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Solar Energy 82 (2008) 820–831

Thermal and environmental assessment of a passive buildingequipped with an earth-to-air heat exchanger in France

Stephane Thiers, Bruno Peuportier *

Centre Energetique et Procedes, Ecole des Mines de Paris, 60 Bd Saint-Michel, 75272 Paris, Cedex 06, France

Received 12 June 2007; received in revised form 20 February 2008; accepted 20 February 2008Available online 20 March 2008

Communicated by: Associate Editor Matheos Santamouris

Abstract

In France, where a division by 4 of the greenhouse gases emissions is aimed from 1990 to 2050, technical solutions are studied in orderto reduce energy consumption while providing a satisfactory thermal comfort level in buildings. A two-dwelling passive building has beencarried out in Formerie (North-West of France), complying the ‘‘Passivhaus” standard. This building, not yet monitored, has been mod-eled using the dynamic simulation software COMFIE, which is dedicated to building eco-design. In order to account for the implementedventilation system, including a heat recovery unit and an earth-to-air heat exchanger, a specific model has been developed and integratedto COMFIE as a new module. In this article, this model is described first. In order to quantify the benefits brought by a passive design,the simulation results are presented for the passive house and a reference house complying with the French thermal regulation for build-ings. The heating load and thermal comfort level of both houses are compared, showing for the passive design a tenfold reduction of theheating load and a clear reduction of summer discomfort. Finally, the environmental assessment – carried out with the life cycle assess-ment tool EQUER – shows the reduction in primary energy consumption, global warming potential and other impacts brought by thepassive house design. Passive house appears to be an adequate solution to improve the environmental performances of buildings in theFrench context.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Thermal simulation; Passive house; Earth-to-air heat exchanger; Life cycle assessment

1. Introduction

The Passive house concept has been formalized by thePassivhaus Institute in Darmstadt, Germany (Feist et al.,2005). Following the Passivhaus standard, such a buildinghas to fulfill three requirements corresponding to a highlevel of performance: heating energy demand1 lower than15 kW h m�2 yr�1, total primary energy demand2 lower

0038-092X/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.solener.2008.02.014

* Corresponding author. Tel.: +33 1 40 51 91 51; fax: +33 1 46 34 24 91.E-mail address: bruno.peuportier@ensmp.fr (B. Peuportier).

1 Useful energy per net floor area within the thermal envelope (treatedfloor area).

2 Non-renewable primary energy per net floor area within thermalenvelope, including heating, domestic hot water, auxiliary and householdelectricity.

than 120 kW h m�2 yr�1, air leakage at 50 Pa lower than0.6 vol h�1 (Feist, 2004). These requirements can beobtained by the combination of various techniques suchas bio-climatic design, high insulation level, high perfor-mance windows, good air tightness and heat recovery ven-tilation. During the heating period, the low heating load ismainly covered by solar and internal gains. During thecooling period, solar protections and passive cooling mayresult in a satisfactory thermal comfort (Breesch et al.,2005). More than 4000 passive buildings have already beenbuilt, mainly in Germany, Austria and Switzerland.

In France, thermal performance of buildings has beenneglected for a long time. The primary energy consumptionfor heating, cooling, lighting, ventilation and domestichot water of the buildings complying with the last French

S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831 821

thermal regulation ‘‘RT2005” (Reglementation Thermique2005, 2006) are set between 80 and 250 kW h m�2. yr�1

according to energy sources, architectural shape and cli-matic zones.3 The regulation also sets maximal values tothermal bridges (between 0.4 and 0.6 W m�1 K�1), to ther-mal conductance in walls (between 0.36 and 0.4 W m�2 K�1)and windows (between 1.8 and 2.1 W m�2 K�1) and to airpermeability (0.8 (m3 h�1) m�2 at 4 Pa for individualdwellings), but no heating energy demand limit. Thesecriteria are far less restrictive than those of the Passivhausstandard. Although, in France, the development oflow-energy buildings is one of the ways to fulfill thenational objectives of reduction by 4 of the CO2 emissionsbetween 1990 and 2050 (Loi POPE, 2005), the passivehouse concept hardly develops. Up to now, only very fewpassive buildings have been built, mainly for experimentalpurposes. Due to an inadequate conception and implemen-tation of unusual techniques, the first attempt to reach thePassivhaus standard in France has failed (CEPHEUS pro-ject in Rennes (Feist et al., 2005)). Moreover, it is essentialto adapt the concept to the French context, particularlybecause thermal comfort in summer is an important issue:there were 15,000 deaths during the 2003 heat wave inFrance.

In early 2007, the company LES AIRELLES4 has com-pleted the construction of the first labeled passive housesin France (Fig. 1). These attached twin houses have beendesigned to comply with the Passivhaus standard. Heatingand domestic hot water are provided by a small heatpump. Cooling is achieved by an earth-to-air heat exchan-ger (ETAHE). Heat losses are minimized by the use of aheat recovery ventilation unit (HRV). These houses areneither occupied nor monitored yet. The Passivhaus stan-dard corresponds to an excellent energy performance, andit is interesting to know its environmental performanceover a life cycle (including the production of the materi-als, the construction and the end of life and not onlythe operation phase). Also it is essential to assess the sum-mer comfort in such buildings. Consequently, the aim ofthis study is to assess the calculated energy consumption,the thermal comfort and the environmental impacts dur-ing the life cycle of this building and to compare theseperformances to those of a standard building complyingwith the French thermal regulation in order to quantifythe energy saved and the reduction of various environ-mental impacts.

2. Method

In a first step, the two alternatives (passive and stan-dard building) have been modeled and simulated usingthe thermal dynamic simulator COMFIE. This software– developed by the CEP at the Ecole des mines de Paris– is a multi-zone simulation tool based upon a finite

3 Ratios based on the net floor area as defined in France.4 See: http://www.lesairelles.fr/.

volume method on which a modal reduction techniqueis applied (Peuportier and Blanc-Sommereux, 1990). Forthe passive building, the ventilation system has been spe-cially modeled, in order to simulate precisely its influenceon temperature and comfort. This model – described inthe following section – has been implemented as acomplement module to the building simulator. Thus, thesimulator has computed the energy loads and the temper-ature in each building for both heating and coolingperiods.

Then, in a second phase, the environmental impactshave been calculated for both passive and standard alterna-tives using the life cycle assessment (LCA) tool EQUER,which is specialized in LCA of buildings (Polster et al.,1996). EQUER is directly linked to COMFIE: it takesCOMFIE results as input data.

3. Model

3.1. Literature review

The ventilation system comprises an ETAHE and aHRV in series with a by-pass for each one (Fig. 2). Theoptimization of a similar ventilation device has alreadybeen studied using a one-zone building-simulation pro-gram (Bojic, 2000) or as a solution for passive houseswithin the CEPHEUS European program (Feist et al.,2005). A lot of models have been proposed for HRVand ETAHE taken separately. Roulet et al. (2001) haveproposed a detailed model of HRV including air leakagesand Juodis (2006) has discussed the evolution of HRVefficiency as a function of the temperature inside thebuilding. Here, we limit the HRV model to the most basicone as later explained. Concerning ETAHE, a lot ofmodels have been developed, notably by Benkert et al.(1997), Bojic et al. (1997), Hollmuller (2002), De Paepeand Janssens (2003), Dibowski (2003), Al Ajmi et al.(2005), Ghosal and Tiwari (2006) or Badescu (2007),and the analysis of these various models leads to clearlyidentify the main physical phenomena occurring in theETAHE. Within the ETAHE, the main heat fluxes areconvection in the pipes and radial conduction throughand around the pipes in the ground. We have estimatedvertical and longitudinal conduction contribution to theglobal heat flux for typical temperature gradients andfound that they represent respectively only 3% and0.03% of the radial conduction heat flux. Thus they canbe neglected. Underground, various phenomena affectthe soil temperature and consequently the performanceof the ETAHE: conduction in the mass of the soil, waterinfiltration, geothermal power, power flowing from theground surface situated above the ETAHE. In the litera-ture, either complex dynamic models include most ofthese phenomena, or simplified static models include theonly internal ones, through different numerical methods(finite elements, finite volume) and for various configura-tions (single or multiple pipe).

Fig. 1. Top: Plan of the building (first floor). Thermal zones are identified with Roman numerals. Bottom left: View of the twin passive houses (LESAIRELLES, EN ACT ARCHITECTURE). Bottom right: 3D representation of the building before simulation.

Fig. 2. Structure of the ventilation device.

822 S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831

3.2. Description of the ventilation device model

The ventilation system is described in Fig. 2. The modelcomprises three main parts corresponding to ETAHE,HRV and regulation. For the ETAHE, the thermalexchange between the soil and the air flowing in the pipes

depends on the structure of the exchanger, the fresh airtemperature and the soil temperature near the pipes. Onthe one hand, the fresh air temperature is given by anymeteorological database, but on the other hand the soiltemperature near the pipes has to be calculated. The modelproposed here therefore includes two submodels: a soil

S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831 823

thermal model and the actual ETAHE thermal model. Thefirst one allows the temperature to be evaluated at anypoint in the soil, without any ETAHE influence (corre-sponding to an ‘‘undisturbed soil” temperature). This tem-perature is supplied as a boundary condition to the secondsubmodel, which represents a multi-pipe heat exchangerand calculates the outlet air temperature along and outof the buried pipes.

The whole model has been coupled to a building-simula-tion tool, in order to help designing ETAHE systems. It isnotably based on Hollmuller’s analytic model (Hollmuller,2002) and GAEA model (Benkert and Heidt, 2000), andincludes both climate and building influence on the groundtemperature.

3.3. Soil thermal model

This first part is based on several studies, such as Miha-lakakou et al. (1997) and Hagentoft (1988). The soil isassumed homogenous and having constant physical prop-erties: thermal conductivity ksoil, density qsoil and specificheat csoil. Time variation of the soil humidity – e.g. dueto water percolation – is neglected and no ground wateris considered, so that the soil can be considered as asemi-infinite solid.

The model is built as the superposition of three indepen-dent phenomena:

1. Propagation by conduction of the thermal signal fromthe ground surface (linked to atmospheric conditions).

2. Heat flow from the slab of a building close to the soilarea under consideration (influence of studied or nearbybuilding).

3. Heat flow from the sub-soil (geothermal flow).

Each of these three phenomena are associated with spe-cific boundary conditions. Due to the linearity of thermalconduction, we can combine them by superposition, asMihalakakou et al. (1995) did.

5 alat = 103 Pa K�1, blat = 609 Pa, clat = 0.0168 K Pa�1 (Penman, 1963).6 For bare soil: saturated: f = 1; moist: f = 0.6–0.8; dry: f = 0.4–0.5;

arid: f = 0.1–0.2. For grass covered soil: this value must be multiplied by0.7. (Penman, 1963).

7 heq is 12.6 W m�2 K�1 for soil sheltered from wind, 20 W m�2 K�1 fora soil fairly exposed to wind and 50 W m�2 K�1 for a soil particularlyexposed to wind (Peuportier and Blanc-Sommereux, 1994).

3.3.1. Propagation of the temperature signal from the soil

surface

We consider a time-dependent sinusoidal temperaturesignal Tsurf, of angular frequency x, of mean value T surf ,of amplitude Asurf and of phase usurf, applied at the surfaceof a semi-infinite solid:

T surfðtÞ ¼ T surf þ Asurf � sinðx � t � usurfÞ ð1Þ

This signal propagating in the solid, the temperature atdepth z under the solid surface is (Marchio and Reboux,2003)

T ðz; tÞ ¼ T surf þ Asurf � exp � zdðxÞ

� �

� sin x � t � usurf �z

dðxÞ

� �ð2Þ

where

dðxÞ ¼ffiffiffiffiffiffiffiffiffi2 � ax

rð3Þ

is the diffusion length of the signal in the solid (m), a beingthe thermal diffusivity of the ground (m2 s�1).

This model applied to the soil, requires to know the tem-perature of the soil surface Tsurf_soil in function of time,during one full year. An energy balance on the soil surfaceis derived from Mihalakakou et al. (1995), but the solutionshave been partly reformulated. This energy balanceincludes various heat flows: conductive flow towards thesoil /cond, radiative flow received from the sun /rad_r, radi-ative flow emitted towards the sky /rad_s and convectiveand latent flows exchanged with air (resp. /conv_sens and/conv_lat) (4),

/cond ¼ ð/rad r � /rad sÞ þ ð/conv sens � /conv latÞ ð4Þ

This balance can be detailed with:

/cond ¼ �ksoil �dT soil

dz

����z¼0

ð5Þ

/rad r ¼ ð1� asurf soilÞ � G ð6Þ/rad s ¼ esoil � r � ðT 4

surf soil � T 4skyÞ ð7Þ

/conv sens ¼ hsurf � ðT amb � T surf soilÞ ð8Þ/conv lat ¼ clat � f � hsurf � ½ðalat � T surf soil þ blatÞ

� rh � ðalat � T amb þ blatÞ� ð9Þ

where asurf_soil is the soil albedo, G the global horizontalsolar radiation (W m�2), esoil the soil emissivity coefficient,r the Stefan–Boltzmann constant (W m�2 K�4), Tsky thesky equivalent temperature, Tamb the ambient air tempera-ture, rh the air relative humidity. alat,blat,clat are empiricalconstants5 and f a dimensionless empirical parameter.6 hsurf

is expressed by the empirical formula depending on thewind average velocity �vwind (m s�1), given by Mihalakakouet al. (1995):

hsurf ¼ 0:5þ 1:2 �ffiffiffiffiffiffiffiffiffi�vwind

pð10Þ

This energy balance is simplified assuming:

/conv sens � /rad s ¼ heq � ðT amb � T surf soilÞ ð11Þ

where heq is the mean value used by the building model ofCOMFIE software.7

From a Fourier series analysis of the climatic drivingforces (G and Tamb) on a fundamental period of 1 yr, thesoil surface temperature can be expressed as the sum of sinefunctions of time ((12) and (13)), where AX,n and uX,n

824 S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831

designate the amplitude and phase of each n harmoniccomponent for any X quantity.

GðtÞ ¼ GþXNh

n¼1

½AG;n � sinðn � x � t � uG;nÞ� ð12Þ

T ambðtÞ ¼ T amb þXNh

n¼1

½AT amb;n � sinðn � x � t � uT amb ;nÞ� ð13Þ

The soil surface temperature is inferred from the energybalance:

T surf soilðtÞ ¼ T surf soil þXNh

n¼1

½Asurf soil;n � sinðn � x � t

� usurf soil;nÞ� ð14Þ

with

T surf soil ¼1� asurf soilð Þ � Gþ hr � T amb þ ðhr � heÞ � blat

alat

he

ð15Þ

and for each n rank harmonic:

tanðusurf soil;nÞ ¼ðhcondðn �xÞ þ heÞ � Y 2;n � hcondðn �xÞ � Y 1;n

�ðhcondðn �xÞ þ heÞ � Y 1;n � hcondðn �xÞ � Y 2;n

ð16Þ

Asurf soil;n ¼Y 1;n � sinðusurf soil;nÞ þ Y 2;n � cosðusurf soil;nÞ

hcondðn � xÞð17Þ

where he, hr, hcond (x), Y1,n and Y2,n are five intermediatevariables.

he ¼ heq þ hsurf � clat � alat � f ð18Þhr ¼ heq þ hsurf � clat � alat � f � rh ð19Þ

hcondðxÞ ¼ksol

dðxÞ ð20Þ

Y 1;n ¼ ð1� albÞ � AG;n � cosðuG;nÞþ hr � AT amb ;n � cosðuT amb ;nÞ ð21Þ

Y 2;n ¼ �ð1� albÞ � AG;n � sinðuG;nÞ� hr � AT amb ;n � sinðuT amb ;nÞ ð22Þ

Actually, the number of accounted harmonic Nh is limited,because the thermal signal is increasingly attenuated indepth as its period is shorter. For a 2 m deep ETAHE –which is a common depth in France due to regulationand economical reasons – limiting to one fundamental har-monic may induce a deviation superior to one degree onthe soil surface temperature in some moments in the year.On the contrary, two or three harmonic components ensurea sufficient precision (Jacovides et al., 1996).

Tsoil, the temperature in the soil is obtained applying thepropagation expression (2) to each harmonic of the groundsurface temperature expression (14):

T soilðz; tÞ ¼ T surf soil þXNh

n¼1

Asurf soil;n � exp � zdðn � xÞ

� ��

� sin n � x � t � usurf soil;n �z

dðn � xÞ

� ��ð23Þ

3.3.2. Building influence

A model of a nearby building influence on a ETAHEhas already been proposed by Benkert and Heidt (2000)in the GAEA software. This model combines a soil thermalmodel and an ETAHE model in a same formalism, usingthe method of conformal mapping transformation of thesoil. This method is applied for one pipe, and correctiveterms are added to account for possible interactionbetween several pipes and for the nearby building influ-ence. We have preferred to model the dynamic behaviourof a larger portion of ground, including several pipes,and to account for the influence of a nearby building whenevaluating the boundary condition for this system. Thus acorrective term is proposed, characterized by a buildinginfluence factor rbdg(r,z). This multiplicative factor dependson the distance from the considered point to the buildingslab center, r, and the depth of this point under the soil sur-face, z – we assume that the slab is in direct contact to thesoil surface. It is a linear combination of a plane tempera-ture profile close to the slab and a spherical temperatureprofile far from the slab, varying with r. This factor isdefined as follows:

– if z 2 ½0; Z0� and r 2 ½0; Z0�; then rbdgðr; zÞ

¼ 1� zZ0

� �� 1� r

Z0

� �

– if r > Z0 or z > Z0; then rbdgðr; zÞ ¼ 0:

where Z0 is the maximum distance beyond which buildinginfluence may be neglected.

The corrective term supposes that the temperaturestraight under the building slab is a constant which corre-sponds to the mean annual temperature of the lower sideof the building slab. This constant T surf bdg is derivedfrom Hagentoft’s formulas of heat loss factor for a slab(Hagentoft, 1988). If the building influence is equal to 0,there is no building influence. If the building influence ishigh the mean soil temperature is close to the temperatureunder the slab and the effect of climatic driving forces isreduced.

3.3.3. Geothermal flow

The contribution of the geothermal flow may only havea significant influence for deep ETAHE or situated in highgeothermal gradient zones, thus it is modeled by a verticaluniform temperature gradient, whose norm is written geo.

3.3.4. Soil temperature expressionCombining the three terms presented above, the soil

temperature expression is finally:

S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831 825

T soilðr; z; tÞ ¼ geo � zþ rbdgðr; zÞ � T surf bdg þ ð1� rbdgðr; zÞÞ

�"

T surf soil þXNh

n¼1

"Asurf soil;n � exp � z

dðn � xÞ

� �

� sin n � x � t � usurf soil;n �z

dðn � xÞ

� �##ð24Þ

3.4. The ETAHE model

This model supposes that the ETAHE is constituted byparallel, identical pipes, set in a horizontal plane andequally spaced one another. Air is supplied by a distributorduct and evacuated by a collector duct. Heat exchange isneglected in these two ducts. We also assume the soil prop-erties are homogeneous around the pipes. In order to inte-grate both dynamic and spatial aspects, the soil and thepipes are modeled using a finite volume method. This repre-sentation contains several cylinder-shaped, coaxial, horizon-tal axed volumes, standing for the air inside the ducts (air),the ducts (duct) and two soil nodes (soil1 and soil2). The soil2node embracing the whole duct layer has been defined inorder to take into account the thermal interaction betweenadjacent ducts. The modeled zone is regularly divided inten vertical sections (Fig. 3).

The temperature in each volume is assumed to be uni-form at any time. As already said, conductive heat transfersin horizontal and vertical directions are neglected. Thus

Fig. 3. Finite volume model (top: vertical sect

only radial conductive heat transfer occurring in verticalplanes are accounted for, and represented by thermal resis-tances. Thermal mass in each volume is represented by thecorresponding heat capacity.

As each pipe is geometrically and physically similar tothe others and as the equation system is linear, the problemis simplified by considering one mean pipe so that the cal-culated temperature of the exiting air is equal to the meantemperature of the air coming out of each pipe. This leadsto a 40 nodes representation. Boundary conditions are airtemperature at first air node entrance and mean soil tem-perature at the top and bottom surfaces of soil2 node(Fig. 3). The numerical model is constituted by a thermalbalance for each longitudinal duct section, for the air node(25) and for each duct, soil1 and soil2 nodes, written as amatrix system (26),

cair � _mduct � DT air ¼ UAair–duct � ðT duct � T airÞ ð25Þ

C � dTdt¼ A � T þ E � U ð26Þ

cair is the air specific heat capacity (J kg�1 K�1), _mduct theair flow through the duct (kg s�1), DTair the air temperaturedifference between the duct section entrance and exit (K),UAair–duct the global air–duct conductance (W K�1), Tduct

the temperature of the duct node, T air the average temper-ature of air node flowing in the duct section. C is thediagonal matrix of heat capacities (J K�1), T is the vectorof node temperatures (K), A is the matrix of heat transfer

ion, bottom: vertical longitudinal section).

826 S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831

between volumes (W K�1), E is the matrix of heat transferbetween volumes and driving forces (W K�1) and U is thevector of the driving forces (K). C, A, E and U representthe mean characteristics of the whole ETAHE, expressedfor one duct. The solving method chosen for this systemof differential equations is modal analysis (Bacot et al.,1984) but due to the low number of equations (three), nomodal reduction is needed. These equations are integratedover a time step (e.g. 1/2 h), allowing the outlet air temper-ature to be evaluated along the simulation period.

Fig. 4. Comparison between simulation and measurements.

3.5. The HRV model

The heat recovery unit allows fresh air to be pre-heatedby exhausting air with a global efficiency eHR. Assumingfresh and exhausting air flows are the same ( _mHRÞ, this glo-bal instantaneous efficiency eHR defined as (27) is a func-tion of Tif, Tix and Tof, respectively the fresh air andexhausting air inlet temperature, and the fresh air outlettemperature. The corresponding heat loss PHR is (28).

eHR ¼T if � T of

T if � T ix

ð27Þ

P HR ¼ _mHR � cair � ðT if � T ixÞ � ð1� eHRÞ ð28Þ

In our model, the global instantaneous efficiency eHR isassumed to be a constant, independent from Tif and Tix.It represents a mean global efficiency.

3.6. Implementation

The dynamic building-simulation software COMFIE,has been developed according to an object-oriented pro-gramming, which facilitates the addition of new modules(Peuportier and Blanc-Sommereux, 1990). The ventilationmodel presented above has been integrated to this soft-ware, so that indoor air temperatures are calculatedaccording to pre-heated (or precooled) fresh air tempera-ture, and flow rate set for each thermal zone. The climaticdata (hourly values of temperature and solar global radia-tion) are used to evaluate the ground temperature and theoutlet air temperature. For each time step, the ETAHEmodel evaluates the heating or cooling power supplied toeach ventilated zone, allowing an energy balance to be per-formed and the temperature to be calculated by the build-ing-simulation module for each thermal zone.

3.7. Validation

The complete ETAHE model has been validated bycomparing simulation results with measurements madeon two buildings in Greater Paris area: the living-roomof an elderly people’s home equipped with a 1.6 m deep,50 m long ETAHE of 8 polyethylene pipes and the officepart of a tertiary building equipped with a 1.6 m deep,25 m long ETAHE of 6 pipes (Thiers and Peuportier,2007). Fig. 4 shows by example the correspondence

between simulated and measured outlet air temperaturesduring a 1-yr period in the case of the elderly people’shome. Maximum deviation is inferior to 2 �C during 98%of the time, which represents the same order of magnitudethan for GAEA results (Benkert and Heidt, 2000). Theoverestimation of the temperature during spring and sum-mer seems to be due to soil thermal regeneration inducedby rainfall. This phenomenon is not accounted for in ourmodel.

The soil thermal model could not be specifically andcompletely validated nor calibrated due to the lack of soiltemperature measurements.

4. Simulation of the passive building

4.1. Description of the passive building

The building is located in Formerie, France, a smalltown at 100 km north of Paris (49.65�N, 1.73�E). It is com-posed of two attached dwelling units designed each for afamily of four people. Each dwelling is two-storied, withan inhabitable area of 132 m2, a garage, a terrace, a bal-cony and a garden. The rooms are the same for both ofthem: a hall, an office, a living-room and a kitchen down-stairs (Fig. 1), and three bedrooms, a bathroom and a sit-ting room upstairs. The orientation is the same for bothdwellings. The timber structure integrates a 22 cm celluloselayer, and an external 15 cm expanded polystyrene insula-tion layer is added. The slab is insulated with 20 cm ofpolystyrene. The attic insulation is formed by 40 cm of cel-lulose. High performance windows (triple glazed) and insu-lated external doors have been carefully set, in order toobtain good air tightness. Table 1 describes the composi-tion of the main building envelope elements. The thermalbridges considered here are 0.1 W m�1 K�1 for the edgeof the concrete slab and attic floor. After a blow door test,the air leakage at 50 Pa has been measured to 0.58 vol h�1,which complies with Passivhaus standard. The ventilationis controlled as follows for each dwelling:

– During the heating period, an earth-to-air heatexchanger (ETAHE) is connected in series with a heatrecovery ventilation device (HRV). The ETAHE iscomposed of one 30 m long polyvinyl chloride pipe

Table 1Description of the passive building envelope elements

Wall Description Insulation thickness (cm) U-value (W m�2 K�1)

External wall Wood structure with polystyrene and cellulose 38 0.12Slab Concrete and polystyrene (on crawl space) 20 0.19Attic Gypsum board and cellulose 40 0.11

Solar factor (–) U-value (W m�2 K�1)

Winter SummerWindows Triple-glazed with external Venetian blind 0.52 0.1 0.71External doors Wood insulated with polystyrene 0 0 0.78

Table 2Description of the standard building envelope elements

Wall Description Insulation thickness (cm) U-value (W m�2 K�1)

External wall Wood structure with polystyrene and cellulose 13 0.35Slab Concrete and polystyrene (on crawl space) 14 0.27Attic Cellulose 22.5 0.2

Solar factor (–) U-value (W m�2 K�1)

Winter SummerWindows Double-glazed with external Venetian blind 0.66 0.132 1.8External doors Wood 0 0 1.5

S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831 827

(diameter: 200 mm) buried at 1.6 m in the soil. TheHRV has a mean global efficiency of 70%. The totalair exchange rate is 0.6 vol h�1.– During the cooling period, the HRV is by-passed andonly the ETAHE is used. The air exchange rate is1.5 vol h�1.

The passive house is equipped with a solar water heaterand a heat pump for space and water heating backup.A 50% solar fraction and a COP of 3 have been assumedrespectively for the solar system and the heat pump, basedupon previous studies.

8 Inside a building, the annual overheating degree-days, defined on ahourly basis, with respect to a temperature limit Tlim are defined asODDT lim

¼ 124 �P8760

h¼1 ðT intðhÞ � T limÞþ, where Tint is the indoor tempera-ture and the superscript ‘+’ designates the positive part of the enclosedquantity.

4.2. Standard building used as a reference

The performance of the passive building describedabove is compared to a reference building assumed to com-ply with the French thermal regulation (ReglementationThermique 2005, 2006). Its characteristics are the same asthe passive building (dimensions, orientation, timber struc-ture), except for insulation, thermal bridges and air tight-ness, which correspond to the regulation reference level,and the standard equipment in French houses is consid-ered: a natural gas boiler for space heating and hot water,with a global efficiency of 78% (including heat generation,control, distribution and emission). The thermal character-istics of the envelope elements are given in Table 2.

The thermal bridges considered here are 0.4 W m�1 K�1

for the edge of the concrete slab, plus a standard heat lossof 0.5 W m�1 K�1 multiplied by the building perimeter,representing all other thermal bridges (openings frame,intermediate level, imperfections). During the whole year,

mechanical ventilation provides a 0.5 vol h�1 air exchange(plus 0.1 vol h�1 infiltration), without ETAHE or HRV.

4.3. Thermal simulation and life cycle assessment

The building geometry has been input using the ALCY-ONE software (Fig. 1). The building model comprises 10thermal zones, 5 for each building.

In order to distinguish heating and cooling behaviors ofthe buildings, two simulations were performed for eachalternative: one for a hot summer period (2003 heat wavein Paris area) and one for a typical heating period (averageyear in Paris area). No air conditioning is considered forboth buildings. Summer discomfort is evaluated usingoverheating degree-days (ODD)8 with respect to a limittemperature, set at 27 �C.

In a second phase, the thermal simulation results as wellas the building description are used to evaluate environ-mental impacts, using the EQUER software (Polsteret al., 1996). Twelve impacts are studied (Table 3). Theconstruction, operation and demolition phases are takeninto account. The various materials composing the ETA-HE, HRV, heat pump and solar panels are not taken intoconsideration in this assessment, assuming that the quanti-ties and related impacts are small compared to the con-struction materials and the operational energy use.

Table 3Impact indicators observed

Impact indicator Unit Legend

Cumulative energy demand GJ ENERGYWater consumption m3 WATERAbiotic depletion potential kg Sb-Eq RESOURCENon-radioactive waste creation t eq WASTERadioactive waste creation dm3 RADWASTEGlobal warming potential at 100 yr

(GWP100)t CO2-Eq GWP100

Acidification potential kg SO2-Eq ACIDIFEutrophication potential kg PO3�

4 -Eq

EUTROPH

Damage caused by the ecotoxic emissionsto ecosystems

PDF m2 yr ECOTOX

Damage to human health DALY HUMHEALTHPhotochemical oxidant formation

potential (Smog)kg C2H4-Eq

O3-SMOG

Odour M m3 ODOUR

Table 4Results of the thermal simulations

Unit Heating load Summer discomfort

kW h yr�1 kW h m�2 yr�1 Degree-days

Passive house 1978 7.5 22Standard house 19,885 75.3 56

828 S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831

5. Results

5.1. Energy consumption and comfort

The annual heating load of the passive building is evalu-ated at 7.5 kW h m�2, which complies with the Passivhausstandard, (15 kW h m�2 yr�1), the heating degree-daysbeing around 2700. It is about twelve times lower com-pared to the standard building (Table 4).

The ETAHE improves the summer comfort comparedto the standard building: 22 ODD27 �C vs. 56 ODD27 �C

for the standard building.Assuming a hot water consumption of 40 l inhabi-

tant�1 day�1, low-energy domestic appliances and a familywith low electricity consumption (2000 kW h per year andper family),9 the final energy consumption would be5404 kW h, therefore 20 kW h/m2 for the passive buildingand 35,219 kW h, therefore 133 kW h/m2 for the standardone. Using the ‘‘Passivhaus” primary energy factors(2.7 kW h primary energy per kW h electricity and1.1 kW h primary energy per kW h gas), the correspondingprimary energy consumption would be respectively55 kW hPE and 171 kW hPE for the passive and standardbuilding. Thus the passive building also complies with thePassivhaus standard, (120 kW h m�2 yr�1).

The actual electricity mix varies according to the season:electric heating and heat pumps increase the peak demand,and induce the use of thermal plants whereas moreconstant electricity usage like hot water and domesticappliances correspond more to a base production. In thisstudy, the considered electricity mixes are given in Table5. The resulting primary energy factor is 3.13 for heatingand 3.18 for other uses. Considering a gas efficiency of1.28 primary kW h/kW hth (from Ecoinvent Database10

Version 1.2), the energy assessment for both buildings

9 Derived from Cabinet Olivier Sidler (1997).10 See: http://www.ecoinvent.ch/.

(Fig. 5) shows that the primary energy consumptionof the passive building (65 kW h m�2 yr�1) is about athird compared to that of the standard house(200 kW h m�2 yr�1).

5.2. Environmental impacts over the building life cycle

The environmental assessment shows that the passivebuilding impacts are lower than those of the standardbuilding, except for the various wastes (Fig. 6). Thisimprovement is especially visible for impacts related tothe use of gas (global warming, resource depletion, acidifi-cation, summer smog. . .). The non-radioactive waste pro-duction is quite the same, due to similar materialquantities in both alternatives. The radioactive waste pro-duction increases (+29%) due to higher global electricityconsumption in the passive building (heat pump) than inthe standard one (gas boiler).

These various impacts for the whole life cycle have beennormalized using average national impacts per inhabitantduring 1 yr for France (Table 6); thus the normalized val-ues are expressed in one same unit: the ‘‘year-inhabitantequivalent” (Popovici, 2006). This is a mean to comparethese impacts on a same scale, which helps to distinguishthe preponderant impacts from the less significant ones.For instance, here, water consumption is high for bothbuilding designs (Fig. 7), showing that the passive designhas a limited influence on it and, consequently, that someadditional measures would be required to reduce signifi-cantly this impact. Fig. 7 shows the importance of energyand global warming potential (GWP) for the standardbuilding and the corresponding reduction induced by thepassive building. The relative contribution of the construc-tion phase to indicators related to energy consumption(energy, GWP) is increased for the passive building(Fig. 8). Nevertheless, for both standard and passive build-ing, the operation phase (including domestic appliances)remains the main contributor in the global environmentalimpact over the whole life cycle, except for waste genera-tion for which the demolition phase is dominant.

6. Discussion

The planned monitoring of the Formerie houses as wellas other passive houses in France will allow a more exten-sive validation process, complementing previous compari-son exercises (Peuportier, 2005): three experimentalvalidation tests (passive test sells in Stuttgart, Cadaracheand Zurich) and several software benchmarks (e.g. IEA

Fig. 6. Comparison of the two alternatives for twelve indicators (refer-ence = standard building). Indicators and unities are derived from(Popovici, 2006).

Fig. 5. Simulated annual energy balance for both buildings.

Table 5Considered average European electricity mixes and corresponding primary energy factors (from Ecoinvent Database Version 1.2)

Production type Nuclear Hydroelectric Gaz Coal Fuel Equivalent primary energy factor

Heating mix 37% 15% 10% 28% 10% 3.13Other uses mix 78% 14% 4% 4% 0% 3.19Primary energy factor 3.52 1.24 3.25 3.45 3.47

Table 6Year-inhabitant average in France for four impacts (Popovici, 2006)

Impacts Year-inhabitant averagein France (1997)

Unit

ENERGY 175 GJ inh�1 yr�1

WATER 339 m3

WASTE 10 t eq inh�1 yr�1

RADWASTE 0.51 dm3 inh�1 yr�1

GWP100 8.68 t CO2-Eq inh�1 yr�1

S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831 829

‘‘Bestest” procedure). The LCA tool EQUER has beencompared to seven other building LCA tools in the frameof the European thematic network PRESCO. The calcu-lated CO2 emissions for a case study were differing by±10% between the tools, but other environmental indica-tors like toxicity are more uncertain. Further work isplanned to progress towards harmonization of themethods.

The present work focuses on the building envelope andthe ETAHE. An average coefficient of performance hasbeen considered for the heat pump. Coupling heat pumpand ETAHE models is under way, allowing a more preciseevaluation of the electricity consumption of such systems inthe future.

The present study has focused on the thermal balance(energy consumption and temperatures) of the building.A more global evaluation of the comfort could constitutea further investigation.

Fig. 7. Contribution of each phase of the life cycle for both alternatives and

7. Conclusion

In order to assess the energy and environmental perfor-mance of a passive house in Greater Paris area, a model has

for four indicators, expressed in year-inhabitant equivalent in France.

Fig. 8. Contribution of each phase of the life cycle for both alternatives and for four indicators, expressed in percent of the total contribution.

830 S. Thiers, B. Peuportier / Solar Energy 82 (2008) 820–831

been developed for innovative ventilation systems and inte-grated in a thermal simulation tool. The model, includingthe main phenomena occurring in the ETAHE, has beenvalidated against experimental results.

Simulations have shown substantial reduction of energyconsumption and summer discomfort for the passive build-ing compared to a standard building. Thermal comfort canbe achieved most of the time using appropriate measures(solar protection, ventilation and possible ETAHE, ther-mal mass).

Moreover, during its life cycle, a passive house allowsthe reduction of most environmental impacts comparedto a standard building, and in particular, the GWP andexhaust of natural resources. Further improvement regard-ing e.g. the choice of building materials and renewable elec-tricity production could be studied in order to improve theenvironmental performance on other aspects (waste, toxic-ity. . .). Night ventilation could be studied to reach a highercomfort level. Therefore, the passive concept seems to be avalid and efficient solution to improve the environmentalperformance of the dwellings in the French context. Thebuilding presented in this article will be monitored, in orderto check simulation results and conclusions.

Acknowledgments

This study has been supported by ADEME (FrenchAgency for Environment and Energy Management),CANADA CLIM (Design and Construction of earth-to-air heat exchangers) and LES AIRELLES (Construc-tion of passive houses).

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