Exergy-based index for assessing the building sustainability

Building and Environment 60 (2013) 202e210

Contents lists available at SciVerse ScienceDirect

Building and Environment journal homepage: www.elsevier.com/locate/buildenv

Exergy-based index for assessing the building sustainability Ahmed El shenawy*, Radu Zmeureanu Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve Blvd. W, Montreal, Quebec H3G 1M8, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 June 2012 Received in revised form 3 October 2012 Accepted 25 October 2012

Over the past decade, efforts have been made in developing the Sustainable Building (SB) assessment tools which enable all stakeholders to be aware of the consequences of various design choices and to assess the building performance. Currently, a large variety of existing SB tools, approaches, rating systems, indices and methods of assessment are available and used in the construction industry. Despite usefulness of existing assessment methods in contributing towards a more sustainable building, some limitations have led towards a scientifically-based SB assessment tool. This paper proposes an exergybased definition of a sustainable building, the calculation method of a new Exergy-based Index of building Sustainability (ExSI), and the rating scale. Finally, the results from five case studies are presented. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Sustainable building Assessment Single index Exergy Energy Emissions

1. Introduction A significant number of environmental problems are caused by or related to the intensive use for materials, water and conventional energy resources through building construction and operations. In Canada, buildings account for 33% of energy production, 50% of extracted natural resources, 25% of landfill waste, 10% of airborne particulates, and 35% of green house gases [1]. The rating systems and assessment methods of the environmental impacts of building were initially conceived of as a way for owners to demonstrate that they are trying to make a difference in environmental impact [2]. The need for assessment systems of environmental impacts received more attention and support from policy makers [3]. Voluntary rating systems became market-driven systems. Some rating systems differentiate the building-related environmental impacts from user-related environmental impacts [4]. Building designers, architects, engineers and researchers use these rating and assessment tools to test design strategies against different sets of criteria. Some of these ratings (e.g., BREEAM) assess the absolute performance whereby others (e.g., LEED) seek to determine the improvement in the design as a percentage [5]. Rating systems and assessment tools started to be specified by public agencies as performance requirements, and as potential incentives for development approval. Rating systems and assessment

* Corresponding author. Tel.: þ1 514 848 2424x7266; fax: þ1 514 848 7965. E-mail address: [email protected] (A. El shenawy). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2012.10.019

methods are currently used by banking, financial and insurance companies as a basis for risk and mortgage appraisals and real estate valuations. Muldavin [6] demonstrates that a green rating creates building value. He elaborates on assessment methods functions and shows that asset managers and real estate directors are now struggling to assess the performance of their properties, identify opportunities for improvement, determine where and when the repair, rehabilitate or replacement is necessary to insure a healthy working for building users, and make necessary changes to consider sustainability issues in their property decisions. The lack of general consensus on the definition of sustainable buildings is a good reason to return to the fundamental definition of sustainable development as given by the Brundtland report [7]. This definition does not identify the current and future needs, and the type and availability of renewable or non-renewable resources that would be used. Since there is no general consensus on the definition of sustainable buildings, the literature review starts with the most currently used methods based on multi-criteria, weights and scores (e.g., BREEAM, LEED and SBTool), followed by a discussion about the potential use of a single index for assessing the progress towards building sustainability. Since our proposed methodology is based on the application of exergy concept, the literature review covers also studies on the use of exergy on the evaluation of sustainability. In this context of lack of a unique metric for sustainable buildings, several assessment methods and tools were developed and are in use by different stakeholders. Forsberg and Malmborg von [3] classified those assessment tools in two categories: qualitative tools

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

and quantitative tools; the first category is based on multi-criteria, weights and scores (e.g., BREEAM, LEED and SBTool), and the second category is based on life cycle assessment with quantitative input and output data of flows of matter and energy. Those assessment methods and tools can be analyzed in terms of structural organization, functional performance, approach, scale, scope, indicators and methods of measurement, weighting factors, and reporting results. For instance, LEED [8] uses fixed benchmarks which are periodically reviewed and eventually modified in order to comply with new standards, while SBTool [9] requires third party to define some user-defined benchmarks to comply with regional applications. LEED allocates equal weights to each criterion, while SBTool allocates weights through a subjective voting process. Some tools use a binomial approach (e.g., in LEED, the building design receives points of performance if certain requirements are met or looses points if it fails to satisfy the predefined requirements) or a rank-based approach (e.g., SBTool evaluates the level of performance using values from 1 to þ5). Those tools assess mainly the relative performance against specific requirements rather than measuring its performance against carrying capacity [10e12]. The use of a single index for assessing the progress towards building sustainability is not a common practice. Two such sustainability metrics are the monetary and biophysical metrics. They utilize a common currency/denominator (e.g., money, land or energy). Pearce et al. [13] argued that money is a useful metric because it is relatively easy to be understood by non-experts and relevant stakeholders. However, Alberti [14] showed that monetary tools are over-dependent on subjective valuations, and inadequate since sustainability assessment goes beyond economic efficiency. Howarth [15] added that discounting is an important controversial part that is performed to compare future values with present ones. Among the biophysical metrics, we mention here (i) the Ecological footprint [16] that uses the area of land as a limiting factor; (ii) the emergy synthesis [17] that converts the value of all ecological and economic aspects of services and commodities in common unit of solar energy; and (iii) the exergy analysis. Exergy has been widely used as a thermodynamic property of a system. The term was introduced in the mid 1950s by Rant [18], while the exergy analysis can be traced to 19th century where the pioneering work of Gibbs and Carnot took place. The exergy analysis can evaluate quantitatively the causes of the thermodynamics imperfection of the process, and the impact of energy resource utilization on the environment. A few examples of such studies are presented below. Kotas [19] and Szargut et al. [20] used the cumulative exergy consumption analysis to the evaluation of depletion of environmental resources induced by product generation. Wall [21] suggested the application of an exergy tax as a first step to decrease environmental destruction and to improve the present resource use. Cornelissen [22] suggested that exergy losses should be minimized to obtain sustainable development; and that environmental effects associated with emissions and resource depletion can be expressed in terms of one exergy-based indicator. Dewulf et al. [23] used a set of three independent sustainability indicators to express the sustainability of technological processes: (i) a for renewable resource utilization, (ii) h for the conversion of the energy in the process, and (iii) x for the environmental compatibility of the process. Those independent sustainability parameters were not combined into one sustainability index. Rosen and Dincer [24] illustrated how sustainability increases and environmental impact decreases as the exergy efficiency of a process increases. Gong and Wall [25] proposed that the thermodynamic condition of a sustainable system is achieved when the input of exergy to any building application is less than the output of exergy over the service life of that application. Dewulf and Van Langenhove [26] considered two sustainability indicators to reflect the

203

integration of the process within the natural ecosystem: the re-use indicator and the recoverability indicator. Cornelissen and Hirs [27] concluded that the exergetic life cycle assessment can be applied to determine the depletion of natural resource. Dincer et al. [28] concluded that the effectiveness of exergy analysis in addressing sustainability issues is substantial. Rosen et al. [29] expressed the sustainability index of fuel resource as the inverse of the depletion number, which is the ratio of exergy lost and exergy input to the system. Section 2 proposes a new exergy-based index of building sustainability, and Section 3 presents the results from a few case studies. The paper concludes on the main results and need for future work.

2. Exergy-based index for assessing building sustainability This paper presents a new prototype framework for the estimation, at conceptual design stage, of the building sustainability over its assumed life span Lservice, which allows the potential design alternatives to be explored in the search for a sustainable alternative. The proposed framework is a combination of three categories with the scope of measuring the building sustainability: (1) Multicriteria assessment that uses the holistic approach to cover all of the building aspects which help the designer to understand the building within its wider context; the SBTool is the selected tool as the initial starting point of this category; (2) Life Cycle Analysis (LCA) that allows the assessment over the life cycle of buildings; ATHENA Impact Estimator is the selected tool for this category, and (3) an exergy-based index is used as single commodity to aggregate the multi-criteria scores into one single score. The new definition of building sustainability is based on the concept of strong sustainability, in opposition to the weak sustainability, which requires that different types of natural capital must be maintained indefinitely for future generations. In this context, the solar radiation, which is renewable and supposed to be available on very large time scale, is that natural capital available for the building construction and operation. The use of exergy of solar radiation brings together the amount of energy received and used, as well as the quality of energy flows. The available solar exergy, which is harvested on the building footprint, is used exclusively to define the maximum natural capital, and the building sustainability is defined with respect to that maximum value. According to this new definition, a building is considered as sustainable if the overall exergy lost, due to construction and operation over the life cycle, is substituted by the available solar exergy. The exergy lost is calculated for all material and energy flows, and for the abatement of environmental impacts generated through the construction of operation of the building. Therefore a 100% sustainable building has the exergy index of sustainability ExSI equal to 100. The new thermodynamically-based index of building sustainability gives the upper limit of theoretical performance that a building can achieve, for instance in the case of strong sustainability by using renewable energy sources only. Although such a performance will never be achieved, the rating scale can measure the distance between any technical or economical solutions and theoretical upper limit. This index is not affected by geo-political or market conditions. The proposed index along with a rating scale would enable the designer: i) to measure the building sustainability within its wider context, in relation with energy and non-energy natural resources;

204

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

ii) to adjust benchmarks to fit the need of temporal and spatial changes in the building; theoretical available solar exergy on the building footprint is used in this paper as single benchmark, however, other renewable sources could be considered; iii) to provide an objective assessment and eliminate the use of subjective weights; iv) to provide a yardstick that can be used to compare buildings in the same city or different countries; and v) to estimate the potential for improving the building performance.

2.1. Overall approach of proposed framework The prototype tool uses currently most data extracted from the SBTool; it assumes that SBTool was already used by a design team for assessing the building performance, and therefore such data is available. However, the authors plan for the future development of a standalone tool. We selected two issues, the energy and resource consumption and the environmental loading, for the first presentation of our proposed methodology. These two issues are among the most influential in the assessment of buildings. Several other issues could be included in the evaluation of building sustainability, some of them could be quantified such as energy use and durability, and others could only be discussed in qualitative terms such satisfaction with indoor environment or social benefits of knowledge generated in buildings. The integration of all issues

contributing to the assessment of such an index of building sustainability is beyond the purpose of this paper. The steps of the proposed methodology are presented in Fig. 1 and commented below: Step 1: Extraction of results from the SBTool for each criterion (e.g., Criterion B1.1 refers to the annualized non-renewable energy embodied in construction materials, see Table 1). In this paper only the energy and resource consumption (B issue), and the environmental loading (C issue) are considered. Our sensitivity analysis proved that these two issues have the highest impact of the total building score. Step 2: Processing of results from SBTool and additional calculations, when needed, to estimate the energy use and emissions for each criterion. Step 3: Calculation of exergy lost for each criterion. Step 4: Calculation of annualized total exergy lost due to the building construction and operation, as a sum of corresponding values for all criteria. Step 5: Calculation of annual exergy index of renewability aex as follows:

aex ¼

Annual available solar exergy $100% Annual exergy lost

(1)

Step 6: Calculation of annual exergy index of building sustainability (ExSI), by using the corresponding index of renewability aex.

Fig. 1. Flowchart of the proposed methodology.

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

ENNsb1:1 ¼ exergy lost for new building components, calculated as follows:

Table 1 Selected criteria of B and C issues as extracted from the SBTool. B

Energy and resource consumption

Units

B1.1

Annualized non-renewable primary energy embodied in construction materials. Annual use of purchased electricity for operations, delivered Use of off-site energy that is generated from renewable sources (delivered) Use of durable materials Re-use of salvaged materials from off-site Use of recycled materials from off-site sources. Use of bio-based products obtained from sustainable sources. Use of potable water for site irrigation. Use of potable water for occupancy needs.

MJ/m2$yr

B1.2 B3.1 B4.4 B4.5 B4.6 B4.7 B5.1 B5.2

% by cost % by cost % by cost % by cost m3/m2 L/pp/day

Environmental loadings

Units

Annualized GHG emissions embodied in construction materials. Annual GHG emissions from all energy used for facility operations. Emissions of ozone-depleting substances during facility operations. Emissions of acidifying emissions during facility operations. Emissions leading to photo-oxidants during facility operations.

kg/m2∙yr

C2.2 C2.3

kg/m2∙yr

ENEb1:1 ¼

kg/m2∙yr

X m

(5)

  * 2 ENN $ 1  AR $n b1:1

(6)

g/m2∙yr

2.2.2. Annual non-renewable delivered exergy lost due to the facility operations (B.1.2) The annual exergy lost is calculated based on the information extracted from SBTool:

This section presents the calculation method of the annual exergy lost for the selected criteria of B and C issues, based on the SBTool results.

EXTotal ¼ EXB þ EXC þ EXb4:7 þ EXb5:1 þ EXb5:2 þ EXc1:1 þ EXc1:2 þ EXc2:1 þ EXc2:2 þ EXc2:3 (2) where, for instance, the subscript b1.1 makes reference to the first criterion B1.1 (Table 1). 2.2.1. Annualized non-renewable exergy lost embodied in construction materials (B.1.1) The total annualized embodied energy ENb1.1, [MJ/yr] for the project as listed by the SBTool is the sum of the total embodied energy for new structural elements ENNsb1:1 and walls ENNwb1:1 , existing structural elements ENEsb1:1 and walls ENEwb1:1 , and heavy materials ENHb1:1 :

ENNsb1:1 þ ENNwb1:1 þ ENEsb1:1 þ ENEwb1:1 þ ENHb1:1 Lservice

EXb1:2 ¼ EXEb1:2 þ EXFb1:2

(3)

(7)

where: EXEb1:2 is the exergy lost due to the electricity delivered and used on-site, and EXFb1:1 is the exergy lost due the use of a natural gas-fired boiler:

EXFb1:2 ¼ Sgen $TKo;a

¼ EXb1:1 þ EXb1:2  EXb3:1 þ EXb4:4 þ EXb4:5 þ EXb4:6

(8)

  _ wgboiler swout; gboiler eswin; gboiler Sgen ¼ m       ENFb1:2 $Anetocc 1  hboiler e $hboiler þ ENFb1:2 $Anetocc $ TKo;a TKflame (9) where: Sgen ¼ the entropy generation within the natural gas-fired boiler, MJ/K yr. 2.2.3. Use of off-site exergy generated from renewable sources (B.3.1) This item includes also the on-site generation of electricity from renewable sources. The exergy is calculated as follows, and subtracted from the total exergy lost:

EXb3:1 ¼ ENb3:1 ¼

The corresponding annualized exergy lost is calculated as follows where:

EXb1:1 ¼

  TKo;a ENNb1:1 $ 1  TKmax

ENNb1:1 ¼ energy used for new building components, TKo,a is the annual average outdoor air temperature, in K; and TKmax is the maximum temperature used in the manufacturing of building materials, in K. The exergy lost corresponds to the embodied energy in building components ENb1.1, which is calculated in terms of building materials and quantities that are specific to the building under analysis. Similar calculations are performed for the exergy lost due to existing structures and walls EXEb1:1 and heavy materials EXHmb1:1 . Gradual reduction in the embodied energy for existing structures and walls is applied by using the amortization rate AR and the age of the existing structural elements n [9]:

g/m2∙yr

2.2. Evaluation of annualized exergy lost for the B and C issues (SBTool)

ENb1:1 ¼

X m

% by energy

C

C2.1

EXNb1:1 ¼

MJ/m2$yr

C1.1 C1.2

205

 EXNsb1:1 þ EXNwb1:1 þ EXEsb1:1 þ EXEwb1:1 þ EXHcb1:1 þ EXHmb1:1 Lservice

X occ

annual purchased % ENEb1:2 $Anetocc $ 100 (10)

(4)

206

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

2.2.4. Use of durable materials (B.4.4) The calculation of the exergy lost due to the recurring embodied energy is composed of three steps: 1) the percentage of initial embodied energy of durable materials (walls and heavy materials) to the total initial embodied energy is estimated from the SBTool data as the ratio between the cost of durable materials and the total construction materials costs; 2) the number of replacements (N) of non-durable materials is calculated using the service life expectation of each material (Mservice) [30] and the assumed building life span (from SBTool) (Lservice); Eqs. (11) and (12) apply to new and existing building materials, respectively; 3) the annualized recurring exergy lost is calculated with Eq. (14):

Nnew ¼

Lservice Mservice

Nexist ¼

(11)

Lservice eðMservice eMexist service Þ Mservice ðMservice eMexist service Þ þ Mservice 

EXnonDUR ¼

1 þ

 X DUR $ EXNwb1:1 $Nnew 100 m

X

EXEwb1:1 $Nexist þ

m

EXb4:4 ¼

(12)

X

TECrecycled ¼

X

TECtotal $q

(20)

occ

EXb4:6 ¼

exc$TECrecycled Lservice

(21)

2.2.7. Use of bio-based products obtained from sustainable sources (B.4.7) The indicator used to assess this criterion is the contribution by cost of bio-based products cost from off-site. It is expected that most of these products will have more benign effect on the environment, will be biodegradable, and have lower disposal and cleanup costs than fossil energy-based products they will replace. Eqs. (22) and (23) are used for the calculation of bio-based products cost TECbio-based and the corresponding exergy lost due to the use of the bio-based products.

TECbiobased ¼

X

TECtotal $m

(22)

occ

EXb4:7 ¼

exc$TECbiobased Lservice

(23)

!

EXHb1:1 $Nnew

(13)

m

EXnonDUR Lservice

(14)

2.2.8. Use of potable water for site irrigation (B.5.1) The annual energy used for water treatment to be used for the site irrigation, landscaped with non-native species Anon-native, excluding stored rainwater or grey water used for this purpose, is calculated using Eq. (24):

EXb5:1 ¼ ENb5:1 ¼ Anonnative $Iirrigationrate $ENrate

where Iirrigation-rate ¼ 2.5 m3/m2, irrigation rate, ENrate ¼ 1.6272 MJ/ m3, the specific energy used for water treatment in Montreal [31]. The exergy lost EXb5.1 is equal to the electrical energy used ENb5.1 in the treatment process.

2.2.5. Re-use of salvaged materials (B.4.5) The annualized exergy lost is calculated as follows:

EXb4:5 ¼

exc$TECsalvage Lservice

(15)

where: exc is the unit exergetic cost (exergy lost per unit capital construction cost), in MJ/$:

exc ¼

EXC TEC

(16)

EXC is the exergy lost due to the initial embodied energy, in MJ, and TEC is the thermoeconomic cost of construction, in $:

EXC ¼ EXNsb1:1 þ EXEsb1:1 þ EXHb1:1 þ EXNwb1:1 þ EXEwb1:1 

TEC ¼ TECtotal  TECsalvaged þ TECrecycled þ TECbiobased TECsalvaged ¼

X ðTECtotal $sÞ

(24)

(17) 

(18) (19)

2.2.9. Exergy use related to the usage of potable water for occupancy needs (B.5.2) The exergy lost due to potable water is calculated in terms of water usage by the occupants of the building under analysis for sanitary needs and irrigation purposes. The specific energy use for the treatment of water is not building-specific but city-specific. The predicted building annual water use (TAPWocc), in m3/yr, is calculated using Eq. (25):

TAPWocc ¼

X occ

P$doperation $

XLpt $Tpd  fix

1000

(25)

where: Lpt ¼ amount of water in litres used per one use per person, Tpd ¼ number of uses per day per person, P ¼ project population, and doper ¼ number of days of operation. Since the electricity is the energy used for the water treatment, the exergy lost is equal to the energy use:

occ

where: s is the proportion of the total cost of materials that are salvaged and refurbished or re-used from on-site or from off-site sources. 2.2.6. Use of recycled materials from off-site sources (B.4.6) The cost of recycled materials TECrecycled is calculated based on percentage of contribution of recycled materials of total building cost, Eq. (20), while annualized exergy consumption is calculated using Eq. (21):

EXocc wtreat ¼ ENocc wtreat ¼ TAPWocc $ENrate

(26)

The annual energy used for the heating of domestic hot water (using either gas water heaters or electric water heaters) is calculated with Eq. (27):

ENhot DHW ¼

  1  $ 1000$TAPWhotocc $Cp $ Tsupply Tinlet EF

(27)

where Tsupply ¼ 55  C, supply water temperature; Tinlet ¼ 10  C, the inlet water temperature from city main; TAPWhot-occ ¼ total annual

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

domestic hot water use for occupancy fixtures and uses, in m3/yr. The annual exergy lost is calculated with Eq. (28) for natural gas water heaters:

EXhotg DHW

TKo;a ¼ ENhotg DHW $ 1  TKflame

!

net usable building area. The value for annual CFC-11eq is the accumulated value of all potential hazard offered by each type of refrigerant as given by Eq. (34). The value obtained is normalized for the net usable building area.

(28)

The annual electricity use for major house appliances (e.g., dishwasher) is calculated as follows:

207

P CFC  11eq ¼

i

ODPN;i $mi Anetocc

(34)

where Oph ¼ operating hours per day for the fixture; Eload ¼ electric demand per fixture, in kW; Dcycle ¼ duty cycle, the proportion of time during which a component or device is in operation; and Nfix ¼ number of fixtures in the building. The annual energy use is calculated as follows:

where CFC-11eq ¼ the total annual equivalent CO2 emissions corresponding to ozone-depleting substance, g/m2∙yr; ODPN,i ¼ steady-state Ozone Depletion Potential for the emitted substance i measured in kg of CFC-11 equivalent per unit mass of substance i, (kg CFC-11eq/kg); and mi ¼ quantity of emitted substance i, kg. The total annualized exergy EXc2.1 corresponding to emission of ozone-depleting substances during facility operations is obtained using Eq. (35).

ENb5:2 ¼ ENocc wtreat þ ENhot DHW þ ENoper fix

(30)

EXc2:1 ¼ CFC  11eq $Anetocc $GWPCFC11 $eabat CO2

(31)

2.2.13. Emissions of acidifying emissions during facility operations (C.2.2) The indicator used to assess this criterion is the annual kg of SO2 eq normalized for the net usable building area Anetocc , given by SBTool or calculated using Eq. (36), while the corresponding annual abatement exergy is calculated using Eq. (37). All calculations are mainly based on primary energy use and taking into account the characteristics of available fuels.

EXoper fix ¼ ENoper fix ¼

X

Oph $Eload $Dcycle $P$doper $Nfix



(29)

fix

(35)

The corresponding exergy lost is calculated with Eq. (31):

EXb5:2 ¼ EXocc wtreat þ EXhotg DHW þ EXoper fix

2.2.10. Annualized exergy lost for the abatement of GHG emissions embodied in construction materials (C.1.1) An estimate of emission profile for the building could be obtained from the fuel breakdown of the energy associated with building materials production, assemblies and process emissions. This information can either be obtained using programs such as ATHENA or using historical data of building stock with similar building construction. Should a comprehensive emission profile not be available, an evaluation of GHG can be made by multiplying the total annualized embodied energy derived in criterion B1.1 by the national or regional average CO2 for the building industry (aaver). The abatement exergy approach is used in this study to assess the environmental impact of emissions for its advantages: (i) easily to apply once abatement exergy is known for each waste emission, (ii) availability of some waste emissions in the literature, and (iii) possibility of adding the corresponding exergy value directly to other exergy lost values of other indicators. The annualized abatement exergy lost corresponding to the emissions embodied in construction materials is calculated as follows:

EXc1:1 ¼ ENb1:1 $aaver $eabat

(32)

where aaver ¼ assumed regional fuel emission value kg CO2 per embodied GJ (e.g., emissions for residential taken from average Canadian building stock values for 1999 (SBTool) [9] is 55 kg CO2/ GJ; eabat ¼ specific abatement exergy (e.g., abatement exergy for CO2 is 5.86 MJ/kg [32]). 2.2.11. Annualized exergy lost GHG emissions for the abatement of GHG emissions due to facility operations (C.1.2)

EXc1:2 ¼ aaver $

X ðENb1:2 $Anetocc Þ$eabat CO2

(33)

P SO2 eq ¼

ACIDi $mi

i

Anetocc

EXc2:2 ¼ SO2 eq $Anetocc $eabat SO2

(36) (37)

where SO2 eq ¼ the total annual sulphur dioxide equivalent corresponding to acidification is the unit of ith environmental burden, kg/yr CO2; ACIDi ¼ potency factor of substance i for acidification as an environmental burden, (kg SO2 eq/kg). 2.2.14. Emissions leading to photo-oxidants during facility operations (C.2.3) This indicator measures the annual ethane equivalent normalized for net usable building area. It is assumed to be proportional to the global warming potential index. The value of GWPC2 H6 is found about 20 [33]. The annual abatement exergy is calculated using Eq. (39).

P C2 H6 eq ¼

PCOPi $mi

i

Anetocc

EXc2:3 ¼ C2 H6 eq $Anetocc $GWPC2 H6 $eabat CO2

(38) (39)

where C2H6 eq ¼ the total annual equivalent C2H6 emissions, kg/yr; PCOPi ¼ potency factor of substance i for photochemical oxidants potential as an environmental burden, (kg C2H6 eq/kg).

occ

2.3. Available solar exergy 2.2.12. Emissions of ozone-depleting substances during facility operations (C.2.1) This criterion assesses the environmental impact based on the predicted annual emissions of CFC-11eq, in gm/m2, normalized for

In this study the solar radiation is considered as the sole sustainable energy source. The annual available solar exergy on the building footprint (BFP) is calculated as follows [34]:

208

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

EXsunhorz ¼

X i

   1 TKoi 4 4TKoi Ii $ 1 þ  $ABFP 3TKsun 3 TKsun

(40)

where TKoi ¼ the average environmental temperature for month i, in K; TKsun ¼ sun temperature, 6000 K [34]; Ii ¼ total incident solar energy per unit area of horizontal surface for month i, and ABFP ¼ building footprint, in m2. 2.4. Exergy Index of Renewability aex The Exergy Index of Renewability, Eq. (1), is the ratio between the annual solar exergy on the building footprint, Eq. (40), and the total annualized exergy lost due to the building construction and operation, Eq. (2). This approach implicitly considers the exergy efficiency of 100% of harvesting the solar exergy. However, the theoretical potential is reduced by losses associated with the conversion from the primary source to the secondary resource. If the total annualized exergy lost is less than or equal to zero, for instance in the case when the renewable exergy converted on-site exceeds or is equal to the exergy used, the exergy index of renewability is set equal to 100%. 2.5. Exergy Index of Sustainability (ExSI) Different rating scales and their corresponding linguistic representation have been used by different authors. The rating scale proposed in this study is an extension of Ref. [35], which was compared against rating scales such as ERHA, STAR POINT, and HERS, using however energy use. The ExSI was developed by imposing the following three constraints: (a) the index should tend to zero when the Exergy Index of Renewability tends to zero, (b) the index should tend to 100 when the Exergy Index of Renewability tends to 100%; in this last case, exergy lost due to the building construction and operation is equal to or less than the available exergy that can be harvested on the horizontal surfaces; and (c) the ExSI of 50 corresponds to the Exergy Index of Renewability (aex) of 50%. The rating scale has an asymptotic variation when the index approaches the two extremes, when the building is either unsustainable or sustainable. Since the ExSI is less sensitive at these two extreme conditions, it can be improved only if the building achieves significant reduction of exergy lost, for a given location and footprint. We agree with one reviewer that underlined that the developers of a rating system should reach the balance between “heavy science” that few people understand, and a simpler approach. The approach proposed in this paper, based on applied thermodynamics, belongs to the first case, which might give more accurate and science-based accounts of sustainability. The simpler approaches such as LEED are based on experience, consensus, and market forces, and are easier accepted by the market. This paper is solely based on applied thermodynamics, however, future developments especially related to the calibration of rating scale should involve those using or modifying the market driving forces. The ExSI is calculated in terms of Exergy Index of Renewability aex as follows:

ExSI ¼

100 1 þ exp ½  l$ðaex  50Þ

(41)

Buildings with identical exergy lost may achieve different ratings if different rating scales are implemented. The proposed rating system uses a continuous unipolar function. The value of parameter l in Eq. (41) represents the strength of the policy

implemented in order to achieve sustainability in the building sector; this parameter determines the slope and spread for the relationship between the sustainability index ExSI and the renewability index aex. The value of l, between 0 and 1, is set by the developer of rating scale based on local or national goals, market penetration of technologies and shareholders surveys. In this study we propose the use of l ¼ 0.11 (Fig. 1) that allows for a Gaussian-type distribution of houses in terms of exergy performance. Implementing restrictive sustainability policy, for instance by using l ¼ 0.9, would exclude many buildings to be considered as sustainable buildings. The proposed rating scale assesses the building sustainability in five categories in terms of the Exergy Index of Sustainability: Sustainable, Exergy efficient, Average exergy efficient, Less than average exergy efficient, and Unsustainable (Table 2). 3. Case studies: results and discussion The proposed assessment method was applied to five case studies. Because of space limitations, the following paragraphs give only a few indications about each case. Readers are encouraged to consult references [9,36,37]. The case study no.1 is a large commercial and residential building located in Ottawa, Ontario, Canada [9] with three occupancy types (apartment, retail, and indoor parking), with total floor area of 11,200 m2, and building foot print of 800 m2. Building life span is considered to be 75 years. Aluminum and glass curtain walls and 30 cm reinforced concrete walls are the main type of building envelope that have been used in the building. The case studies no.2 and no.3 is a two storey house in Montreal, Quebec, Canada [36] with a building footprint of 84 m2 and with a total heated floor area of 208 m2. Case study no.2 is an energyefficient house, built in compliance with current codes using electric baseboard heaters; while case study no.3 is a Net-Zero Energy House with a solar combisystem for heating and domestic hot water, plus photovoltaic panels for electricity. Electric baseboard heaters are used in case no.2 while radiant floor heating system is used in case no.3. Thermal insulation value of above ground walls is 3.52 m2 K/W and 6.25 m2 K/W for case no.2 and case no.3, respectively. House life span is considered to be 40 years. The case study no.4 is a two storey single-detached brick house of 140 m2 heated floor area built in 1910. The case study is located in Kitchener, Ontario, Canada [37]. House life span is considered to be 100 years. Outside bricks account for most of initial embodied energy and the corresponding GHG emissions. The case no.5 is the same house after renovation with increased thermal insulation. Natural gas is the energy source for heating and domestic hot water in both cases. An old furnace is used in case no.4 with an average efficiency of 80%, while a new furnace with 96% efficiency is used in the renovated house. Table 3 presents the annualized exergy lost, calculated for each case study for selected criteria of B and C issues, the Exergy Index of Renewability and the Exergy Index of Sustainability. The available solar energy on the building footprint was extracted from TRNSYS program for given location [38].

Table 2 Proposed rating scale. Rating scale

Range of ExSI value

Sustainable Exergy efficient Average exergy efficient Less than average exergy efficient Unsustainable

96%  ExSI  100% 75%  ExSI  96% 25%  ExSI  75% 4%  ExSI  25% 0%  ExSI  4%

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

209

Table 3 Exergy-based Index of Sustainability calculated for five case studies using the B and C issues from SBTool. Criteria

B1.1 B1.2 B3.1 B4.4 B4.5 B4.6 B4.7 B5.1 B5.2

Annualized non-renewable primary energy embodied in construction materials Annual use of purchased electricity for operations, delivered Use of off-site energy that is generated from renewable sources (delivered) Use of durable materials Re-use of salvaged materials from off-site Use of recycled materials from off-site sources. Use of bio-based products obtained from sustainable sources. Use of potable water for site irrigation. Use of potable water for occupancy needs.

C1.1 Annualized GHG emissions embodied in construction materials. C1.2 Annual GHG emissions from all energy used for facility operations. C2.1 Emissions of ozone-depleting substances during facility operations. C2.2 Emissions of acidifying emissions during facility operations. C2.3 Emissions leading to photo-oxidants during facility operations. Total annualized exergy lost [MJ/yr] Available solar exergy [MJ/yr] Building footprint [m2] Exergy Index of Renewability aex Exergy Index of Sustainability estimated at 100% (theoretical potential) Exergy Index of Sustainability estimated at 40% (technical potential) Exergy Index of Sustainability estimated at 20% (technical potential) Exergy Index of Sustainability estimated at 5% (economical potential)

Annual exergy use [MJ/yr] Case study no.1

Case study no.2

Case study no.3

Case study no.4

Case study no.5

þ

4,391,730

20,625

37,542

3710

2108

þ -

3,819,116 567,424

63,158 0

33,196 40,478

183,541 0

27,507 0

þ þ þ þ þ þ

88,845 1,166,880 114,368 937,817 1627 605,013

0 0 0 0 113 29,195

0 0 0 0 57 7380

0 0 0 0 0 21,860

4542 0 0 0 0 6277

þ þ þ þ þ

1,736,338 2,592,457 15,032 250,652 184 15,152,635 3,715,491 800 25 5.7 1.2 0.7 0.5

8156 20,356 299 4989 4 146,895 381,336 83.7 260 100.0 99.7 55.3 1.7

14,846 10,699 299 4989 4 68,535 381,336 83.7 556 100.0 100.0 100.0 8.0

1985 89,330 402 6703 5 307,537 637,130 140 207 100.0 97.4 28.0 1.3

1628 15,330 402 6703 5 64,502 637,130 140 988 100.0 100.0 100.0 48.3

In the case no.1 of large, multi-storey commercial and residential building, the annualized exergy lost due to energy use for construction and operation is 10,518,756 MJ/yr or about 70% of total annualized exergy lost considered in this study. The annualized exergy used due to purchased electricity accounts for 36% of exergy lost due to the energy used. These proportions are 83,783 MJ/yr (75%) for case no.2, 70,738 MJ/yr (47%) for case no.3, 187,251 MJ/yr (98%) for case no.4, and 34,157 MJ/yr (81%) for case no.5. Traditionally the majority of building assessment methods and rating system have linked the building energy use to its operation and therefore much attention has been dedicated to reduce this energy through technical innovation, regulatory control. However this is usually accompanied by an increasing amount of materials and systems to reduce the energy use in the operation. The results show a large increase in exergy lost due to non-renewable primary energy embodied in construction materials, between case no.2 (20,625 MJ) and case no.3 (37,542 MJ). The contribution of the total annualized exergy lost due to embodied energy in construction materials is increased from 14% (case no.2) to 54.8% (case no.3), and from 1.2% (case no.4) to 3.3% (case no.5). The reduction of exergy lost due to the operation is accompanied with an extra 47% decrease of exergy lost due to emissions. In comparison with annualized exergy lost for building operations, the annual exergy lost due GHG emissions from all energy used for facility operations, represents around 30% in cases no.2 and no.3, and represents around 50% in cases no.4 and no.5. In the case no.1, the available solar exergy can compensate for only 25% of the exergy lost due to the construction and operation of that building. In all other four case study houses, the available solar exergy can entirely compensate for the exergy lost: the Exergy Index of Renewability is equal to 260% for case no.2, 556% for case no.3, 207% for case no.4, and 988% for case no.5. In the last four case studies, the building consumes much less exergy than it could theoretically harvest on its horizontal footprint. The Exergy Index of Sustainability (ExSI) of the large commercial building (case no.1) is equal to 5.7. This result shows that under the ideal conversion of 100% of solar exergy, only 5.7% of the annualized exergy lost is compensated by the incident exergy of

solar radiation on the building footprint. According to the proposed rating scale, the case study building with ExSI ¼ 5.7 receives the qualification of “Less than average exergy efficient” building. This is the maximum value of theoretical index of sustainability that this case study building could achieve. This building will never become a more “sustainable” building, according to the definition proposed in this paper. The case study houses no.2 to no.5 achieve an ExSI ¼ 100, and receive the qualification of “Sustainable” under the proposed definition. If a PV system with the overall system efficiency of 5% [39] is used, the Exergy Index of Sustainability drops from 100 to 1.7 (case no.2), to 8 (case no.3), to 1.3 (case no.4), and to 48.3 (case no. 5). These values correspond to the economical potential of such PV systems. The ExSI is evaluated using PV systems with 20% [40] and 40% [41], which correspond to the technical potential of such PV systems (Table 3). The results emphasize the large difference between the maximum theoretical index of sustainability and the potential for sustainability of current technologies. Two cases have a high potential to become more sustainable: case studies no.3 and no.5.

4. Conclusions The paper proposed a new definition of building sustainability and an exergy-based index for a thermodynamic-based assessment process. The long-term building sustainability is assessed by comparing the annualized exergy lost due the building construction and operation, over the building life time, with the available solar exergy that could be harvested on the building footprint, assumed to be the sole sustainable energy source. The proposed exergy-based index overcomes the limitations of the subjectively defined weights that are allocated in other methods to different criteria used in the assessment of building sustainability. The distinctive characteristic of the proposed framework is the calculation and aggregation of different sustainability dimensions into a single commodity, the exergy.

210

A. El shenawy, R. Zmeureanu / Building and Environment 60 (2013) 202e210

The application of the proposed exergy-based index revealed that large commercial buildings with several floors cannot achieve a high level of strong sustainability by using only the building footprint as the reference surface for harvesting solar energy. This type of building is a candidate for weak sustainability approach, with partial use of non-renewable energy sources. All four residential buildings may achieve the highest theoretical potential of building sustainability, and the highest technical potential by using PV technologies with 40% efficiency. The results of case studies also indicated that there is a large difference between the maximum theoretical index of sustainability (as proposed in this paper) and the potential for sustainability of current PV technologies with 5e20% efficiency. Future work will cover other renewable energy sources and location on the building under investigation, and the case of weak sustainability with partial use of non-renewable energy sources. Acknowledgements The authors acknowledge the financial support received from Egyptian Ministry of Higher Education, and from the Faculty of Engineering and Computer Science of Concordia University. References [1] Lucuik M, Trusty W, Larsson N, Charette R. A business case for green buildings in Canada. In: Canadian green building council. Ottawa: Industry Canada; 2005. [2] Cole RJ. Building environmental assessment methods: redefining intentions and roles. Building Res Inf 2005;33(5):455e67. [3] Forsberg A, Malmborg von F. Tools for environmental assessment of the built environment. Building Environ 2004;39(2):223e8. [4] Blom I, Itard L, Meijer A. Environmental impact of building-related and userrelated energy consumption in dwellings. Building Environ 2011;46(8):1657e69. [5] Lee WL, Burnett J. Benchmarking energy use assessment of HK-BEAM, BREEAM and LEED. Building Environ 2008;43(11):1882e91. [6] Muldavin SR. Value beyond cost savings: how to underwrite sustainable properties. Green Building FC; 2010. [7] WCED. Our common future e Brundtland report. Oxford: Oxford University Press; 1987. [8] LEEDÒ. homepage of LEEDÒ. Online [web page] [accessed 04.2012], http:// www.leedonline.com; 2010. [9] SBTool. GBTool and SBTool Overview [web page] [accessed 04.2012], http:// www.iisbe.org/sbtool; 2009. [10] Lee WL, Burnett J. Benchmarking energy use of building environmental assessment schemes. Energy Buildings 2011;43(11):1882e91. [11] Cooper I. Which focus for building assessment methods e environmental performance or sustainability? Building Res Inf 1999;27(4e5):321e31. [12] Bendewald M, Zhai ZJ. Using carrying capacity as a baseline for building sustainability assessment. Habitat Int 2012:1e11. [13] Pearce DW, Markandya A, Barbier E. Blueprint for a green economy. Earthscan/James & James; 1989. [14] Alberti M. Measuring urban sustainability. Environ Impact Assess Rev 1996; 16(4):381e424.

[15] Howarth RB. Discount rates and sustainable development. Ecol Modell 1996; 92(2):263e70. [16] Wackernagel M, Rees WE. Our ecological footprint: reducing human impact on the earth. New Society Pub; 1996. [17] Brown MT, Herendeen RA. Embodied energy analysis and EMERGY analysis: a comparative view. Ecol Econ 1996;19(3):219e35. [18] Rant Z. Exergy, a new name for technical availability. Forsch Ing Wes 1956; 3(1):38e9. [19] Kotas TJ. The exergy method of thermal plant analysis. London, Boston: Butterworths; 1985. [20] Szargut J, Morris DR, Steward FR. Exergy analysis of thermal, chemical, and metallurgical processes. New York: Hemisphere Publishing; 1988. [21] Wall G. Exergy, ecology and democracy-concepts of a vital society or a proposal for an exergy tax. In: International conference on energy systems and ecology, Krahow, Poland, July 5e9, 1993. p. 111e121. [22] Cornelissen RL. Thermodynamics and sustainable development: the use of exergy analysis and the reduction of irreversibility. University of Twente; 1997. [23] Dewulf J, Van Langenhove H, Mulder J, Van Den Berg MMD, Van Der Kooi HJ, de Swaan Arons J. Illustrations towards quantifying the sustainability of technology. Green Chem 2000;2(3):108e14. [24] Rosen MA, Dincer I. Exergy as the confluence of energy, environment and sustainable development. Exergy: Int J 2001;1:3e13. [25] Gong M, Wall G. On exergy and sustainable development e part 2: indicators and methods. Exergy: Int J 2001;1(4):217e33. [26] Dewulf J, Van Langenhove H. Integrating industrial ecology principles into a set of environmental sustainability indicators for technology assessment. Resour Conserv Recycl 2005;43(4):19e32. [27] Cornelissen RL, Hirs GG. The value of the exergetic life cycle assessment besides the LCA. Energy Convers Manag 2002;43(9):1417e24. [28] Dincer I, Hussain MM, Al-Zaharnah I. Energy and exergy use in public and private sector of Saudi Arabia. Energy Policy 2004;32(14):1615e24. [29] Rosen MA, Dincer I, Kanoglu M. Role of exergy in increasing efficiency and sustainability and reducing environmental impact. Energy Policy 2008;36(1): 128e37. [30] Scheuer C, Keoleian GA, Reppe P. Life cycle energy and environmental performance of a new university building: modeling challenges and design implications. Energy Buildings 2003;35(10):1049e64. [31] Dumas C, personal communication, Montreal, 2012. [32] Dewulf J, Van Langenhove H, Dirckx J. Exergy analysis in the assessment of the sustainability of waste gas treatment systems. Sci Total Environ 2001;273(1e 3):41e52. [33] Zhu H, Gong M, Zhang Y, Wu J. Isothermal vapor-liquid equilibrium data for tetrafluoromethane þ ethane over a temperature range from (179.68 to 210.03) K. J Chem Eng Data 2006;51(4):1201e4. [34] Petela R. Exergy analysis of the solar cylindrical-parabolic cooker. Solar Energy 2005;79(3):221e33. [35] Zmeureanu R, Fazio P, DePani S, Calla R. Development of an energy rating system for existing houses. Energy Buildings 1999;29(2):107e19. [36] Leckner M, Zmeureanu R. Life cycle cost and energy analysis of a Net Zero Energy House with solar combisystem. Appl Energy 2012;88(1):232e41. [37] Bin G, Parker P. Measuring buildings for sustainability: comparing the initial and retrofit ecological footprint of a century home e the REEP House. Appl Energy 2011;93:24e32. [38] Klein SA, Beckman WA, Mitchell JW, Duffie JA, Duffie NA, Freeman TL, et al. TRNSYS 16 e a TRaNsient SYstem simulation program, user manual; 2004. [39] Duran Sahin A, Dincer I, Rosen MA. Thermodynamic analysis of solar photovoltaic cell systems. Solar Energy Mater Solar Cells 2007;91(2):153e9. [40] Hoffmann W. PV solar electricity industry: market growth and perspective. Solar Energy Mater Solar Cells 2006;90(18):3285e311. [41] Spectrolab break 40% efficiency for PV solar panel. http://www.reuk.co.uk/40Percent-Efficiency-PV-Solar-Panels.htm.