How to save money with presence detectors? Part 1 Techniques

Andrey Golovin, Executive Director BIG-RU and KNX Russia golovin@bacnet.ru

When we speak about energy saving solutions in large buildings it is clear that the modern BAS (DDC system) will reduce energy consumption in a building, but very often the building automation system has no precise procedures for controlling the lighting system. Using presence detectors, motion detectors and twilight switches provides many possibilities to increase energy savings efforts in a building.

What is the difference between presence detectors and motion detectors?

There are several misunderstandings in the market about the meanings of motion detectors (MD) and presence detectors (PD). Both are based on the passive infrared technology. Some specialists see no difference and use both terms for many cases.

There is actually a difference. PDs detect the sharp movements of people and are intended to be used mainly in offices with enough windows to provide daylighting. The main purpose of PDs is to support the comfort of the light level and to save energy by switching off the light or by dimming it  when there is no need for artificial light.

MDs are using mainly for walking areas with less daylighting, or none at all: corridors, stairs, technical rooms, archives, etc. Their main purpose is just to save energy. Both MDs and PDs contain:

• a relay (which can switch on/off or dim up/down),
• embedded light sensor (Lux level measurement),
• programmable timer (to switch off the light).

How it works?

People emit heat energy which spectrum is located in the infrared and not visible to the human eye. Sensors detect motion of a warm spot and transform it to an electrical signal. 

Thermal radiation is detected by an optical lens and projected to the infrared sensors. Built-in electronics receiving a signal produces a predefined action (on/off lighting groups, for example).
 
The system of optical lens captures the radiation and thermal data on the infrared sensor. The sensor detection area is divided into active and passive zones. That is why it is very important to locate the sensor in the right place for it to work properly.  With a bigger distance between the installed sensors active and passive zones become wider, and harder to cross zones.

It is important to remember the impact of seasonal effects on the ambient temperature. In the middle of summer the difference in ambient temperature and a human body will be insignificant, while in winter time most of the human body surface is covered by clothes. Also, snow, rain and fog, absorb infrared radiation and can reduce the range of detection (for outdoor sensors).

During the installation of PD or MD we have to consider every interference that might occur:

• Pets (dogs, cats)
• Hot air motions from HVAC devices (air curtains, for example)
• Interferences from electronic devices (hi-fi, TV, computers)
• Artificial lights installed near the sensor
  

And when we are locating PDs in the office area, we need to remember that sensors can’t see through glass walls.

What is inside?

Both PDs and MDs have 3 common elements on board:

• A relay for switching or dimming
• The light sensor to measure LUX level in the working area
• The switch-off timer.
  

The inside components are equal, however the behaviour of the MD and PD differs. Inside the PD the light sensor is measuring light levels all the time. In the MD it measures the light level only during its first activation; then it only checks motion in the room.

The twilight switch has a different construction than detectors. Basically it is a relay module with a light sensor and switched timer. When the light level goes down (lower than set up point), the twilight switch turns on the light, and turns it off in the morning when the light level again crosses this point in the opposite direction. There are some models of twilight switches that have additional timers inside and they use a schedule.

What to choose: PDs or MDs?

In order to make the proper choice we can use the table below:

[an error occurred while processing this directive] In 2009 in Russia we started to follow the German experience of energy savings solutions with MDs and PDs. Since that time we have researched more than 300 buildings and determined the economical effects of using local automation lighting technologies for each building.

Basically lighting takes 10-15% of the total electricity consumption in a building. The problem is that almost no one is actively involved in saving money on lighting; people think that the price per kWh is  too low, and the economical affects will not be sufficient .

This is a mistake. Take any hotel corridor: lights are on 24*7, 365 days a year; people and hotel staff are there only 20% of the time. This means that about 80% of the annual lighting expense could be saved. Besides, when we know where people are in a large facility, we can reduce HVAC electrical  charges, and these savings will be more interesting for a building owner.

In the second part (Projects) of this article I will speak about solutions (based on MDs and PDs) that we tried in different types of buildings and what engineers could propose for the facility management company or building owner.

In the third part (ROI) I would like to share economical statistics. How to save money with presence detectors. We have done several experiments with electricity meters which surprised us with the potential of energy savings for a simple solution.

My Facebook http://www.facebook.com/profile.php?id=100000810691550