Polarized refrigerant oil additives are liquids designed to be introduced into all sorts of refrigeration systems, including air conditioners and chillers. According to vendors, these additives mix with refrigerant oil, enabling more-effective lubrication and improved heat transfer within refrigeration system heat exchangers. Vendors also assert that the use of their products yields energy savings ranging from 5 to 30 percent of overall system energy consumption.

It is not clear that these additives can consistently produce savings in the ranges that vendors claim. The chief cause of this uncertainty is a handful of computer simulations and field tests that produced results seeming to contradict the vendors' claims. One well-documented test by the National Institute of Standards and Technology (NIST) did conclude that a particular additive improved the heat flux in an evaporator using a refrigerant and lubricant combination commonly employed in high-efficiency chillers. However, the researchers that conducted this test did not establish to what extent the improved heat flux would affect overall chiller efficiency. Until these issues are cleared up, the ability of these additives to consistently produce their claimed savings will remain uncertain.

According to additive vendors, there are two key mechanisms by which their products improve the efficiency of refrigeration systems: improved internal heat transfer and improved compressor lubricity. Two main areas of evidence sow doubt about both of these mechanisms.

The first area consists of two different organizations' computer simulations of air-conditioning systems, which showed that large improvements in internal heat transfer led to small improvements in overall system efficiency. One of these simulation studies was prepared for Intertek Testing Services, the international testing, inspection, and certification company that owns the well-known Energy Testing Laboratories brand. This study concluded that for every 10 percent improvement in a unitary air conditioner's internal heat transfer coefficient, the overall efficiency of the simulated air conditioner improved by about 1 percent.

A second simulation study was conducted by Trane, one of the world's largest manufacturers of chillers and other air-conditioning equipment. In this study, researchers at Trane concluded that for every 25 percent improvement in the simulated chiller's internal heat transfer, the overall efficiency of the chiller improved by about 1 percent (Figure 1). Based on these simulations, it is doubtful that additives could produce more than a few percentage points of efficiency gain via this mechanism.

The second area of concern involves evidence from the Copeland Corp., one of the world's leading manufacturers of air-conditioning compressors. According to Copeland, only about 4 percent of power input to a compressor is lost in the form of friction that could be affected by a lubricant additive. Thus, it is doubtful that additives could produce more than a few percentage points of savings via this mechanism.

Although numerous tests of oil additives are available, three tests conducted by independent and highly regarded laboratories, including one by the U.S. Department of Energy's Oak Ridge National Laboratory, reported energy savings that were significantly lower than those claimed by vendors (Table 1). All of these tests were performed on packaged air-conditioning systems, and one even found that the efficiency of an air conditioner was reduced by 6.2 percent due to the introduction of the additive. The highest savings achieved in these selected tests was an efficiency improvement of 2.6 percent. It's not at all clear why the results of these tests differ from tests distributed by additive vendors.

Recently, researchers at NIST tested a chlorine-free additive in a profusely instrumented test cell designed to simulate the conditions in a chiller evaporator. They concluded that the additive would have no effect on a low-pressure chiller but would improve the heat flux in an evaporator in some high-pressure chillers (Table 2). The researchers did not determine how such enhanced heat flux would improve the overall efficiency of the chiller, and given the simulation results presented above, it's likely that the overall chiller efficiency change would be much lower than the improvement in evaporator heat flux. This heat flux improvement would also not extend to any type of refrigeration equipment other than chillers. Chlorine-free additives feature a different chemical formulation than other additives on the market, so it's unknown whether these results would extend to competing additives.

Given all these uncertainties, there is good reason to be concerned that the actual energy savings achieved by polarized refrigerant oil additives could be substantially lower than vendors claim. The science of refrigeration lubricants continues to advance, however. Better lubricants are under development, and research also suggests the potential to develop additives that could predictably improve the performance of existing lubricants.