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In much of the developed world, air conditioning has become a near necessity. In the U.S., more than 90% of newly constructed housing units are centrally air-conditioned [ACHRN, 1999]. Two thirds of all dwelling units in the U.S. have central air conditioning and another one-third have one or more room air conditioners. [Appliance, 1997] Virtually all-commercial building space in the U.S. is air-conditioned. The rapid population growth of the Sun Belt in the U.S. was facilitated, if not enabled, by the universal use of air conditioning. Acceptance of air conditioning varies throughout the developed world, but in general is increasing rapidly in all but the coolest climates. Use of air conditioning for commercial buildings is growing rapidly, even in cooler climates.

The majority of both the existing installed capacity and new production of this air conditioning equipment is unitary equipment. Unitary air conditioning equipment is a broad category of air-to-air air conditioning systems and heat pumps, including:

• Residential central air conditioning systems and heat pumps, both single package and split systems, generally between 5 kW (1 1/2 tons) and 18 kW (5 tons) cooling capacity
• Packaged air-to-air systems and split systems for commercial air conditioning, ranging in cooling capacity from 10 kW (3 tons) too more than 350 kW (100 tons). The ubiquitous commercial rooftop air conditioner falls into this category
• Ductless split systems, both mini-splits for one room and larger systems having multiple indoor evaporator/fan units connected to a single outdoor unit
• Room air conditioners (a.k.a. window air conditioners)

Note that while ductless split systems and room air conditioners are both air to air systems, they are not always classified as "unitary" in published market data. In the TEWI-3 study, TEWI comparisons were developed for the first three of these types of unitary equipment, in a variety of climatic locations and at several different efficiency levels.

In this study, the focus is limited to the first two of the above categories of unitary equipment, and energy and LCCP comparisons are presented for a location whose climate is representative of the U.S. average. The prototypical systems that have been analyzed are two of the example systems used in the TEWI-3 study:

• 10.5 kW (3-ton) residential central air conditioning system (cooling only) or heat pump (heating and cooling). The baseline system (HCFC-22, 10 SEER, 7HSPF) is representative of a significant portion of the current (1999) market, meeting the current NAECA minimum SEER of 10 Btu/Watt-hr. This baseline system is compared to current/future systems using HCFC-22 or alternate refrigerants and with higher efficiencies (12 to 14 SEER), within the likely range of future efficiency requirements and of premium efficiency levels to qualify for electric utility rebate programs. Central air conditioners and heat pumps with efficiencies in this range are available commercially now.
• 26.4 kW (7.5 ton) single package commercial rooftop air conditioner (cooling only). The baseline system (HCFC-22, 10 EER) is representative of a significant portion of the current (1999) market. This baseline system is compared to current/future systems using HCFC-22 or alternate refrigerants and with higher efficiencies (11 EER).

For several decades, virtually all of the unitary air conditioning types described above have operated on an HCFC-22 vapor-compression cycle. As noted previously, some unitary air conditioning products are now being produced with one of two HFC blends, R407C or R410A, and with HFC-134a. While the majority of unitary air conditioning in production today still uses HCFC-22 as the refrigerant, in the post-ODS phase-out context of this study the two HFC blends R407C and R410A are the baseline refrigerants. In the Alternative Refrigerant Evaluation Program (AREP), HFC-134a was also evaluated as a replacement for HCFC-22. HFC-134a requires a larger volume flow rate of refrigerant circulation for a given cooling capacity, requiring a larger compressor displacement and larger diameter tubing throughout, increasing the cost. Consequently, limited unitary equipment is likely to be produced in the future with HFC-134a refrigerant. Alternatives to the vapor compression cycle include:

• Vapor compression cycle with ammonia. Aside from toxicity considerations, ammonia is not well suited for use in the typical, cost-effective, unitary configuration that has evolved. Ammonia attacks copper, which is used extensively for refrigerant tubing, and many of the materials used in hermetic motors. Ammonia has not been evaluated further for unitary equipment in this study. Large commercial unitary equipment conceivably could be replaced with ammonia chillers, which are covered in the chiller section
• Vapor compression cycle with propane, or blends of hydrocarbons, which can provide slightly better performance. Propane has a negligibly low GWP and performs well as a refrigerant, but due to its flammability, the cooling and heating capacity of a propane based air conditioner would need to be coupled to the interior space with a secondary coolant loop. For comparison purposes, a propane/secondary loop system is included in the comparison for residential sized equipment.
• Other cycles such as reverse Brayton and reverse Stirling. Reverse Brayton tends to be considerably lower in efficiency than vapor cycle, while reverse Stirling equipment would likely be considerably more expensive than vapor cycle, without offering any efficiency advantage. Both cycles have been well known for a very long time and neither has made significant inroads in the market for stationary air conditioning equipment. Neither technology is likely to become commercially viable in the foreseeable future.

For the purposes of comparison of the energy consumption of the refrigerant alternatives, the prototypical 3 ton residential systems and the comparable capacity technical alternatives described above have been analyzed for a representative, 1800 sq. ft. residential application in Atlanta. The prototypical 7.5-ton commercial rooftop unit has been analyzed for a representative light commercial application in Atlanta. The results have been calculated using the heating and cooling loads calculated in the TEWI-3 study analysis. In the TEWI-3 study, cooling and heating loads and performance were analyzed for several other locations as well, with similar comparative results.

Tables 6-1 and 6-2 summarize the electric energy consumption of 3 ton central air conditioners in Atlanta and 3 ton heat pumps in Atlanta, respectively. Note that in the NAECA efficiency standards driven market for this equipment, energy consumption does not vary with refrigerant choice, per se. For example, if the future NAECA minimum efficiency were to be 12 SEER (note that the rulemaking to determine this level is ongoing and the new minimum has not been determined yet), a large portion of the air conditioners and heat pumps manufactured would meet this level without exceeding it by much, absent significant market demand for higher efficiency systems. The inherent efficiency characteristics of each refrigerant alternative will impact the design (and cost) required to provide the required minimum level of performance.

The differences in the inherent efficiency characteristics of the fluorocarbon alternatives are due to differences in thermodynamic and heat transport properties, and system operating pressure. Among R-22, R-407C, and R410A, these differences, and differences in refrigerant prices, can lead to "comparatively small" differences in manufacturing cost. An analysis of these differences is beyond the scope of this project, but it is recognized that even "comparatively small" differences in manufacturing cost can have a meaningful impact on gross and net profit margin given the highly competitive nature of a market having more than a half dozen strong manufacturers and scores of smaller manufacturers seeking greater market share.

To use propane as the refrigerant, it is assumed that a secondary loop must be used to deliver the cooling and heating capacity to the building interior. The secondary loop consumes parasitic pumping power and adds a heat transfer temperature difference to the overall thermodynamic lift. To overcome the resulting efficiency loss and meet a minimum efficiency level requires significant offsetting design modifications (e.g., larger heat exchangers, more efficient fan motors) to increase the efficiency. The associated costs, along with other costs associated with the safe use of propane will result in a significant (hundreds of U.S. dollars) increase in manufacturing costs and a larger increase in end-user prices.

Table 6-1: Energy Consumption for a Representative Residential Air Conditioning Application in Atlanta (Cooling Only, Annual Cooling Load 33.8 million Btu, per TEWI-3)

Table 6-2: Energy Consumption for a Representative Residential Heat Pump (Heating and Cooling) Application in Atlanta (Annual heating and cooling loads of 34.8 million Btu and 33.8 million Btu, respectively, per TEWI-3)

The annual electric energy consumption of a 7.5 ton commercial rooftop air conditioner in a typical application in Atlanta is summarized in Table 6-3. The general comments, above, on inherent efficiency differences among refrigerants in residential air conditioning equipment are applicable here as well.

Table 6-3: Energy for Rooftop Air Conditioner in Atlanta

The Life Cycle Climate Performance (LCCP, see Section 1.3) for unitary cooling or unitary cooling and heating is made up of the indirect warming associated with the energy consumption summarized above and the direct warming associated with refrigerant emissions (the warming effect of the refrigerant plus the embodied energy and fugitive emissions associated with manufacturing). The refrigerant charge size and charge loss rates that were assumed in the TEWI-3 study are summarized in Table 6-4. The GWP values are summarized in Table 6-5, which includes both the GWP of the refrigerant and the equivalent GWP of the energy and fugitive emissions associated with manufacturing and transporting the refrigerant.

Table 6-4: Refrigerant Charge Size and Charge Losses for Unitary Equipment

Table 6-5: GWP of Refrigerants and Warming Impact of Energy and Fugitive Emissions During Refrigerant Manufacturing

The LCCP for residential cooling only in Atlanta is summarized in Table 6-6, for R22, R407C, R410A, and propane, at several SEER levels. The LCCP for residential heating and cooling is summarized in Table 6-7.

Table 6-6: LCCP for Residential Air Conditioning in Atlanta (2005 Technology)

Table 6-7: LCCP for Residential Heating and Cooling in Atlanta (2005 Technology)

The LCCP for a single package rooftop in Atlanta is summarized in Table 6-8.

Table 6-8: LCCP for Commercial Rooftop in Atlanta (7.5 Ton Rated Capacity)

The basic observations that can be drawn from the LCCP values in Tables 6-6 through 6-8 are:

• The direct warming effect due to refrigerant emissions is < 5% of the total LCCP, for fluorochemical refrigerants
• Differences in efficiency have a much greater effect on LCCP than the direct effect of refrigerant emissions
• While the direct warming effect with propane is insignificant, and the added energy due to the secondary loop can be offset by larger heat exchangers and other efficiency enhancements, the cost would be increased significantly by on the order of $1,000 to a residential end-user.

With the exception of its high level of flammability, propane appears to be a suitable replacement for R-22 in all respects. Because of the flammability, and the several kg charge size of typical residential unitary equipment, to use propane for residential air conditioning, it is necessary to restrict the propane charge to outdoor equipment, and couple the cooling and heating to the interior with a secondary loop. While this might adequately eliminate the possibility of an explosion occurring within the confined interior space, further fire-safety measures would be needed in the outdoor unit:

• Propane vapor detection and alarm
• Placement of all electrical contacts (and any other potential ignition sources) clear of any potential propane leaks and/or in an enclosure that could contain an ignition - either explosion proof or sealed with a flame arresting vent.
• To reach a given efficiency level, e.g., a NAECA minimum SEER, larger heat exchanger coils and other efficiency enhancements are needed to offset the losses of efficiencies due to the secondary loop.

All of the measures described above are technically feasible and involve well-known technology. However, they would add significantly to the cost. When Lennox evaluated this option in the early 1990's, they concluded that the necessary fire safety measures would add 30% to the cost of a residential central air conditioning system.

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