Methodological issues 8221 Choice of method

Emissions of SF6 from electrical equipment can be estimated in a variety of ways with varying degrees of complexity and data intensity. This section describes good practice for using a Tier 1 method (the default emission-factor approach), a Tier 2 method (the country-specific emission-factor approach), and a Tier 3 method (a hybrid that can use either mass-balance or emission-factor approaches for different life cycle stages, depending on country-specific circumstances). Generally, emissions estimates developed using the Tier 3 method, which is implemented at the facility level, will be the most accurate. Estimates developed using the Tier 1 method will be the least accurate.

As is true for other emission sources, the tier selected will depend on data availability and whether or not the category is key. Figure 8.1, Decision Tree for SF6 from Electrical Equipment, summarises the process for choosing among Tiers 3, 2, and 1. Good practice in choosing between the mass-balance and emission-factor variants of the Tier 3 approach is discussed in detail in Section 1.5 of Chapter 1. This choice will depend both on data availability and on country-specific circumstances. As a first step in assessing the importance of SF6 emissions from electrical equipment and the other categories discussed in this chapter, inventory compilers are encouraged to contact chemical producers and suppliers as well as electrical equipment manufacturers and utilities and/or their industry associations. These organisations can provide basic information on chemical consumption and on equipment stocks and applications that can help the inventory compiler estimate emissions and identify sources that merit further investigation. They can also provide important advice and support in establishing more extensive data collection systems to support Tier 2 and Tier 3 estimates.

2 International Council on Large Electric Systems (CIGRE) has published a guide on SF6 handling, Guide for the Preparation of customized "Practical SF6 Handling Instructions," Task Force B3.02.01, CIGRE Publication No.276, August 2005. (CIGRE, 2005)

Figure 8.1 Decision tree for SF6 from electrical equipment1

Figure 8.1 Decision tree for SF6 from electrical equipment1

Note:

1. In selecting an estimation method, it is good practice also to consider the criteria presented in Table 1.7, Chapter 1, Section 1.5 of this volume for choosing between the mass-balance and emission-factor variants of each tier.

2. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3. It is good practice to contact National/Regional associations of utilities/users and manufacturers to collect, check, and aggregate actual and historical data.

Note:

1. In selecting an estimation method, it is good practice also to consider the criteria presented in Table 1.7, Chapter 1, Section 1.5 of this volume for choosing between the mass-balance and emission-factor variants of each tier.

2. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3. It is good practice to contact National/Regional associations of utilities/users and manufacturers to collect, check, and aggregate actual and historical data.

TIER 1 METHOD - DEFAULT EMISSION FACTORS

The Tier 1 approach is the simplest approach for estimating SF6 and PFC emissions from electrical equipment. (Henceforth in this section, 'SF6' will be used to denote 'SF6 and/or PFCs.') In this method, emissions are estimated by multiplying default regional emission factors by, as appropriate, the SF6 consumption of equipment manufacturers and/or by the nameplate SF6 capacity of the equipment at each life cycle stage beyond manufacturing in the country. The term Installation Emissions may be omitted if (1) installation emissions are not expected to occur (i.e., for closed-pressure equipment) or (2) installation emissions are included in the emission factor for emissions from Manufacturing or Use. Default emission factors are given in Tables 8.2 to 8.4.

It is good practice to use the following equation:

Equation 8.1 Default emission factor method

Total Emissions = Manufacturing Emissions + Equipment Installation Emissions + Equipment Use Emissions + Equipment Disposal Emissions

Where:

Manufacturing emissions = Manufacturing Emission Factor • Total SF6 consumption by equipment manufacturers

Equipment installation emissions = Installation Emission Factor • Total nameplate capacity of new equipment filled on site (not at the factory).

Equipment use emissions = Use Emission Factor • Total nameplate capacity of installed equipment. The 'use emission factor' includes emissions due to leakage, servicing, and maintenance as well as failures

Equipment disposal emissions = Total nameplate capacity of retiring equipment • Fraction of SF6 remaining at retirement

TIER 2 METHOD - COUNTRY-SPECIFIC EMISSION FACTOR METHOD

The Tier 2 method uses the same basic equation as Tier 1, but requires reliable country-specific emission factors for each life cycle stage. Country-specific emission factors will be more accurate because they reflect the unique circumstances in which electrical equipment is used in a given country. In addition, if detailed data for equipment retirement are available, emissions due to retirement can be estimated more accurately. The expression for Equipment Disposal Emissions in the Tier 2 method includes terms accounting for SF6 recovery at retirement and disposal, as follows:

Equation 8.2

Equipment disposal emissions under country-specific emission factor method

Equipment disposal emissions = Total nameplate capacity of retiring equipment • Fraction of SF6 remaining at retirement • (1 - fraction of retiring equipment whose SF6 is recovered • recovery efficiency • fraction of recovered SF6 recycled, reused with no further treatment, or destroyed*)

*This final term is intended to account for emissions during chemical recycling and destruction. Note that to be considered Tier 2, estimates must be developed using only country-specific emission factors.

TIER 3 HYBRID METHOD - EMISSIONS BY LIFE CYCLE STAGE OF EQUIPMENT

The Tier 3 method is the most accurate approach for estimating actual emissions of SF6 from electrical equipment. This method is detailed but flexible, accommodating a wide range of national circumstances. The method is implemented at the facility level and includes separate equations for each phase of the life cycle of equipment, including equipment manufacture, installation, use, and disposal. Depending on the type of equipment, the life cycle stage, and country-specific circumstances, either a mass-balance approach or country-(or facility-) specific emission factors may be used. In general, it is good practice to use the mass-balance approach, except where (1) emission rates from a process are near or below the precision of the measurements required for the mass-balance approach (e.g., 3 percent of nameplate capacity per year or less), (2) equipment is never serviced during its lifetime (as is expected to be the case for sealed pressure equipment), or (3) equipment stocks are growing very rapidly, as may be the case in countries where electrical equipment has been introduced within the last 10-20 years.

The hybrid approach enhances accuracy by permitting use of the mass-balance approach for some processes and life cycle stages and the emission-factor approach for other processes and life cycle stages. However, the combination of different approaches also introduces opportunities for double-counting or overlooking emissions. Inventory compilers need to be aware of this problem and take steps to avoid it. Table 8.1, Avoiding Double-Counting or Overlooking Emissions, provides examples of both the problem and some potential solutions.

The annex to this chapter (Annex 8A) briefly describes an example of the Tier 3 approach as it has been applied in Germany. This example is intended to illustrate rather than prescribe; the precise approach taken by any given country will depend on country-specific circumstances.

Ideally, data are obtained for every equipment manufacturer, electricity transmission and distribution facility (utility), equipment disposer (which may be a manufacturer, electric utility, or other entity), and SF6 recycling or destruction facility in the country, and the emissions of all manufacturers, utilities, disposers, and recycling or destruction facilities are summed to develop the national estimate. The basic equation is:

Equation 8.3 Tier 3 total emissions

Total Emissions = 2 Equipment Manufacturing Emissions

+ 2 Equipment Installation Emissions

+ 2 Equipment Use Emissions

+ 2 Equipment Disposal and Final Use Emissions

+ 2 Emissions from SF6 Recycling and Destruction

Where:

Equipment Manufacturing Emissions at the facility level can be estimated by Equations 8.4A and 8.4B.

Equipment Installation Emissions at the facility level can be estimated by Equations 8.5A and 8.5B.

Equipment Use Emissions at the facility level can be estimated by Equations 8.6A and 8.6B.

Equipment Disposal and Final Use Emissions at the facility level can be estimated by Equations 8.7A and 8.7B.

Emissions from SF6 Recycling and Destruction at the facility level can be estimated by Equations 8.8 and 8.9.

In the above equation, national emissions for each phase are equal to the sum of the emissions of all equipment manufacturers, equipment users, equipment disposers, or SF6 recyclers/destroyers at that phase. In practice, it is not always possible to obtain data for every facility; in these cases countries may use one of the extrapolation methods discussed in Section 8.2.2.3, Choice of Activity Data.

Equipment manufacturing emissions

Equipment manufacturing emissions can be estimated using either a pure mass-balance approach or a mixture (hybrid) of a mass-balance approach for some processes and an emission-factor based approach for others. The pure mass-balance approach is preferred except where a substantial fraction of a manufacturer's emissions come from processes whose emission rates fall below the precision of the measurements required for the mass-balance approach (e.g., 3 percent of nameplate capacity per year or less). In these cases, it is good practice to use emission factors to estimate emissions from the processes with very low emission rates and to use the massbalance approach to estimate emissions from the other manufacturing processes.

Pure mass-balance approach: Using the pure mass-balance approach, the total emissions of each equipment manufacturer can be estimated using the following equation:

Where:

Decrease in SF6 Inventory = SF6 stored in containers at the beginning of the year - SF6 stored in containers at the end of the year

Acquisitions of SF6 = SF6 purchased from chemical producers or distributors in bulk + SF6 returned by equipment users or distributors with or inside of equipment + SF6 returned to site after off-site recycling

Disbursements of SF6 = SF6 contained in new equipment delivered to customers + SF6 delivered to equipment users in containers + SF6 returned to suppliers + SF6 sent off-site for recycling + SF6 destroyed

Hybrid approach: This method first requires that manufacturers separate the gas flows associated with processes for which the mass-balance approach will be used from the gas flows associated with processes for which the emission-factor approach will be used. Emissions from the former can then be estimated using the approach outlined in Equation 8.4A. Emissions from the latter can be estimated by multiplying the total nameplate capacity of equipment undergoing each process (e.g., filling) by the country- or facility-specific emission factor for that process. Total emissions for each manufacturer are then estimated by summing the emissions from both sets of processes, using the following equation:

Equation 8.4B Equipment manufacturing emissions - hybrid

Equipment Manufacturing Emissions = Equation 8.4A

+ ^ Nameplate capacity of equipment undergoing each process *

• Emission factor for that process

* Excluding that covered by Equation 8.4A Equipment installation emissions

Equipment installation emissions may be estimated using either a mass-balance or an emission-factor approach. Again, the mass-balance approach is preferred except where emission rates are very low.

Pure Mass-balance approach: Using the mass-balance approach, the total emissions of each equipment installer can be estimated using the following equation:

Equation 8.5A Equipment installation emissions - pure mass-balance

Equipment Installation Emissions = SF6 used to fill equipment

- Nameplate capacity of new equipment

Hybrid approach: This method first requires that users separate the gas flows associated with equipment for which the mass-balance approach will be used from the gas flows associated with equipment for which the emission-factor approach will be used. Emissions from the former can then be estimated using the approach outlined in Equation 8.5A. Emissions from the latter can be estimated by multiplying the newly installed nameplate capacity of each equipment type by the country- or facility-specific installation emission factor for that type. Total emissions for each installer are then estimated by summing the emissions from both sets of equipments, using the following equation:

Equation 8.5B Equipment installation emissions - hybrid

Equipment Installation Emissions = Equation 8.5A

+ ^ Nameplate capacity of new equipment filled on site* • Installation emission factor

* Excluding that covered by Equation 8.5A Equipment use emissions

Equipment use emissions may be estimated using either a pure mass-balance or a hybrid approach. The pure mass-balance approach is likely to be appropriate for countries where (1) electrical equipment that uses SF6 has been in use for 10-20 years or more, and (2) emissions from sealed-pressure systems are likely to be negligible. The hybrid approach is likely to be appropriate for other countries.

Pure mass-balance approach: Using the pure mass-balance approach, the total emissions of each equipment user can be estimated using the following equation:

Equation 8.6A Equipment use emissions - pure mass-balance

Equipment Use Emissions = SF6 used to recharge closed pressure equipment at servicing

- SF6 recovered from closed pressure equipment at servicing

Hybrid approach: This method first requires that users separate the gas flows associated with equipment for which the mass-balance approach will be used from the gas flows associated with equipment for which the emission-factor approach will be used. Emissions from the former can then be estimated using the approach outlined in Equation 8.6A. Emissions from the latter can be estimated by multiplying the total nameplate capacity of each type of equipment by the country- or facility-specific emission factor for that type of equipment. The emission-factor approach is likely to be more accurate for sealed-pressure equipment everywhere and for all types of equipment in countries where electrical equipment has been used for less than 10-20 years. Total emissions for each user are then estimated by summing the emissions from both sets of equipment, using the following equation:

Equation 8.6B Equipment use emissions - hybrid

Equipment Use Emissions = Equation 8.6A

+ £ Nameplate capacity of equipment installed * • Use emission factor

* Excluding that covered by Equation 8.6A Equipment disposal and final use emissions

Equipment disposal and final use emissions may be estimated using either a pure mass-balance or a hybrid approach, based on country-specific circumstances. In both the pure mass-balance and hybrid approaches, emissions from closed-pressure equipment are estimated using a mass-balance equation. In the pure massbalance approach, emissions from sealed-pressure systems are also estimated using a mass-balance equation. In the hybrid approach, emissions from sealed-pressure systems are estimated using an emission-factor-based term.

Pure mass-balance approach: In countries where the gas-collection infrastructure (including recovery equipment, technician training, and economic or legal incentives to recover) is not very well-developed or widely applied, it is good practice to use the pure mass-balance approach, as follows:

Equation 8.7A

Equipment disposal and final use emissions - pure mass-balance

Disposal and Final Use Emissions = Emissions from closed• pressure equipment

+ Emissions from sealed• pressure equipment (MB)

Where:

Disposal and final use emissions from closed-pressure equipment = Nameplate capacity of retired closed-pressure equipment - SF6 recovered from retired closed-pressure equipment, and

Disposal and final use emissions from sealed-pressure equipment (MB) = Nameplate capacity of retired sealed-pressure systems - SF6 recovered from retired sealed-pressure systems

Note that if the inventory compiler uses the emission-factor approach to estimate 'use emissions' from sealed-pressure equipment, a term should be subtracted from the second equation to avoid double counting. See Table 8.1, Avoiding Double-Counting or Overlooking Emissions: Two Examples, for this term.

Hybrid approach: In countries where the disposal of equipment is well controlled and understood (i.e., where an efficient gas collection infrastructure is in place) and where emissions from use of sealed-pressure equipment are accounted for under 'use' above, the hybrid approach may be used, as follows:

Equation 8.7B Equipment disposal and final use emissions - hybrid

Disposal and Final Use Emissions = Emissions from closed pressure equipment

+ Emissions from sealed pressure equipment (EF)

Where:

Disposal and final use emissions from closed-pressure equipment = Nameplate capacity of retired closed-pressure equipment - SF6 recovered from retired closed-pressure equipment, and

Disposal emissions from sealed-pressure equipment (EF) = [(Nameplate capacity of retired sealed-pressure systems) - (Nameplate capacity of retired sealed-pressure systems • Use emission factor • Lifetime of equipment)] • (1 - fraction of retiring equipment whose SF6 is recovered • recovery efficiency)

As noted above, emissions estimated using the above approach should be periodically checked, e.g., by using a pure mass-balance approach and/or assessing recovery frequency and practices. Inventory compilers should pay particular attention to the fraction of retiring equipment whose SF6 is recovered and to the fraction of the charge that is recovered when recovery is performed ('recovery efficiency'). Even in countries where it is the norm to recover SF6 from retiring equipment, some venting may occur, and the venting of just a few percent of the SF6 in retiring equipment will drive emission rates far above the minimum that is technically achievable and that would otherwise be a reasonable basis for an emission factor.

Emissions from SF6 recycling and destruction

Some SF6 emissions occur after the chemical is recovered. These emissions include (1) emissions associated with recycling of SF6, and (2) emissions associated with the destruction of SF6. (Emissions associated with the shipment of SF6 to off-site recyclers or destruction facilities are considered negligible.) Emissions from recycling of SF6 are generally expected to be small — on the order of less than one percent of the total quantity fed into the recycling process. However, these emissions may be higher if state-of-the art handling equipment and practices are not used. In most cases, recycling is expected to occur on the site of the equipment manufacturer or user. In other cases, recycling may take place at a centralised recycling facility that is not associated with a chemical producer. Finally, recycling may take place on the premises of a chemical producer. Recycling emissions from chemical producers will be accounted for under chemical production (see Section 3.10 of this volume) and should not be included here.

Emissions associated with the destruction of SF6 depend on the destruction efficiency of the process and the quantity of SF6 fed into the process. Given the high stability and dissociation temperature of SF6, the destruction efficiency may be as low as 90 percent. Thus, up to 10 percent of the SF6 fed into the destruction process could be emitted. The quantity of gas fed into the destruction process is generally expected to be small compared to that recycled. However, this may vary from country to country.

It is good practice to develop country-specific emission factors for recycling and destruction that are based on full consideration of country-specific logistics and practices for SF6 recycling and destruction.

Emissions from recycling of SF6 may be estimated using the following equation:

Equation 8.8 Emissions from recycling of SF6*

Emissions from Recycling = Recycling emission factor • Quantity SF6 fed into recycling process

*Emissions from recycling that occurs at chemical production facilities should be excluded.

Emissions from destruction of SF6 may be estimated using the following equation:

Equation 8.9 Emissions from destruction of SF6

Emissions from Destruction = Destruction emission factor • Quantity SF6 fed into destruction process

Table 8.1

Avoiding double-counting or overlooking emissions: two examples

Example 1 - Double Counting

Example 2 - Omission

Situation: An emission-factor approach is used to estimate emissions from sealed-pressure equipment during use, and a mass-balance approach is used to estimate emissions during disposal of sealed-pressure equipment.

Situation: A mass-balance approach is used to estimate emissions during use of closed-pressure equipment, but an emission-factor approach is used to estimate emissions during disposal of closed-pressure.

Potential problem: Emissions during use may be double-counted because some of the SF6 that is found to be missing when the equipment is disposed has already been counted as emitted during use.

Potential problem: Emissions that occur between the final servicing of the equipment and its disposal may be overlooked. These 'final use' emissions may account for a significant fraction of total use emissions, particularly if the equipment is refilled every 10 years or more.

Solution: Subtract lifetime use emissions (Nameplate capacity of retired sealed-pressure systems • Use emission factor • Lifetime of equipment) from emissions during disposal.

Solution: Use the mass-balance approach for both the use and disposal phases of the closed-pressure equipment life cycle.

A special case of the Tier 3 method: the utility-level, pure mass-balance approach

Countries that satisfy the good practice criteria for using the pure mass-balance approach beyond equipment manufacturing (i.e., countries where emissions during equipment installation, use, and disposal account for 3 percent or more of facility-level gas flows, where electrical equipment has been used for 10-20 years or more, and where emissions from sealed-pressure equipment are negligible), may, with little or no loss of accuracy, use a simplified version of the Tier 3 method to estimate emissions during equipment use. When summed together and reformulated in terms of facility-level gas flows, equations 8.5A, 8.6A, and 8.7A result in the following equation:

Equation 8.10 Utility-level mass-balance approach

User Emissions = Decrease in SF6 Inventory + Acquisitions of SF6 - Disbursements of SF6 - Net Increase in the Nameplate Capacity of Equipment

Where:

Decrease in SF6 Inventory = SF6 stored in containers at the beginning of the year - SF6 stored in containers at the end of the year

Acquisitions of SF6 = SF6 purchased from chemical producers or distributors in bulk + SF6 purchased from equipment manufacturers or distributors with or inside of equipment + SF6 returned to site after off-site recycling

Disbursements of SF6 = SF6 contained in equipment that is sold to other entities + SF6 returned to suppliers + SF6 sent off-site for recycling + SF6 destroyed

Net Increase in Nameplate Capacity of Equipment = Nameplate Capacity of New Equipment -Nameplate Capacity of Retiring Equipment

Although the utility-level approach is less detailed than the full life cycle approach, it is simple, and for those countries whose national circumstances permit its use, it provides estimates that are closely related to actual gas loss.

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