is absence detection, which is detailed later.

The list of functionality does not end there. Sensors can be programmed for just about any application you can think of. The trick is to assess the application and choose the most appropriate solution.

Even if occupancy sensing alone is used, the energy savings can be large (see table 1). More planning and thought can improve such savings even further.

So how do you ensure that your customer gets the most cost-effective solution from lighting control incorporating sensors?

Presence and absence

Let’s begin with occupancy detection. Modern presence detectors will detect even the smallest movements such as people working at desks.

Absence detection can yield even larger savings and is often an option on standard occupancy sensors. Using this functionality, the sensor will detect occupancy but will not switch on the lighting. Rather the occupant must make a conscious decision to turn on the lights using a wall switch. The sensor will then turn off the lights when it detects no more movement after a set period.

How you position occupancy sensors is important. They work best when someone walks across the detector beams, not towards them (see figure 1).

The space between occupancy sensors is also important. By increasing the level of overlap you reduce the possibility of ‘dead spots’. An overlap of at least one metre is recommended (see figure 2).

Occupancy sensing is particularly good for areas that are used intermittently such as toilets, corridors, single offices or meeting rooms. It will save energy in other areas, but greater savings are possible using sensors that have more functionality.

Daylight linking

An obvious way of making even greater energy savings is to combine occupancy sensing with daylight linking.

Large open-plan offices, for example, are generally occupied throughout the day. Here the lighting may be rarely switched or dimmed in response to occupancy, but there could well be a lot of natural daylight.

Most people will automatically switch on the lights when they enter a room. They will rarely if ever dim or switch them off if natural light levels rise. This often goes unnoticed and we are happy to work in unnecessarily brightly lit conditions for the task that we are working on.

Occupancy sensors with a photocell can be used for both analogue or switchstart lighting and for digital or high-frequency luminaires.

Switchstart luminaires will typically only switch the lighting off after the light levels have reached a minimum level, which is well above that needed, and then only after a time-out period. This avoids individuals being distracted by the lights continually switching on and off.

Digital lighting solutions can dim the lighting up and down continuously in response to light levels and offer greater energy-saving opportunities. Such savings can be as high as 40% in a large open-plan office, when combined with occupancy sensing. Most of us work in the daytime.

Largest savings

The largest savings from daylight linking will be from the luminaires closest to the windows. As you move into the centre of a large area the savings will become less apparent.

For this reason, one common solution for large open-plan offices, classrooms or other similar areas is to use digital occupancy sensors for the luminaires nearest the windows and then standard occupancy sensors for fittings that are in the centre of the room (see figure 3).

Lighting is a major energy consumer, so the demand for control is increasing rapidly.

Intelligent sensors offer a simple solution that provides sophisticated control. They are a win-win solution for all those involved.

Originally published in EMC September 2009 as "You know it makes sense"