Evaporative-Cooled vs. Air-Cooled Chillers: Kirtland AFB Case Study

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Gabriella Willis
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1 Evaporative-Cooled vs. Air-Cooled Chillers: Kirtland AFB Case Study

2 DISCUSSION TOPICS Brief Overview of Air-Cooled and Evaporative-Cooled Condenser Technologies Pros and Cons Scope and Results of Kirtland AFB Study Conclusions and Recommendations Q&A 2

3 AIR-COOLED CONDENSER BASICS Refrigerant hot gas is cooled and condensed in Air-to-Refrigerant Heat Exchanger (coil). Air-Cooled Condenser efficiency depends on ambient air dry bulb temperature, i.e. the higher OAT is, the more power is required to compress hot gas in order to condense it into liquid. 3

EVAPORATIVE-COOLED CONDENSER BASICS Treated water is sprayed over condenser coil. Part of water is evaporated thus lowering surface temperature of the condenser and the air drawn across it.

4 EVAPORATIVE-COOLED CONDENSER BASICS Treated water is sprayed over condenser coil. Part of water is evaporated thus lowering surface temperature of the condenser and the air drawn across it. Remaining water is collected in drain pan and recirculated back to sprayer. Make-up city water is added to replace evaporated water. Refrigeration system efficiency is increased by 25-50% compared to air-cooled due to lower vapor compression pressure, thus reducing compressor s energy. 4

5 AIR-COOLED AND EVAPORATIVE-COOLED CONDENSER COMPARISON PROS OF AIR-COOLED vs. EVAPORATIVE-COOLED: Smaller cabinet Lighter weight No condenser water piping and treatment Lower equipment cost Lower maintenance cost No water consumption for condenser operation PROS OF EVAPORATIVE-COOLED vs. AIR-COOLED: 20-40% lower electrical consumption of the chiller Quieter operation 5

6 KIRTLAND AFB ENERGY STUDY OBJECTIVE: Increase the energy efficiency of existing facilities on various Air Force Bases in order to reduce the building annual operating cost. STUDY SCOPE: Evaluate life cycle costs of replacement of existing Air-Cooled Chillers with Evaporative-Cooled Chillers at seven (7) buildings. Life Cycle Cost Analysis compared costs associated with Evaporative-Cooled Chillers against Status Quo (air-cooled chillers), and included: Capital Cost Estimate (material and labor) at +/-30% accuracy. Maintenance Cost analysis. Operating Cost Analysis (electricity and water). 6

7 KIRTLAND AFB ENERGY STUDY RESULTS CAPITAL COST COMPARISON FOR EVAPORATIVE AND AIR- COOLED CHILLERS BUILDING NO. CHILLER TONNAGE EVAPORATIVE-COOLED CHILLERS AIR-COOLED CHILLERS $178,000 $74, $178,000 $74, $204,000 $84, $262,000 $113, $279,000 $130, $389,000 $176, $471,500 $235,000 7

8 KIRTLAND AFB ENERGY STUDY RESULTS ANNUAL OPERATING COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS CHILLER TONS ELECTRICITY KWh x 1,000 / COST (chiller only) AIR-COOLED EVAP- COOLED CITY WATER AND SEWER (chiller only) AIR- COOLED EVAP- COOLED MAINTENANCE (chiller only) AIR- COOLED EVAP- COOLED / $3, / $2,240 $0 $323 $1,600 $2, / $3, / $2,240 $0 $323 $1,600 $2, / $4, / $2,400 $0 $353 $1,600 $2, / $2, / $1,280 $0 $196 $2,400 $3, / $5, / $3,120 $0 $463 $2,400 $3, / $8, / $4,480 $0 $665 $2,400 $3, / $16, / $8,640 $0 $1,260 $2,400 $3,200 8

9 KIRTLAND AFB ENERGY STUDY RESULTS 10- YEAR LIFE CYCLE COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS BLDG NO. CHILLER TONNAGE LCC OF EVAPORATIVE- COOLED CHILLERS LCC OF AIR- COOLED CHILLERS COST RATIO OF AIR-COOLED / EVAP-COOLED CHILLERS $301,000 $201,000 67% $301,000 $201,000 67% $350,000 $233,000 62% $462,000 $293,000 64% $513,000 $335,000 65% $757,000 $557,000 74% $1,386,000 $1,191,000 86% 9

10 KIRTLAND AFB ENERGY STUDY RESULTS 10- YEAR LIFE CYCLE COST COMPARISON FOR EVAPORATIVE AND AIR-COOLED CHILLERS 10

11 CONCLUSIONS EVAPORATIVE-COOLED CHILLERS MAY NOT BE THE BEST OPTION, BECAUSE: Mid-range ( tons) Air-Cooled Chillers may cost less than Half of Evaporative-Cooled Chillers, which require: Special corrosion resistant coatings on condenser coils. Water spraying equipment. Larger cabinet size to accommodate water spraying equipment. Water treatment equipment, water and sewer piping. Air-Cooled Chillers are more available and its pricing is more competitive than Evaporative-Cooled Chillers.. Evaporative-Cooled Chillers require a larger foundation, water and drain piping, and water treatment equipment. 11

12 CONCLUSIONS (continued) EVAPORATIVE-COOLED CHILLERS MAY NOT BE THE BEST OPTION, BECAUSE: Evaporative-Cooled Chillers consume a significant amount of Potable Make-Up Water, which is in short supply and is costly in high-desert dry and arid areas. Additional Maintenance Cost to treat Make-Up Water and clean condenser coils, which partially offsets energy savings. Energy Consumption of mid-range chillers is only 10-15% of the Total building energy consumption; therefore the 25-40% Energy Savings for Evaporative-Cooled Chillers amount to only 5-7% of the Total Energy bill reduction 12

13 RECOMMENDATIONS Review and Evaluate All Costs related to installation and operation of Evaporative-Cooled Chillers before making a decision. Capital Energy Water Maintenance 13

14 QUESTIONS AND ANSWERS THANK YOU! Corry Freeman, CEM Staff Mechanical Engineer AMEC 14

15 QUESTIONS AND ANSWERS BACK-UP SLIDES 15

16 BRIEF HISTORY OF EVAPORATIVE COOLING Records of Evaporation Cooling use for human comfort trace back to 2,500 BC. In 16 th Century, Leonardo Da Vinci is known to be first to design a Mechanical Evaporative Cooler a hollow wheel moving through a water bath. In early 20 th Century, Willis Carrier developed a psychrometric chart similar to ones in use today along with the development of a formula that linked the transformation of sensible heat into latent heat during the adiabatic (no external heat input or output) saturation of air. 16

17 TYPES OF EVAPORATION COOLING EQUIPMENT Direct Evaporative Coolers Simply draw air through a moistened wick material and delivers the cooled (but more humid) air to the space to be conditioned. Supply air temperature is limited by outside air wet-bulb temperature. Indirect Evaporative Coolers Operate similarly to a direct evaporative cooler but uses a heat exchanger. Stream of outdoor air passes through the heat exchanger, gets cooled by the heat exchanger cold surfaces, and then is delivered to the space. The benefit of this is that the cooled air does not pick up any humidity as it does in the direct evaporative process. The drawback is the delivered air does not get as cool as in a direct evaporative process because it is limited by the ambient wet bulb temperature and the heat exchanger efficiency. Mechanical refrigeration Uses Vapor Compression Cycle consisting of Compression, Condensing, Expansion, and Evaporation stages. Water is sprayed on condenser coil to increase refrigerant condensing rate, thus saving AC system energy. 17

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