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WSU-TFREC/Postharvest Information Network/Reducing Energy Costs in CA Storage



Reducing Energy Costs in CA Storage


Introduction

Dedication

This work is dedicated to Professor Robert M. Smock, who in 1938 published the results of his research on evaporator fan cycling to save energy. We are grateful for his constant support and encouragement in this project and in all fruit storage programs. Dr. Smock suffered a fatal heart attack on April 22,1986, as he walked across the Cornell campus to his office in the Pomology Department.

Introduction

The concept of reducing cold storage energy use through evaporator fan cycling is not new. In 1938 R.M. Smock, a Pomologist at Cornell University in New York, wrote of his work, "Certainly no differences were indicated in this study which would justify the extra power cost of continuous blower operation." Smock's study indicated that fan operating time could be reduced by 45% with no detrimental effect on fruit quality and condition. Thirty plus years have elapsed, and now the merits of this original research are being rediscovered and applied to modern CA storage in New York.


Storage Technology

Fruit storage technology has changed tremendously since the first cold storages were built in New York State. The handling, cooling, and storage milestones are highlighted in Table 1.

Table 1. Commercial Loading and Cooling Rates for Apples.

Year
Loading Period
Cooling Time
Room Atmosphere
Handling Method
1924
2 to 3 weeks
1 week
Air
Barrels
1938
2 weeks
1 week
Air
Bu. Box
1945
1 week
3 to 4 days
CA
Bu. Box
1965
1 week
2 to 3 days
CA
20 Bu. Box
1986
5 days
overnight
Rapid CA
20 Bu. Box

The industry began to use on-farm refrigeration for apples stored in barrels around 1924. Professor Smock's study involved refrigerated air storage of apples in bushel boxes in 1938. Around 1945, commercial CA storage was begun, and by 1965, the importance of faster cooling was recognized and 20 bushel pallet bins were in widespread use. Currently, commercial operations are striving for overnight cooling and rapid CA.

This technology change has required increased cooling capacity for pull down, which is reflected in larger evaporators and bigger air handling systems. The very first refrigerated rooms relied upon gravity refrigeration and contained no supplemental air circulating equipment. By 1938, forced air blower units delivered 18 air changes per hour and added approximately 108 watts of heat per 1,000 bushels of stored fruit. Current centrifugal blower units have a 28 air-change-per-hour capacity. Direct throw propeller fans on modern evaporators deliver 90 air changes per hour. The present hardware associated with both types of systems adds approximately 375 watts of heat per 1,000 bushels of fruit. The heat added by the fan motors is now 2 to 3 times greater than the heat of respiration of the fruit (130 to 180 W/1,000 bushels).

The energy budget for a modern New York State CA storage is shown in Figure 1. This data is for a 120,000 bushel plant equipped with flooded ammonia refrigeration. The daily electrical requirements for compressors and evaporator fans are shown along with the total for the refrigeration system. The "measured total" includes the relatively small amount of electricity used by water pumps and condenser fans--a quantity which averages approximately 6% of compressor use. The "metered total" is the daily average electrical use calculated from monthly power company bills. Hot water heating, office space heat, and the electricity used by CA burners and scrubbers are included in the "metered total."

Evaporator fan energy is the single largest item of the cold storage electrical budget. From October to mid-April, 750 kWh are used per day for continuous fan operation. Daily compressor and condenser requirements average approximately 500 kWh and 30 kWh, respectively, from mid-November to mid-April. Obviously, if fan cycling were used during the fruit holding period, substantial energy savings could be realized. The potential savings for fan cycling in this system are indicated in Table 2.

Table 2. Daily Energy Use and Potential Energy Savings Through Fan Cycling in 120,000 Bushel CA

Item
Fans On
Continuously
Fans Cycled
50% On, 50% Off
Fans Cycled
25% On, 75% Off
Fans
750 kWh
375 kWh
188 kWh
Compressor
500 kWh
375 kWh
313 kWh
Condensor
30 kWh
23 kWh
19 kWh
 
Total use
(% Savings)
1280 kWh
(0%)
773 kWh
(40%)
520 kWh
(59%)

The data in Table 2 indicate that a total energy savings of 40% is possible if evaporator fans could be turned off half of the time. Total savings approach 60% when fans are off 16 hours out of 24. Based on early research by Smock and later reports by Yost (1984) half-time reduction in fan operation is feasible and causes no detrimental effects on stored fruit quality or condition.


Air Handler Efficiency

During storage trials, we investigated the efficiency of high velocity direct throw air handlers presently used in CA storages. (Efficiency is defined as the quantity of air returning through pallet runner openings compared with the total quantity of air delivered from the evaporator discharge). When efficiency measurements were made in a 30,000 bushel CA room, 70% of the discharge air never passed through the stacks; it simply short-circuited over the top of the bins and back to the evaporator. When air flow was reduced 50% by turning off half the fans, the return air flow decreased only 22%. Since the measured return flow coming out of the stacks was still uniform over the entire room, we concluded that half of the evaporator fans could be safely shut down after field heat was removed and the CA room was sealed.


Control Strategies

The energy crisis of the 1970s prompted eastern growers to begin using evaporator fan cycling to save energy. The first storage operators to employ fan cycling simply turned their refrigeration systems off at night and back on again in the morning during the winter. Research studies (Blanpied 1979, Yost 1980) indicated that temperature variations on the order of + 1 °F from the set point would be expected when systems were turned off for 12 to 14 hours. We found these variations to be no greater than those experienced when refrigeration systems operated continuously. Since fruit quality and condition after CA storage in the cycled rooms was equivalent to that in rooms operated continuously, the cycling practice became widely accepted.

Our fruit storage industry currently employs several control strategies for fan cycling. Manual control is still used by some of the smaller growers. Time clock control is used, and a few new systems employ programmable load controllers to sequence fan and refrigeration operations. Presently, however, the most popular technique is to cycle fans and refrigeration with a solid-state thermostat. This thermostat, accurate to ± 0.25°F, can be set to control only temperature while fans run continuously during loading and pull down. Later the thermostat is switched to control fans and refrigeration together. Remote temperature sensors are installed in the CA rooms where the thermostat is used to control fan cycling.

Initially we were skeptical of using a single thermostat sensor to control all of the fans in the CA room. Our recommendation still calls for remote temperature sensors in all rooms whether fans are cycled or not. We encourage growers to turn at least some of the fans on for 30 minutes each 12 hours during very cold weather. We have recorded fan cycles of up to 36 hours off and 1 hour on when the thermostat alone was used to cycle the fans with the refrigeration.

After 3 years of experience we are not aware of any fruit condition problems resulting from the fan cycling methods described above. Some freezing damage has occurred in open, partially empty CA rooms, because insufficient respiration heat was available. Our biggest concern is that compressor capacity in older plants now far exceeds the load developed in the CA rooms when fans are cycled. During extended cold periods, machines may sit idle for 24 hours or more, and provisions must be made to keep the compressor room warm. Heat reclaim from these compressors is no longer possible during the storage season when the fans are cycled.


Temperature, RH, and Atmosphere Variations

We find that the temperature controller, not the fan cycling practice, is the cause of major temperature variations in the CA rooms. We have documented temperature variations in excess ±2°F in rooms with continuously operated fans controlled by mechanical thermostats. Temperature variations in cycled rooms controlled by the solid state thermostats average ±1°F or less.

Few data exist for relative humidity variations. Some very limited data from new storage facilities indicate cycled rooms yield less defrost water than identical, continuously operated rooms. In theory, this should be the case, but we do not have sufficient data to confirm it for commercial systems.

We are equally short of data on atmosphere concentration variations. In one study of 1 % oxygen storage in a 30,000 bushel CA room, no variation in O2 levels could be detected after the fans were off for 12 hours. This determination was made by inserting sampling lines into 6 stack locations during loading and monitoring the oxygen level with an electronic analyzer.


Energy Savings

Eastern storage operators are sold on fan cycling to save energy and dollars. We have documented the monthly savings by two of our commercial cooperators and present this data in Figures 2 and 3. The "base year" is the 1982-83 storage season, when no fan cycling was used. Conversion to fan cycling was completed in 1984-85; total energy use was reduced nearly 50% in the process. The need for good storage management with fan cycling is indicated in Figure 3, where higher than expected use occurred in 1585-86. This was due to a temporary change in refrigeration plant management from October through February in this storage. The new manager was not "comfortable" with fan cycling practices of the previous year and operated the evaporator fans for a longer period of time each day during that season.

The fan cycling practice holds great promise for energy cost savings. Energy rates in the Northeast currently average 11 cents per kWh and electric costs are a significant part of the total storage budget. Our research has shown that fan cycling results in a 60% savings in energy in commercial CA storage. The simple payback on the fan cycling thermostat controller and remote temperature sensing equipment is currently 4 to 5 months. The value of energy saved is equivalent to 16¢ per bushel of storage capacity. We estimate that the potential value of energy savings due to fan cycling is 1 million dollars annually for our CA industry in New York State.


References

Bartsch, James A. 1982. Consumption and Loss of Energy in Commercial Storage Units. Proceedings of 3d National CA Conference. Timber Press. Beaverton, Oregon.

Blanpied, G. David. 1979. Effect of Blower Operation Upon Temperatures in the CA Room. Unpublished Data.

Smock, Robert M., and S.R. Shapley. 1938. Blower Operation in Farm Cold Storage. Refrigeration Engineering Vol. 36.

Yost, G.E. 1980. Temperature Data from CA Rooms Operated with Intermittent Fan Cycles. Unpublished Data.

Yost, G.E. 1984. Energy Saving Through the Use of Fan and Refrigeration Cycling in Apple Cold Storage. Transactions of the ASAE:497-501.


J.A. Bartsch, Agricultural Engineer, Associate Professor of Agricultural Engineering

Cornell University, Ithaca, NY 14853

Post Harvest Pomology Newsletter, Vol. 4, No. 2
July-August 1986

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