Exergy in the built environment. The added value of exergy in the assessment and development of energy systems for the built environment.

Exergy in the built environment

The added value of exergy

in the assessment and development

of energy systems for the built environment

Exergy in the built environment

The added value of exergy in the assessment and development of

energy systems for the built environment

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 5 november 2013 om 10:00 uur

door

Sabine Charissa JANSEN bouwkundig ingenieur geboren te Amsterdam

Dit proefschrift is goedgekeurd door de promotoren:

Prof.ir. P.G. Luscuere

Prof.dr.ir. A.A.J.F. van den Dobbelsteen

Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof.ir. P.G. Luscuere Technische Universiteit Delft, promotor Prof.dr.ir. A.A.J.F. van den Dobbelsteen Technische Universiteit Delft, promotor Prof.dr.ir. T.M. de Jong Technische Universiteit Delft

Prof.dr.ir. P.M. Herder Technische Universiteit Delft Prof.dr. L.J.F. Hermans Universiteit Leiden

Prof.dr.ir. B.J. Boersma Technische Universiteit Delft

Dr. D. Schmidt Fraunhofer Institut für Bauphysik, Kassel, Duitsland

Ir. A.C. van der Linden heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen.

The research described in this thesis was conducted with the financial support of Agentschap NL (EOSLT02003), which is gratefully acknowledged.

ISBN 978-94-6203-465-5

Copyright  2013 Sabine Jansen

Layout and cover design by Sabine Jansen

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Summary

This doctoral thesis presents research on the added value of exergy for the assessment and

development of energy systems for the built environment. The research consists of a theoretical part, related to the relevant thermodynamic theory applied to extend the existing exergy calculation method for the built environment and a second part on the application of the exergy concept to the assessment of current energy systems for the built environment and the development of improved systems.

Introduction & overview

To develop sustainable energy systems for the built environment the required input of energy resources needs to be reduced and the amount of renewable resource input increased, up to the point where the total required input can be supplied by renewable resources. The current approach for evaluating and developing energy systems for the built environment is according to the energy concept. The energy concept however provides an incomplete presentation of the performance of an energy system by failing to address the difference between various forms of energy.

Exergy is a thermodynamic concept that quantifies the ‘work potential’ of different forms of energy, which can be regarded as the quality of energy. Unlike energy, exergy can be destroyed. In

thermodynamically ideal processes no exergy is destroyed, but in all real processes exergy destruction takes place. The amount of exergy destroyed or lost indicates the thermodynamic improvement potential of a system, which is not revealed using energy analyses. Using the exergy concept in addition to energy is therefore expected to provide valuable additional insight that can contribute to the

development of sustainable energy systems for the built environment. The application of exergy analysis in the assessment and development of energy systems for the built environment however is still relatively new.

Therefore, the objective of the research presented in this thesis is:

 to increase knowledge on the exergy calculation method for energy systems in the built environment.

 to explore and demonstrate the added value of using exergy in addition to energy in the assessment and development of these systems, in order to contribute to the development of sustainable energy systems.

For this research a theory and literature study was combined with case studies, including simplified studies using excel as well as dynamic simulations using the transient software TRNSYS. The scope of this research includes the total energy chain for the built environment, with at one hand the energy demand resulting from user requirements, building characteristics and weather data and at the other the energy resources, both renewable and non-renewable. In-between there are energy system components for energy conversion, distribution, storage and emission, for getting the energy in the right form at the right time and place. The system is illustrated in the figure below.

Additions to the exergy calculation method for the built environment (Chapters 2 and 3)

The additions to the existing exergy analysis methods for the built environment as found in literature consist of the following four topics:

1) Calculation of the exergy of cooling in buildings.

2) A new detailed approach for calculating the exergy demand for heating and cooling. 3) The exergy balance of thermal zones of a building and the influence of the exergy demand

calculation method and the chosen system boundaries on the exergy destruction in the zone. 4) The difference between steady state and dynamic exergy demand calculation.

Chapter 2 presents a thorough discussion on the exergy of ‘cold’. Consequently, two essentially different cooling demand situations are distinguished in chapter 3: (1) cooling demand while Ti>T0,

which can occur due to solar and internal gains, and (2) cooling demand while Ti<T0. In the first case the

cooling demand in fact represents an exergy output (a need to dispose of unwanted ‘warm exergy’) in order to bring the system to a temperature more in balance with the environmental temperature. In the second case the cooling demand represents a required exergy input (which is a ‘true’ exergy demand) since the system (indoor space) must be brought to a temperature further from the

environmental temperature. In temperate climates such as the Dutch maritime climate a ‘true’ cooling demand, in the sense of requiring exergy input, occurs only seldom.

Secondly, this thesis presents a new detailed approach to calculate the exergy demand, defining the exergy demand as the “minimum amount of work needed to obtain the required indoor thermal conditions”. The commonly applied approach assumes all heating or cooling need to be supplied at indoor temperature (Ti), while this detailed approach calculates the exergy demand as the sum of the

exergy needed to preheat or pre-cool the ventilation air plus a remaining exergy input to provide heat at Ti, if necessary. This detailed method results in a lower demand than the commonly applied

approach, since it takes into account that reversible heating of ventilation air – from the environmental temperature T0 to Ti – requires less work than providing heat at constant Ti. Therefore the lowest

required exergy input is in principle achieved by preheating or precooling ventilation air, as opposed to mixing outdoor air with (warmer or cooler) indoor air; in practice this obviously depends on the performance of the actual preheating or pre-cooling system.

The detailed energy and exergy balance schemes presented in chapter 3 quantify the amount of exergy destroyed within a building zone. Furthermore they provide insight in the influence of the applied exergy demand calculation method (detailed or simplified) and of the assumptions and system boundaries used for evaluating the exergy flows of all flows of energy and matter.

Lastly, the study on the difference between the results from dynamic exergy demand calculations versus steady-state exergy demand calculations shows that these differences are significant. In the Dutch climate the dynamically calculated exergy demand of the cases studied is largely between 14-17% higher than the steady state results. The difference depends in the climate as well as on the input parameters that determine the heat demand: for instance, the lower the heating demand due to high internal gains, the larger the difference between dynamic results and steady-state results. For detailed analysis or focus on the exergy losses between demand and heating emission system a dynamic simulation is therefore recommended. For cooling a dynamic approach is always required.

The exergy performance of current energy systems and the added value of exergy (mainly chapter 4)

Chapter 4 presents an exergy assessment of currently existing energy systems for two types of dwelling: A Dutch single family terraced dwelling according to a reference described by SenterNovem (2006) and complying with recent Dutch energy standards, and a social dwelling in a multifamily building built in

the 1960’s in Bilbao, Spain. The case studies are analysed using the dynamic simulation software TRNSYS, by incorporating exergy equations, including the additions to the exergy method as described above.

The Dutch case studies include a detailed analysis of the energy and exergy demand (Case 1), an analysis of the use of heat recovery in this dwelling (Case 2) as well as four different energy systems for this dwelling, based on a condensing boiler (Case 3), a micro-combined heat and power unit (micro CHP, Case 4A), a heat pump (Case 4B) and a heat pump in combination with adapted building properties that meet passive house standards (Case 4C). For the Spanish case the original situation as built in the 1960’s is studied (Case 5A) as well as a situation with standard renovation works (Case 5B).

The state of the art energy systems (Dutch case studies) present overall exergy efficiencies (i.e. for supplying all demands) between 28% and 44%, compared with overall energy efficiencies between 74% and 132%, (values higher than 100% resulting from disregarding the free environmental heat used by the heat pump). The overall exergy efficiencies are relatively high due to the significant contribution of the electricity demand combined with the high efficiency assumed for electricity production. The exergy efficiencies of the energy chain for supplying space heating only vary between 5% and 14%, compared to energy efficiencies between 90% and 400%, representing the boiler-based heating system (Case 3) and the heat pump based system (case 4B) respectively. Hence, the difference between energy and exergy efficiencies is substantial. The added value of the exergy efficiency is that it is a measure of the thermodynamic performance that indicates the thermodynamic ideal improvement potential (i.e. the ideal efficiency is 100%); the energy efficiencies merely represent the ratio of energy input to output, without providing information of what is theoretically possible.

In addition to the overall exergy efficiency, the analysis at component level shows that exergy efficiency () of most components varies greatly from their energy efficiency (): the boiler (=95%, =14%), the micro-combined heat and power (CHP) unit (=100%, =30%), and the heat pump (=410%, =42%). Also the fictive component ‘room air’, representing the transfer of heat from the emission system (e.g. radiator) to the thermal zone and thereby the potential ‘mismatch’ in temperatures, proves to involve large exergy losses, which are absent in the energy approach. The second and possibly more important added value therefore presents the exergy assessment at component level, supporting a better

interpretation of the performance and giving more insight into how (much) the system can be improved.

How to use the exergy concept for developing improved energy systems (mainly chapter 5)

Chapter 5 explores different ways of using the exergy concept to support the development of improved and innovative energy systems for the built environment. As a result the following three approaches are defined and demonstrated using exemplary case studies:

1) Using exergy principles - a qualitative approach to generate the most promising energy concepts 2) Using exergy analysis - a quantitative approach to identify and quantify exergy losses, in order to

minimize these.

3) Using exergy insights to develop innovative ideas.

The exergy principles developed are based on literature and on research done within this thesis, and are divided into A) thermodynamic principles to minimize exergy losses, and B) integrated principles to make optimal use of the situation of a project. The exemplary case study demonstrates the use of these principles for generating energy concepts with a potential for maximum reduction of exergy losses and minimal use of fundamentally exergy-destructive processes. The second, quantitative approach aims at

further reducing exergy losses and thereby the required high-quality energy input of a given energy system or concept configuration (potentially a result from approach 1). As a third option the use of exergy for stimulating innovative ideas is presented. Innovation cannot be enforced but the combination of exergy knowledge, application of exergy principles and exergy analysis can lead to innovative ideas. As an example two new ideas resulting from studies in this thesis are explained and a first test of their validity is performed.

In addition to the approaches several important issues related to the use of exergy for developing improved energy systems are discussed. Firstly, the system components that are essential for developing exergy-smart energy systems are discussed. Secondly, an estimation of the technically possible ideal improvement potential is studied, based in minimal driving forces. Furthermore the use of the exergy approach related to a larger (neighbourhood to regional scale) is discussed and lastly some thought about including exergy in energy legislation are presented.

Closure

By increasing insight into the exergy analysis method of energy systems for the built environment and by demonstrating the added value of using exergy in addition to energy, this thesis aims at contributing to the development of more sustainable energy systems. The exergy efficiency of the current

(traditional and state of the art) energy systems studied is very low (between 3% and 14% for providing space heating) and, even though the ‘technical achievable ideal’ efficiency (estimated between 26% and 44%) is much lower than the theoretical ideal of 100%, there is still room for improvement. The exergy approaches developed and demonstrated in this research are promising ‘tools’ to support the

Samenvatting

Dit proefschrift beschrijft het onderzoek naar de toegevoegde waarde van exergie voor het

beoordelen en ontwikkelen van energiesystemen voor de gebouwde omgeving. Het onderzoek bevat een theoretisch deel en een toegepast deel: het eerste betreft aanvullingen op bestaande

exergieberekeningsmethoden voor de gebouwde omgeving, het tweede deel heeft betrekking op de toepassing van exergie voor de beoordeling van bestaande systemen en de ontwikkeling van

verbeterde systemen.

Inleiding & overzicht

Voor het ontwikkelen van duurzame energiesystemen voor de gebouwde omgeving dient de benodigde energie-input te worden verminderd en de hoeveelheid energie van duurzame bronnen te worden verhoogd, tot het punt waar alle benodigde energie kan worden geleverd door duurzame bronnen. De huidige aanpak voor het beoordelen en ontwikkelen van energiesystemen voor de gebouwde omgeving is gebaseerd op het energie concept. Energie geeft echter geen volledig beeld van de prestatie van een energiesysteem omdat geen onderscheid wordt gemaakt tussen verschillende energievormen.

Exergie is een thermodynamisch concept dat het ‘potentieel om arbeid te produceren’ van verschillende energievormen kwantificeert. Dit is een maat voor de kwaliteit van energie. Exergie kan, in tegenstelling tot energie, worden vernietigd, en dit is in alle werkelijke processen ook in meer of mindere mate het geval. Alleen in thermodynamisch ideale processen wordt geen exergie vernietigd; de hoeveelheid exergie die in een systeem wordt vernietigd is daarmee gelijk aan het thermodynamische

verbeterpotentieel, en dit kan niet worden geïdentificeerd met energieanalyses. Toepassing van exergie zal daarom naar verwachting belangrijk extra inzicht geven dat een bijdrage kan leveren aan de

ontwikkeling van duurzame energiesystemen. In de gebouwde omgeving is de toepassing van exergie echter nog betrekkelijk nieuw.

Het doel van dit promotieonderzoek is daarom:

 Het vergroten van de kennis betreffende de exergieberekeningsmethode voor energiesystemen voor de gebouwde omgeving.

 Het verkennen en aantonen van de toegevoegde waarde van het toepassen van exergie in aanvulling op energie voor de beoordeling en ontwikkeling van energiesystemen voor de gebouwde omgeving, met als doel bij te dragen aan de ontwikkeling van duurzame energiesystemen.

Voor het onderzoek is een theorie- en literatuurstudie gecombineerd met case studies, zowel

eenvoudige (Excel) studies als dynamische simulaties met de software TRNSYS. Het onderzoek bekijkt de gehele energieketen voor de gebouwde omgeving met aan de ene kant de energiebehoefte (gebaseerd op (comfort) eisen van de gebruiker, gebouweigenschappen en weersgegevens), aan de andere kant de beschikbare energiebronnen, en daartussen de systeemcomponenten die nodig zijn voor conversie, opslag en distributie van energie. Dit systeem is hieronder schematisch weergegeven.

Aanvullingen op de exergieberekeningsmethoden voor de gebouwde omgeving (Hst 2 &3)

De aanvullingen op de bestaande exergieberekeningsmethoden voor de gebouwde omgeving hebben betrekking op de volgende vier onderwerpen:

1) Berekening van de exergie van een koelbehoefte in gebouwen.

2) Een nieuwe gedetailleerde methode voor het berekenen van de exergiebehoefte voor koelen en verwarmen van gebouwen.

3) De exergiebalans van de thermische zones in gebouwen en de invloed van de exergiebehoefte-berekening en de gekozen systeemgrenzen op de exergievernietiging in deze zones.

4) Het verschil tussen statische en dynamische berekening van de exergiebehoefte.

In hoofdstuk 3 worden twee wezenlijk verschillende typen koelbehoefte onderscheiden: (1) een koelbehoefte terwijl de binnentemperatuur hoger is dan de buitentemperatuur (Ti>T0), tengevolge van

hoge interne- en zonbelasting, en (2) een koelbehoefte terwijl Ti<T0. In het eerste geval

vertegenwoordigt de koelbehoefte in feite een output van exergie, dat wil zeggen een overschot aan ‘warme exergie’, die het systeem uit moet om de temperatuur van het systeem dichter bij de

referentietemperatuur te brengen en zo de gewenste (maximale) binnentemperatuur te bereiken. In het tweede geval vertegenwoordigt de koelbehoefte een daadwerkelijke exergiebehoefte (exergie input), omdat het systeem op een temperatuur verder van de omgevingstemperatuur gebracht moet worden. In een gematigd klimaat zoals het Nederlandse is de daadwerkelijke koelbehoefte zoals bedoeld in het tweede geval zeer laag.

Ten tweede is in dit onderzoek een gedetailleerde methode voor het berekenen van de exergiebehoefte voor verwarmen en koelen ontwikkeld, waarmee de exergiebehoefte gedefinieerd wordt als “de

minimale hoeveelheid arbeid die nodig is om de gewenste verwarming of koeling te realiseren”. De in de literatuur gebruikelijke exergie-behoefteberekening gaat ervan uit dat alle verwarming of koeling op de binnentemperatuur Ti geleverd wordt. De gedetailleerde methode berekent de exergiebehoefte als

de som van de exergie die nodig is om de ventilatielucht op te warmen of af te koelen, plus de exergie van eventueel resterende verwarming of koeling die op Ti geleverd moet worden. De exergiebehoefte

volgens de gedetailleerde methode is lager omdat ideale opwarming of afkoeling van ventilatie lucht - van omgevingstemperatuur T0 tot binnentemperatuur Ti - minder arbeid kost dan verwarmen of koelen

op Ti. De minimale hoeveelheid arbeid wordt dus in principe bereikt door het voorverwarmen of

voorkoelen van ventilatielucht; in de praktijk hangt de hoeveelheid arbeid natuurlijk af van de eigenschappen van het daadwerkelijke apparaat dat deze voorverwarming of -koeling teweeg moet brengen.

De gedetailleerde energie en exergie balansen in hoofdstuk 3 kwantificeren de hoeveelheid exergie die in de thermische zones van een gebouw vernietigd wordt. Deze exergiebalansen verschaffen inzicht in het verschil tussen de gebruikelijke- en de gedetailleerde exergiebehoefteberekening en in de invloed van de gekozen systeemgrenzen op de hoeveelheid exergievernietiging.

Ten slotte is onderzoek gedaan naar het verschil tussen de exergiebehoefte op basis van statische berekeningen - uitgaande van seizoensgemiddelde temperaturen - en op basis van dynamische berekeningen. Voor een tussenwoning in het Nederlandse klimaat is de exergiebehoefte bepaald, waarbij onder andere isolatiewaarde, ventilatievoud en interne warmtelast zijn gevarieerd. Voor bijna alle varianten blijkt de exergiebehoefte volgens dynamische berekeningen tussen 14% en 17% hoger te zijn dan volgens statische berekeningen. Het verschil tussen statische en dynamische berekeningen is afhankelijk van het klimaat en van de invoervariabelen die de warmtebehoefte bepalen. Bijvoorbeeld

geldt: hoe lager de warmtebehoefte als gevolg van een hoge (interne) warmtelast, hoe groter het verschil tussen dynamische en statische berekeningen. Voor een gedetailleerde analyse, en met name wanneer de exergieverliezen tussen de behoefte en emissiesysteem van belang zijn, wordt een dynamische berekening aangeraden. Voor koeling is in ieder geval een dynamische berekening nodig.

Exergieprestatie van huidige energiesystemen en de toegevoegde waarde van exergie (Hst 4)

In hoofdstuk 4 wordt een aantal huidige energiesystemen geanalyseerd voor twee woningtypen: een voor Nederland kenmerkende tussenwoning (referentiewoning nieuwbouw volgens Senternovem, 2006) en een sociale huurwoning in een appartementencomplex uit 1960 in Bilbao, Spanje. De case studies zijn uitgevoerd met de dynamische simulatiesoftware TRNSYS, waarbij de exergieberekeningen, inclusief de toevoegingen zoals hierboven beschreven, per tijdstap van 0.5h zijn uitgevoerd.

Voor de Nederlandse tussenwoning is een gedetailleerde analyse van de exergiebehoefte gedaan (Case 1), een analyse van de toepassing van warmteterugwinning (HR WTW) in deze woning (Case 2), en zijn vier verschillende energiesystemen onderzocht, gebaseerd op een hoog rendements ketel (HR ketel - Case 3), microwarmte-krachtkoppeling (micro WKK, ook wel HRe ketel genoemd - Case 4A), een combi-warmtepomp (Case 4B) en een combi-combi-warmtepomp in combinatie met aangepaste bouwkundige eigenschappen om te voldoen aan de passiefhuis standaard (Case 4C). Voor de Spaanse woning is de oorspronkelijke situatie (Case 5A) en een situatie met standaard renovaties (Case 5B) bestudeerd. De state-of-the-art energiesystemen (Nederlandse case studies) resulteren in overall exergie-efficiënties (d.w.z. voor de gehele keten en het voorzien in alle energiebehoeften) tussen 28% en 44%, vergeleken met energie-efficiënties tussen 74% en 132% (waarbij efficiënties hoger dan 100% komen doordat de ‘gratis’ omgevingswarmte niet als input wordt meegerekend). De exergie-efficiënties zijn relatief hoog door de veronderstelde hoge efficiëntie voor elektriciteitsproductie in combinatie met het relatief grote aandeel elektriciteitsbehoefte. Voor de energieketens die ruimteverwarming leveren variëren de exergie-efficiënties tussen 5% en 14%, vergeleken met energie-efficiënties tussen 90% en 400%, voor respectievelijk Case 3 (boiler) en Case 4B( combi-warmtepomp). Uit deze resultaten blijkt dat het verschil tussen energie- en exergie-efficiënties zeer groot is. De toegevoegde waarde van exergie is dat dit een maatstaf is voor de thermodynamische prestatie van het systeem en daarmee het ideale verbeterpotentieel aangeeft (de maximale exergie-efficiëntie is 100%); de energie-efficiënties geven weliswaar de verhouding tussen energie output en energie-input weer, maar geven daarbij niet aan wat in het ideale geval theoretisch mogelijk is.

Naast de overall efficiëntie toont de energieanalyse op componentniveau aan dat exergie-efficiëntie ()en energie-efficiëntie () voor de meeste componenten sterk verschillen, bijvoorbeeld voor de HR-ketel (=95%, =14%), de micro-WKK (=100%, =30%), en de warmtepomp (=410%, =42%). Ook bij de fictieve ‘room air’ component die de warmteoverdracht van het emissiesysteem (bijvoorbeeld radiator) naar de ruimte vertegenwoordigt, is er sprake van grote exergieverliezen, die in een energiebenadering afwezig zijn. Een energieanalyse op componentniveau geeft hiermee een betere interpretatie van de prestatie van het systeem als geheel als ook meer inzicht in hoe(veel) het systeem verbeterd kan worden. Energieanalyse op componentniveau heeft daarmee veel toegevoegde waarde.

Hoe kan het exergie concept bijdragen aan de ontwikkeling van verbeterde energiesystemen? (Hst 5)

Hoofdstuk 5 onderzoekt verschillende mogelijkheden om door toepassing van het exergie concept bij te dragen aan de ontwikkeling van verbeterde of innovatieve energiesystemen voor de gebouwde

omgeving. De volgende drie benaderingen zijn onderscheiden en verduidelijkt met voorbeeld case studies:

1) Toepassing van exergieprincipes – een kwalitatieve aanpak om veelbelovende energieconcepten te genereren.

2) Toepassing van exergieanalyse – een kwantitatieve aanpak om exergieverliezen te identificeren en kwantificeren met als doel deze te minimaliseren.

3) Toepassing van exergie inzichten om innovatie te stimuleren.

De exergieprincipes zijn ontwikkeld op basis van literatuurstudie en de case studies uit het

promotieonderzoek. Ze zijn onderverdeeld in A) thermodynamische principes om exergieverliezen te minimaliseren en B) integrale principes om de kansen van een project optimaal te benutten. Een voorbeeld case demonstreert het gebruik van de exergieprincipes voor het genereren van

energieconcepten met maximaal gebruik van exergie-efficiënte processen en minimaal gebruik van fundamenteel verkeerde processen. De tweede – kwantitatieve - benadering is bedoeld om de exergieverliezen van een energieconcept (mogelijk de uitkomst van de eerste benadering) verder te reduceren. Als derde optie wordt de mogelijkheid om met behulp van exergie inzichten tot innovatie te komen genoemd. Innovatie kan niet worden geforceerd maar kan wel worden gestimuleerd door nieuwe inzichten, in dit geval inzicht vanuit exergieprincipes en exergieanalyse. Twee ideeën die zijn voortgekomen uit het promotieonderzoek zijn nader beschreven en onderzocht als voorbeeld studies.

Ten slotte worden in hoofdstuk 5 een viertal belangrijke aspecten in relatie tot de toepassing van exergy beschreven: Als eerste wordt ingegaan op de systeemcomponenten die essentieel zijn voor het

ontwikkelen van exergetisch intelligente energiesystemen. Ten tweede is het technisch haalbare verbeterpotentieel bestudeerd, uitgaande van minimale drijvende krachten. Een derde discussie betreft de toepassing van een exergiebenadering voor projecten op een groter schaalniveau en ten slotte worden enkele ideeën besproken met betrekking tot exergie in het energiebeleid voor de gebouwde omgeving.

Afsluiting

Dit proefschrift hoopt bij te dragen aan de ontwikkeling van duurzame energiesystemen voor de gebouwde omgeving door het vergroten van de kennis over exergieanalysemethoden voor de

gebouwde omgeving en door het aantonen van de toegevoegde waarde van het exergie concept voor de beoordeling en ontwikkeling van deze systemen. The exergie-efficiëntie van huidige (state-of-the-art) energiesystemen is laag (tussen 3% en 14% voor ruimteverwarming) en hoewel de ‘technisch haalbare ideale’ efficiëntie (geschat tussen 26% en 43%) veel lager is dan de ideale 100% is er veel ruimte voor verbetering. De verschillende exergiebenaderingen die in dit proefschrift zijn ontwikkeld en

gedemonstreerd zijn veelbelovende ‘tools’ om bij te dragen aan de ontwikkeling van verbeterde energiesystemen met een verminderde input van hoogwaardige energie.

Nomenclature

A [m2] area

cp [J kg-1 K-1] isobaric heat capacity

E [J] electricity

En [J] energy

Ex [J] exergy

e [kJ/kg] specific exergy

fex [-] exergy Factor (Exergy to energy ratio)

H [J] enthalpy

m [kg] mass

[kg/s] mass flow rate

Pelec [W] electric power

Q [J] heat

Q0 [J] heat transfer at environmental temperature

QH [J] heat transfer to or from the hot reservoir or at temperature higher than T0

QC [J] heat transfer to or from the cold reservoir or at temperature lower than T0

Qs [J] hensible heat

[W] heat transfer rate

S [J/K] entropy

T [K] temperature (°C if explicitly mentioned) T0 [K] temperature of the reference environment

TC [K] temperature of the cold reservoir

TH [K] temperature of the hot reservoir

[K] thermodynamic mean temperature

[K] weighted average outdoor temperature over a given period of time U [W m-2 K-1] heat transfer coefficient

V [m3] volume

W [J] work

Greek symbols

 [-] energy Efficiency

 [W m-1 K-1] thermal conductivity

j [-] frequency distribution of variable j

 [-] exergy Efficiency

Subscripts

0 related to the reference environment Carnot related to the Carnot process

dem demand

dem,c demand for cooling dem,h demand for heating del delivered

dd detailed demand

dd,c related to the part of the detailed demand for cooling that is supplied as cooling at Ti

dd,h related to the part of the detailed demand for heating that is supplied as heating at Ti

dd,c,tot related to the total detailed demand for cooling (dd,c + dd,vent,c) dd,h,tot related to the total detailed demand for cooling (dd,h + dd,vent,h)

dd,vent related to the part of the detailed demand used to heat or cool ventilation air

e outdoor

i indoor inf infiltration inl inlet

int Internal gains

op operative (Temperature) outp output

ret return rev reversible sol solar gains sup supply trans transmission vent ventilation

Abbreviations (also used as subscript)

CHP combined heat and power (Cogeneration) COP coefficient of performance

DHW domestic hot water

EER energy efficiency ratio (referring to the heat pump performance for cooling) EnD energy demand

ExD exergy demand HRU heat recovery unit

HP heat pump

HT high temperature LCA Life Cycle Analysis

LCIA Life Cycle Impact Assessment LNG Liquefied Natural Gas

LT low temperature NG natural gas

NPE Net Primary Energy

PEC Primary Energy Conversion PEF Primary Energy Factor PV Photo Voltaic (energy) SH space heating

ST Solar thermal (energy) TES Thermal energy storage VLT very low temperature

WCED World Commission on Environment and Development ZEB Zero Energy Buildings

Table of Contents

Summary ... 3

Samenvatting ... 7

Nomenclature ...11

Table of Contents ...13

1.

Introduction ...17

2.

Understanding exergy ...37

2.2. General understanding of the exergy concept ... 38

2.3. Thermodynamic basics for exergy analysis of energy systems in the built environment ... 43

2.4. Energy versus exergy ... 66

2.5. Exergy in context ... 71

2.6. Closure ... 75

3.

Applying exergy:

Extending the exergy calculation approach for the built environment ...77

3.1. Introduction ... 77

3.2. State of the art & knowledge gaps ... 79

3.3. Exergy analysis framework and important definitions ... 81

3.4. The exergy of cooling in buildings ... 87

3.5. New detailed exergy demand calculation method ... 92

3.6. Exergy balance and exergy destruction within thermal zones ... 98

3.7. Steady state versus dynamic approach ... 103

3.8. Closure ... 111

4.

Analysing exergy performance:

Case studies of current energy systems for the built environment ... 113

4.1. Introduction ... 113

4.2. Method ... 114

4.3. Case 1: Dutch single family reference dwelling – demand analysis ... 118

4.4. Case 2: Dutch single family dwelling –analysis of heat recovery system ... 129

4.5. Case 3: Dutch single family dwelling – reference energy system ... 138

4.6. Case 4: Dutch single family dwelling – state of the art energy systems ... 146

4.7. Case 5: Spanish social dwelling case... 163

5.

Improving with exergy:

Exploring the potential of the exergy approach to support the development of innovative

and improved energy systems for the built environment ... 173

5.1. Introduction ... 173

5.2. Exploration and literature study ... 174

5.3. Approach 1: Using exergy principles ... 178

5.4. Approach 2: Using exergy analysis ... 194

5.5. Approach 3: Eureka - Innovations from exergy insight ... 201

5.6. Discussions & desired developments ... 211

5.7. Closure ... 222

6.

Overall conclusions and recommendations ... 225

6.1. Introduction ... 225

6.2. Main additions to science ... 226

6.3. Answering the research questions ... 229

6.4. Discussion ... 236

6.5. Recommendations ... 242

6.6. Closure ... 244

References ... 245

Appendices ... 255

A: Properties of Dutch Case studies Chapter 4 (Cases 1 until 4) ... 256

B: Properties of Spanish Case Studies Chapter 4 and 5 ... 276

Acknowledgement ... 289

Curriculum Vitae ... 291

1. Introduction

Overview

Energy is essential for human life and related activities and for the global economy. However, the way energy is currently managed cannot be sustained for a long time anymore: fossil fuels are depleting and the negative consequences of their use to the environment are becoming ever more evident. Of all energy we use the amount used in the built environment accounts for a significant part. Sustainable energy systems need to be developed in order to enable future generations to fulfil their (energy) needs as well.

Exergy is a thermodynamic concept that quantifies the ‘work potential’ of different forms of energy, which can be regarded as the quality of energy. Unlike energy, exergy can be destroyed. In

thermodynamically ideal processes no exergy is destroyed, but in all real processes exergy destruction takes place. The amount of exergy destroyed or lost indicates the thermodynamic improvement potential of a system. In current systems for heating purposes exergy destruction and losses can be up to 95%, although better performing systems are also available. This improvement potential cannot be revealed with energy analyses. Performing an exergy analysis in addition to energy analysis is therefore expected to provide valuable additional insight, thereby contributing to the development of sustainable energy systems for the built environment. However, the application of exergy analysis in the

assessment and development of energy systems for the built environment is still relatively new. Therefore, the objective of this research is to increase knowledge on the exergy analysis of energy systems for the built environment and to explore and demonstrate the added value of using exergy in addition to energy in the assessment and development of these systems, in order to contribute to the development of sustainable energy systems.

The scope of this research includes the total energy chain for the built environment. The starting point is the energy demand of the users of the building. This user demand is the amount of energy that is actually desired by the users in the form that is desired; it includes heating, cooling, domestic hot water (DHW) and electricity. On the other hand there are the energy resources available on earth, both renewable and non-renewable. In-between there are energy system components for energy

conversion, distribution, storage and emission, in order to get the energy in the right form at the right time and place. The required system input is the need for resources, considering these transformation processes and the related energy ‘wasted’. The system is illustrated in figure 1-1.

Figure 1-1: Diagram of the scope of this thesis: the total energy chain for the built environment including user demand, system components and energy resources.

1.1. Background

1.1.1. Sustainability

Our Common Future

Since the 1970s - related to the 1972 report of the Club of Rome (Meadows et al. 1972) and the 1973 oil crisis - most people are aware that human actions influence the environment in such a way that they endanger the possibilities of future generations to meet their needs. In 1987 the World Commission on Environment and Development (WCED) published the report “Our Common Future”, also known as the Brundtland Report, and defined sustainable development as: “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”(WCED 1987). This also implies a development that meets the needs of presently less developed parts of the world, aiming at social equity.

We all depend on one planet to fulfil our needs

Our planet is continuously fed with energy from the sun and disposes of low-temperature heat1 by radiation to the universe (Shukuya and Hammache 2002), but apart from this, no energy or matter enters this earth system.

Therefore2, “if needs are to be met on a sustainable basis the Earth's natural resource base must be conserved and enhanced”, which means that a sustainable situation requires at least that:

 We do “not endanger the natural systems that support life on Earth: the atmosphere, the waters, the soils, and the living beings.”

 “Renewable resources like forests and fish stocks need not be depleted provided the rate of use is within the limits of regeneration and natural growth.”

To which degree we can fulfil our needs is confined by limits. These are not absolute limits, but “limitations imposed by the present state of technology and social organization on the environments’

ability to meet present and future needs and by the ability of the biosphere to absorb the effects of

human activities”.

Definition of sustainability

In line with the definition of sustainable development the following definition for sustainability is given by Van den Dobbelsteen (2004): “sustainability [..] is a state in the world in which needs of all people are fulfilled without restricting the fulfilment of future needs, a state of balanced economy and ecology.” The definition given in the report ‘Caring for the earth’ (IUCN-UNEP-WWF 1991) “sustainability is improving the quality of human life while living within the carrying capacity of supporting eco-systems" also makes explicit that sustainability depends on the environment’s regeneration capacity.

Sustainability and environmental impact

The effect of human activities on the environment can be referred to as the ‘environmental impact’, and this should not exceed the regeneration capacity of the environment. The relation between

environmental impact, affluence (the degree to which we fulfil our needs), population and technology is shown in the following formula, which was developed by Commoner (1971) and reintroduced after the Brundtland Report by Speth (1989) and Ehrlich and Ehrlich (1990):

1

With low-temperature heat a temperature near the environments temperature is meant, which is around 15C.

2

Impact = Population x Affluence x Technology = I = P x A x T

This equation states that the environmental impact (I) is the product of population (P), affluence per person (A), and technology (T), the latter measured in environmental impact per unit of affluence3. Given the need to reduce the total environmental impact by 50% in 50 years from the reference year 1990, while the population will be doubled and affluence per person should on average be increased by a factor of 5, this equation prescribes that technological efficiency should be improved by a factor of 20 by 2040. In other words: technology should be able to produce affluence with 95% less environmental impact4 per unit affluence.

Environmental Impact

To determine the environmental impact of a product or human activity the (environmental) Life Cycle Assessment (LCA) method is generally acknowledged. LCA involves a detailed life cycle inventory (LCI) of the resources used and emissions created for a product or service under analysis and a life cycle impact assessment (LCIA), translating these results into environmental impacts. According to the current ISO standard the environmental impact can be categorised into impact categories (midpoints) or damage categories (endpoints). The midpoint categories include climate change, resource depletion, land use, water use, human and eco-toxic effects, ozone depletion and biodiversity5. These midpoints can be linked to the following three damage categories (endpoints): damage to human health, damage to ecosystem quality and resource depletion (UNEP/SETAC 2011). Weighting of the various midpoint or endpoint categories is an optional step in the assessment; it is needed in case a single indicator is desired (Goedkoop, 2000).

Three pillars of sustainability

In the resolution of the 2005 world summit (UN 2005) the three components of sustainable development – economic development, social development and environmental protection – were mentioned as interdependent and mutually reinforcing pillars. These pillars, also referred to as ‘People, Planet, Profit’ are usually drawn as three equally sized and partly overlapping circles. Another vision on the relationship between these aspects is given by Scott Cato (2009), where human society is

considered as a part of the environment and the economy as a part of human society.

1.1.2. Energy & sustainability

Current unsustainable energy systems

People use energy to fulfil various basic and less basic needs. Energy in the form we need it - such as heat, cold or movement - is ‘produced’6 from primary energy sources as they are available in nature (energy contained in raw materials such as fossil fuels and energy flows such as the sun). The current energy system in the Netherlands is for over 95% based on non-renewable sources (oil, coal, gas and uranium) (Ganzevles and Est 2011).

3

In Dutch this equation is written as D = B x W x M

D = totale druk op het milieu; B = omvang wereldbevolking; W = gemiddelde welvaartsniveau van een wereldburger; M = gesommeerde milieueffecten per eenheid van welvaart

4

In this approach the environmental impact is understood as negative environmental impact, which means it should be minimized. In the Cradle to cradle approach (McDonough and Braungart, 2002) the aim is to increase the positive environmental impact of our human actions.

5

For a full list see the report by UNEP and SETAC (2011). Additionally McDonough and Braungart (2002) also explicitly mention the loss of topsoil as an environmental threat.

6

The word ‘produced is placed between brackets since in fact energy cannot be ‘produced’; it is converted from one form into another, as will be thoroughly explained in chapter 2.

This current way of ‘producing’ energy is not sustainable for several reasons. Firstly, natural resources are depleted at a rate much faster than can ever be regenerated by the earth. With the current use there will be oil for the next 40 years, natural gas for the next 60 years, uranium for 70 years and coal for another 150 years, but these reserves will be ever harder to win, resulting in more costs and more environmental risk (Ganzevles and Est 2011). Secondly, the way we ‘win’ energy already causes severe environmental impact such as damage to land or loss of biodiversity. Furthermore, the conversion of primary energy into the energy we need causes many forms of waste or emissions, such as CO2, now

commonly acknowledged to play a major role in climate change (IPCC 2007), and nuclear waste. In addition to the primary effects of the use of resources the materials needed to convert, distribute and store the energy have a related environmental impact.

Renewable and sustainable

Renewable energy resources can be defined as ‘energy resources that are naturally replenishing but flow-limited’ (EIA 2011). They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time and space. Renewable energy resources include biomass, hydro, geothermal, solar, wind, ocean thermal, wave action, and tidal action.

In principle the earth’s supply of solar energy is by far sufficient to feed the current (and growing) energy use of the whole world (De Swaan Arons et al., 2004). The problem is: this energy needs to be ‘harvested’ and subsequently converted, transported and stored. Some, such as Michiel Haas or

Michael Braungart (Haas, 2011) even argue that there is not an energy problem but a material problem. Renewable sources are therefore not by definition providing sustainable energy. Several issues of

concern should be addressed in order for renewable energy sources to provide sustainable energy:  First of all they should not be used at a rate faster than they are replenished. Even though

renewables are virtually unlimited, given certain boundaries of space and time they are not.  Secondly the harvesting or ‘production’ of renewable sources should not cause environmental

damage, as can be the case with for example hydro-power plants or conventional production of biomass with related soil degradation and water use.

 Thirdly, ‘producing’ renewable energy should not compete with other important spatial needs, such as with land use for the production of food (Stremke 2010, Ganzevles and Est 2011).

 Lastly the materials used to convert, distribute and store renewable energy sources to get them in the desired form at the desired time and place, should also be taken into account. Firstly, materials used should not threaten the earth’s resource base: they should also be renewable or reusable and the negative environmental impact during their total life cycle must be below the regeneration capacity of the earth. After functional use they should either be degradable (i.e. return to the ecological cycle) or reusable (stay within a technological cycle) (McDonough and Braungart 2002). Secondly, the input into these materials (i.e, all the energy needed for manufacturing, installation, transport and decommissioning) should not be more than the energy saved or ‘produced’ by it. In this thesis sustainable energy is therefore considered as renewable energy resources used in a sustainable way, that is: in such a way that their use is within the carrying capacity of supporting eco-systems and thus leaving the earth’s natural resource base available to future generations.

Also efficient use of renewables is important

The importance of eliminating the use of non-renewables is very obvious, but according to the above discussion also efficient use of renewable resources is important. This is also stated in the Brundtland report (WCED 1987) and in a more recent report by the World Wide Fund for Nature (WWF) and Ecofys,

where it is argued that a fully renewable energy future is not possible without reducing the required energy input into the system: “We will not be able to meet the needs of our planet’s nine billion inhabitants if we continue to use it [energy] as wastefully as we do today” (WWF et al., 2011, p. 44). Important arguments for promoting efficient use of renewable energy sources are summarized below:  Also renewables can be scarce given certain boundaries of time and space. Especially sustainable

biomass can be considered as scarce: biomass is only renewable given a certain rate of use.  Reduced energy use can also reduce the environmental impact related to capture or growth of

renewable energy (such as hydro or biomass) as well as that related to use of materials for conversion, storage and distribution. Possible side-effects such as increased use of materials for more efficient systems should however also be considered.

 The social acceptance for renewable energy projects (for example windmills) is not always enough to install large amounts of renewable energy plants.

 More efficient use of renewables can increase the contribution of renewables in a project, indirectly reducing the input of non-renewable sources.

 When renewables are not used in one system they can be used elsewhere or for other purposes, resulting in lower environmental impact elsewhere.

 More efficient use of renewables reduces the required input which means space or resources can be used for other purposes to meet other needs or desires than energy.

Towards sustainable energy systems

A truly sustainable energy system should in the first place be based solely on renewable energy sources, such as wind, sun, and biomass under certain conditions, and secondly, it must be managed in a

sustainable way as explained above. As discussed above this is only possible (or much more feasible) if the required energy input to the system is reduced. Hence, two essential pathways towards sustainable energy systems can be distinguished: the renewability approach on one hand - aiming at an increase of sustainable renewable energy - and the reduction approach on the other7 - aiming at the decrease of required energy input. A sustainable energy system is one where required input meets the sustainable supply, as is illustrated in figure 1-2.

Figure 1-2: reduced energy input meets increased sustainable energy supply (adapted from Gommans, 2012). Reduction of the required energy input can be achieved by less use of energy intensive services or by providing these services more efficiently. An increased use of energy intensive services, as can be undoubtedly expected in developing countries but also in countries like the Netherlands (Ganzevles et al. 2011), will increase the global energy need, even though energy systems are more efficient. This makes the need for highly efficient systems even more pressing.

7

The ‘reduction approach’ is often referred to as ‘energy saving’ or ‘energy conservation’. Since according to the first law of thermodynamics energy is always conserved these terms are avoided in this thesis (see also chapter 2).

1.1.3. Energy systems in the built environment

The built environment (households and services sector) accounts for a significant part of the worlds energy use with around 40% of the ‘final energy consumption’8 in Europe (Eurostat 2010). The built environment is therefore an important sector for developing more sustainable energy systems. In households around 50% of this energy is used for heating (ECN, 2012). In commercial or public buildings also cooling plays a role.

Energy system scheme

Energy systems for a building or for the whole built environment can be described as consisting of the energy demand at one hand, the energy sources at the other, and all ‘transformation’ components between these to transform the energy sources to the desired form, place and time. A scheme of the system has been shown in figure 1-1 and is repeated in figure 1-3. These three elements are briefly described below.

Figure 1-3: Scheme of energy systems for the built environment.

The energy demand

The energy demand represents the amount of energy that is actually desired by the users of the building in the form that is desired; it includes heating, cooling, domestic hot water (DHW) and

electricity. The demand for heating and cooling is determined by the users, the building characteristics and the weather. More specifically, the users desire a certain thermal comfort, and given the building properties and the weather this results in a certain need for heating or cooling. Furthermore the users desire lighting and to make use of various appliances such as computers. For the scope of this thesis this desire is translated into an electricity demand, although this is not the actual demand of the users. The total energy demand is thus defined by the users, the building characteristics and the weather. The relative part of each of the demands varies greatly between buildings, since it depends on the use of the building, (e.g. office or residential buildings), the technical building properties (e.g. the insulation value) and the users. In the figure below the annual values of the different demands are shown for the reference dwelling as used in chapter 4, case 1A. This reference represents a single family terraced house in the Netherlands complying with current Dutch energy standards, based on a reference dwelling provided by Agency NL (Senternovem 2006). The figure shows that even in a relatively well-insulated dwelling more than two thirds of the energy demand is for heating (space heating and domestic hot water).

8

According to the definition by Eurostat (2012): “final energy consumption covers energy supplied to the final consumer's door for all energy uses [...]” It thus refers to the energy in the form it is delivered to the consumer: as electricity, gas, or sometimes as heat. ‘Final energy’ differs from the energy demand as used in this thesis, the latter referring to energy in the form that is actually desired (e.g. heating, cooling, etc.), excluding all energy system components, also building services inside within the buildings.

Figure 1-4: The user energy demand (i.e. the energy demand as defined above, for heating and cooling being independent of the ‘transformation’ components used), with related annual values given for the Dutch single family reference dwelling used in this thesis (Chapter 4, Case 1A).

Energy system components and required system input

Energy system components are needed for energy conversion, distribution and storage of energy, since energy resources are usually not available in the right form and at the right time and place. The energy systems components include components at building level, i.e. the building services such as boilers, heat pumps or air conditioning units, as well as components on a larger – regional, national or global scale, such as electricity grids and electricity production plants or district heating networks. In these processes usually not all the energy input is transformed into the desired output but also some waste is created, which means the required energy input in the system of energy components is larger than the user energy demand. This need for energy is referred to as the energy system input.

In figure 1-4 the required energy input for the reference dwelling and reference energy system as described in chapter 4, case 3, is shown. The energy system is based on a condensing boiler for

supplying space heating and domestic hot water and on electricity supply from a combined cycle power plant with an efficiency of 60%9. (See chapter 4, case 3 for details on the system assumed). As can be seen more than half of the required energy input is used to provide heating (space heating and domestic hot water).

Figure 1-5: The annual energy input required for delivering the demand of the Dutch reference dwelling as shown in figure 1-4, given the reference energy system based on a condensing boiler and a combine cycle power plant, as described in chapter 4, case 3).

9 _{For this case the best practice efficiency for electricity production of 60% is assumed, which can be obtained with}

modern combined cycle power plants (Woudstra 2012, p.126). This efficiency is assumed in order to compare potential improvements as studied in chapter 5 of this thesis with a best practice alternative.

The average national (Dutch) efficiency for electricity production including grid losses according to the Dutch building code (NEN 5128, 2008) is 39%, in which case the part needed for space heating becomes less compared to the total.

Energy resources

At the end of the chain the required input is provided by primary energy resources, which are defined as “energy that has not been subjected to any conversion or transformation process carried out by humans”10. Primary energy resources include renewable and non-renewable resources. Renewable resources include biomass, hydro, geothermal, solar, wind, ocean thermal, wave action, and tidal action. Non-renewable resources include all fossil fuels and uranium.

The environmental impact of energy systems for the built environment

The environmental impact of these energy systems is mainly related to the use of energy resources and materials and to the related production of emissions and waste. Especially for renewable resources the material use can cause most of the environmental impact. In the figure below the effects on

environmental impact are illustrated in an adapted version of figure 1-1.

Figure 1-6: Energy systems for the built environment and the related environmental impact

Towards sustainable energy systems for the built environment

By definition, sustainable energy systems for the built environment can only be based on renewable energy sources managed in a sustainable way and closed material cycles (see the sustainability definition by Dobbelsteen (2004)).

As previously discussed meeting the required energy system input with sustainable energy supply requires two approaches: increasing sustainable energy supply and reducing the required system input. Since the required system input is a result of both the energy demand of the user and the energy system components, the following three means can be distinguished:

 Reducing energy demand caused by the user and the building characteristics. This can be obtained by smarter and more energy efficient users and buildings, for example by applying bioclimatic design principles and by adding insulation, increasing air tightness etc.

 Increasing the efficiency of the energy system components, i.e. components that provide the same service or energy output with a lower input of energy, resulting is a reduced energy system input;  Increasing the use of sustainable energy resources.

10

This definition is slightly adapted from CEN-EN 15603(2008), by adding ‘carried out by humans’ in order to make explicit that the conversions occurring in nature are not considered, i.e. all resources available in nature are considered a primary source, such as biomass, fossil fuels, solar energy etc. The process from solar energy to fossil fuels or biomass is not considered.

The combination of reducing the demand, increasing efficiency and using renewables is needed to reduce and finally eliminate negative environmental impact. For all approaches the material consequences are very important, but these are not the focus of this thesis.

In literature several stepped strategies towards sustainable energy systems for the built environment can be found, of which the Trias Energetica11 (Lysen, 1996) is often used. Recently the New Stepped Strategy12 was developed, excluding the use of fossil fuels (Dobbelsteen, 2008).

The main differences between these strategies and the three means listed in this thesis are firstly that a certain hierarchy between the steps is assumed by the strategies mentioned, while the means

mentioned in this thesis are considered equally important and the preference of one solution over another could be based on other objectives such as costs, material use or preferably overall

environmental impact. A second difference is that in this thesis the efficient use of renewable resources is explicitly mentioned to be of importance as one of the means towards sustainable energy systems, which is not addressed by either the Trias Energetica or the New Stepped Strategy.

Efficient use of renewable energy in the built environment

In line with the discussion in the previous section also the efficient use of renewable energy for systems in the built environment is considered of importance. For the energy systems in the built environment the relevance of efficient use of renewable resources can be much related to the use of space,

especially regarding the aim towards zero energy buildings or zero energy communities. For inefficient energy systems is will be impossible to harvest the total energy need at building or community level, but the more efficient the energy system, the less energy input required. This means for example less solar panels are needed and more likely these will fit on the roof of the building, or more space is left for other desires such as a roof terrace, a beautiful green roof or growing tomatoes. When assuming all the sustainable energy to come from the grid the arguments of the end of section 1.1.2 are applicable.

Current approach

The current approach towards more sustainable energy systems is an energy approach: the focus is on reducing the energy demand and increasing energy efficiency of system components. Current

approaches include for example the passive house concept or zero energy buildings (ZEB). However, as will be explained, the energy efficiency is not a complete representation of the performance of a process; it disregards important thermodynamic losses which are related to the thermodynamic quality or exergy of the form of energy under consideration. These losses can also be reduced in order to reduce the need for high quality energy.

This thesis will use the exergy concept and exergy efficiencies to evaluate the performance of energy systems in the built environment and to contribute to the reduced need for high quality energy sources.

11

The Trias Energetica is a strategy developed in 1996 by Novem (Currently Agency NL). This strategy involves the following three steps: 1) Reduce the demand; 2) Increase the use of renewable resources and 3) use

non-renewable resource efficiently.

12

The ‘New stepped strategy’ developed by Dobbelsteen (2008) and extended to the urban scale by Tillie et al. (2009) excludes the use of fossil fuels - since sustainable energy systems do not use any fossil fuels - and explicitly mentions the use of waste heat, according to the following steps: 1) Reduce demand (using intelligent and bioclimatic design), 2) Reuse waste energy streams and 3) Use renewable energy sources and ensure that waste is reused as food according to the Cradle to Cradle philosophy (Mc Donough and Braungart 2002).

1.1.4. The added value of exergy

A short introduction to energy & exergy

Energy is a concept that includes various forms of energy appearances, important of which are electrical energy, chemical energy, heat and work, the two latter actually being modes of energy transfer. The various forms of energy can be converted into one another, and in this conversion no energy is lost, according to the first law of thermodynamics. From experience however we know that something is lost, since the energy available after conversion cannot be used again for the same process. What is lost is the exergy of the energy, which can be regarded as the thermodynamic quality.

Figure 1-7: Different forms of energy: same amount of energy, different amount of exergy (see also chapter 2). According to the second law of thermodynamics some forms of energy have limited convertibility into others (Baehr and Bergmann 1965); work for example can be totally converted into heat, but heat cannot be totally converted into work, as was discovered by Sadi Carnot in 1824. The energy concept, which is based only on the first law of thermodynamics, does not address the convertibility of a form of energy.

Exergy is the maximum theoretical work obtainable from a form of energy as it comes to equilibrium with a reference environment (adapted from Bejan et al. 1996). It is therefore a measure of the ideal convertibility of this form of energy into work, which is a measure of the thermodynamic potential or quality of the energy.

In ideal processes no exergy is lost, i.e. the sum of the ‘work potential’ of the input energy equals the sum of the ‘work potential’ of the output. This means that ideal conversions can be reversed so that the original situation can be re-obtained. The maximum work is thus obtainable using a reversible process. In all real processes however exergy is lost since a real process requires a driving force in order to take place (i.e. a temperature difference) which is destroyed during the process. The exergy destroyed indicates how far the process is from an ideal process and can also be described as thermodynamic inefficiency or irreversibility.

Thermodynamic improvement potential: a key feature of exergy analysis

The amount of exergy destroyed equals the theoretical ideal thermodynamic improvement potential of an energy conversion (Van Gool, 1997). This improvement potential can be identified and quantified for each step or sub-process of the supply chain as well as of the system as a whole. The exergy losses and improvement potential are not revealed using energy analysis; energy analyses do therefore not give a full representation of the performance of energy processes. As stated by Dincer (2002) “more meaningful efficiencies are evaluated with exergy analysis rather than energy analysis, since exergy efficiencies are always a measure of the approach to the ideal”. Similar statements are made by several other authors such as Wall (1977), Gaggioli and Wepfer (1981), Van Gool (1997), Sciubba (2001) and Torío et al (2011a). The identification and quantification of thermodynamic losses and thereby improvement potential is one of the main added values of the exergy approach.

The exergy of heat

Many forms of energy can in theory be converted totally into work, including potential energy, kinetic energy, chemical & electrical energy. For heat, which is very important for energy systems in general as well as for the built environment, the ideal convertibility is limited and depends on the temperature according to the relation (1-T0/T), where T and T0 represent the temperature of the heat and the

environment respectively (in Kelvin). Heat at environmental temperature is already at equilibrium with the environment and thus has no potential to be converted to work; it therefore contains no exergy. The factor (1-T0/T) is referred to as the exergy factor, which is defined as the ratio between exergy and

energy content. It is a measure of the thermodynamic quality of a form of energy. The exergy factor of heat is illustrated in figure 1-8.

Figure 1-8: Exergy factor of heat for a given reference temperature of 5oC .The negative value of the exergy factor for temperatures at T<T0 means that at T>T0 the exergy transfer is in the opposite direction of the energy transfer by heat. For more explanation see chapter 2.

This graph shows that for infinitely high temperatures, the exergy factor approaches the value of one, which means heat transfer at very high temperatures can theoretically be totally converted to work. When T approaches 0 Kelvin, the exergy factor approaches minus infinity. This means heat transfer at very low temperatures (if available, e.g. cryogenic gases) can produce a large amount of work. It also means that in order to obtain low temperatures a large amount of work has to be supplied.

Low-exergy demand

The energy demand for heating and cooling in the built environment is a demand for heat at near environmental temperatures. As can be seen from figure 1-8 the exergy factor of heat at near

environmental temperatures is very low. This means the demand for heating and cooling is a demand for very ‘low-exergy’ or ‘low-quality’ energy.

Figure 1-9 shows the energy demand and the exergy demand of the reference dwelling as used

previously in paragraph 1.1.3. This demand relates to the demand of the user at building level resulting from the building characteristics, weather data and user behaviour, as previously discussed.

Inefficiencies of energy system components (i.e. boilers and radiators) are not included in the demand.

T₀ -4 -3 -2 -1 0 1 -200 -100 0 100 200 300 400 500 600 700 800 900 1000 Exe rgy f act o r [E x/Q] Temperature (o_C)

Figure 1-9: Exergy demand (at building level) of the Dutch single family reference dwelling used in this thesis. As can be seen from this figure the exergy demand for heating, cooling and domestic hot water is significantly lower than the energy demand, due to the low quality of these demands according to figure 1-8. The exergy factor of electricity is equal to the value of one.

1.1.5. Improvement potential of energy systems for the built environment

The in principle low-exergy demand for heating and cooling in the built environment is often met with high-exergy energy sources using processes with large amounts of exergy destroyed. This very poor exergy performance of most present energy systems for the built environment implies a large improvement potential in the built environment as well.

Figure 1-10 compares the energy performance and exergy performance of the reference dwelling: it shows the energy demand at building level and the exergy demand at building level as well as the energy system input, for which in this example the energy value equals the exergy value13. The difference between the demand and the system input represent the losses in the system components (conversion, storage and distribution components).

Figure 1-10: Thepretical ideal improvement potential of the reference dwelling studied.The exergy losses between the actual exergy demand and the exergy input equal the theoretical ideal improvement potential.

According to this figure the energy losses for heating are almost negligible, which could give the impression that little improvement can be achieved in these systems. However, the exergy losses related to heating are very large, due to the conversion of high quality energy (gas) into low quality

13

The energy system of this example considers natural gas as the final system input. For simplicity the exergy factor of gas is assumed at 1.00 (see chapter 2).

-3 0 3 6 9 12 15 18 21

Heating Cooling Domestic

Hot Water Electricity (G J/y ear )

Energy and exergy demand of Dutch single family reference dwelling

Energy demand (EnD) Exergy demand (ExD) -3 0 3 6 9 12 15 18 21 24 27

Heating Cooling Domestic

Hot Water Electricity (GJ/y e ar )

Theoretical ideal improvement potential: the difference beteen exergy input and exergy demand

Energy demand (EnD) Exergy demand (ExD) improvement potential

system input (gas, energy = exergy)