GLOBAL WARMING IMPACT OF LOW GWP CHILLER REFRIGERANTS

Transcription

1 ID: 123 GLOBAL WARMING IMPACT OF LOW GWP CHILLER REFRIGERANTS KONTOMARIS K. DuPont Fluorochemicals R&D, P.O. Box 80711, Wilmington, Delaware, , USA ABSTRACT This paper compares the global warming impacts of large tonnage chillers for three low GWP refrigerant candidates based on Hydro-Fluoro-Olefins and the incumbent refrigerants they intend to replace. HFO- 1234yf and XP10 are potential replacements for HFC-134a. Developmental refrigerant DR-2 is a potential replacement for HCFC-123. Four representative scenarios were examined defined according to the levels of electricity carbon intensity and refrigerant emissions. Under all scenarios, DR-2 would enable the lowest warming impact and electricity consumption among refrigerants not subject to phase-out. When a midpressure refrigerant is required to meet chiller size restrictions, HFO-1234yf and XP10 would enable warming impact reductions relative to HFC-134a, except in the case of simultaneously high electricity carbon intensity and low refrigerant emissions. HFO-1234yf would enable modestly or significantly lower warming impacts per chiller than XP10. XP10, however, could be adopted earlier and more widely than HFO-1234yf because XP10 could replace HFC-134a in existing chillers without extensive equipment and no safety code modifications. Moreover, XP10 consumes less electricity than HFO-1234yf to deliver a target cooling rate. Case studies are presented where the lowest warming impact is not necessarily achieved with the refrigerant with the lowest GWP, but one having a more advantageous combination of GWP and energy efficiency. 1. INTRODUCTION Air conditioning of large commercial and institutional buildings is commonly provided through electrically driven centrifugal chillers. It consumes a substantial fraction of the electrical energy globally and contributes significantly to global greenhouse gas (GHG) emissions. Contributions to GHG emissions consist of both the direct emissions of refrigerant and the emissions of CO 2 and other greenhouse gases (GHGs) from the generation of the electrical power consumed. Yet most attention is focused solely on the Global Warming Potential (GWP) of the refrigerant, a focus that can lead to less than optimum refrigerant choices. Increasing energy costs and emerging climate protection regulations are placing an evermore demanding set of specifications on refrigerants. Low global warming impact has been added to an already long list of desirable refrigerant attributes including high energy efficiency, high volumetric cooling capacity, low temperature glide, low toxicity, low or no flammability, high chemical stability, compatibility with commercially available lubricants and common materials of equipment construction, no ozone depletion potential, acceptable atmospheric breakdown products, acceptable performance in existing equipment with no or little modification and low cost. Clearly, no refrigerant totally fulfills all requirements. Hydro-Fluoro-Olefins (HFOs) have been identified as a new class of compounds that could enable the formulation of refrigerants with GWPs substantially lower than those of incumbent refrigerants. Three HFObased chiller refrigerant candidates, HFO-1234yf, XP10 (slightly reformulated version of developmental refrigerant DR-11) and developmental refrigerant DR-2, have been recently described in the open literature. They have low to moderate GWP values and vary in their thermodynamic properties and resulting chiller energy efficiency. HFO-1234yf and XP10 may prove suitable replacements for HFC-134a in mid-pressure centrifugal chillers (Kontomaris and Leck (2009), Kontomaris et al. (2010a), Kontomaris et al. (2010b)). DR-2 seems suitable for replacing HCFC-123 in low pressure centrifugal chillers (Kontomaris, 2010). The new refrigerants could either replace the incumbent refrigerants in existing equipment or enable new equipment platforms. They have the potential to be more environmentally sustainable future air conditioning options. Table 1 summarizes key thermo-physical, safety, health and environmental properties of the

2 refrigerants of interest. It also lists theoretical coefficient of performance, COP theo, and volumetric cooling capacity, VCC theo, values calculated for a representative ideal chiller cycle from known fluid thermodynamic properties. Table 1. Basic properties of the refrigerants of interest. Property Formula CH 2 F-CF 3 CF 3 CF=CH 2 Azeotrope CHCl 2 CF 3 undisclosed Toxicity Class (1) A A A (expected) B TBD Flammability (1) none 2L none none none ODP none none none none GWP 1,430 4 near ca. 10 T cr [ o C] P cr [MPa] T b [ o C] Glide [ o C] N/A N/A Negligible N/A N/A (2) COP theo VCC (2) theo [kj/m 3 ] 2, , , (1) According to ASHRAE Standard 34 (2) T evap = 4.44 o C (40 o F), T cond = o C (100 o F), T subc = 5.55 o C (10 o F), T suph = 5.55 o C (10 o F), η is = 0.80 and negligible pipe pressured drops. The global warming impact of a cooling application is one of the criteria used to select among competing refrigerant choices. It is often quantified in terms of the Life Cycle Climate Performance (LCCP) or Total Equivalent Warming Impact (TEWI). These metrics depend on several factors including the GWP of the selected refrigerant, the refrigerant emission rate, the energy efficiency with the selected refrigerant, ambient conditions and the primary energy mix used to generate the electricity consumed. The refrigerant-related factors cannot be controlled independently. Their significance depends on the application and is moderated by the remaining factors. The relative effects of the key warming impact factors have not been clearly established for the new HFO-based chiller refrigerants. The objective of this paper was to compare the TEWI values of air-conditioning chillers operating with HFO-1234yf, XP10 or DR-2 and the incumbent refrigerants, HFC-134a and HCFC-123, under realistic scenarios and quantify the relative significance of the key TEWI determinants. Only water-cooled chillers driven by grid electricity were considered. 2. METHODOLOGY FOR ESTIMATING WARMING IMPACT 2.1. Greenhouse Gas Emission Categories The global warming impact of a chiller application is directly related to the amounts of various GHGs emitted to realize the application over its life-time. Such GHG emissions include: EM EQM : EM RFM : EM NRG : EM RFG : EM EOLrf : EM EOLrr : CO 2 and other GHG emissions resulting from the energy use required for the manufacturing, transportation and installation of the chiller equipment Refrigerant, byproduct, CO 2 and other GHG emissions resulting from the manufacturing, transportation and charging of the refrigerant and its feedstock materials as well as the associated energy use CO 2 and other GHG emissions from the use of energy to operate the chiller (e.g. compressors, condenser water pumps, cooling tower fans, etc.) throughout its useful life Refrigerant continuous, regular or intermittent emissions through the chiller operating life from installation completion to just before chiller retirement Refrigerant emissions at the end of chiller life Emissions of GHGs (other than refrigerant) associated with the recovery, reclamation, recycling and/or disposal of equipment and refrigerant at the end of chiller life

3 An emitted amount of a greenhouse gas other than CO 2 (N 2 O, CH 4, various refrigerants, refrigerant manufacturing byproducts, etc.) is converted to an equivalent amount of CO 2 that would generate the same warming impact by multiplying the emitted gas amount by the gas GWP. GHG emissions associated with new equipment manufacturing, EM EQM, could somewhat depend on the refrigerant choice (e.g. mid-pressure versus low pressure refrigerant). However, they would require scarce chiller manufacturing data for their estimation. Moreover, they would only minimally influence the differences in lifetime chiller impact when comparing new chiller options with various refrigerants. They were neglected in this paper. However it should be noted that neglecting EM EQM overlooks the reduction in warming impact resulting from refrigerant candidates that could be used in existing equipment thus averting the impacts associated with equipment replacement. GHG emissions associated with refrigerant manufacturing, EM RFM, cannot be accurately quantified for new refrigerants whose manufacturing processes are not known in some detail. They could be significant in cases where renewable energy is used to operate a chiller and the refrigerant GWP and emission rate during operation are very small. However, they are usually small relative to the other chiller warming impacts and they can be further minimized by reducing the emissions of refrigerant and byproducts (especially those with high GWPs) during manufacturing. Moreover, with a few exceptions (e.g. HCFC-22), they are comparable for refrigerants of the same chemical class. They were neglected. GHG emissions associated with the energy consumed by the application, EM NRG, depend on the primary energy sources used. Usually, a chiller is operated with power from a regional electrical grid. When nuclear or renewable (e.g. hydroelectric, geothermal, solar, wind, biomass) primary sources are used to generate the grid electricity, virtually no GHGs are emitted. When fossil fuels (e.g. coal, natural gas) are used to generate the grid electricity, varying amounts of GHGs (CO 2, N 2 O and CH 4 ) are emitted. In some cases, chillers are operated with off-the grid power sources, in a variety of configurations, resulting in varying degrees of warming impact. For example, a diesel generator could supply the electrical motor of the chiller compressor or an internal combustion engine driven by natural gas could be directly coupled with the chiller compressor. Use of fossil fuels results in GHG emissions. In another example, a turbine driven by locally available waste, solar or geothermal heat through an Organic Rankine Cycle could be used to either directly drive the chiller compressor or to generate and supply electrical power to the chiller compressor motor, condenser water pumps and cooling tower fans. An equitable TEWI analysis would be configuration- and site-specific, as it would depend on factors such as the origin, quality and possible alternative uses of the available heat or the synchronization between heat availability and cooling demand. The more widespread case of grid power use was assumed in this paper. After the completion of the equipment installation, refrigerant can escape into the atmosphere as a result of various causes: continuous (and to some degree practically unavoidable) slow leakage or permeation, equipment defects, regular equipment servicing, unexpected and occasionally catastrophic events and accidental or deliberate refrigerant venting during storage, handling, and transfer. The aggregate amount of refrigerant emitted from the above causes over the equipment operating life is included in EM RFG. Upon equipment retirement at the end of its useful life, both refrigerant losses and energy consumption are incurred. The amount of the one-time refrigerant loss at the end of equipment life, EM EOLrf, is accounted separately. It depends on chiller charge and local practices, regulations and technician training. The amounts of GHGs (other than refrigerant) emitted for the recovery, reclamation, and recycling (or disposal) of equipment and refrigerant at the end of chiller life are included in EM EOLrr. They are normally much smaller than the amounts associated with lifetime operating energy use and refrigerant losses. Moreover, they are not expected to be strongly dependent on the refrigerant choice. They were neglected Total Equivalent Warming Impact (TEWI) The Life Cycle Climate Performance (LCCP) of a cooling application is defined as the total amount of CO 2, in kg, that would produce a global warming impact equivalent to that of all GHGs emitted in the realization of the application over its lifetime ( cradle to grave ). It is a measure commonly used to compare application alternatives, in general, and refrigerant alternatives, in particular. It includes all the emission

4 categories discussed in the previous section. However, the GHG emissions associated with the operating energy consumed, EM NRG, and the refrigerant emissions during, EM RFG, and at the end of equipment life, EM EOLrf, are usually the dominant contributions to global warming resulting from chiller air conditioning applications. These contributions are added to estimate the Total Equivalent Warming Impact (TEWI) of an application, an easier to estimate and almost as informative a metric as LCCP: TEWI = EM NRG + EM RFG + EM EOLrf (1) 2.3. Energy-Related Emissions, EM NRG The amount of equivalent CO 2 emitted per unit of electricity consumed, referred to as Carbon Intensity (CI), is a metric usually used to characterize the warming impact from electricity use. CI varies regionally, seasonally and daily. Average CI values in various countries, in kgco2-eq/kwh, compiled by the World Resource Institute in 2006, show a large variation, from in Iceland to in India or in Bostwana. The CI values in Switzerland ( kgco2-eq/kwh) and China ( kgco2-eq/kwh) were used in this paper as representative of low and high CI levels, respectively. The equivalent emissions, EM NRG, from electricity consumption, E [kwh], to operate the chiller over its lifetime, were estimated as EM NRG [kgco2-eq] = CI x E (2) The electricity consumed over the chiller lifetime was calculated as E [kwh] = P x HRS x N (3) where P [kw] is the average electric power drawn for chiller operation, HRS [hr/yr] is the number of hours of chiller operation per year and N [yrs] is the chiller life. As in the A.D. Little report of (2002), HRS and N in eq. (3) were assumed as 2,125 hrs/yr and 30 yrs, respectively. The power, P, in eq. (3) is used to lift heat from the evaporator to the condenser and to reject it to the ambient. The heat rejected at the condenser consists of the heat extracted at the evaporator and the heat of compression. The refrigerant COP affects the rate of compression work required to deliver a specified cooling rate and, consequently, the rate at which heat must be rejected at the condenser. It is convenient for estimation purposes to separately account for the power required to cool the condenser, P cd : P=P ch +P cd (4) where P ch is the power for uses other than condenser cooling (e.g. fluid compression, fluid circulation, etc.). Chiller energy efficiency and, therefore, power consumption varies with cooling rate. Typically, a chiller operates at varying cooling rates to meet daily and seasonally varying cooling loads with optimum energy efficiency. Moreover, energy efficiency with a given refrigerant varies with chiller capacity and design (e.g. compressor efficiency, cycle variations such as inclusion of a turbine for recovery of expansion energy, heat exchanger design, surface area and approach temperatures, etc.). The value for P ch in eq. (4) should be representative of the field performance of chillers with a given refrigerant over load and efficiency rating profiles representative of the region of interest. Power consumption values for the incumbent refrigerants, HFC-134a and HCFC-123, were based on the nearly best (close to practical efficiency limits) Integrated Part Load Values (in kilowatt per refrigeration ton or kw/rt] for large tonnage (1,000 RT) centrifugal chillers compiled from manufacturers by A. D. Little in 2002: IPLV HFC-134a =0.48 kw/rt and IPLV HCFC-123 =0.40 kw/rt or COP actual_hfc-134a =3.517/IPLV HFC-134a =7.327 and COP actual_hcfc-123 =3.517/IPLV HCFC-123 = In the absence of chiller measurements, actual COPs for the candidate refrigerants were estimated by adjusting the actual COPs for the incumbent refrigerants they intend to replace according to their respective theoretical COPs from Table 1: COP actual_candidate =COP actual_incumbent x (COP theo_candidate /COP theo_incumbent ) (5) The power drawn by a chiller, with either incumbent or candidate refrigerants, was estimated as: P ch [kw] = Q evap [kw] / COP actual (6)

5 where Q evap is the cooling rate in [kw]. In this paper, Q evap = 3,517 kw (1,000RT). The power, P cd, required to run both the condenser water pumps and the cooling tower fans was estimated according to the procedure in Appendix F of the AFEAS study by Oak Ridge National Laboratory (1997): P cd [kw]= x Q evap [kw] x (1+1/COP actual ) (7) 2.4. Refrigerant Emissions Over Operating Life, EM RFG The warming impact of refrigerant loss during the chiller operating life was estimated as: EM RFG = M r x S ANN x N x GWP (8) where M r [kg] is the refrigerant charge and S ANN [%/yr] is the percentage of the charge emitted (and madeup) annually. Representative charge values for the incumbent refrigerants, HFC-134a and HCFC-123, were based on the values for large tonnage (1,000 RT) centrifugal chillers compiled from manufacturers by A. D. Little in 2002 (0.32 and 0.35 kg/kw, respectively). In the absence of actual data, the charge values for the candidate refrigerants were estimated by adjusting the charge values for the incumbent refrigerants they intend to replace according to their respective volumetric cooling capacities, VCC theo [kj/m 3 ] from Table 1: M r_candidate =M r_incumbent x (VCC theo_incumbent /VCC theo_candidate ) (9) Representative values for the annual emission rate, S ANN, can be found in the A.D. Little report of (2002), Calm (2002) and Calm (2006). They have been dramatically reduced over several decades through improved equipment design and practices. A low value of 0.5 %/yr and a high value of 5 %/yr for S ANN were explored in this paper. Possible differences in emission rates between low and mid-pressure refrigerants were not accounted for Refrigerant Emissions at the End of Chiller Life, EM EOLrf The warming impact of refrigerant losses upon chiller decommissioning was estimated as: EM EOLrf = M r x S EOL x GWP (10) where S EOL [%] is the percentage of charge emitted. Representative values for S EOL, can be found in the A.D. Little report of (2002), Calm (2002) and Calm (2006). A low value of 0.5 % and a high value of 5 % for S EOL were explored in this paper. Possible differences in end-of-life loss rates between low and mid-pressure refrigerants were not accounted for. 3. RESULTS Chiller TEWIs for all refrigerants of interest were calculated under four scenarios specified according to the levels of electricity carbon intensity (CI= or kgco2-eq/kwh) and refrigerant emission rates (S ANN =0.5%/yr and S EOL =0.5% or S ANN =5%/yr and S EOL =5%). The results, normalized with the highest TEWI value (for high carbon intensity, high emission rates with HFC-134a), are shown in Figure 1. The TEWI scales in Figure 1 are different for the high and low CI cases. DR-2 could enable a ca % reduction versus HFC-134a in the high chiller TEWI values associated with high electricity carbon intensity. It would also enable the lowest TEWIs at low CI levels among the refrigerants considered. HFO-1234yf or XP10 could be used to reduce TEWI versus HFC-134a except in regions with simultaneously high CI and low refrigerant emissions. HFO-1234yf would enable more significant TEWI reductions per chiller than XP DISCUSSION The results of this paper show that reducing the electricity carbon intensity remains the most effective means for reducing chiller warming impacts even when low GWP refrigerants are considered. Reducing the carbon intensity from levels representative of China to those of Switzerland would reduce warming impact by over 90%. The carbon intensity of electricity varies widely among countries and is being reduced at varying rates.

6 High CI; High Refrigerant Emissions TEWI/TEWI_max RFG NRG 0.00 High CI; Low Refrigerant Emissions TEWI/TEWI_max RFG NRG 0.00 Low CI; High Refrigerant Emissions TEWI/TEWI_max RFG NRG 0.00 Low CI; Low Refrigerant Emissions TEWI/TEWI_max RFG NRG 0.00 Figure 1. Chiller TEWIs for various refrigerants under four scenarios.

7 Although consensus has not been reached, it seems likely that an extensive decarbonization of electricity globally would be costly and gradual (over several decades) and that low carbon intensity electricity would be, generally, more expensive. Clearly, the search for alternative refrigerants for large tonnage electrical chillers must consider the diverse and changing energy landscape within which the search is taking place, as the objectives of reducing warming impact and electricity consumption are closely coupled. In regions where electricity carbon intensity is and will remain high for the foreseeable future, DR-2 would enable the most substantial reductions in chiller TEWI relative to all refrigerants not subject to phase-out considered in this paper. If a mid-pressure refrigerant is required to meet chiller size restrictions, HFO- 1234yf and XP10 would enable TEWI reductions relative to HFC-134a in regions and applications where refrigerant emissions remain unavoidably high. In regions with both high electricity carbon intensity and low refrigerant emissions, HFO-1234yf and XP10 are not predicted to offer chiller TEWI reductions versus HFC-134a. In regions with low electricity carbon intensity, now and more prevalently in the future, reducing emissions of high GWP refrigerants (primarily) and reducing refrigerant GWP (secondarily) become effective in further reducing chiller TEWI. The advantage of DR-2 versus HFO-1234yf in reducing chiller TEWI diminishes compared to the high carbon intensity case. However, the advantage of DR-2 versus HFO- 1234yf in reducing the consumption of increasingly more expensive electricity remains. If a mid-pressure refrigerant is required to meet chiller size restrictions, HFO-1234yf or XP10 would enable TEWI reductions relative to HFC-134a, especially in regions and applications where refrigerant emissions remain unavoidably high. XP10 would enable more modest TEWI reductions per chiller than HFO-1234yf. However, given its nonflammability and predicted performance proximity to HFC-134a, XP10 could replace HFC-134a in existing chillers with minimal equipment and no safety code modifications. Consequently, XP10 could be adopted earlier and more widely than HFO-1234yf and thus enable significant aggregate warming impact reductions, especially in regions or applications with unavoidably high refrigerant emissions, despite its more modest TEWI reduction per chiller. Moreover, XP10 would consume lower amounts of electricity than HFO-1234yf for a given cooling rate. Finally, XP10 substitution for HFC-134a in existing chillers would avert the warming impacts associated with equipment replacement which have been neglected in the present analysis. The results in this paper suggest certain refrigerant selection guidelines regarding the minimization of chiller warming impacts with apparent validity beyond the specific low GWP candidates considered. When the carbon intensity of the electricity used is high, and in view of the relatively low chiller refrigerant emissions, the refrigerant COP (which is correlated with the fluid boiling point or vapor pressure), is the most influential factor in determining chiller warming impacts, more influential than the refrigerant GWP. For example, DR-2 enables a substantially lower TEWI than HFO-1234yf, despite a slightly higher GWP, because of the higher DR-2 COP. Similarly, TEWIs with HFO-1234yf and XP10 are roughly comparable because the TEWI reduction from the slightly higher COP with XP10 largely balances the TEWI increase as a result of the higher GWP of XP10. A similar cancelation of COP and GWP effects applies also to the TEWI comparison of DR-2 versus HCFC-123, as DR-2 has a somewhat lower COP but substantially lower GWP than HCFC-123. In regions with both high electricity carbon intensity and low refrigerant emissions, the TEWI increases due to the lower predicted COPs of HFO-1234yf and XP10 versus HFC-134a outweigh any TEWI reductions expected due to the substantially lower GWPs of the new refrigerants. Clearly, minimization of GWP alone does not necessarily lead to the refrigerant choice with the least TEWI. When the carbon intensity of the electricity used is low, the significance of the refrigerant GWP relative to its COP with respect to reducing chiller warming impacts is elevated, especially when the refrigerant emissions are high. For example, XP10 enables substantial TEWI reductions versus HFC-134a due to its lower GWP despite its lower COP. On the other hand, the TEWI reductions that would be achieved with DR-2 versus HFO-1234yf become small relative to the high carbon intensity case because the effect of the roughly comparable GWPs of the two fluids outweighs their substantially different COPs. Considering the high and increasing electricity cost, however, restores the significance of refrigerant COP as a key selection criterion under both high and low carbon intensity scenarios.

8 Given its significance in determining both warming impacts and electricity costs, the energy efficiency with the candidate refrigerants must be established through chiller tests over representative load profiles and ambient conditions. Such tests would account for the effects of both the thermodynamic and transport properties of the refrigerant on energy efficiency. Some optimization of chiller design and enhancement of heat transfer surfaces may be required to fairly establish the performance of the new fluids. The case studies in this paper did not consider all important performance, safety and cost factors in detail. However, they demonstrate the need for flexibility in climate protection regulations to allow the selection of refrigerants in every region with optimum trade-offs between desirable attributes so as to ensure safe use and minimize environmental impact with minimum cost. 5. REFERENCES Calm JM. 2002, Emissions and Environmental Impacts from Air-conditioning and Refrigeration Systems, Int. J. Refrig. 25: Calm JM. 2006, Comparative Efficiencies and Implications for Greenhouse Gas Emissions of Chiller Refrigerants, Int. J. Refrig. 29(5): Kontomaris K. 2010, A Low GWP Replacement for HCFC-123 in Centrifugal Chillers: DR-2, ASHRAE & UNEP Conference Road to Climate Friendly Chillers, Moving Beyond CFCs and HCFCs, Cairo, Egypt, 30th September-1st October. Kontomaris K, Leck TJ. 2009, Low GWP Refrigerants for Centrifugal Chillers, ASHRAE Annual Conference, Louisville, Kentucky, June Kontomaris K, Leck TJ, Hughes J. 2010a, Low GWP Refrigerants for Air-conditioning of Large Buildings, 10th REHVA World Congress, Sustainable Energy Use in Buildings, Antalya, Turkey, May Kontomaris K, Leck TJ, Hughes J. 2010b, A Non-flammable, Reduced GWP, HFC-134a Replacement in Centrifugal Chillers: DR-11, 13th International Refrigeration and Air Conditioning Conference at Purdue, Purdue University, Lafayette, IN, July Little AD. 2002, Global Comparative Analysis of HFC and Alternative Technologies for Refrigeration, Airconditioning, Foam, Solvent, Aerosol Propellant, and Fire Protection Applications. Sand F, Baxter VD. 1997, Energy and Global Warming Impacts of HFC Refrigerants and Emerging Technologies, Oak Ridge National Laboratory (sponsored by the U.S. Department of Energy and the Alternative Fluorocarbon Environmental Acceptability Study). World Resource Institute. 2006, Climate Analysis Indicators Tool, 6. NOMENCLATURE GWP : Global Warming Potential (integrated over a 100 year time horizon) COP : Coefficient of Performance VCC : Volumetric Cooling Capacity (kj/m 3 ) T evap : Evaporator Temperature : Condenser Temperature T cond T suph T subc η is RT T cr P cr T b IPLV : Vapor Superheat : Liquid Sub-cooling : Compressor isentropic Efficiency : Refrigeration Ton : Critical Temperature : Critical Pressure : Boiling Point Under Atmospheric Pressure : Integrated Part Load Value; used to describe the energy efficiency of a chiller while operating at various capacities (AHRI Standard 550/ ).