FIELD TEST OF HYBRID ROOFTOP UNIT PHASE 1

Transcription

1 Design & Engineering Services FIELD TEST OF HYBRID ROOFTOP UNIT PHASE 1 Report Prepared by: Design & Engineering Services Customer Service Business Unit Southern California Edison December 2012

2 Acknowledgements Southern California Edison s Design & Engineering Services (DES) group is responsible for this project. It was developed as part of Southern California Edison s HVAC Technologies and System Diagnostics Advocacy (HTSDA) Program under internal project number. Jay Madden, P.E., conducted this technology evaluation with overall guidance and management from Jerine Ahmed. For more information on this project, contact jay.madden@sce.com. Disclaimer This report was prepared by Southern California Edison (SCE) and funded by California utility customers under the auspices of the California Public Utilities Commission. Reproduction or distribution of the whole or any part of the contents of this document without the express written permission of SCE is prohibited. This work was performed with reasonable care and in accordance with professional standards. However, neither SCE nor any entity performing the work pursuant to SCE s authority make any warranty or representation, expressed or implied, with regard to this report, the merchantability or fitness for a particular purpose of the results of the work, or any analyses, or conclusions contained in this report. The results reflected in the work are generally representative of operating conditions; however, the results in any other situation may vary depending upon particular operating conditions. Southern California Edison Page ii

3 Executive Summary According to the California Energy Commission s 2003 Commercial End-Use Survey [1], packaged air conditioning (AC) units cool 65% of commercial spaces and account for a large amount of peak electrical demand loads during hot weather. These units can be retrofitted with systems that lower electrical energy usage and demand by evaporating water to cool air passing over the condenser coils. The purpose of this field assessment is to evaluate the performance of hybrid evaporative cooling technology on package air conditioning units serving a big-box retail store. Electrical consumption, electrical demand, water consumption, and maintenance issues associated with water evaporation are evaluated. Hybrid evaporative-cooling systems are retro-fitted onto 13 rooftop packaged air conditioning units serving the sales floor of a big-box retail store in Palmdale, California. These systems are designed to pre-cool both condenser air and incoming outside air. The systems were operated from September 1 to September 28, 2012 while cooling capacity, compressor run-time, electrical consumption, and water consumption were measured. These measurements were taken at outdoor air temperatures ranging from 80 to 100 F. The original air conditioning systems are also operated, from September 28 to October 26, 2012, and the same performance measurements were recorded. Monitoring system problems prevent measurement of AC unit electrical and water consumption. Phase 2 of this study addresses these monitoring issues. The following performance improvements are observed in Phase 1, when evaporative pre-cooling is applied: Overall store electric meter load reduces 3%, but this value is not statistically significant, given the many factors that affect this load. Refrigerant saturated condensing temperature drops 20%. Condensing pressure drops 25%. After four weeks of operation, dissolved solids accumulate on the evaporative media surfaces of three of the systems, and algae grows in the basins of four of the systems. The operational and monitoring system issues on this project will be corrected and further monitoring will be conducted during the summer of This added testing will provide more detailed AC unit electrical and water consumption data. The longer term water treatment issues will also be observed. We recommend developing an equest computer model of electrical savings provided by this measure, serving different commercial building types in various climate zones. This model would be calibrated with actual measured results of this field assessment. The results of this study suggest that SCE s EE program adopt this technology, but as mentioned earlier, continued monitoring at this site will provide a more definitive recommendation. This study will occur during the summer of Southern California Edison Page iii

4 Acronyms AC ASHRAE Btu Btu/hr CFM COP DB DX EE EMCS HTSDA HVAC lb OSA RA RTU SA SCE SCT SST TxV WB Air Conditioning American Society of, heating, Refrigeration, and Air Conditioning Engineers British Thermal Unit British Thermal Unit/hour Cubic Feet per Minute Coefficient of Performance Dry Bulb Direct expansion Energy Efficiency Energy Monitoring and Control System HVAC Technologies and System Diagnostics Advocacy Heating, Ventilating, and Air Conditioning pound Outside Air Return Air Rooftop Unit Supply Air Southern California Edison Saturated Condensing Temperature Saturated Suction Temperature Thermostatic Expansion Valve Wet Bulb Southern California Edison Page iv

5 Contents EXECUTIVE SUMMARY III INTRODUCTION 1 Evaporative Condensing... 1 Hybrid Evaporative Cooling Technology... 1 Some systems being introduced on the market combine condenser air evaporative pre-cooling with OSA sensible pre-cooling. This technology circulates cooled water through a finned coil, located in the OSA intake stream. OSA cools while rejecting heat into the water. The warmed water then drips through the condenser precooling media and evaporates.background... 1 Evaporative Pre-Cooling Condenser Air... 2 Dual-Cooling Technology... 6 Market Barriers... 7 ASSESSMENT OBJECTIVES 8 TECHNOLOGY/PRODUCT EVALUATION 9 TECHNICAL APPROACH/TEST METHODOLOGY 11 Field Testing of Technology Test Plan Instrumentation Plan RESULTS 15 Data Analysis Condenser Air Pre-cooling Overall Energy Impact Water Usage Water Treatment DISCUSSION 20 System Installation and Operation Energy Savings Water Treatment RECOMMENDATIONS 23 CONCLUSIONS 24 Southern California Edison Page v

6 APPENDIX A SAMPLE REFRIGERATION CALCULATIONS 25 APPENDIX B MONITORING EQUIPMENT 30 REFERENCES 33 Figures Figure 1. Direct Expansion Refrigerant Cycle... 2 Figure 2. Air-Cooled Condenser... 3 Figure 3. Psychrometric Chart Air-Cooled Condenser, Ontario, CA 1% Design Dry-Bulb Temperature... 4 Figure 4. Evaporative Pre-Cooling Condenser Air... 5 Figure 5. Psychrometric Chart Evaporative Condenser, Ontario, CA 1% Design Dry-Bulb Temperature... 6 Figure 6. Dual-Cooling Technology... 7 Figure 7. Assessment Site RTU Schedule... 9 Figure 8. Assessment Site Roof Plan Figure 9. Lennox AC Unit Condenser Configuration Figure 10. Monitoring Points Figure 11. Scale Formation on Evaporative Media Figure 12. Algae Growth in Sump Figure 13. Pressure-Enthalpy Chart for R-22 Refrigerant Tables Table 1. Refrigerant Liquid Temperature, AC-7 and AC Table 2. Refrigerant Compressor Lift, AC Table 3. Cooling Stages, Baseline, 3 Compressor AC Unit Table 4. Cooling Stages, Evaporative Pre-Cooling, 3 Compressor AC Unit Table 5. Cooling Stages, Baseline, 4 Compressor AC Unit Table 6. Cooling Stages, Evaporative Pre-Cooling, 4 Compressor AC Unit Table 7. Dual-Cooling System Pump Power Consumption Table 8. Site Overall Energy Demand, Using Dry Bulb Temperature Table 9. Monitoring Equipment Southern California Edison Page vi

7 Equations Equation 1. Evaporator Heat Rejection Equation 2. Evaporator Heat Rejection, R-22 Refrigerant, 40 F SST, 120 F SCT Equation 3.Evaporator Heat Rejection, R-22 Refrigerant, 40 F SST, 100 F SCT Equation 4. Cooling Capacity Increase, R-22 Refrigerant, 120 F to 100 F SCT Equation 5. Compressor Work Equation 6. Compressor Work, R-22 Refrigerant, 40 F SST, 120 F SCT Equation 7. Compressor Work, R-22 Refrigerant, 40 F SST, 100 F SCT Equation 8. Cooling Work Reduction, R-22 Refrigerant, 120 F to 100 F SCT Equation 9. Refrigeration Cycle Coefficient of Performance Equation 10. COP, R-22 Refrigerant, 40 F SST, 120 F SCT Equation 11. COP, R-22 Refrigerant, 40 F SST, 100 F SCT Equation 12. COP Increase, R-22 Refrigerant, 120 F to 100 F SCT Southern California Edison Page vii

8 Introduction According to the California Energy Commission s 2003 Commercial End-Use Survey [1], approximately 65% of commercial floor area is conditioned by packaged air conditioning (AC) units. These units consist of supply fans, Direct expansion (DX) cooling systems, heating, and air filters. These systems are inexpensive to install and are prevalent in schools, smaller office buildings, retail buildings, and other light commercial applications. Because of prevalence and energy performance, commercial AC units are a large part of the electrical demand when outdoor temperatures are high. EVAPORATIVE CONDENSING DX systems provide cooling by rejecting heat from the indoor conditioned space to the outdoor air. The greater the temperature difference between the rooftop unit s (RTU s) supply air (SA) and outdoor air (OSA) temperatures, the more mechanical energy is required to remove the heat from the conditioned space. By contrast, air conditioning systems serving larger commercial applications take advantage of the dry climate of the western United States by evaporating water to reject heat into the atmosphere. This evaporative process reduces both energy consumption and peak electrical demand by lowering the DX system s condensing temperature. Systems are being introduced to the market that bring the advantages of this evaporative cooling process to the packaged AC system. These systems are designed to be retrofitted onto existing RTUs. They operate by evaporating water over a media in the condenser air stream, cooling the incoming condenser air. For this field test, we study electrical consumption, peak electrical demand, water consumption, AC system performance, and maintenance issues associated with evaporating water. HYBRID EVAPORATIVE COOLING TECHNOLOGY Some systems being introduced on the market combine condenser air evaporative precooling with OSA sensible pre-cooling. This technology circulates cooled water through a finned coil located in the OSA intake stream. OSA cools while rejecting heat into the water. The warmed water then drips through the condenser pre-cooling media and evaporates. Southern California Edison Page 1

9 BACKGROUND EVAPORATIVE PRE-COOLING CONDENSER AIR Commercial AC and refrigeration systems use mechanical energy to move heat from a lower temperature space to a higher temperature heat sink. This heat sink is typically the outdoor environment. The refrigerant DX system works as follows: 1. The low-pressure refrigerant boils in the evaporator, which removes heat from the airstream. This colder air is then delivered to the conditioned space. 2. The compressor, driven by an electric motor or other means, raises the pressure of the refrigerant gas. 3. In the condenser, heat is transferred from the refrigerant gas to the heat sink. This heat sink can be the outdoor air, a water stream, or evaporating water. 4. The thermostatic expansion valve (TxV) reduces the pressure of the refrigerant liquid and repeats the cycle. Figure 1 shows the DX refrigerant cycle. Condenser Heat 3 Outside Air Compressor TxV 4 2 Work Evaporator 1 Inside Air Heat FIGURE 1. DIRECT EXPANSION REFRIGERANT CYCLE Southern California Edison Page 2

10 The amount of energy required by the compressor in step 2 depends upon the temperature at which the refrigerant gas condenses. In RTUs and air-cooled chillers, condensers reject heat from the refrigerant directly into the outside air stream. In these systems, higher outside air temperatures result in higher energy usage by the compressor. Evaporative cooling takes advantage of the outside air s ability to absorb moisture and the heat of vaporization. As water evaporates, heat is absorbed from the surrounding air, refrigerant or the remaining water stream. Cooling towers, evaporative condensers, and closed-circuit fluid coolers use this process. Evaporative condensers operate at a lower temperature than air-cooled condensers, which lowers the energy required by the compressor. Figure 2 shows a typical air-cooled condenser for an AC unit. In this example, we used 1% American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) design cooling conditions for Ontario, California (98 DB, 70 WB). Air is drawn through a condenser coil, where heat from the refrigerant is rejected into the airstream. The condenser fan then discharges this heated air into the atmosphere. CONDENSER COIL CONDENSER FAN 98 F OUTSIDE AIR DISCHARGE FIGURE 2. AIR-COOLED CONDENSER Southern California Edison Page 3

11 DRY BULB TEMPERATURE - F HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR Field Test of Hybrid Rooftop Unit Figure 3 shows the psychrometric process where air is sensibly heated in the condenser. R ASHRAE PSYCHROMETRIC CHART NO.1 NORMAL TEMPERATURE BAROMETRIC PRESSURE: INCHES OF MERCURY Copyright 1992 AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC SEA LEVEL SENSIBLE HEAT TOTAL HEAT R WET BULB TEMPERATURE - F Qs Qt ENTHALPY HUMIDITY RATIO h W ENTHALPY - BTU PER POUND OF DRY AIR 30 SATURATION TEMPERATURE - F 60 90% VOLUME- CU.FT. PER LB. DRY AIR % 60 OSA, Ontario, CA.010 Condenser Discharge 40 70% % % % 30% % 10% RELATIVE HUMIDITY ENTHALPY - BTU PER POUND OF DRY AIR FIGURE 3. PSYCHROMETRIC CHART AIR-COOLED CONDENSER, ONTARIO, CA 1% DESIGN DRY-BULB TEMPERATURE Southern California Edison Page 4

12 Figure 4 shows the process when an evaporative pre-cooler is added to the condenser as follows: Water is introduced to the evaporative media, through which the incoming condenser air flows. The air comes in contact with the media, evaporating some of the water. Sensible heat is removed from this airstream, lowering its dry bulb temperature and carrying the evaporated water away in the airstream. The outdoor air temperature is lowered by 20 F before entering the condenser. SPRAY NOZZLES CONDENSER COIL CONDENSER FAN 98 F OUTSIDE AIR EVAPORATIVE MEDIA PRE-COOLED AIR DISCHARGE 78 F SPRAY PUMP WATER SUMP FIGURE 4. EVAPORATIVE PRE-COOLING CONDENSER AIR Southern California Edison Page 5

13 DRY BULB TEMPERATURE - F HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR Field Test of Hybrid Rooftop Unit Figure 5 is a psychrometric chart that shows the evaporative cooling process, followed by the heat rejection at the condenser. ASHRAE PSYCHROMETRIC CHART NO.1 R NORMAL TEMPERATURE BAROMETRIC PRESSURE: INCHES OF MERCURY Copyright 1992 AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC. SEA LEVEL SENSIBLE HEAT Qs TOTAL HEAT Qt R WET BULB TEMPERATURE - F ENTHALPY HUMIDITY RATIO h W ENTHALPY - BTU PER POUND OF DRY AIR 30 SATURATION TEMPERATURE - F 60 90% Pre-Cool Discharge 14.0 VOLUME- CU.FT. PER LB. DRY AIR Condenser Disch % 60 OSA, Ontario, CA % % % % 30% % 10% RELATIVE HUMIDITY ENTHALPY - BTU PER POUND OF DRY AIR FIGURE 5. PSYCHROMETRIC CHART EVAPORATIVE CONDENSER, ONTARIO, CA 1% DESIGN DRY-BULB TEMPERATURE Lowering the condensing temperature of an AC system s refrigerant affects efficiency in two ways. The lower condensing pressure corresponding to a lower temperature reduces the work done by the compressor. Additionally, a lower condensing pressure allows a greater proportion of the refrigerant to condense. Appendix A provides detailed sample calculations of a refrigeration cycle using R-22, which demonstrates a 39% improvement in COP attainable by reducing the condenser air temperature from 100 F to 80 F. DUAL-COOLING TECHNOLOGY The following process describes the dual-cooling technology used in this study: Incoming condenser air passes through wetted media. The resulting evaporative process cools both the incoming condenser air and the remaining water in the media. Southern California Edison Page 6

14 The cooled water drains into a sump, and is then pumped through a finned coil in the AC unit s OSA intake. Sensible heat is rejected from the OSA into the water loop, lowering the OSA temperature. The warmed water is distributed through the evaporative media, and the cycle starts over. Figure 6 shows a simplified diagram of the process using representative temperatures. OUTSIDE AIR PRE-COOLING COIL 98 F OUTSIDE AIR PRE-COOLED OUTSIDE AIR 88 F SPRAY NOZZLES CONDENSER COIL CONDENSER FAN 98 F OUTSIDE AIR EVAPORATIVE MEDIA 80 F PRE-COOLED AIR DISCHARGE SPRAY PUMP WATER SUMP FIGURE 6. DUAL-COOLING TECHNOLOGY MARKET BARRIERS Ongoing maintenance is a potential market barrier because evaporative processes increase the amount of regular RTU monitoring and maintenance. RTUs are usually located on the building roofs so they do not receive regular maintenance. Regular maintenance may include water treatment measures to prevent scale and algae buildup in the equipment, such as bleeding a portion of the system water to maintain the level of dissolved solids low, chemical treatment, or other non-chemical measures. Water usage is also a potential market barrier. In addition to the cost of the water consumed in the evaporation process and the bleed process, there is an electrical penalty associated with delivering water to the site and treating the bleed water in the sewer system. This penalty becomes part of the site s water cost. Southern California Edison Page 7

15 Assessment Objectives The objective of this field assessment is as follows: Assess the feasibility of retrofitting a dual-cooling system on existing air conditioning systems at a big-box retail store. Measure the electrical energy and electric demand savings provided by evaporatively pre-cooling the condenser air intake of an RTU. Measure the AC units increased cooling capacity when dual-cool technology is used. Observe the OSA pre-cooling of the dual-cool technology. Measure the water usage of an evaporatively pre-cooled condenser air system. Measure the effects of operating the dual-cooling technology on the entire sales area of a big box retail store. Observe the water conditions in the dual-cooling system, including but not limited to, scale build-up and algae growth. Southern California Edison Page 8

16 Technology/Product Evaluation In this assessment, dual-cooling systems were installed on 13 existing RTUs serving a big box retail store in Palmdale, California. The RTUs served the sales floor of this building. They were put into service in We chose this site because the local climate provides a wide range of summer outside air temperatures, which allows us to measure and apply results over a wide range of climate zones in SCE territory. The dual-cool system s manufacturer furnished and installed the dual-cooling technology, the water and waste piping systems, associated electrical work, and system commissioning. Western Cooling Efficiency Center of Davis, California furnished, installed, and operated the monitoring system used to measure the performance of the dual-cooling technology and the baseline AC system. Adding an OSA coil to the existing RTUs affects the OSA air flow. An air balance contractor reset the AC units OSA back to their original flows. Figure 7 shows the existing RTUs that are retrofitted with a dual-cooling system. AIR CONDITIONING UNIT SCHEDULE NOMINAL UNIT # MFGR MODEL # CAPACITY (TONS) COMPRESSOR QTY. OUTSIDE AIR (CFM)* RTU-5 LENNOX LGC180H2BL RTU-7 LENNOX LGC180H2BL RTU-8 LENNOX LGC156H2BL RTU-9 LENNOX LGC240H2BL RTU-10 LENNOX LGC240H2BL RTU-11 LENNOX LGC210H2BL RTU-12 LENNOX LGC240H2BL RTU-13 LENNOX LGC210H2BL RTU-14 LENNOX LGC210H2BL RTU-15 LENNOX LGC210H2BL RTU-16 LENNOX LGC210H2BL RTU-17 LENNOX LGC210H2BL ** RTU-18 LENNOX LGC210H2BL FIGURE 7. ASSESSMENT SITE RTU SCHEDULE * Rounded to the nearest 5 CFM. ** OSA damper broken Southern California Edison Page 9

17 Figure 8 shows the locations of the AC units on the roof. FIGURE 8. ASSESSMENT SITE ROOF PLAN Southern California Edison Page 10

18 CONDENSER AIR PRE-COOLER Field Test of Hybrid Rooftop Unit Technical Approach/Test Methodology FIELD TESTING OF TECHNOLOGY Ten RTUs, RTU-9 through RTU-19, serve the main sales floor, and two additional units. RTU-7 and RTU-8, serve the check-out area. The grocery area is served by RTU-23, which is excluded from this assessment. Each RTU is an air-cooled packaged AC unit, with a constant speed supply fan, DX cooling coil, gas-fired furnace, filter section, and OSA economizer. The DX cooling system contains three to four independent refrigerant circuits. Each air conditioning unit discharged supply air into the store through one four-way ceiling diffuser and returned air from one ceiling register. AC unit cooling and heating is controlled by thermostats mounted on columns in the sales area. The store operating hours are 8:00 AM to 10:00 PM, Monday through Saturday, and 8:00 AM to 9:00 PM on Sunday. AC unit on and off scheduling is provided by the facility s Energy Monitoring and Control System (EMCS). The existing AC units are Lennox model #LGC with two cooling stages. When stage one cooling is requested, refrigerant circuits 1 and 2 are energized. Second stage cooling energizes the remaining one or two refrigerant circuits, depending upon unit size. The AC unit condensers are constructed in a V-configuration. As a result, a condenser air pre-cooler could not be configured to serve condenser coils 3 and 4. These two coils are used on a call for second stage cooling and receive uncooled incoming air. Figure 9 illustrates the condenser configuration of a typical Lennox AC unit serving the project site and the pre-cooler side panel. CONDENSER FANS OSA PRE-COOLING COIL CONDENSER CIRCUIT #2 CONDENSER CIRCUIT #4 RTU CABINET AIR FLOW, COOLING STAGE #1 PRE-COOLER SIDE PANEL (BOTH SIDES) CONDENSER CIRCUIT #1 CONDENSER CIRCUIT #3 AIR FLOW, COOLING STAGE #2 (BOTH SIDES) FIGURE 9. LENNOX AC UNIT CONDENSER CONFIGURATION A one-row pre-cooling coil is installed on the OSA intakes of each tested AC unit. This coil increases the pressure drop across the OSA intake, so the OSA damper minimum position is reset to provide the original designed CFM. Southern California Edison Page 11

19 The dual-cooler has a circulating pump, which is controlled to circulate water through the OSA pre-cooling coil and the evaporative media whenever the OSA temperature is above the 75 F set point (adjustable). The pump operates even if the AC unit is off or if there is no call for cooling. A float valve, located in the pre-cooler sump, opens when the water level decreases. A drain valve was manually adjusted to bleed a set amount of water from the system whenever the circulating pump is running. TEST PLAN The field test evaluates baseline operation and dual-cooling technology as follows: Comparing electrical energy, refrigerant liquid temperature, refrigerant compressor pressure lift, and water usage of units RTU-7 and RTU-8. Compressor pressure lift is the difference between the suction and discharge pressures of the refrigerant compressors; the higher the lift, the harder the compressor works. Comparing the electrical energy of units RTU-10 and RTU-11. Observing the electrical consumption of the entire site. For both baseline and technology performance, the AC systems were monitored during the store s operating hours, when OSA temperature is between 80 F and 100 F. Air conditioning unit system energy usage is compared for the following OSA dry bulb temperature bins: 80º 85º F 85º 90º F 90º 95º F 95º 100º F For each test, the following measurements were recorded: OSA temperature, dry bulb and wet bulb System voltage System kw and kva Compressor kw, cooling stages 1 and 2 Condenser fan kw Return air temperature, dry bulb and wet bulb Supply air temperature, dry bulb and wet bulb Pre-cooler circulating pump kw Pre-cooler water temperature, inlet Pre-cooler water temperature, outlet Make-up water consumption* Refrigerant suction pressures and temperatures* Refrigerant compressor pressures and temperatures* Refrigerant condensing temperature* * - RTU-7 and RTU-8 Southern California Edison Page 12

20 Figure 10 shows the general location of the points monitored for each air conditioning unit. OSA temperatures were measured at one point, for all air conditioning units. Discharge Air Dry Bulb Temp Discharge Air Wet Bulb Temp Supply Air Dry Bulb Temp Supply Duct Supply Air Wet Bulb Temp Supply Fan PDISCH1 PDISCH2 PDISCH3 PDISCH4 TDISCH1 TDISCH2 TDISCH3 TDISCH4 #1 Condenser #2 Fans #3 #4 Condenser Fans kw EVAPORATIVE PRE-COOLER Return Air Dry Bulb Temp Return Duct OSA Return Air Wet Bulb Temp OSA PRE- COOLER Evaporator Coil PSUCTION1 PSUCTION2 PSUCTION3 PSUCTION4 TSUCTION1 TSUCTION2 TSUCTION3 TSUCTION4 kw Compressor 1 Compressor 2 kw Compressor 3 Compressor 4 TCOND1 TCOND3 TCOND2 TCOND4 CIRCULATING PUMP kw F OSA Dry Bulb Temp OSA Wet Bulb Temp Make-up Water Gallons Water Temp In Water Temp Out kwsystem kvasystem Unit Voltage FIGURE 10. MONITORING POINTS Southern California Edison Page 13

21 The effects of the evaporative pre-cooling system on the RTU cooling performance were measured in the following ways: Saturated condensing temperature. Pressure lift or the difference in suction and discharge pressure of the compressors. Overall RTU power, in kw. Compressor run-time, 1st and 2nd stages of cooling. Compressor amperage, 1st and 2nd stages of cooling. Dual-cooling water usage is determined by metering the main water supply to all dualcooling units, as well as separate sub-meters to four individual units, for the duration of the test. The electrical distribution of the test site did not allow our monitoring equipment to measure the electrical usage of all of the AC units, so the overall effect on the store of the dual-cooling system was observed by reviewing 15 minute building meter data. Water conditions, including algae growth and water scale formation, are visually observed and recorded during the test. INSTRUMENTATION PLAN Appendix B provides the list of monitored points, sensors, and sensor accuracy. Data is measured and recorded once per minute. A control module is installed in each RTU to read the monitored points and record data. These data are uploaded to WCEC s project engineer. Southern California Edison Page 14

22 Results SCE operated and monitored the dual-cooling technology on the assessment site from September 1 to September 28, During this period, outside air dry bulb temperatures reached 100 F. On the morning of September 28, 2012, the dual-cooling system was disabled as follows: Turned off the circulating pumps. Turned off the water supplies. Drained the water sumps. Removed the evaporative media. This step was necessary to prevent an airflow restriction that results in lower condenser air flow and/or higher condenser fan energy compared to baseline conditions. The baseline AC system is operated and monitored from September 28 through October 26, During this period, outside air dry bulb temperatures also reached 100 F. DATA ANALYSIS CONDENSER AIR PRE-COOLING The amount of condenser air pre-cooling provided by the technology is indirectly measured by measuring the DX system SCT and condensing pressure. Table 1 summarizes the observed and measured Saturated Condensing Temperature (SCT) of the RTU-7 and RTU-8 refrigerant circuits. TABLE 1. REFRIGERANT LIQUID TEMPERATURE, AC-7 AND AC-8 AC-7 AC-8 OUTLET TEMP. OUTLET TEMP. INCOMING OSA TEMPERATURE ( F) ( F) ( F) BASELINE MEASURE TEMP BASELINE MEASURE TEMP 80 F - 85 F F - 90 F F - 95 F F F Several observations are made from the data collected as follows: The second stage of cooling for AC-7 and AC-8 never energized. The first stage of cooling of AC-8 did not energize for more than a few hours during the baseline test. Monitoring of AC-8 was interrupted on October 14, SCE calculated compressor lift for the active refrigeration systems by subtracting the Southern California Edison Page 15

23 measured discharge pressure from the measured suction pressure. Table 2 summarizes the compressor lifts for baseline and post measure operation. TABLE 2. REFRIGERANT COMPRESSOR LIFT, AC-7 COMPRESSOR LIFT (PSI) OSA TEMPERATURE BASELINE MEASURE % DIFF. 80 F - 85 F F - 90 F F - 95 F F F For a constant cooling load, increased compressor capacity is reflected by reduced compressor run times. Table 3 and Table 4 show the compressor run times for the three compressor AC units, before and after evaporative cooling. TABLE 3. COOLING STAGES, BASELINE, 3 COMPRESSOR AC UNIT COOLING STAGES RUN TIME, % AC-7 AC-8 OSA TEMPERATURE, DRY BULB NO COOLING STAGE #1 STAGE #2 NO COOLING STAGE #1 STAGE # F F F F Southern California Edison Page 16

24 TABLE 4. COOLING STAGES, EVAPORATIVE PRE-COOLING, 3 COMPRESSOR AC UNIT COOLING STAGES RUN TIME, % AC-7 AC-8 OSA DRY BULB TEMPERATURE NO COOLING STAGE #1 STAGE #2 NO COOLING STAGE #1 STAGE # F F F F Table 5 and Table 6 show the compressor run times for the four compressor AC units before and after evaporative cooling. TABLE 5. COOLING STAGES, BASELINE, 4 COMPRESSOR AC UNIT COOLING STAGES RUN TIME, % AC-10 AC-11 OSA DRY BULB TEMPERATURE NO COOLING STAGE #1 STAGE #2 NO COOLING STAGE #1 STAGE # F F F F TABLE 6. COOLING STAGES, EVAPORATIVE PRE-COOLING, 4 COMPRESSOR AC UNIT COOLING STAGES RUN TIME, % AC-10 AC-11 OSA DRY BULB TEMPERATURE NO COOLING STAGE #1 STAGE #2 NO COOLING STAGE #1 STAGE # F F F F Southern California Edison Page 17

25 The dual-cooling system s circulating pumps added an electrical load to the system. In the application tested, the pumps ran continuously whenever the outdoor air temperature exceeded 75 F, regardless of the call for cooling or if the RTU was operating. Each pump consumed 1 amp 115 volts, based on monitored data. These results are shown in Table 7. TABLE 7. DUAL-COOLING SYSTEM PUMP POWER CONSUMPTION AC UNIT PUMP CURRENT (AMPS) PUMP POWER (KW) PUMP OPERATING HOURS AC % AC % AC % AC % We are not able to compare RTU energy usage in kw for baseline and measure conditions because of errors in data output. We are investigating these errors. OVERALL ENERGY IMPACT SCE completed a general review of 15-minute electrical utility data. We compared the baseline air-cooled operation to dual-cooled operation at OSA dry bulb temperatures ranging from 80 to 100 F. Table 8 summarizes the overall performance, based on dry bulb temperature. The evaporative systems serving RTU-15 and RTU-18 did not operate during the test period. If these systems were operating correctly, overall building energy usage may have been less. TABLE 8. SITE OVERALL ENERGY DEMAND, USING DRY BULB TEMPERATURE SITE ELECTRIC METER DEMAND (KW) OSA DRY BULB TEMPERATURE ( F) BASELINE DUAL-COOL DECREASE (%) WATER USAGE SCE placed water meters on the main water supply to all of the dual cool units. Water meters are also placed on the individual water supplies to RTU-7, RTU-8, RTU-10, and RTU-11. These meters send a signal to the on-site monitoring system and also have a physical counter. We did not monitor water consumption because of problems with the monitoring system. Additionally, the installation team did not record the initial values on the flow meters counters, so we were unable to use this method to measure water use. Southern California Edison Page 18

26 WATER TREATMENT After one month of dual cooling operation, SCE removed the evaporative media and visually observed scale formation and algae growth. The evaporative media surfaces show minor amounts of precipitated solids on ¼ of the RTUs, but no precipitates on evaporative media surfaces of the remaining units. Figure 11 shows the extent of scale formation on the outside face and the top of the evaporative media. FIGURE 11. SCALE FORMATION ON EVAPORATIVE MEDIA Algae growth is observed in the sump of the dual-cooling system. Figure 12 indicates the level of growth after one month of operation without water treatment. These observations were shared with the dual-cool system s manufacturer. FIGURE 12. ALGAE GROWTH IN SUMP Southern California Edison Page 19

27 Discussion Observations are made regarding the ease of installation of the evaporative pre-cooling systems, their operation, measured performance improvements, and water treatment. SYSTEM INSTALLATION AND OPERATION The evaporative pre-cooling system was installed on the 13 RTUs at the project site in 4 days, using a qualified mechanical contractor. The contractor and manufacturer surveyed the site for a day prior to this installation. This work included the installation of an entire water distribution system on the roof to serve these units and drainage connections to the existing condensate piping. There was one problem during the installation process. Due to confusion between sub-contractors, the evaporative precooling systems serving RTU-15 and RTU-18 were not placed in operation. The remaining systems operated during the test period with no problems. SCE interviewed the facility manager several times, and he did not report any complaints. We did notice that three units leaked a small amount of water onto the roof. These leaks were reported to the manufacturer for repair. A constant amount of water was bled from each system, to maintain the total dissolved solids level of the system water below a point where solids would precipitate onto the evaporative media. This bleed discharged into the AC unit s condensate drain. The condensate drains for all of the RTUs drained into a common piping system, which discharged indirectly into a mop sink at the rear of the building. Because the weather in Palmdale, CA is arid, the amount of condensate draining into the sink is normally minimal. However, the addition of the bleeds from each evaporative system created a constant flow of water into the mop sink. The mop sink was able to accommodate this flow, even when debris from other sources partially blocked the sink s strainer. The circulating pumps for the evaporative systems operated continuously when the outside air temperature was above 75 F. The pumps did not have capability to shut off when the RTUs were not operating. The evaporative pre-cooling system and the water piping system serving it are vulnerable to freezing conditions. For example, outside air temperature sensors recorded sub-zero temperatures for an eight hour period on the morning of November 12, For this reason, SCE decommissioned the system before the threat of exposure to sub-zero temperatures. SCE drained the water from the systems sumps, piping, and the exposed water piping on the roof. ENERGY SAVINGS The energy savings are as follows: Refrigeration saturated condensing temperatures are reduced by 20% when evaporative pre-cooling is applied to RTU-7 and RTU-8, at outdoor dry bulb temperatures from Smaller SCT reductions are measured on these OSA dry bulb temperatures between DX system pressure lift is reduced by 25% in RTU-7, when evaporative pre-cooling is applied. The RTU-8 DX system did not operate long enough during baseline monitoring to provide results for comparison. The evaporative pre-cooling system did not reduce compressor run-time for the Southern California Edison Page 20

28 four RTUs monitored in this test. The measured run-time in cooling stage 1 for RTU-7, 8, 10, and 11 are almost unchanged from baseline to measure. The cooling stage 1 for RTU-7 and RTU-11 operated almost continuously, while the cooling stages of adjacent units RTU-8 and RTU-10 did not energize often. RTU-7 and RTU-8 serve almost identical cooling loads in the front check-out area of the store. RTU-8 is closer to the entry doors and theoretically subject to a higher cooling load. RTU-10 and RTU-11 served similar areas of the sales floor and are expected to have similar cooling loads. Errors in the data output do not allow us to compare the overall RTU energy usage, between measure and baseline. The system tested in this study included an OSA pre-cooling coil, which was designed to lower the sensible cooling load imposed upon the RTUs. In this facility, the OSA cooling load is a large proportion of the overall cooling load. Other components of the cooling load are people, lighting, wall and roof loads. The retail store has minimal glass area and is built to 2008 energy codes, so the exterior wall and roof do not contribute much to the overall cooling load. Separately testing the effects of the evaporative pre-cooling and the OSA sensible precooling is difficult because it s not possible to operate the condenser air pre-cooler without the OSA pre-cooler, unless bypass piping and valves are added to the system. The OSA pre-cooling does not affect the SCT or the discharge pressure of the refrigerant system, so these two variables can be studied to isolate the effects of the condenser air pre-cooler. A study of overall energy usage derived from applying evaporative pre-cooling to all RTUs serving the sales floor shows a 3% savings. This result is affected by two factors as follows: Space cooling represents approximately 10% of the overall electrical consumption in a big box retail store that has supermarket refrigeration. While other major components of the electrical consumption, like lighting, television displays, and instore product cooler remain fairly constant, there is enough variance in overall load to question whether the observed 3% reduction is statistically significant. The evaporative pre-coolers of RTU-15 and RTU-18, did not operate correctly. If they had been operating correctly, overall energy savings may have been higher. WATER TREATMENT Packaged air-cooled RTUs require a minimal amount of service to provide satisfactory space temperature control. These systems may operate inefficiently for long time periods because they are not serviced until the occupants complain that the space is too hot or cold. HVAC and refrigeration systems that use evaporative cooling have a higher level of continuous maintenance. For example, water contains dissolved solids, which remain after a portion of the water is evaporated. The concentration of these solids increases and, if not addressed, reaches a level where they cannot remain dissolved. At that point, the solids precipitate out as scale on solid surfaces. After four weeks of operation, we saw solids on the evaporative media of four of the systems tested on this site. Figure 11 shows an example of scale formation on the evaporative media. Southern California Edison Page 21

29 To prevent scale formation, we used a constant water bleed to allow a drain valve to release a constant flow of water from the system sump. This water is replaced by fresh incoming water, which dilutes the concentration of solids. The proportion of bleed water required to prevent scaling depends on the concentration of dissolved solids in the incoming water. In larger evaporative systems, like cooling towers, a conductivity meter is used to measure the level of dissolved solids in water. This meter automatically opens a bleed valve when the solids level reaches setpoint. That type of system provides better control of water loss and dissolved solids level than a constant bleed. In this study, we observed scaling on three of the 11 units even though each unit received the water of the same quality and operated under the same conditions. It is possible that the three units had a smaller bleed rate than the others. Biological growth forms in water basins. For evaporative systems, the basin water is normally 80 to 100 F, which provides a hospitable environment for biological growth. The average sump water temperature of the operating pre-cooling system is 72 F, which is lower than what is typically observed in open cooling tower basins. We also observed algae growth in the basins of two operating systems, with the largest concentration in non-operating RTU-15. It is possible that RTU-15 provided a better environment for algae growth because the evaporative system did not work, which allowed the basin water to heat to a higher temperature. Biological growth in the water of evaporative systems is controlled by several methods, including injecting biocides into the water or imposing an electrical charge into the water stream. The system we studied did not provide a way to control biological growth. Southern California Edison Page 22

30 Recommendations The results of this study suggest that SCE s EE program adopt this technology, but we recommend that SCE monitor the test site during hot summer months to provide a conclusive recommendation. Recommended Phase 2 work includes the following: Address the issues in the monitoring system, collect data through summer 2013 and finalize the study. Improve the measurement of water consumption, RTU electrical energy consumption, and compressor electrical consumption to provide definitive results. Calculate unit EER with dual-cooling technology. Review the results of other lab testing and field assessments of this technology, performed by both SCE and PG&E as follows: SCEs recently completed laboratory test of a new Trane RTU. PG&E reports of field assessments performed in the summer of SCE s installation of two new Trane RTUs in a shopping mall office in Ontario, CA. Develop an equest computer model of electrical savings provided by evaporatively pre-cooling condenser air for RTUs, serving different commercial building types in various climate zones. Calibrate this model based on actual measured results obtained from this field assessment. Study reliable, inexpensive water treatment systems for the proposed evaporative technology. Air-cooled RTUs are popular choices for commercial HVAC systems because they require less continual maintenance. In the last few years, commercial RTU manufacturers have introduced larger (60 ton capacity) RTUs into the market, which have evaporative condensers instead of air-cooled condensers. These products feature a water treatment system that controls biological growth and scale formation. This technology can be considered for use with evaporative pre-coolers. Study the effectiveness of OSA sensible pre-coolers. The challenge is providing a sensing method for measuring the leaving air temperature from this pre-cooling coil. This is a challenge because the return air and OSA paths of an RTU typically mix soon after the OSA intake damper. This makes it difficult to isolate the OSA temperature downstream of a pre-cooling coil for measurement. Provide feedback to manufacturer regarding system controls, water treatment, and winterizing. Southern California Edison Page 23

31 Conclusions Adding evaporative pre-coolers to the condenser air stream of RTUs provides a significant 20% to 25% reduction in DX system saturated condensing pressure and temperature. This reduction occurs when the OSA temperatures are 80 to 100 F, which is when peak cooling loads and energy consumption occur. Operating this system on most of the RTUs that serve a large retail facility provide a measurable electrical energy savings. Based on the results of this study, we should consider evaporative pre-cooling for air-cooled RTUs in California. Installing this technology is relatively simple, but sites should consider the following operational issues: Increased maintenance costs for water treatment The ability of the existing facility s cold water system to serve the added equipment. A suitable location to drain bleed water The requirement to winterize the evaporative system to prevent freeze damage, in some locations Controls that prevent pump operation and water bleed when the RTU is not in service Difficulty with the monitoring system and the data produced by it prevented an analysis of the following aspects of this technology: Water consumption. Actual electrical energy savings for individual RTUs Electrical energy reduction of the DX compressors Sensible cooling provided by the OSA pre-cooling coil Corrections to the monitoring system used in this project and a longer test period will provide better data analysis for this technology. Southern California Edison Page 24

32 Appendix A Sample Refrigeration Calculations Lowering the condensing temperature of an AC system s refrigerant affects efficiency as follows: The lower condensing pressure corresponds to a lower temperature which reduces the work done by the compressor The lower condensing pressure allows a greater proportion of the refrigerant to condense. Figure 13 diagrams the cycle for a theoretical R-22 refrigeration system. The red lines indicate the SCT = 120 F, and the blue lines indicate the SCT = 100 F. This diagram graphically represents the increase in cooling capacity and decrease in compressor work associated with reducing the system s condensing temperature. The higher SCT corresponds to an air-cooled AC unit 100 F OSA, while the lower SCT corresponds to an AC unit with pre-cooling lowering entering condenser air temperature to 80 F. FIGURE 13. PRESSURE-ENTHALPY CHART FOR R-22 REFRIGERANT Southern California Edison Page 25

33 Equation 1 calculates the heat rejected in the process of evaporating refrigerant in an ideal refrigeration cycle. EQUATION 1. EVAPORATOR HEAT REJECTION Where: = change in enthalpy, evaporator, = Enthalpy, leaving refrigerant = Enthalpy, entering refrigerant Equation 2 applies the formula in Equation 1 to a system with 40 F SST and 120 F SCT. EQUATION 2. EVAPORATOR HEAT REJECTION, R-22 REFRIGERANT, 40 F SST, 120 F SCT Equation 3 applies the formula in Equation 1 to a system with to 40 F SST and 100 F SCT. EQUATION 3.EVAPORATOR HEAT REJECTION, R-22 REFRIGERANT, 40 F SST, 100 F SCT Southern California Edison Page 26

34 Equation 4 calculates the increase in cooling capacity for a theoretical R-22 system, when decreasing the SCT from 120 F to 100 F. EQUATION 4. COOLING CAPACITY INCREASE, R-22 REFRIGERANT, 120 F TO 100 F SCT Equation 5 calculates the work performed by the compressor in an ideal refrigeration cycle. EQUATION 5. COMPRESSOR WORK Where: = change in enthalpy, compressor, = Enthalpy, leaving compressor = Enthalpy, entering compressor Equation 6 applies the formula in Equation 5 to a system with 40 F SST and 120 F SCT. EQUATION 6. COMPRESSOR WORK, R-22 REFRIGERANT, 40 F SST, 120 F SCT Southern California Edison Page 27

35 Equation 7 applies the formula in Equation 5 to a system with 40 F SST and 100 F SCT. EQUATION 7. COMPRESSOR WORK, R-22 REFRIGERANT, 40 F SST, 100 F SCT Equation 8 calculates the cooling work reduction, R-22 refrigerant, 120 F to 100 F SCT. EQUATION 8. COOLING WORK REDUCTION, R-22 REFRIGERANT, 120 F TO 100 F SCT The coefficient of performance (COP) of a refrigeration system is the ratio of cooling provided to work input. Equation 9 calculates the COP for a theoretical refrigeration system. EQUATION 9. REFRIGERATION CYCLE COEFFICIENT OF PERFORMANCE Southern California Edison Page 28

36 Equation 10 calculates the COP for an R-22 refrigeration 40 F SST and 120 F SCT. EQUATION 10. COP, R-22 REFRIGERANT, 40 F SST, 120 F SCT Equation 11 calculates the COP for the system in Equation 10 with 100 F SCT, and the increase in performance. EQUATION 11. COP, R-22 REFRIGERANT, 40 F SST, 100 F SCT Equation 12 calculates the COP improvement when reducing SCT from 120 F to 100 F. EQUATION 12. COP INCREASE, R-22 REFRIGERANT, 120 F TO 100 F SCT Southern California Edison Page 29

37 Appendix B Monitoring Equipment Table 9 provides information about the monitoring equipment used in this study. TABLE 9. MONITORING EQUIPMENT SENSOR TYPE MAKE/MODEL ACCURACY RTUS T OSA Vaisala HUMICAP HMP110 ±0.36 F 7,8,20 RH OSA Vaisala HUMICAP HMP110 ±1.7% RH 7,8,20 T RA Vaisala HUMICAP HMP110 ±0.36 F 7,8,10,11,20,21 RH RA Vaisala HUMICAP HMP110 ±1.7% RH 7,8,10,11,20,21 T SA Vaisala HUMICAP HMP110 ±0.36 F 7,8,10,11,20,21 RH SA Vaisala HUMICAP HMP110 ±1.7% RH 7,8,10,11,20,21 P SA Dwyer WC = 4-20 ma 7,8,10,11,20,21 P OSA Dwyer WC = 4-20 ma 7,8,10,11 WATER OMEGA FTB 4105 A P ±2% 7,8,10,11 MAIN OMEGA FTB8010B PR ±2% 20 OSA Position RA/OSA Damper Actuator 0 10 Vdc NC 7,8,10,11,20,21 ±0.06%CT C1 100 Ω SA1-RT-B ±0.06% 7,8,20 CT C2 100 Ω SA1-RT-B ±0.06% 7,8,20 Southern California Edison Page 30

38 SENSOR TYPE MAKE/MODEL ACCURACY RTUS CT C3 100 Ω SA1-RT-B ±0.06% 7,8,20 CT C4 100 Ω SA1-RT-B ±0.06% NA T LOW, C1 100 Ω SA1-RT-B ±0.06% 7,8,20 T LOW, C2 100 Ω SA1-RT-B ±0.06% 7,8,20 T LOW, C3 100 Ω SA1-RT-B ±0.06% 7,8,20 T CD OUT 3 W 100 Ω SA1-RT-B ±0.06% 8.20 T HI, C1 100 Ω SA1-RT-B ±0.06% 7,8,20 T HI, C2 100 Ω SA1-RT-B ±0.06% 7,8,20 T HI, C3 100 Ω SA1-RT-B ±0.06% 7,8,20 T CD OUT 3 WO 100 Ω SA1-RT-B ±0.06% 8,20 P LOW, C1 ClimaCheck , 10bar ±1% 7,8,20 P LOW, C2 ClimaCheck , 10bar ±1% 7,8,20 P LOW, C3 ClimaCheck , 10bar ±1% 7,8,20 P LOW, C4 ClimaCheck , 10bar ±1% NA P HI, C1 ClimaCheck , 35bar ±1% 7,8,20 P HI, C2 ClimaCheck , 35bar ±1% 7,8,20 P HI, C3 ClimaCheck , 35bar ±1% 7,8,20 Southern California Edison Page 31