A new comprehensive evaluating method for assessing the sustainability credentials of the central air-conditioning system

Abstract

A new comprehensive evaluation method for assessing the sustainability credential of central air-conditioning systems is proposed in this paper. The method is based on the relative coefficient index (K) of costs and benefits of the air-conditioning system. The paper introduces the concept and defined the parameters of costs (energy consumption costs, economic running costs and environmental impact) and benefits (indoor air quality and thermal comfort) and the mathematical principles for the determination and evaluation of these parameters. The parameters are then ranked with an index value for each component that make up the air-conditioning system, and the total index value would be used for the overall evaluation of the costs and benefits to determine the overall index, K. The feasibility of the method is illustrated by three case studies included in this paper. The evaluation index can identify shortcomings of components in the air-conditioning system and to identify the relative environmental impacts and costs as well as the performance in using the system. The evaluation index would thus allow designers and building managers to target performance improvements and guide designers to optimize functions in terms of costs and benefits to minimize environmental impacts. The index method would provide a rating classification of air-conditioning systems for standard specification and selection in terms of cost effectiveness for appropriate building service required.

Keywords

Central air-conditioning system Relative coefficient index Evaluation index Sustainability credential Rating classification Costs and benefits

Introduction

In concomitant with the growing development of economy and technology, the world’s energy consumption has been increasing significantly. Building energy consumption accounts for about 25% of the national total energy consumption in China and many other developing countries. The energy used for air conditioning accounts for about two thirds of this consumption, and this has become an important government policy target to reduce energy intensity and CO₂ emission in response to climate change.^1,2 There is an urgent need to provide a system that can comprehensively evaluate the sustainability credential of central air-conditioning system, so that we can identify efficient systems to enable energy saving as well as to reduce impact on our environment while still providing affordable air-conditioning function for our social needs such as comfort and air quality in our homes and office environments.^3–5

Some scholars have developed evaluation index systems for assessing central air-conditioning system in relation to their national climates and energy consumption costs. Li and Yang⁶ used a grey multi-hierarchical evaluation method to establish a mathematical model for optimization selection of an air-conditioning scheme, including a consideration of air-source heat pump system, household gas air-conditioning system, air-cooled chiller unit with gas boiler system, water loop heat-pump system, ground source heat pump system and solar-assisted heat pump system. Sun et al.⁷ studied the criterion of validity for large temperature difference between the evaporator and the condenser air-conditioning system. The energy consumption of these air-conditioning systems with large temperature gradient was compared with the conventional air-conditioning systems based on the initial investment. The evaluating indexes were established for these air-conditioning systems, including the basic theory of thermodynamics, the variable condition of the main equipment and the energy consumption rate.

Avgelis and Papadopoulos⁸ use energy consumption, thermal comfort of room occupants and indoor air quality (IAQ) to develop a net present value index to evaluate the performance of air-conditioning system for the rating of optimized air-conditioning systems.

The index systems reported so far in the literature have not considered comprehensively all the parameters to combine the criteria into a coherent rating system. Most studies emphasized one parameter such as energy consumption and developed index for the evaluation and did not associate the various parameters and issue criteria for general assessment of their environmental and sustainability credential, especially the social and economic cost issues. However, these reported evaluation indexes have provided the basis and guidance for our comprehensive evaluation method.

As the society in China is becoming more advanced with higher social, economic and environmental expectations, people have a higher requirement on their living standards, demanding better, more comfortable living environment and working space, while saving energy consumption and reducing environmental pollution for a living environment for their families. The heating, ventilating, air-conditioning (HVAC) system is very important for buildings in sub-tropical and tropical climates and is a major contributor to energy consumption,^2,9–11 and there have also been reports that air conditioners could have an impact on health of occupants.^12–17 As well as the energy consumption and CO₂ emission issue that are associated with the use of HVAC system, the use of refrigerant for air cooling would also pose greenhouse warming (global warming potential, GWP) and stratospheric ozone depletion potential (ODP) problems on the global environment.^18–23 Therefore, there is a requirement to develop a comprehensive system that could link all the parameters into rating issues for an evaluation of air conditioners, and this evaluation system could become a part of a national standard for HVAC systems.

The establishment of a comprehensive evaluation system

Central air-conditioning system mainly consists of a cold–heat source, dispatch system and terminal equipment. The performance of the air-conditioning system can be assessed by five parameters. The impact of each of these parameters can be considered hierarchically.^24,25 The model of the comprehensive evaluation parameters system is illustrated by the diagram given in Figure 1.

Evaluation of central air-conditioning system

The evaluation of the central air-conditioning system can be divided into costs (C) and benefits (B). The ratio of overall benefits and costs can be used to provide a comprehensive evaluation index for the whole life of the central air-conditioning system. Obviously, the greater the ratio is, the better the overall performance. The integrated comprehensive evaluation index (K) is defined by equation (1) as follows

K = \frac{B}{C}

(1)

In equation (1), B refers to the benefits due to an improvement of IAQ and thermal comfort due to the use of the central air-conditioning system; specifically, these benefits are determined by (i) thermal comfort and (ii) IAQ. C is the costs of (a) energy, (b) economic and (c) global environmental impact.

For developing the linkage of indexes, a grading system is required to provide a comprehensive evaluation index for the association of the overall parameters. However, there is a need to evaluate the performances of the five parameters contributing to the index system.

Evaluating the energy consumption cost, economic value and global environmental impact

Coefficient of energy consumption (CEC) has been introduced to evaluate the energy consumption of air-conditioning system.²⁶ The CEC as defined by equation (2) can be derived

β = \frac{CEC' - CEC}{CEC} \times 100 %

(2)

where the CEC′ is the coefficient of energy consumption of the air-conditioning system, and CEC is the coefficient number for judgment grading of energy consumption for the air-conditioning system.²⁶ The coefficient β represents the degree of deviation from the coefficient. CEC is defined as the ratio of total annual energy consumption to the total annual hypothetical load.²⁶ So, the smaller the CEC, the higher the energy efficiency. CEC for the evaluation of central air-conditioning system was introduced in Japan as a part of their building energy regulations,²⁶ and this system has been included in the Japanese Energy Conservation Law^27,28 for the usage in various types of buildings. Table 1 shows the specified CEC grading number for air-conditioning products allowed for use in different types of buildings in Japan.

Table 1.

The specified CEC for air-conditioning products allowed for use in Japan.²⁷

Usage	Office building	Market and restaurant	Hotel	Hospital	School
CEC judgment number	1.5	1.7	2.5	2.5	1.5
CEC (owner direction)	1.4	1.5	2.3	2.3	1.4

The total annual energy consumption of an air-conditioning system (air-conditioning load) includes the equipment’s overall annual energy consumption for cold–heat source, cooling tower, fan, water pump, etc.²⁹ The hypothetical air-conditioning load consists of solar radiation heat, internal load and fresh-air cooling load. The calculation of the air-conditioning load can be defined by equation (3),²⁹ and equation (4) provides the annual air-conditioning load

CL = at + bh - c

(3)

{CL}_{annual} = CL \times m

(4)

where CL is the hypothetical air-conditioning load, W/m²; a, b, c are the calculated coefficients given by the method BIN²⁹; t is the outdoor dry-bulb temperature, ℃; h is the outdoor air enthalpy, kJ/kg; and m is the corresponding time frequency.

BIN is a method for linear data processing, supposing the building envelope load can be transformed into the linear relationship of outdoor temperature as given by the equation (3), then we can calculate the coefficients a, b, c and by multiplying CL by the corresponding time frequency (m), the annual hypothetical air-conditioning load can be derived as equation (4). The calculation of time frequency should use BIN meteorological parameters, which use the annual outdoor dry-bulb temperature of the location where the air-conditioning system is installed to report the temperature at certain time intervals.²⁹

The economic cost of the air-conditioning system should include a consideration of the initial investment cost and annual operating cost. Using the dynamic annual cost method would thus link the investment and annual operating cost, which should also include a consideration of the time factor. The cost can be calculated by equation (5),³⁰ as follows

AC = (K_{0} - K_{L}) \frac{i (1 + i) n}{(1 + i) n - 1} + C + K_{L} i

(5)

In this equation, AC is equal to the overall cost; K₀ refers to initial investment cost; K_L refers to the salvage value after economic life; C is the annual operating cost; i is the benchmark discount rate (depreciation), which generally is about 10%³⁰; and n is the total number of years the system would be used throughout its operation life, taking into account the percentage of its operation.

Total Environmental Warming Impact (TEWI) has been used to determine the direct effect (DE) of emission of refrigerant on GWP and the indirect effect (IE) of CO₂ emission due to energy consumption of the refrigerator.^31,32 The relationship of the GWP with the DE is defined by equation (6), and the energy consumption due to IE is defined by equation (7). Equation (8) defines the combined effect of DE and IE³²

DE = GWP (LN + Ma)

(6)

IE = N \times E \times b

(7)

TEWI = DE + IE

(8)

In equation (6), L is the annual amount of refrigerant leakage; N is the number of years (y) of operating the refrigerator; M is the charged volume (m³) of the refrigerant and a is the refrigerant’s loss (or discharge) rate (m³/h) at the end of life when the refrigerator is disposed or discarded.

In equation (7), E refers to the annual power consumption, kWh/a, of the refrigerator; and b is the mass of CO₂ emissions due to the consumption of 1 kWh electricity, kgCO₂/(kWh).

Evaluating the benefits of thermal comfort and IAQ

There can be many factors that could affect thermal comfort. The predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) are used in the ISO 7730:2005 standard³³ to describe the thermal environment. The recommended scale of PMV and PPD is −0.5 to +0.5. The related equations (9) and (10) for determining PMV and PPD are as follows³⁴

PMV = [0.303 exp (- 0.036 M) + 0.0275] TL

(9)

PPD = 100 - 95 exp [- (0.03353 PMV 4 + 0.2179 PMV 2)]

(10)

where M is the human metabolic rate, W/m²; and TL is the difference between heat production and heat dissipation from the body to the outside.

IAQ can be determined based on an index model^35,36 defined by equation (11), as follows

I = \sqrt{\frac{(max I_{i}) 2 + (\frac{1}{n} \sum I_{i}) 2}{2}}

(11)

where the I_i = C_i/C_vi; I_i is the single factor index and the maxI_i is the maximum I_i; C_i is the measured or predicted concentration (mg/m³) of pollutant, i, and the C_vi is the pollutant concentration limit, mg/m³ of pollutant, i, as specified by the IAQ guidelines. The model can describe the degree of indoor air pollution that could be caused by indoor air pollutants directly. The model can also highlight the impact of the maximum pollution.

A new comprehensive evaluation index system

The evaluation of the parameters (energy consumption cost, economic cost, global impact, IAQ and thermal comfort) has been defined above, and these values can be integrated into a comprehensive, mutually associated index system to determine the overall sustainability credential of the central air-conditioning system.

Details of the grading system

Thermal comfort index Table 2 shows the grading of the thermal comfort index based on the ‘architectural classification of human thermal comfort index PMV and PPD’.³⁴

IAQ index Table 3 shows the grading of the IAQ index, I.³⁵ The IAQ characteristics should be defined by guidelines and standards as stipulated by various countries.^36–46 The characteristics given in Table 3 provide a generalized description.

Table 2.

The thermal comfort index.³⁴

Classification	1	2	3	4
PMV	−0.2 < PMV < +0.2	−0.5 < PMV < +0.5	−0.7 < PMV < +0.7	PMV < −0.7 or PMV > +0.7
PPD (%)	<6	<10	<15	>15
Index	4	3	2	1

PMV: predicted mean vote; PPD: predicted percentage of dissatisfied.

Energy consumption cost index Using the Japanese architecture standards law reference, the index based on the energy consumption coefficient (CEC) is as illustrated in Table 4.

Table 3.

The indoor air quality index.^35–46

I value	Grade	Evaluation	Indoor air quality characteristics	Index
≤0.49	I	Clean	Clean environment for human habitat	5
>0.5 to ≤0.99	II	Acceptable	The level of environmental pollution is below the standard guideline value and is acceptable for human life	4
>1.00 to ≤1.49	III	Low pollution	The concentration level of at least one key environmental pollutant (e.g. total volatile organic compounds (TVOC), formaldehyde, radon, microbes, ammonia, PM₁₀, CO₂, ozone and NO_x) is higher than the standard guideline value and could cause an acute or chronic effect to a sensitive person	3
>1.50 to ≤1.99	IV	High pollution	The concentration levels of some key environmental pollutants (e.g. TVOC (total volatile organic compounds), formaldehyde, radon, microbes, ammonia, PM₁₀, CO₂, ozone and NO_x) are much higher than the standard guideline values and could pose an obvious health effect to especially sensitive person	2
> 2.00	V	Serious pollution	The concentration levels of some key environmental pollutants (e.g. TVOC (total volatile organic compounds), formaldehyde, radon, microbes, ammonia, PM₁₀, CO₂, ozone and NO_x) are much higher than the standard guideline values and 1 or 2 pollutants (e.g. CO, radon, PM_2.5, formaldehyde, benzene and polyaromatic hydrocarbon compounds (PAHs), trichloroethylene, tetrachloroethylene and NO₂) could seriously harm human health (e.g. causes death, cancer or respiratory diseases)⁴⁶	1

Economic cost index

Table 4.

The energy consumption index.

Coefficient β	Index
0%–5%	1
5%–10%	2
10%–15%	3
15%–20%	4
>20%	The design of system needs to be modified

Different systems can correspond to different building areas and air-conditioning loads, so the cost cannot be easily compared. Initial investment and operating cost are often contradictory. In general, an energy efficient central air-conditioning system should have a low operating cost but would often have a large initial investment value. In order to evaluate the cost for equitable comparison, an energy cost coefficient, α is introduced, and this is defined by equation (12), as the ratio of the overall cost (AC) to the total energy consumption (E) per overall areas (A) served by the air-conditioning system

α = \frac{AC}{E \times A} (kJ - 1 m - 2 Yuan)

(12)

For the evaluation of a central air-conditioning system, a critical value, α′ which is the average value of α for the various components that made up the central air-conditioning system. Several components of the central air-conditioning system are included for consideration. Table 5 shows an example of a cost breakdown of an overall air-conditioning system. Other system with additional or different components would have different costs. The Chinese money unit, yuan (¥) is used here but equally other currency unit (e.g. the American dollars ($), British pounds (£), Euro (€), Hong Kong dollars (HK$), or Korean won (₩) relevant to a particular country can also be used.

Table 5.

An example of a cost breakdown of the various components of an air-conditioning system.⁴⁷

Project	Ground source heat pump	Chiller and gas-fired boiler supporting	Chiller and city heat-supply network	Direct-fired LiBr heat and cold water unit	Air-cooled chiller unit
Initial investment, K₀ × 10⁵ Yuan	300	300	350	350	300
Operating cost, C × 10⁵ Yuan	15.37	72.94	33.41	39.74	23.45
The average useful lifetime of the system (n)	20	10	10	10	10
Depreciation value after economic life × 10⁵ Yuan	30	30	35	35	30
AC × 10⁵ Yuan	50.08	119.88	88.175	94.5	70.39
Α × 10⁻⁸kJ⁻¹ m⁻² Yuan	0.192	0.428	0.353	0.487	0.289
The critical value (α′), kJ⁻¹ m⁻² Yuan = 0.3498 × 10⁻⁸

Note: the above costs are converted into the costs of a 10,000 m² air-conditioning area so that we can calculate the α′.

When α is bigger than the critical value, α′, the index value would be 2, otherwise is 1.

The above example should only be considered as a guide and the average value α′, calculated here should not be used as a standard for all central conditioning system. Due to the limited selection of sample quantity and factors to be considered, such as the air-conditioning area and operating time of a building, the actual calculation process, in the range of ± 15% of the average value can be the critical value.

Global environmental costs index The value TEWI is related to the GWP of refrigerants, and the coefficient of performance (COP) of the refrigerator–air conditioner. The bigger the energy consumption coefficient, the smaller would be the performance coefficient. The bigger the performance coefficient of the refrigerator, the smaller the value of TEWI.³² According to the requirements of the Chinese standard, HJ2531-2012, ‘environmental standard product requirements for industrial and commercial refrigeration equipment’,⁴⁸ the ODP and the GWP of any refrigerator should be 0. When meeting this requirement, the value of TEWI is determined by refrigerator’s COP. The requirements of HJ2531-2012 for the refrigeration equipments are shown in Tables 6 to 8.⁴⁸ For other countries, the refrigerator’s COP should be defined by their relevant national standard; e.g. in USA, the 2012 and 2015 ASHRAE Handbook on HVAC and HVAC applications^49,50 and in UK, CIBSE Guide B.⁵¹

Table 6.

The COP requirements for the LiBr absorption chillers.⁴⁸

Type	Heat source	Environmental labelling products		Environmental labelling of low-carbon products
Type	Heat source	Steam consumption of unit cold/(kg/h·kW)	COP/(W/W)	Steam consumption of unit cold/(kg/h·kW)	COP/(W/W)
Steam double-effect	Saturated vapour 0.4 MPa	1.19	–	1.12	–
	Saturated vapour 0.6 MPa	1.11	–	1.05	–
	Saturated vapour 0.8 MPa	1.09	–	1.02	–
Direct-fired double-effect	Fuel gas, fuel oil	–	1.30	–	1.40

COP: coefficient of performance.

Table 7.

The COP requirements for the air-source heat pump water heater.⁴⁸

The type of water heater			COP/(W/W)
The ordinary type	Primary heating		4.4
	Heating hydronic	No water pump	4.4
	Heating hydronic	Have water pump	4.4
The low-temperature type	Primary heating		3.7
	Heating hydronic	No water pump	3.7
	Heating hydronic	Have water pump	3.7

COP: coefficient of performance.

Table 8.

The COP requirements for the water-source heat pump unit.⁴⁸

Type	Rated cooling capacity (m³)/kW	COP/(W/W)^a
Water-to-air
Water ring	–	4.5
Ground water	–	4.0
Ground loop	–	3.5
Water-to-water
Water ring	≤150	4.8
Water ring	>150	5.0
Ground water	≤150	4.2
Ground water	>150	4.5
Ground loop	≤150	3.6
Ground loop	>150	3.9

Other COP of air-conditioning systems could also be used.

COP: coefficient of performance.

The index value attributed to the global environmental impact of a central air-conditioning system is as follows: If the ODP and GWP of the refrigerant is 0 and the unit’s COP meets the requirements of HJ2531-2012, the index value is 1. If the unit does not meet either of these requirements, the index value is 2, and if the unit meets neither of these requirements, the index value is 3.

Case studies

The project no. 1 is an example of central air conditioning in an office building block complex in Xiangtan, Hunan Province, China. The air-conditioning area is about 6800 m². The designed outdoor dry-bulb temperature for summer air conditioning is 35.8℃, and the wet-bulb temperature is 27.7℃. The relative humidity is 81%. The designed outdoor dry-bulb temperature in winter for air conditioning is −3℃, and the relative humidity is 75%. This project used 12 modular air-cooled heat pump units produced by McQuay International. The parameters are shown in Table 9.

Table 9.

Parameters of modular air-cooled heat pump units.

Name	Type	Cooling capacity	Input power of cooling	Heating capacity
Modular air-cooled heat pump unit	MAC230DR	65 kw	19.2 kw	68 kw
Input power of heating	Water flow rate	Compressor	Weight of the operating equipment	Size
19.1 kw	11.2 m³/h	2	520 kg	1990 × 840 × 1840 mm

After installation, the concentration of indoor air pollutants in the central air-conditioned area was measured, and the IAQ index value was evaluated as shown in Table 10.

Table 10.

Pollutants concentrations in the central air-conditioned space of project no.1.

Evaluation	Carbon dioxide	Formaldehyde	Benzene	Ammonia	TVOC
Detected concentration, mg/m³	12	0.08	0.15	0.18	0.55
Evaluation standard,⁵² mg/m³	10	0.10	0.11	0.20	0.60
IAQ sub-index value	1.2	0.8	1.36	0.9	0.92

IAQ: indoor air quality; TVOC: total volatile organic compounds.

The project no. 2 is related to a quadrangle residential court complex in Xiangtan. The air-conditioning area is about 800 m². The outdoor designed dry-bulb temperature for summer air conditioning is 35.5℃, and the wet-bulb temperature is 27.5℃. The relative humidity is 81%. The designed outdoor dry-bulb temperature for winter air conditioning is −3℃, and the relative humidity is 75%. This project used a screw-water source heat pump unit (KNG-80), and the parameters are shown in Table 11.

Table 11.

Parameters of the screw water source heat pump unit of KNG-80.

Water source heat pump	Quantity	Cooling capacity (kW)	Input power of cooling (kW)	Heating capacity (kW)	Input power of heating (kW)
KNG-80	1	82	12.5	87	19

After installation, the concentration of indoor air pollutants in the central air-conditioned space was measured, and the evaluation index was shown in Table 12.

Table 12.

Pollutants concentrations in the central air-conditioned space of project no.2.

Evaluation	Carbon dioxide	Formaldehyde	Benzene	Ammonia	TVOC
Detected concentration, mg/m³	14	0.06	0.15	0.16	0.48
Evaluation standard,⁵² mg/m³	10	0.10	0.11	0.20	0.60
IAQ subindex	1.4	0.6	1.36	0.8	0.8

TVOC: total volatile organic compounds; IAQ: indoor air quality.

The project no. 3 is a house in Shenzhen. The air-conditioning area is about 80 m². The outdoor designed dry-bulb temperature for summer air conditioning is 33℃, and the wet-bulb temperature is 27.9℃. The designed outdoor dry-bulb temperature for winter air conditioning is 6℃, and the relative humidity is 72%. This project used a screw-water source heat pump unit (LSBLGHP610/M), and the parameters are shown in Table 13.

Table 13.

Parameters of the screw water source heat pump unit of LSBLGHP610/M.

Water source heat pump	Quantity	Cooling capacity (kW)	Input power of cooling (kW)	Heating capacity (kW)	Input power of heating (kW)
LSBLGHP610/M	1	608	106	714	145

After installation, the concentration of indoor air pollutants in the central air-conditioned space was measured, and the evaluation index was shown in Table 14.

Table 14.

Pollutants concentrations in the central air-conditioned space of project no.3.

Evaluation	Carbon dioxide	Formaldehyde	Benzene	Ammonia	TVOC
Detected concentration, mg/m³	12	0.08	0.17	0.14	0.42
Evaluation standard,⁵² mg/m³	10	0.10	0.11	0.20	0.60
IAQ subindex	1.2	0.8	1.55	0.7	0.7

IAQ: indoor air quality.

The overall index values evaluated for the three projects are shown in Table 15.

Table 15.

Overall index values for the evaluation of the three project case studies.

Index		Thermal comfort	Indoor air quality	Energy consumption cost	Economical cost	Global environment impact
Project		Thermal comfort	Indoor air quality	Energy consumption cost	Economical cost	Global environment impact
No.1	Calculation value	PMV = 0.125	I = 1.233	β = 2%	α = 0.75 × 10⁻⁸	ODP = 0
	Calculation value	PPD = 4.9957	I = 1.233	β = 2%	α = 0.75 × 10⁻⁸	COP = 3.38
	Index value	4	3	1	2	1
	K₁	1.75
No.2	Calculation value	PMV = 0.236	I = 1.157	β = 3%	α = 0.53 × 10⁻⁸	ODP = 0.05
	Calculation value	PPD = 6.151	I = 1.157	β = 3%	α = 0.53 × 10⁻⁸	COP = 4.3
	Index value	3	3	1	2	2
	K₂	1.2
No.3	Calculation value	PMV = 0.157	I = 1.300	β = 5%	α = 0.68 × 10⁻⁸	ODP = 0
	Calculation value	PPD = 5.135	I = 1.300	β = 5%	α = 0.68 × 10⁻⁸	COP = 4.92
	Index value	4	3	2	2	1
	K₃	1.4

COP: coefficient of performance; ODP: ozone depletion potential; PMV: predicted mean vote; PPD: predicted percentage of dissatisfied. K1, K2 and K3 are derived from equation (1) as a ratio of the benefits against costs of projects no. 1, 2 and 3.

Discussion

A comparison of the three cases shows that the comprehensive evaluation index (K) of the three projects is greater than 1, that is to say the score index for benefits is greater than the score index for costs. So the comprehensive performances of the central air-conditioning systems for these three projects are satisfactory and would have relatively good sustainability credential.

The K index value of project 1 is greater than project 2 and 3, that is to say, the sustainability credential of the air-conditioning system for project 1 is better than project 2 and 3. However, the economic cost of air conditioning in project 1 would need to be improved in comparison to the system used for project no. 2 and 3, and the IAQ and global environmental impact of the system used in project no. 1 and 3 would need to be improved in comparison to the system used for project no. 2. The IAQ is not within the specified limit of the national standard for benzene and carbon dioxide, showing the relatively lower efficiency of the air-conditioning system used in project no. 1. Although the energy consumption of the air-conditioning system of project no. 1 is less than the system used for project no. 2 and 3; the operation cost is higher and the system is demonstrably less efficient in performance than the system used in project no. 2 and 3.

Conclusion

A comprehensive index system has been developed for the evaluation of central air-conditioning systems. The system combined the parameters relating to costs and benefits of the system by integrating the index value of each of these parameters into a grading coefficient index (K) for the evaluation of the sustainability credential of the air-conditioning system.

The index evaluation can identify shortcomings of components of the air-conditioning system and would also identify the relative environmental impacts and costs as well as providing an insight into the benefits of IAQ and comfort in using the system. The system would thus allow designers and building managers to target improvement in the performance of the conditioning system and guide designers to optimize functions of the central air-conditioning system in terms of costs and benefits to minimize environmental impacts.

The comprehensive index can be used as a grading value for rating classification of air-conditioning system for selection for any building development, where the consumers such as the building owners, designers, and building managers would have the benefit to identify the cost-effective system for the appropriate building service operation for their buildings.

Footnotes

Authors’ contribution

All authors contributed equally to the preparation of this manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project was financially supported by the National Natural Science Foundation of China (Grant no.51274098; 51308206); and the State Key Program of National nature Science of China (Grant no.51134005); and the Natural Science Foundation of Hunan Province, China (Grant no. 12JJ8009).

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