How to improve the indoor climate in classrooms?

Proceedings of Conference: Adapting to Change: New Thinking on Comfort
Cumberland Lodge, Windsor, UK, 9-11 April 2010. London: Network for Comfort
and Energy Use in Buildings, http://nceub.org.uk
How to improve the indoor climate in classrooms?
Runa Tabea Hellwig1,2
1
University of Applied Sciences, Faculty of Architecture and Building Sciences; Dep.
Energy Efficient Design and Building Climatology, Postfach 110605, 86031
Augsburg, Germany, [email protected]
2
Fraunhofer Institute for Building Physics, Dept. Indoor Climate and Climatic
Impacts, Fraunhoferstr. 10, 83626 Valley, Germany
Abstract
Natural ventilation controlled by the occupants opening the windows is the main way
to provide fresh air in German schools. Ventilation only during the breaks between
the lessons is insufficient. Therefore a certain amount of continuous airing is required
with regard to both improved indoor air quality and thermal comfort. After a short
description of the special circumstances in classrooms the requirements for the indoor
climate and possible solutions for the improvement of the indoor climate will be
discussed. The aim, the methodological approach and first results of an experimental
study on automated window ventilation will be presented. A precondition for
automation is a well-designed façade with separate openings for supply and exhaust
air. For automation a fuzzy control algorithm is developed which automatically adapts
the opening width of the windows in dependence of the outside temperature and the
indoor air quality and temperature. First tests show that fuzzy control is a suitable
method for automation.
Keywords
Natural Ventilation, Indoor Climate, Retrofitting, School
1. Introduction
Most German classrooms are not equipped with mechanical ventilation systems.
Natural ventilation controlled by the occupants opening the windows is the common
way to provide fresh air. Measurements in real classrooms show that the ventilation
especially in cold season is insufficient because the users do not open the windows
due to cold air coming in from outside (e.g. Hellwig et al. 2008, Hellwig et al. 2009a).
Ventilation only during the breaks between the lessons (short-term airing) is
insufficient. Therefore a certain amount of continuous airing is required with regard to
both improved indoor air quality and thermal comfort. After a short description of the
special circumstances in classrooms the requirements for the indoor climate and
possible solutions for the improvement of the indoor climate will be discussed. The
aim, the methodological approach and first results of an experimental study on
automated window ventilation will be presented.
2. Specific characteristics of the occupancy “classroom”
Classrooms hold specific characteristics because of their occupancy. These
characteristics must be taken into consideration when planning the ventilation system
and the heating/ or cooling system. In the scope of the project "Energy saving, thermal
comfort and good indoor air quality in schools using hybrid ventilation" supported by
the German Federal Ministry for Economic Affairs and Technology the properties of
classrooms in the county Miesbach, Bavaria, Germany regarding geometry, façade
and occupancy are mapped (Hellwig et al. 2009b). In sum 106 classrooms in 22
schools (several primary and grammar schools, 1 vocational school) are visited.
88 percent of the investigated classrooms are built with medium or high thermal mass.
The glazed facade area is between 30 and 60 percent for most of the classrooms. 50
percent of the classrooms do not have a sun shading device. The summer overheating
protection by passive means is insufficient. The classrooms are ventilated by opening
the windows. There are different opening types of the windows and several
combinations of the types in the facades. Tilt and turn windows and the horizontally
pivot-hung type are the most prevalent types. Table 1 shows typical geometrical and
occupancy related parameters of the investigated classrooms and the requirements in
most German federal states.
Table 1: Typical geometrical and occupancy related parameters of classrooms in the county Miesbach, Bavaria,
Germany according to Hellwig et al. 2009b, number of classrooms 106 and minimum or maximum requirements in
most German federal states.
minimum/
range
lower
median
upper
maximum
quartile
quartile
requirements
pupils per class(room)
28 - 33
10 - 40
18
24
29
volume [m³]
76 – 480
188
215
250
volume per pupil [m³/ pers]
≥6
3.8– 23.1
7.6
9.0
11.2
floor space [m²]
20 - 113
59
70
73
floor space per pupil [m² /
1.8 - 2
1.1 – 6.6
2.4
2.8
3.5
pers]
ceiling height [m]
≥3
2.4 – 4.7
3.0
3.1
3.2
There are three typical but quite different seating arrangements in classrooms. The
seating arrangement in classrooms is dependent on the preference of the teacher and
cannot be fixed in the design stage. In 60% of the classrooms a seating arrangement in
rows according to Fig. 1a is found. In this case the tables next to the facade are often
in contact with the facade or the radiator. In 30% of the classrooms tables and chairs
are arranged u-shaped (Fig. 1b). In primary schools (6 to 11 years old) a free seat
arrangement as shown e.g. in Fig. 12c can be found. Therefore the complete floor
space can be regarded as occupancy zone. As a consequence there is no space to
define zones of reduced comfort for air supply.
Due to the high number of persons in one classroom, the required air change rate is
high. Depending on the indoor air quality aimed for and the room volume an air
change rate of 2.5 to 4.5 per hour is required. Compared to dwellings this value is app.
8 times higher. Currently only air change rates in classrooms of 1 to 1.5 per hour are
achieved in most classrooms.
Fig. 1 Seating arrangement in classrooms in the county Miesbach, Bavaria, Germany according to Hellwig et al.
2009b.
Classrooms are characterised by simultaneously occurring external solar heat loads
and internal heat gains from the pupils. The external load for a classroom (see table 2)
in an old school building (old double glazing with U= 3.0 W/(m²K), external shading
with marquee) with east orientation reaches 2.4 kW on a sunny day in the transition
period. This equals the order of magnitude of the value for the internal loads because
of the presence of 30 pupils. The external solar load decreases to 1.4 kW when using
an insulating glazing with a total solar energy transmittance of 0.45.
So far the occupancy hours in German schools are quite low. A typical daily
occupancy period starts at 8:00 and ends at 13:00 h. But this is already in a process of
change and more all-day schools will be established. The duration of one lesson varies
from 45 min, 60 min to 90 min for a double lesson. There are app. 190 school days
per year. Based on an occupancy period from 8:00 to 13:00 the cumulative frequency
distribution of the outdoor air temperature for middle German climate during lessons
is determined (Fig. 2). In 90% of the occupancy period the outdoor air temperature
varies between -3 and 23°C. During the heating-up period of the classrooms in the
morning of course lower outdoor temperature could occur.
For the example of a classroom in an old school building described in table 1 and an
air change rate of 4.2 h-1 at an outdoor temperature of 5°C the overall heat loss is 6
kW with 1 kW transmission heat loss and 5 kW ventilation heat loss. The internal heat
gains cover 2.7 kW of the losses. With low solar radiation in winter of app. 100 W/m²
façade area the radiator has to substitute 2.5 kW. On a sunny day with 300 W/m² the
heating load to be covered by the radiator is 0.9 kW. In case of a lower air change rate
of 2.4 h-1 (1400 ppm CO2) the overall heat losses decreases to 3.9 kW. On a cloudy
day the required heat load is then only 0.4 kW and on a sunny day there are more heat
gains than losses which should lead to a rise in temperature. The example shows that
transmission losses play a secondary role in comparison with the ventilation losses
and that the system “classroom” is quite sensitive to changes.
Often teachers are expected to pay attention to regularly opening of the windows but
the primary task of teachers is to give lessons and to concentrate on the effective
learning. In the case of frequently changes of classrooms and teachers regularly
venting is difficult to organize. Both teachers and pupils adapt to indoor air quality in
a room already after 15 to 20 min as everyone does and cannot detect bad air quality
anymore. Especially during double lessons the break is missing which could remind
the occupants to vent the room.
100
95
Cumulative frequency distribution [%]
Fig. 2 Cumulative frequency distribution of the
outdoor temperature during lessons, test reference
year Würzburg, 190 school days, lessons between
8:00 and 13:00
60
40
20
5
0
-10
-3
0
10
15 23
30
Outdoor air temperature [°C]
3. Requirements concerning the indoor climate in classrooms
In the following the requirements concerning the indoor climate in classrooms are
described. For a better understanding the requirements are calculated for the example
in table 2. The percentage of glazed façade area is derived from the need for natural
lighting in the classroom.
Table 2: Geometric dimensions and properties of the classroom example
example classroom
pupils per class(room)
floor space [m²]
ceiling height [m]
volume [m³]
façade length [m]
room depth [m]
glazed façade area [%]
volume per pupil [m³/ pers]
floor space per pupil [m² / pers]
Buildings materials
30
75
3
225
10
7.5
50
7.5
2.5
low emitting
EN 15251, 2007 defines four categories of indoor climate considering the
expectations of the occupants. For new buildings category II corresponding to a
normal level of expectations is recommended. The operative temperature in
classrooms should range in winter between 20 and 24°C and in summer between 23
and 26°C. In old school buildings category III (winter 19-25°C, summer 22-27°C)
could be applied. The lower value 19°C of the range is in conflict with the German
ordinance on workplaces which requires at least 20°C at workplaces. Following
EN 15251 the adaptive model is probably not applicable. In schools it is possible to
adapt clothing and most German schools are free running buildings in summer. But as
in open plan offices the access to the windows is limited because of the high number
of occupants per window. According to a guideline of the German Ministry of
Transport, Building and Urban Development BMVBS 2008 for office buildings there
must be at least one openable window per two occupants.
Draught caused by high air velocities should be avoided. When designing a
ventilation system the requirements according to ISO 7730, 2005 category B should
be aimed for. With high air change rates these requirements are difficult to fulfil with
both natural ventilation and mechanical ventilation. When designing for a mixed
ventilation system the air velocity should not exceed 0.15 m/s resp. 0.19 m/s at an air
temperature of 20 resp. 24°C. Because of the lower fluctuation range of displacement
ventilation the air velocity can reach 0.19 resp. 0.25 m/s at the mentioned air
temperatures. Natural ventilation using the windows can be regarded as a kind of
displacement ventilation in the case an adequate volume flow can be provided from
the facade. Table 4 summarises the thermal requirements.
The occupants are the main source of pollution and odour in classrooms. The intensity
of odour correlates with the rise of carbon dioxide concentration in the room.
Therefore carbon dioxide is used as a guide value for assessing the ventilation status
of a room. Already in the middle of the 19th century Pettenkofer applied the carbon
dioxide concentration as a benchmark for the indoor air pollution caused by humans.
The German Working Group on Indoor Guideline Values of the Federal Environmental Agency and the States’ Health Authorities (Ad-hoc-Arbeitsgruppe 2008) comments: Although the Pettenkofer-Value (1000 ppm) is often used, the relevance of this
value is not sufficiently clear from today’s view, because the today’s situation indoors
does not correspond to the circumstances in dwellings and to the personal hygiene
habits of the people at the time of Pettenkofer.
Regarding recent studies on the performance of students in dependence of indoor air
quality, the German Working Group on Indoor Guideline Values of the Federal
Environmental Agency and the States’ Health Authorities (Umweltbundesamt 2008)
therefore recommends the following guide values, based on health and hygiene
considerations: concentrations of indoor air carbon dioxide below 1000 ppm are
regarded as harmless, those between 1000 and 2000 ppm as elevated and those above
2000 ppm as unacceptable (table 3). The values correspond to recommendations for
actions. They can be interpreted similar to a traffic light as shown in table 3.
Table 3. Assessment of carbon dioxide concentration and recommendations for actions according to
Umweltbundesamt, 2008.
Carbon dioxide
Hygienic assessment
CO2- meter
Recommendations
concentration [ppm]1)
signal
< 1000
hygienically inoffensive
green
no measures
1000 – 2000
elevated
yellow
check and improve ventilation behaviour,
increase air flow rate
> 2000
hygienically unacceptable
red
check potential for ventilation of the room
take into account advanced ventilation
measures
1)
Absolute carbon dioxide concentration with an assumed outside concentration of 400 ppm.
According to EN 15251 the required air flow rate can be derived from an air flow rate
per person and an air flow rate per floor area depending on the level of emissions
from building materials. For new buildings category II is applicable. For the example
in Table 2 the overall air flow rate calculates to 950 m³/h (264 l/s). This equals to an
air change rate of 4.2 h-1. With an outdoor carbon dioxide concentration of 400 ppm
the indoor concentration reaches 1000 ppm. In case the overall air flow rate is reduced
to 540 m³/h (150 l/s, air change rate: 2.4 h-1) a concentration of 1500 ppm is reached
This is the value which was applied in Germany as a design value for office buildings
for many years.
Classrooms have the purpose to provide a space where learning can take place. To
allow an effective learning process the prior precondition is that the pupils can
understand what the teacher says, thus high speech intelligibility is required. The
three factors the teacher as speaker, the transfer of the spoken words and the listening
and understanding by the pupils is highly influenced by the sound reflection, the
reverberation time and the background noise. To achieve high speech intelligibility
the sound level of the speech has to be considerably higher than the level of
background noise (noise from outside and adjacent rooms, noise from building
services, e.g. ventilation system). According to DIN 18041, 2004 good speech
intelligibility is given in case the difference between the direct speech sound level at
the listener’s place and the overall background noise is between 10 to 20 dB or
between 20 to 30 dB for second language speakers. To keep the background noise
from outside on a low level sufficient noise insulation of the outer walls and windows
is necessary.
The maximum allowed reverberation time depends on the room volume. From DIN
18041, 2004 it can be derived to 0.58 s for the occupied classrooms. For second
language speakers it should be reduced by 20% to 0.47s for the occupied classroom.
In the unoccupied status the reverberation time should be below 0.67 s. Table 4
summarises the most important requirements for the indoor climate using the example
in Table 2.
Table 4: Requirements for classrooms in primary and secondary schools according to EN 15251, ISO 7730, DIN
18041. Airflow rate, air change rate and reverberation time are calculated for the classroom example according to
table 2.
Cate
operative temperature1)
ventilation, low emitting building2)
acoustics3)
gory
winter
summer
air flow rate
air change rate
noise level
reverberation time
°C
°C
m³/h
h-1
dB(A)
s
I
21 - 23
23.5 – 25.5
1350
6.0
30 - 40
0.675)
II
20 - 24
23 - 26
950
4.2
4)
III
20 - 25
22 - 27
540
2.4
1)
Additionally local discomfort phenomena are to be considered (ISO 7730, 2005), Draught Rating DR (Cat B) < 20%
The term “low emitting” is defined in Annex C of EN 15251, 2007
3)
Additionally requirements to the properties of walls, ceilings, doors have to be fulfilled.
4)
Category III defines the lower value with 19°C. This is contrary to the German ordinance on workplaces
5)
The declared reverberation time is valid for unoccupied condition with consideration of the more strict requirements for
second-language speakers resp. for the understanding of difficult texts.
2)
4. Possible solutions
When developing a ventilation concept for classrooms the following requirements
should be considered:
- providing good thermal comfort for most of the school time
- meeting the acoustical requirements for most of the school time
- providing a good indoor air quality for most of the school time
- acceptable adaptation frequency of the ventilation opening for both good
thermal comfort and air quality as well as low distraction potential
- easy to understand and easy to handle ventilation system
-
easy to maintain system considering that the fewest janitors in schools have
the appropriate education for facility management
low running costs
easy to integrate in case of old school buildings to be retrofitted
In principle, classrooms can be ventilated using natural ventilation or mechanical
ventilation. The latter can be divided into central systems and local single ventilation
units. The combination of natural ventilation and mechanically supported systems is
called hybrid ventilation. The most important attribute of a hybrid ventilation system
is an intelligent control system which automatically switches between the operation
modes maintaining the indoor comfort or minimizing the energy demand. A special
case of hybrid ventilation systems are automated windows with a control algorithm.
When applying window ventilation, automated window ventilation or other natural
ventilation systems or pure exhaust air systems with supply air opening in the façade
the outdoor sound level has to be proved. Some producers offer sound insulated
windows with venting openings or supply air openings in the façade. Also the
application of mechanical ventilation helps to avoid high background noise levels
having the origin from traffic. But especially for local ventilation units the low
background noise level of 30 dB or even 35 dB is a challenging task.
Mechanical ventilation systems offer the possibility of heat recovery and provide
preheated air which is an advantage especially in cold season. The air can then be
supplied to the room more comfortably. On the other hand the above mentioned rough
energy balance shows that for German climate in case of occupation of classrooms the
required net heat is very low. For not yet retrofitted classrooms a typical proportion of
transmission losses and ventilation losses is as shown app. 1:5. With a moderate
improving of the insulation (outer walls U = 0.2 W/(m²K), windows U = 1.35
W/(m²K), total solar energy transmission g = 0.45) and the application of a
mechanical ventilation system with heat recovery (80%) the proportion changes to
1:2.2. At an outdoor temperature of 5°C and with an air change rate of 4.2 h-1 the
overall heat losses (transmission and ventilation) add then up to 1.6 kW. The internal
gains heat the room with 2.7 kW. With low winter solar radiation (100 W/m²) already
1.6 kW is the net heat gain for the room which will lead to an increase in temperature
or the heat has to be buffered by the building materials. On a sunny day (300 W/m²)
with a sun shading device in operation 1.3 kW is the net heat gain.
The estimated example shows that it is more a question of comfort to preheat the air.
It shows also that a careful planning of mechanical ventilation systems when applying
in schools is essential. A by-pass for the heat recovery should be planned. Also the
effort for regularly inspection and cleaning of the system is mandatory and cannot be
neglected.
For natural ventilation systems the design of the façade plays an important role. The
question is whether certain positions of openings in the façade can support an
effective but comfortable air change also at low outside temperatures. To develop an
automated window ventilation system for classrooms a control algorithm is required.
To answer both questions a research project is developed at the Fraunhofer Institute
for Buildings Physics. The experimental setup and the first results are described in the
following sections 5 and 6. (see more detailed Steiger et al. 2008, Steiger and Hellwig
2009, Hellwig and Steiger 2010).
5. Automated window ventilation - materials and methods
An experimental setup in a field test laboratory is developed. The field test laboratory
has two classrooms with a room depth of 7.20 m and room height of 3.20 m (Fig. 3).
The rooms are ventilated with single-sided natural ventilation. The windows are
opened and closed by small motors. The façade of the rooms is equipped with 5
façade elements each with 3 tilt windows arranged one above the other (Fig. 3c) or
horizontal pivot-hung windows with 5 façade elements respectively. 24 dummies
emitting continuously heat (75 W sensible heat per person) and carbon dioxide (20 l/h
per person) simulate the students/pupils (Fig 3b). Carbon dioxide is used as a tracer
gas to measure the ventilation efficiency and can be directly translated to the indoor
carbon dioxide concentration in a real classroom. Carbon dioxide is measured at
various positions in the room. Air temperature and air velocities at selected positions
are measured also. The building is well insulated and has triple glazed windows.
Therefore the surface temperatures of the building envelope are almost as high as the
air temperature. The experimental setup is also capable to test the developed
automated window ventilation system with real persons when removing the dummies.
Experiments to investigate the optimum positioning of openings in the façade are
carried out at an outside air temperature between -4 and 6 °C. First experiments
investigating the new fuzzy control algorithm are carried out at an outside air
temperature between 8 and 14°C. Temperature conditions are assessed according to
EN 15251, 2007. The draught rate is calculated according to ISO 7730, 2005. To
assess indoor air quality the carbon dioxide concentration is used. Classification is
carried out according to the guideline for indoor hygiene in schools of the German
Federal Environmental Agency (Umweltbundesamt 2008), see Table 3.
For statistical analysis of the measurement data the decision tree analysis procedure of
the software package GNU R (2008) is used. The method of decision tree analysis is
used to find out the main influencing variables on the air change rate, indoor
temperature and draught rate. The level of significance is defined to α = 0.001.
a)
b)
c)
Figure 3. a) Exterior and b) interior view on the outdoor test facility „schoolhouse“ at Fraunhofer-Institute for
Building Physics. c) Sketch of one façade element with 3 tilt windows.
6. Automated window ventilation - results and discussion
The experiments are divided in two parts. The first experiments investigate the
appropriate positioning of openings in the façade. The second part aims for
developing a suitable control algorithm. The algorithm should control not only the
indoor air quality but also the temperature. It seems likely that occupants complain
rather about uncomfortable temperature than about bad indoor air quality – the current
indoor climate situation in schools is an authentic evidence for this assumption.
Façade design
During the experiments several combinations of the tilt windows and the horizontal
pivot windows with different opening widths are tested. The first test series shows the
advantage of air supply and exhaust opening in different heights of the façade because
there is no optimum opening position for all outside weather conditions. In general
opening arrangements providing good indoor air quality have the tendency not to
ensure good thermal comfort with regard to draught and vertical temperature gradient
all the time. The tilt window type only in one row which is widely used in Germany
provides in most cases inappropriate indoor climate conditions, especially at cold
outside temperatures. Furthermore the amount of fresh air delivered by the tilt
window type is highly fluctuating with the outside wind conditions. More stable
volume flow can be achieved by separating the supply and exhaust openings.
Fig. 4 shows the resulting air change rate for horizontal pivot-hung windows (a) and
two rows of tilted windows (b) under winter conditions. For the pivot windows the
opening width (7 to 60% or 4 to 34 cm) and the number of opened elements (3 or 5)
are varied. With the tilt window type two rows of tilted windows in different positions
(bottom, centre, top), different opening widths (100% or 50% tilt angle) and number
of opened elements (3 or 5) are tested. An air change rate of 2 to 2.5 h-1 and a room
air temperature in the occupational zone between 20 and 24°C are defined as
acceptable. Fig. 4a) shows for the pivot type that the wider the windows are open the
higher is the air change rate. The air change rate is sufficient for 5 elements open and
an opening width higher than 15% and for 3 elements open with an opening width
higher than 30%. Fig 4b) shows a comparable effect for two rows of tilted windows.
All investigated cases provide sufficient air change rate. For the case when there is
only one row of windows tilted no clear dependency of opening area and air change
rate can be seen.
The air temperature is in the accepted range for the pivot type if 5 elements are open
and the opening width is lower than 30% or if 3 elements are open and the opening
width is lower than 60% (Fig. 4c). For the pivot type half of the cases provide
sufficient indoor temperature conditions. For two rows of tilted windows only the
cases with 3 or 5 elements open with centre50% and top windows open and the case
with 3 elements open with centre and top windows open (Fig. 4d). In cases with only
one row of tilted windows the temperature varies in a much broader range (up to 4 K).
When designing an automation algorithm for windows it is important that the variable
most influencing the control variables (air temperature and air quality) can really be
affected by the controller. For example the opening width can be changed easily by a
control algorithm, but in case the wind would have a strong impact on the control
variables the automation algorithm cannot work effectively. Analyzing the data gives
the impression that for some opening constellations the wind has a strong impact.
Therefore the measurement data where explored using the decision tree procedure to
find out which variable acts as an influencing variable on the indoor climate. Table 5
shows that for the pivot type and two rows of tilt windows the most important
influencing variable is the case itself, the second important is either the outside
temperature or the temperature difference between inside and outside. By contrast in
case of only one row of tilt windows the wind velocity plays the most important role
for the value of air change rate and the indoor air temperature. This means that a
control algorithm for one row of tilted windows is very difficult to develop: it needs a
very fast prediction of the expected wind velocity. Therefore tilt windows in one row
are excluded from the following test series and cannot be recommended for effective
control of automated windows.
b)
Air change rate [h-1]
a)
3 elements
5 elements
28
Indoor air temperature [°C]
c)
d)
24
20
16
12
7%
15%
30%
Opening width [%]
60%
center 50%/ center/
top
top
bottom/
top
Case
Figure 4. Air change rate and air temperature in dependence of the investigated cases. a) and c): horizontal pivothung windows; b) and d): two rows of tilted windows. Outdoor temperature range: -6°C to 4°C.
Definition of box-whisker-plot: _ min/ max; whisker 5/95%, box 25/75% and — median
For the design of facades suitable for window automation the following conclusions
can be drawn: a separate opening for supply air and exhaust air in the facade is more
favourable than only one opening for both supply and exhaust air, e.g. horizontal
pivot-hung windows or two rows of tilt windows (Fig. 5b) work better than one row
of tilt windows (Fig. 5a).
Control algorithm
Following the requirements for a control algorithm and the findings from literature
(Dounis et al. 1995, Dounis et al. 1996, Marjanovic and Eftekhari 2004) the control
algorithm should control the indoor air temperature and the indoor air quality.
Actuating variable is the opening width of the windows and the outside temperature is
the disturbance variable. Information about wind velocity is only relevant to close the
windows at very high wind velocity.
Table 5. Variables influencing the target variables air change rate, indoor air temperature or draugth rate. Outdoor
temperature range: -6°C to 4°C. 1 or 2 indicate the most or second important influencing variable respectively.
Underlined: for horizontal pivot-hung windows / Italic underlined: for two rows of tilted windows / Bold: for one
row of tilted windows
Influencing variable
Target variable
CASE
∆T
TA
WV
WD
Air change rate
1/1/2
(2)a) / 2
1
Indoor air temperature
1/1/2
2/2
2/1
Draught rate
1/1/1
2/2/2
(2)b)
CASE:
means for horizontal pivot-hung window the different opening width and for tilt windows the position of openings
and opening width
∆T:
temperature difference between indoor and outdoor air temperature
TA:
outdoor air temperature
WV:
wind velocity
WD:
wind direction
a) only for opening width of 60 and 90%
b) only for small opening width of 7 and 15%
a)
b)
Figure 5. Design of classroom facade: a separate opening for supply air and exhaust air in the facade is more
favourable, e.g. horizontal pivot-hung windows or two rows of tilted windows (b) than only one opening for both
supply and exhaust air e.g. one row of tilted windows (a)
Table 6. Exemplarily rules (pivot windows) to define the function of the fuzzy controller.
Outdoor temperature
:
cold
cold
:
slightly cold
:
slightly warm
:
warm
Indoor temperature
:
too cold
OK and too warm
:
too cold
:
too warm
:
-
Indoor air quality
:
OK and acceptable
unacceptable
:
acceptable
:
:
-
Ventilation
:
close
little
:
minimal
:
maximal
:
maximal
To define the function of the fuzzy controller the empirical knowledge can be used. In
case the outside temperature is very low, the ventilation should be minimal (to avoid
discomfort from being to cold) but sufficient (to provide acceptable air quality).
During mild weather ventilation should be high to provide good indoor air quality.
Because of mild temperatures outside indoor temperature is not the critical target
variable. At warm outside temperature ventilation should reach a maximum to keep
the indoor temperature in comfortable range. The requirement regarding the air
quality is fulfilled at the same time. As a result from the test series a number of rules
have to be defined. Table 6 shows some rules for the pivot type for example. For all
variables used in the rules membership functions are defined. The membership
functions for the control variables are shown in Figure 6. Carbon dioxide is the
indicator for the indoor air quality.
With the mentioned rules and membership functions the fuzzy control is programmed
and tested in the outdoor test bench “schoolhouse” at the Fraunhofer Institute for
Building Physics. During the first tests the control of the heating system was not
integrated in the fuzzy controller. Both the carbon dioxide concentration and the
indoor air temperature are controlled in a sufficient manner and the controller acts
generally stable. But still there is a frequent adaptation of the opening width, which
could be perceived as annoying by the occupants. Therefore the control of the heating
system is now integrated in the control algorithm.
too cold
OK
19
20
too warm OK
26
a)
27
°C
1000
acceptable
1200
unacceptable
1500 2000
ppm
b)
Figure 6. Fuzzification of the control variable: a) membership function for indoor air temperature according to
EN 15251, 2007; b) membership function for carbon dioxide according to the German guideline of the Federal
Environmental Agency (Umweltbundesamt 2008).
The measurement results for carbon dioxide and indoor temperature are shown in
Figure 7. The outside temperature ranges during the test from 8 to 14°C. The carbon
dioxide concentration is measured in the middle of the room in 1.50 m height. The
values vary in an acceptable range. During 6 hours the carbon dioxide concentration
does not exceed 1550 ppm and has a median value of 1250 ppm. The temperatures
opening width
radiator
b)
air temperature
Opening width and radiator [%]
CO2
Indoor air temperature [°C]
a)
Opening width and radiator [%]
Carbon dioxide concentration [ppm]
shown are measured in 0.6 m height and in several distances to the façade, 0.2 to 4 m
distance from the façade. For a limited period the temperature near the façade ranges
from 18 to 19°C, which is to low. This means that either the windows are opened too
wide or the radiators deliver too less heat. The frequency of adaptation of the opening
width is now lower, but still implies an optimization potential.
Figure 7. Carbon dioxide concentration and air temperature with automated window ventilation using pivot-hung
windows and a fuzzy controller (control of radiators integrated in the control algorithms), outside temperature
between 8 and 14°C.
7. Conclusion and Outlook
It is a great challenge is to meet all requirements for indoor climate at the same time
with one design of a classroom. For high air quality and to facilitate comfortable
venting a low person density or big room volume is helpful. But this demands more
sound absorbing measures. The sound absorbing measures – mostly applied on the
ceiling and partly on the wall – again decrease the thermal mass of the room and
decrease the effect of night ventilation. This will lead to higher maximum
temperatures in summer. However, abstaining from sound absorbing measures is not
acceptable because of the ineffective learning and more stress for the teachers. High
air quality should be aimed for. But it has to be discussed how much ecological (in
terms of energy consumption) and economical effort we would spend to provide low
background noise, high thermal comfort and high air quality at all outdoor weather
conditions and all day long.
The question which ventilation system is the most suitable for a classroom cannot be
answered universally. The answer depends on the construction period of the building
(insulation level, air tightness), on the location of the building (noisy or more quiet
surrounding), on the façade design (suitable or not for whole year natural ventilation)
and of course on the investment and running costs aimed for.
It could be shown that automated window ventilation can improve the indoor air
quality compared to manually operated windows considerably. Fuzzy control provides
a suitable algorithm to automate windows in schools. The target variables indoor
temperature and air quality can be controlled satisfactory. First experiments
investigating the performance of automated windows in classrooms indicate that the
positioning of the openings in the façade is a key factor. It is a precondition to have a
well designed façade with separated openings for supply and exhaust air. Comfortable
indoor temperature and acceptable indoor air quality at the same time can then be
provided by variable opening widths of the windows. The opening width should be
automatically adapted to the outside temperature in dependence on the indoor air
quality and thermal comfort. The measurement results show that it is adequate to open
only 3 elements instead of 5 elements. When planning the façade of a classroom this
finding might help saving costs.
The first tests are carried out during relatively mild outside temperatures. Additional
tests at cold outside temperatures are planned. They will be utilized also to further
optimize the behaviour of the controller in terms of the frequency of the adaptation of
the actuating variable and in combination with the heating system.
The results regarding the façade design can also help to improve the ventilation
situation in classrooms with manually operated windows. Appropriate positioning of
the openings in combination with improved adaptability of the openings widths give
the chance for a certain amount of continuous airing with an acceptable thermal
comfort and a good air quality at the same time.
Acknowledgment
This work has been financially supported by the Bundesministerium für Wirtschaft
und Technologie/ Projektträger Jülich under the contract number AZ: 0327387A.
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