Application of AI-based Image Processing for Occupancy Monitoring
in Building Energy Management
Andreas Reichel
a
, Jens Döge
b
, Dirk Mayer
c
and Jan Bräunig
d
Fraunhofer Institute of Integrated Circuits IIS, Division Engineering of Adaptive Systems EAS,
Münchner Str. 16, 01187 Dresden, Germany
Keywords: Vision-based User Recognition, Machine Learning, Building Energy System.
Abstract: Smart Buildings enable significant savings in energy and CO2 emissions by model-predictive methods. The
building users have a considerable influence on the energetic building management. On the one hand, they
dictate the comfort parameters to be set. On the other hand, they generate internal thermal gains through their
presence, affect humidity, consume oxygen and produce carbon dioxide. The more precisely the user behavior
is known, the more precisely and resource-efficiently the room climate control can be adapted to this user
behavior. In this paper, an intelligent vision-based sensor concept is proposed and tested that is capable to
estimate occupancy and activity inside a building. The contribution initially concentrates on functional
buildings, since here, compared to residential buildings, there is an even greater need for use-oriented room
air conditioning, including savings potential.
1 INTRODUCTION
Buildings, and particularly their energy systems, are a
significant contributor to the world-wide CO
2
emissions. Besides conventional measures like
enhanced insulation of walls or windows, digital
technologies are an enabler towards greener buildings.
These smart buildings integrate data from external
sources (e.g. weather forecasts) and sensors with
powerful analytics and control systems to optimize
energy consumption. One of the most important
parameters is occupancy.
Traditionally, building automation systems use
sensors to measure temperature, humidity and CO
2
concentration in order to draw conclusions about the
indoor climate. Such systems usually provide very
inaccurate data on user behaviour, which makes user-
adapted energy-optimised building control with
advanced control strategies hardly feasible.
However, improvements in detection of user
behaviour are still under development. Two questions
need to be answered: technical possibilities and data-
law constraints.
a
https://orcid.org/0000-0002-3971-1585
b
https://orcid.org/0000-0002-2891-984X
c
https://orcid.org/0000-0002-4972-6529
d
https://orcid.org/0000-0002-7282-723X
This paper aims to provide an overview of
integrating a vision-based system into a smart building
data infrastructure.
To this end, general system architectures for smart
buildings are considered, with a focus on the
distribution of data analysis to the edge of the network.
Potential scenarios for the integration of a smart
sensor for occupancy detection into building
automation are discussed. Finally, the set-up and first
results from a feasibility study at a real building are
presented and directions for further research are given.
2 CURENT STATE OF THE ART
2.1 Smart Building Technologies and
System Architectures
Smart buildings integrate digital building control
systems and networked building automation. Using
multiple, distributed sensors for the building energy
system enables advanced building control schemes. In
Reichel, A., Döge, J., Mayer, D. and Bräunig, J.
Application of AI-based Image Processing for Occupancy Monitoring in Building Energy Management.
DOI: 10.5220/0011080600003203
In Proceedings of the 11th International Conference on Smar t Cities and Green ICT Systems (SMARTGREENS 2022), pages 139-146
ISBN: 978-989-758-572-2; ISSN: 2184-4968
Copyright
c
2022 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
139
contrast to preset schedules, the energy system can be
adapted to physical parameters, weather forecast or
occupancy. This can lead to significant energy savings
of 24% (King, 2017). The gathered sensor information
can also be applied to fault detection and diagnosis.
In summary, the main hardware components of a
smart building are:
Sensors to acquire physical parameters
An internal network to connect smart
devices and control systems as well as an
internet connection
Centralized or decentralized computing
capabilities
This infrastructure is complemented by analytics
and control algorithms to extract relevant information
from sensor data, classify the state of components,
predict time series for relevant parameters and, last
but not least, implement control actions. Obviously,
these applications are very well-suited for machine
learning algorithms.
Findings of a recent review study show that most
control architectures are cloud-based (Yaïci, 2021).
This enables using powerful machine learning
algorithms, e.g. deep learning, that require high
efforts for training. However, such an approach needs
a reliable data connection from the building to the
cloud; also, data privacy might be a concern.
An alternative to this is shifting parts of the data
analytics and control to the edge of the network
(Figure 1). Due to computing power and a network
connection being available in nearly every building
automation device, this enables a variety of system
architectures. In most cases, data analytics and
control functions are implemented on a unit that also
serves as a gateway between the sensor-actuator-
network and the internet (Curto Fuentes, 2021). In a
Figure 1: System architecture for deployment of AI to the
edge and the sensor.
building automation system, this would be the central
building automation control platform (Rinaldi, 2020).
Such an approach has several advantages. The
distribution of computing power enhances the
scalability of the system. The control system is more
robust, which is particularly relevant in case of
extreme weather events: If the connection to the cloud
is lost, external information (e.g. weather forecast) is
missing, but the control system is still operating.
Furthermore, the building owner is not forced to share
raw data with a remote cloud server and can
implement higher privacy standards.
Still, smart functions can be shifted further
towards the edge of the network by using smart
sensors (Figure 1). The concept of a smart sensor is
well known for three decades (Najafi, 1991). Their
main characteristics are:
Analog preprocessing of the input signals
(amplification, filtering)
Analog to digital conversion
Bidirectional communication possibility
Protocol based communication interface and
a unique identification (network address)
Possibility to implement data analysis
algorithms, e.g. compressed sensing, auto-
calibration and self-diagnosis
For building automation applications, the digital
networking capabilities alleviate the installation of a
scalable sensor network that can be spread also on
larger facilities. Compression of the acquired signals
can be useful, if a low-bandwidth wireless network is
used and a large number of sensors have to be
connected.
In certain applications, the sensor can also be shut
down into sleep mode, enabling long-term operation
on batteries or even energy harvesting. This is
especially relevant when the sensors are wireless,
which is an attractive solution for retrofitting of
existing buildings.
There are several potential energy sources in
buildings to power energy harvesting systems:
Floor vibrations can transmit power to
piezoelectric oscillators, that also can be used as a
sensor measure to detect occupancy behavior (Jung,
2018). Also, indoor solar panels have been
investigated for powering wireless sensors, e.g.
temperature and light sensors (Fraternali, 2018). Last
but not least, radio frequency (RF) energy harvesting
should be mentioned. (Bjorkqvist, 2018) developed a
harvester that is able to generate power in the micro-
Watt range from surrounding Wifi and cellular
networks in a building.
SMARTGREENS 2022 - 11th International Conference on Smart Cities and Green ICT Systems
140
Usually, sensor platforms detect a variety of
physical parameters. For instance, the BiB (Building
in Briefcase) platform presented by Weekly (Weekly,
2018) integrates temperature, humidity, ambient
light, CO2, infrared motion detection, and inertial
measurement units in a wireless platform. Another
example for a wireless multi-sensor platform was
developed and tested by De Donno et al. (De Donno
2018), acquiring humidity, temperature, CO
2
, air
quality, and occupancy by passive infrared sensors.
Yet, power measurements showed that the device
would operate just about 1 month until a battery
replacement is necessary, i.e. a significant
maintenance effort would be necessary.
Especially for the case of occupancy detection or
other perhaps sensitive personal data, preprocessing
directly at the sensor avoids transmission of raw data
to remote devices, thus increasing the privacy of the
acquired data.
2.2 Inclusion of User Behavior in
Building Automation
For some years there has been an effort to use model
predictive control strategies (MPC) in the field of
building automation, which can save up to 40% of the
energy used compared to conventional control
(Serale, 2018). These include, for example, model-
predictive zone and individual room controllers
(Paschke, 2018; Kelman, 2011; Gwender, 2010), the
design of which, however, requires knowledge of a
prediction model for the air comfort parameters. This
can be generated automatically using recorded
measurement data (Paschke, 2019), but the number of
people present in a room remains unknown. Since air
comfort parameters are significantly influenced by
the number of people, measuring room occupancy is
needed for model identification and thus for the
design of an MPC control strategy.
The recording of high-resolution measurement
data from building occupancy as well as control and
operation of technical systems with the help of
decentralized sensor technology has become an
important part of building control technology and the
planning process in the last 10 years due to the
increasing spread of bus systems and IoT applications
in buildings. In this way, engineers are given a tool to
cope with the increasing complexity of efficient
energy supply systems. In this context, the
implementation of multi-year monitoring campaigns
for system optimization after commissioning has
become established in selective research and
innovation projects.
2.3 Determination of User Behavior
Several options exist to determine the occupancy and
user behavior in-situ as an input to model predictive
building control systems.
A well-established technology are CO
2
sensors, that
directly acquire the quality of the air inside a room as
an important control input to the ventilation.
A major difficulty here is that measurements are
usually taken at only one or very few individual
points. This is particularly problematic in large rooms
(lecture halls, gymnasiums...), as very strong local
differences can occur, which can negatively influence
the functionality of the control system and thus the
energy consumption of the building if the sensor is
placed in an unfavorable position. Further
disadvantages of conventional CO
2
sensor technology
include:
No direct statement about the number of
people present.
Delayed/indirect measurement of human
activity
Relatively high susceptibility to interference
High calibration effort for CO
2
sensor
technology, comparably strong sensor drift
However, intelligent sensing can enhance the
performance of such systems: The number of
occupants can be estimated using an observer that
processes the current CO
2
concentration by a physical
model (Jin, 2015). Another approach is the fusion of
a CO
2
and a light sensor (Huang, 2017).
Besides CO
2
, also other modalities are suitable to
measure occupancy. (Ahmad, 2020) gives a review
on several technologies, including passive infrared
sensors, ultrasound or wireless network connections.
As demonstrated in (Naylor, 2017), integration of
CO
2
with other sensor modalities like Wi-Fi tracking
by machine learning based data fusion can drastically
increase the precision of the occupancy estimation.
The classic optical approach is based on simple
presence or activity sensing using passive IR sensors
(PIR, 2020). Here, one to three individual point
sensors per sensor head with upstream Fresnel optics
detect the movement of people through the room
based on the change in their thermal signatures. This
approach can be used very well to activate light in
small rooms, but even transferring it to larger rooms
is not possible due to the limited resolution. Likewise,
people who do not move "disappear", as they are no
longer detected by the system without a change in
their whereabouts in the room.
Thus, this sensor technology is very inaccurate,
especially in large rooms, and is neither able to
provide information about the actual number of
Application of AI-based Image Processing for Occupancy Monitoring in Building Energy Management
141
people in a certain area of the building, nor
information about their level of activity. It is also not
possible to reliably detect the absence of people, so a
predefined period of inactivity in the room is usually
used as an equivalent.
Alternatively, there are surveillance cameras in
security-critical building sections that transmit or
record real images and thus technically provide the
possibility to also derive information about the actual
number of people and their activity. However, the
computer-aided storage and automatic analysis of
image data must usually be carried out in compliance
with strict data protection guidelines.
Doppler radar-based solutions (Radar, 2020) try
to circumvent this disadvantage, as they react much
more sensitively to movements. Since these sensors
also only provide integral information about larger
areas, they are also not suitable for detecting specific
numbers of people.
According to the state of the art, the use of
systems based on area scan cameras is currently the
only possibility to solve the resolution problem,
however, especially in connection with the automatic
evaluation of the image data, it rightly places very
high demands on data protection. Thus, the output
and storage of real image data and their external
processing is a fundamental problem that is difficult
to solve technically and, moreover, also has to
contend with considerable acceptance problems
among users.
In certain areas with increased security
requirements, such as the gate environment of
airports, autonomous 3D camera systems based on a
stereo arrangement (Stereo 2020) are sometimes used
to count and track people. These systems usually have
a very high accuracy, but can only be used in quite
small areas - usually up to 5m x 5m at typical room
heights - and have to be installed and set up in a
complex way. Due to the two camera heads required
for 3D reconstruction and the high computing effort,
these systems are also quite expensive. The data
protection aspect is the same as with classic camera
solutions.
To prevent using raw image data, that can include
private information, encryption methods have been
proposed that shuffle the image data in the regions
containing persons (Ahmad 2020). Yet, such
algorithms should be implemented directly at the
pixel sensor. This way, an intelligent sensor is
realized that does not transmit privacy-relevant
information.
3 SOLUTION APPROACH AND
FIRST FEASIBILITY STUDY
For the real-world demonstrator the lecture hall center
“Bergstraße” of the TU Dresden was chosen and
equipped. First, in a standard lecture hall, user
behaviour can be recorded with sufficient accuracy
over a certain period of time and processed with
regard to the activity level.
As a key component for the vision-based
occupancy sensor, the vision system-on-chip (VSoC)
depicted in Figure 2 has been used (Döge, 2015).
Figure 2: Chip microphotograph of the Vision System-on-
Chip (VSoC).
This VSoC is capable to analyze image data
directly on the sensor chip, extract information that
can no longer be personalized and output only the
features required for further processing. Based on
this, the embedded image processing system performs
the next sensor-external processing steps only on pre-
selected image descriptors - such as specific gradient
and texture features or local activity levels, and
calculate the approximate number of people in
regions of increased activity. The implemented
activity and occupancy measurement system thus
takes special account of the data protection required
for public buildings.
The auditorium observed is shown in Figure 3 top.
Due to its size and geometry, it has to be observed
from two overlapping perspectives (see Figure 3
bottom) in order to achieve a complete coverage of
the room with the used optics on the one hand and on
the other hand to avoid that the effective subject size
in pixels varies too much out of the camera
perspective (see Figure 4 right). By choosing a height
of the vision system's viewpoint of about 5m and
splitting it into two views for two installed vision
SMARTGREENS 2022 - 11th International Conference on Smart Cities and Green ICT Systems
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systems, the minimum number of pixels per
monitored seat could be slightly improved. However,
the resolution in the rear areas of the room is still too
low for detection and classification of the head-
shoulder region, which is common according to the
state of the art. Dynamic and static methods were
investigated as a basis for occupancy analysis, but all
of them basically work without object detection,
which is potentially problematic in terms of data
protection.
Figure 3: Photography of the lecture hall (top) positioning
of the vision systems (bottom).
In dynamic scene analysis, it is assumed that
people are generally not completely motionless.
Regardless of the lighting situation, the
corresponding regions are only learned as
background on the assumption of the absence of
movements over a certain period of time. Regions of
a defined dimension, in which deviations from the
current background model are detected, are by
definition foreground and thus people.
In static methods, a large number of images of the
unoccupied space are used to train the classifier. The
training data differ mainly in the possible illumination
situations that have been automatically recorded in
advance. An extension of the training data set by data
augmentation is also possible but was not necessary
in the presented example. The trained model of the
room is then used in operation by detecting deviations
from it as people.
Figure 4: Gradient image from the wall-mounted vision
system (left) and marked seats with derived occupancy data
(right).
As a basis for both approaches, various
descriptors feasible on the VSoC were evaluated,
such as:
gradients and angles,
histograms based thereon (Histogram-of-
Oriented-Gradients - HOG), and,
Local Binary Patterns (LBP).
The static approach using principal component
analysis (PCA) based on gradient data proved to be
the most robust to the confounding effects of
illumination. For this purpose, a static background
model is initially trained by dividing a set of training
gradient images into rectangles and, for each
rectangle, calculating the first five principal
components. Using more than five components did
not improve performance any further. At runtime,
each rectangle’s gray-values are projected onto its
respective principal components and back into image
space. An activity map is then obtained from the
absolute difference between this reconstructed image
and the pre-trained background model. Binary
thresholding and morphological operations are used
to close holes and suppress false positives. Including
angular information provided very little added value,
but introduced a significant noise contribution due to
uncertainty in low-contrast areas.
Application of AI-based Image Processing for Occupancy Monitoring in Building Energy Management
143
Figure 5: Detected occupancy levels and estimated error.
The determined active regions were then
converted into an occupancy level using the spatial
resolution illustrated by the size of the seats in Figure
4 (right) as an example. Although this approach is
more of an estimate than a precise count, especially
when the room is very crowded with clusters of
people connected or overlapping, it does provide a
reliable indication that is considerably more accurate
than required in terms of the air turnover and thermal
load of the people present. In particular, considering
random samples at low occupancy levels (less than 10
people), the estimated occupancy differs from the true
occupancy by up to 20–25%. Empty rooms were
consistently detected as empty, though. At higher
occupancy levels (50–100 people), the relative
estimation error was consistently below 5%. For
reference, an example sequence of detected
occupancy and roughly estimated error is depicted in
Figure 5. It can be clearly observed that students start
trickling into the lecture hall at around 04:27 pm, with
an even larger group entering at around 04:35 pm. As
expected, the estimated number of people fluctuates
only mildly until 06:10 pm, when lecture time ends
and students start leaving the lecture hall rather
hastily. A more in-depth evaluation would have
required either acquiring high-resolution gray-value
image data instead of gradient data or attending
events in the lecture hall in-person. The former was
discarded due to data protection concerns raised by
the proprietor of the building. The latter was not
possible because most events taking place at the time
were examinations with strict access control.
The vision-based occupancy sensor system and
the building management system operate without
feedback and independently of each other. The
occupancy percentage determined in the building area
being monitored is assigned to the available building
operating data and statements can then be derived on
comfort and energy optimization of the operational
management. The aerosol load in the rooms is also to
be reduced by adjusting the implemented air volume.
Machine learning algorithms are used for this
optimization task.
4 PERSPECTIVES
The operator of a single functional building or city
district with very heterogenous building usage (e.g.
lecture hall, classrooms and seminar rooms, sports
hall, shopping center, exhibition hall) receives
information about when and where exactly how many
people are in a quarter/building area at what activity
level (walking, sitting, physical work, playing sports,
etc.). For each usage scenario, the system provides
statements on optimal building operating parameters
and target specifications that can be applied manually
by the system operator. As a result, the energy
demand (and thus the CO
2
emissions) for the
building/district area under consideration is reduced
while the required comfort parameters are ensured
through use-specific system operation and
minimisation of aerosol dispersion.
In the future, the system can be expanded in such
a way that, after a pilot phase, manual tracking of the
operating parameters can be rolled out to other
buildings/districts and finally integrated fully
automatically into the building management system.
An OPC-UA interface to commercial building
management systems (BMS) and energy
management systems (EMS) systems, which is
operated together with these systems as a value-added
service, is ideal for this purpose. The operator of a
building gains potentially comprehensive benefits
through such a scenario (Focus on energy and cost
savings):
Long-term forecast
model- and user-specific optimisation
Communication on necessary maintenance
Increase in system availability through wear
prediction adapted to the usage behaviour
The algorithms implemented at the VSoC level
have been tested in a lecture hall with fixed furniture
positions and limited activity of the occupants. Future
research should include improvements towards
rooms with movable furniture (resulting in varying
background images and greater variations in positions
and activities of the occupants.
In addition to the determination of the occupancy
levels of buildings, the presented vision technology
can also be used in a variety of other applications, e.g.
for the analysis of walkways in stores for the
evaluation of interest in presented advertising
measures or for the demand-driven regulation of
ventilation and air-conditioning of booths at trade
fairs. In particular, the large dynamic range of the
sensor with a linear-logarithmic characteristic makes
it predestined, e.g. in combination with other
SMARTGREENS 2022 - 11th International Conference on Smart Cities and Green ICT Systems
144
wavelength ranges, for outdoor use for movement
sensing in public areas (Reichel, 2019).
Due to the high compression rate of the intelligent
sensor from raw images to the estimated number of
occupants, a wireless connection is a possible
improvement. This would facilitate the retrofit
installation of the sensors at hard-to-reach positions
in existing buildings. However, in turn an energy
management on the sensor platform will have to
implemented to enable a long-term operation on
batteries. Even more ambitious would be the
development of a low power system that could be
supplied by energy harvesting technologies.
5 CONCLUSIONS
In this paper, a concept for using machine learning
methods on sensor level to provide a privacy-oriented
vision-based occupancy detection has been proposed
and tested in a first feasibility study.
The article makes it clear that there is great
potential in the application of vision technology in the
building sector with regard to energy saving.
Furthermore, the approach provides an important
building block for intelligent neighbourhood
monitoring, also with the aim of recording the
utilisation concepts in the neighbourhood and
deriving further savings potential from them.
In the perspective, in addition to CO
2
-compatible
modes of operation, it is also necessary to find ways
of reducing the aerosol load individually when there
is a high density of people in order to reduce the
potential for viral contagion.
ACKNOWLEDGEMENTS
On the basis of the TGM.plus project (SAB
Sächsische Aufbaubank), among others in
cooperation with the partner of the SIB (Staatsbetrieb
Sächsisches Immobilien- und Baumanagement),
initial results on image processing-based presence
detection could be elaborated: The aim of the project
was to develop a value-added module for existing
building automation systems. Its application enables
a reduction of resource consumption in technical
building operation through model-based forecasts of
energy consumption and the derivation of appropriate
measures. The basic knowledge created there makes
a significant contribution to this paper.
The authors gratefully acknowledge the ENLIGHT
project (http://www.enlight-project.eu/), funded by
ENIAC and the German Federal Ministry of
Education and Research, for the financial support of
the development of the Vision System-on-Chip.
Last not least the authors wish to thank Dr. Jürgen
Haufe, Chief Scientist at EAS, for initiating the work
and acting as a mentor for the research team.
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