Economic and Exergy Analysis of Alternative Plants for a Zero Carbon Building Complex
Transcript of Economic and Exergy Analysis of Alternative Plants for a Zero Carbon Building Complex
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Energy and Buildings 43 (2011) 787–795
Contents lists available at ScienceDirect
Energy and Buildings
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n b u i l d
Economic and exergy analysis of alternative plants for a zero carbonbuilding complex
Tiziano Terlizzese, Enzo Zanchini ∗
Dipartimento di Ingegneria Energetica, Nucleare e del Controllo Ambientale, Università di Bologna, Viale Risorgimento 2, I-40136, Bologna, Italy
a r t i c l e i n f o
Article history:
Received 5 March 2010
Received in revised form12 November 2010
Accepted 23 November 2010
Keywords:
Zero carbon buildings
Heat pumps
Solar collectors
Dynamic simulation
Exergy analysis
a b s t r a c t
Thefeasibilityof zero carbon emission plantsfor heating, airconditioningand domestic hotwater(DHW)
supply, is analyzed, with respect to conventional plants, for a new residential building complex to be
constructed, in Northern Italy. Two zero carbon plants are considered: the first is composed of air-to-
water heat pumps for space heating and cooling, PV solar collectors, air dehumidifiers, thermal solar
collectors and a wood pellet boiler for DHW supply; in the second, the air-to-water heat pumps are
replaced by ground-coupled heat pumps. Theconventional plant is composed of a condensing gas boiler,
single-apartment air to air heat pumps, and thermal solar collectors. The economic analysis shows that
both zero carbon plants are feasible, and that the air-to air heat pumps yield a shorter payback time. The
exergy analysis confirms the feasibility of both plants, and shows that the ground coupled heat pumps
yield a higher exergy saving.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Since a few decades, improving the energy efficiency of build-
ings, and possibly reach zero energy use for space heating and
cooling and DHW production, is considered as an importanttechni-
cal target both in industrialized and in developing countries; thus,
much research activity in this field has been performedworldwide.
Peippo et al. [1] proposed a procedure for the optimum design
trade-off strategy for solar low energy buildings, and reported
some qualitative results of the procedure for a single family resi-
dential house and a large electricity intensive office building, with
reference to three different climatic zones in Europe. Balaras [2]
audited 8 apartment buildings, located in three climatic zones of
Greece, and showed that a considerable energy saving in heating,
air conditioning, DHW production and lighting can be obtained by
proper retrofit actions. Iqbal [3] studied the feasibility of a zero
energy one family home in Newfoundland, Canada, in which a
grid connected 10 kW wind turbine provides the electric energyfor space and water heating, cooking, lighting and appliances; he
found that the total cost of the wind energy system is about 30%
of the cost of the house. Rijksen et al. [4] studied, both experi-
mentally and through dynamic simulation, the reduction of peak
cooling requirement for an office building obtainable by means
of thermally activated building systems (TABS); TABS have pipes
embedded in the concrete floor, to carry water for heating and
∗ Corresponding author. Tel.: +39 051 2093295; fax: +39 051 2093296.
E-mail address: [email protected] (E. Zanchini).
cooling. Zhao et al. [5] designed and studied numerically a novel
dewpointair conditioningsystems,and Zhao et al. [6] investigated
the feasibility of this system in several China regions. Chan et al.
[7] pointed out advantages and limitations of passive solar heating
and cooling technologies and suggested research guidelines to
improve the economic feasibility of these techniques. Wang et al.
[8], discussed possible solutions for zero energy building design
in UK. They showed that zero energy buildings, in which energy
for heating, air conditioning, DHW, lighting and home appliances
is provided by PV and thermal solar collectors and wind turbines,
are theoretically possible in UK. They also provided optimization
criteria for the building insulation and orientation, but did not
perform an economic or exergy feasibility analysis.
The aim of the present paper is to analyze the economic and
exergy feasibility of zero carbon emission plants for heating, cool-
ing and DHW supply, for a residential building complex planned
for construction in a village close to Bologna, in Northern Italy. The
transmittance of walls and windows is assumed as fixed, and twoalternative zerocarbon plants are designed and studied by dynamic
simulations, performed through TRNSYS 16, and life cycle analysis.
The first plant is based on air-to-air heat pumps and PV collectors,
the second on ground coupled heat pumps and PV collectors. The
economic and exergy payback time of these plants is determined
with respect to a traditional plant, composed of a condensing gas
boiler and single-apartment heat pumps for air conditioning. This
kind of plant is still the most commonly employed for residential
buildings in Northern Italy, where winter loads are important and
air conditioning is usually not provided by the building construc-
tor, but installed by single apartment owners. For all the plants
0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.enbuild.2010.11.019
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788 T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795
Fig. 1. Layout of the building complex.
considered, about 70% of the DHW energy use is supplied by ther-
mal solar collectors. The economic analysis shows that both zero
carbonplants arefeasible, and that the air-toair heat pumps yield a
shorter payback time. On the other hand, the ground coupled heat
pumps appear as preferable from the exergy analysis viewpoint.
2. Description of the building complex and of the plants
The building complex is composed of seven four-apartment
houses and five two-apartment houses; a layout of the complex
is reported in Fig. 1. Each apartment has a heated floor area of
111.41 m2, so that the total heated floor area of the complex (38
apartments) is about 4234m2. Each apartment has two floors.
The ground floor is composed of an entrance hall, a living room, a
bathroom and a garage (unheated). The first floor is composed of a
kitchen with dining room, two bedrooms, a bathroom and a small
terrace. All houses have a timber frame and wooden walls, and
are insulated with wood-derived insulating materials. A view of a
house with 4 apartments and a map of the first floor are illustratedin Fig. 2.
Two alternative zero carbon plants, named Plant A and Plant
B, and a conventional plant, named Plant C are considered. Plant
A is composed of air-to-water heat pumps (AWHPs), with electric
energy supplied by PV collectors, which provide heating and cool-
ing; air dehumidifiers; thermal solar collectors and a wood pellet
boiler, whichprovideDHW. Plant B is similar to Plant A, butAWHPs
are replaced by ground-coupled heat pumps (GCHPs). Plant C is
composed of a central condensing gas boiler for heating and single
apartment heat pumps for air conditioning; DHW is supplied by
thermal solar collectors and by the gas boiler.
The thermal solar plant for DWH, designed by the f -chart
method [9] as illustrated in Section 3, is the same in all cases: it
provides 70% of DHW energy use. In each case, floor radiant pan-
els are employed and fresh air is supplied by a forced ventilation
circuit,providedwith a humiditycontroland heatrecoverysystem.
3. Energy demand for heating, cooling and DHW supply
Thecomponent materialsof the external wall,between thetim-
berpillars,and their thermalproperties are listed in Table1, starting
from outside. Oriented Strand Board (OSB) is manufactured from
waterproof wood strands, that are arranged in cross-oriented lay-ers. For air layers, the effective thermal conductivity is reported in
Table 1, evaluated as
=s
R(1)
where s is the thickness and R is the thermal resistance per unit
area of the layer.
The transmittance of the external wall, evaluated according to
EN ISO 6946:2008, is 0.170 W/(m2 K) in correspondence of the
wood fiber insulation (layers listed in Table 1) and 0.326W/(m2 K)
in correspondence of the timber frame; the latter covers about 10%
of the total wall area, so that the averagetransmittance of the exter-
nal wall is about 0.186 W/(m2 K). In the dynamic simulation, the
thermal resistance of the external surface has been evaluated as a
function of the wind velocity and of the external surface tempera-ture.
Theroofhas a compositionsimilar tothatof theexternalvertical
wall. The wood beams, whichare placed under the roof, provide an
additional thermal resistance. The roof transmittance, evaluated
accordingto ENISO 6946:2008, is 0.15 W/(m2 K) in correspondence
of the timber frame, which covers about 22% of the total roof area,
and0.21W/(m2 K) elsewhere; therefore,the average transmittance
of the roof is about 0.197W/(m2 K).
The heat exchange between building and ground has been
evaluated by considering the real, time-dependent, temperature
distribution in the soil, determined by means of TRNSYS Type
501. The ground is composed of heavy clay with 15% water con-
tent. The following values of the ground thermal conductivity kgd
and heat capacity per unit volume (c p)gd have been considered:kgd =1.70W/(mK); (c p)gd = 2.938MJ/(m3 K) [10]. Double glazed
windows with4 mmthick panes separated by a 16mm thick argon
layer have been considered. The window transmittance, including
frame, is 1.4 W/(m2 K); the frame area is 20% of the total window
area, and the glazed surface solar factor is g = 0.589. Shadowing
effects have been considered to evaluate solar energy gains. The
width of the shading devices placed above the windows (see Fig. 2)
has been designed in order to shelter completely the direct solar
radiation from April 15th to September 15th for windows facing
South.
The heat capacity of internal walls has been taken into account.
The internal heat loads have been evaluated, for each hour, accord-
ing to ISO 13790:2008. The heat loss due to ventilation has been
determined by assuming an air change rate of 0.3h−1
and theemployment of a heat recovery system with efficiency 0.6.
The weather data for Bologna have been considered, with ref-
erence to the typical meteorological year (TMY) determined by
Remundand Kunz [11]; these data are available in the default TRN-
SYS 16 climaticdata packages. For theTMY considered, themonthly
averaged air temperatures are reported in Table 2, while the val-
ues of both the beam and the diffuse solar radiation incident on a
horizontal surface, during each month, are illustrated in Fig. 3.
During winter, theinternal airtemperatureis setat 20 ◦C during
the day and at 18◦C during the night, except for bathrooms, where
it is kept 2 ◦C higher. During summer, the internal air temperature
issetat28 ◦C duringthe day, thecoolingsystem is turnedoff during
the night, while the relative humidity of the internal air is kept at
50% both night and day. The heating and cooling heat loads for the
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T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795 789
Fig. 2. House with 4 apartments: view of the building and map of the first floor.
Table 1
Materials of the external wall: s = thickness [cm]; = thermal conductivity (for
air, effective thermal conductivity) [W/(m K)]; c = heat capacity per unit volume
[MJ/(m3 K)]; e = emissivity.
Material s c
1: Plaster 0.5 0.9 1.638
2: Mineralized wood fiber 5 0.083 0.840
3: Air 4 0.222 0.000
4: Vapour barrier 0.1 0.077 0.034
5: Air 4.5 0.149 0.000
6: Low e layer 0.1 0.071 0.034
7: Mineralized wood fiber 3.5 0.083 0.756
8: OSB 1.2 0.13 1.701
9: Wood fiber 12 0.038 0.105
10: Air 2 0.111 0.000
11: Vapour barrier 0.1 0.071 0.034
12: OSB 1.2 0.13 1.701
13: Mineralized wood fiber 5 0.083 0.756
14: Cellulose–gypsum board 1.3 0.32 1.265
whole building complex, in kW, are illustrated in Fig. 4. The annual
energy need for the whole building complex is: 131.75MWh for
heating, 64.00MWh for cooling, 25.85 MWh for dehumidifying.
The domestic hot water demand has been determined by
employing the national Technical Standard UNI TS 11,300, as is
Table 2
Monthly averaged temperatures of the TMY.
Month Average temperature [◦C]
January 1.72
February 4.35
March 9.43
April 13.84
May 20.19
June 21.55
July 24.45
August 24.12
September 20.96
October 14.44
November 8.39
December 3.88
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790 T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795
0
50
100
150
200
250
300
350
400
November September JulyMayMarchJanuary
Beam
Diffuse
MJ/m2
Fig. 3. Monthly values of beam and diffuse radiation on a horizontal surface for
Bologna, during a typical meteorological year.
0
40
80
120
160
8760730058404380292014600
kW
hours
heating heating
cooling
Fig. 4. Heating (dark gray) and cooling (gray) heat load for the whole building
complex during a typical meteorological year.
prescribed by the Regional Law 156/2008. The result, for the DHW
demand, is 165.66 L per day, per each apartment. By assuming a
temperature rise from 15 ◦C to 40 ◦C, one obtains a total energy
need, forthe wholebuildingcomplex, given by E ndhw =66.70MWh.
The annual energy needs for heating, cooling, dehumidifying, and
DHW supply are summarized in Table 3.
4. Plant sizing and primary energy use
A floor radiant panel heat distributionsystem is adopted, ineach
plant for heating, in Plants A and B also forcooling. The distribution
efficiency, the emission efficiency and the control efficiency have
beenevaluated according to thenational TechnicalStandard UNI/TS
11300; their values are, respectively: Ád = 0.97, Áe = 0.99, Ác =0.99.
For Plant B, double U tube borehole heat exchangers (BHEs)
with the following features have been considered: high density
polyethylene tubes SDR 11 with external diameter 32 mm; bore-
hole diameter 156mm; grout thermal conductivity 1.1 W/(mK),hence borehole thermal resistance 0.095 m K/W. The undisturbed
ground temperature, i.e., the average temperature of the ground
from the soil surface to the BHE bottom (100 m), before the begin-
ning of the BHE field operation, has been assumed equal to 14 ◦C.
The GCHP system has two water tanks: WT1, between BHEs and
Table 3
Annual energy needs for the whole building complex.
Kind of service Energy need (MWh)
Heating 131.75
Cooling 64.00
Dehumidifying 25.85
DHW 66.70
heat pumps; WT2, between heat pumps and radiant panels. The
total length of theBHEs hasbeen designed by iterativesimulations,
performed through TRNSYS. A scheme of Plant B during winter
operation is reported in Fig. 5, where red lines represent warmer
water, blue lines coolerwater,arrows inlinesdenotethe waterflow
direction, and large arrows at the sides of the heat pumps denote
the energy flow direction.
The water tank WT2 is present in all the plants considered. For
this tank, a maximum water temperature equal to 35 ◦C has been
assumed; the latter is sufficient to match the design heat load of
166.9 kW (external temperature – 5 ◦C).
Both for Plant A and for Plant B, two heat pumps, with a heating
power of 79.5 kW each, have been selected, so that the maximum
heating power supplied by the heat pumps is Q maxhp
= 159kW. For
each plant, the coefficient of performance (COP) of the heat pumps
has been evaluated for each hour, by considering the external air
temperature (Plant A) or the water temperature in WT1 (Plant B),
with a constant value of the water temperature in WT2 (35 ◦C).
ForPlant A, theCOP of theair-to-water heat pumps as a function
of the external air temperature and of the supply water temper-
ature provided by the manufacturer has been employed, after a
comparison with available experimental data. A reliable experi-
mentalevaluation of the long term COPof air-to-waterheat pumps
operatingin conditions similar to those considered in thispaper has
beenprovidedby Marcic [12]. Theauthor presents the results of the
monitoring, during the period 1988–1998, of an air-to-water heat
pump installed in 1988 which supplies water at a mean tempera-
tureof40 ◦C.In Fig.6, three plots of theCOP of air-to-air heat pumps
versus the external air temperature are reported: the plot in light
gray refers to the heat pumps considered in this paper, with a sup-
ply water temperature of 35 ◦C; the plot in dark gray refers to the
heat pumps considered in this paper, with a supply water temper-
ature of 40 ◦C; the plot in black refers to the heat pump monitored
by Marcic(supply water temperature 40◦C).The figure shows that,
with reference to the same supply water temperature, the COP of
the heat pumps considered in this paper is about 19% higher than
that measured by Marcic. A recent report available in the literature
[13] showsthat, on account of technological improvement, the per-cent COP increase of air-to-water heat pumps from 1986 to 2004 is
about 25%. Therefore, the COP data provided by the manufacturer
of the air-to-water heat pumps considered in this paper have been
considered as reliable and employed in calculations.
For Plant B, which uses GCHPs, reliable experimental data in
working conditions similar to those employed in this paper are
not available in the literature. Therefore, the COP data provided by
the manufacturer have been employed. For a water temperature
in WT2 equal to 35 ◦C, the COP as a function of the water temper-
ature in WT1 is given by 0.12 T WT1 + 4.4, where T WT1 is the water
temperature in WT1 expressed in degrees Celsius.
The seasonal weighted mean values of the COP obtained are
as follows: for Plant A, COP= 3.81 during the heating period and
COP = 3.60 during the cooling period; for Plant B, COP = 5.32 duringthe heating period, while the heat pumps are not used for cooling
(free cooling).
For Plants A and B, the power supplied to the building is
Q̇ s =Q̇ n
ÁdÁeÁc= Q̇ hp + Q̇ aux, (2)
where Q̇ n is the net thermal power required by the building, Q̇ hp
is the thermal power supplied by the heat pumps and Q̇ aux is the
auxiliary thermal power for heating supplied by the wood pellet
boiler.
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T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795 791
WT1
heat pumps
building complex
PV solar
collectors
thermal solar collectors
WT2
boiler
DHW tank
BHEs
Fig. 5. Scheme of Plant B.
For Plant B, the power extracted from the ground to meet the
winter heat load is given by
Q̇ gd = Q̇ hp
1−
1
COP
(3)
Simulations of the BHEs have been performed through TRNSYS
Type 557, by employing the data obtained with Eq. (3). The total
length of the BHEs has been determined by iterations, in order to
obtaina minimum temperature of WT1 not lower than 4 ◦C. A total
lengthof 4000m has been obtained, whichcorresponds to 40 BHEs
100m deep. A plot of the temperature of WT1 versus time, for aperiod of two years, is reported in Fig. 7. The figure shows that the
temperature of WT1 duringsummer exceeds 18◦C only exception-
ally. Thethermalpower subtracted from thebuilding bythe radiant
panels, with a water inlet temperature of 18 ◦C and an internal air
temperature of 28 ◦C, is 28.9 W/m2.
Simulations of the apartments have been performed by TRNSYS
under the followingconstraints: the maximum heating powerdur-
ingwinter, per unit floor area, is equal to the designheating power,
for each room, and the maximum cooling power per unit floor area
during summer is equal to 28.9W/m2. By means of these simula-
tions, the electric energy required by the heat pump system has
been determined, for Plants A andB. Moreover, the thermal energy
0
1
2
3
4
5
14121086420 °C
COP Present paper, 35 °C
Marcic
Present paper, 40 °C
Fig. 6. COP of air-to-air heat pumps versus external air temperature.
supplied for heating by the wood pellet boiler during one year has
been evaluated. Finally, it has been verified that, for Plant B, the
internal set point temperature (28◦C)is reached insummer byfree
cooling, i.e., sending water directly from WT1 to theradiant panels.
For PlantsA and B, duringsummer nightsthe waterflow in radi-
ant panels is stopped; nevertheless, the internal air temperature is
usually lower than 29 ◦C andexceeds this value only exceptionally.
Thesetemperature conditions and 50% relative humidityhave been
considered as satisfying.
The electric energy consumed by the heat pumps per year, E hp,
has been determined as the integral, during one year of Q̇ hp/COP.The results are: E hp = 55.10 MWh for Plant A (36.38 MWh for heat-
ing and 18.72 MWh for cooling); E hp = 26.05 MWh for Plant B, for
heating (free cooling is adopted).
The electric energy use for water circulation, dehumidification
(Plant A and Plant B) and single apartment air conditioning (Plant
C) has been evaluated as follows.
For the piping system between WT2 and the radiant panels, the
total head loss and flow rate are respectively 69.9 kPa and 8.03L/s.
The estimated electric energy consumption is 5.34MWh per year
for Plants A and B; 3.59 MWh per year for Plant C (where radiant
2
4
6
8
10
12
14
16
18
20
1752014600116808760584029200
hours
°C
Fig. 7. Temperature of WT1 versus time, for a period of 2 years.
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792 T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795
Table 4
Annual electric energy use for Plants A, B and C.
Plant A (MWh) Plant B (MWh) Plant C (MWh)
Heating 36.38 26.05 –
Cooling 18.72 – 20.68
Dehumidifying 12.30 12.30 8.35
Radiant panel pumping 5.34 5.34 3.59
DHW loop pumping 0.05 0.05 0.05
BHE loop pumping – 8.51 –
Total 72.79 52.25 32.67
panels areused only forheating). Theelectricenergy usefor pump-
ing domestic hot water is about 0.05 MWh per year. The electric
energy use for dehumidification, for Plant A and Plant B, has been
evaluated by assuming the COP of air dehumidifiers equal to 2.1;
the result is 12.30 MWh per year.
For Plant C, the electric energy consumed by the single-
apartment heat pumps for cooling and dehumidifying has been
determined by considering the hourly thermal loads evaluated by
TRNSYS and the COP data provided by the constructor. In anal-
ogy with Plants A and B, the thermal loads have been evaluated
by assuming that both temperature and relative humidity are con-
trolled during theday, while onlythe relativehumidityis controlledduring the night. The result is 29.03 MWh per year (20.68MWh for
cooling and 8.35 MWh for dehumidifying), and the weighted mean
value of the COP is 3.10.
For Plant B, also the energy use for the BHE loop pumping must
be considered. The BHE piping system is composed of 8 parallel
loops, each with 5 BHEs piped in parallel. The water flow rate is
20 L per minute, for each BHE. The total head loss, evaluated as
suggested in Ref. [14], is 93.9 kPa, and the estimated energy con-
sumption is 8.51MWh per year. The values of the electric energy
used for heating, cooling, dehumidifying and pumping, for Plants
A, B and C, are summarized in Table 4.
For Plants A and B, the PV collectors have been sized in order to
supplyexactlythe total useof electricenergy reported in Table 4, in
a typical meteorological year,namely 72.79 MWh of electric energyforPlantA, and52.25 MWhof electricenergy forPlantB; the design
software available in Ref. [15] has been employed.The followingPV
system features have beenconsidered: tilt angle 14◦, azimuth angle
−21◦, combined PV system losses 25.5%.The desired energysupply
is obtained by a PV system with 71.2kWp (peak power) for Plant A,
with 51.2 kWp for Plant B. The PV collectors are roof-integrated, in
each house. The total PV collector area is 569.6 m2 for plant A and
409.6m2 for plant B, i.e., about 60 m2 for Plant A and about 43 m2
for Plant B, for a house with four apartments.
For Plants A and B, the auxiliary thermal energy for heating per
year, supplied by the wood pellet boiler, is E aux = 0.11MWh.
For all the plants considered, the thermal energy supplied per
year to the DHW system is
E sdhw =E ndhw
(ÁdÁstÁs)dhw
, (4)
where Ád, Ást and Ás are the distribution, storage and supply effi-
ciencies for the domestic hot water system, which have been
evaluated according to EN 15316-3-1:2007. Their product is 0.89;
thus, since E ndhw = 66.70 MWh, one obtains E sdhw =74.94MWh.
To meet a part of the thermal load E sdhw, single glazed flat
plane thermal solar collectors with a selective absorbing surface
have been chosen, with the following plant features: tilt angle 45◦;
azimuth angle 0◦; F R (˛)0 = 0.824, where F R is the heat removal fac-
tor and (˛)0 is the effective transmittance–absorptance product
at normal incidence; F R U L = 3.66 W/(m2 K), where U L is the overall
heat transfer coefficient; storage volume 75 kg/m2. The plant has
been sized by the f -chart method [9], which allows to determine
0.4
0.5
0.6
0.7
0.8
0.9
140120100806040
Collector area [m2]
f
Fig. 8. Plot of the fraction f of the annual thermal energy use for DHW provided by
solar collectors as a function of the transparent collector area.
the fraction f of the annual thermal energy use for DHW provided
by solar collectors, as a function of monthly thermal loads, climatic
data, collectorperformanceparameters,storagevolume andcollec-
tor area. Clearly, the same climatic data employed for the building
simulation [11] have been used. The total radiation per unit area
incident on the collector surface has been evaluated by TRNSYSType 16. The latter employs hourly data of both direct and diffuse
radiation on a horizontal surface, and thus accounts for the effects
of clouds. In Fig.8, the fraction f of the annualthermalenergyuse for
DHW provided by solar collectors is plotted versus the transparent
collector area. The figure shows that solar collectors provide 70%
of E sdhw with a transparent area of about 87.5 m2. Thermal collec-
tors areplaced onthe roof of a detached plantroom,which contains
WT2and theDHW tank, thewood pellet boilerand the heat pumps
for Plants A and B, the condensing gas boiler for Plant C, and WT1
for Plant B.
For Plants A and B, the total thermal energy supplied by the
wood pellet boiler during one year is given by
E wpb =E
sdhw(1− f )+ E aux
Áwpb, (5)
where f is the fraction of E sdhw supplied by the thermal solar col-
lectors, E aux is the auxiliary thermal energy for heating per year
supplied by the boiler, and Áwpb is the boiler efficiency.
A wood pellet boiler with 200 kW power and an efficiency
equal to 0.92 has been chosen. Indeed, an analysis of the techni-
cal data provided by constructors has shown that the efficiency of
wood pellet boilers produced nowadays ranges from 0.9 to 0.95.
Thermal solar collectors have been sized to yield f = 0.70, so thatE wpb = 24.56MWh. Since electric energy for heat pumps, dehumid-
ifiers andwater circulationis provided by PV collectors, forPlants A
andB E wpb = 24.56MWhis the total primaryenergy useof the build-
ing complex for heating, cooling, dehumidifying and DHW supply.
This consumption corresponds to 5.80 kWh/(m2
year), with zerocarbon emission.
For Plant C, the efficiency of the condensing gas boiler has
been considered as equal to 1.05. The product of the distribution,
the emission and the control efficiency for the heating system is
ÁdÁeÁc = 0.95, and the product of the distribution, the storage and
the supply efficiency for the DHW system is (ÁdÁstÁs)dhw =0.89.
Thus, the total plant efficiency is 1.00 for heating and0.94 for DHW
supply, and the energy use to provide space heating and 30% of the
DHW energy need is 153.04 MWh. This consumption corresponds
to 36.15 kWh/(m2 year), to which the use of 32.67 MWh of electric
energy, for cooling, dehumidifying and pumping, must be added.
The use of primary energy which corresponds to this use of elec-
tricity has beendetermined according to the Resolution EEN 3/08of
the Italian Agency for Electric Energy and Gas (AEEG), which states
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T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795 793
Table 5
Plant costs.
Plant A
Heat pumps 40,000 D
Dehumidifiers 20,500 D
PV solar collectors 341,800 D
Pellet boiler 12,000 D
Total 414,300 D
Plant BBHE, loop and pump 205,200 D
Cold tank 2000 D
Heat pumps 40,000 D
Dehumidifiers 20,500 D
PV solar collectors 245,800 D
Pellet boiler 12,000 D
Total 525,500 D
Plant C
Gas boiler 11,000 D
Air to air heat pumps 114,000 D
Total 125,000 D
that 1 kWh of electric energy corresponds to 2.175kWh of primary
energy; the result is 78.63 MWh of primary energy. Therefore, thetotal use of primary energy per year for plant C is 231.67 MWh.
5. Economic analysis
The economic feasibility of Plants A and B has been analyzed by
comparison with a conventional heating and cooling plant, called
Plant C. The same thermal solar collector system has been consid-
ered, for all plants.
Since a comparative economic analysis of Plants A, B and C has
been performed, the costs of the common components, present in
all plants, have not been considered. These components are: radi-
ant panels, water distribution system to radiant panels, tank WT2,
thermal solar collector system, DHW distribution circuit. The cap-
ital costs of Plants A, B and C, excluding the common components,are reported in Table 5. For the PV systems, a capital cost of 4800D /kWphas beenconsidered.For the BHE systemof Plant B, a costof
50 D /m has been considered for the BHEs (length 4000 m and total
cost 200,000 D ), plus a cost of 2600 D for pipes and pumps and a
cost of 2600 D for labour and machinery use. The table shows that
Plant B is themost expensiveand that theratiobetween thecapital
cost of Plant B and that of Plant A is about 1.27.
The operating costs have been evaluated as follows. The cur-
rent rates of fuels and electricity in Bologna have been considered:
0.23 D /kg for wood pellet; 0.70 D /m3 for natural gas; 0.25 D /kWh
for electricity. The State financial support given for PV electricity
production in Italy has been taken into account: only the annual
difference between the electric energy consumed by the plant and
the electric energy produced by the PV system is paid by the user(zero in this case); all the PV electricity produced is paid by the
State at the rate 0.422 D /kW h, for roof-integrated PV Panels.
ForPlant A and Plant B, an additional maintenance cost hasbeen
considered, with respect to Plant C, by assuming that the additive
maintenance cost is due only to the PV system, because also Plant
C has heat pumps, for summer cooling and dehumidifying. Indeed,
PV systems require periodical maintenance activities such as mod-
ule cleaning, visual checking of the electrical wiring system, and
checking of module watertight seals. An annual maintenance cost
equalto46 D /kWp,whichcorresponds to the average service costof
local maintenance companies, has been assumed. Hence, the addi-
tive maintenance cost has been evaluated as equal to 3300 D /year
for Plant A, and to 2400 D /year for Plant B. The annual operating
costs/incomes for Plants A, B and C are reported in Table 6.
Table 6
Annual cost (income) for energy use (production).
Cost Income
Plant A
Wood pellet 1200 D
PV electricity 30,700 D
Maintenance 3300 D
Annual income 26,200 D
Plant B
Wood pellet 1200 D
PV electricity 22,000 D
Maintenance 2400 D
Annual income 18,400 D
Plant C
Methane 11,200 D
Electricity 8200 D
Annual cost 19,400 D
On account of the uncertainty in the previsions of the cost of
money and on the annual increase of the unit costs of fuels and
electricity, we have performed our economic analysis by assuming
zero cost of money and zero annual increase of fuels andelectricity
costs. The total capital plus operating cost versus time is plotted
in Fig. 9, for each plant, for a period of 20 years. The figure showsthat Plant A is the most convenient. Its payback time, with respect
to Plant C, is about 6 years, while that of Plant B is about 11 years;
moreover it has a total cost always lower than that of Plant B.
Clearly, the results illustrated in Fig. 9 are strongly influenced
by the presence of PV systems with different areas and by the
State incentives to PV electricity production. Therefore, it may be
interesting to perform a comparative economic analysis of Plants
A, B, C, in the absence of PV systems. The results of this analysis
are reported in Fig. 10, and show that Plant A remains the most
convenient, for a time interval of 20 years.
6. Exergy analysis
A comparative exergy analysis of Plants A, B and C has beenperformed. As usual, we will call embodied energy of a plant com-
ponent the exergy loss due to its construction and installation. In
analogy with the economic analysis, the embodied energy of the
commoncomponents of Plants A, B, and C hasnot been considered.
For each plant,the embodied energy of each non-common com-
ponent has been evaluated as follows. For heat pumps, boiler,
dehumidifiers and tanks, the real mass has been considered,
together with the mass fractions of the constituent materials given
in Ref. [16], while the value of the embodied energy of each mate-
rial, per unit mass, has been taken from Ref. [17]. For the high
density polyethylene tubes of BHEs, the real mass has been con-
Fig. 9. Capital plus operating cost versus time, for Plants A, B, C.
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794 T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795
0
100000
200000
300000
400000
500000
600000
20151050
€
years
Plant A
Plant B
Plant C
Fig. 10. Capital plus operating cost versus time, for Plants A, B, C, in the absence of
PV collectors.
sidered and the value of the embodied energy per unit mass has
been taken from Ref. [17], by considering the feedstock energy as
no longer available. The embodied energyof PV collectors has been
evaluated by assuming an embodied energy per unit peak power
equal to 8.5MWh/kWp, as reported in Ref. [18]. Theexergy loss due
to borehole drilling has been evaluated by considering a diesel fuelconsumption of 1 L per each meter of borehole (typical consump-
tion for the soil considered), and by approximating the diesel fuel
exergy with its lower heating value, namely 10.02 kW h/L [19,20].
The values of the embodied energy for the non common compo-
nents of Plants A, B, and C are summarized in Table 7. The table
shows that the total embodied energy for Plant B is greater than
that for Plant A, and that (excluding the components common to
all plants) the ratio between the embodied energy of Plant B and
that of Plant A is about 1.43.
For each plant,the exergyloss due to theplantoperation during
a typical meteorologicalyear has beenevaluated, by approximating
the fuel (methane or wood pellet) exergy with its lower heating
value. With this approximation, the annual exergy use for Plants
A and B is 24.56MWh, while the annual exergy use for Plant C is
231.67MWh, as is shown in Section 3. Plots of the total exergy loss
due to theplant constructionand operationversustime, for a period
of 20 years, are illustrated in Fig. 11 f orPlants A,B and C.Thefigure
shows that the lowest exergyuse after 20 years is obtained by Plant
A.
Table 7
Values of the embodied energy for the non common components of Plants A, B, and
C.
Plant A
Heat pumps 18.6 MWh
Dehumidifiers 3.4 MWh
PV solar collectors 605.2 MWh
Pellet boiler 6.8 MWh
Total 634.0 MWh
Plant B
Heat pumps 18.6 MWh
Cold tank 8.4 MWh
Boreholes 40.1 MWh
BHE pipes 392.7 MWh
Dehumidifiers 3.4 MWh
PV solar collectors 435.2 MWh
Pellet boiler 6.8 MWh
Total 905.2 MWh
Plant C
Gas boiler 8.5 MWh
Air to air heat pumps 38.0 MWh
Total 46.5 MWh
0
500
1000
1500
2000
2500
3000
3500
4000
20151050
MWh
years
Plant A
Plant B
Plant C
Fig. 11. Total (construction+ operation) exergy use versus time, for Plants A, B, C.
0
500
1000
1500
2000
2500
3000
3500
4000
20151050
MWh
years
Plant A
Plant B
Plant C
Fig.12. Total (construction+ operation) exergyuse versustime,for PlantsA, B, C, in
the absence of PV collectors.
The exergy analysis illustrated in Fig. 10 does not yield a direct
comparison between the exergy use of an air-to-water heat pump
system and that of a ground-coupled heat pump system, becausePlant A and Plant B have different PV collector areas.
To obtain a direct comparison, the exergy analysis has been
repeated by excludingthe embodied energy andthe annual exergy
production of the PV system. Clearly, the data for Plant C do not
change. The total embodied energy becomes 28.8 MWh for Plant A
and 470MWh Plant B. The annual exergy use for Plant A is given
by the sum of 24.56 MWh, due to the consumption of wood pellet,
and of the primary energy equivalent of the electric energy use per
year,namely72.79×2.175= 158.32MWh;thetotalis 182.88MWh.
Similarly, for plant B one obtains a total exergy use per year equal
to 24.56+ 52.25×2.175= 138.20 MWh. Plots of the total exergyloss
due to theplant constructionand operationversus time, for a period
of 20 years, in this scenario, are illustrated in Fig. 12. The figure
shows that, in the absence of the PV system, the lowest exergy use
after 20 years is obtained by Plant B. Therefore, the exergy analysis
reveals that ground-coupled heat pump systems yield the lowest
consumption of primary energy sources, even in a ground with a
rather low thermal conductivity (kgd = 1.70W/(m K)), as in the case
considered here.
7. Conclusions
Two alternative zero carbon plants for heating, cooling, humid-
ity control and domestic hot water supply, for a new building
complex in Northern Italy, have been studied by means of the sim-
ulation codeTRNSYS and compared witha conventional plant. Both
plants employ heat pumps which receive electricity by PV panels
and thermal solar collectors for DHW supply. Plant A employs air-
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T. Terlizzese, E. Zanchini / Energy and Buildings 43 (2011) 787–795 795
to-water heat pumps, whereas Plant B employs ground-coupled
heat pumps; they have the same, nearly vanishing, primary energy
use (wood pellet) and different PV collector areas.
The economic analysis has shown that both Plant A and Plant B
are feasible, and that Plant A has a lower financial payback time (6
years) than Plant B (11 years). The exergy analysis has shown that
Plant A yields also a lower total exergy consumption after 20 years
of operation. However, this result is due to the higher PV collector
area employed in Plant A. If the exergyanalysis is repeated without
considering the PV panels, then the lowest exergy consumption
after 20 years is obtained by Plant B.
The results point out that ground-coupled heat pumps ensure
a lower environmental impact than air-to-water heat pumps, but
are economically less feasible, at least in a ground with a low or
medium thermal conductivity. Therefore, a specific financial sup-
port for the installation of ground-coupled heat pumps should be
given by public administrations.
References
[1] K. Peippo, P.D. Lund, E. Vartiainen, Multivariate optimization of designtrade-offs for solar low energy buildings, Energy and Buildings 29 (1999)189–205.
[2] C.A. Balaras, K. Droutsa, A.A. Argiriou, D.N. Asimakopoulos, Potential for
energy conservation in apartment buildings, Energy and Buildings 31 (2000)143–154.
[3] M.T. Iqbal,A feasibility study of a zero energyhome in Newfoundland, Renew-able Energy 29 (2004) 277–289.
[4] D.O. Rijksen, C.J. Wisse, A.W.M van Schijndel, Reducing peak requirements forcooling by using thermally activated building systems, Energy and Buildings42 (2010) 298–304.
[5] X. Zhao, J.M. Lee, S.B. Riffat, Numerical study of a novel counter-flow heat andmassexchangerfor dew pointevaporative cooling, Applied Thermal Engineer-ing 28 (2008) 1942–1951.
[6] X. Zhao, S. Yang, Z. Duan, S.B. Riffat, Feasibility study of a novel dew point airconditioning system for China building application, Building and Environment44 (2009) 1990–1999.
[7] H.Y. Chan, S.B. Riffat, J. Zhu, Review of passive solar heating and cooling tech-nologies, Renewable and Sustainable Energy Reviews 14 (2010) 781–789.
[8] L. Wang, J. Gwilliam, P. Jones, Case study of zero energy house design in UK,Energy and Buildings 41 (2009) 1215–1222.
[9] S.A. Klein, W.A Beckman, J.A. Duffie, A design procedure for solar heating sys-
tems, Solar Energy 18 (1976) 113–127.[10] ASHRAE Handbook – HVAC Applications, Ch. 32.[11] J. Remund, S. Kunz, Meteonorm Version 5, METEOTEST, http://www.
meteotest.com.[12] M. Marcic, Long-term performance of central heat pumps in Slovenian homes,
Energy and Buildings 36 (2004) 185–193.[13] M. Forsén, Heat pumps technology and environmental impact,
Report of the Swedish Heat pump Association and of the Euro-pean Heat pump Association, 2005, http://ec.europa.eu/environment/ecolabel/about ecolabel/reports/hp tech env impact aug2005.pdf .
[14] S.P. Kavanaugh, K. Rafferty, Ground-source heat pumps: design of geothermalsystems for commercial and institutional buildings, ASHRAE (1997).
[15] Joint Research Centre, Institute for Energy, http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php.
[16] G. Cammarata, L. Marletta, Embodied energy versus energy efficiencyin build-ing heating systems, Proceedings of CLIMA 2000/Napoli 2001 World Congress,1, 15–18. (Naples, 2001).
[17] G.P.Hammond, C.I.Jones, Embodiedenergy and carbon in construction materi-als, Proceedings of Institution of Civil Engineers, Energy 161 (2) (2008) 87–98.
[18] J.K. Kaldellis, D. Zafirakis, E. Kondili, Optimum autonomous stand-alone pho-tovoltaic system design on the basis of energy pay-back analysis, Energy 34(2009) 1187–1198.
[19] UNI 10389-1, Heat generators. Flue gases analysis and measurement on site of combustion efficiency, September 2009.
[20] EN 590, Automotive fuels – Diesel – Requirements and test methods, October1st 2009.