Energy assessment of office buildings in China using China building energy codes and LEED 2.2

Energy and Buildings 86 (2015) 514–524

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Energy assessment of office buildings in China using China building energy codes and LEED 2.2 Hua Chen a , W.L. Lee b,∗ , Xiaolin Wang c a b c

Tianjin Key Laboratory of Refrigeration Technology, Tianjin University of Commerce, Tianjin, China Department of Building Services Engineering, Hong Kong Polytechnic University, Hong Kong, Hong Kong School of Engineering, University of Tasmania, Private Bag 65, Hobart 7001, TAS, Australia

a r t i c l e

i n f o

Article history: Received 12 August 2014 Received in revised form 15 October 2014 Accepted 18 October 2014 Available online 28 October 2014 Keywords: Office buildings Baseline criteria China building energy codes LEED Energy and energy cost savings

a b s t r a c t China building energy codes (CBEC) have been introduced for over two decades but little has been publicized in literature. LEED on the contrary is the most publicized building environmental assessment scheme. To enable better understanding of the compliance standards of CBEC, this paper presents the energy performance assessment results (represented by energy and energy cost savings) of three office buildings in China (one in Beijing and two in Shanghai) using the current versions of CBEC and LEED. The energy and energy cost savings of the three buildings were predicted based on hour-by-hour simulations using the weather data and energy tariffs of Beijing and Shanghai where the three studied buildings are located, and their actual building and system characteristics. The study revealed that LEED in general sets more stringent requirements than CBEC in indoor design conditions, building envelope characteristics and air-conditioning system features. Amongst various building end uses, energy use for air-conditioning was found dominating the assessment results, and the use of energy efficient measures not forming part of the baseline criteria, could lead to 2 to 5% reduction in the overall building energy use. The two schemes were benchmarked against BEAM Plus and their weaknesses were also unveiled. © 2014 Elsevier B.V. All rights reserved.

1. Introduction China had a rapid economic growth in the past two decades which led to growing concerns about her energy consumptions and environmental impacts [1]. Buildings are dominant energy consumers in China. In 2006, China’s building sector accounted for more than 25% of total primary energy use. This figure is expected to increase to 35% by 2020. Whilst for environmental impact, Liu et al. [2] developed an exergy assessment model to assess the total environmental impact of a residential building in Chongqing, China for a 50-year building lifecycle. It was found that building energy use accounts for 70 to 80% of the total environmental impact, mainly in the operation phase. Although efforts had been made to curb energy use in buildings since the early 1980s, a survey done in 2000 by the Ministry of Construction (MOC) indicated that little improvement had been made so far [3]. A decision was therefore made by the Chinese government to enhance the energy efficiency of buildings in China; leading to the introduction of a series of new building energy codes [4–10]. These energy codes set the minimum performance

∗ Corresponding author. Tel.: +852 2766 5852; fax: +852 27657198. E-mail address: [email protected] (W.L. Lee). http://dx.doi.org/10.1016/j.enbuild.2014.10.034 0378-7788/© 2014 Elsevier B.V. All rights reserved.

criteria on building envelope design, air-conditioning system features and lighting systems; aiming to reduce 50% of the building energy use as compared with those constructed in early 1980s. Although these building energy codes have been introduced for over ten years, little information is available in the public domain to enable better understanding of their performance criteria. A recent study by Yu et al. [11] found that depending on the reference scenario, mandating building energy codes in China can help reduce building energy use by 13 to 22%. This seems deviate largely from the target of 50%. An earlier study was conducted by the authors [12] to benchmark the Hong Kong and China energy codes on residential buildings. In the study, the Hong Kong Building Environmental Assessment Method (HK-BEAM) and China building codes were applied to evaluate the energy performance of a case study residential building. It was found that if in compliance with the China building energy codes (CBEC), the studied building would be 51.1% better in energy performance as compared to that of the HK-BEAM (earlier version of BEAM Plus). Another study was also conducted by the authors [13] to benchmark the energy assessment of three office buildings in China using the latest version of Leadership in Energy and Environmental Design Scheme (LEED 2.2) and BEAM Plus 1.1 (BEAM Plus). The study revealed that the assessment results of the three buildings were comparable

H. Chen et al. / Energy and Buildings 86 (2015) 514–524 Table 1 Similarities and differences between LEED and CBEC.

Application History First version Latest version Nature Assessment method Criteria

Approach Trade-off items Indoor design conditions Building envelope Building orientation Energy efficient measures Operation schedule

LEED

CBEC

Worldwide

China

1998 (Version 1.0) 2009 (Version 2.2) Voluntary

1986 (JGJ26-86) 2005 (GB50189-2005) Regulatory

Feature-specific (certification) Performance-based (point scoring) Energy cost budget

Feature-specific or Performance-based

No

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

No

Energy budget

despite the differences between the two schemes. Similar benchmarking study has also been conducted by Melo et al. [14]. In the study, a Brazilian regulation on building energy use was benchmarked against ASHRAE Standard 90.1-2007 (ASHRAE 90.1) [15] where LEED is based upon (discussed in a later section). Equivalence was found between the two instruments for commercial buildings, but for residential buildings, ASHRAE 90.1 resulted in higher energy consumption than the Brazilian regulation. Based on the above, questions are raised whether CBEC are stringent enough, and whether office buildings that are in compliance with CBEC are equally good if assessed by LEED. Thus, additional research is needed to benchmark the performance criteria of CBEC with LEED. Though a lot of life-cycle building energy assessment schemes have been developed and implemented elsewhere in the world [16–18], LEED was again chosen because benchmarking performance criteria of office buildings in China using LEED and CBEC would be of interest to most of their building designers and policy makers because: (1) LEED is developed by the US Green Building Council (USGBC) for the US Department of Energy [19–22] and so far is the most recognized building environmental assessment scheme, (2) United States and China each has almost the same geographical area and similar climate zones to enable a fair comparison on the performance standard, (3) LEED is the most-adopted environmental assessment scheme in China. Major building developers in China often undergo LEED assessment to demonstrate the improved environmental performance of their building assets in attracting international investors. This paper compares the similarities and differences of the two instruments (Table 1). The energy performance standards (represented by energy and energy cost savings) of three new office buildings in China (one in Beijing and two in Shanghai) using current versions of LEED and CBEC are also compared. As space heating in Beijing is provided by the urban central heating network, but the same does not apply to Shanghai, to enable a side-by-side comparison of the two schemes, space heating is excluded in this study. The simulation package, HTB2 [23] and BECON [24], was employed to evaluate the energy use for the air-conditioning systems, as well as the whole building. The latest energy tariffs of the two cities (Beijing and Shanghai) were adopted for calculation of the energy costs. The results of this

515

benchmarking study will provide useful information to China policy-makers and building designers for further development of CBEC.

2. Overview of LEED 2.2 (LEED) LEED is a voluntary scheme developed by the US Green Building Council (USGBC) to define and measure green buildings for the US Department of Energy. Points are awarded to provide recognition of green efforts. The pilot version (LEED 1.0) [12] for new constructions was first launched at USGBC Membership Summit in August 1998. The second version (LEED 2.0) [13], revised based on modifications made during the pilot period, was released in 2000. Since then, LEED continues to evolve to respond to the needs of markets and to expand to cover other building types and constructions including: LEED-NC [25] for new construction, LEED-EB [26] for existing building, LEED-Homes for residential dwellings [27], LEED-CI [28] for commercial interiors and LEED-CS [29] for core and shell. The latest version of LEED for new construction (LEED 2.2) was released in February 2010 [22]. Latest versions for other building types including schools, homes, etc. were also released in 2010 [30]. So far LEED is the most recognized building environmental assessment scheme. The scheme has registered projects in progress in 24 different countries, including Canada, Brazil, Mexico, India and China and the World Green Building Council—an affiliation of seven national green building councils, including the United States. In LEED 2.2, the minimum energy performance requirement is to fulfill the prescriptive requirements of relevant codes as defined by the US Department of Energy. To score points, the energy performance of the proposed building has to be in compliance with ASHRAE 90.1-2007 standard [24] which specifies details of the performance-based approach and the energy modeling protocol. The performance-based approach is set based on the performance rating method which is aiming to offer points for the use of more favorable building orientation, additional design measures and strategies. The energy modeling protocol specifies the use of an energy cost budget approach for assessing the energy performance of a proposed design. This approach evaluates the energy performance based on the annual energy cost saving relative to an energy cost budget. In the standard, simulation programs tested according to ASHRAE Standard 140 [31] that can provide detailed hour-by-hour energy analysis of buildings have been specified for the prediction of the annual energy use and budget. The energy tariff where the proposed building is located will be used for calculation of the energy cost, and thus the energy cost saving. A baseline building incorporating with a range of criteria that are specified in the standard is adopted for calculation of the energy cost budget. The baseline criteria vary by climate zones. The climate zones are classified by heating and cooling degree days which can be grouped as very hot-humid/dry; hothumid/dry; warm-humid/dry; mixed-humid/dry; mixed-marine; cool-humid/dry/marine; cold-humid/dry; very cold; and subarctic. The criteria focus on the indoor design conditions, the maximum allowed heat transfer coefficients and shading coefficient for different opaque elements, the maximum allowed window-to-wall ratio, and the air-conditioning system characteristics and performance. Trade-offs are allowed within certain design features including the use of more favorable orientation, better envelope design, better performed air-conditioning equipments, smaller installed lighting power intensities and energy efficient strategies. This is to provide flexibilities in making trade-offs among performances of different envelope assemblies and service systems for buildings.

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3. Overview of China building energy codes (CBEC) In China, energy efficiency efforts were started from the early 1980s in response to the continuous increase in energy use of the residential sector [32], leading to the introduction of a series of building energy standards and codes. The Chinese Ministry of Construction (MOC) is the leading organization responsible for formulating national standards to curb energy use of buildings in China. But considering the wide variations amongst provinces in climatic, energy supply and demand, economic developments, and environmental conditions, 15% deviation from the national standards is allowed to strike a balance between energy efficiency and cost effectiveness. Thus, regional and/or provincial authorities are encouraged to set up local standards to supplement the national standards. The first building energy code “Energy Conservation Standard for New Heating in Residential Buildings JGJ26-86” was introduced in 1986. The revised version, JGJ26-95 sets a higher energy saving standard was issued in December 1995 [33]. In 2001, a series of building energy codes for energy efficiency of residential buildings were introduced. Examples include technical specification for energy conservation renovation of existing heating residential building (JGJ 129-2001) [5], standard for energy efficiency inspection of heating residential Buildings (JGJ 132-2001) [6] and the design standard for energy efficiency of residential buildings in hot summer and cold winter zone (JGJ 134-2001) [7]. All these codes are for cold and very cold zones mainly for the northern part of China. In 2003, a design standard for energy efficiency of residential buildings in hot summer and warm winter zone (JGJ 75-2003) [8] was issued for southern China. Following the introduction of energy codes for residential buildings, those for public and commercial buildings were started to introduce in 2004. The standard for lighting design in buildings (GB50034-2004) [9] was implemented in 2004, mandating the provisions of energy efficient lighting design. In 2005, the design standard for energy efficiency of public buildings (GB501892005) [10] became effective. The standard prescribes design and selection standard for envelope components, and heating and air-conditioning systems. Besides energy codes, the evaluation standard for green building (GB/T 50378 2006) [34] was issued in 2006. It is used to assess the overall environmental performance of residential, commercial and public buildings. The energy-related requirements are cross-referenced to Standard GB50189-2005; thus this standard, together with GB50034-2004, are often collectively referred as the China building energy codes for commercial and public buildings (CBEC). Similar to LEED, CBEC adopts two compliance approaches: the prescriptive and the performance-based approaches. The prescriptive approach also stipulates requirements by climate zones, which include the maximum allowed heat transfer coefficients for different building envelope components and shape factors (ratio of area of building envelope to the floor area), as well as the maximum allowed window-to-wall area ratio by orientations. The climate zones are classified as very cold; cold; hot summer and cold winter; hot summer and warm winter; and moderate zones [35]. However, as compliance with CBEC is regulatory, the use of the performance-based approach is not for scoring points. It is used as an alternative compliance route when the design of a building cannot meet the prescriptive requirements. This alternative approach adopts the energy budget approach of which again is determined by simulation. However, no special requirements have been specified for the simulation program. The energy budget is the maximum allowed heating/cooling energy use of a building determined based on a baseline building whose energy performance barely meets the prescriptive requirements in CBEC. No consideration is given to the energy tariffs. Trade-offs is also allowed among performances of

different envelope assemblies and service systems. Judging from the compliance approaches and criteria adopted, it is noted that CBEC is modeled from LEED [36]. 4. The baseline characteristics In the use of the performance-based approach, both CBEC and LEED have specified a range of characteristics for incorporation into the baseline building for determination of the energy budget. Table 1 compares the baseline characteristics of the two schemes and the proposed design characteristics of three case study buildings (BJ1, SH1 and SH2). As the three case study buildings have been used in an earlier study and details have already been reported [13], to avoid duplication, only characteristics that are essential for illustrating the details of this study are presented in Table 1 and later in Section 5. 4.1. Indoor design conditions Building energy use is substantially affected by the indoor design conditions including: the space set-point temperature (dry bulb) and relative humidity, ventilation rate, occupancy density, and lighting and equipment power intensities [37]. Table 1 illustrates that the baseline indoor design conditions for CBEC are different to the proposed design values. They are in fact threshold values specified in GB50189-2005. Conversely, LEED sets “same as proposed design” values as baseline indoor design conditions and thus their values are identical; with the exception of the lighting power density which is determined based on the maximum allowed lighting power densities according to building or space types without occupancy control (note that it is a coincident that the LEED’s baseline values are same as the design values of the proposed buildings). Accordingly, the use of more energy efficient indoor design conditions can be used as trade-off items in CBEC, but not in LEED. 4.2. Building envelope LEED and CBEC adopts similar parameters in defining the baseline envelope characteristics including the assembly types provided; shading coefficient (SC) of fenestration; and heat transfer coefficients required for the building components including wall, roof, fenestrations and floor construction. As mentioned earlier, the baseline criteria in CBEC vary by climate zones. Stricter requirements are set for the colder zone, and thus smaller heat transfer coefficients for building components and SC for fenestrations are set as baseline for buildings in Beijing (cold zone) than in Shanghai (hot summer and cold winter zone). However, in CBEC, irrespective of the location of the proposed building, the total fenestration area is limited to 70% of the gross wall area; and the baseline heat transfer coefficients of building components and SC of fenestration are set according to the window to wall ratio (WWR) determined by the actual building orientation. The higher the WWR, the smaller the U-values and SC, and thus different baseline values have been set for the two studied buildings in Shanghai to address their difference in WWR. Similar to CBEC, LEED sets stricter requirements for the colder zone, and thus smaller heat transfer coefficients for building components and SC for fenestrations are set as baseline criteria for buildings in Beijing (mixed-dry zone) than in Shanghai (warm-dry zone). The total fenestration area is limited to 40% of the gross wall area or the actual fenestration area whichever is smaller; and is assumed to be uniformly distributed across four distinct building orientations. In respect of building orientation, the baseline building performance under four orientations including its actual

H. Chen et al. / Energy and Buildings 86 (2015) 514–524

orientation plus rotating the proposed building by 90, 180, and 270 degrees and their performance will be equally considered. The above indicates that LEED and CBEC allow trade-off for the use of more favorable envelope design and LEED allows also more favorable building orientation. 4.3. Air-conditioning system LEED specifies the baseline parameters for determining the baseline air-conditioning system performance. It includes the minimum coefficient of performance (COP) of chillers, the type of heat rejection system, the numbers of chiller to be used, the maximum allowed fan power per unit flow rate (W/L/s), and the maximum allowed pump power per unit flow rate. For the use of chilled water system, electric chillers are assumed for the baseline building irrespective of the cooling energy source. The minimum required COP is defined by the type of chiller, the heat rejection method and the capacity range. The maximum allowed fan power is set based on the motor efficiency, the total supply air volume and the type of airside system classified as constant air volume (CAV) or variable air volume (VAV) systems. Chilled water and condenser water pump powers are defined based on their corresponding water flow rate and circulating medium. LEED also specifies for the baseline building the air-conditioning system sizing method, the system selection method (based on the building type, building area and number of floors) and the plant capacity and configurations (based on the total conditioned area). However, they also allow the use of same as proposed design characteristics as baseline characteristics except those energy efficient measures. Similar to LEED, the baseline parameters specified in CBEC for the air-conditioning system include the coefficient of performance (COP) of chillers, fan power per unit flow rate (W/L/s), pump supply efficiency ratio (ER), etc. The criteria for setting the minimum chillers’ COP are same as that for LEED. The maximum allowed fan power per unit flow rate is set based on the type of air-side system (CAV or VAV), and the type of filter adopted. The minimum ER is considered based on the water temperature difference, pump efficiency and the system pressure loss. Other parameters including the type of air-conditioning system adopted, the system sizing method, the plant capacity and the plant configurations are same as proposed design characteristics. CBEC has not mentioned about the use of energy efficient measures. The above indicates that both LEED and CBEC allow the use of more energy efficient equipments and measures as trade-off. 4.4. Building operation schedule The energy benefit of the use of an energy efficient measure is dependent on the simultaneous building and system operation schedules. LEED and CBEC specified the occupancy, lighting, equipment and other operation schedules based on building types, which must be the same for the baseline and proposed design cases. However, in modeling the non-standard energy efficient measures such as natural ventilation, demand control ventilation etc., LEED allows the use of the proposed design schedules but prior approval must be obtained from the rating authority. The same is not allowed in CBEC. For the provision of automatic lighting controls, the lighting schedules remain the same but a power adjustment factor is allowed for the use of different types of control system. 5. The case study buildings Three office buildings in China, one located in Beijing (BJ1) and the other two in Shanghai (SH1 and SH2), were selected as case study buildings for benchmarking the performance standard of

517

CBEC and LEED. The considerations for selection were: (i) the three buildings are comparable in building characteristics and (ii) they are the first group of prime office buildings in China designed and constructed to qualify for LEED certifications [13]. Although the sample size may seem small, considering the limited availability of prime office buildings in Beijing (1929,245 m2 ) [38] and Shanghai (1077,037 m2 ) [39], the three studied buildings are already representing characteristics of 8.7% building stocks of this type. Building and system characteristics, as well as baseline and proposed design parameters of the three studied buildings have already been compared in our previous study [13]. To avoid duplication, only key characteristics are summarized below. 5.1. Building and system characteristics BJ1 is a two-tower building with 22 storeys aboveground and 2 storeys underground. The building height is 108 m above the ground with 168,000 m2 total floor area. 3/F to 22/F are for office use and other floors are for retail purposes. The central chilled water system comprises eight water-cooled centrifugal chillers (each 3500 kW), together with eight cooling towers and condenser water pumps (plus one standby) for chiller heat rejection. Chilled water is distributed through a primary/secondary piping network to variable air volume (VAV) air handling units (AHU) for offices and fan coil units (FCUs) for retails. SH1 is 25 storeys aboveground and 3 storeys underground. The building height is 101.4 m above the ground. 4/F to 25/F are for office use, 1/F to 3/F are for retail use and other floors are car park space. The total floor area is 38,000 m2 . Five air-cooled screw chillers (each 1258 kW) are used in the building. Chilled water is distributed through a single-loop pumping system to FCUs of the whole building. SH2 has the same number of aboveground and underground floors as SH1. The building height is 104.8 m above the ground. 3/F to 25/F are for office use, 1/F and 2/F are for retail use and other floors are car park space. The total floor area of the building is 54,400 m2 . The central chilled water system comprises five water-cooled chillers, together with five cooling towers and condenser water pumps (plus one standby) for heat rejection. Three chillers adopt centrifugal compressors (each 2990 kW) and the other two adopt screw compressors (each 1406 kW). Chilled water is distributed through a primary–secondary piping network to VAV AHUs for offices and FCUs for retails. All the three buildings are provided with occupancy control for lighting system, heat pipe for exhaust air heat recovery and demand control for fresh air supply. 5.2. Design parameters Table 2 summarizes the baseline and proposed design characteristics of the three studied buildings. Between LEED and CBEC, it is noted that LEED in general sets higher baseline values than CBEC, except for indoor set-point temperature, which in CBEC, is 25 ◦ C for retail premises and 26 ◦ C for office premises. But as LEED sets proposed design values as baseline indoor design conditions, to score points in LEED in this aspect is difficult; which can only be achieved by improving the envelope features, air-conditioning system features and lighting control. Amongst the three studied buildings, there are favorable and unfavorable design characteristics as compared to the baseline values in LEED and CBEC. For BJ1, the unfavorable designs are that COP of chillers are lower than the baseline value in LEED though higher than that in CBEC; and the chilled water pump power per unit flow rate is higher than the baseline values in the two schemes. Whilst for favorable design, the fan power and condenser water pump power

518

Table 2 Baseline (by CBEC and LEED) and proposed design characteristics of the three studied buildings. Building

Parameters

BJ1

Temperature (◦ C)

Office Retail

Relative humidity (%) Office Retail Ventilation rate (l/s/person) Office Retail Office Lighting power density Retail (W/m2 ) Office Equipment power density Retail (W/m2 ) Wall heat transfer coefficient (W/m2 K) Roof heat transfer coefficient (W/m2 K) Window heat transfer coefficient (W/m2 K) Office Shading coefficient Retail Office Window-to-wall ratio Retail System type Numbers of chiller and type Chillers’ COP Fan power (W/(l/s)) Chilled Water pump power (W/(l/s)) Condenser Water pump power (W/(l/s)) Lighting control Heat recovery Fresh air demand control Occupancy (m2 /person)

Envelop features

Air-conditioning features

Energy efficient features

Baseline

Design

CBEC

24 24 55–65 12.5 12.5 11 11 10 15 20 20 1.4 0.31 3.3 0.4 0.4 0.6 0.88 VAV and FCU 8 Water-cooled 5.6 0.8 0.54 0.37 Inc. Inc. Inc.

26 25 50–60 4 3 8.3 5.6 11 12 20 13 0.34 0.43 1.9 0.45/0.55 (N) 0.55 0.6 0.7 VAV and CAV 8 Water-cooled 5.1 2.1 0.52 0.52 Nil Nil Nil

SH2

Proposed

Baseline

LEED

Design

CBEC

24 24 55–65 12.5 12.5 11 11 10 15 20 20 0.64 0.37 2.84 0.4 0.4 0.4 0.4 VAV and CAV 8 Water-cooled 6.1 1.9 0.35 0.3 Nil Nil Nil

25.5 26 55–65 12.5 25 10 7 10 15 20 20 0.51 0.51 3.3 0.3 0.78 0.5 0.3 FCU 5 Air-cooled 3.2 0.43 0.47 NA Inc. Inc. Inc.

26 25 40–65 4 3 8.3 5.6 11 12 20 13 1.0 0.7 2.8 0.4/0.5 (N) 0.4/0.5(N) 0.5 0.3 CAV 5 Air-cooled 2.8 2.1 0.52 NA Nil Nil Nil

Proposed

Baseline

LEED

Design

CBEC

LEED

25.5 26 55–65 12.5 25 10 7 10 15 20 20 0.64 0.37 3.41 0.25 0.25 0.4 0.4 CAV 5 Air cooled 2.8 1.9 0.35 NA Nil Nil Nil

24 24 55–65 12.5 12.5 11 11 10 15 20 20 0.34 0.43 1.9 0.4 0.4 0.6 0.88 VAV and FCU 5 Water-cooled 6.7 1.2 0.3 0.49 Inc. Inc. Inc.

26 25 40–65 4 3 8.3 5.6 11 12 20 13 1.0 0.7 2.8 0.4/0.5 (N) 0.4/0.5(N) 0.6 0.7 VAV and CAV 5 Water-cooled 5.1 2.1 0.52 0.52 Nil Nil Nil

24 24 55–65 12.5 12.5 11 11 10 15 20 20 0.64 0.37 3.41 0.25 0.25 0.4 0.4 VAV and CAV 5 Water-cooled 6.1 2.0 0.35 0.3 Nil Nil Nil

H. Chen et al. / Energy and Buildings 86 (2015) 514–524

Indoor design conditions

SH1

Proposed

H. Chen et al. / Energy and Buildings 86 (2015) 514–524

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Table 3 OTTV (W/m2 ) of the baseline and proposed design buildings. Building

BJ1

Premises

Office

Retail

Office

Retail

Office

Retail

43.2 23.8 27.7

75.1 49.5 75.3

37.8 20.7 24.6

23.2 11.0 31.5

37.7 25.0 35.0

55.0 28.3 58.1

Baseline

CBEC LEED

Proposed design

SH1

per their corresponding unit flow rates are lower than the baseline values in both schemes. For SH1, the unfavorable design is that the chilled water pump power per unit flow rate is higher than the baseline value in LEED though lower than that in CBEC. The favorable designs are that COP of chillers is higher and the fan power per unit flow rate is lower than the baseline values in the two schemes. For SH2, the unfavorable design is that the condenser water pump power per unit flow rate is higher than the baseline value in LEED though lower than that in CBEC. The favorable designs as compared to the baseline values of the two schemes are that COP of chillers are higher while the fan power and chilled water pump power per unit flow rate are lower.

SH2

The proposed design and baseline OTTV values of the three studied buildings are presented in Table 3, illustrating that LEED’s baseline values, both for office and retail premises, are smaller than that of CBEC and the proposed design values. This is reasonable because LEED sets more stringent requirements on heat transfer coefficients, WWR and shading coefficient (Table 1). However, between CBEC’s baseline and the proposed design values, it is noted that the proposed design values are higher than the baseline values for the retail premises but lower for the office premises, indicating that Chinese developers tried to trade-off the envelope characteristics between retail and office premises to get higher WWR at the retail premises for aesthetic reason.

6.3. Energy use for various end uses 6. Benchmarking results 6.1. General review of LEED and CBEC Review of the two instruments in the above sections in terms of their backgrounds, metrics and baseline characteristics indicate that they possess a certain level of similarities and differences which are summarized in Table 1.

To benchmark the performance standard of LEED and CBEC, energy simulations by the use of HTB2 [23] and BECON [24] were conducted. There were used because they have been tested according to ASHRAE 140 [31]. The baseline and proposed design parameters were assumed for the three studied buildings to commit the energy simulations. The simulated results are compared in Table 4 and are then used to calculate the percentage reduction of annual energy use for various end uses as shown in the following equation:

6.2. Overall thermal transfer value (OTTV) In LEED and CBEC, heat transfer coefficients, window-to-wall ratios and shading coefficients have been used as the controlled parameters to set the baseline characteristics of the building envelope. But as the values set for the building components are different in the two schemes, there is a need to convert the two performance metrics into a common parameter for a fair comparison. OTTV, being widely used for quantifying the envelope performance in a considerable number of regimes [40], and can be easily calculated based on the building envelope’s characteristics, is therefore chosen as the common parameter. In this study, the calculation was based on Hong Kong’s OTTV code [41], and the climatic data of Beijing and Shanghai were obtained from the meteorological database of the national meteorological center of China [42].

Ei =

Ei,d − Ei,b × 100% Ei,b

(1)

where subscript i represent the different end uses including airconditioning system (AC), lighting installation (LGT), and small power (SPW) and the total building energy use (ALL); and subscripts b and d represent the baseline and proposed design values, respectively. The percentage reductions in annual energy use for various end uses are also summarized in Table 4, indicating that the overall savings and the savings by individual end-uses differ by buildings. Fig 1 relates EAC and EALL of the three studied buildings assessed by LEED and CBEC. It is noted that they are strongly correlated (r2 is equal to 0.99 for LEED and 0.88 for CBEC) to indicate the level

Table 4 Summary of annual energy use and savings of various end-uses. Building

BJ1

SH1

SH2

End-use

AC LGT SPW ALL AC LGT SPW ALL AC LGT SPW ALL

Annual Energy Use (kW h)

Savings

Proposed design

CBEC

LEED

CBEC (%)

LEED (%)

7.98E6 6.11E6 9.78E6 2.39E7 2.35E6 1.27E6 1.98E6 5.60E6 4.18E6 1.95E6 3.55E6 9.68E6

1.16E7 7.19E6 9.38E6 2.82E7 5.54E6 1.33E6 1.92E6 8.79E6 6.29E6 2.0E6 3.4E6 1.17E7

1.09E7 9.44E6 9.78E6 3.01E7 3.92E6 1.84E6 1.98E6 7.74E6 4.49E6 2.93E6 3.55E6 1.09E7

26.8 7.9 −4.3 20.2 57.6 4.5 −3.1 36.3 33.5 2.5 −4.4 17.3

21.4 29.9 0.0 18.9 40.1 31.0 0.0 27.6 6.9 33.4 0.0 11.2

Remarks: AC = air-conditioning; LGT = lighting; SPW = small power equipment; ALL = Overall; E = exponential.

H. Chen et al. / Energy and Buildings 86 (2015) 514–524

(4)

The positive values indicate that the baseline performance requirement of LEED is more stringent than CBEC, in particular the office premises, ranging between 13% and 28% for the three studied buildings. The big range, especially for SH1 and SH2, indicates that there is a need to further tighten the performance standard of CBEC for the cold winter warm summer zone where these two buildings are located.

44.7 (16.5) 258.7 (36.7) 73.1 (26.0) 106.2 (42.7) 276.8 (33.1) 129.4 (39.9) 53.9 (2.8) 403.2 (18.0) 85.6 (6.8) 46.4 (19.6) 285.3 (42.6) 78.4 (31.0) 155.2 (60.8) 356.0 (48.0) 182.5 (57.4) 76.3 (31.3) 505.5 (34.6) 120.0 (33.5)

CBEC

37.3 163.8 54.1 60.9 185.1 77.8 52.4 330.8 79.8 12.9 6.1 11.5 22.4 10.7 20.7 27.5 10.6 24.0 41.2 (−3.4) 85.6 (11.7) 47.1 (0.2) 64.1 (23.2) 132.9 (49.1) 73.4 (29.6) 46.2 (4.5) 86.5 (21.3) 50.9 (22.0)

LEED CBEC

47.3 (9.9) 91.2 (17.1) 53.2 (11.7) 82.6 (40.4) 148.9 (54.5) 92.6 (44.2) 58.4 (30.8) 96.8 (29.6) 67.0 (40.7) SH2

EAC−A,CBEC − EAC−A,LEED × 100% EAC−A,CBEC

(3)

SH1

× 100%

Office Retail Total Office Retail Total Office Retail Total

EAC−pk,CBEC

BJ1

EAC−A,b =

EAC−pk,CBEC − EAC−pk,LEED

42.6 75.6 47.0 49.2 67.7 51.7 44.1 68.1 39.7

(2)

where subscript J represents different end uses including peak power consumption (pk) and annual energy use (A). Subscripts b and d represent the baseline and proposed design values, respectively. Simulated EAC−A,d , EAC−pk,d , EAC−A and EAC−pk for the three studied buildings are summarized in Table 5. It can be seen that EAC−pk and EAC−A are high for SH1 and SH2. This is due to the adoption of energy efficient building envelope, equipments (fans, pumps and chillers), and systems (exhaust air heat recovery and demand side control system). Comparing to the baseline buildings of CBEC; EAC−pk ranges between 9.9 and 54.5%, and EAC−pk ranges between 19.6 and 60.8%. To benchmark the baseline energy performance set by LEED and CBEC, EAC−pk,b and EAC−pk,b of the two schemes, calculated by the use of Eqs. (3) and (4) are compared also in Table 5. EAC−pk,b =

Pro-posed design

× 100%

Baseline (EAC−pk (%))

EAC−J,b

Building

EAC−J,d − EAC−J,b

Table 5 Air-conditioning system energy use of the baseline and proposed design buildings.

EAC−J =

EAC−pk,b (%)

Since the level of savings achieved by air-conditioning system dominates the overall savings, it is important to further analyze the influence of the baseline parameters on the peak power consumption and the annual energy use by the air-conditioning system of the three studied buildings. HTB2 [23] and BECON [24] are again employed for the annual cooling load and annual energy use predictions for the baseline and proposed design buildings. The percentage reductions in peak power consumption (EAC−pk ) and annual energy use (EAC−A ) for air-conditioning are calculated using the following equation:

Pro-posed design

6.4. Air-conditioning system energy use

Annual energy use (kW h/m2 )

of savings achieved by the air-conditioning system dominates the overall savings.

Peak power consumption (kW/m2 )

Fig. 1. Relating % reduction in annual total and annual air-conditioning energy uses.

Baseline (EAC−A (%))

LEED

3.7 9.3 6.8 31.6 22.2 29.1 29.4 20.2 28.7

EAC−pk,b (%)

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521

6.5. Influence of baseline parameters From the above analysis, LEED obviously sets a higher baseline requirement for air-conditioning systems in comparison to CBEC. To identify if their baseline criteria differ largely from each other, and if the performance standards are building-dependent, a ratio (RAC(X) ) is introduced to relate the annual energy use savings (EAC(X) ) achieved by equipment X assessed by the two schemes. A value close to 1 indicates that there is no major difference in baseline criteria in the two schemes. Too large or too small a value indicates a major difference. This ratio also enables bivariate correlation analysis to identify if the performance standard is building-dependent. The ratio is mathematically shown in Eq. (5) and calculated results are summarized in Table 6. RAC(X) =

EAC(x),LEED

(5)

EAC(x),CBEC

where EAC(x), CBEC and EAC(x),LEED are the percentage reduction in annual energy consumption achieved by equipment X when assessed by CBEC and LEED, respectively. It can be seen in Table 6 that RAC(X) ranged between −3.8 and 14 to indicate the baseline criteria differ largely between the two schemes. Correlation of savings achieved by the six equipments were analysed using the bivariate correlation feature in SPSS. The correlation coefficients, significance levels,andthe absolute value of the proximity correlation coefficient

2 RN

of each building

determined by Eq. (6) are shown in Table 7. 2 = RN



r2 (m − 1)

(6)

where r is the correlation coefficient of one building with other buildings, N is the building number (=1 to 3), and m is the number of buildings (3 in this study). The proximity correlation coefficient is to indicate the proximity of one building and other buildings—the higher the value; the closer the link to indicate the constituent equipment savings are buildingindependent. As the system used and thus the equipments installed at SJ1 differ from BJ1 and SJ2, the equipments have to be categorized into two groups in the calculation of the proximity correlation coefficient. Group 1 are AHU and FCU fans, chillers and primary chilled water pumps, and Group 2 are secondary loop pumps, condenser water pumps and cooling towers. 2 of Group 1 equipIt is noted in Table 7 that the computed RN ments for the three buildings are between 0.65 and 0.85, while the same for Group 2 equipments is 0.265. The significance levels are between 0.216 and 0.829. Although there is no definite baseline coefficient to indicate a strong correlation, it is generally accepted that for the range of significance level, a coefficient of 0.5 is of moderate magnitude [43], and can be regarded as a moderate correlation. Thus, bivariate correlation analysis indicates that savings achieved by Group 1 equipments are building-independent. However, no significant correlation was found for Group 2 equipments, indicating that the performance standards set for these equipments vary largely between buildings and thus they are building-dependent. This observation is reasonable because LEED specifies the pumps and cooling tower fan requirements without taking into account the system pressure losses and circulating medium temperature difference.

Fig. 2. Tariffs of Beijing and Shanghai.

[44] (Fig. 2) were computed and compared in Table 8. Note that the tariffs are not a fixed value within a day and thus hourby-hour calculation of savings is needed. Given that only LEED adopts the energy cost saving approach, to evaluate if the benchmarking results will be affected by the tariff system in different cities in China, the energy and energy cost savings of the three studied buildings assessed by LEED and CBEC are compared in Fig. 3. The results show that they correlate well with each other with a coefficient of determination (r2 ) equals 0.99, indicating that the tariff system will not introduce any influence on the energy cost savings as long as the same tariff system is used for calculating the energy budget of the baseline and proposed design cases.

6.7. Influence of energy efficient measures Energy efficient measures including heat recovery, automatic lighting control and demand side control systems were adopted in the three studied buildings to further enhance energy savings. The impact of these measures on the energy use for the proposed design of the air-conditioning systems and the buildings were predicted and summarized in Table 9. The results show that further saving of energy use for air-conditioning is between 5% and 19.5%, which corresponds 2.2 to 5% of the total building energy and energy cost savings.

6.6. Total energy and energy cost saving The total energy and energy cost savings of the three studied building based on the energy tariffs of Beijing [43] and Shanghai

Fig. 3. Relating building energy and energy cost savings (%).

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H. Chen et al. / Energy and Buildings 86 (2015) 514–524

Table 6 Annual energy savings achieved by various air-conditioning equipments. BJ1 (RAC(x) )

Equipment

SH1 (RAC(x) )

CBEC

LEED 48%

35% (1.37) 23% (0.17) 38% (−0.76) −4% (14) 34% (−0.2) −5% (−3.8)

AHU and FCU fans Chillers Primary chilled water pumps Secondary chilled water pumps Condenser water pumps Cooling towers

SH2 (RAC(x) )

CBEC

LEED

72% (0.74) 8% (0.5) 5% (−0.8) – (–) – (–) – (–)

4% −29% −56% −7% 19%

CBEC

53%

LEED

27% (0.74) 25% (0.16) 36% (0.08) 80% (0.26) 17% (−2.9) 4% (0.5)

4% −4% – – –

20% 4% 3% 21% −50% 2%

Table 7 Proximity coefficient (r) matrix. Equipment group

Building

1

BJ1 SH1 SH2 BJ1 SH2

2

BJ1

SH1

 

2 Absolute Proximity Coefficient RN

SH2

r

Sig

r

Sig

r

Sig

1 0.9 0.943 1 0.265

n/a 0.287 0.216 n/a 0.829

0.9 1 0.703 n/a n/a

0.287 n/a 0.503 n/a n/a

0.943 0.703 1 0.265 1

0.216 0.503 n/a 0.829 n/a

0.85 0.65 0.69 0.265 0.265

Remarks: r = proximity correlation coefficients; sig = significance.

Table 8 Building energy use and energy cost savings. Building

Annual energy use (kW h) (energy use saving)

Annual energy cost (RMB) (energy cost saving)

Proposed design

CBEC

LEED

Proposed design

CBEC

LEED

BJ1

2.39E7 5.54E6

SH2

9.67E6

3.01E7 (15.2%) 7.74E6 (28.8%) 1.07E7 (13.0%)

2.25E7

SH1

2.82E7 (20.6%) 9.04E6 (38.7%) 1.35E7 (28.3%)

2.66E7 (20.8%) 7.44E6 (41.5%) 1.23E7 (26.7%)

2.84E7 (15.4%) 5.98E6 (27.4%) 1.02E7 (11.9%)

4.35E6 8.98E6

Remarks: E = exponential.

7. Benchmarking LEED, CBEC and BEAM Plus As mentioned earlier, energy assessment of office buildings in China using LEED 2.2 and BEAM Plus 1.1 [45] was benchmarked in our earlier study [13]. Considering that seeking BEAM Plus certification is increasingly being made a condition for building projects in China, it is an advantage if appropriate benchmarking of the baseline building performance of the three schemes can be established. Accordingly, results of previous works and the above are summarized in Table 10 for the information of policy makers and building designers.

It is worthy to note that previous works have been done to convert the performance metrics of the three schemes into equivalent parameters to enable the comparison in Table 10. To avoid duplications, the methodologies adopted are not repeated in this study. Table 10 summarizes the baseline building performance of the three schemes, illustrating that for the average values of the three studied buildings, LEED’s baseline AC energy use and annual building energy use not only lower than that of CBEC (29.5% and 12.2%), but BEAM Plus as well (33.3% and 15.5%) to conclude that LEED sets the most stringent baseline criteria amongst the three schemes.

Table 9 Energy and energy cost savings by energy efficient measures (EEM). Building

BJ1

EEM

Provided

Not provided

Provided

41.8 (19.5%) 155.0 (5.0%) 143.4 (5.0%)

51.9

77.8 (8.0%) 175.0 (3.8%) 137.4 (4.4%)

2

AC energy use (kW h/m ) Total energy use (kW h/m2 ) Total cost (RMB/m2 )

Remarks: % savings for the provision of EEM are in bracket

SH1

163.2 150.9

SH2 Not provided 84.6 181.9 143.7

Provided 79.8 (5.0%) 184.5 (2.2%) 171.4 (2.3%)

Not provided 84.0 188.7 175.4

H. Chen et al. / Energy and Buildings 86 (2015) 514–524

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Table 10 Baseline building energy use of the three studied buildings by LEED, CBEC and BEAM Plus. Description

AC energy use (kW h/m2 )

LEED CBEC BEAM Plus

Total annual energy use (kW h)

LEED CBEC BEAM Plus

Total annual energy cost (RMB)

Building

Scheme

LEED CBEC BEAM Plus

Average difference

BJ1

SH1

SH2

73.1 78.4 (7.25%) 94.1 (28.73%) 3.01E + 07 2.82E + 07 (−6.31%) 3.36E + 07 (11.63%) 2.84E + 07 2.66E + 07 (−6.34%) 2.88E + 07 (1.41%)

129.4 182.5 (41.04%) 188.6 (45.75%) 7.74E + 06 9.04E + 06 (16.80%) 1.03E + 07 (33.07%) 5.98E + 06 7.44E + 06 (24.41%) 6.76E + 06 (13.04%)

85.6 120 (40.19%) 107.3 (25.35%) 1.07E + 07 1.35E + 07 (26.17%) 1.09E + 07 (1.87%) 1.02E + 07 1.23E + 07 (20.59%) 1.04E + 07 (1.96%)

– 29.5% 33.3% – 12.2% 15.5%

12.9% 5.5%

Remarks: % difference with LEED in bracket.

Between CBEC and BEAM Plus, the baseline performance standard of CBEC is slightly higher, but if take into account the energy tariff, the result is reversed. This is reasonable because the energy tariff in China is not a fixed value within a day.

[6]

[7]

8. Conclusion A side-by-side comparison on the baseline criteria and assessment results of LEED and CBEC were conducted. The comparison was done based on the actual building and system characteristics of three studied buildings. The baseline criteria of LEED were found more stringent than CBEC in building envelope features, indoor design conditions and most of the air-conditioning features, but are comparable to CBEC on the lighting power and equipment power densities. The baseline criteria of the two schemes do not include the use of energy efficient measures. The difference in baseline criteria of the two schemes led to a difference in assessment results to enable identification of the weakness of the two schemes. For CBEC, the assessment results indicated there was a potential to tighten CBEC’s baseline criteria on the cold winter warm summer zone. Whilst for LEED, statistical analysis based on proximity correlation coefficient indicated that the performance standards set for secondary and condenser pumps, as well as cooling tower fans, did not take into account individual building’s characteristics. Further investigations on the breakdown of the energy and energy cost savings indicated that they were dominated by the performance of air-conditioning system, but not the difference in tariff systems between cities. The additional savings achieved by the use energy efficient measures were attractive, amounting to 2.2% to 5%. LEED, CBEC and BEAM Plus, being three most popular schemes in China, were again benchmarked. It was found that LEED set the most stringent baseline criteria. Between CBEC and BEAM Plus, the baseline criteria of CBEC was found slightly higher. References [1] E. Martinor, Word Bank energy project in China: influence on environment protection, Energy Policy 29 (2001) 581–594. [2] M. Liu, B.Z. Li, R. Yao, A generic model of exergy assessment for the environmental impact of building lifecycle, Energy and Buildings 42 (2010) 1482–1490. [3] R. Yao, B.Z. Li, K. Steemers, Energy policy and standards for built environment in China, Renewable Energy 30 (2005) 1973–1988. [4] J. Liang, R.M. Yao, B.Z. Li, Y. Wu, An investigation of the existing situation and trends in building energy efficiency management in China, Energy and Buildings 30 (10) (2007) 1098–1106. [5] China Academy of Building Research, Industrial Standard of the People’s Republic of China (JGJ 129-2001). Technical Specification for Energy

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