Energy performance

Buried pipeEarth tube

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. . . . . .cteristicarth-to-re . . . .ocesses. . . . . .HE . . .e perfoing app

. . . . . . . . . . . . 115

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There is a rising interest in heating and cooling systems based and cooling of buildings [1]. In these systems, the heat pump is

Contents lists available at ScienceDirect

w.e

Renewable and Sustain

Renewable and Sustainable Energy Reviews 28 (2013) 107116i.e. on the ground temperature. In closed loop systems, the heatE-mail addresses: klara.bz@gmail.com, kla_ra@tiscali.it (C. Peretti).on renewable energy sources. The ground source heat pump generally used for both the space heating and cooling; as a matterof fact, the ground acts as a heat source in the winter and a heatsink in the summer. As well known, the energy efciency of theheat pump directly depends on the heat source/sink temperature,

1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.rser.2013.07.057

n Corresponding author. Tel.: +39 49 8276889; +39 366 5433155.1. Introduction (GSHP) systems are a suitable alternative to conventional systemsto reduce the primary energy consumption required for heating6.1. EAHE performance and design: Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1. Building application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1146. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

. . . . . . . . . . . . 114Contents

1. Introduction . . . . . . . . . . . . . . . . .2. Methods . . . . . . . . . . . . . . . . . . . .3. Evaluation of EAHE design, chara

3.1. Working principle of an e3.2. Calculating soil temperatu3.3. Heat and mass transfer pr3.4. Heat transfer equations . .

4. Algorithms for heat transfer in EA4.1. EAHE models to predict th

5. HVAC system coupling and build. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108s and coupling with HVAC systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108air heat exchanger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110in EAHE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110rmance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111lication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Ground coolingGround heatingClara Peretti n, Angelo Zarrella, Michele De Carli, Roberto ZecchinDepartment of Industrial Engineering, University of Padova, Venice Street 1, 35131 Padova, Italy

a r t i c l e i n f o

Article history:Received 30 January 2013Received in revised form28 March 2013Accepted 21 July 2013Available online 14 August 2013

Keywords:Heat exchanger

a b s t r a c t

There is a rising interest in heating and cooling systems based on renewable energy sources. Air heatingor cooling with earth-to-air heat exchangers (EAHE) is one approach for reducing ventilation heat lossesand improving thermal comfort in buildings. A literature research was performed in order to analyze thedesign, characteristics of earth-to-air heat exchangers and whether they could be coupled with HVACsystem coupling. A range of projects was compared in order to collect and summarize design suggestions.

& 2013 Elsevier Ltd. All rights reserved.exchangers (EAHE). A literature review

The design and environmental evaluation of earth-to-air heatjournal homepage: ww lsevier.com/locate/rser

able Energy Reviews

Greek symbols

soil thermal diffusivity [m2/h]p moisture transfer coefcient [m/s] emittance of surface [] thermal conductivity of the soil [W/(m K)]cp volume heat capacity of the soil [J/(m3 K)]v water vapour concentration of the air in EAHE

[kgv/m3]v,sat saturated vapour concentration [kgv/m3]s StefanBoltzmann constant: 5.67108 W/(m2 K4)

Subscripts

d deliveryext externalg ground

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116108pump is coupled with the ground by means of heat exchangers,that can be vertically or horizontally oriented. Copious was thetheoretical and experimental work on GSHP systems [214].

In this paper a particular earth coupled heat exchanger isanalyzed, which is the earth-to-air heat exchanger (EAHE).Due to the high thermal inertia of soil, temperature uctuatesmuch less below the ground than at surface level. At a sufcientdepth, soil temperature is lower than the outdoor temperature insummer and higher in winter. When ambient air is drawn throughburied pipes, the air is cooled in summer and heated in winterbefore it is used for ventilation. An earth-to-air heat exchanger(EAHE) system consists of a network of buried pipes throughwhich air is transported by a fan. In summer the air supplied to abuilding is cooled and dehumidied because the soil temperaturearound the heat exchanger is lower than the ambient temperature.During winter, when the ambient temperature is lower than thesoil temperature, the process is reversed and the air is pre-heated.m air mass ow rate through the tube [kg/s]ma air mass ow rate [kg/s]rp,i internal radius of the pipe [m]rp,o external radius of the pipe [m]t time [s]T temperature [K]Va air volume ow rate [m3/s]z ground depth [m]IlTh

2.

earbuhealsWe

besh convective heat transfer coefcient [W/(m2 K)]total incident solar radiation on the surface [W/m2]latent heat condensation [J/kg]ca specic thermal capacity of air [J/(kg K)]cp specic heat of the air [J/(kg K)]gv condensating/evaporating amount of water vapour

[kg/(m2 s)]aNomenclature

absorptance of surface []e main advantages of the system are discussed in this paper.

Methods

Literature was searched by using the following key terms:th-to-air heat exchanger (EAHE, EAHX, ETAHE, ATEHE),ried-pipe system, preconditioning of air, and ground-coupledat exchanger. Selected proceedings and conference papers wereo searched, as was a number of electronic databases, includingb of Science, PubMed, Sciencedirect and Google Scholar.This paper analyzes the following:

Evaluation of EAHE design, characteristics, and coupling withHVAC systems.EAHE performance: existing models and related algorithmsused to evaluate heat transfer with the surrounding soil andthermal behavior of pipes. Simulation programs are alsodescribed.Case studies: building types, location and different approachesto evaluating EAHE systems.

This literature search enabled us to outline suggestions andt practices for designing future systems, including EAHE.3. Evaluation of EAHE design, characteristics and couplingwith HVAC systems

An EAHE system typically consists of an inlet shaft with ltersand a network of pipes buried in the ground through which air istransported by a fan (Fig. 1).

The network of buried pipes can be installed in open spaces(Fig. 2a and b) or beneath buildings (below the foundation slab) asa grid, serpentine or ring layout. The ring layout is used aroundpaved areas (see Fig. 2(c)) [15].

Open-loop and closed-loop are the two main types of EAHE.Each one takes air from a different point, the former fromoutdoors, the latter from indoors [16].

EAHE components are described in Table 1.

3.1. Working principle of an earth-to-air heat exchanger

m means suctionw surface of the pipeThe principle of using soil thermal inertia dates back to theancient Greeks and Persians [20]. In Medieval Italy, special cavescalled covoli were used to precool or preheat air before it entereda building. The energy performance of EAHE depends on the heattransfer between the soil and air inside a pipe. Fig. 4 illustrates the

Fig. 1. Example of an EAHE.

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116 109main conditions that inuence heat and mass transfer within anEAHE. See Fig. 2 for EAHE types (a, b and c).

Heat is transferred to or from the surrounding soil by conduc-tion through the pipe thickness and by convection with the pipesinternal surface (Fig. 3). Conduction heat transfer is transient andfully three-dimensional in the soil [19]. Condensation and

Fig. 2. Different pipe layou

Table 1Component description.

Material Features

Air inletAir intake tower Stainless steel Dimension: 2 m

High efciency lterPipeSeamless pipes Steel [1719] Resistance coefcients, w

burying depth. Pipes shostable and accessible for

Aluminum [20,21]Plastic: PE, PVC, PPa Copperb

Stony pipes ConcreteBrick

FanAir intake fan Diameter, height and lExhaust fanCondensation management Two types: integrated a

a The most widely used today [22].b Early applications [17].

Fig. 3. Environmental factors and heat-transfer me

Fig. 4. Steps in evaluatingevaporation should also be taken into account. A general diagramcan be drawn up to evaluate EAHE performance (see Fig. 4).

The rst step is to calculate ground temperature on the basis ofclimatic and ground parameters. The second is to calculate outletair parameters, which are inuenced by EAHE characteristics.EAHE models are described in the next part. Once outlet air

ts below ground level.

Function

Introduce and lter outdoor air

all roughness, diameter, length, anduld be anticorrosive, structurallyinspection and cleaning

Transport air and exchangeheat with soil

ter type Take air from outdoorsRemove exhaust air

nd standalone Drain condensation and expel water

chanisms that inuence soil temperature [23].

EAHE performance.

parameters have been calculated, cooling (Qc) and heating poten-tial (Qh) integrated during summer/winter can be calculated withthe follow equations:

Qh _m cp TdTst J 1

Qc _m cp TsTdt J 2where t is the calculation time step [s].

3.2. Calculating soil temperature

When analyzing natural heat ow in a shallow soil region, theground may be considered a semi-innite solid and heat conduc-tion is calculated with Fouriers Law. The daily mean groundsurface temperature follows a harmonic variation, and its yearlyamplitude is more or less equal to that of the air. Kusuda

thermal resistance was set to 0.067 m K/W; the absorptivity andemittance of the ground surface was assumed to be equal to

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116110and Achenbach [24] have mathematically modeled Eq. (3), whichdescribes/calculates the annual sub-surface soil temperature basedon heat conduction theory applied to a semi-innite homogenoussolid [25].

Tgz; t Tm Asurf expz=87600:5

cos 28760

tt0z2

8760

0:5 !" #3

where t is the time elapsed from the beginning of the calendaryear in hours; Tm is the annual mean soil temperature [1C]; Asurf isthe amplitude of surface temperature variation [1C]; and t0 is thephase constant of the lowest average/mean soil surface tempera-ture in hours since the beginning of the year.

The annual mean soil temperature is generally estimated byconsidering the annual average/mean air temperature. This assump-tion is valid when there is no anomalous gradient temperature orsignicant groundwater ow.

The results of Eq. (3) are shown in Fig. 5, which uses thefollowing parameters: soil volume heat capacity equal to 2.07 MJ/(m3 K), soil thermal conductivity equal to 1.8 W/(m K), Tm equalto 13 1C, As equal to 11 1C, and t0 equal to 31 days.

A detailed model has to take into account the interactionbetween the ground and the ambient environment. The groundsurface interacts with the aboveground environment; the rate ofheat entry into the ground surface is a combination of convectionheat transfer with external air, incident solar radiation and radiantheat exchange with the sky. As a consequence, the heat balance forthe ground surface is expressed as follows:

Q conduction into the ground hext TextTg a I s T4gT4sky4

Fig. 5. Example of ground temperature oscillation calculated with the Kusuda and

Achenback model [24].0.8 and 0.9, respectively. The sky temperature Tsky is determinedwith Swinbanks correlation [27]. Fig. 6 illustrates the tempera-tures calculated. Fig. 6ac shows the results when only external airtemperature is applied as a boundary condition, whereas Fig. 6bdshows the ground temperature prole when the radiant heatexchange on the surface is taken into account.

3.3. Heat and mass transfer processes in EAHE

Soil temperatures more than 2 m below the surface are close tothe annual mean temperature, while the inlet air is the warmest ofthe year in summer. As air passes through an EAHE, the convectiveheat transfer takes place between the air and duct surfaces. Theenthalpy and the dry bulb temperature of the air decrease alongthe ow direction. If the duct is long enough and the duct surfacesare cooler than the dew point temperature, condensation maytake place. In Fig. 7, heat and moisture balance are represented ona longitudinal control volume in the pipe.

3.4. Heat transfer equations

Differential Eq. (5) describes the heat balance of owing airwhen latent heat generation is taken into account:

Tax

TaTw;iha2r0maca

gvl2rp;imaca

0 5

Differential Eq. (6) describes the heat conduction around thepipe:

cpTt

2Ty2

2Tz2

6

where cp and are the density, specic heat and the thermalconductivity of the ground. The heat balance for pipe is:

TaTw;i ha 2 rp;i Tw;iTw;o

1=2lp lnrp;o=rp;i7

Moisture transfer: Differential Eq. (8) describes the moisturebalance on the longitudinal element

vx

v v;satTw;i b2rp;i

Va 0 8

4. Algorithms for heat transfer in EAHE

Several theoretical and experimental studies have been madeto evaluate the thermal performance of EAHE. Tzaferis et al. [28]studied heat transfer in EAHE with eight algorithms, which can bedivided into two groups: the rst calculates convective heattransfer from the circulating air to the pipe and then the con-ductive heat transfer from the pipe to the ground and inside theground mass; and the second calculate only the convective heattransfer from the circulating air to the pipe. Tzaferis et al. [28]Short- and long-wave radiant heat exchange at a depth ofbetween 1 and 2 m has a major effect on the top layer of the soil.For this purpose a detailed numerical model based on the nitedifference method was developed. In this model, Eq. (4) expressesthe heat balance of the ground surface and under the ground theheat transfer takes place only by heat conduction. By means of thissimulation tool, the effect of the aboveground environment on thesoil temperature for two Italian cities (Milan in the North andPalermo in the South) was analyzed. This study considers the testreference years [26] of the cities analyzed. The specic convection

2concluded that almost all the proposed algorithms are sufciently

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116 111accthe

4.1

ma

cie

and

Fig.(b)

Fig.urate when predicting the temperature of the outgoing air fromEAHE.

. EAHE models to predict the performance

Many models are today available for evaluating the perfor-nce of EAHE. They can be divided into:

analytical methodsnumerical methods.

Thermal performance (temperature performance and energy ef-ncy) of EAHE can be calculated using a range of approaches [20]:

specic energy supply [kW h/(m2y) and W/m2];heat transfer NTU and hmean;temperature behavior ;energy efciency (COP).

Input parameters for the model can be divided into inexibleexible [15], as illustrated in Table 2.

6. Effect of the short- and long-wave radiant exchange on ground temperature at dand (d) with radiant heat exchange.

7. Heat balance (a) and moisture balance (b) on a longitudinal control volume.Different parametric and numerical models for EAHE have beenpublished in the last 20 years and a wealth of literature hasfocused on EAHE performance. The heat transfer models for EAHEconsidered in this paper are summarized in Table 3 inchronological order.

The model developed by Mihalakakou et al. [29] considers thatthe energy transfer inside the soil is driven by simultaneous heatand moisture transfer gradients along both axial and radialdirections. By superimposing the heat transfer from more thanone duct, Mihalakakou et al. [30] modied the previous model sothat it could simulate multiple-pipe EAHE. The authors dened thedifference between the inlet and outlet air temperatures as theEAHE energy potential.

Sodha et al. [31] proposed an analytical model to determine theannual heating and cooling potential of underground air pipesystems. The model assumes that the thermal properties of thesoil are constant and homogeneous and ignores condensation andevaporation within the pipes. The model considers the inuencethat the pipes have on each other and neglects the hourlyvariations of the time-dependent input parameters (ambient airtemperature, solar radiation, relative humidity).

Bojic et al. [32] developed a method to solve heat transfer inthe soil by horizontally dividing the earth into a number of parallellayers, each with a uniform temperature. Heat transfer betweenthe soil layers is solved with energy balance equations for each soillayer. Two years later, Bojic et al. [18] modied the method bydividing the earth layers into smaller control volumes.

A numerical model for multiple-pipe EAHE was developed andvalidated by Hollmuller and Lachal [33]. The model considerscomplex geometries/designs, more ground characteristics andmore boundary conditions. It uses the nite elements method

ifferent depths: (a) and (c) only external air temperature as a boundary condition,

)])

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116112Table 3Heat transfer models for EAHE.

Authors and method Objective Input

Table 2Input parameters for the model according to VDI 4640Part 4.

Prespecied (inexible) parameters

Location/weatherThermal proprieties of the ground: (E [kg/m3], E, [W/(m K)], cE [k J/(kg KBacklling materialGround water contentGround straticationUsable ground areaGround water temperature and level Building cooling loadfor the resolution. It can also consider water inltration, pressurelosses and the control of air ow direction in the pipes.

Ghosal et al. [34] developed a thermal model to investigate theperformance of EAHE connected to a greenhouse. The model isbased on the following assumptions: the analysis is based onquasi-steady state conditions, air ow is uniform along the lengthof the buried pipes and there is no radiative heat exchangebetween the sides of the buried pipe.

Badescu [17] developed a ground heat exchanger model basedon a numerical transient bidimensional approach. He proposedsegmentation into sections that are perpendicular to the pipes. Ineach section, the heat equation is solved with the volume controlformulation method. The interaction between different sections ismade with the energy balance of the pipe (no lateral heat transferon the ground is considered).

Thiers and Peuportier [35] consider the interaction betweenseveral parallel pipes laid at the same depth. This system uses a

Mihalakakou et al. [29].Transient, implicitnumerical model forsingle pipes

Predict air and soil temperature elds beneatha building

Soil charactemperatu

Mihalakakou et al. [30].Numerical model (formulti parallel pipes)

Describe the simultaneous heat and masstransfer inside the pipe and in the soil. The soils natural thermal stratication is taken intoaccount

Independepipe inlet,temperatu

Sodha [37]. Analyticalmodels

Determine the annual heating and coolingpotential of underground parallel air pipesystems

Environmerelative hucharacteris

Bojic [18]. Numericalmodel for two-pipesystems: time-marching method

Calculate energy transfer from soil to EAHE Soil and psolicitation

Hollmuller and Lachal[33]. Explicitnumerical model formulti-pipe systems

Predict latent and sensible heat exchanges Soil propeair tempertemperatu

Ghosal [34]. Simpliedanalytical model forgreenhouses (solvedwith MATLAB)

Evaluate EAHE performance in terms of thermalload leveling (TLL) and COP

House desconvectiveundergrouthe buried

GHE model [17].Numerical transientbi-dimensionalapproach

Calculate energy provided by heat exchangerduring cold or warm season

Temperaturadiation o

Thiers model [35]. Modalanalysis for theresolution ofdifferential equations

Calculate outlet air temperature duringsimulation period

EAHE shapmodel: soparameter

Convolutive responsefactors method [36].Numerical model

Solve problem of conduction by reducingcomputational time

Soil propesolicitationtemperatu

AGHX model [38]. Quasi3D nite elementsmethod

Analyze energy performance, which dependson a wide range of parameters

Soil type,efciency,Output

Freely selectable (exible) parameters

Flow ratePipe lengthPipe diameterPipe materialInstallation depthDistance between grid pipes or from nearby buildings or underControl modesnite volume formulation with a limited number of meshes andenables quick calculations. Two concentric cylindrical meshes areused for each pipe. If the pipes are fairly well-spaced, anothercylindrical mesh is added; if they are close, the third meshincludes every pipe, so as to take their interactions into account.

One year later Tittelein et al. [36] developed a new numericalmodel for EAHE; heat ux entering the pipe is expressed as afunction of the temperature of the air owing through the pipeand the external solicitations. A heat balance is then applied foreach layer to nd the outlet air temperature.

The accuracy of parametric and numerical models can be testedagainst experimental data for the thermal performance (energyand temperature) of EAHE. The problem with most of the afore-mentioned models is that it may take a long time to calculateexchanger behavior accurately due to the type of mesh required.However, simulation software can be used to reduce time andsimplify EAHE analysis. Several mathematical models for EAHE

teristics, geometries, pipe airre, and mean annual soil temperature

Undisturbed soil temperature; soilmoisture content and circulating-airhumidity

nt variables: pipe radius, distance fromand time. Dependent variables: soilre and moisture content

Air temperature at pipe outlet

ntal parameters: ambient temperature,midity, and earth/soil temperature. Pipetics

Heating and cooling potential [kW h]

ipe characteristics, geometries,of the soil, pipe air temperature

Heat ux through volume elements,temperature in the volume elements

rties, shapes, solicitation of the soil, pipeature, surface soil and pipere, and humidity ratio

Latent and sensible energy exchanges(from air to soil) in kW h

ign and features, solar radiation,heat transfer coefcient fromnd earths surface to owing air insidepipes

Temperature of greenhouse aircombined with earth-to-air heatexchanger

re distribution in soil, incident solarn a horizontal surface, weather data

Yearly energy provided by the groundheat exchanger [kW h/year]

e, outdoor air temperature, soil thermalil characteristics and ambients

Soil temperature as a function of pipedepth, time and distance from the pointconsidered to the left of the foundationslab

rties, composition, annual and dailyof soil, inlet air temperature, surface

re of soil and pipe

Incoming heat ux is the responsefactor, steady-state conductance of thesystem

pipe and system characteristics, fanweather data

Heating and cooling potential

tackle uid ow problems. It provides numerical solutions of/for/inpartial differential equations governing airow and heat transfer in

Featu

Thetemp

Num

OutptimeOutpwint

OutpperfoInputempand

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116 113have been used in simulation software either dedicated to EAHE orto overall building-system analysis. Table 4 shows the character-istics of the most common software.

The EnergyPlus software contains an earth tube mathematicalmodel developed by Lee and Strand [39,40] and it uses a detailedalgorithm to calculate soil temperature variation around a speci-ed earth tube/pipe for every time step of the simulation.

EAHE thermal modeling can be evaluated within the TransientSystem Simulation Software Program (TRNSYS) environment.Researchers have used this software in different ways: Ahmedet al. [48] used the TRNSED application to simplify simulation.Hollmuller and Lachal [43] developed a user-friendly interface withType 460 Air-to-soil heat exchangers. Al-Ajmi et al. [49] used theTRNSYS-IISIBAT program for EAHE model (Type 264) with the sub-soil model (Type 263).

The Widerstands-Kapazitaten-Modell (WKM-LTe, resistancecapacity model) is a calculation model for the simulation ofearth-to-air heat exchangers. The model is similar to the heatand electricity models [44]. This software creates a yearly simula-tion of the ground system with heat recovery and bypass, easilyintegrates weather data, takes into account collective ducts andfunnels, calculates pressure drops in the ground, suggests pipetypes and ground characteristics, takes into account the inuenceof a basement, provides an excel input and output interface, andenables air ventilation way and airow rates to be selected.

The Division of Building Physics and Solar Energy at the Universityof Siegen, Germany, has developed a commercial software, GAEA

Table 4Software for EAHE models.

Description

EnergyPlus Simulation software currently supported bythe United States Department of Energy. Itimplements the complete Heat Balance approach

TRNSYS+Calculation of soiltemperature

TRNSYS is a complete and modular simulationenvironment for the study of dynamic systems

WKM Design tool developed by Huber EnergietechnikAG (Zurich). Commercial software

GAEA (Graphische AuslegungvonErdwarmeAustauschern)

Design tools developed by University of Siegen.Commercial software

L-EwtSim Design tool developed by DLR Koln (AG-Solar).Downloadable freeware

REHAU Awadukt ThermoSoftware GHE 1.03

Design tool developed by REHAU(private tool) for its own in-house use only(Graphische Auslegung von Erdwaerme Austauschern) for the designof EAHE [46]. This software is based on the calculations of heatexchange between the soil, buried pipes and air in the system; it alsotakes into account variations in soil temperature, airow rate andambient air temperature. An optimization routine enables a choicefrom a range of layout variations, which inuences heat gains and cost.A validation study of GAEA was published by Heidt and Benkert [50].

L-EwtSim is a German freeware. No literature details researchusing L-EwtSim.

REHAU Software was developed as an in-house design tool. Themodel calculates heat transfer between adjacent elements indened time steps. For each time step the software takes intoaccount the air ow rate, soil temperature and other inuentialparameters, e.g. ground water. The software was validated againstsimulation results from TRNSYS calculation models and resultsfrom test areas. The tool performs calculations based on pipelength or set target temperature extremes.

Computational Fluid Dynamics (CFD) has been well known as apowerful method to study heat and mass transfer for many years.CFD codes are structured around the numerical algorithms used todiscretized form.FLUENT [51] is one example of CFD software used for EAHE

simulation. The thermal model of an EAHE system was simulatedconsidering a 3D transient turbulent ow with heat transferenabled.

Another CFD software used for EAHE simulations is PHOENICS[52]. This methods solutions imply that the integral conservationof quantities, such as energy, mass and momentum, are exact overany group of control volumes and over the whole calculationdomain. The CFD approach, however, requires a great deal ofcomputational resources and lengthy calculation times over a longperiod.

EAHE are governed by a number of standards and guidelines.European standard EN 15241 [53] proposes a simplied methodfor calculating ground-to-air heat ux. Part 4 of the Germanguidelines VDI 4640 [15] (now under review) provides a systemdescription, environmental aspects and air-hygiene recommenda-tions. VDI 4640 also provides an extensive description of thedesign procedure and distinguishes between small systems forresidential buildings with ow rates up to 1000 m3/h and largesystems for higher ow rates. Piping materials have to meet arange of air-duct hygiene and corrosion requirements.res References

model developed heat transfer and soilerature algorithms into a program that simulates EAHE

[39,40]

erical model that predicts thermal performance of EAHE [29,41,42,43]

ut: temperature trend (inlet/outlet), EAHE operating, heat recovery, winter and summer energy performance

[44,45]

ut: temperature trend (inlet/outlet), EAHE operating time,er and summer energy performance, payback time

[46]

ut: Temperature prole in tube/pipe, temperature trend, energyrmance

[47]

t: Ground and outdoor parameters; air conditioning plan. Output:erature trend (inlet/outlet), operating time, heat recovery, wintersummer energy performance

5. HVAC system coupling and building application

In recent years, mechanically ventilated buildings have seentheir energy efciency increase. Hybrid ventilation, which usesmechanical and natural airow driving forces (alternating orsimultaneous), has been used in many building designs [54]. It isbased on reducing pressure drop in the ductwork by increasing itscross-sectional area so that natural airow driving forces, such asbuoyancy and wind, can be used to reduce fan energy consump-tion. When EAHE are incorporated into a hybrid ventilationsystem, the cross-sectional areas of the duct should be muchlarger than those of the conventional ducts used in mechanicalventilation systems. Combined EAHE and hybrid ventilation isregarded as a new approach to improving building energy ef-ciency (Fig. 8).

Fink et al. [55] describe three types of HVAC integration forofces and administration buildings. The rst, dened comfortcooling (see Fig. 6(a)), is designed to improve comfort. Thesecond, dened space cooling is designed to control roomtemperature (see Fig. 6(b)) in order to avoid exceeding a default

(3)

(4)

(5)

(6)

n: E

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116114temperature. The last type, dened support cooling uses anEAHE to support a conventional cooling system (see Fig. 6(c)).

Boijc [56] studied four devices employing refuse and renewableenergy: an air-to-air heat pump (HP), a heat-recovery exchanger(HRE), an air-to-earth heat exchanger (EAHE), and an air-mixingdevice (MD). A computer was used to study different combinationsin order to nd one that would consume the least energy. Boijcconcluded that the MD and HRE have a major inuence on energysavings but that EAHE have a relatively slight inuence.

For residential buildings, VDI 4640 recommends that ventila-tion systems include an efcient heat recovery unit.

Chlela et al. [57] analyzed an EAHE coupled with heat recoverybalanced ventilation systems in order to evaluate their impact on theenergy consumption and thermal comfort of a detached house. Theauthors concluded that a balanced ventilation system signicantlyreduces a buildings heat demand and thus its CO2 emissions.However, the earth-to-air heat exchanger makes a marginal con-tribution to the energy savings and reducing CO2 emissions. Oneadvantage during the cold season is that the heat recovery unit isprotected against freezing. An EAHE has great potential for improvingthermal comfort in summer, but to ensure good thermal comfortthroughout the summer, it should be combined with other solutions.

5.1. Building application

An EAHE system is suitable for different types of buildings;Table 5 provides a summary and analysis. The rst column containsthe building and climate type, as per Kppens climate classication:Tropical/megathermal climates, Dry (arid and semiarid) climates,

Fig. 8. HVAC system integratioTempPolar

6. D

6.1.

Inwe hsyste

(1)

(2)erate/mesothermal climates, Continental/microthermal climate,climates, and Alpine climates.

iscussion

EAHE performance and design: Considerations

order to summarize advice and suggestions for EAHE design,ave included the following aspects that inuence overallm performance:

Ground temperature is governed by the aboveground envir-onment. The earths surface may be blackened or glazed toincrease the subsurface temperature, but to decrease thesubsurface temperature, the earths surface can be shaded,painted white or wetted with sprayed water.Pipe depth and positioning: pipes can be positioned beneath thebuilding or in the ground outside the building foundation.

(7)

(8)

(9)

(10)the heat stored by the ground is higher in vegetation-coveredsoils for heating but is lower for cooling (in absolute values)Climate and soil composition: the high water content of somesoil typologies improves EAHE performance. Soil must beclosely packed around the pipe to foster heat exchange,consequently compacted clay or sand is recommended [62].Pipe radius: an increase in the buried pipes radius leads to areduction in the convective heat transfer coefcient; this leads toa lower air temperature at the pipe outlet and thus reduces thesystems heating capacity. Moreover, reduced outlet air tempera-ture is associated with increased pipe surface, as the pipe radiusthere are no signifcant advantages in using pipes over 70 mlong [40]. Optimal length depends on climate conditions [62].Soil cover: Mihalakakou [71] states that bare soil surface mayincrease a systems heating capacity. Badescu [17] combinedhis analysis with a cooling mode simulation, concluding thataccount that the pipe depth affects the whole system efciency,as also reported by Esen et al. [13].Pipe length: an increase in the buried pipes length causesoutlet air temperature to rise, which means that the systemspotential heating capacity may also increase [71]. However,(which decreases with depth) and excavation costs (whichincrease with depth). Ascione et al. [62], however, state that3 m is an optimum compromise. We may therefore concludethat optimum depth must be evaluated in each case, taking intoAn increase in soil depth above the pipes provides a considerableincrease in the systems potential heating capacity [71]. Accord-ing to Badescu [17], burying the pipe 2 m below the surface is agood compromise between a small yearly temperature excursionAHE and ventilation unit [54].increases [71]. Typical diameters are 10 cm to 30 cm but may beas large as 1 m for commercial buildings.Pipe material: pipe material has little inuence on summer andwinter performance [17]. The different thermal conductivityvalues scarcely inuence heat exchange, if pipes are the correctdepth and length. Concrete pipes require an additional internalcoating to prevent radon inltrations; furthermore the pipeinterior must be perfectly hygienic, consequently an antimicro-bial coating is recommended [62].Pipe spacing: if parallel pipe systems are used, pipes should bekept approximately 1 m from each other in order to minimizethermal interaction. Greater spacing was not found to bring anyextra benet [66].Pipe number: it depends on air-ow requirements, pipelength and layout.Air velocity: increased air velocity in the pipe leads to a slightdecrease in outlet air temperature. This is mainly due tothe increase in mass ow. High air velocities are not energyefcient [62].

triceasueasuarieasueasueasu

easuic sim

easurements and interpretation+dynamic simulation (TRNSYS and [63]

easumon

buildingGermany: oceanic climate)ion

Commercial building: ofce (Switzerland: oceanic climate) Data measuResidential building (India: humid subtropical climate) Data measuResitrsaUsu

ic sim

Resi ic sim

cleasu

ic simion

C. Peretti et al. / Renewable and Sustainable Energy Reviews 28 (2013) 107116 115ResidScho(11)

(12)

7. C

Tpromto acsummavoidcombinfor

EdeseTheyEurosuch

MEAHEhousenerbuildcond

SchSchSchdential building: detached house (north Italy: humid subtropicalimate)

Data m

ential building: detached house (Kuwait: hot desert climates) Dynamol (Italy: Mediterranean climate) SimulatcliResidential building: three-zones (Comparison between south USA:opicalvannah climate, central USA: humid continental climate, south/westSA:btropical arid climate, and north USA: semi-arid climate)

Dynam

dential buildings: two-dwelling passive buildings (France: oceanicmate)

DynamTable 5Buildings and projects.

Building type and climate Method

Greenhouse (Greek: Mediterranean climate) ParameGreenhouse (India: humid subtropical climate) Data m

Data mand Tiw

Farmhouses (India: humid subtropical climate) Data mLivestock building (Switzerland: oceanic climate) Data mMultifamily and commercial building (Comparison between CentralEurope:oceanic climate, and Southern Europe: Mediterranean climate)

Data m

Commercial buildings: three ofces (Germany: oceanic climate) Data mCommercial building: ofce (Comparison between north Italy: humidsubtropicalclimate, and south Italy: Mediterranean climate)

Dynam

Commercial building: ofce (Belgium: oceanic climate) Data mCOMIS)

Commercial building: ofce (Germany: oceanic climate) Data mEnergy

Commercial buildings: ofces and administration buildings (reference SimulatSeasons: exchanged heat is on average about 1.3 times higheron summer days than on winter days [18].Control mode: it is closely related to building use. The mostconvenient solution for ofce buildings is to use EAHE through-out the day. Thus, a 15-h day is the best solution [62]. The systemshould be bypassed when the outside temperature enters acertain range (for example 1522 1C); the system can be con-trolled by ground temperature or by external air temperature.

onclusions

he European Energy Efciency Building Directive 2010/31/CEotes the adoption of passive strategies for buildings in orderhieve high levels of indoor thermal comfort, especially iner, thus reducing the use of air conditioning systems oring them all together. We looked at EAHE performance andining EAHE with other passive strategies in order to providemation and suggestions for designing and evaluating EAHE.AHE can be installed in different types of climate, such as hotrt, Mediterranean, humid subtropical and oceanic climates.can be designed both for cool climates such as Western

pe (e.g. Germany, Switzerland) and for warmer countries,as India and Kuwait.any types of buildings combine ventilation systems with. For residential buildings an optimal combination is green-es and EAHE. Moreover, in combination with other low-gy cooling techniques (e.g. night cooling) and good thermaling design, EAHE may eliminate the need for an air-itioning system in many cases.

ool (Norway: maritime subarctic climate) Data measuool: university (Greek: Mediterranean climate) Data measuool: laboratory center (China: humid subtropical climate) Data measuulation (COMFIE) [35]

rements and interpretation [68]

ulation (TRNSYS-IISIBAT) in desert climates [49]with GAEA [67]rements and interpretation+dynamic simulation (TRNSYS) [64]itoring [65](WKM and TRNSYS) [55]

rements and interpretation [66]rements and interpretation [67]ulation (EnergyPlus) [40]References

analysis (TRNSYS) [20]rements and interpretation+analysis with MATLAB [34]rements and interpretation+thermal model developed by Ghoshal [58]

rements and interpretation [59]rements and interpretation+non-steady-state heat ow model [60]rements and interpretation+dynamic simulation (TRNSYS) [33]

rements and interpretation+parametric model (NTU method) [61]ulation in a range of Italian climates [62]Simulation combined with data measurement interpretation isthe best way to analyze the real behavior of an EAHE system, but itmust be considered alongside building characteristics and buildingmanagement.

Acknowledgements

Wewould like to thank the European Social Fund (ESF) for theirnancial support of the research project Determination of theenergy efciency of buildings with passive cooling system, withair-to-earth heat exchangers, under which this work wascarried out.

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The design and environmental evaluation of earth-to-air heat exchangers (EAHE). A literature reviewIntroductionMethodsEvaluation of EAHE design, characteristics and coupling with HVAC systemsWorking principle of an earth-to-air heat exchangerCalculating soil temperatureHeat and mass transfer processes in EAHEHeat transfer equations

Algorithms for heat transfer in EAHEEAHE models to predict the performance

HVAC system coupling and building applicationBuilding application

DiscussionEAHE performance and design: Considerations

ConclusionsAcknowledgementsReferences