Full text of DOI:10.1016/S0038-092X(00)00069-4

Solar Energy Vol. 69, No. 4, pp. 351–356, 2000
2000 Elsevier Science Ltd
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THERMODYNAMIC PARAMETERS FOR ENERGY SUSTAINABILITY OF
URBAN AREAS
†
CARLA BALOCCO and GIUSEPPE GRAZZINI
Dipartimento di Energetica ‘S. Stecco’, Via S. Marta 3, Firenze, Italy
Received 31 August 1999; revised version accepted 28 February 2000
Communicated by ANNE GRETE HESTNES
Abstract—This paper presents some thermodynamic indicators useful for energy planning of urban areas, and
for defining the scenarios of integrated low environmental impact energy strategies and actions in an urban
area. It is based on the results of the energy balance of a reference volume obtained by the Geographical
Information System (GIS) techniques applied to the built-up area studied. The defined parameters allow
evaluation of the sustainability of energy use in an urban area and its areal distribution in different building
meshes superimposed on the area considered, using entropy.
reserved.
 2000 Elsevier Science Ltd. All rights
1
. INTRODUCTION
ered. The urban canopy presents a large spatial
variability of the parameters that determine wind
flow and turbulent field structure in the lowest
layers of the atmosphere, different roughness and
heat fluxes and surface temperatures. These as-
pects together greatly influence the atmospheric
diffusion properties, and usually generate local
wind motions such as quarter breezes. The struc-
ture of wind and turbulence field influences the
behaviour of the pollutants emitted and their
diffusion velocity within the urban area. Urban
morphology and topology has significant effects
on climate on a local scale. It influences local
solar irradiation, air and surface temperature, air
humidity and wind speed. High buildings, paved
and cemented areas create an urban canyon,
where the incoming heat energy is trapped and
turbulence zones are generated. Add to these
effects the exhaust from cars, factories, thermal or
refrigeration plants, then urban heat islands need
to be considered.
There are ample possibilities for planning and
design techniques to reduce energy demand and
increase reliance on renewable sources of energy
(
Baeiswyl et al., 1996; Goodchild, 1994). The
basic planning guidelines to ensure solar gains
and natural ventilation are known and often
discussed (Okeil, 1993; Wright, 1996; Jones et al.,
1
997; Shabbir, 1993):
•
consider the connection between the energy
needs of buildings and the local climate (mi-
cro-urban climate);
establish green corridors in urban areas;
produce desirable and energy efficient mi-
croclimate changes;
•
•
•
•
create shelter from hot or cold and strong
winds for particular conditions;
use local breezes or warm/fresh places for the
comfort of the building’s occupants;
build climate responsive buildings;
consolidate the existing city area to make the
best use of land and resources.
•
•
Some recent research defines procedures and
computer models to quantify and predict the
environmental impact of cities (Jones et al.,
These guidelines do not allow a quantitative 1997), while the SATURN Project (AA .V V.,
evaluation in terms of sustainability. Throughout 1998) takes into account different dispersion
this article we use the by now common term models to describe transport velocity and deposi-
‘sustainability’ to mean low environmental impact tion rates of atmospheric pollutants. In some
energy durability. On the other hand, some im- recent work, different wind dynamic models are
portant and interrelated aspects need to be consid- used to calculate urban area roughness (Deaves,
1
981; Melbourne, 1978). In one of these papers,
†
land use maps and land use data from optical
remote sensing systems are used to map rough-
ness length (AA .V V., 1998).
Author to whom correspondence should be addressed. Tel.:
39-0554-796-242; fax: 139-0554-796-342; e-mail:
ggrazzini@ing.unifi.it
1
3
51

3
52
C. Balocco and G. Grazzini
In a recent article, a quality index is used to evaluated for each mesh, by the sum of the
quantify sustainability. The index of sustainability maximum inside building heights, plus 10 m. The
is calculated by a social survey for different last value is the standard height for measuring
typologies of rented houses. It is a subjective wind speed in urban areas and is usually quan-
parameter that takes into account the urban form tified by a ‘roughness profile’, which depends on
in terms of distances between buildings, numbers the height of the frontal area divided by roughness
and kinds of streets inside the built-up area, height z0 (BRE, 1985; Yamamura and Kondo,
different building uses and quantities of services 1993).
and facilities. So it is used to provide a predictive
So the mean wind speed is evaluated as fol-
tool for development of the rented house market lows:
in urban areas (Li and Li, 1996). Sustainability so
v 5 (2u*/0.4) log (z/z )
(1)
z
10
0
defined refers to a socioeconomic approach as
reported in the latest OECD document (OECD,
the ratio (2 u*/0.4) represents the reference wind
speed, where u* is the friction velocity (Deaves,
1
998). Here some indicators, such as CO pro-
2
duction, are introduced to evaluate ‘environmental
sustainability’, but they are then connected with
the GDP. The index so obtained has a poor link
with the physical reality of the environment.
An exergetic index that can be used to gain an
idea of the energy sustainability of urban areas
was defined by Balocco et al. (1998). Starting
from statistical data, the ratio between exergy
needs and effective exergy use of the buildings in
the town of Pisa was calculated. Maps of required
energy and exergy, that allow programming
energy saving actions, were produced.
Taking into account that the analysis is ex-
tremely complex, thermodynamics can provide a
tool for approaching it. The aim of this paper is to
provide some indicators useful for measuring the
energy sustainability of urban areas and defining
planning criteria by using GIS. Real sustainability
can only be obtained if total irreversible entropy
flux, due to human activities, is lower than the
neghentropy flux from the sun. Using energy and
also renewable energy, an irreversible entropy
flux is always produced. It has to be reduced as
much as possible to reach sustainability, in com-
parison with the flux due to solar energy degra-
dation, when absorbed by the earth.
1
981).
The steady state energy balance can be applied
to the control volume, considered as an open
system. Solar radiation and wind are taken into
account in the energy balance equation. To evalu-
ate the inlet and outlet mass flows, simplified
cross sections of the control volume can be
calculated by assuming two equal lateral sections
of the control volume. As a consequence, wind
speed is calculated by its components on the mesh
direction. The mass flow is directly connected to
urban porosity. Then to evaluate the mass flow
balance, the built volume of the cell can be
calculated as a function of the average weighted
height of buildings and as a function of the
porosity factor. The porosity factor is expressed
by control volume V and the average volume of
the buildings inside Vbm:
r
p 5 1 2 (Vbm /V ).
(2)
r
The porosity factor can also be connected to
urban roughness, and can provide a global indica-
tion of the different canopy layers in the built-up
area studied (BRE, 1985). If roughness is calcu-
lated by the standard deviation of building vol-
ume, then:
2
1 / 2
rg 5 [
O
(V 2V ) /(M 2 1)]
b bm
2
. THE METHOD
2
1 / 2
Generally the outline of an urban area is
complex and variable (Gandemer, 1981; Hosker,
5 [
O
(V
b
2V
r
(1 2 p)) /(M 2 1)]
.
(3)
By applying the steady state energy balance to the
control volume, the equation is:
1
995; Wise, 1971). In this case GIS tools are
useful for grid spatial analysis, taking into account
different urban energy typologies and forms. In
energy analysis it is usual to define a control
(
EHs 1 E 1 E ) 5 (E 1 E 1 E )
(4)
g
J
in
d
g
J out
volume to study different processes. Consequent- where EHs is the energy due to the global solar
ly, a control volume, concerning the grid mesh radiation on the mesh, E is the global energy due
g
and including buildings and open spaces, will be to mass flows, E is the energy due to air enthalpy
J
quantified. By using grid dimensions control and E represents the energy losses of buildings.
d
volume is obtained by the area of the mesh
To evaluate these different terms, some param-
multiplied by the reference height: the latter is eters, associated with the climatic conditions of

Thermodynamic parameters for energy sustainability of urban areas
353
2
/ 3
the urban area and others closely connected to the N1 5 Ame /V b
energy losses of buildings, can be used.
0.5
0.5
me
0.5
N2 5 Ame K(T 2 T )/H 5 A KDT/Hs
i
a
s
The first are solar radiation H , air enthalpy J,
(6)
s
0.5
N3 5 Ame K(T 2 T )/H 5 A KDT9/Hs
air mass flow G and temperature difference Dt,
between the average external air temperature and
the building internal one; the second are building
energy coefficient K and global building volume
a
c
s
me
N4 5 GJ/AmeHs
These dimensionless numbers are strictly con-
nected to the building form of the area, and to its
local climate. N1 concerns an urban parameter
because it is close to the built density as utilised
from an urban planning point of view. N2,
connected to the system consumption of energy,
expresses the ratio between the energy losses of
the built area to the environment and energy solar
gain. N3 is also connected to system energy
consumption, but it expresses the energy losses of
the built area to the sky more accurately. N4
concerns a local climatic parameter.
V .
b
Energy coefficient K is the ratio between the
total energy needs of buildings, calculated for the
heating period, the total built volume, and the
difference between the standard temperature in
the buildings (208C) and the average value of the
external air temperature. Local climatic parame-
ters and albedo values are linked to mesh area
building density. Both the temperature differ-
ences, between internal temperature and ambient
temperature t , and between internal temperature
a
The equation between all four numbers can be
expressed as the first element of a power series:
and sky temperature t , are used, because the
c
former concern the thermal dispersions of build-
ings and the latter the energy losses of the urban
system. Air enthalpy J can be easily calculated
b
c
d
N1 5 aN2 N3 N4 .
(7)
using the local climatic data, as a function of Referring to the CEN method (CEN/TC89,
specific air humidity and water vapour saturated 1992), sky temperature can be calculated by
pressure. To estimate air enthalpy and saturated applying a constant difference to the ambient
pressure, the definitions and correlation function temperature. Doing this, the number of indepen-
given by ASHRAE (1997) and expressed by the dent variables decrease and a second equation
average air temperature, can be directly used.
between only three dimensionless numbers can be
The air mass flow, to be used in the mass obtained:
balance of the control volume, is connected to the
porosity of the mesh, and calculated as:
f
g
N1 5 eN2 N4 .
(8)
The connection between energy urban form and
(5) the climatic characteristics of the area gives a new
1
/ 2
G 5 vr(hmeL 1 (Vbhbw
)
)
dimensionless number N29. By referring to the
where r is the air density, the product (hmeL) is expression of the porosity factor, and by using the
the section area of the reference volume by using ratio between the total area of the urban cell and
the basic mesh dimensions; V is the total inside the inside total building volume,
b
built volume of the mesh; hbw is the weighted
Ame /V 5 h /(1 2 p)
(9)
b
me
height of buildings of the mesh. Wind speed v is
the component of the local wind (e.g. the standard
year climatic data) related to the mesh direction
and perpendicular to the section area.
a different expression of N2 can be obtained:
0
.5
N29 5V
KDT(hme
A
me )/[H
A
me(1 2 p)].
(10)
b
s
Solar radiation, air mass flow, air enthalpy, the
difference between the internal ambient tempera-
This last parameter N29 expresses the ratio
ture of the buildings and the external ambient between the energy losses of the built area and the
temperature, the difference between the ambient global solar radiation incoming to the mesh area,
temperature and the sky, energy coefficient K, as taking into account its porosity. It indicates the
defined before, the total mesh area Ame and the energy sustainability of an urban area from the
total built volume of mesh V , are considered as first thermodynamic law point of view, giving an
b
independent variables. Dimensional analysis can idea of the relationship between anthropogenic
be used to search for a function connecting these and solar energy flux.
variables.
Using the second law of thermodynamics, an
Buckingam’s theorem applied to these eight entropy indicator must be given. It can be ex-
independent parameters (AA .V V., 1963), gives pressed by the ratio between the entropy variation
four dimensionless numbers:
due to the total energy losses of buildings DS and
b

3
54
C. Balocco and G. Grazzini
Table 1. Evaluated coefficient and exponent values and errors
the entropy variation due to the solar energy gain
Eq. (7)
a
b
c
d
of the mesh area DS that naturally exists in the
s
0
.0568
1.0003
0.0054
3.6154
0.0869
23.6371
0.1179
environment. If the two entropy variations, DSb
Error
0.0042
and DS , are calculated as:
s
2
x 50.0914
Eq. (8)
e
f
g
DS 5 Q(1/T 2 1/T )
(11)
(12)
b
a
i
7
.9283
0.2575
0.0216
0.0312
0.0566
0.0042
Error
DS 5 ´H A (1/T 2 1/T )
2
s
s
me
a
s
x 50.0935
where ´ is the adsorbent coefficient of the ground,
which often can be taken as 0.8, the ratio is:
ety 5 DS /DS .
(13)
b
s
for all the cells. Seasonal values of the parameters
of the heating period were used to evaluate the
dimensionless numbers. But average monthly
values could also have been used. To determine
the sky temperature, the value directly connected
to ambient temperature variations was utilised
Actually, it is necessary to consider how the
internal building temperature is maintained con-
stant at 208C, by using different energy sources.
Then it is important to evaluate the entropy
generated by building heating if, for example, it
comes from fossil fuels. Referring to a burning
(
CEN/TC89, 1992).
Average external air temperature values, total
temperature T of 1000 K and taking into account
F
solar radiation values and also local wind speed
values, are given by the climatic data of a
standard year for Florence (Alabisio and Sidri,
an average value h for the boiler’s efficiency, the
entropy variation due to heating equipment be-
comes:
1
985).
DS
9
5 Q(1/T 2 1/T )/h
(14)
b
a
F
The values of the coefficients in Eqs. (7) and
2
(
8) and the corresponding error value, and x are
and the parameter is:
ety 5 DS /DSs.
shown in Table 1. Table 2 gives the average value
9
b
(15) and the corresponding standard deviation s, for
F
all the 81 meshes.
Some parameters for seven building meshes are
shown in Table 3.
3
. AN APPLICATION OF THE METHOD
The above presented method has been applied
to one urban area of the city of Florence, Italy.
When urban planning has to start in order to
modify the energy efficiency and sustainability of
The urban area studied belongs to a part of the urban areas, the priority of different interventions
city whose building data (1082 buildings) have has to be fixed. This is necessary to evaluate the
been placed in an Arc/Info archive. Each building effectiveness of different actions. Dimensionless
parameter such as typology, technical and build- parameter N29, which refers to the first law of
ing characteristics (building materials), age, vol- thermodynamics, gives an ordered list of the
ume, form factor (expressed by the ratio between areas, and the difference between meshes is not
the global volume and the total dispersing area) high. As a matter of fact, the ratio between the
and height are included in the data base. The minimum value of N29 and its maximum value is
average monthly and seasonal heating needs, 56%. When considering the two parameters
calculated by a statistical method (Grazzini and gained by applying the second law of thermo-
Balocco, 1997), were also included in the data dynamics, ety and ety , the order of the areas
F
base.
changes and the differences increase. The ratio
A grid structure of regular square meshes was between the minimum value of ety and its maxi-
overlain by the Arc/Info method. Grid resolution
and consequently mesh dimension, in terms of
Table 2. The statistic values of parameters for all the 81
statistical similarity, efficiency and representativi- building meshes; h50.8
ty (Kalton, 1977) were tested. The area was Parameter
divided up into 81 regular square cells each with a
Average value
s
DSb (J/K)
(J/K)
3.71
86.61
1.79
2
00 m side (Balocco and Mercanti, 1998). Then a DS
9
0.51 23
DS (J/K)
137.40
0.0270
0.6303
58,065
0.929
1.53310
s
control volume of the grid mesh was used to
calculate the steady state energy balance.
The urban area distribution of the four dimen-
sionless numbers was studied by calculating them
ety
etyF
N29
p
0.01
0.30
88.13
0.05

Thermodynamic parameters for energy sustainability of urban areas
Table 3. The values of parameters for seven building meshes; h50.8
355
3
p
V (m )
N1
N2
N4
N29
ety
etyF
1
2
3
4
5
6
7
0.983
0.995
0.857
0.992
0.806
0.934
0.978
22,905
5651
49.59
126.08
11.72
78.25
9.56
3675
4124
5814
3665
6500
5924
5721
3026
3085
2621
3669
2421
2755
2961
36,300
40,736
57,427
36,197
64,199
58,513
56,506
0.0031
0.0016
0.0432
0.0021
0.0361
0.0213
0.0088
0.0702
0.0382
0.8410
0.0476
0.8035
0.4968
0.2062
199,313
11,556
270,623
91,840
30,668
19.65
40.83
mum value is 3.7%. The same ratio calculated for calculated by a simplified method disregarding
ety is 4.5%. those terms connected to the entropy variation of
F
With these parameters it is easy to see where humid air, of mass flow rate and then of wind
intervention is necessary to start reducing anth- boundary layer. The sustainability indicator given
ropic impact.
by these two ratios provides a general criterion for
These parameters allow comparison of different evaluating the irreversibilities produced in urban
urban areas by a thermodynamic approach and areas.
then, deduction of an indicator of their energy
sustainability.
They are related to energy consumption and
environmental impact, but they could also be
applied by using entropy analysis from chemistry
and information theory. When including these
analyses they could provide a tool for studying
4
. CONCLUSIONS
The thermodynamic parameters that were given the environmental performance and sustainability
can be directly utilised to compare different of economic development obtained with different
energy uses of existing urban areas. They allow to energy efficiencies and urban policy decisions.
indicate parts of building meshes where it is
possible to intervene, in order to reduce the
ambient impact due to energy losses of buildings.
NOMENCLATURE
These parameters can also be used to define the
2
A
total area of urban mesh (m )
built area or green place extension, and the
facility zones for urban energy planning. These
thermodynamic parameters allow an evaluation of
the quality of an urban area, by referring to the
analysis made on the existing situation.
The variability and spatial distribution of the
N29 parameter gives some indications for reduc-
ing energy consumption and developing efficient
energy use in the built up area.
The two thermodynamic parameters, expressed
using the second law are not only closely con-
nected to energy quality, but also, directly useful
to design different sustainable urban energy
a, b, c, d, e, f, g related to Eqs. (7) and (8)
E
energy (J)
ety
G
h
parameters defined by Eqs. (13) and (15)
air mass flow (kg/s)
height (m)
2
H
J
global solar radiation (J/m )
air enthalpy (J/Kg)
K
building energy coefficient due to global
energy need (J/m K)
3
L
mesh dimension (m)
total number of elements of the population
from 1 to 4, dimensionless numbers
porosity factor
M
N
p
Q
rg
t
heat loss (J)
roughness
temperature (8C)
T
temperature (K)
scenarios. If the energy consumption limits of an u*
friction velocity
average wind speed (m/s)
height of an element (m)
v
urban zone are fixed, as for example in the case of
z
current legal limits, it is possible to study and
verify the energy sustainability of different pro- Greek symbols
jects. By varying the entropy generation due to
some urban and plant designs, the defined ratio
´
absorption coefficient of the ground
heating plant efficiency
h
r
3
air density (kg/m )
2
ety, and in particular ety of entropy generation
s
standard deviation5[o(y 2 Y ) /
F
ie
m
1
/ 2
due to the fossil fuel used by particular building
heating plants, give a measure of energy sus-
tainability. They give an idea of the impact in
terms of entropy produced by using fossil fuels or
different energy sources in comparison with solar
entropy flux.
(M21)]
2
(1 / 2)
x
error5(o(yie 2 Y ) )
m
Subscripts
0
a
b
c
d
referred to roughness
external ambient or referred to dry air
building
sky
On the other hand, entropy variation was
total energy dispersion

3
56
C. Balocco and G. Grazzini
Comit e´ Europ e´ en de Normalisation (1992) DRAFT prEN 832.
F
referred to burning temperature
g
referred to global mass flow
In CEN/TC89, Agosto.
i
internal ambient of building
element
inlet
mean
mesh
outlet
sun
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in
m
me
out
s
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w
referred to enthalpy
weighted
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