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AtzeBoerstrabbabinnenmilieu, | Arjen Rauebbabinnenmilieu, | Louie ChengPureLiving China, |
To
successfully deploy web-based IEQ sensor networks in buildings one needs more
than just accurate sensors. It is important to develop an overall view on what
to measure, why, where, when and how. A general methodology has to be developed
that allows to analyse and present the enormous amount of IEQ data that will be
gathered with indoor air quality and temperature sensors in a way that building
users and decision makers can relate to. In this article we try to answer some rather
essential questions, based upon literature review and the authors’ experience
with several kinds of IEQ sensor networks. The results presented in this paper
can be used to further develop IEQ sensor networks both for academic and more
practical purposes, e.g. the application of sensor networks in the context of
PPP/DBFMO contracts (Public Private Partnerships /Design, Build, Finance
and Maintain-contract).
This
article is partly based on a paper that was presented by the first author at
the 2018 AIVC conference that was held in Juan-les-Pins (France). It
furthermore can be seen as a follow up article of a REHVA journal article on IAQ
monitoring (5) that was written by the third author (published in 2017).
In recent
years, air quality sensor technology has improved considerably, resulting in
smaller sensors that are more and more reliable, accurate and affordable.
Multiple manufacturers for instance offer electronic PM2.5 fine particle
sensors the size of a matchbox, or even smaller, of professional quality.
Meanwhile, internet of things (IOT) technology has taken off. For IAQ practice
this opens a whole new range of possibilities, as ad hoc sensor networks can be
built from wireless IEQ monitor devices without much hassle. Today it is
possible to monitor the indoor environmental quality of multiple rooms in
multiple buildings in real time, from behind the desk, using online monitoring
platform that receive test data from the sensor devices, updated every second
if you wish. For more background information see e.g. Guyot et al. (2017) [1].
The growing
awareness of poor air quality, especially fine particles, as a health threat
boosts the call for such monitoring networks. Mainstream electronics
manufacturers offer consumer grade devices at rather affordable prices,
apparently recognising a market for personal air monitoring. Which helps to
boost the interest from building occupants in indoor climate monitoring.
On a
professional level, building performance labelling programmes such as WELL
require indoor air quality monitoring [2]. Initiated in China, RESET (see www.reset.build) offers a framework for IAQ monitoring that includes standardised
practice, technical quality standards for the test equipment, as well as the
RESET Accredited Professional training and accreditation programme. There are
currently nearly ten types of RESET certified monitoring devices from diverse
manufacturers mentioned on the RESET website and over 100 RESET accredited
professionals worldwide [3].
Any
practitioner who intends to set up an online sensor network will be confronted
by a number of issues, each of which has to be solved. This article discusses a
number of these considerations, especially the more generic ones. Some of them
stem from our own experience (see Figure 1), others are the result of a
workshop held at the NCEUB Windsor Conference 2018 [4].
Figure 1.
Sensor network test site, pilot building The Hague (NL)
The first
question to discuss is: How to explain
to decision makers the added value of measuring with a sensor network compared
to old school, short term, handheld measurements?
Monitors
are critical for developing recognition of an indoor air quality (IAQ) problem,
which then drives improvement. Traditionally, facility managers or building
owners had to commission long and in-depth audits with handheld particle
counters to determine whether there is a problem. However, today, continuous
monitoring of IEQ allows us to quickly, inexpensively, and meaningfully depict the
health performance of a space.
There is a
growing recognition that monitoring is critical to validate performance. In
China, the phrase “PM2.5” was the fourth most searched term on the internet
(per Baidu.com) in 2015. With the easy availability of inexpensive consumer
grade monitors (as low as US$40), it is easy and natural for employees and
tenants to test out their homes and offices. If they discover problems, they
will usually share the information on social media or else challenge their
managers, facility managers, or operations teams. This can either be a PR
nightmare or a marketing, selling or recruitment opportunity.
Monitoring
data enables self-auditing and green building certification, such as BREEAM,
LEED and WELL. Most sophisticated clients want to show the Return on
Investments (ROI) on projects to justify their investment in a healthy
building. They may also want to keep their building or office space performing
at a high level over time. The addition of furnishings, increase of headcount
density, maintenance, outdoor air infiltration and occupant activity all are
actors that impact air quality after commissioning. An unnoticed side effect of
air quality monitoring is a mind shift in involving the facility manager and
operations team in the “care and feeding” of their indoor environment, because
they have a feedback loop now which allows them - and other stakeholders - to
observe cause and effect.
Furthermore,
monitoring enables climate system optimization and automation. Data informed
operation of ventilation, heating and cooling devices can be a very effective
way to improve overall building and building system performance.
The second
question than is: What IAQ and thermal
parameters should be monitored with the sensor network and at what level of
performance?
For
moderate environments (as in most European locations), we consider particulate
matter (PM2.5), carbon dioxide CO2) and temperature (plus possibly
also relative humidity) the most important parameters to be monitored indoors.
Some monitors include a Total Volatile Organic Compound (TVOC) sensor as well,
however our experience is that indoor levels usually stay below detection
levels of these sensors. They may be nice to have in specific situations where
more significant levels are expected, such as in post-renovations or industrial
environments.
Also,
monitors with real-time formaldehyde sensors are starting to emerge, though
common consensus is that these are not yet reliable enough. As far as nitrogen
dioxide sensors are concerned (relevant e.g. at a location with above average
outdoor air pollution) also these are not as affordable and reliable yet as
e.g. fine particle and carbon dioxide sensors.
PM2.5
sensors should be able to provide particle count, not just mass concentration.
Therefore, optical particle counter (OPC) sensors are required with a minimum
measurement range of 0-300µg/m³. Critical considerations include: humidity
compensation, stability, repeatability and accuracy over the ranges likely to
be encountered.
CO2
sensors should also be of the optical (NDIR) type, with a measuring range of at
least 0-2000ppm. Select sensors that have auto-zeroing features and that can be
field replaceable.
Temperature
sensors can be thermocouples, Resistive Temperature Devices (RTD’s) or silicon
diodes, with a temperature range up to 50°C. Though measuring temperatures seem
straightforward, we find many IEQ monitors to be inaccurate, with an offset up
to 2K in off the shelf devices. This may be caused by heat production from
other components within the devices, e.g. the driving fans of the air quality
sensors.
For those
users who may not be sensor professionals, another option for
"pre-certified" monitors is to simply look for third-party certified
monitors. E.g. RESET is a third-party system that establishes specific criteria
for monitoring hardware to reach Grades A (professional), B (building-grade),
and C (consumer).
Some
manufacturers also have produced monitors that include noise and light sensors.
This is something we do not further elaborate upon in this article as the main
focus here is on indoor climate monitoring.
Question
nr. 3 is: How to select the sensors?
Taking into account aspects like measurement range, accuracy and
self-calibration.
Sensors
must be fit for purpose. Most sensors need periodical calibration, e.g. once a
year, whereas other sensors use disposable heads that are periodically
replaced. There are numerous devices on the market and it may be hard to choose
the right one (best value for money). Which one is the best in a specific
situation of course also depends on the accuracy that is needed and e.g. the budget. RESET [3] has tested and approved a limited number
of sensor devices that are considered accurate enough / of B-grade (professional,
however not lab-grade) quality.
The
measurement range is another important issue when selecting sensors. In Table 1, recommended measurement ranges are described for sensors meant for
non-industrial, indoor use.
Table 1. Selection parameters [5]
IAQ parameter | Common sensor technology used | Recommended measurement range (Grade B) | Selection notes |
Particulate Matter (PM) | Optical particle counters (OPC) | 0–300 µg/m³ | Sensors should be able to provide particle count, not just mass concentration. Critical considerations: humidity compensation, stability, repeatability, long term accuracy. Measurement of PM 2.5 or PM 1 has preference over measurement of e.g. PM 10, as the smaller particles are more relevant from a health point of view. |
Carbon Dioxide (CO2) | NDIRs | 0–2000 ppm | CO2 is an indicator of the amount of bio-effluents in the air and allows one to assess the effectiveness of the ventilation system. This is usually the most determining parameter for IAQ related symptoms. Select sensors that have auto-zeroing features and that can be field-replaceable. |
Total Volatile Organic Compounds (TVOC) | Metal Oxide Sensors (MOS); Photo-ionization Detectors (PID) | 0.15–2.00 mg/m³ | Both MOS and PID sensors are indicative only and used mainly to show relative change. They will not usually match lab testing. High chemical levels will also require recalibration. |
Temperature | Thermocouples; Resistive Temperature Devices (RTDs); Silicon diodes | 0–50°C | Many IEQ monitors suffer from inaccuracy due to heat generated by nearby components on same PCB. |
Relative Humidity | Capacitive | 20–90% | Generally, field-replaceable. Important to measure due to impact of humidity on measurements of other parameters (e.g. PM). |
Formaldehyde | Colormetric, electrochemical; chemical | 0.03–0.3 mg/m³ | Currently, there are no real-time technologies known to the authors that reliably match lab analysis. |
A further
question is: What threshold values
should be applied and how to present measurement outcomes graphically so that
e.g. building users understand how (un)healthy/(un)comforTable their
indoor climate is?
The World
Health Organization and e.g. the European commission offer limit values for air
quality [6, 7]. However, more appropriate values may apply for a specific
country, trade or organisation. Furthermore, Occupational Health & Safety
standards may have appropriate guidelines for work situations. RESET [3] also
has defined specific threshold levels, especially for indoor air quality
parameters, see Table 2.
RESET has
both Regular and High Performance categories of
certification. The latter has requirements that are even more stringent for
PM2.5 than LEED v4 or e.g. WELL.
Also, some
might argue that instead of absolute limit values (concentrations) as threshold
values one should evaluate measurement results (esp. air quality) in terms of
maximum allowable Indoor-Outdoor (I/O) ratios (measured indoor concentration
divided by momentary outdoor concentration).
When
presenting the monitoring results, serious health threats should be
distinguished from results that may seem alarming at first sight, such as
incidental exceedance of a threshold value that was meant as a limit for long
term exposure. You want the building occupants to be alarmed only by real
hazards.
Representation
of (continuous) measurement outcomes (e.g. via a dedicated IEQ platform)
normally benefits from intelligent colour coding. That e.g. uses the colour
green to indicate non-harmful pollutant levels, red to indicate harmful
pollutant levels and orange or yellow when exposure levels are in between the
two.
Table 2.
Suggested RESET threshold values [3]
IAQ parameter | Target level (24 h average) | |
Acceptable | High performance | |
Particulate Matter (PM 2.5) | < 35 µg/m³ | < 12 µg/m³ |
Total Volatile Organic Compounds
(TVOC) | < 500 µg/m³ | < 400 µg/m³ |
Carbon Dioxide (CO2) | < 1000 ppm | < 600 ppm |
Carbon Monoxide (CO) | < 9 ppm | – |
Formaldehyde (HCOH) | –** | –** |
* CO sensors are only required in spaces with combustion appliances
** no requirements defined yet
Another
question that one has to answer before a sensor network can be deployed: Is it only necessary to measure air quality and
temperature at several locations indoors, or also the outdoor air quality and
temperature?
Some areas offer
publicly accessible data from sophisticated outdoor measurement stations. This
may be an excellent source of outdoor data, e.g. for local PM2.5
concentrations. Often however, outdoor stations don’t measure what one needs
(e.g. only PM 10 and not PM 2.5). Also, sometime outdoor stations are simply
located too far away from the building that is under investigation (more than
10 KM or so). And when a building is located very close to e.g. a severely
polluting source like a factory of a busy road local exposure is different
anyhow from what the nearby outdoor station of the city or county is measuring.
Therefore,
often it does make sense to include an outside air quality and outside
temperature sensor when setting up an IEQ sensor network in a building. In that
case one can decide to position the outdoor sensors on the roof or so (covered
from rain and shielded from direct sunlight), or one places it in the HVAC air
inlet.
One
considerable advantage of also measuring outdoor levels with the same devices
is that one can very accurately calculate the so called Indoor-Outdoor (I/O)
ratio for all indoor air quality parameters involved. At the same time, it
might make sense to also relate e.g. measured indoor temperatures with the
momentary outdoor climate (e.g. daily maximum temperature).
Furthermore,
one could ask: How many sensors should
one use? And where to place the sensors?
It
obviously does not make sense to install one sensor in a building that has e.g.
1000 building occupants. But how does one decide to how many sensors to use as
part of an IEQ sensor network? Sensors and monitoring devices are becoming more
and more affordable, therefore the deployment of a
substantial number becomes more feasible over time. On the other hand: one can
overdo it too. For example: applying a monitor / sensor box in all spaces
of a building generally speaking is not (cost) effective.
As a
general rule one sensor per 500 m2 of occupied floor space seems to be adequate
(this is in line with the RESET requirements [3]. Plus at least one sensor per
representative room type (e.g. office room vs meeting room vs laboratory
space).
Also, one
has to decide about the location / position of the sensors. Ideally is a
location as close to where people are sitting, standing or lying most of the
time. In an office building for example this implies that sensors are placed on
people’s desks, if possible, at breathing zone height (1 to 1,20 m above floor
level). If this is not possible, second best is a location on a nearby wall
(e.g. next to a wall thermostat). Third best would be a position under the
ceiling. Positions within (false) ceiling or e.g. placement inside ventilation
ducts should be avoided as this will lead to inadequate estimates of building
occupant exposure, unless the purpose is to measure performance of HVAC systems
providing air within a building.
An
important question is further: What
connectivity solution to select?
Generally
speaking, sensor devices are available with Wi-Fi, ethernet or serial
connections for data communication. These may be fine for permanent
installations. However, in non-permanent situations where an external party
sets up a temporary / ad hoc installation, the client is likely to forbid that
the local ethernet or Wi-Fi network is used due to security reasons. In these
cases, a dedicated Wi-Fi network is the most straightforward solution, with one
internet access point that forwards the collected data from multiple Wi-Fi
coupled monitors to the cloud, using the mobile phone network or LoRa. Another option is a decentralised network, where each
monitor has its own sim card. However, this technology is not yet wide spread.
Whichever connectivity solution is chosen, data is collected on a central
server and can be accessed via an online portal where it is stored and can be
accessed for analysis.
A last
question is: Are there any other
important issues that should be addressed?
One
important aspect that often is forgotten is privacy. Sensor networks should be
deployed in such a way that sensitive information is dealt with in accordance
with e.g. European General Data Protection Regulation (GDPR). Apart from that,
one should recognize that ‘technical data’ like e.g. measured CO2
concentrations indoors in fact inform about whether people are present or not
(e.g. in a dwelling). Persons with criminal intentions and hacking competences
might be very interested in these kinds of data, which is why sensor networks
should be designed and operated with not just privacy but also security in
mind.
Another
often forgotten aspect is interface quality. Data gathered with IEQ sensor
networks often are presented via website, smartphones or wall devices in a
non-optimal way. Using overcomplex graphs and infographics or even irrelevant
ones. One should design the overall system in such a way that data is
transformed into information. Explain (graphically) what it means e.g. when the
CO2 concentration is above a certain limit for a considerable amount
of time. Figure 2provides an example on how to graphically
display indoor and outdoor PM2.5 concentrations. In this example chart, the
RESET threshold value is indicated, average and peak values are summarized and
non-working hours are masked.
Make sure
that end-users intuitively understand the information provided and test
interfaces with non-technical people before they are launched officially. The
last thing we need is high tech sensor networks that measure all kinds of
relevant parameters but that produce data that nobody can translate /
understand.
One last
aspect that often is overseen is overall sensor network robustness. In this
context think of questions like: How is the overall system functioning over
time? Are all sensors still working after e.g. one year? Is it necessary to
exchange components every month or every year or over 5-year period? Are there
any alarm signals when there are sensor connectivity issues? Is somebody
responsible for periodical maintenance and periodical quality checks?
Figure 2.
example interface sensor data presentation.
There are
many considerations related to the deployment of IEQ sensor networks.
Especially adequate, continuous measurement of indoor air quality parameters is
still quite a challenge.
Several
aspects should be considered when designing and operating these sensor
networks:
·
added
value of the network to building occupants (and meaning of the data gathered);
·
what
parameters to measure (e.g. just CO2 or also fine particles and volatile
organic compounds);
·
what
threshold values to use and how to present measurement results in relation to
these limits;
·
simultaneous
measurement of (local) outdoor parameters;
·
accuracy,
measurement range, self-calibration and robustness of sensor components;
·
deployment
strategy, amount of sensors per floor and location of
sensor in rooms;
·
connectivity
(Wi-Fi vs ethernet etc).
The results
presented in this paper can be used to successfully deploy IEQ sensor networks
in the field. Which in turn will help to objectify building and building
service system performance.
[1] G. Guyot, M.H. Sherman, I.S. Walker, J.D.
Clark, 2017. Residential smart ventilation: a review. Lawrence Berkeley Lab,
Berkeley (CA), USA. Available online via: http://eta-publications.lbl.gov/sites/default/files/lbnl-2001056.pdf.
[2] IWBI, 2018. WELL Building standard v2
(online version only). International WELL Building Institute, New York (NY),
USA. Available online via: https://www.wellcertified.com/.
[3] RESET, 2018. RESET Healthy Buildings
Standard (online version only). Standard available via:
https://www.reset.build/.
[4] NCEUB, 2018. Proceedings 2018 Windsor
conference. Network for Comfort and Energy Use in Buildings, London, UK.
Proceedings available via: http://windsorconference.com/.
[5] L. Cheng (2017). Indoor air quality
monitoring 2.0; seeing the invisible. REHVA journal, 54, 32-37 (2017).
Available online via: https://www.rehva.eu/fileadmin/REHVA_Journal/REHVA_Journal_2017/RJ3/p.32/32-37_RJ1703_WEB.pdf.
[6] WHO, 2010. WHO guidelines for indoor air
quality: selected pollutants. World Health Organisation, Copenhagen, Denmark.
Document available via:
http://www.euro.who.int/__data/assets/pdf_file/0009/128169/e94535.pdf.
[7] EC, 2017. European Commission, Brussels,
Belgium. European air quality standards. Available online via:
http://ec.europa.eu/environment/air/quality/standards.htm.
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