Life-cycle operational and embodied energy for a generic single-storey office building in the UK

Life-cycle operational and embodied energy for a generic single-storey office building in the UK

Energy 27 (2002) 77–92 www.elsevier.com/locate/energy Life-cycle operational and embodied energy for a generic single-storey office building in the U...

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Energy 27 (2002) 77–92 www.elsevier.com/locate/energy

Life-cycle operational and embodied energy for a generic single-storey office building in the UK Y.G. Yohanis *, B. Norton Centre for Sustainable Technologies, School of the Built Environment, University of Ulster, Newtownabbey, Co. Antrim, BT37 0QB, Northern Ireland, UK Received 22 February 1999

Abstract Increasing energy efficiency makes embodied energy considerations increasingly significant. The energy initially embodied in a building could be as much as 67% of its operating energy over a 25-year period. If additional embodied energy gained over the building life is also included, the total life-cycle energy could be larger than the operating energy over the same period. Currently, embodied energy cannot be predicted accurately due to lack of reliable and accurate data; there is a wide variation in the data available. The variation of life-cycle operational and embodied energy and capital cost as a function of building parameters is explored.  2001 Elsevier Science Ltd. All rights reserved.

1. Introduction The energy embodied in a building is that used to extract, process, manufacture and transport building materials and components. As improvements in the operational energy efficiency of buildings are made, the relative significance of embodied energy forms a higher proportion of the total amount of energy used over the lifetime of a building. Achieving a truly energy-optimised design requires the ability to investigate both operational and embodied energy implications of alternative design options including all inter-related inputs, processes and outputs. The total energy used in a building over its life is the sum of the operational energy and the life-cycle embodied energy as illustrated in Fig. 1, the latter is sum of initial embodied energy, recurring energy and demolition energy. The initial embodied energy increases from zero to a maximum during the construction phase as shown in Fig. 2. During this phase as

* Corresponding author. Tel./fax: +44-2890-36825. E-mail address: [email protected] (Y.G. Yohanis).

0360-5442/02/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 6 1 - 5

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Fig. 1. Indicative components of embodied and operational energy over an illustrative building life cycle.

Fig. 2. Operational and embodied energy as a function of building life. * Initial embodied energy plus recurring embodied energy over 25 years*, 50 years**, and 100 years***. A, Construction Phase; B, Operation Phase.

the building is not occupied, there is no operating energy requirement. Any energy requirement by construction personnel is assumed to be part of the initial embodied energy. During the operation phase, the increase in embodied energy is due to repainting, recarpeting, replacement of lamps and systems, and major periodic modelling and refurbishment due to changes in tenancy

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Table 1 Additional embodied energy and increase in embodied energy during life of a building compared with initial embodied energy [2] Building life (years)

Additional embodied energy (GJ m⫺2) Percentage increase compared with initial embodied energy

25 50 100

2.52 6.32 14.4

59 148 339

or function. Major refurbishment may involve substantial reconstruction amounting to 0.10 to 0.17 GJ m⫺2, 0.13 to 0.23 GJ m⫺2 or 0.17 to 0.34 GJ m⫺2 for basic, medium or top-grade office fit-out, respectively [1]. Estimates for the additional energy associated with typical replacement and repair over various building lives, for the case of a building with a wood structure, are shown in Table 1. Although these figures are illustrative and cannot be applied universally, they nevertheless show clearly that recurring embodied energy is significant in life-cycle energy analysis. At the end of the useful life of a building, energy is used for demolition and transport. This component is very difficult to assess due to difficulty in predicting the useful life of a building, the methods of demolition and the energy implications of any materials and/or component re-use and/or recycling at a future date. Initial embodied energy is estimated to account for about 70% of the total energy used in building construction and about 20% of the total energy requirement for UK industry [2]. For some new well-insulated buildings, embodied energy could be as much as 50% of the operational energy over a 25-year period [2]. Estimates for the initial embodied energy and for the sum of the initial and recurring embodied energy in relation to the operating energy over various building lives are shown in Table 2. The operational and embodied energy of alternative design options can be calculated using the Early Design Model (EDM). EDM — which is summarised in Fig. 3 — is an integrated and simplified energy model based on proven well-established algorithms and, being spreadsheetbased, is simple and flexible [4]. It incorporates generalised mathematical expressions for correlation parameters for the solar energy utilisation factor [5] and a facility for preliminary cost evaluation [6]. It is intended for use in estimating capital cost, embodied energy and operating energy requirement at the earliest stages of design. For given building orientation and weather Table 2 Initial and recurring embodied energy as a percentage of operating energy [3] Building life (years)

Initial embodied energy as a percentage of operating energy

The sum of initial embodied energy and recurring embodied energy as a percentage of operating energy

25 50 100

67 34 17

105 82 71

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Fig. 3. Simplified chart of the early design model (EDM) [4].

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data, building parameters (e.g., overall heat loss coefficient) and derived internal conditions (e.g., comfort temperature and infiltration rate) can be varied to analyse the energy performance and capital cost of a design option with respect to glazing ratio, obstructions, room depth, infiltration, insulation levels, daylight level, installed illuminance and internal heat gain. The aim of this paper is to explore the variation of life-cycle operational and embodied energy and capital cost as a function of building parameters using EDM. In order to have a common basis to compare energy performance and cost, energy predictions from EDM have been converted to cost figures assuming that the cost of energy is £0.05 kW⫺1 h⫺1 and the life of the building is either 60 or 30 years. Comparative, rather than absolute, variations with building parameters are examined, so to simplify the analysis; inflation, discount rate and interest rate variations are ignored. The investigations are, except where indicated otherwise, on a primary energy basis. The cost of operational energy, embodied energy and the economic optimisation of window sizes as functions of glazing ratio are investigated.

2. Base-case building A floor plan and dimensions for the base-case building are shown in Fig. 4. It is a notional open-plan, single-storey building with east-facing and west-facing windows located at 52° latitude. Construction is steel frame, with concrete floors, brick cladding, and a pitched roof with concrete interlocking tiles on timber rafters, steel trussed to roof. Walls, ceiling, floor and windows are assumed to have average reflectance. Thermal insulation meets the requirements of the Northern Ireland Building Regulations [7], and windows are double-glazed throughout. Glazing areas to the offices comprise 42% of the east and west elevations and permit good daylight. It is assumed that the 16 m wide office is naturally ventilated. Heating is provided by a low-temperature hot water heating system. Ceiling-mounted fluorescent lights are used throughout, giving a lighting level of 300 lux. The building and environmental parameters for the base-case building are provided in Appendix A.

Fig. 4.

Floor plan of single-storey base-case building.

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Table 3 Variation in the values of embodied energy for key materials of construction [9] Material

Variation in embodied energy (GJ t⫺1)

Aggregates Cement Bricks Wood Steel

0.02–0.3 3–9.5 0.5–10 0.5–10 10–65

3. Capital cost and initial embodied energy data The data used in the cost model in EDM [4] are median elemental costs of office buildings taken from BCIS cost information [8]. BCIS gives elemental unit costs mainly in terms of cost per unit floor area; these data are used for all elements except external walls, windows and the heating system. The cost model requires costs of windows and external walls on a per unit elemental area basis (i.e., £ per window unit area in m2 and £ per wall unit area in m2), and for heating installations, £ per unit rate of heat loss in Watts. There is a wide variation in the data available for embodied energy as shown in Tables 3 and 4. There are a number of factors that lead to this variation [9–11]: some data are given in primary (i.e., energy considering losses due to refining, distribution and inefficiency of plant) and others in delivered energy (i.e., without considering losses due to refining, distribution and inefficiency of plant) units and energy for transport may or may not be included for the import of raw materials or delivery of final products. The scope of estimates also differs in terms of product specification (e.g., for steel the final product may be a continuously cast item or a sheet or wire), the scope may differ in the way recycled materials are accounted for or if some allowance has been made for the future recyclability of the material; the feedstock (oil, gas or solid fuels) may differ resulting in different process energy and whether these are counted as energy inputs; different primary to delivered energy conversion ratios, different sources of fuel and different manufacturing processes; and older estimates tend to be higher as manufacturing processes become progressively more energy-efficient. For the office building considered here, EDM uses the following elemental analysis: substrucTable 4 Variation in embodied energy values of construction elements per element area [9] Construction element

Upper floors External walls Internal walls Roof

Variation in embodied energy values (GJ m⫺2) Delivered embodied energy

Primary embodied energy

0.3–1.13 0.3–2.7 0.17–0.62 0.42–1.15

0.5–2 0.35–3.6 0.28–1.05 0.7–1.8

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ture, frame (either steel frame or concrete structure), floor, roof, internal wall, external wall, windows, external wall finish, floor finish, ceiling finish and heating. The design and material selection of the substructure depends on the structure and ground conditions at site. Concrete, though a low embodied energy material compared with steel, is used in relatively large quantities. A large mass of steel is also used in reinforced concrete and steel-framed buildings. The variation in the embodied energy values of upper floors and external walls is large (see Table 4). For external walls, this variation also arises from the fact that some external walls are load-bearing while others are not. Embodied energy data are generally given in terms of gigajoules per tonne (GJ t⫺1) of material. However, in order to assess the embodied energy of an element (e.g., external wall or floor) in the context of a building design, the mass of each material (e.g., steel or cement or wood) in a square metre of the element is multiplied by respective embodied energy values. The sum of these individual material components constitutes the initial embodied energy of the element expressed in (GJ m⫺2) element area [9]. For some elements, e.g., steel frame and heating system, it would be more practical from the designer’s point of view if embodied energy values were expressed in terms of gigajoules per unit floor area (GJ m⫺2). This would require one to assess all of the embodied energy in, e.g., the steel frame in a building and divide the same by the total floor area. The embodied energy data used in this work have been estimated from embodied energy data for wall, roof and floor construction systems [12] (see Appendix B) and from data for building elements [9]. 4. Initial embodied energy analysis Embodied energy can be calculated on an industrial sector basis (i.e., the total embodied energy divided by the total material used in a sector, e.g., manufacture of steel) or by process analysis in which the embodied energy of a particular material is tracked from extraction to end-use. The figures produced by each approach differ, particularly for low-volume commodities [13]. The figures used in the study are mainly the result of process analysis but, due to the high production volumes for the materials used, would not be substantially different from those given by input– output analysis. Table 5 shows embodied energy values (in GJ m⫺2 of floor area) from different sources and different countries. Clearly there is a very wide variation. It is not clear whether Table 5 Initial embodied energy values in GJ m⫺2 floor area for office buildings Number of storeys, country

Principal structure

Embodied energy (G m⫺2) Reference

15, Australia 3–8, New Zealand 3–8, New Zealand 3, Canada 3, Canada Not specified, UK 1, UK

Concrete Steel Concrete Steel Concrete Not specified Steel

8.23 6.6 5.6 4.86 4.52 10–18 9.5

[11] [6] [6] [7] [7] [8] This study

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Table 6 Percentage of embodied energy in a structure compared with total embodied energy Steel frame

Concrete frame

Reference

67 25.2 30

61 20.6 23

[6] [7] This study

these data are based on primary or delivered energy basis. There is also ambiguity as to whether the figures include energy embodied in the substructure. It is not also possible to draw universal conclusions on the basis of categorising buildings in terms of number of storeys and type of principal structure (i.e., steel or concrete). Therefore, it is difficult to arrive at universally applicable embodied energy values because of the large variation in the values of embodied energy available to date. Further, the magnitude of these values depends, among other factors, on the method of construction employed and on material selection. The value of 10.5 GJ m⫺2 determined in this work is at the lower end of the range of 10–18 GJ m⫺2 determined in the context of the UK [9]. The embodied energy in the structure of a building is significant as shown in Table 6. There is general agreement between the embodied energy values in the structure, envelope (i.e., floor, roof, external wall, window and internal wall) and finishes as shown in Table 7 between figures determined in this work and previously [2]. The big difference in embodied energy values in the substructure is assumed to be arising from a much lower substructure requirement in the case of the latter. The embodied energy for services in this work is much lower than reported previously [2] as only heating provision is assumed in the former. 5. The dependence of cost on building parameters Fig. 5 shows typical costs of operational energy (i.e., sum of heating, cooling and lighting, on primary energy basis), operational energy plus capital (i.e., the initial cost of the building including the heating system), operational energy plus embodied energy and, finally, operational energy, capital and embodied energy as functions of glazing ratio, which is the ratio of the area of glazing Table 7 Percentage of embodied energy in major elements of office buildings Description

Ref. [2]

This study

Substructure Structure Envelope Finishes Services Construction Total

6.0 25.2 27.0 12.5 22.8 6.5 100

18 30 29 13 10 – 100

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Fig. 5. Typical graph of cost as a function of glazing ratio.

in a wall to the total area of that wall. Capital cost refers to the initial cost of erecting the building and the systems within it. As glazing ratio is increased from 0 to about 15%, the total cost reduces to a minimum value; for glazing ratio values in the range of 15% to about 30%, the total cost remains at the minimum value and for glazing ratio values greater than about 30%, it increases as shown in Fig. 5. The variation of capital and embodied energy costs with respect to the variation of glazing ratio is very small; this is because for this particular building the cost of the windows is overwhelmed by the costs of other elemental components of the building. 6. Window size optimisation Fig. 6 shows operational energy as a function of glazing ratio for an operating life of 30 years and 60 years, and for the case when delivered energy only is considered. For the base case, for an operating life of 60 years, the cost of operational energy is the lowest for a glazing ratio of about 15%; however, when delivered energy only is considered, the cost of operational energy

Fig. 6. Cost of energy as a function of glazing ratio for primary and delivered energy for operating life of 30 and 60 years.

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Fig. 7. Cost of energy as a function of glazing ratio for base case and for infiltration rate of 0.5 air changes per hour.

reduces to the lowest value as glazing ratio is reduced to zero. When primary energy is considered for an operating life of 30 years, the cost of operational energy is the lowest for glazing ratio in the range of 15% to 20%. However, when delivered energy only is considered, the cost of operational energy for an operating life of 30 years does not vary as glazing ratio increases. Cost of operational energy is lower for an operating life of 60 years when delivered energy is considered than for an operating life of 30 years when primary energy is considered. The cost of operational energy for the base case, for an operating life of 30 years, when delivered energy only is considered, reduces gradually to the lowest value as glazing ratio is reduced as shown in Fig. 7. For the same case but an infiltration value of 0.5 air changes per hour, the cost of operational energy reduces down to a minimum value at a glazing ratio of about 15% and remains almost constant at this value. The variation of the cost of embodied energy is almost negligible as glazing ratio is increased. The cost of operational energy is compared for two situations: first, for the base case but with the rate of infiltration reduced to 0.5 air changes per hour, the operating life set at 30 years and considering delivered energy only; and, second, for the same case as above but with the efficiency of the heating plant reduced down to 0.75. The result is shown in Fig. 8. The trends for the two cases are similar: the cost reduces sharply when glazing ratio is reduced, reaching a minimum

Fig. 8. Cost of energy as a function of glazing ratio for primary and delivered energy for different building parameters.

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for a glazing ratio of about 15% and then rising slightly. The situation for the case when installed illumination is reduced to 10 W m⫺2 also follows the same pattern. The situation for the case when the overall heat loss coefficient is reduced to 0.25 W m⫺2 also follows the same pattern and is below the cost of embodied energy for values of glazing ratio lower than about 55%. This infiltration rate and the overall heat loss coefficient fall below values recommended in the UK for practical design [7,14]. For practical applications where the rate of infiltration is about 1.0 air change per hour and the heat loss coefficient is about 0.45 W m⫺2 K ⫺1, the operational energy will be much higher than the embodied energy. 7. Insulation For the base case shown in Appendix A and assuming a typical glazing ratio of 42% and an operating life of 30 years, the level of insulation at which embodied energy equals the lifetime heating energy of the building has been calculated. Fig. 9 shows that there is no such overall heat loss coefficient of the envelope for the base case when primary energy is considered, even for an operating life of 30 years. For the base case, if delivered energy only is considered, the level of insulation at which embodied energy equals lifetime heating energy is about 0.05 W m⫺2 K ⫺1; for an infiltration value of 0.5 and 0.4 air changes per hour, the overall heat loss coefficient is about 0.25 and 0.32 W m⫺2 K ⫺1, respectively, as shown in Fig. 9. The higher the infiltration rate and, therefore, the higher the heating energy requirement, the less impact embodied energy has on building design considerations. The variation of embodied energy with glazing ratio is, for practical purposes, negligible. This is because energy embodied is largely in the substructure and structure as can be seen in Fig. 10, which shows the typical distribution of embodied energy in major building elements. Thus the increase in embodied energy due to the use of, e.g., more insulation will be very small when it is compared with energy embodied in the whole building. Reliable estimates of embodied energy are hampered by a lack of data for initial and recurring embodied energy of office buildings and are very scanty, which does not allow useful generalisable conclusions to be drawn. Work so far carried out indicates that the initial embodied energy in an office building could be as much as 50% of the operating energy over a 25-year period [3]. Whilst

Fig. 9. Cost of annual heating energy and embodied energy as a function of overall heat loss coefficient of building fabric.

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Fig. 10.

Percentage distribution of embodied energy in major building elements.

this, in itself, is significant, the additional consideration of recurring embodied energy over the lifetime of a building further strengthens the case for proper design and selection of materials to reduce overall energy consumption in the construction industry. However, to quantify properly the amount of energy used in buildings over their life, there is a need for a rigorous and consistent collection and collation of initial and recurring embodied energy data. 8. Conclusions Using EDM, it is possible to determine operational energy, capital cost and embodied energy. Irrespective of the value of building parameters, it has been shown that it is important to define clearly the basis of energy consideration, i.e., primary energy or delivered energy. Capital and embodied energy costs as functions of glazing ratio do not vary much; this is because, for the base case considered, the cost of the windows is overwhelmed by the costs of other elemental components of the building. For the particular building considered, when primary energy is considered for an operating life of 30 years, the cost of operational energy is the lowest for glazing ratio in the range of 15% to 30%. However, when delivered energy only is considered, it reduces gradually to the lowest value as glazing ratio is reduced. The cost of operational energy is the lowest for a glazing ratio of about 15% for an operating life of 60 years; however, when delivered energy only is considered, the cost of operational energy reduces to the lowest value as glazing ratio is reduced to zero. The trends for the two cases involving the base case on the one hand and the case when the efficiency of heating has been reduced to 0.75, when delivered energy only is considered, are similar: cost reduces sharply when glazing ratio is reduced, reaching a minimum for a glazing ratio of about 15% and then rising slightly. Reductions in the values of illumination and heat loss

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coefficient also have the same effect. When the heat transfer coefficient and the infiltration rate are reduced to 0.25 W m⫺2 and 0.5 air changes per hour, respectively, the cost of operational energy is below the cost of embodied energy for values of glazing ratio lower than about 55%. However, for the base case, operational energy will be much higher than embodied energy. Appendix A Building and urban characteristics for the base case Description

Value

Description

Value

Latitude Orientation Room width Room depth Room height Window width on one side Windon height Tilt (from the horizontal) Transmission coefficient of glass Obstruction angle Urban horizon angle Height of sill Infilration rate Heating reference temperature Cooling reference temperature Hours per day Incidental gains without lights

52° East/west facing 36.5 m 16.0 m 3.0 m 28.5 m 1.6 m 90° 0.8 90° 0° 0.8 m 1.0 ach 21°C 25°C 10 15 W m⫺2

Illumination U-value of ceiling U-value of floor U-value of window (day) U-value of window (night) U-value of external wall Reflectance of walls Reflectance of ceiling Reflectance of door Reflectance of window Winter ventilation rate Summer ventilation rate Installed illuminance Heating plant efficiency Cooling plant efficiency Cost of electricity Cost of heating

300 lux 0.27 W m⫺2 K ⫺1 0.27 W m⫺2 K ⫺1 3.3 W m⫺2 K ⫺1 3.3 W m⫺2 K ⫺1 0.45 W m⫺2 K ⫺1 0.5 0.7 0.3 0.08 0.0 ach 0.0 ach 15 W m⫺2 0.65 2.0 3.7 unitsa 1.7 unitsa

a

The cost of delivered energy is assumed to be 1 unit. The cost of electricity and heating in primary energy cost basis is assumed to be 3.7 and 1.7 times the delivered energy cost, respectively.

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Appendix B

Figs. B1–B4

Fig. B1. Embodied energy of wall construction systems per element area [12].

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Fig. B2. Embodied energy of roof construction systems per element area [12].

Fig. B3. Embodied energy of floor construction systems per element area [12].

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Fig. B4. Embodied energy of thick concrete slab construction systems per element area [12].

References [1] Howard N, Sutcliffe H. Precious joules. Building 1994;11:48–50. [2] Atkinson C, Hobbs S, West J, Edwards S. Life cycle embodied energy and carbon dioxide emissions in buildings. Industry and Environment 1996;2:29–31. [3] Cole RJ, Kernan PC. Life-cycle energy use in office buildings. Building and Environment 1996;31(4):307–17. [4] Yohanis YG, Norton B. The early design model for prediction of energy and cost performance of building design options. Int J Solar Energy 2000;20(3):207–26. [5] Yohanis YG, Norton B. Utilization factor for building solar-heat gain for use in a simplified energy model. Appl Energy 1999;63(4):227–39. [6] Yohanis YG, Norton B. Estimating at the earliest stages of design the financial cost and operational energy requirement of buildings. J Finan Manag Property Construct 1998;3(2):41–58. [7] The building regulations (Northern Ireland). London (UK): Her Majesty’s Stationary Office, 1991. [8] Building Cost Information Service News, No. 13. London (UK): The Royal Institution of Chartered Surveyors, 1995. [9] Howard NP. Embodied energy and consequential CO2 in construction. In: Energy and Mass Flow in Buildings, Proceedings International Symposium of CIBW67. Vienna, Austria: Centre Scientifique et Technique du Batiment, 1996:161–76. [10] Tucker SN, Treloar GJ. Embodied energy in construction and refurbishment of buildings. In: Buildings and the Environment, Proceedings of International Conference. Garston, UK: Building Research Establishment, 1994:1–8. [11] Buchanan AH, Honey BG. Energy and carbon dioxide implications of building construction. Energy and Buildings 1994;20:205–17. [12] Lawson WR. Embodied energy of building materials, environment design guide. Manuka (Australia): Royal Australian Institute of Architects, 1996. [13] Norton B. Renewable energy, what is the true cost? IEE Power Eng J 1999;13:6–12. [14] The Chartered Institution of Building Services Engineers Guide Volume A: Design Data. London (UK): The Chartered Institution of Building Services Engineers, 1997.