[ << Previous | Contents | Next >> ]

The chilled water or brine provided by chillers is used for commercial building air conditioning and for a wide variety of process cooling applications. Most of the chiller capacity is used for commercial air conditioning applications, and the energy and LCCP analysis herein is restricted to these applications.

The majority of chiller capacity produced each year is vapor compression cycle based, along with a moderate amount of absorption chiller capacity. Chillers are usually referred to by the type of compressor used, with four types now commonplace:

• Centrifugal, primarily large tonnage [above l000 kW (300 tons)]
• Screw (50 - 400 tons)
• Scroll (up to 50 tons)
• Reciprocating (up to 150 tons)

Centrifugal chillers most commonly use a low-pressure refrigerant such as HCFC-123 or a medium pressure refrigerant such as HFC-134a. Higher pressure HCFC-22 is sometimes used in unusually large capacity units. Centrifugal chillers usually are water cooled, so that at design conditions the temperature lift and pressure ratio fall comfortably within the performance capabilities of a single stage compressor.

The other three compressors are all positive displacement compressors, and use either HCFC-22, HFC-134a, R407C or R410A. Both air and water-cooled versions are available.

Table 7-1 summarizes the technology alternatives - both refrigerant and alternative cycle - that are potentially commercially viable. While engine or turbine driven vapor compression is an alternative, it really is the prime mover that is the alternative (to an electric motor). Engine driven vapor compression is subject to the same refrigerant selection issues as conventional electric motor driven vapor compression cycles, therefore engine or turbine driven vapor cycle is not treated here as a separate alternative.

Table 7-1: Chiller Technology Alternatives

Most of the entries in Table 7-1 identify both the refrigerants that are currently in use and alternative refrigerants that can be used after all ozone-depleting refrigerants (i.e., CFCs and HCFCs) are phased out.

HCFC-22 is currently used in a large proportion of positive displacement compressor based chillers and in some larger tonnage centrifugal chillers. These uses predate the Montreal Protocol, but will be phased out as part of the overall HCFC phase-out. In the U.S., HCFC-22 cannot be used in new equipment after Jan 1, 2010.

HFC-134a is currently used in some screw chillers and in many centrifugal chillers. It replaced CFC-12 in these uses, and is more widely used than CFC-12 was for these applications.

HCFC-123 is currently used in "low pressure" centrifugal chillers, having replaced CFC-11 in this use. While it will be phased out along with the other HCFCs, new equipment using HCFC-123 can be manufactured in the U.S. until 2020.

Provided that safety issues are addressed, ammonia (R717) can be used in open drive screw chillers. It is not suited for hermetic motor applications, because of material compatibility consideration, and is not well suited for centrifugal chillers because of its low molecular weight.

R410A is a near azeotropic HFC blend (50/50 wt. % HFC-32 and HFC-125) that is intended for HCFC-22 replacement. Because it is near azeotropic it is suitable for use in the flooded evaporator configuration that is typical of large chillers. R407C is another HFC blend that is intended for HCFC-22 replacement, but it is zeotropic and not suited for use in a flooded evaporator. R-407C has been used in reciprocating chillers with direct expansion evaporators.

HFC-245fa has been developed primarily to replace HCFC-141b as a foam-blowing agent, but it is potentially applicable as a non-ozone-depleting refrigerant to replace HCFC-123 for low-pressure centrifugal chillers. While HCFC-123 has a low ODP and will be available for new equipment for a correspondingly long time, it is in the ASHRAE Std 34 "B" toxicity classification. Chiller manufacturers are interested in finding a suitable, long term - i.e., low toxicity, non-ozone depleting - replacement for HCFC-123. There is still some uncertainty whether HFC-245fa will be commercially produced.

The assessment of energy and LCCP, which follows, is based largely on the TEWI-3 study assumptions and results. In TEWI-3, energy efficiency levels for individual refrigerants were based on the performance of then currently commercially available chillers using each refrigerant and industry input predicting the efficiency levels that are likely to be available in 2005. Estimated efficiencies were arrived at by this method for screw and centrifugal chillers using HCFC-22, HFC-134a, HCFC-123 (centrifugal only), and ammonia (screw compressor only). For the alternative refrigerants HFC-245fa and R-410A, no current product based efficiency estimate is available. To arrive at an estimate of the efficiency level, the theoretical (ideal cycle) efficiencies of HFC-245fa and HCFC-123 are compared and the theoretical efficiencies of HCFC-22 and R-410A are compared. Table 7-2 summarizes the ideal cycle COPs for these refrigerants, based on the REFPROP thermodynamic property subroutines. For comparison, the ideal COPs calculated by ORNL in the TEWI-3 Study for HCFC-22, HCFC-123, HFC-134a, and ammonia are included in this table and agree closely (well within ½%) with the REFPROP based ideal COP values.

Table 7-2: Ideal Cycle COPs for Alternative Refrigerants for Chillers

Comparing theoretical COP values, the COP of HFC-245fa is 2 to 3% less than the COP of HCFC-123. Absent any rationale to the contrary, an HFC-245fa based chiller is assumed to have an IPLV 3% higher than the IPLV of a corresponding HCFC-123 based chiller.

The theoretical cycle COP of R410A is approximately 6% less than the theoretical COP of HCFC-22. As discussed in the previous section, in unitary air conditioners at a roughly equivalent manufacturing cost, the COP with R410A potentially is about 5% higher than with HCFC-22. The difference between theoretical and actual is attributable to the 50% higher pressure and density and the superior heat transport properties of R410A, both of which result in increased refrigerant side heat transfer coefficients. The higher density and pressure of R410A also allows the use of smaller diameter tubing (saving cost, which can then be applied to incremental efficiency improvements) and larger pressure drops (providing increased velocity, increasing the heat transfer coefficient). It isn't clear to what extent these potential performance advantages of R410A in unitary equipment translate into performance advantages for large chillers. For the purposes of this analysis, it is assumed that large chiller efficiencies with R410A are equal to large chiller efficiencies with HCFC-22.

A portion of the large chillers sold are absorption chillers, either steam powered or direct fired. For the purposes of this study, a direct (natural gas) fired, double-effect lithium bromide-water (LiBr-water) chiller is assumed. The assumed seasonal average COP for both 350 ton and 1,000 ton chillers is:

• 1.07 for 1996 technology, based on the HHV of the natural gas consumed to fire to the chiller
• 1.15 for 2005 technology, based on the HHV of the natural gas consumed to fire the chiller

The focus of this section is larger tonnage chillers. Energy consumption and LCCP are compared for centrifugal, screw, and absorption. The annual cooling loads and energy vs. IPLV calculated by the TEWI-3 study are used in this material. The baseline assumptions for energy efficiency used in TEWI-3 has been updated based on discussions with manufacturers and are summarized in Table 7-3.

Table 7-3: Assumed Chiller Energy Efficiency (IPLV) Levels Best Currently Available (for U.S. Applications)

The integrated part load value (IPLV) values in Table 7-3 were based on inputs provided by chiller manufacturers. IPLV values were used to best represent the seasonal performance. Large chillers are offered in a range of efficiency levels. Higher efficiency levels are obtained by using larger heat exchangers in relationship to the capacity, bringing the evaporating and condensing temperatures closer to the leaving water temperature. In many applications, the increased cost of selecting and specifying a chiller at the upper end of the range of available efficiencies is quickly repaid by the resulting energy savings. As a result, many chillers are specified at, or close to, the upper end of the range of available efficiencies. The IPLVs chosen in Table 7-3 represent the best, or nearly the best, efficiency levels available in current (1999) equipment. The market demand for high efficiency large chillers has been sufficient to drive large chiller product offerings close to practical efficiency limits, although further, incremental improvement can be expected in the future. For refrigerants other than those in Table 7-3, the IPLV was estimated based on the ratio of theoretical COPs. In the TEWI-3 study, energy and TEWI were estimated for application to an office building in Atlanta, under the following basic assumptions:

• 2125 annual operating hours
• 30 year equipment life
• 0.65 kg CO₂ /kWhe (power plant CO₂ emissions)
• Condenser side parasitics - water pump and cooling tower fan power consumption - are accounted for, as described in appendix F of the TEWI-3 Report (for both electric and gas absorption chillers).

Annual energy, including the condenser side parasitics, and the corresponding indirect warming effect is plotted vs. IPLV for this Atlanta office application, in Figure 7-1.

Figure 7-1: Annual Energy Use and Lifetime Indirect warming for a Prototypical Office Building in Atlanta, as a Function of IPLV

The annual energy use for the commercially relevant chiller technologies in the hypothetical Atlanta office building is summarized in Table 7-4 for a 350 ton chiller and in Table 7-5 for a 1000 ton chiller, both at current (1999) efficiency levels as discussed above.

Table 7-4: Annual Energy Use of 1200 kW (350 ton) Chiller in Atlanta, Current (1999) Efficiency Levels

Table 7-5: Annual Energy Use of 3500 kW (1000 ton) Chiller in Atlanta, Current (1999) Efficiency Levels

The annual energy consumption levels in Figures 7-4 and 7-5 actually fall within a reasonably narrow range. They are representative of a warmer than average climate, requiring a large number of full-load-equivalent operating hours. The site energy consumption of the double-effect absorption chiller is not directly comparable to the site the energy of the electrically powered chiller. On a primary energy basis, the absorption chiller consumes approximately 60% more energy than the electric chillers.

The preceding analysis assumes, in effect, that the chiller plant consists of one chiller, whose output modulates between zero and full load (the basis for the IPLV). In many installations - one manufacturer estimates 85% of all new installations, currently - the chiller plant consists of multiple chillers, so that a much higher proportion of the operating time of any given chiller is at high - 70% to 100% of full load - capacity levels, and at higher efficiency. While a detailed analysis is beyond the scope of this study, for many, if not most, applications, energy consumption levels potentially are lower than those indicated in Tables 7-4 and 7-5.

The LCCP for the chiller alternatives in the prototypical, Atlanta office building, was calculated by combining the indirect warming due to energy consumption with the direct warming. The direct warming was calculated on the basis of adjusted GWP values to account for the embodied energy and fugitive emissions (as presented in Appendix A). The indirect warming impact is taken from Figure 7-1 at the appropriate IPLV value. The direct warming impact depends specifically on the GWP and embodied energy and fugitive emissions impact of the refrigerant and the lifetime charge loss. The refrigerant charge size is summarized in Table 7-6 and is based on inputs provided by large chiller manufacturers. Charge losses assumed in TEWI-3 are summarized in Table 7-6 and are based on industry input collected by ARI. The GWP values are summarized in Table 7-7.

Table 7-6: Refrigerant Charge and Annual Losses

Table 7-7: GWP and Manufacturing Warming

Based on industry input via ARI, a modern large chiller will on average lose 0.5% of its charge annually to leakage and servicing. This represents a considerable improvement over designs from 10 or more years previously. For this analysis, an average loss of 1.0% annually has been assumed to account for some end of life charge loss and accidental losses in the field. Table 7-8 summarizes the LCCP for the hypothetical 350-ton chiller in an Atlanta office building, at the upper end of currently (in 1999) available efficiency levels. The character of the comparison among technical alternatives is similar for 1000 tons capacity.

Table 7-8: LCCP for 1200 kW (350 ton) Chiller in Atlanta Office Building, Current (1999) Efficiency Level

For all of the alternatives in Table 7-8, the major portion of the LCCP is the indirect warming associated with the energy consumption, with direct warming due to refrigerant emissions only amounting to between 0.2 and 3percent of the total LCCP. In other climates, the annual cooling can be more (e.g., Miami) or less than Atlanta, and the direct/indirect portions of the LCCP will vary accordingly. Because a significant portion of the cooling load of a large building is due to internal loads (lights, office equipment, elevator machinery, people), the cooling load and the corresponding direct impact vary less with climate than is the case with smaller buildings.

In the TEWI-3 Study, TEWIs were calculated for annual charge loss rates up to 4% per year (four times the level assumed here). Even with this high (for current technology, practices, and regulation) loss level, the direct warming is less than 10% of the LCCP.

The LCCP of direct-fired, double-effect LiBr-Water absorption is about 70% higher than the average LCCP for the vapor compression cycle chillers. However, in practice a large portion of these machines are operated to meet peak loads only, as a means of reducing electric demand charges, and, as a result, operate for considerably fewer equivalent full load hours per year.

The LCCP values of the vapor compression alternatives fall within a reasonably narrow range and do not provide a compelling reason to favor one alternative over another.

Large chillers are most commonly located in mechanical equipment rooms, within the building they are air conditioning. If a hazardous refrigerant is used, e.g., ammonia, the equipment room must meet additional requirements typically including minimum ventilation airflows and vapor concentration monitoring.

In many urban code jurisdictions, the use of ammonia as a refrigerant is prohibited outright. For large chillers, the refrigerant charge is too large to allow hydrocarbon refrigerants in chillers located in a mechanical equipment room.

[ << Previous | Contents | Next >> ]