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See also
Project Guide Making sense of detectors
As building operators seek to gain enhanced
control of energy costs, the use of movement detectors linked to lighting
and other services is on the increase.
Here we explain the importance of knowing your
detectors and how this can significantly reduce your costs and provide
payback within a year.
In the building industry there has always been a
tendency to specify a lighting system that provides the client with all
they require, only to be cut back to the bare essentials or, at best, a
lower specification when the rest of the project has exceeded the
budget.
It’s a well known saying in the lighting industry
that lighting is the first budget to be cut and the last thing to be
ordered; and nearly always at the last minute!
Therefore, leaving lighting and other services
running when they are not needed is still one of the most common causes
of energy wastage in buildings. Whilst any system is cheaper to install
at the time of initial building, the retro-fit costs needn’t be as high
as some are today. Ironically, once the building is completed and the
handover has occurred, the tendency to leave the lighting system alone
for a few months, or even years, is a common one.
Nevertheless, occupancy detectors, using presence
or absence detection, are now being deployed more widely in buildings
both at initial build and in refurbishment. The installation of even the
lowest performing types of these items can reduce ongoing energy costs.
However, just because they perform the same basic
function, this doesn’t mean that all detectors are the same. They can vary
greatly in both their functionality and controllability. Choosing the
right detector for each space can make all the difference to achieving the
maximum return on investment; after all, the additional costs of
installation, wiring, commissioning and maintenance can be severely
affected when using the wrong product.
There are two types of detector in common use –
passive infra-red (PIR) and microwave. PIR detectors work by passively
detecting a rapid change in the infra-red spectrum within their field of
view. Microwave detectors actively transmit energy into the space and
measure the reflected energy from the background. When a person, or item,
enters or moves in the space, the pattern of the reflected energy
changes and this is detected as a change in the Doppler pattern.
In all cases, it’s important that the detector can
distinguish between human movement and other movements, so that false
responses are reduced, at worst, or eliminated. This requires in-built
programming to analyze the size and pattern of the movement – something
that is lacking from lower quality detectors.
The choice of detector should also be dictated by
the space it’s controlling – a decision that should be based on
performance rather than price. In particular, range and direction make a
significant difference to the performance of the system and making the
right choices can also minimize the number of detectors required for
effective control.
Corridors are a case in point. People will often
install a basic detector with 360o coverage in a corridor and, while this
will work in most cases, it’s far from being the best solution. By using
an opposing radial detector with a range of eight metres in each
direction, provided more accurate coverage and also reduced the number
of detectors needed to cover the space. In fact, with the right choice
of detector, many corridors can be served perfectly adequately by a
single detector.
Similarly, in open plan spaces there are
considerable benefits from being able to control lighting in zones to
respond to changing occupancy patterns. To achieve this effectively it
may be necessary to use a close cell detector with very precise adjustment
capabilities, or a detector with wide coverage which is narrowed down to
enhance the local sensitivity. Better still, use a PIR which detects
‘true presence’ like the IDW or IDS office detector and not just merely
claim to have presence detection. The IDW or IDS can detect a person
sitting still at a computer desk up to three metres away from the
detector
and walking up to 18 metres away in any direction around a 360o plain.
Where detectors are being retrofitted to an
existing lighting system, they are most likely to be operating on a
stand alone basis rather than simply sending signals to a lighting
management system. In these cases, the detectors need to have the
capabilities to deliver the required control functions themselves so a
basic detector that just switches lighting on and off will not fit the
bill. For example, to achieve maximum energy savings it is generally
desirable to use an integrated movement detector and light detector, so
that lighting is only turned on when ambient light levels need to be
supplemented.
However, there are some installations where
linking to a full Building Management System is required. By using
potential free detectors, the BMS can be triggered by the detector in one of
several ways but the BMS is responsible for the power control. For
example, IDW or IDS detectors to link in with a full BMS where the
detectors
are just set to send a pulse trigger to the BMS, even though they could
have been set to any number of seconds between 1 and 900, or any number
of minutes from 1 to 15. By using intelligent, easy to install,
potential free detectors the client saves money on excessive adaptation of
the BMS, fewer ramping cards, etc because of the flexibility of the
detector.
Sometimes, where the level of activations equate
to the lights to be on during darkness for 80 - 90% of the time, the
saving created by turning the lights off for 10 - 20% of darkness may be
less of a saving than may be originally thought and a simple photocell
will ensure long lamp and control gear life.
These are just a few examples of how detectors can
vary in the levels of performance and functionality they can offer and
why it’s so important to consider what is required of each space. Taking
the time to understand the lighting control needs of the space and
choosing detectors accordingly will make all the difference to the energy
savings that are achieved through the life of the installation. Careful
and correct consideration of the type of detector and its specific
location will also lead to other savings:
-
Time (preparation, installation,
commissioning and ongoing maintenance)
-
Materials (cable, connectors and interface
cards for BMS)
-
Improved lamp and control gear life
-
Reduced carbon footprint!
In summary, installing fewer, but better quality,
detectors will ensure you save more money than you think. Typical payback
is under two years but if you use the correct advice and products you
can get this down to as low as six to 12 months!
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