Strengths and weaknesses of the massbalance approach

The mass-balance approach tracks the amount of new chemical introduced into the country, facility, or stock of equipment (at the application or sub-application level) each year. This approach then accounts for the share of this new chemical that is used to fill new equipment capacity or to replace destroyed gas. The consumption that cannot be accounted for is assumed either to replace emitted gas or to be emitted itself.

The mass-balance approach has the important advantage of reflecting actual emissions at the place where they occurred, capturing differences not only among types of facilities and equipment, but among individual facilities and pieces of equipment. Thus, the mass-balance approach is likely to be more accurate where emission rates vary across equipment and facilities, and to some extent, where emission rates vary over time. Because emission rates frequently do vary, often unpredictably, it is good practice to use the mass-balance approach rather than the emission-factor approach as long as (1) accurate activity data for the mass-balance approach are available, and (2) neither of the drawbacks described below applies to the process or equipment whose emissions are being estimated.

The mass-balance approach has two drawbacks. First, the accuracy of the approach is limited by the precision of mass-, density-, and pressure-measuring devices, which tends to fall around ±1 or 2 percent. If the emission rate from a process (such as equipment installation) is in this range (i.e., 3 percent of nameplate capacity per year or less), then the mass-balance approach will be inaccurate for that process.

Second, the mass-balance approach detects some emissions after they occur, sometimes several years later. This is because equipment that leaks slowly can operate for years or even decades with less than a full charge. This time lag can sharply reduce accuracy where servicing is infrequent and/or stocks are growing quickly. This is likely to be the case for (1) types of equipment that are almost never refilled during their lifetimes (e.g., sealed-pressure electrical equipment and hermetically sealed air-conditioning and refrigeration equipment, such as household refrigerators), and (2) countries that have only recently begun using electrical equipment containing SF6 and/or air conditioning and refrigeration equipment containing HFCs. In the latter case, the mass-balance approach will significantly underestimate emissions during the first few years of equipment use, because chemical consumption for refilling equipment will be close to zero until the first set of equipment is refilled for the first time. For electrical equipment, this may not occur until 10 to 20 years after the introduction of the equipment into the country, depending on the leak rate of the equipment. For air-conditioning and refrigeration equipment, this may not occur until 5 to 20 years after the introduction of the equipment, again depending on the leak rate of the equipment.

Figures 1.4 and 1.5 illustrate the 'lag error' associated with the mass-balance approach for these two situations. Figure 1.4 focuses on the error that can occur when countries have only recently begun using electrical equipment containing SF6 or air-conditioning equipment containing HFCs. In this example, equipment is serviced (refilled) every 10 years and has a lifetime of 30 years. Annual equipment sales are assumed to remain constant, but the total stock of equipment grows until the lifetime of the equipment is reached. For illustrative purposes, leaks are assumed to make up 100 percent of emissions (e.g., emissions at equipment installation, servicing and disposal are assumed to be zero).7

Figure 1.4 Apparent versus Actual Leaks; No growth in annual sales of equipment (10-yr service, 30-yr life)

1.20

Figure 1.4 Apparent versus Actual Leaks; No growth in annual sales of equipment (10-yr service, 30-yr life)

1.20

0.00

0.00

11 16 21 26 31 Year Since Chemical Introduced

-Actual Leaks

-Apparent Leaks

-Apparent Leaks/ Actual Leaks

In Figure 1.4, after the chemical is first introduced into the equipment, emissions ('Actual Leaks') grow rapidly as the bank of chemical in the equipment stock doubles in the second year, triples in the third, and quadruples in the fourth. However, sales of the chemical for refilling ('Apparent Leaks') remain close to zero until year 11, when the equipment installed in year 1 is recharged for the first time. In year 21, sales jump again, as, for the first time, two sets of equipment are serviced. When equipment begins to retire, apparent leaks rise to equal actual leaks (resulting in a ratio of 1.0), and the lag error disappears.

Figure 1.5 describes the same situation as Figure 1.4, except in this case, annual equipment sales are assumed to grow by 5 percent per year. The relationship between apparent and actual leaks is very similar to that shown in Figure 1.4 until the equipment begins to retire. At that point, apparent leaks rise, but they never quite equal actual leaks. Instead, the relationship between apparent leaks and actual leaks stabilizes at a constant, equilibrium value, 0.78 for this scenario.

7 In this example, the nameplate capacity of the equipment sold each year is assumed to equal 1 000 tonnes, and the leak rate is assumed to equal one percent per year. Note, however, that the relationship between apparent and actual leaks is actually independent of the sizes of the annual sales and the leak rate.

Figure 1.5 Apparent versus Actual Leaks; 5% growth in annual sales of equipment (10-yr service, 30-yr life)

0.90

Figure 1.5 Apparent versus Actual Leaks; 5% growth in annual sales of equipment (10-yr service, 30-yr life)

0.90

11 16 21 26 31 Year Since Chemical Introduced a cc

-Actual Leaks

-Apparent Leaks

■Apparent Leaks/ Actual Leaks

36 41

In general, if the average time between refilling events is R, then the mass balance approach will yield a very poor estimate of emissions until R+1 years have passed since the chemical was introduced into the country. The accuracy of the estimate will fluctuate in following years, reaching a maximum once the equipment begins to retire.8

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment