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.

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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



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




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.




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.




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




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.




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-




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.

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Economic and Exergy Analysis of Alternative Plants for a Zero Carbon Building Complex