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2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and
Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
Potential of passive design strategies using the free-running temperature
L. Rosales, M. E. Hobaica
Universidad Central de Venezuela, Venezuela
C. Ghiaus, F. Allard
Universiy of La Rochelle, France
ABSTRACT
The main decisions concerning thermal comfort and energy efficiency of buildings are made in the early stages
of the architectural design. Common practice relies on
general indications based on climate analysis and heuristic rules about orientation, thermal mass and so forth.
Such an approach usually demands assessment by simulation, deriving in an empirical process of “check and
see”. Another approach would be to think of the building
as an adaptive system with a potential to ensure thermal
comfort. By decoupling it into its main elements (i.e.,
climate, comfort criteria and the building itself) and
considering the building as a result of design strategies
producing specific energy fluxes, it becomes possible
to assess the potential of each strategy in a particular
context. The starting point is the free-running temperature of a sketch of the building. The frequency distribution of degree-time is obtained from this temperature
and the one ensuing from a design strategy. With this
result it is possible to assess the potential of each strategy and their combinations. An example is shown for
the tropical climate of Venezuela in the case of using
natural ventilation, shading, solar reflection, insulation
and thermal mass.
geometry and thermal mass. Dynamic simulation needs
as inputs the geometry of the building, which makes it
more adapted for the final stages. The results are usually
given in time series and total energy load or consumption. Energy performance can also be assessed using
simpler steady state methods based on degree-days or
temperature bins, appropriate if the building operation and the efficiency of HVAC systems are constant
(ASHRAE, 2001). It is common for architects to start
with simplified methods for the primary design decisions and confirm them later by simulation or steady
state methods, a practice which leads inevitably to an
empirical process of “check and see”.
Another approach would be to consider the building as
a system that should have the potential to ensure indoor
comfort. The indoor temperature is the result of the balance of energy fluxes and accumulation. Arguably, the
main gains are solar and internal, the main losses are
by advection and conduction, and the energy storage
takes place in the thermal mass. Design strategies and
architectural elements modify the scale of each process and therefore the outcome of the balance. Putting
the problem this way permits assessing the potential of
such elements and strategies.
1. INTRODUCTION
2. INITIAL SKETCH AND FREE-RUNNING TEMPERATURE
The existing methods for finding out the most suitable
architectural strategies to achieve thermal comfort and
minimize energy consumption can be classified in simplified guidelines, expert advice based on experience
and building simulation based decisions (deWit and Augenbroe, 2002). Simplified guidelines rely on empirical studies or on climate analysis collated with human
comfort requirements and common design strategies.
Among these are the building bioclimatic charts, which
are widely used (Olgyay, 1963; Givoni, 1967; Szokolay,
1986). Like expert advice, theses methods are useful
mainly in the first stages of design, when the most important decisions regarding comfort and energy saving
are made. Although the results are not very accurate,
they give general indications for building orientation,
The initial sketch is the building at the first stage of
evaluation, i.e. the reference from which the effect of
the design strategies will be assessed. It can be a primal
draft or a building having already some strategies and
expected to be evaluated for additional ones. It can also
be an existing building meant for refurbishing.
The free-running temperature is defined as the temperature of a free-running building without indoor climate
controls, i.e. without heating, cooling and with the minimal air-changes per hour (Ghiaus, 2003). The free-running temperature can be obtained by simulation or, in
the case of an existing building, by measurements or by
using its relation with the load curve used in ASHRAE
energy estimation methods (Ghiaus, 2006). The difference between the free-running temperature and the out-
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2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and
Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
door temperature may be considered independent from
outdoor temperature, since it is basically the ratio between the heat gains and the overall heat transfer coefficient. Within intervals with no major changes in solar
angles, the dispersion due to random disturbances like
occupancy, ventilation and solar gains can be set aside,
especially when occupancy is constant and the building is airtight. With these assumptions, it may be accepted that, for a given month of the year, the difference
between the free-running temperature and the outdoor
temperature is a function of the hour of the day (Figure
1). As a consequence, the free-running temperature can
be used instead of balance point temperature in degreedays or bin methods (Ghiaus and Allard, 2006).
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ited by a lower acceptable temperature, Tcl, and an upper
acceptable temperature, Tcu.
4. DOMAINS
Setting up a passive design strategy shifts the free-running temperature, Tfr, to a new indoor temperature,
Tst. At a specific hour, this shift can be negative (Tst <
Tfr) or positive (Tst > Tfr). In the first case the strategy
cools down the building (passive cooling) and in the
second, it heats it up (passive heating). Figure 2 shows
the domains for mechanical cooling, comfort and heating when Tst < Tfr. These domains can be expressed by
binary value functions, as follows.
Before setting up the passive strategy, the condition for
mechanical cooling was:
(1)
For this condition, the degree-hour for mechanical cooling is:
(2)
In the case of a passive design strategy, the condition for
mechanical cooling becomes:
(3)
For this new condition, the degree-hour needed to be
balanced by mechanical cooling is:
Figure 1. Visualization of the difference between indoor and outdoor temperature in a free-running building.
3. COMFORT CRITERIA
ASHRAE 55 and ISO 7730 standards specify the temperature range for which people having low activity feel
the environment thermally adequate. It is accepted that
these standards perform properly for moderate environments and HVAC controlled buildings. However field
studies have shown that people adapt to the environment and that comfort in naturally ventilated buildings
has larger seasonal ranges. As a result, linear correlations linking an optimal comfort temperature to mean
outdoor temperature have been proposed (de Dear et al.,
1997; Brager and de Dear, 1998; Nicol and Humphreys,
2002). Figure 2 shows a correlation proposed by Brager
and de Dear (1998). A neutral zone is defined by a range
which extends both sides of the optimum temperature,
corresponding to 90% acceptability. This zone is delim-
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(4)
The difference between Eqs. (2) and (4) gives the degree-hour that the strategy saves for mechanical cooling.
In contrast, the strategy can be inappropriate for low
outdoor temperatures, when heating may become necessary. It follows that it would be suitable to control it,
in order to move back toward the free-running building
condition.
The condition for which the free-running building must
be heated is:
(5)
And the degree-hour for heating the free-running building is:
(6)
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2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and
Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
The difference between Eqs. (8) and (6) gives the heating saved by controlling the passive strategy. This control may be achieved by behavioural or technological
means and depends on the possibilities of the strategy.
For an extended number of days (e.g., 365), the sum
of the degree-hour given by Eqs. (2), (4), (6) and (8) in
bins of outdoor temperature gives the degree-hour distribution as a function of outdoor temperature. Putted in
a common form, this would be:
Figure 2. Domains for mechanical cooling, comfort and heating
for the free-running building, before and after the use of a passive
design strategy (Tst < Tfr).
The condition for which the building after using the
passive strategy must be heated is:
(9)
where
DH ( j ): is the total degree-hour for bin j of outdoor
temperature
ΔT ( i, j ): is the temperature difference for day i and bin j
δ: is the condition being evaluated by Eqs. (2), (4), (6)
and (8)
The integral of the degree-hour distributions obtained
for each condition by applying Eq. (9) gives the total
degree-hours for the hour being analyzed and through
the time interval.
An analogous reasoning can be made when the strategy
heats up the building (Tst > Tfr).
5. EXAMPLE
And the degree-hour for heating is:
(7)
(8)
To illustrate the method, a simple house was simulated. Two different locations were chosen: Caracas and
Maracaibo, which are the most populated cities of Venezuela. Both cities have the same latitude (10.5º N) and
Table 1. Annual degree-hours at 3 h, 9 h, 15 h and 21 h for two locations of Venezuela of same latitude (10.5ºN) and different altitudes:
Caracas: 950 m ; Maracaibo: 60 m.
Free-running
Caracas
3h
9h
15 h
21 h
Maracaibo
3h
9h
15 h
21 h
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Shading Natural ventilation
Thermal mass
Solar reflection Insulation
Cooling
0
0
0
0
0
7
Heating
-201
-560
-932
-82
-482
0
Cooling
1
0
0
0
0
420
Heating
-57
-233
-315
-40
-180
0
Cooling
1021
51
161
0
104
1399
Heating
0
-3
-9
-8
0
0
Cooling
0
0
0
0
0
19
Heating
-55
-240
-399
-26
-186
0
Cooling
32
0
0
111
1
1112
Heating
0
0
-3
0
0
0
Cooling
502
132
161
381
210
2087
Heating
0
0
-1
0
0
0
Cooling
2223
1003
1274
453
1210
2590
Heating
0
0
0
0
0
0
Cooling
152
12
3
373
21
1254
Heating
0
0
0
0
0
0
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2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and
Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
their seasonal variations are small. However, Caracas
altitude is 950 m and has a warm humid/moderate climate (average temperature 22.5 ºC) while Maracaibo
altitude is 60 m and has a warm-hot humid climate (average temperature 27.5 ºC). The house was conceived
concerning basic geometry, considering that walls and
roof are conductive with dark finishes and that the windows are single glazed and are closed (i.e., the house
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is tight). The internal loads are constant and consist
of home equipment and 5 occupants. It is intended to
analyze the potential of five design strategies in broad
terms: shading, natural ventilation, thermal mass, insulation and solar reflection. The hypotheses were: a) the
best shading corresponds to a building receiving only
diffuse solar radiation, b) the best possible ventilation
corresponds to a relatively high ventilation rate of 50
Figure 3. Degree-hour distribution at 15 h and 21 h for (a) Caracas and (b) Maracaibo.
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2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and
Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece
ach, c) the higher thermal mass consists of walls and roof
having a 10 h time lag, d) the optimal solar reflection
consists of highly reflective walls, roof and windows,
and e) the best insulation corresponds to non-conductive light-weigh walls and roof. It is important to note
that these hypotheses can be argued or that the evaluation can target other types of strategies concerning specific elements, e.g., broad or particularly orientated lees.
Figure 3 shows the degree-hour distributions at 15 h and
21 h in both cities throughout 365 days for the free-running building and for the same building having settled
the design strategies separately. Table 1 shows the results of integrating these curves along with those obtained for 3 h and 9 h.
The results give a primary indication about which design strategies may have the priority for achieving comfort and minimizing energy consumption. For example,
in Caracas, cooling can be completely achieved at 15
h by maximizing thermal mass, with the benefit that it
brings low heating requirements at 21 and 3 h. If maximizing thermal mass is not practical, optimizing shading or natural ventilation appear to be good solutions,
particularly if behavioural or technological adaptation
is foreseen. As observed, at 15 h, natural ventilation
fully covers the cooling requirements for at least half
a year. It has also the advantage that it is easy to adjust
at night by simply closing windows. On the other hand,
in Maracaibo, there is no need for heating, no matter
the strategy used. Maximizing thermal mass appears to
produce lower cooling requirements at 15 h, but they
become higher at 9 h, 21 h and 3 h. Shading and natural
ventilation emerge as better solutions, as their potential
for cooling drops only in the afternoon. It also comes
out that maximizing insulation can be very unfavourable in both locations. However, it is well known that
insulation is a good strategy in warm sunny climates
if applied partially or in combination with other strategies. Indeed, the potential of any partial application of
theses strategies or any combination of them can be assessed using the same approach.
6. CONCLUSIONS
Assessing the potential of design strategies depends on
building thermal behaviour, comfort criteria and climate. In the early stages of design, the interest consists
in deciding on correct solutions founded on broad data.
Usually this is done by simplified guidelines, which
give general indications for basic strategies such as orientation, ventilation or thermal mass. A more specific
approach is to use the concept of free-running temperature, defined as the temperature of a free-running building without indoor climate controls. If air-change rate
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is minimal and heat gains are moderately stable, the
difference between indoor and outdoor temperature can
be considered independent from outdoor temperature. It
follows that having an initial draft of the building any
further change on its thermal characteristics will produce at a same hour a constant shift of the indoor temperature. The potential of any design strategy can then
be assed by the frequency distribution of the differences
between the indoor temperature and the comfort limits
as a function of outdoor temperature. This method gives
much more information than the common simplified
methods. The procedure can be used at any stage of the
design process with the condition that at least a primary
sketch of the building is available. It can also be used
for assessing the potential of specific architectural elements or combinations of design strategies.
ACKNOWLEDGMENT
This study was supported by France-Venezuela Programme Ecos-Nord (Project Ispaven-9269).
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