1 MINIMISATION DES BESOINS 1.1 BESOINS EN CHAUFFAGE · température unitaire. Si on inclue dans ce déperditions le réchauffage de l’air neuf introduit dans les locaux, on obtient

TAREB Principes des énergies renouvelables

1 MINIMISATION DES BESOINS

1.1 BESOINS EN CHAUFFAGE

«L’énergie la moins polluante est celle qu’on ne consomme pas» (citation Ademe, Agence de l'environnement et de la maîtrise de l'énergie). Derrière cette évidence se trouve une des clés du développement durable dans le domaine du bâtiment : les économies d’énergie. Quelque soit la ressource énergétique utilisée, la minimisation des besoins est la première étape de la conception d’un bâtiment s’inscrivant dans le cadre de la haute qualité environnementale.

La mise en place en 1974 d’une réglementation thermique a contribué de façon importante à la réduction des besoins en énergie de chauffage des constructions neuves (textes de 1974, 1982, 1989 et 2000).

La consommation d’énergie des habitations a ainsi été divisée par trois environ en une trentaine d'années (Fig 5.1).

Fig 5.1 – Influence des réglementation sur la consommation (D'après documents Ademe / A.F.M.E)

Actuellement l'ordre de grandeur de la consommation d'énergie pour le chauffage d'un bâtiment d'habitation en France (conforme à la RT 2000) est de 80 à 90 [kW.h/an.m²]. Les évolutions réglementaires prévues d'ici 2010 porteront cette valeur autour de 65 à 70 [kW.h/an.m²].

On sait qu'on peut aller beaucoup plus loin. Il est en effet techniquement possible de construire des bâtiments d'habitation dont la consommation de chauffage est inférieure à 15 [kW.h/an.m²], (certification "Passiv Haus" des maisons individuelles allemandes) et même 10 [kW.h/an.m²] (Fig 5.2).

Chapitre 5 Energie Confort et Bâtiments

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Fig 5.2 – Consommation selon les règlementations (Cahiers Techniques du Bâtiment)

Ces technologies ont été validées sur plusieurs centaines de constructions. Déjà, le programme Minergie des autorités suisses (Fig 5.3) valide des maisons individuelles atteignant la limite de 38 [kW.h/an.m²] alors que la construction classique atteint 100 [kW.h/an.m²] (SIA 380/1) et que 55 [kW.h/an.m²] est déjà considéré comme une faible consommation énergétique.


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Fig 5.3 – Programme Minergie (Liebard et De Herdre)

La conception d’un bâtiment économe en énergie nécessite une étroite liaison entre la conception architecturale et la conception des équipements techniques. La modélisation des transferts thermiques permet d’avoir une approche prévisionnelle des besoins en énergie pour le chauffage et la climatisation des locaux. Le bilan énergétique d’un bâtiment est particulièrement sensible à deux facteurs : la qualité thermique de l’enveloppe et l’utilisation des apports solaires.

1.2 QUALITE THERMIQUE DE L'ENVELOPPE

Il s’agit de diminuer les déperditions de chaleur vers l’extérieur (situation hivernale). L’isolation thermique des parois permet d’obtenir ce résultat :

• pour les parois opaques, en utilisant des matériaux alvéolaires et des doublages en mousse de synthèse, fibres végétales ou fibres minérales,

• pour les parois vitrées, en utilisant des vitrages doubles, avec traitement de surface dans le domaine des rayonnements à grande longueur d’onde (V.R.I : Vitrages à Isolation Renforcée).


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Briques type “monomur” (Sturm)

Isolation intérieure (Rockwool)


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La valeur du coefficient U (conductance globale en [W/m 2.K]) caractérise en régime permanent la qualité thermique de ces parois. La réglementation thermique RT 2000 impose des valeurs maximales du coefficient U des parois, selon la zone climatique concernée (Fig 5.4).

Fig 5.4 – Exigences réglementaires (Guide RT 2000)

Un soin particulier dans la conception de l’enveloppe doit être accordé à la réduction des ponts thermiques (Fig 5.5), qui peuvent représenter une part importante des transferts de chaleur (jusqu'à 40% selon le C.S.T.B, Centre Scientifique et Technique du Bâtiment).


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Fig 5.5 – Ponts thermiques (Schöck- Rutherma)


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Ponts thermiques en photo infra- rouge (Schöck)

Correcteur de pont thermique (Schöck- Rutherma)


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Une forte compacité de la construction (rapport de la surface d’enveloppe au volume habitable) est également favorable à la réduction des déperditions thermiques (Fig 5.6).

Fig 5.6 – Influence de la compacité sur le périmètre exposé

Il en est de même pour l'utilisation des mitoyennetés (Fig 5.7).


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Fig 5.7 – Influence de la mitoyenneté des maisons

La qualité environnementale de certains matériaux d'isolation est actuellement remise en cause (fabrication, émission de polluants, fin de vie), alors que des matériaux respectueux de l'environnement trouvent accès à une part du marché :

• fibres végétales (chanvre par exemple),

• laine de mouton, etc...

Les briques multi - alvéolées assemblées à joints minces permettent aussi de satisfaire aux besoins d'isolation sans nécessiter de doublage.

L'énergie utilisée pour la fabrication des matériaux est également un facteur de sélection (Fig 5.8). Ainsi, les fabricants de laines minérales (Filmm) déclarent que l'énergie (et le CO2) à considérer pour la fabrication des fibres est équivalente à seulement 1 mois d'économie d'énergie engendrée par leur mise en œuvre dans le bâtiment.


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Matériau Conductivité[W/m.K]

Energie incorporée[kW.h/m 3]

Fin de vie

polystyrène 0,028 à 0,035 450 à 850 non recyclablecontient HCFC

polyuréthane 0,025 à 0,030 1000 à 1200 non recyclablecontient HCFC

fibres de polyester 0,035 600laines minérales 0,035 à 0,040 150 à 250 peu recyclableverre cellulaire 0,035 à 0,048 1600 recyclable en

remblaisperlite vermiculite 0,045 à 0,080 230 recyclable en

vracargile expansée 0,10 à 0,11 300 recyclable en

vrac ou en remblais

bois feutré 0,042 à 0,07 12,5 compostage ou combustion

laine de cellulose 0,035 à 0,040 6 compostageliège expansé 0,032 à 0,045 90 recyclable

Fig 5.8 – Energie incorporée et fin de vie des isolants (D'après J.P Oliva)

La qualité globale de l’enveloppe peut être estimée par le coefficient UBAT

[W/m 2.K] qui rapporte toutes les déperditions à l’unité de surface de l'enveloppe, pour un écart de température unitaire entre l’intérieur et l’extérieur.

On peut également utiliser le coefficient G1 [W/m 3.K] qui rapporte les déperditions de l'enveloppe à l'unité de volume du bâtiment pour un écart de température unitaire. Si on inclue dans ce déperditions le réchauffage de l’air neuf introduit dans les locaux, on obtient le coefficient G [W/m 3.K].

1.3 UTILISATION DES APPORTS SOLAIRES

L’installation de chauffage doit pouvoir satisfaire à chaque instant aux besoins en chaleur des locaux. Cependant, une partie des déperditions est déjà compensée :

• par les apports internes aux locaux : éclairage, occupants, activités,

• par les apports solaires : l’énergie solaire, située dans le domaine des rayonnements visibles (courte longueur d’onde), pénètre dans les locaux à travers les parois vitrées. A l’issue des réflexions et absorptions successives sur les parois des locaux, cette énergie finit par être


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complètement dégradée en chaleur.

Le processus de dégradation de l’énergie solaire en chaleur passe par plusieurs étapes (Fig 5.9) :

• pénétration du rayonnement solaire dans les locaux,

• absorption du rayonnement solaire par les parois intérieures,

• échauffement des parois intérieures,

• émission des parois par rayonnement de grande longueur d’onde et convection avec l’air intérieur.

Fig 5.9- Dégradation du rayonnement solaire entrant

La phase d’échauffement des parois intérieures conduit à un phénomène d’amortissement et de retard entre l’énergie solaire entrante (apport solaire) et la chaleur restituée aux locaux (charge solaire). C’est la manifestation de «l’inertie thermique» des locaux.

Les parois vitrées sont caractérisées par leur facteur solaire (Fig 5.10). Le facteur solaire est le rapport entre l’énergie thermique cédée au local et


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l’énergie solaire incidente au vitrage. Les parois opaques participent aussi aux apports solaires, cependant leur contribution aux apports solaires est généralement très minoritaire.

Fig 5.10 – Facteur solaire et coefficient U (Saint Gobain)

Une utilisation optimale des apports solaires est un point essentiel de l’architecture économe en énergie. Comme aucun équipement spécifique n’est nécessaire (on dit souvent que le meilleur capteur solaire est une fenêtre orientée au sud), et u’aucune circulation de fluide n’est requise (hors d'une ventilation correcte des locaux), on parle généralement de «solaire passif».

L’analyse des apports solaires met en évidence le rôle de plusieurs paramètre :

• l’énergie incidente : l’énergie disponible dépend évidemment du site géographique considéré (ensoleillement hivernal), de l’environnement immédiat (effet des ombres portées), mais surtout de la surface et de l’orientation des parois vitrées,

• la nature des vitrages : plus la vitre est transparente au rayonnement solaire, plus la quantité d’énergie entrante est importante. Un double vitrage possède un coefficient U plus faible qu’un vitrage simple (donc moins de déperditions), par contre la présence de deux vitres diminue son facteur solaire (donc moins d’apports de chaleur). Un compromis entre coefficient de transfert et facteur solaire doit donc être trouvé.


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Sur l'ensemble de la période de chauffage d'un bâtiment, le bilan énergétique d'un vitrage peut faire apparaître (Fig 5.11) :

• soit une perte d'énergie devant être compensée par le système de chauffage,

• soit un gain en énergie participant au chauffage des locaux.

Bilan d'un vitrage en Ile de France sur une saison de chauffage [kW.h/m 2]Sud S.E/S.O E/O Nord

Simple(U=4,95 [W/m 2.K])

- 75 - 86 - 137 - 203

Double(U=2,95 [W/m 2.K])

+41 +30 - 22 - 87

Double + volets +81 +70 +19 - 47Double V.I.R(U=1,8 [W/m 2.K])

+107 +96 +45 - 21

Double V.I.R + volet(U=1,5 [W/m 2.K])

+125 +114 +62 - 4

Huisserie bois, RCL=0,7, sans masque

Fig 5.11 – Bilan thermique d'un vitrage (Ademe / Arene / GdF)

La «surface sud équivalente» représente la surface d’une baie (non vitrée) située dans une paroi verticale orientée au sud, qui laisserait pénétrer la même quantité d’énergie solaire que l’ensemble des parois vitrées du bâtiment. La surface sud équivalente quantifie donc la surface de captation du bâtiment.

Une étude de l'Asder (Association Savoyarde de Développement des Energies Renouvelables) menée sur une maison de conception classique de 100 [m²] située en Savoie montre que les besoins en chauffage peuvent diminuer de 25% uniquement sous l'influence de l'orientation.

1.4 SURCHAUFFE ET INERTIE THERMIQUE

A) Situation hivernale

Même en plein hiver, il peut arriver que la charge solaire soit supérieure aux besoins en chaleur d’un local (Fig 5.12). Dans ce cas, on constate une surchauffe du local (émetteurs de chaleur arrêtés sous l’action de la régulation thermostatique) qui présente au moins deux inconvénients :

• cette surchauffe peut devenir inacceptable vis à vis du confort thermique des occupants,


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• cette surchauffe correspond à un gaspillage de la ressource solaire.

Fig 5.12 – Surchauffe par charge solaire

Toute l’énergie solaire disponible n’est donc pas utilisable. La partie des apports solaires qui participe effectivement au chauffage des locaux est d’autant plus importante que :

• l’inertie thermique des locaux est forte,

• les déperditions sont importantes.

Comme on ne souhaite évidemment pas augmenter les déperditions, on constate l’importance de l’inertie thermique sur le comportement des locaux (Fig 5.13). Le plancher a un rôle privilégié dans cette inertie, car souvent c’est lui qui reçoit l’impact direct de l’ensoleillement et réalise la première absorption du rayonnement solaire. Cela explique l’intérêt des constructions sur terre plain et l’influence du revêtement de sol (couleur, résistance thermique).


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Fig 5.13 – Influence de l'inertie (C.S.T.B)

X=apports solaires / besoins en chaleur F=apports solaires utilisables sans surchauffe/besoins en chaleur

En régime thermique non permanent, un matériau de construction est caractérisé par deux coefficients (Fig 5.14) :

• la diffusivité ac

= λρ , qui caractérise la faculté du matériau à propager

une perturbation thermique,

• l’effusivité b c= λρ , qui caractérise la faculté du matériau à stocker

l’énergie calorifique.

Matériau Conductivit Masse Chaleur Diffusivit Effusivité


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é[W/m.K]

volumique

[kg/m 3]

massique

[J/kg.K]

é[m2/s]

[J/m 2.K.s0.5

]

Béton 1.65 2200 1000 75.10 - 08 1905Béton cellulaire 0.21 600 1000 35.10 - 08 355Plâtre 0.25 800 1000 31 10 - 08 447Laine de roche 0.041 30 1030 130 10 - 08 36Laine de verre 0.039 15 1030 250 10 - 08 25Polystyrène 0.047 10 1450 320 10 - 08 26Polyuréthane 0.037 50 1400 53 10 - 08 51Calcaire 1.7 2100 1000 81 10 - 08 1889Sol argileux 1.5 1500 2000 50 10 - 08 2121Brique 0.69 1800 1000 38 10 - 08 1114Acier 50 7800 450 1400 10 -

08

13248

Aluminium 230 2700 880 9700 10 -

08

23377

Bois 0.18 650 1600 17 10 - 08 433

Fig 5.14 – Propriétés des matériaux (D'après valeurs règles TH- U)

Idéalement, une enveloppe de bâtiment devrait donc comporter :

• coté extérieur : des matériaux à faible diffusivité pour l’isoler des sollicitations climatiques,

• coté intérieur : des matériaux à forte effusivité pour obtenir une inertie importante.

Si l’on considère la notion de «profondeur de pénétration» par conduction dans un matériau :

δπ

= aT

on constate que pour une périodicité de 24 heures, une épaisseur de l’ordre de 10 à 15 [cm] de matériau fortement diffusif suffit généralement à assurer le stockage de la chaleur.

La recherche d'une inertie thermique importante est favorable au locaux chauffés en permanence. Dans le cas d'un chauffage intermittent, l'inertie peut rendre moins importante l'économie d'énergie envisageable.

B) Situation estivale


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En l'absence de climatisation, la surchauffe des locaux risque également de survenir en été. Si les locaux sont climatisés, l’appel de puissance et la consommation des équipements seront très importants.

La réduction estivale de la charge solaire est donc indispensable. Cela peut être réalisé par la mise en œuvre :

• de vitrages à faible valeur du facteur solaire,

• de protections solaires adaptées (Fig 5.15),

• d'une inertie thermique importante.


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Fig 5.15 – Ombrage d'un auvent (Modélisation sur logiciel Codyba)

On remarque que l’inertie thermique des locaux est ici favorable été comme hiver. Par contre, la conception du bâtiment doit concilier des conditions apparemment contradictoires :

• favoriser les apports solaires en hiver,

• minorer les apports solaires en été.


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Ombrage (Revue Systèmes Solaires)

La modélisation du comportement thermique des bâtiment met à disposition du concepteur des outils d’évaluation et d’aide à la conception des bâtiments, permettant d’atteindre un niveau de performance élevé vis à vis des besoins en énergie et du respect des critères de confort thermique.

1.5 ESPACES TAMPONS ET SERRES

Une protection de l’enveloppe du bâtiment peut être obtenue en lui accolant certains locaux annexes non chauffés (garage par exemple). Ces locaux trouvent une température d’équilibre comprise entre la température extérieure et la température des locaux chauffés. Les transferts de chaleur à travers la paroi mitoyenne est donc fortement minorée par rapport à une situation la mettant en contact direct avec l’extérieur (Fig 5.16).


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Fig 5.16 – Local tampon

Ce type de conception architecturale est très efficace si le local tampon protège une paroi particulièrement exposée aux intempéries (paroi nord, paroi sous vent dominant). Il est à la base de la conception de maisons "bioclimatiques" (Fig 5.17).


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Fig 5.17 – Architecture “bioclimatique” (Asder / Pierre Rieussec)


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Architecture “bioclimatique” (Asder / Pierre Rieussec)

L’équilibre énergétique du local tampon en régime permanent établi conduit à la détermination du coefficient de réduction de température :

EXTINT

INT

θθθθτ

−−=

avec

τ =+

+ +D Q

D A QPE V

PE PM V

0 34

0 34

,

,

Dans cette relation où θ est la température du local tampon, on a :

• DPE : déperditions à travers les parois du local tampon vers l’extérieur, pour 1 [K] d’écart de température,

• APM : apports à travers la paroi mitoyenne, pour 1 [K] d’écart de température,

• QV : débit volumique d’air neuf, aux conditions standards, traversant le local tampon.


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S’il est largement vitré et exposé à l’ensoleillement, le local tampon peut atteindre une température élevée, même en hiver, et devenir source de chaleur pour les locaux à chauffer. Il s’agit alors d’une serre (Fig 5.18).

Fig 5.18 – Serre intégrée (A.F.M.E)

Le système de ventilation du bâtiment fait généralement transiter par la serre une partie du débit d’air neuf prélevé à l’extérieur. La chaleur disponible dans la serre est alors utilisée sous forme d’un préchauffage de l’air neuf (Fig 5.19).

Fig 5.19 – Serre et ensoleillement

La mise ne œuvre d'une serre peut diminuer sensiblement les besoins en chauffage (Fig 5.20). Cependant, des dispositions doivent être prises pour qu’en situation estivale la serre n’augmente pas la charge thermique des locaux contiguës (protection solaire, ventilation indépendante).


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en épis en appui encastréeConsommation

chauffage[kW.h/an]

8612 8469 8469

Apports solaires[kW.h/an]

1500 1579 1186

Apports utiles[kW.h/an]

61 81 174

Fig 5.20 – Influence d'une serre sur la consommation (Le Moniteur / Guide des recommandations)

2 SYSTEMES SOLAIRES ACTIFS

2.1 PRINCIPES

L’utilisation de l’énergie solaire peut également se faire par la mise en œuvre d’équipements spécifiques plus ou moins intégrés à l’architecture du bâtiment. On parle alors de «systèmes solaires actifs».

Dans tous les cas, un dispositif capteur transmet l’énergie calorifique à un fluide (eau chaude, air chaud). Ce fluide participe ensuite au chauffage des locaux, généralement en collaboration avec un système de chauffage d’appoint.

En effet, le caractère irrégulier de la ressource solaire conduit à prévoir une seconde source d’énergie calorifique :

• chauffage électrique d’appoint,

• chaudière à combustible fossile en relais sur le circuit d’eau chaude,

• chauffage au bois.

Un dispositif de stockage de l’énergie solaire peut contribuer à lisser la disponibilité de la ressource. Le stockage est chargé en chaleur quand l’énergie solaire est excédentaire, il est déchargé lorsque l’énergie solaire manque (Fig


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5.20). L’inertie thermique des locaux est une première forme de stockage, mais il peut aussi s’agir d’un stockage alimenté par les capteurs :

• réservoir d’eau chaude,

• modules à changement de phase,

• stockage dans un lit de galets pour les capteurs à air chaud.

Fig 5.20 – Chauffage solaire à air avec stockage et appoint électrique (Communauté Européenne)

Stockage de chaleur dans des blocs en béton (Communauté Européenne)

L’inconvénient de ces systèmes de stockage est d’une part le surcoût engendré, et d’autre par la place nécessaire à leur implantation.

La ressource en ensoleillement varie selon la latitude et la climatologie locale. En France, la durée d’insolation varie de 1500 à 3000 heures selon la région (2270 heures à La Rochelle). Cependant l’intérêt d’un chauffage solaire est


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sensiblement identique du nord au sud (taux de couverture entre 30 et 70% selon l’installation), une plus longue durée d’utilisation compensant au nord de la Loire un ensoleillement plus faible. Une économie d’énergie de l’ordre de 400 à 500 [kW.h/m²] est envisageable dans des conditions économiques viables.

De nombreux systèmes actifs ont été mis au point, la complexité n’étant pas forcément un gage de performance élevée.

2.2 MUR CAPTEUR

On est à la limite entre les systèmes actifs et les systèmes passifs (Fig 5.21). La conception des murs capteurs est déjà ancienne (mur «Trombe»). Cette solution présente trois avantages .

• peu d’éléments mobiles sont mis en œuvre, ce qui conduit à une fiabilité élevée pour une maintenance minimale,

• un stockage à court terme est incorporé au système,

• l’intégration architecturale est assez facile à réaliser.

Fig 5.21 _ Mur capteur “Trombe” (A.F.M.E)

Par contre, la surface de captation envisageable est très liée à la disposition des locaux et au proche environnement du bâtiment. Elle reste souvent limitée et insuffisante (Fig 5.22).


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Fig 5.22 – Mur capteur (Simulations d'après Zalewski, Lassue, Duthoit et Chantant)

Bilan énergétique pour la saison de chauffage

(6)[kW.h/m²]

Trombe(1)

Isolé(2)

Composite(3)

Non ventilé(4)

Mur nuU=0,47

[W/m².K](5)

Carpentrasorientation sud (7)

263 241 189 217 - 23

Trappeorientation sud (8)

170 152 124 142 - 36

Conduction retard(9) proportion

4 [h] 4055 %

6 [h] 4515 %

7 [h]10 %

5 [h]100 %

Convection retard(10) proportion

1 [h] 1045 %

1 [h] 0585 %

5 [h] 4590 %

Fig 5.22 bis– Bilan thermique d'un mur capteur (Simulations d'après Zalewski, Lassue, Duthoit et Chantant)

La gestion d'un mur capteur nécessite la manipulation des volets d'air et du store de protection (Fig 5.23). On peut automatiser cette manipulation, mais elle reste souvent effectuée par l'utilisateur des locaux, avec plus ou moins d'assiduité.


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Fig 5.23 – Utilisation d'un mur capteur (Revue Cahiers Techniques du Bâtiment)


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2.3 PLANCHER SOLAIRE DIRECT

Des capteurs solaires à eau chaude (capteurs plans) alimentent en eau chaude à basse température un plancher chauffant à tubes plastiques noyés dans du béton (Fig 5.24 et Fig 5.25).

Fig 5.24 – Plancher solaire direct (Clipsol)


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Fig 5.25 – Chauffage solaire à eau chaude

Un régime à basse température est nécessaire à un fonctionnement du plancher respectant les règles de confort (température de surface maximale 28 [°C]) et à l’autorégulation. Il est aussi favorable à un rendement de captation élevé (Fig 5.26).

Fig 5.26 – Capeur solaire à eau (A.F.M.E)

Comme dans le cas d’un mur capteur, une capacité de stockage à court terme fait partie intégrante du dispositif grâce à la masse du plancher (Fig 5.27). L'épaisseur est de 12 à 15 [cm], jusqu’à 30 [cm].


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Fig 5.27 – Effet de l'inertie de la dalle (Ademe / Asder / Cler)

Quand l’énergie solaire n’est plus disponible, une chaudière (Fig 5.28) peut assurer le réchauffage de l’eau (appoint intégré), à moins que l’on utilise une installation de chauffage (électrique par exemple) en parallèle avec le plancher solaire (appoint non intégré). Cette dernière solution est plus coûteuse mais d’une gestion plus simple.


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Fig 5.28- Chauffage d'appoint

La conception du bâtiment reste classique et le chauffage peu différent des solutions habituelles utilisant un plancher chauffant. Par contre l’intégration architecturale de la surface de captation est souvent difficile.

L’ordre de grandeur de la surface de captation est de 12 à 20 [m2] pour 100 [m2] habitables. L’économie d’énergie est d’environ 350 à 400 [kW.h/m 2], ce qui correspond selon le site à un taux de couverture de 30 à 60% des besoins en chauffage. En été, les capteurs sont utilisés pour l’eau chaude sanitaire, ou pour le réchauffage d’une piscine.

2.4 CAPTEUR SUR POMPE A CHALEUR


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Le système de chauffage utilise de l’eau chaude à basse température, pour alimenter un plancher chauffant ou des radiateurs adaptés. Dans les systèmes à détente directe, l’eau est chauffée au condenseur d’une pompe à chaleur (P.A.C) dont l’évaporateur est constitué par un capteur aileté, généralement sans couverture. On peut cependant utiliser un évaporateur sur circuit d'eau intermédiaire (Fig 5.29).

Fig 5.29- Pompe à chaleur solaire

En l’absence d’ensoleillement, on est en présence d’un évaporateur statique à air. En présence d’ensoleillement, la température de l’évaporateur se trouve augmentée. Le rendement thermodynamique de la machine s’accroît, on peut considérer que la P.A.C transfert à l’eau l’énergie solaire reçue par le capteur. Le rendement de captation est élevé car le capteur fonctionne à basse température.


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Absorbeurs de P.A.C solaire (Revue C.F.P)

Ce dispositif présente l’inconvénient d’être plus complexe (machine thermodynamique) et de ne pas éliminer les inconvénients des pompes à chaleur air- eau (givrage, chute de C.O.P). Par contre il améliore sensiblement le coefficient de performance moyen de la P.A.C, laquelle constitue déjà un système à haute efficacité énergétique.

Cependant, du point de vue environnemental, l’utilisation d’un fluide frigorigène peut conduire à certaines réserves. En effet, si le problème de la nocivité vis à vis de la couche d’ozone semble résolu par les H.F.C, leur forte contribution à l’effet de serre reste un problème important. De plus, le fonctionnement du compresseur peut rendre son intégration difficile pour des raisons acoustiques. Le développement de machines à absorption de petite puissance sera peut être une solution de remplacement.

3 CONFORT D'ETE

3.1 REDUCTION DES APPORTS

Le confort thermique d’été correspond à une demande de plus en plus importante des utilisateurs, aussi bien dans les bâtiments du secteur tertiaire que dans les bâtiments d’habitation. Ceci peut conduire à de fortes consommations énergétiques si la conception du bâtiment et de l’installation est insuffisante.

De manière globale, la climatisation conduit :

• à des pointes de consommation électrique estivales pouvant conduire à


34


la remise en service de centrales thermiques,

• à l'amplification des micro- climats urbains par dégagement de chaleur aux condenseurs des machines frigorifiques.

La conception thermique du bâtiment peut diminuer les besoins en climatisation des locaux, et même éviter le recours à la climatisation dans de nombreux cas. Dans ce but, on peut mettre en œuvre :

• une réduction des charges internes, par une gestion efficace de l’éclairage artificiel et des appareils (ordinateurs avec mise en veille automatique),

• une réduction des apports solaires, par l’utilisation de protections solaires et de vitrages à faible facteur solaire,

• une inertie thermique permettant d’éviter les pointes de température intérieure par stockage thermique dans la structure du bâtiment.

Au cours d’une journée, la température intérieure moyenne d’un bâtiment non climatisé, soumis a l’évolution d’allure sinusoïdale de la température extérieure, aux apports solaires et aux apports internes, peut difficilement être inférieure à la valeur de la température extérieure moyenne. Cependant une conception thermique adaptée peut conduire à des valeurs maximales de la température intérieure acceptables vis à vis des conditions de confort.

Le déstockage de l’énergie accumulée peut être effectué par une surventilation nocturne, particulièrement dans le cas les locaux à utilisation intermittente. Le passage du taux de renouvellement d’air de 1 [Vol/h] à 20 [Vol/h] a évidemment une répercussion sur la conception et le dimensionnement de l’installation de ventilation.

3.2 CLIMATISATION CLASSIQUE

L’optimisation du fonctionnement des systèmes de climatisation, notamment par l’utilisation du «free cooling» (mise à profit de l'air extérieur chaque fois que sa température est inférieure à celle de l'air intérieur) permet une diminution des consommations, de même que la conception de systèmes procédant par transfert entre zones (P.A.C sur boucle d’eau par exemple).

D'autre part, l'amélioration des performances du matériel porte sur plusieurs aspects :

• meilleure efficacité énergétique des compresseurs (compresseurs spiro-orbitaux, détendeurs électroniques),


35


• régulation plus fine des machines frigorifiques (variation de vitesse "inverter"),

• gestion technique centralisée des installations.

En application de la directive européenne du 22 mars 2002, les climatiseurs font l'objet d'un marquage obligatoire de leur efficacité énergétique. Afin d'informer et de motiver les acheteurs pour les appareils économes en énergie, un étiquetage normalisé les classe en 7 catégories (Fig 5.30).


36


Fig 5.30 – Classification des climatiseurs (Revue Clim Pratique)

Malgré ces améliorations, les systèmes classiques de climatisation restent gros consommateurs d'énergie, majoritairement électrique.

3.3 CLIMATISATION ALTERNATIVE


37


Parmi les nombreux dispositifs permettant une amélioration du confort d’été sans mettre en œuvre un système de climatisation avec production frigorifique, on peut citer :

• les systèmes évaporatifs, le refroidissement étant obtenu par le prélèvement de la chaleur latente de vaporisation. On peut utiliser une pulvérisation d’eau sans reliquat directement dans l’ambiance à traiter, soit un refroidissement de l’air soufflé. Dans ce dernier cas, la pulvérisation d’eau peut être effectuée dans l’air neuf, avec l’inconvénient de le conduire à une humidité proche de la saturation, ou dans l’air extrait avant rejet, avec l’inconvénient de nécessiter ensuite un échangeur entre l’air extrait et l’air neuf (Fig 5.31),

• les conduits enterrés, l’air soufflé étant refroidi par son passage dans des conduits placés à une profondeur assurant une température suffisamment basse et stable (Fig 5.32).

Fig 5.31 – Rafraîchissement évaporatif indirect


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Fig 5.32 – Puits canadien

4 EAU CHAUDE SANITAIRE

4.1 BESOINS

L’eau chaude sanitaire, utilisée pour les besoins ménagers et la toilette, représente une consommation importante :

• en eau (potable) : le volume d’eau consommé est très influencé par le comportement des utilisateurs. De plus, la consommation n’est pas uniforme dans le temps, elle est généralement maximale durant le week- end, et pendant l’hiver. Selon les auteurs, les calculs prévisionnels sont basés sur une consommation moyenne de 100 à 150 litres d’eau chaude (à 60 [°C]) pour un logement standard de type 4 (trois chambres). On observe cependant une augmentation constante des besoins en eau, et spécialement en eau chaude, de l’ordre de 3 à 4% par an (Enquête Gaz de France),

• en énergie : le réchauffage de l’eau sanitaire représente près de 20% de la consommation d’énergie finale dans les secteur résidentiel (d'après l' Observatoire de l‘énergie). Au rythme d'évolution actuel, on atteindra bientôt une consommation de 1000 [kW.h/an] par personne (Fig 5.33 et 5.34).


39


Fig 5.33 – Evolution des besoins énergétiques pour l'E.C.S (Revue Plein soleil / C.N.R.S-EcoDev)

Eau Chaude Sanitaire : ratio de consommation énergétique d'une

maison1973 92 [kW.h/m²]1980 108 [kW.h/m²]1995 118 [kW.h/m²]

Fig 5.34 – Besoins énergétiques en E.C.S pour une maison (Enquête C.E.R.E.N)

Divers dispositifs permettent de réduire la consommation d'eau chaude, par exemple les robinets thermostatiques, les douchettes à forte efficacité, etc.

Le production d’eau chaude sanitaire peut être réalisée :

• de manière individuelle (chauffe- eau, chaudière mixte),

• de manière collective, avec réseau de distribution.

La production individuelle a l’avantage de limiter les pertes (pas de réseau) et de responsabiliser les utilisateurs (économie d’eau et d’énergie). Par contre l’appareil de production occupe de la place dans l’appartement et peut y générer du bruit. Ce choix est cohérent si le chauffage est individuel.

La production collective à l’avantage de décharger l’utilisateur de tout entretien. Les pertes de distribution peuvent être compensées par un meilleur rendement de la production de chaleur (du moins en période hivernale). La


40


mise en place de compteurs divisionnaires d’eau chaude individualise la consommation, mais il faut veiller à ce que l’alimentation en eau chaude soit rapide (disponibilité immédiate sans effet de purge de canalisation). Ce choix est cohérent si le chauffage est collectif.

4.2 PRODUCTION

A) Modes de préparation

Qu’elle soit collective ou individuelle, la production d’eau chaude peut être :

• instantanée : elle est produite au fil de la demande,

• à accumulation totale : la totalité de l’eau est préparée la nuit afin d’être disponible pour la durée de la journée,

• à semi accumulation : seule une partie du volume consommé est préparé de nuit, au moins une seconde période de réchauffage du réservoir est prévue de jour,

• semi instantanée : la préparation est instantanée pour les petits débits, et complétée par le puisage dans une réserve de petit volume pour les appels de pointe.

B) Chauffe- eau solaire

La part des énergies renouvelables dans la production d’eau chaude sanitaire reste faible, mais un peu plus importante que pour le chauffage. Cela s’explique par la relative simplicité de la production d’E.C.S par énergie solaire.

En France, un capteur orienté au sud et incliné d’un angle égal la latitude du lieu, reçoit une énergie solaire annuelle variant de 1200 à 1800 [kW.h/m²]. Cette énergie peut largement contribuer au chauffage de l’eau chaude sanitaire.

Un chauffe eau solaire individuel (C.E.S.I) est constitué :

• d’un capteur plan. Le niveau de température requis (au moins 50 [°C]) conduit à utiliser des capteurs à couverture vitrée,

• d’un réservoir de 100 litres environ, associé par exemple à 2 [m²] de capteurs,

• d’un court réseau de circulation entre le réservoir et le capteur,

• d’un chauffage d’appoint, le plus souvent électrique,

• d’un dispositif de régulation et de répartition entre les deux énergies


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disponibles.

Capteur solaire pour chauffe- eau (Agence Méditerranéenne de l'Environnement)

Dans sa version la plus simple, c’est directement l’eau sanitaire qui circule par thermosiphon dans le capteur (Fig 5.35).

Fig 5.35 – Chauffe eau solaire à thermosiphon

Ce choix technologique n’est pas possible si le gel du capteur est à craindre. Dans ce cas, on doit utiliser un circuit intermédiaire à eau glycolée, qui réchauffe l’eau sanitaire à travers la paroi d’un échangeur de chaleur (Fig 5.36), ou prévoir un dispositif de vidange automatique.


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Fig 5.36 – Chauffe eau solaire à circulation forcée

Chauffe eau solaire à thermosiphon (E.d.F / Giordano)

Un taux de couverture des besoins de 40 à 60% est facilement obtenu. Par exemple, une étude de l’Ademe montre un taux de couverture de 60 à 75 % selon le site choisi (en France) pour 4 [m²] de surface de captation, inclinée à 45° et orientée vers le sud associée à un réservoir de 160 litres.

Le ratio minimum est d’au moins 1 [m²] pour 100 litres (Fig 5.37).


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Occupants (2)

1 ou 2 3 ou 4 5 ou 6 7 et plus (3)

Volume réservoirE.C.S +appoint (4)

100 à 250litres

250 à 400litres

400 à 550litres

550 à 650litres

Surface capteurs

peu ensoleilléensoleillé

très ensoleillé

couverture de50 à 70% des besoins à 45 [°C](50 à 60 litres/pers)

2 à 3 3 à 5,5 4 à 7 5 à 72 à 2,5 2 à 4 3 à 5,5 3,5 à 72 à 2,5 2 à 3,5 2,5 à 4,5 3,5 à 6

Fig 5.37 – Dimensionnement d'un chauffe eau solaire (Revue Sycodes)

Un surdimensionnement de la surface de captation augmente peu le taux de couverture mais rend son coût inacceptable.

Les solutions adoptées en préparation collective sont du même type. Il est cependant nécessaire de séparer plus complètement la chauffage solaire de l’eau et la fourniture d’appoint, par exemple grâce à deux réservoirs associés en série (Fig 5.38).

Fig 5.38 – Eau chaude sanitaire solaire collective (Revue Vecteur Gaz)


44


Eau chaude sanitaire solaire (A.F.M.E)

Le plan d'aide aux chauffe- eau solaires individuels a pour objectif d'atteindre un rythme annuel de 30000 unités à partir de 2006 en encourageant la pose de 350 000 [m²] de capteurs entre 1999 et 2006. En collectif, l'aide à 50 000 [m²] devrait conduire à un taux annuel de 15 000 [m²] installés.

5 Passive cooling systems

5.1 Introduction

The energy consumption for cooling may be reduced with the use of renewable energy systems or techniques. Those systems can be divided into three major categories:

1. passive systems

2. hybrid systems or techniques

3. low energy systems or techniques

5.2 Passive systems or techniques


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Passive systems or techniques are those that can reduce energy consumption without the use of any mechanical system. The reduction of energy consumption is achieved through the use of renewable energy sources or geothermal energy.

5.2.1 Passive radiative cooling

Radiative cooling is described as the heat loss by long- wave radiation emission from a body towards another body of lower temperature, which acts as a heat sink. Passive radiative cooling happens naturally, since the temperature of the space around a building is usually lower than the temperature of its surfaces, so the building envelope radiates, enhancing the heat loss from the indoor environment.

During the hot season the building envelope has a higher temperature than the environment and thus radiates towards the sky and gets cooler, enhancing heat loss from the interior of the building. The performance of radiative cooling depends on the temperature difference between the long wave sky temperature and the building skin temperature. Another parameter that influences the heat exchange between them is the material of the external building skin. In order to improve the effectiveness of radiative cooling the following are proposed:

A. Use of reflective paints The simplest way to cool a building is to paint its external surfaces with white paint. Radiative cooling takes place through long- wave radiation. White and black paints have almost the same emissivity in long- wave radiation, so radiation rates during the night do not differ. The advantage of white painted surfaces is that they absorb less solar radiation than black ones during daytime. So, the surface temperature is lower and it can easily be cooled during the night.

B. Installation of a movable insulation material on the roofA movable insulating material is installed on the building’s roof. It can be operated either manually or automatically. During daytime, the insulating material covers the roof in order to reduce the heat absorption by solar radiation. In reverse, during the night, it allows exposure of the roof's thermal mass to the sky. The advantage of this system is that its operation is reversed during the winter. The disadvantage is the additional cost of the system, especially for the automatic version. Also, the problem of cooling multi - storey buildings remains.

5.2.2 Evaporative cooling

Evaporative cooling is the process that uses the effect of evaporation as a natural heat sink. The air is cooled, since part of its sensible heat is absorbed as the latent heat necessary to evaporate water. Evaporative cooling can be direct or indirect. In direct evaporative cooling, the water content of the cooler air increases since air is in contact with the evaporated water. In indirect evaporative cooling, evaporation occurs inside a heat exchanger, so the water content of the cooled air remains unchanged.


46


Evaporative cooling systems can be classified in two ways. The first classification is related to the contact (direct) or not (indirect) of the cooled air with the evaporated water. The second classification is performed according to the energy required to produce evaporation. If evaporation occurs naturally then the systems are called passive evaporative systems, but if evaporation is due to a current of air mechanically induced by electric ventilators, the systems are called hybrid evaporative.

Passive direct cooling systemsThis category includes the use of vegetation for evapo- transpiration, the use of fountains, sprays, pools and ponds. Trees and other plants transpire moisture in order to reject their sensible heat. A normal size deciduous tree evaporates 1460kg of water during a sunny summer day. The corresponding energy consumption is 870MJ, which is the cooling effect of five air- conditioners.

The rate of evaporation from a wetted surface depends on the air velocity and the difference between the water vapour pressure and the air pressure next to the moist surface. Evaporative cooling is effective in areas that have a wet bulb temperature below 21°C.

Another variation of passive radiative cooling is the use of a tower where water contained in a jar or in pads or spray is precipitated. Ambient air introduced into the tower is cooled by evaporation and then transferred inside the building. The tower operates as a reverse solar chimney.

Passive indirect cooling systems


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Fig.5.39 Building with evaporative cooling tower


These systems include mainly the following techniques:

A. Roof sprinklingThis is a very interesting way to apply evaporative cooling, since the main part of external indirect heat gains comes from the roof.

Spraying systems consist of an all- copper network. Instead of spraying uncontrolled amounts of water on the roof, the system injects water according to the temperature and evaporation rates. The goal is to optimize cooling, while using as little water as possible.

B. Roof pondsWater ponds are constructed over non- insulated flat roofs. The water surface must be shaded during daytime to avoid excessive water heating. The wet bulb temperature should be lower than 20°C for the system to be efficient. However the system has some limitations:

• Roofs must be capable of supporting a significant weight.

• Their application is limited to single storey buildings.

• The roof must be waterproof.

C. Moving water films In this application, the evaporation process is enhanced by an increase in the relative velocity between the air and the water surface. The cooled water is stored in the basement of the building and then is circulated inside the building space and cools it.

5.2.3 Solar control (Fixed shading devices, glazing)

Solar control can be achieved either with a fixed shading device, or with a special type of glazing, or a combination of both. Ideally they reduce solar gains during the hot period, without affecting them during the winter period. The use of shading devices improves visual comfort, while reducing energy consumption needed for cooling. Shading devices can be either external or internal.

The use of external shading devices is more efficient where the aim is to minimize solar gains and provide an acceptable visual environment. The following are some types of fixed shading devices:

• Overhang: As well as shading, this provides rain protection to walls and openings. In addition, it has little effect on the view or natural ventilation. Overhangs are proposed for south oriented openings.


48

Fig.5.40 South façade with overhang


• Side fins: These affect the view and the potential natural ventilation. They are proposed for east and west oriented openings.

• Louvres: These affect the view and the potential natural ventilation, but they may provide security.

• Curtains (draperies) and fabric shades. The following table gives the shading factor for curtains and blinds:

Table 5.43 Shading factor for curtains and blinds, [3]

Shading device Shading factor

Internal drapery

- light 0.44

- dark 0.65

Internal venetian blinds

- angle 0º 0.38

- angle 45º 0.41

• External and internal venetian blinds

• Any combination of them.


49

Fig.5.41 Façade with side fin

Fig.5.42 External vertical louvres

Fig.5.44 External venetian blinds


The use of special types of glazing may reduce solar gains and/or improve visual conditions (eg avoidance of glare). Some of these glazing types are:

• Reflective: Highly reflective glazing with grey color .

• Tinted: Highly reflective coloured glazing.

• Low emissivity glazing: Surface 2 or 3 has a special coating with a low emissivity factor.

5.2.4 Heat exchange with ground by direct contact

When part of the building envelope is in contact with the ground, then the building exchanges thermal energy with the ground by conduction. During the cooling period, the earth may act as a heat sink and, during the winter period, as a heat source. The ground temperature depends on the depth, soil type, soil coverage and the soil moisture.


50

Fig.5.45 Recessed window

Fig.5.47 Double clear low emissivity glazing

1 2 3 4

Air g

ap12

mm

Glazing6mm

Glazing5mm

Low – E coating

1 2 3 4

Air g

ap12

mm

Glazing6mm

Glazing5mm

Low – E coating

Fig.5.46 Internal venetian blinds


5.2.5 Solar chimneys

A typical solar chimney has a tubular shape and is constructed with a high conductivity material, while the external facade of the chimney should be exposed to solar radiation. Also, the bottom of the solar chimney should be sloped with an outfall in order to avoid odour and mould from possible condensation.

The air in the solar chimney is deliberately heated by incident solar radiation and so the air mass with higher temperature moves upward due to the stack effect. The construction of a solar chimney is quite inexpensive and simple.

During the cold period and the daytime the dampers at the upper and lower parts of the chimney are closed and the air mass enclosed by the chimney is heated by incident solar radiation. During the night, warm air is provided through the open dampers to the building space, while it is replaced by fresh air.

During the hot period and the daytime, while the dampers are open, the solar chimney works as a cost- free ventilator.

Generally speaking, a solar chimney is unable to meet the needs for fresh air in a variable building environment (different number of people, different activities), while its operation depends on the damper controls.

Fig.5.48 Solar chimney

5.3 Hybrid systems


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5.3.1 Radiative cooling

Radiative cooling is based on heat loss by long- wave radiation emission from a body towards another body of lower temperature, which acts as a heat sink. In hybrid systems the radiator is not the building envelope, but a metal plate. A fluid is cooled while circulating under the metal plate, assisted by a fan or pump.

Description of an air coolerAn air cooler consists of a horizontal rectangular duct. The top of the duct is the radiator, which consists of a metal plate.

The main elements of an air cooler are the radiator plate and its coating, which is usually ordinary matt black paint. Under the plate there is a gap of 4cm where the air to be cooled circulates. Also, between the casing of the cooler and the gap, there is an insulation layer to avoid heat losses.

Regarding the coating, in order to maximize net radiative losses during the night, it must not absorb incoming atmospheric long- wave radiation, while it should emit strongly in the range of the atmospheric window. So, an ideal coating must have high emissivity in the 8 to 13μm wavelength and a high reflectivity above and below this band. Oxides and carbonates of titanium, aluminum, calcium are appropriate coatings.

Normally, a radiative cooler is uncovered, or at least unglazed. However, some radiative coolers are covered with a windscreen. Initially, the temperature of the radiator is higher than the ambient temperature, so the radiator loses heat by radiation and convection. Later the radiator becomes cooler than the ambient air so at this moment, convection starts heating the radiator. The role of the windscreen is to reduce the heat gains by convection from the warm ambient air to the radiator. When a windscreen is used, attention should be paid to avoid dew formation, since this will negatively affect the cooler’s performance.

The characteristics of a typical radiative air cooler could be the following:

Length: 10 – 15mWidth: 1mThickness of the radiator plate: 2 mmEmissivity of the coating: 0.90Width of the duct: 1 – 3 cmTransmittance of the windscreen (if used): 0.75 within the long- wave

range.

System operationThe radiator plate cools because of its exposure to the sky vault during the night. Air that circulates in the gap is cooled by convection and then is injected into the space to be cooled. An air cooler can operate in an open (cooling the atmospheric air and then injecting it) or in a closed loop (extracting air from the building and re- injecting it).


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5.3.2 Evaporative cooling

Direct hybrid air coolersThe principal element of a direct air humidifier is a porous material saturated with water. Air circulates through this porous pad by means of a ventilator and evaporates part of the water. The main elements of this system are:

• A water pump

• A ventilator

• A wetted surface on which evaporation occurs (saturation efficiency: 0.6- 0.9).

• A water injection system

• A collection system of the droplets

• A water tank

• A water valve to keep a constant water level in the tank.

The advantage of these systems is their low operating cost and the main disadvantage is the humidity control inside the building.

Indirect hybrid air coolersHybrid indirect evaporative cooling systems use a heat exchanger. Air circulates through the first circuit where evaporation occurs, while the air is cooled through the secondary circuit. The temperature of the air in the secondary circuit is decreased, while the humidity ratio does not change.

There is a limitation regarding the indoor wet bulb temperature, which should be lower than the outdoor dry bulb temperature.

The evaporative cooler’s main components are:

• The heat exchanger

• Two ventilators

• Two filters

• A water pump

• A water injection system

• A water tank

The saturation efficiency ranges between 60 and 80 per cent and is defined as follows:

where the superscripts “prim” and “sec” denote the primary and the secondary circuit, the subscripts “in” and “out” the inlet and outlet and “db” and “wb” the dry- bulb and the wet- bulb temperature.


53


In general, evaporative coolers offer an interesting alternative for cooling. Their energy consumption is very low, being simply for the operation of air fans. Indirect evaporative cooling systems have an advantage over direct evaporative cooling systems, as there is no danger for excessive air humidification.

5.3.3 Earth heat exchanger

An earth heat exchanger is a buried pipe, through which a fluid circulates by means of electric fans or pumps. The fluid may be air or a water solution.

Earth heat exchangers can be used combined with a HVAC system for pre-heating or pre- cooling the working fluid. It is a simple construction with low to medium construction cost.

5.3.4 Rock bed storage


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Fig.5.49 Indirect evaporative cooler

Fig.5.50 Horizontal buried pipes

(with permission from .E.A.CY. S.A.)Θ


The rock bed storage system is a heat storage system. A system like this consists of a heat source, a storage media, a transfer fluid and a fan or pump. Usually the heat source is a solar collector (or an array of solar collectors) and the transfer fluid is air. During the day, the air is heated through the solar collector and afterward exchanges energy with the storage media. The air circulation is forced by the fan. The storage media may be small rocks, broken bricks or other material. The porosity of the material should be as uniform as possible, so that the temperature of the storage media is almost the same as all the thermal mass. The storage media is usually situated in a cellar, in order to minimize heat losses.

During the night time the air flow is reversed. The physical procedures that take place are mainly convection and conduction.

5.3.5 Solar control (Moveable shading devices, glazing)

In order to optimize solar control the use of movable shading devices and/or special glazing is advisable. The advantage of movable shading devices compared with fixed is that they can be adjusted according to weather conditions. The most common types of moveable shading devices are external horizontal louvres and awnings.

• Louvres: These affect the view and potential natural ventilation, but they may provide security.

• External and internal movable shading devices, such as venetian blinds and rollers.

• Any combination of them, such as overhangs made by metallic grating and external movable rollers.


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Fig.5.52 External venetian blinds

Fig.5.51 Skylight with internal roller connected with BMS


Also some special types of glazing may act as moveable shading devices. Such a glazing type is variable transmittance glazing: The solar or visual transmittance and the overall solar energy performance of the glazing may take different values.

5.4 Low energy active systems

These are mechanical systems which do not use any kind of renewable energy sources and their use results in lower energy consumption for various reasons. Such systems are:

5.4.1 Waste heat recovery system

Ventilation losses may be reduced using a waste heat recovery system. Such systems are usually used in order to reduce heating load but they may provide energy savings for cooling too. The energy savings for cooling are significant when the required air changes per hour are high and the building is air-conditioned. Such kinds of buildings are offices, restaurants, museums.

Waste heat recovery systems consist of two ventilators and a heat exchanger. Generally heat exchangers are devices in which a fluid stream exchanges thermal energy by convection with another fluid stream. The fluid may be gas, liquid or vapour. Fluid A, the one that has the desired temperature, exchanges


56

Fig.5.53 External shading made by fixed metallic grating and movable rollers

Fig.5.54 Variable transmittance glazings


energy with Fluid B with the use of solid material or another independent fluid. In buildings, one can assume that Fluid A is the internal, outgoing air and Fluid B the fresh, incoming air. Figure 5.61 shows the above procedure in a schematic way.


57

Fig.5.55 Representation of an air heat exchanger unit


5.4.2 Ceiling fans

The use of ceiling fans does not reduce energy losses. Nor do they use renewable energy sources in order to provide the necessary energy for cooling, but they do achieve reduced energy consumption for cooling, increasing the thermal comfort temperature and acceptable humidity levels. The thermal comfort temperature can be increased by up to 2.0ºC.

The nominal power of a ceiling fan is in the range of 60–100 Watts. So, the energy consumption of the fan must be subtracted from the possible energy saving. Table 1 contains recommended ceiling fan diameters for different room sizes. Generally it is proposed that one ceiling fan is used for every 9- 25m² . Finally, the recommended distance between the blades and the floor is 2.10–2.75m, while the recommended blades to ceiling clearance is 0.30–0.75m.

Table 5.57. Ceiling fan sizing chart, [1]

Largest dimension of room

Minimum fan diameter (m)

3.5m or less 0.90m

3.5 – 5.0m 1.20m

5.0 – 5.5m 1.32m

5.5 – 6.0m 1.42m

6.0m or more 2 fans

5.5 References

1. Passive Building Design – A Handbook of Natural Climatic Control, Bansal N.K., Elsevier, 1994

2. Natural Cooling Techniques, M. Santamouris – D. Assimakopoulos, 19953. Thermal Analysis for Summer Comfort in Buildings, M. Santamouris – D.

Assimakopoulos, 1995


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Fig.5.56 Ceiling fan in classroom


6 WIND POWER

6.1 Potential of wind power

6.1.1 Present situation

• over 50% energy usage in buildings (London 75%)

• UK target of 10% renewable energy by 2010

• UK target of 20% renewable energy by 2020

• wind energy is the major likely source of renewable energy

6.1.2 Wind resources

6.1.3 Wind resource vs demand


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Fig.5.58 Wind resource: W Europe

Fig.5.60 Wind resource versus demand

Fig.5.59 Wind resource: UK


6.1.4 The options

• Offshore large scale (>100kW)

• Onshore large scale (>100kW)

• Onshore medium/small /micro scale

• Building mounted/integrated: medium/small /micro scale

6.2 Large scale installations

6.2.1 Economies of scale

Power in wind = 0.5 A v 3

where:

ρ = air density

A = swept area of rotor = rπ 2

v = wind speed

Power increases with ….

• cube of wind speed

• square of rotor radius

• height (natural logarithmic relationship)

… therefore locate large diameter rotors on high towers in areas of high wind speed

6.2.2 UK provision

• 86 onshore wind farms

• 1066 wind turbines

• 683MW rated capacity


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Fig.5.61 Onshore, large scale Fig.5.62 Building mounted turbines

Fig.5.63 Large- scale, off shore


• provide power for 440,000 homes

• energy balance/pay back is typically 2 months

• 1.5M tons reduction in CO2

6.2.3 Comparative costs

6.3 Installations near buildings

6.3.1 Medium/small scale installations

• These are traditionally installed at a distance from any obstacle of at least 10 to 20 times the height of that obstacle or on a tower at least twice the obstacle’s height

• …. or installed on open land of minimum size 64m x 64m for a turbine capable of powering a whole house (AWEA)

• These requirements preclude most UK properties


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Fig.5.64 Estimated cost of renewable and fossil fuel technologies in 2003 (UK)


Fig.5.65 Criteria for siting turbines near obstacles

6.3.2 Reasons for avoiding buildings

• Wind speed increases more slowly with height over ‘rough’ ground (e.g. built - up areas) than over open fields

• Interaction of wind with buildings creates turbulent and high shear flows around buildings

• Potentially reduced energy yield due to lower wind speeds plus potential damage from increased turbine loads due to turbulence


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Fig.5.66 Difference of wind speed between rural and urban sites

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Air flow around buildings can be turbulent, depending on conditions:

6.3.3 Reasons for choosing buildings

• Height advantage without the need for large towers

• Disturbed flows around buildings can locally increase wind speeds

• Energy yields may therefore be increased relative to open sites


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Fig.5.68 Wind speed in built - up areas

Fig.5.67 Turbulent airflow around buildings


Depending on conditions, air flow around buildings can also be clean:

Wind speed can be enhanced in the area above a building by comparison with the site with no building.


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Fig.5.69 Clean airflow around buildings

no above building building

Fig.5.70 Sample vertical wind speed distribution above a building


6.4 Integrating wind turbines with buildings

Possibilities for integrating wind turbines with buildings include:

• stand- alone wind turbines in the vicinity of buildings

• installation onto buildings

• full integration with buildings, where the turbines drive the architectural form

6.4.1 Types of turbines

6.4.1.1 Horizontal Axis Wind Turbines (HAWT)

• are efficient (ideally up to ~50%), but ...

• are susceptible to high turbulent loads and shear

• .....and potentially noisier, especially with turbulence and shear

6.4.1.2 Vertical axis wind turbines (VAWT)

• are less efficient, but ...

• are much less susceptible to high turbulent and shear loads

• and are quieter


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Fig.5.71 Examples of HAWT


6.4.2 Building mounted turbines

6.4.2.1 Characteristics

• Typically in 1kW to 10kW rated power range

• Typically achieve annual average of 30% rated power

• Average UK household uses 4,377kWh/year – can be achieved by a 1.7kW turbine at 30%

• Average US household uses 10,392kWh/year – can be achieved by a 4.0kW turbine at 30%

• Unlikely to be capable of providing total power requirements of multi -occupancy buildings but could, for example, power lighting only


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Fig.5.72 Examples of VAWT

Fig.5.73 Examples of building mounted turbines


6.4.2.2 Noise and vibration issues for building mounted turbines

• Potential noise to neighbours

• Potential building induced vibrations

• Noise and vibration levels both wind speed and wind direction dependent

• Lack of certified noise data

• HAWT noise output increases with (installation dependent) inflow turbulence – difficult to specify meaningful noise certification test methodology

• HAWT ‘swish’ character can increase with high wind shear conditions

6.4.3 Building integrated turbines

Turbines can drive the architectural form when fully building integrated.

6.4.3.1 Types

• Diffuser concept

• 'Bluff body' concept


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Fig.5.75 'Bluff body' concept (TU Delft, after Mertens)

Fig.5.74 Diffuser concept (TU Delft, after Mertens)


6.4.3.2 Potential energy yield of building integrated turbines

6.4.3.3 Noise and vibration issues for building integrated turbines

•Potential noise to neighbours (typical 95dB(A) to 105dB(A) SWL for a 30m – 40m diameter rotor)

•Potential building induced vibrations

•Noise output will be installation specific

•HAWT noise output increases with inflow turbulence

• HAWT ‘swish’ character can increase with high wind shear conditions

• Concern over HAWT rotor/stator interactions and potential low frequency ‘thump’

• Need for sealed windows and forced ventilation (increased energy usage) on rooms close to rotors


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Fig.5.77 (after Blanch)

Multi-Turbine Twin Tower Buildinglocated in Dublin

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Naturally Ventilated Air-Conditioned

Fig.5.76 (CCLRC)


7 INTEGRATION OF PHOTOVOLTAICS IN BUILDINGS

7.1 BENEFITS OF PHOTOVOLTAICS

Photovoltaic systems have many attractive features in addition to useful local environmental, socio- economic, architectural and technical benefits.

Photovoltaic energy is environmentally benign. Globally it represents an important component of our energy future and helps prevent depletion of valuable natural resources. Locally, solar electricity can be produced almost anywhere and on any scale and can thus make a considerable contribution to sustainability immediately and in the longer term.

Photovoltaics can already offer a number of economically viable and competitive applications within the built environment. Photovoltaics can be part of new, attractive and prestigious design approaches to meet social and cultural demands. It also creates new employment and emerging business opportunities as well as raising social awareness about sustainability, energy saving and commitment to environmental protection.

Photovoltaic systems are both versatile in application and modular in structure and fit well into buildings and other structures. Integrating photovoltaic elements has a double benefit - they can produce electricity and replace construction materials - be it for building components or for parking meters. As a matter of fact, cities offer numerous opportunities for photovoltaic deployment.

The operation of photovoltaic systems is technically reliable, generates energy virtually free of emissions (02, NOx and SOx), needs little maintenance and recovers the energy used in production of the cells several times over This makes photovoltaics politically interesting within the Local Agenda 21 as well as international initiatives such as the Kyoto Protocol or the European White Paper on Energy.

The world of solar electricity is growing and changing rapidly. Setting up photovoltaic projects or photovoltaic policy means joining a market which is doubling in size every three years. Grid- connected photovoltaic systems are gaining importance, especially in Europe. The biggest potential application for photovoltaics in Europe is as embedded generators installed in the built environment and connected to the local electricity network. Building- integrated photovoltaic systems are expected to account for about 50% of the global photovoltaic market share by the year 2010. In Europe the figures are even higher. Stand- alone photovoltaic systems can supply energy for a great variety of modem structures and remote applications


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7.2 PHOTOVOLTAIC (PV) SYSTEMS

Photovoltaic (PV) systems have been installed on many types of buildings in a wide variety of ways. These range from the installation of systems onto house roofs to integration into the façades of commercial buildings or into roof lights of industrial buildings. All these types of systems have been installed in the UK and the number of installations is increasing. Building integrated grid-connected systems has been identified as potentially the most promising route for wide- scale deployment of photovoltaics. PV is the only renewable, electricity- generating technology suitable for widespread deployment in urban areas.

The expected lifetime of a PV module is over 20 years. The fact that it has no moving parts increases its lifetime and reliability and reduces maintenance requirements. The lifetime of a PV system as a whole is therefore likely to be determined by the lifetime of other components such as the support structure, inverter or wiring. An important aim of testing and commissioning is to ensure that the lifetime of the system as a whole is not shortened by poor quality components or installation methods and that components which intrinsically have shorter lifetimes are incorporated in a manner allowing for easy replacement.

It should be recognised that the PV system may also be performing an additional function as part of the weatherproofing of the walls or roof. As such it will be vital to the client that rain, wind and snow are prevented from entering the structure.

It is recommended that all PV systems are monitored to provide data on the performance of the system and verify to its owners and operators that it is working as expected. Monitoring also helps with the identification and diagnosis of any problems affecting the energy output of the system. It is particularly important to monitor the operation of a PV system, as compared with other systems installed a building, because its operation is so inconspicuous. PV systems make no noise, have no moving parts, and the building occupants will see no difference when power is being supplied by the


70

Fig.5.78 PV integrations in buildings: greenhouse, façade and coating


grid or by the PV system. Hence it is easy to overlook the successful smooth operation of the system and not to notice whether any problems occur.

7.3 ARCHITECTURAL INTEGRATION

Building- integrated photovoltaic (BIPV) systems not only produce electricity, they also form part of the building. A BIPV system is an integral component of the building envelope as well as a solar electric energy system that generates electricity for the building. The components of a solar system are thus multifunctional construction materials. The primary element of a BIPV system is the PV module. Individual solar cells are interconnected and encapsulated on various materials to form a module. These are electrically connected in series to form individual strings. Several strings are then interconnected in parallel to form a PV array. Direct or diffuse light falling on the solar cells induces the photovoltaic effect, generating DC electricity. This DC electricity can be used, stored in a battery system, or fed into an inverter that transforms and synchronises the DC electricity into AC. This can be then used in the building or exported to the grid via a grid- connection.

A wide variety of BIPV systems are currently on the market. Most of these can be grouped into two main categories: façade systems and roofing systems.

Façade systems include curtain wall products, spandrel panels, and glazing. Roofing systems include tiles, shingles, standing seam products, and skylights. This section illustrates how PV modules can be incorporated into the design of aesthetically desirable integrated building components (such as awnings) and entire structures (such as bus shelters). BIPV is sometimes the optimal method of installing renewable energy systems in urban, built - up areas where undeveloped land is both scarce and expensive. The fundamental first step in any BIPV application is to maximise energy efficiency within the buildings. Holistically designed BIPV systems will reduce a building’s energy consumption from the electric utility grid by generating electricity on site and in their role as structural components. Roof and wall systems can, for example, be designed to improve insulation and thus heating or cooling demand. Windows, skylights, and façades can be designed to increase the natural light in interior spaces. PV awnings can be designed to reduce unwanted glare and heat gain.

This integrated approach, which brings together energy conservation, energy efficiency, building envelope design, and PV technology and system design, maximises energy savings and maximises the benefits of BIPV system use.

As an ever increasing number of architects and designers gain experience in integrating photovoltaic systems into the built environment, this relatively new technology will blend into the nation’s urban and rural landscapes. BIPV will continue to demonstrate itself as an economically preferable, environmentally


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benign and aesthetically pleasing way of generating electricity for commercial, institutional, and many other kinds of buildings.

7.3.1 PV on buildings

Photovoltaic systems in the built environment can offer a number of advantages. No additional land is required, and no separate support structure is necessary. The PV array can displace conventional building materials, hereby omitting material and installation costs. Electricity is generated at the point of use, which avoids transmission and distribution losses and reduces the utility's transmission and distribution capital and maintenance costs. Finally, building integrated PV systems can enhance the aesthetic and architectural quality of the building due to the innovative, 'green' and self- sufficiency image of the PV system. Overall, it is expected that successful integration of PV systems into the built environment will contribute significantly to the future spread of PV.


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Currently, in Italy, electricity produced using PV is more expensive than that from the grid. Therefore it makes better financial sense to use any electricity generated by a BIPV system within the building itself, rather than to export it to the grid. The proportion of electricity from the PV system which can be used on site depends on the size of the array and the magnitude and pattern of demand.

A wide range of building types, from schools, offices and hotels to industrial buildings, can use PV. Office blocks have good PV potential because their electricity demand is significant all year round (including in the summer) and because demand is highest between 9am and 5pm. The match between supply by PV and demand is therefore very good.

Domestic buildings on the other hand, although they are occupied seven days a week, tend to use energy mainly at night. Nonetheless, individuals (and perhaps electricity suppliers) are likely to be interested in their PV potential. Commercial and industrial buildings with large available roof areas also offer significant scope for PV systems.


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Fig.5.79 Different types of cells


Although the costs of PV energy generation have significantly reduced in recent years, PV is still considered as an (economically) expensive method of electricity production. Depending on the assumed system lifetime, the interest rate and other constraints, the costs given today vary between 0.5- 0.80 € per kWh. If no subsidies for market introduction are granted, these costs are generally considered too high.

A solution to this problem is to replace conventional elements of the building envelope (roof tiles, metal roofs, façade elements, windows, shading devices, etc.) with PV modules. This replacement leads to a dual function of PV technology: building element and electricity generation. If the costs of the conventional building element are deducted from the overall PV system costs the specific costs of electricity generation (in EURO/kWh) can be significantly reduced.


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Fig.5.80PV applications in Freiburg, Germany


Within this activity, nine PV systems were planned and technically designed for public buildings in Italy. These considered the application types and the selection of components with sizing for achieving a good performance on the system level. The following sections briefly describe considerations for technical specifications of the PV systems.

7.3.1.1 Building location and orientation

The topography of the site and its wind regime should be considered in the design of a PV system. Wind should be made use of as part of ventilation strategies. This is especially important in summer, where wind can be used to improve comfort during the day, to reduce the temperature of the PV modules and to assist night- time cooling.

In terms of architectural design, shading by buildings, chimneys, trees, poles or by part of the array itself should be avoided. An estimation of any losses due to shading, especially in urban areas, should be made.

Azimuthal orientation of the solar system is very important, although some flexibility for designers does exist. In the northern hemisphere optimal positioning is at +/ - 20° of due south; this allows 95% of available energy to be collected, depending on the tilt angle, which must also be optimised depending on geographical location.

The figure shows the energy yield losses at the different orientations in Middle European Countries. It shows that in an installation of PV modules into a south- facing vertical façade the gain of energy is 70% and losses 30%.

7.3.1.2 System design


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Fig.5.81 Energy yield at different tilt orientations


Building- integrated PV systems play the same role as the traditional wall and roof cladding elements that they replace and consequently they must address all the normal design issues:

• Appearance

• Weather tightness and protection

• Material

• Wind loading

• Construction safety

• Cost

In addition to these there are more particular aspects associated with electricity production:

• Avoidance of self- shading

• Ventilation to remove and possibly reuse any heat generated, avoiding high module temperatures and improving efficiency

7.3.2 Systems

There are three main system types of BIPV:

• Roof- based systems

• Façade systems

• Sunshades and sunscreens

It may be easier to integrate PV aesthetically and functionally into a roof than a wall.


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Fig.5.82 Roof system


7.3.2.1 Building- integrated roof systems

Roofs have advantages as sites for PV systems: • The roof slope can be selected for high performance. • They are often free from shading.

Roof types and possible ways of using PV


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Fig.5.83 Rooflight system

Fig.5.84 Facade system


Roof systems are likely to be easier to ventilate than façade systems. Also any unwanted gains are likely to have less effect than for façade systems, if they are above occupancy height. For sloping roof designs one approach is to have a sub- frame for mounting the PV modules onto the roof structure. This allows an air space (100mm if possible) between the modules and the roof structure, (which incorporates the insulation). For many saw- toothed roof designs, opening north facing skylights can aid heat removal.

Sloping roof examples

1. PV modules integrated in the roof, substituting for conventional roof material

2. Ventilated PV roof, which could be installed on an existing roof creating ventilation in the PV structure

3. Small PV installation on an existing roof

4. Photovoltaic tiles

Flat roof examples

PV integration with skylights

Standard PV installation on the roof

7.3.2.2 Use of heat from PV modules


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During operation, the back of a PV panel heats up. This waste heat must be removed if a rise in temperature of the PV cells and therefore a drop in efficiency is to be prevented. This heat is potentially of value during the heating season. It is possible to use the heat directly or to recover it via a ducting system - both a prototype ventilated façade (9) and a demonstration building have used these (10). However, the economical viability of such ducting systems is questionable. This is particularly the case for highly energy- efficient buildings, which often have very low space heating demands. (For example, an office, which is in operation during the day, may need no additional heat other than that from the occupants and other passive gains, when external temperatures are above 8°C or 9°C.) Obviously the simplest, least expensive ways of using the heat from the back of the PV modules are most likely to be viable and to be adopted. Other possibilities of using the waste heat exist. For example, PV panels could incorporate water pipes linked to space or domestic hot water systems but these methods tend to be at greater cost and of increased complexity.

Outside the heating season or, more precisely, whenever no additional heat is required, it is important that heat produced by the modules does not cause overheating and contribute to the building’s cooling load. Therefore the building’s ventilation strategy should be considered. Thought should be given to ensure that, in conditions of little wind in summer, heat from the modules does not lead to discomfort.


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7.3.2.3 Façades and curtain walling systems

Façades have significant potential as methods of integrating PV. Most PV cladding can be considered to be panes of glass to which PV cells are applied and so the existing and extensive experience of glazed façades can be called upon. In addition, modules can be easily incorporated into other proven systems such as rain- screen cladding. The table below lists the main systems available.

Curtain walling systems are a well- established technology used in numerous prestige projects such as city centre offices. The mullion/transom stick system is the most common. Window areas are normally double- glazed and non- window areas are either opaque glass or insulated metal panels. PV modules can be incorporated easily as factory-assembled double- glazed units. The outer pane might be laminated glass- PV-resin- glass and the inner pane, glass, with a sealed air gap between; the overall thickness of the module would typically be under 30mm.

Numerous design options are available. For example, a façade could consist of a combination of transparent glazed areas and opaque PV panels or it could have PV modules, which themselves consist of opaque areas and transparent areas.

Careful consideration needs to be given to the junction box positions and cable routing.

Façade integrations Roof and façade integrations


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Fig.5.85 Façade integration


Façade PV integration1. External serrated shading devices2. shading devices3. Vertical façade integration

Table 5.86 Façade systems for PV


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Fig.5.87 External roofing

Fig.5.88 Coating systems


7.3.3 Key points

• PV systems need to be considered as an integral part of the energy strategy of a building.

• Appearance and aesthetics are key issues.

• PV systems make a positive contribution to the environment.

• It is essential that shading by topographical features, other buildings, or features of the PV building itself be minimized so as not to impair performance.

• There should be a good match between the building energy demand pattern and the energy available from the PV array.

• PV modules need to be adequately ventilated so as to lower temperatures and thus maintain good performance.

• There is a wide range of architectural methods of successfully integrating PV systems into buildings, including roof and façade systems.

• The performance of a PV system is affected by its orientation.

7.4 ISSUES RELATED TO PV INSTALLATIONS

7.4.1 Issues relating to the use of PV on buildings

The development of the PV industry in Italy lags behind other countries in certain areas. While there is now good design assistance material in the form of design guidelines and computer- based optimisation tools, as well as guidance for project commissioning, testing and monitoring, the procurement stage is not well served with material to help organisations in the process of implementing a PV system.

There is a need for the standards covering performance of modules and related equipment to be incorporated into Italian standards documentation. In areas where no standards exist, such as manufacturing defects and appearance, these need to be formulated.

There are many issues relating to the integration of PV on a building, which arise throughout the course of a project. These include the following:

o Planning Legislation

o Grid Connection and Surplus Power

o Optimising PV Systems

o Standards


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o Maintenance

o PV Building Products Costs

Each of the above listed issues is briefly described in the following sections.

7.4.2 Planning legislation

Local authorities generally support projects directed toward environmental strategies and PV . Published projects are all integrated in public buildings to be realised with funding from public administration.

Goals for sustainability are increasingly part of local Plans and reflect the Government’s commitment to environmental measures through Local Agenda 21.

7.4.3 Grid connection and surplus power

In many European countries, it is possible for PV systems to produce energy surplus to the building’s requirements for export to the grid.

At the moment it is not possible to sell surplus electricity produced by a PV system on a building to the national grid in Italy. The Italian Government may follow other European countries for the privatisation of ENEL (the Italian national electricity producer) in 2004 so that PV buildings will be able to satisfy not only their individual needs, but will also be able to export surplus electricity thus making a useful contribution to the country’s electricity demand.

7.4.4 Optimising PV systems

This was briefly mentioned in an earlier section. Optimisation involves optimisation of tilt and azimuth angles. It is also necessary to prevent over-heating of the modules and to keep the modules clean and free from dust etc.


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Fig.5.89 Residential buildings in UK


7.4.5 Standards

Designers need simple standards and codes of practice to cover component manufacture and installation. Current Italian standards relate to:

• Module appearance, including bubbles in the resin in glass/glass modules, alignment of cells, acceptability of faulty cells and alignment and positioning of conducting wire;

• Non- electrical safety issues including handling, durability and hazards arising from fire;

• Support systems such as PV-integrated curtain walls, roof lights etc.

7.4.6 Maintenance

Maintenance requirement of PV systems is very low. However, providing safe access for maintenance to a PV array is very important and has frequently been overlooked until too late in the design process.

7.4.7 PV Building Products Costs

Recently, most photovoltaic elements for building integration have been moderately more costly than their conventional building material counterparts. The electricity generated from BIPV systems cannot, in the near term, compete on cost basis with that supplied by conventional means. However, BIPV costs are expected to decrease over the next 10- 15 years to the point where the additional cost of a PV building component is offset by the value of the electricity supplied over its operating period. A significant portion of the cost decrease will be attributed to the expected increases in market size.

When considering the cost of a general PV system, it is common to quote the cost in relation to rated power output. This is not the case for a BIPV system where comparisons with conventional building materials expressed as per unit area, are considered. This is often done without factoring in the cost savings from the energy the BIPV system provides. Earlier results of five case studies undertaken in the UK in 1997, comparing BIPV product costs to conventional building materials, showed approximate costs as listed in the table below:

Table 5.88 Installed cost in Euro/m 2

PV curtain walling, glass/glass crystalline modules 1,200PV curtain walling, glass/glass thin- film amorphous modules 450PV rain screen cladding 900PV roofing tiles 800PV modules on a pitched roof 1,100


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7.4.8 Requirements

Certification is required for:

• type- tested components

• factory tests

• electrical installation.

• Warranties for the components and the installed PV system.

• Method of integration and fixing into the building fabric and any associated tests eg water penetration tests or tests on the adequacy of the base material the system is to be mounted on.

• Materials to be used for fixings and mountings taking into account strength and corrosion- resistance requirements and the avoidance of any material incompatibility problems.

• The standards the electrical installation is expected to meet eg BS 7671 Requirements for Electrical Installations (IEE 16th Edition) which covers design, installation, inspection and testing, or a requirement that the electrical installation should be installed by a NICEIC (National lnspection Council for Electrical Installation Contracting) registered company.

• Electrical parameters to be tested (test points, test conditions and tolerances)


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Fig.5.90 Heliotrop, Solar House, Freiburg


for acceptance.

• Requirements for earthing, lightning and overvoltage protection and appropriate test and commissioning methods.

• Safe and accessible positioning of disconnection points for use in testing and maintenance.

• Access requirements for testing and commissioning, monitoring, maintenance and repair.

• The grid connection requirements of the REC and witness testing requirements by the REC and client/client representatives.

• Acceptable PV array performance, for example, kWh under specified test conditions or kWh over a period. lnverter performance eg efficiency, maximum power point (MPP) tracking capability, harmonics.

• Handover information requirements.

• Requirements for the monitoring system — level of monitoring required, parameters to be monitored, data to be stored and calculated, reports to be generated and commissioning requirements for the monitoring system.

7.5 PV MODULES AND ENERGY YIELD

PV modules can be integrated in different ways. It depends on the result and solution which is required. PV modules can be used as retro- fit elements, shading devices and also as transparent elements. For the design of all PV systems to be implemented into the public buildings standard, available PV modules and BOS ('balance of systems') components have been selected. PV modules that are available with frames and as laminates are more convenient for integration into the building. In cases where daylight is required, transparent PV modules can be selected. For all PV modules technical specifications are required, both to configure the PV system and also to perform a simulation of the plant. DC cables were selected, which minimize resistance losses.

7.5.1 Sizing

The size of the PV array depends on different requirements. The surface of the installation area and the orientation play key roles. For maximum energy output from the system, it is important that no shading occurs on the module. This might minimize the PV array, but the PV modules are the most expensive part of a PV system and, therefore, it is important not to place modules where shading occurs. For selecting the PV modules the application type is essential.

PV modules can also be used as transparent elements for daylight, as at the JPS-Presse and Vertriebs GmbH building in Meckenheim, Germany which includes fully integrated transparent PV modules.


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The energy yield of a PV system depends on the orientation of the tilt angles of the PV array in the application.

7.5.2 Power Losses

PV systems are affected by several performance parameters which influence the overall system efficiency and operational system lifetime, technical performance, safety aspects, installation and maintenance costs. The following section describes considerations and weaknesses of PV systems with central system concept and string inverters.

In PV systems with centralised power control, mismatch losses occur because of connecting the PV modules in series to achieve the required voltage and current to the DC bus. In PV plants with different PV module orientation it is common to install string inverters.

Large diameter cables for connecting the PV modules also cause resistance losses. Large PV systems require a high DC bus current and the costs for cabling can increase dramatically.

In particular , where building integrated PV modules are used in cities, PV modules are occasionally shaded by other buildings or construction elements which affects the power output of the string. This can lead to an increase of


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Fig.5.91Different integration solutions for different applications


10% of power losses (measured within the German MuD programme and ‘1000 Roofs’ programme performed by WIP).

7.5.3 Safety and maintenance

In large PV systems with a high DC bus voltage, problems have been observed. The high DC voltage could cause problems such as leakage from the PV modules between the frame and solar cells. This has been observed at several PV plants installed world- wide.

In addition, arcing can also appear and causing severe problems in the production of energy from the PV array and even exploding of complete PV arrays.

If replacement or maintenance of PV modules is necessary, electrical shocks are possible, caused by the DC bus. Here, safety also plays an important role.

Safety regulations and quality certifications exist at national and international levels. The intention here is to provide an overview of regulations and quality requirements.

Maintenance, repair, or replacement of PV system components should only be performed under defined safety regulations and qualifications of technical staff.

• Work should be performed only under voltage- free conditions. It should be possible to disconnect the PV array from the inverter.

• DC disconnection switch and AC isolation should be possible and assured.

• Safety Class II. The standard tests are applied to any PV modules.

• The electrical protection of implementing a by- pass diode into a string of series connected PV modules is essential in conventional system types.

7.5.4 Installation

The installation of PV modules in conventional systems requires the series connection and then connection of individual strings to the power bus. According to Italian Standards, the system voltage limit is 600 Vdc.

7.5.5 Monitoring and metering

Metering can be a practical way to provide transition support for small- scale grid- connected PV generators. A single meter is used to measure both the export of electricity to the grid and also the import of electricity from the grid.


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7.5.6 System configuration and selection of the inverters

Current, voltage and power of the PV modules provide the basis for connecting PV modules and for the number of PV modules to be connected in series to achieve a minimum of losses and keep below the limit of 600 Vdc system voltage. The inverter should provide a voltage range which is adequate to meet requirements for connection in a string or the total DC voltage (central inverter concept).

7.5.7 Protection devices

Protection is provided on the DC side, which is a switch to protect the inverter from excessive voltage and to allow disconnection of the PV array when replacement of the inverter is necessary. On the AC side, the grid has to be protected from peaks and other disturbances, which is usually regulated by a national standard (Italian standards set by the utility).

7.6 PHOTOVOLTAIC MARKET, TARGETED ACTION FACILITATING PV MARKET PENETRATION IN ITALY

7.6.1 Introduction and background


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Fig.5.92 Building integrated PV with transparent PV modules

(Source: Saint Gobain Glas Solar)


Photovoltaic systems are the most innovative and promising of the different technologies exploiting solar energy because they convert sunlight into electricity. It is a very attractive technology for architects because of its modular characteristics, simplicity, reliability and low maintenance demand, as well as its potential for technological development.

Electrical energy generation, directly from solar radiation, can contribute to providing people, whether they live in cities or in the most remote places of the Earth, with a reliable energy supply. Since PV systems operate silently and without environmental pollution they have an important role in sustainable development.

The rate of technological progress in the PV industry and the scale of economies - tied to a future market expansion - bring substantial cost reductions. Costs however, remain the main barrier to widespread diffusion of this technology.

The current Italian PV market concentrates on applications not covered by other systems. The demand for urban and grid- connected large scale applications is created institutionally, i.e. by political will and public opinion, rather than by real demands.

However, there is a lack of available information concerning the applications of PV and therefore no specific strategy for increasing the number of PV installations in Italy.

Once local government bodies are made aware of the advantages of PV systems and are presented with the range of opportunities that PV systems offer, they could boost the sales of PV systems by directing part of their budgets towards acquisition and installation of suitable PV systems on public buildings.

This would encourage the installation of further systems on corporate and residential buildings. It would add to the growing awareness of the need for sustainable social- economic development and could contribute to boosting sales and improving the PV market.


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7.6.2 Component Selection and Product Certification

Testing and commissioning requirements will be minimised if appropriate product certification and guarantees are specified and obtained for the PV system components. These guidelines assume that the certificates and guarantees listed below are obtained and hence further testing is not required to demonstrate those aspects covered by product certification or guarantee. Standards and certification requirements are continually being introduced and upgraded and it is recommended that the designer check that the references given are still current and whether any additional standards have been introduced.

It is recommended that PV modules and laminates should be certified as having been type- tested as specified in IEC 1215 [6] for crystalline silicon PV modules or IEC 1646 171 for thin film silicon modules. These tests are designed to ensure modules can withstand harsh weather conditions over a prolonged period whist retaining their electrical integrity. Tests can be conducted by the European Solar Test Installation at the European Commission’s Joint Research Centre at Ispra, or a similar recognised test institutional.

The power output from the PV modules should be guaranteed to remain within a stated range of their nominal output power for a certain length of time. (Crystalline silicon modules are normally guaranteed to remain within 10 % of their rated power output for ten years while a five or six year period is the norm for thin film modules.)

Inverters should be certified to meet relevant standards on electromagnetic compatibility (EMC) [81, 191]and harmonic content of AC outpu t [101]. Type- test requirements for inverters are currently under preparation. Once available, these tests should ensure that an inverter meets REC requirements. The inverter should be covered by a guarantee from the supplier or


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Fig.5.93 Products for integration


manufacturer. Guarantees for inverters, and similar electrical equipment, are normally for one or two years.

The PV system should also be guaranteed against faulty components or installation by the installer for a minimum of one year. The guarantee should cover the electrical installation and the building integration.

It should be recognised that the PV system may also be performing an additional function as part of the weatherproofing of the walls or roof. As such it will be essential to the client that rain, wind and snow is prevented from entering the structure.

7.7 MONITORING PV SYSTEMS

The designer and client must consider the aims of the monitoring programme and decide whether a comprehensive monitoring survey will be conducted for a period of time after the project is first commissioned or if a simple monitoring regime, i.e. a long- term performance review of the system, is sufficient. The monitoring programme can then be tailored to meet the agreed aims and the resources available for monitoring.

Personnel responsible for the eventual running of the monitoring programme should be identified, and the resources available for monitoring agreed, before the monitoring programme is finalised. The installation of sophisticated monitoring equipment is a complicated process and data collection and analysis can be time- consuming. Data should not be collected for its own sake.

All projects should monitor the long- term performance of the PV system in order to record what the system is delivering both in energy and monetary terms and to ensure that system performance is not degrading. Normally this regime would be started as the comprehensive monitoring programme was coming to an end. However, for those projects which elect not to undertake a comprehensive monitoring programme, the long- term monitoring should be started straight away and continued for as long as the PV system is operational.

The designer will need to consider the specific requirements of the monitoring system such as provision of measurement points. It is also necessary to consider who will be responsible for the installation and commissioning of the monitoring system and whether this will be done by those responsible for commissioning the PV system.

7.8 CONCLUSIONS


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Within only a few years, photovoltaic systems have made an essential step forward towards aesthetic integration into the built environment. Today PV is flexible in shape, function and colour. Application areas are roofs, façades, and windows. Engineers are now in a position to respond to almost every wish of architects but the costs are a very important factor. This movement is largely driven by smaller companies active in the PV business.

National and European PV programmes contribute significantly towards decreasing system costs. The German programmes (100 000 Roofs PV programme and REL) led to cost reduction of about 7% in the year 2002. They also contribute to advancing the technology development with better performances at system level, as each kWh that is fed into the public grid is paid by the utilities.

8 BATIMENT AUTONOME EN ENERGIE

8.1 OBJECTIF

L’évolution ultime des techniques de réduction des besoins en énergie et d’utilisation d’énergies renouvelables est la conception d’un bâtiment complètement autonome en énergie, c’est à dire ne nécessitant aucun raccordement à un réseau de distribution d’énergie, ni utilisation de combustibles.

Les ressource d’énergie disponibles :

• solaire thermique,

• solaire photovoltaïque,

• vent,

utilisées conjointement à un stockage (thermique et/ou électrique) peuvent satisfaire aux besoins d’un habitat économe en énergie. Le solaire photovoltaïque utilise notamment la technique du «toit solaire» (Fig 9.1.a). Une surface de 10 [m²] délivre une puissance maximale de 1 [kW] et permet de collecter de l’ordre de 1100 [kW.h] (zone d’ensoleillement moyen en France).


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Fig 5.94 – Production d'électricité raccordée au résau (V.Fristot)

Toit solaire photovoltaïque (V.Fristot)

Des expérimentations mettent en évidence la faisabilité de telles conceptions, respectueuses à la fois des conditions de confort intérieur et de l’environnement extérieur. Les technologies utilisées sont cependant assez complexes, avec accumulateurs électriques, machines thermodynamiques ou


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thermochimiques. Le coût de construction et l’acceptabilité des contraintes techniques de fonctionnement sont les deux points principaux à considérer lors de la conception d’un tel bâtiment.

L’analyse énergétique globale sur l’ensemble du cycle de vie doit être examinée avec soin, en prenant notamment en compte les énergies nécessaires à la fabrication des composants et à leur élimination après usage.

Par exemple, les panneaux photovoltaïques de première génération était incapables de restituer pendant leur durée de vie de 30 ans, l’équivalent de l’énergie qui avait été nécessaire à leur élaboration. L’amélioration des technologies a fait passer à 4 ans, puis 2 ans, la période de restitution.

Le terme de "bâtiment autonome en énergie" est donc mieux adapté qu’une traduction littérale de "zero energy building", car une dépense d'énergie a été nécessaire à leur construction. On rencontre également le terme de "maison à énergie positive", puisqu'en fin de vie, elle aura fourni plus d'énergie qu'elle n'en a nécessité (par exemple grâce à un toit photovoltaïque raccordé au réseau).

8.2 EXEMPLES

De nouvelles technologies, pour certaines issues des techniques spatiales (vitrages à très hautes performances, pile à combustible par exemple) sont susceptibles de contribuer à la faisabilité des constructions autonomes en énergie, à l’échelle industrielle.

On peut noter les références de quelques réalisations récentes :

• maison « Hybrid Z » (MSK Corp & Misawa) : toit solaire intégral photovoltaïque de 12,5 [kW] en raccordement réversible au réseau national,

• maison « Jestico & Whiles » : solaire passif et cellules photovoltaïques, éolienne et micro centrale hydraulique locale, bilan de 180000 [kW.h] dont 35000 en P.V,

• maison « Darmstadt- Kranichstein » : 145 [m²] habitables, pour un bilan de 15 [kW.h/m²] en chauffage et 45 [kW.h] en E.C.S et besoins domestiques. Les cellules P.V utilisent un gisement solaire de annuel de 1000 [kW.h/m²] sur un plan horizontal, le stockage étant réalisé par voie électro- chimique (hydrolyse de l’eau et combustion lente de l’hydrogène),

• bâtiment de bureaux à Delft : les quatre étages nécessitent une énergie d'appoint quasi nulle grâce à une utilisation judicieuse des apports internes et solaires.


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Maison autonome en énergie (Misawa Corp.)

Bibliographie

Eau chaude et chauffage solaireR.Espic, J.P.Isoardi, M.MoreauSedit

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Solaire actif et passifC.CardonnelEditions Parisiennes

Chauffage solaire et habitatCabiro, Fauré et RouxEdisud

Energétique du bâtimentC.A.RouletPresses Universitaires Romandes


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Solar Energy HousesHestnes, Hasting, SaxhofJames and James

Passive cooling in buildingsM.SantamourisJames and James

Solar energy and housing designSimos YannasArchitectural Association Publications

Architecture climatiqueP.Lavigne, P.Bregon, P.Fernandez, A.ChateletPYC Editions

Guide de l'architecture bioclimatiqueA.LiebardEd Systèmes Solaires

Guide de l'énergie solaire passiveA.MazriaEd Parentèses

Architecture solaire en EuropeCommission des Communautés EuropéennesEdisud

L'isolation écologiqueJ.P.OlivaTerre Vivante

Qualité environnementale des bâtimentsCollection Connaître pour agirAdeme

Eco- conception des bâtimentsBruno PeuportierPresses de l'Ecole des Mines

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Documents

1 MINIMISATION DES BESOINS 1.1 BESOINS EN CHAUFFAGE · température unitaire. Si on inclue dans ce déperditions le réchauffage de l’air neuf introduit dans les locaux, on obtient