Download Natural night ventilation and thermal inertia

Natural night ventilation and thermal inertia
0. - Douzane, J-M. Roucoult and T. Langlet
Laboratoire de Thermique du Batiment - JUT Genie Civil
Universite de Picardie Jules Verne, Amiens, France
Abstract
The objective of this study is to propose a simplified characterization of the thermal
inertia, as part of the implantation of a system of summer refreshment by night cooling
ventilation. On the low of a previous study, realized with the help of the modal
analysis, the interactions between the thermal inertia of a building and the night
cooling ventilation are clarified. Then, it is shown that the notion of useful thermal
mass is here maladjusted to take into account the thermal inertia of the building and
that it would suit to substitute it an approached calculation of the main time-constant of
the building.
More, the necessity to add a parameter characterizing the rapid dynamics of the air
temperature of a zone is justified. Finally, a characterization of the thermal inertia
based on the three criteria calculation is proposed. An approached value of the timeconstant during the period of night cooling and out of this period and an approached
value of the height of the line associated to the rapid dynamics.
Keywords: time-constant, night cooling ventilation, pilot study, rapid dynamics,
summer refreshment, thermal inertia.
1 Introduction
Taking into account the thermal inertia during the conception of a building remains a
delicate problem. In fact, the thermal inertia is not accessible directly ; alone its
consequences are observable. Its effects can be very different according to the nature
of the outdoor thermal solicitations to which the system is submitted and the position
of observation of the response that we have choseen [ 1,2].
However, at the beginning of a project, the designer has to use simplified criteria so
as to direct its study. The useful thermal mass or an approached value of the main
time-constant are generally used to anticipate effects of the thermal inertia. But, these
global criteria do not take into account the nature of thermal solicitations and appear
often insufficient [ 31.
In previous works [4, 51, we have proposed a simplified characterization, of the
thermal inertia of a one-zone building, that can be used in the pilot project and takes
into account the nature of thermal solicitations. The aim of this paper is to apply these
works to the natural night cooling ventilation of buildings.
2 The summer refreshment by night cooling ventilation
This process of refreshment, consists in increase the air exchange rate during the
unoccupied period of night in order to eliminate the heat that was stored in the building
mass during the day. Blondeau and al [6] have shown that some sites are more
favourable to the implantation of a night cooling ventilation. Indeed, a raised value of
the difference of the average temperature between the day and the night gives a best
efficiency to this technique. These same authors have proven also that the night
ventilation will give best result if the building has relatively important solar
protections. An experiment directed by Sperandio and al [7] has shown that the good
distribution of the fresh air in the building increases the global efficiency of the
system ; this technique will be therefore more efficient for the small volumes.
In order to avoid discomforts in the beginning of the occupied period, this technique
requires adapted scenarios of regulation. More, the utilization of an air conditioning in
complement of this system is generally not desirable, because the air conditioner
modifies the past thermal behaviour of the building therefore annihilates a susceptible
gain part to be realized by the night ventilation.
It is obvious that potentialities of a night cooling ventilation are closely linked to the
thermal inertia of the building, but we often forget that the increased air exchange rate
modifies characteristics of the initial thermal system and therefore its thermal inertia.
In a previous study [8], we have analyzed the role of the air exchange rate on the
dynamics of the temperature of a zone.
In the following paragraph, we will use results of this study, in as part of the
implantation of a night cooling ventilation. We will release some necessary parameters
in order to take into account the thermal inertia of a building in pilot study.
3 The air exchange rate and the thermal inertia
3.1 The thermal inertia of a system
The thermal behaviour of a system in dynamic state is completely defined by the
totality of these eigenvalues and corresponding eigenmnctions that are these intrinsic
characteristics. They have elsewhere allowed to give the first definition of the thermal
inertia [9]. Eigenvalues set the time scale of all dynamic phenomena to which the
thermal system is submitted. The form and the amplitude of the eigenfunctions allow
to analyze the action of each mode on the different components of the system.
Natural modes, that are in infinite number, can be classified summarily in rapid
modes and in slow modes. Among these, some are dominant and others weaks. In fact,
alone some modes sufficed to reconstitute correctly the dynamic state. The slow
dynamic is usually dominated by several modes of large time-constants ; the first one
is generally the most influential. This first mode translates in fact the importance of
the coupling by the air of walls delimiting the zone. The study of natural modes shows
also that there exists always a dominant rapid mode associated to the time-constant rc,
around of which we will be able to amalgamate the rapid dynamics. This mode will be
all the more rapid that the coupling air - walls will be strong.
We show that the notion of useful thermal mass, that characterizes in fact only the
low dynamic, is here maladjusted to take into account the thermal inertia of a building
in the case of the installation of a night cooling ventilation
3.2 Action of the air exchange rate on time-constants
We have shown in [S] that when the air exchange rate increases, all natural times
corresponding to the natural modes decrease and have as limit a local value of a wall
of the system. Especially the global first time-constant tends to the largest timeconstant of walls.
As an example, the table 1 gives the twelve first time-constants of a cell for air
exhange rates of 1, 5 and 10 volumes per hour. The cell has a volume of 200 m3
insulated by the interior with 2 cm of insulator. The external wall has a surface of
150 m2 and the cell contains 60 m2 internal walls of 8 cm of concrete.
Mode number
1
2
3
4
5
6
7
8
9
10
11
12
approached z1
Air exchange rate (volume per hour)
1
5
10
53.61
32.85
25.01
6.38
6.36
6.35
0.801
0.783
0.769
0.562
0.562
0.562
0.206
0.204
0.202
0.168
0.167
0.167
0.092
0.091
0.091
0.077
0.077
0.077
0.054
0.052
0.05 1
z, = 0.046
0.044
0.044
0.044
=
0.040
5
?Jr = 0.035
0.032
0.032
0.03 1
49.87
24.68
15.13
,
Table 1: Evolution of the time-constants (hours) as a function of the air exchange rates
In fact, for higher air exchange rates, the low dynamic will be marked by a disorderly
time evolution of the walls. Indeed, the air do not insures its role of coupling between
walls. These latters will be therefore thermically dissociated. The notion of useful
thermal mass, that supposes implicitly a good coupling air - walls, appears in this case
maladjusted to translate modifications of the thermal inertia as a function of the air
exchange rate. Van der Mass and al [lo] had already noticed it on experimental tests
realized on a building of the Leso in Lausanne.
An approached calculation of the time-constant of the dominant slow mode,
integrating variations of the air exchange rate, would allow a best taken into account of
this phenomenon as it is shown in the last line of the table 1. It would suit here to
undertake this calculation during the period of night ventilation and out of this period.
1
air exchange rate = 1 volume/hour
017
8
%
Time-constant (hours)
099
air exchange rate = 5 volumes/hour
tn
-s
07
E
3
05
'
1
Time-constant (hours)
air exchange rate = 10 volumes/hour
3
z
E
“0
097
1
05
I
k B
0,
0
d
Time-constant (hours)
Figure 1: Response spectrums of the indoor air temperature of a cell,
with interior insulation, as a function of the air exchange rate
3.3 Action of the air exchange rate on the temperature response of a zone
During the implantation of a night cooling ventilation, it is important to understand the
air temperature response of a zone under the action of the outdoor temperature
solicitation.
At this end, the figure 1 represents the spectrums of the air temperature response of a
zone for different air exchange rates. We can notice that the importance of the first
mode is less and less pronounced when the air exchange rate increases contrary to the
dominant rapid mode z, (see § 3.1) that have an action more important on the air
temperature response of the zone. For a rate of 1 volume per hour, the rapid response
represents 4 % of the rise to the steady state, on the other hand it happens to 35 % for a
rate of 10 volumes per hour.
In period of night ventilation, the rapid dynamic represents an important part of the
air temperature response of a zone. It proves therefore necessary to attach to the
characterization of the thermal inertia a parameter that takes into account the rapid
response of the temperature of zone in period of night ventilation.
4 Simplified criteria taken into account the thermal inertia
According to the conclusions of the previous paragraph, we propose to characterize the
thermal inertia by the three following parameters :
z approached value of the time-constant characterizing the low dynamic during the
occupied period of no-ventilation.
LC~ approached value of the time-constant characterizing the low dynamic during the
unoccupied period of night ventilation.
r the part share taken by the rapid dynamic in the air temperature response of the zone
during the unoccupied period of night ventilation.
4.1 Calculation of approached time-constants
This calculation can be carried out by using a property of temperature fields
demonstrated by Sicart [9]. When a temperature field has the form of a natural mode,
the ratio of the energy contained in the system by the outgoing flow is equal to the
time-constant associated to this mode. This property allows an approached calculation
of the time-constant of the dominant mode that has a close form to the one of the
steady state associated to the convective power solicitation of heating. The
methodology of this calculation is detailed in [ 1,4].
4.2 Calculation of the parameter r
We can use as indicatory r an approached value of the height of the line (i.e the
contribution of the natural mode), associated to the rapid dynamics, of the spectrum of
the air temperature response of a zone to an outdoor temperature solicitation [4].
This criterion r can be calculated by noticing that the part taken by the rapid
dynamics in the air temperature response of a zone can be approached by the ratio of
the heat flow brought to the air by the flow exchanged with walls at the outdoor air
temperature.
Ca.Qr
‘= Ca.Qr+Kv.Sv+~hi,A,
k
with
Ca the calorific capacity of the air (Wh / m30C)
Q7 the air exchange rate (m3/h)
Kt the the heat transmission coefficient of glazings (W/m2”C)
SV the surface of glazings (m2)
.
hz xc the superficial exchange coefficient of the wall (k> (W/m2”C)
A k the surface of the wall (k> (m2)
The thermal resistance of the interior insulating is integrated in the coefficient hik .
For the example quoted in the paragraph 3.2, z = 49.9 h, z, = 15.1 h and r = 0.365
5 Conclusion
We have shown, in this paper that the notion of useful thermal mass is insufficient to
take into account the thermal inertia, in pilot study, in the case of the presence of a
night cooling ventilation. It would suit to substitute it the approached calculation of the
dominant time-constant of the low dynamic that while remaining simple, presents the
advantage to integrate dynamic characteristic variations of the thermal system during
the variation of the air exchange rate.
More, it is indispensable to add there a parameter characterizing the rapid dynamics
during the period of night ventilation, what can be made simply by an evaluation of the
height of the line associated to the dominant rapid mode.
Finally, to be really usable, this method will have to be linked to weather report
conditions of the site and to the solar protections rate.
References
1 . Lefebvre, G. (1989) Caracterisation de l’inertie thermique d’un batiment par analyse
modale. Revue generule de thermique, No. 332-333. pp. 501-512.
2 . Neirac, F.P. (1989) Approche theorique et experimentale des modeles reduits du
comportement thermique des batiments. These de doctorat, Ecole des Mines de
Paris.
3 . Depecker, P., Brau, J. et Rousseau, S. (1982) Pertinence des modeles simplifies
pour la description du comportement des batiments en regime thermique variable.
Annales de l!Lnstitut Technique du Bitiment et des Travaux Publics, No. 404. pp.
139-150.
4 . Douzane, 0. (1995) Contribution a la prevision du comportement thermique
dynamique d’un batiment monozone a l’aide de criteres approches ; applications
aux methodes de dimensionnement. These de doctorat, Universite de Picardie Jules
Verne.
5 . Roucoult, J.M., Douzane, 0. et Langlet, T. (1994) Caracterisation simplifiee de
l’inertie thermique d’un batiment monozone. Revue G&&ale de Thermique,
no 390 -391. pp.406-413.
6 . Blondeau, P. , Sperandio, M. et Sandu, L. (1994) Potentialites de la ventilation
nocturne pour le rafrailchissement des batiments du sud de 1’Europe. European
Conference on energy performance and indoor climate in buildings, Lyon, tome 3,
pp. 854-859.
7 . Sperandio, M., Blondeau, P. et Allard, F. (1995) Experience de surventilation
nocturne pour l’amelioration des conditions de confort en etc. Rencontre
Association Universitaire de Genie Civil, Nantes, pp. 183-l 87.
8 . Roucoult, J-M., Douzane, 0. et Langlet, T. (1996) Analyse du comportement en
regime dynamique d’un Batiment monozone soumis a une surventilation de nuit.
Colloque de la Societe Francaise des Thermiciens, Toulouse, pp.336-342.
9 . Sicart, J. (1984) Analyse modale appliquee a la thermique, fondements analytiques
et application au batiment. These de docteur-ingenieur, Paris VI.
10. Van der mass, J., Florentzos, F., Rodriguez, J.A. et Jaboyedoff, P. (1994) Passive
cooling by night ventilation. European Conference on energy performance and
indoor climate in buildings, Lyon, Tome 3. pp. 646-65 1.