Residential and Commercial Space Cooling and Refrigeration

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For space cooling (which includes heat pumps) and refrigeration, the dominant technologies are those based on the vapor compression cycle. These technologies are powered by electricity, making it important to optimize system components and design for energy efficiency. This dual impact of vapor compression systems on climate change - the direct impact from refrigerant losses and the indirect impact due to electricity usage to power the system - led to development of the Total Equivalent Warming Impact (TEWI) methodology to quantify the combined impact in units of CO2 [123]. Direct emissions are converted to CO2 equivalents using GWPs of the different refrigerants, and indirect emissions are estimated based on electricity usage and the CO2 emissions associated with electricity generation. Whether direct or indirect effects dominate the TEWI calculation depends on the system design. For household refrigerators with small refrigerant losses, indirect effects dominate, while for supermarket systems with large refrigerant charges and high refrigerant losses, direct effects dominate. As of year 2000, the direct emissions of refrigerants are estimated to be responsible for 13% of the warming already in place [124].

Presently, the primary refrigerants sold in the marketplace are hydrochlorofluo-rocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) which have significantly higher 100-year GWPs (GWP100) than CO2, which has, by definition, a GWP100 of 1 (see Table 12.3). Due to the Montreal Protocol, the market is transitioning out of HCFCs and into HFCs. In 2008, EPA estimated that emissions from refrigeration and air conditioning systems represented 86% of the combined HFC and PFC emissions from all sectors in the U.S. economy [125]. A majority of those were HFC-134a emissions from motor vehicle air conditioners (MVACs). The European Union has signed a directive which bans the use of HFC-134a in new MVAC systems beginning in 2011. Replacement refrigerant must have a GWP100 of less than 150, and the primary candidate is CO2 (R-744), a "natural" refrigerant.

Natural refrigerants are those which occur in nature's biological and chemical cycles without human intervention and include CO2, ammonia, hydrocarbons (such as propane, butane, and isobutane), air, and water. For space conditioning and refrigeration, the natural refrigerants of primary interest are CO2, ammonia, and hydrocarbons (GWP100 <20). Although natural refrigerants have no ozone depletion potential and low GWPs, they are not without other environmental concerns including toxicity, flammability, or in some cases lower operating efficiencies.

Systems using CO2 will operate at higher pressures and in some designs may have lower operating efficiencies. Thus it will be important to understand the TEWI consequences of substituting a less efficient CO2 system for a more efficient HFC system [126]. The carbon dioxide used as a refrigerant is generally of industrial or scientific grade, and is typically recovered from the waste streams of industrial processes.

Table 12.3 100-Year global warming potential (GWP100) values for various refrigerants on a per unit mass basis

Table 12.3 100-Year global warming potential (GWP100) values for various refrigerants on a per unit mass basis

Refrigerant

Compound

100

HFC-134a

1,1,1,2-Tetrafluoroethane

1,300

HCFC-22

chcif2

1,700

R-32

Methylene fluoride

550

R-125

chf2cf3

3,400

R-407c

Mixture of HFC-134a, R32, and R125

1,800

R-407a

Mixture of R32 and R125

2,100

R-717

nh3

<1

R-744

CO2

1

Ammonia systems are typically equally to slightly more efficient than HFC systems. However, ammonia has the potential to impact human health due to its toxicity at concentrations above 300 ppm. Permissible exposure limits are 25-35 ppm. It is also moderately flammable, which is a concern for human safety. Ammonia releases can also impact ecosystems through the formation and deposition of ammonium hydroxide. It can also combine with sulfates to form secondary PM. However for these last two impacts to be measurable it may require a catastrophic loss of the ammonia charge. While ammonia is considered a natural chemical, it is manufactured through chemical processes. Thus increased production of ammonia to meet refrigerant demands will increase the environmental impacts associated with ammonia manufacturing.

Hydrocarbons are also slightly more efficient than HFCs, but are highly flammable. Use of hydrocarbons as refrigerants has been limited to systems with very small refrigerant charge, such as household refrigerators. In systems requiring larger charges, they are used in secondary loop systems, a design where the hydrocarbon refrigeration cycle cools a secondary fluid in the safety of an equipment room, and then the secondary fluid is piped to the occupied area to deliver the cooling. The addition of this secondary loop reduces the overall efficiency of the process and thus requires careful evaluation of the TEWI consequences. In addition, the hydrocarbon refrigerants may also play a role in the atmospheric formation of ozone or PM. It is questionable whether the levels of leakage would be significant enough to measurably impact ozone or PM concentrations. However, this potential should be considered when evaluating potential refrigerant alternatives.

As has been mentioned before, life-cycle evaluations of mitigation technologies will be an important aspect of determining their environmental acceptability. For natural refrigerants, the life-cycle energy usage is very favorable. The embedded energy required to reclaim, clean, liquefy and transport refrigerant-grade carbon dioxide is estimated to be 1 kg CO2eq per kg. The ammonia production process has a carbon equivalent of 2 kg CO2eq per kg of ammonia. This is in contrast to the fluo-rocarbon production process which is about 9 kg CO2eq per kg of fluorocarbon refrigerant (Arthur D. Little [127]).

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